Synthesis, Structure, and Photophysical Properties of Luminescent Platinum(II) Complexes Containing Cyclometalated 4-Styryl-Functionalized 2-Phenylpyridine Ligands
Résumé
A series of new luminescent cyclometalated platinum(II) complexes functionalized with various substituted styryl groups on the cyclometallating ligand [Pt(CN-ppy-4-styryl-R)(OO-(O)CCR'CHCR'C(O))] (ppy-4-styryl-R = E-4(4-(R)styryl-2-phenylpyridine) (3, R' = Me (acac); 4, R' = tBu (dpm); R = H, OMe, NEt2, NO2) have been prepared. All complexes undergo an E−Z photoisomerization process in CH2Cl2 solution under sunlight, as monitored by 1H NMR. The solid-state structures of 3-OMe, 3-NEt2, 3-NO2, and 4-OMe have been determined by X-ray diffraction studies and compare well with optimized geometries obtained by density functional theory (DFT) calculations. The orbital pictures of 3-H, 3-OMe, and 3-NO2 are very similar, the highest occupied molecular orbital (HOMO) being highly Pt(5d) metal-based. For 3-NMe2, an additional contribution from the amino-styryl fragment leads to a decreased metal parentage of the HOMO, suggesting a predominantly ILCT character transition. Complexes 3-H, 3-OMe, and 3-NO2 show a low-energy band (350−400 nm) assigned to predominantly charge-transfer transitions. The amino derivative 3-NEt2 displays a very strong absorption band at 432 nm, tentatively assigned to a mixture of ILCT (Et2N → CHCH*) and metal-to-ligand charge-transfer (MLCT) (dπ(Pt) → π*) transitions. Complexes 3 are weakly luminescent in CH2Cl2 solution at room temperature; the low intensity may be due to a competitive quenching through the E−Z photoisomerization process. All complexes exhibit similar structured emission bands under these conditions ( around 520 nm), independent of the nature of the styryl-R group. In a frozen EPA glass (77 K), the spectrum of the representative complex 3-H exhibits two sets of vibronically structured bands (460−560, 570−800 nm; λmax = 596 nm), due to the presence of two emitting species, the E and Z isomers, which have significantly different triplet excited-state energies. The other three complexes show similar behavior to 3-H at 77 K, but the lower-energy emission bands are progressively red-shifted in the order H < OMe < NO2 < NEt2 (e.g., for 3-NEt2, = 658 nm; τ = 26 μs). The very large red-shift compared to related unsubstituted complexes (e.g., to [Pt(CN-ppy)(OO-acac)]) is the result of the extension of the π-conjugated system and the electronic effects of substituent R.