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The solvatochromic behavior of two donor-π bridge-acceptor (D-π-A) compounds based on the 2-(3-boryl-2-thienyl)thiazole π-linker and indandione acceptor moiety are investigated. DFT/TD-DFT calculations were performed in combination with steady-state absorption and emission measurements, along with electrochemical studies, to elucidate the effect of two different strongly electron-donating hydrazonyl units on the solvatochromic and fluorescence behavior of these compounds. The Lippert–Mataga equation was used to estimate the change in dipole moments (Δµ) between ground and excited states based on the measured spectroscopic properties in solvents of varying polarity with the data being supported by theoretical studies. The two asymmetrical D-π-A molecules feature strong solvatochromic shifts in fluorescence of up to ~4300 cm\(^{−1}\) and a concomitant change of the emission color from yellow to red. These changes were accompanied by an increase in Stokes shift to reach values as large as ~5700–5800 cm\(^{−1}\). Quantum yields of ca. 0.75 could be observed for the N,N-dimethylhydrazonyl derivative in nonpolar solvents, which gradually decreased along with increasing solvent polarity, as opposed to the consistently reduced values obtained for the N,N-diphenylhydrazonyl derivative of up to ca. 0.20 in nonpolar solvents. These two push–pull molecules are contrasted with a structurally similar acceptor-π bridge-acceptor (A-π-A) compound.
We employ transient absorption from the deep-UV to the visible region and fluorescence upconversion to investigate the photoinduced excited-state intramolecular proton-transfer dynamics in a biologically relevant drug molecule, 2-acetylindan-1,3-dione. The molecule is a ß-diketone which in the electronic ground state exists as exocyclic enol with an intramolecular H-bond. Upon electronic excitation at 300 nm, the first excited state of the exocyclic enol is initially populated, followed by ultrafast proton transfer (≈160 fs) to form the vibrationally hot endocyclic enol. Subsequently, solvent-induced vibrational relaxation takes place (≈10 ps) followed by decay (≈390 ps) to the corresponding ground state.
Ever since the discovery of dye self-assemblies in nature, there have been tremendous efforts to exploit biomimetic supramolecular assemblies for tailored artificial photon processing materials. This feature necessarily has resulted in an increasing demand for understanding exciton dynamics in the dye self-assemblies. In a sharp contrast with pi-type aggregates, however, the detailed observation of exciton dynamics in H-type aggregates has remained challenging. In this study, as we succeed in measuring transient fluorescence from Frenkel state of π-stacked perylene tetracarboxylic acid bisimide dimer and oligomer aggregates, we present an experimental demonstration on Frenkel exciton dynamics of archetypal columnar π-π stacks of dyes. The analysis of the vibronic peak ratio of the transient fluorescence spectra reveals that unlike the simple π-stacked dimer, the photoexcitation energy in the columnar π-stacked oligomer aggregates is initially delocalized over at least three molecular units and moves coherently along the chain in tens of femtoseconds, preceding excimer formation process.
Single crystals of three at bay area tetraphenoxy‐substituted perylene bisimide dyes are grown by vacuum sublimation. X‐ray analysis reveals the self‐assembly of these highly twisted perylene bisimides (PBIs) in the solid state via imide–imide hydrogen bonding into hydrogen‐bonded PBI chains. The crystallographic insights disclose that the conformation and sterical congestion imparted by the phenoxy substituents can be controlled by ortho‐substituents. Accordingly, whilst sterically less demanding methyl and isopropyl substituents afford double‐stranded PBI chains of complementary P and M atropo‐enantiomers, single hydrogen‐bonded chains of homochiral PBIs are observed for the sterically more demanding ortho‐phenyl substituents. Investigation of the absorption and fluorescence properties of microcrystals and thin films of these PBIs allow for an unambiguous interpretation of these exciton systems. Thus, the J‐aggregates of the double‐stranded crystals exhibit a much larger (negative) exciton coupling than the single‐stranded one, which in contrast has the higher solid‐state fluorescence quantum yield.
Temperature-responsive luminescent solar concentrators (LSCs) have been fabricated in which the Förster resonance energy transfer (FRET) between a donor–acceptor pair in a liquid crystalline solvent can be tuned. At room temperatures, the perylene bisimide (PBI) acceptor is aggregated and FRET is inactive; while after heating to a temperature above the isotropic phase of the liquid crystal solvent, the acceptor PBI completely dissolves and FRET is activated. This unusual temperature control over FRET was used to design a color-tunable LSC. The device has been shown to be highly stable towards consecutive heating and cooling cycles, making it an appealing device for harvesting otherwise unused solar energy.
Many dyes suffer from fast non-radiative decay pathways, thereby showing only short-lived excited states and weak photoluminescence. Here we show a pronounced fluorescence enhancement for a weakly fluorescent merocyanine (MC) dye by being co-facially stacked to other dyes in hetero-folda-trimer architectures. By means of fluorescence spectroscopy (lifetime, quantum yield) the fluorescence enhancement was explained by the rigidification of the emitting chromophore in the defined foldamer architecture and the presence of a non-forbidden lowest exciton state in H-coupled hetero-aggregates. This folding-induced fluorescence enhancement (FIFE) for specific sequences of π-stacked dyes points at a viable strategy toward improved fluorophores that relates to the approach used by nature in the green fluorescent protein (GFP).
Three different perfluoroalkylated borafluorenes (\(^{F}\)Bf) were prepared and their electronic and photophysical properties were investigated. The systems have four trifluoromethyl moieties on the borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo‐aryl moieties. They differ with regard to the para substituents on their exo‐aryl moieties, being a proton \(^{F}\)Xyl\(^{F}\)Bf, \(^{F}\)Xyl: 2,6‐bis(trifluoromethyl)phenyl), a trifluoromethyl group (\(^{F}\)Mes\(^{F}\)Bf, \(^{F}\)Mes: 2,4,6‐tris(trifluoromethyl)phenyl) or a dimethylamino group (p‐NMe\(_{2}\)‐\(^{F}\)Xyl\(^{F}\)Bf, p‐NMe\(_{2}\)‐\(^{F}\)Xyl: 4‐(dimethylamino)‐2,6‐bis(trifluoromethyl)phenyl), respectively. All derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron‐deficient derivative \(^{F}\)Mes\(^{F}\)Bf was also chemically reduced and its radical anion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 μs; however, the underlying mechanisms responsible for this differ. The donor‐substituted derivative p‐NMe\(_{2}\)‐\(^{F}\)Xyl\(^{F}\)Bf exhibits thermally activated delayed fluorescence (TADF) from a charge‐transfer (CT) state, whereas the \(^{F}\)Mes\(^{F}\)Bf and FXylFBf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition‐dipole moments.
In TFIIH the Arch domain of XPD is mechanistically essential for transcription and DNA repair
(2020)
The XPD helicase is a central component of the general transcription factor TFIIH which plays major roles in transcription and nucleotide excision repair (NER). Here we present the high-resolution crystal structure of the Arch domain of XPD with its interaction partner MAT1, a central component of the CDK activating kinase complex. The analysis of the interface led to the identification of amino acid residues that are crucial for the MAT1-XPD interaction. More importantly, mutagenesis of the Arch domain revealed that these residues are essential for the regulation of (i) NER activity by either impairing XPD helicase activity or the interaction of XPD with XPG; (ii) the phosphorylation of the RNA polymerase II and RNA synthesis. Our results reveal how MAT1 shields these functionally important residues thereby providing insights into how XPD is regulated by MAT1 and defining the Arch domain as a major mechanistic player within the XPD scaffold.
We report the direct imidization of naphthalene and perylene dicarboxylic anhydrides/esters with bulky ortho,ortho‐diaryl‐ and ortho,ortho‐dialkynylaniline derivatives. This imidization method uses n‐butyllithium as a strong base to increase the reactivity of bulky amine derivatives, proceeds under mild reaction conditions, requires only stoichiometric amounts of reactants and gives straightforward access to new sterically crowded rylene dicarboximides. Mechanistic investigations suggest an isoimide as intermediary product, which was converted to the corresponding imide upon addition of an aqueous base. Single‐crystal X‐ray diffraction analyses reveal dimeric packing motifs for monoimides, while two‐side shielded bisimides crystallize in isolated molecules without close π–π‐interactions. Spectroscopic investigations disclose the influence of the bulky substituents on the optical properties in the solid state.
The 2- and 2,7- substituted para-N-methylpyridinium pyrene cations show high-affinity intercalation into ds-DNAs, whereas their non-methylated analogues interacted with ds-DNA/RNA only in the protonated form (at pH 5), but not at physiological conditions (pH 7). The fluorescence from non-methylated analogues was strongly dependent on the protonation of the pyridines; consequently, they act as fluorescence ratiometric probes for simultaneous detection of both ds-DNA and BSA at pH 5, relying on the ratio between intensities at 420 nm (BSA specific) and 520 nm (DNA specific), whereby exclusively ds-DNA sensing could be switched-off by adjustment to pH 7. Only methylated, permanently charged pyrenes show photoinduced cleavage of circular DNA, attributed to pyrene-mediated irradiation-induced production of singlet oxygen. Consequently, the moderate toxicity of these cations against human cell lines is strongly increased upon irradiation. Detailed studies revealed increased total ROS production in cells treated by the compounds studied, accompanied by cell swelling and augmentation of cellular complexity. The most photo-active 2-para-N-methylpyridinium pyrene showed significant localization at mitochondria, its photo-bioactivity likely due to mitochondrial DNA damage. Other derivatives were mostly non-selectively distributed between various cytoplasmic organelles, thus being less photoactive.