Intramolecular triplet energy transfer in benzophenone-norbornadiene dyads linked by flexible spacers

The photochemistry of bichromphoric compounds with benzophenone (BP) and norbornadiene (NBD) chromophores linked by various lengths of flexible spacers, BP-(CH2)(n)-NBD, was examined both in fluid solution and in organic glass. Time-resolved spectroscopic measurements in room temperature fluid solution indicate that the lifetime of the triplet state of the BP chromophore is shortened by the NBD group. Selective excitation of the BP chromophore results in the isomerization of the NBD group to quadricyclane. These observations suggest that intramolecular triplet energy transfer occurs in BP-(CH-(2))(n)-NBD. The rate constant of such transfer for the bichromophoric coumpound with 6 connecting atoms is ca. 6 x 10(6) s(-1), and that for the compound with a 14-atom spacer decreases only by a factor 1.5. The large rate constants for long molecules and its less dependence on spacer length are interpreted in terms of the fact that rapid conformational equilibria allow a sufficient fraction of the molecules self-coiling up within the lifetime of the BP triplet state so that the two end chromophores are close enough for orbital overlap and through space energy transfer. In organic glass at 77 K phosphorescence lifetime measurements show that the intramolecular triplet energy transfer rate constants drop to ca. 10(2) s(-1) for the compounds with short spacers and become negligible for the long molecules. These results are attributed to the ability of the low-temperature matrix to prevent the chromophores from achieving conformations conducive to good orbital overlap.