Electron spin polarized triplet state energy transfer in mixed organic single crystals at low temperatures

Abstract

Transfer of photoexcitation from phenazine guest molecules to anthracene guest molecules in diphenyl single host crystals at 1.7°K, producing anthracene triplet species, was investigated by electron paramagnetic resonance methods. The triplet spin lattice relaxation rates were slow enough compared to depopulation rates so that preferential populating of the individual triplet sublevels of phenazine and anthracene could be observed. It was found that the long axis zero field state was populated fastest in both donor and acceptor, indicating that spin polarization was conserved in the transfer. This information was used along with several other experimental results to elucidate the nature of the processes responsible for the phenazine to anthracene transfer. It was concluded that the transfer took place either by means of a Dexter exchange process between triplet phenazine and unexcited anthracene, or by production of diphenyl triplet excitons from phenazine excited singlet species, and subsequent exciton migration and population of anthracene acceptor triplet traps. This latter route was suggested by the observation that singlet excitation of phenazine populated diphenyl‐h₁₀ triplet traps in a mixed diphenyl‐d₁₀/h₁₀ host crystal.