Enhanced Second Harmonic Generation Response via Rational Design of Iodinated π-Conjugated Pyridine Group

Crystal materials constructed by π-conjugated units possess unique advantages due to their significant polarization anisotropy and hyperpolarizability, thereby attracting increasing attention in the field of nonlinear optical (NLO) crystals. Herein, two NLO compounds, i.e., 2,4-dihydroxypyridine (I) and 3-iodo-2,4-dihydroxypyridine (II), were successfully crystallized using the solution method, and both of the crystals belong to the non-centrosymmetric (NCS) space group P2₁2₁2₁. I exhibits a moderate second harmonic generation (SHG) response (1.1 × KDP) with large birefringence (0.256@1064 nm), while new iodinated II exhibits enhanced SHG response (1.7 × KDP) by a more favorable arrangement. Besides, theoretical calculations have been performed to understand the origin of their optical properties.