Sensitivity and Resolution Development of Spiropyran‐based Molecular Photoswitches

Abstract

The phenylazo moiety and its donor‐ and acceptor‐substituted derivatives are studied as effective auxochromes to improve their sensitivity and resolution for distinguishing between the spiro (SP; OFF) and mero (ON) forms in molecular photoswitching applications. Thus, 13 azospiropyran derivatives were synthesized and their spectroscopic and photokinetic behaviors were studied. The quality of photochromic reactions of the synthesized photochromic compounds were compared using a dose–response model. Interestingly, by replacing the nitro group in 6‐nitrospiropyran (ε = 0.42 × 104 M−1 cm−1) with a simple phenylazo moiety, the SP form is still colorless and the color intensity of the merocyanine (MC) form is improved desirably by extending the conjugation length (1a, ε = 1.35 × 104 M−1 cm−1). The presence of a hydrophilic OH group or a CH3 group at the para position of phenylazo moiety revealed more or less the same photochromic properties as 1a. The OCH3 group substituted at position 6 of the phenylazo moiety at the para position of the azobenzene moiety effectively increased the photochromic properties with the maximum k‐value for SP to MC switching. Meanwhile, Cl, Br, COOH, and NO2 groups at the para position of the azobenzene moiety revealed the reduction in photochromic properties compared to 1a.

Reversible Phase Transition of Porous Coordination Polymers

“…Photoswitched materials have attracted much attention because they possess advantages of high spatial and temporal precision and are environmentally friendly.S ome photoswitchable molecules can transfer from one isomeric form to another through reversible photochemical reactions, such as azobenzene (AZB), [50] spiropyran, [51,52] vinyl, [53] and diarylethene groups. [54] Typically,t he AZB group can undergo reversible trans-cis isomerization upon UV irradiation or heat.…”

Monolithic Spiropyran-Based Porous Polysilsesquioxanes with Stimulus-Responsive Properties

“… 12 − 25 To name some recent examples, Sumaru et al reported the possibility to use spiropyran-functionalized poly( N -isopropylacrylamide) (pNIPAAm) for the recognition of cyclodextrin, 26 Cao et al described the possibility to capture or release cells on or from spiropyran-modified Janus particles by UV or visible light irradiation, 27 and several publications exploited the different colorations of SP for chemical drawing purposes or mechanochemical studies (e.g., in stress/strain sensing and damage reporting). 28 − 32 Further examples for SP-based dynamic materials can be found in an excellent review recently given by R. Klajn. 33 …”

“…In the SP form, both parts are not in conjugation and the isomer mainly absorbs in the UV region, whereas the isomer undergoes a 6-π electrocyclic ring opening reaction by an external stimulus and forms a highly colored merocyanine (MC) product, which can be protonated to the yellow colored MCH + isomer . In recent literature studies, a rising number of publications about the incorporation of SPs in diverse materials can be found. − To name some recent examples, Sumaru et al reported the possibility to use spiropyran-functionalized poly­( N -isopropylacrylamide) (pNIPAAm) for the recognition of cyclodextrin, Cao et al described the possibility to capture or release cells on or from spiropyran-modified Janus particles by UV or visible light irradiation, and several publications exploited the different colorations of SP for chemical drawing purposes or mechanochemical studies (e.g., in stress/strain sensing and damage reporting). − Further examples for SP-based dynamic materials can be found in an excellent review recently given by R. Klajn …”