The ultraviolet transitions of 4-(dimethylamino)pyridine and its derivatives: 3-methyl-4-(dimethylamino)pyridine and 3,5-dimethyl-4-(dimethylamino)pyridine were investigated by electronic absorption and magnetic circular dichroism (MCD) spectroscopy, and by quantum chemical calculations. The ortho-methylation in the pyridine ring creates a steric hindrance to coplanarity. The resulting changes in the strength of the dimethylamino substitutent are reflected in the MCD spectra. Near-UV experimental data indicate a presence of three low-energy transitions, assigned to 1Lb, 1La and 1(n,pi*) excited states. These results are corroborated by TD-DFT and INDO/S calculations.

Contrary to 4-(dimethylamino)pyridine (DMAP), which revealed solvent-dependent dual fluorescence, its sterically hindered ortho-methylated derivatives: 3-methyl-4-(dimethylamino)pyridine and 3,5-dimethyl-4-(dimethylamino)pyridine show single charge transfer fluorescence band. The electronic structures of the lowest excited states and molecular geometries were investigated by stationary and kinetic absorption and fluorescence spectroscopy as well as time-dependent density functional response theory (TD-DFT) and semiempirical (INDO/S) calculations. The results can be interpreted in terms of the TICT state model.y Dedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.

4-(Dimethylamino)pyridine (DMAP) shows solvent-dependent dual fluorescence from the initially excited state B* and a highly polar TICT state A*. Room-temperature time-resolved picosecond fluorescence investigations prove the bimodal kinetics of the excited-state electron transfer reaction B*-->A* in polar aprotic media. In medium polarity solvents (such as ethyl acetate) two emitting states of DMAP are shown to reach equilibrium within 50 ps. Both emitting states originate from the same ground state. The rate of excited-state charge separation depends on polarity and proton donating ability of the surrounding medium. The effects of temperature on the quantum yields of both fluorescences of DMAP in polar aprotic media indicate the transition from the kinetic regime (at low temperatures) to the equilibrium regime (at high temperatures). The kinetic behaviour of the dual luminescence of DMAP in protic solvents is more complex than in aprotic ones. In alcohols an efficient nonradiative channel competes with excited-state charge separation.