On the orientation of substituents in N-methyl- and N, N-dimethyl- anilines in the excited state Narasimha H Ayachit & G. Neeraja Rani Department of Physics, SDM College of Engineering & Technology, Dharwad, Karnataka, India.

ABSTRACT  The orientation of amino group in aniline was considered to change to change to 180 degrees in the excited state from 38 degrees in the ground state by Suppan, to determine the excited electric dipole moment using solvatochromic shifts. Ayachit freed the problem of assumption of considering the orientation in the excited state. This method led to not only to the better calculation of excited state dipole moments also gave a novel method of calculating the orientation of substituents in mono substituted benzenes in their excited states. This method was shown to work successfully in some other cases In this paper the method has been applied to N-methyl- and N, N- dimethyl- anilines and the orientation of the substituents in the excited state seems to follow the same changes as in amino group in aniline.

Introduction  The excited state electric dipole moment estimation of molecules has gained importance in view of insight it gives in the optical aspects.  Aniline is one interesting system as it being a molecular model for aromatic amines, which are of great industrial importance in the pharmaceutical and other chemical industries which depend on the photochemical and photo physical aspects in the excited states.  Extensive studies which are both experimental and theoretical have focused on the aspect of electronic and vibrational properties in the ground and excited states as interaction between the ring and the amino group in aniline causes significant geometric change upon electro excitation. ……Continued

  The orientation of amino group makes an angle of 38 degrees. This angle is assumed to change to 180 degrees upon excitation affecting the symmetry of the molecule.   There are several studies on the first excited state of aniline and the corresponding state in its derivatives involving stark effect/microwave/electron diffraction/rotational band contour analysis, which speak, of change in the orientation of amino group.   ab initio calculations also predict the above change in the orientation.   The interpretation of electronic - vibrational spectra of the aniline and it derivates have been comfortable with assuming large change of structure on excitation. Similar attempt is also made in phenylisocynate. ……… Continued

  All these studies aim at determining the excited state electric dipole moment and are complicated and difficult. Of all the methods the solvatochromic shift method is found to be simplest. The orientation of amino group in aniline was considered to change to 180 degrees in the excited state from 38 degrees in the ground state by Suppan, to determine the excited electric dipole moment using solvatochromic shifts.   However, in the present paper a simple method proposed by the author is presented which determines the orientation of the dipole moment in the excited state using the solvatochromic shifts in fluorescence and absorption.   The method, which was applied to first excited state of aniline, is extended to substituents N-methyl- and N, N- dimethyl- anilines in the corresponding excited states. The results are presented.

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. ν s = ν 0 + (A+B+C)f(n s ) + E[f(D s ) - f(n s )] + F[f(D s ) - f(n s )] 2

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. Further by taking solvents of nearly same refactive index, which are slightly polar, the following equations are formulated.

The equation cited below for the difference in frequency in absorption for two different solvents 1 and 2, which is -Δν 1-2 (abs) = [(μ g.Δ μ g-e )/ hca 0 3 ] Δ [f(D s )] (1) The above equation for fluorescence will be, (with ae being the cavity radius in the excited state), -Δν 1-2 (flu) = [(μ e.Δ μ g-e )/ hca e 3 ] Δ [f(D s )] (2)

By defining ζ and Φ, as the orientation of dipole moments of a molecule in its ground and excited states respectively with respect to the direction of charge transfer and as defined in the following figure 1, θ is the angle between two dipole moments.

When a molecule gets electronically excited (by having different dipole moment and orientation), the electron will be raised to a new electronic level in much less time than it takes for the whole molecule to rearrange itself with solvent environment. Thus, immediately upon excitation the molecule can be assumed to be in the same environment in the excited state as in the ground state and hence one can safely assume a e = a 0. In the present work the values of μ e are calculated by assuming a e = a 0.

From the equations (1) and (2) the relation between θ,ζ and Φ can be obtained as Φ = 180 – tan-1 {[- Δν 1-2 (abs) / Δν 1-2 (flu)] tan (180 – ζ)} and θ = Φ - ζ If ζ is known θ and Φ can be calculated.

Application: In this paper the method has been applied to N-methyl- and N, N-dimethyl- anilines ( by taking solvent shifts in absorption and fluorescence available in literature) and the orientation of the substituents in the excited state seems to follow the same changes as in amino group in aniline which is evident from the following table.

MoleculeΔν 1-2 (abs) / Δν 1-2 (flu) ζ in degrees Φ in degrees Aniline N-methyl- aniline N,N- dimethyl- aniline

References P.Suppan, Chemical Physics Letters 94, 272(1983). 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). P. Suppan and G. Guerry Butty, Journal of Luminescence, 33, 335(1985).

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. Thank you