Solid-state photoswitching in arylazopyrazole-embedded polydimethylsiloxane composite thin films

Abstract

Arylazopyrazoles (AAPs) show significant potential as a new family of molecular photoswitches owing to their efficient reversible trans -to- cis photoisomerization behavior and the high thermal stability of their metastable cis -isomer. In this study, AAPs have been used for the fabrication of solid-state photoswitchable polydimethylsiloxane (PDMS) based composite thin films. The thin films were prepared by using PDMS as a polymer matrix and different concentrations of AAPs as chromophores at 150 ​°C via spin-coating. The photoswitching behavior of the AAPPDMS composite films and the pristine AAP samples induced by irradiation with specific wavelength of light were investigated. We found that the as prepared AAPPDMS composite films showed rapid and near-quantitative (>98%) reversible trans -to- cis isomerization upon alternating irradiation with UV (λ ​= ​365 ​nm) and green (λ ​= ​525 ​nm) light which is comparable to the isomerization behavior of the pristine AAP chromophores in solution. This indicates that the excellent photoswitching property of the AAPs is preserved in the solid-state of the AAP-PDMS composite films. The results also show that the optical properties of the AAP-PDMS composite films can be tuned by using different ratios of the AAP chromophores and exposure to UV-light irradiation. Additionally, the thin films were tested for their photo-actuation behavior by UV–vis spectroscopy. Irradiation of the thin film with alternating 365 ​nm UV and 525 ​nm green light lead to a slight reversible bending behavior. This is presumably caused by the light-induced conformational change of the AAP moiety embedded within the PDMS matrix and the soft nature of the PDMS. This methodology provides a new approach for exploring the fabrication of polymers with enhanced mechanical behavior and solidstate photoswitching properties. Arylazopyrazoles (AAPs) show significant potential as a new family of molecular photoswitches owing to their efficient reversible trans -tocis photoisomerization behavior and the high thermal stability of their metastable cis - isomer. In this study, AAPs have been used for the fabrication of solid-state photoswitchable polydimethylsiloxane (PDMS) based composite thin films. The thin films were prepared by using PDMS as a polymer matrix and different concentrations of AAPs as chromophores at 150 ​°C via spin-coating. The photoswitching behavior of the AAP-PDMS composite films and the pristine AAP samples induced by irradiation with specific wavelength of light were investigated. We found that the as prepared AAP-PDMS composite films showed rapid and near-quantitative (>98%) reversible trans -to- cis isomerization upon alternating irradiation with UV (λ ​= ​365 ​nm) and green (λ= 525 ​nm) light which is comparable to the isomerization behavior of the pristine AAP chromophores in solution. This indicates that the excellent photoswitching property of the AAPs is preserved in the solid-state of the AAP-PDMS composite films. The results also show that the optical properties of the AAP-PDMS composite films can be tuned by using different ratios of the AAP chromophores and exposure to UV-light irradiation. Additionally, the thin films were tested for their photo-actuation behavior by UV–vis spectroscopy. Irradiation of the thin film with alternating 365 ​nm UV and 525 ​nm green light lead to a slight reversible bending behavior. This is presumably caused by the light-induced conformational change of the AAP moiety embedded within the PDMS matrix and the soft nature of the PDMS. This methodology provides a new approach for exploring the fabrication of polymers with enhanced mechanical behavior and solid-state photoswitching properties.