| dc.description.abstract | The concept of molecular switches has garnered enormous interest because of the vital role of these switches as components in molecular electronic/photonic devices, signal transmission and optical communication. Compounds possessing switchable(on/off) nonlinear optical (NLO) properties can serve as important building blocks in the formation of molecular devices in the optoelectronic field. Alteration of the electronic charge distribution in the components of an NLO-active molecule can substantially change the second order NLO response (which is quantified by first hyperpolarizability, β) and can be exploited for constructing molecular switches. For designing an effective switch, a molecule must be stable in two or more forms and must exhibit significantly contrasting β in each form. Among various approaches that use external stimuli to change the electronic properties and thereby β, the one that utilizes the reversible redox properties of metal atoms/ligands in metallorganic complexes is preferred, due to minimal structural distortion. Multivalent (MV) metals can be stabilized in different oxidation states and their donor/acceptor properties are changed accordingly. This in turn is expected to change β significantly. In this thesis, I have investigated the redox switching of β of various metallorganic complexes containing Ru, Re and Fe centers by in-situ second harmonic light scattering (SHLS). The experimental results have been rationalized by DFT and TDDFT calculations of linear and nonlinear electronic properties.
In chapter 1, I have introduced the second order nonlinear optical response of metallorganic complexes, various factors influencing β and different strategies used to increase the β value of molecules and materials. The basic principle of SHLS experiments has also been described
in this chapter. The scope and motivation of this work have been presented at the end of this chapter. In the next chapter the experimental details of the SHLS technique used for the measurement, the techniques used for the characterization of the linear optical properties of the complexes and the method for theoretical calculations have been discussed. In chapter 3, the reversible redox switching of first hyperpolarizability of a mono metallic and a bimetallic ruthenium complexes has been described. I have synthesized [Ru(acac)2(CH3CN)2] and [(acac)2Ru-bptz-Ru(acac)2] complexes and measured their first hyperpolarizabilities as a function of in-situ electrochemical oxidation/reduction of the Ru metal centers. As a result of the oxidation of ruthenium from Ru(II) to Ru(III), the molecular hyperpolarizability of both the mono- and bi-metallic complexes increased. The mixed-valence bimetallic complex exhibits a high β value which has been ascribed to the increase in the change in dipole moments associated with an intervalence charge transfer (IVCT) transition. I have demonstrated that the change in hyperpolarizability is reversible with respect to the oxidation state of the metal centers and both the complexes are stable for several cycles of redox switching. DFT and TDDFT calculations supporting the experimental data are also presented.
In chapter 4, ligand (1,2-pyridinecarboxaldehydeazine, pyca-pyca) based redox switching of β of monometallic, Re(CO)3(pyca–pyca)Cl and bimetallic, Re(CO)3Cl(pyca–pyca)Re(CO)3Cl rhenium complexes have been studied in the following chapter. Ligand centered one electron reduction led to increase in β for both the rhenium complexes. Among all the complexes, the highest value of β has been realized for the two electron reduced bimetallic rhenium complex due to the presence of an intra ligand charge transfer (ILCT) with large change in dipole moment and two-photon resonance enhancement due to the overlap between metal to ligand charge transfer transition (MLCT) and SH wavelength. The DFT and TDDFT calculations were able to reproduce the experimentally observed trend in β. As all the electrochemical
processes are reversible and the complexes are stable, I achieved electrochemical switching of SH signal for multiple cycles.
In the 5th chapter, I have presented the effect of oxidation potential on β of di and tetraferocenyl complexes. The asymmetrically substituted diferrocenyl thiophene complex show a modest β value whereas the symmetrical tetraferrocenyl complex exhibits extremely low β value. The stepwise oxidation of Fe+2 to Fe+3 didn’t increase the β value significantly in the bimetallic complex. Although the electrochemical oxidation and reduction processes are reversible as seen in the cyclic voltammetry experiments, I was not able to obtain the redox switching of β due to small differences in β values of the immediate oxidized/reduced complexes.
Next, in chapter 6, the β values of two monometallic and a trimetallic Ru complexes with respect to the oxidation states of the metal centers were investigated. The trimetallic complex was synthesized by coupling two different monometallic units in 1:2 ratio through cyanopyridine ligands. The oxidation of the metal center from +2 to +3 in the monometallic complexes increased their β values with respect to the unoxidized complexes due to an increase in the change in dipole moments between the ground and first excited states. Both monometallic compounds showed switching of β with respect to the oxidation/reduction potential. In case of the trinuclear complex, one of the MV complex exhibited a large β value due to large change in dipole moment associated with IVCT charge transition in the NIR region. However, due to the lack of resolution between two sequential redox processes, electrochemical switching of β was inconclusive. The final chapter of the thesis is the concluding chapter in which broad conclusions based on the work done in this thesis have been drawn and possible future directions based on the conclusions drawn are proposed | en_US |
