Two-photon transitions in quadrupolar and branched chromophores: experiment and theory

New Multifunctional Bipyrimidine-Based Chromophores for NLO-Active Thin-Film Preparation

New synthesized bipyrimidine-based chromophores presenting alkoxystyryl donor groups carrying aliphatic achiral and chiral chains in the 4 position, connected to electron-accepting 2,2-bipyrimidine cores have been synthesized. Their linear and nonlinear optical (NLO) properties were investigated as well as their mesomorphic properties by various techniques (light-transmission measurements, polarized-light optical microscopy, differential scanning calorimetry measurements and two-photon excited fluorescence). The derivatives with achiral linear carbon chains were found to exhibit liquid-crystal properties with the formation smectic phases over large temperature ranges, which were confirmed by small-angle X-ray scattering analysis via stacking models. The nonlinear optical properties in the solid state for derivatives with C14 and the citronellol chains have been studied by wide-field second-harmonic generation and multi-photon fluorescence imaging, confirming centrosymmetry for these achiral mesogens and their excellent third-order nonlinearity whereas the chiral compound exhibits non-centrosymmetric organization resulting in a strong Second Harmonic Generation at the crystal state.

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.

In Silico Designing of Thieno[2,3-b]thiophene Core-Based Highly Conjugated, Fused-Ring, Near-Infrared Sensitive Non-fullerene Acceptors for Organic Solar Cells

The performance of organic solar cells (OSCs) has been improving steadily over the last few years, owing to the optimization of device fabrication, fine-tuning of morphology, and thin-film processing. Thiophene core containing fused ring-type non-fullerene acceptors (NFAs) achieved significant proficiency for highly efficient OSCs. Quantum chemical computations are utilized herein with the motive of suggesting new NIR sensitive, highly efficient low-band gap materials for OSCs. A series of extended conjugated A-π-D-π-A architectured novel fused-ring NFAs (FUIC-1-FUIC-6) containing thieno[2,3-b]thiophene-based donor core are proposed by substituting the end-capped units of synthesized molecule F10IC. Different properties including frontier molecular orbital analysis, density of states analysis, transition density matrix analysis, excitation energy, reorganizational energies of both holes (λ_h) and electrons (λ_e), and open-circuit voltage (V _oc) were performed employing the density functional theory approach. Charge transfer analysis of the best-designed molecule with the donor complex was analyzed to comprehend the efficiency of novel constructed molecules (FUIC-1-FUIC-6) and compared with the reference. End-caped acceptor alteration induces the reduction of the energy gap between HOMO-LUMO (1.88 eV), tunes the energy levels, longer absorption in the visible and near-infrared regions, larger V _oc, smaller reorganizational energies, and binding energy values in designed structures (FUIC-1-FUIC-6) in comparison to reference (FUIC). The designed molecules show the best agreement with the PTBT-T donor polymer blend and cause the highest charge from the HOMO to the LUMO orbital. Our findings predicted that thieno[2,3-b] thiophene-based newly designed molecules would be efficient NFAs with outstanding photovoltaic characteristics and can be used in future applications of OSCs.

Tris(4'-Nitrobiphenyl)amine─An Octupolar Chromophore with High Two-Photon Absorption Cross-Section and Its Application for Uncaging of Calcium Ions in the Near-Infrared Region

Compounds with high two-photon absorption (2PA) performance in the near-infrared region have attracted great attention because of their application in the material and biological science. In this study, we have developed a simple and novel octupolar chromophore, tris(4'-nitrobiphenyl)amine 1, with three nitro peripheral groups attached to a triphenylamine core via biphenyl linkers. A mono-branched analogue 2 has also been prepared to investigate the effects of octupolar and dipolar systems on photophysical and 2PA behaviors. Compound 1, despite having a much simpler structure than the previous three-branched scaffolds, exhibits comparable σ₂ values, reaching 1330 GM at 730 nm and 900 GM at 820 nm in toluene. Combined with an outstanding σ₂/MW ratio (2.2 GM g^-1 mol) and a high fluorescence quantum yield (0.51), 1 displays potential as a promising two-photon (2P) probe for bioimaging. Subsequently, the ethylene glycol tetraacetic acid-substituted derivatives featuring octupolar (3 and 5) or dipolar (4 and 6) character have been synthesized and their one-photon (1P) and 2P photochemical reactions have been examined. Finally, 1P- and 2P-triggered uncaging of Ca²⁺ from these calcium chelators has been confirmed.

Ultrafast Evaluation of Two-Photon Absorption with Simplified Time-Dependent Density Functional Theory

This work presents the theoretical background to evaluate two-photon absorption (2PA) cross-sections in the framework of simplified time-dependent density functional theory (sTD-DFT). Our new implementation allows the ultrafast evaluation of 2PA cross-sections for large molecules based on a regular DFT ground-state determinant as well as a variant employing our tight-binding sTD-DFT-xTX flavor for very large systems. The method is benchmarked against higher-level calculations for trans-stilbene and typical fluorescent protein chromophores. For eGFP, a quadrupolar chromophore and its branched version, the flavine mono-nucleotide, and the iLOV protein, we compare sTD-DFT 2PA spectra to experimental ones. This includes extension and testing of our all-atom quantum chemistry methodology for the evaluation of 2PA for a system of ∼2000 atoms, providing striking agreement with the experimental spectrum.

Two-Photon Absorption Cooperative Effects within Multi-Dipolar Ruthenium Complexes: The Decisive Influence of Charge Transfers

One- and two-photon characterizations of a series of hetero- and homoleptic [RuL_3-n(bpy)_n]²⁺ (n = 0, 1, 2) complexes carrying bipyridine π-extended ligands (L), have been carried out. These π-extended D−π−A−A−π−D-type ligands (L), where the electron donor units (D) are based on diphenylamine, carbazolyl, or fluorenyl units, have been designed to modulate the conjugation extension and the donating effect. Density functional theory calculations were performed in order to rationalize the observed spectra. Calculations show that the electronic structure of the π-extended ligands has a pronounced effect on the composition of HOMO and LUMO and on the metallic contribution to frontier MOs, resulting in strikingly different nonlinear properties. This work demonstrates that ILCT transitions are the keystone of one- and two-photon absorption bands in the studied systems and reveals how much MLCT and LLCT charge transfers play a decisive role on the two-photon properties of both hetero- and homoleptic ruthenium complexes through cooperative or suppressive effects.

Frequency-Upconverted Stimulated Emission by Up to Six-Photon Excitation from Highly Extended Spiro-Fused Ladder-Type Oligo(p-phenylene)s

Frequency-upconverted fluorescence and stimulated emission induced by multiphoton absorption (MPA) have attracted much interest. As compared with low-order MPA processes, the construction of high-order MPA processes is highly desirable and rather attractive, yet remains a formidable challenge due to its inherent low transition probability. We report the observation of the first experimental frequency-upconverted fluorescence and stimulated emission by simultaneous six-photon excitation in an organic molecular system. The well-designed organic conjugated system based on cross-shaped spiro-fused ladder-type oligo(p-phenylene)s (SpL-z, z=1-3) manifests reasonably high MPA cross-sections and brilliant luminescence emission simultaneously. The six-photon absorption cross-section of SpL-3 with an extended π-conjugation was evaluated as 8.67×10^-169  cm¹²  s⁵  photon^-5 . Exceptionally efficient 2- to 6-photon excited stimulated emission was achieved under near-infrared laser excitation.

Effects of Hydroxyl Group on the Interaction of Carboxylated Flavonoid Derivatives with S. Cerevisiae α-Glucosidase

Introduction

Carboxyalkyl flavonoids derivatives are considered as effective inhibitors in reducing post-prandial hyperglycaemia.

Methods

Combined with Density Functional Theory (DFT) and the theory of Atoms in Molecules (AIM), molecular docking and charge density analysis are carried out to understand the molecular flexibility, charge density distribution and the electrostatic properties of these carboxyalkyl derivatives.

Results

Results show that the electron density of the chemical bond C14-O17 on B ring of molecule II increases while O17-H18 decreases at the active site, suggesting the existence of weak non-covalent interactions, most prominent of which are H-bonding and electrostatic interaction. When hydroxyl groups are introduced, the highest positive electrostatic potentials are distributed near the B ring hydroxyl hydrogen atom and the carboxyl hydrogen atom on the A ring. It was reported that quercetin has a considerably inhibitory activity to S. cerevisiae α-glucosidase, from the binding affinities, it is suggested that the position and number of hydroxyl groups on the B and C rings are also pivotal to the hypoglycemic activity when the long carboxyalkyl group is introduced into the A ring.

Conclusion

It is concluded that the presence of three well-defined zones in the structure, both hydrophobicity alkyl, hydrophilicity carboxyl and hydroxyl groups are necessary.

Photophysics, Electrochemistry and Efficient Electrochemiluminescence of Trigonal Truxene-Core Dyes

We synthesized and characterized a series of dyes built from a spirofluorene or truxene core. The quadrupolar spirofluorene system is the initial building unit for the design and preparation of more complex star-shaped dyes consisting of a truxene core bearing three di- or triphenylamine moieties with or without a thiophene connector. Their photophysical, electrochemical, and electrochemiluminescence (ECL) properties were first investigated in solution. Structure/activity relationships were derived and rationalized by comparing the quadrupolar system and trigonal truxene-core derivatives using computational studies. The photophysical and redox characteristics are drastically tuned by the introduction of a thiophene bridge and electron-donor substituents at their terminal branches. These comparative studies show the essential role of the stability of both radical cations and anions to obtain efficient ECL dyes. The stabilization of the radicals is directly related to the charge delocalization due to the π-conjugation by the thiophene bridge. The brightest ECL is achieved by annihilation and coreactant (benzoyl peroxide) pathways with the blue-emitting truxene dye, which is 2- and 4.5-times greater than that of the quadrupolar compound and reference [Ru(bpy)₃ ]²⁺ emitter, respectively. Such an extensive study on these extended π-conjugated molecules presenting different core structures may guide the design and synthesis of new ECL dyes with a strong efficiency.

Branching effect on the linear and nonlinear optical properties of styrylpyrimidines

The branching effect on the photophysical properties of styrylpyrimidines is studied experimentally and theoretically.

Tuning of hyperpolarizability, and one- and two-photon absorption of donor-acceptor and donor-acceptor-acceptor-type intramolecular charge transfer-based sensors

The present work aims to study the effect of solvent as well as arrangement of donor-acceptor groups on linear and non-linear optical (NLO) response properties of two experimentally studied intramolecular charge-transfer (ICT)-based fluorescent sensors. One of them (molecule 1) is a donor-acceptor (D-A) system with hemicyanine and dimethylanilino as electron withdrawing and donating groups, respectively, while the other one (molecule 3) is molecule 1 fused with a boron-dipyrromethene (BODIPY) moiety. BODIPY acts as the electron acceptor group of molecule 2 that as well consists of dimethylanilino as the electron donor. Density functional theory (DFT) as well as time-dependent DFT has been employed to optimize the geometry of the molecules, followed by computation of dipole moment (μ), static first hyperpolarizability (β_total), and one- and two-photon absorption (TPA) strengths. The results reveal that dipole moment as well as total static first hyperpolarizability (β_total) of the studied molecules is dominated by the respective components in the direction of charge transfer. The ratio of vector component of first hyperpolarizability (β_vec) to β_total also supports the unidirectional charge transfer in the studied systems. In molecule 3, which is a donor-acceptor-acceptor (D-A-A)-type system, the BODIPY moiety is found to play a major role in controlling the NLO response over the other acceptor group. Solvents are also found to play an important role in controlling the linear as well as NLO response of the studied systems. A significant increase in the first hyperpolarizability as well as TPA cross-section of the studied molecules is predicted due to an increase in the dielectric constant of the medium. The results presented are expected to provide a clue in tuning the NLO response of many ICT-based chromophores, especially those with D-A-A arrangements.

Dendrimeric Nanoparticles for Two-Photon Photodynamic Therapy and Imaging: Synthesis, Photophysical Properties, Innocuousness in Daylight and Cytotoxicity under Two-Photon Irradiation in the NIR

The synthesis and the photophysical properties of a new class of fully organic monodisperse nanoparticles for combined two-photon imaging and photodynamic therapy are described. The design of such nanoparticles is based on the covalent immobilization of a dedicated quadrupolar dye that combines excellent two-photon absorbing (2PA) properties, fluorescence and singlet oxygen generation ability, in a phosphorous-based dendrimeric architecture. First, a bifunctional quadrupolar dye bearing two different grafting moieties, a phenol function and an aldehyde function, was synthesized. It was then covalently grafted through its phenol function to a phosphorus-based dendrimer scaffold of generation 1. The remaining aldehyde functions were then used to continue the dendrimer synthesis up to generation 2, introducing finally 24 water-solubilizing triethyleneglycol chains at its periphery. A dendrimer confining 12 photoactive quadrupolar units in its inner scaffold and showing water solubility was thus obtained. Interestingly, the G1 and G2 dendrimers retain some fluorescence as well as significant singlet oxygen production efficiencies while they were found to show very high 2PA cross-sections in a broad range of the NIR biological spectral window. Hydrophilic dendrimer G2 was tested in vitro on breast cancer cells, first in one- and two-photon microscopy, which allowed for visualization of their cell internalization, then in two-photon photodynamic therapy. While being nontoxic in the dark and, more importantly, under exposure to daylight, dendrimer G2 proved to be a very efficient cell-death inducer only under two-photon irradiation in the NIR.

Diphenylamino-substituted tristyryl vs. triphenyl isocyanurates: improved conjugation has minimal impact on two-photon absorption

Planarization and extension of the central π-system does not increase two-photon absorption cross-section.

The role of intramolecular charge transfer and symmetry breaking in the photophysics of pyrrolo[3,2-b]pyrrole-dione

This paper reveals structurally unique π-expanded pyrrolo[3,2-b]pyrrole and its non-typical photophysical behaviour.

Triphenylamine-based allylidenemalononitrile chromophores: synthesis, and photophysical and second-order nonlinear optical properties

A series of NLO chromophores based on a mono-, di- or tri-substituted triphenylamine core and allylidenemalononitrile fragments has been designed.

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.

Interplay of twist angle and solvents with two-photon optical channel interference in aryl-substituted BODIPY dyes

Channel interference plays a crucial role in understanding the physics behind multiphoton absorption processes. In this work, we study the role of channel interference and solvent effects on the two-photon absorption in aryl-substituted boron dipyrromethene (BODIPY) dyes, a class of intramolecular charge-transfer (ICT) molecules. For this purpose, we consider fourteen dyes of this class with various donor/acceptor substitutions at the para position of the phenyl ring and with or without methyl (-CH₃) substitution on the BODIPY moiety. The presence of a methyl group on the BODIPY moiety affects the dihedral angle significantly, which in turn affects the one- (OPA) and two-photon absorption (TPA) properties of the molecules. Among the molecules studied, the one having the strong electron-donating dimethylamino group and no methyl substitution at the BODIPY moiety is found to have the highest TPA cross section. Our few-state model analysis shows that the large TPA activity of this molecule is due to the all positive contributions from different channel interference terms. Change in dielectric constant of the medium is found to have a profound impact on both the magnitude and sign of the channel interference terms. The magnitude of destructive channel interference gradually decreases with decreasing solvent polarity and becomes constructive in a low-polarity solvent. We also study the effect of rotating the phenyl ring with respect to the BODIPY moiety on the TPA activity. In the gas phase and in different solvents, we found that channel interference is changed from destructive to constructive on twisting the molecule. These results are explained by considering different dipole-, energy- and angle-terms appearing in the expression of a two-state model.

A biocompatible macromolecular two-photon initiator based on hyaluronan

The possibility of the direct encapsulation of living cells via two-photon induced photopolymerization enables the microfabrication of hydrogel scaffolds with high initial cell loadings and intimate matrix-cell contact. While highly efficient water-soluble two-photon initiators based on benzylidene ketone dyes have been developed, they exhibit considerable cyto- and phototoxicity. To address the problem of photoinitiator migration from the extracellular matrix into the cytosol, a two-photon initiator bound to a polymeric hyaluronan backbone (HAPI) was synthesized in this work. HAPI exhibited a distinct improvement of cytocompatibility compared to a reference two-photon initiator. Basic photophysical investigations were performed to characterize the absorption and fluorescence behavior of HAPI. Laser scanning microscopy was used to visualize and confirm the hindered transmembrane migration behavior of HAPI. The performance of HAPI was tested in two-photon polymerization at exceedingly high printing speeds of 100 mm s-1 producing gelatin-based complex 3D hydrogel scaffolds with a water content of 85%. The photodamage of the structuring process was low and viable MC3T3 cells embedded in the gel were monitored for several days after structuring.

Photoinitiators for two-photon polymerisation: effect of branching and viscosity on polymerisation thresholds

The effect of the photoinitiator branching on polymerisation thresholds in a series of triphenylamine ethyl ester derivatives is studied.

Ultrafast electronic energy relaxation in a conjugated dendrimer leading to inter-branch energy redistribution

Dendrimers are arrays of coupled chromophores, where the energy of each unit depends on its structure and conformation.

Excited-state localization and energy transfer in pyrene core dendrimers with fluorene/carbazole as the dendrons and acetylene as the linkages

Multi-scale theoretical model and spectra simulation for dendrimers combining TD-DFT/DFT and semi-empirical methods.

Solvent and branching effect on the two-photon absorption properties of push–pull triphenylamine derivatives

The photophysical and two-photon absorption (2PA) properties of two tri-podal molecules and of their quadrupolar and dipolar counterparts are reported for a series of solvents with varying polarity.

Photophysical properties of octupolar triazatruxene-based chromophores

The intramolecular charge transfer properties of tribranched chromophores related to their TPA properties are explored by estimating the TPA essential factors.

Solvatofluorochromic, non-centrosymmetric π-expanded diketopyrrolopyrrole

A donor–acceptor type π-expanded diketopyrrolopyrrole behaves as non-centrosymmetric as far as linear optical properties are concerned but as ‘pseudo-symmetric’ for two-photon absorption.

An internal charge transfer-dependent solvent effect in V-shaped azacyanines

New V-shaped non-centrosymmetric dyes, possessing a strongly electron-deficient azacyanine core, have been synthesized based on a straightforward two-step approach. The key step in this synthesis involves palladium-catalysed cross-coupling of dibromo-N,N'-methylene-2,2'-azapyridinocyanines with arylacetylenes. The resulting strongly polarized π-expanded heterocycles exhibit green to orange fluorescence and they strongly respond to changes in solvent polarity. We demonstrate that differently electron-donating peripheral groups have a significant influence on the internal charge transfer, hence on the solvent effect and fluorescence quantum yield. TD-DFT calculations confirm that, in contrast to the previously studied bis(styryl)azacyanines, the proximity of S1 and T2 states calculated for compounds bearing two 4-N,N-dimethylaminophenylethynyl moieties establishes good conditions for efficient intersystem crossing and is responsible for its low fluorescence quantum yield. Non-linear properties have also been determined for new azacyanines and the results show that depending on peripheral groups, the synthesized dyes exhibit small to large two-photon absorption cross sections reaching 4000 GM.

A theoretical study of a series of novel two-photon nitric oxide (NO) fluorescent probes based on BODIPY

The PET probing mechanism and intrinsic two-photon absorption properties of the studied molecules are rationally explained.

A cyclometallated fluorenyl Ir(iii) complex as a potential sensitiser for two-photon excited photodynamic therapy (2PE-PDT)

A cyclometallated fluorenyl Ir(iii) complex that exhibits enhanced triplet oxygen sensing properties for two photon photodynamic therapy is reported.

Two-photon absorption in CdSe colloidal quantum dots compared to organic molecules

We discuss fundamental differences in electronic structure as reflected in one- and two-photon absorption spectra of semiconductor quantum dots and organic molecules by performing systematic experimental and theoretical studies of the size-dependent spectra of colloidal quantum dots. Quantum-chemical and effective-mass calculations are used to model the one- and two-photon absorption spectra and compare them with the experimental results. Currently, quantum-chemical calculations are limited to only small-sized quantum dots (nanoclusters) but allow one to study various environmental effects on the optical spectra such as solvation and various surface functionalizations. The effective-mass calculations, on the other hand, are applicable to the larger-sized quantum dots and can, in general, explain the observed trends but are insensitive to solvent and ligand effects. Careful comparison of the experimental and theoretical results allows for quantifying the range of applicability of theoretical methods used in this work. Our study shows that the small clusters can be in principle described in a manner similar to that used for organic molecules. In addition, there are several important factors (quality of passivation, nature of the ligands, and intraband/interband transitions) affecting optical properties of the nanoclusters. The larger-size quantum dots, on the other hand, behave similarly to bulk semiconductors, and can be well described in terms of the effective-mass models.

Rational design of small indolic squaraine dyes with large two-photon absorption cross section

Small organic dyes with large two-photon absorption (TPA) cross sections (δ) are more desirable in many applications compared with large molecules. Herein, we proposed a facile theoretical method for the fast screening of small organic molecules as potential TPA dyes. This method is based on a theoretical analysis to the natural transition orbitals (NTOs) directly associated with the TPA transition. Experimental results on the small indolic squaraine dyes (ISD) confirmed that their TPA cross sections is strongly correlated to the delocalization degree of the NTOs of the S₂ excited states. Aided by this simple and intuitive method, we have successfully designed and synthesized a small indolic squaraine dye (ISD) with a remarkable δ value above 8000 GM at 780 nm. The ISD dye also exhibits a high singlet oxygen generation quantum yield about 0.90. The rationally designed TPA dye was successfully applied in both two-photon excited fluorescence cell imaging and in vivo cerebrovascular blood fluid tracing.

Multiphoton excited fluorescent materials for frequency upconversion emission and fluorescent probes

Recent progress in developing various strategies for exploiting efficient MPA fluorophores for two emerging technological MPA applications including frequency upconversion photoluminescence and lasing as well as 2PA fluorescence bioimaging and biosensing are presented. An intriguing application of MPA frequency-upconverted lasing offers opportunity for the fabrication of high-energy coherent light sources in the blue region which could create new advantages and breakthroughs in various laser-based applications. In addition, multiphoton excitation has led to considerable progress in the development of advanced diagnostic and therapeutic treatments; further advancement is anticipated with the emergence of various versatile 2PA fluorescence probes. It is widely appreciated that the two-photon excitation offers significant advantages for the biological fluorescence imaging and sensing which includes higher spatial resolution, less photobleaching and photodamage as well as deeper tissue penetration as compared to the one-photon excited microscopy. To be practically useful, the 2PA fluorescent probes for biological applications are required to have a site-specificity, a high fluorescence quantum yield, proper two-photon excitation and subsequent emission wavelengths, good photodecomposition stability, water solubility, and biocompatibility besides large 2PA action cross-sections.