Catherine E. McCusker

Robust Cuprous Phenanthroline Sensitizer for Solar Hydrogen Photocatalysis

The Cu(I) metal-to-ligand charge-transfer complex, [Cu(dsbtmp)2](+) (dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline), exhibits outstanding stability as a visible-light-absorbing photosensitizer in hydrogen-evolving homogeneous photocatalysis. In concert with the Co(dmgH)2(py)Cl water reduction catalyst and N,N-dimethyl-p-toluidine sacrificial donor in 1:1 H2O:CH3CN, this Cu(I) sensitizer remains active even after 5 days of visible-light-pumped (λex = 452 ± 10 nm) hydrogen evolution catalysis. Deuteration studies illustrate that the hydrogen produced from this composition does indeed originate from aqueous protons.

Design of a Long-Lifetime, Earth-Abundant, Aqueous Compatible Cu(I) Photosensitizer Using Cooperative Steric Effects

A new homoleptic Cu(I) photosensitizer, [Cu(dsbtmp)2](+) (dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline), designed to exhibit cooperative steric hindrance, unexpectedly produced strong photoluminescence (Φ = 1.9-6.3%) and long excited state lifetimes (τ = 1.2-2.8 μs) in a broad range of coordinating and noncoordinating solvents. The combination of the 2,9-sec-butyl substituents with the neighboring 3,8-methyl groups led to a Cu(I) complex with small degrees of ground and excited state distortion ultimately producing a molecule with robust metal-to-ligand charge transfer photophysics largely insulated from solvent interactions, reversible redox chemistry serving as a strong excited state reductant, along with impressive thermodynamic and photochemical stability in solution.

Ligand-localized triplet-state photophysics in a platinum(II) terpyridyl perylenediimideacetylide.

The synthesis, electrochemistry, and photophysical behavior of a Pt(II) terpyridyl perylenediimide (PDI) acetylide (1) charge-transfer complex is reported. The title compound exhibits strong (ε ≈ 5 × 10(4) M(-1)cm(-1)) low-energy PDI acetylide-based π-π* absorption bands in the visible range extending to 600 nm, producing highly quenched singlet fluorescence (Φ = 0.014 ± 0.001, τ = 109 ps) with respect to a nonmetalated PDI model chromophore. Nanosecond transient absorption spectroscopy revealed the presence of a long excited-state lifetime (372 ns in 2-methyltetrahydrofuran) with transient features consistent with the PDI-acetylide triplet state, ascertained by direct comparison to a model Pt(II) PDI-acetylide complex lacking low-energy charge-transfer transitions. For the first time, time-resolved step-scan FT-IR spectroscopy was used to characterize the triplet excited state of the PDI-acetylide sensitized in the title compound and its associated model complex. The observed red shifts (∼30-50 cm(-1)) in the C═O and C≡C vibrations of the two Pt(II) complexes in the long-lived excited state are consistent with formation of the (3)PDI acetylide state and found to be in excellent agreement with the expected change in the relevant DFT-calculated IR frequencies in the nonmetalated PDI model chromophore (ground singlet state and lowest triplet excited state). Formation of the PDI triplet excited state in the title chromophore was also supported by sensitization of the singlet oxygen photoluminescence centered at ∼1275 nm in air-saturated acetonitrile solution, Φ((1)O(2)) = 0.52. In terms of light emission, only residual PDI-based red fluorescence could be detected and no corresponding PDI-based phosphorescence was observed in the visible or NIR region at 298 or 77 K in the Pt(II) terpyridyl perylenediimideacetylide.

Mono- and dinuclear cationic iridium(III) complexes bearing a 2,5-dipyridylpyrazine (2,5-dpp) ligand.

The synthesis, X-ray structures, photophysical, and electrochemical characterization of mono- (1) and dinuclear (2) cationic iridium(III) complexes bearing a 2,5-dipyridylpyrazine (2,5-dpp) ancillary ligand are reported. Upon the complexation of a first equivalent of iridium, the photoluminescence shifts markedly into the deep red (λem = 710 nm, ΦPL = 0.9%) compared to other cationic iridium complexes such as [Ir(ppy)2(bpy)]PF6. With the coordination of a second equivalent of iridium, room temperature luminescence is completely quenched. Both 1 and 2 are luminescent at low temperatures but with distinct excited state decay kinetics; the emission of 2 is significantly red-shifted compared to 1. Emission both at 298 and 77 K results from a mixed charge-transfer state. Density functional theory (DFT) calculations and electrochemical behavior point to an electronic communication between the two iridium complexes.

Orange-to-blue and red-to-green photon upconversion with a broadband absorbing copper(I) MLCT sensitizer

A synthetically facile and earth abundant Cu(I) metal-to-ligand charge transfer sensitizer was successfully incorporated into two distinct photochemical upconversion schemes, affording red-to-green and orange-to-blue wavelength conversions.

Dondorff Rings: Synthesis, Isolation, and Properties of 60°‐Directed Bisterpyridine‐Based Folded Tetramers

Square feat: The synthesis, isolation, and characterization of five novel bisterpyridine-based metallomacrocycles, possessing a folded tetrameric configuration is reported (see figure). The initial dimeric building block with the stable linear {tpy-Ru(II)-tpy} connectivity circumvents the formation of the thermodynamically favored molecular triangles.

Transient absorption dynamics of sterically congested Cu(I) MLCT excited states.

Subpicosecond through supra-nanosecond transient absorption dynamics of the homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizers including the benchmark [Cu(dmp)2](+) (dmp =2,9-dimethyl-1,10-phenanthroline) chromophore, as well as [Cu(dsbp)2](+) (dsbp =2,9-di(sec-butyl)-1,10-phenanthroline and [Cu(dsbtmp)2](+) (dsbtmp =2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) were investigated in dichloromethane and tetrahydrofuran solutions. Visible and near-IR spectroelectrochemical measurements of the singly reduced [Cu(dsbp)2](+) and [Cu(dsbtmp)2](+) species were determined in tetrahydrofuran, allowing for the identification of redox-specific phenanthroline-based radical anion spectroscopic signatures prevalent in the respective transient absorption experiments. This study utilized four different excitation wavelengths (418, 470, 500, and 530 nm) to elucidate dynamics on ultrafast times scales spanning probe wavelengths ranging from the UV to the near-IR (350 to 1450 nm). With the current time resolution of ∼150 fs, initial excited state decay in all three compounds was found to be independent of excitation wavelength. Not surprisingly, there was little to no observed influence of solvent in the initial stages of excited state decay in any of these molecules including [Cu(dmp)2](+), consistent with results from previous investigators. The combined experimental data revealed two ranges of time constants observed on short time scales in all three MLCT chromophores and both components lengthen as a function of structure in the following manner: [Cu(dsbtmp)2](+) < [Cu(dsbp)2](+) < [Cu(dmp)2](+). The molecule with the most inhibited potential for distortion, [Cu(dsbtmp)2](+), possessed the fastest ultrafast dynamics as well as the longest excited state lifetimes in both solvents. These results are consistent with a small degree of excited state distortion, rapid intersystem crossing, and weak vibronic coupling to the ground state. The concomitant systematic variation in both initial time constants, assigned to pseudo-Jahn-Teller distortion and intersystem crossing, suggest that both processes are intimately coupled in all molecules in the series. The variability in these time scales illustrate that strongly impeded structural distortion in Cu(I) MLCT excited state enables more rapid surface crossings in the initial deactivation dynamics.

Efficient Visible to Near-UV Photochemical Upconversion Sensitized by a Long Lifetime Cu(I) MLCT Complex

The current investigation compares the photochemical upconversion sensitization properties of two long lifetime Cu(I) metal-to-ligand charge transfer (MLCT) chromophores to 3 distinct anthryl-based triplet acceptors. The sensitizers [Cu(dsbtmp)2](PF6) (1, dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline) and [Cu(dsbp)2](PF6) (2, dsbp = 2,9-di(sec-butyl-1,10-phenanthroline) were selectively excited in the presence of anthracene, 9,10-diphenylanthracene (DPA), and 9,10-dimethylanthracene (DMA) in degassed dichloromethane solutions. In all instances, triplet energy transfer was observed from selective excitation of the Cu(I) MLCT chromophore to each respective anthryl species. The bimolecular triplet-triplet energy transfer quenching rate constants were extracted from dynamic Stern-Volmer analyses in each case, yielding values below the diffusion limit in dichloromethane. However, the Stern-Volmer quenching constants (KSVs) were sizable enough (up to ∼2300 M(-1) with 1 as a sensitizer) to support efficient photochemical upconversion. As such, visible to near-UV photochemical upconversion was observed in every instance, along with the anticipated quadratic-to-linear incident light power dependence when pumping at 488 nm. The latter verified that it is indeed sensitized triplet-triplet annihilation responsible for the generation of the anthryl-based singlet fluorescence. Photochemical upconversion quantum efficiencies were evaluated using a relative actinometric method as both a function of incident light power density as well as anthryl acceptor/annihilator concentration. When 1 was used as the sensitizer, upconversion quantum yields as large as 9.2% and 17.8% were observed for DMA and DPA, respectively. Finally, the combination of 1 with DMA was shown to be quite robust, showing no obvious signs of decomposition during 12 h of continuous 488 nm photolysis.

Metal Coordination Induced π-Extension and Triplet State Production in Diketopyrrolopyrrole Chromophores

Triplet state photophysics has been generated in two distinct diketopyrrolopyrrole (DPP) chromophores terminated with either phenyl (1) or thienyl (2) spacers, when sandwiched between two Ir(III) complexes using bipyridyl linkers. Upon coordination of the bpy-DPP-bpy subunit resulting in its planarization, the π-conjugation in the DPP chromophore formally extends and was manifested as a substantial red shift in the absorption and fluorescence profiles of 1 and 2. Low energy excitation of these dinuclear metal complexes produced strongly quenched singlet fluorescence, generated quite intense long-lived (τ ∼ 3 μs) absorption transients in the red, sensitized (1)O(2) photoluminescence centered at 1270 nm in aerated solutions, and yielded low temperature near-IR phosphorescence in 1 centered at 950 nm.

Excited state equilibrium induced lifetime extension in a dinuclear platinum(II) complex.

Covalently linking two square planar platinum(II) centers using two pyrazolate bridging ligands allows the filled dz(2) orbitals on each Pt center to overlap, producing a Pt-Pt σ interaction and new low energy dσ* → π* metal-metal-to-ligand charge transfer (MMLCT) transitions terminating on an appropriate π-acceptor ligand such as 2-phenylpyridine (ppy). In an effort to extend the lifetime of the associated MMLCT excited state, we decided to append piperidinyl naphthalimide (PNI) chromophores to the 2-phenylpyridine charge transfer ligands. This structural modification introduces low-lying PNI-based triplet states serving as long-lived triplet population reservoirs, thermally capable of repopulating the charge transfer state at room temperature (RT), thereby extending its excited state lifetime. Specifically, [Pt(PNI-ppy)(μ-Ph2pz)]2 (1), where PNI-ppy is N-(2-phenylpyridine)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide and Ph2pz is 3,5-diphenylpyrazolate, was synthesized and structurally characterized. The static and dynamic photophysical behavior of 1 was directly compared to the MMLCT complex [Pt(ppy)(μ-Ph2pz)]2 (2), lacking the PNI substituents, as well as the naked PNI-ppy ligand 3, intended to independently model the MMLCT and NI excited state properties, respectively. Ultimately, experimental evidence for the presence of both the (3)PNI and (3)MMLCT excited states in 1 were revealed at RT in nanosecond transient absorbance and time-resolved photoluminescence spectroscopy, respectively. Temperature-dependent transient absorption spectroscopy permitted the extraction of an energy gap of 1740 cm(-1) between the MMLCT and PNI triplet states in 1 along with the time constants associated with the interconversions between the various excited states resident on this complex chromophore, ultimately decaying back to the ground state with a time constant of 65 μs at RT.