The Synthesis and Spectroscopic Properties of Some Rhenium(I) and Copper(I) Polypyridyl Complexes

Abstract

This thesis details a study of Re(I) and Cu(I) metal polypyridyl complexes based on different polypyridyl ligand systems, with the aim of understanding the nature of their excited state properties. This project is focussed primarily on the synthesis and characterisation of the polypyridyl ligands and their Re(I) and Cu(I) complexes, but also presented are spectroscopic and computational chemistry results.

A series of sulfur-substituted dppz ligands and their {Re(CO)3Cl} complexes are reported in chapter 3. The complexes [Re(dppzS2CS)(CO)3Cl] and [Re(dppz(Sdec)2)(CO)3Cl] display electronic transitions atypical of Re-dppz complexes. Using TD-DFT calculations and resonance Raman studies, the unusual transition for these complexes is assigned as a charge transfer from both the metal and sulfur portions to the phenazine LUMO, an intraligand/metal-to-ligand charge-transfer transition. [Re(dppzS2CS)(CO)3Cl] and [Re(dppz(Sdec)2)(CO)3Cl] both display interesting photophysical properties, emitting from a ligand-centered state in polar solvents such as DMSO, however emitting from an MLCT state in relatively non-polar solvents such as toluene. Transient absorption and emission experiments reveal the emissive states have lifetimes of less than 10 ns, they also reveal the presence of a non emissive (dark) excited states with much longer excited-state lifetimes; up to 8.5 μs in toluene for [Re(dppz(Sdec)2)(CO)3Cl].

The TRIR spectra of [Re(dppzS2CS)(CO)3Cl] reveal the existence a MLCT-phz state that decays within 5 ns to an excited state consistent with partial oxidation of the Re centre. For [Re(dppzSdec)2)(CO)3Cl] the TRIR reveal initial population of a LC state which decays after 1600 ns into a state that is consistent with partial oxidation of the Re metal centre. The observation of bands consistent with partial oxidation of the metal centre, is consistent with an IL/MLCT THEXI state. In chapter 4, the complete family of mono, bi and trinuclear {Re(CO)3Cl} complexes of HATN-Me6 are reported. The syn and anti isomers of binuclear [(Re(CO)3Cl)2(μ-HATN-Me6)] and trinuclear [(Re(CO)3Cl)3(μ-HATN-Me6)] complexes were successfully separated. This is the first example of {Re(CO)3Cl} polypyridyl complexes where both the syn and anti isomers have been synthesised and subsequently separated The isomers may be differentiated by their 1H NMR spectra, separated by column chromatography and assigned by X-ray crystallography. Intense π→π* and MLCT absorptions in the visible region dominate the electronic spectra of the compounds. The time-resolved infrared spectroscopy of the compounds confirms that the lowest excited state is MLCT in nature. Separation of the isomers of the bi and trinuclear complexes finally confirms that the orientation of the {Re(CO)3Cl} moieties have only subtle influences on the spectroscopic properties of the compounds. Four bromo-substituted dppz ligands, and their {Re(CO)3Cl} and {Re(CO)3py}+ complexes are presented in chapter 5. The crystal structures of [Re(dppzBr2)(CO)3Cl] and [Re(dppzBr)(CO)3py]PF6 are presented and discussed. The electrochemistry reveals the electron-withdrawing nature of the substituents on the dppz phz MO. The dppzBrCF3 and pppBr have the most positive reduction potentials of the ligands. Binding of the {Re(CO)3Cl} stabilises the reduction potential by 240 to 290 mV, and substituting the chloride ligand for the pyridine ligand further stabilises the reduction potential by 100-120 mV. A series of Cu(I) heteroleptic complexes based on 6,6’-dimesityl -2,2’-bipyridine are presented in chapter 6. X-ray crystal-structure studies reveal the complexes as a whole adopt Cu(I) geometries that are best described as distorted trigonal pyramidal, due to the intramolecular π interactions between the mesityl ring and the adjacent ligand. UV-vis absorption spectra of the complexes reveal MLCT absorptions that are generally red-shifted from the respective homoleptic complexes. The oxidation potentials of the complexes reflect the steric crowding around the Cu(I) coordination sphere. The non-emissive nature of the complexes in CH2Cl2 solution is presumably due to the bipyridine-mesityl ligand being too flexible to sustain the flattened geometry required for the long-lived triplet state associated with emissive Cu(I) polypyridyls.