Structure-property relationship and photoinduced birefringence of the azo and azo-azomethine dyes thin films in PMMA matrix
Introduction
The materials containing > A = B< chromophore groups are a class of optical materials extensively investigated in the past decade due to their potential applications in optical information storage devices, as optical switches and sensors, in polarization holography, and in photonics [[1], [2], [3]]. Depending on the structure of the chromophore groups, they are classified as (Fig. 1): (i) diazobenzenes containing N=N- group; (ii) azomethines with CH=N- group; (iii) stilbenes containing > C=C< bond. The photophysical properties of the azobenzene derivatives (ABs), for example containing N=N- (azo) chromophore group undergoes reversible trans-cis-trans photoisomerization cycle. The main characteristic UV-VIS absorption bands are near at λmax ∼440 nm related to the n→π* transition or S1 excited state and at λmax ∼340 nm related to the π→π* transition or S2 excited state [[1], [2], [3], [4], [5]].
It is well known that most azobenzenes isomerize and exhibit the effect of photoorientation. Due to photoorientation with linearly polarized light the azobenzene molecules preferentially absorb light polarized along the long axis of the molecule. In practice, this means that the absorption of molecules perpendicular to the polarization axis of the incident light is insignificant compared to the molecules positioned along the axis. Repeated cycling between the trans (E) and cis (Z) isomers, which is highly efficient in azo chromophores, results in reorientation perpendicular to the polarization direction of incident light [[6], [7], [8], [9]]. Generally, one of the most popular technologies used for recording of information is the one based on induction and subsequent detection of the changes generated by external agents in the magnetic or optical properties of the storage media [10]. In the case of azo thin films, the role of an external agent is played by the light illuminating them at appropriate wavelength, generating changes in the refractive index or in the coefficient of absorption as a result of photoizomerization of the azobenzene molecule accompanied by molecular orientation. In a typical kinetic curve showing the variation of the photoinduced birefringence in thin films of azo compounds under laser irradiation the following stages can be distinguished: the writing process (phase A), the relaxation of the chromophores (phase A-B) and the erasing process (phase B) achieved by illuminating with circularly polarized light or by heating. Before starting the illumination with the writing beam the azo material contains highly asymmetric molecules which are inherently anisotropic. As they are oriented randomly and all orientations are equally probable, the entire layer is isotropic. On irradiation with linearly polarized light (during phase A) the azo molecules isomerize from trans (E) to cis (Z) and orient predominately perpendicular to the polarization of the incident light inducing anisotropy of the index of refraction (known as birefringence, Δn). In phase A the photoinduced birefringence increases with the time of exposure until reaching certain saturation level (Δnmax). An exponential growth of the refractive index modulation (ΔnA) can be observed (eq. (1)):where Δnmax is the maximal value of the induced birefringence during phase A, t is the time between turning on and off the writing beam (t0 ≤ t ≤ t1), t0 is the time at which the writing beam has been turned on, t1 is the time of switching off the acting light and τn,1 is a characteristic time constant. The time required to reach the maximum birefringence during phase A depends on several factors such as the size of the azobenzene molecules, the thickness of the film, the wavelength of the excitation light and its intensity. The series of trans-cis-trans isomerizations continue until all the molecules are oriented perpendicular to the polarization of the incident light. In phase A-B some of the azo molecules undergo cis-trans isomerization due to photoinduced relaxation processes and interactions with the host polymer for example. In this phase an exponential decrease of Δnmax to ΔnPlateau is observed (eq. (2)).where ΔnPlateau is the attained during phase A-B stable stationary level of the photoinduced birefringence and τn,2 is a characteristic time constant.
In phase B, after illuminating with circularly polarized light, the molecules return to their initial, isotropic state and the birefringence kinetics can be described mathematically by an exponential decay (eq. (3)):where t2 is the time of turning on the erasing beam and τn,3 is a characteristic time constant [11,12].
Such optically induced birefringence can be observed by measuring the polarization state of a probe beam that passes through the azo and azomethine film. In our previous investigation three 4-aminoazobenzene (Azo) and three azo-azomethine (AAM) dyes have been theoretically and spectrally characterized in order to understand their photochromic behaviour [13,14]. In the present work we have prepared composite “host-guest” films, where the host is PMMA polymer and the “guest” is the dye. The photoinduced birefringence of these “host-guest” films was investigated using pump lasers with wavelengths of 355 nm and of 442 nm in the region of the absorbance band of the chromophores used. Due to the presence of two chromophore groups in the molecular backbone, the AAM dyes exhibit potentially useful physical and photochromic properties. The vapour deposited 100 nm nanosized films were analysed through UV-VIS and Fluorescence spectroscopy with the aim of finding application of the produced Azo and AAM thin films in PMMA matrix as data storage devices.
Section snippets
Materials used
The synthesis of the 4-aminoazobenzenes (Azo-1, Azo-2 and Azo-3) and azo-azomethines (AAM 1, AAM 2 and AAM 3) was described in our previous works [13,14]. The PMMA and DCM were purchased from Sigma Aldrich. The structures of the dyes are presented on Fig. 2.
Vapour deposition of nanosized films
The thin films of Azo and ААМ dyes were prepared by evaporation from Knudsen type vessels on quartz substrates with vacuum pressure 6 × 10−6 mbar. The temperature of evaporation was kept between 80 and 100 °C. Temperature of substrate was
Spectral characterization
The spectral features of the photoactive N=N- and CH=N- groups depend on the nature of substitutes in the aromatic rings. The electron withdrawing (EW) and electron donating (ED) groups cause unsymmetrical electron (“push-pull”) distributions on the molecular backbone, resulting in decrease the energy of the π→π* and n→π* electron transitions. In order to reduce the physical interaction between the molecules (π-π stacking, intermolecular bonding, etc.) nanosized vapour deposited films of the
Conclusions
The spectral features of the dyes were evaluated by UV-VIS and Fluorescence spectroscopy as vapour deposited 100 nm nanosized films. Absorption spectra have shown that π→π* transitions of the N=N- and CH=N- groups appear in the range 340–380 nm. The presence of electron withdrawing (EW) and electron donating (ED) groups cause unsymmetrical electron (“push-pull”) distributions on the molecular backbone, resulting in decrease of energy of the π→π* and n→π* electron transitions. This indicates
Conflicts of interest
The authors declared that the article content has no conflicts of interest.
Acknowledgments
This work was financial supported by the Bulgarian National Scientific Fund project ДН 08/10 of the Ministry of Education and Science, Bulgaria.
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