A dye-photoswitch dyad (consisting of BODIPY and DTE) has been studied spectroscopically. In this system, the fluorescent dye can be quenched via FRET by closing the photoswitch, resulting in a photomodulated fluorescence. CdSe/ZnS quantum dot (QD)/dyad complexes have been prepared to investigate the suitability of QDs as antennas to extend the excitation range of this system. Illumination experiments show that the photoswitch retains its functionality even after the dyad is attached to the QD surface. Furthermore, photoluminescence (PL) measurements show a very efficient FRET from the QD to the dyad. Transient absorption experiments reveal signals indicative of a direct FRET from the QD to the closed switch and two successive FRET processes - from the QD to the dye and subsequently from the dye to the closed switch. With this integrated spectroscopic approach, we demonstrated that QDs are well suited for enhancing photomodulated fluorescence.

NBN- and BNB-doped phenalenyls are isoelectronic to phenalenyl anions and cations, respectively. They represent a pair of complementary molecules that have essentially identical structures but opposite properties as electron donors and acceptors. The NBN-phenalenyls 1-4 considered here were prepared from N,N'-dimethyl-1,8-diaminonaphthalene and readily available boron-containing building blocks (i. e., BH3 & sdot;SMe2 (1), p-CF3-C6H4B(OH)(2) (2), C6H5B(OH)(2) (3), or MesBCl(2)/iPr(2)NEt (4)). Treatment of 1 with 4-Me2N-2,6-Me2-C6H2Li gave the corresponding NBN derivative 5. The BNB-phenalenyl 6 was synthesized from 1,8-naphthalenediyl-bridged diborane(6), PhNH2, and MesMgBr. A computational study reveals that the photoemission of 1, 4, and 5 originates from locally excited (LE) states at the NBN-phenalenyl fragments, while that of 2 is dominated by charge transfer (CT) from the NBN-phenalenyl to the p-CF3-C6H4 fragment. Depending on the dihedral angle theta between its Ph and NBN planes, compound 3 emits mainly from a less polar LE (theta >55(degrees)) or more polar CT state (theta <55(degrees)). In turn, the energetic preference for either state is governed by the polarity of the solvent used. An equimolar aggregate of the NBN- and BNB-phenalenyls 3 and 6 (in THF/H2O) shows a distinct red-shifted emission compared to that of the individual components, which originates from an intermolecular CT state.

Abstract The binding and release of ligands from their protein targets is central to fundamental biological processes as well as to drug discovery. Photopharmacology introduces chemical triggers that allow the changing of ligand affinities and thus biological activity by light. Insight into the molecular mechanisms of photopharmacology is largely missing because the relevant transitions during the light-triggered reaction cannot be resolved by conventional structural biology. Using time-resolved serial crystallography at a synchrotron and X-ray free-electron laser, we have captured the release of azo-combretastatin A4 and the resulting conformational changes in tubulin. Nine structural snapshots from 1 ns to 100 ms complemented by simulations show how cis-to-trans isomerization of the azobenzene bond leads to a switch in ligand affinity, opening of an exit channel, and collapse of the binding pocket upon ligand release. The resulting global backbone rearrangements are related to the action mechanism of tubulin-binding drugs against gout, cancer, and COVID-19.

Life support systems for long-term space missions or extraterrestrial installations have to fulfill major functions such as purification of water and regeneration of atmosphere as well as the generation of food and energy. For almost 60 years ideas for biological life support systems have been collected and various concepts have been developed and tested. Microalgae as photosynthetic organisms have played a major role in most of these concepts. This review deals with the potentials of using eukaryotic microalgae for life support systems and highlights special requirements and frame conditions for designing space photobioreactors especially regarding illumination and aeration. Mono- and dichromatic illumination based on LEDs is a promising alternative for conventional systems and preliminary results yielded higher photoconversion efficiencies (PCE) for dichromatic red/blue illumination than white illumination. Aeration for microgravity conditions should be realized in a bubble-free manner, for example, via membranes. Finally, a novel photobioreactor concept for space application is introduced being parameterized and tested with the microalga Chlamydomonas reinhardtii. This system has already been tested during two parabolic flight campaigns.

Photochromic switches are essential for the control and manipulation of nanoscale reactions and processes. The expansion of their application to aqueous environments depends strongly on the development of optimized water-soluble photoswitches. Here we present a femtosecond time-resolved investigation of the photochromic reactions (transition between the open and the closed form) of a water-soluble indolylfulgimide. We observe a pronounced effect of the protic nature of water as a solvent on the ultrafast ring-opening reaction. Typically, the excited state of the closed form has a larger dipole moment than the ground state, which leads to stabilization of the excited state in polar solvents and hence a lifetime (3 ps) longer than in non-polar solvents (2 ps). However, in water, despite the increased solvent polarity and the increased excited state dipole moment, the opposite trend for the excited state lifetime is observed (1.8 ps). This effect is caused by the opening of a new excited state deactivation pathway involving proton transfer reactions.

The ultrafast ring-opening reaction of photochromic fulgides proceeds via conical intersections to the ground state isomers involving activation barriers in the excited state. The coherent oscillations observed in the femtosecond transient absorption signal of a methyl-substituted indolylfulgide were analysed in the framework of vibrational wavepackets to expose a dominant low-frequency mode at ∼80 cm(-1). The quantum chemical calculations in the relaxed excited state geometry of this fulgide revealed that the experimentally observed vibrational normal mode has a dominant contribution to the relevant ring-opening reactive coordinate.

We report on the influence of intramolecular hydrogen bonding between the 2′OH group of ribose and the N3 of adenine in 2-(1-ethynylpyrene)-adenosine (PyA) on the ultrafast dynamics, by comparing PyA with its deoxy derivate (PydA).

Excimer formation and dissociation in 9,10-dichloroanthracene is investigated in solution phase (low/high concentration) and monocrystalline state (α/β structure) at room temperature by ultrafast absorption spectroscopy combined with streak-camera fluorescence experiments. The transition from the monomeric to the excimeric state is accompanied by a red-shift of the fluorescence and a blue-shift of an excited state absorption band. Excimer formation in solution occurs in the 1ns time regime. In the pre-arranged crystalline state of the β-form a strong acceleration of excimer formation to about 1ps is observed. The slower formation time in solution is attributed to a diffusion process before the two molecules arrange in an ideal excimer geometry. The crystalline structure of the β-form does not allow to reach the optimal geometry, leading to significantly smaller spectral shifts than in solution. For the α-form spectral dynamics typical for the excimer formation are not observed.

The photo-physical properties of 2-(1-ethynylpyrene)-adenosine (PyA), a fluorescent probe for RNA dynamics, were examined by solvation studies. The excited-state dynamics display the influence of the vicinity on the spectral features. Combining improved transient absorption and streak camera measurements along with a new analysis method provide a detailed molecular picture of the photophysics. After intramolecular vibrational energy redistribution (IVR), two distinct states are observed. Solvent class (protic/aprotic) and permittivity strongly affect the properties of these states and their population ratio. As a result their emission spectrum is altered, while the fluorescence quantum yield and the overall lifetime remain nearly unchanged. Consequently, the hitherto existing model of the photophysics is herein refined and extended. The findings can serve as basis for improving the information content of measurements with PyA as a label in RNA.

We examine the photoinduced excited state dynamics of pyrene modified adenosine, a versatile probe for folding and hybridization of ribonucleic acids. Measurements in different solvents revealed complex ultrafast dynamics, but high robustness since the overall fluorescence quantum yield (Φ f ) is hardly affected. The result is a strong fluorescent RNA-probe whose spectral properties change in a defined way upon environmental changes.

The origin of coherent oscillations in CdSe quantum dots and in the CdSe/methylviologen electron transfer system is studied. In CdSe/methylviologen coherent phonons are triggered by the electron transfer from the quantum dot to methylviologen.

The dynamics of a water soluble spiropyran is investigated by means of femtosecond transient absorption spectroscopy in the visible and infrared spectral range revealing an ultrafast reversible switching behavior under aqueous conditions with a high fatigue resistance.

We observe the CdSe longitudinal-optical ground-state phonon in the electron transfer system composed of CdSe quantum dots and methylviologen directly by femtosecond absorption spectroscopy. A significant phase shift indicates that the coherent oscillations are triggered by an ultrafast charge migration, which is the consequence of an electron transfer from the photoexcited quantum dot to the molecular acceptor methylviologen. In contrast, the observed coherent phonons in isolated quantum dots stem from the frequency modulation of the quantum dot excited-state spectrum. From the probe wavelength dependence of the longitudinal-optical phonons in the electronic ground state and excited state it is possible to determine a biexciton binding energy of 35 meV.

Femtosecond X-ray diffraction allows for real-time mapping of structural changes in condensed matter on atomic length and timescales. Sequences of diffraction patterns provide both transient geometries and charge-density maps of crystalline materials. This article reviews recent progress in this field, the main emphasis being on experimental work done with laser-driven hard X-ray sources. Both Bragg diffraction techniques for bulk and nanostructured single crystals as well as the recently implemented powder diffraction from polycrystalline samples are discussed. In ferroelectric superlattice structures, coherent phonon motions and the driving stress mechanisms are observed in real time. In molecular crystals charge-transfer processes and the concomitant changes of the lattice geometry are analyzed.

Intramolecular charge transfer in aminobenzonitrile (DIABN) single crystals is studied by a combination of ultrafast techniques. The intramolecular dynamics is probed by time-resolved spectroscopy in the UV/Vis and mid-IR. The intermolecular structural response is investigated by transient X-ray scattering and transmission experiments. A theoretical analysis shows that diffuse scattering and dipole solvation determine the X-ray data. The observed dynamics results from charge transfer reaction leading to strong local changes of molecular dipole moments and related structural rearrangements in the crystal.

The photodynamics of single crystals of 4-(diisopropylamino)benzonitrile (DIABN) have been studied by femtosecond UV/Vis and mid-IR absorption spectroscopy. The observed spectroscopic response, especially in the UV/Vis, strongly depends on the optical excitation conditions. For the excitation light in resonance with the absorption of DIABN, broad and structureless transient spectra are observed, which decay non-exponentially within pico- to nanoseconds. For off-resonance excitation in the very red wing of the absorption band a stimulated emission signal is detected, which decays in ∼10 ps matching the known behavior of the non-interacting chromophore in solution. The 10 ps decay is due to an intramolecular charge transfer process which is supported by femtosecond IR spectroscopy. The observed dependence on the excitation wavelength is assigned to a change in the density of excited chromophores and therefore bears relevance for time resolved X-ray diffraction experiments.

Femtosecond photoexcitation of organic chromophores in a molecular crystal induces strong changes of the electronic dipole moment via intramolecular charge transfer as is evident from transient vibrational spectra. The structural response of the crystal to the dipole change is mapped directly for the first time by ultrafast x-ray diffraction or diffuse scattering. Changes of diffracted and transmitted x-ray intensity demonstrate an angular rearrangement of molecules around excited dipoles following the 10 ps kinetics of charge transfer and leaving lattice plane spacings unchanged. Transient x-ray scattering is governed by solvation, masking changes of the chromophore molecular structure.

The photo-induced ring-opening reaction C→E of a photochromic indolyl-fulgimide is investigated by sub-picosecond pump–probe spectroscopy in the visible and infrared spectral range. The thermally stable C-isomer shows a pronounced absorption in the visible spectral range without spectral overlap with the absorption of the E- and Z-isomer. The comparison of infrared and visible transient absorption data allows to assign the observed exponential 2.1ps dynamics to the ring-opening reaction and the non-exponential 10–40ps component to the cooling of the hot molecules. The transient absorption spectra taken 80ps after photo-excitation are identical with the steady state difference spectra.

The motion of vibrational wave packets in the system oxazine 1 in methanol is investigated by spectrally resolved transient absorption spectroscopy. The spectral properties of the probe pulse from 600 to 700 nm were chosen to cover the overlap region where ground-state bleach and stimulated emission signals are detected. The spectral phase of the pump pulse was manipulated by a liquid crystal display based pulse-shaping setup. Chirped excitation pulses of negative and positive chirp can be used to excite vibrational modes predominantly in the ground or excited state, respectively. To distinguish the observed wave packets in oxazine 1 moving in the ground or excited state, spectrally resolved transient absorption experiments are performed for various values of the linear chirp of the pump pulses. The amplitudes of the wave packet motion show an asymmetric behavior with an optimum signal for a negative chirp of -0.75 +/- 0.2 fs/nm, which indicates that predominantly ground-state wave packets are observed.

Four distinct stages (motion on an excited state surface, internal conversion, vibrational cooling and slow structural re-arrangements) of the photoisomerization of an azobenzene are identified by means of femtosecond fluorescence, absorption, and IR techniques.