Exploration of nonlinear optical enhancement in acceptor-π-donor indacenodithiophene based derivatives via structural variations: a DFT approach

Herein, a series of indacenodithiophene-based derivatives (TNPD1-TNPD6) were designed having D-π-A architecture via end capped acceptor modulation of a reference molecule (TNPR) to investigate nonlinear optical (NLO) behavior. Quantum chemical calculations were accomplished to examine electronic, structural and optical properties utilizing a density functional theory (DFT) approach at M06 functional with 6-311G(d,p) basis set. For this, natural bond orbitals (NBOs), density of states (DOS), frontier molecular orbitals (FMOs), transition density matrix (TDM) and non-linear optical (NLO) analyses were performed for TNPR and TNPD1-TNPD6. The structural modifications revealed a significant electronic contribution in tuning the HOMOs and LUMOs of the derivatives with lowered energy gaps and wider absorption spectra. FMOs findings revealed that compound TNPD5 was found with the lowest energy gap (1.692 eV) and with the highest softness (0.591 eV-1) among the derivatives. Furthermore, a UV-Vis study also disclosed that maximum absorption (λmax = 852.242 nm) was exhibited by TNPD5 in chloroform solvent. All the derivatives exhibited significant NLO results; in particular, TNPD5 showed the highest first hyper-polarizability (βtot = 4.653 × 10-27 esu) and second hyper-polarizability (γtot = 9.472 × 10-32 esu). These DFT findings revealed that the end-capped substituents play a key role in enhancing the NLO response of these push-pull chromophores and the studied derivatives can be utilized as efficient NLO materials.

Excited-state symmetry breaking in quadrupolar pull-push-pull molecules: dicyanovinyl vs. cyanophenyl acceptors

A significant number of quadrupolar dyes behave as their dipolar analogues when photoexcited in polar environments. This is due to the occurrence of excited-state symmetry breaking (ES-SB), upon which the electronic excitation, initially distributed over the whole molecule, localises preferentially on one side. Here, we investigate the ES-SB properties of two A-D-A dyes, consisting of a pyrrolo-pyrrole donor (D) and either cyanophenyl or dicyanovinyl acceptors (A). For this, we use time-resolved vibrational spectroscopy, comparing IR absorption and femtosecond stimulated Raman spectroscopies. Although dicyanovinyl is a stronger electron-withdrawing group, ES-SB is not observed with the dicyanovinyl-based dye even in highly polar media, whereas it already takes place in weakly polar solvents with dyes containing cyanophenyl accepting groups. This difference is attributed to the large electronic coupling between the D-A branches in the former dye, whose loss upon symmetry breaking cannot be counterbalanced by a gain in solvation energy. Comparison with analogues of the cyanophenyl-based dye containing different spacers reveals that interbranch coupling does not so much depend on the distance between the D-A subunits than on the nature of the spacer. We show that transient Raman spectra probe different modes of these centrosymmetric molecules but are consistent with the transient IR data. However, lifetime broadening of the Raman bands, probably due to the resonance enhancement, may limit the application of this technique for monitoring ES-SB.

Polymersomes with Red/Near-Infrared Emission and Reactive Oxygen Species Generation

In photodynamic therapy (PDT), the uses of nanoparticles bearing photosensitizers (PSs) can overcome some of the drawbacks of using a PS alone (e.g., poor water solubility and low tumor selectivity). However, numerous nano-formulations are developed by physical encapsulation of PSs through Van der Waals interactions, which have not only a limited load efficiency but also some in vivo biodistribution problems caused by leakage or burst release. Herein, polymersomes made from an amphiphilic block copolymer, in which a PS with aggregation-induced emission (AIE-PS) is covalently attached to its hydrophobic poly(amino acid) block, are reported. These AIE-PS polymersomes dispersed in aqueous solution have a high AIE-PS load efficiency (up to 46% as a mass fraction), a hydrodynamic diameter of 86 nm that is suitable for in vivo applications, and an excellent colloidal stability for at least 1 month. They exhibit a red/near-infrared photoluminescence and ability to generate reactive oxygen species (ROS) under visible light. They are non-cytotoxic in the dark as tested on Hela cells up to concentration of 100 µm. Benefiting from colloidal stability, AIE property and ROS generation capability, such a family of polymersomes can be great candidates for image-guided PDT.

AIE Polymer Micelle/Vesicle Photocatalysts Combined with Native Enzymes for Aerobic Photobiocatalysis

Biocatalytic transformation has attracted increasing attention in the green synthesis of chemicals due to the diversity of enzymes, their high catalytic activities and specificities, and environmentally benign conditions. Most redox enzymes in nature are dependent on nicotinamide cofactors like β-nicotinamide adenine dinucleotide (NAD⁺)/reduced nicotinamide adenine dinucleotide (NADH). The use of solar energy, especially visible light, in the regeneration of cofactors through the combination of photocatalysis and biocatalysis provides an extraordinary opportunity to make complete green processes. However, the combination of photocatalysts and enzymes has been challenged by the rapid degradation and deactivation of the enzymatic material by photogenerated reactive oxygen species (ROS). Here, we design core-shell structured polymer micelles and vesicles with aggregation-induced emission (AIE) as visible-light-mediated photocatalysts for highly stable and recyclable photobiocatalysis under aerobic conditions. NAD⁺ from NADH can be efficiently regenerated by the photoactive hydrophobic core of polymer micelles and the hydrophobic membrane of polymer vesicles, while the enzymatic material (glucose 1-dehydrogenase) is screened from the attack of photogenerated ROS by the hydrophilic surface layer of polymer colloids. After at least 10 regeneration cycles, the enzyme keeps its active state; meanwhile, polymer micelles and vesicles maintain their photocatalytic activity. These polymer colloids show the potential to be developed for the implementation of industrially relevant photobiocatalytic systems.

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.

First theoretical probe for efficient enhancement of optical nonlinearity via structural modifications into phenylene based D-π-A configured molecules

The design of nonlinear optical (NLO) materials using conjugated molecules via different techniques is reported in the literature to boost the use of these systems in NLO. Therefore, in the current study, designed phenylene based non-fullerene organic compounds with a D-π-A framework were selected for NLO investigation. The initial compound (PMD-1) was taken as a reference and its seven derivatives (PMDC2-PMDC8) were made by introducing different acceptor moieties into the chemical structure of PMD-1. To explain the NLO findings, frontier molecular orbital (FMO), transition density matrix (TDM), density of states (DOS), natural bond orbital (NBO) and UV-Vis study of the title compounds was executed by applying the PBE1PBE functional with the 6-311G(d,p) basis set. The descending order of band gaps (E gap) was reported as PMDC7 (2.656) > PMDC8 (2.485) > PMD-1 (2.131) > PMDC3 (2.103) > PMDC2 (2.079) > PMDC4 (2.065) > PMDC5 (2.059) > PMDC6 (2.004), in eV. Global reactivity parameters (GRPs) were associated with E gap values as PMDC6 with the lowest band gap showed less hardness (0.0368 E h) and high softness (13.5785 E h). The UV-Vis investigation revealed that the maximum λ max (739.542 nm) was exhibited by PMDC6 in dichloromethane (DCM) as compared to other derivatives. Additionally, natural bond orbital (NBO) based findings revealed that PMDC6 exhibited the highest stability value (34.98 kcal mol-1) because of prolonged hyper-conjugation. The dipole moment (μ), average linear polarizability 〈α〉, first hyperpolarizability (β tot) and second hyperpolarizability (γ tot) were evaluated for the reference and its derivatives. Consequently, among the designed compounds, the highest β tot (4.469 × 10-27 esu) and γ tot (5.600 × 10-32 esu) values were shown by PMDC6. Hence, it's concluded from said results that these structural modifications proved PMDC6 as the best second and third order NLO candidate for various applications like fiber optics, signal processing and data storage.

Watching Excited-State Symmetry Breaking in Multibranched Push-Pull Molecules

The emissive properties of symmetric molecules containing several donor-acceptor branches are often similar to those of the single-branched analogues. This is due to the at least partial localization of the excitation on one branch. Detailed understanding of this excited-state symmetry breaking (ES-SB) requires the ability to monitor this process in real time. Over the past few years, several spectroscopic approaches were shown to enable visualization of ES-SB and of its dynamics. They include the detection of new vibrational or electronic absorption bands associated with transitions that are forbidden in the symmetric excited state. Alternatively, ES-SB can be detected by observing transitions that become weaker or vanish upon localization of the excitation. Herein, we discuss these different approaches as well as their merits and weaknesses.

Effect of Symmetry Breaking in Excited Quadrupole Molecules on Transition Dipole Moment

Manifestations of charge transfer symmetry breaking in excited quadrupolar molecules in optical spectra are theoretically studied. The molecules are supposed to have π-conjugated structures of A-π-D-π-A or D-π-A-π-D character, where electron acceptors (A) or electron donors (D) are identical. A theory describing the effect of symmetry breaking and solvent fluctuations on the dipole moments of optical transitions associated with absorption by a quadrupolar dye in the ground and excited states, as well as fluorescence, is developed. Simple equations describing the influence of the symmetry breaking extent on the transition dipole moments are found. The orientational solvent fluctuations are predicted to decrease the transition dipole moment of the ground state absorption. The decrease does not exceed 10%. A considerably larger effect of symmetry breaking and the solvent fluctuations on the emission dipole moment is found. Equations describing dependencies of the transition dipole moment associated with excited state absorption on the solvent polarity and the parameters of the dye are derived. The scale of the changes in the transition dipole moments due to symmetry breaking in the excited state are determined. The influence of the polar solvent fluctuations is also taken into account. The theoretical findings are shown to be consistent with the available experimental data.

Symmetry Breaking in an Excited Quadrupolar Acridine-Dione Derivative Driven by Hydrogen Bonding

An acridine-dione derivative (3,3,11,11-tetramethyl-8,16-diphenyl-3,4,8,10,11,12,13,16-octahydroacridino[4,3-c]acridine-1.9(2H,5H)dion) with quadrupolar motif has been synthesized and its stationary and transient spectra have been measured. Stationary absorption and fluorescence spectra as well as nonstationary spectra show no signs of symmetry breaking (SB) in aprotic solvents, even of high polarity. The specific features of SB are revealed in alcohol solvents through a considerable red shift of stationary fluorescence spectra and the appearance of a new excited state absorption band in transient absorption spectra. SB is due to the formation of asymmetric strong hydrogen bonds, mainly on one side of the molecule. An unexpected regularity of symmetry breaking is found in mixtures of aprotic dimethylformamide and protic methanol, where methanol acts as a fluorescence quencher. It is revealed that there is no quenching as long as the methanol concentration is less than the critical value of 9 M. This leads to the conclusion that SB in such mixtures is possible only if the concentration of the protic solvent exceeds a certain threshold value.

Excited state dynamics and exciton diffusion in triphenylamine/dicyanovinyl push-pull small molecule for organic optoelectronics

Triphenylamine-based small push–pull molecules have recently attracted substantial research attention due to their unique optoelectronic properties. Here, we investigate the excited state de-excitation dynamics and exciton diffusion in TPA-T-DCV-Ph-F small molecule, having simple chemical structure with asymmetrical architecture and end-capped with electron-withdrawing p-fluorodicyanovinyl group. The excited state lifetime in diluted solutions (0.04 ns in toluene and 0.4 ns in chloroform) are found to be surprisingly shorter compared to the solid state (3 ns in PMMA matrix). Time-dependent density functional theory indicates that this behavior originates from non-radiative relaxation of the excited state through a conical intersection between the ground and singlet excited state potential energy surfaces. Exciton diffusion length of ~ 16 nm in solution processed films was retrieved by employing time-resolved photoluminescence volume quenching measurements with Monte Carlo simulations. As means of investigating the device performance of TPA-T-DCV-Ph-F, we manufactured solution and vacuum processed bulk heterojunction solar cells that yielded efficiencies of ~ 1.5% and ~ 3.7%, respectively. Our findings demonstrate that the short lifetime in solutions does not hinder per se long exciton diffusion length in films thereby granting applications of TPA-T-DCV-Ph-F and similar push–pull molecules in vacuum and solution processable devices.

Excitation/detection energy controlled anisotropy dynamics in asymmetrically cyano substituted tri-podal molecules

In the present work, the photophysical properties of two series of asymmetrical tri-podal molecules are studied, in order to determine the dependence of energy localization/delocalization phenomena on excitation and detection wavelength, by means of steady state, femtosecond time-resolved fluorescence and anisotropy spectroscopy. The molecules bear triphenylamine as an electron donating core and an acetylenic or olefinic π-conjugated bridge. At the periphery, they are substituted by no, one, two or three -CN groups used as electron acceptors. Thus, the compounds with only one or two -CN groups are asymmetrically substituted. As a comparison, the photophysics of their dipolar and quadrupolar analogues is also presented. The steady state absorption spectra of the asymmetrical tri-podal compounds exhibit a broadening and a low energy shoulder due to the splitting of the excited states. The fluorescence spectra are more red-shifted in the tri-podal molecules with a single -CN group, providing the first evidence of its mostly dipolar nature. Time-resolved anisotropy measurements by using different excitation and detection wavelengths provide clear evidence that the asymmetrical tri-podal molecules with one or two -CN groups behave like octupolar molecules upon high-energy excitation (the initial anisotropy is found 0.1-0.15), while upon low-energy excitation they reveal a behavior expected for linear dipolar or V-shaped quadrupolar molecules (the initial anisotropy is very close to 0.4 and 0.17, respectively). The symmetrical tri-podal compounds with no or three cyano groups, exhibit an anisotropy depolarization time of 2.5 ps attributed to energy hopping. The amplitude of this energy hopping component is wavelength dependent and increases as the excitation is shifted towards the long wavelength edge.

An aggregation-induced emission dye-powered afterglow luminogen for tumor imaging

Semiconducting polymer (SP)-based afterglow luminogens are showing increasing potential for in vivo imaging because of their long-life luminescence and the associated benefits (e.g., zero-autofluorescence background and high signal-to-noise ratio). However, such organic afterglow luminescence agents are still rare and their application is usually limited by their relatively low afterglow intensity and short afterglow duration. Herein, we report an aggregation-induced emission (AIE) dye-powered SP afterglow luminogen by leveraging on the unique characteristics of an AIE dye to circumvent the concentration-quenching effect, enhance afterglow intensity and prolong afterglow duration. The underlying working mechanism is investigated by a series of experiments and it is found that the AIE dye provides sufficient 1O2 to excite SPs and form massive amounts of high-energy intermediates, and then the SP intermediates emit photons that can activate the AIE dye to generate 1O2 and simultaneously trigger the energy transfer process between the SPs and AIE dye, resulting in a deep-red emission. It is this closed-loop of "photon-1O2-SP intermediates-photon" that provides the afterglow emission even after the cessation of the excitation light. The as-prepared luminogen shows good performance in in vivo tumour imaging. This study demonstrates the advantages of AIE-facilitated afterglow luminescence and discloses its mechanism, and hopefully it could inspire the development of other innovative designs for cancer theranostics.

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.

Excited-State Symmetry Breaking in an Aza-Nanographene Dye

The photophysics of a structurally unique aza-analogue of polycyclic aromatic hydrocarbons characterized by 12 conjugated rings and a curved architecture was studied in detail. The combined experimental and computational investigation reveals that the lowest excited state has charge-transfer character, in spite of the absence of any peripheral electron-withdrawing groups. The exceptionally electron-rich core comprised of two fused pyrrole rings is responsible for it. The observed strong solvatofluorochromism is related to symmetry breaking occurring in the emitting excited state, leading to a significant dipole moment (13.5 D) in the relaxed excited state. The anomalously small fluorescence anisotropy of this molecule, which is qualitatively different from what is observed in standard quadrupolar dyes, is explained as due to the presence of excited states that are close in energy but have different polarization directions.

The Effect of Branching on the One- and Two-Photon Absorption, Cell Viability, and Localization of Cationic Triarylborane Chromophores with Dipolar versus Octupolar Charge Distributions for Cellular Imaging

Two different chromophores, namely a dipolar and an octupolar system, were prepared and their linear and nonlinear optical properties as well as their bioimaging capabilities were compared. Both contain triphenylamine as the donor and a triarylborane as the acceptor, the latter modified with cationic trimethylammonio groups to provide solubility in aqueous media. The octupolar system exhibits a much higher two-photon brightness, and also better cell viability and enhanced selectivity for lysosomes compared with the dipolar chromophore. Furthermore, both dyes were applied in two-photon excited fluorescence (TPEF) live-cell imaging.

Intramolecular Cooperative and Anti-Cooperative Effect on the Two-Photon Absorption Cross Section in Triphenylamine Derivatives

The intramolecular cooperative effect in branched molecules is a consequence of the interaction and extent of electronic coupling among the different axes of charge transfer. Such an effect is the key to obtain remarkable nonlinear optical response in molecular systems. Here we show that triphenylamine derivative molecules containing only two branches present the strongest electronic interaction between them at the excited state, generating exponential enhancement of the 2PA cross section. The primary factor for such behavior was ascribed to the substantial extent and interaction of the π-electron delocalization promoted by the strong electron-donating and acceptor antisymmetrical groups present in each branch. However, for the three-branch molecules we observed an anticooperative effect, i.e., the 2PA cross section decreases as compared to the one-branch structure as we normalized the signal by the effective π-electron number in each molecule.

Raposo MM, Martínez-Máñez R, Sancenón F. N,N-Diphenylanilino-heterocyclic aldehyde-based chemosensors for UV-vis/NIR and fluorescence Cu(ii) detection

Cu(ii) coordination with aldehyde-containing probes induced the appearance of NIR bands coupled with remarkable colour changes.

Branching effect on triphenylamine-CF3 cyanostilbenes: enhanced emission and aggregation in water

Aggregation-induced emission and gelation behavior of symmetric and asymmetric triphenylamines

Wavelength-optimized Two-Photon Polymerization Using Initiators Based on Multipolar Aminostyryl-1,3,5-triazines

Two-photon induced polymerization (2PP) based 3D printing is a powerful microfabrication tool. Specialized two-photon initiators (2PIs) are critical components of the employed photosensitive polymerizable formulations. This work investigates the cooperative enhancement of two-photon absorption cross sections (σ_2PA) in a series of 1,3,5-triazine-derivatives bearing 1-3 aminostyryl-donor arms, creating dipolar, quadrupolar and octupolar push-pull systems. The multipolar 2PIs were successfully prepared and characterized, σ_2PA were determined using z-scan at 800 nm as well as spectrally resolved two-photon excited fluorescence measurements, and the results were compared to high-level ab initio computations. Modern tunable femtosecond lasers allow 2PP-processing at optimum wavelengths tailored to the absorption behavior of the 2PI. 2PP structuring tests revealed that while performance at 800 nm is similar, at their respective σ_2PA-maxima the octupolar triazine-derivative outperforms a well-established ketone-based quadrupolar reference 2PI, with significantly lower fabrication threshold at exceedingly high writing speeds up to 200 mm/s and a broader window for ideal processing parameters.

Development of colloidally stable carbazole-based fluorescent nanoaggregates

Fluorescent nanomaterials require high colloidal stability for effective use in imaging and sensing applications. We herein report the synthesis of carbazole-based organic fluorescent nanoaggregates, and demonstrate the superior colloidal stability of alkyl-substituted dye aggregates over their non-alkylated analogs. The role of alkyl chains in self-assembly and stability of such nanoaggregates is discussed based on both experimental and molecular dynamics simulation data, and spectral characteristics of the precursor dyes and their aggregates are described. The obtained results provide new insights on development of colloidally stable organic fluorescent nanomaterials with low polydispersity.

Aggregates of quadrupolar dyes for two-photon absorption: the role of intermolecular interactions

We present a theoretical investigation of small aggregates of quadrupolar (A-π-D-π-A or D-π-A-π-D) charge-transfer dyes, with attention focused on the role of intermolecular interactions in determining their optical properties. We tackle the theoretical issue by adopting essential-state models (ESMs), which describe an isolated molecule in terms of a minimal number of electronic states, corresponding to the resonance structures. ESMs quite naturally describe intermolecular interactions relaxing the dipolar approximation and accounting for molecular polarizabilities. The approach is applied to curcuminoid and squaraine dyes, two families of chromophores with weak and strong quadrupolar character, respectively. The method is validated against experiment and for curcuminoids also against time-dependent density functional theory. ESMs rationalize the strong ultra-excitonic effects recurrently observed in the experimental optical spectra of aggregates of highly polarizable quadrupolar dyes, offering a valuable tool to exploit the supramolecular design of material properties.

Specific Monitoring of Excited-State Symmetry Breaking by Femtosecond Broadband Fluorescence Upconversion Spectroscopy

Most quadrupolar molecules designed for large two-photon absorption cross section have been shown to undergo symmetry breaking upon excitation to the S₁ state. This was originally deduced from their strong fluorescence solvatochromism and later visualized in real time using transient infrared spectroscopy. For molecules not containing clear IR marker modes, however, a specific real-time observation of the symmetry breaking process remains lacking. Here we show that this process can be resolved using broadband fluorescence upconversion spectroscopy by monitoring the instantaneous emission transition dipole moment. This approach is illustrated with measurements performed on two quadrupolar molecules, with only one of them undergoing excited-state symmetry breaking in polar solvents.

Solute-Solvent Interactions and Excited-State Symmetry Breaking: Beyond the Dipole-Dipole and the Hydrogen-Bond Interactions

Symmetry breaking of the excited state of a linear quadrupolar acceptor-donor-acceptor molecule was investigated using time-resolved infrared spectroscopy in 55 solvents allowing the influence of several solute-solvent interactions to be examined separately. No symmetry breaking was found in nonpolar solvents irrespective of their refractive index, indicating that differences in dispersion interactions between the two arms of the molecule do not suffice to induce an asymmetric distribution of the excitation. However, symmetry breaking was observed in nondipolar but quadrupolar solvents like benzene to an extent that can be as large as that found in medium dipolar solvents like THF. Whereas larger symmetry breaking occurs in the most dipolar solvents, the strongest are observed in protic solvents due to hydrogen bonding. Strong evidence of the formation of halogen bonds in the excited state is also presented, confirming the idea of symmetry-breaking-induced asymmetrical photochemistry.

Triphenylamine-Based Push-Pull Molecule for Photovoltaic Applications: From Synthesis to Ultrafast Device Photophysics

Small push-pull molecules attract much attention as prospective donor materials for organic solar cells (OSCs). By chemical engineering, it is possible to combine a number of attractive properties such as broad absorption, efficient charge separation, and vacuum and solution processabilities in a single molecule. Here we report the synthesis and early time photophysics of such a molecule, TPA-2T-DCV-Me, based on the triphenylamine (TPA) donor core and dicyanovinyl (DCV) acceptor end group connected by a thiophene bridge. Using time-resolved photoinduced absorption and photoluminescence, we demonstrate that in blends with [70]PCBM the molecule works both as an electron donor and hole acceptor, thereby allowing for two independent channels of charge generation. The charge-generation process is followed by the recombination of interfacial charge transfer states that takes place on the subnanosecond time scale as revealed by time-resolved photoluminescence and nongeminate recombination as follows from the OSC performance. Our findings demonstrate the potential of TPA-DCV-based molecules as donor materials for both solution-processed and vacuum-deposited OSCs.

Femtosecond to nanosecond studies of octupolar molecules and their quadrupolar and dipolar analogues

The fs–ps anisotropy dynamics of octupolar, quadrupolar and dipolar molecules with different π-bridges.

Direct Visualization of Excited-State Symmetry Breaking Using Ultrafast Time-Resolved Infrared Spectroscopy

Most symmetric quadrupolar molecules designed for two-photon absorption behave as dipolar molecules in the S1 electronic excited state. This is usually explained by a breakup of the symmetry in the excited state. However, the origin of this process and its dynamics are still not fully understood. Here, excited-state symmetry breaking in a quadrupolar molecule with a D-π-A-π-D motif, where D and A are electron donating and accepting units, is observed in real time using ultrafast transient infrared absorption spectroscopy. The nature of the relaxed S1 state was found to strongly depend on the solvent polarity: (1) in nonpolar solvents, it is symmetric and quadrupolar; (2) in weakly polar media, the quadrupolar state observed directly after excitation transforms to a symmetry broken S1 state with one arm bearing more excitation than the other; and (3) in highly polar solvents, the excited state evolves further to a purely dipolar S1 state with the excitation localized entirely on one arm. The time scales associated with the transitions between these states coincide with those of solvation dynamics, indicating that symmetry breaking is governed by solvent fluctuations.

Optical and photophysical properties of anisole- and cyanobenzene-substituted perylene diimides

Substituent and solvent effects on the one- and two-photon absorption and the photophysical properties of eight bay-substituted perylene-diimides are reported and analyzed.

Dye-sensitized-solar-cells based on calix[4]arene scaffolds

Calix[4]arene-based dyes open the possibility to prepare more D–π–A branched dyes for increasing light-current conversión efficiencies.

Two-dimensional electronic-vibrational spectra: modeling correlated electronic and nuclear motion

2D-EV spectra of solvated organic dyes modeled in terms of a reduced set of electronic diabatic states.

Star shaped ferrocenyl substituted triphenylamines

This manuscript reports design and synthesis of star shaped ferrocenyl substituted triphenylamine conjugates (Fc-TPA) 3a–3c by the Pd-catalyzed Sonogshira cross-coupling reaction.

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.