New heptazines bearing strongly electron-donating substituents with quasi-degenerate 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 quasi-degenerate singlet and triplet CT states.