Interaction of N-(2-Methyl Thio Phenyl)-2-Hydroxy-1-Naphthaldimine with Tin Dioxide Nanoparticles: A Spectroscopic Approach

Suvetha Rani Jayaprakash, Ramakrishnan Veerabahu

Department of Laser Studies, School of Physics, Madurai Kamaraj University, Madurai – 625 021, India.

DOI: 10.4236/ajac.2012.38069                

Abstract

The interaction of N-(2-methyl thiophenyl)-2-hydroxy-1-naphthaldimine (NMTHN) with tin dioxide nanoparticles (SnO₂ NPs) has been investigated by spectroscopic tools such as absorption and fluorescence spectroscopy. Absorption spectroscopy reveals the formation of ground state complex. Fluorescence spectroscopy has been used to study the signatures of fluorescence quenching. SnO₂ NPs are found to quench the intrinsic fluorescence of NMTHN via static and dynamic quenching. The deviation from linearity in the Stern-Volmer plot has been observed.

Keywords

N-(2-Methyl Thiophenyl)-2-Hydroxy 1-Naphthaldimine; Tindioxide Nanoparticles; Optical Absorption; Fluorescence Quenching

1. Introduction

Owing to their potential application in solar energy conversion, semiconductor nanoparticles have been extensively studied from both experimental and theoretical viewpoints [1-4]. Variety of techniques including spectroscopy, microscopy and X-ray techniques has been used to characterize the properties of nanoparticles. Most of the studies have been focused on their equilibrium properties such as absorption and emission, particle shape, surface structure, inter particle interaction, self assembly, and formation of superlattices [5]. Tindioxide (SnO₂) is a n-type metal oxide semiconductor with the wide bandgap of 3.6 eV. Because of its remarkable electrical, optical and electrochemical properties, SnO₂ serves a wide range of applications in solar cells, catalytic supporting materials, transparent electrodes and solid state chemistry [6]. The higher electron mobility (~100 - 200 cm²·v^–1·s^–1) of SnO₂ NPs proposes a faster diffusion of photo induced electrons in SnO₂ and its larger band gap would be creating fewer oxidation holes in the valence band. Thus low sensitivity of SnO₂ to UV degradation facilitates the long-term stability of dye senstitized solar cells. The low isoelectric point (at pH 4 - 5) of SnO₂ leads to less absorption of the dye with acidic carboxyl groups [7,8]. The ability of organic dyes to sensitize large band gap semiconductor materials has been used for the design of light energy conversion devices [9]. Schiff bases play an important part in the development of co-ordination chemistry. The common structural feature of these compounds is the azomethine group with a general formula RHC=N-R’, where R and R’ are alkyl, aryl, cyclo alkyl or heterocyclic group which may be variously substituted. They are easily prepared in general by the condensation reaction of primary amines with carbonyl compounds [10]. In the field of co-ordination chemistry, Schiff bases from 2-hydroxy 1-napthaldehyde have been used as chelating ligands. N-H…O (keto form) and N…H-O (enol form) are the two types of intra molecular hydrogen bonds in Schiff bases. Both types of hydrogen bonds were found in the aldimine compounds derived from 2-hydroxy 1-Naphthaldehyde [11]. The existence of tautomerism between these two types of bonds created a great interest in 2-hydorxy Schiff base ligands. The ortho hydroxyl naphthalidene anilines show two bands above 400 nm in the visible region which are assigned to the keto form. In naphthaldimines both forms keto/enolimino are possible and O-H…N or N-H…O intra molecular hydrogen bonds can occur [12]. If combined with chelating activities, Schiff base may become a promising dye sensitizer in molecular photovoltaic cells [13].

Semiconducting oxides such as TiO₂ and SnO₂ directly interact with the excited dye molecules thus inducing heterogeneous electron transfer at the semiconductor/dye interface. This interesting property of sensitizing dyes is useful in the design of photochemical solar cells.

A process which decreases the fluorescence intensity of a given substance is known as quenching. It may also result from a photo-induced electron transfer process between the excited dye and the nanoparticles. Our group has studied spectral investigation of NMTHN by silver nanoparticle using fluorescence quenching [14]. Although there are many studies on the photochemical and fluorescence behavior of organic dyes on SnO₂ thin film, there is none for SnO₂ NPs on the fluorescence quenching of Schiff base in methanol medium [15-18]. In the present study, using optical absorption and fluorescence emission techniques the effect of SnO₂ NPs on NMTHN has been investigated.

2. Experimental

2.1. Materials

All chemicals that are used in this work were obtained from Merck with 99.9% purity and also used without further purification. The procedure of synthesis of NMTHN (Figure 1) is described as follows [14].

2-hydroxy 1-napthldehyde (1.72 g, 10 mmol) was dissolved in alcohol and treated with an alcoholic solution of 2-(methylthio) aniline (1.39 g, 10 mmol). The content of the above solution was kept at room temperature over night. The formation of Schiff base took place slowly with good yield. The pure brownish yellow crystals of the Schiff base was filtered, washed with alcohol and dried.

SnO₂ nanoparticles used in this study were synthesized as follows: Solution of SnCl₂·5H₂O (4.5126 g) of 0.1 M was prepared in the de-ionized water (200 ml) to get a mixed aqueous solution. To this mixed aqueous solution, ammonia solution was added into drop wise under vigorous stirring to get the pH value of the solution in the range of 8 - 9. Now, the precipitate had been formed at the bottom of the glass beaker. For about 2 hours the precipitate was kept at room temperature for ageing and then washed with deionized water. The washing was repeated for a number of 5 - 6 times. The resulting precipitate was heated at 80˚C for about 5 hours. The dried precipitate was kept at 105˚C for 4 hours, and it was loaded into the alumina crucible. Then for about

Figure 1. Molecular structure of N-(2-methyl thiophenyl)-2- hydroxy-1-napthaldimine (NMTHN).

5 hours in air it was annealed in a muffle furnace at 600˚C to enhance the crystallinity of SnO₂ NPs and the product appeared as white in color after the heat treatment. The particle size of the resultant product was found as 160 nm using micro Raman spectroscopic technique [19].

2.2. Apparatus

At room temperature using 1cm path length rectangular quartz cell by means of UV-Vis absorption spectrophotometer (Shimadzu UV 2450) and Spectrofluorophotometer (Shimadzu RC 5301-PC), steady state optical absorption and fluorescence emission spectra of the samples were recorded. While the concentration of SnO₂ NPs ranged from 0.5 to 0.9 mM, the concentration of NMTHN in methanol was 0.01 mM throughout the experiment and was precisely maintained the same in all samples. The optical absorption and fluorescent measurements have been repeated for five times for each set of samples. It was noticed that the data are reproducible with an accuracy of ±0.1 nm. And hence, there is a good reliability of the data.

3. Results

3.1. Absorption Spectroscopy

Figure 2 shows the UV-Vis absorption spectrum of NMTHN in methanol. The absorption maximum occurs at 441 nm. The entire spectrum undergoes a hyper chromic effect with a little spectral shift (Figure 3), with each addition of SnO₂ NPs concentration.

Conflicts of Interest

The authors declare no conflicts of interest.

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