Photochromic Schiff Base Research Articles

A thermochromic salicylaldehyde Schiff bases derivative with AIE properties due to twisted structure

Great attention has been attracted to the salicylaldehyde Schiff base due to its thermochromic and photochromic characteristics in the solid state. However, the boundary between photochromic and thermochromic salicylaldehyde Schiff base structures is still unclear, and few studies have been conducted to investigate their thermochromic behavior and structures in depth and systematically. In the present work, we have synthesized a novel salicylaldehyde Schiff base derivative (2,2′-((1E,1′E)-((2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diyl)-bis(azaneylylidene))bis(methaneylylidene))diphenol, BTB) with twisted structure, which possesses both thermochromic and aggregation-induced emission (AIE) characteristics, and systematically investigated its thermochromic properties, enol-keto isomerization, AIE properties, aggregated state, and excited state intramolecular proton transfer (ESIPT) process. Interestingly, in our study, we found that the color of the BTB molecule changed gradually during the heating process, and it can be found that the thermochromism of BTB is produced by the reversible equilibrium shift of the enol form and the keto forms due to the temperature change by the color difference test, the in-situ IR spectra and the DSC curves. Finally, the pathway of BTB proton transfer was analyzed through quantum chemical calculations, and it was found that enol ketone isomerization is achieved through single excited state intramolecular proton transfer, rather than the expected double excited state intramolecular proton transfer.

Fabricating a photochromic Schiff base into a wearable cellulose sensor and a smart textile for ultraviolet radiation monitoring outdoors

A dual-channel photochromic Schiff base HBA is reported herein, which undergoes a reversible tautomerism in response to UV exposure, showing visible changes in both color and fluorescence. The spectral behavior, fatigue resistance, sensitivity are studied. The quantification of photochromic fluorescence/color change is solved by applying color spaces and chromatic aberration. HBA is further fabricated into low-cost wearable sensor and smart textile, enabling visual feedback under UV radiation. It demonstrates a low-costing platform for naked-eye-based dosimetry of outdoor UV radiations, providing an early warning for excessive UV exposure.

Photochromic Single-Component Organic Fulgide Ferroelectric with Photo-Triggered Polarization Response.

Organic photochromic compounds have been widely investigated for optical memory storage and switches. Very recently, we pioneeringly discovered optical control of ferroelectric polarization switching in organic photochromic salicylaldehyde Schiff base and diarylethene derivatives, differently from the traditional ferroelectrics. However, the study of such intriguing photo-triggered ferroelectrics is still in its infancy and relatively scarce. In this manuscript, we synthesized a pair of new organic single-component fulgide isomers, (E and Z)-3-(1-(4-(tert-butyl)phenyl)ethylidene)-4-(propan-2-ylidene)dihydrofuran-2,5-dione (1E and 1Z). They undergo prominent photochromism from yellow to red. Interestingly, only polar 1E has been proven to be ferroelectric, while the centrosymmetric 1Z does not meet the basic requirement for ferroelectricity. Besides, experimental evidence shows that the Z-form can be converted to the E-form by light irradiation. More importantly, the ferroelectric domains of 1E can be manipulated by light in the absence of an electric field, benefiting from the remarkable photoisomerization. 1E also adopts good fatigue resistance to the photocyclization reaction. As far as we know, this is the first example of organic fulgide ferroelectric reported with photo-triggered ferroelectric polarization response. This work has developed a new system for studying photo-triggered ferroelectrics and would also provide an expected perspective on developing ferroelectrics for optical applications in trap future.

Fabricating a photochromic benzonitrile Schiff base into a low-cost reusable paper-based wearable sensor for naked-eye dosimetry of UV radiations

We report in this study a photochromic benzonitrile Schiff base, (E)-4-((2-hydroxy-4-methoxybenzylidene)amino)benzonitrile (HMBAB). The molecular design, synthesis, aggregation-induced emission (AIE) as well as the quantum chemical calculations were outlined. In particular, HMBAB would undergo a reversible tautomerism in response to UV exposure, exhibiting remarkable changes in both absorption and emission: the compound shows yellow color and green-yellow luminescence; after UV exposure, the changes into orange-red while the luminescence is dramatically quenched, accompanied by a large bathochromic-shift. In addition, the photochromic state can be fully recovered via thermal treatment. Such reversible dual-channel photochromism was investigated using UV–vis reflectance spectroscopy and colorimeter, wherein a gradient change with time and a high fatigue resistance in cycle use was recorded. The photochromism is quantified by well-established RGB and Lab color space, in which the color change can be accurately analyzed by the chromatic aberration (ΔE*Lab). Sensitivity test gives a two-stage linear relation between ΔE*Lab and UV intensity, by which a limit of detection (LOD) as low as 67 μW/cm2 is obtained. HMBAB was further fabricated into a paper-based wearable sensor, capable of being integrated into a chest card or a bracelet. It exhibits various degrees of color change in different sunlight environments, which can be readily observed by naked eyes, providing an early warning for high-dose UV radiations.

A photochromic salicylaldehyde hydrazone derivative based on C[dbnd]N isomerization and ESIPT mechanisms and its detection of Al3+ in aqueous solution

A simple photochromic Schiff base was successfully prepared by the condensation of salicylaldehyde and benzoyl hydrazine. This compound has reversible photochromic properties based on isomerization and ESIPT mechanisms. In organic solvents, after irradiation with 365 nm UV light for 2 min, the absorption peak at 367 nm of the compound showed a significant decrease, while a double absorption peak appeared at 418 nm and 438 nm, accompanied by a significant change of the solution color from colorless to yellow. The compound can also complex with Al3+ at the molar ratio of 2:1 in the water solution (acetonitrile/water, v/v, 1:99), resulting in significantly enhanced fluorescence of the compound, so as to achieve fluorescence detection of Al3+ in living cells and water samples.

Reversible photochromic tetraphenylethene-based Schiff base: Design, synthesis, crystal structure and applications as visible light driven rewritable paper and UV sensor

“Paper” plays an important role in the development and spread of civilization. However, massive use of paper has led to nonnegligible negative influence on environment. Recently, great progresses have been made in the fields of rewritable paper that has enormous potential in alleviating the increasing demand for paper. Therefore, we report a new photochromic tetraphenylethene-based Schiff base derivative 2-ethoxy-6-(((4-(1,2,2-triphenylvinyl)phenyl)imino)-methyl)phenol (TPENOEt). TPENOEt exhibits reversible photochromic behavior, fast response, simple synthetic step, lower cost and excellent fatigue resistance. The bleaching rate of TPENOEt is tunable by longer wavelength light, white light irradiation and elevated temperature. The fluorescence of TPENOEt is weak in solid state, which increases obviously after grinding with a pestle accompanied by reduced photochromic efficiency and longer response time. These results provide a new view to get insight into the relationship between packing style and fluorescence/photochromism property of Schiff base derivatives. TPENOEt is successfully applied as a visible light driven rewritable paper, 410 nm violet light can serve as a pen, while 420–590 nm visible light and white light is employed as the eraser. The recorded information will be erased in about 15 s after white light irradiation and will be self-erased in 20 min in daylight and in about 15 days in dark. Moreover, TPENOEt can also serve as a UV strength sensor for naked eye detection of UV radiation pollution.

6-[(R)-(2-Hydroxy-1-phenylethyl)aminomethylidene]-4-(2-phenyldiazen-1-yl)cyclohexa-2,4-dien-1-one

The title chiral photochromic Schiff base compound, C21H19N3O2, was synthesized from (R)-(−)-2-phenylglycinol and salicylaldehyde of azobenzene derivative. The molecule exhibits the keto–amine tautomeric form and displays characteristic features of azobenzene derivatives. The diazenyl group adopts a trans (E) conformation, with N=N bond length of 1.260 (2) Å. The hydroxy group is involved in an intermolecular O—H...O hydrogen bond.

Crystal structure of 2-{(R)-[1-(4-bromo-phen-yl)eth-yl]imino-meth-yl}-4-(phenyl-diazen-yl)phenol, a chiral photochromic Schiff base.

The title chiral photochromic Schiff base compound, C21H18BrN3O, was synthesized from (R)-(+)-1-(4-bromo­phen­yl)ethyl­amine and the salicyl­aldehyde of an azo­benzene derivative. The mol­ecule corresponds to the phenol–imine tautomer, the C=N and N—C bond distances being 1.285 (3) and 1.470 (3) Å, respectively. The diazenyl group adopts a trans form, with an N=N distance of 1.256 (3) Å. The hy­droxy group is involved in intra­molecular O—H⋯N hydrogen bonding. In the crystal, C—H⋯π inter­actions consolidate the crystal packing of one-dimensional chains, which exhibits short inter­molecular Br⋯C contacts of 3.400 (3) Å.

Chemionics Study of Newly Synthesized Photochromic Schiff Base Compound

A new Schiff base compound 4((o-tolylimino)methyl)-3-methyl-1-phenyl-1H-pyrazol-5-ol(4TM-MPP) was synthesized and characterized by elemental analyses, LC-MS, FTIR, and 1H-NMR spectra. The compound exhibits photochromic property under 365 nm UV-light irradiation, which can be observed with solid state reflectance spectra. It suggests that the photoisomerization is due to photoinduced intermolecular hydrogen bond, which results in enol to keto transformation. The chemionics study is carried out using (Ni+2, H+), (Ag+, H+), and (Na+, H+, and OH−) using the UV absorption spectra in solution, which shows logic behavior of INHIBIT, NOR, and XOR, respectively.

Proton Transfer Assisted Charge Transfer Phenomena in Photochromic Schiff Bases and Effect of –NEt2 Groups to the Anil Schiff Bases

Photochromic Schiff bases 5-diethylamino-2-[(4-diethylamino-benzylidene)-hydrazonomethyl]-phenol (DDBHP) and N,N'-bis(4-N,N-diethylaminosalisalidene) hydrazine (DEASH) with both the proton and charge transfer moieties have been synthesized, and their photophysical properties such as excited state intramolecular charge transfer (ICT) and proton transfer (ESIPT) processes have been reported on the basis of steady-state and time-resolved spectral measurement in various solvents. The ground-state six-membered intramolecular hydrogen bonding network at the proton transfer site accelerates the ESIPT process for these compounds. Both the compounds show large Stokes-shifted emission bands for proton transfer and charge transfer processes. The hydrogen bonding solvents play a crucial role in these photophysical processes. Excited-state dipole moment of DDBHP and DEASH calculated by the solvatochromic method supports the polar character of the charge transfer excited state. Introduction of -NEt(2) groups to the reported salicylaldehyde azine (SAA) Schiff base results an increase in fluorescence lifetime from femtosecond to picosecond time scale for the proton transfer process.

ChemInform Abstract: Tautomeric Schiff Bases: Iono‐, Solvato‐, Thermo‐ and Photochromism

AbstractReview: 177 refs.

Tautomeric Schiff bases: Iono-, solvato-, thermo- and photochromism

The review article summarizes the recent data about novel tautomeric iono-, solvato-, thermo- and photochromic Schiff bases. This contribution focuses on the factors determining tautomeric equilibrium and on the investigations into the proton transfer mechanisms.

Photo-deactivation pathways of a double H-bonded photochromic Schiff base investigated by combined theoretical calculations and experimental time-resolved studies

The photophysics of N,N'-bis(salicylidene)-p-phenylenediamine (BSP) is analyzed both theoretically and experimentally. The alternative intramolecular proton-transfer reactions lead to three different tautomers. We performed DFT and TDDFT calculations to analyze the topography of the reactions connecting the three tautomers. Deactivation paths through a Conical Intersection (CI) region are also analyzed to explain the low fluorescence quantum yield of the phototautomers. The complex molecular structure of BSP provides a large number of deactivation paths, almost all of them energetically available following the initial photoexcitation. Femtosecond (fs) time-resolved emission studies in solution and flash photolysis experiments (nano to millisecond regime) were performed to get detailed information on the time domain of the full photocycle. The picture that emerges by combining theoretical and experimental results shows a very fast (less than 100 fs) photoinduced single proton transfer process leading to a phototautomer where a single proton has moved. This species may deactivate through a low-energy CI leading in about 20 ps to a rotameric form in the ground state that has a lifetime of several tens of microseconds in solution. This process competes with another deactivation path taking place prior to the proton-transfer reaction which involves a low-energy CI leading to a rotamer of the enol structure. In the flash photolysis studies, the rotamer of the enol structure was directly identified by the positive transient absorption band in the 250-260 nm and its lifetime in n-hexane (10 ms) is almost 3 orders of magnitude longer than the lifetime of the photochrome (around 40 μs). Our findings do not exclude a double proton transfer reaction in the excited enol form to give a tautomer in less than 100 fs during the first (impulsive) phase of the reaction which reverts back to the photoproducts of the simple proton transfer in 1-3 ps.

4-Phenyldiazenyl-2-[(R)-(1-phenylethyl)iminomethyl]phenol

The title chiral photochromic Schiff base compound, C21H19N3O, was synthesized from (R)-1-phenyl­ethyl­amine and the salicylaldehyde of an azobenzene derivative. The mol­ecule corresponds to the phenol–imine tautomer, the C=N and N—C bond distances being 1.279 (3) and 1.477 (3) Å, respectively. An intra­molecular O—H⋯N hydrogen bond occurs. The diazenyl group adopts a trans form with an N=N distance of 1.243 (3) Å.

A chiral photochromic Schiff base: (R)-4-methoxy-2-[(1-phenylethyl)iminomethyl]phenol

The title chiral photochromic Schiff base compound, C16H17NO2, was synthesized from (R)-1-phenyl­ethyl­amine and 5-methoxy­salicylaldehyde. The mol­ecule of the title compound exists in the phenol–imine tautomeric form. The dihedral angle between the two aromatic rings is 62.61 (11)°. An intra­molecular O—H⋯N hydrogen bond with an O⋯N distance of 2.589 (2) Å is observed. The crystal packing is stabilized by C—H⋯π inter­actions involving the aromatic ring.

Influence of Intermolecular Hydrogen Bonding on the Photochromic Cycle of the Aromatic Schiff Base N,N′-Bis(salicylidene)-p-phenylenediamine in Solution

A photochromic symmetric Schiff base, N,N'-bis(salicylidene)-p-phenylenediamine, has been studied by means of stationary and time-resolved spectroscopic absorption and emission techniques in the UV-vis spectral range with particular attention to the role of intermolecular hydrogen bonds. They are found to be responsible for the solvent-assisted excited-state proton transfer (the time constant of about 400 fs) in very strongly protic solvents and the photochrome deactivation (the time constant from 0.5 micros to 3 ms) in both protic and nonprotic solvents. Moreover, formation of an isomer that competes with the photochromic cycle in solution is also observed.

Photochromism and hydrolysis of aromatic Schiff base N,N′-bis(salicylidene)-p-phenylenediamine (BSP) studied in heterogeneous environments

A photochromic symmetric Schiff base, N,N′-bis(salicylidene)-p-phenylenediamine has been studied by means of stationary spectroscopic absorption and emission techniques in the IR and UV-vis spectral range in heterogeneous environments: microporous and mesoporous molecular sieves and micellar systems. The intermolecular hydrogen bonds between the molecule studied and silanol groups in mesoporous material have led initially to the stabilization of the cis-keto tautomer and later to a very effective hydrolysis. Persisting two-step color changes have been found to occur after the UV irradiation of the molecule studied encapsulated in microporous and mesoporous molecular sieves.

Spectroscopic and photophysical properties of salicylaldehyde azine (SAA) as a photochromic Schiff base suitable for heterogeneous studies

The photochromic cycle of the salicylaldehyde azine (SAA) has been investigated by means of the stationary and time-resolved UV–vis spectroscopy in a number of differently interacting solvents and micellar systems. The primary enol form of this Schiff base is much more energetically stabilized than in the other aromatic Schiff bases, which is reflected in remarkable resistance of SAA to the hydrolysis process. The fluorescence decay measured in highly viscous solvents as well as in micellar systems has proved the existence of different conformers of the cis-keto tautomer. Three different routes of the photochrome ( trans-keto tautomer) decay have been also observed.

A selective colorimetric chemosensor based on calixarene framework for lanthanide ions-Dy 3+ and Er 3+

A novel photochromic Schiff base derivative, 5,17- N, N′-(5,5′-di-methoxyl-benzlaldehyde)diimine-25,27-dipropoxy-calix[4]arene has been synthesized through incorporation of two imine groups into the upper rims of calix[4]arene. The experiments showed that the addition of Dy 3+ or Er 3+ ions to compound 4 results in the solution color change, but other metal ions caused less change. It indicated that the synthetic receptor can selectively recognize Dy 3+ and Er 3+ ion by naked eye over other lanthanide cations. Thus, these systems can be applied as chemosensor for Dy 3+ and Er 3+ ion in analytical chemistry.

Multistate/multifunctional switches based on photochromic Schiff base

The salicylidene Schiff base derivative, namely, N-salicylidene-( S)-α-naphthylethylamine (SNEA) has been synthesized to study the characterization of multistate/multifunctional switches. Upon the stimulations of optical inputs (UV light and visible light) and chemical inputs (pH and Zn 2+), SNEA undergoes reversible photochromism, deprotonation and complexation reactions. In this case, four chemical species are involved. These interconversions of SNEA between four states have been systematically investigated by the absorption and the emission spectra. Spectroscopic studies indicate that the photochromic switch, pH switch and fluorescent switch can be realized using the single molecular entity of SNEA.