Xiangguo Guan

Light-emitting platinum(II) complexes supported by tetradentate dianionic bis(N-heterocyclic carbene) ligands: towards robust blue electrophosphors

The synthesis, structures and photophysical properties of the charge-neutral Pt(II) complexes (1–6) and their Pd(II) (7) and Ni(II) (8) congeners supported by tetradentate dianionic bis[phenolate-(N-heterocyclic carbene)] ligands are described. The X-ray crystal structures of two solvatomorphs of 2, which has p-F substituents on the tetradentate ligand, have been determined. The photophysical properties of all the complexes were examined. In THF solutions, 1–4 display deep blue phosphorescence (λmax = ∼440–460 nm, Φe = 3–18% and τ = 0.5–3.5 μs). In solutions at room temperature, 5–8 show profoundly different luminescence properties from being virtually non-emissive (Φe < 10−3) for 6–8 to highly emissive (Φe = 15%) with much red-shifted phosphorescence (λmax = ∼530 nm) and a long emission lifetime (τ = 47.2 μs) in the case of 5. Time-dependent density functional theory (TDDFT) calculations reveal that the tetradentate bis(phenolate-NHC) ligands in 1–4 provide a rigid scaffold for preserving a tightly bound Pt(II) in a square-planar coordination geometry in the T1 as in the S0 states and the blue emission is derived from the T1 state having predominant ligand (πAr–O)-to-ligand (π*NHC) charge transfer (LLCT) character. A switch of orbital parentage from LLCT to ligand-centred (LC) π–π* is responsible for the long emission lifetime and vibronically structured emission displayed by 5 when compared to that of 1–4 and 6. Both femtosecond time-resolved fluorescence (fs-TRF) and nanosecond time-resolved emission (ns-TRE) measurements were conducted on 2 and 4 to directly probe the excited-state dynamics after photoexcitation. Excellent thermal stability of the fluorine-free complex 4 and its higher emission quantum yield (relative to 1 and 3), and using 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) as host material, led to the fabrication of highly efficient deep blue OLEDs with peak current efficiency of 24 cd A−1 and white organic light-emitting devices (WOLEDs) with peak current efficiency of 88 cd A−1.

Strongly Luminescent Gold(III) Complexes with Long‐Lived Excited States: High Emission Quantum Yields, Energy Up‐Conversion, and Nonlinear Optical Properties

Strongly Luminescent Gold(III) Complexes with Long-Lived Excited States: High Emission Quantum Yields, Energy Up-Conversion, and Nonlinear Optical Properties Photochemistry : A series of emissive gold(III) complexes with fluorene-containing cyclometalating ligands exhibits strong phosphorescence and long-lived excited states with emission quantum yields and lifetimes up to 58 % and 305 ms, respectively. These complexes can sensitize energy up-conversion of 9,10-diphenylanthracene (DPA; see picture) and display rich two-photon absorption properties (TPA; TTA = triplet–triplet annihilation). Angewandte Chemie

Blue electrophosphorescent organoplatinum(II) complexes with dianionic tetradentate bis(carbene) ligands

Robust charge-neutral Pt(II) complexes containing dianionic tetradentate bis(N-heterocyclic carbene) ligands exhibit intense blue phosphorescence in fluid solutions and in polymer films, and have been vacuum-deposited as a phosphorescent dopant in organic blue-light-emitting diodes.

Structurally robust phosphorescent [Pt(O^N^C^N)] emitters for high performance organic light-emitting devices with power efficiency up to 126 lm W−1 and external quantum efficiency over 20%

A series of robust, bulky and strongly emissive platinum(II) complexes supported by tetradentate O^N^C^N ligands with tert-butyl groups (1–4), a bridging tertiary amine (5) or a biphenyl group with a spiro linkage (6) at the periphery of the [O^N^C^N] ligand scaffold have been prepared. Their photophysical properties were examined by absorption and emission spectroscopy, density functional theory calculations, and ultra-fast time-resolved emission measurements. These complexes display emission quantum yields of up to 95%, with emission maxima λmax in the range of 522 to 570 nm, and have a good thermal stability of up to Td > 423 °C. Notably, the kq values of 4–6 are in the range of 8.5 × 106 to 2.0 × 107 mol−1 dm3 s−1, smaller than those (∼108 to 109 mol−1 dm3 s−1) of other reported Pt(II) complexes. The bulky groups at the periphery of the [O^N^C^N] ligand disfavour intermolecular interactions and hence excimer formation in solutions. These complexes are good light-emitting materials (dopants) for OLEDs, since the triplet–triplet annihilation (TTA) and concentration quenching effect arising from intermolecular interactions can be minimized even at a high dopant concentration. The efficiency of the devices fabricated with 4–6 increased with dopant concentration up to a high level of 10% with no extra emitting component or significant shift in the CIE observed. The maximum power efficiency (PE) values achieved for the 5 (yellow-emitting) and 6 (green-emitting) based devices were 118 and 126 lm W−1, respectively. These PE values are the highest among the reported Pt(II)-OLEDs and comparable to those of the best reported Ir(III)-OLEDs without the out-coupling technique. Complex 7 is structurally analogous to, but less bulky than 3–6 and is prone to giving excimer emission in the solid state. A high PE of up to 55.5 lm W−1 and external quantum efficiency of up to 25.1% have been realized in the white OLEDs fabricated with 7 as a single emitting material. These values are comparable with those of the best reported WOLEDs based on a single emitting material.

Long‐Lived Excited States of Zwitterionic Copper(I) Complexes for Photoinduced Cross‐Dehydrogenative Coupling Reactions

Four heteroleptic copper(I) complexes containing phenanthroline and monoanionic nido-carborane-diphosphine ligands have been prepared and structurally characterized by various spectroscopic techniques and X-ray diffraction. These complexes exhibit intense absorptions in the visible range and excited-state lifetimes on the microsecond scale. Their application in visible-light-induced cross-dehydrogenative coupling reactions was investigated. Preliminary studies showed that one of the four copper(I) complexes is an efficient catalyst for photoinduced oxidative C-H functionalization using oxygen as oxidant. Furthermore, α-functionalized tertiary amines were obtained in good-to-excellent yields by light irradiation (λ>420 nm) of a mixture of our Cu(I) complex, tertiary amines, and a variety of nucleophiles (nitroalkane, acetone, or indoles) under aerobic conditions. Electron paramagnetic resonance measurements provided evidence for the formation of superoxide radical anions (O2(-⋅)) rather than singlet oxygen ((1)O2) during these photocatalytic reactions.

A resonance Raman spectroscopic and CASSCF investigation of the Franck–Condon region structural dynamics and conical intersections of thiophene

Resonance Raman spectra were acquired for thiophene in cyclohexane solution with 239.5 and 266 nm excitation wavelengths that were in resonance with ∼240 nm first intense absorption band. The spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mostly along the reaction coordinates of six totally symmetry modes and three nontotally symmetry modes. The appearance of the nontotally symmetry modes, the C-S antisymmetry stretch +C-C=C bend mode ν(21)(B(2)) at 754 cm(-1) and the H(7)C(3)-C(4)H(8) twist ν(9)(A(2)) at 906 cm(-1), suggests the existence of two different types of vibronic-couplings or curve-crossings among the excited states in the Franck-Condon region. The electronic transition energies, the excited state structures, and the conical intersection points (1)B(1)/(1)A(1) and (1)B(2)/(1)A(1) between 2 (1)A(1) and 1 (1)B(2) or 1 (1)B(1) potential energy surfaces of thiophene were determined by using complete active space self-consistent field theory computations. These computational results were correlated with the Franck-Condon region structural dynamics of thiophene. The ring opening photodissociation reaction pathway through cleavage of one of the C-S bonds and via the conical intersection point (1)B(1)/(1)A(1) was revealed to be the predominant ultrafast reaction channel for thiophene in the lowest singlet excited state potential energy hypersurface, while the internal conversion pathway via the conical intersection point (1)B(2)/(1)A(1) was found to be the minor decay channel in the lowest singlet excited state potential energy hypersurface.

Water‐ and Acid‐Mediated Excited‐State Intramolecular Proton Transfer and Decarboxylation Reactions of Ketoprofen in Water‐Rich and Acidic Aqueous Solutions

We present an investigation of the decarboxylation reaction of ketoprofen (KP) induced by triplet excited-state intramolecular proton transfer in water-rich and acidic solutions. Nanosecond time-resolved resonance Raman spectroscopy results show that the decarboxylation reaction is facile in aqueous solutions with high water ratios (water/acetonitrile ≥50%) or acidic solutions with moderate and strong acid concentration. These experimental results are consistent with results from density functional theory calculations in which 1) the activation energy barriers for the triplet-state intramolecular proton transfer and associated decarboxylation process become lower when more water molecules (from one up to four molecules) are involved in the reaction system and 2) perchloric acid, sulfuric acid, and hydrochloric acid can shuttle a proton from the carboxyl to carbonyl group through an initial intramolecular proton transfer of the triplet excited state, which facilitates the cleavage of the C-C bond, thus leading to the decarboxylation reaction of triplet state KP. During the decarboxylation process, the water molecules and acid molecules may act as bridges to mediate intramolecular proton transfer for the triplet state KP when KP is irradiated by ultraviolet light in water-rich or acidic aqueous solutions and subsequently it generates a triplet-protonated carbanion biradical species. The faster generation of triplet-protonated carbanion biradical in acidic solutions than in water-rich solutions with a high water ratio is also supported by the lower activation energy barrier calculated for the acid-mediated reactions versus those of water-molecule-assisted reactions.

Photochemistry of Iodoform in Methanol: Formation and Fate of the Iso‐CHI2‐I Photoproduct

Ultrafast population and structural dynamics due to the iso-CHI(2)-I isomer product formed upon UV excitation of iodoform (CHI(3)) in solution is monitored by femtosecond transient absorption with deep-UV through near-IR probing and picosecond transient resonance Raman spectroscopy. Iso-CHI(2)-I is found to be a major photochemical product regardless of excitation wavelength (266 and 350 nm) and choice of solvent (methanol, acetonitrile, and cyclohexane), and is produced in 50% quantum yield upon 266 nm excitation of CHI(3) in CH(3)OH. The isomer remains stable up to at least several nanoseconds in C(6)H(12) and CH(3)CN, but undergoes decay with a 740 ps lifetime in CH(3)OH simultaneously with the formation of an iodide ion. In agreement with the experiments, MP2 calculations suggest that iso-CHI(2)-I readily reacts with CH(3)OH via O-H insertion/HI elimination reactions.

Bis(sulfonylimide)ruthenium(VI) Porphyrins: X‐ray Crystal Structure and Mechanism of CH Bond Amination by Density Functional Theory Calculations

The X-ray crystal structure of [Ru(VI) (NMs)2 (tmp)] (Ms=SO2 - p-MeOC6 H4 ; tmp=5,10,15,20-tetramesitylporphyrinato(2-)), a metal sulfonylimide complex that can undergo alkene aziridination and C-H bond amination reactions, shows a Ru=N distance of 1.79(3) Å and Ru-N-S angle of 162.5(3)°. Density functional theory (DFT) calculations on the electronic structures of [Ru(VI) (NMs)2 (tmp)] and model complex [Ru(VI) (NMs)2 (por(0) )] (por(0) =unsubstituted porphyrinato(2-)) using the M06L functional gave results in agreement with experimental observations. For the amination of ethylbenzene by the singlet ground state of [Ru(VI) (NMs)2 (por(0))], DFT calculations using the M06L functional revealed an effectively concerted pathway involving rate-limiting hydrogen atom abstraction without a distinct radical rebound step. The substituent effect on the amination reactivity of ethylbenzene by [Ru(VI) (NX)2 (por(0) )] (X=SO2 -p-YC6 H4 with Y=MeO, Me, H, Cl, NO2 ) was examined. Electron-withdrawing Y groups lower the energy of the LUMOs of [Ru(VI) (NX)2 (por(0))], thus facilitating their interaction with the low-lying HOMO of the ethylbenzene C-H bond and hence increasing the reactivity of [Ru(VI) (NX)2 (por(0) )]. DFT calculations on the amination/aziridination reactions of [Ru(VI) (NSO2 C6 H5 )2 (por(0) )] with pent-4-enal, an aldehyde substrate bearing acyl, homoallylic, and allylic C-H bonds and a C=C bond, revealed a lower reaction barrier for the amination of the acyl C-H bond than for both the amination of the other C-H bonds and aziridination of the C=C bond in this substrate.

Direct observation of an isopolyhalomethane O–H insertion reaction with water: Picosecond time-resolved resonance Raman (ps-TR3) study of the isobromoform reaction with water to produce a CHBr2OH product

Picosecond time-resolved resonance Raman (ps-TR3) spectroscopy was used to obtain the first definitive spectroscopic observation of an isopolyhalomethane O-H insertion reaction with water. The ps-TR3 spectra show that isobromoform is produced within several picoseconds after photolysis of CHBr3 and then reacts on the hundreds of picosecond time scale with water to produce a CHBr2OH reaction product. Photolysis of low concentrations of bromoform in aqueous solution resulted in noticeable formation of HBr strong acid. Ab initio calculations show that isobromoform can react with water to produce a CHBr2(OH) O-H insertion reaction product and a HBr leaving group. This is consistent with both the ps-TR3 experiments that observe the reaction of isobromoform with water to form a CHBr2(OH) product and photolysis experiments that show HBr acid formation. We briefly discuss the implications of these results for the phase dependent behavior of polyhalomethane photochemistry in the gas phase versus water solvated environments.