The synthesis of aminoazobenzenes and the effect of intermolecular hydrogen bonding on their photoisomerization
Introduction
In the past years, azo dyes have been attracting intensive interest for their potential use in optical data storage [1], optical switching [2], polarization holography [3], [4], optical modulation [5], nonlinear optics [6], and photolabile surfactants [7]. Recently, most works are investigating the photoinduced motions resulting from photoisomerization of the azo moieties, which are connected to the side chain of the polymer or doped into the polymer matrix [8], [9].
It is well known that the substituents of azo chromophores play an important role in molecular motion during photoisomerization. Many efforts have been made to investigate the motion of azobenzene derivatives by introducing substituents, such as chloro atom into the 2′-position [1], naphthalene [10], [11] or carbazole moiety [12] into the azobenzene chromophore, even extending the chromophore by two azo moieties [13]. According to the classification by Rau [14], azobenzene derivatives can be divided into three groups based on their photochemical behavior. One of these is aminoazobenzene group. They possessed longer cis lifetimes than “pseudostilbene” group, although their extinction coefficient is smaller than that of pseudostilbenes. Consequently, aminoazobenzenes are also treated as good candidates for photoinduced birefringence materials when a blue or green laser is used as a pump source [14]. Moreover, potential hydrogen bondings in azobenzenes also influence their properties. Intramolecular hydrogen bonding (H-bonding), formed by introducing o-hydroxyl phenyl moiety into azobenzene, has significant effect on liquid crystalline properties [15]. It is also reported that azobenzenes with intramolecular H-bonding possess a longer cis isomer lifetime and have potential application in photoswitching [16].
Our attention was given to the intermolecular H-bonding effect on the photochemical behavior of aminoazobenzenes. In this paper, two kinds of aminoazobenzene derivatives Ia–c and IIa–c were designed and synthesized to understand the substituents' effect on the photoisomerization process (Scheme 1). The photoisomerization behaviors of the polymer matrix (PMMA) doped with Ia–c and IIa–c were investigated by varying the irradiating laser power and the doping concentration of azobenzenes. Furthermore, the substituent and concentration effects on the rate constant of photoisomerization are discussed.
Section snippets
Materials
4-Acetylaminoaniline, phenol, sodium nitrite, hydrochloric acid (36–38%), sodium hydroxide, tetrahydrofuran and chloroform-d were purchased from Beijing Chemical and Reagent Company. 3-Chloropropan-1-ol and 6-chlorohexan-1-ol were obtained from J&K Chemicals. All the reagents and solvents were used as-received without further purification.
Synthesis and characterizations
Compounds Ia–c and IIa–c used in this paper were synthesized according to previously reported procedure [17], [18]. 1H NMR spectra were recorded on a Varian
IR spectra
Infrared spectrum presented rich information about the azobenzene-doped PMMA film. As shown in Fig. 1A, Ib in KBr exhibited the N–H stretching vibration of aggregated amine at 3317 cm−1. The N–H band shifted to 3379 cm−1 in PMMA, which indicates the intermolecular H-bonding formation between amino group of Ib and C
O in PMMA. Similar H-bonding was reported between carbonyl group and hydroxyl group [19]. This H-bonding can be indirectly verified by a similar IR band of Ib in THF at 3358 cm−1
Dynamics
The predominant photochemistry of azobenzenes involves reversible photoisomerization from a thermally stable trans form to a cis form; the backward reaction can occur both photochemically and thermally (Eq. (1)). When the forward and backward reaction rates are equal, the system reaches a photostationary state at equilibrium.According to the dynamics function the integrated rate constant for approach to the equilibrium is given by Eq. (2).
Conclusion
Two series of azobenzenes were synthesized and their photoisomerization behaviors in the polymer matrix (PMMA) were investigated by UV–vis spectra. The rate constants were calculated according to dynamics equations for reversible photoisomerization. Aminoazobenzenes Ia–c exhibited faster photoisomerization and had larger integration rate constants than the corresponding acetylamino derivatives IIa–c. Azobenzenes with a longer alkyl chain (Ic, IIc) showed a faster photoisomerization rate than
Acknowledgements
We acknowledge financial support from “One Hundred Oversea Talent” program of Chinese Academy of Sciences (CAS) and CREST program, Japan Science and Technology Agency (JST).
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