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Chiral second‐harmonic generation from small organic molecules at surfaces

The molecules of life are chiral. Advanced materials for heterogeneous enantioselective catalysis and biomolecular sensing and recognition will need nanoscale chiral structures, and methods of characterizing them. Optical circular dichroism (CD) is the main technique for probing chiral molecules in solution. Amino acids are levorotatory, but CD cannot detect chirality in ultrathin films of these small molecules at surfaces, as the response is too weak. Chiral second‐harmonic generation (SHG) from ultrathin layers of larger, more polarizable organic molecules has been detected at silica surfaces, where the presence of electronic resonances enhanced the signal. Here chiral SHG is reported from ultrathin films of 1,1′‐binaphthalene‐2,2′‐diol (binol) and cysteine. The SH response from the different chiral enantiomers was measured away from resonance, using unamplified femtosecond excitation at 800 nm, by rotating a quarter‐wave plate polarizer, and detecting the p‐polarized SH output. The SH response from the molecules is measurable and differs significantly between the enantiomers. Simultaneous fitting of the polarizer rotation plots, using a simple phenomenological model, produces reasonable agreement with the experimental results. This exploratory work shows that SHG has sufficient sensitivity to detect the chirality of small molecules adsorbed on surfaces.