Role of Hydrogen Bonding in Crystal Structure and Luminescence Properties of Melem Hydrates

In recent years, carbon nitride (CN) compounds, such as g-C₃N₄ and melem, have attracted attention as new visible light-driven photocatalysts with a variety of functions, including water splitting, organic decomposition, and dark photocatalysis. The building unit of these materials is the heptazine ring, and molecules with this structure have attracted considerable attention as luminescent materials. Melem is an organic molecule with amino groups at the three termini of its heptazine ring. Melem exhibits near-UV (NUV) emission with high quantum yield via thermally activated delayed fluorescence (TADF). Materials exhibiting TADF can achieve highly efficient luminescence without the use of heavy metals, generating interest in their potential as luminescent materials for organic electroluminescent devices. Compared to materials that emit in the visible-light region, there are few reports on TADF materials such as melem that exhibit NUV emissions. Melem hydrate is easily obtained by hydrothermal treatment of melem. Unlike melem crystals, melem hydrate (Mh) has a porous structure because of a hydrogen-bond network formed between melem and water molecules. To date, only one type of Mh has been well-investigated. Mhs are expected to exhibit novel properties, such as photocatalysis, molecular adsorption, and highly efficient NUV emission. Mh also provides an opportunity to investigate how hydrogen bonds between the melem molecule and crystal water affect the TADF NUV emissions. This provides clues to the mechanism of the TADF action exhibited by other melem compounds. In this study, we focus on a new melem hydrate with a parallelogram shape, Mhp, first reported by Dai et al. in 2022. The crystal structure of Mhp reportedly differs from that of Mh; however, the Mhp crystal structure has not been determined to date, and its physical properties have not been investigated. Therefore, in this study, we reexamined the conditions for growing single crystals of Mhp and succeeded in growing samples that could be used to measure physical properties. We also determined its crystal structure and investigated the role in crystal formation of the hydrogen bonds between melem and water molecules. We evaluated the thermal behavior and optical properties and discussed their correlation with the crystal structure. Similar to melem, Mhp displayed NUV luminescence in its photoluminescence (PL) spectrum. This luminescence was found to have high quantum yield and delayed fluorescence. At low temperatures, the PL of Mhp dramatically increased at a wavelength of approximately 350 nm. This behavior was attributed to a significant change in the hydrogen-bond network between melem and water molecules in the Mhp crystal at low temperatures. We found that distortion of the melem molecule in the excited state at low temperatures was suppressed by its strong hydrogen bonds with water molecules. As a result, the displacement of the atomic nuclei of the atoms that make up the melem molecules in the excited state produced by light absorption is small, and in the de-excitation process, radiative transitions to low-energy vibrational levels are promoted. At the same time, nonradiative deactivation was suppressed, resulting in high fluorescence quantum efficiency. The results of this research provide deep insight into the role of hydrogen bonds in the optical properties of hydrate crystals that exhibit highly efficient luminescence, including TADF.