Photochromism of a Diarylethene Having an Azulene Ring

Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha's Rule

Fluorescence exclusively occurs from the lowest excited state of a given multiplicity according to Kasha's rule. However, this rule is not obeyed by a handful of anti-Kasha fluorophores whose underlying mechanism is still understood merely on a phenomenological basis. This lack of understanding prevents the rational design and property-tuning of anti-Kasha fluorophores. Here, we propose a model explaining the photophysical properties of an archetypal anti-Kasha fluorophore, azulene, based on its ground- and excited-state (anti)aromaticity. We derived our model from a detailed analysis of the electronic structure of the ground singlet, first excited triplet, and quintet states and of the first and second excited singlet states using the perturbational molecular orbital theory and quantum-chemical aromaticity indices. Our model reveals that the anti-Kasha properties of azulene and its derivatives result from (i) the contrasting (anti)aromaticity of its first and second singlet excited states (S1 and S2, respectively) and (ii) an easily accessible antiaromaticity relief pathway of the S1 state. This explanation of the fundamental cause of anti-Kasha behavior may pave the way for new classes of anti-Kasha fluorophores and materials with long-lived, high-energy excited states.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

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.

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.

Development of Heterocycle-Substituted and Fused Azulenes in the Last Decade (2010-2020)

Azulene derivatives with heterocyclic moieties in the molecule have been synthesized for applications in materials science by taking advantage of their unique properties. These derivatives have been prepared by various methods, involving electrophilic substitution, condensation, cyclization, and transition metal-catalyzed cross-coupling reactions. Herein, we present the development of the synthetic methods, reactivities, and physical properties for the heterocycle-substituted and heterocycle-fused azulenes reported in the last decade.

Cyclization from Higher Excited States of Diarylethenes Having a Substituted Azulene Ring

The cyclization reaction of diarylethenes having an azulene ring occurs only via higher excited states. Novel diarylethenes having an azulene ring with a strong donor or acceptor were synthesized and examined in these reactions. A derivative having an electron-donating 1,3-benzodithiol-2-ylidenemethyl group at the 1-position of the azulene ring showed photochromism, whereas neither a derivative having a π-conjugated electron-donating group at the 3-position of the azulene ring nor derivatives having a π-conjugated electron-withdrawing group at the 1- or 3-position of the azulene ring showed any photochromism. The photoreactivities of these compounds were explained by calculating forces and bond orders on the excited states using density functional theory (DFT) and time-dependent (TD)-DFT.

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.

Organic co-crystals of 1,3-bis(4-pyridyl)azulene with a series of hydrogen-bond donors

1,3-Bis(4-pyridyl)azulene has been employed as a hydrogen bond acceptor to construct two-component organic cocrystals.

Synthesis of azulene-substituted benzofurans and isocoumarins via intramolecular cyclization of 1-ethynylazulenes, and their structural and optical properties

The preparation of azulene-substituted benzofurans and isocoumarins was established by two types of intramolecular cyclization reaction of 1-ethynylazulenes.

Poly[2(6)-aminoazulene]: synthesis, photophysical properties, and proton conductivity

Dimeric aminoazulene and poly[2(6)-aminoazulene] are synthesized by Buchwald–Hartwig coupling of the corresponding monomeric carbamatoaminozulenes followed by hydrolysis.

Optical Properties of Diarylethenes with TD-DFT: 0-0 Energies, Fluorescence, Stokes Shifts, and Vibronic Shapes

This contribution is an investigation of both the structures and optical properties of a set of 14 diverse, recently synthesized diarylethenes using Time-Dependent Density Functional Theory (TD-DFT) at the ωB97X-D/6-31G(d) level of theory. The linear response (LR) and state-specific (SS) versions of the Polarizable Continuum Model (PCM) have been adopted to account for the bulk solvation effects and their relative performances were critically accessed. It is shown, for the first time in the case of nontrivial diarylethenes, that TD-DFT provides good agreement between the experimental absorption-fluorescence crossing points (AFCPs) and their theoretical counterparts when a robust model accounting for both geometrical relaxation and vibrational corrections is used instead of the vertical approximation. On the other hand, the theoretical estimates for the Stokes shifts based on the vertical transition energies were found to be in disagreement with respect to experiment, prompting us to simulate the absorption/emission vibronic band shapes. It is proved that difficulties associated with the breakdown of the harmonic approximation in Cartesian coordinates exist for the investigated system, and we show how they can be at least partially overcome by means of a vertical approach including Duschinsky effects. Our results provide a valuable basis to rationalize the experimental vibronic structure of both emission and absorption bands and are expected to be a significant asset to the understanding of the optical properties of diarylethene derivatives.

A new multi-addressable molecular switch based on a photochromic diarylethene with a 6-aryl[1,2-c]quinazoline unit

A novel diarylethene with a 6-aryl[1,2-c]quinazoline unit has been synthesized via a nucleophilic reaction. Its photochromism and fluorescence exhibited multi-addressable behaviors by the stimulation of both light irradiation and acid/base. Addition of trifluoroacetic acid to the solution of the diarylethene resulted in notable absorption spectral change, and the protonated form also possessed excellent photochromic properties. Meanwhile, its emission intensity was enhanced remarkably and the emission peak redshifted with a notable color change from dark blue to bright green. The changes could be recovered to the initial state by neutralizing with triethylamine. Consequently, a logic circuit was constructed with the diarylethene by using the fluorescence intensity at 482nm as output and acid/base as inputs.

Synthesis of 2- and 6-thienylazulenes by palladium-catalyzed direct arylation of 2- and 6-haloazulenes with thiophene derivatives

Preparation of 2- and 6-thienylazulene derivatives was established by the palladium-catalyzed direct cross-coupling reaction of 2- and 6-haloazulenes with the corresponding thiophene derivatives in good yield.

Visible-light triggered photoswitching systems based on fluorescent azulenyl-substituted dithienylcyclopentenes

Novel azulenyl functionalized dithienylethenes were synthesized, and their photochromic reactivity, fluorescent and electrochemical properties were investigated.

Perfluorocyclohexene bridges in inverse DiArylEthenes: synthesis through Pd-catalysed C-H bond activation, experimental and theoretical studies on their photoreactivity

The palladium-catalysed direct di-heteroarylation of 1,2-dichloroperfluorocyclohexene with a variety of heteroarenes gives rise in to a new family of 1,2-di(heteroaryl)perfluorocyclohexenes. These derivatives do not exhibit photoreactivity and this unexpected outcome is explained by calculations demonstrating the lack of reactive isomers.

Potential energy surfaces and quantum yields for photochromic diarylethene reactions

Photochromic diarylethenes (DAEs) are among the most promising molecular switching systems for future molecular electronics. Numerous derivatives have been synthesized recently, and experimental quantum yields (QYs) have been reported for two categories of them. Although the QY is one of the most important properties in various applications, it is also the most difficult property to predict before a molecule is actually synthesized. We have previously reported preliminary theoretical studies on what determines the QYs in both categories of DAE derivatives. Here, reflecting theoretical analyses of potential energy surfaces and recent experimental results, a rational explanation of the general guiding principle for QY design is presented for future molecular design.