Synthesis and investigation of spectroscopic properties and excited state electron transfer reactions of Pt(II) NCN type complexes
The world energy needs are always increasing, and an over-intensive use of fossil fuels has caused deleterious problems such as global warming. Maximum efforts need to be put in studying the ways to harness energy from clean renewable energy resources. Platinum NCN complexes are known to possess very high emission quantum yields with very high triplet lifetimes in the range of 2-6 s in deaerated organic solvents. Most of the studies pertaining to the photochemistry of Platinum NCN molecules have been done in the field of OLEDs. The goal of this research was to study their excited state electron transfer reactions. The focus of this dissertation is 3-fold: (1) Synthesis of 1,3-dipyridylbenzene (NCN) ligands. (2) Synthesis of platinum(II) NCN complexes and study of their spectroscopic and electrochemical properties. (3) Investigation of photoinduced electron transfer reactions of Platinum NCN complexes. The first chapter is an introduction to the past and the current energy scenario. World energy needs, consumption and availability is discussed. Further, the role of fossil fuels as a primary energy source, their effects on nature and the importance of the use of renewable energy sources such as solar energy is discussed. This leads to the discussion of the basics of photochemistry and the applications of some of the principles will be discussed as necessary in the following chapters. Chapter 2 focuses on the Platinum complex chemistry and the syntheses of bispyridylbenzene ligands and the Platinum NCN complexes. In the introduction, properties of the platinum metal center are discussed. Square planar complexes of platinum with different type of bidentate and tridentate ligands are shown and their photophysical properties are compared. The axial interactions of some of the square planar platinumcomplexes are also discussed. Further, Platinum NCN complex chemistry is discussed in detail, highlighting the awesome photophysical properties of these type of complexes. This is followed by a detailed discussion of the synthesis of the NCN ligands using a [3+3]-type condensation reaction of O-acetyl oximes and ,-unsaturated aldehydes through synergistic and the platinum complexes. Pure ligands were synthesized in good yields and were characterized extensively using various NMR methods. The platinum complexes were synthesized and characterized by NMR and mass spectrometry. Crystal structures of 2 platinum complexes were obtained. First half of chapter 3 focuses on the spectroscopic and electrochemical properties of the synthesized Platinum NCN complexes. The studies involved measurement of spectroscopic properties, redox potentials and spectroelectrochemical studies of the platinum complexes. All the complexes are intensely luminescent with emission quantum yields ranging from 10 -60%,, and triplet emission lifetimes ranging from 1-6 s in deaerated chloroform and 200 ns - 20 s in de-aerated N,N’-Dimethylformamide (DMF). All the complexes display 2 reversible reductions between -1.5 to -2 V vs Ag/AgCl. Electroanalytical investigations of the oxidation of square planar Pt(II) complexes, including NCN complexes, generally exhibit chemical irreversibility unless one or more coordinated ligands have reversible one electron oxidation. This leads to the discussion of the next part of chapter 3 which is determination of 1 electron oxidation potentials of these platinum NCN complexes on a faster time-scale using the Rehm - Weller approach. Oxidation potentials of PtPhNCNCl (PhNCN = 1,3-di(4-phenyl-2-pyridyl)benzene) and PtDMAPhNCNCl (DMAPh=1,3 di(4-(4-dimethylamino)phenyl-2-pyridyl)benzene) were determined using this approach. Using methyl viologen as a quencher, transient absorptionexperiments were done on PtPhNCNCl and PtDMAPhNCNCl which facilitated determination of the 1 electron oxidized spectra of these complexes. It was found out that the PtPhNCNCl complex generates hydrogen if photolyzed in the presence of a reductive quencher and water. Chapter 4 involves hydrogen generation studies by photolyzing PtPhNCNCl complex in presence of triethylamine as the electron donating quencher and water as the proton donor in DMF. Photolysis was performed using 3 light sources: A PTI spectrofluorometer, and home-built Blue and UV LED cylinders. The PtPhNCNCl acts as a catalyst and a chromophore and, as such, it is a single component chromophore-catalyst system. After 2 hours of photolysis on the blue LED photoreactor with a photon flux of 4.4*1018 quanta/min, the turnover number is 22 per mole of catalyst and 45 mmoles of hydrogen were generated with a quantum yield of 4.6 %. The UV LED cylinder which facilitated higher number of excited states, resulted in the photodecomposition of the complex after 10 minutes of irradiation. A mechanism was proposed, whereby a 1 electron reduced species is generated followed by the protonation of that species. Control studies were performed. Various experiments were performed to study the mechanism and prove it. Transient absorption spectroscopy along with spectroelectrochemistry was performed to investigate the claims about the species formed during photolysis. Using transient absorption spectroscopy, spectral signature of a 1 electron reduced species was observed which was supported by results of spectroelectrochemical studies. Time dependent -DFT calculations were done on the one electron reduced PtPhNCNCl. No conclusive evidence of the type species formed after 1 electron reduction was found experimentally. This could be a part of the future study on this project.