Luminescent Trityl‐based Diradicaloids: A Theoretical and Experimental Assessment of Charge‐Resonance in Low‐Lying Excited States
Davide Mesto, Michele Orza, Brunella Bardi, Angela Punzi, Imma Ratera, Jaume Veciana, Gianluca M. Farinola, Anna Painelli, Francesca Terenziani, Davide Blasi, Fabrizia Negri
Abstract
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 π-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.