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

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.