Thermal Z-E isomerization of azobenzenes. The pressure, solvent, and substituent effects

Abstract

The rates of thermal Z-E isomerization of 4-(dimethylamino)-4'-nitroazobenzene were measured in a variety of solvents, and the Kirkwood plot revealed that the rate constant increase is much larger in polar solvents than expected from the continuum theory. The result can be reasonably explained by the intervention of rotational isomerization which proceeds via a highly polar transition state. The large negative activation volumes support it. It is also shown that a strong electron-donating dialky lamino group makes the rotational transition state stable enough to compete with the inversional one without assistance from the electron-attracting substituent. On the contrary, an electron-attracting group like nitro increases the stability of the inversion transition state more than that of the transition state for the rotational isomerization.

Kinetics of the Z-E isomerization of monosubstituted azobenzenes in polar organic and aqueous micellar solvents

“…However, the presence of only one strong electron-donating group, such as the dialkylamino group, bonded to one phenyl ring is sufficient, 11 in polar solvents, to make the isomerization via rotation competitive with the isomerization via inversion. This changeover of the mechanism of Z-E isomerization upon changing the substituents on the phenyl ring has been confirmed 12 by nonlinear Hammett plots.…”

Molecularly Imprinted Polymers with Stimuli-Responsive Affinity: Progress and Perspectives

“…Those photo-sensitive materials commonly contain photosensitive chromophores, such as azobenzene [106], stilbene [107], spiropyran and spitooxazines [108] and fugides [109]. Amongst the numerous photosensitive chromophores, the UV-visible photo-induced trans-cis isomerization of azobenzene and its derivatives is probably the most extensively studied for its chemical stability, large change in dipole [110], and high quantum yield [111]. The azobenzene chromophore exhibits two isomeric states, the thermodynamically more stable trans-isomer and the meta-stable cis-isomer.…”

Nonlinear Absorption of Light in Materials with Long-lived Excited States

“…If these molecules don't return to their ground state immediately, when new photons fall on the sample they can't be absorbed anymore in the initial layers and therefore propagate through the sample with a sub-exponential law. So, the effective photo-bleaching of the first layers allows a further propagation of light into the sample and this leads to nonlinear phenomena which are interesting both from the theoretical [Andorn 1971, Berglund 2004, Statman & Janossi 2003, Corbett & Warner 2007 and from the experimental point of view [Meitzner & Fischer 2002, Barrett et al 2007, Van Oosten et al 2005, Van Oosten et al 2007]. The aim of this chapter is to explore the effect that this phenomenon has on the typical absorption measurements which are commonly performed on these kinds of molecules.…”

“…The rate constants of these two processes are usually different, thermal isomerisation being slower. The microscopic mechanism that leads to the isomerisation is still not clear, but there are suggestions for both rotational and inversional [Asano & Okada 1984] mechanisms may be competing. The isomerisation of azobenzene can be monitored by UV-Vis (ultraviolet and visible light) spectroscopy, because the two trans and cis compounds have different absorption spectra in this range: the trans isomer absorbs around 365 nm, while the cis isomer at around 440 nm [Rau 1990].…”