Different effects of oligomerization on frontier-orbital gaps and bright excitations in heptazine-and triazine-based carbon nitride models

Understanding how molecular structure affects light absorption is important for the study of carbon nitride building blocks. Here, density functional theory and time-dependent density functional theory calculations were used to compare six heptazine-and triazine-based molecular models with different sizes and connectivities. The optimized heptazine-based structures were largely planar, whereas the triazine-based models showed greater deviations from planarity. The two families exhibited different changes in their frontier-orbital gaps. In the gas phase, the heptazinebased gap changed only slightly from the first to the second model and decreased further in the third, giving values of 4.32, 4.29, and 3.87 eV. The corresponding triazine-based values were 3.74, 4.80, and 4.67 eV. In contrast, the first bright excitation energies were 5.16, 3.54, and 3.28 eV for the heptazine-based models and 4.74, 3.95, and 3.96 eV for the triazine-based models, using an oscillator-strength threshold of 0.01. Analysis of the low-lying states shows that the lowest singlet excitation and the first bright transition can respond differently to oligomerization. CPCM calculations at fixed geometries revealed additional changes in excitation energies and intensities. These results show why frontier-orbital gaps, excited-state energies, and transition strengths must be considered together when comparing the optical properties of finite carbon nitride models.