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.     

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.     

Thermodynamics and Conformations in the Formation of Excited States and Their Interconversions for Twisted Donor‐Substituted Tridurylboranes

We synthesized a series of donor‐substituted tridurylboranes containing different types and number of chromophores including 1‐pyrene (PB1–3), 3‐carbazole (CBC1–3), or substituted p‐carbazol‐N‐phenyl (CBN3a–c) as various donor–acceptor (D–A) molecules. The photophysical and electrochemical properties of these twisted D–A molecules were investigated by means of UV/Vis absorption and fluorescence spectroscopy as well as cyclic voltammetry (CV). Solvent polarity, viscosity, and temperature effects on the fluorescence emission reveal the existence of three types of excited states, and their equilibria and interconversions between three excited states. In increasing order of the charge‐separated extent and the conformational change, three excited states are the locally excited (LE) state, the more planar intramolecular charge‐transfer (ICT) state, and the more twisted ICT (TICT) state as compared to the ground state. The TICT state undergoes a conformational change with a higher energy barrier over the ICT state. The solvent polarity effect on the state conversion is opposite to the viscosity effect, and temperature effects derive from its resulting changes of polarity and viscosity. For example, the increase of the polarity of the solvent results in excited‐state conversions from the LE state to the ICT state, and/or from the ICT to the TICT state, and an increased viscosity leads to the opposite conversions. On the basis of electrochemical and spectral data, thermodynamics of a possible ICT process were estimated, and correlated with the excited‐state character. Finally, three excited states have been characterized by the conformation, the photophysical properties, and the thermodynamics of the ICT processes.     

The electronic origin of the dual fluorescence in donor-acceptor substituted benzene derivatives

The origin of the dual fluorescence of DMABN (dimethylaminobenzonitrile) and other benzene derivatives is explained by a charge transfer model based on the properties of the benzene anion radical. It is shown that, in general, three low-lying electronically excited states are expected for these molecules, two of which are of charge transfer (CT) character, whereas the third is a locally excited (LE) state. Dual fluorescence may arise from any two of these states, as each has a different geometry at which it attains a minimum. The Jahn-Teller induced distortion of the benzene anion radical ground state helps to classify the CT states as having quinoid (Q) and antiquinoid (AQ) forms. The intramolecular charge transfer (ICT) state is formed by the transfer of an electron from a covalently linked donor group to an anti-bonding orbital of the pi-electron system of benzene. The change in charge distribution of the molecule in the CT states leads to the most significant geometry change undergone by the molecule which is the distortion of the benzene ring to a Q or AQ structure. As the dipole moment is larger in the perpendicular geometry than in the planar one, this geometry is preferred in polar solvents, supporting the twisted intramolecular charge transfer (TICT) model. However, in many cases the planar conformation of CT excited states is lower in energy than that of the LE state, and dual fluorescence can be observed also from planar structures.     

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.     

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。     

Time resolved spectroscopy of 2-(dimethylamine)fluorene. Solvent effects and photophysical behavior

The effect of different solvents on the fluorescent properties of 2-(dimethylamine)fluorene (DAF) were studied. In aprotic solvents we detected a strongly emissive intramolecular charge transfer (ICT) state that decayed by intersystem crossing to triplet. In proton-accepting solvents DAF exhibits in the excited state an intramolecular proton transfer. An ionized species is postulated, which simultaneously twists to a rotated conformation in the excited state. Thus, the specific solvent interactions supplement but do not replace the twist mechanism and accompany the charge transfer accepted as the prerequisite for twisted intramolecular charged transfer (TICT) state formation.     

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.     

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.     

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     

Excited state intramolecular charge transfer in N,N-heterocyclic-4-aminobenzonitriles: a DFT study

The excited state intramolecular charge transfer (ICT) reaction in a series of N,N-heterocyclic 4-aminobenzonitriles is investigated theoretically by a combination of density functional theory and multi-reference configuration interaction (DFT/MRCI). Experimentally, increasing ICT emission is observed with increasing ring size. Formation of both a planar and twisted ICT (PICT and TICT) state are energetically unfavorable in the small systems due to high inversion barriers. With increasing ring size, the TICT state is more stabilized than the PICT state. A good agreement of the computed TICT state dipole moment is found with experimental values. The red-shifted fluorescence of all systems is explained by the TICT model due to both arguments.     

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.     

Orientation of a TICT probe trapped in the peripheral confined water created by ionic surfactant envelope around silver nanoparticles

Ionic surfactants are known to aggregate around the surface of a nanoparticle as a single layer in premicellar and a double layer in micellar concentrations. This motif of arrangement indicates the development of a layer of confined water of lower polarity than bulk water around the surface of the nanoparticle. We have demonstrated the behavior of a twisted intramolecular charge transfer (TICT) probe, trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT), in the confined aqueous layer developed at the surface of spherical silver nanoparticles (Ag NPs) at and above the critical micellar concentrations (CMC) of a cationic and an anionic surfactant, namely, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). It is observed that the presence of charged surfactant head groups affects DMASBT differentially in the ground and the excited states. In presence of CTAB, DMASBT turns over in the excited state and interacts with the Ag NP surface, whereas in SDS the probe remains in its original orientation during the interaction. Steady-state and time-resolved fluorescence spectral studies provide enough evidence for orientation of the TICT probe in the peripheral water of Ag NP created by the surfactants. The results were confirmed by steady-state anisotropy measurements. The data show the difference between the properties of the confined peripheral water and the bulk aqueous environment. The TICT probe, DMASBT, is proved to be an excellent marker for the phenomenon.     

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.     

Non-covalent interactions of coumarin dyes with cucurbit[7]uril macrocycle: modulation of ICT to TICT state conversion

Non-covalent interaction of coumarin laser dyes, namely coumarin-1 (C1), coumarin-481 (C481) and coumarin-6H (C6H), with a versatile macrocyclic host molecule cucurbit[7]uril (CB7), has been investigated in aqueous solution using photophysical methods. Steady-state and time-resolved fluorescence studies illustrate significant enhancements/modifications in the fluorescence yields, lifetimes and spectral features of C1, C481 and C6H on interaction with CB7, and are assigned to 1 : 1 complex formation between the dyes and the CB7 host. The complex formation is mainly driven by charge-dipole interaction, as evident from the binding constant values (K ~ 10(4)-10(5) M(-1)). The large changes in the excited state behaviour of C1 and C481 as compared to C6H in the presence of CB7 indicate that CB7 binds C1 and C481 through the encapsulation of the 7-N,N'-diethylamino group of the dyes and the structural rigidity imposed by this interaction dramatically alters the excited state properties of the dyes by preventing the conversion of their emissive intramolecular charge transfer (ICT) state to the non-radiative twisted intramolecular charge transfer (TICT) state. The present results direct towards the probable supramolecular approach using water soluble macrocyclic CB7, in the development of aqueous dye laser systems in the blue-green region.     

Solvent effect on excited state potential energy surfaces of Thioflavin T. Qualitatively different results by TDDFT and SA-2-CASSCF methods

Thioflavin T (ThT) is a viscosity-sensitive fluorescent dye and its emission intensity undergoes a significant enhancement upon binding to DNA or amyloid fibrils. This fluorescence light-up feature has been attributed earlier to restriction of structural rearrangements in the excited state that are coupled to an intramolecular charge transfer (ICT) reaction. In this work TDDFT (using B3LYP and CAM-B3LYP functionals) and SA-2-CASSCF calculations were carried out to obtain relaxed excited-state potential energy surfaces (PES) along twisting φ and wagging δ angles that describe mutual orientation of benzothiazole (BTZ) and dimethylaniline (DMA) fragments in ThT. For isolated ThT molecule both methods predict that during structural rearrangements of the initially excited Franck-Condon state, besides twisting along C C bond which connects BTZ and DMA fragments, a considerable wagging motion is expected to occur. Account for solvent effect using polarized continuum model showed qualitative differences in the excited state PES features calculated by SA-2-CASSCF and TDDFT methods. Single-reference TDDFT calculations failed to describe solvation of TICT state and predicted increase of its energy in more polar media.     

Intramolecular Charge Transfer of Carotene‐porphyrin‐fullerene Triad: Sequential or Superexchange Cechanism

As an excellent artificial photosynthetic reaction center, the carotene (C)‐porphyrin (P)‐fullerene (F) triad was extensively investigated experimentally. To reveal the mechanism of the intramolecular charge transfer (ICT) on the mimic of photosynthetic solar energy conversion (such as singlet energy transfer between pigments, and photoinduced electron transfer from excited singlet states to give long‐lived charge‐separated states), the ICT mechanisms of C‐P‐F triad on the exciton were theoretically studied with quantum chemical methods as well as the 2D and 3D real space analysis approaches. The results of quantum chemical methods reveal that the excited states are the ICT states, since the densities of HOMO are localized in the carotene or porphyrin unit, and the densities of LUMO are localized in the fullerene unit. Furthermore, the excited states should be the intramolecular superexchange charge transfer (ISCT) states for the orbital transition from the HOMO whose densities are localized in the carotene to the LUMO whose densities are localized in the fullerene unit. The 3D charge difference densities can clearly show that some excited states are ISCT excited states, since the electron and hole are resident in the fullerene and carotene units, respectively. From the results of the electron‐hole coherence of the 2D transition density matrix, not only 3D results are supported, but also the delocalization size on the exciton can be observed. These phenomena were further interpreted with non‐linear optical effect. The large changes of the linear and non‐linear polarizabilities on the exciton result in the charge separate states, and if their changes are large enough, the ICT mechanism can become the ISCT on the exciton.     

Time-dependent density functional theory study on hydrogen-bonded intramolecular charge-transfer excited state of 4-dimethylamino-benzonitrile in methanol

The time-dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen-bonded intramolecular charge-transfer (ICT) excited state of 4-dimethylaminobenzonitrile (DMABN) in methanol (MeOH) solvent. We demonstrated that the intermolecular hydrogen bond C[triple bond]N...H-O formed between DMABN and MeOH can induce the C[triple bond]N stretching mode shift to the blue in both the ground state and the twisted intramolecular charge-transfer (TICT) state of DMABN. Therefore, the two components at 2091 and 2109 cm(-1) observed in the time-resolved infrared (TRIR) absorption spectra of DMABN in MeOH solvent were reassigned in this work. The hydrogen-bonded TICT state should correspond to the blue-side component at 2109 cm(-1), whereas not the red-side component at 2091 cm(-1) designated in the previous study. It was also demonstrated that the intermolecular hydrogen bond C[triple bond]N...H-O is significantly strengthened in the TICT state. The intermolecular hydrogen bond strengthening in the TICT state can facilitate the deactivation of the excited state via internal conversion (IC), and thus account for the fluorescence quenching of DMABN in protic solvents. Furthermore, the dynamic equilibrium of these electronically excited states is explained by the hydrogen bond strengthening in the TICT state.     

Review: fluorescent cyclodextrins for molecule sensing

Cyclodextrins (CDs), which are spectroscopically inert, were converted into fluorescent CDs by modification with one or two fluorophores. Many fluorescent CDs changed the fluorescent intensities upon addition of guest compounds, causing the locational change of the fluorophore mostly from inside to outside of the CD cavities. On this basis, the fluorescent CDs were used as fluorescent chemosensors for molecule recognition. Modified CDs bearing two naphthalene or pyrene moieties exhibit intramolecular excimer emission and their guest-responsive excimer intensity variations were used for molecule sensing. Fluorescent CDs bearing a dansyl moiety decreased the fluorescence intensity upon guest addition, reflecting the environmental change around the fluorophore from the hydrophobic interior of the CD cavities to bulk water solution. Modified CDs bearing a p-N,N-dimethylaminobenzoyl (DMAB) moiety exhibit dual emissions from nonpolar planar (NP) and twisted intramolecular charge transfer (TICT) excited states, and the TICT emission intensity was useful for sensing molecules. A biotin-bound DMAB system was also constructed, and the presence of the protein (avidin) was found to enhance the NP fluorescence. This avidin-bound DMAB system showed higher sensitivities and stronger binding ability for guest species than the system without avidin.     

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.