Charge Transfer and Intraligand Excited State Interactions in Platinum-Sensitized Dithienylethenes

Amplified photomodulation of a bis(dithienylethene)-substituted phosphine

Phosphine ligands play a crucial role in homogeneous catalysis, allowing fine-tuning of the catalytic activity of various metals by modifying their structure. An ultimate challenge in this field is to reach controlled modulation of catalysis in situ, for which the development of phosphines capable of photoswitching between states with differential electronic properties has been proposed. To magnify this light-induced behavior, in this work we describe a novel phosphine ligand incorporating two dithienylethene photoswitchable moieties tethered to the same phosphorus atom. Double photoisomerization was observed for this ligand, which remains unhindered upon gold(I) complexation. As a result, the preparation of a fully ring-closed phosphine isomer was accomplished, for which amplified variation of phosphorus electron density was verified both experimentally and by computational calculations. Accordingly, the presented molecular design based on multiphotochromic phosphines could open new ways for preparing enhanced photoswitchable catalytic systems.

Ytterbium(III) Complex with Photochromic Ruthenium(II) Acetylide Ligand: All Visible Light Photoswitching of NIR Luminescence

We report a ruthenium(II) bisacetylide complex bearing a photochromic dithienylethene (DTE) acetylide arm and a coordinating bipyridyl on the trans acetylide unit. Its coordination with Yb(TTA)₃ centers (TTA = 2-thenoyltrifluoroacetonate) produces a bimetallic complex in which the dithienylethene isomerization is triggered by both ultraviolet (UV) light absorbed by the DTE unit and 450 nm excitation in a transition of the organometallic moiety. The redox behavior arising from the ruthenium(II) bisacetylide system is fully investigated by cyclic voltammetry and spectroelectrochemistry, revealing a lack of stability of the DTE-closed oxidized state preventing effective redox luminescence switching. On the other hand, the photoswitching of ytterbium(III) near-infrared (NIR) emission triggered by the photochromic reaction is fully operational. The electronic structure of this complex in its different states characterized by strong electronic coupling between the DTE and the ruthenium(II)-based moieties leading to metal-assisted photochromic behavior were rationalized with the help of time-dependent density functional theory (TD-DFT) calculations.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

Two cyclometalated Pt(ii) complexes showing reversible phosphorescence switching due to grinding-induced destruction and crystallization-induced formation of supramolecular dimer structure

Two Pt(ii) complexes 1 and 2 reveal similar supramolecular dimer structure, in which two [Pt(dfppy/ppy)(pbdtmi)]+ cations connect each other through the π⋯π stacking interaction. Thus these complexes show reversible phosphorescence switching by grinding and crystallization with toluene.

Light-Triggered Metal Coordination Dynamics in Photoswitchable Dithienylethene-Ferrocene System

The C₂-symmetric photochromic molecule 3, containing dithienylethene (DTE) and ferrocene units connected by an alkyne bridge, represents a unique probe where a metal (Hg²⁺) binds with the central DTE moiety. Both photoisomerized states of 3 (open, 3o; closed, 3c) are found to interact with Hg²⁺ ion by the S atoms of the DTE core; however, the binding constants (from a UV-vis study) and DFT calculations suggest that the open isomer (3o) binds with the metal ion more strongly than that of the closed isomer (3c). Notably, the course of metal binding does not perturb the inherent photoisomerization properties of the DTE core and the photoswitchability persists even in the metal-coordinated form of 3, however, with a comparatively slower rate. The quantum yields for photocyclization (Φ_o→c) and photocycloreversion (Φ_c→o) in the free form are 0.56 and 0.007, respectively, whereas the photocyclization quantum yield in the Hg²⁺ complexed species is 0.068, 8.2 times lower than the photocyclization quantum yield (Φ_o→c) of free 3o. Thus, the rate of photoisomerization can be modulated by a suitable metal coordination to the DTE core. The dynamics of photoswitchability in the metal-coordinated form of DTE has been explored by experimental means (UV-vis and electrochemical studies) as well as quantum chemical calculations.

Switchable second-order nonlinear optical response of platinum-sensitized dithienylethenes

The electronic structure and the nonlinear optical (NLO) properties of a series of platinum-sensitized dithienylethenes (DTEs) were investigated by using the density functional theory (DFT) method. DFT calculations reveal that the second-order NLO properties of complexes significantly increase with the DTE ligand being directly linked with terpyridine–Pt(II) complexes. Due to the good [Formula: see text]-conjugated characteristics, closed-ring complexes possess much larger second-order NLO properties than the corresponding open-ring complexes. The computational [Formula: see text] values are in the order of 3c ([Formula: see text] esu) [Formula: see text] 3o ([Formula: see text] esu) [Formula: see text] 4c ([Formula: see text] esu) [Formula: see text] 4o ([Formula: see text] esu) [Formula: see text] 1 ([Formula: see text] esu) [Formula: see text] 2c ([Formula: see text] esu) [Formula: see text] 2o ([Formula: see text] esu). Among all calculated platinum-sensitized dithienylethenes, 3c has the largest second-order NLO properties. The 4c and 4o have almost the same [Formula: see text] values and the [Formula: see text] value of 4o is slightly larger than that of 2o because the ether bonds can significantly prevent charge transfer within the complexes.

The phosphorescence properties of a series of diarylethene-containing platinum complexes: the effect of ligand photoisomerization

Benefitting from isomerization in the ancillary-ligand diarylethene, a control of phosphorescence efficiency becomes available.

Contrasted photochromic and luminescent properties in dinuclear Pt(ii) complexes linked through a central dithienylethene unit

Two unprecedented photochromic complexes, Pt-A and Pt-B, have been characterised.

Heteroleptic Ir(iii) and Pt(ii) complexes based on 2-(2,4-difluorophenyl)-pyridine and bisthienylethene BrLH: the influence of the metal center on structures, luminescence and photochromism

Based on bisthienylethene BrLH, [Ir(dfppy)₂(BrL)]·3CH₃OH (1) and [Pt(dfppy)(BrL)]·CH₃OH (2) have been prepared. The two complexes are significantly different in structure, luminescence and photochromic behavior, due to their different metal centers.

C^C* cyclometalated platinum(II) N-heterocyclic carbene complexes with a sterically demanding β-diketonato ligand – synthesis, characterization and photophysical properties

Neutral cyclometalated platinum(ii) N-heterocyclic carbene complexes [Pt(C^C*)(O^O)] with C^C* ligands based on 1-phenyl-1,2,4-triazol-5-ylidene and 4-phenyl-1,2,4-triazol-5-ylidene, as well as acetylacetonato (O^O = acac) and 1,3-bis(2,4,6-trimethylphenyl)propan-1,3-dionato (O^O = mesacac) ancillary ligands were synthesized and characterized. All complexes are emissive at room temperature in a poly(methyl methacrylate) (PMMA) matrix with emission maxima in the blue region of the spectrum. High quantum efficiencies and short decay times were observed for all complexes with mesacac ancillary ligands. The sterically demanding mesityl groups of the mesacac ligand effectively prevent molecular stacking. The emission behavior of these emitters is in general independent of the position of the nitrogen in the backbone of the N-heterocyclic carbene (NHC) unit and a variety of substituents in 4-position of the phenyl unit, meta to the cyclometalating bond.

Visible-Light-Activated Molecular Switches

The ability to influence key properties of molecular systems by using light holds much promise for the fields of materials science and life sciences. The cornerstone of such systems is molecules that are able to reversibly photoisomerize between two states, commonly referred to as photoswitches. One serious restriction to the development of functional photodynamic systems is the necessity to trigger switching in at least one direction by UV light, which is often damaging and penetrates only partially through most media. This review provides a summary of the different conceptual strategies for addressing molecular switches in the visible and near-infrared regions of the optical spectrum. Such visible-light-activated molecular switches tremendously extend the scope of photoswitchable systems for future applications and technologies.

Enhanced bi-stability in a ruthenium alkynyl spiropyran complex

Attachment of a ruthenium alkynyl unit to a spiropyran ligand extends the lifetime of the merocyanine form of the complex more than twenty-fold.

Photoswitching of the triplet excited state of diiodobodipy-dithienylethene triads and application in photo-controllable triplet-triplet annihilation upconversion

Dithienylethene (DTE)-2,6-diiodoBodipy triads were prepared with the aim to photoswitch the triplet excited state of the 2,6-diiodoBodipy moiety. Bodipy was selected due to its low T1 state energy level to avoid sensitized photocyclization of DTE, which is very often encountered in DTE photoswitches, so that the photochemistry of DTE and the organic chromophore can be addressed independently. This is the first time that DTE was covalently connected with an organic triplet photosensitizer. For the triad with DTE-o structure, selective photoexcitation into the diiodoBodipy part did not initiate photocyclization of DTE-o. Upon photoirradiation at 254 nm, thus the DTE-o → DTE-c transformation, the intersystem crossing (ISC) of 2,6-diiodoBodipy moiety was competed by the photoactivated resonance energy transfer (RET), with Bodipy as the intramolecular energy donor and DTE-c as energy acceptor. The fluorescence of Bodipy was quenched and the triplet state lifetime of Bodipy was reduced from 105.1 to 40.9 μs. The photoreversion is O2-independent, but can be greatly accelerated upon selective photoexcitation into the diiodoBodipy absorption band (at 535 nm). We concluded that ISC is not outcompeted by RET. The photoswitching of the triplet state was transduced to the singlet oxygen photosensitizing, as well as triplet-triplet annihilation upconversion.

Second-order NLO switches from molecules to polymer films based on photochromic cyclometalated platinum(II) complexes

Novel photochromic dithienylethene-based platinum(II) complexes (C^N^N)Pt(C≡C-DTE-C6H4-D) ((C^N^N) = 4,4'-di(n-hexyl)-6-phenyl-2,2'-bipyridine; D = H, NMe2) were prepared and characterized. Their excellent photochromic properties allow the photoinduced switching of their second-order nonlinear optical properties in solution, as measured by the EFISH technique, due to formation of an extended π-conjugated ligand upon suitable electromagnetic radiation. Insights into the electronic structures of the complexes and the nature of their excited states have been obtained by DFT and TD-DFT calculations. These novel Pt(II) complexes were nanoorganized in polymer films which were poled, affording new materials characterized by a good second-order NLO response that can be easily switched, with an excellent NLO contrast. To the best of our knowledge, our compounds allowed designing the very first examples of switchable NLO polymer films based on metal complexes.