Combined Photopolymerization and Localized Photochromism by Aza-Diarylethene and Hemiindigo Synergy

Abstract

Molecular photoswitches produce light-controlled changes at the nanometer scale and can therefore be used to alter the states and behavior of materials in a truly bottom-up fashion. Here an escalating photonic complexity of material property control with light is shown using a recently developed aza-diarylethene in combination with hemiindigo (HI) photoswitches. First, aza-diarylethene can be used as a photoswitch in polystyrene (PS) to reversibly inscribe relief-type 3D structures into PS. Second, aza-diarylethene can further be used as a photoinitiator for light-induced polymerization of methyl acrylate (MA), demonstrating for the first time light-controlled chemical reactivity control with its zwitterionic switching state. Third, aza-diarylethene and HIs are implemented into aza-diarylethene polymerized MA, generating photochromic polymers. At the fourth level, a binary mixture allows to synergize aza-diarylethene-induced photopolymerization with localized photochromism changes of the simultaneously entrapped functional HI. With such multilevel light response, the utility of this particular photoswitch combination for applications in advanced photonic materials is demonstrated.

Keywords: aza‐diarylethene; hemiindigo; light‐responsive materials; photochromism; photopolymerization.

Figure 1

Advanced light‐controlled materials using HIs 1–8 and aza‐diarylethene 9 as photoresponsive components. a) Schematic overview of HI 1–8 switching modes. b) Schematic overview of aza‐diarylethene 9 switching modes. c) Four levels of advanced photoresponsive materials using HI and aza‐diarylethene. Level one: Mixing of aza‐diarylethene 9 with polystyrene (PS) enables reversible photochromic inscription and 3D relief imprinting. Level two: Photopolymerization of methyl acrylate (MA) using aza‐diarylethene 9 as photoinitiator. Level three: Sequential multi‐photoresponses by first photopolymerization of MA with aza‐diarylethene 9 followed by addition of HIs to the resulting poly‐(methyl)‐acrylate (PMA) delivering photochromic materials. Level four: Simultaneous multi‐photohotochromic behavior in a dual mixture of aza‐diarylethene 9, HI 7, and MA. Selective photopolymerization initiated with aza‐diarylethene 9 yields a colored transparent PMA polymer in which HI 7 photochromism is retained and localized.

Figure 2

Level one photoresponsive materials. a) The addition of aza‐diarylethene 9 to PS enables localized reversible photochromism and 3D relief inscription in a solid material. b) Photochromic polymer with 9 in PS showing reversible writing and erasing. c) Photoswitching of open 9 (black) to closed 9 isomers (yellow) in PS. d) Reversible photoinduced 3D relief formation upon photoswitching of 9. The FAU University Seal was inscribed into the material in a haptic way using a photomask.

Figure 3

Level two photoresponsive materials. a) Aza‐diarylethene 9 as photoinitiator for radical polymerization of MA. b) Irradiation of two samples with 365 nm light for 2 h 40 min (or with 365 nm + 450 nm light for 1 h 5 min). The left sample vial contains 0.72 mg of open 9 in 1 mL MA, and the right sample vial contains solely 1 mL MA. After the specified irradiation duration, the sample containing compound 9 shows the characteristic color of the closed Z‐9 isomer and is polymerized. The sample containing solely MA did not polymerize. After storing both samples for 14 h in the dark at 23 °C, a post‐curing effect happened to the compound 9 carrying sample. The sample containing solely MA did not show any changes.

Figure 4

Level three photoresponsive materials part 1: a) Sequential photopolymerization and photochromism introduction using aza‐diarylethene 9. Aza‐diarylethene 9 was used first as a photoinitiator for MA polymerization and subsequently fresh 9 was added again as photochrome to the obtained PMA. b) UV/vis spectra showing irradiation of open 9 (black) with light of 365 nm (260 mW) leads to closed Z‐9 generation (gray to yellow) followed by consumption of closed Z‐9 (yellow dotted) in the course of the polymerization progress until full polymerization occurred and only remaining open 9 (dark gray) is visible. The related UV/vis cuvette is also depicted, as well as a reference cuvette containing only MA after irradiation with light of 365 nm for 6 h 36 min. c) UV/vis absorption spectra recorded during photoswitching of open 9 (black) to closed Z‐9 (gray to yellow) in PMA with light of 275 nm during 1 h 48 min. After 3 h 23 min photodegradation occurred (gray dotted). d) Reversible T‐type photoswitching in PMA with light of 365 nm from open 9 to closed Z‐9 (yellow square) and reversion to open 9 after the addition of CH₂Cl₂ at 23 °C.

Figure 5

Level three photoresponsive materials part 2. a) Sequential MA photopolymerization using aza‐diarylethene 9 followed by photochromism introduction by adding HIs 1–7 to the finished PMA. UV/vis spectroscopic monitoring of the localized light responses of HI photoswitches 1–7 within photochromic PMA polymers is shown together with the corresponding photochromic PMA/HI polymers in petri dishes. The inner squares within the petri dishes contain enriched bathochromic isomers, while the surroundings contain the hypsochromic isomers. b) Schematic representations of the molecular structures of HIs 1–7. c) Photochromic polymer with HI 1. d) Photochromic polymer with HI 2. e) Photochromic polymer with HI 3. f) Photochromic polymer with HI 4. g) Photochromic polymer with HI 5. h) Photochromic polymer with HI 6. i) Photochromic polymer with HI 7.

Figure 6

Level four photoresponsive materials. a) MA photopolymerization using aza‐diarylethene 9 in the presence of HI 7, which retains its photochromic function and allows for spatially resolved light‐addressing. b) Upon 365 nm irradiation, 9 undergoes zwitterion formation. Prolonged irradiation unleashes radical formation that initiates MA polymerization via the initial attack of the Michael system. HI 7 is not affected during this polymerization process and stays fully functional proving its exceptional robustness. Reversible visible light photoswitching with 490 and 625 nm of 7 in the resulting polymer allows to inscribe and erase information afterwards several times.