Crystal Structures and Luminescence Properties of Platinum(II) Complexes Containing 3,3'-Biisoquinoline

Strategy for Achieving Long-Wavelength Near-Infrared Luminescence of Diimineplatinum(II) Complexes

Although many strategies have been used to help design effective near-infrared (NIR) luminescent materials, it is still a huge challenge to realize long-wavelength NIR luminescence of diimineplatinum(II) complexes in the solid state. Herein, we have successfully achieved long-wavelength NIR luminescence of a family of diimineplatinum(II) complexes based on a new strategy that combines a one-dimensional (1D) "Pt wire" structure with the electronic effect of the substituent. The structures of six solvated diimineplatinum(II) complexes based on 4,4-dichloro-2,2'-bipyridine or 4,4-dibromo-2,2'-bipyridine and 4-substituted phenylacetylene ligands have been determined, namely, 1·¹/₂toluene, 2·¹/₂THF, 3·¹/₈toluene, 4·¹/₂THF, 5·¹/₈CH₂Cl₂, and 6·¹/₄toluene. All of them crystallize in the monoclinic space group C2/c or C2/m and stack in the 1D "Pt wire" structure. In the solid state, six complexes exhibited unusual long-wavelength metal-metal-to-ligand charge-transfer luminescence that peaked at 984, 1044, 972, 990, 1022, and 935 nm, respectively. Interestingly, 2·¹/₂THF has the shortest Pt···Pt distance and the longest emission wavelength among the six complexes. As far as we know, the luminescence of 2·¹/₂THF at 1044 nm is the longest emission wavelength among known diimineplatinum(II) complexes. Systematic studies revealed that good molecular planarity, suitable substituent position, weak hydrogen-bond-forming ability of the substituents, appropriate molecular bending, and weakening of the interaction between solvated molecules and platinum molecules are conducive to the construction of a 1D "Pt wire" structure of the diimineplatinum(II) complex. Furthermore, the emission energy of the complex is mainly determined by the strength of the Pt-Pt interaction and electronic effect of the substituent.

1D polymeric platinum cyanoximate: a strategy toward luminescence in the near-infrared region beyond 1000 nm

We report the synthesis and properties of the first representative of a new class of PtL2 complexes with ambidentate mixed-donor cyanoxime ligands [L = 2-cyano-2-oximino-N,N'-diethylaminoacetamide, DECO (1)]. Three differently colored polymorphs of "Pt(DECO)2" (3-5) were isolated, with the first two being crystallographically characterized. The dark-green complex [Pt(DECO)2]n (5) spontaneously forms in aqueous solution via aggregation of yellow monomeric complex 3 into the red dimer [Pt(DECO)2]2 (4), followed by further oligomerization into coordination polymer 5. A spectroscopic and light-scattering study revealed a "poker-chips"-type 1D polymeric structure of 5 in which units are held by noncovalent metallophilic interactions, forming a Pt---Pt wire. The polymer 5 shows a broad absorption at 400-900 nm and emission at unusually long wavelengths in the range of 1000-1100 nm in the solid state. The near-infrared (NIR) emission of polymer 5 is due to the formation of a small amount of nonstoichiometric mixed-valence Pt(II)/Pt(IV) species during synthesis. A featureless electron paramagnetic resonance spectrum of solid sample 5 recorded at +23 and -193 °C evidences the absence of Pt(III) states, and the compound represents a "solid solution" containing mixed-valence Pt(II)/Pt(IV) centers. Exposure of KBr pellets with 5% 5 to Br2 vapors leads to an immediate ∼30% increase in the intensity of photoluminescence at 1024 nm, which confirms the role and importance of mixed-valence species for the NIR emission. Thus, the emission is further enhanced upon additional oxidation of Pt(II) centers, which improves delocalization of electrons along the Pt---Pt vector. Other polymorph of the "Pt(DECO)2" complex--monomer--did not demonstrate luminescent properties in solutions and the solid state. An excitation scan of 5 embedded in KBr tablets revealed an emission only weakly dependent on the wavelength of excitation. The NIR emission of quasi-1D complex 5 was studied in the range of -193 to +67 °C. Data showed a blue shift of λmax and a simultaneous increase in the emission line intensity with a temperature rise, which is explained by analogy with similar behavior of known quasi-1D K2[Pt(CN)4]-based solids, quantum dots, and quantum wells with delocalized carriers. The presented finding opens a route to a new class of platinum cyanoxime based NIR emissive complexes that could be used in the design of novel NIR emitters and imaging agents.

A non-doped phosphorescent organic light-emitting device with above 31% external quantum efficiency

The demonstrated square-planar Pt(II)-complex has reduced triplet-triplet quenching and therefore a near unity quantum yield in the neat thin film. A non-doped phosphorescent organic light-emitting diode (PhOLED) based on this emitter achieves (31.1 ± 0.1)% external quantum efficiency without any out-coupling, which shows that a non-doped PhOLED can be comparable in efficiency to the best doped devices with very complicated device structures.

THEORETICAL COMPUTATIONAL STUDIES ON ELECTRONIC STRUCTURES, SPECTROSCOPIC PROPERTIES AND NITROGEN HETEROATOM EFFECT OF A SPECIES OF ASYMMETRICAL DIIMINE LIGAND PLATINUM(II) COMPLEXES

Electronic structures and spectroscopic properties of a series of platinum(II) complexes based on the C-linked asymmetrical diimine ligand (2-pyridyl-pyrazole (1), 2-pyridyl-1,2,4-triazole (2), 2-pyridyl-tetrazole (3), 2-pyrazine-pyrazole (4) have been studied by the time-dependent density functional theory calculations with polarizable continuum model. The ground- and excited-state structures were optimized by the density functional theory and single-excitation configuration interaction methods, respectively. The calculated structures and spectroscopic properties are in agreement with the corresponding experimental data. The results of the spectroscopic investigations revealed that the lowest-energy absorptions have1,3metal-to-ligand charge transfer (MLCT)/1,3single ligand centered charge transfer (ILCT) mixing characters. The highest-occupied molecular orbitals (HOMOs) of 1–4 are composed of Pt ( d yz) and azole, while the lowest-unoccupied molecular orbitals (LUMOs) are mainly localized upon the pyridyl-azolate ligand (70% on the pyridine segment). From 1 to 3, the molecular orbital (MO) energies of HOMO and LUMO are decreased and the HOMO energies are changed more remarkably. This is caused by that the conjugation of the azolate segment of the ligand are enhanced through introducing more N heteroatoms into this segment. As a result of MO energy change, the lowest-energy absorptions are blue-shifted in the order 1 < 2 < 3. With the replacement of pyridyl by pyrazine, the HOMO energy of 4 is comparable to 1, but the LUMO energy is decreased by 0.8 eV, and the lowest-energy absorptions are red-shifted to 2.36 eV. Otherwise, the phosphorescent emissions of these complexes have the 3MLCT/3ILCT characters, and should originate from the lowest-energy absorptions. The emissions of 1–4 are red-shifted in the order 3 < 2 < 1 < 4. The heteroatom effect is suitable for tuning the spectra of this kind of materials.

Emissive Pt(II) complexes bearing both cyclometalated ligand and 2-pyridyl hexafluoropropoxide ancillary chelate

A new series of mono-cyclometalated Pt(II) complexes 1-4 with chelating 2-pyridyl hexafluoropropoxide as the ancillary ligand were synthesized. Single crystal X-ray diffraction studies were examined, giving evidence for the occurrence of pi pi stacking between the cyclometalated ligands, but a lack of intermolecular Pt ... Pt interaction. Among these complexes, the benzo[h]quinoline analogue 2a shows the greatest degree of pi pi stacking, which is also confirmed by the observation of additional, large Stokes shifted emission attributed to the aggregated counterparts in solid thin film. All these Pt(II) complexes are highly emissive in solid state, whereas except for 4-phenylquinazoline analogues 4a and 4b, complexes 1-3 in solution are subject to dominant radiationless deactivation induced, in part, by the solvent collision to the square planar Pt(II) framework. Electroluminescent OLEDs employing 2-phenylpyridine analogue 1b as the dopants were fabricated, rendering satisfactory performance data suited for future improvement.

Fe(II), Ru(II) and Re(I) complexes of endotopic, sterically non-hindering, U-shaped 8,8'-disubstituted-3,3'-biisoquinoline ligands: syntheses and spectroscopic properties

The redox behaviour, optical-absorption spectra and emission properties of U-shaped and elongated disubstituted biisoquinoline ligands and of derived octahedral Fe(ii), Ru(ii), and Re(i) complexes are reported. The ligands are 8,8'-dichloro-3,3'-biisoquinoline (1), 8,8'-dianisyl-3,3'-biisoquinoline (2), and 8,8'-di(phenylanisyl)-3,3'-biisoquinoline (3), and the complexes are [Fe(3)(3)](2+), [Fe(2)(3)](2+), [Ru(1)(phen)(2)](2+), [Ru(2)(3)](2+), [Ru(3)(3)](2+), [Re(2)(py)(CO)(3)](+), and [Re()(py)(CO)(3)](+). For the ligands, the optical properties as observed in dichloromethane are in line with expectations based on the predominant (1)pipi* nature of the involved excited states, with contributions at lower energies from (1)npi* and (1)ILCT (intraligand charge transfer) transitions. For all of the Fe(ii), Ru(ii), and Re(i) complexes, studied in acetonitrile, the transitions associated with the lowest-energy absorption band are of (1)MLCT (metal-to-ligand charge transfer) nature. The emission properties, as observed at room temperature and at 77 K, can be described as follows: (i) the Fe(ii) complexes do not emit, either at room temperature or at 77 K; (ii) the room-temperature emission of the Ru(ii) complexes (phi(em) > 10(-3), tau in the micros range) is of mixed (3)MLCT/(3)LC character (and similarly at 77 K); and (iii) the room-temperature emission of the Re(i) complexes (phi(em) approximately 3 x 10(-3), tau < 1 ns) is of (3)MLCT character and becomes of (3)LC (ligand-centered) character (tau in the ms time scale) at 77 K. The interplay of the involved excited states in determining the luminescence output is examined.

Platinum(II) Complexes with Pyridyl Azolate-Based Chelates: Synthesis, Structural Characterization, and Tuning of Photo- and Electrophosphorescence

A new series of luminescent platinum(II) azolate complexes with a formula of [Pt(NwedgeN)(2)], in which NwedgeN = mppz (1), bppz (2a), bzpz (2b), bmpz (2c), bqpz (2d), fppz (3a), hppz (3b), bptz (4), hptz (5), were synthesized, and their photophyscial properties were examined. Single-crystal X-ray diffraction studies of 2c and 3b revealed a planar molecular geometry, in which the NwedgeN chelates adopt a trans configuration and show notable interligand C-H...N hydrogen bonding within the complex. Interesting intermolecular interactions were observed in the solid state. Complex 2c formed a slipped-stack structure with a Pt...Pt separation distance of 6.432 Angstroms, while complex 3b showed a columnar stacking with the molecules oriented in an alternating order in relation to the chain axis, giving a much reduced Pt...Pt distance of 3.442 Angstroms. The lowest absorption band for all complexes revealed strong state mixings between the singlet and triplet (MLCT and intraligand pipi) manifolds. Complexes 1 and 2 showed mixed (3)MLCT and (3)pipi phosphorescence in fluid solution. While radiationless deactivation was apparently dominant for complexes 3-5 in solution, resulting in rather weak emission, strong phosphorescence was observed in the room-temperature solid state with the peak wavelength being significantly red shifted compared to that in solution. The emission nature has been tentatively assigned to be (3)MMLCT in character. OLED devices with a multilayer configuration of ITO/NPB/CBP:2a/BCP/Alq(3)/LiF/Al were successfully fabricated using a CBP layer doped with various amount of 2a, ranging from 6 to 100%, as the emitting layer. A substantial red shift with increasing doping concentrations was observed in electroluminescence. With a neat film of 2a, the device showed a green emission with lambda(max) at 556 nm and an external QE of approximately 1.6% at a driving current of 20 mA. Similarly, for the device using a neat film of 3a, an electroluminescence centered at 616 nm with a slightly reduced external QE of approximately 2.1% was recorded. Aggregation of platinum(II) complexes in the solid state was proposed to account for the large red-shift in electroluminescence.

Luminescent Langmuir-Blodgett films of platinum(II) complex [Pt(L18)Cl](PF6) (L18 = 2,6-bis(1-octadecylbenzimidazol-2-yl)pyridine)

A novel amphiphilic Pt complex containing 2,6-bis(1-octadecylbenzimidazol-2-yl)pyridine (L18), [Pt(L18)Cl](PF6), has been synthesized. The complex exhibits concentration-dependent absorption and emission spectra in solution. With increasing the concentration of the Pt complex, we observed a new absorption band centered at 550 nm derived from a metal-metal d sigma* to ligand pi* charge transfer (MMLCT) transition and the corresponding broad emission centered at 650 nm. The Pt complex is surface-active, and the surface pressure-area isotherm reveals three phase transitions. The three phases correspond to one liquid-expanding phase and two solid-condensed phases, respectively, with different intermolecular overlap in the "flat-on" orientation at the air-water interface. Without additives such as fatty acids, the complex forms a stable and reproducible Langmuir-Blodgett (LB) multilayer film above a surface pressure of 15 mN m-1. Strong emission from the LB films, even monolayer, was observed. Comparing the relative emission intensity of the MMLCT band for transferred LB monolayer film with that for cast films, we concluded that Pt-Pt interactions are suppressed in the LB film. Instead, the emission at 600 nm arising from the ligand-ligand pi-pi interacted excited state became dominant. The results would provide the insight into the control of molecular ordering for planar Pt complexes from the viewpoint of characteristic excited states.

Syntheses and electronic spectroscopy of [PtL(L')][ClO4] complexes (HL = 6-phenyl-2,2'-bipyridine; L' = pyridine, 4-aminopyridine, 2-aminopyridine, and 2,6-diaminopyridine)

Four cyclometalated Pt(II) complexes, [PtL(L')][ClO4] [HL = 6-phenyl-2,2'-bipyridine; L' = pyridine (1), 4-aminopyridine (2), 2-aminopyridine (3), 2,6-diaminopyridine (4)], were designed and synthesized to probe intramolecular N...Pt interactions. The crystal structures of the compounds show that the pyridine ligands are almost perpendicular to the planes of the molecules. In addition, the pendant NH2 groups of the 2-aminopyridine and 2,6-diaminopyridine ligands are close to the metal centers in complexes 3 and 4, with the Pt-N(H2) distances (3.065(3)-3.107(3) A) significantly shorter than the sum of the van der Waals radii of Pt and N. These compounds were also studied by electronic spectroscopy. All the complexes display intense intraligand pi-->pi* transitions at 200-340 nm (epsilon = 10(4)-10(3) M-1 cm-1) and moderately intense (epsilon approximately 10(3) M-1 cm-1) metal (Pt)-to-ligand (pi*) charge-transfer (MLCT) transitions. For 1 and 2, the MLCT transitions occur at approximately 390 nm, but the MLCT transition of 4 is exceptionally low in energy (492 nm). The low-temperature emission spectra of the complexes in frozen EMD glass indicate that 3 pi pi* is the emissive excited state for 1 and 2 but the emission of 3 is from a 3MLCT excited state. On the basis of the spectroscopic results, the order of energy of the MLCT excited states is established as 1 approximately 2 > 3 > 4. It is proposed that the red shifts of the MLCT transitions in 3 and 4 are due to increased electron-donating abilities of the ancillary pyridine ligands and intramolecular interactions between the orbitals of amine nitrogen lone pairs. Crystal data for the complexes are as follows. 1: triclinic P1, Z = 2, a = 8.7917(2) A, b = 10.6398(3) A, c = 11.9592(3) A, alpha = 107.130(1) degrees, beta = 92.522(1) degrees, gamma = 111.509(1) degrees. 2.CH3CN: triclinic P1, Z = 2, a = 7.0122(4) A, b = 12.9653(8) A, c = 14.0283(9) A, alpha = 107.3100(10) degrees, beta = 102.7640(10) degrees, gamma = 91.6320(10) degrees. 3.CH3CN: triclinic P1, Z = 2, a = 7.6459(1) A, b = 10.8433(1) A, c = 14.8722(2) A, alpha = 99.383(1) degrees, beta = 93.494(1) degrees, gamma = 101.385(1) degrees. 4.CH3CN: triclinic P1, Z = 2, a = 7.862(2) A, b = 10.977(3) A, c = 14.816(5) A, alpha = 99.34(2) degrees, beta = 92.64(2) degrees, gamma = 104.11(2) degrees.