Phenylbenzothiazole-Based Platinum(II) and Diplatinum(II) and (III) Complexes with Pyrazolate Groups: Optical...

Based on 2-phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me₂N-pbt), mononuclear [Pt(pbt)(R'₂-pzH)₂]PF₆ (R'₂-pzH = pzH 1a, 3,5-Me₂pzH 1b, 3,5-ⁱPr₂pzH 1c) and diplatinum (Pt^II-Pt^II) [Pt(pbt)(μ-R'₂pz)]₂ (R'₂-pz = pz 2a, 3,5-Me₂pz 2b, 3,5-ⁱPr₂pz 2c) and [Pt(Me₂N-pbt)(μ-pz)]₂ (3a) complexes have been prepared. In the presence of sunlight, 2a and 3a evolve, in CHCl₃ solution, to form the Pt^III-Pt^III complexes [Pt(R-pbt)(μ-pz)Cl]₂ (R = H 4a, NMe₂ 5a). Experimental and computational studies reveal the negligible influence of the pyrazole or pyrazolate ligands on the optical properties of 1a-c and 2a,b, which exhibit a typical ³IL/³MLCT emission, whereas in 2c the emission has some ³MMLCT contribution. 3a displays unusual dual, fluorescence (¹ILCT or ¹MLCT/¹LC), and phosphorescence (³ILCT) emissions depending on the excitation wavelength. The phosphorescence is lost in aerated solutions due to sensitization of ³O₂ and formation of ¹O₂, whose determined quantum yield is also wavelength dependent. The phosphorescence can be reversibly photoinduced (365 nm, ∼ 15 min) in oxygenated THF and DMSO solutions. In 4a and 5a, the lowest electronic transitions (S₁-S₃) have mixed characters (LMMCT/LXCT/L'XCT 4a and LMMCT/LXCT/ILCT 5a) and they are weakly emissive in rigid media. The ¹O₂ generation property of complex 3a is successfully used for the photooxidation of p-bromothioanisol showing its potential application toward photocatalysis.