Intramolecular Energy Transfer to trans-Stilbene

This study examines the photophysics of a series of transition-metal complexes that contain a (diimine)Re I (CO) 3 (py) chromophore covalently linked to trans -stilbene via a semirigid amide spacer. In this series of complexes moderately exothermic triplet−triplet energy transfer from the 3 dπ* charge-transfer excited state of the diimine−Re chromophore to trans- stilbene is competitive with “normal” radiative and nonradiative decay of the 3 dπ* state. The rate constant for intramolecular energy transfer ( k EnT ) was determined as a function of temperature from 190 to 290 K for three complexes in which the driving force (Δ E EnT ) varies from ca. −29 to −38 kJ mol -1 . The temperature-dependence data indicate that energy transfer is characterized by a low activation energy ( E a ≤2 kJ mol -1 ) and a low-frequency factor ( A ≈10 6 s -1 ). The results are interpreted by a mechanism in which energy transfer from the 3 dπ* state to trans- stilbene occurs at nearly the optimal driving force (i.e., Δ E EnT ≈λ, where λ is the reorganization energy). However, owing to the moderate separation distance between the Re(I) center and trans -stilbene, the exchange coupling matrix element ( V TT ) is small, leading to low A values.