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.     

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.     

Photo-excited intramolecular charge transfer and excited state intramolecular proton transfer behaviors for HPIBT system: A theoretical investigation

Given facile synthetic route and excellent photo stability, excited state intramolecular proton transfer (ESIPT)-active luminous materials have gained more and more attention. Here, we focus on photo-induced excitation process and the ESIPT reaction process for the novel 5-benzothiazol-2-yl-6-hydroxy-2-methyl-isoindole-1,3-dione (HPIBT) molecule. On the level of chemical geometries and infrared spectra, we verify that O─H⋯N of HPIBT should be enhanced. We find that a proton is likely to be attracted by enhanced electronic densities around N, that is, charge transfer impetus ESIPT trend. Combing potential energy curves and searching for transition state, we clarify the ultrafast ESIPT mechanism of HPIBT due to a low barrier, which legitimately explains previous experimental characteristics.     

Electronic polarization reversal and excited state intramolecular charge transfer in donor/acceptor ethynylpyrenes

An attempt to tune the electronic properties of pyrene (Py) by coupling it with a strong electron donor (-PhNMe2, DMA)/acceptor (anthronitrile, AN) through an ethynyl bridge has been undertaken. A moderate electron donor (iPrOPh-, IPP)/acceptor (2-quinolinyl, 2Q) has also been incorporated, and all four molecules were studied with reference to a neutral molecule, namely, 1-phenylethynylpyrene (PhEPy). All the arylethynylpyrenes (ArEPy's) have been thoroughly characterized, and their electronic properties were studied by absorption and emission spectral properties of these ArEPy's. The electrochemical characteristics were also studied for arriving at the electrochemical band gap which has been compared with the HOMO-LUMO energy gap derived from the photophysical measurements and theoretical calculations performed by density functional theory (DFT) using B3LYP/6-31G basis sets. The results obtained from experimental and theoretical studies are critically discussed.     

Role of excited state intramolecular charge transfer in the photophysical properties of norfloxacin and its derivatives

The photophysical properties of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and some of its derivatives have been studied to evaluate the role of the free carboxylic acid and the nonprotonated piperazinyl group in the behavior of the 1,4-dihydro-4-oxoquinoline ring. Steady state and time-resolved fluorescence measurements at different pHs provide clear evidence in favor of singlet excited-state deactivation of NFX and its N(4')-methyl derivative pefloxacin (PFX) via intramolecular electron transfer from the N(4') atom of the piperazinyl ring to the fluoroquinolone (FQ) main system. This is a very efficient, energy-wasting pathway, which becomes dramatically enhanced in basic media. Acetylation at N(4') (as in ANFX) decreases the availability of the lone pair, making observable its fluorescence and the transient absorption spectrum of its triplet excited state even at high pH. It also reveals that the geometry of FQs changes from an almost sp3 hybridization of the N(1') of the piperazinyl substituent in the ground state to nearly sp2 in the singlet excited state (rehybridization accompanied by intramolecular charge transfer, RICT); accordingly, the singlet energy of ANFX is significantly lower than that of NFX and PFX. The fluorescence measurements using acetonitrile as a polar nonprotic organic solvent further support deactivation of the singlet excited state of nonacetylated NFX derivatives via intramolecular electron transfer from the N(4') atom.     

Ultrafast excited state dynamics of fucoxanthin: excitation energy dependent intramolecular charge transfer dynamics

Carotenoids containing a carbonyl group in conjugation with their polyene backbone are naturally-occurring pigments in marine organisms and are essential to the photosynthetic light-harvesting function in aquatic algae. These carotenoids exhibit spectral characteristics attributed to an intramolecular charge transfer (ICT) state that arise in polar solvents due to the presence of the carbonyl group. Here, we report the spectroscopic properties of the carbonyl carotenoid fucoxanthin in polar (methanol) and nonpolar (cyclohexane) solvents studied by steady-state absorption and femtosecond pump-probe measurements. Transient absorption associated with the optically forbidden S(1) (2(1)A) state and/or the ICT state were observed following one-photon excitation to the optically allowed S(2) (1(1)B) state in methanol. The transient absorption measurements carried out in methanol showed that the ratio of the ICT-to-S(1) state formation increased with decreasing excitation energy. We also showed that the ICT character was clearly visible in the steady-state absorption in methanol based on a Franck-Condon analysis. The results suggest that two spectroscopic forms of fucoxanthin, blue and red, exist in the polar environment.     

An intramolecular charge transfer state of carbonyl carotenoids: implications for excited state dynamics of apo-carotenals and retinal

Excited state dynamics of two apo-carotenals, retinal and 12'-apo-β-carotenal, were studied by femtosecond transient absorption spectroscopy. We make use of previous knowledge gathered from studies of various carbonyl carotenoids and suggest that to consistently explain the excited-state dynamics of retinal in polar solvents, it is necessary to include an intermolecular charge transfer (ICT) state in the excited state manifold. Coupling of the ICT state to the A(g)(-) state, which occurs in polar solvents, shortens lifetime of the lowest excited state of 12'-apo-β-carotenal from 180 ps in n-hexane to 7.1 ps in methanol. Comparison with a reference molecule lacking the conjugated carbonyl group, 12'-apo-β-carotene, demonstrates the importance of the carbonyl group; no polarity-induced lifetime change is observed and 12'-apo-β-carotene decays to the ground state in 220 ps regardless of solvent polarity. For retinal, we have confirmed the well-known three-state relaxation scheme in n-hexane. Population of the B(u)(+) state decays in <100 fs to the A(g)(-) state, which is quenched in 440 fs by a low-lying nπ* state that decays with a 33 ps time constant to form the retinal triplet state. In methanol, however, the A(g)(-) state is coupled to the ICT state. This coupling prevents population of the nπ* state, which explains the absence of retinal triplet formation in polar solvents. Instead, the coupled A(g)(-)/ICT state decays in 1.6 ps to the ground state. The A(g)(-)/ICT coupling is also evidenced by stimulated emission, which is a characteristic marker of the ICT state in carbonyl carotenoids.     

Stereo-differentiation in the excited state behaviour of naphthalene-thymine dyads

Using two diastereomeric dyads containing naphthalene and thymine units, significant chiral discrimination has been found in the photophysical processes involving the naphthalene excited states: singlet deactivation by hydrogen bonding molecules, singlet-singlet energy transfer from thymine and triplet decay.     

气相水杨酸激发态分子内质子转移特性分析

采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)研究了气相水杨酸(SA)分子的激发态氢键动力学过程。通过对水杨酸分子基态和激发态结构的优化,以及对其稳态吸收和发射光谱特性、前线分子轨道、红外振动光谱和势能曲线的计算分析,阐明水杨酸分子内质子转移可在激发态下自发地发生,导致其激发态可存在烯醇式和酮式两种异构体结构,并揭示了这种质子转移源于分子内电荷转移的激发态氢键的加强机制。     

Effect of nonproximate atomic substitution on excited state intramolecular proton transfer

The central C atom of the OCCCO skeleton of the malonaldehyde molecule is replaced by N, and the effects upon the intramolecular H-bond and the proton transfer are monitored by ab initio calculations in the ground and excited electronic states. The H-bond is weakened in the singlet and triplet states arising from n→π* excitation in both molecules, which is accompanied by a heightened barrier to proton transfer.³ππ* behaves in the same manner, but the singlet ππ* state has a stronger H-bond and lower barrier. Replacement of the central C atom by N strengthens the intramolecular H-bond. Although the proton transfer barrier in the ground state of formimidol is lower than in malonaldehyde, the barriers in all four excited states are higher in the N-analog. The latter substitution also dampens the effect of the n→π* excitation upon the H-bond and increases the excitation energies of the various states, particularly ππ*. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 129–138, 1998     

Photoinduced charge transfer in short-distance ferrocenylsubphthalocyanine dyads

Two new ferrocenylsubphthalocyanine dyads with ferrocenylmethoxide (2) and ferrocenecarboxylate (3) substituents directly attached to the subphthalocyanine ligand via the axial position have been prepared and characterized using NMR, UV-vis, and magnetic circular dichroism (MCD) spectroscopies as well as X-ray crystallography. The redox properties of the ferrocenyl-containing dyads 2 and 3 were investigated using the cyclic voltammetry (CV) approach and compared to those of the parent subphthalocyanine 1. CV data reveal that the first reversible oxidation is ferrocene-centered, while the second oxidation and the first reduction are localized on the subphthalocyanine ligand. The electronic structures and nature of the optical bands observed in the UV-vis and MCD spectra of all target compounds were investigated by a density functional theory polarized continuum model (DFT-PCM) and time-dependent (TD)DFT-PCM approaches. It has been found that in both dyads the highest occupied molecular orbital (HOMO) to HOMO-2 are ferrocene-centered molecular orbitals, while HOMO-3 as well as lowest unoccupied molecular orbital (LUMO) and LUMO+1 are localized on the subphthalocyanine ligand. TDDFT-PCM data on complexes 1-3 are consistent with the experimental observations, which indicate the dominance of π-π* transitions in the UV-vis spectra of 1-3. The excited-state dynamics of the dyads 2 and 3 were investigated using time-correlated single photon counting, which indicates that fluorescence quenching is more efficient in dyad 3 compared to dyad 2. These fluorescence lifetime measurements were interpreted on the basis of DFT-PCM calculations.     

Possible mechanisms of intermolecular charge transfer and electron transfer processes in the excited electronic state

Mechanisms of intermolecular charge transfer and electron transfer processes in the electronically excited states of solute molecules have been discussed in relation to the exciplex formation and fluorescence quenching reactions in solution. A new model for the electron transfer process has been proposed and studied by the quantum mechanical method. Some naive and intuitive concepts of the electron transfer process have been given a more rigorous theoretical basis. An experiment which can test this model has been suggested. Furthermore, the possible connections among the very weak CT complex formation, exciplex formation and the electron transfer reaction have been discussed in general on the basis of the theoretical considerations.

---

Zusammenfassung Mechanismen für den intermolekularen Ladungs- und Elektronenübergang bei gelösten Molekülen in elektronisch angeregten Zuständen werden im Zusammenhang mit der Bildung von Exiplexen und der Fluoreszenzlöschung diskutiert. Für den Elektronenübergang wird ein neues Modell vorgeschlagen, das quantenmechanisch untersucht wird. Dadurch wird einigen einfachen und intuitiven Vorstellungen zum Elektronenübergang eine breitere theoretische Grundlage gegeben. Zur Überprüfung des Modells wird ein Experiment vorgeschlagen. Ferner werden auf der Grundlage theoretischer Überlegungen mögliche Zusammenhänge zwischen der Bildung eines sehr schwachen charge transfef-Komplexes, der Bildung eines Exiplexes und dem Elektronenübergang diskutiert.

---

Résumé Les mécanismes de transfert de charge intermoléculaire et de transfert d'électrons dans les états électroniques excités de molécules solutées sont discutés en relation avec la formation d'exciplex et les réactions d'extinction de fluorescence en solution. On propose et on étudie quantiquement un nouveau modèle pour les processus de transfert d'électrons. Il donne une base théorique plus rigoureuse à certains représentations naïves et intuitives du transfert d'électron. On suggère une expérience pour étudier la validité de ce modèle. Enfin les rapports possibles entre la formation de complexes CT très faibles, la formation d'exciplex et la réaction de transfert d'électrons a été discutée de façon générale sur la base de considérations théoriques.

     

A non-adiabatic quantum-classical dynamics study of the intramolecular excited state hydrogen transfer in ortho-nitrobenzaldehyde

Ab initio surface-hopping dynamics calculations have been performed to simulate the intramolecular excited state hydrogen transfer dynamics of ortho-nitrobenzaldehyde (o-NBA) in the gas phase from the electronic S(1) excited state. Upon UV excitation, the hydrogen is transferred from the aldehyde substituent to the nitro group, generating o-nitrosobenzoic acid through a ketene intermediate. The semiclassical propagations show that the deactivation from the S(1) is ultrafast, in agreement with the experimental measurements, which detect the ketene in less than 400 fs. The trajectories show that the deactivation mechanism involves two different conical intersections. The first one, a planar configuration with the hydrogen partially transferred, is responsible for the branching between the formation of a biradical intermediate and the regeneration of the starting material. The conversion of the biradical to the ketene corresponds to the passage through a second intersection region in which the ketene group is formed.     

DFT based computational study on the excited state intramolecular proton transfer processes in o-hydroxybenzaldehyde

Potential energy (PE) curves for the intramolecular proton transfer in the ground (GSIPT) and excited (ESIPT) states of o-hydroxybenzaldehyde (OHBA) were studied using DFT-B3LYP/6-31G(d) and TD-DFT-B3LYP/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer in this compound. Excited states PE calculations support the ESIPT process in OHBA. The contour PE diagram and the variation of oscillator strength along the proton transfer co-ordinate support the dual emission in OHBA. Our calculations also support the experimental observations of Nagaoka et al. [S. Nagaoka, U. Nagashima, N. Ohta, M. Fujita, T. Takemura, J. Phys. Chem. 92 (1988) 166], i.e. normal emission of the title compound comes from S(2) state and the red-shifted proton transfer band appears from the S(1) state. ESIPT process has also been explained in terms of HOMO and LUMO electron density of the enol and keto tautomer of OHBA and from the potential energy surfaces.     

A ratiometric fluorescent probe for magnesium employing excited state intramolecular proton transfer

A simple fluorescent sensor has been developed for the ratiometric recognition of Mg²⁺ in semi-aqueous solution at pH 7.0. The sensor, a Schiff base, undergoes Excited State Intramolecular Proton Transfer (ESIPT) to generate a keto tautomer with proficient Mg²⁺ binding capability. The sensor displays good selectivity over other metal ions including alkali/alkali earth ions and can measure Mg²⁺ ion concentration between 2.0 and 30.0 μM. The binding stoichiometry was established as 2:1 (host:guest) with an association constant (K₂₁) of (1.4 ± 0.1) × 10⁴ M⁻². The sensor could potentially be used to detect conditions such as hypermagnesaemia.     

Competing excited state intramolecular proton transfer pathways from phenol to anthracene moieties

Four new 9-(2'-hydroxyphenyl)anthracene derivatives 7-10 were synthesized and their potential excited state intramolecular proton transfer (ESIPT) reaction investigated. Whereas 7 reacted via the anticipated (formal) ESIPT reaction (proton transfer to the 10-position of the anthracene), derivatives 8-10 reacted via ESIPT to both 9- and 10-positions, giving rise to two types of intermediates, quinone methides (e.g., 29) and zwitterions (e.g., 30). These intermediates are trapped by solvent (water or methanol) giving addition products that can readily revert back to starting material. However, on extended photolysis, the products that are isolated can best be rationalized as being due to competing elimination and intramolecular cyclization of zwitterions 30 and 37. These results show that it is possible to structurally tune ESIPT in (hydroxyphenyl)anthracenes to either result in a completely reversible reaction or give isolable anthracene addition or rearrangement products.     

Time-resolved spectroscopy of the metal-to-metal charge transfer excited state in dinuclear cyano-bridged mixed-valence complexes

Visible pump-probe spectroscopy has been used to identify and characterize short-lived metal-to-metal charge transfer (MMCT) excited states in a group of cyano-bridged mixed-valence complexes of the formula [LCo(III)NCM(II)(CN)(5)](-), where L is a pentadentate macrocyclic pentaamine (L(14)) or triamine-dithiaether (L(14S)) and M is Fe or Ru. Nanosecond pump-probe spectroscopy on frozen solutions of [L(14)Co(III)NCFe(II)(CN)(5)](-) and [L(14S)Co(III)NCFe(II)(CN)(5)](-) at 11 K enabled the construction of difference transient absorption spectra that featured a rise in absorbance in the region of 350-400 nm consistent with the generation of the ferricyanide chromophore of the photoexcited complex. The MMCT excited state of the Ru analogue [L(14)Co(III)NCRu(II)(CN)(5)](-) was too short-lived to allow its detection. Femtosecond pump-probe spectroscopy on aqueous solutions of [L(14)Co(III)NCFe(II)(CN)(5)](-) and [L(14S)Co(III)NCFe(II)(CN)(5)](-) at room temperature enabled the lifetimes of their Co(II)-Fe(III) MMCT excited states to be determined as 0.8 and 1.3 ps, respectively.     

Potential energy functions for an intramolecular proton transfer reaction in the ground and excited state

We describe the development of empirical potential functions for the study of the excited state intramolecular proton transfer reaction in 1-(trifuloroacetylamino)-naphtaquinone (TFNQ). The potential is a combination of the standard CHARMM27 force field for the backbone structure of TFNQ and an empirical valence bond formalism for the proton transfer reaction. The latter is parameterized to reproduce the potential energies both in the ground and the excited state, determined at the CASPT2 level of theory. Parameters describing intermolecular interactions are fitted to reproduce molecular dipole moments computed at the CASSCF level of theory and to reproduce ab initio hydrogen bonding energies and geometries for TFNQ-water bimolecular complexes. The utility of this potential energy function was examined by computing the potentials of mean force for the proton transfer reactions in the gas phase and in water, in both electronic states. The ground state PMF exhibits little solvent effects, whereas computed potential of mean force shows a solvent stabilization of 2.5 kcal mol⁻¹ in the product state region, suggesting proton transfer is more pronounced in polar solvents, consistent with experimental findings. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Contribution to the Fernando Bernardi Memorial Issue.     

Presence and absence of excited state intramolecular charge transfer with the six isomers of dicyano-N,N-dimethylaniline and dicyano-(N-methyl-N-isopropyl)aniline

The excited state behavior of the six m,n-dicyano-N,N-dimethylanilines (mnDCDMA) and m,n-dicyano-(N-methyl-N-isopropyl)anilines (mnDCMIA) is discussed as a function of solvent polarity and temperature. The dicyano moiety in these electron donor (D)/acceptor (A) molecules has a considerably larger electron affinity than the benzonitrile subgroup in 4-(dimethylamino)benzonitrile (DMABN). Nevertheless, the fluorescence spectra of the mnDCDMAs and mnDCMIAs in n-hexane all consist of a single emission originating from the locally excited (LE) state, indicating that a reaction from LE to an intramolecular charge transfer (ICT) state does not take place. The calculated energies E(ICT), obtained by employing the reduction potential of the dicyanobenzene subgroups and the oxidation potential of the amino substituents trimethylamine (N(Me)(3)) and isopropyldimethylamine (iPrNMe(2)), are lower than E(LE). The absence of an LE → ICT reaction therefore makes clear that the D and A units in the dicyanoanilines are not electronically decoupled. In the polar solvent acetonitrile (MeCN), dual (LE + ICT) fluorescence is found with 24DCDMA and 34DCDMA, as well as with 24DCMIA, 25DCMIA, and 34DCMIA. For all other mnDCDMAs and mnDCMIAs, only LE emission is observed in MeCN. The ICT/LE fluorescence quantum yield ratio Φ'(ICT)/Φ(LE) in MeCN at 25 °C is larger for 24DCDMA (1.2) than for 34DCDMA (0.35). The replacement of methyl by isopropyl in the amino substituent leads to a considerable increase of Φ'(ICT)/Φ(LE), 8.8 for 24DCMIA and 1.4 for 34DCMIA, showing that the LE ? ICT equilibrium has shifted further toward ICT. The appearance of an ICT reaction with the 2,4- and 3,4-dicyanoanilines is caused by a relatively small energy gap ΔE(S(1),S(2)) between the two lowest excited singlet states as compared with the other m,n-dicyanoanilines, in accordance with the PICT model. The observation that the ICT reaction is more efficient for 24DCMIA and 34DCMIA than for their mnDCDMA counterparts is mainly caused by the fact that iPrNMe(2) is a better electron donor than N(Me)(3): E(D/D(+)) = 0.84 against 1.05 V vs SCE. That ICT also occurs with 25DCMIA, notwithstanding its large ΔE(S(1),S(2)), is due to the substantial amino twist angle θ = 42.6°, which leads to partial electronic decoupling of the D and A subgroups. The dipole moments μ(e)(ICT) range between 18 D for 34DCMIA and 12 D for 25DCMIA, larger than the corresponding μ(e)(LE) of 16 and 11 D. The difference between μ(e)(ICT) and μ(e)(LE) is smaller than with DMABN (17 and 10 D) because of the noncollinear arrangement of the amino and cyano substituents (different dipole moment directions). The dicyanoanilines that do not undergo ICT, have LE dipole moments between 9 and 16 D. From plots of ln(Φ'(ICT)/Φ(LE)) vs 1000/T, the (rather small) ICT reaction enthalpies ΔH could be measured in MeCN: 5.4 kJ/mol (24DCDMA), 4.7 kJ/mol (24DCMIA), and 3.9 kJ/mol (34DCMIA). With the mnDCDMAs and mnDCMIAs only showing LE emission, the fluorescence decays are single exponential, whereas for those undergoing an LE → ICT reaction the LE and ICT picosecond fluorescence decays are double exponential. In MeCN at 25 °C, the decay times τ(2) have values between 1.8 ps for 24DCMIA and 4.6 ps for 34DCMIA at 25 °C. Longer times are observed at lower temperatures. Arrhenius plots of the forward and backward ICT rate constants k(a) and k(d) of 25DCMIA in tetrahydrofuran, obtained from the LE and ICT fluorescence decays, give the activation energies E(a) = 4.5 kJ/mol and E(d) = 11.9 kJ/mol, i.e., ΔH = -7.4 kJ/mol. From femtosecond transient absorption spectra of 24DCDMA and 34DCDMA at 22 °C, ICT reaction times τ(2) = 1/(k(a) + k(d)) of 1.8 and 3.1 ps are determined. By combining these results with the data for the fluorescence decays and Φ'(ICT)/Φ(LE), the values k(a) = 49 × 10(10) s(-1) (24DCDMA) and k(a) = 23 × 10(10) s(-1) (34DCDMA) are calculated. An LE and ICT excited state absorption is present even at a pump/probe delay time of 100 ps, showing that an LE ? ICT equilibrium is established.