Fluorescence quenching of DMB by aniline in benzene–acetonitrile mixture

. The fluorescence quenching of coumarin dye namely 4-(2, 6-dibromo-4-methyl-phenoxymethyl)-benzo[h]chromen-2-one [DMB] has been studied by aniline, in a different solvent mixture of benzene (BN) and acetonitrile (AN) at room temperature. The quenching is found to be appreciable and shows positive deviation from linearity in the Stern-Volmer (S-V) plots for all the solvent mixtures. The various rate parameters responsible for fluorescence quenching have been determined using a sphere of action static quenching model and finite sink approximation model. The magnitudes of these rate parameters indicate that positive deviation in the S-V plot is due to both static and dynamic quenching processes.


Introduction
Derivatives of coumarin are well-known for their photochemical and photophysical properties, as well as for their interesting second-order nonlinearities [1]. They are extensively studied due to their widespread industrial use as dye lasers [2]. Now a day, there has been drive to synthesize coumarin-based organic dyes for use in high-efficiency dye-sensitized solar cells (DSCs) [3][4]. Coumarin and its derivatives have attracted significant interest in pharmaceutical research areas such as anti-inflammatory, hepatoprotective, antiviral, anticarcinogenic and anticoagulant activities [5][6].
The fluorescence quenching of organic molecules in solution by various quenchers like aniline, carbon tetrachloride, halides ions, ethyltriocarbonate, CdSe nanoparticles, oxygen etc has been studied by several investigators [7][8][9][10][11][12]. In all most all cases the experimental results follow the linear Stern-Volmer equation given by K SV =k q  0 called S-V constant. Where I 0 , I, k q ,  0 and [Q] are the fluorescence intensities in the absence and presence of the quencher, bimolecular quenching rate parameter, lifetime of the excited solute molecule in the absence of quencher and quencher concentration respectively. But in few cases, it has been observed that the experimental results show positive deviation from a linear S-V relation [13][14][15]. This positive deviation was attribute to various processes like singlet to triplet excitation, static and dynamic quenching, the formation of charge transfer complexes both at the ground and excited states. Apart from this, the polarity of the solvent medium and the range of quencher concentration are also except to play a part in this mechanism.
In the present work, we have studied the steady state fluorescence quenching of the newly synthesised coumarin derivative DMB using aniline as a quencher in a different mixture of benzene and acetonitrile at room temperature. The various rate constants responsible for fluorescence quenching mechanisms have been estimated using a sphere of action static quenching model and a finite sink approximation model.

Materials and methods
The coumarin dye DMB was synthesized according to the literature [16] and its molecular structure is shown in Fig. 1. The solvents were obtained from the SD-fine chemical limited Indian. The quencher aniline was double distilled and tested for its purity before use. The solutions were prepared using benzene and acetonitrile in different proportions by volume with a fixed solute concentration of 1x10 -

Results and discussion
Initially, the fluorescence intensity I 0 was measured without quencher concentration and then the fluorescence intensity I was measured at different quencher concentrations in a fixed solute concentration. Practical values were reproducible within 5% of the experimental errors. The S-V plots I 0 /I versus [Q] for DMB are shown in Fig. 2 were found to be non-linear, shows positive deviation, it may be concluded that the quenching is not purely collisional in other word's it may be due to either the ground state complex formation or static quenching process. The deviations from the linearity of S-V plots were explained using the sphere of action static quenching model. According to this, the static quenching was explained by introducing an additional factor W in the linear S-V plots and the fraction W depends on the quencher concentration [Q]. (2) The graph of [1-(I/I 0 )]/[Q] versus (I/I 0 ) for DMB are shown in Fig. 3, from these graphs we can easily estimate the values of K sv and W by least-square fit method.
Frank and Wawilow [17] have suggested that the instantaneous quenching results at the instances in a randomly distributed system when a quencher happens to reside within a sphere of action with volume of ′ and kinetic distance r.
i.e. ′ =  The dielectric constant of solvent mixtures and various quenching rate parameters calculated from sphere of action static quenching models namely S-V constant (K sv ) bimolecular quenching rate parameters (k q ), range (W), static quenching constant (V) and kinetic distance (r) for DMB in different solvent mixtures are shown in Table 1. The radii of the solute (R S ) and a quencher (R Q ) were estimated by Edward method [18] and are given at the bottom of Table 1, from these values the sum of the molecular radii R (R S + R Q ) is determined, and it is referred as encounter distance. These values are compared with the values of 'r' in order to verify whether the reaction is due to sphere of action static quenching model. From Table 1 we observed that values of kinetic distances r are greater than the encounter distances R. Therefore, according to Andre et al. [19] and Zeng et al. [20] the static effect takes place irrespective of ground state complex formation, provided reactions are limited by diffusion indicating the sphere of action static models holds good.   Table 2. From 0 sv K , it is possible to find out the distance parameter Rˈ. The value of 4πN'DRˈ are estimated using the experimentally determined values of R ˈ and D are shown in Table 2. According to Joshi et al. [22] the bimolecular reactions are said to be diffusion limited if the values of k q are greater than 4πNˈDRˈ. From the Table 2, it can be seen that for all the solvent mixtures the values of k q are greater than 4πN'DRˈ, which is an excepted result for diffusion limited reaction.  Table 2. The various quenching rate parameters calculated from finite sink approximation model.

Conclusions
From the above discussion we observed that (1) The S-V plots show the positive deviation leading to high value of k q it indicates the efficiency of fluorescence quenching. From the above facts, we conclued that quenching phenomenon in DMB is diffusion limited and both static and dynamic quenching processes are playing a role.