Involving Synergy of Green Light and Acidic Responses in Control of Unimolecular Multicolor Luminescence

Azulene as an ingredient for visible-light- and stimuli-responsive photoswitches

The azulene molecule features a unique combination of optical, luminescence, and stimuli-responsive properties. This makes the azulene motif a promising functional group to be introduced in photoswitches. Recent challenges in the field of photochromic compounds require the development of new approaches to molecules that are switched by visible light (400-760 nm), are proton responsive and have advanced luminescent properties. Merging azulene with photoswitches opens prospects for fulfilling these requirements. Herein, we highlight recent results on the application of this hydrocarbon motif in various photochromic systems, such as stilbenes, diarylethenes, and azobenzenes.

Azulene-A Bright Core for Sensing and Imaging

Azulene is a hydrocarbon isomer of naphthalene known for its unusual colour and fluorescence properties. Through the harnessing of these properties, the literature has been enriched with a series of chemical sensors and dosimeters with distinct colorimetric and fluorescence responses. This review focuses specifically on the latter of these phenomena. The review is subdivided into two sections. Section one discusses turn-on fluorescent sensors employing azulene, for which the literature is dominated by examples of the unusual phenomenon of azulene protonation-dependent fluorescence. Section two focuses on fluorescent azulenes that have been used in the context of biological sensing and imaging. To aid the reader, the azulene skeleton is highlighted in blue in each compound.

Proton-Gated Ring-Closure of a Negative Photochromic Azulene-Based Diarylethene

Proton-responsive photochromic molecules are attractive for their ability to react on non-invasive rapid optical stimuli and the importance of protonation/deprotonation processes in various fields. Conventionally, their acidic/basic sites are on hetero-atoms, which are orthogonal to the photo-active π-center. Here, we incorporate azulene, an acid-sensitive pure hydrocarbon, into the skeleton of a diarylethene-type photoswitch. The latter exhibits a novel proton-gated negative photochromic ring-closure and its optical response upon protonation in both open and closed forms is much more pronounced than those of diarylethene photoswitches with hetero-atom based acidic/basic moieties. The unique behavior of the new photoswitch can be attributed to direct protonation on its π-system, supported by 1H NMR and theoretical calculations. Our results demonstrate the great potential of integrating non-alternant hydrocarbons into photochromic systems for the development of multi-responsive molecular switches.

Engineering stable radicals using photochromic triggers

Long-standing radical species have raised noteworthy concerns in organic functional chemistry and materials. However, there remains a substantial challenge to produce long-standing radicals by light, because of the structural dilemmas between photoproduction and stabilization. Herein, we present a pyrrole and chloride assisted photochromic structure to address this issue. In this well-selected system, production and stabilization of a radical species were simultaneously found accompanied by a photochemical process in chloroform. Theoretical study and mechanism construction indicate that the designed π-system provides a superior spin-delocalization effect and a large steric effect, mostly avoiding possible consumptions and making the radical stable for hours even under an oxygen-saturated condition. Moreover, this radical system can be applied for a visualized and quantitative detection towards peroxides, such as 2,2,6,6-tetramethylpiperidine-1-oxyl, hydrogen peroxide, and ozone. As the detection relies on a radical capturing mechanism, a higher sensing rate was achieved compared to traditional redox techniques for peroxide detection.

Long-standing radical species have raised noteworthy concerns in organic chemistry and but there remains a substantial challenge to produce long-standing radicals by light. Here, the authors demonstrate a stable dithienylethene derived photochromic radical for detection of peroxides and ozone.

Azulene-Derived Fluorescent Probe for Bioimaging: Detection of Reactive Oxygen and Nitrogen Species by Two-Photon Microscopy

Two-photon fluorescence microscopy has become an indispensable technique for cellular imaging. Whereas most two-photon fluorescent probes rely on well-known fluorophores, here we report a new fluorophore for bioimaging, namely azulene. A chemodosimeter, comprising a boronate ester receptor motif conjugated to an appropriately substituted azulene, is shown to be an effective two-photon fluorescent probe for reactive oxygen species, showing good cell penetration, high selectivity for peroxynitrite, no cytotoxicity, and excellent photostability.