INTRAMOLECULAR ENERGY-TRANSFER TO TRANS-STILBENE

This study examines the photophysics of a series of transition-metal c omplexes that contain a (diimine)Re-I(CO)(3)(py) chromophore covalentl y linked to trans-stilbene via a semirigid amide spacer. In this serie s of complexes moderately exothermic triplet-triplet energy transfer f rom the 3d pi charge-transfer excited state of the diimine-Re chromop hore to trans-stilbene is competitive with ''normal'' radiative and no nradiative decay of the 3d pi state. The rate constant for intramolec ular energy transfer (k(EnT)) was determined as a function of temperat ure 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-de pendence 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-f requency factor (A approximate to 10(6) s(-1)). The results are interp reted by a mechanism in which energy transfer from the 3d pi state to trans-stilbene occurs at nearly the optimal driving force (i.e., Delt a E-EnT approximate to lambda, where lambda is the reorganization ener gy). 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.