Achieving the Birefringence Optimization by Methyl Modulation in Organic Nonlinear Optical Crystals

Non-centrosymmetric organics are promising nonlinear optical (NLO) candidates, which always exhibit a short UV absorption cutoff edge, and a strong second harmonic generation (SHG) response, but an overlarge birefringence. Here, in order to optimize the birefringence, the methyl modulation of π-conjugated organic planar units is proposed and implemented with the urea structure as a template. Thus, N-methylurea and N,N'-dimethylurea crystals are obtained by partially replacing hydrogen atoms with methyl groups. This replacement reduces hydrogen donors and weakens interchain hydrogen bonds, which facilitates the decreasing density and the parallel arrangement of π-conjugated planar units. Hence, both N-methylurea and N,N'-dimethylurea crystals exhibit not only an optimized birefringence (N-methylurea ≈0.099 and N,N'-dimethylurea ≈0.072 at 546 nm) but also an enhanced SHG response (N-methylurea ≈1.2 × β-BaB₂O₄ and N,N'-dimethylurea ≈1.9 × β-BaB₂O₄), while maintaining a short UV absorption cutoff edge. Therefore, this work provides a novel strategy for the structural design and performance modulation of organic NLO crystals.