Solving the “Magic Angle” Challenge in Determining Molecular Orientation Heterogeneity at Interfaces

Abstract

It is critical to determine conformations of molecular monolayers in order to understand and control their functions and properties, such as efficiencies of self-assembly-based biosensors and turnover frequency of surface-bound electrocatalysts. However, surface molecules of the monolayers can adopt conformations with many different orientations. Thus, it is necessary to describe the orientations of surface molecular monolayers using both mean tilt angle and orientational distribution, which together we refer as orientation heterogeneity. Orientation heterogeneity is difficult to measure. In most cases, in order to calculate the mean tilt angle, it is assumed that the orientational distribution is narrow. This assumption causes ambiguities in determining the mean tilt angle, and loss of orientational distribution information, which is known as the "magic angle" challenge. Using heterodyne two-dimensional vibrational sum frequency generation (HD 2D VSFG) spectroscopy, we report a novel method to solve the "magic angle" challenge, by simultaneously measuring mean tilt angle and orientational distribution of molecular monolayers. We applied this new method to a CO2 reduction catalyst/gold interface and found that the catalysts formed a monolayer with a mean tilt angle between its quasi-C3 symmetric axis and the surface normal of 53°, with 5° orientational distribution. The narrow orientational distribution indicates that the surface molecules are rigid, which sample only limited configurations for facilitating a reaction, because of the short anchoring groups. Although applied to a specific system, this method is a general way to determine the orientation heterogeneity of an ensemble-averaged molecular interface.3

Sum frequency generation vibrational spectroscopy (SFG-VS) for complex molecular surfaces and interfaces: Spectral lineshape measurement and analysis plus some controversial issues

“…Therefore, in such cases the Bloch assumption can give a reasonable description of the vibrational dynamics [112,135,136]. Molecules in the monolayer at the surfaces are usually packed and ordered with limited orientational fluctuation and disorder [31,76,137]. Polarization SFG-VS measurement have shown that even the water molecules at the top-layer of the air/water interface is well-ordered [83,138].…”

“…Thus, the vibrational spectral lineshape based on Bloch dynamics model will be explicitly discussed along with the development of the HR-BB-SFG-VS in this review. However, spectral diffusion might be significant for interfacial species, particularly water species at aqueous interfaces [140,141], and the explorations of the higher order SFG-VS spectroscopy, such as 2D-SFG-VS, are important and shall bring in new details of the surface spectral lineshape and dynamics of the surface interactions [26,28,107,137,142].…”

“…In short, in addition to understanding and utilizing the uniqueness of the surface/interface selectivity of SFG-VS, it is also useful and helpful to put SFG-VS in the context of a unified view with other vibrational spectroscopies. In such a way, concepts, techniques and methodologies developed in other multidimensional nonlinear spectroscopy can be borrowed and utilized to advance SFG-VS studies, as it has been making exciting advances in recent years [26,28,107,137].…”

Development of ultrafast broadband electronic sum frequency generation for charge dynamics at surfaces and interfaces

“…Previous attempts were mainly concentrated on vibrational SFG (VSFG) experiments. 22,26 In particular, recent efforts in a femtosecond broadband IR source have revolutionized VSFG experiments in terms of both ultrafast time scale 30,39,54,[62][63][64][65][66] and high-resolution spectrum. 51,67 On the other hand, the development of electronic sum frequency generation (ESFG) is much left behind.…”

Surface-Sensitive and Surface-Specific Ultrafast Two-Dimensional Vibrational Spectroscopy

“…2D SFG at solid-liquid interfaces has a broad range of applications and has been used to study surface-specific signals of CO adsorbed to Platinum electrodes 135 , surface-bound peptides 121 and DNA strands 249 , amyloid fibers 250 as well as metal-carbonyl electro-catalysts 49,251,252 .…”