Azobenzenes donor-acceptor substituted

Table 2.3 Xmax values for a series of donor-acceptor substituted azobenzenes (16a-f)...

Donor/acceptor substituted azobenzenes, which have large second-and third-order nonlinear optical properties, show a fast thermal Z-F (cis-trans) conversion. 

Electronic state calculations for azobenzene in early papers suffered from the inability of older methods to take into account the mixing of (n,7t ) and (7t,7C ) states. New calculations using ab-initio methods are successful, even in mastering donor/acceptor substituted azobenzenes. A survey of calculations in connection with the isomerization mechanism will be given in Section 1.6. 

The prototype molecule for donor/acceptor substitution is 4-dimethyl-amino-4 -nitroazobenzene. Here, the n- n band is shifted far to the red due to the charge transfer character of the transition. The band has few vibrational features, and its energy is influenced by the polarity of the solvent. The weak n Tt band cannot be seen under the intense it —> it band (Figure 1.13). Most commercial azo dyes are pseudo-stilbenes rather than azobenzene-type molecules. 

For pseudo-stilbene-type molecules, the question of the mechanism of thermal isomerization was taken up again in the early eighties by Whitten et al. and later by Kobayashi et al., " who, on the basis of their isomerization experiments with donor/acceptor-substituted azobenzenes in polar solvents, postulate rotation. Asano and coworkers infer from the isomeriza-... 

Based on their isomerization experiments in polar solvents, Whitten et al. stated that thermal Z—> E isomerization of donor/acceptor-substituted azobenzenes proceeds by rotation. Later, Shin and Whitten modified this point of view and saw a dual mechanism active dependent on solvent and donor strength, Asano and coworkers,from their experiments under pressure, inferred a dependence of the mechanism on solvent and even parallel reaction paths along these mechanistic coordinates. 

Shin, D.-M., and Whitten, D. G. (1988). Solvent induced mechanism change in charge-transfer molecules. Inversion versus rotation paths for the Z E isomerization of donor-acceptor substituted azobenzenes. /. Am. Chem. Soc. 100, 5206-5208. 

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... 

Azobenzene-containing polymers are reported in the literature at a rate of about 150 papers/year for the last five years. There are two main directions of interest the first relates to the donor-acceptor substituted azobenzenes as the structure generating second-order polarizability and - by noncentrosymmetrical alignment - second order nonlinear optical properties in poled films. The second direction of interest is related to the photoinduced isomerization between the trans (more stable) and cis configurations of the azobenzene. This photoisomerization has a wealth of unexpected and useful consequences which were noted only in the last decade. Both these directions appeared because of the availability of lasers, which on the one hand allowed the observation of the nonlinear optical properties and on the other hand... 

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. 

Azobenzenes do not emit noticeably at room temperature, but all three types show fluorescence in strong acids at 77K." Modern experimental techniques, however, allow one to study the very weak fluorescence of azobenzene and azobenzene functionalized molecules even at room temperature. Corner et al. observed a transient after laser flash photolysis of donor/acceptor-substituted azobenzenes which they assigned to the lowest energy triplet state of the E-isomer. Recent studies were directed at examining the behavior of the transient absorption of the singlet excited states of both geometric isomers with respect to time (see Section 89.4). 

Contrary to the Z-forms of protonated azobenzenes, the Z-forms of donor/acceptor di-substituted azobenzenes isomerize very quickly at room temperature. To investigate this reaction, the Z-form is produced by flash excitation. The isomerization of these pseudo-stilbenes is strongly dependent on the polarity of the solvent. " For instance, 4-diethyIamino-4 -nitroazobenzene isomerizes with k g = 0.007 s in hexane, but with kz, g = 600 s in N-methylformamide. 4-Anilino-4 -nitroazobenzene in cyclohexane obeys the relation In kz, E = 22( 3) - 72( 9)T0V(8.314 T). Schanze et al. " have established two linear relationships between the free-activation enthalpy of isomerization and the Kosower Z values of aprotic and protic solvents. Sanchez and de Rossi report a strong pH sensitivity of... 

Other compounds contaiimig a nitrobenzene unit like substituted benzamides ", azobenzenes, A -benzyMeneanilinesi" and donor-acceptor cyclophanes " have been investigated by PES. The PE spectrum of l-methyl-3-nitro-2-phenylindole (68) has been measured . 

Three novel organic optical materials, 4 -(N,N-dihydroxyethylamino)-4-(pyridine-4-vinyl) stilbene, N- ((4- N, N-dihydroxyethylamino)benzylidene) -4- (pyridine-4-vinyl) aniline, and 4 -(N,N-dihydroxyethylamino)-4-(pyridine-4-vinyl)azobenzene, were synthesized [105]. Tolunitriles reacted vdth donor-substituted aromatic aldehydes in high yielding reactions of the synthesis of donor-acceptor cyanostilbenes without the need of inert atmosphere. The keys to this reaction were the use of anhydride DMF solvent and the phase transfer agent - tris(3,6-dioxaheptyl)amine (TDA). High yields of stilbenes were also obtained with amino-substituted aromatic aldehydes [106]. 

Electron-donor and electron-acceptor substituents selectively interact with different ring orbitals. Compare the HOMO and LUMO of azobenzene with the corresponding orbitals of the two substituted molecules. Which orbitals show signficant substituent contributions What are the nature of these contributions, bonding or antibonding Try to relate this to the effect which the substituents have on orbital energies and on the HOMO-LUMO gap in azobenzene. 

---