Exploiting Rigid Inorganic π-Conjugated Units and Deprotonatable Organic π-Conjugated Dimers for Enhanced Linear and Nonlinear Optical Properties

The design of nonlinear optical (NLO) crystals has captivated the interest of chemists, materials scientists, and physicists for many years. The organic–inorganic hybrid π–π stacking is an effective method to design high-performance NLO crystals. Herein, rigid inorganic π-conjugated units (CO₃ and NO₃, the symbol is π) and organic π-conjugated dimers (biguanide group, the symbol is π^) are employed for stacking, resulting in the compounds of NH[C(NH₂)₂]₂(NO₃)₂ and {N[C(NH₂)₂]₂}₂CO₃ through π--π^ interaction. Both compounds exhibit strong linear optical properties—transparency: 318 nm and birefringence: 0.122@550 nm for the former; transparency: 258 nm and birefringence: 0.232@550 nm for the latter—and NLO effects (3.5 × KH₂PO₄ (KDP) and 5 × KDP, respectively). Theoretical calculations reveal that strong linear and NLO properties mainly derive from the interactions of multiple hydrogen bonding and π--π^ stacking. Interestingly, deprotonatable N[C(NH₂)₂]₂ organic π-conjugated dimers demonstrate higher polarizability anisotropy and hyperpolarizability than their protonated NH[C(NH₂)₂]₂ counterparts, resulting in enhanced macroscopic linear and NLO performance. This offers an opportunity to understand π–π stacking and provides a new strategy for enhancing the linear and NLO properties of the material.