Excited State Charge Research Articles

Structure-property relationships for three indoline dyes used in dye-sensitized solar cells: TDDFT study of visible absorption and photoinduced charge-transfer processes

The electronic structures of three D-A-π-A indoline dyes (WS-2, WS-6, and WS-11) used in dye-sensitized solar cells (DSSCs) were studied by performing quantum chemistry calculations. The coplanarity of the A-π-A segment and distinct noncoplanarity of the indoline donor part of each dye were confirmed by checking the calculated geometric parameters. The relationships between molecular modifications and the optical properties of the dyes were derived in terms of the partial density of states, absorption spectrum, frontier molecular orbital, and excited-state charge transfer. 3D real-space analysis of the transition density (TD) and charge difference density (CDD) was also performed to further investigate the excited-state features of the molecular systems, as they provide visualized physical pictures of the charge separation and transfer. It was found that modifying the alkyl chain of the bridge unit near the acceptor unit is an efficient way to decrease dye aggregation and improve DSSC efficiency. Inserting a hexylthiophene group next to the donor unit leads to a complicated molecular structure and a decrease in the charge-transfer ability of the system, which has an unfavorable impact on DSSC performance.

Excited State Charge Redistribution and Dynamics in the Donor-π-Acceptor Flavin Derivative ABFL

Chromophores containing a donor-π-acceptor (D-π-A) motif have been shown to exhibit many interesting photophysical properties. The lowest electronic transition of a flavin derivative containing this motif, azobenzylflavin (ABFL), has previously been shown to be highly sensitive to solvent environment and hydrogen bonding ligands. To better understand this sensitivity, we have investigated the excited state charge redistribution and dynamics of ABFL in a low-dielectric, non-hydrogen bonding solvent by steady-state Stark and femtosecond optical transient absorption spectroscopies. The Stark measurements reveal the difference dipole moment, Δμ01, between the ground and first excited states to be 22.3 ± 0.9 D. The direction of Δμ01 in the molecular frame was assigned with the aid of TD-DFT and finite field calculations, verifying the hypothesis that electron density moves from the diethylaniline donor to the flavin acceptor in the excited state. The magnitude of the difference dipole moment was used to estimate the hyperpolarizability of ABFL, β0 = 720 × 10(-30) esu. Subsequent excited state decay via charge recombination was shown to take place in a few picoseconds. The data was best fit to a kinetic model composed of a sub-picosecond internal conversion step from S2→S1, followed by a 5 ps decay to the ground state. A competing process involving formation of an additional long-lived state from S1 was also observed. Cyclic voltammetry shows one oxidation and two reduction waves and is completely reversible. This analysis lays the groundwork for developing new flavin dyads with the desired excited electronic state properties for applications such as nonlinear optical devices, molecular electronics applications, or dye-sensitized solar cells.

Photophysics of indole-2-carboxylic acid (I2C) and indole-5-carboxylic acid (I5C): Heavy atom effect

In this study the effect of carboxylic group substitution in the 2 and 5 position of indole ring on the photophysics of the parent indole chromophore has been studied. The photophysical parameters crucial in triplet state decay mechanism of aqueous indole-2-carboxylic acid (I2C) and indole-5-carboxylic acid (I5C) have been determined applying our previously proposed methodology based on the heavy atom effect and fluorescence and phosphorescence decay kinetics [Kowalska-Baron et al., 2012]. The determined time-resolved phosphorescence spectra of I2C and I5C are red-shifted as compared to that of the parent indole. This red-shift was especially evident in the case of I2C and may indicate the possibility of hydrogen bonded complex formation incorporating carbonyl CO, the NH group of I2C and, possibly, surrounding water molecules. The possibility of the excited state charge transfer process and the subsequent electronic charge redistribution in such a hydrogen bonded complex may also be postulated. The resulting stabilization of the I2C triplet state is manifested by its relatively long phosphorescence lifetime in aqueous solution (912μs). The relatively short phosphorescence lifetime of I5C (56μs) may be the consequence of more effective ground-state quenching of I5C triplet state. This hypothesis may be strengthened by the significantly larger value of the determined rate constant of I5C triplet state quenching by its ground-state (4.4×108M−1s−1) as compared to that for indole (6.8×107M−1s−1) and I2C (2.3×107M−1s−1). The determined bimolecular rate constant for triplet state quenching by iodide kqT1 is equal to 1×104M−1s−1; 6×103M−1s−1 and 2.7×104M−1s−1 for indole, I2C and I5C, respectively. In order to obtain a better insight into iodide quenching of I2C and I5C triplet states in aqueous solution, the temperature dependence of the bimolecular rate constants for iodide quenching of the triplet states has been expressed in Arrhenius form. The linearity of the obtained Arrhenius plots clearly indicated the existence of one temperature-dependent non-radiative process for the de-excitation of I2C and I5C triplet state in the presence of iodide. This process may be attributed to the solute-quenching by iodide and, most probably, proceeds via reversibly formed exciplex. The activation energies obtained from linear Arrhenius plots (1.89kcal/mol for I5C; 2.55kcal/mol for I2C) are smaller as compared to that for diffusion controlled reactions in aqueous solution (about 4kcal/mol), which may indicate the great importance of the electrostatic interactions between solute and iodide ions in lowering the energy barrier needed for the formation of the triplet-quencher complex. Based on the theoretical predictions (at the DFT(CAM-B3LYP)/6-31+G(d,p) level of theory) and careful analysis of the obtained FTIR spectra it may be concluded that in the solid state I2C and I5C molecules form associates by intermolecular NH⋯OC and OH⋯OC hydrogen bonding interactions, whereas the existence of intramolecular NH⋯OC interactions in the solid state of I2C and I5C is highly unlikely.

The influence of temperature on red-edge excitation effects in liquid solutions of N,N′-dimethylaminobenzonitrile

A systematic study of new photophysical and photochemical processes in solutions is continued by the example of a recently found phenomenon of redistribution of the intensities of two fluorescence bands of N,N′-Dimethylaminobenzonitrile (DMABN) in polar solutions at room temperatures under selective irradiation by light with different photon energies in the region of the long-wavelength absorption band. The effects observed are explained using data of quantum-mechanical calculations, which reveal that solutions of these systems are very likely to contain rotational isomers with different orientations of the dimethylamino group with respect to the plane of the benzonitrile residue. The excited-state charge transfer reactions in these rotamers occur in different ways and, hence, with different rates, because of which the intensity ratio of recorded fluorescence bands is different for different wavelengths of selective excitation. In this study, the influence of the temperature on the red-edge excitation effect observed in the fluorescence of DMABN solutions in acetonitrile is studied in the temperature range of 274–313 K using the previously used selective excitation method. It is found that these effects manifest themselves at any temperature within this range, but are especially strong at 313 K. The parameters of the dual fluorescence that are most sensitive for recording of the considered effects are determined, and the obtained temperature dependences are interpreted.

Synthesis and characterization of new rhodamine dyes with large Stokes shift

Two new rhodamine dyes (Rh Q-H, Rh Q-Me) containing 1, 4-diethyl-1, 2, 3, 4-tetrahydroquinoxaline as an effective electron donor are designed and synthesized. The structures of the novel compounds are confirmed by 1H NMR, 13C NMR and ESI. Due to an excited-state intramolecular charge transfer (ICT), the new dyes exhibit longer absorption (>580 nm) and emission (>640 nm) compared with the model compounds, rhodamine 101 and rhodamine 6G. The new rhodamine dyes show large Stokes shift of 40–50 nm in commonly used solvents. Notably, when measured in a mixture of H2O/EtOH solution, significant stokes shift of 65–68 nm are achieved, which is among the largest Stokes shifts ever reported for rhodamine dyes.

Re and Br X-ray Absorption Near-Edge Structure Study of the Ground and Excited States of [ReBr(CO)3(bpy)] Interpreted by DFT and TD-DFT Calculations

X-ray absorption spectra of fac-[ReBr(CO)3(bpy)] near the Re L3- and Br K-edges were measured in a steady-state mode as well as time-resolved at 630 ps after 355 nm laser pulse excitation. Relativistic spin-orbit time-dependent density functional theory (TD-DFT) calculations account well for the shape of the near-edge absorption (the ″white line″) of the ground-state Re spectrum, assigning the lowest-lying transitions as core-to-ligand metal-to-ligand charge transfer from Re 2p(3/2) into predominantly π*(bpy) molecular orbitals (MOs) containing small 5d contributions, followed in energy by transitions into π* Re(CO)3 and delocalized σ*/π* MOs. Transitions gain their intensities from Re 5d and 6s participation in the target orbitals. The 5d character is distributed over many unoccupied MOs; the 5d contribution to any single empty MO does not exceed 29%. The Br K-edge spectrum is dominated by the ionization edge and multiple scattering features, the pre-edge electronic transitions being very weak. Time-resolved spectra measured upon formation of the lowest electronic excited state show changes characteristic of simultaneous Re and Br electronic depopulation: shifts of the Re and Br edges and the Re white line to higher energies and emergence of new intense pre-edge features that are attributed by TD-DFT to transitions from Re 2p(3/2) and Br 1s orbitals into a vacancy in the HOMO-1 created by electronic excitation. Experimental spectra together with quantum chemical calculations provide a direct evidence for a ReBr(CO)3 → bpy delocalized charge transfer character of the lowest excited state. Steady-state as well as time-resolved Re L3 spectra of [ReCl(CO)3(bpy)] and [Re(Etpy)(CO)3(bpy)](+) are very similar to those of the Br complex, in agreement with similar (TD) DFT calculated transition energies as well as delocalized excited-state spin densities and charge changes upon excitation.

Computational study of the excited state properties of modified RNA nucleobases

In this work, we present time dependent density functional theory (TD-DFT) computations of the photophysical properties for a recently synthesized family of emissive RNA nucleobases (see: D. Shin, R.W. Sinkeldam, Y. Tor, Journal of the American Chemical Society 133 (2011) 14912–14915). These modified analogues are obtained by replacing the imidazole moiety of the RNA nucleobases with thiopene and represent a complete alphabet of emissive and isomorphic analogues derived from one heterocylic nucleus. An extensive study of absorption and emission wavelengths as well as the excited state charge transfer character for these molecules was conducted at the TD-DFT/6-311++G(2df,2p) level of theory employing the CAM-B3LYP, B3LYP and PBE0 functionals in water and dioxane. The theoretical results reveal good agreement with the reported experimental data. The nature of the low-lying excited states are compared and contrasted with their naturally occurring RNA nucleobase counterparts.

Excited-state charge coupled proton transfer reaction via the dipolar functionality of salicylideneaniline

Abstract Based on design and synthesis of salicylideneaniline derivatives (1a–1d), we demonstrate a prototypical system to investigate the excited-state intramolecular charge transfer (ESICT) coupled excited-state intramolecular proton transfer (ESIPT) reaction via the dipolar functionality of the molecular framework. In solid and aprotic solvents 1a–1d exist mainly as E conformers that possess an intramolecular six-membered-ring hydrogen bond. Compounds 1a–1c exhibit a unique proton-transfer tautomer emission, while compound 1d exhibits remarkable dual emission due to the different solvent-polarity environment between ESICT and ESIPT states. Time-dependent density functional theory (TDDFT) calculations are reported on these Schiff bases in order to rationalize their electronic structure and absorption spectra.

A guanidine derivative of naphthalimide with excited-state deprotonation coupled intramolecular charge transfer properties and its application

A new fluorophore based on guanidine substituted 1,8-naphthalimide was synthesized and characterized. In aqueous solution, the guanidine group undergoes deprotonation/protonation with a pKa of ∼8.5 in the ground-state and ∼0.9 in the excited-state. The emission of its protonated and deprotonated forms exhibits a large Stokes shift (Ex/Em: 350/460 nm and 400/580 nm) due to the excited-state intramolecular charge transfer (ICT) process. The protonated form of this fluorophore exhibits dual fluorescence emission (Em: 460 and 580 nm; Ex: 350 nm) that is contributed to by an excited-state deprotonation coupled ICT process. The emission properties of this fluorophore are strongly dependent on the solvent environment, which make it possible to tune the luminescence of the materials made using this fluorophore. The absorption and emission spectra of this fluorophore respond to fluoride ions ratiometrically, showing the potential application as a fluoride ion sensor.

Investigation of Excited State Charge Redistribution of the Reduced Anionic Flavin in DNA Photolyase and Simple Solvents by Stark Spectroscopy

The two-electron reduced anionic form of flavin adenine dinucleotide (FADH−) is the catalytically active redox cofactor in DNA photolyase. This enzyme repairs UV-induced cyclobutane pyrimidine dimers (CPD) via a photoinduced electron transfer (PET) reaction, in which the FADH− is excited with blue light. The UV-visible absorption spectra of reduced anionic flavins, characterized decades ago, show a peak at ∼350 nm with a broad shoulder at ∼420 nm. We have shown, through earlier linear dichroism experiments, that these absorption bands arise from two distinct electronic transitions, a controversial result at the time. Here we have used Stark spectroscopy as a complementary approach to substantiate this result. The sensitivity of the amplitude and band shape of the Stark spectra unambiguously resolves the broad absorption band structure into two separate transitions; in fact, an analysis of the Stark data is not possible under the assumption of a single electronic transition. The magnitude of the difference dipole moment of the low energy S0→S1 transition (∼420 nm) is about three times smaller than the S0→S2 transition (∼350 nm). These experimental assignments were supported with TD-DFT calculations which help to assign the direction of charge redistribution. A picture emerges in which the xylene moiety of the isoalloxazine ring becomes electron-rich upon excitation into either absorption band. Interestingly, this part of the isoalloxazine ring is in close proximity to the bound CPD, suggesting that this is the site for PET to the CPD.

Quantum chemical study on excited states and charge transfer of oxazolo[4,5-b]-pyridine derivatives

The excited-state intramolecular charge transfer of four oxazolo[4,5-b]pyridine derivatives with different electron donating and electron withdrawing groups was investigated using the time-dependent density functional theory. The vertical excitation energies and the electronic structures were explored. Their distinct properties of absorption and fluorescence spectra in solvent phase were explained according to the electronic coupling matrix elements calculated by the Mulliken-Hush theory. The substituent on the oxazolo[4,5-b]pyridines will remarkably change their spectra properties and increase the first excited-state dipole moments. The effect of protonation on the absorption and fluorescence spectra was also investigated systematically. Our study suggests that the present method is feasible to explain charge transfer excitation and predict the properties of absorption and emission spectra in the studied systems.

Redox Modulation of Flavin and Tyrosine Determines Photoinduced Proton-coupled Electron Transfer and Photoactivation of BLUF Photoreceptors

Photoinduced electron transfer in biological systems, especially in proteins, is a highly intriguing matter. Its mechanistic details cannot be addressed by structural data obtained by crystallography alone because this provides only static information on a given redox system. In combination with transient spectroscopy and site-directed manipulation of the protein, however, a dynamic molecular picture of the ET process may be obtained. In BLUF (blue light sensors using FAD) photoreceptors, proton-coupled electron transfer between a tyrosine and the flavin cofactor is the key reaction to switch from a dark-adapted to a light-adapted state, which corresponds to the biological signaling state. Particularly puzzling is the fact that, although the various naturally occurring BLUF domains show little difference in the amino acid composition of the flavin binding pocket, the reaction rates of the forward reaction differ quite largely from a few ps up to several hundred ps. In this study, we modified the redox potential of the flavin/tyrosine redox pair by site-directed mutagenesis close to the flavin C2 carbonyl and fluorination of the tyrosine, respectively. We provide information on how changes in the redox potential of either reaction partner significantly influence photoinduced proton-coupled electron transfer. The altered redox potentials allowed us furthermore to experimentally describe an excited state charge transfer intermediately prior to electron transfer in the BLUF photocycle. Additionally, we show that the electron transfer rate directly correlates with the quantum yield of signaling state formation.

Linear-Response and Real-Time Time-Dependent Density Functional Theory Studies of Core-Level Near-Edge X-Ray Absorption

We discuss our implementation and application of time-dependent density functional theory (TDDFT) to core-level near-edge absorption spectroscopy, using both linear-response (LR) and real-time (RT) approaches. We briefly describe our restricted excitation window TDDFT (REW-TDDFT) approach for core excitations, which has also been reported by other groups. This is followed by a detailed discussion of real-time TDDFT techniques tailored to core excitations, including obtaining spectral information through delta-function excitation, postprocessing time-dependent signals, and resonant excitation through quasi-monochromatic excitation. We present results for the oxygen K-edge of water and carbon monoxide; the carbon K-edge of carbon monoxide; the ruthenium L3-edge for the hexaamminerutheium(III) ion, including scalar relativistic corrections via the zeroth order regular approximation (ZORA); and the carbon and fluorine K-edges for a series of fluorobenzenes. In all cases, the calculated spectra are found to be in reasonable agreement with experimental results, requiring only a uniform shift ranging from -4 eV to +19 eV, i.e., on the order of a few percent of the excitation energy. Real-time TDDFT visualization of excited state charge densities is used to visually examine the nature of each excitation, which gives insight into the effects of atoms bound to the absorbing center.

Charge Transfer from an Aromatic Adsorbate to a SemiconductorTiO2Surface Probed on the Femtosecond Time Scale with Resonant Inelastic X-Ray Scattering

The excited state charge transfer dynamics of an aromatic adsorbate to a rutile TiO(2) surface is studied using a core-hole clock (CHC) implementation of resonant inelastic x-ray scattering (RIXS) on the low femtosecond time scale. Comparing the abundance of the elastic and inelastic radiative N 1s core-hole decay channels provides a method for probing ultrafast injection of electrons resonantly excited to unoccupied molecular orbitals that lie above the conduction band of the substrate on the time scale of the core-hole lifetime. The method is analogous to the CHC implementation of resonant photoemission, but in the case of RIXS it is the emitted soft x-ray photons that are measured rather than photoelectrons, leaving the probed molecules in a neutral rather than ionized final state. The results reveal a charge transfer time of 2.8±1.5 fs from the lowest unoccupied molecular orbital +1 state of adsorbed bi-isonicotinic acid to the conduction band of the TiO(2) substrate.

Ultrafast photo-induced charge transfer unveiled by two-dimensional electronic spectroscopy

The interaction of exciton and charge transfer (CT) states plays a central role in photo-induced CT processes in chemistry, biology, and physics. In this work, we use a combination of two-dimensional electronic spectroscopy (2D-ES), pump-probe measurements, and quantum chemistry to investigate the ultrafast CT dynamics in a lutetium bisphthalocyanine dimer in different oxidation states. It is found that in the anionic form, the combination of strong CT-exciton interaction and electronic asymmetry induced by a counter-ion enables CT between the two macrocycles of the complex on a 30 fs timescale. Following optical excitation, a chain of electron and hole transfer steps gives rise to characteristic cross-peak dynamics in the electronic 2D spectra, and we monitor how the excited state charge density ultimately localizes on the macrocycle closest to the counter-ion within 100 fs. A comparison with the dynamics in the radical species further elucidates how CT states modulate the electronic structure and tune fs-reaction dynamics. Our experiments demonstrate the unique capability of 2D-ES in combination with other methods to decipher ultrafast CT dynamics.

Electrochemically induced charge transfer in platinum–silicon heterosystems

AbstractElectrochemical cyclovoltammetry experiments were carried out on the platinum surface of a platinum–silicon thin‐film device in sulfuric acid. The internal electronic barrier of the device allows the separation of ground state and excited state charge transfer events released by the electrochemical surface reaction. The traces of the device current show similar features as the cyclovoltammogram and can be discussed in terms of small deviations from the equilibrium state of the thin platinum film. The device current plotted as a function of the electrode potential shows a well‐defined shift of the baseline when hydrogen is adsorbed. This baseline shift may be originated by an adsorbate‐induced change of the chemical potential of the platinum film, thus influencing the internal field in the device (up to values of 23 mV).magnified imagePlot of the electrochemical current and the device current as functions of the electrode potential.

Photoinduced electron transfer in folic acid investigated by ultrafast infrared spectroscopy.

Conformational control of excited-state intramolecular electron transfer (ET) in folic acid (FA) has been investigated using femtosecond time-resolved infrared (TRIR) spectroscopy. Ultrafast excited-state ET between the pterin and the 4-aminobenzoyl subunits of FA is observed for the anionic form (at pH 10.0). An ET lifetime of 2.5 ps is estimated from Marcus theory for FA in the "U" conformation, in close agreement with the observed lifetime of 2.0 ps. Return to the ground state through the reverse ET reaction happens almost as rapidly, within 5 ps, resulting in rapid quenching of the singlet excited state. In mixed water:dimethyl sulfoxide solvent, ET becomes more unfavorable as FA adopts a more open conformation, thereby increasing the effective donor-acceptor distance and reducing the coupling energy. In contrast, no ET is observed for the cationic form of FA at low pH (6.0). In this case, the initial singlet excited state is localized on the pterin moiety of FA, and the excited-state charge distribution evolves with time. The charge redistribution in the pterin that occurs with intersystem crossing to the triplet state is characterized by changes in the transient IR spectrum. The excited-state lifetime is much longer in the absence of an ET quenching pathway. These results provide new insight into the mechanism of photodegradation and toxicity of FA. Ultrafast intramolecular ET in closed conformations of FA rapidly quenches the excited state and prevents efficient triplet state formation. Thus, conformations of FA that allow ultrafast intra-ET and rapid quenching of the singlet excited state play a key role in inhibiting pathological pathways following photoexcitation of FA.

Photophysical study of some 3-benzoylmethyleneindol-2-ones and estimation of ground and excited states dipole moments from solvatochromic methods using solvent polarity parameters

► Photophysical studies of 3-benzoylmethyleneindol-2-ones in a series of different solvents. ► Ground and excited states dipole moments. ► Analysis of the ground and excited state processes. ► Study of pH effect on 3-benzoylmethyleneindol-2-ones. 3-Benzoylmethyleneindol-2-ones, isatin based chalcones containing donor and acceptor moieties that exhibit excited-state intramolecular charge transfer, have been studied in different solvents by absorption and emission spectroscopy. The excited state behavior of these compounds is strongly dependent on the nature of substituents and the environment. These compounds show multiple emissions arising from a locally excited state and the two states due to intramolecular processes viz. intramolecular charge transfer (ICT) and excited state intramolecular proton transfer (ESIPT). Excited-state dipole moments have been calculated using Stoke-shifts of LE and ICT states using solvatochromic methods. The higher values of dipole moments obtained lead to support the formation of ICT state as one of the prominent species in the excited states of all 3-benzoylmethyleneindol-2-ones. The correlation of the solvatochromic Stokes-shifts with the microscopic solvent polarity parameter ( E T N ) was found to be superior to that obtained using bulk solvent polarity functions. The absorption and florescence spectral characteristics have been also investigated as a function of acidity and basicity (Ho/pH) in aqueous phase.

Excited-state intramolecular proton transfer and charge transfer in 2-(2′-hydroxyphenyl)benzimidazole crystals studied by polymorphs-selected electronic spectroscopy

Crystal structures of polymorphs of 2-(2'-hydroxyphenyl)benzimidazole (HPBI), Forms α and β, are analyzed by X-ray crystallography. The fluorescence excitation (FE) and fluorescence spectra of the polymorphs are separately observed at temperatures 77-298 K. It has been found that the electronic spectra of the two crystal forms are significantly different from each other. Photo-excitation of the enol forms in Forms α and β induces the excited-state intramolecular proton transfer (ESIPT) to produce the S(1) state of the keto forms. In the FE spectra of Forms α and β, the S(1) ← S(0) (ππ*) transition of the keto form is observed in the 360-420 nm region in addition to that of the enol form in the 250-420 nm region. In the FE spectrum of Form β a new band peaking at 305 nm is observed, which is assigned to the S(1) ← S(0) transition of a non-planar enol form based on the observation of dual fluorescence in the UV and visible regions and quantum chemical calculation on the transition energy against the twisted angle between the benzimidazole and hydroxyphenyl rings. The fluorescence quantum yield (φ(T)) for the keto form is remarkably dependent on polymorphs at room temperature; φ(T) = 0.53 for Form α is much larger than φ(T) ≤ 0.23 for Form β. At 77 K the φ(T) values for Forms α and β increase to 0.67 and ≤0.57, respectively. The changes in the φ(T) values are associated with the intramolecular charge transfer (ICT) state. The potential barrier height between the S(1)-keto and S(1)-ICT states is significantly lower for Form β than for Form α. At 77 K the S(1)-keto → S(1)-ICT process followed by S(1)-ICT → S(0)-keto internal conversion is significantly suppressed in Form β. We compare difference in the dynamics between Forms α and β in the electronic ground and excited states.

Effect of temperature on symmetry breaking excited state charge separation: restoration of symmetry at elevated temperature

With an increase in temperature, an unprecedented restoration of symmetry in the symmetry breaking excited state charge transfer is observed in a geminal pair of near-orthogonally connected perylenimide dimers. Such restoration of symmetry could be attributed to the interchromophoric planarization and/or loss of solvation asymmetry at elevated temperature resulting in enhanced fluorescence quantum yield.