Supersonic jet spectroscopy and intramolecular electronic energy transfer in naphthalene-(CH2)n-anthracene bichromophoric molecules

The spectroscopy and intramolecular electronic energy transfer (EET) process for a series of molecules, 9-anthryl-(CH2)(n)-1-naphthyl, n = 1, 3, and 6 (AnN), was investigated in a supersonic jet. For all bichromophoric molecules, the excitation spectra of the anthracene chromophore was slightly red-shifted with respect to bare anthracene and showed low-frequency vibrational progressions associated with the relative motion of the two chromophores. For AlN, when the naphthalene moiety is excited, the emission contains only anthracene-like fluorescence. From the homogeneous broadening of the absorption lines, the EET rate is estimated to be 1.5 ps. For n = 3 (A3N) and n = 6 (A6N), only the fluorescence excitation spectra of the anthracene moiety is observed. This suggests that the EET rate in A3N and A6N is much faster than that in the A1N system. Molecular geometry computations using molecular mechanics methods were applied to the interpretations of experimental results. In the case of A3N and A6N, a face-to-face conformer can be formed in which exchange coupling can be effective in enhancing the EET rate. In A1N, the bridge is sufficiently rigid that a face-to-face conformer cannot be formed.