A Study of Intramolecular Energy Transfer in Conformationally Rigid molecules with stereoisomerically oriented donor and acceptor groups

Abstract The indanono naphthaleno compounds 1, 3, 5 and 7 exhibit, on both irradiation in the 1Lb band (315 nm) and the n → π* transition (> 340 nm) in EPA at 77° K, neither fluorescence from naphthalene nor phosphorescence from indanone, but exclusively phosphorescence from naphthalene, and quenching in liquid solution with 1,3‐pentadiene results in triplet energy transfer from the naphthaleno group only. The naphthalene phosphorescence exhibited by the ketones (1, 3, 5, 7) has an enhanced quantum efficiency with respect to that on direct excitation of the corresponding hydrocarbons (2, 4, 6, 8) and more strongly in the exo than in the endo orientation. In order to account for the energy wasting in the intramolecular triplet energy transfer in the endo compounds, a transfer route via a weak triplet donor‐acceptor exciplex, specific to through‐space interaction in the endo configuration (providing for additional radiationless T → S energy dissipation through vibrational coupling), competing with an efficient through‐σ‐bond exchange transfer mechanism operative in both configurations is proposed.