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

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, (C₅H₆NO)⁺(CH₃SO₃)^- (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 Pna2₁ room temperature phase (RTP) to the centrosymmetric P2₁/c phase, caused by the rotation of the (C₅H₆NO)⁺ and (CH₃SO₃)^- 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 (C₅H₆NO)⁺ primarily dictates both the SHG coefficient (d_ij) and birefringence (▵n_(Zeiss) = 0.216 vs ▵n_(cal.) = 0.202 at 546 nm; Δn_(Immersion) = 0.210 vs ▵n_(cal.) = 0.198 at 589.3 nm), while the band gap (E_g) is influenced synergistically by (C₅H₆NO)⁺ and (CH₃SO₃)^-. Additionally, 4HPMS-RTP also exhibits mechanochromism upon grinding as well as an aggregation-enhanced emission in a mixture of acetone and water.