Wolfgang Freyer

Two-photon excitation of alkyl-substituted magnesium phthalocyanine: radical formation via higher excited states

Abstract The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t 4 -PcMg) in solution and microheterogeneous systems (liposomes and micelles) were investigated. Radical cation formation occurs in chloroform during UV excitation in the presence of an electron acceptor (CBr 4 ). The same result is achieved by two-step absorption in the singlet manifold using pulsed excitation at λ exc =670 nm, which is of interest from the viewpoint of photon delivery through the therapeutic window of tissues. To obtain a deeper insight into the photophysics leading to radical cation formation via the higher excited singlet state, the transient spectra and singlet—singlet absorption cross-sections were determined. In addition to strong excited state absorption within the spectral range of the Q x -band, relatively large absorption cross-sections were also found in regions with low ground state absorption. The importance of these transitions for an effective two-colour excitation regime is discussed with regard to new start mechanisms for photodynamic laser tumour theraphy.

Fluorescence Studies of Melanin by Stepwise Two-Photon Femtosecond Laser Excitation

Fluorescence of synthetic melanin in the solvents H2O, KOH, ethylene glycol monomethyl ether, and dimethyl sulfoxide has been excited by two-photon absorption at 800 nm, using 120-fs pulses with photon flux densities of ≥1027 cm−2.S−1. Compared to the one-photon (400-nm)-induced fluorescence of melanin, the overall spectral shape is red-shifted and shows a strong environment sensitivity. The decay of the two-photon-induced fluorescence (TPF) of melanin is three-exponential, with a shortest main component of about 200 ps. The results of the TPF studies in line with the unique light absorption property of melanin of a monotonously decreasing absorption spectrum between the near UV-region and the near infrared region indicate that the TPF is realized via stepwise absorption of two 800-nm photons. In comparison to the simultaneous absorption of two photons, the stepwise process needs lower photon flux densities to get a sufficient population of the fluorescent level. This stepwise process offers new possibilities of selective excitation of melanin in skin tissue in a spectral region where there is no overlap with any absorption of another fluorescent tissue component. The first results with different samples of excised human skin tissue (healthy, nevus cell nevi, malignant melanoma) suggest that fluorescence excited in this way yields information on malignant transformation.

Femtosecond two-photon excited fluorescence of melanin.

Abstract. Fluorescence of synthetic melanin in dimethyl sulfoxide has been excited by two‐photon absorption at 800 run, using 120 fs pulses with photon flux densities 1027 cm 2 s1. The shortest main component of the three‐exponential decay of fluorescence is 200 ± 2 ps. The overall spectral shape is red‐shifted with respect to the 400 nm excited fluorescence. Two‐photon excited melanin fluorescence also has been measured from excised samples of healthy human skin tissue. Because of the selectivity of melanin excitation via resonant two‐photon absorption, it is hypothesized that fluorescence excited in this way may yield information on malignant transformation.

Structure and excitonic coupling in self-assembled monolayers of azobenzene-functionalized alkanethiols.

Optical properties and the geometric structure of self-assembled monolayers of azobenzene-functionalized alkanethiols have been investigated by UV/visible and near edge X-ray absorption fine structure spectroscopy in combination with density-functional theory. By attaching a trifluoro-methyl end group to the chromophore both the molecular tilt and twist angle of the azobenzene moiety are accessible. Based on this detailed structural analysis the energetic shifts observed in optical reflection spectroscopy can be qualitatively described within an extended dipole model. This substantiates sizable excitonic coupling among the azobenzene chromophores as an important mechanism that hinders trans to cis isomerization in densely packed self-assembled monolayers.

Dual and multiple fluorescence mechanisms of p-dimethylaminobenzaldehyde and its trimethylene-bridged double molecule

Abstract p-Dimethylaminobenzaldehyde (DMABA, I) and its N-to-N trimethylene-bridged double molecule (III) exhibit dual or multiple fluorescences. Several mechanisms were proved to be responsible for the long-wave fluorescence bands: intramolecular solvent-assisted relaxation in the excited state (I and III); ground state aggregation at low temperatures (I in nonpolar solvents); excimer formation (III; and I only at high concentrations in some solvents). Intramolecular interaction in the ground state of III prepares an excimer-like structure. The dimer of DMBA aside of its own emission, may relax and emit an excimer-like fluorescence.

Spectroscopic properties of potential sensitizers for new photodynamic therapy start mechanisms via two-step excited electronic states

Three substituted tetraazaporphyrins, octa‐(4‐tert‐butylphenyl)‐tetrapyrazinoporphyrazine, tetra‐(4‐tert‐butyl)phthalocyanine and tetra‐(4‐tert‐butyi)phthalocyanatomagnesium (t4‐PcMg), were spectroscopically checked in solutions and liposomes with respect to suitability as potential sensitizers of a possible new start mechanism for photodynamic therapy (PDT) from a stepwise excited higher singlet state. This PDT start mechanism was recently proposed to overcome the problem of O2 (1δ2)‐caused cutaneous phototoxicity in PDT.

Synthese von Metallkomplexen des Tetra-(2,3-anthra)-tetraazaporphins und Vergleich ihrer Elektronenabsorptionsspektren mit denen anderer anellierter Tetraazaporphinsysteme

Copper, nickel and aluminium derivatives of tetra-2,3-anthratetraazaporphin bearing different substituents in the anthracen part have been prepared. The absorption spectra of these substances in different solvents are given and compared with metal complexes of other linearly annellated tetraazaporphins. In the series of vanadyl complexes of tetraazaporphin (λmax 597 nm), tetra-[1,2-(4-tert-butyl)-benzo]-tetraazaporphin (λmax 698 nm), tetra-[2,3-(6-tert-butyl)-naphtho]-tetraazaporphin (λmax 807 nm), tetra-2,3-(anthra)-tetraazaporphin (λmax 932 nm), tetra-2,3-(tetraceno)-tetraazaporphin (λmax 1055 nm), the absorption maxima of the Q-band are shifted per annellated benzene ring about 100 nm to longer wave lengths.

High-power fifth-harmonic generation of femtosecond pulses in the vacuum ultraviolet using a Ti:sapphire laser.

We demonstrate the generation of fifth-harmonic pulses at 161 nm, with an energy of up to 600 nJ and 160 fs pulse duration from a Ti:sapphire laser at 1 kHz repetition rate by four-wave difference-frequency mixing in argon-filled waveguides. The efficiency is greatly improved by coupling to higher-order transverse modes, as well as by coating the inner surface of the waveguide. A numerical model of the process yields an understanding of the main effects influencing the harmonic generation.

Mixed self-assembled monolayers of azobenzene photoswitches with trifluoromethyl and cyano end groups

Mixed self-assembled monolayers (SAMs) of alkanethiolates carrying azobenzene chromophores with either a trifluoromethyl or a cyano substituent have been studied. High-resolution x-ray photoelectron spectroscopy proves that the ratio of adsorbed molecules can be arbitrarily adjusted via the molar fractions in solution. As a function of these molar fractions core level shifts are observed which are attributed to local work-function changes. By simulating the electric dipole field distribution, the continuous core level shifts are ascribed to a homogeneous mixture of molecules with different end groups adsorbed on adjacent lattice sites. Near-edge x-ray absorption fine structure measurements reveal formation of well-ordered SAMs. Despite the difference in dipole moment of the end groups, the molecular tilt and twist angles are identical for both single-component SAMs and a 1:1 mixed SAM.

Photochemistry of Anthracene-9,10-endoperoxide

The wavelength dependence of the photochemistry of anthracene-9,10-endoperoxide (APO) in acetonitrile was quantitatively investigated at 5 degrees C, with excitation varied from 240 to 450 nm. Anthracene (AC) and a diepoxide (DE) were identified as the main primary photoproducts. After short exposure times DE was at all wavelengths the dominating photoproduct, while AC was only formed for lambda <or= 320 nm. The maximum AC quantum yield of 29% was reached at 270 nm. Anthraquinone (AQ) and a bicyclic acetal (BA) were identified as the main secondary products. Formation of AQ and BA occurred both from DE and from ground-state APO. Formation of BA from ground-state APO involved excited DE or BA itself, while formation of BA from DE required UV excitation of DE. Room-temperature thermolysis of APO only produced AQ. For lambda <or= 310 nm the total photochemistry quantum yield was, within error margins, constant and close to unity. Between 300 and 450 nm, the tail of the APO absorption spectrum, a more or less monotonic decrease of the total photochemistry quantum yield was observed.