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

Abstract

Based on 2-phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me₂N-pbt), mononuclear [Pt(pbt)(R'₂-pzH)₂]PF₆ (R'₂-pzH = pzH <b>1a</b>, 3,5-Me₂pzH <b>1b</b>, 3,5-ⁱPr₂pzH <b>1c</b>) and diplatinum (Pt^II-Pt^II) [Pt(pbt)(μ-R'₂pz)]₂ (R'₂-pz = pz <b>2a</b>, 3,5-Me₂pz <b>2b</b>, 3,5-ⁱPr₂pz <b>2c</b>) and [Pt(Me₂N-pbt)(μ-pz)]₂ (<b>3a</b>) complexes have been prepared. In the presence of sunlight, <b>2a</b> and <b>3a</b> evolve, in CHCl₃ solution, to form the Pt^III-Pt^III complexes [Pt(R-pbt)(μ-pz)Cl]₂ (R = H <b>4a</b>, NMe₂ <b>5a</b>). Experimental and computational studies reveal the negligible influence of the pyrazole or pyrazolate ligands on the optical properties of <b>1a</b>-<b>c</b> and <b>2a</b>,<b>b</b>, which exhibit a typical ³IL/³MLCT emission, whereas in <b>2c</b> the emission has some ³MMLCT contribution. <b>3a</b> 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 <b>4a</b> and <b>5a</b>, the lowest electronic transitions (S₁-S₃) have mixed characters (LMMCT/LXCT/L'XCT <b>4a</b> and LMMCT/LXCT/ILCT <b>5a</b>) and they are weakly emissive in rigid media. The ¹O₂ generation property of complex <b>3a</b> is successfully used for the photooxidation of <i>p</i>-bromothioanisol showing its potential application toward photocatalysis.