Mononuclear rhenium(i) complexes incorporating 2-(arylazo)phenyl benzyl thioethers: synthesis, structure, spectral, DFT and TDDFT studies

Abstract

The stoichiometric reaction of [Re(CO)₅Cl] with 2-(arylazo)phenyl benzyl thioethers [ArNNC₆H₄SCH₂Ph: Ar = Ph, 4-chlorophenyl, 4-nitrophenyl] (L¹–L³) in a ratio of 1 : 1 in dry toluene under refluxing condition afforded the Re(I) complexes of general formula fac-[Re(L)(CO)₃Cl] in excellent yields. The ligands used in this present work are 2-(phenylazo)phenyl benzyl thioether (L¹), 2-(4-chlorophenylazo)phenyl benzyl thioether (L²) and 2-(4-nitrophenylazo)phenyl benzyl thioether (L³). The elemental analysis and ESI mass spectroscopic measurements ensure the formation of desired complexes. The crystal structure of the complexes [Re(L¹)(CO)₃Cl], 1 and [Re(L²)(CO)₃Cl], 2 were determined by X-ray diffractometry study. The molecular structures observed in the solid state were retained in the solution (¹H and ¹³C NMR spectra). The ground and excited-state geometries, NMR, absorption, and luminescent properties of three Re(I) complexes were examined by DFT and TDDFT methods. The introduction of an electron withdrawing group at the para position of the phenyl ring attached to the NN bond varies the frontier molecular orbital energies, compositions and optical properties of these molecules significantly. The lowest lying triplet excited is associated with an admixture of the ³MLCT and ³ILCT excited state having a cis conformation of the azoaryl moiety. The emission like transition having mixed ³MLCT and ³ILCT nature was characterized by natural transition orbital (NTO) and spin density difference map analysis. The presence of the electron withdrawing NO₂ group leads to the low quantum yield through enhanced non-radiative deactivation. The cis orientation at the lowest lying triplet excited state (T₁) may imply some kind of photoinduced isomerization in the ligand frame and as a result there may be more than one emitting species in the solution and hence bi-exponential decay nature was observed for all the complexes.