d → f Energy Transfer in a Series of Ir^III/Eu^III Dyads: Energy-Transfer Mechanisms and White-Light Emission

An extensive series of blue-luminescent iridium(III) complexes has been prepared containing two phenylpyridine-type ligands and one ligand containing two pyrazolylpyridine units, of which one is bound to Ir^III and the second is pendant. Attachment of {Ln(hfac)₃} (Ln = Eu, Gd; hfac = anion of 1,1,1,5,5,5,-hexafluoropentanedione) to the second coordination site affords Ir^III/Ln^III dyads. Crystallographic analysis of several mononuclear iridium(III) complexes and one Ir^III/Eu^III dyad reveals that in most cases the complexes can adopt a folded conformation involving aromatic π stacking between a phenylpyridine ligand and the bis(pyrazolylpyridine) ligand, but in one series, based on CF₃-substituted phenylpyridine ligands coordinated to Ir^III, the steric bulk of the CF₃ group prevents this and a quite different and more open conformation arises. Quantum mechanical calculations well reproduce these two types of “folded” and “open” conformations. In the Ir^III/Eu^III dyads, Ir → Eu energy transfer occurs with varying degrees of efficiency, resulting in partial quenching of the Ir^III-based blue emission and the appearance of a sensitized red emission from Eu^III. Calculations based on consideration of spectroscopic overlap integrals rule out any significant contribution from Förster (dipole–dipole) energy transfer over the distances involved but indicate that Dexter-type (exchange) energy transfer is possible if there is a small electronic coupling that would arise, in part, through π stacking between components. In some cases, an initial photoinduced <i>electron</i>-transfer step could also contribute to Ir → Eu energy transfer, as shown by studies on isostructural iridium/gadolinium model complexes. A balance between the blue (Ir-based) and red (Eu-based) emission components can generate white light

d→f Energy Transfer in Ir(III)/Eu(III) Dyads: Use of a Naphthyl Spacer as a Spatial and Energetic “Stepping Stone”

A series of luminescent complexes based on {Ir­(phpy)₂} (phpy = cyclometallating anion of 2-phenylpyridine) or {Ir­(F₂phpy)₂} [F₂phpy = cyclometallating anion of 2-(2′,4′-difluorophenyl)­pyridine] units, with an additional 3-(2-pyridyl)-pyrazole (pypz) ligand, have been prepared; fluorination of the phenylpyridine ligands results in a blue-shift of the usual ³MLCT/³LC luminescence of the Ir unit from 477 to 455 nm. These complexes have pendant from the coordinated pyrazolyl ring an additional chelating 3-(2-pyridyl)-pyrazole unit, separated via a flexible chain containing a naphthalene-1,4-diyl or naphthalene-1,5-diyl spacer. Crystal structures show that the flexibility of the pendant chain allows the naphthyl group to lie close to the Ir core and participate in a π-stacking interaction with a coordinated phpy or F₂phpy ligand. Luminescence spectra show that, whereas the {Ir­(phpy)₂(pypz)} complexes show typical Ir-based emissionalbeit with lengthened lifetimes because of interaction with the stacked naphthyl groupthe {Ir­(F₂phpy)₂(pypz)} complexes are nearly quenched. This is because the higher energy of the Ir-based ³MLCT/³LC excited state can now be quenched by the adjacent naphthyl group to form a long-lived naphthyl-centered triplet (³nap) state which is detectable by transient absorption. Coordination of an {Eu­(hfac)₃} unit (hfac = 1,1,1,5,5,5-hexafluoro-pentane-2,4-dionate) to the pendant pypz binding site affords Ir–naphthyl–Eu triads. For the triads containing a {Ir­(phpy)₂} core, the unavailability of the ³nap state (not populated by the Ir-based excited state which is too low in energy) means that direct Ir→Eu energy-transfer occurs in the same way as in other flexible Ir/Eu complexes. However for the triads based on the­{Ir­(F₂phpy)₂} core, the initial Ir→³nap energy-transfer step is followed by a second, slower, ³nap→Eu energy-transfer step: transient absorption measurements clearly show the ³nap state being sensitized by the Ir center (synchronous Ir-based decay and ³nap rise-time) and then transferring its energy to the Eu center (synchronous ³nap decay and Eu-based emission rise time). Thus the ³nap state, which is energetically intermediate in the {Ir­(F₂phpy)₂}–naphthyl–Eu systems, can act as a “stepping stone” for two-step d→f energy-transfer

Simultaneous Surface-Enhanced Raman Scattering with a Kerr Gate for Fluorescence Suppression

The combination of surface-enhanced and Kerr-gated Raman spectroscopy for the enhancement of the Raman signal and suppression of fluorescence is reported. Surface-enhanced Raman scattering (SERS)-active gold substrates were demonstrated for the expansion of the surface generality of optical Kerr-gated Raman spectroscopy, broadening its applicability to the study of analytes that show a weak Raman signal in highly fluorescent media under (pre)resonant conditions. This approach is highlighted by the well-defined spectra of rhodamine 6G, Nile red, and Nile blue. The Raman spectra of fluorescent dyes were obtained only when SERS-active substrates were used in combination with the Kerr gate. To achieve enhancement of the weaker Raman scattering, Au films with different roughnesses or Au-core-shell-isolated nanoparticles (SHINs) were used. The use of SHINs enabled measurement of fluorescent dyes on non-SERS-active, optically flat Au, Cu, and Al substrates

Electrochemistry, Chemical Reactivity, and Time-Resolved Infrared Spectroscopy of Donor–Acceptor Systems [(Q^<i>x</i>)Pt(pap^<i>y</i>)] (Q = Substituted <i>o</i>‑Quinone or <i>o</i>‑Iminoquinone; pap = Phenylazopyridine)

The donor–acceptor complex [(^O,NQ^2–)­Pt­(pap⁰)] (<b>1</b>; pap = phenylazopyridine, ^O,NQ⁰ = 4,6-di-<i>tert</i>-butyl-<i>N</i>-phenyl-<i>o</i>-iminobenzoquinone), which displays strong π-bonding interactions and shows strong absorption in the near-IR region, has been investigated with respect to its redox-induced reactivity and electrochemical and excited-state properties. The one-electron-oxidized product [(^O,NQ^•–)­Pt­(pap⁰)]­(BF₄) ([<b>1</b>]­BF₄) was chemically isolated. Single-crystal X-ray diffraction studies establish the iminosemiquinone form of ^O,NQ in [<b>1</b>]⁺. Simulation of the cyclic voltammograms of <b>1</b> recorded in the presence of PPh₃ elucidates the mechanism and delivers relevant thermodynamic and kinetic parameters for the redox-induced reaction with PPh₃. The thermodynamically stable product of this reaction, complex [(^O,NQ^•–) Pt­(PPh₃)₂]­(PF₆) ([<b>2</b>]­PF₆), was isolated and characterized by X-ray crystallography, electrochemistry, and electron paramagnetic resonance spectroscopy. Picosecond time-resolved infrared spectroscopic studies on complex <b>1b</b> (one of the positional isomers of <b>1</b>) and its analogue [(^O,OQ^2–)­Pt­(pap⁰)] (<b>3</b>; ^O,OQ = 3,5-di-<i>tert</i>-butyl-<i>o</i>-benzoquinone) provided insight into the excited-state dynamics and revealed that the nature of the lowest excited state in the amidophenolate complex <b>1b</b> is primarily diimine-ligand-based, while it is predominantly an interligand charge-transfer state in the case of <b>3</b>. Density functional theory calculations on [<b>1</b>]^<i>n</i>+ provided further insight into the nature of the frontier orbitals of various redox forms and vibrational mode assignments. We discuss the mechanistic details of the newly established redox-induced reactivity of <b>1</b> with electron donors and propose a mechanism for this process

Electrochemistry, Chemical Reactivity, and Time-Resolved Infrared Spectroscopy of Donor–Acceptor Systems [(Q^<i>x</i>)Pt(pap^<i>y</i>)] (Q = Substituted <i>o</i>‑Quinone or <i>o</i>‑Iminoquinone; pap = Phenylazopyridine)

The donor–acceptor complex [(^O,NQ^2–)­Pt­(pap⁰)] (<b>1</b>; pap = phenylazopyridine, ^O,NQ⁰ = 4,6-di-<i>tert</i>-butyl-<i>N</i>-phenyl-<i>o</i>-iminobenzoquinone), which displays strong π-bonding interactions and shows strong absorption in the near-IR region, has been investigated with respect to its redox-induced reactivity and electrochemical and excited-state properties. The one-electron-oxidized product [(^O,NQ^•–)­Pt­(pap⁰)]­(BF₄) ([<b>1</b>]­BF₄) was chemically isolated. Single-crystal X-ray diffraction studies establish the iminosemiquinone form of ^O,NQ in [<b>1</b>]⁺. Simulation of the cyclic voltammograms of <b>1</b> recorded in the presence of PPh₃ elucidates the mechanism and delivers relevant thermodynamic and kinetic parameters for the redox-induced reaction with PPh₃. The thermodynamically stable product of this reaction, complex [(^O,NQ^•–) Pt­(PPh₃)₂]­(PF₆) ([<b>2</b>]­PF₆), was isolated and characterized by X-ray crystallography, electrochemistry, and electron paramagnetic resonance spectroscopy. Picosecond time-resolved infrared spectroscopic studies on complex <b>1b</b> (one of the positional isomers of <b>1</b>) and its analogue [(^O,OQ^2–)­Pt­(pap⁰)] (<b>3</b>; ^O,OQ = 3,5-di-<i>tert</i>-butyl-<i>o</i>-benzoquinone) provided insight into the excited-state dynamics and revealed that the nature of the lowest excited state in the amidophenolate complex <b>1b</b> is primarily diimine-ligand-based, while it is predominantly an interligand charge-transfer state in the case of <b>3</b>. Density functional theory calculations on [<b>1</b>]^<i>n</i>+ provided further insight into the nature of the frontier orbitals of various redox forms and vibrational mode assignments. We discuss the mechanistic details of the newly established redox-induced reactivity of <b>1</b> with electron donors and propose a mechanism for this process

Ultrafast Intramolecular Charge Separation in a Donor–Acceptor Assembly Comprising Bis(η⁵‑cyclopentadienyl)molybdenum Coordinated to an Ene-1,2-dithiolate-naphthalenetetracarboxylicdiimide Ligand

The first example of a Donor-spacer-Acceptor tryad, based upon a molybdenum-ene-1,2-dithiolate unit as the Donor and a naphthalene-diimide as the Acceptor, has been synthesized and its photophysical properties investigated. Synthesis required the preparation of a new pro-ligand containing a protected ene-1,2-dithiolate bound through a phenyl linkage to a naphthalenetetracarboxylicdiimide (NDI) group. Deprotection of this pro-ligand by base hydrolysis, followed by reaction with [Cp₂MoCl₂], produced the new dyad [Cp₂Mo­(SC­(H)­C­(C₆H₄–NDI)­S)] (<b>2</b>). Electrochemical studies showed that <b>2</b> can be reversibly oxidized to [<b>2</b>]⁺ and reduced to [<b>2</b>]⁻, [<b>2</b>]^2–, and [<b>2</b>]^3–. These studies, augmented by UV/vis, IR, and electron paramagnetic resonance (EPR) spectra of electrochemically generated [<b>2</b>]⁺ and [<b>2</b>]⁻, show that the highest occupied molecular orbital (HOMO) of <b>2</b> is ene-1,2-dithiolate-based and the lowest unoccupied molecular orbital (LUMO) is NDI-based; these conclusions are supported by density functional theory (DFT) calculations for the electronic ground state on a model of <b>2</b> which also showed that these two parts of the molecule are electronically distinct. The dynamics of the excited states of <b>2</b> in CH₂Cl₂ solution were investigated by picosecond time-resolved IR spectroscopy following irradiation by a 400 nm ∼120 fs laser pulse. These investigations were complemented by an ultrafast transient absorption spectroscopic study from 420 to 760 nm of the nature of the excited states of <b>2</b> in CH₂Cl₂ solution following irradiation by a 383 nm ∼120 fs laser pulse. These studies showed that irradiation of <b>2</b> at both 400 and 383 nm leads to the formation of the [(Cp)₂{Mo­(dt)}⁺-Ph-{NDI}⁻] charge-separated state as a result of a cascade electron transfer initiated by the formation of an ¹NDI* excited state. ¹NDI* rapidly (ca. 0.2 ps) forms the local charge transfer state [Cp₂Mo­(dt)-{Ph}⁺-{NDI}⁻] which has a lifetime of about 1.7 ps and decays to produce the ground state and the charge-separated state [(Cp)₂{Mo­(dt)}^+·-Ph-{NDI}⁻]; the latter has an appreciable lifetime, about 15 ns in CH₂Cl₂ at room temperature

Dynamics of Ground and Excited State Vibrational Relaxation and Energy Transfer in Transition Metal Carbonyls

Nonlinear vibrational spectroscopy provides insights into the dynamics of vibrational energy transfer in and between molecules, a crucial phenomenon in condensed phase physics, chemistry, and biology. Here we use frequency-domain 2-dimensional infrared (2DIR) spectroscopy to investigate the vibrational relaxation (VR) and vibrational energy transfer (VET) rates in different solvents in both the electronic ground and excited states of Re­(Cl)­(CO)₃(4,4′-diethylester-2,2′-bipyridine), a prototypical transition metal carbonyl complex. The strong CO and ester CO stretch infrared reporters, located on opposite sides of the molecule, were monitored in the 1600–2100 cm^–1 spectral region. VR in the lowest charge transfer triplet excited state (³CT) is found to be up to eight times faster than in the ground state. In the ground state, intramolecular anharmonic coupling may be solvent-assisted through solvent-induced frequency and charge fluctuations, and as such VR rates are solvent-dependent. In contrast, VR rates in the solvated ³CT state are surprisingly solvent-insensitive, which suggests that predominantly intramolecular effects are responsible for the rapid vibrational deactivation. The increased VR rates in the excited state are discussed in terms of intramolecular electrostatic interactions helping overcome structural and thermodynamic barriers for this process in the vicinity of the central heavy atom, a feature which may be of significance to nonequilibrium photoinduced processes observed in transition metal complexes in general

Anion-Mediated Photophysical Behavior in a C₆₀ Fullerene [3]Rotaxane Shuttle

By addressing the challenge of controlling molecular motion, mechanically interlocked molecular machines are primed for a variety of applications in the field of nanotechnology. Specifically, the designed manipulation of communication pathways between electron donor and acceptor moieties that are strategically integrated into dynamic photoactive rotaxanes and catenanes may lead to efficient artificial photosynthetic devices. In this pursuit, a novel [3]­rotaxane molecular shuttle consisting of a four-station bis-naphthalene diimide (NDI) and central C₆₀ fullerene bis-triazolium axle component and two mechanically bonded ferrocenyl-functionalized isophthalamide anion binding site-containing macrocycles is constructed using an anion template synthetic methodology. Dynamic coconformational anion recognition-mediated shuttling, which alters the relative positions of the electron donor and acceptor motifs of the [3]­rotaxane’s macrocycle and axle components, is demonstrated initially by ¹H NMR spectroscopy. Detailed steady-state and time-resolved UV–vis–IR absorption and emission spectroscopies as well as electrochemical studies are employed to further probe the anion-dependent positional macrocycle–axle station state of the molecular shuttle, revealing a striking on/off switchable emission response induced by anion binding. Specifically, the [3]­rotaxane chloride coconformation, where the ferrocenyl-functionalized macrocycles reside at the center of the axle component, precludes electron transfer to NDI, resulting in the switching-on of emission from the NDI fluorophore and concomitant formation of a C₆₀ fullerene-based charge-separated state. By stark contrast, in the absence of chloride as the hexafluorophosphate salt, the ferrocenyl-functionalized macrocycles shuttle to the peripheral NDI axle stations, quenching the NDI emission via formation of a NDI-containing charge-separated state. Such anion-mediated control of the photophysical behavior of a rotaxane through molecular motion is unprecedented

Visible Light-Driven O₂ Reduction by a Porphyrin–Laccase System

Several recent studies have shown that the combination of photosensitizers with metalloenzymes can support a light-driven multielectron reduction of molecules such as CO₂ or HCN. Here we show that the association of the zinc tetramethylpyridinium porphyrin (ZnTMPyP⁴⁺) photosensitizer with the multicopper oxidase (MCO) laccase allows to link the oxidation of an organic molecule to the four electrons reduction of dioxygen into water. The enzyme is photoreduced within minutes with porphyrin/enzyme ratio as low as 1:40. With a 1:1 ratio, the dioxygen consumption rate is 1.7 μmol L^–1 s^–1. Flash photolysis experiments support the formation of the triplet excited state of ZnTMPyP⁴⁺ which reduces the enzyme to form a radical cation of the porphyrin with a <i>k</i>_ET ≈ 10⁷ s^–1 M^–1. The long-lived triplet excited state of the ZnTMPyP⁴⁺ (τ₀ = 0.72 ms) accounts for a substantial electron-transfer quantum yield, ϕ_ET = 0.35. Consequently, the enzyme-dependent photo-oxidation of the electron donor occurs with a turnover of 8 min^–1 for the one-electron oxidation process, thereby supporting the suitability of such enzyme/sensitizer hybrid systems for aerobic photodriven transformations on substrates. This study is the first example of a phorphyrin-sensitized four-electron reduction of an enzyme of the MCO family, leading to photoreduction of dioxygen into water

Femtosecond to Millisecond Dynamics of Light Induced Allostery in the <i>Avena sativa</i> LOV Domain

The rational engineering of photosensor proteins underpins the field of optogenetics, in which light is used for spatiotemporal control of cell signaling. Optogenetic elements function by converting electronic excitation of an embedded chromophore into structural changes on the microseconds to seconds time scale, which then modulate the activity of output domains responsible for biological signaling. Using time-resolved vibrational spectroscopy coupled with isotope labeling, we have mapped the structural evolution of the LOV2 domain of the flavin binding phototropin <i>Avena sativa</i> (AsLOV2) over 10 decades of time, reporting structural dynamics between 100 fs and 1 ms after optical excitation. The transient vibrational spectra contain contributions from both the flavin chromophore and the surrounding protein matrix. These contributions are resolved and assigned through the study of four different isotopically labeled samples. High signal-to-noise data permit the detailed analysis of kinetics associated with the light activated structural evolution. A pathway for the photocycle consistent with the data is proposed. The earliest events occur in the flavin binding pocket, where a subpicosecond perturbation of the protein matrix occurs. In this perturbed environment, the previously characterized reaction between triplet state isoalloxazine and an adjacent cysteine leads to formation of the adduct state; this step is shown to exhibit dispersive kinetics. This reaction promotes coupling of the optical excitation to successive time-dependent structural changes, initially in the β-sheet and then α-helix regions of the AsLOV2 domain, which ultimately gives rise to Jα-helix unfolding, yielding the signaling state. This model is tested through point mutagenesis, elucidating in particular the key mediating role played by Q513