Persistent Charge-Separated States in Self-Assembled Twisted Nonsymmetric Donor–Acceptor Triads

Abstract

Organic materials which can self-assemble into higher-order superstructures have extensive applications in artificial light-harvesting systems, solution-processable bulk-heterojunction solar cells, and photofunctional devices owing to their unique charge transport properties. In this report, we demonstrate a self-assembled nonsymmetric donor–acceptor triad (TAN) composed of triphenylamine (T), anthracene (A), and naphthalimide (N) units, for achieving long-lived charge separation via aggregation. Steric hindrance imposed by diisopropyl groups of naphthalimide and the propeller-shaped triphenylamine unit obstructs planarization in TAN. The quantum theory of atoms in molecules demonstrated the presence of synergistic C–H···π, C–H···H–C, π–π, and C–O···O–C interactions between the adjacent TAN units in the crystalline state, leading to significant electronic coupling (Hab). The nonplanar geometry of the TAN triad in the monoclinic space group dictates a unique antiparallel arrangement between the adjacent TAN units along the b-axis. Solvent polarity dependent Lippert–Mataga and spin density distribution analyses of TAN established the presence of charge transfer interactions in the molecule. Solvent polarity dependent nanosecond and femtosecond transient absorption measurements of TAN revealed ca. 108-fold enhancement in the lifetime of the charge-separated state in the aggregated TAN in CHCl3 (τcra ≈ 11 μs) and THF (τcra ≈ 11.20 μs) when compared to that in monomeric TAN (τcrm < 110 fs) in CH3CN. Extension in lifetime of the charge-separated state in self-assembled TAN could be due to the synergetic effect induced by delocalization of charge carriers across alternate slipped antiparallel (Hab = 13 meV) and antiparallel dimers (Hab = 10 meV) of TAN and the presence of triplet charge-separated states.