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Title:  Quadratic Optical Nonlinearity And Geometry Of 1:1 Electron Donor Acceptor Complexes In Solution 
Authors:  Ghosh, Sampa 
Advisors:  Das, Puspendu K 
Keywords:  Molecular Structure Optical Nonlinearity Molecular Complexes  Geometry Electron Donor Acceptor (EDA) Complexes HyperRayleigh Scattering Electron Donor Acceptor Complexes  Hyperpolarizability EDA Complexes  Geometry 1:1 Complexes Quadratic Nonlinearity 
Submitted Date:  1Jun2008 
Series/Report no.:  G22343 
Abstract:  The knowledge of geometry of molecular complexes formed via molecular association in solution through weak interactions is always important to understand the origin of stability and function of an array of molecules, supramolecular assemblies, and macromolecular networks. Simple 1:1 molecular complexes are very useful in this regard as they provide a model to understand both the nature of these interactions and their structural implications. Several weak noncovalent forces from long range (van der Waal’s, electrostatic, induction, dispersion) to short range (charge transfer) govern the geometry, that is, relative orientation of the two molecules in such a complex. On one hand, we find 1:1 electron donor acceptor (EDA) complexes such as naphthalenetetracyanobenzene, hexamethylbenzenechloranil etc. which stack parallel or in slipped parallel geometry in their crystals. On the other, benzene dimer has been found to stabilize in T shaped geometry in all its three physical states. In this thesis, I focus on 1:1 EDA complexes in solution. A good volume of literature is available which deals with the optical studies on the formation of such complexes. It has been suggested that the nature of the intermolecular interactions stabilizing these complexes in the gas phase or in their crystals is modified by the presence of solventsolute interactions in solution thus bringing in difference in the solution geometry. However, the existing experimental techniques, both optical and magnetic, are unable to determine the exact geometries of 1:1 EDA complexes in solution. This opens an opportunity to probe their geometry in solution.
The quadratic nonlinearity or first hyperpolarizability (β) of a molecule is a measure of the change in dipole moment (or polarization) in the second order of the applied electrical field and thus has a purely electronic origin. It is a tensorial property and can be resolved in components along the three dimensions. The number of β components and the nonlinear optical anisotropies in a typical donoracceptor type dipolar molecule, defined as (equation) (where1, 2, 3 axes define the molecular frame, 1 being the direction along the principal axis of symmetry and pointing from the acceptor toward the donor), are determined by the symmetry /structure of the molecule. It has been shown theoretically that the 1:1 EDA complexes possess large hyperpolarizabilities. In the case of pNA dimers calculation revealed that the geometry of the dimer and its symmetry is important for obtaining the correct estimate of β from its tensorial components. Therefore, it should be possible to use the values of tensorial β components to construct the unknown geometry of such complexes. Experimentally macroscopic depolarization ratios (D and D′) in the laboratory fixed frame (XYZ, X being the direction of polarization and Z the direction of propagation of the incident light), are measured from the polarization resolved intensities of second harmonic scattering from molecules in solution using the hyperRayleigh scattering technique. The depolarization ratios are correlated to the anisotropy parameters, u and v through a coordinate transformation. In this thesis I, have first, characterized the quadratic nonlinear optical property of a variety of 1:1 electron donor acceptor complexes and used the values of u and v obtained from depolarized hyperRayleigh scattering to deduce their geometry in solution.
Chapter 1 provides an introduction to the 1:1 electron donor acceptor complexes, their relevance to chemistry and biology. It also contains an introduction to nonlinear optical processes in molecules. The objective of the present work and scope of the investigation carried out in this thesis is presented in this chapter.
Chapter 2 describes the details of the experimental polarization resolved HRS technique. The geometrical model adopted for the analysis of the HRS data has also been introduced and the method of analysis has been described in detail in this chapter.
Chapter 3 presents the measurement of β values of two series of 1:1 EDA complexes of variously substituted methylbenzenes donors with tetrachloropbenzoquinone (CHL) and dicyanodichloropbenzoquinone (DDQ) acceptors at 1064 nm. In agreement with recent theoretical results we find large first hyperpolarizabilities for these complexes. The β values are greater than that of the typical pushpull molecule pnitroaniline (pNA). We also find that in general β decreases with decrease in the donor strength.
Chapter 4 presents the β values for the two series of EDA complexes of CHL and DDQ acceptors at 1907 nm. The values of β are less in magnitude at 1907 nm than that at 1064 nm which is due to the dispersion effect in β.
In Chapter 5 and 6, it is described how depolarized hyperRayleigh scattering can be utilized to probe geometries of 1:1 complexes in solution. Chapter 5 concentrates mainly on 1:1 EDA complexes of CHL and DDQ and TCNB (tetracyanobenzene), while chapter 6 contains examples of other 1:1 molecular complexes where the noncovalent interactions are much weaker, such as in benzenenaphthalene, benzenemethoxybenzene, benzenehexafluorobenzene and benzenechlorobenzene pairs. We find the geometry of 1:1 EDA complexes in solution in terms of tilt angle (θ) and twist angle (ϕ) between the donor and acceptor pairs. The angle θ varies from 29°47° for different pairs of EDA complexes, while ϕ varies within 34° and 38°. We find that the geometry of 1:1 EDA complexes in solution is different (twisted and tilted cofacial and twisted ‘V’) from those in the crystalline or gaseous states (cofacial), if known. We find that both benzenenaphthalene and benzenechlorobenzene pairs assume twisted ‘T’ shape geometry with θ = 82° and 85°, respectively, and φ = 38°, while benzenehexafluorobenzene assumes a twisted ‘V’ shape. A strong solvent effect is seen in the geometry of the benzene methoxybenzene complex. The tilt angle is 55° when chloroform is used as a solvent and it is 82° without chloroform.
Chapter 7 is the concluding chapter where the main work done in this thesis is summarized and future directions are presented. 
URI:  http://hdl.handle.net/2005/711 
Appears in Collections:  Inorganic and Physical Chemistry (ipc)

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