Phenoxyl-imidazolyl Radical Complex Research Articles

Stepwise Photochromism of Large Macrocycles Incorporating Two Negative Photochromic Units.

Macrocyclic photochromic molecules incorporating multiple photochromic units are known to exhibit cooperative and nonlinear photochromic reactions among distinct photochromic components. While extensive research has concentrated on positive photochromic molecules, this study presents a pioneering attempt in synthesizing macrocyclic photochromic molecules that integrate negative photochromic units. Binaphthyl-bridged phenoxyl imidazolyl radical complex, BN-PIC, exhibits unique negative photochromism in which the thermally stable colored isomer converts to the metastable colorless isomer via a short-lived biradical upon visible-light irradiation. Macrocyclic biphotochromic molecules incorporating two BN-PIC units were synthesized and the effects of ring strain on the photochromic properties including the photoconversion efficiencies and the rates of the thermal reverse reaction were investigated. The photokinetic study of these macrocyclic biphotochromic molecules demonstrated that the structural distortion of the ring caused by the isomerization of one photochromic unit significantly influenced the photoisomerization efficiency and the rate of the thermal reverse reaction of the other photochromic unit.

Photochromic dinuclear iridium(III) complexes having phenoxyl-imidazolyl radical complex derivatives.

We demonstrate that the phenoxyl-imidazolyl radical complex (PIC), which is a rate-tunable fast photoswitch, can be used as a ligand that directly coordinates with iridium (III) ions. The iridium complexes show the characteristic photochromic reactions originating from the PIC moiety, whereas the behaviour of transient species is substantially different from that of the PIC.

Controlling Diradical Character of Photogenerated Colored Isomers of Phenoxyl-Imidazolyl Radical Complexes.

We propose a rational method for evaluating the diradical character of the photochromic phenoxyl-imidazolyl radical complex (PIC) derivatives based on their radical-radical coupling reaction rates. PIC consists of an imidazole ring, a phenoxyl ring, and a bridging unit that structurally connects them. The C-N bond formed between the imidazole and phenoxyl rings can be dissociated photochemically in a homolytic manner. The photochromism of PIC differs significantly from other photochromic molecules in that the transient colored open-ring isomer has a diradical character. The colored open-ring isomer returns promptly to the initial colorless closed-ring isomer by the intramolecular radical recombination reaction. By changing the aromaticity and substitution position of the bridging unit, it is possible to control the degree of contribution of the open-shell diradical and closed-shell quinoidal structures to the open-ring isomer. Systematic investigation of the photochromic reactions of several PIC derivatives revealed that the half-life of the open-ring isomers reflects the diradical character. Thus, the radical recombination reaction rate of the open-ring isomer of the PIC derivatives is an excellent parameter of the diradical character.

Enhancement of Negative Photochromic Properties of Naphthalene-Bridged Phenoxyl-Imidazolyl Radical Complex.

Negative photochromism has increased attention as a light-switch for functional materials. A development of fast photochromic molecules has been also expected because a rapid thermal back reaction within a millisecond time scale is useful for real-time switching. Herein, we synthesized the derivatives of the naphthalene-bridged phenoxyl-imidazolyl radical complex (Np-PIC) showing the negative photochromism to demonstrate the efficient strategy to increase the visible light sensitivity and to control the thermal back reaction rates. The distances of the C-C bond of the transient 2,4'-isomer shows good agreement with the thermodynamic stability, leading to the control of the thermal back reaction rate. We revealed the cyclic voltammetry and the DFT calculations are efficient to predict the characters of the HOMO and LUMO. The introduction of the electron-withdrawing dicyanoquinodimethane group is efficient to induce the photochromic reaction with increased visible-light sensitivity by the expansion of the π-conjugation. The results will give an important insight for the future development of fast-responsive negative photochromic molecules.

Photochromic Radical Complexes That Show Heterolytic Bond Dissociation.

Photochromic materials have been widely used in various research fields because of their variety of photoswitching properties based on various molecular frameworks and bond breaking processes, such as homolysis and heterolysis. However, while a number of photochromic molecular frameworks have been reported so far, there are few reports on photochromic molecular frameworks that show both homolysis and heterolysis depending on the substituents with high durability. The biradicals and zwitterions generated by homolysis and heterolysis have different physical and chemical properties and different potential applications. Therefore, the rational photochromic molecular design to control the bond dissociation in the excited state on demand expands the versatility for photoswitch materials beyond the conventional photochromic molecular frameworks. In this study, we synthesized novel photochromic molecules based on the framework of a radical-dissociation-type photochromic molecule: phenoxyl-imidazolyl radical complex (PIC). While the conventional PIC shows the photoinduced homolysis, the substitution of a strong electron-donating moiety to the phenoxyl moiety enables the bond dissociation process to be switched from homolysis to heterolysis. This study gives a strategy for controlling the bond dissociation process of the excited state of photochromic systems, and the strategy enables us to develop further novel radical and zwitterionic photoswitches.

Photochromic Reaction by Red Light via Triplet Fusion Upconversion

Red or near-infrared (NIR) light responsive molecules have received much attention for biological and material applications because potentially harmful UV light for materials and cells is not required for the photochemical reactions. Although some molecular designs for photochromic molecules to increase the photosensitivity to red or NIR light have been reported, the strategies are limited to the extension of π-conjugation length and the utilization of charge transfer transition or energy and electron transfers. Triplet fusion is an attractive tool to cause chemical reactions by converting low-energy excitation light to high-energy upconversion light. However, the efficient use of the high energy of upconversion light is difficult because almost all reported triplet fusion systems rely on reabsorption of upconversion light. Here, we demonstrated red-light-driven photochromism via the triplet fusion of a phenoxyl-imidazolyl radical complex, Pery-RPIC, that has a covalently bonded perylene as an annihilator unit. The femtosecond time-resolved absorption and fluorescence spectroscopy revealed that this photochromic reaction proceeds by the highly efficient singlet energy transfer from the annihilator unit to the photochromic unit. This strategy can be applied not only to the development of visible and NIR light responsive photochromic system but also to various photochemical reactions.

Excited state dynamics for visible-light sensitization of a photochromic benzil-subsituted phenoxyl-imidazolyl radical.

Visible-light sensitized photoswitches have been paid particular attention in the fields of life sciences and materials science because long-wavelength light reduces photodegradation, transmits deep inside of matters, and achieves the selective excitation in condensed systems. Among various photoswitch molecules, the phenoxyl-imidazolyl radical complex (PIC) is a recently developed thermally reversible photochromic molecule whose thermal back reaction can be tuned from tens of nanoseconds to tens of seconds by rational design of the molecular structure. While the wide range of tunability of the switching speed of PIC opened up various potential applications, no photosensitivity to visible light limits its applications. In this study, we synthesized a visible-light sensitized PIC derivative conjugated with a benzil unit. Femtosecond transient absorption spectroscopy revealed that the benzil unit acts as a singlet photosensitizer for PIC by the Dexter-type energy transfer. Visible-light sensitized photochromic reactions of PIC are important for expanding the versatility of potential applications to life sciences and materials science.

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.

Visible light intensity dependent negative photochromism of a binaphthyl-bridged phenoxyl-imidazolyl radical complex.

We herein report a novel biphotochromic molecule composed of two fast negative photochromic phenoxyl-imidazolyl radical complex units showing incident light intensity dependence of the photochemical reaction. The electronic coupling between the two fast negative photochromic units gives a non-linear photoresponse; weak- and intense-visible light irradiation proceed the one-photon and two-photon photochemical reactions, respectively. The solution of the stepwise negative photochromic molecule changes to colorless only by excitation with intense visible light.

Electrochromism of fast photochromic radical complexes forming light-unresponsive stable colored radical cation.

We demonstrated the electrochromism of photochromic radical complexes containing triaryl imidazole: fast photoswitchable pentaarylbiimidazole (PABI) and the phenoxyl-imidazolyl radical complex (PIC). Cyclic voltammetry and spectroelectrochemistry revealed that PABI and PIC generate the highly stable radical cation by one-electron oxidation accompanied by a color change from colorless to green. The stability of the radical cation is strongly affected by the dihedral angle between the imidazole ring and the phenyl ring at the 2-position of the imidazole ring.

Reversible Valence Photoisomerization between Closed-Shell Quinoidal and Open-Shell Biradical Forms.

We report here a kinetic study on the thermal equilibrium process between the biradical form and the quinoidal form starting from the singlet biradical form alone. A photochromic phenoxyl-imidazolyl radical complex repeatedly generates biradical species upon UV light irradiation, and the following thermal equilibrium process responsible for valence isomerization from the open-shell singlet biradical to the closed-shell quinoidal form is observed in the microsecond time region. The thermodynamic parameters for the equilibrium process were determined for the first time by nanosecond laser flash photolysis. We also found that visible-light excitation to the equilibrium state causes valence photoisomerization from the quinoidal to the biradical form, which returns thermally to the quinoidal form.

Structurally and electronically modulated spin interaction of transient biradicals in two photon-gated stepwise photochromism.

The development of two-photon induced photochromic compounds is important for advanced photoresponsive materials. The utilization of the long-lived transient states or species for two-photon absorption is one of the efficient strategies to realize the advanced photochemical behavior beyond a one-photon photochemical reaction. We have synthesized bi-photochromic compounds composed of two photochromic phenoxyl-imidazolyl radical complex units. The biphotochromic compounds generate two biradical units when the two photochromic units absorb photons with a stepwise manner. The interaction between the two biradicals through the central bridging phenyl ring is the key feature to control the stepwise photochromic reaction. Here, we introduced aromatic spacers in order to modulate the distance and the dihedral angle between the biradical units. The color and the rate of the thermal back reaction of the stepwise photochromism can be regulated by the control of the central bridging part. These results give important insights to develop desirable advanced photoresponsive compounds.

Rate-Tunable Stepwise Two-Photon-Gated Photoresponsive Systems Employing a Synergetic Interaction between Transient Biradical Units.

The cooperative interaction between photons and molecules, recently termed as the "photosynergetic" effect, is crucial to develop advanced photofunctional materials beyond a one-photon reaction of a single chromophore. The two-photon absorption is one of the attractive processes for the efficient utilization of photons. Especially, the nonlinear response of the two-photon absorption process is of interest not only to realize temporal and spatial control of reactions but also to develop the rewritable optical memory media and smart optical devices responding to the intensity of light. The stepwise two-photon-induced photochromism, which involves a short-lived transient species as an intermediate state, is one of the advanced photoresponsive compounds. The key feature of the stepwise two-photon-induced photochromism is an effective electronic interaction between the photogenerated transient chromophores. Here, we designed bis(phenoxyl-imidazolyl radical complex) (bisPIC) derivatives, which are composed of a couple of photochromic units and absorb two photons in a stepwise manner. The stepwise photochromic properties were investigated in detail by using double pulse laser flash photolysis and time-resolved Fourier transform infrared (TR-FTIR) spectroscopy. The one-photon reaction leads to the generation of a short-lived biradical species, which absorbs an additional photon and generates two electronically coupled biradical units, resulting in the formation of the long-lived quinoid species. The short-lived biradical species and the long-lived quinoid species of each bisPIC derivatives show the significantly different absorption spectra and rates of the thermal back reactions. These results indicate the colors and the lifetimes of the transient species can be systematically changed by switching the wavelength and intensity of the excitation light. The development of an excitation light threshold system based on the fast-switchable photochromic compounds will give important insights not only for the development of functional photoresponsive materials but also for the fundamental research using the cooperative excitation and interaction between photochromophores.

Highly durable photochromic radical complexes having no steric protections of radicals.

Steric protection groups are usually necessary for stable radicals. However, here, we developed novel photochromic radical complexes which generate sterically unprotected imidazolyl and phenoxyl radicals upon UV light irradiation based on the phenoxyl-imidazolyl radical complex (PIC) framework. These photochromic compounds show excellent durability against repeated irradiation of intense nanosecond laser pulses even in polar protic solvents, such as ethanol.

Fast Photochromism Involving Thermally-Activated Valence Isomerization of Phenoxyl-Imidazolyl Radical Complex Derivatives.

Open-shell biradicals have received considerable attention in material science because of their high two-photon absorption cross sections and broad and high absorptive features over the visible region. However, the instability of the biradical caused by the open-shell nature was one of the drawbacks; therefore, novel radical compounds which can suppress unwanted reactions by tuning the open-shell features are desired to expand the versatility of the radical compounds. Here, we report a novel radical-dissociation-type photochromic compound whose photochromic reaction involves a valence isomerization from the open-shell biradical to closed-shell quinoidal forms by using a phenoxyl-imidazolyl radical complex framework. The valence isomerization from the biradical to quinoid forms effectively tunes the open-shell feature in time and drastically changes the spectral features, which were revealed by time-resolved Fourier transform infrared spectroscopy. This novel fast photochromic property not only is important for fundamental spin chemistry but also expands the versatility of the radical compounds for novel advanced photofunctional materials.

Fast Negative Photochromism of 1,1′-Binaphthyl-Bridged Phenoxyl–Imidazolyl Radical Complex

Negative photochromism, in which a thermally stable colored form isomerizes to the transient colorless form by light irradiation and the back reaction occurs thermally, is advantageous in its applications for photoswitching materials because visible light can cause the photochromic color change of the materials. Moreover, the photochromic color change can be induced even on the inside of the materials due to the absence of the reabsorption of the visible excitation light by the photogenerated colorless species. While several negative photochromic compounds have been reported, the time scales of the back reaction are still slower than minutes, and no available fast responsive negative photochromic compounds have been reported. Here, we developed a negative photochromic 1,1'-binaphthyl-bridged phenoxyl-imidazolyl radical complex (BN-PIC) which enables fast photoswitching by visible light. The stable colored BN-PIC shows instantaneous decoloration by visible light irradiation, and the photogenerated colorless form thermally reverts to the initial colored form with a half-life of 1.9 s at room temperature. BN-PIC can also cause the drastic change in the chiroptical properties by the photochromic reaction, and the rate of the thermal back reaction is affected by the chirality of the solvent. Since the negative photochromic reaction can occur on the inside of the materials, the fast negative photochromism is expected to have an impact in the fields of photoresponsive materials of solid states and molecular aggregates.

Stealth fast photoswitching of negative photochromic naphthalene-bridged phenoxyl-imidazolyl radical complexes.

Naphthalene-bridged phenoxyl-imidazolyl radical complex (Np-PIC) is a novel fast switchable negative photochromic compound, which shows the thermal back reaction in the millisecond time scale. Upon UV light irradiation, Np-PIC shows the hypochromic effect in the UVA region due to there being less conjugation in the transient isomer. By replacing the phenoxyl unit with a naphthoxyl unit, the molecular structure has an asymmetric carbon, leading to fast chiroptical switching. This simple molecular design will be a good candidate for the future development of negative photochromic compounds.

Photochromic phenoxyl-imidazolyl radical complexes with decoloration rates from tens of nanoseconds to seconds.

We report a novel photochromic molecular system, phenoxyl-imidazolyl radical complex (PIC), in which both a phenoxyl radical site and an imidazolyl radical site are reversibly and simultaneously generated upon UV light irradiation. PIC consists of the three parts: an aromatic linker, a diarylimidazole moiety, and a 4H-cyclohexadienone ring. Upon UV light irradiation, the C-N bond between the 4H-cyclohexadienone ring and the imidazole ring in the colorless closed-ring isomer of PIC undergoes a homolytic cleavage, leading to the formation of the transient colored open-ring isomer. Based on the substituents on the imidazoyl/4H-cyclohexadienone rings and the nature of the aromatic linker, the half-life of the colored open-ring isomer can be varied between tens of nanoseconds and seconds. PIC derivatives containing a 1,2-phenylene linker exhibit high fatigue resistance toward repeated photochromic reactions. Analysis using laser flash photolysis reveals that the absorption spectra of the open-ring isomers are not readily rationalized by a straightforward superposition of the spectra of the two component radical fragments and the photogenerated radicals are electronically coupled through the aromatic linker. Furthermore, the open-ring isomer can be treated as a hybrid of the pure open-shell biradical and closed-shell quinoid resonance structures.