Photophysics and Applications of Triazole Complexes

1,2,3-Triazoles have proven to be highly successful in ligand design due to their facile synthesis owing to the advent of the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The Cu(I) species necessary for the CuAAC reaction can be generated in situ with simple purification of the resulting 1,4-disubstituted products and has proven to be an outstanding method to prepare triazole-containing ligands.

The present work focuses on the synthesis, characterization and photophysical investigation of triazole-containing transition metal complexes and their possible use in applications, such as LECs and biological cell imaging. To explore this wide topic different types of 1,2,3-triazole containing ligands including 2-(1-benzyl-1H-1,2,3-triazol-4-yl)pyridine as a bidentate ligand, 2,6-bis(1-phenyl-1H-1,2,3-triazol-4-yl)pyridine as a tridentate ligand and 1,4-bis((4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)methyl)benzene as a bridging ligand. were prepared and used with Re(I), Os(II), Ru(II) and Ir(III) transition metals.

Chapter 2 deals with rhenium complexes [Re(CO)3(Bn-pytz)(X)]+/0 (X = Cl or substituted pyridine). The photophysical investigation indicates that the emission can be further shifted towards the blue by replacing the Cl- ligand with pyridine. Two of these complexes were utilised in preliminary tests as the phosphor in light-emitting electrochemical cell LEC devices.

In Chapter 3 a series of osmium(II) complexes [Os(bpy)3−n(Bn-pytz)n](PF6)2 (where bpy = bipyridine, n = 1, 2, 3) were prepared and characterised. The progressive replacement of bpy by Bn-pytz leads to blue-shifted UV-visible electronic absorption and emission spectra. Successful separation of the fac and mer isomers of [Os(Bn-pytz)3](PF6)2 complexes was achieved, which exhibits phosphorescence (λem = 615 nm, in degassed acetonitrile). [Os(Bn-pytz)3][PF6]2 was shown to exhibit significant quenching of luminescence intensity in the presence of oxygen (Ksv = 83 atm-1). The water soluble chloride form of the complexes were prepared and were subjected to preliminary cellular uptake and luminescence imaging microscopy studies. The results from these studies reveal that the [Os(Bn-pytz)3]2+ is successfully taken up by two cancer cell lines. Bright emission from [Os(Bn-pytz)3]2+ can be seen by confocal microscopy with localisation at the lysosomes, however, no photodynamic therapy (PDT) activity is observed.

Described in Chapter 4 is the synthesis, characterisation and photophysical investigation of an osmium(II) 2,6-bis(1-phenyl-1H-1,2,3-triazol-4-yl)pyridine (btzpy) complex. From photophysical investigations [Os(btzpy)2](PF6)2 exhibits phosphorescence (λem = 595 nm, τ = 937 ns, φem = 9.3% in degassed acetonitrile). The complex undergoes significant oxygen-dependent quenching of emission with a 43-fold reduction in luminescence intensity between degassed and aerated acetonitrile solutions (Ksv = 110 atm-1), demonstrating its capability as a singlet oxygen sensitiser. The water soluble chloride form of the complex was prepared on the basis of the photophysical properties and was subject to preliminary cellular uptake and luminescence microscopy imaging studies. The complex easily entered the HeLa and U2OS cancer cell lines with mitochondrial localisation observed with intense emission permitting imaging at reduced concentrations up to 1 μM. Long-term dose toxicity results show low toxicity in HeLa cells with LD50 >100 μM.

Described in Chapter 5 is the synthesis and characterisation of a range of dinuclear supramolecular complexes, containing a bridging ligand incorporating the 1,2,3-triazole moiety. The homonuclear complexes ([Ir(ppy)2-L-Ir(ppy)2](PF6)2, [Ru(bpy)2-L-Ru(bpy)2](PF6)4, [Os(bpy)2-L-Os(bpy)2](PF6)4 and [Os(Bn-pytz)2-L-Os(Bn-pytz)2](PF6)4 and heteronuclear complex ([Ru(bpy)2-L-Ir(ppy)2](PF6)3 (where L = 1,4-{bis-4-(pyrid-2-yl)-1,2,3-triazol-1-yl methyl}benzene) were prepared. Photophysical studies show that the dinuclear species display greater luminescence intensities than their mononuclear model complexes. The emission spectra of the heteronuclear complex [Ru(bpy)2-L-Ir(ppy)2](PF6)3, exhibits features similar to those observed in the emission spectra of the mononuclear complexes [Ir(ppy)2-L](PF6) and [Ru(bpy)2-L](PF6)2. This indicates dual emission occurs from both the Ir- and Ru-centred chromophores (φem = 0.33% in acetonitrile). Interestingly the emission from the Ru-centred chromophore seems greatly enhanced relative to the emission of the Ru mononuclear analogue. This indicates partial energy transfer from Ir to Ru. The dual emission results in near white light emission and could be used for the devolpment of single component white light emitting phosphors.