Dual fluorescence and intramolecular charge transfer (ICT) are observed with aminobenzonitriles when two excited state levels (Sl and S2(CT) in DMABN) have an energy gap sufficiently small for vibronic coupling: a solvent-induced pseudo-Jan-Teller effect. It is argued that the N-inversion of the amino group acts as a promoting mode. These conclusions are based on a comparison of absorption spectra and photostationary and time-resolved fluorescence data. Dual fluorescence is also observed with MMD, in which the dimethylamino group is twisted towards a perpendicular configuration with respect to the phenyl ring.
Monomer and excimer fluorescence decays of 1,3-di( 1 -pyrenyl)propane (!Py(3)lPy) in «-heptane are measured at different temperatures by using time-correlated single-photon counting (SPC). The decays at 61 °C are triple-exponential with decay times and amplitudes independent of the number of total counts, excitation wavelength, and width of the excitation pulse. At -43 °C and lower temperatures, single-exponential monomer decays are obtained, next to triple-exponential excimer records. Aggregates of lPy(3)lPy are not detected. These results can be described by the three-state model, DMD, comprising one group of rapidly interconverting excited-state monomers (M*) and two excimers (D*). No evidence for distributed decay times is found down to a time resolution of 0.1 ns/channel and 2000 channels. In support of the DMD model, molecular mechanics calculations backed by *H NMR measurements are presented, showing that only two monomer conformen of lPy(3)lPy are populated in the ground state. The interconversion between these two monomers in the excited state is too fast to be resolved by the present SPC measurements.
The fragmentation reactions of the cation radicals of a series of remote diamine-substituted pinacols have been investigated. The cation radicals were generated upon photooxidation with excited 2,6,9,10-tetracyanoanthracene in acetonitrile. The products are consistent with cleavage of the central C-C bond. The rate constants for fragmentation were determined both from steady-state quantum yield studies and from time-resolved measurements. In general, the rate constants for fragmentation increase with increasing stability of the radical and cation products. However, the results of temperature dependence studies clearly demonstrate that conformational effects play an important role in the transition state. In some cases, these conformational effects can result in changes in the reactivity order expected from purely thermodynamic considerations.
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