Charge-Transfer Crystals with Ionic Triplet States: An ESR and ODMR Study of HMB-TCNB and HMBTCPA

Abstract

Esr and zero-field Odmr measurements were performed for charge-transfer single crystals of hexamethylbenzene (HMB) with tetracyanobenzene (TCNB) and of HMB with tetrachlorophthalic anhydride (TCPA). Both systems exhibit very high ionic (charge-transfer) character of the triplet excitation as deduced from the small S₁,–T₁ separation and small fine structure parameters of |D| = |0.0285.|E| = 0.0066 cm ^I (HMB-TCNB) and |D| = 0.01525, |E| = 0.00342 cm⁻¹ (HMB-TCPA). By comparison with the respective parameters of the neutrally excited acceptors a charge-transfer character is determined as e² _CT = 90% and 100%, respectively. Delocalized states (triplet excitons) are detected in HMB-TCNB in a narrow temperature range (∼ 140 K).