Investigation of thermochromism in a series of side-chain, liquid-crystalline, azobenzene-containing polymers

Abstract

The thermochromic properties of a series of liquid crystalline polymethacrylates, containing azobenzene side-chains with variable spacer lengths, were investigated. Annealing the amorphous polymer thin films above the glass transition temperature results in a rearrangement of the azobenzene moieties, causing a hypsochromic shift in the electronic absorption spectra. A detailed investigation of the spectral shift was performed by in situ UV–vis spectroscopy and indicated the formation of H-type aggregates by the side-chain chromophores above the glass transition temperature. The rate at which the hypsochromic shift occurs is faster for polymers with shorter spacers since their high glass transition temperature results in a higher thermal energy during the thermochromic effect. Experimentally determined activation energies show that the aggregation occurs primarily due to side-chain relaxation (β-relaxation) and main-chain relaxation (α-relaxation). Further annealing above the isotropization temperature resulted in the onset of deaggregation and in most cases showed that the chromophores were freed from the ordered state.Key words: liquid-crystalline polymer, thermochromic properties, chain relaxation, aggregation, thin films, azobenzene mesogens.

Tailoring Linear and Nonlinear Optical Properties of a Side‐Chain Liquid Crystalline Azo‐Polymethacrylate

“…The polymer film spectrum could be roughly described by a band at about 430 nm associated with trans isomers (analogous to the one in solution) and an extra absorption band with maximum at about 380 nm. Similar absorption spectra, with a peak in the higher energy side of the band associated with the trans isomers (430 nm), have been reported by several authors and assigned to anti‐parallel H azo aggregates 26–30. So, we associate the absorption band with the peak at about 380 nm present in the film spectrum with the aggregation of azo groups.…”

“…Similar absorption spectra, with a peak in the higher energy side of the band associated with the trans isomers (430 nm), have been reported by several authors and assigned to anti-parallel H azo aggregates. [26][27][28][29][30] So, we associate the absorption band with the peak at about 380 nm present in the film spectrum with the aggregation of azo groups. Figure 3a also shows the vertical and horizontal polarized absorption spectra of the film obtained at normal (08) and oblique (458) incidence.…”

Photoinduced Chiral Nematic Organization in an Achiral Glassy Nematic Azopolymer

“…[37] The spectra of the thin films can be fitted with Gaussian-type absorption bands, keeping all parameters constant except the amplitudes of the peaks. [37][38][39] The relative amounts of the chromophores in the different arrangements were determined as the areas of the corresponding peaks and are quoted as percentages in Figure 3. The peak fitting procedure is not a real quantitative determination but is a valuable tool to determine relative amounts of the aggregates.…”

Electronic Spectroscopy of a C7H4+ Isomer in a Neon Matrix: Methyltriacetylene Cation

“…A recent example of this has been the investigation of the electronic spectra of C n H 4 þ (n ¼ 4, 6, 8) isomers and some of their neutral counterparts in neon matrices. [15,16] In this contribution, the technique has been applied to characterise a C 7 H 4 þ isomer. The ions are produced in a low voltage discharge through a mixture of diacetylene and propyne.…”