Chiral second‐harmonic generation from small organic molecules at surfaces

Abstract

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 0 -binaphthalene-2,2 0 -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 ppolarized 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.

Spectroscopy for model heterogeneous asymmetric catalysis

“…As we demonstrated for thin films of 1,1′‐bi‐2‐naphtol on glass, analyzing the polarization of the second harmonic beam generated by the sample can determine the optical activity of thin molecular films with high precision . It was also revealed that, in agreement with theory, the chiroptical effects are enhanced by orders of magnitude for nonlinear processes …”

“…58 It was also revealed that, in agreement with theory, the chiroptical effects are enhanced by orders of magnitude for nonlinear processes. [150][151][152][153][154] Alternatively, using circularly polarized light for s-SHGS and comparing the efficiency of second harmonic generation for left and right circularly polarized light, the circular dichroism (CD) of the sample can be determined. Doing so, the anisotropy factor of the sample, defined as the difference of the signals for the signals recorded with left and right circularly polarized light divided by the average of the two, can be extracted according to the following equation.…”

The origin of off-resonance non-linear optical activity of a gold chiral nanomaterial

“…To investigate the origin of the SHG we used a method previously applied to chiral molecular layers. [24][25][26][27][28][29][30] This involves measuring the dependence of the s-or p-polarised components of the SH emission on the orientation of the linearly polarised incident light with respect to the scattering plane (denoted by the angle q). The OA-SHG measurements were performed using a system described in detail elsewhere [31][32][33]41 and which utilises a Nd:YAG pulsed laser (Quanta Ray Coherent Ltd) (8 ns, 1064 nm).…”

“…Hence the properties of the substrates can be assumed to be homogeneous and the plane wave based formalism used to describe molecular systems is valid for the metamaterials studied, and the microscopic origins of the SHG response can be determined. [20][21][22][23][24] Optically active second harmonic generation (OA-SHG), utilising a range of measurement methodologies, has been widely used to probe the structure, including chirality, of (bio)molecular materials, [24][25][26][27][28][29][30] and adsorbate-induced chiral perturbations of the electronic structure of achiral metal surfaces. [31][32][33] The non-linear optical activity displayed by chiral metamaterials has been investigated less extensively.…”

“…36 The nanostructures studied in the present work display chirality on a signicantly longer length scale, hundreds of nm, than the molecular and supramolecular systems previously investigated with non-linear techniques. [24][25][26][27][28][29][30] Since the dimensions of the nanostructures are comparable with the wavelength of the fundamental and SH light, intuitively one may expect the relative contribution of non-localised higher multipole excitations to non-linear optical activity to be increased compared to molecular/supramolecular systems. This is the question that the current study seeks to address.…”