Remarkable Second Harmonic Generation Response in (C5H6NO)+(CH3SO3)−: Unraveling the Role of Hydrogen Bond in Thermal Driven Nonlinear Optical Switch

Abstract Heat‐activated second harmonic generation (SHG) switching materials are gaining interest for their ability to switch between SHG on and off states, offering potential in optoelectronic applications. The novel nonlinear optical (NLO) switch, (C 5 H 6 NO) + (CH 3 SO 3 ) − (4‐hydroxypyridinium methylsulfonate, 4HPMS), is a near‐room‐temperature thermal driven material with a strong SHG response (3.3 × KDP), making it one of the most potent heat‐stimulated NLO switches. It offers excellent contrast of 13 and a high laser‐induced damage threshold (2.5 × KDP), with reversibility > 5 cycles. At 73 °C, 4HPMS transitions from the noncentrosymmetric Pna 2 1 room temperature phase (RTP) to the centrosymmetric P 2 1 / c phase, caused by the rotation of the (C 5 H 6 NO) + and (CH 3 SO 3 ) − due to partially thermal breaking of intermolecular hydrogen bonds. The reverse phase change exhibits a large 50 °C thermal hysteresis. Density functional theory (DFT) calculations show that (C 5 H 6 NO) + primarily dictates both the SHG coefficient ( d ij ) and birefringence (▵ n (Zeiss) = 0.216 vs ▵ n (cal.) = 0.202 at 546 nm; Δ n (Immersion) = 0.210 vs ▵ n (cal.) = 0.198 at 589.3 nm), while the band gap ( E g ) is influenced synergistically by (C 5 H 6 NO) + and (CH 3 SO 3 ) − . Additionally, 4HPMS‐RTP also exhibits mechanochromism upon grinding as well as an aggregation‐enhanced emission in a mixture of acetone and water.