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The X-ray crystallographic, optical spectroscopic, and electrochemical properties of a newly synthesized class of boron-diindomethene (BDI) dyes and their tetrahydrobicyclo precursors (bc-BDP) are presented. The BDI chromophore was designed to show intensive absorption and strong fluorescence in an applicationary advantageous spectral range. Its modular architecture permits fusion of a second subunit, for example, a receptor moiety to the dye's core to yield directly linked yet perpendicularly prearranged composite systems. The synthesis was developed to allow facile tuning of the chromophore platform and to thus adjust its redox properties. X-ray analysis revealed a pronounced planarity of the chromophore in the case of the BDIs, which led to a remarkable close packing in the crystal of the simplest derivative. On the other hand, deviation from planarity was found for the diester-substituted bc-BDP benzocrown that exhibits a butterfly-like conformation in the crystal. Both families of dyes show charge- or electron-transfer-type fluorescence-quenching characteristics in polar solvents when equipped with a strong donor in the meso-position of the core. These processes can be utilized for signaling purposes if an appropriate receptor is introduced. Further modification of the chromophore can invoke such a guest-responsive intramolecular quenching process, also for receptor groups of low electron density, for example, benzocrowns. In addition to the design of various prototype molecules, a promising fluoroionophore for Na+ was obtained that absorbs and emits in the 650 nm region and shows a strong fluorescence enhancement upon analyte binding. Furthermore, investigation of the remarkable solvatokinetic fluorescence properties of the butterfly-like bc-BDP derivatives suggested that a second intrinsic nonradiative deactivation channel can play a role in the photophysics of boron-dipyrromethene dyes.
Chemical and electrochemical interaction mechanisms of metal-reducing bacteria with gold surfaces
(2017)
The ubiquity of biofilms is accompanied by significant health and safety risks. Clarifying the interaction mechanisms of sessile bacteria with metal and metal oxide surfaces is of great interest to advance the development of new control strategies and novel anti-fouling surfaces which mitigate the detrimental effects of bio-fouling.
The Shewanella genus is a faculatively aerobic, Gram-negative, exoelectrogenic bacterium known to demonstrate exceptional adaptability in respiratory capacities. Shewanella is often found in microbial communities within aquatic and sedimentary settings. During aerobic conditions, Shewanella utilizes oxygen as a terminal electron acceptor. During anaerobic conditions, respiration occurs via reduction of alternative terminal electron acceptors such as oxidized metals via extracellular electron transfer mechanisms involving electron shuttle molecules released by the bacteria.
The purpose of this work is to investigate the mechanisms of bacterial attachment and subsequent biofilm formation on metallic surfaces. Using Shewanella sp. model biofilms, surface enhanced Raman spectroscopy (SERS) complemented with electrochemical techniques have been utilized to study this dynamic interaction. Additionally, to study the process of diffusion of extracellular electron shuttles, a model system comprised of calcium cross-linked alginate to mimic the architecture of bacterial extracellular polymeric substances has been used. Inert model substrates with consistent uniformity and high surface enhancement factor have been fabricated via the deposition of gold nano-islands on thin glass slides. Moreover, it has permitted modification of the surface chemistry and surface energy via surface functionalization using organothiol self-assembled monolayers. The presentation will summarize our results on the interplay between the initial settlement rate and biofilm formation kinetics of bacteria on metallic substrates with variable environmental conditions such as electrolyte composition, pH, in addition to surface characteristics such as hydrophobicity/hydrophilicity and external polarization.
The synthesis and characterization of the new dinuclear dipeptide [(EtOOC-tpy)Ru(tpy-NHCO-tpy)Ru(tpy-NHCOCH3)]4+ 34+ of the bis(terpyridine)ruthenium amino acid [(HOOC-tpy)Ru(tpy-NH2)]2+ 12+ are described, and the properties of the dipeptide are compared to those of the mononuclear complex [(EtOOC-tpy)Ru(tpy-NHCOCH3)]2+ 42+ carrying the same functional groups. 34+ is designed to serve a high electronic similarity of the two ruthenium sites despite the intrinsic asymmetry arising from the amide bridge. This is confirmed via UV–vis absorption and NMR spectroscopy as well as cyclic voltammetry. 42+ and 34+ are emissive at room temperature, as expected. Moreover, 34+ exhibits dual emission from two different triplet states with different energies and lifetimes at room temperature. This is ascribed to the presence of a unique thermal equilibrium between coexisting [RuII(tpy-NHCO-tpy·)RuIII] and [RuIII(tpy-NHCO-tpy·)RuII] states leading to an unprecedented excited-state RuIIRuIII mixed-valent system via the radical anion bridge tpy-NHCO-tpy·. The mixed-valent cation 35+, on the other hand, shows no measurable interaction of the RuIIRuIII centers via the neutral bridge tpy-NHCO-tpy (Robin–Day class I). Reduction of 34+ to the radical cation 33+ by decamethylcobaltocene is bridge-centered as evidenced by rapid-freeze electron paramagnetic resonance spectroscopy. Interestingly, all attempts to observe 33+ via NMR and UV–vis absorption spectroscopy only led to the detection of the diamagnetic complex 3-H3+ in which the bridging amide is deprotonated. Hence 3-H3+ (and 4-H+) appear to reduce protons to dihydrogen. The ease of single and double deprotonation of 42+ and 34+ to 4-H+, 3-H3+, and 3-2H2+ was demonstrated using a strong base and was studied using NMR and UV–vis absorption spectroscopies. The equilibrating excited triplet states of 34+ are reductively quenched by N,N-dimethylaniline assisted by hydrogen bonding to the bridging amide.
In a systematic approach we synthesized a new series of fluorescent probes incorporating donor–acceptor (D-A) substituted 1,2,3-triazoles as conjugative π-linkers between the alkali metal ion receptor N-phenylaza-[18]crown-6 and different fluorophoric groups with different electron-acceptor properties (4-naphthalimide, meso-phenyl-BODIPY and 9-anthracene) and investigated their performance in organic and aqueous environments (physiological conditions). In the charge-transfer (CT) type probes 1, 2 and 7, the fluorescence is almost completely quenched by intramolecular CT (ICT) processes involving charge-separated states. In the presence of Na+ and K+ ICT is interrupted, which resulted in a lighting-up of the fluorescence in acetonitrile. Among the investigated fluoroionophores, compound 7, which contains a 9-anthracenyl moiety as the electron-accepting fluorophore, is the only probe which retains light-up features in water and works as a highly K+/Na+-selective probe under simulated physiological conditions. Virtually decoupled BODIPY-based 6 and photoinduced electron transfer (PET) type probes 3–5, where the 10-substituted anthracen-9-yl fluorophores are connected to the 1,2,3-triazole through a methylene spacer, show strong ion-induced fluorescence enhancement in acetonitrile, but not under physiological conditions. Electrochemical studies and theoretical calculations were used to assess and support the underlying mechanisms for the new ICT and PET 1,2,3-triazole fluoroionophores.
The fluorescence quenching of 1,5-bis[4-(diethylamino)phenyl]penta-1,4-dien-3-one (1), 1-[4-(diethylamino)phenyl]-5-phenylpenta-1,4-dien-3-one (2) and chalcone 3 (1-[4-(diethylamino)phenyl]but-1-en-3-one) was studied in acetonitrile using ferrocene and three commercially available derivatives, 1,1?-diacetylferrocene, acetylferrocene and 1,1?-dimethylferrocene. The spectroscopic and electrochemical characteristics of all the compounds were investigated. The feasibility of the quenching process was calculated, showing in particular that electron transfer was thermodynamically possible in every case. The steady-state fluorescence of the three organic dyes was then measured in the presence and in the absence of the ferrocenyl derivatives and the data were corrected for inner-filter effects. 1,1?-Diacetylferrocene and acetylferrocene proved to be the most efficient quenchers, and among the three dyes investigated, chalcone 3 was the most sensitive to the presence of the ferrocenyl derivatives. However, the fluorescence decay times of the ketocyanines were not affected by the presence of the ferrocene derivatives, indicating that quenching was not diffusion-controlled but rather the consequence of the formation of a ground state complex.
Photoactive metal complexes employing Earth‐abundant metal ions are a key to sustainable photophysical and photochemical applications. We exploit the effects of an inversion center and ligand non‐innocence to tune the luminescence and photochemistry of the excited state of the [CrN6] chromophore [Cr(tpe)2]3+ with close to octahedral symmetry (tpe=1,1,1‐tris(pyrid‐2‐yl)ethane). [Cr(tpe)2]3+ exhibits the longest luminescence lifetime (τ=4500 μs) reported up to date for a molecular polypyridyl chromium(III) complex together with a very high luminescence quantum yield of Φ=8.2 % at room temperature in fluid solution. Furthermore, the tpe ligands in [Cr(tpe)2]3+ are redox non‐innocent, leading to reversible reductive chemistry. The excited state redox potential and lifetime of [Cr(tpe)2]3+ surpass those of the classical photosensitizer [Ru(bpy)3]2+ (bpy=2,2′‐bipyridine) enabling energy transfer (to oxygen) and photoredox processes (with azulene and tri(n‐butyl)amine).
Metalloligands L1 and L2 consisting of directional bis(terpyridine)ruthenium(II) units and bipyridine moieties were constructed by amide formation. From these metalloligands two Ru–Pt heterobimetallic complexes 1 and 2 were derived by a building-block method by means of platination with [PtCl2(dmso)2]. Both bimetallic complexes 1 and 2 feature metal-to-ligand charge transfer (MLCT) absorptions, and emission occurs at room temperature in fluid solution from ³MLCT(Ru) states in all cases. Energy transfer from platinum to ruthenium is observed in 2 but not in 1 (light harvesting). The one-electron-reduced species [1] and [2] were prepared by reduction of 1 and 2 with decamethylcobaltocene. EPR spectra and DFT calculations reveal that the spin density is localized at the tpy–CO/Ru (tpy = terpyridine) site in [1], whereas it is centered at bpy–CO/Pt (bpy = 2,2'-bipyridine) in [2]. Efficient photoinduced electron transfer from triethanolamine to 1 and 2 is feasible by excitation at 500 nm [MLCT(Ru)].