Azobenzenes, light absorption

Solvent can alter a dye s color. One inteipretation is that light absorption moves an electron from one part of the molecule to another with a resulting change in overall polarity. Examine the HOMO and LUMO of azobenzene, 4-hydroxyazobenzene and 4-amino-4 -nitroazobenzene. Which, if any, of the molecules would be expected to change color in different solvents How does excitation change the polarity of these molecules Explain how you reached your conclusions. 

Figure 16 shows a change in the absorption spectrum of the LB film of APT(8-12) with UV (365 nm) and visible (436 nm) photoirradiation. The strong band around 360 nm is due to the trans isomer of azobenzene. The absorption due to the local excitation of TCNQ polarized along the long axis is located at about 315 nm but is indiscernible in this spectrum since the transition moment of this band is oriented almost perpendicular to the film surface and the electric field of the light is parallel to the film surface [149]. 

Polarized light absorption orients both isomers of photisomerizahle chromo-phores, and quantified photo-orientation both reveals the symmetrical nature of the isomers photochemical transitions and shows how chromophores move upon isomerization. Photo-orientation theory has matured by merging optics and photochemistry, and it now provides analytical means for powerful characterization of photo-orientation by photoisomerization. In azobenzenes, it was found that the photochemical quantum yields and the rate of the cis—>trans thermal isomerization strongly influence photo-... 

Fig. 1 Schematic representation of the reversible trans-cis-trans photo-isomerization of azobenzene. Upon absorption of UV light, a trans-to-cis conversion is induced with visible (blue) light, a cis-to-trans conversion occurs until an equilibrium between both isomers is reached. The thermodynamically less stable cis isomer can thermally return to the trans isomer...

Local temperature increases resulting from light absorption contribute to the shrinkage of this stretched rubber, similar to that observed in the contraction behavior of swollen gels with azobenzene chromophores, as described earlier. Such an effect could result from insufficient heat exchange in the solid polymer sample and be dependent on the specific heat capacity and the density of the polymer. The experimental difficulty in measuring simultaneously in situ temperature increases in the rubber and contraction under irradiation leaves an uncertainty in the interpretation of the photocon-... 

This facile reaction involves a modest change in the absorption of visible light, largely because of the visible absorption band of <7 -azobenzene [1080-16-6] having a larger extinction coefficient than azobenzene [17082-12-1]. Several studies have examined the physical property changes that occur upon photolysis of polymeric systems in which the azobenzene stmcture is part of the polymer backbone (17). 

A typical photochemical isomerization of the azobenzene amphiphile was found in an ethanol solution. A trans isomer converted to a cis isomer with ultraviolet irradiation. Back reaction from cis to trans was accelerated when a weak n-n absorption band of the cis isomer at ca.450nm was excited (Figure 21a). An alternative irradiation of uv and visible light to the ethanol solution gave reversible changes of the ji-ji transition between 355nm and 325nm attributed to the trans and cis isomers, respectively. 

HOMO (bottom) andLUMO (top) of azobenzene show where an electron is removed and where it is added upon absorption of light. 

With the irradiation of UV, the trans-to-cis photoisomerization of the azobenzene proceeds, which is seen by a decrease in the absorption assigned to the trans isomer and a simultaneous increase in the absorption assigned to the cis isomer in the region 400 to 500 nm. The conversion to the cis isomer in the photostationary state is estimated to be about 25% from the change in the absorbance of the trans isomer, which serves as a measure of the cis content. The formed cis isomer isomerizes back to the trans isomer with the irradiation of visible light. 

Isomerization can be induced by light in both directions or by heat in the Z — E direction. The reverse thermal reaction is not observed at normal temperatures. Any one of the elementary reactions can be missing. Z-azobenzene in solution has a thermal Z E activation enthalpy AH 96 kJ moT and a half life time of 2 to 3 days at room temperature. Thus, the thermal reaction is irrelevant for the photoisomerization at usual irradiation intensities (for comparison Z-stilbene has Eg 180 kJ moT is liquid, and is kinetically stable). On the other hand, one of the photoreactions may not be active (e.g., when an irradiation wavelength is selected where one form does not absorb or when the quantum yield is too small). Inspection of Figure I.IB shows that E- and Z-azobenzene have virtually no spectral region without overlapping absorption. 

Polyamide (6) was irradiated with a single 20 ns flash (530 nm) in A. iV-dimethyl-ac tamide. The kinetics of the cis to trans isomerization of backbone azobenzene residues was followed by time resolved optical absorption, and the subsequent conformation change of the total polymer chain by time resolved light scattering. Before each laser e eriment, the polymer was brought to a compact conformation by eontiimous ultraviolet irradiation, and then the unfolding proce was traced by the laser flash photolysis method. 

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