Chemical Tuning of Second Harmonic Generation Efficiency in Aminothiophene-Based Molecular Microcrystals

We report on a chemical way to tune the efficiency of second harmonic generation (SHG) by aminothiophene-based molecular crystals (MCs) of a fixed noncentrosymmetric crystal structure. Through the multistep synthesis of three substituted pyrrolo[1,2-a]thieno[3,2-e]pyrimidines, we design a set of MCs of a similar Pna21 space group, providing stable (over 30 min of continuous generation) SHG in a controlled visible spectral range. Substitution of the donor methyl component of the MC with neutral hydrogen and acceptor chlorine leads to an approximately 400-fold increase in SHG efficiency due to 70% growth of the MC's hyperpolarizability. The reported chemical strategy, thereby, opens the way to design MCs and tune their SHG efficiency, being competitive with commercial nonlinear optical crystals.