ELECTRONIC-STRUCTURE AND MOLECULAR-CONFORMATION IN THE EXCITED CHARGE-TRANSFER SINGLET-STATES OF 9-ACRIDYL AND OTHER ARYL DERIVATIVES OF AROMATIC-AMINES

Donor (D)-acceptor (A) compounds containing aromatic amine as an elect ron donor and acridine as an acceptor show a low-energy CT absorption band which undergoes a red shift with increasing solvent polarity. Sol vatochromic effects on the spectral position and profile of the statio nary fluorescence spectra clearly indicate the CT character of the emi tting singlet states of all the compounds studied. A band-shape analys is of the CT absorption and emission spectra leads to the quantities r elevant for the electron transfer in the Marcus inverted region. The c omparative determination of the electronic transition dipole moments c orresponding to the (CT)-C-1 <-- S-0 absorption and the radiative char ge recombination (CT)-C-1 --> S-0 (M-abs and M-flu, respectively) made possible the estimation at the electronic coupling elements V-0 and V -1 between the (CT)-C-1 state and the ground state S-0 or the locally excited (LE)-L-1 state lying most closely in energy, respectively. To describe the properties of the excited (CT)-C-1 slate of aryl derivati ves of aromatic amines, the significant contributions of both of the a bove interactions together with the solvent induced changes of V-0 and V-1 have to be taken into account. Ln low polarity solvents the confo rmation of these compounds in the fluorescent (CT)-C-1(f) state is mor e planar than that in the ground state (and in the unrelaxed Franck-Co ndon (CT)-C-1(a) excited state), whereas in highly polar environment t he compounds do not undergo any significant conformational changes acc ompanying the excited-state charge separation. The experimental and co mputational results led us to propose a simple model which allows one to predict the photophysical behavior of a particular D-A compound fro m the properties of its donor and acceptor moieties.