Engineering hydrogen bonding to align molecular dipoles in organic solids for efficient second harmonic generation

Area of Science:

• Materials Science
• Organic Chemistry
• Nonlinear Optics

Background:

• Organic molecules offer vast design possibilities for nonlinear optical (NLO) materials, particularly for second harmonic generation (SHG).
• A major challenge is the tendency of organic molecules to form nonpolar crystals, hindering macroscopic polarization and NLO properties.

Purpose of the Study:

• To design a new organic molecule motif capable of forming asymmetric organic solids with enhanced NLO properties.
• To control crystal packing through hydrogen bonding and protonation to achieve polar alignment of molecular dipoles.

Main Methods:

• Synthesis of a conjugated polar organic molecule featuring a triple bond connecting electron-donating (thiophene) and electron-withdrawing (pyridine) rings.
• Controlled protonation to modulate hydrogen bonding and influence crystal packing.
• X-ray diffraction analysis to determine crystal space groups and molecular arrangement.

Main Results:

• Two distinct crystal packing motifs were achieved: a nonpolar P1̄ space group and a polar P1 space group.
• The P1 polar packing resulted in aligned molecular dipoles and high macroscopic polarization.
• The material in the P1 polar phase exhibited efficient second harmonic generation, with an intensity approximately three times that of potassium dihydrogen phosphate.

Conclusions:

• Crystal engineering via controlled hydrogen bonding within a single molecular backbone is an effective strategy for tuning macroscopic NLO properties.
• The developed molecule motif demonstrates significant potential for creating high-performance organic NLO materials for applications like second harmonic generation.