Controlling triplet-pair formation in acene-bridged trimers through locally excited-charge-transfer state mixing.

Abstract

Singlet fission (SF) offers a pathway to surpass conventional efficiency limits in photovoltaics, yet how bridge-chromophore coupling governs locally excited/charge-transfer state (LE-CT) mixing and thereby the multiexciton yield remains elusive. We design three systems, TIPS-BT0, TIPS-TAT, and Encap-TAT, that progressively increase bridge-chromophore coupling. Steady-state spectroscopy and transient absorption reveal that TIPS-BT0 and TIPS-TAT undergo efficient SF, forming correlated triplet pairs (1TT) and, in TIPS-TAT, long-lived triplets. By contrast, Encap-TAT loses vibronic structure and relaxes ultrafast into a broad, red-shifted intramolecular exciplex-like (Ex) state. Quantum chemical analysis shows that the Ex state originates from strong hybridization between local excitations (LEs) and bridge-chromophore CT configurations, driven by large electron and hole transfer integrals and a small LE-CT energy gap. Such strong LE-CT hybridisation suppresses well-defined 1TT formation, favouring intramolecular Ex emission and enhancing photoluminescence quantum yield (≃80.5% vs. ≃38.5% for TIPS-TAT). These findings establish a unifying framework in which the balance between multiexciton generation and exciplex emission in acene-bridged trimers is governed by the degree of LE-CT mixing. Tuning bridge energetics and LE-CT offsets enables deliberate routing of excited-state pathways toward efficient triplet production or bright exciplex emission, providing actionable design rules for next-generation singlet-fission and optoelectronic architectures.