New Heptazines Bearing Strongly Electron-Donating Substituents with Quasidegenerate Singlet and Triplet Excited States: An Experimental and Computational Study

New heptazine-based compounds bearing phenoxazine substituents were synthesized from 2,5,8-tri(3,5-diethyl-1H-pyrazolyl-1-yl)heptazine, replacing pyrazolyl groups with one, two, or three phenoxazines to yield compounds H1, H2, and H3. Given the strong electron-accepting properties of heptazine (Hz) and the donor strength of phenoxazines, these compounds exhibit low-lying singlet and triplet states with pronounced charge-transfer (CT) character. Their excited-state electronic structures were investigated using electronic absorption and emission spectroscopies, electrochemical methods, and ab initio calculations. UV–vis absorption spectra in solution, compared with computed vertical excitation energies, reveal strong transitions from bright locally excited 1ππ* states of the Hz core and bright low-lying singlet CT states. Computations predict a singlet–triplet inversion in vertical excitation spectra, with the lowest singlet state of Hz below the lowest triplet CT states, though S1 and T1 are nearly degenerate in H3. Considering 0–0 transition energies, CT states are the lowest in energy in all three compounds, with nearly degenerate singlet and triplet CT states. These results are confirmed by steady-state and time-resolved spectroscopy in thin films. Notably, bright locally excited singlet emission from Hz is observed in solution, violating Kasha’s rule, while in thin films, luminescence arises from quasidegenerate singlet and triplet CT states.