Chemical Tuning\nof Second Harmonic Generation Efficiency\nin Aminothiophene-Based Molecular Microcrystals

We report on a chemical way to tune the efficiency of\nsecond harmonic\ngeneration (SHG) by aminothiophene-based molecular crystals (MCs)\nof a fixed noncentrosymmetric crystal structure. Through the multistep\nsynthesis of three substituted pyrrolo[1,2-<i>a</i>]thieno[3,2-<i>e</i>]pyrimidines, we design a set of MCs of a similar <i>Pna</i>21 space group, providing stable (over 30 min of continuous\ngeneration) SHG in a controlled visible spectral range. Substitution\nof the donor methyl component of the MC with neutral hydrogen and\nacceptor chlorine leads to an approximately 400-fold increase in SHG\nefficiency due to 70% growth of the MC's hyperpolarizability.\nThe\nreported chemical strategy, thereby, opens the way to design MCs and\ntune their SHG efficiency, being competitive with commercial nonlinear\noptical crystals.