Helge Lemmetyinen

Electronic energy harvesting multi BODIPY-zinc porphyrin dyads accommodating fullerene as photosynthetic composite of antenna-reaction center

Efficient electronic energy transfer (EET) in the newly synthesized dyads comprised of zinc porphyrin covalently linked to one, two or four numbers of boron dipyrrin (BDP) entities is investigated. Both steady-state and time-resolved emission as well as transient absorption studies revealed occurrence of efficient singlet-singlet energy transfer from BDP to zinc porphyrin with the time scale ranging between 28 and 48 ps. A decrease in time constants for energy transfer with increasing the number of BDP units is observed revealing better antenna effect of dyads bearing higher number of boron dipyrrin entities. Further, supramolecular triads to mimic the antenna-reaction center functionality of photosynthetic reaction center have been successfully constructed by coordinating fulleropyrrolidine appended with an imidazole ligand to the zinc porphyrin. The structural integrity of the supramolecular triads was arrived by optical, computational and electrochemical studies. Free energy calculations revealed possibility of photoinduced electron transfer from singlet excited zinc porphyrin to fullerene, and the preliminary transient absorption studies involving pump-probe technique are supportive of occurrence of electron transfer from (1)ZnP* to fullerene in the supramolecular triads.

Edge-on Langmuir-Blodgett multilayers derived from disc-shaped multiyne mesogens☆

Abstract Langmuir-Blodgett (LB) multilayers were prepared from disc-shaped multiyne mesogens based on amphiphilic alkyl pentakis (aryl-ethynyl) benzene ethers. The two compounds used are characterized by five hydrophobic flexible chains and one hydrophilic substituent at the terminal position of the alkoxy chain. The LB films were analyzed by X-ray scattering and spectroscopic measurements. An edge-on arrangement of the two discotic pentaalkynes within Y -type bilayers with a different packing density proved to be possible for the LB films of both compounds.

Study of photocycle and spectral properties of bacteriorhodopsin in Langmuir-Blodgett films

Abstract Langmuir-Blodgett (LB) films of bacteriorhodopsin (BR) in soya-phosphatidylcholine (PC) matrix were prepared on quartz plates and the photocycle and spectral properties of the films were studied. The transient absorption spectrum of BR LB film was measured by the nanosecond laser flash photolysis method in a wavelength range from 380 to 650 nm. The absorption maxima were at 540 and 415 nm for the L and M states respectively. In water the rate of decay of the L state was 2.3 × 10 4 s −1 and the rates of formation and decay of the M state were 2.3 × 10 4 s −1 and 3.0 × 10 2 s −1 respectively. The photocycle reaction was not observed in nitrogen-dried BR LB films, but when the films were put back in water, even after being stored for 8 months in dry conditions, the phenomenon could be observed. The energy transfer from trytophan to BR retinal takes place in both dry and wet LB films. The films were relatively stable to changing physical conditions as well as to laser excitation and no changes were observed following repeated photocycles. Molecular surface potentials of BR-soya-PC and soya-PC Langmuir films as a function of mean molecular area decreased towards zero with increasing surface pressure.

A kinetic study of monomer and excimer fluorescence of pyrene lecithin in Langmuir-Blodgett films

Abstract The fluorescence of Langmuir-Blodgett films of 1-palmitoyl-2-pyrene-decanoylphosphatidylcholine in matrices composed of dipalmitoyl-, 1-palmitoyl-2-oleyl- and dioleylphosphatidylcholine was studied quantitatively at different concentrations and temperatures. Stationary quantum yields, time-resolved life-times and spectral data were used to determine the kinetics of pyrene fluorescence. Possible schemes for monomer and excimer decays are presented and rate parameters for the individual steps are evaluated. The main paths for the monomer decay involve two kinds of non-interacting singlet species. The first is quenched by interaction with another pyrene moiety or an aggregate in the ground state and does not form excimers. The other pyrene moiety is aggregated in such a way that only a small rotational motion is required for it to attain a dimeric configuration which might be a precursor of an excimer. The formation of pyrene excimer is mainly determined by a reorganization, diffusional rotation in or out of the plane of the pyrene moieties in the lattice. Evidence for energy migration in the lattice in the excimer formation process is presented. The excimer is kinetically distinguishable from the monitored monomeric species. Relative intensities of excimer emission are considerably reduced when the temperature decreases, indicating diffusional excimer formation at higher temperatures. Activation energies for excimer formation are evaluated at different temperatures and concentrations. Evidence for the reorganization of the molecular lattice by light and temperature is presented.

Laser spectroscopic study of the nucleophilic photosubstitution of 4-chloroanisole and 4-fluoroanisole in aqueous solutions

Abstract The nucleophilic photosubstitution reactions, photocyanation and photohydrolysis, of 4-chloroanisole (ACl) and 4-fluoroanisole (AF) have been studied by time resolved spectroscopy and by measurements of transient photoconductivity in anhydrous and aqueous acetonitrile and tert-butyl alcohol solutions. The mechanism involves in its initial step the formation of a triplet state transient complex composed of an excited and a ground state aromatic molecule. This complex dissociates into radical ions only in the presence of water and this step determines the quantum yield of the product. The formation of a solvated electron is ruled out as a step in the substitution mechanism. Attack of the nucleophile on the radical cation yields a neutral radical from which the reaction product is formed in a single step. The transient species which have been observed spectroscopically are 3ACl*, 3AF*, [AClδ−⋯δ+ ACl], ACl+ , AF+ , AClOH·, AClCN·, AFOH· and AFCN·.

The orientation of quinquethiophene in langmuir-blodgett films

Abstract Langmuir-Blodgett multilayer films consisting of 20–60 mol% α- quinquethiophene in arachid acid were deposited on quartz substrates. The orientation of α-quinquethiophene molecules in layers was determined by polarized absorption and fluorescence methods. The results showed that they stand predominantly with their long axes perpendicular to the plane of the layer.

Temperature effects on excited state protolytic dissociation in aqueous solutions, micelles and liposomes: influence of phase transitions in lipid bilayers

Abstract The kinetics of the excited state protolytic dissociation of 1- and 2-naphthol, hydroxy- and chloro-substituted naphthols (ArOH) in aqueous solutions, micelles and liposomes was studied as a function of temperature (over the range 283–353 K) by using steady state and time-resolved fluorescence measurements. The fluorescence data suggest kinetic non-equivalence of ArOH undergoing excited state protolytic dissociation in liposomes. Naphthol molecules in lipid membranes can be subdivided into two main fractions which have quite different reactivities, i.e. rate constants of proton transfer. In suspensions of dipalmitoylphosphatidylcholine (DPPC) liposomes the fluorescence spectra and kinetics were complicated by incomplete solubilization of ArOH in lipid membranes. The temperature dependences of the rate constants of 1- and 2-naphthol photodissociation in water, micelles and bilayer membranes of egg lecithin are described by the Arrhenius equation. The increase in activation energy and prefactor observed in surfactant assemblies as compared with aqueous solutions can be explained by an increase in the enthalpy and entropy of the protolytic equilibrium for weak acid dissociation in less polar media. An abrupt change in the dissociation rate constant was observed near the temperature of the main phase transition in DPPC liposomes.

A flash photolysis study of all-trans-retinal in langmuir-blodgett films and in liposomes

Abstract The triplet-triplet absorption decay and changes in the ground-state absorption spectra of all-trnas-retinal (ATR) has been studied in Langmuir-Blodgett films (LB films), in liposomes and in hexane solution. In addition all-trans-retinal acid (ATRA) has been studied in LB films and in hexane. In LB films the decay of the triplet absorption of ATR is bi-exponential. When the number of excitation pulses increases, the relative fraction of one decay component decreases and the decay of the triplet state changes to mono-exponential. In liposomes and hexane the decay of the triplet state is independent of the number of excitation pulses and is bi-exponential for liposomes but mono-exponential in hexane solution. The changes are due to a photopolymerization reaction via the triplet state. For ATR the polymerization can also take place by thermal processes. The triplet absorption of ATRA cannot be observed in LB films or in hexane, but the polymerization takes place in LB films under the influence of light.

Excited-state proton transfer reactions of long-chain derivatives of naphthols in solutions and Langmuir—Blodgett films

Abstract LB films of stearic acid mixed with long-chain substituted naphthols (2-stearoyl-1-naphthol, 2-octadecyl-1-naphthol, 1-octadecyl-2-naphthol) were prepared. Intramolecular excited-state proton transfer in 2-stearoyl-1-naphthol was found to proceed with a high rate constant (⪢10 10 s −1 ) both in LB films (over the temperature range 77–300 K) and in ethanol or hexane solutions. Possible reasons for the absence of intermolecular excited-state proton transfer between alkyl naphthols and the anion of stearic acid in LB films are discussed.

Fluorescence of pyrene intramolecular excimers in Langmuir-Blodgett films

Abstract Fluorescence spectra and fluorescence decay kinetics of intramolecular excimers of di(1-pyrenylmethylene)adipate (DPA) and di(1-pyrenylmethylene)ether (DPE) were studied in Langmuir-Blodgett (LB) monolayers consisting of stearic acid (SA) and dipalmitoylphosphatidylcholine (DPPC). Two types of excimers with different lifetimes and different lifetime dependences on temperature were found for both of the compounds. No quenching of excimer fluorescence by oxygen was observed in LB films with SA as the matrix compound. When DPPC was used as a matrix only one type of excimer was quenched by oxygen. Photolysis of DPA and DPE LB films resulted in a dramatic decrease in excimer fluorescence, but negligible changes in the pyrene absorption band.