Photoswitching Using Visible Light: A New Class of Organic Photochromic Molecules

Abstract

A versatile new class of organic photochromic molecules that offers an unprecedented combination of physical properties including tunable photoswitching using visible light, excellent fatigue resistance, and large polarity changes is described. These unique features offer significant opportunities in diverse fields ranging from biosensors to targeted delivery systems while also allowing non-experts ready synthetic access to these materials. In the field of adaptable and responsive materials, the ability of organic photochromic compounds to reversibly undergo changes in spectral absorption, volume, and solubility is of particular importance for applications in energy storage and chemical sensing and for controlling the conformation and activity of biomolecules.1 These switches are particularly valuable because their property changes are triggered by light, the most widely available, non-invasive, and environmentally benign external stimulus. Significantly, light also provides unique opportunities for spatial and temporal resolution.Among the classes of organic photochromic materials, azobenzenes, spiropyrans and diarylethenes have received the most attention because of their excellent performance and broad utility. Specifically, azobenzene has been extensively employed for its change in volume, resulting from a trans to cis isomerization, upon irradiation.2 Similarly, the change in spectral properties of spiropyrans and diarylethenes upon photoswitching has been exploited in a number of applications, 3 with spiropyran exhibiting the added benefit of a solubility switch, or a conversion from a hydrophobic to a hydrophilic form, upon irradiation. 4 Despite their ubiquity and broad utility, these privileged classes of photochromes typically all require the use of high-energy UV light to trigger their photochemical reactions. This hinders their potential use in biomedical applications and material science because UV light can be damaging to healthy cells and results in degradation for many macromolecular systems. Fatigue resistance is also a primary concern for UV-based photochromic switches.A common design principle to address this problem is to make synthetic modifications to these known classes of photochromic compounds that enable the use of visible light.5 Herein, we describe a conceptually different approach in which we designed a new class of visible light activated photochromes, termed donor−acceptor Stenhouse adducts (DASAs).6 These derivatives switch from a conjugated, colored, and hydrophobic form to a ring-closed, colorless, and zwitterionic structure on irradiation with visible light and have high fatigue resistance under ambient conditions ( Figure 1). In addition, the potential of this photoswitch in materials science is highlighted through the synthesis of a functional amphiphile that displays light-mediated micelle disassembly and cargo release.Our interest in the cascade rearrangements of activated furans 7 and a pioneering report by Honda 8 played a critical role in our development of t...

Tunable Visible and Near Infrared Photoswitches

“…In initial work, we described DASAs containing alkyl-based amine "donors" (electron-rich) and Meldrum's or barbituric acid "acceptors" (electron-deficient) that display visible light absorption, high fatigue resistance, and a significant polarity and color change upon switching. 32 However, with these first-generation DASAs, wavelength tunability was limited to 545 and 570 nm for the Meldrum's and barbituric acid derivatives, respectively. 31 Additionally, reversible photoswitching was only possible in nonpolar solvents such as toluene, and reversible switching could not be observed in solid-supported matrices.…”

Solvent Effects on the Actinic Step of Donor–Acceptor Stenhouse Adduct Photoswitching

“…[7][8][9][10][11][12] Applications differ markedly in the environment the photoswitch is exposed to,b ei td ifferent solvents, [13,14] matrices or surfaces,a nd understanding how ag iven photoswitch behaves in various environments is crucial for its success in any applications. Donor-acceptor Stenhouse adducts (DASAs,F igure 1a) were introduced in 2014 [17,18] and feature important advantages as compared to traditional photoswitches,i ncluding visible light responsiveness [11,19,20] and negative photochromism. [21] Moreover,t heir modular architecture [22] allows for afine-tuning of properties.…”

A temperature-mapping molecular sensor for polyurethane-based elastomers

“…7,8 The molecule is shown in Figure 1(a) and behaves as an inverse photoswitch. The closed, colorless form is zwitterionic and upon heating opens to a conjugated, colored system.…”