Influence of triphosphine ligand coordination geometry in Mn(I) hydride complexes [(PPP)(CO)MnH] on their kinetic hydricity.

Octahedral Mn(I) complexes bearing tridentate donor ligands [(LL'L'')(CO)MnX] have recently emerged as major players in catalytic (de)hydrogenation processes. While most of these systems are still based on structurally rigid pincer scaffolds imposing a meridional coordination mode, for some more flexible tridentate ligands a facial arrangement of donor moieties becomes possible. Accordingly, the geometry of the corresponding Mn(I) hydrides [(LL'L'')(CO)MnH] directly involved in the catalytic processes, namely the nature of the donor extremity located in the -position of the hydride (CO and L for - and -configurations, respectively) may influence their hydride transfer ability.

Photoisomerization Paths of α,ω-Diphenylpolyenes: Reaction Rate Dependence on Temperature, Excitation Wavelength, and Deuteration.

The photoisomerization rate of -stilbene (tS) and trans-trans-diphenylbutadiene (ttD) is studied in solution and compared to that in jet/gas. Rice-Ramsperger-Kassel-Marcus (RRKM) theory correctly predicts the tS rate in jet, = exp(-/) with = 1398 cm, and = 1.8 ps corresponding to frequency ν = 60 cm of the reactive mode, being the molecular temperature.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes intermolecular bimetallic cooperation.

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (M⋯M') have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate the synergetic action of Lewis acidic (M) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br).

-to- isomerization triggers hydride transfer from Mn(I) complex -[(dppm)Mn(CO)H].

Low-temperature IR and NMR studies combined with DFT calculations revealed the mechanistic complexity of apparently simple reactions between Mn(I) complex -[(dppm)Mn(CO)H] and Lewis acids (LA = PhC, B(CF)) involving the formation of so-far elusive meridional hydride species -[(dppm)Mn(CO)H⋯LA] and unusual dearomatization of the PhC cation upon hydride transfer.

Substitution pattern dependent behavior of the singlet excited states in symmetrically fluorinated biphenyls.

Biphenyls are important basic chromophore systems that offer a possibility to study the effects of chemical substitution on the lower-lying excited states without complications from photoisomerization or other side processes. For several symmetric biphenyls, pristine biphenyl (bP0), 4,4'-difluorobiphenyl (bP2), 2,3,5,6,2',3',5',6'-octafluorobiphenyl (bP8), and perfluorobiphenyl (bP10), we report stationary and ultrafast solution-phase spectra rationalized with the aid of computations by means of the XMCQDPT2 multi-configuration perturbation theory and TDDFT. Polyfluorination tends to broaden the gap between the nearly degenerate S + S pair of states and the S state in bP8 and bP10, yet relaxation from any sheet of the S-S manifold leads through a system of state crossings to the same stationary points in S.

Type-I Energy Level Alignment at the PTCDA-Monolayer MoS Interface Promotes Resonance Energy Transfer and Luminescence Enhancement.

Van der Waals heterostructures consisting of 2D semiconductors and conjugated molecules are of increasing interest because of the prospect of a synergistic enhancement of (opto)electronic properties. In particular, perylenetetracarboxylic dianhydride (PTCDA) on monolayer (ML)-MoS has been identified as promising candidate and a staggered type-II energy level alignment and excited state interfacial charge transfer have been proposed. In contrast, it is here found with inverse and direct angle resolved photoelectron spectroscopy that PTCDA/ML-MoS supported by insulating sapphire exhibits a straddling type-I level alignment, with PTCDA having the wider energy gap.

Insights into Charge Transfer at an Atomically Precise Nanocluster/Semiconductor Interface.

The deposition of an atomically precise nanocluster, for example, Ag (SR) , onto a large-band-gap semiconductor such as TiO allows a clear interface to be obtained to study charge transfer at the interface. Changing the light source from visible light to simulated sunlight led to a three orders of magnitude enhancement in the photocatalytic H generation, with the H production rate reaching 7.4 mmol h  g .

Evaluating the Solvent Stark Effect from Temperature-Dependent Solvatochromic Shifts of Anthracene.

The solvent Stark effect on the spectral shifts of anthracene is studied with temperature-dependent solvatochromic measurements. The Stark contribution Δv to the absorption shift Δv in polar solvents is measured to be Δv =(53±35) cm , in reasonable agreement with dielectric continuum theory estimate of 28 cm , whereas the major shift Δv ∼300 cm presumably originates from the solute quadrupole. We pay attention to the accurate correction of Δv for the nonpolar contribution that is crucial when the shifts are modest in magnitude.

Atomically Precise Bimetallic Nanoclusters as Photosensitizers in Photoelectrochemical Cells.

The atomically precise bimetallic nanocluster (NC), Au Ag (PhCC) (SPy) Cl (1) (Py=pyridine), was employed for the first time as a stable photosensitizer for photoelectrochemical applications. The sensitization of TiO nanotube arrays (TNA) with 1 greatly enhances the light-harvesting ability of the composite because 1 shows a high molar extinction coefficient (ϵ) in the UV/Vis region. Compared to a more standard Au (SG) -TNA (2-TNA; SG=glutathione) composite, 1-TNA shows a much better stability under illumination in both neutral and basic conditions.

Tuning Stilbene Photochemistry by Fluorination: State Reordering Leads to Sudden Polarization near the Franck-Condon Region.

Spontaneous polarization of a nonpolar molecule upon photoexcitation (the sudden polarization effect) earlier discussed for 90°-twisted alkenes is observed and calculated for planar ring-fluorinated stilbenes, trans-2,3,5,6,2',3',5',6'-octofluorostilbene (tF2356) and trans-2,3,4,5,6,2',3',4',5',6'-decafluorostilbene (tF23456). Due to the fluorination, Franck-Condon states S and S are dominated by the quasi-degenerate HOMO-1 → LUMO and HOMO-2 → LUMO excitations, while their interaction gives rise to a symmetry-broken zwitterionic S state. After optical excitation of tF2356, one observes an ultrafast (∼0.

Effect of a Tertiary Butyl Group on Polar Solvation Dynamics in Aqueous Solution: Femtosecond Fluorescence Spectroscopy.

We monitor the time-dependent Stokes shift (TDSS) of fluorescence from the zwitterionic probe N-methyl-6-oxyquinolinium betaine in water. A spectral relaxation time τ = 0.57 ps (at 20.

Perpendicular State of an Electronically Excited Stilbene: Observation by Femtosecond-Stimulated Raman Spectroscopy.

In the photoisomerization path of stilbene, a perpendicular state P on the S potential energy surface is expected just before internal conversion through a conical intersection S/S. For decades the observation of P was thwarted by a short lifetime τ in combination with slow population flow over a barrier. But these limitations can be overcome by ethylenic substitution.

Sensitized Two-NIR-Photon Z→E Isomerization of a Visible-Light-Addressable Bistable Azobenzene Derivative.

Two-NIR-photon-triggered Z→E isomerization of an azobenzene was accomplished by covalently linking a two-photon-harvesting triarylamine antenna to a thermally stable ortho-fluorinated azobenzene derivative. The obtained photoswitch is fully addressable with visible and NIR light by using one-photon and two-photon excitation, respectively, with the latter offering enhanced penetration depth and improved spatial resolution.

Two-State Intramolecular Charge Transfer (ICT) with 3,5-Dimethyl-4-(dimethylamino)benzonitrile (MMD) and Its Meta-Isomer mMMD. Ground State Amino Twist Not Essential for ICT.

From X-ray structure analysis, amino twist angles of 90.0° for 2,4-dimethyl-3-(dimethylamino)benzonitrile (mMMD), 82.7° for 4-(di-tert-butylamino)benzonitrile (DTABN), and 88.

β-Carotene Revisited by Transient Absorption and Stimulated Raman Spectroscopy.

β-Carotene in n-hexane was examined by femtosecond transient absorption and stimulated Raman spectroscopy. Electronic change is separated from vibrational relaxation with the help of band integrals. Overlaid on the decay of S1 excited-state absorption, a picosecond process is found that is absent when the C9 -methyl group is replaced by ethyl or isopropyl.

Resonance Femtosecond-Stimulated Raman Spectroscopy without Actinic Excitation Showing Low-Frequency Vibrational Activity in the S2 State of All-Trans β-Carotene.

Raman scattering with stimulating femtosecond probe pulses (FSR) was used to observe vibrational activity of all-trans β-carotene in n-hexane. The short-lived excited electronic state S2 was accessed in two ways: (i) by transient FSR after an actinic pulse to populate the S2 state, exploiting resonance from an Sx ← S2 transition, and (ii) by FSR without actinic excitation, using S2 ↔ S0 resonance exclusively and narrow-band Raman/broad-band femtosecond probe pulses only. The two approaches have nonlinear optical susceptibilities χ((5)) and χ((3)), respectively.

Dissociation of a strong acid in neat solvents: diffusion is observed after reversible proton ejection inside the solvent shell.

Strong-acid dissociation was studied in alcohols. Optical excitation of the cationic photoacid N-methyl-6-hydroxyquinolinium triggers proton transfer to the solvent, which was probed by spectral reconstruction of picosecond fluorescence traces. The process fulfills the classical Eigen-Weller mechanism in two stages: (a) solvent-controlled reversible dissociation inside the solvent shell and (b) barrierless splitting of the encounter complex.

Presence and absence of excited state intramolecular charge transfer with the six isomers of dicyano-N,N-dimethylaniline and dicyano-(N-methyl-N-isopropyl)aniline.

The excited state behavior of the six m,n-dicyano-N,N-dimethylanilines (mnDCDMA) and m,n-dicyano-(N-methyl-N-isopropyl)anilines (mnDCMIA) is discussed as a function of solvent polarity and temperature. The dicyano moiety in these electron donor (D)/acceptor (A) molecules has a considerably larger electron affinity than the benzonitrile subgroup in 4-(dimethylamino)benzonitrile (DMABN). Nevertheless, the fluorescence spectra of the mnDCDMAs and mnDCMIAs in n-hexane all consist of a single emission originating from the locally excited (LE) state, indicating that a reaction from LE to an intramolecular charge transfer (ICT) state does not take place.

Dual photochemistry of anthracene-9,10-endoperoxide studied by femtosecond spectroscopy.

The dual photochemistry of anthracene-9,10-endoperoxide (APO) was investigated in a fs UV pump-supercontinuum probe experiment, along with anthracene (AC) and anthraquinone (AQ) for comparison. Excitation of APO at 282 nm leads to 100% product formation by two competing photoreaction channels. Cycloreversion generates with a ∼25% quantum yield (QY) (1)O(2) and AC vibrationally excited in the singlet electronic ground state (hot AC).

Dual fluorescence and ultrafast intramolecular charge transfer with 6-N,N-dialkylaminopurines. A two-state model.

6-N,N-Dimethyl-9-methyladenine (DMPURM) and 6-N,N-dimethyladenine (DMPURH) show dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state in solvents of different polarity over extended temperature ranges. The fluorescence quantum yields are very small, in particular those of LE. For DMPURM in acetonitrile (MeCN) at 25 °C, for example, Φ'(ICT) = 3.