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.

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.

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.

The excited state dipole moments mue(ICT) and mue(LE) of the dual fluorescent molecules N-phenylpyrrole (PP), N-(4-cyanophenyl)pyrrole (PP4C) and N-(3-cyanophenyl)pyrrole (PP3C) are determined from solvatochromic and thermochromic measurements. It is shown that the best results are obtained when the solvatochromic as well as the thermochromic analysis of the spectral shifts is made relative to 4-(dimethylamino)benzonitrile (DMABN) as the model compound. Direct thermochromic experiments with PP4C, PP3C and DMABN in diethyl ether lead to reasonable results, but unrealistically large dipole moments mue(ICT) are found for PP, PP4C, PP3C and DMABN in acetonitrile, ethyl cyanide and n-propyl cyanide. The mue(ICT) values obtained for the N-phenylpyrroles from the thermochromic analysis in these solvents relative to DMABN (17 D) do not depend on solvent polarity: 13 D for PP, 15 D for PP4C and PP3C. The spectral shifts for the LE emission of the N-phenylpyrroles and aminobenzonitriles are much smaller than those for the ICT fluorescence, resulting in relatively small values for mue(LE). With PP and N-(4-methylphenyl)pyrrole (PP4M) the problem arises that one of the two values calculated by solving the quadratic equation for mue(LE) in the solvatochromic and thermochromic analysis cannot be discarded on photophysical or molecular grounds, as is the case for the other molecules. The experimental data for mue(ICT) of PP and PP4C are compared with theoretical values calculated for coplanar (PICT) and perpendicular (TICT) conformations of the pyrrole and phenyl or cyanophenyl groups. The experimental ICT dipole moment of PP4C has a value in between the theoretical results for mue(PICT) and mue(TICT), whereas the data for PP tend to favour the TICT configuration. It appears that in the LE state of PP and PP4M a negative charge remains on the pyrrole moiety, whereas a charge reversal takes place for the LE state of PP3C and the ICT state of PP, PP4C and PP3C.