Fluorescence azobenzenes

They also investigated the quenching of the azobenzene dendrimer fluorescence by Eosin Y (2/,4/,5/,7/-tetrabromofluorescein dianion) [35]. It was concluded that Eosin Y was hosted in these dendrimers and Z-form dendrimers were more efficient hosts than E-form dendrimers. 

Tsuda K, Dol GC, Gensch T, Hofkens J, Latterini L, Weener JW, MeijerEW, De SchryverEC. Fluorescence from azobenzene functionalized poly(propylene imine) dendrimers in self-assembled supramolecular structures. J Am Chem Soc 2000 122 3445-3452. 

The rigidity introduced by O-bridge in fluorescein makes the molecule highly fluorescent. Similarly, azobenzene is nonfluorescent, whereas diazaphenanthrene is capable of fluorescence. 

POPAM dendrimers bearing up to 32 photoswitchable azobenzene groups at the periphery were used as host compounds for eosin Y (2, 4, 5, 7 -tetrabromo-fluorescein dianion) (see also Section 5.1.2). One reason for the choice of this dye was that it shows strong fluorescence which should be influenced by being enveloped by the dendrimer. The other reason was that the energy of its lowest triplet state is higher than that of the lowest triplet state of azobenzene, leading... 

Kikuchi, T. Narita, M. and Hamada, F. (2002) Synthesis of bis dansyl-modified /f-cyclodextrin dimer linked with azobenzene and its fluorescent molecular recognition, J. Incl. Phenom. 44, 329-333. 

In nature, polypeptides with amphiphilic structures are known to form transmembrane channels formed by an assembly of several helices, so as to present their polar faces inward and their apolar faces outward. In view of such behavior, the photochromic amphiphilic polypeptide was incorporated into a cationic bilayer membrane composed of dipalmitoyl phosphatidyl choline.11201 Fluorescence and microscopic measurements provided evidence that the polypeptide was able to form bundles of helical molecules analogous to their natural counterparts, which acted as transmembrane channels for K+ ions. Irradiation, and the consequent transacts isomerization of the azobenzene link, caused a bending of the molecular structure and a destabilization of the transmembrane bundles. Therefore, formation of ion permeable channels would be favored or inhibited depending on whether the azo moiety... 

Rotational Raman spectra of N2 and O2 Resonance fluorescence spectrum of Br2 Vibrational-rotational spectra of CD3H and CH3D Spectrophotometric study of stabihty of metal ion-EDTA complexes Kinetics of the H2 + I2 = 2HI reaction in the gas phase Weak-acid catalysis of BH4 decomposition Photochemistry of the cis-trans azobenzene interconversion Isotope effect on reaction-rate corrstants... 

Poly(propyleneamine) dendrimers of generations 1 and 4 (89) functionalized with azobenzene groups were investigated as hosts for eosin Y (eosin = 2, 4, 5, 7 -tetrabromofluorescein dianion) in DMF solution [159]. The peripheral azobenzene groups can be switched by light excitation from the E to the Z form. The fluorescent excited state of eosin is reductively quenehed by the tertiary amine units present in the dendrimer structure. This electron transfer quenching takes place with a static... 

In strongly acidic media azobenzene is protonated and irradiation leads to a cis trans photostationary state of the cations. Prolonged irradiation induces a photochemical cyclodehydrogenation with the formation of benzo[c]cinnoline. Protonated azobenzene also shows intense fluorescence . 

Some azobenzenes that are locked against rotation by bulky substituents in all four ortho positions may show fluorescence when frozen rigidly at 77 K 2,2 ,4,4 ,6,6 -hexaisopropyl 2,2 -difflethyl-4,4 6,6 -tetra-tert-butyl azobenzene belong to this series. Azobenzenophanes 7 to 13 do not emit, even at 77 K this is the expectation for card-packed dimers. 

Tsuda et al. found the same fluorescence at = 600 nm in a giant vesicle in a card-packed azobenzene arrangement. The noisy appearance of their fluorescence trace (if not due to the technique of confocal laser miGroscopy), however, suggests a very low emission intensity. Both Shinomura and Kunitake and Tsuda et al. report time-dependent orientation phenomena on Z-E isomerization in the supramolecular arrangement, which is reflected in the fluorescence intensity. So the former general statement that azobenzene-type azo molecules do not emit needs to be modified. 

On the other hand, azobenzene can quench the fluorescence of other moleetdes. This has been investigated in molecules containing both a fluorescing and an azobenzene unit. It was found that the E-form is about 3 times, or even up to 13 times, as effective a quencher as the Z-form. The influence of the environment on such bichromophoric molecules was studied by Eisenbach et al. ... 

The short pulse duration combined with the high photon density of ps-and fs-lasers have provided the means to study the properties of the excited states by emission and transient absorption measurements. Fluorescence of the lowest and higher excited states of azobenzene can be detected, but most work is being directed toward the dynamics of isomerization. Because questions about the isomerization mechanism are prominent in this field, this work will be discussed in Section 1.6 The Isomerization Mechanism. 

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 K with quantum yields of about 0.1. Inclusion of azobenzene in the channels of AIPO4-5 crystals provides complexation of the n-electrons and space confinement. This leads to emission by protonated azobenzene at room temperature. For their cyclopalladated azobenzenes, Ghedini et al. " report quantum yields of ca. 1T0 and lifetimes of ca. 1 ns. In contrast, donor/acceptor pseudo-stilbenes, if emitting at low temperatures or when adsorbed to surfaces, are weak emitters. In textile chemistry, it has long been known that azo dyes adsorbed to fibers may show fluorescence. ... 

In a very new report, Fujino et al. challenge the two-isomerization-mechanism concept on the basis of their time-resolved and time-integrated femtosecond fluorescence measurements of B-azobenzene following excitation of the (7t,7t ) State. They use the extremely weak fluorescence (cf. Figure 1.8) as an indicator for the population of the emitting state. From the ratios of their measured fluorescence lifetimes (S2 0.11 ps Sp 0.5 ps) and the radiative lifetimes deduced from the (absorption-spectra-based) oscillator strengths, they determine the fluorescence quantum yields 2.5310 for the emission and 7.5410" for the Si—>So emission. By comparison... 

Monoprotonation of azobenzenes is asymmetric. a (n,7t ) state still exists, but it is expected to shift to high energy, and the molecules are very stilbene-like. Rotation would be the isomerization mode. This concept is supported by the strong temperature dependence of fluorescence Azobenzene-... 

Hamai, S., and Hirayama, F. (1984). S2 —> Sg fluorescence of azobenzene. Book of Abstracts, Annual Symposium cm Photochemistry (Japan), 315-316. 

Fujino, T., Arzhantsev, S. Y., and Tahara, T. (2001). Femtosecond time-resolved fluorescence study of photoisomerization of trans-azobenzene. J. Chem. Phys. A, 105, 8123-8129. 

Autret, M., Le Plouzennec, M., Molnet, C., and Sirnonneaux, G. (1994). Intramolecular Fluorescence Quenching in Azobenzene-substituted Porphyrins. J. Chem. Soc., Chem. Commun. 1169-1170. 

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