Excited-state Potentials Research Articles

Nanosecond Intra-Ionic Chloride Photo-Oxidation.

Transition-metal photocatalysts capable of oxidizing chloride are rare yet serve as an attractive means to controllably generate chlorine atoms, which have continued to garner the interest of researchers for notable applications in photoredox catalysis and solar energy storage. Herein, a new series of four Ir-photocatalysts with different dicationic chloride-sequestering ligands were synthesized and characterized to probe the relationship between chloride binding affinities, ion pair solution structures, and rate constants for chloride photo-oxidation in acetonitrile at room temperature. The substituents on the quaternary amines of dicationic bipyridine ligands had negligible effects on the photocatalyst excited-state reduction potential, yet dramatically influenced the affinity for chloride binding, indicating that synthetic design can be utilized to independently tune these important properties. An inverse correlation was observed between the equilibrium constant for chloride ion pairing and the rate constant for intra-ionic chloride oxidation. Exceptions to this trend suggest structural differences in the ion-paired solution structures, which were probed by 1H NMR binding experiments. This study provides new insights into light-induced oxidation of ion-paired substrates, a burgeoning approach that offers to circumvent diffusional constraints of photocatalysts with short excited-state lifetimes. Ground-state association of chloride with these photocatalysts enables intra-ionic chloride oxidation on a rapid nanosecond timescale.

Mechanical Photoluminescence Excitation Spectra of a Strongly Driven Spin-Mechanical System

We report experimental studies of a driven spin-mechanical system, in which a nitrogen-vacancy ($\mathrm{N}\text{\ensuremath{-}}$V) center couples to out-of-plane vibrations of a diamond cantilever through the excited-state deformation potential. Photoluminescence excitation studies show that in the unresolved sideband regime and under strong resonant mechanical driving, the excitation spectra of a $\mathrm{N}\text{\ensuremath{-}}$V optical transition feature two spectrally sharp peaks, corresponding to the two turning points of the oscillating cantilever. In the limit that the strain-induced frequency separation between the two peaks far exceeds the $\mathrm{N}\text{\ensuremath{-}}$V zero-phonon linewidth, the spectral position of the individual peak becomes sensitive to minute detuning between the mechanical resonance and the external driving force. For a fixed optical excitation frequency near the $\mathrm{N}\text{\ensuremath{-}}$V transition, $\mathrm{N}\text{\ensuremath{-}}$V fluorescence as a function of mechanical detuning features resonances with a linewidth that can be orders of magnitude smaller than the intrinsic linewidth of the mechanical mode. This enhanced sensitivity to mechanical detuning can potentially provide an effective mechanism for mechanical sensing, for example, mass sensing via measurements of induced changes in the mechanical oscillator frequency.

Nonadiabatic Potential Energy Surfaces for a Molecule on a Surface as Found by Constrained Complete Active Space Theory.

In order to study electron-transfer mediated chemical processes on a metal surface, one requires not one but two potential energy surfaces (one ground state and one excited state) as in Marcus theory. In this letter, we report that a novel, dynamically weighted, state-averaged constrained CASSCF(2,2) (DW-SA-cCASSCF(2,2)) can produce such surfaces for the Anderson impurity model. Both ground and excited state potentials are smooth, they incorporate states with a charge transfer character, and the accuracy of the ground state surface can be verified for some model problems by renormalization group theory. Future development of gradients and nonadiabatic derivative couplings should allow for the study of nonadiabatic dynamics for molecules near metal surfaces.

A Stable Triphenylamine-Based Zn(II)-MOF for Photocatalytic H2 Evolution and Photooxidative Carbon–Carbon Coupling Reaction

Efficient charge transfer has always been a challenge in heterogeneous MOF-based photoredox catalysis due to the poor electrical conductivity of the MOF photocatalyst, the toilless electron-hole recombination, and the uncontrollable host-guest interactions. Herein, a propeller-like tris(3'-carboxybiphenyl)amine (H3TCBA) ligand was synthesized to fabricate a 3D Zn3O cluster-based Zn(II)-MOF photocatalyst, Zn3(TCBA)2(μ3-H2O)H2O (Zn-TCBA), which was applied to efficient photoreductive H2 evolution and photooxidative aerobic cross-dehydrogenation coupling reactions of N-aryl-tetrahydroisoquinolines and nitromethane. In Zn-TCBA, the ingenious introduction of the meta-position benzene carboxylates on the triphenylamine motif not only promotes Zn-TCBA to exhibit a broad visible-light absorption with a maximum absorption edge of 480 nm but also causes special phenyl plane twists with dihedral angles of 27.8-45.8° through the coordination to Zn nodes. The semiconductor-like Zn clusters and the twisted TCBA3- antenna with multidimensional π interaction sites facilitate photoinduced electron transfer to render Zn-TCBA a good photocatalytic H2 evolution efficiency of 27.104 mmol·g-1·h-1 in the presence of [Co(bpy)3]Cl2 under visible-light illumination, surpassing many non-noble-metal MOF systems. Moreover, the positive enough excited-state potential of 2.03 V and the semiconductor-like characteristics of Zn-TCBA endow Zn-TCBA with double oxygen activation ability for photocatalytic oxidation of N-aryl-tetrahydroisoquinoline substrates with a yield up to 98.7% over 6 h. The durability of Zn-TCBA and the possible catalytic mechanisms were also investigated by a series of experiments including PXRD, IR, EPR, and fluorescence analyses.

Iron Photoredox Catalysis-Past, Present, and Future.

Photoredox catalysis of organic reactions driven by iron has attracted substantial attention throughout recent years, due to potential environmental and economic benefits. In this Perspective, three major strategies were identified that have been employed to date to achieve reactivities comparable to the successful noble metal photoredox catalysis: (1) Direct replacement of a noble metal center by iron in archetypal polypyridyl complexes, resulting in a metal-centered photofunctional state. (2) In situ generation of photoactive complexes by substrate coordination where the reactions are driven via intramolecular electron transfer involving charge-transfer states, for example, through visible-light-induced homolysis. (3) Improving the excited-state lifetimes and redox potentials of the charge-transfer states of iron complexes through new ligand design. We seek to give an overview and evaluation of recent developments in this rapidly growing field and, at the same time, provide an outlook on the future of iron-based photoredox catalysis.

Unveiling Photodegradation and Photosensitization Mechanisms of Unconjugated Pterins.

Unconjugated pterins are ubiquitous molecules participating in countless enzymatic processes and potentially involved in the photosensitization of singlet oxygen, amino acids, and nucleotides. Following electronic excitation with UV-A light, some of these pterins degrade, producing hydrogen peroxide as the main side product. This process, that is known to take place in vivo, contributes to oxidative stress and melanocyte destruction in vitiligo. In this work, we present for the first time mechanistic insight to the formation of transient triplet species which simultaneously trigger Type I and Type II photosensitizing processes and the initiation of degradation processes. Our calculations reveal that photodegradation of 6-biopterin, which accumulates in the skin of vitiligo patients, leads to 6-formylpterin via retro-aldol reaction, and subsequently to 6-carboxypterin through a water-mediated aldehyde oxidation. Additionally, we show that the changes in the photosensitizing potential of these systems with pH come from the modulation of their excited-state redox potentials.

DFT Studies on a Metal Oxide@Graphene-Decorated D-π1-π2-A Novel Multi-Junction Light-Harvesting System for Efficient Dye-Sensitized Solar Cell Applications.

Graphene nanocomposites have emerged as potential photoanode materials for increased performance of the dye-sensitized solar cells (DSSCs) via charge transfer. Various metal-oxide-decorated graphene nanocomposites have widespread applications in energy devices, such as solar cells, fuel cells, batteries, sensors, electrocatalysis, and photocatalysis. However, the possible role of these composites in DSSC applications has largely remained unexplored. Herein, we studied a Sb2O3-decorated graphene-D-π1-π2-A sensitized TiO2 nanocomposite (dye-(TiO2)9/Sb2O3@GO) as a model multi-junction light-harvesting system and examined the impact of various π-bridges on the optical and photovoltaic properties of the push-pull dye system employed in this light-harvesting system. We have shown that by changing the spacer unit, the light sensitivity of nanocomposites can be varied from visible to near-infrared wavelengths. Furthermore, with the integration of metal-oxide-decorated graphene nanocomposites on D-π1-π2-A systems and D-π-A systems, composite photoelectrodes displayed better optical and photovoltaic characteristics with an enhanced absorption spectrum in the wavelength range of 800-1000 nm. The performance of the D-π1-π2-A system has been evaluated in terms of various photovoltaic parameters such as the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gaps, excited-state oxidation potential (E dye *), free energy of electron injection (G inject), total reorganization energy (λtotal), and open-circuit voltage (V oc). This work throws light on the current trends and the future opportunities in graphene-metal oxide nanocomposite-based DSSCs for better harvesting of the solar spectrum and better performance of solar devices.

Formation and degradation of strongly reducing cyanoarene-based radical anions towards efficient radical anion-mediated photoredox catalysis

Cyanoarene-based photocatalysts (PCs) have attracted significant interest owing to their superior catalytic performance for radical anion mediated photoredox catalysis. However, the factors affecting the formation and degradation of cyanoarene-based PC radical anion (PC•‒) are still insufficiently understood. Herein, we therefore investigate the formation and degradation of cyanoarene-based PC•‒ under widely-used photoredox-mediated reaction conditions. By screening various cyanoarene-based PCs, we elucidate strategies to efficiently generate PC•‒ with adequate excited-state reduction potentials (Ered*) via supra-efficient generation of long-lived triplet excited states (T1). To thoroughly investigate the behavior of PC•‒ in actual photoredox-mediated reactions, a reductive dehalogenation is carried out as a model reaction and identified the dominant photodegradation pathways of the PC•‒. Dehalogenation and photodegradation of PC•‒ are coexistent depending on the rate of electron transfer (ET) to the substrate and the photodegradation strongly depends on the electronic and steric properties of the PCs. Based on the understanding of both the formation and photodegradation of PC•‒, we demonstrate that the efficient generation of highly reducing PC•‒ allows for the highly efficient photoredox catalyzed dehalogenation of aryl/alkyl halides at a PC loading as low as 0.001 mol% with a high oxygen tolerance. The present work provides new insights into the reactions of cyanoarene-based PC•‒ in photoredox-mediated reactions.

Quantitative prediction of excited-state decay rates for radical anion photocatalysts.

We present a computational approach for predicting key properties of organic radical anions, including excited-state lifetimes and redox potentials. The approach shows good agreement with experimental data and has potential for in silico screening to facilitate the rational design of photocatalysts.

Energy level tuning of push-pull porphyrin sensitizer by trifluoromethyl group for dye-sensitized solar cells

We have introduced trifluoromethyl groups into meta-positions of two meso-phenyl rings of a push-pull-type porphyrin dye (ZnP-CF[Formula: see text] to modulate the energy levels of the porphyrin dye for dye-sensitized solar cells (DSSCs). The light-harvesting ability of ZnP-CF[Formula: see text] is almost comparable to those of reference porphyrins where the trifluoromethyl groups are replaced with methyl (ZnP-CH[Formula: see text] or tert-butyl groups (YD2). We revealed that the introduction of the electron-withdrawing trifluoromethyl groups is effective to lower the excited-state oxidation potential of the porphyrin (–0.80 V vs NHE), which is consistent with the theoretical calculation. Meanwhile, DSSCs with ZnP-CF[Formula: see text] exhibited a lower power conversion efficiency ([Formula: see text] of 5.95% than DSSCs with ZnP-CH[Formula: see text] ([Formula: see text] = 7.33%) and YD2 ([Formula: see text] = 8.97%) because of the severe aggregation tendency of ZnP-CF[Formula: see text] arising from the trifluoromethyl groups. In addition, the insufficient steric bulkiness of the trifluoromethyl and methyl groups relative to tert-butyl groups would result in the lower short circuit current and open circuit voltage for ZnP-CF[Formula: see text] and ZnP-CH[Formula: see text] due to fast charge recombination between electrons in the conduction band of TiO2 and I[Formula: see text]in the electrolyte solution. Overall, introducing both trifluoromethyl groups and bulky substituents into a porphyrin core would be necessary to boost cell performance.

Balancing Panchromatic Absorption and Multistep Charge Separation in a Compact Molecular Architecture

A panchromatic triad and a charge-separation unit are joined in a crossbar architecture to capture solar energy. The panchromatic-absorber triad (T) is comprised of a central free-base porphyrin that is strongly coupled via direct ethyne linkages to two perylene-monoimide (PMI) groups. The charge-separation unit incorporates a free-base or zinc chlorin (C or ZnC) as a hole acceptor (or electron donor) and a perylene-diimide (PDI) as an electron acceptor, both attached to the porphyrin via diphenylethyne linkers. The free-base porphyrin is common to both light-harvesting and charge-separation motifs. The chlorin and PDI also function as ancillary light absorbers, complementing direct excitation of the panchromatic triad to produce the discrete lowest excited state of the array (T*). Attainment of full charge separation across the pentad entails two steps: (1) an initial excited-state hole/electron-transfer process to oxidize the chlorin (and reduce the panchromatic triad) or reduce the PDI (and oxidize the panchromatic triad); and (2) subsequent ground-state electron/hole migration to produce oxidized chlorin and reduced PDI. Full charge separation for pentad ZnC-T-PDI to generate ZnC+-T-PDI- occurs with a quantum yield of ∼30% and mean lifetime ∼1 μs in dimethyl sulfoxide. For C-T-PDI, initial charge separation is followed by rapid charge recombination. The molecular designs and studies reported here reveal the challenges of balancing the demands for charge separation (linker length and composition, excited-state energies, redox potentials, and medium polarity) with the constraints for panchromatic absorption (strong electronic coupling of the porphyrin and two PMI units) for integrated function in solar-energy conversion.

Visible-light-Induced synthesis of imidazolidines through formal [3 + 2] cycloaddition of aromatic imines with N,N,N',N'-tetramethyldiaminomethane

Imidazolidine is a saturated heterocycle with a cyclic aminal core that can be used to develop new pharmacological drugs. Heteroleptic cyclometalated Ir(III) complex photosensitizers have been shown to enable the unprecedented synthesis of imidazolidines through a formal [3 + 2] photocycloaddition of aromatic imines with N,N,N',N'-tetramethyldiaminomethane. Herein, a reaction mechanism based on quenching experiments, quantum yield, ground-state redox potentials, and excited-state potentials is proposed.

Synthesis, Electrochemistry, Photophysics, and Photochemistry of a Discrete Tetranuclear Copper(I) Sulfido Cluster.

A tetranuclear copper(I) complex, [Cu4{μ-(Ph2P)2NH}4(μ4-S)](PF6)2 (1), was synthesized. It was found to display intense and long-lived phosphorescence in the solid and solution states. The lowest-energy excited state was assigned as ligand-to-metal charge transfer (LMCT) [S2- → Cu4] mixed with some metal-centered (ds/dp) character. In addition, the phosphorescent state of this complex was found to be quenched by pyridinium acceptors via an oxidative electron-transfer quenching process. An excited-state reduction potential of -1.74 V versus saturated salt calomel electrode was estimated through oxidative quenching studies with a series of structurally related pyridinium acceptors, indicative of its strong reducing power in the excited state. From the transient absorption difference spectrum of the tetranuclear copper(I) sulfido complex and 4-(methoxycarbonyl)-N-methylpyridinium hexafluorophosphate, in addition to the characteristic absorption of the pyridinyl radical at ca. 395 nm, two absorption bands at ca. 500 and 660 nm were also observed. The former was assigned as an LMCT absorption [S2- → Cu4] and the latter as an intervalence charge-transfer transition, associated with the mixed-valence species CuI/CuI/CuI/CuII.

Tuning the Ground- and Excited-State Redox Potentials of Octahedral Hexanuclear Rhenium(III) Complexes by the Combination of Terminal Halide and N-Heteroaromatic Ligands.

The present study reports that the ground- and excited-state Re6(23e)/Re6(24e) redox potentials of an octahedral hexanuclear rhenium(III) complex can be controlled by systematically changing the number and type of the N-heteroaromatic ligand (L) and the number of chloride ions at the six terminal positions. Photoirradiation of [Re6(μ3-S)8Cl6]4– with an excess amount of L afforded a mono-L-substituted hexanuclear rhenium(III) complex, [Re6(μ3-S)8Cl5(L)]3– (L = 4-dimethylaminopyridine (dmap), 3,5-lutidine (lut), 4-methylpyridine (mpy), pyridine (py), 4,4′-bipyridine (bpy), 4-cyanopyridine (cpy), and pyrazine (pz)). The bis- and tris-lut-substituted complexes, trans- and cis-[Re6(μ3-S)8Cl4(lut)2]2– and mer-[Re6(μ3-S)8Cl3(lut)3]−, were synthesized by the reaction of [Re6(μ3-S)8Cl6]3– with an excess amount of lut in refluxed N,N-dimethylformamide. The mono-L-substituted complexes showed one-electron redox processes assignable to E1/2[Re6(23e)/Re6(24e)] = 0.49–0.58 V versus Ag/AgCl. The ground-state oxidation potentials were linearly correlated with the pKa of the N-heteroaromatic ligand [pKa(L)], the 1H NMR chemical shift of the ortho proton on the coordinating ligand, and the Hammett constant (σ) of the pyridyl-ligand substituent. The series of [Re6(μ3-S)8X6–n(L)n]n−4 complexes (n = 0, X = Cl, Br, I, or NCS; n = 1–3, X = Cl) showed a linear correlation with the sum of the Lever electrochemical parameters at the six terminal ligands (ΣEL). The cyclic voltammograms of the mono-L-substituted complexes (L = bpy, cpy, and pz) showed one-electron redox waves assignable to E1/2(L0/L–) = −1.28 to −1.48 V versus Ag/AgCl. Two types of photoluminescences were observed for the complexes, originating from the cluster core-centered excited triplet state (3CC) for L = dmap, lut, mpy, and py and from the metal-to-ligand charge-transfer excited triplet state (3MLCT) for L = bpy, cpy, and pz. The complexes with the 3CC character exhibited emission features and photophysical properties similar to those of ordinary hexanuclear rhenium complexes. The emission maximum wavelength of the complexes with 3MLCT shifted to the longer wavelength in the order L = 4-phenylpyridine (ppy), bpy, pz, and cpy, which agreed with the difference between E1/2[Re6(23e)/Re6(24e)] and E1/2(L0/L–). The calculated oxidation potential of the excited hexanuclear rhenium complex with the 3CC character was linearly correlated with pKa(L), σ, and ΣEL. The ground- and excited-state oxidation potentials were finely tuned by the combination of halide and L ligands at the terminal positions.

Building Kohn–Sham Potentials for Ground and Excited States

We analyze the inverse problem of Density Functional Theory using a regularized variational method. First, we show that given k and a target density $$\rho $$ , there exist potentials having kth bound mixed states which densities are arbitrarily close to $$\rho $$ . The state can be chosen pure in dimension $$d=1$$ and without interactions, and we provide numerical and theoretical evidence consistently leading us to conjecture that the same pure representability result holds for $$d=2$$ , but that the set of pure-state v-representable densities is not dense for $$d=3$$ . Finally, we present an inversion algorithm taking into account degeneracies, removing the generic blocking behavior of standard ones.

Green-Light-Driven Fe(III)(btz)3 Photocatalysis in the Radical Cationic [4+2] Cycloaddition Reaction

Green-light-driven FeIII(btz)3 photocatalysis for the radical cationic [4+2] cycloaddition of terminal styrenes and nucleophilic dienes has been investigated. The Fe-MIC (mesoionic carbene) complex forms a ligand-to-metal charge-transfer transition state with relatively high excited-state reduction potentials that can selectively oxidize terminal styrene derivatives. Unique multisubstituted cyclohexenes and structurally complex biorelevant cyclohexenes were constructed, highlighting the usefulness of this mild and practical first-row transition metal complex system.

Ultrafast photo-induced processes in complex environments: The role of accuracy in excited-state energy potentials and initial conditions

Light induces non-equilibrium time evolving molecular phenomena. The computational modeling of photo-induced processes in large systems, embedded in complex environments (i.e., solutions, proteins, materials), demands for a quantum and statistical mechanic treatment to achieve the required accuracy in the description of both the excited-state energy potentials and the choice of the initial conditions for dynamical simulations. On the other hand, the theoretical investigation on the atomistic scale of times and sizes of the ultrafast photo-induced reactivity and non-equilibrium relaxation dynamics right upon excitation requests tailored computational protocols. These methods often exploit hierarchic computation schemes, where a large part of the degrees of freedom are required to be treated explicitly to achieve the right accuracy. Additionally, part of the explicit system needs to be treated at ab initio level, where density functional theory, using hybrid functionals, represents a good compromise between accuracy and computational cost, when proton transfers, non-covalent interactions, and hydrogen bond dynamics play important roles. Thus, the modeling strategies presented in this review stress the importance of hierarchical quantum/molecular mechanics with effective non-periodic boundary conditions and efficient phase-sampling schemes to achieve chemical accuracy in ultrafast time-resolved spectroscopy and photo-induced phenomena. These approaches can allow explicit and accurate treatment of molecule/environment interactions, including also the electrostatic and dispersion forces of the bulk. At the same time, the specificities of the different case studies of photo-induced phenomena in solutions and biological environments are highlighted and discussed, with special attention to the computational and modeling challenges.

Born–Oppenheimer potentials for Π, Δ, and Φ states of the hydrogen molecule

We report on accurate variational calculations of the Born–Oppenheimer potential for excited states of the hydrogen molecule with , and Φ symmetries. The obtained potential energy curves reach the relative precision of or better along internuclear distances of 0.01–20 au. Calculations rely on the recursive evaluation of two-centre two-electron molecular integrals with exponential functions in arbitrary precision arithmetics. Our results for most of the states are the first-ever reported, and for the previously calculated states constitute an improvement by several orders of magnitude.

Phenoxazine‐Sensitized CO2‐to‐CO Reduction with an Iron Porphyrin Catalyst: A Redox Properties‐Catalytic Performance Study

AbstractWe have evaluated six phenoxazine derivatives as visible light photosensitizers for the photochemical reduction of CO2 to CO with an iron porphyrin catalyst in organic media. The phenoxazine core was functionalized with electron‐donating or ‐withdrawing groups to modify the photophysical properties. Both singlet and triplet excited state potentials of the sensitizers spanned several hundred mV range and the ground state oxidation potentials spanned 250 mV. We observed that no correlation can be established between the production of CO and the excited state potential of the phenoxazine, which determines the driving force for electron transfer to the catalyst. On the contrary, a clear correlation can be made between the oxidation potentials of the phenoxazine and the production of CO. This observation indicates that the process was limited by photosensitizer regeneration and highlights the fact that electron transfers not directly related to the catalyst activation could play a key role in homogeneous photocatalytic systems.

Solving the Puzzle of Unusual Excited-State Proton Transfer in 2,5-Bis(6-methyl-2-benzoxazolyl)phenol.

2,5-Bis(6-methyl-2-benzoxazolyl)phenol (BMP) exhibits an ultrafast excited-state intramolecular proton transfer (ESIPT) when isolated in supersonic jets, whereas in condensed phases the phototautomerization is orders of magnitude slower. This unusual situation leads to nontypical photophysical characteristics: dual fluorescence is observed for BMP in solution, whereas only a single emission, originating from the phototautomer, is detected for the ultracold isolated molecules. In order to understand the completely different behavior in the two regimes, detailed photophysical studies have been carried out. Kinetic and thermodynamic parameters of ESIPT were determined from stationary and transient picosecond absorption and emission for BMP in different solvents in a broad temperature range. These studies were combined with time-dependent- density functional theory quantum-chemical modeling. The excited-state double-well potential for BMP and its methyl-free analogue were calculated by applying different hybrid functionals and compared with the results obtained for another proton-transferring molecule, 2,5-bis(5-ethyl-2-benzoxazolyl)hydroquinone (DE-BBHQ). The results lead to the model that explains the difference in proton-transfer properties of BMP in vacuum and in the condensed phase by inversion of the two lowest singlet states occurring along the PT coordinate.