Luminescent and electronic properties of stilbenoid NCN-pincer PtII compounds

A series of novel 4,4'-disubstituted organic-organometallic stilbenes were synthesized, that is, the 4'-substituted stilbenoid- NCN-pincer platinum(II) complexes [PtCl(NCN-R-4)] (NCN-R-4 = [C6H2(CH2NMe2)2-2,6-R-4]- in which R = C2H2C6H4-R'-4' with R' = NMe2, OMe, SiMe3, H, I, CN, NO2) (1-7). In these compounds the PtCl grouping can be considered to be present as a donor substituent. Their synthesis involved a Horner-Wadsworth-Emmons reaction of [PtCl(NCN-CHO-4)] (9) with the appropriate phosphonate ester derivatives (8a-g). Under these reaction conditions, the C-Pt bond in aldehyde 9 was not affected, and the platinated stilbene products were obtained in 53-90% yield. The solidstate structures of complexes 1, 2 and 5-7 were determined by single-crystal X-ray diffraction, which revealed interesting bent conformations for 2, 5 and 7. Linear correlations were found between both the 13C{1H} (C ipso to Pt) and the 195Pt{1H} NMR chemical shift and the Hammett σp value of the R' substituent; therefore, these NMR shifts can be used as a qualitative probe for the electronic properties of the delocalized π-system to which it is connected. The platinum-stilbene complexes were investigated for charge-transfer properties in solvents of different polarity. The luminescent properties, shown by donor-acceptor complexes 1, 6 and 7, were investigated by fluorescence spectroscopy, and the complexes showed positive solvatochromism, which indicates dipolar character of the excited state. The excited state lifetimes, which were in the picosecond range, and the quantum yields (ranging from 0.002 to 0.2) were also determined for these complexes. It was established that the presence of the transition metal favours nonradiative decay from the excited state to the ground state.