PHOTOINDUCED ELECTRON-TRANSFER AND INTRAMOLECULAR FOLDING IN A TRICARBONYLRHENIUM (BI)PYRIDINE BASED DONOR CROWN/ACCEPTOR ASSEMBLY - DEPENDENCE ON SOLVENT/

The title assembly diplays an emissive rhenium-to-pyridine charge-tran sfer state that is partially quenched by electron transfer to an attac hed acceptor (nitrobenzene). Quenching is preceded by intramolecular f olding (J. Ant. Chem. Sec. 1993, 115, 2048). Variable-temperature quen ching measurements can be used to determine the characteristic tempera ture, T-tr, above which unfolded photoexcited state conformations beco me favored over folded conformations. Similar information for the grou nd state can be obtained from variable-temperature NMR measurements. S tudies in eight solvents show that excited state folding is (1)enthalp ically favored but entropically disfavored (all solvents), (2) correla ted (via T-tr) with the inverse dielectric strength of the solvent, an d (3) enhanced in comparison to folding in the electronic ground state (studies in one solvent). The combined evidence points to a folding r eaction that is driven by optimization of localized Coulombic interact ions. Optimization of solvent cohesive interactions, however, may poss ibly also play a role in the folding reaction.