Solid‐State Emission of the Anthracene‐o‐Carborane Dyad from the Twisted‐Intramolecular Charge Transfer in the Crystalline State

The emission process of the o ‐carborane dyad with anthracene originating from the twisted intramolecular charge transfer (TICT) state in the crystalline state is described. The anthracene‐o ‐carborane dyad was synthesized and its optical properties were investigated. Initially, the dyad had aggregation‐ and crystallization‐induced emission enhancement (AIEE and CIEE) properties via the intramolecular charge transfer (ICT) state. Interestingly, the dyad presented the dual‐emissions assigned to both locally excited (LE) and ICT states in solution. From the mechanistic studies and computer calculations, it was indicated that the emission band from the ICT should be attributable to the TICT emission. Surprisingly, even in the crystalline state, the TICT emission was observed. It was proposed from that the compact sphere shape of o ‐carborane would allow for rotation even in the condensed state.     

Restriction of Molecular Twisting on a Gold Nanoparticle Surface

To understand the photophysical properties of intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states on a gold nanoparticle (Au NP) surface, we have designed and synthesized a new coumarin molecule (C3) that exists both as ICT and TICT states in its excited state in a polar environment. On a Au NP surface, an excited C3 molecule only exists as an ICT state owing to restricted molecular rotation of a diethylamino group; as a result, no conversion from the ICT to TICT state was observed. Selection of the preferential state of a molecule with dual emitting states can be helpful for selected biological applications.     

环糊精诱导胶束形成的TICT荧光探针法研究

环糊精与表面活性剂的相互作用研究已有诸多报导，但主要涉及环糊精与表面活性剂的包络物的稳定常数和其中主客体的化学剂量比［１，２］，至于环糊精对表面活性剂胶束化性质的影响则少见涉猎，这或许与研究方法有关，分子内扭转电荷转移激发态涉及到一个完整的电荷转移，具有很高的极性，其荧光特性显示出显著的介质性质敏感性［３］，因此将ＴＩＣＴ荧光探针法用于环糊精－表面活性剂相互作用研究可能会提供一些新的信息，本文的结     

TICT formation in para- and meta-derivatives of N-phenylpyrrole

The photophysical properties of m- and p-cyano N-phenylpyrrole (m- and p-PBN) are compared. Both compounds show highly red-shifted and strongly forbidden emission in polar solvents, assigned to a charge transfer state. The forbidden nature is indicative of very weak coupling between the two pi-systems, and a twisted emissive structure is suggested (TICT state). Comparison to quantum chemical calculations indicates that the twisted structure possesses an antiquinoid distortion of the benzonitrile group, i.e., the central bonds in the ring are lengthened instead of shortened. m-PBN is the first meta compound which shows a CT emission assignable to a TICT state. It differs from p-PBN by a less exergonic formation of the CT state from the LE/ICT quinoid state. Consequently, it shows only single LE/ICT fluorescence in nonpolar alkane solvents, whereas p-PBN shows dual fluorescence in this solvent (LE/ICT and TICT).     

Twisting dynamics in the excited singlet state of Michler's ketone

Ultrafast relaxation dynamics of the excited singlet (S(1)) state of Michler's ketone (MK) has been investigated in different kinds of solvents using a time-resolved absorption spectroscopic technique with 120 fs time resolution. This technique reveals that conversion of the locally excited (LE) state to the twisted intramolecular charge transfer (TICT) state because of twisting of the N,N-dimethylanilino groups with respect to the central carbonyl group is the major relaxation process responsible for the multi-exponential and probe-wavelength-dependent transient absorption dynamics of the S1 state of MK, but solvation dynamics does not have a significant role in this process. Theoretical optimization of the ground-state geometry of MK shows that the dimethylanilino groups attached to the central carbonyl group are at a dihedral angle of about 51 degrees with respect to each other because of steric interaction between the phenyl rings. Following photoexcitation of MK to its S1 state, two kinds of twisting motions have been resolved. Immediately after photoexcitation, an ultrafast "anti-twisting" motion of the dimethylanilino groups brings back the pretwisted molecule to a near-planar geometry with high mesomeric interaction and intramolecular charge transfer (ICT) character. This motion is observed in all kinds of solvents. Additionally, in solvents of large polarity, the dimethylamino groups undergo further twisting to about 90 degrees with respect to the phenyl ring, to which it is attached, leading to the conversion of the ICT state to the TICT state. Similar characteristics of the absorption spectra of the TICT state and the anion radical of MK establish the nearly pure electron transfer (ET) character of the TICT state. In aprotic solvents, because of the steep slope of the potential energy surface near the Franck-Condon (FC) or LE state region, the LE state is nearly nonemissive at room temperature and fluorescence emission is observed from only the ICT and TICT states. Alternatively, in protic solvents, because of an intermolecular hydrogen-bonding interaction between MK and the solvent, the LE region is more flat and stimulated emission from this state is also observed. However, a stronger hydrogen-bonding interaction between the TICT state and the solvent as well as the closeness between the two potential energy surfaces due to the TICT and the ground states cause the nonradiative coupling between these states to be very effective and, hence, cause the TICT state to be weakly emissive. The multi-exponentiality and strong wavelength-dependence of the kinetics of the relaxation process taking place in the S1 state of MK have arisen for several reasons, such as strong overlapping of transient absorption and stimulated emission spectra of the LE, ICT, and TICT states, which are formed consecutively following photoexcitation of the molecule, as well as the fact that different probe wavelengths monitor different regions of the potential energy surface representing the twisting motion of the excited molecule.     

The role of hydrogen bonding in excited state intramolecular charge transfer

Intramolecular charge transfer (ICT) that occurs upon photoexcitation of molecules is a vital process in nature and it has ample applications in chemistry and biology. The ICT process of the excited molecules is affected by several environmental factors including polarity, viscosity and hydrogen bonding. The effect of polarity and viscosity on the ICT processes is well understood. But, despite the fact that hydrogen bonding significantly influences the ICT process, the specific role of hydrogen bonding in the formation and stabilization of the ICT state is not unambiguously established. Some literature reports predicted that the hydrogen bonding of the solvent with a donor promotes the formation of a twisted intramolecular charge transfer (TICT) state. Some other reports stated that it inhibits the formation of the TICT state. Alternatively, it was proposed that the hydrogen bonding of the solvent with an acceptor favors the TICT state. It is also observed that a dynamic equilibrium is established between the free and the hydrogen bonded ICT states. This perspective focuses on the specific role played by hydrogen bonding of the solvent with the donor and the acceptor, and by proton transfer in the ICT process. The utility of such influence in molecular recognition and anion sensing is discussed with a few recent literature examples in the end.     

Dual fluorescence of omeprazole: effect of solvents and pH

Absorption, steady state fluorescence and time-resolved fluorescence spectra of omeprazole (OMP) have been studied in solvents of different polarity and pH. With an increase in the polarity of the solvents, blue shift is observed in the longer wavelength whereas red shift is noticed in the shorter wavelength band. The dual emission observed in non-polar solvents suggests that the energy of the twisted intramolecular charge transfer (TICT) state is lower than that of the locally excited (LE) state. The normal Stokes-shifted band originates from the LE state, and the large Stokes-shifted band is due to the emission from a TICT state. The Stokes shift of OMP is correlated with various solvent polarity scales like E_T(30) and f?(D,n).     

Intramolecular charge-transfer state formation of 4-(N,N-dimethylamino)benzonitrile in acetonitrile solution: RISM-SCF study

Intramolecular charge-transfer (ICT) state formation of 4-(N,N-dimethylamino)benzonitrile in acetonitrile solution is studied by the reference interaction site model self-consistent field (RISM-SCF) method. Geometry optimizations are performed for each electronic state in solution with the complete-active-space SCF wave functions. Dynamic electron correlation effects are taken into account by using the multiconfigurational quasidegenerate perturbation theory. Two-dimensional free energy surfaces are constructed as the function of the twisting and wagging angles of the dimethylamino group for the ground and locally excited (LE) states. The calculated absorption and fluorescence energies are in good agreement with experiments. The validity of the twisted ICT (TICT) model is confirmed in explaining the dual fluorescence, and the possibility of the planar ICT model is ruled out. To examine the mechanism of the TICT state formation, a "crossing" seam between the LE and charge-transfer (CT) state surfaces is determined. The inversion of two electronic states occurs at a relatively small twisting angle. The effect of solvent reorganization is also examined. It is concluded that the intramolecular twisting coordinate is more important than the solvent fluctuation for the TICT state formation, because the energy difference between the two states is minimally dependent on the solvent configuration.     

Time‐dependent density functional theory study on the hydrogen bonding‐induced twisted intramolecular charge‐transfer excited states of 2‐(4′‐N,N‐dimethylaminophenyl)imidazo[4,5‐b]pyridine

In this work, the time‐dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen‐bonded intramolecular charge‐transfer excited state of 2‐(4′‐N,N‐dimethylaminophenyl)imidazo[4,5‐b]pyridine (DMAPIP) in methanol (MeOH) solvent. All the geometric conformations of the ground state and locally excited (LE) state and the twisted intramolecular charge‐transfer (TICT) state for isolated DMAPIP and its hydrogen‐bonded complexes have been optimized. At the same time, the absorption and fluorescence spectra of DMAPIP and the hydrogen‐bonded complexes in different electronic states are also calculated. We theoretically demonstrated for the first time that the intermolecular hydrogen bond formed between DMAPIP and MeOH can induce the formation of the TICT state for DMAPIP in MeOH solvent. Therefore, the two components at 414 and 506 nm observed in the fluorescence spectra of DMAPIP in MeOH solvent were reassigned in this work. The fluorescence peak at 414 nm is confirmed to be the LE state. Furthermore, the red‐shifted shoulder at 506 nm should be originated from the hydrogen‐bonded TICT excited state. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.     

Ab initio configuration interaction study on electronically excited 4-dimethylamino-4′-cyanostilbene

Ab initio configuration interaction calculations have been performed to examine the electronic structures of both trans-4-dimethylamino-4′-cyanostilbene (DCS) and four types of perpendicularly twisted DCSs, trans-DCS is predominantly excited into the S₁ state out of low-lying excited states. The S₁ state is an intramolecular charge-transfer (ICT) state in which the dipole moment is about twice as large as that in S₀. The excited DCS at the 4-dimethylanilino twisted conformation, which becomes S₁ in polar solvents, has a very much larger dipole moment than that in S₁ to trans-DCS. This means that the geometrical structure of the twisted ICT (TICT) is the 4-dimethylanilino twisted form, not the dimethylamino twisted one which is well know from the TICT structure of 4-dimethylaminobenzonitrile. Received: 16 December 1998 / Accepted: 19 March 1999 / Published online: 9 September 1999     

Photophysical properties of coumarin-7 dye: role of twisted intramolecular charge transfer state in high polarity protic solvents

Photophysical studies on coumarin-7 (C7) dye in different protic solvents reveal interesting changes in the properties of the dye on increasing the solvent polarity (Deltaf; Lippert-Mataga solvent polarity parameter) beyond a critical value. Up to Deltaf approximately 0.31, the photophysical properties of the dye follow good linear correlations with Deltaf. For Deltaf > approximately 0.31, however, the photophysical properties, especially the fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)) and nonradiative rate constants (k(nr)), undergo large deviations from the above linearity, suggesting an unusual enhancement in the nonradiative decay rate for the excited dye in these high polarity protic solvents. The effect of temperature on the tau(f) values of the dye has also been investigated to reveal the mechanistic details of the deexcitation mechanism for the excited dye. Studies have also been carried out in deuterated solvents to understand the role of solute-solvent hydrogen bonding interactions on the photophysical properties of the dye. Observed results suggest that the fluorescence of the dye originates from the planar intramolecular charge transfer (ICT) state in all the solvents studied and the deviations in the properties in high polarity solvents (Deltaf > approximately 0.31) arise due to the participation of a new deexcitation channel associated with the formation of a nonfluorescent twisted intramolecular charge transfer (TICT) state of the dye. Comparing present results with those of a homologous dye coumarin 30 (C30; Photochem. Photobiol., 2004, 80, 104), it is indicated that unlike in C30, the TICT state of the C7 dye does not experience any extra stability in protic solvents compared to that in aprotic solvents. This has been attributed to the presence of intramolecular hydrogen bonding between the NH group (in the 3-benzimidazole substituent) of the C7 dye and its carbonyl group, which renders an extra stability to the planar ICT state, making the TICT state formation relatively difficult. Qualitative potential energy diagrams have been proposed to rationalize the differences observed in the results with C7 and C30 dyes in high polarity protic solvents.     

Drastic changes in the fluorescence properties of NBD probes with the polarity of the medium: involvement of a TICT state?

The spectroscopic and photophysical properties of two 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) derivatives with different substituents on the nitrogen group are reported in 18 solvents. The solvatochromic shifts were analysed by correlating with polarity scales. The results, together with the help of modified neglect of diatomic overlap (MNDO) calculations, enable the polarity of the ground and first singlet excited states to be determined. Experiments based on variations in temperature and viscosity establish that the two probes undergo different de-excitation pathways. The possibilities of internal rotation leading to a twisted intramolecular charge transfer (TICT) state in the case of diethylamino-NBD are discussed. A study in binary solvent mixtures outlines specific solvent—solute interactions. Appropriate restrictions are emphasized on the utilization of NBD probes in biological fields.     

Evidence for excited state intramolecular charge transfer in benzazole-based pseudo-stilbenes

Two azo compounds were obtained through the diazotization reaction of aminobenzazole derivatives and N,N-dimethylaniline using clay montmorillonite KSF as catalyst. The synthesized dyes were characterized using elemental analysis, Fourier transform infrared spectroscopy, and (13)C and (1)H NMR spectroscopy in solution. Their photophysical behavior was studied using UV-vis and steady-state fluorescence in solution. These dyes present intense absorption in the blue region. The spectral features of the azo compounds can be related to the pseudo-stilbene type as well as the E isomer of the dyes. Excitation at the absorption maxima does not produce emissive species in the excited state. However, excitation around 350 nm allowed dual emission of fluorescence, from both a locally excited (LE, short wavelength) and an intramolecular charge transfer (ICT, long wavelength) state, which was corroborated by a linear relation of the fluorescence maximum (ν(max)) versus the solvent polarity function (Δf) from the Lippert-Mataga correlation. Evidence of TICT in these dyes was discussed from the viscosity dependence of the fluorescence intensity in the ICT emission band. Theoretical calculations were also performed in order to study the geometry and charge distribution of the dyes in their ground and excited electronic states. Using DFT methods at the theoretical levels BLYP/Aug-cc-pVDZ, for geometry optimizations and frequency calculations, and B3LYP/6-311+G(2d), for single-point energy evaluations, the calculations revealed that the least energetic and most intense photon absorption leads to a very polar excited state that relaxes non-radioactively, which can be associated with photochemical isomerization.     

Extensive Reduction in Back Electron Transfer in Twisted Intramolecular Charge‐Transfer (TICT) Coumarin‐Dye‐Sensitized TiO2 Nanoparticles/Film: A Femtosecond Transient Absorption Study

We report the synthesis, characterization, and optical and electrochemical properties of two structurally similar coumarin dyes ( C1 and C2 ). These dyes have been deployed as sensitizers in TiO₂ nanoparticles and thin films, and the effect of molecular structure on interfacial electron‐transfer dynamics has been studied. Steady‐state optical absorption, emission, and time‐resolved emission studies on both C1 and C2 , varying the polarity of the solvent and the solution pH, suggest that both photoexcited dyes exist in a locally excited (LE) state in solvents of low polarity. In highly polar solvents, however, C1 exists in an intramolecular charge‐transfer (ICT) state, whereas C2 exists in both ICT and twisted intramolecular charge‐transfer (TICT) states, their populations depending on the degree of polarity of the solvent and the pH of the solution. We have employed femtosecond transient absorption spectroscopy to monitor the charge‐transfer dynamics in C1 ‐ and C2 ‐sensitized TiO₂ nanoparticles and thin films. Electron injection has been confirmed by direct detection of electrons in the conduction band of TiO₂ nanoparticles and of radical cations of the dyes in the visible and near‐IR regions of the transient absorption spectra. Electron injection in both the C1 /TiO₂ and C2 /TiO₂ systems has been found to be pulse‐width limited (<100 fs); however, back‐electron‐transfer (BET) dynamics has been found to be slower in the C2 /TiO₂ system than in the C1 /TiO₂ system. The involvement of TICT states in C2 is solely responsible for the higher electron injection yield as well as the slower BET process compared to those in the C1 /TiO₂ system. Further pH‐dependent experiments on C1 ‐ and C2 ‐sensitized TiO₂ thin films have corroborated the participation of the TICT state in the slower BET process in the C2 /TiO₂ system.     

4-[2-(9,10-二氰基蒽)乙烯基] -N,N-二甲基苯胺的三荧光发射

从分子水平进行电子转移,电荷分离的研究是十分重要的,它不仅是自然界光合作用的基本过程,也是现代高新技术中的一个关键问题。近年来分子内含电子给体与电子受体的D-A化合物一直引起人们的极大兴趣。这些化合物能发生光致分子内电子转移,使其激发态分子的偶极矩远大于基态,它们的发射光谱对介质的粘度及极性十分敏感,随分子结构的变化而变化,展现出特有的光电性质,可利用作为非线性光学材料、光电转换材料以及荧光探针等。     

Photophysical Properties of Coumarin‐30 Dye in Aprotic and Protic Solvents of Varying Polarities¶

Experimental results on various photophysical properties of coumarin‐30 (C30) dye, namely, Stokes' shift (Δv), fluorescence quantum yield (τ_f), fluorescence lifetime (τ_f), radiative rate constant (k_f) and nonradiative rate constant (k_nr), as obtained using absorption and fluorescence measurements have been reported. Though in most of the solvents the properties of C30 show more or less linear correlation with the solvent polarity function, Δf= [(ε ‐ 1)/(2ε+ 1) ‐ (n² ‐ 1)/ (2n²+ l)], they show unusual deviations in nonpolar solvents at one end and in high‐polarity protic solvents at the other end. From the solvent polarity and temperature effect on the photophysical properties of the dye, following inferences have been drawn: ( 1 ) in nonpolar solvents, the dye exists in a nonpolar structure, where its 7‐NEt₂ substituent adopts a pyramidal configuration and the amino lone pair is out of resonance with the benzopyrone π cloud; ( 2 ) in medium to higher polarity solvents, the dye exists in a polar intra‐molecular charge transfer structure, where the 7‐NEt₂ group and the 1,2‐benzopyrone moiety are in the same plane and the amino lone pair is in resonance with the benzopyrone π cloud; ( 3 ) in protic solvents, the dye‐solvent intermolecular hydrogen bonding influences the photophysical properties of the dye; and ( 4 ) in high‐polarity protic solvents, the excited C30 undergoes a new activation‐controlled nonradiative deexcitation process because of the involvement of a twisted intra‐molecular charge transfer (TICT) state. Contrary to most other TICT molecules, the activation barrier for this deexcitation process in C30 is observed to increase with solvent polarity. A rational for this unusual behavior has been given on the basis of the solvent polarity‐dependent stabilization and crossing of relevant electronic states and the relative propensity of interconversion among these states.     

Photophysical studies of dipolar organic dyes that feature a 1,3-cyclohexadiene conjugated linkage: the implication of a twisted intramolecular charge-transfer state on the efficiency of dye-sensitized solar cells

A detailed study of the synthesis and photophysical properties of a new series of dipolar organic photosensitizers that feature a 1,3‐cyclohexadiene moiety integrated into the π‐conjugated structural backbone has been carried out. Dye‐sensitized solar cells (DSSCs) based on these structurally simple dyes have shown appreciable photo‐to‐electrical energy conversion efficiency, with the highest one up to 4.03 %. Solvent‐dependent fluorescence studies along with the observation of dual emission on dye 4 b and single emission on dyes 4 a and 32 suggest that dye 4 b possesses a highly polar emissive excited state located at a lower‐energy position than at the normal emissive excited state. A detailed photophysical investigation in conjunction with computational studies confirmed the twisted intramolecular charge‐transfer (TICT) state to be the lowest emissive excited state for dye 4 b in polar solvents. The relaxation from higher‐charge‐injection excited states to the lowest TICT state renders the back‐electron transfer process a forbidden one and significantly retards the charge recombination to boost the photocurrent. The electrochemical impedance under illumination and transient photovoltage decay studies showed smaller charge resistance and longer electron lifetime in 4 b ‐based DSSC compared to the DSSCs with reference dyes 4 a and 32 , which further illustrates the positive influence of the TICT state on the performance of DSSCs.     

Comprehensive Photophysical Behaviour of Ethynyl Fluorenes and Ethynyl Anthracenes Investigated by Fast and Ultrafast Time‐Resolved Spectroscopy

Detailed investigations by time‐resolved transient absorption and fluorescence spectroscopies with nano‐ and femtosecond time resolutions are carried out with the aim of characterising the lowest excited singlet and triplet states of three ethynyl fluorenes ( 1 – 3 ) and three ethynyl anthracenes ( 4 – 6 ) in solvents of different polarity. The solvent is found to modify the deactivation pathways of the lowest excited singlet state of compounds 1 – 4 , thus changing their fluorescence, intersystem crossing and internal conversion efficiencies. The fluorescence and triplet yields gradually decrease, while the internal conversion quantum yield increases upon increasing the solvent dielectric constant. These experimental results, coupled with the marked fluorosolvatochromic effect, point to the involvement of an emitting state with a charge‐transfer (CT) character, strongly stabilised by polar solvents. This is proved by ultrafast spectroscopic studies in which two transients, distinguished by characteristic spectral shapes assigned to locally excited (LE) and CT states, are detected, the CT state being the longer lived and fluorescent one in highly polar solvents. The intramolecular LE→CT process, operative in highly polar media, becomes particularly fast (up to ≈300 fs) in the case of the NO₂ derivative 1 . No push–pull character is found for 5 and 6 , which exhibit different photophysical behaviour; indeed, the solvent polarity does not modify significantly the dynamics of the lowest excited singlet states. Quantum mechanical calculations at the TDDFT level are also used to determine the state order and nature of the lowest excited singlet and triplet states and to rationalise the different photophysical behaviour of fluorine and anthracene derivatives, particularly concerning the intersystem crossing process.     

Fast intramolecular charge transfer with a planar rigidized electron donor/acceptor molecule

The planar rigidized molecule fluorazene (FPP) undergoes fast reversible intramolecular charge transfer (ICT) in the excited state, with a reaction time of 12 ps in the polar solvent ethyl cyanide at -45 degrees C. The ICT state of FPP has a dipole moment mu(e)(ICT) of 13 D, much larger than that of the locally excited state LE (1 D). The ICT behavior of FPP is similar to that of its flexible counterpart N-phenylpyrrole (PP), for which mu(e)(ICT) = 12 D. These results show that intramolecular charge transfer to a planar ICT state can occur efficiently. In designing ICT systems capable of rapid switching, it is therefore important to realize that large amplitude motions such as those necessary for the formation of a twisted intramolecular charge transfer (TICT) state are not required.