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

It has been shown for the first time that the Pt-IV complex cis-[Pt(N boolean AND C-tolpy)(2)Cl-2] (tolpy=2-(4-tolyl)pyridinyl) can be prepared in a one-pot reaction from K-2[PtCl4], although analogous complexes containing 2,5-bis(4-dodecyloxyphenyl)pyridine (=HL) could be prepared using existing routes. The resulting complexes cis-[Pt(N boolean AND C-L)(2)Cl-2] are liquid crystals and small-angle X-ray scattering suggests formation of a lamellar mesophase. Surprisingly, heating [Pt(kappa(2)-N boolean AND C-L)(2)Cl(kappa(1)-N boolean AND C-LH)] also leads to a mesomorphic compound, which results from thermally induced oxidation to cis-[Pt(N boolean AND C-L)(2)Cl-2] and what is presumed to be another geometric isomer of the same formula. The Pt-IV complexes are quite strongly luminescent in deoxygenated solution, with phi approximate to 10 % and show vibrationally structured emission spectra, lambda(max)(0,0)=532 nm, strongly displaced to the red compared to cis-[Pt(N boolean AND C-tolpy)Cl-2]. Long luminescence lifetimes of 230 mu s 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 boolean AND C-ppy)X-2], 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.