Core-twisted aromatics exhibit enhanced intersystem crossing upon photoexcitation when compared to their planar analogs.

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.

We report a nonparallel stacked arrangement of donor–acceptor (D–A) pairs for prolonging the lifetime of photoinduced charge-separated states. Hydrogen–hydrogen steric repulsion in naphthalimide-naphthalene (NIN) dyad destabilizes the planar geometry between the constituent units in solution/ground state. Sterically imposed nonplanar geometry of the dyad allows the access of nonparallel arrangement of the donor and acceptor stacks having triclinic space group in the crystalline state. Antiparallel trajectory of excitons in nonparallel D–A stacks can result in lower probability of geminate charge recombination, upon photoexcitation, thereby resulting in a long-lived charge-separated state. Upon photoexcitation of the NIN dyad, electron transfer from naphthalene to the singlet excited state of naphthalimide moiety results in radical ion pair intermediates that survive >10,000-fold longer in the aggregated state (τcra > 1.2 ns) as compared to that of monomeric dyad (τcrm < 110 fs), monitored using femtosecond transient absorption spectroscopy.

Ultrafast intersystem crossing of carbonylpyrenes in chloroform was investigated by femtosecond pump-probe spectroscopy. When compared to the dominant fluorescence decay pathway in pyrene, carbonyl functionalized pyrenes display near-unity triplet formation upon photoexcitation. The excited singlet state (S) undergoes rapid intersystem crossing (k) concomitantly with internal conversion (k) to lower excited singlet states (S) within a timescale of 5-11 ps (1/τ = k + k). Furthermore, intersystem crossing from lower excited singlet states (S) proceeds through coupling with receiver triplet states, eventually leading to high triplet quantum yields (Φ = 97%; tetraacetylpyrene). Followed by internal conversion in the triplet manifolds, phosphorescence decay on a microsecond timescale is observed from the emitter triplet state.

The design, synthesis, and evaluation of multifunctional dithieno[3,2‐b;2′,3′‐d]thiophene (DTT) trimers is described. Twisted push‐push‐pull or donor‐donor‐acceptor (DDA) trimers composed of one DTT acceptor and two DTT donors show strong mechanochromism in lipid bilayer membranes. Red shifts in excitation rather than emission and fluorescence recovery with increasing membrane order are consistent with planarization of the twisted, extra‐long mechanophores in the ground state. The complementary pull‐pull‐pull or AAA trimers with deep σ holes all along the scaffold are not mechanochromic in membranes but excel with submicromolar anion transport activity. Anion transport along membrane‐spanning strings of chalcogen‐bond donors is unprecedented and completes previous results on transmembrane cascades that operate with equally unorthodox interactions such as halogen bonds and anion‐π interactions.