Free energy and entropy changes in vertical and nonvertical triplet energy transfer processes between rigid and nonrigid molecules. A laser photolysis study

Abstract

The free energy and entropy changes associated with intermolecular triplet energy transfer (TT) processes were determined by direct measurement of the equilibrium constant using a laser flash photolysis technique. 10,10-Dimethylanthrone and 9,9-dimethylfluorene were synthesized and used as a "rigid" donor and acceptor, respectively, in comparison with 4-methylbenzophenone, a "nonrigid" donor, and 4-methylbiphenyl, a "nonrigid" acceptor, in the TT reactions. Both the nonrigid donor and the nonrigid acceptor lose entropy while going from the twisted ground state to the planar triplet state. The AS values for 4-methylbenzophenone and 4-methylbiphenyl are 0.5-1.0 and 2.5-3.0 gibbs/mol, respectively.

Cis−Trans Photoisomerization of the 1,6-Diphenyl-1,3,5-hexatrienes in the Triplet State. The Quantum Chain Mechanism and the Structure of the Triplet State

“…Examples of reversible intermolecular triplet excitation transfer were first published in 1964, 56,57 and studies of the temperature dependence of equilibrium constants for such processes appeared more than 20 years later. 58,59 As expected, ∆S is nearly zero when donor and acceptor molecules have rigid structures that allow little, if any, change in conformational freedom upon undergoing S 0 -T 1 transitions. 58,59 Close correspondence in the equilibrium geometries of the final and initial states of donor and acceptor ensure that vertical coupled transitions are involved in the two partners.…”

Triplet Energy Transfer through the Walls of Hemicarcerands: Temperature Dependence and the Role of Internal Reorganization Energy

“…As the “inverted region” is entered, the transfer rates begin to decelerate with increasing temperature, and negative values of activation energy are obtained. 14b, In this work, the dependence of the rate constants on temperature served as the crucial diagnostic in determining whether a reaction is in the “normal” or “inverted” region, and as an aid in partitioning the reorganization energy between the high- and low-energy modes. While the importance of the structural relaxation in triplet energy transfer was recognized in the past, we believe that this is the first systematic investigation of how the internal reorganization energy of different classes of acceptors affects the effective Franck−Condon factors and the dependence of the rate of electronic energy transfer on the driving force.…”

A Large Entropic Term Due to Water Rearrangement is Concomitant with the Photoproduction of Anionic Free-Base Porphyrin Triplet States in Aqueous Solutions†

“…The entropy change for the triplet-triplet energy transfer process has been analyzed in several fundamental papers (1)(2)(3). An important conclusion has been that the entropy change for the triplet energy transfer reaction in organic solvents is associated with intrinsic geometric changes in the molecular species.…”

“…An important conclusion has been that the entropy change for the triplet energy transfer reaction in organic solvents is associated with intrinsic geometric changes in the molecular species. For example, entropy changes determined by direct measurement of the temperature dependence of the equilibrium constant between triplet energy donors and acceptors by nanosecond laser flash photolysis were attributed to intrinsic changes in the flexible partner in the triplet-triplet energy transfer process (2). Thus, when a nonrigid energy acceptor, such as 4-methylbiphenyl, or donor, such as 4-methylbenzophenone, goes from the ground to the triplet state or from the triplet to the ground state, respectively, the so-called rigid partner, e.g., 10,10-dimethylanthrone as energy donor and 9,9fluorene as energy acceptor, was assumed to have a negligible entropic change, and conclusions were drawn about the conformational changes in the flexible partner giving rise to the entropic changes (2).…”