THERMODYNAMICS OF INTRAMOLECULAR ELECTRON-TRANSFER IN ALKANE SOLVENTS

The thermodynamics of the intramolecular charge recombination, electron transfer reaction between a dimethoxynaphthalene donor and a 1,2-dicarbomethoxyethylene acceptor, separated by a rigid three-bond spacer, have been determined in alkane solvents using picosecond optical calorimetry (POC) and fluorescence spectroscopy. The molar enthalpy and volume changes of the charge recombination are -288 +/- 12 kJ/mol and 14 +/- 4 mL/mol respectively. The determination of these quantities from the POC data is described. The free energy change for the reaction was determined in three ways: from an analysis of the charge transfer (CT) emission band, from the calorimetric data, and from redox potentials plus Coulombic corrections. The results from the first two approaches are in reasonable agreement, Delta G degrees = -289 kJ/mol. The redox potential approach also generates similar free energy changes provided ''proper guesses'' of the ion radii and separation are used. Experimental determination of the entropy change upon charge recombination presents the greatest challenge. The temperature dependence of the CT emission band's first moment may be accurately determined but is related both to the entropy change and to the temperature dependence of the reorganization energies, d lambda/dT. Using an estimate of the entropy change, derived from the volume of reaction, and the temperature dependence of the CT band first moment produces an estimate of d lambda/dT that is twice the magnitude of Delta S degrees. The thermodynamic values determined in this investigation are compared with predictions of simple continuum models and with results from related charge transfer systems.