L. Viaene

Synthesis and spectroscopic properties of phthalocyanine dimers in solution

Abstract A soluble dimer of a tetra-(methoxy)-tetra-(octyloxy)-substituted silicon phthalocyanine has been prepared. Optical absorption spectra of the compound in solution showed a pronounced solvatochromism, characterized by a rather sharp peak with maximum at 640 nm in aromatic solvents and a broad absorption band consisting of up to five individual peaks for non-aromatic solutions such as THF or chloroform. Nanosecond transient absorption spectra exhibited a long-lived photoinduced absorption band at 520 nm which can be assigned to triplet absorption. Large quantum yields for intersystem crossing ranging between 30% and 40% were observed. These findings are compared to the results on the monomeric and polymeric compounds and discussed in the framework of exciton coupling.

References for laser-induced opto-acoustic spectroscopy using UV excitation

Abstract In order to extend the use of laser-induced opto-acoustic spectroscopy (LIOAS) to the investigation of the photophysics of molecules which absorb UV radiation, it is necessary to have suitable reference substances which absorb in this wavelength region. In this paper, the use of (2-amino)-benzophenone, (2-hydroxy)-benzophenone quenched by potassium iodide, dihexyloxatrimethinecyanine iodide, dioctadecylmonomethineoxacyanine perchlorate, iron (III)-acetylacetonate, ferrocene and p -( N,N -dimethylamino)-nitroaniline as references for LIOAS experiments is investigated. When the energy of the laser pulse is less than 20 μJ, the amount of heat, integrated over 250 ns, is (within experimental error) equal to the energy of the laser pulse absorbed by the ground state for all systems, with the exception of a solution of benzophenone and potassium iodide in ethanol. At laser pulse energies higher than 20 μJ, positive or negative deviations from a linear relationship between the energy of the laser pulse and the amplitude of the photoacoustic signal are observed. These deviations are due to the accurrence of biphotonic processes and the depletion of the ground state respectively. For the solution of benzophenone and potassium iodide in ethanol the biphotonic processes also occur at pulse energies below 20 μJ.

Observation of α-terthiophene excited dimer fluorescence in aqueous solutions of γ-cyclodextrin

Observation of .alpha.-terthiophene excited dimer fluorescence in aqueous solutions of .gamma.-cyclodextrin

Direct proof of electron transfer in a rigid first generation triphenyl amine core dendrimer substituted with a peryleneimide acceptor

The combination of nanosecond transient absorption experiments and single photon timing experiments proved the occurrence of an electron transfer process in the triphenyl amine core dendrimer, N1P1, by demonstrating the presence of an ion-pair absorption for N1P1 in solvents of medium polarity. By means of femtosecond transient absorption measurements the rise time of this ion-pair absorption dominated by the radical anion absorption could be determined, resulting in a value of 180 ps in MeTHF and 138 ps in THF. Furthermore, in femtosecond fluorescence upconversion as well as in monochromatic femtosecond transient absorption, a few ps component was resolved which was assigned to a vibrational and solvent relaxation process of the locally excited singlet state of the peryleneimide.

Non-radiative decay processes of the intramolecular charge transfer state in a rigid bichromophoric system

Abstract For the rod-shaped bichromophoric comppound 1-phenyl-4-(1-naphthylemthylene) piperidine a systematic study of the non-radiative decay processes of the charge transfer state has been carried out by means of single photon timing, phosphorescence and transient absorption measurements and laser-induced optoacoustic spectroscopy. These results can tentatively be explained by a significant admixture of the less polar locally excited state configuration in the wave function of the emissive charge transfer state, especially in the low polarity region. Due to this configuration interaction, an efficient radiationless decay channel via twisting of the ethylene bond in the locally excited state of the vinylnaphthalene chromophore becomes accessible for the charge transfer state.

Enhanced intersystem crossing in 3-(1-pyrenyl)propylbromide

Abstract Using opto-acoustic spectroscopy, transient absorption spectroscopy and time-resolved fluorescence spectroscopy, it could be demonstrated that the rate constant for intersystem crossing of 3-(1-pyrenyl)-propylbromide is twice that the model compound 1-methylpyrene. The effect of the intramolecular interaction between bromine and pyrene on the rate constant of intersystem crossing leads to a value that is still several orders of magnitude smaller than the rate constant for the intramolecular excimer formation of 1,3-di(2-pyrenyl)-propane or of intramolecular exciplex formation in 1-N,N-dimethylamino-3-(1-pyrenyl)-propane. This suggests that the contribution of the interaction between bromine and pyrene to the rate constant for intersystem crossing is considerably below the values expected for a process whose rate is limited by the rate of folding of the propyl chain.

Diffuse-reflectance laser flash photolysis of 16-(1-pyrene)-hexadecanoic acid adsorbed on silica

The absorption and fluorescence of 16-(1-pyrene)-hexadecanoicacid adsorbed on silica have been investigated. Time-resolved transient diffuse reflectance spectra were recorded following pulsed nanosecond laser excitation at 355 nm of pyrene, 1-methylpyrene and 16-(1-pyrene)-hexadecanoicacid adsorbed on silica. In addition to a rapidly decaying transient, absorbing at 420 nm assigned as the triplet state, and of the radical cation, absorbing at 460 nm, another long living transient species absorbing at 420 nm was observed for 16-(1-pyrene)-hexadecanoic acid. The decay is reversible but complete recovery takes several hours. Although no definitive assignment could be made for this transient several possibilities are discussed. The radical cations of the investigated molecules are formed by a biphotonic process. The non-exponential decay of the radical cations could be analyzed in the framework of a Gaussian distribution of free energy barriers.

α-Terthiophene in non-ionic Triton X-100 micelles: biphotonic creation of its radical cation

Abstract α -Terthiophene ( α T) has been photophysically and photochemically investigated in a non-ionic micellar environment. The distribution of α T amongst micelles is poissonian, this distribution determining its photophysical and photochemical reactivity. Neither ground-state α T, nor its triplet, migrates between micelles while the radical cation does so via the aqueous bulk. From transient absorption measurements, with different laser energies, it is concluded that the creation of the radical α T · + is biphotonic, contrary to what has been reported for homogeneous solutions.

Local properties of adsorption sites probed by the emission of fluorophores of different size and shape

Abstract The photophysics of aromatic amines, having a polar excited state, adsorbed on silica is discussed based on the results obtained by stationary and time-resolved fluorescence experiments. The stationary results suggest a correlation between the molecular shape and size and the effective polarity of the adsorption site. Time-resolved fluorescence emission experiments carried out at different emission wavelengths indicate non-exponential fluorescence decays of the adsorbed molecules. These decays could be analysed globally assuming a Gaussian distribution of decay rates. Using the standard deviation of the decay rate as a measure of the width of the distribution of the local properties of the adsorption sites, these data substantiate the interpretation of the stationary fluorescence experiments.

Photophysical properties of anthanthrone in the presence of electron donors

Abstract In order to obtain more information on the hole-injecting properties of the excited state of anthanthrone, the photophysical properties of the excited singlet and triplet states of this molecule were investigated. The excited singlet state is situated at 20 480 cm−1 in benzene and its energy is slightly dependent on the solvent properties. On the assumption that all non-radiative decay of the singlet is due to intersystem crossing, the triplet energy is estimated to be 13 000 cm−1 in toluene using laser-induced opto-acoustic spectroscopy. Using laser flash photolysis it is observed that the triplet state can be quenched by tertiary aromatic amines. In benzonitrile the quenching leads to the formation of the radical anion of anthanthrone and the radical cation of the amine, whereas in benzene, for all quenchers investigated except N,N,N′,N′-tetramethylphenylenediamine, the semiquinone radical of anthanthrone is formed. When the oxidation potential of the amine is decreased the rate constant for the quenching of the triplet state of anthanthrone is increased in benzene and benzonitrile.