Metal-to-ligand Charge Transfer Excited State Research Articles

Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups.

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)3](PF6)2, where dab is 4,4'-dicarbaldehyde-2,2'-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CH3CN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (ε = 22,000 M-1 cm-1) and an infrared (IR) band at 1712 cm-1 assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, ϕPL = 0.048, and an excited state lifetime, το = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, kr = 9.1 × 104 s-1 and knr = 1.8 × 106 s-1. Pulsed visible light excitation yielded transient UV-vis and IR absorption spectra consistent with an MLCT excited state; relaxation occurred with the maintenance of two isosbestic points in the visible region, and a lifetime that agreed with that measured by time-resolved PL. Cyclic voltammetry studies in a CH3CN solution with 0.1 M TBAPF6 electrolyte revealed a quasi-reversible oxidation, E°(RuIII/II) = +1.25 V vs. Fc+/0, and three sequential one-electron reductions at -1.10, -1.25, and -1.54 V vs. Fc+/0. An excited state reduction potential of E°(Ru*2+/+) = +0.89 V vs. Fc+/0 was estimated with the Rehm-Weller expression. Titration of tetrabutylammonium iodide, TBAI, into a CD3CN solution of [Ru(dab)3](PF6)2 resulted in significant shifts in the aldehyde H atom and 3,3'-biypridyl resonances that were analyzed with a 1:1 equilibrium model, from which Keq = 460 M-1 was extracted, increasing to 5800 M-1 when the solvent was changed to acetone-d6. Iodide titrations resulted in a significant quenching of the [Ru(dab)3]*2+ lifetime and quantum yield in both CH3CN and acetone solvents. In CH3CN, the quenching was mainly dynamic and well described by the Stern-Volmer model, from which a quenching rate constant, kq, of 4.5 × 1010 M-1 s-1 and an equilibrium constant, Keq, of 8.3 × 103 M-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a Keq of 1.2 × 104 M-1 and a higher kq of 9.2 × 1010 M-1 s-1.

Impact of Diimine Ligand on Singlet-to-Triplet Charge Transfer Electroabsorption in Iron and Ruthenium Complexes.

The electroabsorption and absorption spectra of eight homoleptic complexes of the general form [M(LL)3]2+ where M = Ru, Fe, and LL = 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and 4,4',-(R)2-bpy where R = -OCH3, -CF3, were quantified at 77 K in a butyronitrile glass. Intense metal-to-ligand charge transfer (MLCT) absorption bands were evident in the visible region. Electroabsorption spectra measured with applied electric fields >0.2 MV/cm were analyzed by the two-state Liptay model. Significant light-induced dipole moment changes of = 4-13 D were found consistent with a metal-to-ligand charge transfer (MLCT) excited state comprised an electron localized on a single diimine ligand, [MIII(LL-)(LL)2]*2+, in the initially formed Franck-Condon excited state. A low energy feature evident in the electroabsorption spectra was assigned to a direct singlet-to-triplet MLCT excited state. The identity of the diimine ligand had an unexpected and large impact on these transitions. Analysis relative to the higher energy absorption provides a comparison of spin-allowed and disallowed transitions for first- and second-row transition metal complexes. With the notable exception of [Fe(CF3bpy)3]2+, the change in dipole moment for the 3MLCT excited states was less than or equal to that of the 1MLCT excited states. The charge transfer distances for the iron complexes were generally larger than those for the Ru complexes, a behavior attributed to a smaller degree of iron-diimine coupling in the ground state. A striking result was the sensitivity of the extinction coefficient and spectral profile of the low energy electroabsorption assigned to the identity of the diimine ligand; data that suggests electronic coupling with ligand localized triplet states and high spin metal centered states must be considered when modeling the Franck-Condon excited state.

Controlling the Photophysical Properties of a Series of Isostructural d6 Complexes Based on Cr0, MnI, and FeII.

Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes.

Enhanced Visible Light Absorption in Heteroleptic Cuprous Phenanthrolines.

This work presents a series of Cu(I) heteroleptic 1,10-phenanthroline chromophores featuring enhanced UVA and visible-light-harvesting properties manifested through vectorial control of the copper-to-phenanthroline charge-transfer transitions. The molecules were prepared using the HETPHEN strategy, wherein a sterically congested 2,9-dimesityl-1,10-phenanthrolne (mesPhen) ligand was paired with a second phenanthroline ligand incorporating extended π-systems in their 4,7-positions. The combination of electrochemistry, static and time-resolved electronic spectroscopy, 77 K photoluminescence spectra, and time-dependent density functional theory calculations corroborated all of the experimental findings. The model chromophore, [Cu(mesPhen)(phen)]+ (1), lacking 4,7-substitutions preferentially reduces the mesPhen ligand in the lowest energy metal-to-ligand charge-transfer (MLCT) excited state. The remaining cuprous phenanthrolines (2-4) preferentially reduce their π-conjugated ligands in the low-lying MLCT excited state. The absorption cross sections of 2-4 were enhanced (εMLCTmax = 7430-9980 M-1 cm-1) and significantly broadened across the UVA and visible regions of the spectrum compared to 1 (εMLCTmax = 6494 M-1 cm-1). The excited-state decay mechanism mirrored those of long-lived homoleptic Cu(I) phenanthrolines, yielding three distinguishable time constants in ultrafast transient absorption experiments. These represent pseudo-Jahn-Teller distortion (τ1), singlet-triplet intersystem crossing (τ2), and the relaxed MLCT excited-state lifetime (τ3). Effective light-harvesting from Cu(I)-based chromophores can now be rationalized within the HETPHEN strategy while achieving directionality in their respective MLCT transitions, valuable for integration into more complex donor-acceptor architectures and longer-lived photosensitizers.

Electronic Structures and Photoredox Chemistry of Tungsten(0) Arylisocyanides.

ConspectusThe high energy barriers associated with the reaction chemistry of inert substrates can be overcome by employing redox-active photocatalysts. Research in this area has grown exponentially over the past decade, as transition metal photosensitizers have been shown to mediate challenging organic transformations. Critical for the advancement of photoredox catalysis is the discovery, development, and study of complexes based on earth-abundant metals that can replace and/or complement established noble-metal-based photosensitizers.Recent work has focused on redox-active complexes of 3d metals, as photosensitizers containing these metals most likely would be scalable. Although low lying spin doublet ("spin flip") excited states of chromium(III) and metal-to-ligand charge transfer (MLCT) excited states of copper(I) have relatively long lifetimes, the electronic excited states of many other 3d metal complexes fall on dissociative potential energy surfaces, owing to the population of highly energetic σ-antibonding orbitals. Indeed, we and other investigators have shown that low lying spin singlet and triplet excited states of robust closed-shell metal complexes are too short-lived at room temperature to engage in bimolecular reactions in solutions. In principle, this problem could be overcome by designing and constructing 3d metal complexes containing strong field π-acceptor ligands, where thermally equilibrated MLCT or intraligand charge transfer excited states might fall well below the upper surfaces of dissociative 3d-3d states. Notably, such design elements have been exploited by investigators in very recent work on redox-active iron(II) systems. Another approach, one we have actively pursued, is to design and construct closed-shell complexes of earth-abundant 5d metals containing very strong π-acceptor ligands, where vertical excitation of 5d-5d excited states at the ground state geometry would require energies far above minima in the potential surfaces of MLCT excited states. As this requirement is met by tungsten(0) arylisocyanides, these complexes have been the focus of our work aimed at the development of robust redox-active photosensitizers.In the following Account, we review recent work on homoleptic tungsten(0) arylisocyanides. Originally reported by our group 45 years ago, W(CNAr)6 complexes have exceptionally large one- and two-photon absorption cross-sections. One- or two-photon excitation produces relatively long-lived (hundreds of nanoseconds to microsecond) MLCT excited states in high yields. These MLCT excited states, which are very strong reductants with E°(W+/*W0) = -2.2 to -3.0 V vs Fc[+/0], mediate photocatalysis of organic reactions with both visible and near-infrared (NIR) light. Here, we highlight design principles that led to the development of three generations of W(CNAr)6 photosensitizers; and we discuss likely steps in the mechanism of a prototypal W(CNAr)6-catalyzed base-promoted homolytic aromatic substitution reaction. Among the many potential applications of these very bright luminophores, two-photon imaging and two-photon-initiated polymerization are ones we plan to pursue.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII.

Many coordination complexes and organometallic compounds with the 4d6 and 5d6 valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d6 complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr0, MnI, FeII, and CoIII compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Employing Long-Range Inductive Effects to Modulate Metal-to-Ligand Charge Transfer Photoluminescence in Homoleptic Cu(I) Complexes

Four Cu(I) bis(phenanthroline) photosensitizers formulated from a new ligand structural motif (Cu1-Cu4) coded according to their 2,9-substituents were synthesized, structurally characterized, and fully evaluated using steady-state and time-resolved absorption and photoluminescence (PL) measurements as well as electrochemistry. The 2,9-disubstituted-3,4,7,8-tetramethyl-1,10-phenanthroline ligands feature the following six-membered ring systems prepared through photochemical synthesis: 4,4-dimethylcyclohexyl (1), tetrahydro-2H-pyran-4-yl (2), tetrahydro-2H-thiopyran-4-yl (3), and 4,4-difluorocyclohexyl (4). Universally, these Cu(I) metal-to-ligand charge transfer (MLCT) chromophores display excited-state lifetimes on the microsecond time scale at room temperature, including the three longest-lived homoleptic cuprous phenanthroline excited states measured to date in de-aerated CH2Cl2, τ = 2.5-4.3 μs. This series of molecules also feature high PL quantum efficiencies (ΦPL = 5.3-12% in CH2Cl2). Temperature-dependent PL lifetime experiments confirmed that all these molecules exhibit reverse intersystem crossing and display thermally activated delayed PL from a 1MLCT excited state lying slightly above the 3MLCT state, 1050-1490 cm-1. Ultrafast and conventional transient absorption measurements confirmed that the PL originates from the MLCT excited state, which remains sterically arrested, preventing an excessive flattening distortion even when dissolved in Lewis basic CH3CN. Combined PL and electrochemical data provided evidence that Cu1-Cu4 are highly potent photoreductants (Eox* = -1.73 to -1.62 V vs Fc+/0 in CH3CN), whose potentials are altered solely based on which heteroatoms or substituents are resident on the 2,9-appended ring derivatives. It is proposed that long-range electronic inductive effects are responsible for the systematic modulation observed in the PL spectra, excited-state lifetimes, and the ground state absorption spectra and redox potentials. Cu1-Cu4 quantitatively follow the energy gap law, correlating well with structurally related cuprous phenanthrolines and are also shown to triplet photosensitize the excited states of 9,10-diphenylanthracene with bimolecular rate constants ranging from 1.61 to 2.82 × 108 M-1 s-1. The ability to tailor both photophysical and electrochemical properties using long-range inductive effects imposed by the 2,9-ring platforms advocates new directions for future MLCT chromophore discovery.

Pseudo-Octahedral Iron(II) Complexes with Near-Degenerate Charge Transfer and Ligand Field States at the Franck-Condon Geometry.

Increasing the metal-to-ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's π* orbital energy and by increasing the ligand field splitting. In the homo- and heteroleptic complexes [Fe(cpmp)2 ]2+ (12+ ) and [Fe(cpmp)(ddpd)]2+ (22+ ) with the tridentate ligands 6,2''-carboxypyridyl-2,2'-methylamine-pyridyl-pyridine (cpmp) and N,N'-dimethyl-N,N'-di-pyridin-2-ylpyridine-2,6-diamine (ddpd) two or one dipyridyl ketone moieties provide low energy π* acceptor orbitals. A good metal-ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six-membered chelate rings. The push-pull ligand cpmp provides intra-ligand and ligand-to-ligand charge transfer (ILCT, LL'CT) excited states in addition to MLCT excited states. Ground and excited state properties of 12+ and 22+ were accessed by X-ray diffraction analyses, resonance Raman spectroscopy, (spectro)electrochemistry, EPR spectroscopy, X-ray emission spectroscopy, static and time-resolved IR and UV/Vis/NIR absorption spectroscopy as well as quantum chemical calculations.

Next Generation Cuprous Phenanthroline MLCT Photosensitizer Featuring Cyclohexyl Substituents.

A new long-lived, visible-light-absorbing homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizer, [Cu(dchtmp)2]PF6 (dchtmp = 2,9-dicyclohexyl-3,4,7,8-tetramethyl-1,10-phenanthroline), has been synthesized, structurally characterized, and evaluated in terms of its molecular photophysics, electrochemistry, and electronic structure. Static and time-resolved transient absorption (TA) and photoluminescence (PL) spectroscopy measured on the title compound in CH2Cl2 (τ = 2.6 μs, ΦPL = 5.5%), CH3CN (τ = 1.5 μs, ΦPL = 2.6%), and THF (τ = 2.0 μs, ΦPL = 3.7%) yielded impressive photophysical metrics even when dissolved in Lewis basic solvents. The combined static spectroscopic data along with ultrafast TA experiments revealed that the pseudo-Jahn-Teller distortion and intersystem crossing dynamics in the MLCT excited state displayed characteristics of being sterically arrested throughout its evolution. Electrochemical and static PL data illustrate that [Cu(dchtmp)2]PF6 is a potent photoreductant (-1.77 V vs Fc+/0 in CH3CN) equal to or greater than all previously investigated homoleptic Cu(I) diimine complexes. Although we successfully prepared the cyclopentyl analog dcptmp (2,9-dicyclopentyl-3,4,7,8-tetramethyl-1,10-phenanthroline) using the same C-C radical coupling photochemistry as dchtmp, the corresponding Cu(I) complex could not be isolated due to the steric hindrance presented at the metal center. Ultimately, the successful preparation of [Cu(dchtmp)2]+ represents a major step forward for the design and discovery of novel earth-abundant photosensitizers made possible through a newly conceived ligand synthetic strategy.

Synthesis of a Ru(II) Complex with a Naphthoquinone-Annelated Imidazole Ligand Exhibiting Proton-Responsive Redox and Luminescent Behavior

A mononuclear ruthenium complex, [RuII(L)(bpy)2](PF6), with a naphthoquinone-annelated imidazole ligand HL (2-(pyridin-2-yl)-1H-naphtho[2,3-d]imidazole-4,9-dione) was synthesized and structurally characterized. Electrochemical study reveals that the Ru complex shows four reversible redox waves at +0.98 V, −1.13 V, −1.53 V, and −1.71 V versus SCE in acetonitrile, which are assigned to Ru(II)/Ru(III), L−/L•2−, and two bpy/bpy•− redox couples, respectively. The redox potential of Ru(II)/Ru(III) was positively shifted upon the addition of trifluoromethanesulfonic acid due to protonation of the L− moiety, leading to stabilization of the Ru 4d orbital. In UV-vis absorption measurements for the Ru complex in acetonitrile, a metal-to-ligand charge transfer (MLCT) band was observed at 476 nm, which was shifted to 450 nm by protonation, which might be due to a decrease in the electron delocalization and stabilization of the π orbitals in L−. The blue shift of the MLCT band by protonation was associated with a shift of an emission band from 774 nm to 620 nm, which could be caused by the decreased electronic delocalization in the MLCT excited state. These electrochemical and spectroscopic changes were reversible for the protonation/deprotonation stimuli.

Near-IR Charge-Transfer Emission at 77 K and Density Functional Theory Modeling of Ruthenium(II)-Dipyrrinato Chromophores: High Phosphorescence Efficiency of the Emitting State Related to Spin-Orbit Coupling Mediation of Intensity from Numerous Low-Energy Singlet Excited States.

Efficient charge-transfer (CT) phosphorescence in the near-IR (NIR) spectral region is reported for four substituted Ru-(R-dipyrrinato) complexes, [Ru(bpy)2(R-dipy)](PF6), where bpy is 2,2'-bipyridine and the substituent R is phenyl (ph), 2,4,6-trimethylphenyl, 4-carboxyphenyl (HOOC-ph), or 4-pyridinyl. The experimentally determined phosphorescence efficiency, ιem(p) = kRAD(p)/(νem(p))3 (where kRAD(p) and νem(p) are the phosphorescence rate constant and the phosphorescence frequency, respectively), of the [Ru(bpy)2(R-dipy)]+ complexes was approximately double that of [Ru(bpy)(Am)4]2+ complexes (Am = ammine ligand) in the NIR region. Density functional theory (DFT) modeling indicated two strikingly different electronic configurations of the triplet emitting state (Te) in the two types of complexes. The Te of [Ru(bpy)2(R-dipy)]+ complexes shows a CT-type corresponding to the metal-to-ligand charge transfer (MLCT)-(Ru-(R-dipy)) and the ππ*-(R-dipy) moiety configurations, and the Te state in the [Ru(bpy)(Am)4]2+ complexes corresponds to an approximately MLCT excited state consisting of mostly MLCT-(Ru-bpy) with a minimal ππ*(bpy) contribution. DFT modeling also indicated that the low-energy singlet excited states in the Te geometry (Sn(T)) of the [Ru(bpy)2(ph-dipy)]+ complex consist of numerous CT-Sn(T)-type states of the Ru-dipy and Ru-bpy moieties, whereas the [Ru(bpy)(Am)4]2+ ions show quite simple MLCT-Sn(T)-type states of the Ru-bpy moiety. Based on experimental observations, DFT modeling, and the plain spin-orbit coupling (SOC) principle, we conclude that the remarkably high ιem(p) amplitudes of the [Ru(bpy)2(R-dipy)]+ complexes relative to those of [Ru(bpy)(Am)4]2+ complexes can be attributed to the relatively substantial contribution of intrinsic SOC-mediated intensity stealing from the numerous low-energy CT-type Sn(T) states.

Synthesis, Photophysical, Photochromic, and Photomodulated Resistive Memory Studies of Dithienylethene-Containing Copper(I) Diimine Complexes.

A series of dithienylethene-containing copper(I) diimine complexes have been synthesized and structurally characterized. Systematic studies on their photophysics, electrochemistry, and photochromism have been carried out. The photoinduced color changes of the copper(I) complexes have been achieved by photoexcitation into the metal-to-ligand charge-transfer (MLCT) absorption bands, indicating the photosensitization of light-induced cyclization by the 3MLCT excited state. In addition, by an increase in either the steric bulkiness around the copper(I) center or the structural rigidity of the complexes, the quantum efficiencies of photoluminescence and photocyclization can be effectively enhanced because of suppression of the flattening distortion of the complexes at the MLCT excited state. Furthermore, one of the complexes has been employed as an active component in the fabrication of solution-processed resistive memory devices. Notable lowering of the switching threshold voltage of the binary memory devices has been realized through photocyclization of the dithienylethene-containing copper(I) system.

Stark Spectroscopic Evidence that a Spin Change Accompanies Light Absorption in Transition Metal Polypyridyl Complexes.

The "Franck-Condon" (FC) excited state is the first state created when a molecule absorbs a visible photon. Here we report Stark and visible absorption spectroscopies that interrogate the FC state of rigorously diamagnetic [M(bpy)3]2+ complexes, where bpy is 2,2'-bipyridine and M = Fe, Ru, and Os. Direct singlet-to-triplet metal-to-ligand charge transfer (MLCT) transitions are evident in the 550-750 nm region of the absorbance spectrum of [Os(bpy)3]2+, yet are poorly resolved or absent for [Ru(bpy)3]2+ and [Fe(bpy)3]2+. In the presence of a strong 0.4-0.8 MV/cm electric field, well-resolved transitions are observed for all the complexes in this same spectral region. In particular, an electroabsorption feature at 633 nm (15 800 cm-1) provides compelling evidence for the direct population of a high spin [Fe(bpy)3]2+* MLCT excited state. Group theoretical considerations and Liptay analysis of the Stark spectra revealed dramatic light-induced dipole moment changes in the range [Formula: see text] = 3-9 D with the triplet transitions consistently showing shorter charge transfer distances. The finding that the spin of the initially populated FC excited state differs from that of the ground state, even with a relatively light first row transition metal, is relevant to emerging applications in energy up-conversion, dye sensitization, spintronics, photoredox catalysis, and organic light emitting diodes (OLEDs).

Formation of organometallic xenon complexes in conventional fluids: A time-resolved infrared (TRIR) study of the photochemistry of W(CO)5(4AcPyr) (4-AcPyr=4-Acetylpyridine) in perfluoromethylcyclohexane (PFMCH)

The photochemistry and photophysics of W(CO)5(4-Acpy) in perfluoromethylcyclohexane (PFMCH) has been investigated using fast time-resolved infrared spectroscopy (TRIR). Analysis of these TRIR data shows that rradiation (400 nm) of W(CO)5(4-Acpy) in PFMCH results in the formation of a metal-to-ligand charge transfer (MLCT) excited state which decays (τ =330 ns) to reform mainly the parent and a small amount of a new photoproduct. The production of this new photoproduct is supported by the use of global analysis and assigned to the formation of W(CO)5(PFMCH). W(CO)5(PFMCH) is not stable and reacts with residual water to form (τ = 5 μs) W(CO)5(H2O). In the presence of Xe, irradiation of W(CO)5(4-Acpy) again results in formation of an MLCT excited state but under these conditions it decays slightly faster (τ =245 ns) which is consistent with the formation of W(CO)5(Xe). Global analysis supports the formation of W(CO)5(Xe) which is more stable than W(CO)5(PFMCH) and reacts more slowly (τ = 12 μs) to form W(CO)5(H2O).

Excited-State Dipole Moments of Homoleptic [Ru(bpy')3]2+ Complexes Measured by Stark Spectroscopy.

The visible absorption and Stark spectra of five [Ru(4,4'-(R)2-2,2'-bipyridine)3](PF6)2 and [Ru(bipyrazine)3](PF6)2 complexes, where R = CH3O-, tert-butyl-, CH3-, H-, or CF3-, were obtained in butyronitrile glasses at 77 K as a function of an applied electric field in the 0.2-0.8 MV/cm range. Analysis of the metal-to-ligand charge-transfer (MLCT) absorption and Stark spectra with the Liptay treatment revealed dramatic light-induced dipole moment changes, [Formula: see text]. The application of a two-state model to the [Formula: see text] values provided metal-ligand electronic coupling, HDA = 3100-4500 cm-1. The ground state of these complexes has no net dipole moment and with the RuII center as the point of reference, the dipole moment changes were reasonably assigned to the dipole present in the initially formed MLCT excited state. Further, the excited-state dipole moment was sensitive to the presence of electron-donating (MeO-, tert-butyl-, CH3-) or -withdrawing (CF3-) substituents and was correlated with the substituent Hammett parameters. Hence, the data show for the first time that substituents on the bipyridine ligands, which are often introduced to tune formal reduction potentials, can also induce significant changes in the excited-state dipole, behavior that should be taken into consideration for artificial photosynthesis applications.

Ultrafast Dynamics of the Metal-to-Ligand Charge Transfer Excited States of Ir(III) Proteo and Deutero Dihydrides.

For decades, transition metal hydrides have been at the forefront of numerous photocatalytic reactions leveraging either photoacid or photohydride generation. Of upmost importance is the nature of the M-H bond itself, which is typically the major site of photochemical reactivity, particularly in Ir(III) hydrides featuring metal-to-ligand charge transfer (MLCT) excited states. As a departure point for understanding the fundamental spectroscopy and photophysics of the MLCT excited states of Ir(III) diimine hydrides, cis-[Ir(bpy)2H2]+ (bpy = 2,2'-bipyridine) and its deuterated analogue cis-[Ir(bpy)2D2]+ were prepared and investigated. The robust nature of these molecules enabled detailed solution-based photophysical studies using ultrafast transient absorption and infrared spectroscopy, executed without the generation of permanent photoproducts. Static Fourier transform infrared and Raman spectra (λex = 785 nm) of these two molecules revealed weak but measurable Ir-H and Ir-D stretching vibrations centered at 2120 and 1510 cm-1, respectively. Short-lived (τ = 25 ps) MLCT excited states were observed for both cis-[Ir(bpy)2H2]+ and cis-[Ir(bpy)2D2]+ following femtosecond pulsed laser excitation at 480 nm in visible and near-IR transient absorption experiments. A similar time constant was measured for the in-phase and out-of-phase Ir-H stretching modes of the triplet excited state between 1900 and 2200 cm-1 using transient IR spectroscopy. The Ir-D stretching modes in the MLCT excited state were masked by bpy-localized vibrations rendering quantitative evaluation of these modes difficult. The time-resolved infrared data were consistent with density functional theory calculated mid-IR difference spectra in both of these molecules, yielding quantitative matches to the measured IR difference spectra. The information presented here provides valuable insight for understanding the primary photophysical events and transient absorption and IR spectroscopic signatures likely to be encountered throughout metal hydride photochemistry.

Dye Excited States Oriented Relative to TiO2 Surface Electric Fields

Two ruthenium compounds were anchored to mesoporous nanocrystalline TiO2 thin films to probe local electric fields generated by TiO2 reduction or cation adsorption in acetonitrile electrolytes. The metal-to-ligand charge-transfer (MLCT) excited states were well formulated as [RuIII(dtb)2(dcb–)]2+* and [RuIII(btfmb)(dcb)(btfmb–)]2+*, where dtb is 4,4′-(tert-butyl)2-2,2-bipyridine, dcb is 4,4′-(CO2H)2-2,2-bipyridine, and btfmb is 4,4′-(CF3)2-2,2-bipyridine. The MLCT excited state was orientated toward the TiO2 surface linker (antiparallel to the field) for Ru(dtb)2(dcb–)*/TiO2 and away from the surface for Ru(btfmb)(dcb)(btfmb–)*/TiO2. Reduction of the TiO2 in an electrochemical cell resulted in a blue shift of the Ru(dtb)2(dcb–)*/TiO2 photoluminescence spectrum, while the Ru(btfmb)(dcb)(btfmb–)*/TiO2 spectrum was either unchanged or was red-shifted. Accompanying these spectral shifts were bimodal changes in the PL intensity and excited-state lifetime that first increased and then decreased as the quasi-Fer...

Enhancing the Visible-Light Absorption and Excited-State Properties of Cu(I) MLCT Excited States.

A computationally inspired Cu(I) metal-to-ligand charge transfer (MLCT) chromophore, [Cu(sbmpep)2]+ (sbmpep = 2,9-di(sec-butyl)-3,8-dimethyl-4,7-di(phenylethynyl)-1,10-phenanthroline), was synthesized in seven total steps, prepared from either dichloro- or dibromophenanthroline precursors. Complete synthesis, structural characterization, and electrochemistry, in addition to static and dynamic photophysical properties of [Cu(sbmpep)2]+, are reported on all relevant time scales. UV-Vis absorption spectroscopy revealed significant increases in oscillator strength along with a concomitant bathochromic shift in the MLCT absorption bands with respect to structurally related model complexes (ε = 16 500 M-1 cm-1 at 491 nm). Strong red photoluminescence (Φ = 2.7%, λmax = 687 nm) was observed from [Cu(sbmpep)2]+, which featured an average excited-state lifetime of 1.4 μs in deaerated dichloromethane. Cyclic and differential pulse voltammetry revealed ∼300 mV positive shifts in the measured one-electron reversible reduction and oxidation waves in relation to a Cu(I) model complex possessing identical structural elements without the π-conjugated 4,7-substituents. The excited-state redox potential of [Cu(sbmpep)2]+ was estimated to be -1.36 V, a notably powerful reductant for driving photoredox chemistry. The combination of conventional and ultrafast transient absorption and luminescence spectroscopy successfully map the excited-state dynamics of [Cu(sbmpep)2]+ from initial photoexcitation to the formation of the lowest-energy MLCT excited state and ultimately its relaxation to the ground state. This newly conceived molecule appears poised for photosensitization reactions involving energy and electron-transfer processes relevant to photochemical upconversion, photoredox catalysis, and solar fuels photochemistry.

Recent Applications of [Cu(dap)2]Cl in Visible Light-Mediated Photoredox Catalysis

Since 2008, visible light-mediated photoredox catalysis has enjoyed a renaissance in organic synthesis. [1–5] The most commonly employed metal-based photoredox catalysts typically utilise second- and third-row transition metals featuring polypyridyl ligand systems such as [Ru(bpy) 3 ]Cl 2 (1). [1] In contrast, the synthetic applications of copper-based photoredox catalysts have received much less attention. [6] A class of photoactive homoleptic, phenanthroline-based copper(I) complexes, including [Cu (dap) 2 ]Cl (2), were first prepared some 35 years ago (Chart 1). [7,8] This class of complexes share several interesting properties that can be exploited to facilitate chemical transformations. The absorption of a photon in the visible range leads to a metal-to-ligand charge transfer (MLCT) excited state. Subsequent intersystem crossing (ISC) results in spin inversion and allows access to long-lived triplet photoexcited states. [9] In its photoexcited state, complex 2 is a strong reductant (-1.43 V versus saturated calomel electrode) capable of reducing organic acceptor molecules via single electron transfer (SET) processes. [8] The ensuing CuIIspecies can undergo a second SET process to oxidise an organic donor molecule, which reduces the catalyst back to its ground state and closes the catalytic cycle. [1] The coordinatively unsaturated nature of these copper(I) complexes typically provides lower coordinative stability relative to saturated octahedral complexes (e.g. 1). Also, in the ground state, these copper(I) complexes typically exhibit a distorted tetrahedral coordination geometry. Upon photoexcitation, oxidation of the metal centre occurs as a result of the population of the MLCT excited state. This causes a flattening distortion that results in a more square-planar coordination geometry. These two factors are primarily responsible for reducing excited state lifetimes. The present focus article aims to highlight recent applications of [Cu(dap) 2 ]Cl in organic synthesis.

Ligand manipulation of charge transfer excited state relaxation and spin crossover in [Fe(2,2'-bipyridine)2(CN)2

We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2′-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2′-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2− in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6–2N]2N-4, where N = 1–3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.