Abstract
A series of 4,4′-disubstituted organic−organometallic stilbenes, i.e., 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′ = NPh2, NMe2, OMe, SiMe3 H, I, CN, NO2) (1−8), were studied for their electronic, electrochemical, and NLO properties. Complex 7 was also chemically oxidized using Cu(II)Cl2,
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yielding the [Pt(IV)Cl3(NCN(C2H2C6H4-CN-4′)-4)] complex 11. In contrast to 1 and 7, 11 did not show luminescent properties in solution at room temperature. In the solid state (X-ray crystallography) 11 has an overall bent structure, with the pincer moiety and the cyano group beneath the plane of the central double bond. Stilbenoid pincer complex 1 (R′ = NPh2) was also studied with UV/vis spectroscopy in a series of different solvents. The compound shows an intense low-energy transition band, appearing at a lower energy (λmax = 374–379 nm) than 2 (R′ = NMe2), which contains a stronger donor group. The UV/vis absorption data of a selected series of stilbenoid pincer platinum complexes were interpreted in terms of a qualitative orbital model based on DFT and TD-DFT calculations. The trends observed in the transition energies correlate well with the donor/acceptor properties of the substituents. It was shown that the HOMO–LUMO gap of the stilbenoid pincer platinum compounds decreases when going from an electron-neutral group (R′ = H) toward an electron-donating (R′ = NMe2) or -accepting (R′ = NO2) group. The fluorescence properties of 1 were also investigated using fluorescence and laser spectroscopy, showing that the metal center favors nonradiative decay from the excited state to the ground state. For 7 (R′ = CN), solid-state 13C CP/MAS NMR measurements were performed, and a value of 1J(13C,195Pt) = 1003 ± 15 Hz was obtained for the Cipso carbon, reflecting π-contributions in the carbon−metal bond. The cyclic voltammetry study on the complexes revealed an irreversible oxidation from Pt(II) to Pt(IV). In general the oxidation potential is influenced by the electronic character of the R′ substituent. The influence of the para R′ substituent on the hyperpolarizability of the compound was investigated using the femtosecond (frequency-resolved) HRS technique. For 1−7 good βzzz values were measured (βzzz ranging from (164–1324) × 10−30 esu) with the highest value for 6 (R′ = I). The higher βzzz values were found for the compounds that contain a low-energy UV/vis absorption band and/or a large absorption coefficient. Donor–acceptor complexes 7 and 8 were successfully applied in an organic light-emitting diode (OLED). In the electroluminescence spectrum of 7 (R′ = CN) a low-energy emission band was observed at 643 nm. This band also appeared in the solid-state luminescence spectra of 7, however, only at low temperatures (298–15 K). The emission band was assigned to result from a triplet excited state. The present study shows that the introduction of a NCN-pincer platinum fragment in a donor–acceptor stilbenoid molecule leads to complexes that have promising NLO properties. These compounds can now successfully be applied in OLED devices, although the performances of the devices have to be improved to become interesting for practical applications.
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