Stark, Charles W.Arak, JohannaTrummal, AleksanderUudsemaa, MerleSildoja, Meelis-MaitPahapill, JuriRebane, Aleks2025-01-102024-060277-786Xhttps://scholarworks.montana.edu/handle/1/19114Triamino-heptazines (TAH's) comprise the fundamental building blocks of graphitic carbon nitride, an alluring material with promising applications in optoelectronics. However, the core D3h molecular symmetry enforces a forbidden lowest-energy excited singlet state, making it a challenge to characterize via conventional spectroscopy. Here, we measure oneand two-photon absorption spectra of an acidic form of triamino-heptazine, 3H-TAH, and use reversible acid/base titration to further probe the symmetry of the low-energy transitions in aqueous solution, which suggests the molecular base structure is dimelem. Two-photon absorption reveals two distinct low-energy transitions in acidic conditions, both of which are one-photon forbidden. The lowest energy state additionally becomes one-photon allowed in basic conditions. Spectroscopic changes can be described according to chromophore symmetry switching, with C3h, D3h, or Cs point group symmetry in respective acidic, neutral, or basic environments.en-USCopyright SPIE 2024https://www.spiedigitallibrary.org/article-sharing-policiestwo-photon absorptionforbidden transitiontriamino-heptazinedimelemsymmetry switchingArticle10.1117/12.3022312