Synthesis, Mesomorphism, and Photophysics of 2,5-Bis(dodecyloxyphenyl)pyridine Complexes of Platinum(IV)

Abstract: It has been shown for the first time that the PtIV complex cis-[Pt(N^C-tolpy)2Cl2] (tolpy=2-(4-tolyl)pyridinyl) can be prepared in a one-pot reaction from K2[PtCl4], although analogous complexes containing 2,5-bis(4-dodecyloxyphenyl) pyridine (=HL) could be prepared using existing routes. The resulting complexes cis-[Pt(N^C-L)2Cl2] are liquid crystals and small-angle X-ray scattering suggests formation of a lamellar mesophase. Surprisingly, heating [Pt(k2-N^CL) 2Cl(k1-N^C-LH)] also leads to a mesomorphic compound, which results from thermally induced oxidation to cis- [Pt(N^C-L)2Cl2] and what is presumed to be another geometric isomer of the same formula. The PtIV complexes are quite strongly luminescent in deoxygenated solution, with f 10% and show vibrationally structured emission spectra, lmax(0,0)=532 nm, strongly displaced to the red compared to cis-[Pt(N^C-tolpy)Cl2]. Long luminescence lifetimes of 230 ms are attributed to a lower degree of metal character in the excited state accompanying the extension of conjugation in the ligand. There is no significant difference between the emission properties of the bromo- and chloro-complexes, in contrast with the known complexes cis-[Pt(N^Cppy) X2], where the quantum yield for X=Br is some 30 times lower than for X=Cl (ppyH=2-phenylpyridine). The lower energy of the excited state in the new complexes probably ensures that deactivating LLCT/LMCT states remain thermally inaccessible, even when X=Br.