Multicolor Chromism from a Single Chromophore through Synergistic Coupling of Mechanochromic and Photochromic Subunits

We report a versatile mechanophore exhibiting a vividly detectable, light-regulable multicolor mechanochromism. Such optical features rely on the synergistic coupling of mechanochromic bis-rhodamine (Rh) and photochromic bisthienylethene (BTE). Poly(methyl acrylate)s incorporating this bis-mechanophore can be mechanically activated under sonication. The relative distribution of the two distinctly colored and fluorescent Rh ring-opening products is altered with different magnitudes of applied force. Orthogonal use of the photochromic reaction of the BTE core can strengthen the mechanochromism and gate the mechanofluorescence in polymers. Due to increased conjugation offered by the BTE linker, both force- and light-induced optical signals display high contrast. Combined DFT simulated and experimental results reveal that the three subunits (two Rhs and one BTE) in this chromophore are activated sequentially, thus generating switchable three-colored forms and gradient optical responses.

Competitive Photoisomerization and Energy Transfer Processes in Fluorescent Multichromophoric Systems

Multichromophoric systems showing both fluorescence and photoisomerization are fascinating, with complex interchromophoric interactions. The experimental and theoretical study of a series of compounds, bearing a variable number of 4-dicyanomethylene-2-tert-butyl-6-(p-(N-(2-azidoethyl)-N-methyl)aminostyryl)-4H-pyran (DCM) units are reported. The photophysical properties of multi-DCM derivatives, namely 2DCM and 3DCM, were compared to the single model azido-functionalized DCM, in the E and Z isomers. The (EE)-2DCM and (EEE)-3DCM were synthesized via the click reaction. Steady-state spectroscopy and photokinetics experiments under UV or visible irradiation indicated the presence of intramolecular energy transfer processes among the DCM units. Homo- and hetero-energy transfer processes between adjacent chromophores were confirmed by fluorescence anisotropy and decays. Molecular dynamics simulations for 2DCM were carried out and analyzed using a Markov state model, providing geometrical parameters (orientation and distance between chromophores) and energy transfer efficiency. This work contributes to a better understanding and rationalization of multiple energy transfer processes occuring within multichromophoric systems.

Rewritable acidochromic papers based on oxazolidine for anticounterfeiting and photosensing of polarity and pH of aqueous media

Oxazolidine is a new category of stimuli-chromic organic compounds with unique characteristics in response to polarity, pH changes, water, light, and metal ions that were well-known as solvatochromism, acidochromism, hydrochromism, photochromism, and ionochromism, respectively. Therefore, oxazolidine derivatives have been developed for their potential applications in chemosensors, anticounterfeiting, and rewritable hydrochromic papers. In this study, various oxazolidine derivatives containing hydroxyl and naphthalene substituted groups were synthesized by using two different indolenine compounds. The synthesized oxazolidine derivatives were used for investigation of solvatochromism in different solvents, and also acidochromism in various pHs by using UV–Vis and fluorescence spectroscopies. In addition, the oxazolidine derivatives were coated on cellulosic papers using a layer-by-layer strategy to develop rewritable acidochromic papers for printing of security tags on cellulosic papers by using acidic and alkaline solutions as water-based inks. Therefore, the developed rewritable acidochromic papers could be used as security papers.

High response photochromic films based on D-A diarylethenes and their application in holography

A set of photochromic dithienylethenes bearing amino and nitro groups are synthesised and embedded at high concentrations in a polymer matrix (Cellulose Acetate Butyrate, CAB) to produce films showing a large reversible modulation of the complex refractive index in the Vis-NIR spectral range, thanks to an interesting combination of remarkable response at the molecular level and very high load capability in the chosen matrix. The photochromic derivatives are characterized in solution and in CAB films by means of electronic and vibrational spectroscopy, complemented by DFT calculations. Both the real and imaginary part of the refractive index are determined by spectroscopic ellipsometry. The modulation of the refractive index in the near infrared is in the range 0.02–0.04. These are very large values for such kinds of systems and they are due to a favourable combination of very large solubility of the derivatives in CAB and a high polarisability change. As for the change in transparency in the visible, contrast values larger than 10³ are easily achieved. Based on such films, holograms are written and reconstructed with a very high fidelity and efficiency.

Photochromic diarylethenes with D–A structure are good candidates in holography thanks to their very large modulation of the complex refractive index.

Efficient green light-excited switches based on dithienylethenes with BF2-doped π-conjugated systems

A series of dithienylethenes 1a-1d with BF₂-doped π-conjugation have been successfully developed. Upon alternating irradiation with green light and red/NIR light, they show efficient photochromism and fluorescence switching behaviors in toluene and PMMA film. Moreover, the fluorescence of 1a can be switched effectively in living cells.

Is energy transfer limiting multiphotochromism? answers from ab initio quantifications

Dithienylethenes (DTEs) can be assembled to form supramolecular multiphotochromic systems that are highly functional molecular architectures of potential interest for building complex optoelectronic devices. Yet even simple DTE dimers relying on an organic linker may suffer from a partial photoactivity, i.e., only one of the two switches does isomerise. Experimentally, this limited photochromism has been attributed to an excited state energy transfer (EET) between the two DTEs of the multimer; this EET taking place instead of the desired photoinduced cyclisation of the DTE. However, no clear evidences of this phenomenon have been provided so far. In this work we propose the first rationalisation of this potential parasite photoinduced event using a computational approach based on Time-Dependent Density Functional Theory (TD-DFT) for the calculation of the electronic coupling in DTE dimers. Besides quantifying EET in several systems, we dissect the role of through-bond and through-space mechanisms on this process and clarify their dependence on both the nature and length of the bridge separating the two photochromes. The theoretical data obtained in this framework are in full agreement with the experimental outcomes and pave the way toward a molecular design of coupled, yet fully functionals, DTE-based multiswitches.

A fluorescent photochromic diarylethene based on naphthalic anhydride with strong solvatochromism

A diarylethene molecule consisting of an 1,8-naphthalic anhydride and two 2,5 dimethylthiophene which exhibits reversible fluorescence switching capacity as well as solvatochromism with red shift of the fluorescence maximum by more than 150 nm.

Theoretical design of solvatochromism switching by photochromic reactions using donor-acceptor disubstituted diarylethene derivatives with oxidized thiophene rings

We have designed several diarylethene derivatives with oxidized thiophene rings and donor-acceptor substituents, which show the solvatochromism switching by photochromic reactions, using a time-dependent density functional theory (TD-DFT) method using the polarizable continuum model (PCM). It is found that in the UV-vis spectral region examined only the open-ring isomers exhibit the solvatochromism, while the closed-ring isomers do not. The mechanism of the solvatochromism behavior and its switching process are clarified from the viewpoint of the charge-transfer (CT) excitation from the donor to the acceptor substituents. We demonstrate that this CT excitation can be controlled by choosing appropriate pairs of the donor and the acceptor substituents on the basis of the orbital correlation diagram between the diarylethene derivatives and the donor-acceptor substituents, which is constructed from the topologies and the orbital energies of the molecular orbitals primarily contributing to the excitations.

Designing efficient photochromic dithienylethene dyads

Aiming at designing more efficient multiphotochromes, we investigate with the help of ab initio tools the impact of the substitution on a series of dimers constituted of two dithienylethene (DTE) moieties, strongly coupled to each other through an ethynyl linker. The electronic structure and the optical properties of a large panel of compounds, substituted on different positions by various types of electroactive groups, have been compared with the aim of designing a dyad in which the three possible isomers (open-open, closed-open, closed-closed) can be reached. We show that appending the reactive carbons atoms of the DTE core with electroactive groups on one of the two photochromes allows cyclisation to be induced on a specific moiety, which leads to the formation of the desired closed-open isomer. Substituting the lateral positions of the thiophene rings provides further control of the topology of the frontier molecular orbitals, so that the electronic transition inducing the second ring closure stands out in the spectrum of the intermediate isomer.

Dye chemistry with time-dependent density functional theory

In this perspective, we present an overview of the determination of excited-state properties of "real-life" dyes, and notably of their optical absorption and emission spectra, performed during the last decade with time-dependent density functional theory (TD-DFT). We discuss the results obtained with both vertical and adiabatic (vibronic) approximations, choosing relevant examples for several series of dyes. These examples include reproducing absorption wavelengths of numerous families of coloured molecules, understanding the specific band shape of amino-anthraquinones, optimising the properties of dyes used in solar cells, mimicking the fluorescence wavelengths of fluorescent brighteners and BODIPY dyes, studying optically active biomolecules and photo-induced proton transfer, as well as improving the properties of photochromes.

Multiphotochromic molecular systems

We review molecular compounds encompassing several photochromic units with a focus on their functionalities.

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.

Near-infrared photochromic behavior in a donor-acceptor type diarylethene modulated by the cyanide anion

Two new near-infrared (NIR) photochromic compounds, containing both electron-donating (D) and accepting groups (A), have been developed and their NIR photochromic and fluorescence performances can be modulated by the cyanide anion. Such a colorimetric cyanide sensor has been achieved based on this donor-photochromic bridge-acceptor system.

Single molecule multiphotochromism with diarylethenes

In single photochromes, the two isomers that are interconverted in photoinduced reactions can serve as on and off states in a molecular switching device. The addition of several photochromic moieties onto a single molecule can allow the processing of more complex logical patterns. For example, an asymmetric triad could, in principle, store a byte, rather than a bit, of data. Because of the potential impact of multiphotochromic molecules in many research areas, over the past decade several groups have synthesized these coupled structures. The targets are easily addressable molecules that display increased contrast between the on and off states and in which all isomers have significantly distinguishable optical signatures. In this Account, we provide an overview of the multiswitchable molecular systems that incorporate at least one diarylethene group, which is the most successful thermally stable (P-type) organic photochrome. Up to this point, most systems have presented significant limitations. First of all, the reversibility of the processes is hindered by several side reactions more frequently than for single photochromes. Second, switching one part of the compound impedes the photoreactivity of other fragments in approximately 50% of the cases, and maximizing the electronic communication increases the probability of partial activity. In addition, most of the few synthesized operative systems only demonstrate cumulative absorption spectra rather than new features. Finally, it is impossible to selectively induce a chosen conversion because one wavelength might trigger several processes. We also emphasize the promising successes of asymmetric diarylethene dimers and trimers and molecules that combine two families of photochromes, such as diarylethene added to fulgimide or phenoxy-naphthacenequinone. In that framework, theoretical simulations offer complementary tools to investigate these structures, both to obtain structure/property relationships and to propose paths for the design of more efficient molecules. However, due to the size of the systems, researchers can only apply semiquantitative models. The investigation of the absorption spectra of the photochromes with time-dependent density functional theory (TD-DFT), the analysis of the topology of the LUMO + n (typically n = 1) of the closed-open hybrid, and an estimate of the steric stress in the hypothetical (ground-state) closed-closed structure serve as a useful combination of parameters to obtain initial insights regarding the photocyclization of the different open diarylethene groups. Nevertheless, because a first-order qualitative approach does not explore the potential energy surface of the photoexcited states, it remains inadequate for the investigation of some molecules.