Measurement Of Dissociation Constant (Ka) And Partition Coefficient (KP) Of Weak Organic Acids From Their First Hyperpolarizabilities

Abstract

Weak organic acids and their salts are slowly becoming recognized as an important class of materials for second-order nonlinear optics (NLO) due to their high second harmonic generation (SHG). The main advantages in using weak organic acids and their salts for NLO are:

The dipolar interaction that provides a strong driving force for centrosymmetric crystallization in neutral compounds is countered by the Coulombic interactions, thus favoring noncentrosymmetric space group with good SHG efficiency, e.g., p-hydroxy benzoic acid (1.2 × U), sulfamic acid (1.5 × U), and 4-(4'-aminophenyl) azobenzoic acid (1 × U).

Substituted benzoic and cinnamic acids have high β in spite of the cut-off wavelength being shorter than 400 nm, which makes these materials potentially useful in developing diode lasers (820 nm).

Large second-order susceptibility (χ(2) = 50 pm/V, for sulfophthalein dyes).

In order to investigate the microscopic hyperpolarizability (β) of these ionic molecules in solution, only the hyper-Rayleigh scattering (HRS) technique can be employed. In this thesis I demonstrate that the HRS technique, which is normally used for the measurement of the first hyperpolarizability (β) of neutral, ionic and octupolar molecules, can also be used to obtain dissociation constant (Ka) of a weak organic acid in different protic solvents and partition coefficient (kp) between two immiscible solvents. A general introduction for nonlinear optics along with the various techniques to probe microscopic second-order nonlinearity is presented in the first chapter. At the end of the chapter, the importance of the present work is highlighted. The experimental set-up for the HRS technique has been described in detail in chapter 2. Also the theoretical analysis necessary to obtain β from the measured signal has been discussed. Various computational schemes for calculation of β's in ionic and neutral molecules in the gas phase as well as in the presence of solvent have been highlighted in chapter 3. Methods to compute static (in the absence of a frequency-dependent field) and dynamic (in the presence of laser light) β's have also been presented. In chapter 4, first hyperpolarizability of para-, meta- and ortho-substituted benzoic acids is described. The magnitude of the measured value of ⟨β⟩ (⟨βz⟩) varies between two extreme limits: the molecular hyperpolarizability of the acid form (βa) at the lower and the basic form (βb) at the higher sides. The degree of dissociation (α) of the acid in a solvent is related to the measured hyperpolarizability, βm, by the equation. Theoretical estimate of β by Austin Model 1 (AM1)/Finite Field (FF), ab initio/self-consistent reaction field (SCRF)/FF and the intermediate neglect of differential overlap package of Zerner employing the correction vector (ZINDO/CV)/SCRF are also compared with the experimental values. The agreement between the ZINDO values and experimental β is within ±10%. From the measured βa, βb and βm values, we have calculated dissociation constant (Ka = α²C/(1-α)) of the benzoic acids by using equation (1). Measured β's for ortho-, meta- and para-substituted benzoic acids are compared with the values reported by other methods in the literature. Measurement of dissociation constant of substituted cinnamic acids has been described in chapter 5. Ka's have been measured by three different methods, namely the HRS, spectroscopic and potentiometric methods. The values obtained by these three methods are critically examined. The advantages and drawbacks of these various methods for equilibrium constant determination are also discussed. First hyperpolarizability of various sulfophthalein dyes has been discussed in chapter 6. Since these dyes can exist in three different forms (neutral, singly charged acidic and doubly charged basic forms), we have measured βa and βb only for the monosodium salts (singly charged form). Dissociation constants have been measured for all the sodium salts of the dyes and compared with the literature values (if available) or values measured by the absorption spectroscopic technique. In chapter 7, we demonstrate that the HRS technique can be employed to measure the partition coefficient (kp) of substituted benzoic acids in a mixture of two immiscible solvents [water/toluene (1:1 v/v) and water/chloroform (1:1 v/v)]. Ka values are compared with the values measured earlier by other techniques. The advantages offered by the HRS technique for partition coefficient measurement are also discussed.