Dynamics of Intramolecular Triplet Exciton Transfer Using Electron Spin Resonance

Abstract

The rate of intramolecular triplet exciton transfer in a series of congeneric dimeric systems composed of two biphenylene subunits linked by an insulating tetravalent Group IVa atom is studied as a function of temperature using electron spin resonance. A first-order density matrix theory including the transfer process is developed. The transfer model assumes a coupling between the ``high-field'' monomer magnetic substates with the same mg only (T2 process), without inducing transitions between states of different mg (T1 processes). By comparing computer simulated line shapes with the experimental spectra, activation energies for the dynamic process are extracted. These energies are attributed to low-frequency torsional modes which are active in vibronically coupling the monomer sites. A discussion based on electron exchange and on vibronic coupling is given, and an order of magnitude calculation of the transfer probability based on the resonance excitation transfer mechanism yields numbers agreeing satisfactorily with the transfer rates obtained from experiment.