Search results for "Photoexcitation"

Bowl-shape electron donors with absorptions in the visible range of the solar spectrum and their supramolecular assemblies with C 60

2012

We describe the synthesis, electronic, optical and photophysical properties of a family of three electron-donor bowl-shaped organic molecules that absorb light in the whole range of the visible spectrum (up to 800 nm in one case), and associate C60 in solution with binding constants in the range of 104–102 M−1 as measured from both UV-vis and fluorescence titrations in several solvents. These molecules are π-extended derivatives of tetrathiafulvalene, based on a truxene core to which two or three units of dithiole are covalently attached. The inclusion of the bulky dithiole groups is responsible for their bowl-shape geometry, which allows them to associate with C60, and their electron-donor…

Photoexcitation of the P4480 State Induces a Secondary Photocycle That Potentially Desensitizes Channelrhodopsin-2

2018

Channelrhodopsins (ChRs) are light-gated cation channels. In spite of their wide use to activate neurons with light, the photocurrents of ChRs rapidly decay in intensity under both continuous illum...

2020

Phytochrome proteins control the growth, reproduction, and photosynthesis of plants, fungi, and bacteria. Light is detected by a bilin cofactor, but it remains elusive how this leads to activation of the protein through structural changes. We present serial femtosecond X-ray crystallographic data of the chromophore-binding domains of a bacterial phytochrome at delay times of 1 ps and 10 ps after photoexcitation. The data reveal a twist of the D-ring, which leads to partial detachment of the chromophore from the protein. Unexpectedly, the conserved so-called pyrrole water is photodissociated from the chromophore, concomitant with movement of the A-ring and a key signaling aspartate. The chan…

Damming an electronic energy reservoir: ion-regulated electronic energy shuttling in a [2]rotaxane

2021

We demonstrate the first example of bidirectional reversible electronic energy transfer (REET) between the mechanically bonded components of a rotaxane. Our prototypical system was designed such that photoexcitation of a chromophore in the axle results in temporary storage of electronic energy in a quasi-isoenergetic “reservoir” chromophore in the macrocycle. Over time, the emissive state of the axle is repopulated from this reservoir, resulting in long-lived, delayed luminescence. Importantly, we show that cation binding in the cavity formed by the mechanical bond perturbs the axle chromophore energy levels, modulating the REET process, and ultimately providing a luminescence read-out of c…

Self-trapped exciton luminescence in crystalline α-quartz under two-photon laser excitation

2003

Abstract The luminescence of pure crystalline α-quartz is studied under pulsed ArF laser excitation. The luminescence parameters obtained correspond well with those of self-trapped excitons (STEs) in α-quartz, indicating that the excitation process is two-photon. The efficiency of two-photon excitation is of the same order of magnitude as the one-photon excitation of sodium salicylate. The STE luminescence decay kinetics and their temperature dependence under photoexcitation were recorded with higher accuracy than previously. Changes in the decay kinetics with temperature are explained by the splitting of the STE triplet state in zero magnetic field and are analyzed with the assumption of t…

Breaking the semi-quinoid structure: spin-switching from strongly coupled singlet to polarized triplet state.

2014

2,7-TMPNO (4,5,9,10-tetramethoxypyrene-2,7-bis(tert-butylnitroxide)) was found to exist in semi-quinoid form with unprecedented strong intramolecular magnetic exchange interaction of 2 J/kB = 1185 K operating over a distance of 10 A. Structural transformations with the activation energy of ΔEeq = 949 K were observed by varying the temperature, from more quinoid structure at low temperature to more biradicaloid structure at higher temperature. Moreover, this molecule undergoes a transient spin transition from singlet to polarized triplet state upon photoexcitation revealed by TREPR spectroscopy. The spin Hamiltonian parameters were determined to be S = 1, g = 2.0065, D = -0.0112 cm(-1), and …

Consequences of the One-Electron Reduction and Photoexcitation of Unsymmetric Bis-imidazolium Salts

2012

Coupling of uronium salts with in situ generated N-heterocyclic carbenes provides straightforward access to symmetrical [4](2+) and unsymmetrical bis-imidazolium salts [6](2+) and [9](2+) . As indicated by cyclic and square-wave voltammetry, [6](2+) and [9](2+) can be (irreversibly) reduced by one electron. The initially formed radicals [6](.+) and [9](.+) undergo further reactions, which were probed by EPR spectroscopy and density functional calculations. The final products of the two-electron reduction are the two carbenes. Upon irradiation with UV light both [6](2+) and [9](2+) emit at room temperature in solution but with dramatically different characteristics. The different fluorescenc…

Long lived photoinduced charges in donor-acceptor anthraquinone substituted thiophene copolymers

2006

The photoinduced charge-transfer properties of a series of polyalkylthiophene copolymers, carrying anthraquinone substituents covalently linked to the conjugated backbone, have been studied in the solid state by photoinduced absorption (PA) and light-induced electron spin resonance (LESR) spectroscopy. The measurements indicate the formation of metastable charges arising from the photoinduced electron transfer from the polythiophene backbone to the anthraquinone moieties. At low temperatures (below 200 K), long-lived persistent charges are formed, exhibiting lifetimes that extend for several minutes; their recombination kinetics has been studied by following the formation and decay of the P…

Optically Detected Magnetic Resonance of Chlorophyll Triplet States in Water-Soluble Chlorophyll Proteins from Lepidium virginicum: Evidence for Exci…

2018

Optically detected magnetic resonance of triplet states populated by photoexcitation in water-soluble chlorophyll proteins (WSCPs) from Lepidium virginicum has been performed using both absorption and fluorescence detection. Well resolved triplet-singlet (T-S) spectra have been obtained and interpreted in terms of electronic interactions among the four chlorophylls (Chls), forming two dimers in the WSCP tetramer. Localization of the triplet state on a single Chl leads to a redistribution of the oscillator strength in the remaining three Chls of the complex. By comparing the spectra with those obtained on a substoichiometric WSCP complex containing only 2 Chls per protein tetramer, we proved…

Extraction dynamics of electrons from magneto-optically trapped atoms

2017

Pulsed photoionization of laser-cooled atoms in a magneto-optical trap (MOT) has the potential to create cold electron beams of few meV bandwidths and few ps pulse lengths. Such a source would be highly attractive for the study of fast low-energy processes like coherent phonon excitation. To study the suitability of MOT-based sources for the production of simultaneously cold and fast electrons, we study the photoionization dynamics of trapped Cs atoms. A momentum-microscope-like setup with a delay-line detector allows for the simultaneous measurement of spatial and temporal electron distributions. The measured patterns are complex, due to the Lorentz force inducing spiral trajectories. Ray-…