The triplet-sensitized (by the solvent acetone) as well as the direct (lambda ex=300-320 nm) photochemical decarboxylation of N-phthaloylated gamma-aminobutyric acid (GABA) derivatives are versatile and high-yielding routes to benzopyrrolizidines via intramolecular electron transfer initiated decarboxylation followed by radical coupling. The ss-mono- and ss,ss'-disubstituted N-phthaloyl GABA derivatives 7 a-7 g, respectively, were applied as substrates. Decarboxylative photocyclization yielded hydroxy benzopyrrolizidines 8 a-8 g in high chemical yields and with moderate diastereoselectivities from the ss-monosubstituted substrates. The analogous alpha-substituted GABA derivatives 11 a-11 c were also applied as potential substrates for memory of chirality effects. The reaction quantum yields of the photodecarboxylation reactions for the parent GABA derivative 13 and for the new substrates 7 h and 11 a were determined by the quantum yield determination system (QYDS) and showed a remarkable concentration dependency indicating aggregation at higher substrate concentrations. Inhibition studies on the atherogenic human serine hydrolase cholesterol esterase showed derivatives 8 a and 8 d to exhibit a hyperbolic mode of inhibition with moderate IC50 values of about 60-80 mu M. Reagent and catalyst-free conversion of numerous GABA derivatives into biologically active pyrroloisoindolinones is achieved by direct or triplet-sensitized photolyses in aqueous solvent mixtures with high quantum yields. image

Despite the great achievements of advanced photoredox catalysis for organic-synthetic reactions, the literature is rather vague with respect to reaction quantum yields – the number of product molecules per absorbed photon. This stands in contrast to the clear and commonly used chemical yield as standard parameter to quantify the efficiency of chemical reactions. We applied an opto-electronic device to measure the reaction quantum yields of a reference reaction in a rapid and facile way, which revealed that this parameter cannot be regarded as a single, isolated value. A so far undescribed strong dependence of the reaction quantum yield on the incident light power and the irradiation time was revealed. The light input even decides on the interplay of the closed photoredox catalytic cycle and the radical chain propagation. The reaction kinetics were modelled in full detail to obtain important insight into the general description of photoredox catalytic mechanisms. the of absolute reaction function irradiation Reported values of the reaction quantum yield for this tot single snapshots at given irradiation times and light input. that the power of the light decides about the mechanism of the photocatalytic In the light power influences the competition between an open shell, propagation mechanism (predominant at incident light than 1 photoredox of photoredox

Photoredox catalysts are integral components of artificial photosystems, and have recently emerged as powerful tools for catalysing numerous organic reactions. However, the development of inexpensive and efficient earth-abundant photoredox catalysts remains a challenge. We here present the photochemical and photophysical properties of a Ni-Mabiq catalyst ([NiII(Mabiq)]OTf (1); Mabiq = 2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N6)-and of a Zn-containing analogue ([ZnII(Mabiq)OTf] (2))-using steady state and time resolved optical spectroscopy, time-dependent density functional theory (TDDFT) calculations, and reactivity studies. The Ni and Zn complexes exhibit similar absorption spectra, but markedly different photochemical properties. These differences arise because the excited states of 2 are ligand-localized, whereas metal-centered states account for the photoactivity of 1. The distinct properties of the Ni and Zn complexes are manifest in their behavior in the photo-driven aza-Henry reaction and oxidative coupling of methoxybenzylamine.

Efficiency and performance of light triggered molecular motors are crucial features that need to be mechanistically understood to improve the performance and enable conscious property tailoring for specific applications. In this work, three different hemithioindigo-based molecular motors are investigated and all four steps in their complete unidirectional rotation are unraveled fully quantitatively. Transient absorption spectroscopy across twelve orders of magnitude in time is used to probe the fs nuclear motions up to the ms thermal kinetics, covering the timeframe of the whole motor rotation. The newly known full mechanisms allow simulation of the motor systems to scrutinize their performance at realistic illumination conditions. This highlights the importance of photoisomerization quantum yields for the rotation speed. The substitution pattern in close proximity to the rotation axle influences the excited and ground state properties. Reduction of electron donation and concomitant increase of steric hindrance leads to faster photoisomerization reactions with quasi-ballistic behavior, but also to a slight decrease in the quantum efficiency. The expected decelerating effects of increased sterics are primarily manifested in the ground state. A promising approach for next-generation hemithioindigo motors is to elevate electron donation at the rotor fragment followed by an increase of steric hindrance.

The hydrogen bond network accounts for many of the extraordinary physical properties of liquid water and ice. Its vibrational dynamics are quite complex in their entirety but can be accessed in detail by investigating small groups of only a few water molecules. Here, aqueous salt hydrates turned out to be an exceptional model system for water molecules arranged in well-defined geometrical structures that can be accessed by means of femtosecond spectroscopy of the OH stretching vibration. In this study, we find striking resemblance between the vibrational properties of three water molecules connected via strong hydrogen bonds in the trihydrate of LiNO3 and those of ordinary ice I-h. As in ice, the vibrations of the hydrate water molecules show ultrafast excited state dynamics that are strongly accelerated when proceeding from deuterated to neat H2O samples. The latter is analyzed by means of an additional relaxation channel that is due to Fermi resonance between the OH stretching vibration and the bend overtone accompanied by delocalization of the vibration over neighboring water molecules in the H2O species. Moreover, in the hydrate and ice samples severe spectral broadening is examined when comparing fundamental and excited state absorption bands. Here, proton delocalization along the strong hydrogen bonds is given as a possible underlying mechanism.

We report the generation of extremely broadband and inherently phase-locked mid-infrared pulses covering the 5 to 11 µm region. The concept is based on two stages of optical parametric amplification starting from a 270-fs Yb:KGW laser source. A continuum seeded, second harmonic pumped pre-amplifier in β-BaB2O4 (BBO) produces tailored broadband near-infrared pulses that are subsequently mixed with the fundamental pump pulses in LiGaS2 (LGS) for mid-infrared generation and amplification. The pulse bandwidth and chirp is managed entirely by selected optical filters and bulk material. We find an overall quantum efficiency of 1% and a mid-infrared spectrum smoothly covering 5-11 µm with a pulse energy of 220 nJ at 50 kHz repetition rate. Electro-optic sampling with 12-fs long white-light pulses directly from self-compression in a YAG crystal reveals near-single-cycle mid-infrared pulses (32 fs) with passively stable carrier-envelope phase. Such pulses will be ideal for producing attosecond electron pulses or for advancing molecular fingerprint spectroscopy.

Summary form only given. For many applications the second and third harmonic of a laser system is required and quite often high pulse energies are desired - therefore efficiency of the SHG and THG processes should be high. In the literature high efficiencies can be found frequently. Values up to 60 % and higher for SHG are reported for the femtosecond pulse regime and BBO as nonlinear medium. We now find that pushing to such high efficiency leads to distorted beam profiles and broadened, structured spectra of the harmonics due to the Kerr effect in the nonlinear medium. Our calculations show that for a common collimation / focusing geometry of SHG, very high nonlinear phase shifts are acquired when the crystal gets close to the focus - see Fig. I. The nonlinear phase shift can lead to strong self-focusing even in extremely thin crystals. B-Integrals higher than I0 are reached easily. Consequently the divergence in the crystal is strongly changed and the phase matching angle in the crystal can readily be lost. This limits the doubling efficiency and leads to strongly distorted beam profiles as well as spectra.

Summary form only given. Femtosecond pulses are primarily generated at 800 or 1030 nm in Ti:sapphire or Yb based systems. To generate fully tunable pulses over the full visible and NIR range, optical parametric amplifiers and in particular white light seeded noncollinear optical parametric amplifiers (NOPA) are now widely used. For highest energy pulses this scheme is expanded to OPCPAs. Energy and beam pointing stability is a crucial topic in the application of all these sources. We investigated - to the best of our knowledge for the first time - how the beam pointing instability of the pump laser source translates to the NOPA output.

Broadly tunable picosecond pulses in the UV for nonlinear microscopy and lifetime measurements are not yet readily available. Complex synchronously pumped optical parametric oscillators with subsequent frequency doubling are typically used. We show that direct second harmonic generation of a visible picosecond supercontinuum source at 78 MHz renders pulses easily tunable from 250 to 430 nm. We find that an unexpectedly large numerical aperture and the use of thick crystals increase the efficiency of the frequency doubling process dramatically. The observed spectral width and efficiency are nearly two orders of magnitude larger than predicted by conventional theory. With broadband achromatic doubling, a 130 nm wide spectrum is achieved. Pulse durations of 17–35 ps are found in the UV and an average power between 1 and 70 μW. This qualifies the setup for most UV-based microscopic investigations. As first application, the fluorescence lifetime of two differing conformations of 2-(2′-hydroxyphenyl) benzothiazole is measured.

Sunlight is the ultimate driver of biosynthesis but photochemical steps late in biosynthetic pathways are very rare. They appear to play a role in the formation of certain furanocembranoids isolated from Caribbean corals. One of these compounds, intricarene, has been suspected to arise from an intramolecular 1,3-dipolar cycloaddition involving an oxidopyrylium. Here we show, by a combination of experiments and theory, that the oxidopyrylium forms under photochemical conditions and that its cycloaddition occurs via a triplet state. The formation of a complex by-product can be rationalized by another photochemical step that involves a conical intersection. Our work raises the question whether intricarene is biosynthesized in the natural habitat of the corals or is an artefact formed during workup. It also demonstrates that the determination of exact irradiation spectra, in combination with quantum chemical calculations, enables the rationalization of complex reaction pathways that involve multiple excited states.

We present an efficient concept for generating carrier-envelope phase stable pulses tunable between 1.6 and 2.0 μm with durations down to 8.2 fs based on nonlinear frequency conversion. As a first application we measure the high nonlinearity of multiphoton photoemission from a nanoscale metal tip.

The bandwidth of ultrafast pulses in the UV is limited by the finite acceptance bandwidth of the nonlinear crystals used for their generation. For fundamental laser pulses it is well established that spectral broadening can be used to overcome intrinsic bandwidth limits. We show that self-phase modulation of UV pulses in bulk materials leads to large spectral broadening and allows for a significant reduction of the pulse duration. We find that for pulse energies in the range of a few μJ, a thin crystal is favorable due to the strong dispersion in the UV and the limitations set by self-focusing. In contrast to spectral broadening in gaseous media, the self-focus has to lie outside the crystal to avoid beam break up. We focus UV pulses into a 1 mm thick CaF2 crystal. For moderately short input pulses, a shortening factor up to 2.4 is achieved: the 120 fs long third harmonic output of a Ti:sapphire amplifier is compressed down to 50 fs FWHM. For a central wavelength of 315 nm, we generate pulses as short as 14.9 fs after compression with an UV pulse shaper. In both cases the resulting beam shape is close to Gaussian and fully usable for spectroscopic experiments. We use the pulses in a collinear 2D-UV experiment and clearly resolve vibronic off-diagonal peaks of the S2 1B2u vibronic progression of pyrene.

Difference frequency generation between broadband visible noncollinear optical parametric amplifier (NOPA) pulses and the fundamental pump laser pulses allows the generation of ultrashort infrared pulses with passively stabilized carrier-envelope phase. A simple prism compressor for the visible NOPA pulses is sufficient to generate few-cycle pulses in the infrared and no additional compression is needed. We theoretically investigate the concept, explain the principles, and demonstrate it for high repetition rate, long pulse durations, and various wavelengths by applying it to a Ti:sapphire and an Yb:KYW-based laser systems. For the latter sub-15 fs phase stable pulses around 1.8 μm with an energy of 100 nJ are obtained at 100 kHz repetition rate.

Fully tunable deep UV pulses of 30 fs duration are generated in an all-solid-state scheme from 200 fs long pump pulses of a Ti:sapphire amplifier. The tunability comes from a noncollinear optical parametric amplifier with tailored spectral width. The output pulses are frequency-doubled in a 50 mu m BBO crystal and subsequently mixed in a 32 mu m BBO crystal with part of the Ti:sapphire output. The compression is solely performed in the visible. This makes the setup extremely simple and efficient. Less than 400 mu J pump energy suffice to obtain submicrojoule deep UV pulses. The strategy for the bandwidth and chirp management is explained in detail. (c) 2012 Optical Society of America

We investigate the reaction kinetics of ultrafast excited state intramolecular proton transfer (ESIPT) and discuss the possible origins of the process: tunneling of the reactive proton, vibrationally enhanced tunneling, and multidimensional wave packet dynamics of the entire system. Comparison of the measured kinetics for the protonated and the deuterated form of 2-(2′-hydroxyphenyl)benzothiazole (HBT) to numerical simulations allows us to ascribe the characteristic 50 fs time found for the ESIPT solely to a ballistic wave packet motion along skeletal coordinates that mainly affect the donor acceptor distance. Tunneling is not found to be decisive.

We investigate the effects of encapsulation on the dynamics after photoinduced bond cleavage of a diphenylmethyl phosphonium salt in acetonitrile reverse micellar nanopools by femtosecond UV/Vis transient absorption. The small volume of the nanopool is just large enough to accommodate one precursor molecule and therefore eliminates the effects of diffusion present in bulk solution. The tight environment keeps the fragments together and prolongs the time for geminate recombination to occur. We therefore observe an enhanced yield of this bimolecular reaction of the ground state photofragments.

We study the charge separation and recombination of two arylamino core-substituted naphthalene diimides by transient absorption. Both compounds show a 3ps relaxation from the initially excited partial to the full charge transfer state. Quantum chemical calculations show that this process is associated with a twisting of the arylamino substituent. In the twisted conformation the energy gap between ground and excited state is 0.7eV for two amino core-substituents and 0.5eV for one amino and one chloro core-substituent. The larger energy gap leads to a six-fold increase in the lifetime of the fully charge separated state from 11 to 70ps.

The beam attenuation by two-photon absorption in thin crystals and glass plates is utilized for directly measuring the intensity autocorrelation of UV femtosecond pulses without the need for an auxiliary pulse. We give a full description of the newly developed setup for operation from the blue down to the deep UV. The conditions that must be met to achieve reliable measurements are investigated. The choice of the two-photon-absorbing material governs the attainable wavelength range, the material thickness determines the shortest pulse that can be reliably characterized and high intensities influence the derived pulse duration due to saturation effects. The performance of the UV autocorrelator is demonstrated for pulses with durations below 20 fs, with energies of 3 nJ and with central wavelengths from the visible down to 195 nm. 2-Hz update rates are achieved at the 1-kHz repetition rate of the laser. The wavelength dependence of the two-photon-absorption coefficient of BBO is determined by z -scan measurements and we find that it decreases much faster at longer wavelengths than is expected from the linear absorption spectrum.