Structural symmetry breaking in octupolar tetrastyrylpyrazines and their dipole moments in equilibrium ground and Franck-Condon excited state

Abstract

From electrooptical absorption measurements (EOAM) follows that the dipole moment of octupolar tetrastyrylpyrazines in the equilibrium ground state is large and that the change of dipole moments upon transition to the excited Franck–Condon state is significant. Obtained results unambiguously testify to structural symmetry breaking in the studied octupolar tetrastyrylpyrazines. Molecular mechanics and semi-empirical calculations evidence that tetrastyrylpyrazines have non-planar configurations. The non-planar geometry of the molecules causes large μg values and significant change of dipole moment Δaμ after excitation.Due to large μg and Δaμ values the maxima of the first absorption band and fluorescence band of the studied tetrastyrylpyrazines in various solvents shift to the long-wavelength region with an increasing solvent polarity. Both absorption and fluorescence spectra of the dyes do not change the shape during bathochromic shift in the solvents. This phenomenon evidences about the defining role of universal van der Waals interactions of dyes with the environment, namely dispersive, orientational, and inductive nature.