Lone Pairs of Carbogenic Center Enable Thermally Activated Delayed Fluorescence from Carbon to Ligand Charge Transfer Excited States

Carbogenic compounds of the type L→C(0)←L, where L represents various ligands such as phosphines or carbenes, and C(0) denotes zerovalent carbon with two orthogonal lone pairs, are widely utilized in organic and coordination chemistry; however, in-depth photophysical and optical properties remain largely unknown. In the following work, we report detailed luminescent properties of one of the most prominent examples of carbogenic compounds, hexaphenylcarbodiphosphorane (CDP). Combined spectroscopic/TD-DFT studies demonstrate that both lone pairs of CDP participate in the observed visible light absorption and emission, which are associated with carbon lone pairs →π*(PPh3) carbon-to-ligand charge transfer states (CLCTs). The CLCT exhibits a considerably decoupled electron hole pair, resulting in a small S1-T1 gap and thermally assisted delayed fluorescence (TADF). Moreover, two lone pairs teamed up around C(0) provide numerous close-lying σ- and π-CLCT singlet and triplet states, which promote extremely fast rates of exergonic intersystem crossing up to 1.27×10^9 s-1, competitive with some 3d and 4d metal based TADF luminophores. These results open fresh directions for innovative design of novel luminescent materials for high-end photonic applications, utilizing lone pairs of carbogenic centers as donors of electron density in the excited states.