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 pi-pi stacks at an interplanar separation of 3.7 angstrom. Slip-stacked arrangement of PDI units in PDI-Br-4 arises due to the presence of bulky bromine atoms. Femtosecond pump-(p)robe measurements of monomeric PDI-Br-4 in toluene reveal ultrafast intersystem crossing (tau(ISC) < 110 fs) when excited at 400 nm. Triplet quantum yield ((Phi(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.