Photochromic bipyridyl metal complexes: Photoregulation of the nonlinear optical and/or luminescent properties

The combination of transition metals and ligands featuring photochromic units is receiving much attention and opens up new perspectives for the design of metal-based photoswitchable molecules. This account summarizes some of our recent works made in the area of photochromic organometallic and coordination compounds, which have been used for the photomodulation of the quadratic non-linear optical (NLO) properties, as well as of the photoregulation of the emission properties of the resulting systems. For this purpose, we have designed new chromophores combining dithienylethene (DTE)-based bipyridine ligands with different metallic fragments (Re^I, Fe^II, Ru^II, Ir^III, Cu^I, Zn^II), giving rise to multi-photochromic metal complexes containing from two to six DTE units, and studied the photocontrol of both NLO and luminescence properties.     

Photoisomerization of PtII Complexes Containing Two Different Photochromic Chromophores: Boron Chromophore versus Dithienylethene Chromophore

In order to examine competitive photoisomerization, a series of novel photochromic Pt^II molecules that contain both dithienylethene (DTE) and B(ppy)Mes₂ units (ppy=2-phenylpyridine, Mes=mesityl) were successfully synthesized and fully structurally characterized. Their photochromic properties were examined by UV/Vis, emission and NMR spectroscopy. It was found that the DTE unit in all three compounds is the preferred photoisomerization site, exhibiting reversible photochromism with irradiation. The B(ppy)Mes₂ unit does not undergo photoisomerization in these molecules, but likely enhances the photoisomerization quantum efficiency of the DTE moiety through the antenna effect. Extended irradiation with UV light leads to the rearrangement of the ring-closed isomers of DTE. TD-DFT computational studies indicate that the DTE photocyclization proceeds via a triplet pathway through an efficient energy transfer process.     

Photochromic metal complexes: photoregulation of both the nonlinear optical and luminescent properties

A series of dithienylethene (DTE)-containing 2,2'-bipyridine ligands and their zinc(II) diacetate, zinc(II) dichloro, rhenium(I) tricarbonyl bromo, and ruthenium(II) bis(bipyridine) complexes have been designed and synthesized, and their photochromic, photophysical, and quadratic nonlinear optical properties have been studied. Upon UV irradiation at 350 nm, the ligands and complexes undergo ring closure of the DTE units, with a good to excellent photocyclization yield. In the case of the Re(I) and Ru(II) complexes, the photocyclization of the DTE units can also be triggered using visible light, upon excitation into the metal-to-ligand charge-transfer (MLCT) bands at 400 and 490 nm, respectively. Molecular quadratic nonlinear optical (NLO) responses of the complexes have been determined by using either the electrical field induced second harmonic generation (EFISH) or harmonic light scattering (HLS) technique at 1910 nm. These studies reveal a large increase of the second-order NLO activity after UV irradiation and subsequent formation of the ring-closed isomers. This efficient enhancement clearly reflects the delocalization of the π-electron system and the formation of strong push-pull chromophores in the closed forms. The combination of the photochromic DTE-based bipyridine ligand with luminescent Re(I) and Ru(II) fragments also allows the photoregulation of the emission, leading to an efficient quenching of the ligand-based 77 K luminescence and demonstrating that the photocontrol of two optical properties, linear and nonlinear, could be achieved by using the same photochromic ligand.     

Highly efficient visible-light driven photochromism: developments towards a solid-state molecular switch operating through a triplet-sensitised pathway

We introduce a new highly efficient photochromic organometallic dithienylethene (DTE) complex, the first instance of a DTE core symmetrically modified by two Pt(II) chromophores [Pt(PEt(3))(2)(C≡C)(DTE)(C≡C)Pt(PEt(3))(2)Ph] (1), which undergoes ring-closure when activated by visible light in solvents of different polarity, in thin films and even in the solid state. Complex 1 has been synthesised and fully photophysically characterised by (resonance) Raman and transient absorption spectroscopy complemented by calculations. The ring-closing photoconversion in a single crystal of 1 has been followed by X-ray crystallography. This process occurs with the extremely high yield of 80%--considerably outperforming the other DTE derivatives. Remarkably, the photocyclisation of 1 occurs even under visible light (>400 nm), which is not absorbed by the non-metallated DTE core HC≡C(DTE)C≡CH (2) itself. This unusual behaviour and the high photocyclisation yields in solution are attributed to the presence of a heavy atom in 1 that enables a triplet-sensitised photocyclisation pathway, elucidated by transient absorption spectroscopy and DFT calculations. The results of resonance Raman investigation confirm the involvement of the alkynyl unit in the frontier orbitals of both closed and open forms of 1 in the photocyclisation process. The changes in the Raman spectra upon cyclisation have permitted the identification of Raman marker bands, which include the acetylide stretching vibration. Importantly, these bands occur in the spectral region unobstructed by other vibrations and can be used for non-destructive monitoring of photocyclisation/photoreversion processes and for optical readout in this type of efficiently photochromic thermally stable systems. This study indicates a strategy for generating efficient solid-state photoswitches in which modification of the Pt(II) units has the potential to tune absorption properties and hence operational wavelength across the visible range.     

Photocontrolled Intramolecular Charge/Energy Transfer and Fluorescence Switching of Tetraphenylethene‐Dithienylethene‐Perylenemonoimide Triad with Donor–Bridge–Acceptor Structure

Photochromic 1,2‐dithienylethene (DTE) derivatives with a high thermal stability and fatigue resistance are appealing for optical switching of fluorescence. Here, we introduce a donor–photochromic bridge–acceptor tetraphenylethene‐dithienylethene‐perylenemonoimide (TPE‐DTE‐PMI) triad, in which TPE acts as the electron donor, PMI as the electron acceptor, and DTE as the photochromic bridge. In this system, the localized and intramolecular charge transfer emission of TPE‐DTE‐PMI with various Stokes shifts have been observed due to the photoinduced intramolecular charge transfer in different solvents. Upon UV irradiation, the fluorescence quenching resulting from photochromic fluorescence resonance energy transfer in TPE‐DTE‐PMI has been demonstrated in solution and in solid films. The fluorescence on/off switching ratio in polymethylacrylate film exceeds 100, a value much higher than in polymethylmethacrylate film, thus indicating that the fluorescence switching is dependent on matrices.     

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.     

Photoregulated fluorescence switching in axially coordinated tin(IV) porphyrinic dithienylethene

Photochromic fluorophore Sn(TTP)(DTE)2 , in which two phenolic derivatives of 1,2-dithienylethene are axially coordinated to (5,10,15,20-tetratolylporphyrinato)tin(IV) in trans position, has been synthesized and fully characterized by various spectroscopic methods. We have also investigated the photoregulated fluorescence switching behavior of Sn(TTP)(DTE)2 . The fluorescence of the porphyrin macrocycle in Sn(TTP)(DTE) 2 greatly depends on the state of the 1,2-dithienyletene photochromic switch. In the open state (Sn(TTP)(o-DTE)2), the porphyrin exhibits high fluorescence intensity at 609 and 664 nm when excited at 410 nm. When the photocyclization reaction was carried out by irradiating Sn(TTP)(o-DTE)2 with the UV light (approximately 365 nm), the fluorescence intensity of the porphyrin macrocycle decreased. Back irradiation with visible light at wavelengths greater than 500 nm regenerated Sn(TTP)(o-DTE)2 and almost restored the original fluorescence spectrum. The fluorescence intensity of the porphyrin fluorophore is efficiently regulated by photochromic switching between Sn(TTP)(o-DTE)2 and Sn(TTP)(c-DTE)2 in several cycles, clearly demonstrating that the Sn(TTP)(DTE)2 can act as a system for reversible data processing using fluorescence as the detection method.     

A Versatile Dithienylethene‐Functionalized Ph‐Diazabutadiene (DAB) Ligand: From Photoswitchable Main‐Group Molecules to Photochromic Polymers

Within the past decade photochromic materials, specifically dithienylethenes (DTEs), have received increased interest because of their ability to function as potential photoswitchable molecular devices and optical memory storage systems. Current research in this area has focused on incorporating organic architectures to functionalize the DTE framework and alter the resulting photophysical properties; however, their syntheses are often not trivial. In this context, we have designed a simple and versatile diimine ( 2 ) containing adjacent 2,5‐dimethyl(thienyl) rings in the backbone. This redox active diimine ( 2 ) acts as a precursor to a novel photochromic ligand and has been used to coordinate to both boron and phosphorus elements, along with the synthesis of a phosphorane‐side‐chain functionalized polymer without further functionalization to the parent DTE framework. A study of the resulting photochromic properties of these compounds revealed that 1) the UV‐visible absorption spectra of the closed‐ring isomer were dependent of the element present in the N,N′‐chelating pocket and 2) incorporating the dithienylethene into a side‐functionalized phosphorane polymer greatly increased the closed‐/open‐ring reversibility and decreased the formation of by‐products.     

Recent progress of luminescent metal complexes with photochromic units

Photo-responsive molecules have been studied extensively because of their light irradiation abilities that enable modulation of certain physical and chemical properties in emerging molecular electronic and photonic devices. For advanced photonic applications, photochromic metal complexes that have photochromic units as the photo-responsive ligand are highly desirable, as they allow improvement of the photochromic properties and their photo-switching functionality. This article focuses on recent progress in luminescent metal complexes with photochromic units. Luminescence-switching properties of photochromic metal complexes depend on characteristic electronic transitions. The electronic transitions of photochromic metal complexes can be divided into three categories: (1) π–π* transition of the ligand, (2) metal to ligand charge transfer (MLCT) in transition-metal complex, and (3) f–f transition in lanthanide complex. Luminescence modulation using various metal complexes with photochromic units has been studied extensively in recent years, and various applications for future molecular switching devices are expected in the field of advanced photonics. Based on the literature and our studies on luminescent metal complexes with photochromic units, we report on the recent progress of luminescent metal complexes with photochromic units.     

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.     

Core‐Modified Rubyrins Containing Dithienylethene Moieties

Two stable core‐modified rubyrins bearing one and two dithienylethene (DTE) units ( 1 and 2 ) have been synthesized. With one “closed‐form” DTE unit, 1 shows aromaticity associated with its conjugated circuit of 26 π‐electrons. In contrast, rubyrin 2 containing one “open‐form” DTE unit has nonaromatic properties.     

二芳基乙烯衍生物与DNA结合的分子动力学研究

基于分子动力学方法, 对2种旋转异构的二芳基乙烯(DTE, dithienylethene)衍生物(DTE1和DTE2)与不同分子结构DNA结合过程的热力学与动力学特征进行模拟, 结果发现, DTE1, DTE2与DNA分子采用小凹槽结合(MiGB)的模式结合时所需能量最低, 存在的分子间库仑能与范德华相互作用能最小, 说明该结合模式最稳定; 由于空间位阻作用, 互为旋转异构体的2个DTE衍生物与DNA作用表现出截然不同的结合行为, DNA对DTE衍生物具有明显的对映异构体选择性; DTE衍生物与DNA分子作用位点的选择性直接与构成位点的碱基对相关.     

Synthesis,Photochromic, and Computational Studies of Dithienylethene‐Containing β‐Diketonate Derivatives and Their Near‐Infrared Photochromic Behavior Upon Coordination of a Boron(III) Center

A series of dithienylethene‐containing 1‐thienyl‐3‐aryl‐propane‐1,3‐diones (aryl=phenyl (Ph), thienyl (Th), and 4,5‐bis(2,5‐dimethylthiophen‐3‐yl)thiophen‐2‐yl (DTE‐Th)) and the corresponding boron(III) diketonates, (O^O)BR₂ (R=F, C₆F₅, and Ph), have been designed and synthesized. Their photophysical, electrochemical, and photochromic properties have been studied. Upon coordination of a boron(III) center, the closed forms of the dithienylethene‐containing β‐diketonates show near‐infrared response and the photochromic behavior was also found to be affected by the aryl substituents at the 3‐position of the β‐diketonates. Moreover, computational studies have been performed that help to provide an understanding of the effect of substituents on the photophysical and photochromic properties.     

Spectrokinetic studies on new bi-photochromic molecules containing two naphthopyran entities

A range of new bi-photochromic molecules containing two identical (3a-d) or two distinct naphthopyran units (6a-d), linked through the phenyl substituents located on the sp³ hybridised pyran ring carbon atom, using conjugated and non-conjugated spacers, have been synthesised from bis-propynols and (substituted)naphthols. Study of the spectrokinetic properties of these compounds under near UV-vis continuous irradiation conditions revealed that the two naphthopyran units are stimulated independently leading to open forms with higher colourabilities but without affecting the individual bleaching kinetics. Compared to the individual photochromic components and to model mono-photochromes it was observed that the nature of the bridge has a small effect on the photochromic properties of each system.     

Gold(I)-coordination triggered multistep and multiple photochromic reactions in multi-dithienylethene (DTE) systems

The preparation, characterization, and photochromic properties of a mononuclear gold(I) complex (1oo) with two identical DTE-acetylides and a dinuclear gold(I) complex (2ooo) with both DTE-acetylide and DTE-diphosphine are described. Both gold(I) complexes exhibit multistep and multiple photocyclization/cycloreversion reactions. Particularly, four-state and four-color photochromic switch is successfully achieved for the dinuclear gold(I) complex upon irradiation with appropriate wavelengths of light. In contrast, fully ring-closed form is unattained through multiple photocyclization for the two corresponding model organic compounds coupling with the same DTE units as gold(I) complexes but without gold(I)-participation. It is demonstrated that coordination of gold(I) ion to DTE-acetylides exerts indeed a crucial role in achieving stepwise and selective photocyclization and cycloreversion reactions for both gold(I) complexes, in which the coordinated gold(I) atom acts as an effective "barrier" to prohibit intramolecular energy transfer between multi-DTE moieties.     

Diarylethene-containing cyclometalated platinum(II) complexes: tunable photochromism via metal coordination and rational ligand design

The synthesis, characterization, electrochemistry, photophysics and photochromic behavior of a new class of cyclometalated platinum(II) complexes [Pt(C(∧)N)(O(∧)O)] (1a-5a and 1b-5b), where C(∧)N is a cyclometalating 2-(2'-thienyl)pyridyl (thpy) or 2-(2'-thienothienyl)pyridyl (tthpy) ligand containing the photochromic dithienylethene (DTE) unit and O(∧)O is a β-diketonato ligand of acetylacetonato (acac) or hexafluoroacetylacetonato (hfac), have been reported. The X-ray crystal structures of five of the complexes have also been determined. The electrochemical studies reveal that the first quasi-reversible reduction couple, and hence the nature of lowest unoccupied molecular orbital (LUMO) of the complexes, is sensitive to the nature of the ancillary O(∧)O ligands. Upon photoexcitation, complexes 1a-3a and 1b-3b exhibit drastic color changes, ascribed to the reversible photochromic behavior, which is found to be sensitive to the substituents on the pyridyl ring and the extent of π-conjugation of the C(∧)N ligand as well as the nature of the ancillary ligand. The thermal bleaching kinetics of complex 1a has been studied in toluene at various temperatures, and the activation barrier for the thermal cycloreversion of the complex has been determined. Density functional theory (DFT) calculations have been performed to provide an insight into the electrochemical, photophysical and photochromic properties.     

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.     

Charge transfer and intraligand excited state interactions in platinum-sensitized dithienylethenes

The photophysical behavior for two photochromic Pt-terpyridine acetylide complexes containing pendant dithienylethenes (DTEs) bound to the metal through the alkynyl linkage is presented. Selective excitation of the Pt complex with visible light resulted in the metal-sensitized ring closing of the DTE unit. The central purpose of this study was to understand how excited state interactions govern the photophysics by correlating differences in the linkage of the two components with differences in the intramolecular energy transfer processes that occur between the Pt complex and the DTE. A series of model complexes without photochromic ligands were prepared and studied to elucidate the contributions of the triplet metal-to-ligand charge transfer and triplet intraligand states. It is demonstrated that reducing the orbital overlap of the metal-based and intraligand states by lengthening the linkage and eliminating a conjugated pathway is effective at dramatically decreasing the efficiency of intramolecular energy transfer. This is evidenced by the appearance of Pt-terpyridine based phosphorescence and a significant decrease in the observed rate of metal-sensitized ring closing of the DTE.     

Design,synthesis, photochromism and electrochemistry of a novel material with pendant photochromic units

In the present work, the synthesis, photochromism and electrochemistry of a novel material 1, 1-(4-[3,4-bis(2,5-dimethyl-3-thienyl)cyclopent-3-en-1-yl]phenyl)-2,5-di-2-thienyl-1H-pyrrole, with pendant dithienylethene (DTE) photochromic units are described. It should be noted that the system 1 can be reversibly and efficiently switched between open (1o) and closed (1c) states by light in both solution and in the solid poly(methyl methacrylate) matrix. It is also noteworthy that the two isomers (1o and 1c) of this novel system 1 can be smoothly polymerized on ITO by electrochemical means. Surprisingly, the DTE unit in 1 does not retain its photochemical switching properties after immobilization onto ITO. The morphology of the polymer film was investigated by AFM analysis. Furthermore, it was found that the polymer exhibited remarkable electrochromic features that can be switched from green in the neutral state to violet state under applied external potentials without disturbing the photochromic units.     

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.