Birefringence Modulation by Phenyl and Acyl Groups in Urea-Based Crystals

Birefringent crystals are important optical functional materials widely used in light polarization modulation. Generally, the larger the birefringence, the higher the modulation efficiency. Thus, it is necessary to develop crystals with a large birefringence. Herein, two planar groups, phenyl and acyl, were adopted to generate strong optical anisotropy. Then, three urea-based crystals, N-phenylurea, N,N′-diphenylurea, and acetylurea, were found, all of which exhibit a large birefringence of 0.169, 0.266, and 0.315 (at 546 nm), respectively. Although the birefringence improvement is facilitated by phenyl groups, it is restricted by their incomplete coplanar arrangement, while it achieves the maximum by acyl groups and their coplanar arrangement with urea. Moreover, these crystals all exhibit a wide ultraviolet transparency window. Therefore, this work develops three birefringent crystal candidates and also provides some new insights, which will facilitate the exploration of novel birefringent crystals.