Synthesis, characterisation and theoretical study of ruthenium 4,4′-bi-1,2,3-triazolyl complexes: Fundamental switching of the nature of S ₁ and T ₁ states from MLCT to MC

The series of complexes [Ru(bpy) _3-n(btz) _n][PF ₆] ₂ (bpy = 2,2′-bipyridyl, btz = 1,1′-dibenzyl-4,4′-bi-1,2,3-triazolyl, 2n = 1, 3n = 2, 4n = 3) have been prepared and characterised, and the photophysical and electronic effects imparted by the btz ligand were investigated. Complexes 2 and 3 exhibit MLCT absorption bands at 425 and 446 nm respectively showing a progressive blue-shift in the absorption on increasing the btz ligand content when compared to [Ru(bpy) ₃][Cl] ₂ (1). Complex 4 exhibits a heavily blue-shifted absorption spectrum with respect to those of 1-3, indicating that the LUMO of the latter are bpy-centred with little or no btz contribution whereas that of 4 is necessarily btz-centred. DFT calculations on analogous complexes 1′-4′ (in which the benzyl substituents are replaced by methyl) show that the HOMO-LUMO gap increases by 0.3 eV from 1′-3′ through destabilisation of the LUMO with respect to the HOMO. The HOMO-LUMO gap of 4′ increases by 0.98 eV compared to that of 3′ due to significant destabilisation of the LUMO. Examination of TDDFT data show that the S ₁ states of 1′-3′ are ¹MLCT in character whereas that of 4′ is ¹MC. The optimisation of the T ₁ state of 4′ leads to the elongation of two mutually trans Ru-N bonds to yield [Ru(κ ²-btz)(κ ¹-btz) ₂] ²⁺, confirming the ³MC character. Thus, replacement of bpy by btz leads to a fundamental change in the ordering of excited states such that the nature of the lowest energy excited state changes from MLCT in nature to MC.