Photoexcitation azobenzene

TRPES has been recently reviewed and details of the experimental method and its interpretation can be found elsewhere [5], Trans-azobenzene was introduced via a helium supersonic molecular beam into the interaction region of a magnetic bottle photoelectron spectrometer. The molecules were photoexcited by a tunable femtosecond laser pulse (pump pulse) with a wavelength of 280-350nm. After a variable time delay, the excited molecules were ionized by a second femtosecond laser pulse (probe pulse) with a wavelength of 200 or 207nm. The emitted photoelectrons were collected as a function of pump-probe time delay and electron kinetic energy. 

Photoisomerisations of azobenzene units have been found to proceed particularly clearly and to be well suited for experimental studies. The thermodynamically more stable E-isomer can be transformed photophysically into the Z-form, which can return to the E-form on photoexcitation or thermal treatment (Fig. 5.20). 

Terazima, N., Takezaki, M., Yamaguchi, S., and Hirota, N. (1998). Thermalization after photoexcitation to the state of trans-azobenzene in solution. J. Chem. Phys. 109, 603-609. 

Aromatic azo compounds are well-known to exhibit trans cis photoisomerization upon photoexcitation [129]. The volume change and AH from the photoisomerization of azobenzene and azobenzene derivatives were recently measured by the time-resolved TG technique discussed in Section III.D [130] (Scheme 9). 

Recently, the thermalization rates after the photoexcitation of azobenzene were reported by using the transient absorption and the acoustic grating signal. The thermalization process is discussed in Section IV. 

Figure 26. (a) Absorption change after the photoexcitation of azobenzene (solid line) and the time dependence of the temperature after the excitation (dotted line). (b) Difference of the absorption change in (a) from that of 300 K [160],... 

Geometrical isomerisations involving the azobenzene chromophore continue to attract attention. Low temperature flash photolysis studies of 4-aminoazobenzene and 4-(N,N-dimethylamino)azobenzene in an ethanol glass on the microsecond timescale indicate that for both compounds the photoexcited Z-isomer produces two species. One of these is the JF-isomer the other is a short-lived intermediate, tentatively identified as a zwitterionic species, which decays to form the E-isomer. The donor-acceptor substituted azobenzene sodium 4-(4 -aminophenyl-azo)benzoate has been reported to complete the E- to Z-photoisomerisation... 

We have studied pMEA, pMAEA and pDRlM in comparison, in order to determine what is the importance of Rau s classification on the photoinduced birefringence. These three polymers have increasing dipole moments, and their comparison clearly indicate that the pseudostilbene -type azobenzenes are the best candidates for photoinduced orientation. Their absorbance in the visible range of the spectrum allows the use of lower power lasers (514 nm), the coincidental absorbances of the cis and trans isomers allows photoexcitation of both trans-cis and cis-trans isomerization processes. Both are necessary for orientation, and the lower the polarity of the azobenzene, the slower the cis-trans thermal isomerization process. The levels, rates and stabilities of the photoinduced birefringence, all are hi er for pDRlM in comparison with the other two, as is the efficiency of the process. Almost all our research is concentrated on the donor-acceptor substituted azobenzenes. 

Poly(azobenzene) and its derivatives have applications in optical devices [17]. A novel polyaniline containing azo groups was synthesized by the HRP-catalyzed oxidative coupling of 4,4 -diaminoazobenzene (Scheme 1). The polymerization was carried out at pH 6.0 in tris buffer with 70% yield. The polymer analyzed by GPC (80 000, PD = 4.8) was soluble in DMSO and DMF. Azo groups were detected in the main chains as well as in the side chains. Photoexcitation studies indicated that cis-trans isomerization of the chromophore may be the result of structmal constraints in the polymer [ 18). Photodynamic properties of the azo functionahzed polyaniline in their relaxed or constrained conformations were quite different. 

Figure 8.12. (Top) Molecular structures of a commercial dye DRl, mehtyl-red (MR) dye, and azobenzene liquid crystal (ALC). (Middle) Molecular structural changes associated with trans-cis isomerization of azomolecules upon optical illumination. Bottom diagram shows the excitation energy versus the molecular coordinate associated with these photoexcited processes and the various trans-cis cross sections.

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