H. Port

Fulgides as switches for intramolecular energy transfer

In a molecular unit which is a donor-fulgimide-acceptor triade, the possibility of an intramolecular energy transfer from donor to acceptor depends upon the isomer configuration of the photochromic fulgimide. It is shown that a switching on and off of this intramolecular energy transfer is possible by photoinduced shifting of the fulgimide levels between a trap and an antitrap function.

Anthryloligothienylporphyrins: Energy Transfer and Light‐Harvesting Systems

In the new trichromophoric supermolecule 1, a quantitative intramolecular energy transfer (>98 %) occurs from the anthracene donor to the emitting porphyrin acceptor through the oligothiophene chain after selective UV excitation of anthracene. Energy-transfer properties and fluorescence quantum yields are independent of the oligothiophene chain length. Attachment of further anthrylquinquethienyl chromophores to the porphyrin ring results in an increase in the intensity of characteristic absorption and fluorescence bands.

Excited-state proton transfer in a benzotriazole photostabilizer investigated by femtosecond spectroscopy

Abstract The proton transfer in 2-(2′-hydroxy-5′-methylphenyl)benzotriazole is studied in nonpolar solvents, where two major ground state conformers are identified. After femtosecond excitation of the predominant species, the hydroxylic proton is transferred to the benzotriazole moiety on a time scale of 100 fs. The excited state of the resulting keto tautomer is deactivated by internal conversion to the electronic ground state within 200 fs. The subsequent back-reaction to the ground state of the enol tautomer also occurs in the subpicosecond regime. Keto emission in the red spectral range is observed for the first time in nonpolar solvents at room temperature.

The two-step excimer formation in perylene crystals

Abstract The time dependence, rise and decay, of the fluorescence of α-perylene following ps-pulse excitation as investigated as a function of temperature (4.2–300 K). A kinetic scheme for the coupled relaxation process of the Y state as precursor and the excimeric E state is established and the complete set of kinetic parameters is determined. The analysis is supported by comparison with perdeuterated α-perylene and also with the monomeric crystal form β-perylene.

Optical properties of amorphous ultrathin films of perylene derivatives

Abstract Ultrathin films of PTCDA and MePTCDI (submono/multilayers with thickness d = 0.01 – 10 ML), UHV-deposited on He-cooled quartz substrate, are studied in situ by cw and ps time-resolved fluorescence spectroscopy. The absorption spectrum, measured via photoexcitation, shows a monomer-like shape independent of film thickness, indicating an amorphous structure. The monotonic red-shift with increasing d reflects enhancement of isotropic intermolecular interaction. The fluorescence of ultrathin films consists of a single, monomeric component (M) exhibiting mirror symmetry to the absorption. With increasing film thickness a lower energy excimer-like component (E) emerges, gaining intensity at the expense of M due to energy transfer to E traps. The two components can be separated by time-resolved spectroscopy. Individual molecular behaviour can be studied in the limit of ultrathin films (

Relaxation pathways and fs dynamics in a photoswitchable intramolecular D → A energy transfer system

Abstract A photoswitchable energy transfer system (donor–switch–acceptor) is examined in a polymer environment using time-resolved pump and probe spectroscopy. The system consists of a photochromic fulgide derivative acting as switching unit, an anthracene group as energy donor and a coumarin moiety as acceptor. The general value of the concept has already been proven by different spectroscopic methods. To get a deeper insight into the complex energy relaxation process, both the supermolecule and all its subunits are studied separately. The role of the switch is analysed by comparing spectra and transients of both isomeric forms.

Mid-infrared recognition of the reversible photoswitching of fulgides

Abstract IR absorption spectra of E- and C-isomers of several photochromic fulgide compounds were examined. As a characteristic feature, the spectra of all C-isomers show peaks that are completely missing in the spectra of the corresponding E-isomers. Photoisomerization reactions of fulgides can, therefore, be monitored and their isomerization state can be detected by measuring the IR absorption at the characteristic wavenumbers. It is verified that mid-IR irradiation using tunable IR diode lasers does not induce any photoreaction. Thus, for the application of fulgides as optical recording media or photoswitches, narrowband mid-IR absorption measurement provides a method for entirely non-destructive readout.

Optical homogeneous linewidths in organic glasses below 1 K: a hole-burning study of free-base porphin in 2-methyltetrahydrofuran

Abstract Photochemical hole-burning has been used to determine the temperature dependence of the homogeneous linewidth Γ hom of the 0-0 S 1 - S 0 transition of free-base porphin (H 2 P) in 2-methyltetrahydrofuran (MTHF) from T ≈ 5 K down to 0.4 K. A T 1.30±0.05 dependence for Γ hom is observed which extrapolates to the lifetime-limited value (≈ 10 MHz) at T ≈ 0.05K.

Design, synthesis and assembly of new thiophene-based molecular functional units with controlled properties

Abstract Individual aspects in the field of ‘molecular electronics’ are addressed using new materials which are based on conjugated thiophenes as key building blocks. Among then, functionalized polythiophenes represent prototype compounds in which the three-dimensional network of ‘molecular wires’ can interact with the specific functional groups. Corresponding structurally defined oligothiophenes serve as model compounds and allow the correlation of various properties with structure. In addition, novel properties and superior processibility of these oligomers lead to interesting applications. Light emitting diodes and metal/organic/metal nanostructures are constructed by evaporation of thin oligomer films. A clear dependence of electroluminescence and electrical transport properties on chain length is found. Finally, the supramolecular assembly of functionalized (oligo)thiophenes is achieved by oriented adsorption on single crystal surfaces and self-organization in highly ordered LB films.

Two-component optical spectra in thin PTCDA films due to the coexistence of α- and β-phase

Abstract Thin crystalline PTCDA films deposited under UHV conditions on various substrates at temperatures 10–373 K were investigated in-situ by emission and excitation spectroscopy after being annealed. Morphology and crystal structure of these films were examined by atomic force microscopy and X-ray measurements. In optical and structural investigations two spectral components were distinguished and correlated with the two known crystal phases of perylene-3,4:9,10-tetracarboxylic-dianhydride (PTCDA) in any annealed film. The relative amount of the phases depends on substrate temperature and film thickness. These effects can be made responsible for previously reported thickness-dependent energy shifts and spectral changes, which were attributed to quantum confinement.