New heptazines bearing strongly electron-donating substituents with quasi-degenerate singlet and triplet excited states: an experimental and computational study

New compounds based on heptazine core bearing phenoxazines substituent were synthesized, starting from the easily accessible 2,5,8-tri(3,5-diethyl-1H-pyrazolyl-1-yl)heptazine. The pyrazolyl groups were substituted by one, two, or three phenoxazines, yielding compounds H1, H2, H3. Heptazine (Hz) being an exceptionally strong electron acceptor and the phenoxazines being strong electron donors, these novel compounds are expected to exhibit low-lying singlet and triplet states of pronounced charge-transfer (CT) character. The excited-state electronic structures of these chromophores were investigated with electronic absorption and emission spectroscopies, electrochemical methods, and ab initio electronic-structure calculations. The UV-Vis absorption spectra in the solution were assigned by comparison with computed vertical electronic excitation energies. The absorption spectra exhibit strong transitions to the bright locally-excited 1ππ* state of the Hz core as well as to bright low-lying singlet CT states. The calculations predict that the vertical excitation spectra of all three compounds exhibit singlet-triplet inversion, that is, the lowest singlet state of the Hz core is lower in energy than the lowest triplet states of CT character, albeit the S1 and T1 states are nearly degenerate in H3. When the 0-0 transition energies are considered, on the other hand, the CT states are lowest in energy in all three compounds, and singlet and triplet CT states are nearly degenerate, which is confirmed by stationary and time-resolved spectroscopy in thin solid films. Remarkably, clear emission from the bright locally-excited singlet state of the Hz core is observed in solution in violation of Kasha’s rule. Conversely, luminescence is predicted to occur in thin solid films from the lowest quasi-degenerate singlet and triplet states of CT character.