Properties of the lowest singlet states of dimethylaminobenzonitrile in liquid dielectrics: Quantum-mechanical calculations

摘要

The energies of the ground (SO) and first two excited (S1 and S2) singlet states, their electric dipole moments, and the oscillator strengths for different conformers of dimethylaminobenzonitrile having different orientations of the dimethyl group are calculated for different solvents assuming a universal character of the intermolecular interactions. The results obtained show that, as the polarity of the solvent increases, both the energy gap between the S1 and S2 levels, which determines their vibronic interaction, and the barrier for the nonradiative S-1 -> S-2 transition, which can be interpreted as the barrier for the reaction of charge transfer, strongly decrease. In strongly polar solvents, the inversion angle along the twist coordinate, which determines the mutual position of the first two singlet states, decreases from 55 degrees in free molecules to similar to 16 degrees in a solvent with a permittivity of 47, which radically facilitates charge transfer. The dipole moments of all the conformations in the singlet states are shown to depend considerably on the dielectric properties of the solvent, demonstrating a strong polarizability of the dimethylaminobenzonitrile molecule. The strongest increase in the dipole moments with increasing solvent polarity is observed for the planar geometry in the S, state and for the perpendicular orientation of the dimethyl group in the S2 state. An important result is that the spectral properties of different conformations, first of all, the energies of the 0-0 transitions, are different, which leads to the necessity of taking into account this factor for the correct description of the light absorption and photoreactions in systems with charge transfer. In the calculations, the semiempirical software packages HyperChern 5.0 and MOPAC are used.