EXCITED-STATE CONFORMATIONAL DYNAMICS OF FLEXIBLY AND SEMIRIGIDLY BRIDGED ELECTRON DONOR-ACCEPTOR SYSTEMS IN SOLUTION - INFLUENCE OF TEMPERATURE AND SOLVENT VISCOSITY

The fluorescent behavior of a series of donor-bridge-acceptor systems was studied in nonpolar solvents of different viscosity as a function of temperature. The D/A units in these systems are held apart in the g round state by a saturated hydrocarbon bridge, which was either a flex ible trimethylene chain or a semirigid piperidine ring. Photoexcitatio n of the semirigidly bridged systems containing a ''strong'' 4-cyanona phthalene acceptor leads to long-range electron transfer forming an in itial extended-charge-transfer (ECT) species which subsequently transf orms into a folded dipolar species (compact-charge-transfer (CCT) stat e, similar to a tight polar exciplex) due to the Coulomb attraction (' 'harpooning mechanism''). From fluorescence decay rates of the ECT and CCT species folding rates were determined at different temperatures, which were analyzed in terms of activation energies needed for the con formational change. The activation energies for the piperidine bridged systems were typically 4-6 kcal/mol, consistent with the barrier for a chair to boat inversion of the piperidine ring (11-12 kcal/mol) bein g lowered considerably by the concomitant gain in Coulombic energy. No clear viscosity effect was found. In the flexibly bridged system with a ''weak'' naphthalene acceptor long-range electron transfer appears not to occur at least not in the low-polarity solvents employed and in stead a conformational change precedes charge transfer. However, the f lexibly bridged DA systems with a ''strong'' acceptor also appear to f ollow the ''harpooning'' mechanism. In this case low activation energi es (2-4 kcal/mol) were found for the folding process, which were attri buted to solvent viscosity only because the steric barrier imposed by the trimethylene chain is completely compensated by the gain in Coulom bic energy accompanying the ECT --> CCT folding process. The effect of viscosity on the activation energies found for the harpooning mechani sm is discussed within the framework of Kramers' theory.