A Photoswitchable Metallocycle Based on Azobenzene: Synthesis, Characterization, and Ultrafast Dynamics.

The novel photoswitchable ligand 3,3'-Azobenz(metPA)2 (1) is used to prepare a [Cu2(1)2](BF4)2 metallocycle (2), whose photoisomerization was characterized using static and time-resolved spectroscopic methods. Optical studies demonstrate the highly quantitative and reproducible photoinduced cyclic E/Z switching without decay of the complex. Accordingly and best to our knowledge, [Cu2(1)2](BF4)2 constitutes the first reversibly photoswitchable (3d)-metallocycle based on azobenzene. The photoinduced multiexponential dynamics in the sub-picosecond to few picosecond time domain of 1 and 2 have been assessed. These ultrafast dynamics as well as the yield of the respective photostationary state (PSSZ = 65 %) resemble the behavior of archetypical azobenzene. Also, the innovative pump-probe laser technique of gas phase transient photodissociation (τ-PD) in a mass spectrometric ion trap was used to determine the intrinsic relaxation dynamics for the isolated complex. These results are consistent with the results from femtosecond UV/Vis transient absorption (fs-TA) in solution, emphasizing the azobenzene-like dynamics of 2. This unique combination of fs-TA and τ-PD enables valuable insights into the prevailing interplay of dynamics and solvation. Both analyses (in solution and gas phase) and quantum chemical calculations reveal a negligible effect of the metal coordination on the switching mechanism and electronic pathway, which suggests a non-cooperative isomerization process.

Electronic spectroscopy of homo- and heterometallic binuclear coinage metal phosphine complexes in isolation.

Binuclear coinage metal phosphine complexes are examined under ion trap isolation in order to elucidate their noncovalent binding, structural properties and intrinsic electronic spectra. Our survey shows an intriguing order of electronic transitions obtained by in situ synthesis and mass-spectrometrically supported UV photodissociation spectroscopy on a series of six isolated homo- and heterobinuclear complexes of type [MM'(dcpm)2]2+ (M, M' = CuI, AgI, AuI; dcpm = bis(dicyclohexyl-phosphino)methane). This approach provides the unique opportunity to study all possible coinage metal interactions within a fixed ligand framework. A successive blue-shift (33 700-38 500 cm-1; 297-260 nm) of the lowest-energy bright electronic transition energy in gas phase was observed in the order of Cu2 < CuAu < CuAg < Au2 < AgAu < Ag2. This order was reproduced by quantum chemical calculations using a scalar-relativistic GW-Bethe-Salpeter-equation (GW-BSE) approach. Theory ascribes the electronic bands of all complexes to metal-centered 1MC(dσ*-pσ) transitions revealing a strengthening of metal-metal' (M-M') binding upon excitation, in agreement to mass spetrometric results. A test of the correlation of transition energies with M-M' distance by quantum chemical calculations of single point energies as a function of intermetallic distance indicates qualitative agreement with experimental results. However, the experimentally observed high sensitivity of spectroscopic shifts towards metal composition cannot be accounted for solely by M-M' distance variation. The differences in electronic transitions are qualitatively rationalized by the varying (n + 1)s (n = 3, 4, 5) orbital contributions (increase from Cu2via CuAu/CuAg to Au2/AgAu/Ag2) within the nd(n + 1)s/p-hybridization for the ground electronic state of the different complexes, whereas the excited state (of (n + 1)p orbital character) shows significantly less variation in energy. In particular, the observed spectroscopic and mass spectrometric sequence for the Ag/Au complexes is traced back to the interplay of Pauli repulsion and variation in metal-ligand bond strength within the orbital hybridization model.

Chromone-methanol clusters in the electronic ground and lowest triplet state: a delicate interplay of non-covalent interactions.

Chromone offers two energetically almost equivalent docking sites for alcohol molecules, in which the hydroxyl group is hydrogen bonded to one of the free electron pairs of the carbonyl O atom. Here, the delicate balance between these two competing arrangements is studied by combining IR/R2PI and UV/IR/UV spectroscopy in a molecular beam supported by quantum-chemical calculations. Most interestingly, chromone undergoes an efficient intersystem crossing into the triplet manifold upon electronic excitation, so that the studies on aromatic molecule-solvent complexes are for the first time extended to such a cluster in a triplet state. As the lowest triplet state (T1) is of ground state character, powerful energy decomposition approaches such as symmetry-adapted perturbation theory (SAPT) and local energy decomposition using the domain-based local pair natural orbital coupled-cluster method (DLPNO-CCSD(T)/LED) are applied. From the theoretical analysis we infer for the T1 state a loss of planarity (puckering) of the 4-pyrone ring of the chromone unit, which considerably affects the interplay between different types of non-covalent interactions at the two possible binding sites.

Time-Resolved Spectroscopy and Electronic Structure of Mono-and Dinuclear Pyridyl-Triazole/DPEPhos-Based Cu(I) Complexes.

Chemical and spectroscopic characterization of the mononuclear photosensitizers [(DPEPhos)Cu(I)(MPyrT)]0/+ (CuL, CuLH) and their dinuclear analogues (Cu2 L', Cu2 L'H2 ), backed by (TD)DFT and high-level GW-Bethe-Salpeter equation calculations, exemplifies the complex influence of charge, nuclearity and structural flexibility on UV-induced photophysical pathways. Ultrafast transient absorption and step-scan FTIR spectroscopy reveal flattening distortion in the triplet state of CuLH as controlled by charge, which also appears to have a large impact on the symmetry of the long-lived triplet states in Cu2 L' and Cu2 L'H2 . Time-resolved luminescence spectroscopy (solid state), supported by transient photodissociation spectroscopy (gas phase), confirm a lifetime of some tens of µs for the respective triplet states, as well as the energetics of thermally activated delayed luminescence, both being essential parameters for application of these materials based on earth-abundant copper in photocatalysis and luminescent devices.

Metal-to-Metal Distance Modulated Au(I)/Ru(II) Cyclophanyl Complexes: Cooperative Effects in Photoredox Catalysis.

The modular synthesis of Au(I)/Ru(II) decorated mono- and heterobimetallic complexes with π-conjugated [2.2]paracyclophane is described. [2.2]Paracyclophane serves as a rigid spacer which holds the metal centers in precise spatial orientations and allows metal-to-metal distance modulation. A broad set of architectural arrangements of pseudo -geminal, -ortho, -meta, and -para substitution patterns were employed. Metal-to-metal distance modulation of Au(I)/Ru(II) heterobimetallic complexes and the innate transannular π-communication of the cyclophanyl scaffold provides a promising platform for the investigations of structure-activity relationship and cooperative effects. The Au(I)/Ru(II) heterobimetallic cyclophanyl complexes are stable, easily accessible, and exhibit promising catalytic activity in the visible-light promoted arylative Meyer-Schuster rearrangement.

On the Hydrogen Oxalate Binding Motifs onto Dinuclear Cu and Ag Metal Phosphine Complexes.

We report the binding geometries of the isomers that are formed when the hydrogen oxalate ((CO2 )2 H=HOx) anion attaches to dinuclear coinage metal phosphine complexes of the form [M1 M2 dcpm2 (HOx)]+ with M=Cu, Ag and dcpm=bis(dicyclohexylphosphino)methane, abbreviated [MM]+ . These structures are established by comparison of isomer-selective experimental vibrational band patterns displayed by the cryogenically cooled and N2 -tagged cations with DFT calculations of the predicted spectra for various local minima. Two isomeric classes are identified that feature either attachment of the carboxylate oxygen atoms to the two metal centers (end-on docking) or attachment of oxygen atoms on different carbon atoms asymmetrically to the metal ions (side-on docking). Within each class, there are additional isomeric variations according to the orientation of the OH group. This behavior indicates that HOx undergoes strong and directional coordination to [CuCu]+ but adopts a more flexible coordination to [AgAg]+ . Infrared spectra of the bare ions, fragmentation thresholds and ion mobility measurements are reported to explore the behaviors of the complexes at ambient temperature.

Photoinitiated Charge Transfer in a Triangular Silver(I) Hydride Complex and Its Oxophilicity.

The photoexcitation of a triangular silver(I) hydride complex, [Ag3 (µ3 -H)(µ2 -dcpm)3 ](PF6 )2 ([P](PF6 )2 , dcpm=bis(dicyclohexylphosphino)methane), designed with "UV-silent" bis-phosphine ligands, provokes hydride-to-Ag3 single and double electron transfer. The nature of the electronic transitions has been authenticated by absorption and photodissociation spectroscopy in parallel with high-level quantum-chemical computations utilizing the GW method and Bethe-Salpeter equation (GW-BSE). Specific photofragments of mass-selected [P]2+ ions testify to charge transfer and competing pathways resulting from the unique [Ag3 (µ3 -H)]2+ scaffold. This structural motif of [P](PF6 )2 has been unequivocally verified by 1 H NMR spectroscopy in concert with DFT and X-ray diffraction structural analysis, which revealed short equilateral Ag-Ag distances (dAgAg =3.08 Å) within the range of argentophilic interactions. The reduced radical cation [P]. + exhibits strong oxophilicity, forming [P+O2 ].+ ,which is a model intermediate for silver oxidation catalysis.

Bimetallic d10 -Metal Complexes of a Bipyridine Substituted N-Heterocyclic Carbene.

The hybrid ligand 3-(2,2'-bipyridine-6-ylmethyl)-1-mesityl-1H-imidazolylidene (NHCBipy ) featuring both carbene and N-donor sites, was selectively complexed with various d10 metal cations in order to examine its coordination behavior with regard to homo and heterometallic structures. Respective silver complexes can be obtained by the silver oxide route and are suitable transmetallation reagents for the synthesis of gold(I) compounds. Starting from the mononuclear complexes [(NHCBipy )AuCl], [(NHCBipy )Au(C6 F5 )] and [(NHCBipy )2 Au][ClO4 ], open-chain as well as cyclic heterobimetallic complexes containing Cu+ , Ag+ , Zn2+ , Cd2+ , and Hg2+ were synthesized. Furthermore, the homobimetallic species [(NHCBipy )2 M2 ][ClO4 ]2 (M=Cu, Ag) were obtained. All bimetallic compounds were fully characterized including single-crystal X-ray analysis. Their photoluminescence (PL) properties were investigated in the solid state at temperatures between 15 and 295 K and compared with those of the mononuclear species. There is a clear difference in PL properties between the open chain and the cyclic heterobimetallic complexes. The latter species show different PL properties, depending on the metals involved. In addition, collision-induced dissociation (CID) experiments were performed on electrosprayed cations of the cyclic heterobimetallic compounds, to compare the metal binding at the carbene and N-donor sites.

Photodynamics and Luminescence of Mono- and Tri-Nuclear Lanthanide Complexes in the Gas Phase and in Solution.

Lanthanide ions (DyIII , EuIII ) are stabilized by coordination with two Schiff base ligands in compounds [Dy{H3 L}2 ](NO3 )(EtOH)(H2 O)8 (1) and [Eu{H3 L}2 ](NO3 )(H2 O)8 (3) (H4 L, 2,2'-{[(2-aminoethyl)imino]bis[2,1-ethanediyl-nitriloethylidyne]}bis-2-hydroxy-benzoic acid). The latter is reported here for the first time. Both luminescence and ultrafast photodynamics after photoexcitation via a ligand absorption band (∼400 nm) have been studied. In solution, only the [Eu{H3 L}2 ]+ ([3]+ ) complex displays the typical lanthanide emission lines, whereas in gas phase both, [Dy{H3 L}2 ]+ ([1]+ ) and [3]+ , show their corresponding transitions depending on excitation energy. The ultrafast excited state dynamics, obtained in gas phase and in solution, are assigned to excited state intramolecular proton transfer processes in the ligands. The antenna ligand moiety of these complexes provides pockets for stabilization of two MnII ions so that we additionally investigated the photophysical behavior of the corresponding tri-nuclear (NHEt3 )2 [Ln{MnL}2 ](ClO4 )(H2 O)2 (Ln=DyIII , EuIII ) compounds (2, 4). Interestingly, the related complexes do not show lanthanide emission, neither in solution nor in gas phase. Transient data in solution and gas phase suggests an efficient quenching of the ligand's electronically excited state by strong interaction with the MnII ions. This effect could possibly be developed further into a design principle for luminescence-based sensing devices for metal cations.

Gas-Phase Photoluminescence and Photodissociation of Silver-Capped Hexagold Clusters.

We report on the radiative and nonradiative deactivation pathways of selected charge states of the stoichiometric hexagold phosphine-stabilized ionic clusters, [(C)(AuDppy)6Ag2·(BF4) x](4- x)+ with x = 2 and 3 (Dppy = diphenylphosphino-2-pyridine), combining gas-phase photoluminescence and photodissociation with quantum chemical computations. These clusters possess an identical isostructural core made of a hyper-coordinated carbon at their center octahedrally surrounded by six gold ions, and two silver ions at their apexes. Their luminescence and fragmentation behavior upon photoexcitation was investigated under mass and charge control in an ion trap. The experimental and computational results shed light on the electronic states involved in the optical transitions as well as on their core, ligand, or charge transfer character. Gas-phase results are discussed in relation with condensed phase measurements, as well as previous observations in solution and on metal-organic frameworks. The monocationic species ( x = 3) is found to be less stable than the dicationic one ( x = 2). In the luminescence spectrum of the monocationic species, a shoulder at short wavelength can be observed and is assigned to fragment emission. This fragment formation appears to be favored for the monocation by the existence of a low lying singlet state energetically overlapping with the triplet state manifold, which is populated quickly after photoexcitation.

Exploring the Gas-Phase Activation and Reactivity of a Ruthenium Transfer Hydrogenation Catalyst by Experiment and Theory in Concert.

This study elucidates structures, activation barriers, and the gas-phase reactivity of cationic ruthenium transfer hydrogenation catalysts of the structural type [(η6-cym)RuX(pympyr)]+. In these complexes, the central ruthenium(+II) ion is coordinated to an η6-bound p-cymene (η6-cym), a bidentate 2-R-4-(2-pyridinyl)pyrimidine ligand (pympyr) with R = NH2 or N(CH3)2, and an anion X = I-, Br-, Cl-, or CF3SO3-. We present infrared multiple-photon dissociation (IR-MPD) spectra of precursors (before HCl loss) and of activated complexes (after HCl loss), which elucidates C-H activation as the key step in the activation mechanism. A resonant two-color IR-MPD scheme serves to record several otherwise "dark" bands and enhances the validity of spectral assignments. We show that collision-induced dissociation (CID)-derived activation energies of the [(η6-cym)RuX(pympyr)]+ (R = N(CH3)2) complexes depend crucially on the anion X. The obtained activation energies for the HX loss correlate well with quantum chemical activation barriers and are in line with the HSAB concept. We further elucidate the reaction of the activated complexes with D2 under single-collision conditions. Quantum mechanical simulations substantiate that the resulting species represent analogues for hydrido intermediates formed after abstraction of H+ and H- from isopropanol, as postulated for the catalytic cycle of transfer hydrogenation by us before.

Pump-Probe Fragmentation Action Spectroscopy: A Powerful Tool to Unravel Light-Induced Processes in Molecular Photocatalysts.

We present a proof of concept that ultrafast dynamics combined with photochemical stability information of molecular photocatalysts can be acquired by electrospray ionization mass spectrometry combined with time-resolved femtosecond laser spectroscopy in an ion trap. This pump-probe "fragmentation action spectroscopy" gives straightforward access to information that usually requires high purity compounds and great experimental efforts. Results of gas-phase studies on the electronic dynamics of two supramolecular photocatalysts compare well to previous findings in solution and give further evidence for a directed electron transfer, a key process for photocatalytic hydrogen generation.

Self-pairing of 1-methylthymine mediated by two and three Ag(I) ions: a gas phase study using infrared dissociation spectroscopy and density functional theory.

Metal base pairs of Ag(I) cations and 1-methylthymine (1MT) or deprotonated 1-methylthymine (1MT-H) are produced and analyzed by electrospray ionization mass spectrometry (ESI-MS). Mass-selected ions of type [Ag2(1MT)(1MT-H)](+) and [Ag3(1MT-H)2](+) are interrogated by infrared multiple-photon dissociation (IRMPD) in an ion trap in the range of 1200-3700 cm(-1). Supporting spectroscopic data were obtained from the investigation of the analogous 2'-deoxy-thymidine complexes which exhibit advantageously high fragment yields. By comparison with calculated linear IR spectra (obtained by density functional theory, DFT) we assign the structures and the possible isomeric forms of these metal base pairs and their dependence on the number of mediating Ag(I) ions. Based on the observed Ag(+)/1MT complexes and related polarizable continuum model DFT calculations we describe the probable formation pathways in aqueous solution. The present findings pave the way for subsequent UV investigations of the multi-metal mediated base pairs.

Time-resolved photoelectron spectroscopy of a dinuclear Pt(II) complex: Tunneling autodetachment from both singlet and triplet excited states of a molecular dianion.

Time-resolved pump-probe photoelectron spectroscopy has been used to study the relaxation dynamics of gaseous [Pt2(µ-P2O5H2)4 + 2H](2-) after population of its first singlet excited state by 388 nm femtosecond laser irradiation. In contrast to the fluorescence and phosphorescence observed in condensed phase, a significant fraction of the photoexcited isolated dianions decays by electron loss to form the corresponding monoanions. Our transient photoelectron data reveal an ultrafast decay of the initially excited singlet (1)A2u state and concomitant rise in population of the triplet (3)A2u state, via sub-picosecond intersystem crossing (ISC). We find that both of the electronically excited states are metastably bound behind a repulsive Coulomb barrier and can decay via delayed autodetachment to yield electrons with characteristic kinetic energies. While excited state tunneling detachment (ESETD) from the singlet (1)A2u state takes only a few picoseconds, ESETD from the triplet (3)A2u state is much slower and proceeds on a time scale of hundreds of nanoseconds. The ISC rate in the gas phase is significantly higher than in solution, which can be rationalized in terms of changes to the energy dissipation mechanism in the absence of solvent molecules. [Pt2(µ-P2O5H2)4 + 2H](2-) is the first example of a photoexcited multianion for which ESETD has been observed following ISC.

Investigation by two-color IR dissociation spectroscopy of Hoogsteen-type binding in a metalated nucleobase pair mimic.

Two Ag(I) ions, deprotonated 1-methyl-thymine (1MT-H)(-) and 1,3-dideaza-adenine (DDA), self-assemble in methanolic solution to a cationic [(Ag)2(1MT-H)(DDA)](+) complex as identified by electrospray ionization mass spectrometry. Assignment of vibrational bands and identification of the silver coordination pattern arise from comparison of one- and two-color Infrared Multiple Photon Dissociation (IRMPD) spectra (1000-4000 cm(-1)) of isolated and trapped complexes to calculated spectra from density functional theory. This approach reveals two structurally and energetically close isomers that resemble a metalated Hoogsteen-like binding motif. The two color IR/IR double resonance scheme proved in particular useful to observe weakly absorbing or weakly fragmenting vibrational modes.

From a Dy(III) single molecule magnet (SMM) to a ferromagnetic [Mn(II)Dy(III)Mn(II)] trinuclear complex.

The Schiff base compound 2,2'-{[(2-aminoethyl)imino]bis[2,1-ethanediyl-nitriloethylidyne]}bis-2-hydroxy-benzoic acid (H(4)L) as a proligand was prepared in situ. This proligand has three potential coordination pockets which make it possible to accommodate from one to three metal ions allowing for the possible formation of mono-, di-, and trinuclear complexes. Reaction of in situ prepared H(4)L with Dy(NO(3))(3)·5H(2)O resulted in the formation of a mononuclear complex [Dy(H(3)L)(2)](NO(3))·(EtOH)·8(H(2)O) (1), which shows SMM behavior. In contrast, reaction of in situ prepared H(4)L with Mn(ClO(4))(2)·6H(2)O and Dy(NO(3))(3)·5H(2)O in the presence of a base resulted in a trinuclear mixed 3d-4f complex (NHEt(3))(2)[Dy{Mn(L)}(2)](ClO(4))·2(H(2)O) (2). At low temperatures, compound 2 is a weak ferromagnet. Thus, the SMM behavior of compound 1 can be switched off by incorporating two Mn(II) ions in close proximity either side of the Dy(III). This quenching behavior is ascribed to the presence of the weak ferromagnetic interactions between the Mn(II) and Dy(III) ions, which at T > 2 K act as a fluctuating field causing the reversal of magnetization on the dysprosium ion. Mass spectrometric ion signals related to compounds 1 and 2 were both detected in positive and negative ion modes via electrospray ionization mass spectrometry. Hydrogen/deuterium exchange (HDX) reactions with ND(3) were performed in a FT-ICR Penning-trap mass spectrometer.

Ring-puckering motion in cyclopentene studied by time-resolved rotational coherence spectroscopy and ab initio calculations.

The ring-puckering vibration in cyclopentene was studied by rotational time-resolved femtosecond degenerate four-wave mixing (fs DFWM) spectroscopy. The fs DFWM spectra of cyclopentene were measured both in a supersonic expansion and in a gas cell at room temperature. The room temperature fs DFWM spectrum has been satisfactorily reproduced by a fitted simulation based on a one-dimensional model for the ring-puckering vibration. This has allowed for the determination of energetic parameters of the ring-puckering motion such as the energy barrier to ring inversion of 274(+12/-20) cm(-1) and the equilibrium ring-puckering angle of 24.3 degrees . The derived dependences of the rotational constants A and B on the puckering angle resemble very closely those obtained by microwave spectroscopy. In addition, previous theoretical estimates of the ring inversion barrier of cyclopentene were improved by performing high level ab initio calculations. Zero-point vibrational energy correction was found to be essential for an accurate evaluation of the puckering potential. Altogether, this study provides a proof-of-principle of the applicability of the fs DFWM technique for investigating large amplitude intramolecular motions.

The structure of adenine monohydrates studied by femtosecond multiphoton ionization detected IR spectroscopy and quantum chemical calculations.

We present femtosecond multiphoton ionization detected infrared spectra of jet-cooled monohydrates of adenine and 9-methyladenine. By quantum chemical vibrational analysis and comparison with available literature data we identified two isomers of adenine hydrate with one water molecule hydrogen-bonded to either the amino or the N9-H group. These two monohydrates revealed different fragmentation patterns in the ion depletion spectra, indicating isomer specific intermolecular dynamics. This different behaviour is discussed in terms of competing electronically excited state relaxation and dissociation processes.

Pseudorotation in pyrrolidine: rotational coherence spectroscopy and ab initio calculations of a large amplitude intramolecular motion.

Pseudorotation in the pyrrolidine molecule was studied by means of femtosecond degenerate four-wave mixing spectroscopy both in the gas cell at room temperature and under supersonic expansion. The experimental observations were reproduced by a fitted simulation based on a one-dimensional model for pseudorotation. Of the two conformers, axial and equatorial, the latter was found to be stabilized by about 29 +/- 10 cm(-1) relative to the former one. The barrier for pseudorotation was determined to be 220 +/- 20 cm(-1). In addition, quantum chemical calculations of the pseudorotational path of pyrrolidine were performed using the synchronous transit-guided quasi-Newton method at the MP2 and B3LYP levels of theory. Subsequent CCSD(T) calculations yield the energy preference of the equatorial conformer and the barrier for pseudorotation to be 17 and 284 cm(-1), respectively.