Nonparallel Stacks of Donor and Acceptor Chromophores Evade Geminate Charge Recombination

We report a nonparallel stacked arrange merit 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 hi radical ion pair intermediates that survive >10,000-fold longer in the aggregated state (tau(a)(cr) > 1.2 ns) as compared to that of monomeric dyad (tau(m)(cr) < 110 fs), monitored using femtosecond transient absorption spectroscopy.