Second harmonic generation from symmetry breaking stimulated by mixed organic cations in zero-dimensional hybrid metal halides.

Mixing cations with different chemical properties to induce the generation of asymmetric structures is a new approach for tuning the optical properties of hybrid organic-inorganic metal halides (HOIMHs). In this study, zero-dimensional (C 9 N 3 H 15 )(C 9 H 13 SO)MBr 6 (M = Bi/Sb, [C 9 N 3 H 15 ] 2+ = [(C 4 N 2 H 10 )(C 5 NH 5 )] 2+ and [C 9 H 14 SO] + = [CH 3 (C 6 H 4 )OS(CH 3 ) 2 ] + ) are synthesized. Two different cations cause both compounds to crystallize in the polar space group P 2 1 2 1 2 1 , thus resulting in significant phase matchable second harmonic generation under a 1064 nm laser excitation. Thus, (C 9 N 3 H 15 )(C 9 H 13 SO)BiBr 6 and (C 9 N 3 H 15 )(C 9 H 13 SO)SbBr 6 exhibit intensities that are approximately 1.8 and 1.7 times that of KH 2 PO 4 , respectively. The results of density functional theory calculations show that both (C 9 N 3 H 15 )(C 9 H 13 SO)BiBr 6 and (C 9 N 3 H 15 )(C 9 H 13 SO)SbBr 6 exhibit direct bandgaps of 2.95 and 2.81 eV, respectively. Additionally, because of the distortion of the inorganic octahedra, (C 9 N 3 H 15 )(C 9 H 13 SO)SbBr 6 exhibited bright yellow emission at room temperature, which is attributed to n s 2 fluorescence emission. We believe that the symmetry of the HOIMH crystal structure can be broken by introducing spatially differentiated bifunctional organic cations, which consequently enables even-order nonlinear activities.