SERS spectroscopy and quantum-chemical calculations of stilbene amino derivatives in conditions of adsorption on silver nanoparticles

How is the Observation of High-Order Overtones and Combinations Elucidated by the Charge-Transfer Mechanism in SERS?

“…In our previous studies, the resonance Raman mechanism for DAT was investigated at FC and FCHT levels in refs 13 and 14. Nevertheless, to confirm our explanation and extend it to other cases, we improved the previous calculation 11 of DAS, which was based on the FC approach, in the current work. The calculations based on the FC and FCHT approach with timedependent path integral technique were performed for BPE, DPA, and DAS with the Ag 6 cluster at two levels of approximation: vertical gradient (VG) and adiabatic Hessian after step (AHAS).…”

“…Symmetric 4,4′-diamino derivatives of stilbene and tolane have molecular excitations nearly close to the charge-transfer excitation. Experimental and theoretical studies on the SERS spectra of these stilbene and tolane derivatives adsorbed on silver nanoparticles have been conducted previously, − , revealing high-intensity overtones and combination bands in the spectral range of 3000–6000 cm –1 . , However, for dipyridyl stilbene- and tolane-like compounds, SERS spectra were obtained experimentally using a 488 nm laser, focusing on the range of 500–2500 cm –1 , but high-order overtones were not observed. To unravel the primary reason behind the absence of overtones in comparison to amine derivatives, both experimental and theoretical UV–Vis absorption spectra and SERS spectra were obtained for the dipyridyl compounds.…”

“…Compounds containing amine groups or pyridyl moieties capable of binding to silver and gold surfaces through their lone pairs are attractive candidates for investigating the static and dynamic charge-transfer mechanisms in SERS. Symmetric Experimental and theoretical studies on the SERS spectra of these stilbene and tolane derivatives adsorbed on silver nanoparticles have been conducted previously, [11][12][13]33 revealing high-intensity overtones and combination bands in the spectral range of 3000−6000 cm −1 . 12,14 However, for dipyridyl stilbene-and tolane-like compounds, SERS spectra were obtained experimentally using a 488 nm laser, focusing on the range of 500−2500 cm −1 , but high-order overtones were not observed.…”

“…However, the predominant role of the electromagnetic mechanism (EM) makes it difficult to separate the SERS-CT enhancement and study this mechanism independently. In previous studies, we studied experimentally and theoretically the SERS-CT mechanism for stilbene and tolane compounds with singular charge-transfer states. − These compounds show bright, low-energy molecular transitions close to the plasmonic excitations (with a separation of less than 1.0 eV) and undergo enhancement through the resonance Raman (RR) mechanism. This molecular state can also couple with the charge-transfer excitations with the vibronic coupling term.…”