Long-range corrected time-dependent density functional study on fluorescence of 4,4'-dimethylaminobenzonitrile

Dual fluorescence of 4,4(')-dimethylaminobenzonitrile (DMABN) was theoretically investigated on the basis of long-range corrected time-dependent density functional theory. Excited-state geometry optimization states and single-point energy calculations with and without solvent effect were carried out. It has been explained that DMABN emits dual fluorescence only in polar solvents through locally excited (LE) and charge transfer (CT) states. It was, however, concluded from this study that although the main spectrum of dual fluorescence in acetonitrile solvent is clearly due to twisted intramolecular CT fluorescence, small secondary fluorescence in acetonitrile may also emanate from CT fluorescence during the DMABN twisting process. This conclusion is supported by an experimental interpretation on polarization spectroscopy. It was also found that the optimized DMABN geometries have certain wagging angles for the CT state and no wagging angle for the LE state. This may support an early experimental hypothesis that the dual fluorescence of DMABN is induced by the wagging mode due to vibronic coupling between LE and CT states. Consequently, the authors propose a fluorescence mechanism of DMABN in gas phase and in acetonitrile solvent: the main absorption proceeds to the CT state in both situations. In gas phase, single fluorescence is chiefly emitted from the LE state through the internal conversion from CT to LE states. Dual fluorescence in acetonitrile solvent may only be emitted from the CT state.     

Dynamics of ultrafast intramolecular charge transfer with 4-(dimethylamino)benzonitrile in acetonitrile

The kinetics of the intramolecular charge-transfer (ICT) reaction of 4-(dimethylamino)benzonitrile (DMABN) in the polar solvent acetonitrile (MeCN) is investigated by fluorescence quantum yield and picosecond time-correlated single photon counting (SPC) experiments over the temperature range from -45 to +75 degrees C, together with femtosecond Sn <-- S1 transient absorption measurements at room temperature. For DMABN in MeCN, the fluorescence from the locally excited (LE) state is strongly quenched, with an unquenched to quenched fluorescence quantum yield ratio of 290 at 25 degrees C. Under these conditions, even very small amounts of the photoproduct 4-(methylamino)benzonitrile (MABN) severely interfere, as the LE fluorescence of MABN is in the same spectral range as that of DMABN. The influence of photoproduct formation could be overcome by a simultaneous analysis of the picosecond and photostationary measurements, resulting in data for the activation barriers Ea (5 kJ/mol) and Ed (32 kJ/mol) of the forward and backward ICT reaction as well as the ICT reaction enthalpy and entropy: DeltaH (-27 kJ/mol) and DeltaS [-38 J/(mol K)]. The reaction hence takes place over a barrier, with double-exponential fluorescence decays, as to be expected in a two-state reaction. From femtosecond transient absorption down to 200 fs, the LE and ICT excited state absorption (ESA) spectra of DMABN in n-hexane (LE) and in MeCN (LE and ICT) and also of 4-aminobenzonitrile in MeCN (LE) are obtained. For DMABN in MeCN, the quenching of the LE and the rise of the ICT ESA bands occurs with a single characteristic time of 4.1 ps, the same as the ICT reaction time found from the picosecond SPC experiments at 25 degrees C. The sharp ICT peak at 320 nm does not change its spectral position after a pump-probe delay time of 200 fs, which suggests that large amplitude motions do not take place after this time. The increase with time in signal intensity observed for the LE spectrum of DMABN in n-hexane between 730 and 770 nm, is attributed to solvent cooling of the excess excitation energy and not to an inverse ICT --> LE reaction, as reported in the literature.     

Pentacyano-N,N-dimethylaniline in the excited state. Only locally excited state emission, in spite of the large electron affinity of the pentacyanobenzene subgroup

Pentacyano-N,N-dimethylaniline (PCDMA) does not undergo an intramolecular charge transfer (ICT) reaction, even in the strongly polar solvent acetonitrile (MeCN), in clear contrast to 4-(dimethylamino)benzonitrile (DMABN). Within the twisted ICT (TICT) model, this is unexpected, as the electron affinity of the pentacyanobenzene moiety of PCDMA is much larger than that of the benzonitrile subgroup in DMABN. According to the TICT model, the energy of the ICT state of PCDMA would be 2.05 eV (～16550 cm(-1)) lower than that of DMABN, on the basis of the reduction potentials E(A(-)/A) of pentacyanobenzene (-0.29 V vs saturated calomel electrode (SCE)) and benzonitrile (-2.36 V vs SCE), more than enough to compensate for the decrease in energy of the locally excited (LE) state of PCDMA (E(S(1)) = 19990 cm(-1)) relative to that of DMABN (E(S(1)) = 29990 cm(-1)). This absence of a LE → ICT reaction shows that the TICT hypothesis does not hold for PCDMA in the singlet excited state, similar to what was found for DMABN, N-phenylpyrrole, and their derivatives. In this connection, the six dicyano-substituted dimethylanilines are also discussed. The energy gap ΔE(S(1),S(2)) between the two lowest singlet excited states is, at 7170 cm(-1) for PCDMA in MeCN, considerably larger than that for DMABN (2700 cm(-1) in n-hexane, smaller in MeCN). The absence of ICT is therefore in accord with the planar ICT (PICT) model, which considers a sufficiently small ΔE(S(1),S(2)) to be an important condition determining whether an ICT reaction will take place. The fluorescence quantum yield of PCDMA is very small: Φ(LE) = 0.0006 in MeCN at 25 °C, predominantly due to LE → S(0) internal conversion (IC), as the intersystem crossing yield Φ(ISC) is practically zero (<0.01). From the LE fluorescence decay time of 27 ps for PCDMA in MeCN at 25 °C, a radiative rate constant k(f)(LE) = 2 × 10(7) s(-1) results, comparable to the k(f)(LE) of DMABN (6.5 × 10(7) s(-1)) and 2,4,6-tricyano-N,N-dimethylaniline (TCDMA) (1.2 × 10(7) s(-1)) in this solvent, but clearly larger than the k'(f)(ICT) = 0.79 × 10(7) s(-1) of DMABN in MeCN. The IC reaction with PCDMA in MeCN at room temperature, with a rate constant k(IC) of 3.6 × 10(10) s(-1), is much faster than with TCDMA (25 × 10(7) s(-1)) and DMABN (1.3 × 10(7) s(-1), in n-hexane). This is connected with the nonzero (37°) amino twist angle of PCDMA, which leads to a decrease of the effective LE-S(0) energy gap. The femtosecond excited state absorption (ESA) spectra of PCDMA in MeCN at 22 °C are similar to the LE ESA spectra of TCDMA and DMABN and are therefore attributed to the LE state, confirming that an ICT reaction does not occur. The decay of the LE ESA spectra of PCDMA is single exponential, with a decay time of 22 ps, in reasonable agreement with the LE fluorescence decay time of 27 ps at 25 °C. The spectra decay to zero, showing that there is no triplet or other intermediate.     

Intramolecular charge transfer in 4-aminobenzonitriles does not necessarily need the twist

In electron donor/acceptor species such as 4-(dimethylamino)benzonitrile (DMABN), the excitation to the S(2) state is followed by internal conversion to the locally excited (LE) state. Dual fluorescence then becomes possible from both the LE and the twisted intramolecular charge-transfer (TICT) states. A detailed mechanism for the ICT of DMABN and 4-aminobenzonitrile (ABN) is presented in this work. The two emitting S(1) species are adiabatically linked along the amino torsion reaction coordinate. However, the S(2)/S(1) CT-LE radiationless decay occurs via an extended conical intersection "seam" that runs almost parallel to this torsional coordinate. At the lowest energy point on this conical intersection seam, the amino group is untwisted; however, the seam is accessible for a large range of torsional angles. Thus, the S(1) LE-TICT equilibration and dual fluorescence will be controlled by (a) the S(1) torsional reaction path and (b) the position along the amino group twist coordinate where the S(2)/S(1) CT-LE radiationless decay occurs. For DMABN, population of LE and TICT can occur because the two species have similar stabilities. However, in ABN, the equilibrium lies in favor of LE, as a TICT state was found at much higher energy with a low reaction barrier toward LE. This explains why dual fluorescence cannot be observed in ABN. The S(1)-->S(0) deactivation channel accessible from the LE state was also studied.     

What determines the rate of excited-state intramolecular electron-transfer reaction of 4-(N,N'-dimethylamino)benzonitrile in room temperature ionic liquids? A study in [bmim][PF6

The kinetics of excited-state intramolecular electron-transfer reaction and dynamics of solvation of the intramolecular charge transfer (ICT) state of 4-(N,N'-dimethylamino)benzonitrile (DMABN) was studied in 1-butyl-3-methylimidazloium hexafluorophosphate, [bmim][PF(6)], by monitoring the dual fluorescence of the system. The picosecond time-resolved emission spectra (TRES) of DMABN exhibit decay of the locally excited (LE) emission intensity and shift of the ICT emission peak position with time, thus capturing the kinetics of evolution of the ICT state from the LE state and solvent relaxation of the ICT state. These results show that the LE→ICT transformation rate is determined not by the slow dynamics of solvation in ionic liquid, but is controlled mainly by the rate of structural reorganization of the molecule, which accompanies the electron-transfer process in this polar viscous medium. Even though both solvent reorganization around photo-excited DMABN and structural rearrangement of the molecule are dependent on the viscosity of the medium, it is the latter process that contributes to the viscosity dependence of the LE→ICT transformation.     

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.     

Dynamics of ultrafast intramolecular charge transfer with 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in n-hexane and acetonitrile

The intramolecular charge transfer (ICT) reaction of 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in n-hexane and acetonitrile (MeCN) is investigated by picosecond fluorescence experiments as a function of temperature and by femtosecond transient absorption measurements at room temperature. NTC6 in n-hexane is dual fluorescent from a locally excited (LE) and an ICT state, with a quantum yield ratio Phi'(ICT)/Phi(LE) of 0.35 at +25 degrees C and 0.67 at -95 degrees C, whereas in MeCN mainly an ICT emission is observed. From the temperature dependence of Phi'(ICT)/Phi(LE) for NTC6 in n-hexane, an LE/ICT enthalpy difference DeltaH of -2.4 kJ/mol is determined. For comparison, 1-isopropyl-6-cyano-1,2,3,4-tetrahydroquinoline (NIC6) is also investigated. This molecule does not undergo an ICT reaction, because of its larger energy gap DeltaE(S1,S2). From the molar absorption coefficient epsilonmax of NTC6 as compared with other aminobenzonitriles, a ground-state amino twist angle theta of approximately 22 degrees is deduced. The increase of epsilonmax between n-hexane and MeCN indicates that theta decreases when the solvent polarity becomes larger. Whereas single-exponential LE fluorescence decays are obtained for NIC6 in n-hexane and MeCN, the LE and ICT decays of NTC6 in these solvents are double exponential. For NTC6 in n-hexane at -95 degrees C, with a shortest decay time of 20 ps, the forward (ka=2.5x10(10) s(-1)) and backward (kd=2.7x10(10) s(-1)) rate constants for the LE<-->ICT reaction are determined from the time-resolved LE and ICT fluorescence spectra. For NTC6 in n-hexane and MeCN, the excited-state absorption (ESA) spectrum at 200 fs after excitation is similar to the LE(ESA) spectra of NIC6 and 4-(dimethylamino)benzonitrile (DMABN), showing that LE is the initially excited state for NTC6. These results indicate that the LE states of NTC6, NIC6, and DMABN have a comparable molecular structure. The ICT(ESA) spectrum of NTC6 in n-hexane and MeCN resembles that of DMABN in MeCN, likewise indicating a similar ICT structure for NTC6 and DMABN. From the decay of the LE absorption and the corresponding growing-in for the ICT state of NTC6, it is concluded that the ICT state originates from the LE precursor and is not formed by direct excitation from S0, nor via an S2/ICT conical intersection. The same conclusion was made from the time-resolved (picosecond) fluorescence spectra, where there is no ICT emission at time zero. The decay of the LE(ESA) band of NTC6 in n-hexane occurs with a shortest time tau2 of 2.2 ps. The ICT reaction is much faster (tau2 = 0.82 ps) in the strongly polar MeCN. The absence of excitation wavelength dependence (290 and 266 nm) for the ESA spectra in MeCN also shows that LE is the ICT precursor. With NIC6 in n-hexane and MeCN, a decay or growing-in of the femtosecond ESA spectra is not observed, in line with the absence of an ICT reaction involving an S2/ICT conical intersection.     

The tetrafluoro analogue of DMABN: anomalous fluorescence and mechanistic considerations

Absorption and emission properties of DMABN-F4, the tetrafluoro analogue of DMABN, have been investigated and compared with the parent compound. Unlike in DMABN, this new compound exhibits only a highly solvatochromic and strongly red-shifted charge transfer (CT) fluorescence and is characterized by the absence of an LE band even in nonpolar solvents. This evidences the faster formation of CT in the excited state as compared to DMABN. The low quantum yield values of DMABN-F4 suggest that the high rate of nonradiative decay takes place via internal conversion (IC) rather than intersystem crossing (ISC) as no phosphorescence is observed in rigid glass solvents at 77 K in contrast to DMABN. The emission transition moment and radiative rate constant values of DMABN-F4 in medium and highly polar solvents point to a forbidden emission in the excited state similar to that of DMABN. Electronic structure and twist potentials were also studied by quantum chemical calculations using ab initio and semiempirical methods. In contrast to DMABN, the dimethylamino group in DMABN-F4 is found to be twisted by around 30-50 degrees, but the photophysics are concluded to be analogous to DMABN with the addition of a very fast IC channel.     

Reaction coordinate analysis of the S1-S0 internal conversion of azulene

The reaction coordinate of the S(1)-S(0) internal conversion of azulene has been analyzed using ab initio complete active space self-consistent field method. The stable geometry in S(0) (S(0) geometry) takes a bond-equalized structure where all the peripheral skeletal bond distances are similar to an aromatic CC bond distance. The transannular bond is similar to a normal C-C single bond. The first event upon electronic excitation into S(1) at S(0) geometry is characterized by the following two simultaneous changes in the skeletal bonds; the transannular bond in S(1) increases its double bond character and the aromaticity of the peripheral bonds disappears. In consequence, the most stable azulene in S(1) (S(1) geometry) has a biradical character. To reach the conical intersection between S(1) and S(0) (S(1)S(0)-CIX) where radiationless relaxation takes place, the seven-membered ring greatly deviates from a planar structure. After a transition into S(0) at S(1)S(0)-CIX, the bond-equalized structure is recovered immediately and then the nonplanarity decreases so that azulene again takes the stable planar S(0) geometry. In order to deepen the understanding of the S(1)-S(0) internal conversion, the dipole moments along the reaction coordinate have been analyzed.     

Skeletal relaxation effect on the charge transfer state formation of 4-dimethylamino,4'-cyanostilbene

Ab initio complete active space self-consistent field (CASSCF) calculations combined with polarized continuum model (PCM) have been performed to examine the charge transfer (CT) state formation of trans-4-dimethylamino,4'-cyanostilbene (DCS) in a solvent. In a polar solvent, the globally stable geometry in S1 takes a twisted conformation where the electron-donating dimethylanilino group is highly twisted against the other part of the electron-withdrawing 4-cyanostyryl group. In addition, skeletal relaxation where the aromatic benzene rings turn to be a nonaromatic quinoid structure is essential to stabilize the CT state. In a nonpolar solvent, the stable geometry in S1 takes a nontwisted conformation, though the skeletal relaxation is also an essential factor. By means of the free energy decomposition analysis, it is found that the stable CT geometry which depends on solvent polarity mainly comes from two factors: the linkage bond between the dimethylanilino and the 4-cyanostyryl group and the electrostatic interaction. In a polar solvent, the linkage bond has a single bond character to slightly prevent the torsional motion. This twist geometrically assists the charge separation so as to reinforce the electrostatic interaction. In consequence, the twisted internal CT (TICT) conformation is stable. In a nonpolar solvent, on the other hand, a nontwisted CT state is stable because the linkage bonds greatly increase a double bond character so as to prevent the torsional motion, while the electrostatic interaction is not so enhanced even by the geometrical twist.     

Intramolecular charge transfer with the planarized 4-aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6)

Fast and efficient intramolecular charge transfer (ICT) and dual fluorescence is observed with the planarized aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in a series of solvents from n-hexane to acetonitrile and methanol. Such a reaction does not take place for the related molecules with 1-isopropyl (NIC6) and 1-methyl (NMC6) groups, nor with the 1-alkyl-5-cyanoindolines with methyl (NMC5), isopropyl (NIC5), or tert-butyl (NTC5) substituents. For these molecules, a single fluorescence band from a locally excited (LE) state is found. The charge transfer reaction of NTC6 is favored by its relatively small energy gap DeltaE(S(1),S(2)), in accordance with the PICT model for ICT in aminobenzonitriles. For the ICT state of NTC6, a dipole moment of around 19 D is obtained from solvatochromic measurements, similar to micro(e)(ICT) = 17 D of 4-(dimethylamino)benzonitrile (DMABN). For NMC5, NIC5, NTC5, NMC6, and NIC6, a dipole moment of around 10 D is determined by solvatochromic analysis, the same as that of the LE state of DMABN. For NTC6 in diethyl ether at -70 degrees C, the forward ICT rate constant (1.3 x 10(11) s(-1)) is much smaller than that of the back reaction (5.9 x 10(9) s(-1)), showing that the equilibrium is on the ICT side. The results presented here make clear that ICT can very well take place with a planarized molecule such as NTC6, when DeltaE(S(1),S(2)) is sufficiently small, indicating that a perpendicular twist of the amino group relative to the rest of the molecule is not necessary for reaching an ICT state with a large dipole moment. The six-membered alicyclic ring in NMC6, for example, prevents ICT by increasing DeltaE(S(1),S(2)) relative to that of DMABN.     

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.     

Ultrafast intramolecular charge transfer and internal conversion with tetrafluoro-aminobenzonitriles.

The five 2,3,5,6-tetrafluoro-4-aminobenzonitriles XABN4F with a dimethyl-amino (DMABN4F), diethyl-amino (DEABN4F), azetidinyl (AZABN4F), methyl-amino (MABN4F) or amino (ABN4F) group undergo ultrafast intramolecular charge transfer (ICT) at room temperature, in the polar solvent acetonitrile (MeCN) as well as in the nonpolar n-hexane. ICT also takes place with the corresponding non-fluorinated aminobenzonitriles DMABN, DEABN and AZABN in MeCN, whereas for these molecules in n-hexane only minor (DMABN, DEABN) or no (AZABN) ICT fluorescence is detected. For the secondary (MABN) and primary (ABN) amines, an ICT reaction does not occur, which makes ABN4F the first electron donor/acceptor molecule with an NH(2) group for which ICT is observed. The ICT state of the XABN4Fs has a dipole moment of around 14 D, clearly smaller than that of DMABN (17 D). This difference is attributed to the electron withdrawing from the CN group to the phenyl ring, exerted by the four F-substituents. The reaction from the initially prepared locally excited (LE) to the ICT state in n-hexane proceeds in the sub-picosecond time range: 0.35 ps (DMABN4F), 0.29 ps (DEABN4F) and 0.13 ps (AZABN4F), as determined from femtosecond transient absorption measurements. In the highly polar solvent MeCN, an ICT reaction time of around 90 fs is observed for all five XABN4Fs, irrespective of the nature of their amino group. This shows that with these molecules in MeCN the ICT reaction rate is limited by the solvent dielectric relaxation time of MeCN, for which a value of around 90 fs has been reported. It is therefore concluded that, during this ultrashort ICT reaction, a large-amplitude motion such as a full 90 degrees twist of the amino group is unlikely to occur in the XABN4Fs. The ICT state of the XABN4Fs is strongly quenched via internal conversion (IC), with a lifetime tau'(0) (ICT) down to 3 ps, possibly by a reaction passing through a conical intersection made accessible due to a deformation of the phenyl group by out-of-plane motions induced by vibronic coupling between low-lying pisigma* and pipi* states in the XABN4Fs.     

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。     

Fluorescence of crystalline 4-(dimethylamino)benzonitrile. Absence of dual fluorescence and observation of single-exponential fluorescence decays

The fluorescence spectrum of crystals grown from newly synthesized 4-(dimethylamino)benzonitrile (DMABN), measured from 25 down to −112 °C, consists of a single emission band originating from a locally excited (LE) state. The fluorescence decay of the DMABN crystals is single exponential at all temperatures investigated. These results show that intramolecular charge transfer (ICT) does not occur in crystalline DMABN. The additional red-shifted emission bands and multiexponential fluorescence decays previously reported for DMABN crystals are attributed to a minor amount of the impurity 4-(dimethylamino)benzaldehyde, the synthetic precursor of commercial DMABN.     

Triplet state dipole moments of aminobenzonitriles

The triplet state dipole moments mu(T) of a series of 4-amino- and 3-aminobenzonitriles in cyclohexane, benzene, and 1,4-dioxane are recalculated from previously published [J. Phys. Chem. 1992, 96, 10809] time-resolved microwave conductivity data, on the basis of newly measured intersystem crossing yields. For 4-(dimethylamino)benzonitrile (DMABN), the following values are now determined for mu(T): 8.3 D (cyclohexane), 8.9 D (benzene), and 9.7 D (1,4-dioxane), as compared with the previously reported dipole moment of 12 D for the first and the last solvent. With the other aminobenzonitriles, similar mu(T) data are obtained, between 6.9 D for 4-aminobenzonitrile (ABN) in n-hexane and 10.0 D for 4-(di-n-decylamino)benzonitrile (DDABN) in 1,4-dioxane. The increase of mu(T) observed for all aminobenzonitriles when going from cyclohexane via benzene to 1,4-dioxane may indicate that their triplet dipole moments become larger with increasing solvent polarity. The present mu(T) of DMABN, between 8.3 and 9.7 D, although larger than the ground state dipole moment mu(0) of 6.6 D, is somewhat smaller than that of the locally excited (LE) state (9.9 D) but considerably smaller than the dipole moment of the intramolecular charge transfer (ICT) state (17 D). By comparing these mu(X) data with the frequency (CN) of the cyano vibration in each state, it appears that at least for DMABN in the triplet state (CN) is not a reliable indication of the extent of charge transfer as compared with the other states S0, LE, and ICT.     

Theoretical prediction of the S1-S0 internal conversion of 6-cyanoazulene

Ab initio complete active-space self-consistent field (CASSCF) and second-order Multireference M?ller-Plesset perturbation (MRMP2) calculations were performed to examine the S1-S0 internal conversion of 6-cyanoazulene (6CNAZ). The azulene skeletons of 6CNAZ in S0 and S1 have features that resemble those of azulene. The stable geometry in S0 is characterized by (i) a C2v structure, (ii) an aromatic bond-equalized structure in which all the peripheral skeletal bond distances resemble an aromatic CC bond distance, and (iii) a single bond character of the transannular bond. The stable geometry in S1 is characterized by a nonaromatic C2v structure. Contrary to similarities of the stable geometries in S0 and S1 between 6CNAZ and azulene, the conical intersection (S1/S0-CIX) of 6CNAZ is different from that of azulene. The S1/S0-CIX of 6CNAZ takes a planar structure, whereas that of azulene takes a nonplanar structure in the seven-membered ring (Amatatsu, Y.; Komura, K. J. Chem. Phys. 2006, 125, 174311/1-8). On the basis of those computational findings, we predict the photochemical behavior of 6CNAZ in the S1-S0 internal conversion.     

Computational study of thioflavin T torsional relaxation in the excited state

Quantum-chemical calculations of the Thioflavin T (ThT) molecule in the ground S0 and first excited singlet S1 states were carried out. It has been established that ThT in the ground state has a noticeable nonplanar conformation: the torsion angle phi between the benzthiazole and the dimethylaminobenzene rings has been found to be approximately 37 degrees. The energy barriers of the intramolecular rotation appearing at phi = 0 and 90 degrees are quite low: semiempirical AM1 and PM3 methods predict values approximately 700 cm-1 and ab initio methods approximately 1000-2000 cm(-1). The INDO/S calculations of vertical transitions to the S1(abs) excited state have revealed that energy ES1(abs) is minimal for the twisted conformation with phi = 90 degrees and that the intramolecular charge-transfer takes place upon the ThT fragments' rotation from phi = 0 to 90 degrees. Ab initio CIS/RHF calculations were performed to find optimal geometries in the excited S1 state for a series of conformers having fixed phi values. The CIS calculations have predicted a minimum of the S1 state energy at phi approximately 21 degrees; however, the energy values are 1.5 times overestimated in comparison to experimental data. Excited state energy dependence on the torsion angle phi, obtained by the INDO/S method, reveals that ES1(fluor) is minimal at phi = approximately 80-100 degrees, and a plateau is clearly observed for torsion angles ranging from 20 to 50 degrees. On the basis of the calculation results, the following scheme of photophysical processes in the excited S1 state of the ThT is suggested. According to the model, a twisted internal charge-transfer (TICT) process takes place for the ThT molecule in the excited singlet state, resulting in a transition from the fluorescent locally excited (LE) state to the nonfluorescent TICT state, accompanied by torsion angle phi growth from 37 to 90 degrees. The TICT process effectively competes with radiative transition from the LE state and is responsible for significant quenching of the ThT fluorescence in low-viscosity solvents. For viscous solvents or when the ThT molecule is located in a rather rigid microenvironment, for example, when it is bound to amyloid fibrils, internal rotation in the dye molecule is blocked due to steric hindrance, which results in suppression of the LE --> TICT quenching process and in a high quantum yield of fluorescence.     

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.     

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.