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 3d pi* charge-transfer excited state of the diimine-Re chromophore to trans-stilbene is competitive with "normal" radiative and nonradiative decay of the 3d pi* 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 (Delta 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 less than or equal to 2 kJ mol(-1)) and a low-frequency factor (A approximate to 10(6) s(-1)). The results are interpreted by a mechanism in which energy transfer from the 3d pi* state to trans-stilbene occurs at nearly the optimal driving force (i.e., Delta E-EnT approximate to lambda, where lambda 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.