Tuning the Photoisomerization of a N^C‐Chelate Organoboron Compound with a Metal?Acetylide Unit

To examine the impact of metal moieties that have different triplet energies on the photoisomerization of B(ppy)Mes₂ compounds (ppy=2‐phenyl pyridine, Mes=mesityl), three metal‐functionalized B(ppy)Mes₂ compounds, Re‐B , Au‐B , and Pt‐B , have been synthesized and fully characterized. The metal moieties in these three compounds are Re(CO)₃(tert‐Bu₂bpy)(C?C), Au(PPh₃)(C?C), and trans‐Pt(PPh₃)₂(C?C)₂, respectively, which are connected to the ppy chelate through the alkyne linker. Our investigation has established that the Re^I unit completely quenches the photoisomerization of the boron unit because of a low‐lying intraligand charge transfer/MLCT triplet state. The Au^I unit, albeit with a triplet energy that is much higher than that of B(ppy)Mes₂, upon conjugation with the ppy chelate unit, substantially increases the contribution of the π→π* transition, localized on the conjugated chelate backbone in the lowest triplet state, thereby leading to a decrease in the photoisomerization quantum efficiency (QE) of the boron chromophore when excited at 365 nm. At higher excitation energies, the photoisomerization QE of Au‐B is comparable to that of the silyl–alkyne‐functionalized B(ppy)Mes₂ ( TIPS‐B ), which was attributable to a triplet‐state‐sensitization effect by the Au^I unit. The Pt^II unit links two B(ppy)Mes₂ together in Pt‐B , thereby extending the π‐conjugation through both chelate backbones and leading to a very low QE of the photoisomerization. In addition, only one boron unit in Pt‐B undergoes photoisomerization. The isomerization of the second boron unit is quenched by an intramolecular energy transfer of the excitation energy to the low‐energy absorption band of the isomerized boron unit. TD‐DFT computations and spectroscopic studies of the three metal‐containing boron compounds confirm that the photoisomerization of the B(ppy)Mes₂ chromophore proceeds through a triplet photoactive state and that metal units with suitable triplet energies can be used to tune this system.     

Insights into the Photoinduced Isomerization Mechanisms of a N,C–Chelate Organoboron Compound: A Theoretical Study

As the first discovered organoboron compound with photochromic property, B(ppy)Mes₂ (ppy=2-phenylpyridine, Mes=mesityl) displays rich photochemistry that constitutes a solid foundation for wide applications in optoelectronic fields. In this work, we investigated the B(ppy)Mes₂ to borirane isomerization mechanisms in the three lowest electronic states (S₀, S₁, and T₁) based on the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT) calculations. Our results show that the photoisomerization in the S₁ state is dominant, which is initiated by the cleavage of the B-C_ppy bond. After overcoming a barrier of 0.5 eV, the reaction pathway leads to a conical intersection between the S₁ and S₀ states (S₁/S₀)_x, from which the decay path may go back to the reactant B(ppy)Mes₂ via a closed-shell intermediate (Int1-S₀) or to the product borirane via a biradical intermediate (Int2-S₀). Although triplet states are probably involved in the photoinduced process, the possibility of the photoisomerization in T₁ state is very small owing to the weakly allowed S₁→T₁ intersystem crossing and the high energy barrier (0.77 eV). In addition, we found the photoisomerization is thermally reversible, which is consistent with the experimental observations.     

Regulation of Charge Delocalization in a Heteronuclear Fe2Ru System by a Stepwise Photochromic Process

Heteronuclear complexes FeCp₂?DTE?C?C?Ru(dppe)₂Cl ( 1 o ; dppe=1,2‐bis(diphenylphosphino)ethane, Cp=cyclopentadienyl, DTE=dithienylethene) and FeCp₂?DTE?C?C?Ru(dppe)₂?C?C?DTE?FeCp₂ ( 2 oo ), with redox‐active ferrocenyl and ruthenium centers separated by a photochromic DTE moiety, were prepared to achieve photoswitchable charge delocalization and Fe???Ru electronic communication. Upon UV‐light irradiation of 2 oo , the Fe???Ru heterometallic electronic interaction is increasingly facilitated with stepwise photocyclization, 2 oo → 2 co → 2 cc ; this is ascribed to the gradual increase in π‐conjugated systems. The near‐infrared absorptions in mixed‐valence species [ 2 oo ]⁺/[ 2 co ]⁺/[ 2 cc ]⁺ are gradually intensified following the conversion of [ 2 oo ]⁺→[ 2 co ]⁺→[ 2 cc ]⁺, which demonstrates that the extent of charge delocalization shows progressive enhancement with stepwise photocyclization. As revealed by electrochemical, spectroscopic, and theoretical studies, complex 2 exhibits nine switchable states through stepwise photochromic and reversible redox processes.     

Structural Dynamics and Stereoselectivity of Chiral Benzylideneamine N,C-Chelate Borane Photo–Thermal Isomerization

New chiral N,C-chelate organoboron compounds based on benzylideneamines (bza) with the general formula of B(bza-R)Mes₂ (R=H or Me; Mes=mesityl) are reported. A chiral substituent group R- or S-CH(CH₃)Ph (Ph=phenyl) was introduced to the imine center, which imposed a previously unobserved pseudo- or axial-chirality on the BMes₂, creating distinct diastereomers. NMR spectroscopic studies established that the diastereomers undergo slow exchange in solution at ambient temperature. The chiral N,C-chelate B(bza-R)Mes₂ molecules undergo photoisomerization in the same manner as their non-chiral analogues, generating chiral BN-cyclooctatriene (BN-COT) derivatives. Most significantly, by tracking the photoisomerization with circular dichroism (CD) and ¹H NMR spectra along with time-dependent density functional theory (TD-DFT) computational studies, the photoisomerization was established to proceed in a highly stereoselective manner, that is, one diastereomer converts exclusively to the corresponding diastereomer product in the photoreaction.     

Single boryl isomerization in silyl-bridged photochromic diboryl dyes

Silyl-bridged dimers of a ppy-BMes(2) (ppy = 2-phenylpyridine, Mes = mesityl) photochrome were found to undergo photochromic switching involving a single boryl unit only. A through-space intramolecular energy transfer was found to be responsible for the single-chromophore isomerization phenomenon and fluorescence quenching. Steric congestion in the diboryl molecules was found to have an impact on photoisomerization quantum efficiency.     

Crystal structure and photochromism of 1-phenyl-3-methyl-4-benzyl-5-one-pyrazole S-methyl thiosemicarbazone

A new organic photochromic compound containing pyrazolone-ring photochromic functional unit: 1-phenyl-3-methyl-4-benzyl-5-one pyrazole S-methyl thiosemicarbazone (PMBP-smtsc) was synthesized. The photochromic properties and photochemical kinetics of PMBP-smtsc have been studied by UV reflectance spectra under irradiation of 365 nm light. The crystal structure analyses of photocolored product show the photochromism is due to the photoisomerization from enol form to keto form through an intermolecular proton transfer.     

Two-photon absorption of photochromic diarylethene and its application to rewritable holographic recording

A new photochromic diarylethene (1a) has been prepared. Both its photochromic behavior and nonlinear optical properties are investigated. 1a shows excellent ring-opening (λ_max = 386 nm) and ring-closing (λ_max = 652 nm) photoisomerization with UV-Vis light irradiation. With 800 nm femtosecond pulsed laser irradiation, 1a shows two-photon-induced photoisomerization, and a two-photon absorption cross-section (σ = 423×10⁵⁰ cm⁴·s per photon) is obtained by using two-photon induced fluorescence method. The applications of two-photon absorption of 1a to holographic recording has been also investigated. A two-photon induced micro-pattern is recorded on the diarylethene 1a-PMMA film with an femtosecond laser of 800 nm, 100 fs, 1 kHz, 50 mW.     

Fluorescence and morphology modulation in a photochromic diarylethene self-assembly system

Switchable supramolecular self-assemblies on the basis of interaction between melamine group containing photochromic diarylethene unit (DTE) and naphthalimide derivate (1) were designed and fabricated. 1 can gelate several aprotic solvents with different morphologies. The gel turned into partial gel in ethyl acetate with the addition of DTE as a guest molecule. Both the absorption and fluorescence spectra of the assembly can be reversibly switched by alternating UV/visible light irradiation. Meanwhile, the morphology of the coassembly of 1(2)·DTE changed to film from original pieces of gel 1 in ethyl acetate. When 1(2)·DTE was irradiated by UV light, the film morphology was converted into aggregated flakes. Moreover, the surface wettability of the complex can also be switched by light irradiation. The photochromic diarylethene unit is able to modulate the fluorescence and morphology of the assembled system only by virtue of light irradiation. Therefore, these results provide further insights into fluorescence and morphology controlling, especially application in upscale smart responsive materials.     

Theoretical Study on Photoisomerization Mechanisms of Diphenyl-Substituted N,C-Chelate Organoboron Compounds

N,C-chelate organoboron compounds are widely employed as photoresponsive and optoelectronic materials due to their efficient photochromic reactivity. It was found in experiments that two diphenyl-substituted organoboron compounds, namely B(ppy)Ph₂ (ppy=2-phenylpyridyl) and B(iba)Ph₂ (iba=N-isopropylbenzylideneamine), show distinct photochemical reactivity. B(ppy)Ph₂ is inert on irradiation, whereas B(iba)Ph₂ undergoes photoinduced transformations, yielding BN-cyclohepta-1,3,5-triene via a borirane intermediate. In this work, the complete active space self-consistent field and its second-order perturbation (CASPT2//CASSCF) methods were used to investigate the photoinduced reaction mechanisms of B(ppy)Ph₂ and B(iba)Ph₂. The calculations showed that the two compounds isomerize to borirane in the same way by passing a transition state in the S₁ state and a conical intersection between the S₁ and S₀ states. The energy barriers in the S₁ state of 0.54 and 0.26 eV for B(ppy)Ph₂ and B(iba)Ph₂, respectively, were explained by analyzing the charge distributions of minima in S₀ and S₁ states. The results provide helpful insights into the excited-state dynamics of organoboron compounds, which could assist in rational design of boron-based photoresponsive materials.     

Photo-modulated wide-spectrum chromism in Eu3+ and Eu3+/Tb3+ photochromic coordination polymer gels: application in decoding secret information

Photo-switching emission of photochromic materials has paramount importance in the field of optoelectronics. Here, we report synthesis and characterization of a dithienylethene (DTE) based photochromic low molecular weight gelator (LMWG) and self-assembly with lanthanide (Eu³⁺ and Tb³⁺) ions to form a photochromic coordination polymer gel (pcCPG). Based on DTE ring opening and closing, the TPY-DTE gel shuttles from pale-yellow coloured TPY-DTE-O to dark blue coloured TPY-DTE-C and vice versa upon irradiating with UV and visible light, respectively, and both the photoisomers show distinct optical properties. Furthermore, integration of Eu³⁺ and Tb³⁺ lanthanides with TPY-DTE resulted in red and green emissive Eu-pcCPG (Q.Y. = 18.7% for the open state) and Tb-pcCPG (Q.Y. = 23.4% for the open state), respectively. The photoisomers of Eu-pcCPG exhibit photo-switchable spherical to fibrous reversible morphology transformation. Importantly, an excellent spectral overlap of the Eu³⁺ centred emission and absorption of DTE in the closed form offered photo-switchable emission properties in Eu-pcCPG based on pcFRET (energy transfer efficiency >94%). Further, owing to the high processability and photo-switchable emission, the Eu-pcCPG has been utilized as invisible security ink for protecting confidential information. Interestingly, mixed Eu³⁺/Tb³⁺ pcCPG exhibited photo-modulated multi-spectrum chromism reversibly where the colour changes from yellow, blue, and red to green and vice versa under suitable light irradiation.

A lanthanide based photochromic coordination polymer gel (pcCPG) material has been developed which showed photomodulated colour change based on pcFRET and has the potential to be employed for decoding secret information.     

A Magneto‐optical Molecular Device: Interplay of Spin Crossover,Luminescence, Photomagnetism,and Photochromism

A bis(pyrazolylpyridyl) ligand, L, containing a central photochromic dithienylethene spacer predictably forms a ferrous [Fe₂L₃]⁴⁺ helicate exhibiting spin crossover (SCO). In solution, the compound [Fe₂L₃](ClO₄)₄ ( 1 ) preserves the magnetic properties and is fluorescent. The structure of 1 is photo‐switchable following the reversible ring closure/opening of the central dithienylethene via irradiation with UV/visible light. This photoisomerization switches on and off some emission bands of 1 and provides a means of externally manipulating the magnetic properties of the assembly.     

A Magneto‐optical Molecular Device: Interplay of Spin Crossover,Luminescence, Photomagnetism,and Photochromism

A bis(pyrazolylpyridyl) ligand, L, containing a central photochromic dithienylethene spacer predictably forms a ferrous [Fe₂L₃]⁴⁺ helicate exhibiting spin crossover (SCO). In solution, the compound [Fe₂L₃](ClO₄)₄ ( 1 ) preserves the magnetic properties and is fluorescent. The structure of 1 is photo‐switchable following the reversible ring closure/opening of the central dithienylethene via irradiation with UV/visible light. This photoisomerization switches on and off some emission bands of 1 and provides a means of externally manipulating the magnetic properties of the assembly.     

Achieving Enhanced Photochromic Properties of Diarylethene through Host-Guest Interaction in Aqueous Solution

Diarylethene (DTE) has been widely used in fluorescence probes, molecular logic gates, optical data-storage devices owing to the excellent photochromic property, while constructing high-performance photochromic DTE in aqueous media remains a big challenge. Herein we present several host-guest systems formed between cucurbit[n]uril (CB[n], n=7, 8, 10) and two water-soluble DTE derivatives 1 and 2 . It was found that host-guest interactions not only affect the photophysical properties of photochromic guests, but also make great differences on the photoreaction process. Different host-guest binding behaviors also lead to different effects on the photochromic properties of guests. In the presence of CB[n], both 1 and 2 showed enhanced emission and higher fluorescence quenching ratio at photostationary state. Besides, CB[10] ⋅1 exhibited faster response rate in cyclization reaction and better photofatigue resistance than free 1 in aqueous solution, while the supramolecular assembly of (CB[8])_n ⋅ ( 2 )_n showed slower response rate in both directions of the reversible photoreaction. Besides, the photofatigue resistance of 2 can be greatly improved through binding with CB[7]. Our results suggest that host-guest interactions could be an efficient way to improve photochromic properties of DTE in aqueous solution.     

Dithienylethene metallodendrimers with high photochromic efficiency

It has been challenging to achieve multi-photochromic systems without affecting the individual photoswitching properties of the constituent units. Herein, we present the design and synthesis of a new family of platinum-acetylide dendrimers containing up to twenty-one photochromic dithienylethene(DTE)units that exhibit both high photochromic efficiency and individual switching properties. Upon irradiation with ultraviolet(UV) and visible(vis) light, the resultant metallodendrimers display high co...     

Influence of helical twisting power on the photoswitching behavior of chiral azobenzene compounds: applications to high-performance switching devices

Five photochromic chiral azobenzene compounds and one nonphotochromic chiral compound were synthesized and characterized by IR, 1H NMR spectroscopy, and elemental analysis. Cholesteric liquid crystalline phases were induced by mixing of the nonphotochromic chiral compound and one of the photochromic chiral azobenzene compounds in a host nematic liquid crystal (E44). The helical pitch of the induced cholesteric phase was determined by Cano's wedge method and the helical twisting power (HTP) of each sample was thus determined. The helical twisting powers of azobenzene compounds were decreased upon UV irradiation, due to trans-->cis photoisomerization of azobenzene molecules. Among the azobenzene compounds synthesized in our study, Azo-5, with isomannide (radical) as chiral photochromic dopant, showed the highest HTP and contrast ratio (Tmax/Tmin). Photoswitching between compensated nematic phase and cholesteric phase was achieved through reversible trans<-->cis photoisomerization of the chiral azobenzene molecules through irradiation with UV and visible light, respectively. Transmission rates (contrast ratios) increased with decreasing helical pitch length in the induced cholesteric phase. The influence of helical twisting power on the photoswitching behavior of chiral azobenzene compounds is discussed in detail.     

Understanding of Photo-Induced Reversible Rearrangement from Borepin to Borirane

The first reversible photoisomerization between a borepin and a borirane was reported in the photo-induced reactions of B(npy)Ar₂ (npy=2-(naphthalen-1-yl) pyridine, Ar=phenyl or electron rich aryl; S. Wang, et al. Angew. Chem. Int. Ed. 2019 , 58, 6683–6687). In this work, the detailed mechanisms of the unprecedented reversible photoisomerization between the borepin (compound a ) and the borirane (compound b ) of B(npy)Ph₂ in the first excited singlet (S₁) state and the ground (S₀) state were studied by carrying out calculations with the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT). The calculation results show that photoexcitation of a -S₀ at 365 nm and b -S₀ at 450 nm populate their S₁ state with evident charge transfer characteristics. The photoisomerization is triggered in the S₁ state and ends in the S₀ state, at which the intersection points in a (S₁/S₀)_x intersection seam participate in and promote phenyl migration and ring-closure processes. Furthermore, we reveal that the not large energy difference (less than 0.6 eV) and similar conjugation properties of π electrons between a -S₀ and b -S₀ are responsible for their unique photo-reversible reactivity, compared with those of the isomers of the thermally reversible compound B(ppy)Mes₂. Our results contribute to an understanding of the excited-state reactivity of organoboron compounds and will be useful to support the design of new boron-based photo-responsive materials.     

A Mononuclear Iron(II) Spin-Crossover Molecule Decorated by Photochromic Azobenzene Group

Aiming at constructing photoresponsive spin crossover (SCO) behavior, herein we designed a new ligand Abtz (Abtz = (E)-N-(4-((E)-phenyldiazenyl)phenyl)-1-(thiazol-4-yl)methanimine) which was decorated by a photochromic azobenzene group. Based on this photochromic ligand, a mononuclear Fe(II) SCO molecule [Fe(Abtz)₃](BF₄)₂·(EAC)₂ (1, EAC = ethyl acetate) was successfully synthesized and showed a complete one-step SCO behavior. Under continuous UV light and blue-light exposure, the cis–trans photoisomerization of both ligand Abtz and compound 1 in the liquid phase was confirmed through UV–Vis spectra. Moreover, the ¹H-NMR spectra of Abtz reveal a trans–cis conversion ratio of 37%. Although the UV–Vis spectra reveal the photochromic behavior for 1 in the solution phase, the SCO behavior in the liquid state is absent according to the variable-temperature Evans method, suggesting the possible decomposition. Moreover, in the solid state, the cis–trans photoisomerization of both Abtz and 1 was not observed, due to the steric hindrance.     

A Diazabutadiene stabilized Nickel(0) Cyclooctadiene Complex: Synthesis,Characterization and the Reaction with Diphenylacetylene

The reaction of [Ni(COD)₂] with one equivalent of DAB^Mes (DAB^Mes = (2,4,6‐Me₃C₆H₂)N=C(Me)‐C(Me)=N(2,4,6‐Me₃C₆H₂)) affords a mixture of the compound [Ni(DAB^Mes)₂] ( 2 ) and starting material [Ni(COD)₂]. The crystallographically characterized, diamagnetic complex 2 can be obtained in a stoichiometric reaction of [Ni(COD)₂] and two equivalents of DAB^Mes. This reaction can be accelerated by addition of 1‐chloro‐fluorobenzene or methyl iodide. In the presence of 1‐chloro‐fluorobenzene, [Ni(DAB^Mes)(COD)] ( 3 ) is available via reaction of [Ni(COD)₂] and one equivalent of DAB^Mes. The crystallographically characterized complex 3 reacts with diphenylacetylene to afford [Ni(DAB^Mes)(Ph‐C≡C‐Ph)] ( 4 ). A long‐wavelength absorption band in the UV‐Vis spectrum of this compound has to be assigned to a mixed MLCT/LL′CT transition, as quantum chemical calculations reveal.     

Amphiphilic Diblock Copolymer with Dithienylethene Pendants: Synthesis and Photo‐Modulated Self‐Assembly

In this work, an amphiphilic diblock copolymer (PEG₄₃‐b‐PSDTE₂₉) bearing photochromic dithienylethene (DTE) pendants is synthesized by reversible addition fragmentation chain transfer radical polymerization. The diblock copolymer was characterized by spectroscopic methods and gel permeation chromatography. The analyses proved the well‐defined structure and narrow molecular weight distribution of the diblock copolymer. The DTE pendants could undergo reversible photoisomerization between their open and closed forms in solution when irradiated with UV and visible light as indicated by ¹H NMR and UV‐vis spectroscopy. Hollow vesicle‐like structures were formed by gradually adding deionized water to the colorless PEG₄₃‐b‐PSDTE_29open (DTE in open form) tetrahydrofuran solution. Under the same conditions, the aggregates formed in the blue PEG₄₃‐b‐PSDTE_29close (DTE in closed form) solution were colloidal spheres with solid interiors. The isomerization of DTE pendants could cause the deformation of the vesicle‐like structures. The above results demonstrate a kind of novel photo‐modulated self‐assembly behavior of the amphiphilic diblock copolymer, which could be used for drug‐delivery and other applications.

     

Hydroxypyridinechromene and Pyridinechalcone: Two Coupled Photochromic Systems

Substitution of the phenyl group in 2‐hydroxychalcones by a 4‐pyridine unit dramatically changes the network of chemical reactions of this compound: trans‐chalcone‐type ( Ct ), cis‐chalcone‐type ( Cc ), and a hemiketal (hydroxy‐4‐pyridinechromene) ( B ) and their protonated forms are formed, but the presence of a flavylium‐type cation could not be detected even at very acidic pH values. Moreover, whereas in 2‐phenyl‐2‐benzopyrylium compounds B and Cc are generally elusive species whose kinetic processes in aqueous solutions occur on the sub‐second timescale, in the present compound these species equilibrate on a timescale four orders of magnitude lower. Complete characterization of the equilibrium and kinetics of the reaction network could thus be achieved by ¹H NMR spectroscopy and UV/Vis spectrophotometry. The network of chemical reactions exhibits cis–trans photoisomerization, as well as photochromism between the hemiketal and the chalcone‐type species. The irradiation of Ct in MeOH/H₂O (1:1) at 365 nm produces B almost quantitatively through two consecutive photochemical reactions: Ct → Cc photoisomerization followed by Cc → B photo ring closure with a global quantum yield of 0.02. On the other hand, irradiation of B at 254 nm leads to a photostationary state composed by 80 % Ct and 20 % B , with a quantum yield of 0.21.