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

Zerovalent carbon 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, have been 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 zerovalent carbon compounds, hexaphenylcarbodiphosphorane ( CDP ). Combined spectroscopic/TD-DFT studies demonstrate that lone pairs of CDP participate in the observed visible light absorption and emission, which are associated with carbon lone pairs → π*(PPh 3 ) carbon-to-ligand charge-transfer (CLCT) states. The CLCT state exhibits a considerably decoupled electron–hole pair, resulting in a small S 1 –T 1 gap and thermally activated delayed fluorescence (TADF). Moreover, two nearly degenerate lone pairs on the C 0 center provide multiple close-lying σ- and π-CLCT singlet and triplet states, which promote extremely fast rates of exergonic intersystem crossing (S 1 → T 1 ) up to 1.27 × 10 9 s –1, competitive with some 3d- and 4d-metal-based TADF luminophores. These results open fresh directions for the innovative design of novel luminescent materials for high-end photonic applications, utilizing lone pairs of zerovalent carbon centers as donors of electron density in the excited states.