Intramolecular Energy Transfer to <i>trans</i>-Stilbene

Abstract

This study examines the photophysics of a series of transition-metal complexes that contain a (diimine)ReI(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 3dπ* charge-transfer excited state of the diimine−Re chromophore to trans-stilbene is competitive with “normal” radiative and nonradiative decay of the 3dπ* state. The rate constant for intramolecular energy transfer (kEnT) was determined as a function of temperature from 190 to 290 K for three complexes in which the driving force (ΔEEnT) varies from ca. −29 to −38 kJ mol-1. The temperature-dependence data indicate that energy transfer is characterized by a low activation energy (Ea ≤2 kJ mol-1) and a low-frequency factor (A ≈106 s-1). The results are interpreted by a mechanism in which energy transfer from the 3dπ* state to trans-stilbene occurs at nearly the optimal driving force (i.e., ΔEEnT ≈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 (VTT) is small, leading to low A values.