Light-induced Excited State Research Articles

Photocurrent Generation and Polarity Switching in Electrochemical Cells through Light-induced Excited State Proton Transfer of Photoacids and Photobases.

Light is a common source of energy in sustainable technologies for photocurrent generation. To date, in such light-harvesting applications, the excited electrons generate the photocurrent. Here, we introduce a new mechanism for photocurrent generation that is based on excited state proton transfer (ESPT) of photoacids and photobases that can donate or accept a proton, respectively, but only after excitation. We show that the formed ions following ESPT can either serve as electron donors or acceptors with the electrodes, or modify the kinetics of mass transport across the diffuse layer, both resulting in photocurrent generation. We further show that control of the current polarity is obtained by switching the irradiation between the photoacid and the photobase. Our study represents a new approach in photoelectrochemistry by introducing ESPT processes, which can be further utilized in light-responsive energy production or energy storage.

Atomic Scale Evidence of the Switching Mechanism in a Photomagnetic CoFe Dinuclear Prussian Blue Analogue.

Molecular complexes based on Prussian Blue analogues have recently attracted considerable interest for their unique bistable properties combined to ultimately reduced dimensions. Here, we investigate the first dinuclear FeCo complex exhibiting both thermal and photomagnetic bistability in the solid state. Through an experimental and theoretical approach combining local techniques-X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD), and ligand field multiplet calculations-we were able to evidence the changes occurring at the atomic scale in the electronic and magnetic properties. The spectroscopic studies were able to fully support at the atomic level the following conclusions: (i) the 300 K phase and the light-induced excited state at 4 K are both built from FeLSIII-CoHSII paramagnetic pairs with no apparent reorganization of the local structure, (ii) the 100 K phase is composed of FeLSII-CoLSIII diamagnetic pairs, and (iii) the light-induced excited state is fully relaxed at an average temperature of ≈50 K. In the paramagnetic phase at 2 K, XAS and XMCD reveal that both Fe and Co ions exhibit a rather large orbital magnetic moment (0.65 μB and 0.46 μB, respectively, under an external magnetic induction of 6.5 T), but it was not possible to detect a magnetic interaction between spin centers above 2 K.

Alternative Electron-Transfer Channels Ensure Ultrafast Deactivation of Light-Induced Excited States in Riboflavin Binding Protein

Flavoproteins, containing flavin chromophores, are enzymes capable of transferring electrons at very high speeds. The ultrafast photoinduced electron-transfer (ET) kinetics of riboflavin binding protein to the excited riboflavin was studied by femtosecond spectroscopy and found to occur within a few hundred femtoseconds [ Zhong and Zewail, Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11867-11872 ]. This ultrafast kinetics was attributed to the presence of two aromatic rings that could transfer the electron to riboflavin: the side chains of tryptophan 156 and tyrosine 75. However, the underlying ET mechanism remained unclear. Here, using a hybrid quantum mechanical-molecular dynamics approach, we perform ET dynamics simulations taking into account the motion of the protein and the solvent upon ET. This approach reveals that ET occurs via a major reaction channel involving tyrosine 75 (83%) and a minor one involving tryptophan 156 (17%). We also show that the protein environment is designed to ensure the fast quenching of the riboflavin excited state.

XAFS Analysis for Light-induced Excited State of Supported Vanadium Oxide Catalyst for Activation of Molecular Oxygen

XAFS Analysis for Light-induced Excited State of Supported Vanadium Oxide Catalyst for Activation of Molecular Oxygen

Substitution-controlled excited state processes in heteroleptic copper(I) photosensitizers used in hydrogen evolving systems.

Four different heteroleptic [Cu(N^N)(P^P)]PF6 complexes, which combine classical bidentate diimine ligands and sterically demanding diphosphine ligands, are studied by a combination of ultrafast time-resolved spectroscopy and quantum chemical calculations. The light-induced excited state processes, accompanied by a structural change, are discussed with respect to the application of these complexes as a new class of noble-metal-free photosensitizers in proton reducing systems. In particular, the influence of different substituents in the ligand backbone on the photophysical properties is highlighted.

Photoexcitation and Relaxation Properties of a Spin-Crossover Solid in the Case of a Stable High-Spin State

The molecular solid [Fe(II)L(2)](ClO(4))(2).CH(3)CN where L is 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine provides a stable high-spin (HS) state at low temperature. Photoexcitation and subsequent relaxation have been studied using light-induced excited state spin trapping [LIESST(H --> L)] in the 700-850 nm range, determination of T(LIESST), relaxation curves at different temperatures, and temperature dependence of the light-induced spin equilibrium under constant irradiation. The measured photoinduced population of the metastable low-spin (LS) state (<30%) was drastically limited by the concomitant L --> H photoprocess. The absence of static light-induced thermal hysteresis and the stretched exponential shape of the relaxation curves respectively revealed the absence of sizable interactions and a large spreading of the activation energies attributed to the ligand flexibility. The whole data set has been simulated using a linear rate equation, with a simplified correction for the bulk extinction of light in the powder sample.

Structural studies of light-induced excited states

Over the past few years there has been a marked increase in the number of structural studies carried out on light-induced excited states. Until now, however, there has been no systematic approach to the irradiation of samples, which can make data collection difficult and results unreliable. This paper presents a device for mounting a laser on a CCD diffractometer; this device not only enables the collection of data without any constraints on the data collection strategy, but also simplifies alignment of the laser and can be left in place permanently.

Dynamics of light-induced excited state quenching of ferrioxalate complexes by peroxides. Fast kinetics events and interaction with toxic pollutants

The behaviour of the trioxaloferrate (Fe(ox) 3 3−) complex on laser illumination and its quenching in the presence of H 2O 2 were investigated by laser photolysis. At neutral Ph, a short (approximately 30 μs) and a longer (approximately 1 ms) lifetime were observed for transient Fe(ox) 3 3−. The lifetime decreases as the pH becomes more acidic. In light-induced processes, the intensity of the light used is one of the determining reaction parameters and this effects is reported in detail. The excited states of the Fe oxalate complexes induced by laser photolysis are quenched by H 2O 2. The decay observed can be fitted in a simplified way by bi-exponential function. The transient species Fe(ox) 3 3−∗ observed on laser photolysis does not undergo excited state annihilation, but ground state collisional quenching. The fast formation of a hydroxylated phenol adduct is shown by kinetic laser spectroscopy. A detailed account of the kinetics and spectral characteristics of the transients is presented for these reactions. This allows a scheme to be suggested for the intervention of these radicals in agreement with the experimental observations. The results obtained implicate the OH radical in the degradation of pollutants involving Fe complexes in the presence of H 2O 2.

Two-electron atoms in superintense radiation fields: Dichotomy and stabilization.

The high-frequency Floquet theory describing the interaction of a two-electron atom with a linearly polarized laser field is applied to the case when the characteristic parameter ${\mathrm{\ensuremath{\alpha}}}_{0}$=${\mathit{E}}_{0}$${\mathrm{\ensuremath{\omega}}}^{\mathrm{\ensuremath{-}}2}$ a.u. is large, corresponding to the dichotomy regime of the one-electron problem. We first revisit this case and extend the large-${\mathrm{\ensuremath{\alpha}}}_{0}$ energy-level formula obtained earlier to higher order in ${\mathrm{\ensuremath{\alpha}}}_{0}^{\mathrm{\ensuremath{-}}1}$. We then prove the existence of a dichotomy regime also for the two-electron atom, characterized by the two electrons being situated in disjoint electronic clouds separated by an average distance of 2${\mathrm{\ensuremath{\alpha}}}_{0}$. We obtain the first four terms in the expansion of the related energy-level formula in fractional powers of ${\mathrm{\ensuremath{\alpha}}}_{0}^{\mathrm{\ensuremath{-}}1}$. The coefficients entering this expansion have been expressed in terms of the eigenvalues of a nonseparable Schr\odinger equation containing the end-point potential and of integrals over its eigenfunctions. The equation was solved using the finite element method. An infinite sequence of levels emerges. In the case of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ this implies the existence of a large number of light-induced excited states, some of them corresponding to two-electron excitations not subject to autodetachment. Finally, we prove that in the dichotomy regime a two-electron atom undergoes stabilization and that the ionization rates are essentially twice those for a one-electron atom with the same nuclear charge. \textcopyright{} 1996 The American Physical Society.

Photophysics of the 2-(2′-hydroxyphenyl)perimidine: On the fluorescence of the enol form

The photophysical behaviour of 2-(2′-hydroxyphenyl)perimidine (6), which possesses a strong intramolecular hydrogen bond is evidenced as due to the non-transferred enol form. This compound is highly stable to light, with a photodegradative quantum yield in degassed cyclohexane of 7 × 10 −6, suggesting that the involvement of a lightinduced excited state intramolecular proton transfer (ESIPT) process in which the phenolic proton is transferred from the enol form to the neighbouring nitrogen atom is less important for conferring photostability than has hitherto been supposed for such type of systems.