Controlling the non-resonant chemical mechanism of SERS using a molecular photoswitch

Abstract

In this work we present a detailed investigation of the Raman properties of a dithienylethene photoswitch interacting with a small gold cluster (Au(19)(+)) using time-dependent density functional theory (TD-DFT). The enhancement mechanism (CHEM) due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized for this system. We demonstrate that it is possible to control the CHEM enhancement by switching the photoswitch from its closed form to its open form. The open form of the photoswitch is found to be the strongest Raman scatterer when adsorbed on the surface whereas the opposite is found for the free molecule. This trend is explained using a simple two-state approximation. In this model the CHEM enhancement scales roughly as (omega(X)/omega(e)(4)), where omega(X) is the HOMO-LUMO gap of the free molecule and omega(e) is an average excitation between the HOMO of the photoswitch and the LUMO of the metal. We propose that the ability of this photoswitch to switch reversibly from open to closed will make it an excellent probe to control the CHEM enhancement of SERS.

Theoretical investigation on surface‐enhanced Raman evidence for conformation transition of dimercaptoazobenzene adsorbed on gold nanoclusters

“…It is clearly displayed that a fair correlation between the model enhancements and the DFT calculated integrated enhancements EF int , although the two‐state model deviates by as much as a factor of 4. The deviations between the model and the DFT results may be due to the fact that for the chemisorbed systems, a single is not enough to describe the system due to strong orbital mixing and more than two states should be used in describing the chemical enhancements, as previously reported for the benzenethiols chemisorbed on the gold clusters . In spite of these deviations, the model well predicts the change trend in the CHEM enhancement for the trans‐DMAB and cis‐DMAB adsorbed on the cluster Au n .…”

“…In addition, it is demonstrated that Raman signal intensities of the adsorbed trans‐DMAB and cis‐DMAB for the Au 19 –DMAB–Au 19 junctions, are further enhanced as compared to the corresponding DMAB–Au 19 complexes and the chemical enhancement of the trans conformation is much larger than that of the cis conformation, which is described by the induced polarizability. In recent years, some researchers employed SERS spectroscopy to probe the molecular configurations and investigate the strong electronic coupling between functionalized molecule and metal to better understand chemical contribution to SERS. We expect that our findings could yield new insights into detecting the photoswitchable azobenzenes anchored on the metal substances and understanding chemical contribution to SERS activity tuned by their conformation transitions.…”

“…The relative Raman intensities depend sensitively on the size, charge, binding site, and orientation of the cluster with respect to the molecule. Thus, numerous researchers have examined many factors that control CHEM effects in SERS, by employing small metal clusters and electronic structure methods . Very recently, Morton et al systematically studied the CHEM enhancement of meta ‐ and para ‐substituted pyridines interacting with a Ag 20 clusters and correlated magnitude of the enhancement with the energy difference between the highest occupied energy level (highest occupied molecular orbital (HOMO)) of the metal and the lowest unoccupied energy level (lowest unoccupied molecular orbital (LUMO)) of the molecule.…”

“…Very recently, Morton et al systematically studied the CHEM enhancement of meta ‐ and para ‐substituted pyridines interacting with a Ag 20 clusters and correlated magnitude of the enhancement with the energy difference between the highest occupied energy level (highest occupied molecular orbital (HOMO)) of the metal and the lowest unoccupied energy level (lowest unoccupied molecular orbital (LUMO)) of the molecule. Despite these studies, to our knowledge, there are few reports on effects of molecular conformation transition on the CHEM enhancement in SERS …”

Surface-Enhanced Raman Scattering-Based Immunoassay Technologies for Detection of Disease Biomarkers

“…The enhancement factor contributing to SERS depends on the magnitude of the localized electromagnetic field as ~E 4 [24]. The chemical mechanism also contributes to signal enhancement for certain molecules, with charge transfer in molecules at the metal–molecule interface [25,26]. Regardless of the mechanism, SERS strategy provides a “fingerprint-like” Raman spectrum that is unique to the excited molecule used.…”