The charge transfer state of excited bianthryl and a derivative: solvatochromism, emission CT spectra broadening in homogeneous solvents Original Research Article

The photophysical properties of a recently synthesized molecule, the bis-9,9′-(2-ethyl-10-hexyl)anthryl (BOA), have been studied with the parent molecule, the 9,9′-bianthryl (BA), in solvents of various polarity. The comparative analysis of the photophysical behaviour of these two probes has been achieved in order to emphasise environmental effects of the alkyl substituent on the aromatic chromophore. The results obtained in perfluoroalkane, reported for the first time in this paper, suggest that the charge transfer state (CT) of BA or BOA contributes to the fluorescence emission even in non polar solvents like cyclohexane. The fluorescence spectra of these two probes in perfluoroalkane have been introduced as standard spectra of the locally excited state form (LE) and used to obtain a new decomposition of the other overall fluorescence spectra into two emission components: one coming from the CT state and the second from the initial excited singlet state (LE). We have analysed the solute-solvent interactions not only in term of Stokes shift but also using the inhomogeneous band broadening parameter. The vibrational band decomposition found its validation in the agreement observed between the thermodynamic and spectroscopic properties of both excited states. A correlation between the inhomogeneous broadening and the Stokes shift of the charge transfer spectrum is exhibited. The existence of up to 50% of the CT state even in non polar solvents like cyclohexane has been shown. The dipole moment of the CT state of BA and of BOA can be estimated in the range of 6.2 to 10.3 D depending on the solvatochromic model used. It has been shown that the alkyl substituent of BOA destabilises the CT state and only four methylene groups of each lateral chain of BOA contribute to the local environment of the aromatic chromophore.