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

ABSTRACT The structure–property relationship investigations constitute a fundamental paradigm for the rational design and synthesis of high‐performance functional materials. Herein, four guanidinopropionates (C 4 H 10 N 3 O 2 )X (X = Cl– (GPC), Br − (GPB), HSeO 3 − (GPSe), and NH 2 SO 3 − (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, Δ n ) 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 × KH 2 PO 4 ). 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.