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Dynamics of the electronically excited state of 9,9′-bianthryl (BA)–acetone complexes was studied in a free jet. The BA–acetone complexes were produced by supersonic expansion of BA/acetone/He mixtures. The laser induced fluorescence spectrum of the BA–acetone complexes showed several bands which were ascribed to the complexes containing different number of acetone molecules. The number of acetone molecules attaching to BA was determined by mass-selected resonance-enhanced multiphoton ionization (REMPI) spectra for each band. The Stokes shift and lifetime were measured for each band, i.e., for the BA–acetone complex with a specific number of ‘‘solvent’’ molecules. These measurements revealed that there are two kinds of BA–acetone complexes; one giving the broad laser-induced fluorescence (LIF) spectrum and largely redshifted fluorescence, and the other yielding the structured LIF and the sharp fluorescence spectrum similar to that of bare BA. The Stokes shift and the lifetime of the former complex increased with increasing number of solvated acetone molecules, whereas those of the latter complex are insensitive to the degree of solvation. These findings can be explained in terms of the importance of both the ‘‘symmetry breaking’’ and the ‘‘polar microscopic solvation’’ in forming the so-called twisted intramolecular charge–transfer state.

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