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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 ground state by a saturated hydrocarbon bridge, which was either a flexible trimethylene chain or a semirigid piperidine ring. Photoexcitation of the semirigidly bridged systems containing a “strong” 4-cyanonaphthalene acceptor leads to long-range electron transfer forming an initial extended-charge-transfer (ECT) species which subsequently transforms into a folded dipolar species (compact-charge-transfer (CCT) state, 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 conformational 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 (1 1 - 12 kcal/mol) being 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 instead a conformational change precedes charge transfer. However, the flexibly bridged DA systems with a “strong” acceptor also appear to follow the “harpooning” mechanism. In this case low activation energies (2-4 kcal/ mol) were found for the folding process, which were attributed to solvent viscosity only because the steric barrier imposed by the trimethylene chain is completely compensated by the gain in Coulombic energy accompanying the ECT - CCT folding process. The effect of viscosity on the activation energies found for the harpooning mechanism is discussed within the framework of Kramers’ theory.

the journal of physical chemistry

Excited-State Conformational Dynamics of Flexibly and Semirigidly Bridged Electron Donor-Acceptor Systems in Solution. Influence of Temperature and Solvent Viscosity