Photochromic benzo[b]phosphole oxide with excellent thermal irreversibility and fatigue resistance in the thin film solid state via direct attachment of dithienyl units to the weakly aromatic heterocycle

1,3-Dipolar cyclisation reactions of nitriles with sterically encumbered cyclic triphosphanes: synthesis and electronic structure of phosphorus-rich heterocycles with tunable colour

We describe the synthesis, solid state and electronic structures of a series of tunable five-membered cationic and charge-neutral inorganic heterocycles featuring a P3CN core. 1-Aza-2,3,4-triphospholenium cations [(PR)3N(H)CR']+, [1R]+ (R' = Me, Ph, 4-MeOC6H4, 4-CF3C6H4) were formed as triflate salts by the formal [3 + 2]-cyclisation reactions of strained cyclic triphosphanes (PR)3 (R = t Bu, 2,4,6-Me3C6H2 (Mes), 2,6- i Pr2C6H3 (Dipp), 2,4,6- i Pr3C6H2 (Tipp)) with nitriles R'CN in the presence of triflic acid. The corresponding neutral free bases (PR)3NCR' (2R) were readily obtained by subsequent deprotonation with NEt3. The P3CN cores in 2R show an envelope conformation typical for cyclopentenes and present as yellow to orange compounds in the solid state as well as in solution depending on both substituents R and R' in (PR)3NCR'. The P3CN cores in [1R]+ show a significant deviation from planarity with increasing steric bulk of the R groups at phosphorus, which results in a decrease in the HOMO-LUMO gap and distinct low-energy UV-Visible absorption bands. This allows access to colours spanning red, blue, indigo, and magenta. TD-DFT calculations provide valuable insight into this phenomenon and indicate an intramolecular charge-transfer from the HOMO located on the P3 framework to the N[double bond, length as m-dash]C-R'-based LUMO in the cationic species. The cations [1R]+ represent rare examples of phosphorus-rich heterocycles with tunable colour, which can be incorporated into polymers by post-polymerization modification to afford coloured polymers, which demonstrate utility as both proton and ammonia sensors.

Synthesis and Characterization of a [1,2,6]Diazaphosphonine Oxide: An Example of a Photoswitchable Phosphorus-Containing Cyclic Azobenzene

We report herein the synthesis and characterization of a phosphorus-containing cyclic azobenzene as a new photoswitchable scaffold. This backbone reveals high bidirectional photoswitching yields and high thermal stability for both isomers, with t1/2 > 90 days at 60 °C. Both E- and Z-isomers have been characterized by UV-vis spectroscopy and X-ray crystallography.

Photochromic dithienylethene-containing four-coordinate boron(III) compounds with a spirocyclic scaffold

A new series of four-coordinate boron compounds bearing a photochromic dithienylethene-containing C^C ligand and an ancillary N^C ligand have been successfully designed and synthesised. These compounds exhibit reversible photochromism upon photoexcitation with percentage conversions of 71-96% and readily tuneable photocycloreversion quantum yields by convenient modification of the ancillary ligand to turn on the thermally activated upconversion from the lower-lying unreactive excited state to the higher-lying photoreactive excited state.

Photocontrolled multiple-state photochromic benzo[b]phosphole thieno[3,2-b]phosphole-containing alkynylgold(I) complex via selective light irradiation

Photochromic materials have drawn growing attention because using light as a stimulus has been regarded as a convenient and environmental-friendly way to control properties of smart materials. While photoresponsive systems that are capable of showing multiple-state photochromism are attractive, the development of materials with such capabilities has remained a challenging task. Here we show that a benzo[b]phosphole thieno[3,2‑b]phosphole-containing alkynylgold(I) complex features multiple photoinduced color changes, in which the gold(I) metal center plays an important role in separating two photoactive units that leads to the suppression of intramolecular quenching processes of the excited states. More importantly, the exclusive photochemical reactivity of the thieno[3,2‑b]phosphole moiety of the gold(I) complex can be initiated upon photoirradiation of visible light. Stepwise photochromism of the gold(I) complex has been made possible, offering an effective strategy for the construction of multiple-state photochromic materials with multiple photocontrolled states to enhance the storage capacity of potential optical memory devices.

C-centered radical-initiated cyclization by directed C(sp3)–H oxidative functionalization

C(sp3)–H functionalization is attracting constant attention. This review emphasizes C-centered radicals initiated cyclization strategies by directed C(sp3)–H oxidative functionalization since 2012.

Extending the Legible Time of Light-Responsive Rewritable Papers with a Tunable Photochromic Diarylethene Molecule

Inkless printing based on rewritable papers has recently made great progress because it can improve the utilization rate of papers, which is of great significance for saving resources and protecting the environment. Among them, light-responsive rewritable papers (LRPs) are a hot research topic because light is clean, easily available, wavelength and intensity adjustable, and noncontacting. However, the photochromic material, as the imaging substance of LRPs, is easily affected by environmental conditions, resulting in insufficient time to read the information. In view of this, we designed and constructed an acid/base tunable diarylethene molecular system that can effectively adjust the photochromic properties by reversibly changing the electron density of the diarylethene photoreaction center through protonation and demonstrated its potential as an imaging material with a longer legible time. What makes us more satisfied is that the acidification can not only extend the legible time of carrying information but also bring a clear and stable absorption/fluorescence imaging dual mode, which can better reflect details and improve contrast. Therefore, we believe that this tunable photochromic diarylethene molecule is a potential imaging material for the development of new LRPs.

Dithienylethene-Based Photochromic Siloles: A Straightforward and Divergent Synthetic Strategy

A straightforward synthetic methodology for the preparation of photochromic siloles based on the dithienylethene motif is developed. It relies upon an efficient palladium-catalyzed annulation reaction of a 2,3-bis(3-thienyl)-silirene with terminal alkynes in mild conditions. The reaction is functional group-tolerant and can be performed in high yields with a variety of functional terminal alkynes. It can even be extended to a polymeric polypropargylmethacrylamide (PPMA) substrate affording the corresponding photochromic polymer with different degree of photochromic unit incorporation by simply adjusting the polymer/ silirene stoichiometric ratio.

2,3-Diarylbenzo[b]arsole: Structural Modification and Polymerization for Tuning of Photophysical Properties

2,3-Diarylbenzo[b]arsoles were synthesized from zirconacycles and diiodophenylarsine. The structural modification to the luminophore was attained through diarylacetylene precursors, Suzuki-Miyaura coupling, and oxidation of the arsenic atom. The emission properties were controlled according to these modifications. The 2,3-diarylbenzo[b]arsoles showed aggregation-induced emission enhancement; the stronger emission was observed in the solid states than in solutions. In addition, Suzuki-Miyaura polycondensation and olefin metathesis polymerization produced main- and side-chain polymers, respectively. The resultant polymers showed different emission behaviors such as aggregation caused quenching and aggregation induced emission enhancement.

2-Arylbenzo[b]arsoles: an experimental and computational study on the relationship between structural and photophysical properties

Benzo[b]arsole derivatives, being arsenic analogues of indole, were synthesized by utilizing a safely prepared arsenic precursor. The structural and photophysical properties of the obtained 2-arylbenzo[b]arsoles were experimentally and computationally studied in comparison with those of 1,2,5-triarylarsoles and 9-phenylarsafluorene. It was found that the emission color and/or quantum yield were significantly affected by substituents on the luminescent center and metal-coordination to the arsenic atom. This is the first study on the structure-property relationship of benzo[b]arsole derivatives.

Synthesis, Photophysical, Photochromic, and Photomodulated Resistive Memory Studies of Dithienylethene-Containing Copper(I) Diimine Complexes

A series of dithienylethene-containing copper(I) diimine complexes have been synthesized and structurally characterized. Systematic studies on their photophysics, electrochemistry, and photochromism have been carried out. The photoinduced color changes of the copper(I) complexes have been achieved by photoexcitation into the metal-to-ligand charge-transfer (MLCT) absorption bands, indicating the photosensitization of light-induced cyclization by the ³MLCT excited state. In addition, by an increase in either the steric bulkiness around the copper(I) center or the structural rigidity of the complexes, the quantum efficiencies of photoluminescence and photocyclization can be effectively enhanced because of suppression of the flattening distortion of the complexes at the MLCT excited state. Furthermore, one of the complexes has been employed as an active component in the fabrication of solution-processed resistive memory devices. Notable lowering of the switching threshold voltage of the binary memory devices has been realized through photocyclization of the dithienylethene-containing copper(I) system.

Synthesis strategies for non-symmetric, photochromic diarylethenes

Diarylethenes (DAEs) represent an important class of photochromes with notable characteristics, like thermally irreversible photoisomerization and high fatigue resistance. Structural diversification of the DAE scaffold has enabled further refinement of photochromic properties and realization of new applications, ranging from advanced materials to tools for studying biological systems. In particular, methods for synthesizing non-symmetric DAE scaffolds, which are typically more challenging to synthesize than their symmetric counterparts, have grown over the past 20 years. These developments are surveyed in this review, with discussion of how access to these compounds has contributed to the improvement of photochromic properties and paved the way for exploring new applications of DAEs. First, non-symmetric DAE structures are classified and their uses and applications are overviewed. Subsequent sections discuss the main strategies that have been used to access non-symmetric DAEs with examples illustrating the impact of non-symmetric DAEs in the growing field of light-controlled molecular systems.

Regulation of an Ambient-Light-Induced Photocyclization Pathway (Norrish-Yang Versus 6π) by Substituent Choice

Photocyclization, irrespective of whether multiple steps (e.g., Norrish-Yang cyclization) or a single concerted step (e.g., 6π photocyclization) are involved, is an intramolecular photochemical process resulting in the formation of one new single bond to afford a ring system. In particular, visible-light-induced photocyclization offers a green and sustainable route to organic cyclic compounds that are difficult to access by thermal reactions. Herein, we describe the ambient light-induced intramolecular photocyclization of a series of donor/acceptor chromophores 1 d-3 d containing two types of photoresponsive motifs, namely an electron-deficient BF₂ -chelated ketone fused with an electron-rich thiophene, and probe the solution-phase and solid-state photochromic performance of these compounds. The results reveal that simple variation of R substituents on the diaryl moiety allows one to control the intramolecular photocyclization mechanism with high photochemical selectivity, e.g., under ambient light, methyl-substituted 1 d and 2 d undergo reversible 6π photocyclization, whereas ethyl-substituted 3 d exclusively undergoes irreversible Norrish-Yang photocyclization. Single-crystal X-ray analysis of Norrish-Yang cyclization products reveals the formation of four pairs of conformational enantiomers differing in the dihedral angle between benzothiophene and the BF₂ core, namely (±)N-3 d@68°, (±)N-3 d@-77°, (±)N-3 d@-78°, and (±)N-3 d@-102°. The UV/Vis absorption spectra of 1 d-3 d cover a broad visible-light region (380-572 nm), while DFT and TD-DFT calculations reveal that absorption in this region is dominated by the charge-transfer (CT) transition from the thiophene-centered HOMO to the LUMO of the electron-deficient π-conjugated BF₂ -chelated unit and the n→π* and π→π* transitions within the latter unit. The spatial separation of the HOMO and LUMO of these dyes promotes triplet-state generation and self-photosensitizes intramolecular photocyclization in the visible-light region. Three-dimensional time-resolved and steady-state emission spectra of 3 d show that the Norrish-Yang photocyclization takes place within milliseconds with excellent conversion efficiency (96 %).

Quinoline-containing diarylethenes: bridging between turn-on fluorescence, RGB switching and room temperature phosphorescence

Simple structural modifications using oxidation and methylation of a quinoline-containing diarylethene result in dramatic variation of photophysical properties. Turn-on fluorescence, room temperature phosphorescence (RTP) and red-green-blue (RGB) switching were achieved in three different related compounds. Photoswitchable diarylethenes (DAEs) that exhibit turn-on fluorescence are in high demand for super-resolution microscopy, and the development of purely organic phosphorescent materials in the amorphous state is attractive but challenging. The findings reported here provide a novel toolkit for designing turn-on fluorescence DAEs for super-resolution microscopy and extending the scope of amorphous RTP materials. More importantly, we bridge between these two fundamentally significant photochemical and photophysical phenomena, and reveal structure-property relationships between DAE photochromism and RTP.

Photochromic Barbiturate Pendant-Containing Benzo[b]phosphole Oxides with Co-Assembly Property and Photoinduced Morphological Changes

A series of novel barbiturate pendant-containing benzo[b]phosphole oxides has been demonstrated to exhibit photochromic and co-assembly properties. Both the open form and the photogenerated closed form of the diarylethene-based benzo[b]phosphole oxides have displayed color changes with new appearance of the bathochromic-shifted lowest-energy absorption bands upon addition of the H-bonding ditopic receptor that is complementary to the barbiturate groups, probably resulting in the formation of a supramolecular polymer. The co-assembly mixtures have displayed dissociation and association processes upon heating and cooling, respectively. Moreover, both the unassembled and co-assembled states of the benzo[b]phosphole oxide have exhibited photoinduced coloration, leading to a near-infrared absorption behavior. Multiaddressable states of the co-assembly mixture with multiple colors have been achieved via the combination of photochromism and thermochromism. More interestingly, photoinduced morphological changes from extensive porous network structures to ring-like aggregates have been observed for the co-assembly upon photoirradiation. The present work provides important insight for further developments of the multiaddressable systems and the supramolecular polymers with photo- and thermo-responsive behaviors, paving the way for the future design of photochromic materials with interesting photocontrollable functions.

Intramolecular Phosphacyclization: Polyaromatic Phosphonium P-Heterocycles with Wide-Tuning Optical Properties

Rationally designed cationic phospha-polyaromatic fluorophores were prepared through intramolecular cyclization of the tertiary ortho-(acene)phenylene-phosphines mediated by CuII triflate. As a result of phosphorus quaternization, heterocyclic phosphonium salts 1 c-3 c, derived from naphthalene, phenanthrene, and anthracene cores, exhibited very intense blue to green fluorescence (Φem =0.38-0.99) and high photostability in aqueous medium. The structure-emission relationship was further investigated by tailoring the electron-donating functions to the anthracene moiety to give dyes 4 c-6 c with charge-transfer character. The latter significantly decreases the emission energy to reach near-IR region. Thus, the intramolecular phosphacyclization renders an ultra-wide tuning of fluorescence from 420 nm (1 c) to 780 nm (6 c) in solution, extended to 825 nm for 6 c in the solid state with quantum efficiency of approximately 0.07. The physical behavior of these new dyes was studied spectroscopically, crystallographically, and electrochemically, whereas computational analysis was used to correlate the experimental data with molecular electronic structures. The excellent stability, water solubility, and attractive photophysical characteristics make these phosphonium heterocycles powerful tools in cell imaging.

Dibenzo[b,e]phosphindolizines synthesized by a ring-closing metathesis of benzo[b]phospholes with two vinyl tethers

Dibenzo[b,e]phosphindolizines 1 were successfully synthesized by a ring-closing metathesis of benzo[b]phosphole chalcogenides 2a-2c with two vinyl tethers (for chalcogen analogs 1a-1c), and by a deselenization reaction (for lone-pair analog 1d). The structures, properties, and bowl inversion derived from a phosphorus atom were fully investigated.

Electron donor and acceptor functionalized dithienylethenes: effects of charge density on photochromic properties

Electron-donating triphenylamine and/or electron-withdrawing pyromellitic diimide (PMDI) are functionalized on dithienylethene (DTE) and three novel photochromic materials have been designed and successfully synthesized. All the compounds display reversible photochromism due to the molecular switching between ring-closed isomers upon UV light irradiation and ring-open isomers upon exposure to visible light. Thus they can be applied as an anti-counterfeiting ink. Moreover, the study of the photoswitching kinetics reveals that both the ring-closing and ring-opening reactions are first-order reactions. Further charge population analysis discovers that the electron densities of the substituents at the DTE core have a dramatic influence on the photochromic properties. The incorporation of electron-donating triphenylamine groups at the α-position of the thiophene rings in the DTE unit facilitates the ring-closing reaction upon UV light irradiation. In contrast, the substitution of an electron-withdrawing PMDI unit in the DTE unit is beneficial to the ring-opening reaction upon irradiation of visible light. This work may help to understand the photochromism of DTE derivatives and provide a pathway for designing DTE-based photochromes with more or less sensitivity to UV or visible light.

Supramolecular Assembly of Phosphole Oxide Based Alkynylplatinum(II) 2,6-Bis(N-alkylbenzimidazol-2'-yl)pyridine Complexes-An Interplay of Hydrophobicity and Aromatic π-Surfaces

A new class of phosphole oxide based alkynylplatinum(II) 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine (bzimpy) complexes were synthesized and characterized. Their self-assembly was driven by hydrophobic-hydrophobic and π-π stacking interactions. The self-assembly properties were also investigated by UV/Vis absorption spectroscopy, which revealed that the alkyl-chain length of the bzimpy moiety and the π-surface area of the alkynyl ligand have significant influence on the overall self-assembly process. The alkyl-chain length also affected the morphological structures of the aggregates, which were studied by transmission electron microscopy and scanning electron microscopy.

Multiphotochromism in an Asymmetric Ruthenium Complex with Two Different Dithienylethenes

An asymmetric bis(dithienylethene-acetylide) ruthenium(II) complex trans-Ru(dppe)₂(L1o)(L2o) (1oo) incorporating two different dithienylethene-acetylides (L1o and L2o) was designed to modulate multistate photochromism in view of the well separated ring-closing absorption bands between L1o and L2o. Upon irradiation with appropriate wavelengths of light, complex 1 undergoes stepwise photocyclization and selective photocycloreversion to afford four states (1oo, 1co, 1oc, and 1cc). As a contrast, symmetric complexes trans-Ru(dppe)₂(L1o)₂ (2oo) and trans-Ru(dppe)₂(L2o)₂ (3oo) with two identical dithienylethene-acetylides were synthesized, and the corresponding photochromic behavior was investigated. The photochromic properties of the oxidized species (1oo⁺/1co⁺/1oc⁺/1cc⁺, 2oo⁺/2co⁺/2cc⁺, and 3oo⁺/3co⁺/3cc⁺) were also investigated. The ring-closing absorption bands of one-electron oxidized species 1oo⁺, 2oo⁺, and 3oo⁺ show obvious blue shifts relative to those of 1oo, 2oo, and 3oo, respectively. The ring-closing absorption bands in both neutral and oxidized species grow progressively following oo → oc/co → cc and oo⁺ → oc⁺/co⁺ → cc⁺. As revealed by spectroscopic, electrochemical, and computational studies, complex 1 displays eight switchable states through stepwise photocyclization, selective cycloreversion, and a reversible redox process.

Visible Light-Responsive DASA-Polymer Conjugates

A modular synthesis of Donor-Acceptor Stenhouse Adduct (DASA) polymer conjugates is described. Pentafluorophenyl-ester chemistry is employed to incorporate aromatic amines into acrylate and methacrylate copolymers, which are subsequently coupled with activated furans to generate polymers bearing a range of DASA units in a modular manner. The effect of polymer glass transition temperature on switching kinetics is studied, showing dramatic rate enhancements in going from a glassy to a rubbery matrix. Moreover, tuning the DASA absorption profile allows for selective switching, as demonstrated by ternary photopatterning, with potential applications in rewriteable data storage.

Copper-mediated phospha-annulation to attain water-soluble polycyclic luminophores

The P-heterocyclic fluorophores, obtained via Cu(ii)-mediated cyclization, are suitable for cell imaging due to their stability, water solubility and low toxicity.