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 bulkheterojunction 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 (H ab ). 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. 10 8 -fold enhancement in the lifetime of the charge-separated state in the aggregated TAN in CHCl 3 (τ cr a ≈ 11 μs) and THF (τ cr a ≈ 11.20 μs) when compared to that in monomeric TAN (τ cr m < 110 fs) in CH 3 CN. 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 (H ab = 13 meV) and antiparallel dimers (H ab = 10 meV) of TAN and the presence of triplet charge-separated states.

Unsolicited Photoexcited-State Pathways Relegate the Long-Lived Charge Separation in Self-Assembled Nucleobase–Arene Conjugate

“…To avoid the distortion arising from the inner filter effect, the concentration-dependent steady-state measurements were carried out in a cuvette of 1 mm path length. 24 Solution state relative quantum yield measurements were performed using quinine sulfate in 0.1 M H 2 SO 4 as the standard (λ ex = 350 nm, reported quantum yield Φ f = 0.577). 25 Lifetime measurements were carried out in an IBH picosecond time correlated single photon counting (TCSPC) system.…”

Of Twists and Curves: Electronics, Photophysics, and Upcoming Applications of Non‐Planar Conjugated Organic Molecules

“…ICT as well as high ordered assembly further aids to generate long‐lived charge‐separated (CS) states through delocalization/hopping of charge carriers (hole/electron). This concept is ably demonstrated by the nonsymmetric donor‐donor‐acceptor triad (TAN 48, Figure b) reported by Hariharanet al. 48 exhibited a significantly long‐lived charge separation in aggregated states (half‐life of charge recombination, τ a cr =11 μs in CHCl 3 ) compared to that in monomeric state ( τ m cr <110 fs in CH 3 CN) due to the sequential intra‐ and intermolecular electron transfer or delocalization in the domain (monomer) linked aggregates.…”

Photoactive organic material discovery with combinatorial supramolecular assembly

“…54 FMOs can be tuned by functionalizing the organic semiconductors with electron donating or withdrawing substituents, 55,56 however, changes to molecular structure en route to tailoring FMOs will inevitably affect packing geometry, and, consequently, the electronic coupling between donors and acceptors. There are many examples of organic donor-acceptor systems that undergo photoinduced CT, and the ways in which they bring together the components involve noncovalent assembly, 57 polymer blending, 58,59 covalently linking, 60 and inorganic bonding to form covalent organic frameworks (COFs). 61,62 An example of a combinatorial system in which forming contiguous conduction pathways is also considered in conjunction with charge transfer to create active layers for OPVs as critical design criteria is the work of Jin et al 63 In this study, they describe a system in which metallophthalocyanine donors and diimide acceptors (Fig.…”