Joerg Daub

Phenol/phenolate-dependent on/off switching of the luminescence of4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes

Boradiazaindacenes 1 and 2 bearing phenolic subunits show deprotonation/protonation dependent fluorescence off/on-switching; the lack of emission of the phenolate forms is attributed to photoinduced electron transfer (PET) between the phenolate and indacene subunits.

Chiral discrimination with a fluorescent boron–dipyrromethene dye

The optically active binaphthalene boron–dipyrromethene (BDP) conjugate 1 shows chiral discrimination towards the enantiomers of 1-phenylethylamine by distinguishable quenching rates of the BDP fluorescence.

Functionalized conducting polymers with polyazulene backbone [1]

Redox-active anthraquinone and ferrocene functional groups covalently linked to electronically conducting polyazulenes (PA) are accessible by electropolymerization of the C-2 substituted azulenes 1 and 2. The polymer films were characterized by electrochemical and spectroelectrochemical methods.

Excitation energy density dependent fluorescence behaviour of a regioselectively functionalized tetraphenylporphyrin–cellulose conjugate

Abstract The fluorescence behaviour of tetraphenylporphyrin appended cellulose strands in chloroform is studied by weak cw excitation and intense picosecond laser pulse excitation (wavelength λ L =347.15 nm, duration Δt L =35 ps). With rising pump pulse energy density, a reduction of the fluorescence efficiency and a shortening of the fluorescence signal decay time are observed. The energy density dependent fluorescence quenching is analysed in terms of intra-strand singlet exciton–singlet exciton annihilation. The annihilation constant is determined and a critical singlet exciton number density ( N ex , ann =4.8×10 19 cm −3 ) as well as a critical fraction of excited repeat units ( f RU,ann =0.1) for annihilation are determined. Additionally the excited-state absorption cross-section, σ ex,L , at the pump laser wavelength, λ L , is determined by energy density dependent transmission measurements and is found to be approximately equal to the ground-state absorption, σ L , at λ L . The small-signal fluorescence quantum distribution, fluorescence quantum yield ( φ F =0.072), and degree of fluorescence polarisation ( P F =0.04) are determined by cw fluorimetry. The emitting exciton size is found to be equal to the repeat-unit size of the cellulose strands.

Mimicking dye-based functions of natural blue-light photoreceptors by studying photoinduced energy and electron transfer in a pyrene–isoalloxazine(flavin)–phenothiazine triad

An artificial system containing phenothiazine as electron donor, isoalloxazine as flavinoid redox-mediator and pyrene as antenna has been built up in order to model photoinduced energy and electron transfer processes of natural blue-light photoreceptors.

Fulvene mit inverser Ringpolarisation, 17 [1] Heptafulvene und Heptafulvenradikalkationen: Ringverengung durch Abspaltung eines C1-Fragments bei der Einelektronen-Oxidation von Bis(dimethylamino)dibenzoheptafulven

Abstract Disubstituted dibenzoheptafulvenes 4 are obtained from dibenzocycloheptene 8 via the carboxylic acid amide 10, and are characterized by spectroscopic methods and cyclic voltammetry. These dibenzoheptafulvenes are nonplanar, geometric enantiomorphy is shown by 1H-NMR of 12-dim ethyl-am ino-12-siloxydibenzoheptafulvene 4c. Oxidation o f 12,12-bis(dim ethylamino)dibenzo[a,d]-heptafulvene 4a with Ag(I), I2 or Cu(II) leads to the radical cation 7. In solution 7 is converted to dibenzocycloheptatriene and anthracene amidinium salts 18 and 19. The ratio o f these two amidinium salts depends on the reaction conditions. H-radical trapping from the medium is proposed for the formation of 18, while 19 must be formed under elimination of a C1-unit.

Photodynamics of pyrene-flavin and phenothiazine-flavin dyads

A pyrene-flavin (isoalloxazine) dyad (PFD) and a phenothiazine-phenylene-isoalloxazine dyad (PPF), dissolved in dichloromethane are characterized by absorption and emission spectroscopy. These dyads are model compounds for flavin based blue-light photoreceptors. Absorption cross-section spectra, fluorescence quantum distributions, fluorescence quantum yields, and fluorescence decay times are determined. The absorption spectra of the dyads resemble the superposition of the absorption spectra of the constituents (1-methylpyrene, isoalloxazine, and phenylphenothiazine). Photo-excitation of the flavin moiety causes fluorescence quenching by reductive electron transfer in thermodynamic equilibrium with the exited flavin subunit. The charge-separated states recover by charge recombination. Photo-excitation of the pyrene or phenylphenothiazine moiety causes oxidative electron transfer with successive recombination, and additionally Förster-type energy transfer and Dexter-type energy transfer. For PFD in dichloromethane the rates of reductive electron transfer and oxidative electron transfer were determined to be (5 ps)-1 and (77 ps)-1, respectively, and a charge recombination time of about 16 ps was found. For PPF in dichloromethane the rates of reductive electron transfer and oxidative electron transfer were determined to be (700 fs)-1 and (100 ps)-1, respectively. The HOMO level position of the pyrene radical cation subunit relative to the HOMO level of the excited isoalloxazine subunit is determined from the delayed fluorescence emission of the PFD sample.