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

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-Me(2)pzH 1b, 3,5-(i)Pr(2)pzH 1c) and diplatinum (Pt-II-Pt-II) [Pt(pbt)(mu-R '(2)pz)](2) (R '(2)-pz = pz 2a, 3,5-Me(2)pz 2b, 3,5-(i)Pr(2)pz 2c) and [Pt(Me2N-pbt)(mu-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)(mu-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 (IL)-I-3/(MLCT)-M-3 emission, whereas in 2c the emission has some (MMLCT)-M-3 contribution. 3a displays unusual dual, fluorescence ((ILCT)-I-1 or (MLCT)-M-1/(LC)-L-1), and phosphorescence ((ILCT)-I-3) emissions depending on the excitation wavelength. The phosphorescence is lost in aerated solutions due to sensitization of O-3(2) and formation of O-1(2), whose determined quantum yield is also wavelength dependent. The phosphorescence can be reversibly photoinduced (365 nm, similar to 15 min) in oxygenated THF and DMSO solutions. In 4a and 5a, the lowest electronic transitions (S-1-S-3) have mixed characters (LMMCT/LXCT/L'XCT 4a and LMMCT/LXCT/ILCT 5a) and they are weakly emissive in rigid media. The O-1(2) generation property of complex 3a is successfully used for the photooxidation of p-bromothioanisol showing its potential application toward photocatalysis.