Electronic Structure and Molecular Conformation in the Excited Charge Transfer Singlet States of 9-Acridyl and Other Aryl Derivatives of Aromatic Amines

Abstract

Donor (D)−acceptor (A) compounds containing aromatic amine as an electron donor and acridine as an acceptor show a low-energy CT absorption band which undergoes a red shift with increasing solvent polarity. Solvatochromic effects on the spectral position and profile of the stationary fluorescence spectra clearly indicate the CT character of the emitting singlet states of all the compounds studied. A band-shape analysis of the CT absorption and emission spectra leads to the quantities relevant for the electron transfer in the Marcus inverted region. The comparative determination of the electronic transition dipole moments corresponding to the 1CT ← S0 absorption and the radiative charge recombination 1CT → S0 (Mabs and Mflu, respectively) made possible the estimation at the electronic coupling elements V0 and V1 between the 1CT state and the ground state S0 or the locally excited 1LE state lying most closely in energy, respectively. To describe the properties of the excited 1CT state of aryl derivatives of aromatic amines, the significant contributions of both of the above interactions together with the solvent induced changes of V0 and V1 have to be taken into account. In low polarity solvents the conformation of these compounds in the fluorescent 1CT(f) state is more planar than that in the ground state (and in the unrelaxed Franck−Condon 1CT(a) excited state), whereas in highly polar environment the compounds do not undergo any significant conformational changes accompanying the excited-state charge separation. The experimental and computational results led us to propose a simple model which allows one to predict the photophysical behavior of a particular D−A compound from the properties of its donor and acceptor moieties.