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

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.     

Structural effects on intramolecular exciplexes of 1-amino-3-anthryl-(9)-propanes

The photophysical behavior of six 1-(N-aryl-N-alkyl)-amino-3-anthryl-(9)-propanes was studied by steady-state and time-resolved fluorescence measurements in methylcyclohexane/ isopentane (MP). Structurally, these compounds are most suitable for the formation of intramolecular exciples as shown by the high exciplex quantum yields. The rate constants and the activation energies show that the rate of the exciplex formation depends on steric effects of the N-alkyl group and on the donor effect of the N-aryl group. From the rise curves of the exciplex emission it follows that also in nonpolar solvents exciplexes with a conformation deviating from the favored sandwich structure can be formed. The solvent effect on the fluorescence properties was investigated.     

EXCITED STATE BEHAVIOR OF PHYTOCHROME Pr

The primary excited state processes of phytochrome, the plant chromoprotein responsible for most photomorphogenetic responses, are investigated by picosecond intensity-dependent transmission measurements. At sufficient high excitation intensity (starting at photon flux densities of about 1′10²⁵ cm^?2 s^?l) a moderate bleaching of the absorption is observed. For the quantitative analysis of the experimental data a rate equation system approach for the energy levels responsible for the ultrafast transformation P_r→ lumi-R and the photon transport through the sample was utilized. The following was found for the excited state parameters of the phytochrome P_r: (1) an excited state absorption of the same order of magnitude as the ground state absorption (cross section: S?_exc= 4′10^?16 cm^?2 at 650 nm), (2) a fast relaxation of the higher excited singlet state on the femtosecond time scale.     

Steady state and laser spectroscopic investigations on the intramolecular exciplex formation of 1-(N-p-anisyl-N-methyl)-amino-3-anthryl-(9)-propane

The photophysical processes of 1-(N-p-anisyl-N-methyl)-amino-3-anthryl-(9)-propane, I, in methylcyclohexane/isopentane at different temperatures were investigated by steady state and time resolved fluorescence measurements. Structurally, I has a higher tendency to intramolecular exciplex formation than 1-p-dimethylaminophenyl-3-anthryl-(9)-propane, a compound of an isomeric type studied by Mataga and Eisenthal.     

Instantaneous Fluorescence Quantum Yield—A New Quantity with Characteristic “Fingerprints” for Excited-State Processes

The instantaneous fluorescence quantum yield _ins—a new quantity for fluorescence studies defined as the ratio of the fluorescence intensity to the optical density, both measured at the moment of the maximum of the exciting pulse—proves to be a very sensitive function for excited-state processes. Dependent on the excitation intensity _ins exhibits characteristic features (maxima/minima) indicating, for example, excited-state absorptions and annihilation processes. _ins is therefore more informative as the intensity dependence of the usually utilized fluorescence yield, the information content of which is restricted because this function is hardly structured. In the paper the influences of specific molecular parameters (excited-state absorption cross section, annihilation constant) on _ins are given, problems of the experimental accessibility of _ins are discussed, and an experimental setup for determination of this new quantity is presented. The application of the method is demonstrated for identification of excited-state absorptions of organic molecules in solution.     

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.     

Determination of the fluorescence decay time of the I aggregates of pseudoisocyanine

The fluorescence decay time of the I aggregates of pseudoisocyanine is measured by means of an optical gate. It averages 30 ps.     

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

The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t₄-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 (CBr₄). 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.     

One- and Two-Photon-Induced Photochemistry of Modified Palladium Porphyrazines Involving Molecular Oxygen

Abstract— Octaphenyltetraanthraporphyrazinato palladium undergoes a self-sensitized photoreaction in the presence of oxygen to form a substituted palladium phthalocyanine with four endoperoxide bridges. This compound exhibits photophysical behavior similiar to palladium tetra-tert- butyl-phthalocyanine. The phthalocyanine-palladium complex with four endoperoxide bridges ejects molecular oxygen when excited by consecutive two-photon absorption in the Q-band region at 662 nm. This photocyclo-reversion, which produces palladium porphyrazines bearing a diminished number of endoperoxide bridges, can occur up to four times per initial molecule. Irradiation of these photoproducts in the presence of oxygen produces substituted palladium phthalocyanine containing four endoperoxide groups.