Solvatochromism and Preferential Solvation of D-Π-A
Push-Pull Dipolar Solutes in Pure and Binary Mixture of
Solvents: Understanding the Role of Specific
Solute-Solvent Interactions

Abstract

In view of the routine use of solvent and solvent mixtures in many chemical and industrial processes, considerable research has been carried out to better understand the chemistry of solute-solvent interaction. Investigations on solute-solvent interactions as well as properties of solvents are commonly performed using solvatochromic probes. Among several solvatochromic probes, compounds with Donor-Pi-Acceptor (D-π-A) push-pull molecular systems with inherent intramolecular charge transfer (ICT) character are gaining wide interest. Study of solvatochromism sheds light on the molecular interaction between the species of interest or the probe (i.e. reactant, activated complex or product in chemical reactions and enzyme, drug or other substrate in biological processes) with the solvent. However in mixed solvent systems, it is difficult to make predictions regarding molecular interaction between the species of interest (solute) and solvents based on the properties of pure solvents. Rather complex behavior is observed as a function of liquid composition owing to the possibility of preferential solvation (PS) when the local composition of solvent mixtures in the immediate neighborhood of the solute (solvation shell or cybotactic region) differs from that of the bulk composition. In practice, experiments are usually required to characterize a particular probe in a particular medium for a specific purpose.
Owing to the importance of Donor-Pi-Acceptor (D-π-A) push-pull molecular systems and the fact that structurally different probes can unveil different aspects of solute-solvent and solvent-solvent interactions in pure and mixed solvent systems, the present dissertation entitled, “Solvatochromism and preferential solvation of D-π-A push-pull dipolar solutes in pure and binary mixture of solvents: Understanding the role of specific solute-solvent interactions” aimed at employing different simple solvatochromic D-π-A push-pull dipolar probes to unveil complex specific and non-specific solute-solvent interaction in pure and binary mixtures of solvents. In this thesis attempt has also been made to propose possible location of solvent components in binary solvent mixtures (BSMs) based on experimental studies as well as theoretical calculations. Four different solvatochromic probes namely p-nitroaniline (PNA), 4- (p-Nitrophenylazo) phenol (NPAP), 4-(2-nitrobenzylideneamino) phenol (4-NBAP) and 4-(4- nitrobenzylideneamino) phenol (2-NBAP) are used for the purpose. All these D-π-A probes contain either an amino (–NH2) group or a hydroxyl (-OH) group as donor and nitro (-NO2) group as acceptor moiety and thus have characteristic strong ICT transition. These specific donor and acceptor groups are also chosen due to their inherent characteristic as HBD and HBA properties respectively, which can orient the solvent molecules at specific sites through solute-solvent hydrogen bonding (H-Bonding) interaction.
To achieve the objectives, (i)solvatochromism of these probes is studied in a range of pure solvents (nonpolar, dipolar aprotic and polar protic of varying polarity) (ii) combine influence of diverse solvent properties on the solvatochromism and solvation pattern of these probes are examined by quantitative linear solvation energy relationship (LSER) (iii) preferential solvation of these probes is investigated in binary solvent mixtures comprising of HBA and HBD solvents using preferential solvation models namely the model of Suppan, the Bagchi-Chatterjee model and the model of Bosch and Roses (iv) strength of H-bonding on the extent of PS is investigated using deuterated chloroform in the PS of these probes in CHCl3-dioxane BSMs (v) possible orientation of solvent molecules at specific sites through solute-solvent hydrogen bonding (H-Bonding), dipole-dipole and apolar (hydrophobic) interaction is proposed based on experimental studies as well as theoretical calculations.
The present thesis is divided into six chapters.
Chapter 1 contains a brief overview about the solvation and preferential solvation of solvatochromic probes in pure and mixed solvent systems. It also focuses on various methods used to study preferential solvation of dipolar solutes. Solvation models used to describe the event of preferential solvation in binary mixtures of solvents. Scope of the work along with the objective of the present thesis has also been presented in this chapter.
Chapter 2 deals with the detailed description of materials and methods used in the present research work.
Chapter 3 describes the investigation on solvatochromism of PNA in pure and binary mixtures of solvents. The solvatochromism as well as the event of preferential is explained on the basis of the interaction of PNA with the solvent molecules by means of both specific and non-specific solute-solvent interaction. Multiparametric linear regression approach is used for quantitative description of contribution of different solvent properties towards the observed effect. Location specific preferential solvation through solute-solvent hydrogen bonding is proposed.
Chapter 4 depicts the investigation on solvatochromism and preferential solvation of 4- (p-Nitrophenylazo) phenol (NPAP) azo dye in neat and binary mixtures of solvents. Hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) solvent pairs such as chloroform with different dipolar aprotic solvents and dioxane with polar protic solvents are used to describe the preferential solvation pattern and location of solvent molecules in the solvation of sphere. Solute-solvent and solvent-solvent hydrogen bonding being responsible for the preferential solvation of NPAP is explored.
Chapter 5 gives an account on reversal in solvatochromism in the solvation of 4-(2-nitrobenzylideneamino) phenol (2-NBAP) and 4-(4- nitrobenzylideneamino) phenol (4-NBAP) Schiff Bases. Preferential solvation of 4-NBAP studied in chloroform-HBA binary solvent mixtures using Suppan’s model of dielectric enrichment, Bagchi-Chatterjee model and the model of Bosch and Rosés confirms strong solute-solvent and solvent-solvent hydrogen bonding. Location specific preferential solvation is proposed based on other experimental evidences and theoretical calculations.
Chapter 6 presents a brief summary and the future scope of the present research work.
The results presented herein should constitute an invaluable addition to explain solvatochromism of D-π-A push-pull dipolar systems in pure and mixed solvent systems, and can be applied in explaining solute-solvent interaction and solvent organization in complex chemical systems with multiple functional groups and can find applications in explaining intricate molecular events such as enzymatic reactions, absorption of drugs and nutrients at target sites, molecular recognition, host-guest interactions, analysis and optimization of analytical separations. A good understanding of these factors can lead to the design of D-π-A push-pull molecules with targeted properties for application in specific areas