Solvent- and conformation-dependent electron transfer interactions in flexible biaromatic compounds: the case of 9-(dimethylanilino) phenanthrene

Absorption- and solvent-dependent fluorescence transition moments (Ma and Mf, respectively) and energies of two differently twisted 9-(dimethylanilino)phenanthrenes are determined and compared with phenanthrene properties and with quantum chemical calculations (AM1 and CNDO/S-CI) to derive the electronic and molecular structure of the first excited singlet state (S1). The increase of Mf on increasing solvent polarity can be attributed to a solvent-induced change of the S1 nature from the phenanthrene 1Lb to 1La type. The observed relation Mf>Ma points to enhanced coupling of the zeroth-order electron transfer state with the ground state due to a relaxation towards planarity in accordance with the calculation. Further increase of solvent polarity leads to a strong decrease of Mf below Ma for the more twisted compound explainable with enhanced electron transfer interactions at the expense of 1La character associated with a narrower perpendicular rotamer distribution in S1.