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

The novel photoswitchable ligand 3,3'-Azobenz(metPA) (1) is used to prepare a [Cu(1)](BF) 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, [Cu(1)](BF) constitutes the first reversibly photoswitchable (3d)-metallocycle based on azobenzene.

Darkness inhibits autokinase activity of bacterial bathy phytochromes.

Bathy phytochromes are a subclass of bacterial biliprotein photoreceptors that carry a biliverdin IXα chromophore. In contrast to prototypical phytochromes that adopt a red-light-absorbing Pr ground state, the far-red light-absorbing Pfr-form is the thermally stable ground state of bathy phytochromes. Although the photobiology of bacterial phytochromes has been extensively studied since their discovery in the late 1990s, our understanding of the signal transduction process to the connected transmitter domains, which are often histidine kinases, remains insufficient.

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.

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)] (CuL, CuLH) and their dinuclear analogues (Cu L', Cu L'H ), 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 Cu L' and Cu L'H . 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.

Ultrafast proton release reaction and primary photochemistry of phycocyanobilin in solution observed with fs-time-resolved mid-IR and UV/Vis spectroscopy.

Deactivation processes of photoexcited (λ = 580 nm) phycocyanobilin (PCB) in methanol were investigated by means of UV/Vis and mid-IR femtosecond (fs) transient absorption (TA) as well as static fluorescence spectroscopy, supported by density-functional-theory calculations of three relevant ground state conformers, PCB, PCB and PCB, their relative electronic state energies and normal mode vibrational analysis. UV/Vis fs-TA reveals time constants of 2.0, 18 and 67 ps, describing decay of PCB*, of PCB* and thermal re-equilibration of PCB, PCB and PCB, respectively, in line with the model by Dietzek et al.

Wavelength-specific optoacoustic-induced vibrations of the guinea pig tympanic membrane.

Significance: Optoacoustic-induced vibrations of the hearing organ can potentially be used for a hearing device. To increase the efficiency of such a hearing device, the conversion of the light energy into vibration energy within each type of irradiated tissue has to be optimized.

Aim: To analyze the wavelength-dependency of optoacoustic-induced vibrations within the tympanic membrane (TM), and to define the most efficient and best-suited optical stimulation parameters for a novel auditory prosthesis.

Real-time observation of molecular flattening and intersystem crossing in [(DPEPhos)Cu(i)(PyrTet)] via ultrafast UV/Vis- and mid-IR spectroscopy on solution and solid samples.

The primary photo-induced processes in the mononuclear, heteroleptic Cu(i) complex [(DPEPhos)Cu(PyrTet)] (1), relevant for OLED applications, were investigated in various solvents and in solid state samples via femtosecond (fs) time resolved UV/Vis and fs time resolved mid-IR transient absorption spectroscopy (TA) with MLCT excitation around 340 nm. UV/Vis fs-TA on 1 in solution reveals (i) a severe blue-shift of excited state absorption on the time scale of a few picoseconds (τ) that is not observed in solids, and (ii), on the time scale of several tens of picoseconds (τ), a process with very similar dynamics in all samples. Mid-IR fs-TA in solution indicates structural changes with τ.

Exploring the Vibrational Side of Spin-Phonon Coupling in Single-Molecule Magnets via Dy Nuclear Resonance Vibrational Spectroscopy.

Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) using the Mössbauer isotope Dy has been employed for the first time to study the vibrational properties of a single-molecule magnet (SMM) incorporating Dy , namely [Dy(Cy PO) (H O) ]Br ⋅2 (Cy PO)⋅2 H O ⋅2 EtOH. The experimental partial phonon density of states (pDOS), which includes all vibrational modes involving a displacement of the Dy ion, was reproduced by means of simulations using density functional theory (DFT), enabling the assignment of all intramolecular vibrational modes. This study proves that Dy NRVS is a powerful experimental tool with significant potential to help to clarify the role of phonons in SMMs.

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

The photoexcitation of a triangular silver(I) hydride complex, [Ag (μ -H)(μ -dcpm) ](PF ) ([P](PF ) , dcpm=bis(dicyclohexylphosphino)methane), designed with "UV-silent" bis-phosphine ligands, provokes hydride-to-Ag 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] ions testify to charge transfer and competing pathways resulting from the unique [Ag (μ -H)] scaffold.

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

Lanthanide ions (Dy , Eu ) are stabilized by coordination with two Schiff base ligands in compounds [Dy{H L} ](NO )(EtOH)(H O) (1) and [Eu{H L} ](NO )(H O) (3) (H 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.

Vibrational Coherence Controls Molecular Fragmentation: Ultrafast Photodynamics of the [AgCl] Scaffold.

The recently introduced pump-probe fragmentation action spectroscopy reveals a unique observation of excited state vibrational coherence (430-460 fs) in the isolated metal complex [Ag(Cl)(dcpm))] (dcpm = bis(dicyclohexylphosphino)methane) containing the [AgCl] scaffold. After photoexcitation by an XMCT transition (260 nm) in an ion trap, an unexpected correlation between specific fragment ions (loss of HCl/Cl vs loss of dcpm) and the phase of the wave packet is probed (1150 nm). Based on ab initio calculations, we assign the primary electronically excited state and ascribe the observed coherences (72-78 cm) to contain predominantly Ag-Ag stretch character.

A luminescent PtFe spin crossover complex.

A heterotrinuclear [PtFe] spin crossover (SCO) complex was developed and synthesized employing a ditopic bridging bpp-alkynyl ligand L and alkynyl coordinated Pt terpy units: [Fe(L-Pt)](BF) (1). We identified two different types of crystals of 1 which differ in their molecular packing and the number of co-crystallized solvent molecules: 1H (1·3.5CHCl in P1[combining macron]) and 1L (1·10CHCl in C2/c); while 1L shows a reversible SCO with a transition temperature of 268 K, the analogous compound 1H does not show any SCO and remains blocked in the HS state.

Spectroscopic, Structural, and Kinetic Investigation of the Ultrafast Spin Crossover in an Unusual Cobalt(II) Semiquinonate Radical Complex.

A comprehensive spectroscopic and structural investigation of [Co (l-N  tBu )(dbsq)][B(p-C H Cl) ] (1, l-N  tBu =N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane, dbsq =3,5-di-tert-butylsemiquinonate), the first known octahedral complex with a low-spin (ls) Co semiquinonate ground state, is reported. Above 200 K, solids as well as solutions of 1 exhibit thermally induced spin-crossover (SCO) from the ls to the high-spin (hs) Co semiquinonate state instead of the frequently observed valence tautomerism from ls Co catecholate to hs Co semiquinonate.

Spectroscopic Investigation on the Primary Photoreaction of Bathy Phytochrome Agp2-Pr of Agrobacterium fabrum: Isomerization in a pH-dependent H-bond Network.

Bathy phytochrome Agp2 from Agrobacterium fabrum exhibits an unusually low pKa =7.6 in the Pr state in contrast to a pKa >11 in the Pfr state, indicating a pH-dependent charge distribution and H-bond network in the Pr chromophore binding pocket around neutral pH. Here, we report on ultrafast UV/Vis absorption spectroscopy of the primary Pr photoisomerization of Agp2 at pH 6 and pH 9 and upon H2 O/D2 O buffer exchange.

Ultrafast deactivation of bilirubin: dark intermediates and two-photon isomerization.

Bilirubin is a neurotoxic product responsible for neonatal jaundice, which is generally treated by phototherapy. The photoreaction involves ultrafast internal conversion via an elusive intermediate and Z-E isomerization with minor yield (less than 3% in solution). The structure of the intermediate remains unclear.

Femtosecond dynamics in the lactim tautomer of phycocyanobilin: a long-wavelength absorbing model compound for the phytochrome chromophore.

Transient UV/Vis absorption spectroscopy is used to study the primary dynamics of the ring-A methyl imino ether of phycocyanobilin (PCB-AIE), which was shown to mimic the far-red absorbance of the Pfr chromophore in phytochromes (R. Micura, K. Grubmayr, Bioorg.

ESIPT and photodissociation of 3-hydroxychromone in solution: photoinduced processes studied by static and time-resolved UV/Vis, fluorescence, and IR spectroscopy.

The spectral properties of fluorescence sensors such as 3-hydroxychromone (3-HC) and its derivatives are sensitive to interaction with the surrounding medium as well as to substitution. 3-HC is a prototype system for other derivatives because it is the basic unit of all flavonoides undergoing ESIPT and is not perturbed by a substituent. In this study, the elementary processes and intermediate states in the photocycle of 3-HC as well as its anion were identified and characterized by the use of static and femtosecond time-resolved spectroscopy in different solvents (methylcyclohexane, acetonitrile, ethanol, and water at different pH).

Unusual spectral properties of bacteriophytochrome Agp2 result from a deprotonation of the chromophore in the red-absorbing form Pr.

Phytochromes are widely distributed photoreceptors with a bilin chromophore that undergo a typical reversible photoconversion between the two spectrally different forms, Pr and Pfr. The phytochrome Agp2 from Agrobacterium tumefaciens belongs to the group of bathy phytochromes that have a Pfr ground state as a result of the Pr to Pfr dark conversion. Agp2 has untypical spectral properties in the Pr form reminiscent of a deprotonated chromophore as confirmed by resonance Raman spectroscopy.

Transient IR spectroscopy and ab initio calculations on ESIPT in 3-hydroxyflavone solvated in acetonitrile.

Femtosecond polarization resolved UV/Vis and mid-infrared spectroscopy was used to thoroughly identify and characterize the relevant elementary chemical and physical processes in the photocycle of 3-hydroxyflavone (3-HF) in solution. In one set of experiments with the polar aprotic solvent acetonitrile-d(3), for the first time excited state intramolecular proton transfer (ESIPT), vibrational cooling/relaxation and rotational diffusion could be separated, and furthermore mid IR vibrational spectra of 3-HF excited states in solution phase were obtained. UV/Vis transient absorption data yield the time constant τ(Rot) = 22 ps for rotational diffusion and the time constant τ(VR) = 8.

Vibrational properties of the polymeric spin crossover (SCO) Fe(II) complexes [{Fe(4-amino-1,2,4-triazole)3}X2]n: a nuclear inelastic scattering (NIS), Raman and DFT study.

The vibrational properties of the cationic spin crossover (SCO) coordination polymers [{Fe(4-amino-1,2,4-triazole)(3)}(+2)](n) containing the anions chlorine, methanosulfonate and 1-naphthalenesulfonate have been studied via nuclear inelastic scattering of synchrotron radiation (NIS) as well as by Raman spectroscopy. Although the different anions have a strong influence on the spin crossover temperature, they have little effect on the positions of the spin marker bands in the NIS and Raman spectra. By comparing the line positions of the NIS spin marker bands with those observed by Raman spectroscopy, it has been possible to distinguish vibrations symmetry (A(u) or A(g)) because modes of A(u) and A(g) symmetries are NIS active, but only the A(g) modes are Raman active.

Vibrational mode analysis of isotope-labeled electronically excited riboflavin.

Isotope-labeled riboflavin in DMSO was employed in conjunction with femtosecond time-resolved infrared vibrational spectroscopy and quantum chemical calculations to analyze and assign the electronically excited state vibrational modes of the isoalloxazine unit as a prototype for the cofactors in flavin binding blue-light receptors. Using the riboflavin (13)C-analogues RF-2-(13)C and RF-4,10a-(13)C, the carbonyl vibrations, in particular, were studied. Various quantum chemical models were applied that take into account a polarizable environment or the impact of hydrogen bonds.

Excited-state dynamics of protochlorophyllide revealed by subpicosecond infrared spectroscopy.

To gain a better understanding of the light-induced reduction of protochlorophyllide (PChlide) to chlorophyllide as a key regulatory step in chlorophyll synthesis, we performed transient infrared absorption measurements on PChlide in d4-methanol. Excitation in the Q-band at 630 nm initiates dynamics characterized by three time constants: τ₁ = 3.6 ± 0.

Fe(II) complex with the octadentate btpa ligand: a DFT study on a spin-crossover system that reveals two distinct high-spin states.

Density functional theory calculations (DFT) were performed for the spin-crossover system [Fe(btpa)]2+ (btpa = N,N,N',N'-tetrakis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine), and for the predominantly low-spin [Fe(b(bdpa))]2+ complex (in the solid state) (b(bdpa) = N,N'-bis(benzyl)-N,N'-bis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine). The calculations confirmed that the former complex exhibits two high-spin isomers of the complexes, i.e.

Primary photoinduced protein response in bacteriorhodopsin and sensory rhodopsin II.

Essential for the biological function of the light-driven proton pump, bacteriorhodopsin (BR), and the light sensor, sensory rhodopsin II (SRII), is the coupling of the activated retinal chromophore to the hosting protein moiety. In order to explore the dynamics of this process we have performed ultrafast transient mid-infrared spectroscopy on isotopically labeled BR and SRII samples. These include SRII in D(2)O buffer, BR in H(2)(18)O medium, SRII with (15)N-labeled protein, and BR with (13)C(14)(13)C(15)-labeled retinal chromophore.

Ultrafast protein conformational alterations in bacteriorhodopsin and its locked analogue BR5.12.

Bacteriorhodopsin, reconstituted with a sterically "locked" retinal chromophore, BR5.12, has frequently been used to elucidate elementary photoinduced processes in the native pigment bacteriorhodopsin. In this work, the vibrational response of BR5.