Photoresponse of new azo pyridine functionalized poly(2-hydroxyethyl methacrylate-co-methyl methacrylate)

Abstract

A new azo polymer containing photoisomerizable azo pyridine functionalities was synthesized via Mitsunobu reaction of 4-(4-hydroxyphenylazo)pyridine with poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (p(HEMA-co-MMA)) for creating new photochromic materials. The resulting polymer with azo pyridine side groups was characterized for structural, thermal, and optical properties. UV-vis, ¹H NMR and IR spectroscopies confirmed that all hydroxyl groups in p(HEMA-co-MMA) were substituted with azo dye. The obtained azo copolymer exhibited high thermal stability (around 240 °C) and a glass transition temperature (113 °C), promising for applications. The trans-to-cis isomerization upon UV irradiation and the thermal back reaction of the azo chromophore in the copolymer in the solid state was studied. A photostationary state with 50% content of cis-isomers upon 6 min of UV irradiation was reached, and during 48 h dark relaxation at ambient temperature, all cis-isomers converted to the trans form. Additionally, the possibility of efficient photogeneration of surface relief gratings with high amplitude of azo copolymer surface modulation was demonstrated.

Figure 1

Chemical synthesis route of azo(HEMA-co-MMA).

Figure 2

Normalized UV–Vis curves for azo(HEMA-co-MMA) in NMP solution (c = 10^–5 mol/l) and the solid state. Thickness of the film was ca. 300 nm.

Figure 3

Curves of cis–trans isomerization for azo(HEMA-co-MMA) (a) whole range of spectra; (b) in the range of absorption characteristic for π-π*. The 365 nm excitation radiation was marked as a dashed line. (c) Normalized content of cis-isomer in the function of time.

Figure 4

Evolution of the 0th, + 1st, -2nd order diffraction efficiencies of the SRG inscribed in the azo(HEMA-co-MMA) layer during irradiation with a light polarization pattern.

Figure 5

(a) The topography profile of the azo(HEMA-co-MMA) surface measured after sample irradiation with a light polarization pattern. (b) The optical microscopy image referring to the center of the generated SRG. (c) The photograph of the sample surface with the inscribed SRG observed under white light; a piece of graph paper on the left provides the scale.

Figure 6

Photograph of diffraction pattern (bottom row) observed on the screen behind the azo(HEMA-co-MMA) when the 690 nm beam was incident on the 1200 nm-deep SRG compared to the photograph of the non-diffracted 690 nm beam spot (top row) observed on the screen when the probe beam was incident on the sample outside the SRG area.