State In Polar Solvents Research Articles

Symmetry Breaking Charge Separation in Far-Red Absorbing Bisstyryl-Bodipy Dimers

Symmetry breaking charge transfer is one of the important photo-events occurring in photosynthetic reaction centers that is responsible for initiating electron transfer leading to a long-lived charge-separated state and has been successfully employed in light-to-electricity converting optoelectronic devices. Here, we report the synthesis of far-red absorbing and emitting BODIPY-dimer to undergo symmetry-breaking charge transfer leading to charge-separated states of appreciable lifetimes in polar solvents. Compared to its monomer analog, both steady-state and time-resolved fluorescence originating from the S1 state of the dimer revealed quenching which increased with an increase in solvent polarity. The electrostatic potential map from DFT and the time-dependent DFT calculations suggested the existence of a quadrupolar type charge transfer state in polar solvents, and the singlet excited state to be involved in the charge separation process. The electrochemically determined redox gap to be smaller than the energy of the S1 state supporting thermodynamic feasibility of the envisioned symmetry-breaking charge transfer and separation. The spectrum of the charge-separated state arrived from spectroelectrochemical studies, revealing diagnostic peaks helpful for transient spectral interpretation. Finally, ultrafast transient pump-probe spectroscopy provided conclusive evidence of diabatic charge separation in polar solvents by far-red pulsed laser light irradiation. The measured lifetime of the final charge-separated states was found to be 165 ps in dichlorobenzene, 140 ps in benzonitrile, and 43 ps in dimethyl sulfoxide, revealing their significance in light energy harvesting, especially from the less-explored far-red region.

The photochemistry and photophysics of benzoyl-carbazole.

Benzoyl-carbazole and its derivatives are considered a platform for exploring processes such as room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF). They have also been reported to exhibit dual emission, but there is a great spectral variability in the relative intensity of the emission bands reported in different studies. To better understand the fundamental photophysical properties, we set to explore BCz and its perfluorinated derivative F5BCz using spectroscopy and quantum chemical simulations. We find that the reported dual fluorescence in solution and in films results from a photochemical process (photo-Fries rearrangement), producing carbazole among other products, explaining the variation in the reported emission spectra. In addition, BCz exhibits solvent dependent TADF, which is explained by the stabilization of the charge transfer S1 state in polar solvents. F5BCz undergoes an efficient photochemical process (Mallory reaction) from its single state to produce highly fluorescent product c-F5BCz, in 40% isolated yield. This photoreactivity also proceeds in films under ambient conditions, which have significant implications on the applications of BCz-based materials for optoelectronic applications.

Single-molecule detection of a terrylenediimide-based near-infrared emitter.

Environment-sensitive fluorescent agents with near-infrared (NIR) emission are in great demand owing to their applications in biomedical and quantum technologies. We report a novel NIR absorbing (λ Abs max = 734 nm) and emitting (λ Fl max = 814 nm) terrylenediimide (TDI) based donor-acceptor chromophore (TDI-TPA4), exhibiting polarity-sensitive single-photon emission. By virtue of the charge transfer (CT) character, ensemble level measurements revealed solvatochromism and NIR emission (ϕ Fl = 26.2%), overcoming the energy gap law. The CT nature of the excited states is further validated by state-of-the-art fragment-based excited state theoretical analysis. To mimic the polarity conditions at the single-molecule level, TDI-TPA4 was immobilized in polystyrene (PS; low polar) and poly(vinyl alcohol) (PVA; high polar) matrices, which enables tuning of the energy levels of the locally excited state and charge-separated (CS) state. Minimal blinking and prolonged survival time of the TDI-TPA4 molecule in the PS matrix, in contrast to the PVA matrix, possibly confirms the implication of the energy gap law and polarity sensitivity of TDI-TPA4. The existence of the CT state in nonpolar and CS state in polar solvents was confirmed by transient absorption measurements in the femtosecond regime. The current work sheds light on the design principle for NIR single-photon emitting organic chromophores for deep tissue imaging and probing the nanoscale heterogeneity.

Symmetry breaking charge transfer leading to charge separation in a far-red absorbing bisstyryl-BODIPY dimer.

Symmetry breaking charge transfer is one of the important photo-events occurring in photosynthetic reaction centers that is responsible for initiating electron transfer leading to a long-lived charge-separated state and has been successfully employed in light-to-electricity converting optoelectronic devices. In the present study, we report a newly synthesized, far-red absorbing and emitting BODIPY-dimer to undergo symmetry-breaking charge transfer leading to charge-separated states of appreciable lifetimes in polar solvents. Compared to its monomer analog, both steady-state and time-resolved fluorescence originating from the S1 state of the dimer revealed quenching which increased with an increase in solvent polarity. The electrostatic potential map from DFT and the time-dependent DFT calculations suggested the existence of a quadrupolar type charge transfer state in polar solvents, and the singlet excited state to be involved in the charge separation process. The electrochemically determined redox gap being smaller than the energy of the S1 state supported the thermodynamic feasibility of the envisioned symmetry-breaking charge transfer and separation. The spectrum of the charge-separated state arrived from spectroelectrochemical studies, revealing diagnostic peaks helpful for transient spectral interpretation. Finally, ultrafast transient pump-probe spectroscopy provided conclusive evidence of diabatic charge separation in polar solvents by far-red pulsed laser light irradiation. The measured lifetime of the final charge-separated states was found to be 165 ps in dichlorobenzene, 140 ps in benzonitrile, and 43 ps in dimethyl sulfoxide, revealing their significance in light energy harvesting, especially from the less-explored far-red region.

High-efficiency multicolor fluorescent indoleninonaphthooxazole derivatives with adjustable excitation

Solvatochromic fluorophores are one of the most investigated dyes because they exhibit multi-color luminescence properties as the polarity of the solvent changes. It remains a challenge to regulate solvatochromism with a large emission shift and maintain high fluorescence efficiency from apolar to polar solvents. Herein, we report a new type of solvatochromic fluorophore that exhibits both multicolor solvatochromism and stable high fluorescence efficiency in polar solvents. The unique solvatochromic properties of indoleninonaphthooxazole derivatives are realized by constructing a single carbon-bond-linked coplanar scaffold containing indolenine and naphthooxazole components with different substituents. We propose a possible mechanism to explain the unique solvatochromism. Excited USTS-3 and USTS-4 exhibit LE state in non-polar solvent and different HLCT states in polar solvents with both high luminescence efficiency. And the excitation of USTS-3 and USTS-4 is determined by aggregation states of molecular USTS-3 and USTS-4 under different concentrations, which exhibits different relaxations but share the same luminescence decay pathway. Benefitting from the unique luminescence properties of indoleninonaphthooxazole derivatives, they exhibit potential applications in solvent polarity detection, bioimaging, anticounterfeiting, electroluminescence, and lighting in different matrix. Our results also provide a new perspective on designing novel fluorophores based on indolenine and naphthoxazole moieties.

Solvent Polarity Governs Ultrafast Structural Dynamics: A Case Study of 4-Dimethylamino-4′-carbomethoxydiphenylacetylene

The compound 4-dimethylamino-4′-carbomethoxydiphenylacetylene (DACM-DPA) is a donor-π-acceptor system that exhibits significant charge transfer character in the excited state in polar solvents. Femtosecond transient absorption measurements in n-hexane, acetonitrile, and methanol exhibit excited-state lifetimes of >2 ns, ∼500 ps, and ∼30 ps, respectively. Femtosecond Raman measurements of DACM-DPA elucidated the intricate structural dynamics during the excited-state evolution by revealing distinct vibrational signatures of DACM-DPA in these solvents. In the case of n-hexane solvent, vibrational bands at 2075 cm–1 (C≡C) and 1600 cm–1 (C═C) stretch exist throughout the excited-state lifetime. However, in the case of polar solvents, the amplitudes of vibrational frequencies at 2098 cm–1 (C≡C), 1618 cm–1 (C═C, acetonitrile), and 1603 cm–1 (C═C, methanol) diminish in a time period of 2.5 ps (acetonitrile) and 6.5 ps (methanol) along with the appearance of new bands at 2082 cm–1 (cumulene stretch) and 1582 cm–1 (quinoid stretch), clearly revealing the formation of an intramolecular charge-separated state. Furthermore, these experimental observations are corroborated by computations using DFT and TD-DFT.

N-Alkylhydantoins as New Organogelators and Their Ability to Create Thixotropic Mixed Molecular Organogels.

The author reported molecular organogels using N-alkylhydantoins as new low-molecular-weight gelators for the first time, and thixotropic mixed molecular organogels using a set of N-alkylhydantoin gelators with different alkyl chain lengths. These homologous compounds with different alkyl chains are found to form macroscopic crystals or solution states in polar solvents, but form homogeneous organogels in non-polar solvents, such as n-octane and squalane. Although there is no significant increase in the minimum gelation concentration of the mixed molecular gels using squalane as a solvent, these mixed molecular organogels show improved mechanical properties, especially in their thixotropic behavior, which is not observed in the single N-alkylhydantoin gels. Furthermore, they exhibit reversible thixotropic behavior with quick recovery of the gel state in a minute by quantitatively measuring dynamic viscoelasticity measurements of rheometry of mixed molecular gels. Based on the morphological observations of the xerogels, the self-assembling fibers of the gelators become finer, indicating an increase in the density of the mesh structure inside the gel, which could explain its thixotropic behavior. These thixotropic mixed molecular gels may be applicable to ointment base materials, because they are gelled with squalane oil.

Null Exciton-Coupled Chromophoric Dimer Exhibits Symmetry-Breaking Charge Separation.

A comprehensive understanding of the structure-property relationships in multichromophoric architectures has pushed the limits for developing robust photosynthetic mimics and molecular photovoltaics. The elusive phenomenon of null exciton splitting has gathered immense attention in recent years owing to the occurrence in unique chromophoric architectures and consequent emergent properties. Herein, we unveil the hitherto unobserved null exciton coupling assisted highly efficient photoinduced symmetry-breaking charge separation (SB-CS) in a Greek cross (+)-oriented spiro-conjugated perylenediimide dimer (Sp-PDI2). Quantum chemical calculations have rationalized the infrequent manifestation of null exciton coupling behavior in Sp-PDI2. Negligible contribution of long-range Coulombic and short-range charge-transfer mediated coupling renders a monomer-like spectroscopic signature for Sp-PDI2 in toluene. The Greek cross (+)-arranged Sp-PDI2 possesses a selective hole-transfer coupling, facilitating the ultrafast dissociation of null excitons and evolution of the charge-separated state in polar solvents. Radical cationic and anionic spectroscopic signatures were characterized by employing femtosecond transient absorption spectroscopy. The substantial hole transfer electronic coupling and lower activation energy barrier of Sp-PDI2 accelerated the charge separation rate. The rate of charge recombination (CR) markedly decelerated due to falling into the inverted region of the Marcus parabola, where the driving force of CR is larger than the total reorganization energy for CR. Hence, the ratio of the rates for SB-CS over CR of Sp-PDI2 exhibited an unprecedently high value of 2647 in acetonitrile. The current study provides impeccable evidence for the role of selective charge filtering in governing efficient SB-CS and thereby novel insights towards the design of biomimics and advanced functional materials.

Self-Assembled Nickel-4 Supramolecular Squares and Assays for HER Electrocatalysts Derived Therefrom.

Solid-state structures find a self-assembled tetrameric nickel cage with carboxylate linkages, [Ni(N2S'O)I(CH3CN)]4 ([Ni-I]40), resulting from sulfur acetylation by sodium iodoacetate of an [NiN2S]22+ dimer in acetonitrile. Various synthetic routes to the tetramer, best described from XRD as a molecular square, were discovered to generate the hexacoordinate nickel units ligated by N2Sthioether, iodide, and two carboxylate oxygens, one of which is the bridge from the adjacent nickel unit in [Ni-I]40. Removal of the four iodides by silver ion precipitation yields an analogous species but with an additional vacant coordination site, [Ni-Solv]+, a cation but with coordinated solvent molecules. This also recrystallizes as the tetramer [Ni-Solv]44+. In solution, dissociation into the (presumed) monomer occurs, with coordinating solvents occupying the vacant site [Ni(N2S'O)I(solv)]0, ([Ni-I]0). Hydrodynamic radii determined from 1H DOSY NMR data suggest that monomeric units are present as well in CD2Cl2. Evans method magnetism values are consistent with triplet spin states in polar solvents; however, in CD2Cl2 solutions no paramagnetism is evident. The abilities of [Ni-I]40 and [Ni-Solv]44+ to serve as sources of electrocatalysts, or precatalysts, for the hydrogen evolution reaction (HER) were explored. Cyclic voltammetry responses and bulk coulometry with gas chromatographic analysis demonstrated that a stronger acid, trifluoroacetic acid, as a proton source resulted in H2 production from both electroprecatalysts; however, electrocatalysis developed primarily from uncharacterized deposits on the electrode. With acetic acid as a proton source, the major contribution to the HER is from homogeneous electrocatalysis. Overpotentials of 490 mV were obtained for both the solution-phase [Ni-I]0 and [Ni-Solv]+. While the electrocatalyst derived from [Ni-Solv]+ has a substantially higher TOF (102 s-1) than [Ni-I]0 (19 s-1), it has a shorter catalytically active lifespan (4 h) in comparison to [Ni-I]0 (>18 h).

Photophysical, Thermal and Structural Properties of Thiophene and Benzodithiophene-Based Copolymers Synthesized by Direct Arylation Polycondensation Method.

Three low-band-gap copolymers based on isoindigo acceptor units were designed and successfully synthesized by direct arylation polycondensation method. Two of them were benzodithiophene (BDT)-isoindigo copolymers (PBDTI-OD and PBDTI-DT) with 2-octlydodecyl (OD) and 2-decyltetradecyl (DT) substituted isoindigo units, respectively. Thiophene donor and DT-substituted isoindigo acceptor units were copolymerized to synthesize PTI-DT. The copolymers have a broad absorption range that extends to over 760 nm with a band gap ≈1.5 eV. The photophysical property studies showed that the BDT-based copolymers have non-polar ground states. Their emission exhibited the population of the intramolecular charge transfer (ICT) state in polar solvents and tightly bound excitonic state in non-polar solvents due to self-aggregation. On the contrary, the emission from the thiophene-based copolymers was only from the tightly bound excitonic state. The thermal decomposition temperature of the copolymers was above 380 °C. The X-ray diffraction pattern of the three copolymers showed a halo due to stacking. A second, sharper peak was observed in the BDT-based copolymer with a longer side chain on the isoindigo unit (PBDTI-DT), and the thiophene-based copolymers with PTI-DT, exhibiting a better structural order.

Syntheses and photophysical properties of natural dehydroabietic acid-based ligands and their zinc complexes

Two Schiff base ligands (2a and 2b) and their zinc complexes (3a and 3b) have been designed and facilely synthesized through simple condensation reactions using compound 1 derived from natural dehydroabietic acid as the precursor. The larger dihedral angles of the ligands shown by the single crystal X-ray diffraction analysis indicate their stronger molecular motion, which is in consistence with their low fluorescence efficiency that is verified by the photophysical measurements. After coordination with zinc ions, the fluorescence intensity of the zinc complexes is larger, which might be explained by the structural rigidification via coordination, thereby reducing the energy loss from molecular motion. Furthermore, complex 3b exhibits a specific negative solvatochromic effect, and the absorption of its maximum wavelength shows a hypsochromic shift with increased polarity. Such solvatochromic phenomena can be explained by the synergy of intramolecular charge transfer and the proximity effect that may increase the energy difference between the ground state and the excited state in polar solvents. This work not only discusses their relationship between photophysical properties and structures, but also provides a new natural source for further research in fluorescent materials.

A Near‐Infrared Emissive π‐Conjugated Polymer Consisting of an Excited‐State Intramolecular Proton Transfer Unit

AbstractNear infrared (NIR)‐emissive polymers are attractive luminescent materials due to their utilities in biological, optoelectronic, and telecommunication applications. Herein we report the use of a dialkylamine‐strapped dithienylpyrrole (DTP) as an excited‐state intramolecular proton transfer (ESIPT) building block for producing NIR‐emissive π‐conjugated polymers. The study of a series of model compounds showed that the extension of π‐conjugation with a moderately electron‐accepting comonomer unit, dithientyl‐substituted thiazolo[5,4‐d]thiazole (TzTz), effectively stabilizes the zwitterionic ESIPT state while increasing the oscillator strength for the electronic transition from the ESIPT state. Consequently, a polymer consisted of DTP and TzTz units exhibited a significantly red‐shifted emission, exceeding 750 nm, from the ESIPT state in polar solvents.

Determining the dispersion stability of black phosphorus colloids by 3D light scattering

Understanding the dispersion state of 2D black phosphorus (BP) nanosheets is of great significance for the application of BP based materials. However, the dispersion state of BP colloids cannot be analyzed by the conventional methods based on the optical signal detecting due to the intensive light absorption of BP in the ultraviolet-visible region and the high turbidity caused by the aggregation of pristine BP nanosheets. In this work, 3D light scattering has been proven to be an effective technique to characterize the dispersion characteristics of nanocolloids with intensive light absorption and limited dispersity by suppressing multiple scattering. The effect of the concentration, the temperature and the solvent on the dispersion stability of BP nanosheets has been systematically studied by modulated 3D cross-correlation. It has been shown that measurements at smaller angles can obtain better autocorrelation functions. The BP colloids exhibit excellent colloidal stability when the concentration is below 250 μgmL−1, above which aggregation tends to form. Increasing the temperature has shown to result in the formation of aggregation in the colloids. BP nanosheets possess better dispersion state in polar solvents than in nonpolar solvents, and the aggregation process in nonpolar solvents can be monitored by the slow relaxation mode of 3D light scattering. This study indicates that 3D light scattering paves way to analyze the dispersion dynamics of various 2D nanoparticle colloids with intensive light absorption and poor dispensability.

Synthesis and Characterization of Liquid-Crystalline Tetraoxapentacene Derivatives Exhibiting Aggregation-Induced Emission.

A series of new tetrakis(dialkoxyphenyl) dicyanotetraoxapentacene derivatives (1 a-c) were prepared by reaction of the appropriate terphenyl diols with tetrafluoroterephthalonitrile in good yields. Compounds 1 b and 1 c, which bear hexyloxy and decyloxy side chains, exhibited columnar hexagonal mesophases, as shown by polarized optical microscopy, variable-temperature powder X-ray diffraction, and differential scanning calorimetry. Single-crystal X-ray diffraction of methoxy-substituted 1 a revealed that the dicyanotetraoxapentacene core is highly planar, consistent with the notion that these molecules are able to stack in columnar mesophases. A detailed photophysical characterization showed that these compounds exhibit aggregation-induced emission in solution, emission in nonpolar solvents, weak emission in polar solvents, and strong emission in the solid state both as powder and in thin films. These observations are consistent with a weakly emissive charge-transfer state in polar solvents and a more highly emissive locally excited state in nonpolar solvents.

Twisted Intramolecular Charge Transfer States in Trinary Star-Shaped Triphenylamine-Based Compounds.

Excited state dynamics of trinary star-shaped dendritic compounds with triphenylamine arms and different cores were studied by means of time-resolved fluorescence and transient absorption. Under optical excitation, nonpolar C3 symmetry molecules form polar excited states localized on one of the molecular substituents. Conformational excited state stabilization of molecules with an electron-accepting core causes a formation of twisted internal charge transfer (TICT) states in polar solvents. A low transition dipole moment from TICT state to the ground state causes very weak fluorescence of those compounds and strong dependence on the solvent polarity. The compound formed from the triphenylamine central core and identical arms also experiences excited state twisting, however, weakly sensitive to the solvent polarity.

Formation of planarized intramolecular charge-transfer state in dichlorotriazinyl-pyrene fluorescent probe: TD-DFT and resonance Raman study

This study is focused on explanation of the remarkable photophysical behaviour of the 1-(4,6-dichloro-1,3,5-triazin-2-yl)-pyrene (PyTC2) compound which has been introduced as a fluorescent polarity probe. This compound exhibits large solvatochromic red-shift of fluorescence emission band while maintaining high fluorescence quantum yield and monoexponential decay kinetics throughout the whole solvent polarity scale. As the semi-empirical calculations reported in the original paper have not revealed any excited state possessing a high dipole moment, it has been suggested that the red-shift originates from planarization of the emitting excited state in polar solvents in contrast to unchanged twisted geometry in non-polar solvents. However, both the extent of the red-shift and the disappearance of the vibronic structure in polar solvents indicate that the emission originates from an excited state with high dipole moment and that the semi-empirical methods may not be sufficient to describe the emitting state of this molecule correctly. Thus, we have performed TD-DFT calculations including the potential energy surface scans. According to these calculations and scans, the emission takes place from a planarized intramolecular charge-transfer (ICT) state. This is in good agreement with all aspects of the observed fluorescence behaviour of PyTC2. Independent experimental evidence for the ICT has been provided by analysis of resonance Raman intensities where bands corresponding to enhanced normal modes residing on triazine and the stretching mode between pyrene and triazine moieties have been identified. The formation of the photoinduced ICT together with easy and inexpensive preparation make this compound and its derivatives candidate as push-pull building blocks for the design of advanced functional materials.

Ultrafast Photoinduced Charge Separation Leading to High-Energy Radical Ion-Pairs in Directly Linked Corrole-C60 and Triphenylamine-Corrole-C60 Donor-Acceptor Conjugates.

Closely positioned donor-acceptor pairs facilitate electron- and energy-transfer events, relevant to light energy conversion. Here, a triad system TPACor-C60 , possessing a free-base corrole as central unit that linked the energy donor triphenylamine (TPA) at the meso position and an electron acceptor fullerene (C60) at the β-pyrrole position was newly synthesized, as were the component dyads TPA-Cor and Cor-C60. Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady-state fluorescence studies showed efficient energy transfer from (1) TPA* to the corrole and subsequent electron transfer from (1) corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron-transfer products, the corrole radical cation (Cor(⋅+) in Cor-C60 and TPA-Cor(⋅+) in TPACor-C60) and fullerene radical anion (C60(⋅-)), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, kCS , was found to be about 10(11) s(-1), suggesting the occurrence of an ultrafast charge-separation process. Interestingly, although an order of magnitude slower than kCS , the rate of charge recombination, kCR , was also found to be rapid (kCR ≈10(10) s(-1)), and both processes followed the solvent polarity trend DMF>benzonitrile>THF>toluene. The charge-separated species relaxed directly to the ground state in polar solvents while in toluene, formation of (3) corrole* was observed, thus implying that the energy of the charge-separated state in a nonpolar solvent is higher than the energy of (3) corrole* being about 1.52 eV. That is, ultrafast formation of a high-energy charge-separated state in toluene has been achieved in these closely spaced corrole-fullerene donor-acceptor conjugates.

Photoinduced intramolecular charge transfer in trans-2-[4'-(N,N-dimethylamino)styryl]imidazo[4,5-b]pyridine: effect of introducing a C[double bond, length as m-dash]C double bond.

The spectral characteristics of trans-2-[4'-(N,N-dimethylamino)styryl]imidazo[4,5-b]pyridine (t-DMASIP-b) have been investigated using absorption and fluorescence techniques, and compared with 2-(4'-N,N-dimethylamino)imidazo[4,5-b]pyridine (DMAPIP-b). The study reveals that introduction of a C[double bond, length as m-dash]C double bond strongly perturbs the photophysics of the system. Unlike DMAPIP-b, t-DMASIP-b emits a single emission in aprotic and protic solvents. The emission occurs from the locally excited state in nonpolar solvents and from a planar intramolecular charge transfer (PICT) state in polar solvents. Multiple linear regression analysis suggests that among the different solvent parameters, the dipolar interaction contributes more to the stabilization of the system in both the ground and excited states. Theoretical calculations suggest that, unlike in DMAPIP-b, proton coupled twisted intramolecular charge transfer (TICT) emission does not occur in t-DMASIP-b. The higher quantum yield obtained in the viscous solvent glycerol is attributed to the restriction of the twisting of the olefinic bond. The photoirradiation of t-DMASIP-b shows that isomerization takes place in all solvents, including viscous glycerol. The theoretically simulated potential energy surface shows that isomerization occurs via a phantom state, which is a nonradiative process. The rise in temperature favors the photoisomerization, thus, the fluorescence quantum yield decreases. The prototropic study indicates that, unlike in DMAPIP-b, the protonation takes place at different places to form the monocations.

Symmetry- and solvent-dependent photophysics of fluorenes containing donor and acceptor groups.

Three two-photon absorption (2PA) dyes (donor-π-donor (DPA2F), donor-π-acceptor (AF240), and acceptor-π-acceptor (BT2F); specifically, D is Ph2N-, A is 2-benzothiazoyl, and the π-linker is 9,9-diethylfluorene) are examined in a variety of aprotic solvents. Because the 2PA cross section is sensitive to the polarity of the local environment, this report examines the solvent-dependent linear photophysics of the dyes, which are important to understand before probing more complex solid-state systems. The symmetrical dyes show little solvent dependence; however, AF240 has significant solvatochromism observed in the fluorescence spectra and lifetimes and also the transient absorption spectra. A 114 nm bathochromic shift is observed in the fluorescence maximum when going from n-hexane to acetonitrile, whereas the lifetimes increase from 1.25 to 3.12 ns. The excited-state dipole moment for AF240 is found to be 20.1 D using the Lippert equation, with smaller values observed for the symmetrical dyes. Additionally, the femtosecond transient absorption (TA) spectra at time zero show little solvent dependence for DPA2F or BT2F, but AF240 shows a 52 nm hypsochromic shift from n-hexane to acetonitrile. Coupled with the solvatochromism in the fluorescence and large excited-state dipole moment, this is attributed to formation of an intramolecular charge-transfer (ICT) state in polar solvents. By 10 ps in AF240, the maximum TA in acetonitrile has shifted 30 nm, providing direct evidence of a solvent-stabilized ICT state, whose formation occurs in 0.85-2.71 ps, depending on solvent. However, AF240 in nonpolar solvents and the symmetrical dyes in all solvents show essentially no shifts due to a predominantly locally excited (LE) state. Preliminary temperature-dependent fluorescence using frozen glass media supports significant solvent reorganization around the AF240 excited state in polar solvents, and may also support a twisted intramolecular charge-transfer (TICT)-state contribution to the stabilization. Finally, time-dependent density functional theory calculations support ICT in AF240 in polar media and also allow prediction of the 2PA cross sections in the 0-0 band, which are much larger for AF240 than the symmetrical dyes.

Tuning Interfacial Charge Separation by Molecular Twist: A New Insight into Coumarin-Sensitized TiO2 Films

The 7-aminocoumarin class of dyes exhibits significant molecular twisting in their electronic excited state, which has important consequences on the reaction mechanism involving the intramolecular charge-transfer (ICT) state. In this work, the relationship between the molecular structure of coumarin dye and charge separation on TiO2 surface has been explored using 7-(dialkylamino)coumarin-3-carboxylic acid dyes C343 and D1421. The alkylamino group in the form of tethered constraint geometry of the julolidyl ring of coumarin C343 and rotation free geometry of 7-N,N′-diethylamino moiety of coumarin D1421 compel planar and twisted ICT (TICT) excited states in polar solvents, respectively. The femtosecond transient absorption studies show that both the charge-transfer states participate in directional electron injection from coumarin dye into the conduction band of the TiO2 semiconductor substrate. The TICT state formation in the excited state minimizes the donor and acceptor orbital overlap and causes slow b...