SERS and DFT study of silver nano particle induced dark isomerisation in 1H-2(Phenylazo) imidazole Washim Hossain a , M. Ghosh b , C. Sinha c , Dilip K. Debnath d , Uttam K. Sarkar a,⇑ a Department of Physics, Malda College, Malda, WB, India b Department of Spectroscopy, IACS, Jadavpur, WB, India c Department of Chemistry, Jadavpur University, Kolkata, WB, India d Department of Chemistry, Malda College, Malda, WB, India article info Article history: Received 27 July 2013 In final form 7 September 2013 Available online 18 September 2013 abstract Raman spectra and SERS of 1H-2(Phenylazo) imidazole (PaiH) adsorbed on silver nano particles are reported. Monomolecular layer is formed at a concentration of 5 Â 10 À6 M. A trans-to-cis isomerisation of PaiH is suggested by the cis-signature peak at 570 cm À1 . In absorption spectra a single p–p / band at 358 nm is observed at higher concentrations whereas the p–p / and the n–p / bands appear at 370 nm and 456 nm, respectively, at a concentration of 5 Â 10 À6 M. This is in support of the hypothesis of trans-to-cis-isomerisation with lowering of concentration. DFT calculations are shown. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Inherently low Raman scattering cross-section even at the level of single molecule, is enormously enhanced when the concerned molecule is adsorbed on noble metals in general and on silver and gold nano particles in particular [1–5]. This phenomenon is referred to as Surface Enhanced Raman Spectroscopy (SERS) and widely used in chemistry, biology, physics and material science [6–10]. Mainly two mechanisms are known to contribute to this large enhancement: (a) the electromagnetic enhancement (EME) originating from surface plasmon polaritons [4,11] which causes overall enhancement of Raman spectrum and (b) the chemical enhancement (CE) that results from the change of electronic struc- tures of molecule adsorbed on metal surfaces. The CE contributes to the selective enhancement of Raman bands where a charge transfer between the molecule and metal is involved [12–16]. Although SERS active metal nanoparticles have been prepared in different procedures to achieve varied surface character, silver and gold sols have found extensive application since the first use in 1979 [17]. Raman as well as SERS techniques are usually com- plemented with theoretical methods for the computation of molec- ular structures, such as Density Functional Theory (DFT) which provides a promising cost effective approach for calculating vibra- tional spectra of large molecules [18,19]. In this Letter, we report Raman spectra and SERS of 1-H-2(Phenylazo) imidazole (PaiH) adsorbed on silver nano parti- cles, for the first time. Arylazo imidazoles constitute an interesting class of heterocyclic azo compounds as a potential switching group in biological applications and in coordination chemistry. These molecules with N-heteroatom have azoimine –N@N– functional group which has interesting properties including electrochemical behaviour, stabilization of low valent metal redox state [20] due to its p-acidity and the presence of low lying azo centred p / molec- ular orbitals [21], formation of azo-metal complexes [22], and acti- vation of the C–H function of the pendant aryl group [20] in metal co-ordinate state. Because of the imidazole group and the azo group metal complexes of these molecules are formed which act as a molecular switch [23]. The trivalent imidazole nitrogen offers a site for functionalization of the compound [24] and consequently imidazole is a ubiquitous and essential group in biology especially as a metal coordination site. The main objective of the present letter is to study the behav- iour of the PaiH molecule in the environment of silver nano parti- cles. UV–Vis spectroscopic study of PaiH in methanol solution and adsorbed on colloidal silver particles has also been carried out for better understanding of the mechanism involved in the electronic interaction between the molecule and the metal particles. p-acidic character of the PaiH molecule is estimated. Theoretical modelling of the experimental observations has been attempted in terms of DFT calculation. 2. Experimental The PaiH molecule was prepared according to the reported pro- cedure [25]. Spectroscopic grade methanol, purchased from Kanto Chemical Industries, was used as solvent for the PaiH molecule. AgNO 3 and NaBH 4 were purchased from Aldrich chemical Co. Stable silver sols were prepared by reducing aqueous solution of AgNO 3 by NaBH 4 according to the process described in earlier re- port [26]. Distilled and deionised water obtained from Milli-Q-Plus system of M/S Millipore Corporation, USA was used for the 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.09.033 ⇑ Corresponding author. Fax: +91 3512 220516. E-mail address: chairman_uks@yahoo.com (U.K. Sarkar). Chemical Physics Letters 586 (2013) 132–137 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett