Anion Size‐Dependent Conformational Folding of Guanidinopropionate: Enabling Tunable Birefringence and Balanced Second‐Harmonic Generation Performance

The structure-property relationship investigations constitute a fundamental paradigm for the rational design and synthesis of high-performance functional materials. Herein, four guanidinopropionates (C₄H₁₀N₃O₂)X (X = Cl- (GPC), Br⁻ (GPB), [HSeO₃]⁻ (GPSe), and [NH₂SO₃]⁻ (GPMS)) were reported. Anion-size-dependent modulation of the C2─C3 single bond rotation (linking guanidino and carboxylic π-conjugated groups) induces a reduction of the dihedral angle between the two π-conjugated planes (α) from 175° to 65°. With such a coplanarity reduction, the optical anisotropy property (birefringence, Δn) decreases from an excessive 0.20 in GPC to a favorable 0.11 in GPMS. Remarkably, GPMS also exhibits a large band gap (5.63 eV) and an enhanced second-harmonic generation (SHG) response (3.3 × KH₂PO₄). DFT reveals these four conformations are nearly degenerated, and the HOMO-LUMO gap, polarizability anisotropy, and hyperpolarizability across these four conformers are broadly tunable. This work first demonstrates how conformational folding control of flexible π-conjugated units effectively modulates and optimally balances three critical yet mutually restrictive parameters essential for high-performance SHG materials.