EFFECTS OF POSITIONS OF DONOR AND ACCEPTOR TYPE SUBSTITUENTS ON GROUND-STATE AND EXCITED-STATE CHARGE-TRANSFER - ELECTROCHROMISM OF SOME BENZENE-DERIVATIVES

4-NO2, 3-NO2, and 3,5-di-NO2 substituents as acceptors (A) relative to 1-NH2, 1-OMe, and 1-C = CH as donors (D) on the benzene ring have been examined in order to understand the relation between position of D and A moieties relative to each other and the charge distribution in the ground and excited states. Ground-state dipole moments (mu-g) [obtained from dielectric measurements] in each case follow the order 4-NO2 > 3,5-di-NO2 > 3-NO2. On the other hand, the excited-state dipole moments (mu-e) and the change in dipole moment (DELTA-mu) [obtained from electrooptic measurements] show a different trend, i.e., 4-NO2 > 3-NO2 > 3,5-di-NO2. In the excited state the charge migration from the donor site to the acceptor site in the meta isomer has been found to be comparable to that in the para isomer, and this supports the unusual meta effect found in various photochemical reactions. The observed changes have been interpreted with simple molecular orbital calculations, and a regular trend has been found in each case studied.