Regulating the coplanarity of π-conjugated units through hydrogen bonding in FAHC2O4 and FAH2C3N3S3 crystals
Yi-Chang Yang, Rui-Xi Wang, Li-Ming Wu*, Ling Chen*
Chin. J. Struct. Chem., 2025, 44(10), 100714. DOI: 10.1016/j.cjsc.2025.100714
October 15, 2025
Birefringence; Hydrogen bond; Coplanarity; Formamidinium; π-Conjugated unit
ABSTRACT
The donor-acceptor hydrogen bonding strategy has been proposed to enforce coplanar packing of anisotropic π-conjugated units, thereby maximizing the material’s achievable birefringence. Herein, employing this strategy, we successfully obtain two highly coplanar birefringent crystals, FAHC2O4 and FAH2C3N3S3 (FA+: CH5N2+, formamidinium). FAHC2O4 shows a wide bandgap (4.20 eV), while FAH2C3N3S3 exhibits a narrower bandgap (2.96 eV) due to the involvement of sulfur atom. Both crystals display notable birefringence in their respective material classes: 0.275@546 nm and 0.504@546 nm, respectively. X-ray crystallography and computational studies attribute the pronounced birefringence to their π-conjugated moieties and their near-coplanar configurations. Comparative analysis of FAHC2O4 and FAH2C3N3S3 further establishes that the hydrogen bond strength directly influences the molecular coplanarity degree. These findings provide new insights for applying the donor-acceptor hydrogen bonding strategy in the rational design of high-performance birefringent materials.
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