Uwe Pischel

Molecular logic devices (half-subtractor, comparator, complementary output circuit) by controlling photoinduced charge transfer processes

The fluorescence properties of a naphthalene derivative with a methoxy and a pyridyl substituent were investigated, including the influence of protons, electron-donating amines, and dihydrogenphosphate anions. The naphthalene derivative shows strong fluorescence with a maximum at 353 nm, which can be quenched by amines through a diffusion-controlled photoinduced electron transfer. Protonation of the pyridyl residue leads to the observation of a red-shifted broad emission at 470 nm, which has been assigned to an internal charge transfer state. The interpretation of the fluorescence behaviour in the presence or absence of the chemical inputs leads to the realisation of unimolecular logic circuits, which are able to perform advanced arithmetic operations such as subtraction (XOR/INH combination) and comparison (XNOR/INH combination). Furthermore, a complementary output circuit (INH/IMP combination) was implemented. The implicated logic gates can be reconfigured, either by application of different input sets or by varying the fluorescence output observation wavelength. The system works in an all-fluorescence output mode, which leads to a superposition of the gates. This is of particular advantage as it allows reading out without any time lag, which is a commonly encountered problem in silicon circuitry based on electrical signals.

A molecular tool kit for the variable design of logic operations (NOR, INH, EnNOR)

A simple set of five components was used to design molecular logic gates based on phthalimide-sensitised Tb(III) luminescence, including the first report of an enabled NOR (EnNOR) gate.

Zirconium organophosphonates as photoactive and hydrophobic host materials for sensitized luminescence of Eu(III), Tb(III), Sm(III) and Dy(III)

Novel luminescent materials based on lanthanides [Eu(III), Tb(III), Sm(III), and Dy(III)] and a mixed zirconium phenyl- and m-sulfophenyl phosphonate as the host matrix, have been prepared and photophysically characterized. Powder X-ray diffraction, 31P-MAS-NMR spectroscopy, IR spectroscopy, and diffuse-reflectance UV spectroscopy revealed the specificity of these materials such as a layered structure, the presence of phenyl groups in the galleries, and the ability to absorb lanthanide ions introduced by simple ion exchange. Characteristic line-shaped long-lived luminescence [ca. 0.27 ms for Eu(III), 0.80 ms for Tb(III), and 0.05 ms for Sm(III)] was observed for different lanthanide ions, and was demonstrated to be generated by an antenna-induced energy transfer process to the metal. The overall luminescence quantum yield of europium- or terbium-loaded materials was measured to be ca. 0.3%. A detailed analysis of the Eu(III) luminescence spectrum and H2O/D2O exchange experiments indicated the presence of ca. 3 water molecules around each lanthanide. Based on the high maximum coordination numbers of lanthanides, up to 9 for free Eu(III) in aqueous solutions, and the presence of only monodentate sulfonate binding sites, more water could be expected. This observation is explained by the rather hydrophobic microenvironment in the interlamellar space of the materials, due to pendant organic moieties, that is, the phenyl groups.

Investigation of Polar and Stereoelectronic Effects on Pure Excited-state Hydrogen Atom Abstractions from Phenols and Alkylbenzenes†

Abstract The fluorescence quenching of singlet-excited 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) by 22 phenols and 12 alkylbenzenes has been investigated. Quenching rate constants in acetonitrile are in the range of 108–109 M−1s−1 for phenols and 105–106 M−1s−1 for alkylbenzenes. In contrast to the quenching of triplet-excited benzophenone, no exciplexes are involved, so that a pure hydrogen atom transfer is proposed as quenching mechanism. This is supported by (1) pronounced deuterium isotope effects (kH/kD ca 4–6), which were observed for phenols and alkylbenzenes, and (2) a strongly endergonic thermodynamics for charge transfer processes (electron transfer, exciplex formation). In the case of phenols, linear free energy relationships applied, which led to a reaction constant of ρ = −0.40, suggesting a lower electrophilicity of singlet-excited DBO than that of triplet-excited ketones and alkoxyl radicals. The reactivity of singlet-excited DBO exposes statistical, steric, polar and stereoelectronic effects on the hydrogen atom abstraction process in the absence of complications because of competitive exciplex formation.

Intramolecular electron transfer in diastereomeric naphthalene–amine dyads: a fluorescence and laser flash photolysis study

Two dyads containing a naphthalene-like chromophore linked to a pyrrolidine-derived moiety, namely (S,S)- and (R,S)-NPX-PYR, have been synthesised by esterification of (S)- or (R)-naproxen (NPX) with (S)-N-methyl-2-pyrrolidinemethanol (PYR) and submitted to photophysical studies (steady-state and time-resolved fluorescence, as well as laser flash photolysis). The emission spectra of the dyads in acetonitrile were characterised by a typical band centred at 350 nm, identical to that of the reference compound (S)-NPX. However the intensities were clearly different, revealing a significant intramolecular quenching in the dyads, as well as a remarkable stereodifferentiation (factor of 1.6). Accordingly, the fluorescence lifetimes of the two dyads were different from each other and markedly shorter than that of (S)-NPX. The quenching mechanism is intramolecular electron transfer, that is thermodynamically favoured. Exciplex formation, that is nearly thermoneutral, does not compete efficiently. The electron transfer rate constants for (S,S)- and (R,S)-(NPX-PYR) were 1.8 x 10(8) and 2.8 x 10(8) s(-1), respectively. By contrast, no significant intramolecular quenching was observed for the excited triplet states (lambda(max)= 440 nm), generated by laser flash photolysis; this is in agreement with the fact that intramolecular electron transfer is thermodynamically disfavoured, due to the lower energy of excited triplets.

Intramolecular singlet–singlet energy transfer in antenna-substituted azoalkanes

Two novel azoalkane bichromophores and related model compounds have been synthesised and photophysically characterised. Dimethylphenylsiloxy (DPSO) or dimethylnaphthylsiloxy (DNSO) serve as aromatic donor groups (antenna) and the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as the acceptor. The UV spectral window of DBO (250-300 nm) allows selective excitation of the donor. Intramolecular singlet-singlet energy transfer to DBO is highly efficient and proceeds with quantum yields of 0.76 with DPSO and 0.99 with DNSO. The photophysical and spectral properties of the bichromophoric systems suggest that energy transfer occurs through diffusional approach of the donor and acceptor within a van der Waals contact at which the exchange mechanism is presumed to dominate. Furthermore, akin to the behaviour of electron-transfer systems in the Marcus inverted region, a rate of energy transfer 2.5 times slower was observed for the system with the more favourable energetics, i.e. singlet-singlet energy transfer from DPSO proceeded slower than from DNSO, although the process is more exergonic for DPSO (-142 kJ mol(-1) for DPSO versus-67 kJ mol(-1) for DNSO).

Stereoselective fluorescence quenching by photoinduced electron transfer in naphthalene-amine dyads

Intramolecular chiral recognition in electron-transfer-induced fluorescence quenching has been observed for diastereomeric dyads composed of a naphthalene chromophore and an amine.