Intramolecular energy transfer to trans-stilbene

This study examines the photophysics of a series of transition-metal complexes that contain a (diimine)Re¹-(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 (k_EnT) was determined as a function of temperature from 190 to 290 K for three complexes in which the driving force (ΔE_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 ≈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., ΔE_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.