Extendable Synthesis of Organic Cations for In Situ Construction of Hybrid Metal Halides with Near‐Unity Photoluminescence and Strong Second Harmonic Generation

The A‐site organic components of organic‐inorganic hybrid metal halides (OIHMHs) significantly impact their crystal structure and optoelectronic properties. However, chemical modification of A‐site cations has been mostly limited to commercial organic precursors, which restricts the structural variability of OIHMHs for optimal functionalities. Herein we have proposed an extendable synthesis approach to the direct procurability of various organic cations with desireable structures for the in situ construction of a library of OIHMH materials. The template condensation reaction between dimethyl sulfoxide and acetone derivatives yields A‐site organic cations with exquisite control of modularization and regioselectivity within the OIHMH crystallization system. The as‐fabricated OIHMHs demonstrated highly efficient linear optical photoluminescence or nonlinear optical second harmonic generation, promising potential applications in photonic devices. This in situ synthetic strategy offers a structural extension of OIHMHs and establishes a fundamental methodological platform for screening functional OIHMH materials.