Search

Growing interest in the use of first-row transition metal complexes in a number of applied contexts—including but not limited to photoredox catalysis and solar energy conversion—underscores the need for a detailed understanding of their photophysical properties. A recent focus on ligand-field photocatalysis using cobalt(III) polypyridyls in particular has unlocked unprecedented excited-state reactivities. Photophysical studies on Co(III) chromophores in general are relatively uncommon, and so here we carry out a systematic study of a series of Co(III) polypyridyl complexes in order to delineate their excited-state dynamics. Compounds with varying ligand-field strengths were prepared and studied using variable-temperature ultrafast transient absorption spectroscopy. Analysis of the data establishes that the ground-state recovery dynamics are operating in the Marcus inverted region, in stark contrast to what is typically observed in other first-row metal complexes. The analysis has further revealed the underlying reasons driving this excited-state behaviour, thereby enabling potential advancements in the targeted use of the Marcus inverted region for a variety of photolytic applications.

Increasing interest in sustainable chemistry coupled with the quest to explore new reactivity has spurred research on first-row transition metal complexes for potential applications in a variety of settings. One of the more active areas of research is photoredox catalysis, where the synthetically tunable nature of their electronic structures provides a rich palette of options for tailoring their reactivity to a desired chemical transformation. Understanding the mechanism of excited-state reactivity is critical for the informed development of next-generation catalysts, which in turn requires information concerning the propensity of their electronic excited states to engage in the desired electron or energy transfer processes. Herein we provide direct evidence of the highly oxidizing nature of the lowest-energy ligand-field (LF) excited state of a first-row d6-low-spin Co(III) photosensitizer [Co(4,4′-Br2bpy)3]3+(where 4,4′-Br2bpy is 4,4′-dibromo-2,2′-bipyridine). The redox potential associated with the LF excited state of the Co(III) complex was bracketed by performing bimolecular quenching studies by using a series of simple organic electron donors. Time-resolved absorption spectroscopy confirmed a dynamic quenching process attributed to reductive quenching of the lowest-energy ligand-field excited state of the Co(III) chromophore. Analysis of the Stern–Volmer plots for each chromophore-quencher pair revealed a limiting value of Ered*∼ 1.25 V vs Fc/Fc+for the metal-centered excited state, which is significantly stronger than that of more commonly employed transition metal-based photoredox agents such as [Ru(bpy)3]2+(Ered*= 0.32 V vs Fc/Fc+) and [Ir(ppy)2(bpy)]+(Ered*= 0.27 V vs Fc/Fc+). These results suggest that this class of chromophores could find utility in applications requiring the activation of oxidatively resistant organic substrates for photoredox catalysis.

The development of chromophores based on earth-abundant transition metals whose photophysical properties are dominated by their charge-transfer excited states has inspired considerable research over the past decade. One challenge associated with this effort is satisfying the dual requirements of a strong ligand field and chemical tunability of the compound’s absorptive cross-section. Herein we explore one possible approach using a heteroleptic compositional motif that combines both of these attributes into a single compound. With the parent complex [Fe(phen)3]2+(1; where phen is 1,10-phenanthroline) as the starting material, replacement of one of the phen ligands for two cyanides to obtain Fe(phen)2(CN)2(2) allows for conversion to [Fe(phen)2(C4H10N4)]2+(3), a six-coordinate Fe(II) complex whose coordination sphere consists of two chelating polypyridyl ligands and one bidentate carbene-based donor. Ground-state absorption spectra of all three compounds exhibit 1A1→ 1MLCT transition(s) associated with the phen ligands that are relatively insensitive to the identity of the third counterligand(s). Ultrafast time-resolved electronic absorption measurements revealed lifetimes for the MLCT excited states of compounds 1and 2of 180 ± 30 and 250 ± 90 fs, respectively, values that are typical for iron(II)-based polypyridyl complexes. The corresponding kinetics for compound 3were substantially slower at 7.4 ± 0.9 ps; the spectral evolution associated with these dynamics confirms that these kinetics are tracking the MLCT excited state and, more importantly, are coupled to ground-state recovery from this excited state. The data are interpreted in terms of a modulation of the relative energies of the MLCT and ligand-field states across the series, leading to a systematic destabilization of metal-localized ligand-field excited states such that the low-energy portions of the charge-transfer and ligand-field manifolds are at or near an energetic inversion point in compound 3. We believe these results illustrate the potential for a modular, orthogonal approach to chromophore design in which part of the coordination sphere can be targeted for light absorption while another can be used to tune electronic-state energetics.

Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy)x(bpy)3–x]2+(CN-Me-bpy = 4,4′-dicyano-5,5′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine, and x= 1–3, abbreviated as 12+, 22+, and 32+) undergo four (12+) or five (22+and 32+) successive one-electron reduction steps between −1.3 and −2.75 V versus ferrocenium/ferrocene (Fc+/Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 22+and 32+being separated by 150–210 mV; reduction of the unsubstituted bpy ligand in 12+and 22+occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Me-bpy ligand shifts the ν(C≡N) IR band by ca. −45 cm–1, enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2+, 3+, and 30show distinct ν(C≡N) features due to the presence of both CN-Me-bpy and CN-Me-bpy•–, confirming that each reduction is localized on a single ligand. NIR spectra of 10, 1–, and 2–exhibit a prominent band attributable to the CN-Me-bpy•–moiety between 6000 and 7500 cm–1, whereas bpy•–-based absorption occurs between 4500 and 6000 cm–1; complexes 2+, 3+, and 30also exhibit a band at ca. 3300 cm–1due to a CN-Me-bpy•–→ CN-Me-bpy interligand charge-transfer transition. In the UV–vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 12+is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(C≡N) IR band is shifted by −38 cm–1and 5–7-fold-enhanced relative to the ground state.

Photoredox catalysis employing ruthenium- and iridium-based chromophores have been the subject of considerable research. However, the natural abundance of these elements are among the lowest on the periodic table, a fact that has led to an interest in developing chromophores based on earth-abundant transition metals that can perform the same function. There have been reports of using FeII-based polypyridyl complexes as photocatalysts, but there is limited mechanistic information pertaining to the nature of their reactivity in the context of photoredox chemistry. Herein, we report the results of bimolecular quenching studies between [Fe(tren(py)3)]2+(where tren(py)3= tris(2-pyridyl-methylimino-ethyl)amine) and a series of benzoquinoid acceptors. The data provide direct evidence of electron transfer involving the lowest-energy ligand-field excited state of the Fe(II)-based photosensitizer, definitively establishing that Fe(II) polypyridyl complexes can engage in photoinduced redox reactions but by a mechanism that is fundamentally different than the MLCT-based chemistry endemic to their second- and third-row congeners.

Design-specific control over excited-state dynamics is necessary for any application seeking to convert light into chemical potential. Such control is especially desirable in iron(ii)-based chromophores, which are an Earth-abundant option for a wide range of photo-induced electron-transfer applications including solar energy conversion1and catalysis2. However, the sub-200-femtosecond lifetimes of the redox-active metal-to-ligand charge transfer (MLCT) excited states typically encountered in these compounds have largely precluded their widespread use3. Here we show that the MLCT lifetime of an iron(ii) complex can be manipulated using information from excited-state quantum coherences as a guide to implementing synthetic modifications that can disrupt the reaction coordinate associated with MLCT decay. We developed a structurally tunable molecular platform in which vibronic coherences—that is, coherences reflecting a coupling of vibrational and electronic degrees of freedom—were observed in ultrafast time-resolved absorption measurements after MLCT excitation of the molecule. Following visualization of the vibrational modes associated with these coherences, we synthetically modified an iron(ii) chromophore to interfere with these specific atomic motions. The redesigned compound exhibits a MLCT lifetime that is more than a factor of 20 longer than that of the parent compound, indicating that the structural modification at least partially decoupled these degrees of freedom from the population dynamics associated with the electronic-state evolution of the system. These results demonstrate that using excited-state coherence data may be used to tailor ultrafast excited-state dynamics through targeted synthetic design.

TiO2-based dye-sensitized solar cells employing Fe(2,2′-bipyridine-4,4′-dicarboxylic acid)2(CN)2(F2CA) have been studied by spectroscopic, electrochemical, photoelectrochemical, time-resolved spectroscopic, and computational methods in an effort to assess how intercalating and passivating electrolyte additives influence photovoltaic performance in this class of devices. Ru(2,2′-bipyridine-4,4′-dicarboxylic acid)2(NCS)2(N3)-sensitized titanium dioxide (TiO2) devices were also studied by similar methods to provide a comparative standard. Ground-state spectroscopic and electrochemical measurements show that the dye electronic structure was influenced in a nonsystematic way by the presence of various electrolyte components such as lithium (Li+), tetrabutylammonium (TBA+), and pyridyl additives, whereas the conduction band edge of TiO2was impacted in a systematic way. An order-of-magnitude increase in short-circuit current (Jsc) and cell efficiency (η) was observed for F2CA-sensitized devices with our selected electrolytes as compared to electrolyte compositions that are typically employed. These substantial gains in F2CA-sensitized device performance correlated with conduction band edge stabilization attributed to electrolyte cation intercalation. Time-resolved spectroscopic and computational studies explain how stabilization of the conduction band edge, which allows for better dye-conduction band overlap, increases the injection yield for F2CA–TiO2assemblies. These studies serve to elevate our understanding of how electrolyte composition may affect injection and photovoltaic performance in sensitizer assemblies with short-lived charge-separated excited states.

We have employed a range of ultrafast X-ray spectroscopies in an effort to characterize the lowest energy excited state of [Fe(dcpp)2]2+(where dcpp is 2,6-(dicarboxypyridyl)pyridine). This compound exhibits an unusually short excited-state lifetime for a low-spin Fe(II) polypyridyl complex of 270 ps in a room-temperature fluid solution, raising questions as to whether the ligand-field strength of dcpp had pushed this system beyond the 5T2/3T1crossing point and stabilizing the latter as the lowest energy excited state. Kα and Kβ X-ray emission spectroscopies have been used to unambiguously determine the quintet spin multiplicity of the long-lived excited state, thereby establishing the 5T2state as the lowest energy excited state of this compound. Geometric changes associated with the photoinduced ligand-field state conversion have also been monitored with extended X-ray absorption fine structure. The data show the typical average Fe-ligand bond length elongation of ∼0.18 Å for a 5T2state and suggest a high anisotropy of the primary coordination sphere around the metal center in the excited 5T2state, in stark contrast to the nearly perfect octahedral symmetry that characterizes the low-spin 1A1ground state structure. This study illustrates how the application of time-resolved X-ray techniques can provide insights into the electronic structures of molecules—in particular, transition metal complexes—that are difficult if not impossible to obtain by other means.

Ultrafast time-resolved electronic and infrared absorption measurements have been carried out on a series of Ru(II) polypyridyl complexes in an effort to delineate the dynamics of vibrational relaxation in this class of charge transfer chromophores. Time-dependent density functional theory calculations performed on compounds of the form [Ru(CN-Me-bpy)x(bpy)3-x]2+(x= 1–3 for compounds 1–3, respectively, where CN-Me-bpy is 4,4′-dicyano-5,5′-dimethyl-2,2′-bipyridine and bpy is 2,2′-bipyridine) reveal features in their charge-transfer absorption envelopes that allow for selective excitation of the Ru(II)–(CN-Me-bpy) moiety, the lowest-energy MLCT state(s) in each compound of the series. Changes in band shape and amplitude of the time-resolved differential electronic absorption data are ascribed to vibrational cooling in the CN-Me-bpy-localized 3MLCT state with a time constant of 8 ± 3 ps in all three compounds. This conclusion was corroborated by picosecond time-resolved infrared absorption measurements; sharpening of the CN stretch in the 3MLCT excited state was observed with a time constant of 3.0 ± 1.5 ps in all three members of the series. Electronic absorption data acquired at higher temporal resolution revealed spectral modulation over the first 2 ps occurring with a time constant of τ = 170 ± 50 fs, in compound 1; corresponding effects are significantly attenuated in compound 2and virtually absent in compound 3. We assign this feature to intramolecular vibrational redistribution (IVR) within the 3MLCT state and represents a rare example of this process being identified from time-resolved electronic absorption data for this important class of chromophores.

Coherence phenomena arise from interference, or the addition, of wave-like amplitudes with fixed phase differences. Although coherence has been shown to yield transformative ways for improving function, advances have been confined to pristine matter and coherence was considered fragile. However, recent evidence of coherence in chemical and biological systems suggests that the phenomena are robust and can survive in the face of disorder and noise. Here we survey the state of recent discoveries, present viewpoints that suggest that coherence can be used in complex chemical systems, and discuss the role of coherence as a design element in realizing function.

A study of a series of six-coordinate Co(III) complexes has been carried out to quantify spectroscopic parameters for a range of ligands that are commonly employed to realize strong charge-transfer absorptions in low-spin, d6systems. Identification of any three ligand-field transitions allows for the determination of the splitting parameter (10 Dq) as well as the Racah Band Cparameters for a given compound. The data revealed a relatively small spread in the magnitude of 10 Dq, ranging from ca. 23 000 cm–1in the case of [Co(pyrro-bpy)3]3+(where pyrro-bpy is 4,4′-dipyrrolidinyl-2,2′-bipyridine) to ca. 26 000 cm–1for [Co(terpy)2]3+(where terpy is 2,2′:6′,2″-terpyridine). Significantly, trends across the series suggest that polypyridyl ligands behave as net π-donors when interacting with Co(III), in contrast to the net π-accepting character they exhibit when bound to second- and third-row metals. The influence of strong σ donation associated with carbene-based ligands was evident from the data acquired for [Co(BMeImPy)2]3+(where BMeImPy is 3,3′-(pyridine-2,6-diyl)bis(1-methyl-1H-3-imidazolium)), where a 10 Dq value of ca. 30 000 cm–1was determined. Spectroscopic data were also analyzed for [Fe(bpy)3]2+using the results on [Co(bpy)3]3+as a reference point. A value for 10 Dq of 21 000 cm–1was estimated, indicating a reduction in the ligand-field strength of ca. 3000 cm–1upon replacing Co(III) with Fe(II). We suggest that this approach of taking advantage of the blueshift of the charge-transfer feature in Co(III) complexes to reveal otherwise obscured ligand-field bands can be a useful tool for the development of new ligand systems to expand the photofunctionality of first-row transition-metal-based chromophores.

BRAFV600EKis a frequent mutationally active tumor-specific kinase in melanomas that is currently targeted for therapy by the specific inhibitor PLX4032. Our studies with melanoma tumor cells that are BRAFV600EKand BRAFWTshowed that, paradoxically, while PLX4032 inhibited ERK12 in the highly sensitive BRAFV600EK, it activated the pathway in the resistant BRAFWTcells, via RAF1 activation, regardless of the status of mutations in NRAS or PTEN. The persistently active ERK12 triggered downstream effectors in BRAFWTmelanoma cells and induced changes in the expression of a wide-spectrum of genes associated with cell cycle control. Furthermore, PLX4032 increased the rate of proliferation of growth factor-dependent NRAS Q61L mutant primary melanoma cells, reduced cell adherence and increased mobility of cells from advanced lesions. The results suggest that the drug can confer an advantage to BRAFWTprimary and metastatic tumor cells in vivo and provide markers for monitoring clinical responses.

The electron injection dynamics of dye-sensitized TiO 2-based solar cells have been investigated to determine the effects of replacing the I 3−/I −redox system by non-redox-active supporting electrolytes. TiO 2films were sensitized with Ru(dcbpy) 2(NCS) 2, where dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine (the “N3” dye), and placed in contact with either M(ClO 4) or M(I 3−/I −) solutions (M = Li +or (n-C 4H 9) 4N +); cells that contained I 3−/I −were fully functional solar cells whose steady-state photocurrents were directly measured. In (n-C 4H 9) 4N +-containing solutions, significant differences were observed between the measured kinetics when ClO 4−was replaced by the redox-active I 3−/I −system. In particular, a ps time scale loss of the metal-to-ligand charge-transfer excited-state of the N3 dye, associated with electron injection, that was observed in cells containing either LiClO 4or [(n-C 4H 9) 4N]ClO 4was absent in fully functional solar cells that contained [(n-C 4H 9) 4N]I/I 2. These results underscore the importance of performing kinetics measurements on this class of solar cells under operational conditions if one is to obtain reliable correlations between the dynamics data and the steady-state performance metrics of the solar cell devices.

A variety of synthetic procedures have been employed that allow access to three new tetranuclear [M4(µ3-O)2]8+ (M 4;= 4;MnIII or FeIII) complexes. The complex [Fe4O2Cl2(O2CMe)6(bpy)2] displays an unusual structural asymmetry in its core that can be described as a hybrid of the bent (butterfly) and planar dispositions of four M atoms seen previously in such compounds. Magnetochemical and 57Fe Mössbauer studies on this compound showed that its structural asymmetry has little influence on these properties compared with the more symmetric types. The complex [NBun4][Fe4O2(O2CMe)7(pic)2] has been prepared by an unusual chelate substitution in [NBun4][Fe4O2(O2CMe)7(bpy)2] that does not disrupt the core or the bridging carboxylate groups.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The rapid synthesis of 4-aryl-2,2'-bipyridines is described leading to some previously reported compounds in good yields in addition to some new functionalized bipyridines.

Renal artery stenosis causing hypertension may be the sequel of blunt nonpenetrating abdominal trauma. Early recognition of such renal artery injury is essential. In the case reported, late recognition of unilateral traumatic renal artery stenosis led to surgical correction of the lesion and lasting cure of hypertension.

For Abstract see ChemInform Abstract in Full Text.

For Abstract see ChemInform Abstract in Full Text.