On the ground and excited state dipole moments of planar vs. twisted nitroaniline analogues

Abstract

Planar and conformationally twisted p-nitroaniline (PNA) and N,N-dimethyl-p-nitroaniline (DMPNA) systems have been subjected to electric field study (electrochromism) to understand their charge transfer characteristics in the ground and excited states. The observed values of ground and excited state dipole moments suggest that twisting of either the donor (D) with respect to the acceptor (A) or vice versa can have a large impact on the resultant charge distribution, particularly on the excited state charge distribution. These observations remarkably support the TICT hypothesis and minimum overlap rule, as proposed by Grabowski et al. Simple molecular orbital calculations provide adequate explanation for the observed changes in the dipole moment and reasonable agreement has been found in each case studied. Key words: electrochromism, dipole moments, charge transfer.