Synthesis, Structure, and Photophysical Properties of Luminescent Platinum(II) Complexes Containing Cyclometalated 4-Styryl-Functionalized 2-Phenylpyridine Ligands

Abstract

A series of new luminescent cyclometalated platinum(II) complexes functionalized with various substituted styryl groups on the cyclometallating ligand [Pt(C/\N-ppy-4-styryl-R)(O/\O-(O)CCR'CHCR'C(O))] (ppy-4-styryl-R = E-4(4-(R)styryl-2-phenylpyridine) (3, R' = Me (acac); 4, R' = (t)Bu (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-NO 2 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 --> CH=CH) and metal-to-ligand charge-transfer (MLCT) (dpi(Pt) --> pi) 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; lambda(max) = 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, lambda(max)(em) = 658 nm; tau = 26 micros). The very large red-shift compared to related unsubstituted complexes (e.g., to [Pt(C/\N-ppy)(O/\O-acac)]) is the result of the extension of the pi-conjugated system and the electronic effects of substituent R.

Fluorine-containing quinoline and quinoxaline styryl derivatives: synthesis and photophysical properties

“…20 As a whole, rather low luminescence intensity of the styryl derivatives of heterocycles in solutions is related to the occurrence of (E) (Z) isomerization. 21, 22 In films or in the solid state the quantum yield can increase by several orders of magnitude.…”

Tuning the Emission Colour of Triphenylamine‐Capped Cyclometallated Platinum(II) Complexes and Their Application in Luminescent Oxygen Sensing and Organic Light‐Emitting Diodes

“…[43,[46][47][48][49][50][51][52] The photophysics of lumophores can be better understood by revealing the electronic structures of the excited states. [2][3][4]18,19,43] In this work we carried out theoretical cal-culations on the complexes and the photophysical properties, such as the excitation and emission properties, were investigated with time-dependent DFT (TDDFT) calculations. The main objective of the theoretical calculations was to investigate the electronic structures of the low-lying excited states and to correlate them with their emission properties.…”

Synthesis, structures and spectroscopic properties of platinum complexes containing orthometalated 2-phenylpyridine

“…Thus the emission colors (i.e. DE) in orthometalated 2-phenylpyridine complexes of platinum(II) can be tuned from blue-green to orange-red by appropriately substituting either the phenyl [17] and/or pyridyl [17,18] rings of 2-phenylpyridine. Substitution by an electron-withdrawing and -donating group in the phenyl ring leads to blue and red shifts, respectively in the emission spectra [17].…”

“…For instance, [Pt(ppy)Cl 2 ] À (Hppy = 2-phenylpyridine) is not luminescent in solution at room temperature whereas [Pt(ppy)(Hppy)Cl] is luminescent under similar conditions [19]. Much of the studies on role of photophysical properties of orthometalated 2-arylpyridine type ligand complexes of platinum(II) are concerned with b-diketonate derivatives [17,18,20,21].…”