Triplet Generation in a Terrylene Bisimide Cyclophane by External Heavy Atom Effect of Encapsulated Brominated Pyrene Guest

Generating triplet states in dye molecules that do not undergo conventional intersystem crossing is of interest for a variety of applications. In this work, the photophysical processes that occur in a terrylene bisimide (TBI) cyclophane and in its host–guest complex with 1,6-dibromopyrene (Ka = 1.5 × 104 M–1 in dichloromethane at 293 K) are studied. First, the pristine cyclophane was studied by transient absorption (TA) spectroscopy measurements in nonpolar toluene and polar dichloromethane. For toluene, the TA data indicate only two relaxation pathways, fluorescence (with a fluorescence quantum yield of 40%) and internal conversion to the ground state. In the more polar solvent dichloromethane, the pristine TBI cyclophane undergoes symmetry breaking charge separation (SBCS) after excitation to the S1 state based on clearly identified TBI radical anion and radical cation spectral signatures, with subsequent relaxation to the ground state without any evidence for triplet state population. In contrast, for the complex with dibromopyrene, the formation of a long-lived triplet state (a lifetime of τ = 8.47 μs) is formed from the SBCS state, which is attributed to the external heavy atom effect.