Excited state electric dipole moment of two substituted indoles through solvatochromic shifts G. Neeraja Rani & Narasimha H.Ayachit Department of Physics, SDM College of Engineering & Technology, Dharwad, Karnataka, India.

ABSTRACT The determination of excited state electric dipole moment through solvatochromic shifts is one of the easiest approaches to understand the molecular structure in the excited state. These studies have gained importance due to their application in photo science, especially if they are of biological importance. In view of this the excited state electric dipole moments of two substituted indoles which are of biological importance are determined and reported here. The fluorescence shifts have been used and the results found seems to be more consistent in comparison with the one calculated through absorption shifts. The results presented are also discussed. A qualitative estimate of the orientation of the dipole moments in ground and excited state are also presented and discussed. Of the several methods proposed, the one proposed from N.H.Ayachit, N.H.Ayachit et al and N.H.Ayachit & G.Neeraja Rani is used in view of the several advantages it has.

Introduction The use of solvatochromic shifts for the determination of the dipole moment of electronically excited state of a molecule has gained importance for an understanding of their structure in that state due to the fast progress in photo science. The different methods proposed using solvatochromic shifts in most cases are found to be the modification of McRae’s equation. The solvatochromic shift method in general although not gives that accurate values of the dipole moments of a molecule, they give fairly acceptable values from the use of proper method like proper choice of solvents.

The dipole moment values obtained using the solvatochromic shifts can improve by the proper choice of solvents and a method, for example, use of slightly polar solvents, which are having almost same refractive indices, and use of fluorescence shifts rather than absorption data wherever available. The fluorescence data in different solvents is expected to give better estimates of dipole moments in the excited state than the absorption data in view of the fact that fluorescence shifts are larger compared to those in absorption. The improved estimate of the dipole moment means a better understanding of the electron charge distribution, which plays an important role in giving insight into photochemical reaction.

With other methods failing especially in case of large molecules solvatochromic shift method is only method for the estimation of excited state electric dipole moments in the excited states of a molecule. Further, the use of a particular method to determine the dipole moment in the excited state, out of the several methods available which use solvatochromic shifts is usually decided by looking into the available data of solute molecule in its ground state, like Onsager cavity radius, orientation of the dipole moment in its ground state etc and capability to give fluorescence/absorption. In the case where the orientation of dipole moment with respect to its symmetry axis is difficult to comprehend and with methods available to estimate Onsager cavity radius, the method proposed by Ayachit et. al. seems to be the better one.

The estimation of the excited state electric dipole moments of substituted indoles have found to be of great importance due to the biological relevance they have. The biological activity of compounds mainly dependent on their molecular structure, the insight about which one can have from their ground and excited state dipole moments. Thus a better estimation of dipole moment means a better understanding of molecular structure. Sharma et al have estimated the excited electric dipole moment of some substituted indoles using combination of fluorescence and absorption data and without taking into account the change in the orientation of dipole moment on excitation. In the present paper the method proposed by Ayachit et al is employed for both absorption shifts and emission shifts separately and the results are presented and discussed.

Theory and procedure The displacement of electronic absorption and luminescence spectra are related to solvent interaction. These interactions can be non-specific, when they depend only on multiple and polarizability properties of solute and solvent molecules. When a liquid solvent surrounds a molecule, each state of it is stabilized by an energy known as salvation energy. The medium i.e solvent can affect the solute molecule by its viscosity and also by its polarity. The number of electronic states which are defined for a molecule are determined through Schrodinger wave equation Eψ =Hψ, where ψ being the wave function. The wave function gives the information regarding the nature of a state. By employing the quantum mechanical second order perturbation theory and using Onsager model, McRae has given an equation for the frequency of maximum solute absorption / fluorescence in a solvent, ν s and is as below.

a s = ν 0 + (A+B+C)f(n s ) + E[f(D s ) - f(n s )] + F[f(D s ) - f(n s )] (1) Where ν 0 is the corresponding vapor phase frequency of ν s, f(n s ) = ( n s 2 -1)/ (2n s 2 +1) is a function of solvent refractive index (n s ), f(D s ) = (D s – 1)/( 2D s + 1) is a function of solvent dielectric medium (D s ), (A+B) is a measure of dispersive effect, and h and c are Planck’s constant and velocity of light respectively. C is either (μ e 2 – μ g 2 )/ hca 0 3 or (μ g 2 – μ e 2 )/ hca 0 3 depending on whether ν s is measured through absorption spectra or emission spectra respectively. Similarly, E is either (μ g.Δ μ g-e )/ hca 0 3 or (μ e.Δ μ e-g )/ hca 0 3 depending upon the spectra under study, with μ e, μ g being dipole moment in the excited state and ground state respectively with Δ μ e-g = μ e - μ g and a 0 is the Onsager cavity radius. The last term in equation (1) and (A+B) can be neglected due to their small contribution towards shift.

With this, Suppan formulated the equation cited below for the difference in frequency in absorption for two different solvents 1and 2, which is -Δν 1-2 = [(μ g.Δ μ g-e )/ hca 0 3] Δ [f(D s ) - f(n s )] [(μ e 2 – μ g 2 )/ hca 0 3 ] Δf(n s ) 1-2 The above equation for fluorescence will be, (with ae being the cavity radius in the excited state), -Δν 1-2 = [(μ e.Δ μ g-e )/ hca e 3 ] Δ [f(D s ) - f(n s )] [(μ g 2 – μ e 2 )/ hca e 3 ] Δf(n s ) 1-2 The above equation was written, by Ayachit et.al in the form,

[(X / C 1 ) + ( Y / C 2 ) ] = (2), Here, X = [ Δ [f(D s ) - f(n s )] 1-2 ] / -Δν 1-2 and Y = [ Δf(n s ) 1-2 ] / -Δν 1-2, with C 1 = hca 0 3 /(μ g.Δ μ g-e ) and C 2 = hca 0 3 / (μe 2 – μg 2 ) in case of absorption studies and C 1 = hca e 3 / (μ e.Δ μ g-e ) and C 2 = hca e 3 / (μ g 2 – μ e 2 ) in fluorescence studies.

The values of C 1 and C 2 can be obtained using either a graphical method or a least square fit. The values of X and Y are of the order of exp(-4) and hence it forces to plot the graph of quantities which are greater by a factor exp(-4), which may lead to points being scattered leading erroneous values of C 1 and C 2. Hence in the present study C 1 and C 2 have been calculated using a least squares fit. From the values of C 1 and C 2 μ e and the angle between μ e and μ g namely θ can be determined by writing μ e.Δ μ g-e = μ e μ g cos θ- (μ e )².

Result and discussion. The values of excited state electric dipolemoments determined for the two indoles in the present work using absorption shifts and fluorescence shifts are presented in table 1 along with the available dipolemoment values in ground state and excited state as reported by Sharma et al. In this table are also given the Onsagar cavity radius.

Table 1 Results for 5-Hydroxy indole and 5-Hydroxy indole 3-Acetic Acid (rep) Moleculeμ *μ *μ e (abs)*μ e (fluo)*Θ(fluo)**μe*μe* 5HI HI3AA *in D ** in Degree s

From the table it can be seen that the values of excited state dipolemoments are much less than the reported earlier however greater than the ground state dipolemoments of the molecules. The larger values observed in the earlier work may because of the fact that

The solvents of very high dielectric constants have been used in the earlier work and hence neglecting the higher terms or considering them as constant may not be suitable. When high dielectric constant solvents are used non specific interactions come in to effect whose contribution can not be neglected. Orientation of the dipolemoment has to be taken in to account. Not much reliable absorption data are used. In view of the above the dipolemoments determined in the present work seems to be reliable representing a Π * Π transition. In the present work also it may be seen that θ could not be determined using absorption data are clear indication towards the following observations.

Emission spectra are usually more informative than the absorption spectra as emission state arise out of energy of more relaxed excited states. In view of this the quantities determined using solvatochromic shifts in fluorescence become more important for the description of the intramolecular charge transfer in molecular excited states. This is evident from the fact that, the shift in fluorescence spectrum is not necessarily accompanied by a shift in the absorption spectra, if exits it need not be in proportion to the one observed in fluorescence. There are examples for shifts in fluorescence to occur with no shifts in the absorption spectrum of a solute.

Since the excited state of a molecule is much different from its ground state, it may be more or less depending upon the transition involved, the ground state dipole moment may slightly change from solvent to solvent, but the excited state may be much different. Thus it is expected that little changes in absorption while large in the fluorescence spectrum with change in solvent. In the different equations used for the determination of excited state dipole moments one or more assumptions are made in obtaining the simplified equation from the original McRae’s equation. The estimations carried out using absorption shifts will thus not be that reliable compared with that calculated with emission spectra. The shifts observed are dependent on the nature of solvent (polar or non-polar), dielectric constant etc.

Acknowledgement Authors are thankful to the management of SDM College of Engineering and Technology, Dharwad, Karnataka, India, for their constant encouragement and financial support and also TEQUIP of ministry of HRD, Govt. of India, India.

References: N.H.Ayachit, Chemical Physics Letters 164, 272(1989). N.H.Ayachit, D.K.Deshpande, M.A. Shashidhar & K. Suryanarayana Rao, Spectrochimica Acta, 42A, 585, 1405(1986). N.H.Ayachit & G.Neeraja Rani, Physics and Chemistry of Liquids, 45, 41(2007). Neera Sharma, Sapan K.Jain, Ramesh C.Rastogi, Spectrochimica Acta Part A 66(2007)

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