Photoelectrochromic properties of a spirobenzopyran derivative

Light-Triggered Sustainable Defect-Passivation for Stable Perovskite Photovoltaics

The generation of photoinduced defects and freely moving halogen ions is dynamically updated in real time. Accordingly, most reported strategies are static and short-term, which make their improvements in photostability very limited. Therefore, seeking new passivation strategies to match the dynamic characteristics of defect generation is very urgent. Without newly generated defects, a passivation molecule should exist in the configuration that would not become the initiation sites for defect generation. With newly generated defects, the passivation molecule should transfer into the other configuration that possesses the passivation sites. Herein, a classical photoisomeric molecule, spiropyran, is adopted, whose pre- and post-isomeric forms meet the requirements for two different configurations, to realize the state transition once the photoinduced defects appear during subsequent operation and dynamic capture for continuous renewal of defects. Consequently, spiropyrans work as light-triggered and self-healing sustainable passivation sites to realize continuous defect repair. The target devices retain 93% and 99% of their initial power conversion efficiencies after 456 h aging under ultraviolet illumination and 1200 h aging under full-spectrum illumination, respectively. This work provides a novel concept of sustainable passivation strategy to realize continuous defect-passivation and film-healing in perovskite photovoltaics.

Hydrogen-Bonded Dimeric Capsules with Appended Spiropyran Units: Towards Controlled Cargo Release

We report the synthesis of unprecedented tetra-urea derivatives of calix[4]arene and calix[4]pyrrole containing four spiropyran (SP) units at their upper rim. We investigate the photo- and acid-induced isomerization of the monomeric and homo-dimeric tetra-ureas derivatives using UV-Vis and ¹ H NMR spectroscopies. At micromolar concentration, irradiation of the samples with 365 nm light induces changes in their absorption spectra that are consistent with SP→merocyanine (MC) isomerization. However, analogous experiments at millimolar concentration do not produce noticeable changes in the ¹ H NMR spectra. The addition of triflic acid to micromolar and millimolar solutions of the tetra-ureas produces the quantitative isomerization of the SP units to the protonated merocyanine form (E-MCH⁺ ) and the simultaneous disassembly of the capsular dimers to form ill-defined aggregates. The neutralization of the acid solutions resets the SP form. Under these acid/base treatment conditions, the controlled release of the included guest and the reassembly of the all-SP tetra-urea dimers occurs at different extents depending on its calix[4]arene or calix[4]pyrrole scaffold.

Solid Multiresponsive Materials Based on Nitrospiropyran-Doped Ionogels

The application of molecular switches for the fabrication of multistimuli-responsive chromic materials and devices still remains a challenge because of the restrictions imposed by the supporting solid matrices where these compounds must be incorporated: they often critically affect the chromic response as well as limit the type and nature of external stimuli that can be applied. In this work, we propose the use of ionogels to overcome these constraints, as they provide a soft, fluidic, transparent, thermally stable, and ionic-conductive environment where molecular switches preserve their solution-like properties and can be exposed to a number of different stimuli. By exploiting this strategy, we herein pioneer the preparation of nitrospiropyran-based materials using a single solid platform that exhibit optimal photo-, halo-, thermo-, and electrochromic switching behaviors.

Electrochemical Ring-Opening and -Closing of a Spiropyran

The bistability of molecular switches is an essential characteristic in their use as functional components in molecular-based devices and machines. For photoswitches, light-driven switching between two stable states proceeds via short-lived changes of the bond order in electronically excited states. Here, bistable switching of a ditertbutyl-substituted spiropyran photoswitch is instead demonstrated by oxidation and subsequent reduction in an overall four-state cycle. The spiropyran structure chosen has reduced sensitivity to the effect of secondary electrochemical processes such as H⁺ production and provides transient access to a decreased thermal Z–E isomerization barrier in the one electron oxidized state, akin to that achieved in the corresponding photochemical path. Thus, we show that the energy needed for switching spiropyrans to the merocyanine form on demand, typically delivered by a photon, can instead be provided electrochemically. This opens up further opportunities for the utilization of spiropyrans in electrically controlled applications and devices.

Photo- and Electro-Driven Molecular Switching System of Aryl-Bridged Photochromic Radical Complexes

Fast photochromic molecules have received much interest in the potential application as a real-time switching trigger in material and biological chemistry. Pentaarylbiimidazole (PABI) and phenoxyl-imidazolyl radical complex (PIC) are one of the fast photochromic molecules based on imidazolyl radicals. Because the photochromic reaction of these fast photochromic molecules proceeds from the optically forbidden S₁ state, it is difficult to estimate the excitation energy to induce the photochromic reactions by spectroscopic techniques. In this study, we performed the electrochemical measurements for PABI and PIC to investigate the electronic properties and to determine the S₀-S₁ transition energies. In addition, we also revealed that the electrochemical reduction of PABI and PIC generates the radical anion which spontaneously shows the C-N bond breaking reaction to produce the radical species. The initial photochromic dimer is reproduced by the reversible oxidation of the anion species. This characteristic photochromic and electrochromic properties can be applicable to the photowritable electrochromic devices with high spatial resolution.

The evolution of spiropyran: fundamentals and progress of an extraordinarily versatile photochrome

Spiropyrans have played a pivotal role in the emergence of the field of chromism following their discovery in the early 20th century, with almost ubiquitous use in materials applications especially since their photochromism was discovered in 1952.

Photochromic properties of polyoxotungstates with grafted spiropyran molecules

The first systems associating in a single molecule polyoxotungstates (POTs) and photochromic organic groups have been elaborated. Using the (TBA)4[PW11O39{Sn(C6H4I)}] precursor, two hybrid organic-inorganic species where a spiropyran derivative (SP) has been covalently grafted onto a {PW11Sn} fragment via a Sonogashira coupling have been successfully obtained. Alternatively, a complex containing a silicotungstate {PW11Si2} unit connected to two spiropyran entities has been characterized. The purity of these species has been assessed using several techniques, including (1)H and (31)P NMR spectroscopy, mass spectrometry, and electrochemical measurements. The optical properties of the hybrid materials have been investigated both in solution and in the solid state. These studies reveal that the grafting of SPs onto POTs does not significantly alter the photochromic behavior of the organic chromophore in solution. In contrast, these novel hybrid SP-POT materials display highly effective solid-state photochromism from neutral SP molecules initially nonphotochromic in the crystalline state. The photoresponses of the SP-POT systems in the solid state strongly depend on the nature and the number of grafted SP groups.

Strategies for the development of gadolinium-based 'q'-activatable MRI contrast agents

The emergence and rapid development of activatable contrast agents (CAs), whose relaxivity changes in response to the variation of a specific marker in the surrounding physiological microenvironment, have expanded the scope of MRI beyond anatomical and functional imaging to also convey information at the cellular and molecular level. The essence of an activatable MRI CA is the difference in relaxivity before and after a change in a physiological variable: the larger the difference, the better the CA. In this review, strategies for the design of activatable gadolinium CAs, with a switching mechanism based on the modulation of hydration (q), sensitive to common variables in the physiological microenvironment, such as pH, light, redox and metal ions, are illustrated and discussed.

Activatable T₁ and T₂ magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI) has become one of the most important diagnosis tools available in medicine. Typically MRI is not capable of sensing biochemical activities. However, recently emerged activatable MRI contrast agents (CAs), whose relaxivity is variable in response to a specific parameter change in the surrounding physiological microenvironment, potentially allow for MRI to indicate biological processes. Among the various factors influencing the relaxivity of a CA, the number of inner-sphere water molecules (q) directly coordinated to the metal center, the residence time of the coordinated water molecule (τ (m)), and the rotational correlation time representing the molecular tumbling time of a complex (τ (R)) contribute strongly to the relaxivity of an activatable CA. Tuning the ligand structure and properties has been the subject of intensive research for activatable MR CA designs. This review summarizes a variety of activatable MRI CAs sensitive to common variables in microenvironment in vivo, i.e., pH, luminescence, metal ions, redox, and enzymes, etc., with emphasis on the influence of ligand design on parameters q, τ (m), and τ (R).

Synthesis and characterization of a redox- and light-sensitive MRI contrast agent

A redox- and light-sensitive, T(1)-weighted magnetic resonance imaging (MRI) contrast agent which tethers a spiropyran(SP)/merocyanine(MC) motif to a Gd-DO3A moiety was synthesized and characterized. When in the dark, the probe is in its MC form which has an r(1) relaxivity of 2.51 mM(-1)s(-1) (60MHz, 37°C). After irradiation with visible light or mixing with NADH, the probe experiences an isomerization and the r(1) relaxivity decreased 18% and 26%, respectively. Additionally, the signal intensity in MRI showed an observable decrease after the compound was mixed with NADH.

Photochromically-controlled, reversibly-activated MRI and optical contrast agent

The contrast agent which tethers a spiropyran group to a Gd-DO3A moiety has higher relaxivity and fluorescence intensity in the dark; the relaxivity and fluorescence intensity decrease after irradiation with visible light.

Understanding Electrochromic Processes Initiated by Dithienylcyclopentene Cation-Radicals

Simple photochromic dithienylethylenes with either a perfluoro or a perhydro cyclopentene ring, and a variety of substituents (chlorine, iodine, trimethylsilyl, phenylthio, aldehyde, carboxylic acid, and ethynylanisyl), have been prepared and their electrochemical behavior was explored by cyclic voltammetry. All dithienylethylenes present two-electron irreversible oxidation waves in their open form, but the cation-radical of the open isomers can follow two different reaction pathways: dimerization or ring closure, whereas the halogen derivatives follow a dimerization mechanism, the presence of donor groups, such as the phenylthio-substituted compound, promote an efficient oxidative ring closure following an ECE/DISP mechanism. Electrochromic properties are also found in the corresponding ring-closed isomers. Depending on the substituents on the thiophene ring, and the perfluro or perhydro cyclopentene ring, open isomers can be obtained from oxidation (chemical or electrochemical) of the corresponding ring-closed isomers via an EC mechanism. This reaction pathway is favored by the presence of electron-withdrawing groups in the molecule. For all these compounds, closed or open, the oxidation lies between 0.8 and 1.5 V vs SCE, and provokes a permanent modification of the color, even after an oxidation-reduction cycle. This could be qualified as "electrochromism with memory". On the other hand, the ring-closed electron-rich isomers (E degrees < 0.8 V), which show reversible waves at the cation-radical or even dication level, give rise to "true electrochromism", for which no structural changes are observed. The experimental study was completed by theoretical calculations at the DFT level, using B3LYP density functional, which gave information on the total energy, the geometry, and the electronic structures of several representative compounds, either in the neutral form or in the cation-radical state. These results are important for the potential design of photochromic systems, such as three-state conjugated systems and photoelectrical molecular switching devices.