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

TL;DR

This study explores how hydrogen bonds in melem hydrate crystals affect their near-UV luminescence and TADF properties.

Contribution

The paper reveals how hydrogen bonding in a new melem hydrate (Mhp) influences its crystal structure and enhances luminescence efficiency.

Findings

Mhp exhibits high quantum yield and delayed fluorescence in NUV emission.

Hydrogen bonds suppress molecular distortion in the excited state, promoting efficient luminescence.

Low-temperature PL increases due to changes in the hydrogen-bond network.

Abstract

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.

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