Time-resolved spectroscopy of intramolecular energy transfer in a rigid spiran

Abstract

The kinetics of intramolecular energy transfer has been studied in the rigid spiran, spiro[9,10-dihydro-9-oxoanthracene-10,2′-5′,6′-benzindan], using picosecond time-resolved spectroscopy. The rate constant for triplet–triplet energy transfer from the donor (anthrone) to the acceptor (naphthalene) chromophore is found to be ∼3×1010 s−1. Low temperature microwave-induced delayed phosphorescence measurements on the spiran show that triplet–triplet energy transfer from the anthrone populates the sublevels of the naphthalene acceptor state randomly, rather than the Tz (z=long axis) sublevel selectively. The latter prediction arises from the known sublevel selectivity for intersystem crossing in anthrone and the geometrical arrangement of the chromophores in the rigid spiran. The randomness of the triplet–triplet energy transfer process is consistent with the rate constant for energy transfer which limits the donor lifetime to a value below the spin precession period in the donor triplet state. The results are discussed in terms of the Dexter theory for triplet–triplet energy transfer.