Luminescent Trityl-based Diradicaloids: A Theoretical and Experimental Assessment of Charge-Resonance in Low-Lying Excited States

The tris(2,4,6-trichlorophenyl)methyl radical (TTM) has inspired the synthesis of several luminescent diradicaloids, providing an extraordinary opportunity to control the nature of the low-lying excited states by fine-tuning the diradical character. However, the photophysical properties of TTM-derived diradicals remain not fully understood yet. Here we present a combined theoretical and experimental investigation to elucidate the origin of their luminescence. The theoretical analysis explores a series of symmetric TTM-derived diradicals with singlet ground state and increasingly longer pi-conjugated spacers between radical moieties, focussing on the nature of the lowest excited electronic states governing their photophysics. The study is complemented by a complete spectroscopic characterization of the TTM-TTM diradical, synthesized using a novel, simpler and more efficient procedure exploiting the unique reactivity of TTM. The diradicals feature two novel low-lying excited states, absent in TTM, arising from charge resonance (CR) between the radical units. The lowest CR state is characterized by the H,H -> L,L double excitation (DE) and is a dark state for symmetric diradicals. The CR nature explains the blue-shifted emission observed by increasing the distance between the radical centres as seen in TTM-ph-TTM. This insight suggests different design strategies to improve the luminescence properties of TTM-derived diradicals.