Effect of Symmetry Breaking in Excited Quadrupole Molecules on Transition Dipole Moment

Manifestations of charge transfer symmetry breaking in excited quadrupolar molecules in optical spectra are theoretically studied. The molecules are supposed to have π-conjugated structures of A-π-D-π-A or D-π-A-π-D character, where electron acceptors (A) or electron donors (D) are identical. A theory describing the effect of symmetry breaking and solvent fluctuations on the dipole moments of optical transitions associated with absorption by a quadrupolar dye in the ground and excited states, as well as fluorescence, is developed. Simple equations describing the influence of the symmetry breaking extent on the transition dipole moments are found. The orientational solvent fluctuations are predicted to decrease the transition dipole moment of the ground state absorption. The decrease does not exceed 10%. A considerably larger effect of symmetry breaking and the solvent fluctuations on the emission dipole moment is found. Equations describing dependencies of the transition dipole moment associated with excited state absorption on the solvent polarity and the parameters of the dye are derived. The scale of the changes in the transition dipole moments due to symmetry breaking in the excited state are determined. The influence of the polar solvent fluctuations is also taken into account. The theoretical findings are shown to be consistent with the available experimental data.