Access to Triplet Excited State in Core-Twisted Perylenediimide

Solvent-free crystal structure of N,N -bis(propylacetyl)-1,6,7,12-tetrabromoperylene-3,4:9,10-bis(dicarboximide), PDI-Br 4, obtained by X-ray diffraction reveals the core-twisted perylene motif having π–π stacks at an interplanar separation of 3.7 Å. Slip-stacked arrangement of PDI units in PDI-Br 4 arises due to the presence of bulky bromine atoms. Femtosecond pump–probe measurements of monomeric PDI-Br 4 in toluene reveal ultrafast intersystem crossing (τ ISC < 110 fs) when excited at 400 nm. Triplet quantum yield (Φ T ) of 19 ± 1% and 105 ± 5% for PDI-Br 4 in toluene and vapor-annealed polycrystalline 60 nm thick film respectively are estimated from nanosecond transient absorption measurements. Quantum chemical calculations show that the combined effects of heavy atom and core-twist in PDI-Br 4 can activate the intersystem crossing by altering the singlet–triplet energy gap. Enhanced quantum yield accounts for the singlet fission mediated generation of triplet excited state in the PDI-Br 4 thin film.