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-Br4, 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-Br4 arises due to the presence of bulky bromine atoms.
Femtosecond pump–probe measurements of monomeric PDI-Br4 in toluene reveal ultrafast intersystem crossing (τISC < 110 fs) when excited at 400 nm. Triplet quantum yield
(ΦT) of 19 ± 1% and 105 ± 5% for PDI-Br4 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-Br4 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-Br4 thin
film.