Design of organic molecules with large two-photon absorption cross sections

Nonlinear Optical Properties of Bis(dehydrobenzoannuleno)benzenes: An Experimental and Computational Approach

Given their molecular properties and electronic structure, graphyne and graphdiyne are promising materials with numerous applications in different fields of material science. Dehydrobenzoannules (DBAs) are candidates that can serve as building blocks for synthesizing and designing new 2D carbon allotropes; however, only a few graphynes have been produced on a practical scale. Herein, we present our investigation of three DBAs, which serve as a model to understand the relationship between the structure and property, contributing to 2D carbon allotropes' rational design and synthetic effort. We performed entangled and classical two-photon absorption at 790 nm, revealing that minor structural changes within acetylenic units significantly impact the magnitudes of the entangled and classical two-photon cross sections. Later, we deconvolved the excited-state dynamics through femtosecond transient absorption, and the lifetimes on the nanosecond time scale were measured using time-correlated single-photon counting. Finally, electronic structure calculations were performed to compute the oscillator strength and energy associated with electronic transitions between the ground and excited states and among the excited states. The results reveal that the remarkable difference in nonlinear optical properties among the DBAs, despite their structural similarities, stems from the transition dipole moment associated with transitions among excited states.

Synthesis, structure and photophysical properties of truxene and truxene-triindole compounds

Multi-branched π-conjugated organic molecules, due to their fascinating structures and unique optical properties, exhibit potential applications in optoelectronics. Herein, series of 4-N,N-diphenylaminostyryl substituted multi-branched truxene compounds (namely TN1, TN2 and TN3) and a truxene-triindole compound (namely N3T3) were synthesized. These compounds are characterized by effective π-extension and remarkably enhanced two-photon absorption (TPA) compared to their less π-conjugated analogues. For truxene compounds, the more the number of the branch, the large the TPA efficiency, and the C3-symmetric three-branched TN3 exhibit the largest TPA. The aromatic triindole unit proves to be a better two-photon fluorophore compared with truxene. The results conclude that the influence of π-conjugation, molecular planarity and intramolecular charge-transfer (ICT) to TPA is far more pronounced than to linear optical properties.

Excitonic Dark States in Molecular Monolayer Crystals

The tightly bound excitons and strong dipole-dipole interactions in two-dimensional molecular crystals enable rich physics. Among them, superradiance (SR), the spontaneous coherent emission from bright excitons, has sparked considerable interest in quantum-information applications. In addition, optically forbidden states (dark exciton states) have potential to both achieve Bose-Einstein condensation and modulate exciton dynamics. Here, we report a unique series of dark exciton states in highly crystalline organic monolayers (MLs) via two-photon excitation spectroscopy (TP-PLE). These dark exciton states convert to the emissive, delocalized exciton states that undergo room temperature SR. Using a vibronic exciton model, we show that these dark exciton states are mixed character states of Frenkel exciton (FE) and charge transfer exciton (CTE) with majority intralayer CTE character (>99.9%) and weak coupling to the emissive FE states. We observe significantly higher photochemical stability of MLs under two-photon excitation, which we attribute to the suppression of exciton-exciton annihilation.

Renal-Clearable Organic Probes From D-A-D Type Aza-BODIPY Fluorophores for Multiphoton Deep-Brain Imaging

Bright near-infrared (NIR) fluorescent probes play an important role in in vivo optical imaging. Here, renal-clearable nanodots prepared from Aza-BODIPY are reported fluorophores for multiphoton brain imaging. The design of donor-acceptor-donor (D-A-D) type conjugated structures endowed the fluorophores with large three-photon absorption cross-section for both 1620 and 2200 nm excitation. The side chain modification and lipid encapsulation yield ultrasmall nanodots (≈4 nm) and a high fluorescence quantum yield (≈0.35) at 720 nm emission in the aqueous phase. The measured three-photon action cross-section of a single Aza-BODIPY fluorophore in the nanodots is ≈30 times higher than the commonly used Sulforhodamine 101 dye. Three-photon deep brain imaging of subcortical structures is demonstrated, reaching a depth of 1900 µm below the brain surface in a live mouse study. The nanodots enabled blood flow measurement at a depth of 1617 µm using line scanning three-photon microscopy (3PM). This work provides superior fluorescent probes for multiphoton deep-brain imaging.

Calibration of several first excited state properties for organic molecules through systematic comparison of TDDFT with experimental spectra

Time-dependent density functional theory (TDDFT) is a powerful computational tool for investigating excitation properties in organic electronics, and it holds significant potential for high-throughput virtual screening (HTVS) in this field. While most benchmarks focus on excitation energies, less attention has been paid to evaluating the accuracy of computed oscillator strengths and exciton reorganization energies against experimental data. In this work, we provide a systematic approach to evaluate in parallel the accuracy of these three quantities on the basis of a suitable fitting of the experimental absorption spectra of 71 molecules in solution. After considering 18 computational methodologies, the results from the M06-2X/def2-TZVP/PCM method demonstrate the strongest correlation with experimental data across the desired properties. For HTVS, the M06-2X/6-31G(d)/PCM method appears to be a particularly convenient choice among all methodologies due to its balance of computational efficiency and accuracy. Our results provide an additional benchmark needed before employing TDDFT methods for the discovery and design of organic electronic molecules.

Azaacene Diradicals Based on Non-Kekulé Meta-Quinodimethane with Large Two-Photon Cross-Sections in the Infrared Spectral Region

Non-Kekulé quinoidal azaacences m-A (1 a,b) were synthesized and compared to their para- and ortho-quinodimethane analogues. m-A display high diradical characters (1 b: y0 = 0.88) due to their meta-quinodimethane (m-QDM) topology. Electron paramagnetic, nuclear magnetic resonance spectroscopies and supraquantum interference device measurements in combination with quantum-chemical calculations revealed singlet ground states for m-A with singlet-triplet gaps ΔEST (0.13-0.25 kcal mol-1) and thermally populated triplet states. These non-Kekulé structures are over all void of zwitterionic character and possess record high two-photon absorption cross sections over a broad spectral range in the near-infrared.

Balancing Brightness and Photobasicity: Modulating Excited-State Proton Transfer Pathways in Push-Pull Fluorophores for Biological Two-Photon Imaging

Push-pull fluorophores with donor-π-acceptor architectures are attractive scaffolds for the design of probes and labels for two-photon microscopy. Such fluorophores undergo a significant charge-delocalization in the excited state, which is essential for achieving a large two-photon absorption cross-section and brightness. The polarized excited state may, however, also facilitate excited-state proton transfer (ESPT) pathways that can interfere with the probe response. Herein, we employed steady-state and time-resolved spectroscopic studies to elucidate whether ESPT is responsible for the pH-dependent emission response of the Zn(II)-selective fluorescent probe chromis-1. Composed of a push-pull architecture with a pyridine ring as the acceptor, the chromis-1 fluorophore core acts as a photobase that promotes ESPT upon acidification. Although the pKa of the pyridine acceptor increases more than six orders of magnitude upon excitation, the photobasicity is not sufficient to deprotonate solvent water molecules under neutral conditions. Rather, the pH-dependent emission response is caused by the pendant bis-isonicotinic acid chelating group which upon protonation facilitates an excited-state intramolecular proton transfer to the pyridine acceptor. A simple permutation of the core pyridine nitrogen from the para- to the ortho-position relative to the thiazole substituent was sufficient to reduce the excited-state basicity by two orders of magnitude without compromising the two-photon excited brightness. These results highlight the importance of choosing the appropriate fluorophore core and chelating moiety for minimizing pH-dependent responses in the design of fluorescent probes for biological imaging.

Aminocarbonyl Fluorophores with a Strong Emissive Inverted Solvatochromism

Three aminocarbonyls were synthesized, and their emissive spectral behavior recorded at various solvent polarities showed marked inverted solvatofluorochromism. The emission energy inversion occurs at moderate solvent polarities and was found to be triggered by a change in the solute-solvent interaction responsible for the stabilization of the highly zwitterionic excited state of the dyes, from dipolarity to acidity.

Spatially selective actuation of liquid-crystalline polymer films through two-photon absorption processes

Soft materials that respond to external stimuli are promising candidates for next-generation actuators with human-friendly nature1,2. Among various stimuli to induce strain, light offers spatial selectivity, which allows versatile motion of a continuous body. However, spatial selectivity of photoactuation has been limited in two dimension due to the predominant absorption of photons by chromophores near a light source in accordance with Beer-Lambert law. Here, we report the deformation of crosslinked liquid-crystalline polymer films triggered by two-photon absorption. The films containing azotolane moieties show photoinduced deformation upon irradiation with fs laser pulses through two-photon absorption. The direction of photoinduced bending is controlled by depth-selective excitation with a focused laser beam. Furthermore, the mode of deformation is transformed from bending to twisting by irradiating spots near an edge of the film. Inhomogeneous photoirradiation with high spatial selectivity allows an infinite variation of three-dimensional motions even apart from preprogrammed behavior, which would be advantageous especially in application to microactuators.

Theoretical Investigation of Novel Nitrogen-Heterocyclic Iridium(III) Polypyridyl Complexes as Photosensitizers for Two-Photon Photodynamic Therapy

Two-photon photodynamic therapy (TP-PDT) has become a major cancer treatment due to its larger tissue penetration depth, good spatial selectivity, and less damage to normal cells. In this contribution, a series of novel photosensitizer molecules (Ir-2, Ir-2-1∼Ir-2-4) have been designed based on the experimentally demonstrated photosensitizer [Ir(ppy)2(osip)] (PF6) by fine tuning the π-conjugated structure and introducing different nitrogen-heterocyclic substituents. The electronic structures, one- and two-photon absorption spectra, triplet excited state lifetime, solvation-free energy, and photosensitizing performance were evaluated by means of density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results suggested that the molecule Ir-2, incorporating thiophene as the π-connecting group, exhibits a higher probability of triplet state formation, enhanced two-photon absorption cross-section, and prolonged triplet state lifetime. Furthermore, the four designed nitrogen-heterocyclic complexes Ir-2-1∼Ir-2-4 demonstrate favorable photosensitizing properties, with two-photon absorption cross-sections reaching up to 110 GM and triplet excited state lifetimes exceeding 1000 μs for Ir-2-4.

Two-Photon Absorption Strengths of Small Molecules: Reference CC3 Values and Benchmarks

We present a large dataset of highly accurate two-photon transition strengths (δTPA) determined for standard small molecules. Our reference values have been calculated using the quadratic response implementation of the third-order coupled cluster method including iterative triples (Q-CC3). The aug-cc-pVTZ atomic basis set is used for molecules with up to five non-hydrogen atoms, while larger molecules are assessed with aug-cc-pVDZ; the differences due to the basis sets are discussed. This dataset, encompassing 82 singlet transitions of various characters (Rydberg, valence, and double excitations), enables a comprehensive benchmark of smaller basis sets and alternative wavefunction methods when Q-CC3 calculations become beyond reach as well as time-dependent density functional theory (TD-DFT) approaches. The evaluated wavefunction methods include quadratic response and equation-of-motion CCSD approximations, Q-CC2, and second-order algebraic diagrammatic construction in its intermediate state representation (I-ADC2). In the TD-DFT framework, a set of five commonly used exchange-correlation functionals are evaluted. This extensive analysis provides a quantitative assessment of these methods, revealing how different system sizes, response intensities, and types of transitions affect their performances.

Perspective on Water-Soluble Two-Photon Initiator for Two-Photon Polymerization

Two-photon polymerization (TPP) as an unparalleled technology empowers the rapid prototyping of customized three-dimensional (3D) micro/nanostructures, garnering noticeable interest in tissue engineering, drug delivery, and regenerative medicine. These applications have a high requirement on the biocompatibility and integrity of 3D structures. Therefore, it is important to develop two-photon initiator with good water-solubility, initiation efficiency, and biocompatibility. Here, we share our insights into the development of a water-soluble two-photon initiator (WTPI) and applications from the material and manufacturing perspective. We highlight the nonlinear optical properties and the synthesis of WTPI through three pathways. Then we further demonstrate the applications of the TPP technique in the aqueous phase in the fields of tissue engineering, 4D printing, and ceramic manufacturing. Finally, a general conclusion and outlook are provided for the future development and application of WTPI.

Design and synthesis of a 2,5-Diarylthiophene chromophore for enhanced near-infrared two-photon uncaging efficiency of calcium ions

The design and synthesis of two-photon-responsive chromophores have recently garnered significant attention owing to their potential applications in materials and life sciences. In this study, a novel π-conjugated system, 2-dimethylaminophenyl-5-nitrophenylthiophene derivatives, featuring a thiophene unit as the π-linker between the donor (NMe2C6H4-) and acceptor (NO2C6H4-) units was designed, synthesized, and applied for the development of two-photon-responsive chromophores as a photoremovable protecting group in the near-infrared region. Notably, the positional effect of the nitro group (NO2), meta versus para position, was observed in the uncaging process of benzoic acid. Additionally, while the para-isomer exhibited a single fluorescence peak, a dual emission was detected for the meta-isomer in polar solvents. The caged calcium ion (Ca2+) incorporating the newly synthesized thiophene unit exhibited a sizable two-photon absorption cross-section value (σ2 = 129 GM at 830 nm). Both one-photon and two-photon photoirradiation of caged calcium ions successfully released calcium ions, indicating the potential utility of 2,5-diarylthiophene derivatives in future biological studies.

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

Enhancing Two-Photon Excited Fluorescence of Metal-Organic Framework Single Crystals through Modulation of Inorganic Nodes

Regulation of the two-photon excited fluorescence (TPEF) emission intensity and wavelength of metal-organic framework (MOF) crystals with similar constitutions presents a significant challenge. In this study, two MOFs, Zn-BTPPA and Cd3-BTPPA, were constructed using tetrakis(1,1'-biphenyl-4-carboxylic acid)-1,4-benzenediamine (H4BTPPA) as the organic ligand and mononuclear Zn and trinuclear Cd3 inorganic nodes, respectively. The incorporation of H4BTPPA within the MOF structures enables effective TPEF emission in both Zn-BTPPA and Cd3-BTPPA. The TPEF results show that Zn-BTPPA and Cd3-BTPPA exhibited strong emissions at 523 and 463 nm, respectively, when excited with a 780 nm laser. Moreover, Zn-BTPPA and Cd3-BTPPA exhibited much higher two-photon absorption cross sections, approximately 4.9 and 5.2 times higher than that of the reported dinuclear MOF, Cd2-BTPPA, with a similar composition, respectively. With different inorganic nodes, the stacking of chromophores, π···π interactions, and ligand geometry were found to correlate with the enhanced TPEF in Cd3-BTPPA and the blue-shifted TPEF in Zn-BTPPA. This work serves as an inspiration for designing efficient TPEF MOF materials based on the structure-property relationship.

Buckybowl Molecular Tweezers for Recognition of Fullerenes

Buckybowl tweezers are a relatively young research area closely associated with the development of non-planar polycyclic aromatic systems and supramolecular chemistry. Since the appearance of the first prototypes in the early 2000s, the tweezers have undergone evolutionary changes. Nowadays they are able to effectively interact with fullerenes and the results opened up prospects for development in the field of sensing, nonlinear optics, and molecular switchers. In the present study, examples of corannulene-based and other buckybowl tweezers for the recognition of C60 and C70 fullerenes were summarized and analyzed. The main structural components of the tweezers were also reviewed in detail and their role in the formation of complexes with fullerenes was evaluated. The revealed structural patterns should trigger the development of novel recognition systems and materials with a wide range of applications.

Two-photon Dye-Based Fluorogenic Organic Nanoparticles as Intracellular Thiols Sensors

Optical bioimaging is an ever-growing field that benefits both from the fast progress of optical instrumentation and modalities, and from the development of light-emitting probes. The efficacy of molecular fluorescent dyes is crucial, yet hindered by limited brightness and hydrophilicity. Addressing these challenges, self-stabilized fluorogenic organic nanoparticles only made of pure dyes (dFONs) are introduced in this work. Comprising thiol-sensitive fluorogenic chromophores, these dFONs exhibit enhanced brightness exclusively in the presence of biological thiols, notably glutathione, overcoming the need for water-solubilizing moieties. Importantly, these nanoparticles demonstrate large fluorescence and one- and two-photon brightness, enabling sensitive bioimaging of intracellular thiols at micromolar concentrations. Notably, only the pristine fluorogenic nanoparticles can penetrate the cells and does not require to wash the cells before imaging, emphasizing their unique role as dye carriers, fluorogenic probes and ease of use. This work highlights the transformative potential of dFONs in advancing optical bioimaging, paving the way for the use of dFONs not just as tracers, but also now as biosensors and ultimately in the future as biomarkers.

Amphiphilic coumarin-based probes for live-cell STED nanoscopy of plasma membrane

Plasma membranes are vital biological structures, serving as protective barriers and participating in various cellular processes. In the field of super-resolution optical microscopy, stimulated emission depletion (STED) nanoscopy has emerged as a powerful method for investigating plasma membrane-related phenomena. However, many applications of STED microscopy are critically restricted by the limited availability of suitable fluorescent probes. This paper reports on the development of two amphiphilic membrane probes, SHE-2H and SHE-2N, specially designed for STED nanoscopy. SHE-2N, in particular, demonstrates quick and stable plasma membrane labelling with negligible intracellular redistribution. Both probes exhibit outstanding photostability and resolution improvement in STED nanoscopy, and are also suited for two-photon excitation microscopy. Furthermore, microscopy experiments and cytotoxicity tests revealed no noticeable cytotoxicity of probe SHE-2N at concentration used for fluorescence imaging. Spectral analysis and fluorescence lifetime measurements conducted on probe SHE-2N using giant unilamellar vesicles, revealed that emission spectra and fluorescence lifetimes exhibited minimal sensitivity to lipid composition variations. These novel probes significantly augment the arsenal of tools available for high-resolution plasma membrane research, enabling a more profound exploration of cellular processes and dynamics.

Self-stacked small molecules for ultrasensitive, substrate-free Raman imaging in vivo

Raman spectroscopy using surface-enhanced Raman scattering (SERS) nanoprobes represents an ultrasensitive and high-precision technique for in vivo imaging. Clinical translation of SERS nanoprobes has been hampered by biosafety concerns about the metal substrates used to enhance Raman signals. We report a set of small molecules with bis-thienyl-substituted benzobisthiadiazole structures that enhance Raman signal through self-stacking rather than external substrates. In our technique, called stacking-induced charge transfer-enhanced Raman scattering (SICTERS), the self-stacked small molecules form an ordered spatial arrangement that enables three-dimensional charge transfer between neighboring molecules. The Raman scattering cross-section of SICTERS nanoprobes is 1350 times higher than that of conventional SERS gold nanoprobes of similar particle size. SICTERS outperforms SERS in terms of in vivo imaging sensitivity, resolution and depth. SICTERS is capable of noninvasive Raman imaging of blood and lymphatic vasculatures, which has not been achieved by SERS. SICTERS represents an alternative technique to enhance Raman scattering for guiding the design of ultrasensitive substrate-free Raman imaging probes.

Highly and Deep Red-Luminescent Bisphenylamine-Appended Benzocarcogendiazole Fluorophores

Benzocarcogendiazole units have been frequently utilized for optoelectronics such as organic solar cells because of their robustness, rigidity, and band-gap tunability based on their strong electron-withdrawing properties. Focusing on the luminescent characteristics, these molecules have been utilized to demonstrate highly sensitive chromisms because of the potential of charge transfer. Here, we demonstrate deep red-emissions in bis(4-tert-butylphenyl)amine-appended benzocarcogendiazole-based donor-acceptor-donor (D-A-D) fluorophores, namely 1 and 2. Because benzocarcogendiazole and bis(4-tert-butylphenyl)amine serve as strong electron acceptor and donor, respectively, strong intramolecular charge transfer (ICT) enables long wavelength of photoluminescence (PL) even in the small molecular weight. Although photoluminescence (PL) in long wavelength tends to exhibit quite low absolute PL quantum efficiency (ΦPL), the values of solutions 1 and 2 are quite high (up to 50 %). According to X-ray crystallographic characterizations and DFT calculations, these high ΦPL values are attributable to the segregated π-planes of benzocarcogendiazole units, which is induced by the bulky substituents of bis(4-tert-butylphenyl)amines.

Noninvasive in vivo microscopy of single neutrophils in the mouse brain via NIR-II fluorescent nanomaterials

In vivo microscopy of single cells enables following pathological changes in tissues, revealing signaling networks and cell interactions critical to disease progression. However, conventional intravital microscopy at visible and near-infrared wavelengths <900 nm (NIR-I) suffers from attenuation and is typically performed following the surgical creation of an imaging window. Such surgical procedures cause the alteration of the local vasculature and induce inflammation in skin, muscle and skull, inevitably altering the microenvironment in the imaging area. Here, we detail the use of near-infrared fluorescence (NIR-II, 1,000-1,700 nm) for in vivo microscopy to circumvent attenuation in living tissues. This approach enables the noninvasive visualization of cell migration in deep tissues by labeling specific cells with NIR-II lanthanide downshifting nanoparticles exhibiting high physicochemical stability and photostability. We further developed a NIR-II fluorescence microscopy setup for in vivo imaging through the intact skull with high spatiotemporal resolution, which we use for the real-time dynamic visualization of single-neutrophil behavior in the deep brain of a mouse model of ischemic stroke. The labeled downshifting nanoparticle synthesis takes 5-6 d, the imaging system setup takes 1-2 h, the in vivo cell labeling takes 1-3 h, the in vivo NIR-II microscopic imaging takes 3-5 h and the data analysis takes 3-8 h. The procedures can be performed by users with standard laboratory training in nanomaterials research and appropriate animal handling.

Detour to success: photoswitching via indirect excitation

Photoswitchable molecules that undergo nanoscopic changes upon photoisomerisation can be harnessed to control macroscopic properties such as colour, solubility, shape, and motion of the systems they are incorporated into. These molecules find applications in various fields of chemistry, physics, biology, and materials science. Until recently, research efforts have focused on the design of efficient photoswitches responsive to low-energy (red or near-infrared) irradiation, which however may compromise other molecular properties such as thermal stability and robustness. Indirect isomerisation methods enable photoisomerisation with low-energy photons without altering the photoswitch core, and also open up new avenues in controlling the thermal switching mechanism. In this perspective, we present the state of the art of five indirect excitation methods: two-photon excitation, triplet sensitisation, photon upconversion, photoinduced electron transfer, and indirect thermal methods. Each impacts our understanding of the fundamental physicochemical properties of photochemical switches, and offers unique application prospects in biomedical technologies and beyond.

Advancing Central Nervous System Drug Delivery with Microtubule-Dependent Transcytosis of Novel Aqueous Compounds

The challenge of delivering therapeutics to the central nervous system due to the restrictive nature of the blood-brain barrier (BBB) is a substantial hurdle in neuropharmacology. Our research introduces a breakthrough approach using microtubule-dependent transcytosis facilitated by novel aqueous compounds. We synthesized a series of red-emitting pyran nitrile derivatives. The molecular structure of compounds, photophysical properties, and water solubility were characterized. BBB permeability of BN1 was assessed in an in vitro BBB model. The transmembrane transport mechanism was next analyzed. The derivative was injected in the wild-type mouse for evaluation of brain penetration and biodistribution in the brain. We further investigated the potential of BN1-functionalized BBB-nonpenetrated silica nanoparticles for brain targeting. This compound demonstrated an ability to form endosomes within the phospholipid layer, thus enabling efficient penetration of the BBB via microtubule-mediated transcytosis, as evidenced in vitro model. This was further confirmed by in vivo experiments that BN1 displays the excellent BBB penetration and retained in brain parenchyma. Furthermore, BBB-impermeable mesoporous silica nanoparticle codelivery system markedly enhanced the transport efficiency to the brain in vivo by BN1-functionalized. These findings indicate that our designed aqueous molecules not only are capable of traversing the BBB but also serve as a viable new strategy for central-nervous-system-targeted drug delivery.

Near-infrared BODIPY photosensitizers for two-photon excited singlet oxygen generation and tumor cell photodynamic therapy

In this paper, two near-infrared BODIPY photosensitizers, Id-BDPI and Cz-BDPI, were obtained by modifying the indole and carbazole aromatic heterocycles in the core of BODIPY. The maximum absorption wavelengths of Id-BDPI and Cz-BDPI were 694 nm and 722 nm, and their singlet oxygen yields were 48% and 48.4%, respectively. In the simulated tumor cell photodynamic therapy, Id-BDPI and Cz-BDPI could effectively inhibit the growth of A549 tumor cells under near-infrared light. Meanwhile, the lysosomal co-localization coefficients of Id-BDPI and Cz-BDPI with A549 tumor cells were 0.94 and 0.89, respectively, showing high lysosomal targeting ability and biocompatibility. The two-photon absorption cross sections measured at 1050 nm by the Z-scanning method were 661.8 GM and 715.6 GM, respectively, and Cz-BDPI was further successfully applied to two-photon fluorescence imaging and two-photon excited singlet oxygen generation in zebrafish. The above results indicate that the introduction of aromatic heterocycles can effectively enhance the photodynamic efficacy of BODIPY photosensitizers, and the larger two-photon absorption cross section also brings potential for two-photon photodynamic therapy applications.

Exceptional three- to six-photon absorption at organometallic dendrimers

The light-intensity dependence of multi-photon absorption (MPA) affords outstanding spatial control. Furthermore, compared to the higher-energy photons needed for analogous linear absorption, the lower-energy photons involved in MPA often correspond to important wavelengths, such as those of the biological and telecommunications "windows". It is therefore of crucial importance to develop molecules that exhibit outstanding MPA cross-sections. However, although progress has been made with two-photon absorption, there is currently a dearth of efficient instantaneous n-photon absorbers (n > 2), a key reason being the scarcity of structure-property studies required to understand higher-order MPA. We herein report systematically-varied metallodendrimers up to third-generation in size, together with their nonlinear absorptive responses over the spectral range 600-2520 nm. We show that the dendrimers exhibit exceptional instantaneous three- to six-photon absorption cross-sections, with maximal values increasing with dendrimer generation and installation of solubilizing group, and we report that changing the groups at the dendrimer periphery can shift the wavelengths of the nPA maxima. We also describe time-dependent DFT studies that have facilitated assignment of the key linear and nonlinear transitions and disclosed the crucial role of the metal in the outstanding MPA performance.

Functional microfluidics: theory, microfabrication, and applications

Microfluidic devices are composed of microchannels with a diameter ranging from ten to a few hundred micrometers. Thus, quite a small (10−9–10−18 l) amount of liquid can be manipulated by such a precise system. In the past three decades, significant progress in materials science, microfabrication, and various applications has boosted the development of promising functional microfluidic devices. In this review, the recent progress on novel microfluidic devices with various functions and applications is presented. First, the theory and numerical methods for studying the performance of microfluidic devices are briefly introduced. Then, materials and fabrication methods of functional microfluidic devices are summarized. Next, the recent significant advances in applications of microfluidic devices are highlighted, including heat sinks, clean water production, chemical reactions, sensors, biomedicine, capillaric circuits, wearable electronic devices, and microrobotics. Finally, perspectives on the challenges and future developments of functional microfluidic devices are presented. This review aims to inspire researchers from various fields—engineering, materials, chemistry, mathematics, physics, and more—to collaborate and drive forward the development and applications of functional microfluidic devices, specifically for achieving carbon neutrality.

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene derivative

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene (fDAE) derivative were investigated by time-resolved absorption and fluorescence spectroscopies. Prescreening with quantum chemical calculation predicted that a derivative with methylthienyl groups (mt-fDAE) in the closed-ring isomer has a two-photon absorption cross-section larger than 1000 GM, which was experimentally verified by Z-scan measurements and excitation power dependence in transient absorption. Comparison of transient absorption spectra under one-photon and simultaneous two-photon excitation conditions revealed that the closed-ring isomer of mt-fDAE populated into higher excited states deactivates following three pathways on a timescale of ca. 200 fs: (i) the cycloreversion reaction more efficient than that by the one-photon process, (ii) internal conversion into the S1 state, and (iii) relaxation into a lower state (S1' state) different from the S1 state. Time-resolved fluorescence measurements demonstrated that this S1' state is relaxed to the S1 state with the large emission probability. These findings obtained in the present work contribute to extension of the ON-OFF switching capability of fDAE to the biological window and application to super-resolution fluorescence imaging in a two-photon manner.

Development of a Two-Photon-Responsive Chromophore, 2-(p-Aminophenyl)-5,6-dimethoxy-1-(hydroxyinden-3-yl)methyl Derivative, as a Photoremovable Protecting Group

Photoremovable protecting groups (PPGs) are powerful tools that are widely used to investigate biological events in cells. An important requirement for PPGs is the efficient release of bioactive molecules by using visible to near-infrared light in the biological window (650-1350 nm). In this study, we report a new two-photon (2P)-responsive PPG, 2-(p-aminophenyl)-5,6-dimethoxy-1-(hydroxyinden-3-yl)methyl, with a donor-π-donor cyclic stilbene structure. The 2P cross section was approximately 40-50 GM at ∼700 nm. The quantum yield of the uncaging process of caged benzoate was greater than 0.7, demonstrating that the 2P uncaging efficiency was approximately 30 GM at around 700 nm. This newly developed 2P-responsive chromophore can be used in future biological experiments. The mechanism of the photo-uncaging reaction via the carbocation intermediate was elucidated using transient absorption spectroscopy and product analysis.

Study of two-photon absorption and excited-state dynamics of coumarin derivatives: the effect of monomeric and dimeric structures

Intramolecular charge transfer (ICT) and π-electron delocalization are two key factors affecting the nonlinear optical absorption of organic molecules. To clarify the different influences of ICT and π-electron delocalization on two-photon absorption (TPA) and excited-state absorption (ESA), monomeric coumarin C1 and dimeric coumarin C2 are synthesized and studied. Transient absorption spectroscopy (TAS) analysis of these coumarin derivatives in solvents of varying polarities describes the polarity-dependent excited-state dynamics and reveals the ESA signals of the charge transfer state (CTS) and local excited state (LES) with different spectral features. Femtosecond broadband Z-scan experiments indicate that dimeric coumarin C2 has a more significant TPA response than monomeric coumarin C1 in the near-infrared region. Natural transition orbital (NTO) analysis further theoretically characterizes the electron transition feature induced by TPA. Our results reveal that the TPA of these coumarin derivatives can be significantly enhanced by expanding π-electron delocalization, but their ESA is mainly dominated by ICT performance. This study indicates that coumarin derivatives will exhibit extremely broad application prospects in the field of ultrafast optical limiting (OL) through reasonable molecular design.

Phosphorus(V) Tetraazaporphyrin with an Intense, Broad CT Band in the Near-IR Region

In phosphorus tetraazaporphyrins (PTAPs), the Q- and charge-transfer (CT) bands appear as a result of configuration interaction between their excited states. On the basis of this concept, a PTAP with an intense, broad CT band in the near-IR region has been rationally designed and realized by introducing eight diphenylaminophenyl (dPAP) groups. The order of the CT and Q-bands in ascending energy was supported by magnetic circular dichroism (MCD) spectroscopy and theoretical calculations. An intense two-photon absorption was also found in the deep near-IR region.

Near-Infrared Light-Driven Photocatalysis with an Emphasis on Two-Photon Excitation: Concepts, Materials, and Applications

Efficient utilization of sunlight in photocatalysis is widely recognized as a promising solution for addressing the growing energy demand and environmental issues resulting from fossil fuel consumption. Recently, there have been significant developments in various near-infrared (NIR) light-harvesting systems for artificial photosynthesis and photocatalytic environmental remediation. This review provides an overview of the most recent advancements in the utilization of NIR light through the creation of novel nanostructured materials and molecular photosensitizers, as well as modulating strategies to enhance the photocatalytic processes. A special focus is given to the emerging two-photon excitation NIR photocatalysis. The unique features and limitations of different systems are critically evaluated. In particular, it highlights the advantages of utilizing NIR light and two-photon excitation compared to UV-visible irradiation and one-photon excitation. Ongoing challenges and potential solutions for the future exploration of NIR light-responsive materials are also discussed.

Aggregation-Induced Fluorescence Upconversion of Pyrene under Low Fluence: In Solutions and Polymeric Nanoparticles

In this study, aggregation-induced photon upconversion (iPUC) is demonstrated in the small polyaromatic molecule, pyrene. In binary-solvent mixtures, water, which induces the aggregation of polyaromatic molecules, assisted in triplet-triplet annihilation-based upconversion. No upconverted emission was observed in a dry solvent. Although upconverted emission in the absence of a triplet sensitizer was assigned to pyrene-aggregate-induced sensitization, the presence of a triplet sensitizer enhanced the upconversion efficiency. This experimental finding was further simulated to explore the possibility of iPUC in the condensed-phase polymer matrix. We studied 2-aminoethyl methacrylate hydrochloride-polystyrene copolymer nanoparticles embedded with the molecular upconversion system. The nanoparticle iPUC agreed with the proposition that water domains were present in polymer nanoparticles and helped aggregate pyrene in the host polymer. Despite the low systemic upconversion efficiency, this study provides a method for achieving fluorescence upconversion in relatively simple systems.

Formulation and Implementation of Frequency-Dependent Linear Response Properties with Relativistic Coupled Cluster Theory for GPU-Accelerated Computer Architectures

We present the development and implementation of relativistic coupled cluster linear response theory (CC-LR), which allows the determination of molecular properties arising from time-dependent or time-independent electric, magnetic, or mixed electric-magnetic perturbations (within a common gauge origin for the magnetic properties) as well as taking into account the finite lifetime of excited states in the framework of damped response theory. We showcase our implementation, which is capable to offload the computationally intensive tensor contractions characteristic of coupled cluster theory onto graphical processing units, in the calculation of (a) frequency-(in)dependent dipole-dipole polarizabilities of IIB atoms and selected diatomic molecules, with a particular emphasis on the calculation of valence absorption cross sections for the I2 molecule; (b) indirect spin-spin coupling constants for benchmark systems such as the hydrogen halides (HX, X = F-I) as well the H2Se-H2O dimer as a prototypical system containing hydrogen bonds; and (c) optical rotations at the sodium D line for hydrogen peroxide analogues (H2Y2, Y = O, S, Se, Te). Thanks to this implementation, we are able to show the similarities in performance, but often the significant discrepancies, between CC-LR and approximate methods such as density functional theory. Comparing standard CC response theory with the flavor based upon the equation of motion formalism, we find that for valence properties such as polarizabilities, the two frameworks yield very similar results across the periodic table as found elsewhere in the literature; for properties that probe the core region, such as spin-spin couplings, on the other hand, we show a progressive differentiation between the two as relativistic effects become more important. Our results also suggest that as one goes down the periodic table, it may become increasingly difficult to measure pure optical rotation at the sodium D line due to the appearance of absorbing states.

Crystallography, Charge Transfer, and Two-Photon Absorption Relations in Molecular Cocrystals for Two-Photon Excited Fluorescence Imaging

Two-photon excited fluorescence imaging requires high-performance two-photon absorption (TPA) active materials, which are commonly intramolecular charge transfer systems prepared by traditional chemical synthesis. However, this typically needs harsh conditions and new methods are becoming crucial. In this work, based on a collaborative intermolecular charge transfer (inter-CT) strategy, three centimeter-sized organic TPA cocrystals are successfully obtained. All three cocrystals exhibit a mixed stacking arrangement, which can effectively generate inter-CT between the donor and acceptor. The ground and excited state characterizations compare their inter-CT ability: 1,2-BTC > 2D-BTC > 1D-BTC. Transient absorption spectroscopy detects TCNB•- , indicating that the TPA mechanism arises from molecular polarization caused by inter-CT. Meanwhile, 1,2-BTC exhibits the highest excited-state absorption and the longest excited-state lifetime, suggesting a stronger TPA response. First-principles calculations also confirm the presence of inter-CT interactions, and the significant parameter Δµ which can assess the TPA capability indicates that inter-CT enhances the TPA response. Besides, cocrystals also demonstrate excellent water solubility and two-photon excited fluorescence imaging capabilities. This research not only provides an effective method for synthesizing TPA crystal materials and elucidates the connection between inter-CT ability and TPA property but also successfully applies them in the fields of multi-photon fluorescence bioimaging.

Evaluation of experimental, computational, molecular docking and dynamic simulation of flucytosine

Flucytosine (5-fluorocytosine), a fluorine derivative of pyrimidine, has been studied both experimentally and quantum chemically. To obtain the optimized structure, vibrational frequencies and other various parameters, the B3LYP method with a 6-311++G(d,p) basis set was used. Atom-in-molecule theory was used to calculate the binding energies, ellipticity and isosurface projection by electron localization of the molecule (AIM). In addition, the computational results from IR and Raman were compared with the experimental spectra. NBO analysis was used to analyze the donor and acceptor interactions. To know the reactive region of the molecule, the molecular electrostatic potential (MEP) and Fukui functions were determined. The UV-Vis spectrum calculated by the TD-DFT/PCM method was also compared with the experimentally determined spectrum. The HOMO-LUMO energy outcomes proved that there was a good charge exchange occurring within the molecule. With DMSO and MeOH as the solvents, maps of the hole and electron density distribution (EDD and HDD) were produced in an excited state. An electrophilicity index parameter was looked at to theoretically test the bioactivity of the compound. To find the best ligand-protein interactions, molecular docking was also carried out with various receptor proteins. In order to verify the inhibitory potency for the receptor protein complex predicted by docking and molecular dynamic simulation studies, the binding free energy of the receptor protein complex was calculated. Using the MM/GBSA technique, we determined the docked complex's binding free energy. To confirm the molecule's drug similarity, a biological drug similarity investigation was also executed.Communicated by Ramaswamy H. Sarma.

A general design of pyridinium-based fluorescent probes for enhancing two-photon microscopy

Two-photon absorbing fluorescent probes have emerged as powerful imaging tools for subcellular-level monitoring of biological substances and processes, offering advantages such as deep light penetration, minimal photodamage, low autofluorescence, and high spatial resolution. However, existing two-photon absorbing probes still face several limitations, such as small two-photon absorption cross-section, poor water solubility, low membrane permeability, and potentially high toxicity. Herein, we report three small-molecule probes, namely MSP-1arm, Lyso-2arm, and Mito-3arm, composed of a pyridinium center (electron-acceptor) and various methoxystyrene "arms" (electron-donor). These probes exhibit excellent fluorescence quantum yield and decent aqueous solubility. Leveraging the inherent intramolecular charge transfer and excitonic coupling effect, these complexes demonstrate excellent two-photon absorption in the near-infrared region. Notably, Lyso-2arm and Mito-3arm exhibit distinct targeting abilities for lysosomes and mitochondria, respectively. In two-photon microscopy experiments, Mito-3arm outperforms a commercial two-photon absorbing dye in 2D monolayer HeLa cells, delivering enhanced resolution, broader NIR light excitation window, and higher signal-to-noise ratio. Moreover, the two-photon bioimaging of 3D human forebrain organoids confirms the successful deep tissue imaging capabilities of both Lyso-2arm and Mito-3arm. Overall, this work presents a rational design strategy in developing competent two-photon-absorbing probes by varying the number of conjugated "arms" for bioimaging applications.

Enhanced NLO response and switching self-focussing in benzodiazepine derivative with -NO2 and -Br substitution

Optoelectronic and the cubic nonlinear optical properties of 4-(4-Bromophenyl)-2-(4-nitrophenyl)-2, 3-dihydro-1H-1, 5-benzodiazepine have been studied. Z-scan technique was used for the third-order nonlinear optical measurements namely, nonlinear absorption, nonlinear refraction, and optical power limiting behaviour employing an Nd: YAG laser of 532 nm wavelength having 5 ns Gaussian pulses. B3LYP/6-311 ++ G (d, p) level of theory was employed for structural optimization, vibrational wavenumber, frontier molecular orbitals, natural bond orbital and population analysis. The MOLVIB programme was used to perform unambiguous vibrational assignments based on potential energy distribution values acquired from normal coordinate analysis. B3LYP and CAM-B3LYP hybrid functions have been employed at the DFT level to calculate the theoretical second-order hyperpolarizability. The substitution of -NO2 and -Br in this benzodiazepine compound enhances the second-order hyperpolarizability (γ) to the order of 10-34 esu and, switching of self-defocussing to self-focussing phenomenon. The HOMO-LUMO and optical band gap analysis illustrates that polarizing nature of the molecule vary with substituents. The obtained results indicate that this compound has potential applications in optoelectronics and photonics.

Synthesis, spectroscopic characterization, density functional theory study, antimicrobial and antioxidant activities of curcumin and alanine-curcumin Schiff base

In this work, a novel Schiff-base derived from curcumin and L-Alanine was synthesized under microwave conditions in excellent yield. The structural characterization has been carried out from their elemental analyses, FTIR, UV-Vis and 13C-NMR and 1H-NMR spectral techniques. The Schiff base (Cur-Ala) and curcumin (Cur) have been screened for their antimicrobial activity toward some pathogens clinically important microorganisms: Bacillus subtilis, Escherichia coli and Staphylococcus aureus, Aspergillus niger and Candida albicans. Result found that the Schiff base was more active than the curcumin. The antibacterial and antifungal activities of Cur-Ala can be attributed to its greatest dipole moment, as shown by theoretical calculations. Also, the antioxidant activity of Schiff base and curcumin were studied by DPPH, cupric ion reducing antioxidant capacity and o-phenanthroline techniques. Results indicate that Cur-Ala and Cur show more antioxidant activities than the standard antioxidants (BHT and BHA). Quantum chemical parameter calculations of Cur-Ala and Cur have been investigated by DFT using B3LYP/6-31G (d,p) basis set method to calculate the optimized structure, atomic charges, MESP, global reactivity descriptors and thermomolecular proprieties of both molecules.Communicated by Ramaswamy H. Sarma.

Two-photon nanoprobes based on bioorganic nanoarchitectonics with a photo-oxidation enhanced emission mechanism

Two-photon absorption (TPA) fluorescence imaging holds great promise in diagnostics and biomedicine owing to its unparalleled spatiotemporal resolution. However, the adaptability and applicability of currently available TPA probes, which act as a critical element for determining the imaging contrast effect, is severely challenged by limited photo-luminescence in vivo. This is particularly a result of uncontrollable aggregation that causes fluorescence quenching, and inevitable photo-oxidation in harsh physiological milieu, which normally leads to bleaching of the dye. Herein, we describe the remarkably enhanced TPA fluorescence imaging capacity of self-assembling near-infrared (NIR) cyanine dye-based nanoprobes (NPs), which can be explained by a photo-oxidation enhanced emission mechanism. Singlet oxygen generated during photo-oxidation enables chromophore dimerization to form TPA intermediates responsible for enhanced TPA fluorescence emission. The resulting NPs possess uniform size distribution, excellent stability, more favorable TPA cross-section and anti-bleaching ability than a popular TPA probe rhodamine B (RhB). These properties of cyanine dye-based TPA NPs promote their applications in visualizing blood circulation and tumoral accumulation in real-time, even to cellular imaging in vivo. The photo-oxidation enhanced emission mechanism observed in these near-infrared cyanine dye-based nanoaggregates opens an avenue for design and development of more advanced TPA fluorescence probes.

Effects of Two Electron-Donating and/or -Withdrawing Substituents on Two-Photon Absorption for Diphenylacetylene Derivatives

Two-photon absorption for diphenylacetylene (DPA) derivatives with two substituents (-OMe and/or -NO₂) at the 4,4'-position was investigated experimentally and theoretically. The two-photon absorption spectra and the two-photon absorption cross-sections σ⁽²⁾ for DPA derivatives were obtained by optical-probing photoacoustic spectroscopy (OPPAS). The simulated two-photon absorption spectra of the DPA derivatives, obtained with the time-dependent density functional theory within the Tamm-Dancoff approximation, agreed well with the experimental ones. The mechanisms for enhancement of the σ⁽²⁾ for centrosymmetric and non-centrosymmetric DPA derivatives were found to be different. The large σ⁽²⁾ for centrosymmetric molecules (DPA-OMeOMe and DPA-NO₂NO₂) results from the magnitude of the transition dipole moment, while for non-centrosymmetric molecules (DPA-OMeNO₂), it is enhanced by the smaller detuning energy. Information on two-photon absorption properties of DPA derivatives obtained in this study will be important for the molecular design of two-photon absorption materials.

Difluoroborate-based bichromophores: Symmetry relaxation and two-photon absorption

Given potential applications of multiphoton absorbers, in the present work we have studied the symmetry-relaxation effects in one- and two-photon absorption spectra in two bichromophore systems based on difluoroborate core linked by biphenylene or bianthracene moieties. We have employed a palette of experimental methods (synthesis, one- and two-photon spectroscopy, X-ray crystallography) and state-of-the-art computational methods to shed light on how symmetry relaxation, a result of twisting of building blocks, affects one- and two-photon absorption of the two studied fluorescent dyes. Electronic-structure calculations revealed that the planarity of central biphenyl moiety, as well as deviations from planarity up to 30-40 deg., ensure maximum values of two-photon transition strengths. Perpendicular arrangement of phenylene units in biphenylene moiety leads to 20% drop in the two-photon transition strengths. More detailed studies demonstrated that equilibrium structures of both compounds in chloroform solution show very different values of two-photon absorption cross sections at absorption band maxima, i.e. 224 GM for and 134 GM for biphenyle and bianthracene linkers, respectively. The latter value is in good agreement with experimental value obtained using Z-scan method. The difference in two-photon absorption cross section between both compounds can be rationalized based on equilibrium geometry differences, i.e. interplanar angle is 35 deg and 91 deg in the case of biphenylene and bianthracene moiety, respectively. It is thus not beneficial to introduce conformationally locked central linker based on bianthracene moiety.

Physical Mechanism of One-Photon Absorption, Two-Photon Absorption, and Electron Circular Dichroism of 1,3,5 Triazine Derivatives Based on Molecular Planarity

We provide a method to regulate intramolecular charge transfer (ICT) through distorting fragment dipole moments based on molecular planarity and intuitively investigate the physical mechanisms of one-photon absorption (OPA), two-photon absorption (TPA), and electron circular dichroism (ECD) properties of the multichain 1,3,5 triazine derivatives o-Br-TRZ, m-Br-TRZ, and p-Br-TRZ containing three bromobiphenyl units. As the position of the C-Br bond on the branch chain becomes farther away, the molecular planarity is weakened, with the position of charge transfer (CT) on the branch chain of bromobiphenyl changing. The excitation energy of the excited states decreases, which leads to the redshift of the OPA spectrum of 1,3,5-triazine derivatives. The decrease in molecular plane results in a change in the magnitude and direction of the molecular dipole moment on the bromobiphenyl branch chain, which weakens the intramolecular electrostatic interaction of bromobiphenyl branch chain 1,3,5-triazine derivatives and weakens the charge transfer excitation of the second step transition in TPA, leading to an increase in the enhanced absorption cross-section. Furthermore, molecular planarity can also induce and regulate chiral optical activity through changing the direction of the transition magnetic dipole moment. Our visualization method helps to reveal the physical mechanism of TPA cross-sections generated via third-order nonlinear optical materials in photoinduced CT, which is of great significance for the design of large TPA molecules.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

Halide perovskites (HPs) are potential game-changing materials for a broad spectrum of optoelectronic applications ranging from photovoltaics, light-emitting devices, lasers to radiation detectors, ferroelectrics, thermoelectrics, etc. Underpinning this spectacular expansion is their fascinating photophysics involving a complex interplay of carrier, lattice, and quasi-particle interactions spanning several temporal orders that give rise to their remarkable optical and electronic properties. Herein, we critically examine and distill their dynamical behavior, collective interactions, and underlying mechanisms in conjunction with the experimental approaches. This review aims to provide a unified photophysical picture fundamental to understanding the outstanding light-harvesting and light-emitting properties of HPs. The hotbed of carrier and quasi-particle interactions uncovered in HPs underscores the critical role of ultrafast spectroscopy and fundamental photophysics studies in advancing perovskite optoelectronics.

Fabrication of Two-Dimensional Platelets with Heat-Resistant Luminescence and Large Two-Photon Absorption Cross Sections via Cooperative Solution/Solid Self-Assembly

The combination of solution self-assembly, which enables primary morphological control, and solid self-assembly, which enables the creation of novel properties, can lead to the formation of new functional materials that cannot be obtained using either technique alone. Herein, we report a cooperative solution/solid self-assembly strategy to fabricate novel two-dimensional (2D) platelets. Precursor 2D platelets with preorganized packing structure, shape, and size are formed via the living self-assembly of a donor-acceptor fluorophore and volatile coformer (i.e., propanol) in solution phase. After high-temperature annealing, propanol is released from the precursor platelets, and new continuous intermolecular hydrogen bonds are formed. The new 2D platelets formed retain the controllable morphologies originally defined by the solution phase living self-assembly but exhibit remarkable heat-resistant luminescence up to 200 °C and high two-photon absorption cross sections (i.e., >19,000 GM at 760 nm laser excitation).

Red fluorescent benzothiadiazole derivative loaded in different nanoformulations: Optical properties and their use in bio-imaging

Fluorophores with optimized nonlinear optical properties have become prominent as contrast labels in laser scanning microscopy (LSM). The purpose of this work is to report on a novel benzothiadiazole derivative, namely 4,7-bis(5-((9,9-dioctyl-9H-fluoren-2-yl)ethynyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (EFBT) and its optical performance when it is loaded into organic nanostructures intended as labels for LSM. Four different nanostructured labels were prepared: i) EFBT-loaded silica nanoparticles (SiNPs); ii) folate-bioconjugated SiNPs (SiNPs-FA); iii) EFBT-loaded PEGylated nanoparticles (NPs-PEG); and iv) EFBT-loaded folate-terminated PEGylated nanoparticles (NPs-PEG-FA). All these nanostructures are reported through a comparative study of their linear and nonlinear optical properties, including their performance as exogenous label agents in the cervical cancer cell line HeLa. This assessment of the performance of a specific fluorophore loaded into different nanostructured matrices (labels), and fairly compared under the same characterization conditions, including the LSM settings, is less common while previous reports had focused in comparing silica and PEGylated nanoparticles but loaded with different fluorophores. The results show that the internal molecular organization into each type of organic nanostructure impacted differently the properties of EFBT, where the silica matrix tend to preserve the optical performance of the fluorophore by preventing intermolecular interactions; in contrast, PEGylated nanoparticles favored molecular interactions and introduced non-radiative decay channels that degrades drastically the optical performance. Nevertheless, the use of functionalized ends entities produced a better cellular label uptake with PEGylated that with silica nanoparticles. In overall, the NPs-PEG-FA label produced the best HeLa imaging.

Reverse Engineering Caged Compounds: Design Principles for their Application in Biology

Light passes through biological tissue, and so it is used for imaging biological processes in situ. Such observation is part of the very essence of science, but mechanistic understanding requires intervention. For more than 50 years a "second function" for light has emerged; namely, that of photochemical control. Caged compounds are biologically inert signaling molecules that are activated by light. These optical probes enable external instruction of biological processes by stimulation of an individual element in complex signaling cascades in its native environment. Cause and effect are linked directly in spatial, temporal, and frequency domains in a quantitative manner by their use. I provide a guide to the basic properties required to make effective caged compounds for the biological sciences.

Derivatized Benzothiazoles as Two-Photon-Absorbing Organic Photosensitizers Active under Near Infrared Light Irradiation

Homogeneous organic photocatalysis typically requires molecular photosensitizers absorbing in the ultraviolet-visible (UV/vis) region, because UV/vis photons possess the sufficient energy to excite those one-photon-absorbing photosensitizers to the desired excited states. However, UV/vis light irradiation has many potential limitations, especially for large-scale applications, such as low penetration through reaction media, competing absorption by substrates and co-catalysts, and incompatibility with substrates bearing light-sensitive functionalities. In fact, these drawbacks can be effectively avoided if near infrared (NIR) photons can be utilized to drive the target reactions. Herein, we report two benzothiazole-derived compounds as novel two-photon-absorbing (TPA) organic photosensitizers, which can function under NIR light irradiation using inexpensive LED as the light source. We demonstrate that by judicially modulating the donor-π-acceptor-π-donor-conjugated structure containing a bibenzothiazole core and imine bridges, excellent two-photon absorption capability in the NIR region can be achieved, approaching 2000 GM at 850 nm. Together with large quantum yields (∼0.5), these benzothiazole-derived TPA organic photosensitizers exhibit excellent performance in driving various O₂-involved organic reactions upon irradiation at 850 nm, showing great penetration depth, superior to that upon blue light irradiation. A suite of photophysical and computational studies were performed to shed light on the underlying electronic states responsible for the observed TPA capability. Overall, this work highlights the promise of developing Ru/Ir-free organic photosensitizers operative in the NIR region by taking advantage of the two-photon absorption mechanism.

Multiscale Modeling Unravels the Influence of Biomembranes on the Photochemical Properties of Embedded Anti-Oxidative Polyphenolic and Phenanthroline Chelating Dyes

The embedding of caffeate methyl ester, the flavonoids luteolin and quercetin, and the o-phenanthroline and neocuproine in a liquid disordered lipid bilayer has been studied through extensive atomistic calculations. The location and the orientation of these bio-active antioxidants are explained and analyzed. While the two phenanthrolines strongly associate with the lipid tail region, the other three compounds are rather found among the head groups. The simulations showcase conformational changes of the flavonoids. Through the use of a hybrid quantum mechanics-molecular mechanics scheme and supported by a profound benchmarking of the electronic excited-state method for these compounds, the influence of the anisotropic environment on the compounds' optical properties is analyzed. Influences of surrounding water molecules and of the polar parts of the lipids on the transition dipole moments and excited-state dipole moments are weighted with respect to a change in conformation. The current study highlights the importance of the mapping of molecular interactions in model membranes and pinpoints properties, which can be biomedically used to discriminate and detect different lipid environments.