Broken Symmetry in Excited States of 9,9′‐Bianthryl

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Abstract
The dual fluorescence in polar solvents of 9,9′‐bianthryl (BA) and of the electronically perturbed BA‐derivatives 10‐chloro‐BA (BACl), BA‐10‐carboxaldehyde (BAA) and N‐(9‐anthryl)‐carbazole (C9A) as well as of N‐(4‐cyanophenyl)‐carbazole (CBN) is analysed and interpreted in terms of a simplified microstructural solvent interaction model. Dipole moments μT relative to μT (CBN) of the fluorescent charge transfer state have been derived from the solvatochromic band shifts in nitrile and alcohol homologous solvent series and are similar for BA, BACl and C9A. This implies that the charge transfer in the excited state occurs to a maximum degree even in the symmetric molecule BA. – QCFF/PI‐potential energy curves have been calculated and are used to discuss these findings. BAA fluoresces in alcohols only, μT‐compared to the other compounds studied being somewhat smaller probably because of specific H‐bond interactions with the solvent. – Using relative fluorescence quantum yields and energetic considerations the dependence of the excited states equilibrium on both solvent polarity and solute perturbation is discussed.