Azobenzenes spectroscopy

The biaxial orientation in photoaddressable azobenzene films has been observed recently by polarized Raman spectromicroscopy [64]. Here, IR spectroscopy has been advantageously used as a complementary technique to measure the order... 

A series of aggregation structures of bilayer forming azobenzene amphiphiles, CnAzoCmN+Br, both in single crystals and cast films was determined by the X-ray diffraction method and uv-visible absorption spectroscopy. From the relationship between chemical structures and their two-dimensional supramolecular structure, factors determining the molecular orientation in bilayer structure were discussed. Some unique properties based on two-dimensional molecular ordering were also discussed. 

The mixed liposomal solutions were prepared by the ethanol-injection method(13) in order to obtain completely transparent solutions. It is interesting to note that miscibility of the photochromic amphiphiles with DPPC depend on the position of bulky azobenzene. If azobenzene is incorporated close to the end of long alkyl chain, a stable mixed bilayer state cannot be formed. On the other hand, when the azobenzene moiety is located near the head group or at the center of the hydrocarbon tail, the azobenzene amphiphiles are successfully incorporated into the bilayer membrane. No individual micelle formation nor phase separation in the bilayer was observed at 25 °C by absorption spectroscopy. However, the microstructure of the mixed liposomes depends on the type of azobenzene amphiphiles. 

In this paper we will present recent experiments on different types of azobenzene molecules with transient visible and IR-spectroscopy which show that ultrafast structural changes of peptide molecules occur on the time-scale of 10 ps. 

There are interesting addition compounds of [AuBr2(S2CNBu2)] with trans-stilbene and rram-azobenzene,563 the electrical resistivity of [Au(S2CNR2)2][TCNQ] has been studied,564 and the mechanism of decomposition of gold(III) dithiocarbamates has been investigated using photoelectron spectroscopy.5 ... 

Heilbroner and co-workers found the UV spectra of A-protonated 1,2-diazocine 8 and M-protonated (Z)-azobenzene were very similar. This and other spectroscopic and chemical evidence indicated that upon protonation (Z)- and ( )-azobenzene retain their configuration and are classical rather than bridged or nonclassical cations (60HCA1890). Haselbach and co-workers studied protonated 8 (and other azo compounds) using electron spectroscopy for chemical analysis (ESCA) and concluded that the cation has classical structure 69 as opposed to a nonclassical structure (70) (72HCA705). 

The hetero-dimerization behavior of dye-modified -cyclodextrins with native CDs was investigated by means of absorption and induced circular dichroism spectroscopy in aqueous solution [43], Three types of azo dye-modified /i-CDs show different association behavior, depending on the positional difference and the electronic character of substituent connected to the CD unit in the dye moiety. p-Methyl Red-modified fi-CD (1), which has a 4-(dimethylamino)azobenzene moiety connected to the CD unit at the 4 position by an amido linkage, forms an intramolecular self-complex, inserting the dye moiety in its / -CD cavity (Figure 13). 1 also associates with native a-CD by inserting the dye residue into the a-CD cavity. The association constants for such hetero-dimerization are 198 M"1 at pH 1.00 and 305 M 1 at pH 6.59, which are larger than the association constants of 1 for / -CD (43 M 1 at pH 1.00). 

A photochromic polymer containing azobenzene units has also been prepared by modification of a naturally occurring microbial poly(E-L-lysine) (Scheme 5, Structure IX), and investigated by means of absorption and circular dichroism spectroscopy.1431 The structure of this polymer, however, does not correspond to those of polypeptides, which are poly(amide)s of a-amino acids, and therefore the results cannot be discussed in terms of the typical polypeptide structures (a-helix, P-structure, random coil) and their standard CD spectra. 

As mentioned above, the sol-to-gel phase transitions can be induced reversibly by trans-cis photoisomerization of the azobenzene groups. UV irradiation (330 < X < 380 nm) transforms a part of the trans isomers to the cis. As a consequence of the structural change, the gel state is switched to the sol. Visible irradiation (at X > 460 nm) isomerizes the cis isomers to their trans form and allows the gel to be re-established. The reversible photocontrol of the sol-to-gel phase transition can be monitored by CD spectroscopy. 

McGeorge, G., Harris, R. K., Chippendale, A. M. and Bullock, J. F. (1996). Con-formahonal analysis by magic-angle spinning NMR spectroscopy for a series of polymorphs of a disperse azobenzene dyestuff. J. Chem. Soc. Perkin Trans., 1733-8. [158]... 

To understand the photoresponsive properties of azobenzene and its molecular family, it is necessary to discuss their spectroscopy and the mechanistic options of isomerization. A review of the spectroscopic properties of azo compounds appeared in 1973. The isomerization properties of azobenzene were reviewed for several periods. Wyman covered the literature up to 1954, Ross and Blanc up to 1969, and Rau up to 1988. The pre nt standalone review is restricted to the spectroscopy and isomerization of simple aromatic azo compounds. It is meant to serve as a basis for the detailed treatments of complex photoresponsive systems in the following chapters of this monograph. 

Few publications on the spectroscopic and isomerization properties of simple azo compounds have appeared in the last 15 years, as compared to the decades before. There is, however, one exception Ultrashort time-resolved spectroscopy of azobenzene and its relatives has opened new access to the dynamics following pico- and femtosecond excitation. The results are most relevant for the mechanisms of the photophysical and photochemical processes, which in azoaromatic compounds primarily are isomerizations. There is, however, a host of newer investigations into the isomerization of azobenzene and its family that are directed to applications in photoswitchable systems and devices. Some of them are relevant for the understanding of the parent molecules and therefore are included in this chapter. 

Triplet state data for azobenzene-type azo compounds are very limited. Direct absorption of a 0.51 mol solution in C7H15J in 5 cm cells has not been detectable. Neither has phosphorescence been detected. The energy of triplet states has been located only by chemical spectroscopy, i.e., the quenching of other molecules triplet states by azobenzene. Ronayette et found two relevant triplet states at about 196 and 180 kj moH... 

The relevant vibrations for this review are the N=N and C-N (Ph-N) stretching vibrations and, perhaps, torsional vibrations around the C-N bond. The E-azobenzene molecule has a center of inversion, and therefore the N=N vibration is infrared-inactive, but Raman-active, and has been found to be at 1442 cm". By IR spectroscopy, Kiibler et al. located the symmetric C-N stretching vibration at 1223 cm" in E- and at 866 cm in Z-azobenzene. The N=N vibration in Z-azobenzene is at 1511 cm" (in KBr pellets). These numbers are confirmed by newer work Biswas and Umapathy report 1439 and 1142 cm for the N=N and C-N vibrations (in CCE), and Fujino and Tahara found nearly identical results (1440 cm" and 1142 cm ). A thorough vibrational analysis of the E-isomer is given by Amstrong et al. The vibrations in the (n,7t ) excited state are very similar 1428 cm" and 1130 cm"h... 

This suggestion, based on intuitive penetration into then nonvisible geometries of azobenzene, has met critical theoretical examination by calculations of the potential energy surfaces and critical experimental examination that comes from ultra-short spectroscopy. 

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