Nonparallel Stacks of Donor and Acceptor Chromophores Evade Geminate Charge Recombination

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.