Chensheng Ma

Luminescent Organogold(III) Complexes with Long‐Lived Triplet Excited States for Light‐Induced Oxidative CH Bond Functionalization and Hydrogen Production

All that glitters is gold: highly phosphorescent gold(III) complexes with extended π-conjugated cyclometalating ligands exhibit rich photophysical and photochemical properties. They act as efficient photocatalysts/photosensitizers for oxidative functionalizations of secondary and tertiary benzylic amines and homogeneous hydrogen production from a water/acetonitrile mixture.

A doorway state leads to photostability or triplet photodamage in thymine DNA

Ultraviolet irradiation of DNA produces electronic excited states that predominantly eliminate the excitation energy by returning to the ground state (photostability) or following minor pathways into mutagenic photoproducts (photodamage). The cyclobutane pyrimidine dimer (CPD) formed from photodimerization of thymines in DNA is the most common form of photodamage. The underlying molecular processes governing photostability and photodamage of thymine-constituted DNA remain unclear. Here, a combined femtosecond broadband time-resolved fluorescence and transient absorption spectroscopies were employed to study a monomer thymidine and a single-stranded thymine oligonucleotide. We show that the protecting deactivation of a thymine multimer is due to an ultrafast single-base localized stepwise mechanism where the initial excited state decays via a doorway state to the ground state or proceeds via the doorway state to a triplet state identified as a major precursor for CPD photodamage. These results provide new mechanistic characterization of and a dynamic link between the photoexcitation of DNA and DNA photostability and photodamage.

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

Development of a Broadband Picosecond Infrared Spectrometer and its Incorporation into an Existing Ultrafast Time-Resolved Resonance Raman, UV/Visible, and Fluorescence Spectroscopic Apparatus

We have constructed a broadband ultrafast time-resolved infrared (TRIR) spectrometer and incorporated it into our existing time-resolved spectroscopy apparatus, thus creating a single instrument capable of performing the complementary techniques of femto-/picosecond time-resolved resonance Raman (TR3), fluorescence, and UV/visible/infrared transient absorption spectroscopy. The TRIR spectrometer employs broadband (150 fs, ∼150 cm−1 FWHM) mid-infrared probe and reference pulses (generated by difference frequency mixing of near-infrared pulses in type I AgGaS2), which are dispersed over two 64-element linear infrared array detectors (HgCdTe). These are coupled via custom-built data acquisition electronics to a personal computer for data processing. This data acquisition system performs signal handling on a shot-by-shot basis at the 1 kHz repetition rate of the pulsed laser system. The combination of real-time signal processing and the ability to normalize each probe and reference pulse has enabled us to achieve a high sensitivity on the order of ΔOD ∼ 10−4–10−5 with 1 min of acquisition time. We present preliminary picosecond TRIR studies using this spectrometer and also demonstrate how a combination of TRIR and TR3 spectroscopy can provide key information for the full elucidation of a photochemical process.

Organic triplet excited states of gold(I) complexes with oligo (o- or m-phenyleneethynylene) ligands : conjunction of steady-state and time-resolved spectroscopic studies on exciton delocalization and emission pathways

A series of mononuclear and binuclear gold(I) complexes containing oligo(o- or m-phenyleneethynylene) (PE) ligands, namely [PhC≡C(C(6)H(4)-1,2-C≡C)(n-1)Au(PCy(3))] (n = 2-4, 4a-c), [μ-{C≡C-(1,2-C(6)H(4)C≡C)(n)}{Au(PCy(3))}(2)] (n = 1-6, 8, 5a-g), [PhC≡C(C(6)H(4)-1,3-C≡C)(n-1)Au(PCy(3))] (n = 2-4, 6a-c), and [μ-{C≡C-(1,3-C(6)H(4)C≡C)(n)}{Au(PCy(3))}(2)] (n = 1, 2, 7a,b), were synthesized and structurally characterized. Extensive spectroscopic measurements have been performed by applying combined methods of femtosecond transient absorption (fs-TA), fs time-resolved fluorescence (fs-TRF), and nanosecond time-resolved emission (ns-TRE) coupled with steady-state absorption and emission spectroscopy at both ambient and low (77 K) temperatures to directly probe the temporal evolution of the excited states and to determine the dynamics and spectral signatures for the involved singlet (S(1)) and triplet (T(1)) excited states. The results reveal that S(1) and T(1) both feature ligand-centered electronic transitions with ππ* character associated with the phenyl and acetylene moieties. The (3)ππ* emission of the PE ligands is switched on by the attachment of [Au(PCy(3))](+) fragment(s) due to the heavy-atom effect. T(1)((3)ππ*) was found to form with nearly unity efficiency through intersystem crossing (ISC) from S(1)((1)ππ*). The ISC time constants were determined to be ∼50, 35, and 40 ps for 4b and 6a,b, respectively. Dual emission composed of fluorescence from S(1) and phosphorescence from T(1) were observed for most of the complexes except 5a and 7a, where only phosphorescence was found. The fluorescence at ambient temperature is accounted for by both the short-lived prompt fluorescence (PF) and long-lived delayed fluorescence (DF, lifetime on microsecond time scale). Explicit evidence was presented for a triplet-triplet annihilation mechanism for the generation of DF. Ligand length and substitution-dependent dynamics of T(1) are the key factors governing the dual emission character of the complexes. By extrapolation from the plot of emission energy against the PE chain length of the [Au(PCy(3))](+) complexes with oligo(o-PE) or oligo(m-PE) ligands, the triplet emission energies were estimated to be ∼530 and ∼470 nm for poly(o-PE) and poly(m-PE), respectively. Additionally, we assign the unusual red shifts of 983 cm(-1) from [PhC≡CAu(PCy(3))] (1) to [μ-{1,3-(C≡C)(2)C(6)H(4)}{Au(PCy(3))}(2)] (7a) and 462 cm(-1) from 7a to [μ(3)-{1,3,5-(C≡C)(3)C(6)H(3)}{Au(PCy(3))}(3)] (8) in the phosphorescence energies to excitonic coupling interactions between the C≡CAu(PCy(3)) arms in the triplet excited states. These complexes, together with those previously reported [Au(PCy(3))](+) complexes containing oligo(p-PE) ligands ( J. Am. Chem. Soc. 2002 , 124 , 14696 - 14706 ), form a collection of oligo(phenyleneethynylene) complexes exhibiting organic triplet emission in solution under ambient conditions. The remarkable feature of these complexes in exhibiting TTA prompted DF in conjunction with high formation efficiency of T(1)((3)ππ*) affords an opportunity for emission spectra to cover a wide range of wavelengths. This may have implication in the development of PE-based molecular materials for future optical applications.

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.

Picosecond time-resolved resonance Raman observation of the iso-CH2I–I photoproduct from the “photoisomerization” reaction of diiodomethane in the solution phase

We report a preliminary picosecond Stokes and anti-Stokes time-resolved resonance Raman (267 nm pump and 400 nm probe excitation wavelengths) investigation of the initial formation and vibrational cooling of the iso-CH2I–I photoproduct species produced after ultraviolet excitation of diiodomethane in room temperature solutions. A comparison of the picosecond resonance Raman spectra with previously reported nanosecond transient resonance Raman spectra and density functional theory computations shows that the iso-CH2I–I photoproduct species is predominantly responsible for the ∼385 nm transient absorption band observed from several picoseconds to nanoseconds after ultraviolet excitation of diiodomethane in the solution phase. Similar results were obtained in both nonpolar solution (cyclohexane solvent) and polar solution (acetonitrile) solvent. The picosecond resonance Raman spectra confirm that the iso-CH2I–I photoproduct species is formed vibrationally hot within several picoseconds and then subsequently ...

Water‐Soluble Luminescent Cyclometalated Gold(III) Complexes with cis‐Chelating Bis(N‐Heterocyclic Carbene) Ligands: Synthesis and Photophysical Properties

A new class of cyclometalated Au(III) complexes containing various bidentate C-deprotonated C^N and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized and characterized. These are the first examples of Au(III) complexes supported by cis-chelating bis-NHC ligands. [Au(C^N)(bis-NHC)] complexes display emission in solutions under degassed condition at room temperature with emission maxima (λmax ) at 498-633 nm and emission quantum yields of up to 10.1 %. The emissions are assigned to triplet intraligand (IL) π→π* transitions of C^N ligands. The Au(III) complex containing a C^N (C-deprotonated naphthalene-substituted quinoline) ligand with extended π-conjugation exhibits prompt fluorescence and phosphorescence of comparable intensity with λmax at 454 and 611 nm respectively. With sulfonate-functionalized bis-NHC ligand, four water-soluble luminescent Au(III) complexes, including those displaying both fluorescence and phosphorescence, were prepared. They show similar photophysical properties in water when compared with their counterparts in acetonitrile. The long phosphorescence lifetime of the water-soluble AuIII complex with C-deprotonated naphthalene-substituted quinoline ligand renders it to function as ratiometric sensor for oxygen. Inhibitory activity of one of these water-soluble Au(III) complexes towards deubiquitinase (DUB) UCHL3 has been investigated; this complex also displayed a significant inhibitory activity with IC50 value of 0.15 μM.

Picosecond time-resolved resonance Raman observation of the iso-CH2Cl–I and iso-CH2I–Cl photoproducts from the “photoisomerization” reactions of CH2ICl in the solution phase

We report a preliminary picosecond Stokes time-resolved resonance Raman investigation of the initial formation and subsequent decay of the photoproduct produced following 267 nm excitation of CH2ClI in acetonitrile solution. Density-functional theory computations were done for several probable photoproduct species. Comparison of these computational results and results from a recent femtosecond transient absorption study to our present picosecond resonance Raman spectra indicate that the iso-CH2Cl–I species is mainly produced and associated with the ∼460 nm transient absorption band. The iso-CH2Cl–I species appears to decay and form appreciable amounts of the more stable iso-CH2I–Cl species that is associated with a ∼370 nm transient absorption band after a few hundred ps.