Two-photon absorption properties of proquinoidal D-A-D and A-D-A quadrupolar chromophores

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.

Diagrammatic theory of magnetic and quadrupolar contributions to sum-frequency generation in composite systems

Second-order nonlinear processes like Sum-Frequency Generation (SFG) are essentially defined in the electric dipolar approximation. However, when dealing with the SFG responses of bulk, big nanoparticles, highly symmetric objects, or chiral species, magnetic and quadrupolar contributions play a significant role in the process too. We extend the diagrammatic theory for linear and nonlinear optics to include these terms for single objects as well as for multipartite systems in interaction. Magnetic and quadrupolar quantities are introduced in the formalism as incoming fields, interaction intermediates, and sources of optical nonlinearity. New response functions and complex nonlinear processes are defined, and their symmetry properties are analyzed. This leads to a focus on several kinds of applications involving nanoscale coupled objects, symmetric molecular systems, and chiral materials, both in line with the existing literature and opening new possibilities for original complex systems.

Alkoxy-Substituted Quadrupolar Fluorescent Dyes

Polar and polarizable π-conjugated organic molecules containing push-pull chromophores have been investigated extensively in the past. Identifying unique backbones and building blocks for fluorescent dyes is a timely exercise. Here, we report the synthesis and characterization of a series of fluorescent dyes containing quadrupolar A-D-A constitutions (where A = acceptor and D = donor), which exhibit fluorescence emission at a variety of different wavelengths. We have investigated the effects of different electron-withdrawing groups, located at both termini of a para-terphenylene backbone, by steady-state UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium units are also introduced during the construction of the quadrupolar backbones. Depending on the quadrupolarity, fluorescence emission wavelengths cover from 380 to 557 nm. Time-resolved absorption and emission spectroscopy reveal that the photophysical properties of those quadrupolar dyes result from intramolecular charge transfer. One of the dyes we have investigated is a symmetrical box-like tetracationic cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic to cells up to a loading concentration of 1 μM, has been employed in live-cell imaging. When taken up by cells, the tetrachloride emits a green fluorescence emission without any hint of photobleaching or disruption of normal cell behavior. We envision that our design strategy of modifying molecules through the functionalization of the quadrupolar building blocks as chromophores will lead to future generations of fluorescent dyes in which these A-D-A constitutional fragments are incorporated into more complex molecules and polymers for broader photophysical and biological applications.

A universal strategy for the fabrication of single-photon and multiphoton NIR nanoparticles by loading organic dyes into water-soluble polymer nanosponges

The development of optical organic nanoparticles (NPs) is desirable and widely studied. However, most organic dyes are water-insoluble such that the derivatization and modification of these dyes are difficult. Herein, we demonstrated a simple platform for the fabrication of organic NPs designed with emissive properties by loading ten different organic dyes (molar masses of 479.1-1081.7 g/mol) into water-soluble polymer nanosponges composed of poly(styrene-alt-maleic acid) (PSMA). The result showed a substantial improvement over the loading of commercial dyes (3.7-50% loading) while preventing their spontaneous aggregation in aqueous solutions. This packaging strategy includes our newly synthesized organic dyes (> 85% loading) designed for OPVs (242), DSSCs (YI-1, YI-3, YI-8), and OLEDs (ADF-1-3, and DTDPTID) applications. These low-cytotoxicity organic NPs exhibited tunable fluorescence from visible to near-infrared (NIR) emission for cellular imaging and biological tracking in vivo. Moreover, PSMA NPs loaded with designed NIR-dyes were fabricated, and photodynamic therapy with these dye-loaded PSMA NPs for the photolysis of cancer cells was achieved when coupled with 808 nm laser excitation. Indeed, our work demonstrates a facile approach for increasing the biocompatibility and stability of organic dyes by loading them into water-soluble polymer-based carriers, providing a new perspective of organic optoelectronic materials in biomedical theranostic applications.

Strong absorber vs. strong emitter in extended π-conjugated systems: a carbo-benzene – benzothiadiazole chromophore

In the search for photoluminescent carbo-benzenes, with the view to measuring the two-photon absorption cross-section by the TPEF method, thienylbenzothiadiazole groups are envisaged as intrinsic fluorophoric substituents through acetylenic linkers.

Resonance-Enhanced Two-Photon Absorption and Optical Power Limiting Properties of Three-Dimensional Perylene Bisimide Derivatives

Push-pull organic structures characterized by an intramolecular charge transfer (ICT) process and π-electron delocalization are potentially interesting luminescent materials. A series of three-dimensional o-carborane-containing perylene bisimide derivatives (PBIs) were synthesized, and their optical properties were systematically investigated to illustrate the stereo effect, especially on the two-photon absorption (2PA) and optical power limiting (OPL) properties. Open-aperture Z-scan curves showed that all four PBIs displayed strong and broad two-photon absorptivities based on the resonance-enhanced phenomenon. The maximum degenerate two-photon absorption cross section (δ_2PA) increased with the number of PBI substituents. The derivative CB-PBI possessed a δ_2PA value of ∼2400 GM at 650 nm, a significant enhancement in comparison with that of the parent PBI (∼719 GM), ascribed to the present stereo effect. When the aromatic-donating units changed from naphthyl and pyrenyl to PBI, the generated multidimensional intramolecular charge transfer (ICT) from the aromatic units to the o-carborane cage contributed to the 2PA processes. All of the fluorophores exhibited excellent optical power limiting (OPL) performances as well as a minimum limiting threshold of ∼4.98 mJ/cm² for CB-PBI. These significant results not only allow us to get deep insight into the nature of the fundamental stereo effect and nonlinear optical (NLO) response involved but also guide us toward the design of new multifunctional luminescent materials.

Prospects for More Efficient Multi-Photon Absorption Photosensitizers Exhibiting Both Reactive Oxygen Species Generation and Luminescence

The use of two-photon absorption (TPA) for such applications as microscopy, imaging, and photodynamic therapy (PDT) offers several advantages over the usual one-photon excitation. This creates a need for photosensitizers that exhibit both strong two-photon absorption and the highly efficient generation of reactive oxygen species (ROS), as well as, ideally, bright luminescence. This review focuses on different strategies utilized to improve the TPA properties of various multi-photon absorbing species that have the required photophysical properties. Along with well-known families of photosensitizers, including porphyrins, we also describe other promising organic and organometallic structures and more complex systems involving organic and inorganic nanoparticles. We concentrate on the published studies that provide two-photon absorption cross-section values and the singlet oxygen (or other ROS) and luminescence quantum yields, which are crucial for potential use within PDT and diagnostics. We hope that this review will aid in the design and modification of novel TPA photosensitizers, which can help in exploiting the features of nonlinear absorption processes.

Two-Photon Fluorescent Nanomaterials and Their Applications in Biomedicine

In recent years, two-photon excited (TPE) materials have attracted great attentions because of their excellent advantages over conventional one-photon excited (OPE) materials, such as deep tissue penetration, three-dimensional spatial selectivity and low phototoxicity. Also, they have been widely applied in lots of field, such as biosensing, imaging, photo-catalysis, photoelectric conversion, and therapy. In this article, we review recent advances in vibrant topic of two-photon fluorescent nanomaterials, including organic molecules, quantum dots (QDs), carbon dots (CDs) and metal nanoclus-ters (MNCs). The optical properties, synthetic methods and important applications of TPE nanomaterials in biomedical field, such as biosensing, imaging and therapy are introduced. Also, the probable challenges and perspectives in the forthcoming development of two-photon fluorescent nanomaterials are addressed.

Unlocking the action mechanisms of molecular nonlinear optical absorption for optical conjugated polymers under aggregation states

Controlling the molecular microstructure and the molecular aggregation state under different conditions to improve the MNOA performance of OCPs.

Diphenyl sulfone-based A–π-D–π-A dyes as efficient initiators for one-photon and two-photon initiated polymerization

Diphenyl sulfone-based symmetrical A–π-D–π-A dyes as efficient initiators for one-photon and two-photon initiated polymerization.

Investigations of Thienoacene Molecules for Classical and Entangled Two-Photon Absorption

Investigations of the optical effects in thienoacene chromophores with different central atoms were performed. These chromophores provide a basis for the comparison of the linear, two-photon, and entangled two-photon properties in organic molecules with varying degrees of dipolar or quadrupolar character. Linear absorption and emission as well as time-dependent density functional theory calculations were performed for the chromophores investigated. Measurements of the classical two-photon absorption (TPA), entangled two-photon absorption (ETPA), as well as entangled two-photon fluorescence were experimentally performed for the four chromophores. Electronic structure calculations were utilized to provide estimates of the classical two-photon absorption coefficients. The results of the measured entangled two-photon cross sections were compared with theoretical estimates for the molecules investigated. It is found that the dipole (transition or permanent) pathway can have an effect on the trends in the entangled photon absorption process in dipolar organic chromophores. This study helps predict the properties of the entangled two-photon effect in chromophores with different dipolar and quadrupolar character.

Laser-Induced Antibacterial Activity of Novel Symmetric Carbazole-Based Ethynylpyridine Photosensitizers

In this study, two kinds of novel carbazole-based ethynylpyridine salts: 3,6-bis[2-(1-methylpyridinium)ethynyl]-9-pentyl-carbazole diiodide (BMEPC) and 3,6-bis[2-(1-methylpyridinium)ethynyl]-9-methyl-carbazole diiodide (BMEMC) have been employed as photosensitizers owing to their excellent antibacterial activity. These molecules possess symmetric A-π-D-π-A-type structures, which would bring in the unique optical properties. The inhibition zone measurement of a gradient concentration from 0 to 100 μM showed BMEPC and BMEMC photoinduced antibacterial activity against Escherichia coli. Diameters of zone of inhibition were up to 15 and 14 mm under laser irradiations. Under the exposure of the laser of 442 nm with a power density of 20 mW/cm2, the minimum inhibitory concentrations (MICs) of BMEPC on E. coli were between 3.5 and 6.9 μM and that of BMEMC were between 9.4 and 18.8 μM, respectively. In the dark experiments as a control, the MIC value is between 6.9 and 13.8 μM for BMEPC, whereas it is between 187.5 and 225.0 μM for BMEMC. By the comparison of the MIC values of BMEPC and BMEMC with laser irradiation and in dark, the laser-induced toxicity on bacteria is more evident, though both of the derivatives have dark toxicity. With the laser irradiation duration of 30 s and 10 min for BMEPC and BMEMC, respectively, the survival rate of E. coli approximates zero. An antibacterial mechanism has been proposed based on the electron paramagnetic resonance characterization, which indicates that a nitride radical is generated under laser irradiation. The carbazole-based ethynylpyridine photosensitizers would provide high potential for further applications in photodynamic therapy.

A combined theoretical and experimental investigation on the influence of the bromine substitution pattern on the photophysics of conjugated organic chromophores

Two-photon photosensitizers with heavy atom substituents were synthesized and evaluated through a combined photophysical and computational study.

Symmetry Breaking in Pyrrolo[3,2-b]pyrroles: Synthesis, Solvatofluorochromism and Two-photon Absorption

Five centrosymmetric and one dipolar pyrrolo[3,2-b]pyrroles, possessing either two or one strongly electron-withdrawing nitro group have been synthesized in a straightforward manner from simple building blocks. For the symmetric compounds, the nitroaryl groups induced spontaneous breaking of inversion symmetry in the excited state, thereby leading to large solvatofluorochromism. To study the origin of this effect, the series employed peripheral structural motifs that control the degree of conjugation via altering of dihedral angle between the 4-nitrophenyl moiety and the electron-rich core. We observed that for compounds with a larger dihedral angle, the fluorescence quantum yield decreased quickly when exposed to even moderately polar solvents. Reducing the dihedral angle (i.e., placing the nitrobenzene moiety in the same plane as the rest of the molecule) moderated the dependence on solvent polarity so that the dye exhibited significant emission, even in THF. To investigate at what stage the symmetry breaking occurs, we measured two-photon absorption (2PA) spectra and 2PA cross-sections (σ_2PA ) for all six compounds. The 2PA transition profile of the dipolar pyrrolo[3,2-b]pyrrole, followed the corresponding one-photon absorption (1PA) spectrum, which provided an estimate of the change of the permanent electric dipole upon transition, ≈18 D. The nominally symmetric compounds displayed an allowed 2PA transition in the wavelength range of 700-900 nm. The expansion via a triple bond resulted in the largest peak value, σ_2PA =770 GM, whereas altering the dihedral angle had no effect other than reducing the peak value two- or even three-fold. In the S₀ →S₁ transition region, the symmetric structures also showed a partial overlap between 2PA and 1PA transitions in the long-wavelength wing of the band, from which a tentative, relatively small dipole moment change, 2-7 D, was deduced, thus suggesting that some small symmetry breaking may be possible in the ground state, even before major symmetry breaking occurs in the excited state.

Charge Transfer Through Dithieno[2,3-a:3',2'-c]phenazine: Effect of Substitution Pattern on the Optoelectronic Properties of Regioisomeric Luminophores

Two series of regioisomeric luminophores that contained a dithieno[2,3-a:3',2'-c]phenazine (DTP) unit as an electron acceptor have been designed and synthesized. To investigate the effect of substitution pattern on the optoelectronic properties of these luminophores, electron donors (N,N-dihexylaniline or N,N-dihexyl-4-vinylaniline) were incorporated at the 2,5-, 8,11-, and 9,10-positions of the DTP unit. We found that the optoelectronic properties of the regioisomeric luminophores were greatly affected by the substitution pattern: functionalization at the 8,11-positions of the DTP unit was superior to the other two substitution patterns in extending the effective π-conjugation and strengthening the intramolecular charge-transfer interactions. Moreover, the insertion of vinyl groups between the DTP and N,N-dihexylaniline units narrowed the energy band-gap for isomers 4 and 5. However, hypsochromically shifted absorption and photoluminescence maxima were observed for isomeric luminophore 6, in which electron donors were substituted at the 2,5-positions of the DTP unit. These results should facilitate greater understanding of the structure-property relationships in regioisomeric semiconductors and present a new way to design optoelectronic materials with effective substitution patterns.

A two excited state model to explain the peculiar photobehaviour of a flexible quadrupolar D–π–D anthracene derivative

The excited state deactivation of the investigated compound involves two distinct fluorescent states. ¹A* → ¹B* transition is accompanied by structural motions, with the ¹B* state exhibiting a certain ICT character.

Novel A–(π–D–π–A)1–3 branched fluorophores displaying high two-photon absorption

Novel A–(π–D–π–A)₃ compounds bearing pyridine end groups are apparently effective in achieving large two-photon responses owing to strong charge transfer.

The application of double click to synthesize a third-order nonlinear polymer containing donor–acceptor chromophores

A new linear polymer containing the dialkylaniline-substituted electron-rich alkynes in the side chains was designed and synthesized by copper-catalyzed azide–alkyne cycloaddition (CuAAC).

Pyrrolo[3,2-b]pyrroles-From Unprecedented Solvatofluorochromism to Two-Photon Absorption

A combined experimental and theoretical study of the two-photon absorption (2PA) properties of a series of quadrupolar molecules possessing a highly electron-rich heterocyclic core, pyrrolo[3,2-b]pyrrole, is presented. In agreement with quantum-chemical calculations, large 2PA cross-section values, σ2PA ≈10(2) -10(3)  GM (1 GM=10(50)  cm(4)  s photon(-1) ), are observed at wavelengths of 650-700 nm, which correspond to the two-photon allowed but one-photon forbidden transitions. The calculations also predict that increased planarity of this molecule through removal of two N-substituents leads to further increase in the σ2PA values. Surprisingly, the most quadrupolar pyrrolo[3,2-b]pyrrole derivative, containing two 4-nitrophenyl substituents at positions 2 and 5, demonstrates a very strong solvatofluorochromic effect, with a fluorescence quantum yield as high as 0.96 in cyclohexane, whereas the fluorescence vanishes in DMSO.

Design of diethynyl porphyrin derivatives with high near infrared fluorescence quantum yields

A design strategy for (porphinato)zinc-based fluorophores that possess large near infrared fluorescence quantum yields is described. These fluorophores are based on a (5,15-diethynylporphinato)zinc(II) framework and feature symmetric donor or acceptor units appended at the meso-ethynyl positions via benzo[c][1,2,5]thiadiazole moieties. These (5,15-bis(benzo[c][1′,2′,5′]thiadiazol-4′-ylethynyl)-10,20-bis[2′,6′-bis(3″,3″-dimethyl-1″-butyloxy)phenyl]porphinato)zinc(II) (4), (5,15-bis[4′-(N,N-dihexylamino) benzo[c][1′,2′,5′]thiadiazol-7′-ylethynyl]-10,20-bis[2′,6′-bis(3″,3″-dimethyl-1″-butyloxy)phenyl]porphinato)zinc(II) (5), (5,15-bis([7′-(4″-n-dodecyloxyphenylethynyl)benzo[c][1′,2′,5′]thiadiazol-4′-yl]ethynyl)-10,20-bis[2′,6′-bis(3″,3″-dimethyl-1″-butyloxy)phenyl]porphinato)zinc(II) (6), (5,15-bis([7′-([7″-(4″ ′-n-dodecyloxyphenyl)benzo[c][1″,2″,5″]thiadiazol-4″-yl]ethynyl)benzo[c][1′,2′,5′]thiadiazol-4′-yl]ethynyl)-10,20-bis[2′,6′-bis(3″,3″-dimethyl-1″-butyloxy)phenyl]porphinato)zinc(II) (7), 5,15-bis ([7′-(4″-N,N-dihexylaminophenylethynyl)benzo[c][1′,2′,5′]thiadiazol-4′-yl]ethynyl)-10,20-bis[2′,6′-bis(3″,3″-dimethyl-1″-butyloxy)phenyl]porphinato)zinc(II) (8), and (5,15-bis([7′-(4″-N,N-dihexylaminophenylethenyl)benzo[c][1′,2′,5′]thiadiazol-4′-yl]ethynyl)-10,20-bis[2′,6′-bis(3″,3″-dimethyl-1″-butyloxy)phenyl]porphinato)zinc(II) (9) chromophores possess red-shifted absorption and emission bands that range between 650 and 750 nm that bear distinct similarities to those of the chlorophylls and structurally related molecules. Interestingly, the measured radiative decay rate constants for these emitters track with the integrated oscillator strengths of their respective x-polarized Q-band absorptions, and thus define an unusual family of high quantum yield near infrared fluorophores in which emission intensity is governed by a simple Strickler–Berg dependence.

Photoinduced symmetry-breaking intramolecular charge transfer in a quadrupolar pyridinium derivative

We report here a joint experimental and theoretical study of a quadrupolar, two-branched pyridinium derivative of interest as a potential non-linear optical material. The spectral and photophysical behaviour of this symmetric system is greatly affected by the polarity of the medium. A very efficient photoinduced intramolecular charge transfer, surprisingly more efficient than in the dipolar asymmetric analogue, is found to occur by femtosecond resolved transient absorption spectroscopy. TD-DFT calculations are in excellent agreement with these experimental findings and predict large charge displacements in the molecular orbitals describing the ground state and the lowest excited singlet state. The theoretical study also revealed that in highly polar media the symmetry of the excited state is broken giving a possible explanation to the fluorescence and transient absorption spectra resembling those of the one-branched analogous compound in the same solvents. The present study may give an important insight into the excited state deactivation mechanism of cationic (donor-π-acceptor-π-donor)(+) quadrupolar compounds characterised by negative solvatochromism, which are expected to show significant two-photon absorption (TPA). Moreover, the water solubility of the investigated quadrupolar system may represent an added value in view of the most promising applications of TPA materials in biology and medicine.

Synthesis of dithienosilole-based highly photoluminescent donor-acceptor type compounds

Highly photoluminescent acceptor-donor-acceptor (A-D-A) and donor-acceptor (D-A) type compounds with a dithienosilole unit as the donor and perfluorotolyl or dimesitylboryl group(s) as the acceptor were prepared by the reaction of lithiated dithienosilole derivatives with perfluorotoluene or fluorodimesitylborane, respectively. The resulting A-D-A and D-A type compounds showed red-shifted UV absorption and PL bands compared to those of simple dithienosiloles having no acceptor units, reported previously, and were highly photoluminescent in the solid state as well as in solution. Solvatochromic behaviour that would arise from the intramolecular donor-acceptor interaction were observed for the D-A type compounds with respect to the UV absorption and PL spectra. In addition, it was found that bis(dimesitylboryl)dithienosilole and (dimesitylboryl)(methylthio)dithienosilole responded to coexisting fluoride anions, leading to clear UV absorption and PL spectral changes in solutions.

Nonlinear optical properties for a class of hexa-peri-hexabenzocoronene chromophores: a computational investigation

The systematic investigation of the linear and nonlinear optical properties on such a class of hexa-peri-hexabenzocoronene (HBC) chromophores is of significance for rationally designing two-photon absorption (TPA) materials. The results indicate that increasing the strength of electron-donating or accepting terminal groups leads to bathochromic-shift of the absorption band and enhancement of the TPA cross section (δ(max)). For the molecules with fluorinated methylene and cyano substituents, replacement of a double bond by a triple bond in the conjugated linker produces the increase of δ(max), owing to the lower bond-length alternation and better rigidity of phenylene-ethynylene. In contrast, for the molecules with nitro groups, the similar replacement results in a slight decrease of δ(max) because N-HBC-E-NO2 has excellent planarity architecture and effective electronic coupling. The TPA spectra are red-shifted and the δ(max) values are enhanced as the number of branches increases. Thus, a solvatochromism effect has a positive influence on the TPA response of the nitryl compounds due to larger polarization of the nitro moiety. We have shed light on the linear relationship between the first hyperpolarizability and δ(max). These HBC derivatives hold potential as high-performance nonlinear optical materials.