Experimental Confirmation of the Importance of Orientation in the Anomalous Chiral Sensitivity of Second Harmonic Generation

Abstract

Macromolecular interactions were demonstrated to yield large chiroptical effects in second harmonic generation measurements of ultrathin surface films. Second harmonic generation (SHG) has recently shown to be several orders of magnitude more sensitive to chirality in oriented systems than common linear methods, including absorbance circular dichroism (CD) and optical rotary dispersion (ORD). Numerous mechanisms have been developed to explain this anomalous sensitivity, with a general emphasis on understanding the molecular origins of the chromophore chirality. In this work, orientational effects alone are shown to be the dominant factor for generating large SHG chiral dichroic ratios in many surface systems. Three distinct uniaxial surface films of SHG-active achiral chromophores oriented at chiral templated surfaces were observed to yield chiral dichroic ratios as great as 40% in magnitude.

Detection of chiral sum frequency generation vibrational spectra of proteins and peptides at interfaces in situ

“…According to the peak assignments of secondary structures of proteins, the peaks at 1,630 and 1,690 cm Ϫ1 should be contributed from ␤-sheet structures (66 -69). We believe that this characteristic must be due to the fact that ␤-sheet structures can more easily have ordered twist angles at the interface, thereby generating strong chiral signals for some vibrational modes, as suggested by Simpson and coworkers (35,36). Our work here clearly indicates that the chiral SFG spectra should provide further structural information about interfacial fibrinogen molecules.…”

“…Electronic resonance enhancement is used to detect chiral SFG vibrational spectra generated from the molecular chirality, because under electronic resonance the asymmetric Raman tensor components will be greatly enhanced by the electronicvibrational coupling (18). Strong SHG and SFG signals can be observed from chiral molecules; however, Simpson and coworkers (33)(34)(35)(36)(37) demonstrated by using SHG that achiral molecules adsorbed on a surface or interface may generate detectable nonlinear optical chiral signals. They concluded that macromolecular orientational effects can dominate the chiroptical responses of uniaxial systems.…”

Molecular Origins of the Remarkable Chiral Sensitivity of Second‐Order Nonlinear Optics

“…[7,[10][11][12][13][14][15] More recently, theoretical and experimental evidence suggests that macromolecular orientational effects alone can dominate the chiroptical responses of uniaxial systems without the need for multiple coupled oscillators or even chiral chromophores at all (Figure 1 c). [7,8,[42][43][44][45][46] [a] Prof. G. J. Simpson Department of Chemistry Purdue University, West Lafayette IN 47907 Fax:(+ 1) 765-494-0239 E-mail: gsimpson@purdue.edu Figure 1.…”

“…The generation of macromolecular chiroptical activity in chiral assemblies of achiral chromophores (referred to as structural rotary power) is reasonably well-understood in absorbance measurements of chiral media, [71,72] but has only relatively recently been applied to the interpretation of chiroptical effects in SHG and SFG of oriented systems. [7,8,42,43,45,46,62] Although predictions of possible chiroptical effects in SHG from oriented assemblies of planar chromophores with C 2v symmetry were reported as early as 1987, [73] most of the work in characterizing the role of molecular orientation in driving chiroptical phenomena in SHG and SFG of uniaxial films and interfaces has been performed within the last few years. [7,8,42,43,45,46,62] In both planar chromophores with C 2v symmetry and in "two-state" chromophores, in which the nonlinear optical properties of the chromophore are reasonably well-described by considering interactions between the ground state and a single excited state, only four elements within the molecular tensor are significant in electronically resonant SHG and SFG: b zzz , b zxx , b xzx , and b xxz .…”

“…[34][35][36][37][38][39][40][41] Additionally, the novel symmetry properties of SHG and SFG in chiral media suggest that new nonlinear optical materials can be constructed that exploit these chiroptical effects. [42][43][44][45][46] A critical bottleneck in all of these emerging inquiries is the development of intuitive models for connecting the macroscopic chiroptical response with the molecular and supramolecular structure. A brief review of recent fundamental advances in the area is presented.…”