Intramolecular energy transfer between chromophores separated by a rigid steroid bridge: I. Singlet and triplet energy transfer in 3β-(1-naphthyl)acetoxy-Δ5-androsten-17-one

Intramolecular transfer of both singlet and triplet electronic excitation between the naphthalene moiety and the carbonyl group in 3β-(1-naphthyl)acetoxy-Δ5-androsten-17-one was studied. Irradiation with 280 nm light selectively leads to excitation of the naphthalene moiety. Transfer of the singlet excitation from the naphthalene moiety to the carbonyl group occurs by dipole-dipole resonance interaction. The measured rates for such energy transfer in several solvents agree with that calculated from Förster equation. After efficient intersystem crossing, the carbonyl group transfers triplet excitation back to the naphthalene moiety via electron exchange mechanism. The combination of singlet and triplet energy transfer with the high efficiency of intersystem crossing of carbonyl group results in the phosphorescence enhancement of naphthalene moiety.