Intramolecular Photocyclization Research Articles

Free Radical-Mediated Intramolecular Photocyclization of AIEgens Based on 2,3-Diphenylbenzo[b]thiophene S,S-Dioxide

As an important category of photochemical reactions, photocyclization is regarded as an ideal entry point for building intelligent photoresponsive materials. Herein, a series of aggregation-induced emission luminogens (AIEgens) with sensitive photoresponsive behavior are developed based on 2,3-diphenylbenzo[b]thiophene S,S-dioxide (DP-BTO), and the impacts of substituents with different electronic structures are investigated. The comprehensive experimental and computational characterizations reveal that their photoresponsive activity is resulted from triplet diradical-mediated intramolecular photocyclization, followed by dehydrogenation to yield stable polycyclic photoproducts. This photocyclization process is active in solution but suppressed in the solid state, and thus can act as a supplementary nonradiative decay channel for the excited state to contribute to AIE effect. Moreover, the generated triplet diradical intermediates upon light irradiation can effectively inhibit the growth of S. aureus, indicative of their promising application as antibacterial agents. This work provides an in-depth mechanistic description about the photocyclization of DP-BTO derivatives and furnishes a perspective on the correlation of photochemical decay and photophysical property.

Tuned Intra‐ and Intermolecular Photoreactions of Tridentate Cyanostilbenes with Distinct Aggregated‐State Photomechanical and Dispersed‐State Photochromic Behaviors

AbstractDeveloping a new strategy to realize on‐demand photoreactions with distinguishable signals is highly desirable. Herein, by simply tuning the molecular state, tuned photoreactions with different dynamic signals based on a series of tridentate cyanostilbene molecules (TCS1‐5) are realized. The results show that in the crystalline state, the ordered and densely molecular packing facilitates the successive intermolecular photocycloaddition between adjacent cyanostilbene molecules and causes accumulated stresses within the crystals, which further induces variable photo‐induced mechanical motions such as peeling, bending, twisting, and so on. In the monodispersed solution state, however, the larger distances between TCS1‐5 molecules afford a photo‐induced radical‐assisted intramolecular photocyclization process and give rise to multiple domino photochromic and photofluorochromic behaviors. Based on their distinct yet controllable thermostability and radical colors, photochromic films are fabricated from TCS1@PMMA and TCS1/TFA@PMMA for Morse code and rewritable photopatterning applications, showing excellent photochromism. By coating TCS3 on a piece of plastic bag, the macroscopic mechanical motion is achieved by amplifying the collective molecular motion from the nanoscale up to the macroscopic dimension. The molecular state‐based strategy reveals the regularity between photoreactions and molecular packing and further provides an opportunity to rapidly access controllable photoresponsive systems.

High-contrast fluorescence modulation driven by intramolecular photocyclization and protonation of bithienylpyridine functionalized α-cyanostilbene

We report herein a light-inducible fluorescence conversion and an acidochromism based on the bithienylpyridine functionalized α-cyanostilbene luminogen (E-CTP). The thienylcyanostyrene unit in this luminogen endows E-CTP AIEE property and 365 nm light-induced fluorescence conversion with high contrast in color (Δλem = 85 nm) and strong intensity. The fluorescence conversion of this molecular system might be driven by an intramolecular photocyclization rather than the commonly occurred intermolecular photodimerization for the α-cyanostilbene system as indicated by UV, PL, TCL-MS as well as NMR spectra measurements. Due to the bithienyl-pyridine in its skeleton, the optoelectronic properties of this luminogen can also be tuned by protonation. The luminogen shows 6 fold fluorescence enhancement and 110 nm emission red-shift in solution upon addition of trifluoroacetic acid (TFA). The optical feature of the reversible recoverability for E-CTP under acid-base stimuli in film was utilized to prepare security paper. Moreover, cooperating the acidochromism with the photo-induced fluorescence conversion of E-CTP, multicolor light-painting pattern was achieved.

Syntheses of Anthracene-Centered Large PAH Diimides and Conjugated Polymers.

Polycyclic aromatic hydrocarbon (PAH) structures with suitable electron-withdrawing groups are useful building blocks for developing optical and electron-transporting materials. Here, we report the application of a double benzannulation process to the syntheses of PAH diimides with enlarged π-frameworks featuring a central anthracene moiety. The preparations are realized by copper-catalyzed [4+2] cycloaddition of ethynyl-substituted aromatic dicarboximide to 2,5-bis(phenylethynyl)terephthalaldehyde, followed by intramolecular photocyclization or direct arylation via Heck cross coupling. A central symmetric benzo[1,2-k:4,5-k']-bis(fluoranthene)-3,4,12,13-tetracarboxyl diimide (BFDI) is acquired, with the single crystal structure revealing its completely planar polycyclic skeleton. Such a shape-persistent PAH expectedly exhibits a tendency to stack face-to-face and forms J-aggregates. Moreover, BFDI can be difunctionalized site-selectively at the reactive 9 and 10 positions of the anthracene unit and then applied to prepare conjugated polymers. When coupled with 1,4-diketopyrrolo[3,4-c]-pyrrole (DPP) via thiophene and dithiophene linkers, two polymers with significantly broadened absorption bands extended to the near-infrared regime are obtained, evidencing the effective π-conjugative extension ability of BFDI unit.

Scissor-Shaped Photochromic Dyads: Hierarchical Self-Assembly and Photoresponsive Property.

Unique relationships between hierarchically organized biological nanostructures and functions have motivated chemists to construct sophisticated artificial nanostructured systems from small and simple synthetic molecules through self-assembly. As one of such sophisticated systems, we have investigated scissor-shaped photochromic dyads that can hierarchically self-assemble into discrete nanostructures showing photoresponsive properties. We synthesized various azobenzene dyads and found that these dyads adopt intramolecularly folded conformation like a closed scissor, and then self-assemble into toroidal nanostructures by generating curvature. The toroids further organize into nanotubes and further into helical supramolecular fibers depending on the nature of alkyl substituents. All of these nanostructures can be dissociated and reorganized through the photoisomerization of azobenzene units. On the other hand, the introduction of stilbene chromophores instead of azobenzenes leads to one-dimensional supramolecular polymerization, which upon the intramolecular photocyclization of stilbene chromophores shifts to curved self-assembly leading to helicoidal fibers with distinct supramolecular chirality.

An efficient approach to diarylethene-amino acid photochromic fluorescent hybrids

• An effective approach to creating photo-controlled diarylethene-amino acid hybrids. • X-ray diffraction study, spectral luminescent and photochromic properties. • Thermally and photochemically reversible intramolecular photocyclization. • Dependence of the photoinduced form stability on the structure. • Modulation of emission under successive irradiation with light of different wavelengths. An effective approach to the synthesis of diarylethene-amino acid hybrids DE-Gly, DE-AABA and DE-DAA ( 5a-d - 7-a-d ) was developed via condensation of furan-2,5-dione-based diarylethenes (DE) and ethyl esters of glycine (Gly), α-aminobutyric acid (AABA) and D-aspartic acid (DAA) with moderate to good yields. According to X-ray diffraction data, DE-Gly hybrid 5a exists in an antiparallel conformation with a distance of 4.549 Å between the reactive carbon atoms C(1)-C(11), potentially capable of forming a single bond, which is suitable for the conrotatory photocyclization reaction allowed by the Woodward-Hoffman rules. The structures of the obtained compounds were proved by 1 H, COZY, HSQC, HMBC and 13 C NMR spectroscopy. The hybrids DE-Gly, DE-AABA and DE-DAA absorb at 443–456 nm, which corresponds to their existence in a ring-open form O, and display fluorescence at 531–612 nm. Irradiation with light of 436 nm results in their rearrangement into ring-closed colored nonfluorescent isomers C. Backwards re-opening occurs under the action of visible light (λ > 500 nm). Spectral kinetic investigation of 5a,c revealed that the efficiency of photocyclization, as well as the emission quantum yield, decreases with the growth of solvent polarity. The ring-closed isomer C of sterically hindered 5a (R 2 = R 3 = Me) is stable at room temperature, whereas for 5c (R 3 = H), ring opening readily occurs with the speed increasing with the polarity of the solvent. The internal emission of sterically hindered hybrids 5a, 6a and 7a (R 2 = R 3 = Me) is reversibly modulated in a binary response with good fatigue resistance under successive irradiation with light of 436 and 540 nm.

Exploration of high-performance light-conversion agents based on cyanostilbene and phenanthrenecarbonitrile backbones: E/Z and position isomerism, high-contrast Michael addition reaction activity and intramolecular photocyclization

High-contrast Michael addition and intramolecular photocyclization, excellent photostability and significantly enhanced transmittance.

Photoinduced Hydroarylation and Cyclization of Alkenes with Luminescent Platinum(II) Complexes.

Photoinduced hydroarylation of alkenes is an appealing synthetic strategy for arene functionalization. Herein, we demonstrated that aryl radicals generated from electron-deficient aryl chlorides/bromides could be trapped by an array of terminal/internal aryl alkenes in the presence of [Pt(O^N^C^N)] under visible-light (410 nm) irradiation, affording anti-Markovnikov hydroarylated compounds in up to 95 % yield. Besides, a protocol for [Pt(O^N^C^N)]-catalyzed intramolecular photocyclization of acrylanilides to give structurally diverse 3,4-dihydroquinolinones has been developed.

Photoinduced Hydroarylation and Cyclization of Alkenes with Luminescent Platinum(II) Complexes

AbstractPhotoinduced hydroarylation of alkenes is an appealing synthetic strategy for arene functionalization. Herein, we demonstrated that aryl radicals generated from electron‐deficient aryl chlorides/bromides could be trapped by an array of terminal/internal aryl alkenes in the presence of [Pt(O^N^C^N)] under visible‐light (410 nm) irradiation, affording anti‐Markovnikov hydroarylated compounds in up to 95 % yield. Besides, a protocol for [Pt(O^N^C^N)]‐catalyzed intramolecular photocyclization of acrylanilides to give structurally diverse 3,4‐dihydroquinolinones has been developed.

Phototransformations of acrylamide derivatives of piperazine-substituted polyfluorinated chalcones

Phototransformations of acrylamide derivatives of the piperazino-substituted polyfluorochalcones (APPFC) were studied by UV, IR, Raman, and NMR spectroscopy. It has been shown that two types of photochemical reactions occur in the films of APPFC: free-radical polymerization of acrylic fragments and cyclodimerization of the carbon-carbon double bond in a chalcone. It was shown that in polyfluorochalcones PFCh-1, PFCh-2, trans-cis isomerization of chalcone occurs with the subsequent formation of dimers. Along with this, a competitive reaction of intramolecular photocyclization occurs in PFCh-2 via the dehydrofluorination. Based on MALDI-TOF mass spectra, the structural formulas of the possible dimeric and oligomeric fragments of polyfluorochalcones formed in the films were proposed. The obtained spectral data indicate that, according to the efficiency of the formation of photocrosslinked films, studied polyfluorochalcones can be arranged in the following order: TAFCh >> PFCh-1 > > PFCh-2.

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

A series of donor/acceptor chromophores 1d-3d containing two types of photoresponsive motifs, namely an electron-deficient BF2-chelated ketone fused with an electron-rich thiophene have been synthesized and their photochromic performance are investigated. 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 1d and 2d undergo 6π photocyclization, whereas ethyl-substituted 3d exclusively undergoes 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. The UV-vis absorption spectra of 1d-3d cover a broad visible light region (380-572 nm), while theoretical 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. The spatial separation of the HOMO and LUMO of these dyes promotestriplet state generation and self-photosensitizes intramolecular photocyclization in the visible light region. Three-dimensional time-resolved and steady-state emission spectra of 3d show that the Norrish-Yang photocyclization takes place within milliseconds with excellent conversion efficiency (96%).

UV-detecting dual-responsive strips based on dicyanoacetate-containing hexaphenylbutadiene with aggregation-induced emission characteristic

An ultraviolet (UV) radiation is a double-edged sword. The right amount can kill bacteria on the skin and promote the metabolism of calcium and phosphorus, which further stimulate the hematopoietic and immune functions. However, an excessive UV radiation will burn the skin and can even cause skin cancer. Therefore, a method based on naked-eye judgment should be developed to detect the intensity of UV radiation. Herein, the aggregation-induced emission (AIE) characteristics of two hexaphenyl-1,3-butadiene-based isomers containing dicyanoacetate, ZZ-HPB-NC and EE-HPB-NC were elucidated by single-crystal structure analysis. As the ZZ-HPB-NC, with a looser packing structure, is more suitable for intramolecular photocyclization, the ZZ-HPB–NC–based strips were sensitive to UV radiation. This is because the crystallization of ZZ-HPB-NC can be easily induced on the paper fibers, followed by intramolecular cyclization under UV radiation. As the illumination time increases, the color of the strips changes from yellow to green under a hand-held UV lamp. This approach makes it easy to estimate the intensity of the UV radiation by the naked eyes. The dual-responsive sensitivity is in direct proportion to the intensity of the UV radiation. The detection limit of the ZZ-HPB–NC–based strips at 254 nm UV intensity was estimated to be 8.2 μW/cm2. Thus, the ZZ-HPB–NC–based strips can be used as a simple and inexpensive technique to detect the intensity of the UV lamps in hospitals, nursery schools, and public areas, avoiding the harmful effects of the UV lamps to human eyes and skins.

Computational design of an intramolecular photocyclization reaction with state-selective reactivity: a strategy for indole synthesis

We disclose a photoinduced photocyclization reaction of carbonyls and tertiary amines under photocatalyst-free conditions, providing a green entry to indoles.

Fluorine-Containing Dibenzoanthracene and Benzoperylene-Type Polycyclic Aromatic Hydrocarbons: Synthesis, Structure, and Basic Chemical Properties.

Intramolecular photocyclization of stilbene derivatives (Mallory reaction) is one of the efficient methods for building polycyclic aromatic hydrocarbon (PAH) frameworks, and is also expected to be applicable to synthesis of fluorine-containing PAHs (F-PAHs). In this study, dibenzoanthracene-type (4a) and benzoperylene-type (4b) F-PAHs were synthesized using the Mallory reaction of the 1,4-distyrylbenzene-type π-conjugated molecule (3a), which was prepared by addition-defluorination of available octafluorocyclopentene (OFCP) and aryllithium in three steps. The structure of 4a originating from π–π interaction was characterized by X-ray crystallographic analysis. The absorption maxima of UV-Vis spectra and emission maxima of photoluminescence spectra of the PAHs were positioned at a longer wavelength compared to those of the corresponding unsubstituted PAHs, presumably due to the electron-withdrawing nature of perfluorocyclopentene (PFCP) units. The effect of PFCP units in F-PAHs was also studied by time-dependent density functional theory (TD-DFT) calculation.

Synthesis, Structure and Photophysical Properties of Pyrene–based [5]Helicenes: an Experimental and Theoretical Study

AbstractPyrene‐cored [5]helicenes were prepared by a facile, efficient Wittig reaction and an intramolecular photocyclization reaction utilising 2,7‐di‐tert‐butylpyrene‐4‐carbaldehyde and naphthalene/pyrene‐based phosphorus ylides. Optical properties based on UV−vis absorption and fluorescence spectra were investigated. X‐ray crystallography revealed that the pyrene‐based [5]helicenes exhibited strong face‐to‐face π−π interactions and edge‐to‐face π−π interactions. HOMO and LUMO energies and molecular orbitals were also studied by density functional theory (DFT) calculations. This study has revealed that the torsion angle of the helical structure plays a role in determining the π−π interactions and the frontier molecular orbital energy levels. Thus, pyrene‐based helicenes need to be considered when one designs new highly efficient organic light‐emitting diodes and organic semiconductor materials.

Development of Photoactivated Fluorescent N-Hydroxyoxindoles and Their Application for Cell-Selective Imaging.

Photoactivatable fluorophores are essential tools for studying the dynamic molecular interactions within important biological systems with high spatiotemporal resolution. However, currently developed photoactivatable fluorophores based on conventional dyes have several limitations including reduced photoactivation efficiency, cytotoxicity, large molecular size, and complicated organic synthesis. To overcome these challenges, we herein report a class of photoactivatable fluorescent N-hydroxyoxindoles formed through the intramolecular photocyclization of substituted o-nitrophenyl ethanol (ONPE). These oxindole fluorophores afford excellent photoactivation efficiency with ultra-high fluorescence enhancement (up to 800-fold) and are small in size. Furthermore, the oxindole derivatives show exceptional biocompatibility by generating water as the only photolytic side product. Moreover, structure-activity relationship analysis clearly revealed the strong correlation between the fluorescent properties and the substituent groups, which can serve as a guideline for the further development of ONPE-based fluorescent probes with desired photophysical and biological properties. As a proof-of-concept, we demonstrated the capability of a new substituted ONPE that has an uncaging wavelength of 365-405 nm and an excitation/emission at 515 and 620 nm, for the selective imaging of a cancer cell line (Hela cells) and a human neural stem cell line (hNSCs).

Combined aggregation induced emission (AIE), photochromism and photoresponsive wettability in simple dichloro-substituted triphenylethylene derivatives.

A dichloro-substituted triphenylethylene derivative (TrPECl2) with aggregation-induced emission (AIE), photochromism and photoresponsive wettability has been synthesized. The new compound shows fast-response photochromic behaviour with good ON/OFF repeatability by utilizing a proposed stilbene-type intramolecular photocyclization in the solid state. Compared with the more usual diphenylethylene derivatives, the photochromic properties of the triphenylethylene derivative are much more striking and easier to achieve. The triphenylethylene derivative also displays AIE properties leading to strong fluorescence in the solid state. Therefore, both the ultraviolet-visible absorption and fluorescence emission are drastically changed during the photochromic processes. Furthermore, the morphology of the TrPECl2 microcrystalline surface could be controlled by irradiation. The wettability of the surface could be drastically decreased with contact angles of a water droplet changing from 73° to 118°. The triphenylethylene derivative with a simple molecular structure is, therefore, attractive for multifunctional materials.

ChemInform Abstract: Amino Azaxylylenes Photogenerated from o‐Amido Imines: Photoassisted Access to Complex Spiro‐Poly‐Heterocycles.

AbstractThe intramolecular photocyclization of amino azaxylylenes derived from o‐amido imines, e.g. compounds (I), (IV), and (VII), leads to mixtures of [4 + 4] and [4 + 2] adducts.

Photoinduced Intramolecular formal [4 + 2] Cycloaddition of Aryl-Substituted o-Vinylstyryl-2-oxazoles To Form Benzo[f]quinoline Derivatives: Experimental Results and Theoretical Interpretation.

A new approach to benzo[f]quinoline derivatives has been found by an effective formal [4 + 2] photocycloaddition process from novel aryl-substituted o-vinylstyryl-2-oxazoles. All of the o-vinylstyryl-2-oxazoles were synthesized by a multicomponent Wittig reaction from the diphosphonium salt of α,α'-o-xylene dibromide, formaldehyde, and 5-tolyl-, 4-phenyl-5-methyl-, and 4,5-diphenyloxazole-2-carbaldehydes. TD-DFT calculations revealed that the intramolecular photocyclization in 2-(2-vinylstyryl)oxazoles to form benzo[f]quinoline derivatives proceeds on the S1 PES via a stepwise pathway, namely by 10π followed by 6π ring closure. On that path the existence of an S0/S1 conical intersection was indicated. The reactivity of the photocyclization steps depends on the substitution pattern at positions 4 and 5 of the oxazole ring, where the aryl group in position 5 deactivates the reaction.

Practical Syntheses of Both Enantiomers of the Conformationally Restricted GABA Analogue cis‐(2‐Aminocyclobutyl)acetic Acid

AbstractTwo efficient routes have been established for the preparation of both enantiomers of cis‐(2‐aminocyclobutyl)acetic acid, a conformationally restricted analogue of GABA. Both procedures converged on the racemic N‐tert‐butoxycarbonyl derivative of the target compound, which was resolved through chiral derivatization with an oxazolidinone auxiliary, which also allowed determination of the absolute configuration of the new compounds. The first route involved the homologation of cis‐2‐aminocyclobutanecarboxylic acid, whereas the second route employed an intramolecular photocyclization protocol, which provided an expedient, cis‐selective access to the lactam form of the target structure.