Excited-state triplet−doublet spin dynamics in compact donor–acceptor–radical triads

Abstract

Compact naphthalimide-based donor–acceptor–radical triads, together with their corresponding radical-free donor–acceptor dyads, were prepared as a tunable platform for manipulating long-lived excited-state spin dynamics in closely coupled organic multispin systems. Structural analysis and DFT calculations support a compact, orthogonal geometry, which preserves the distinct functions of the donor, acceptor, and radical fragments while keeping them close enough for efficient excited-state interactions. By combining optical spectroscopy with time-resolved electron paramagnetic resonance (TREPR), we observed a consistent progression across the series. The sequence starts from the localized triplet state of the naphthalimide chromophore and extends to the donor-containing dyads, where triplet locally excited (3LE) and triplet charge-separated (3CS) states coexist. Changing the donor from phenothiazine to phenoxazine increases the charge-transfer character, and increasing solvent polarity further stabilizes the charge-separated state. When TEMPO is attached, the proximal radical shortens the excited-state lifetimes, modifies triplet-state formation, and produces a distinct spin-polarization pattern, consistent with electronic coupling between the TEMPO moiety and the photoexcited naphthalimide chromophore. TREPR measurements show that the radical-containing dyads and triads are best described as triplet–doublet (TD) systems in the D/J-dominant regime and that the donor affects both the sign and the magnitude of the triplet–radical exchange interaction. Comparison of the matched radical-free and radical-containing series shows that donor identity sets the balance between localized and charge-transfer excited states, solvent polarity controls the stabilization of the charge-separated state, and the proximal radical governs spin-population redistribution and relaxation. These findings establish a unified picture of intersystem crossing pathways and excited-state dynamics in compact organic multispin assemblies.