OPTICAL, ZERO-FIELD ODMR AND EPR STUDIES OF THE TRIPLET-STATES FROM SINGLET FISSION IN BIPHENYL-TCNQ AND BIPHENYL-TETRAFLUORO-TCNQ CHARGE-TRANSFER CRYSTALS

Biphenyl-TCNQ and biphenyl-tetrafluoro-TCNQ crystals, when excited with visible light, give rise to triplet excitons which were investigated by optical, EPR and zero-field (ZF) ODMR spectroscopies in a wide temperature range. The analysis of spin polarization carried by EPR and ZF ODMR spectra at room temperature enabled us to reveal the singlet depletion path S --> (T, T) which is thermally activated. The activation energies are DELTAE = 1940 cm-1 for B-TCNQ and DELTAE = 1720 cm-1 for B-TCNQF4. From the relation 2E(T) = E(S) + DELTAE the energies of the triplet state in the CT complexes were derived: E(T) = 9770 +/- 100 cm-1 for B-TCNQ and E(T) = 8695 +/- 100 cm-1 for B-TCNQF4. It was also established that the triplet species detected by ODMR at 1.8 K are populated by ISC.