Photocyclization Process Research Articles

Oxidative and non-oxidative photo-cyclization reaction of pyrazoles

A series of 5-(dimethoxyphenyl)-1,3-diphenylpyrazoles containing two methoxy groups on the different C5-aryl ring positions were synthesized and the electronic and steric effects, especially the position of the methoxy substitutions on the outcome of their photoreaction were investigated. Furthermore, the effect of the type and nature of solvent (chloroform or cyclohexane) in the progress of the photoreaction are also investigated. The photochemical results indicated the formation of two different phenanthridine products depending on the used solvent. Based on the proposed photo-induced electron-transfer mechanism in CHCl3 solution, the methoxy group on the ortho-position is eliminated during the photo-cyclization process and a chlorine atom is added to one of the allowed C5-aryl ring positions. While the observed photo-cyclization reaction in cyclohexane solvent occurs via complexation mechanism, which strongly requires the presence of the methoxy group on the ortho-position. In this case, complexation of the ortho-methoxy group to the N1-phenyl ring started the cyclization process, resulting in the formation of the phenanthridine photoproducts by elimination of this group. The phenanthridine products of both photoreactions are characterized by the analysis of their FT-IR, 1H NMR, 13C NMR, UV–Vis and Mass spectra. In addition, DFT computational studies were carried out to support the results of the experimental study.

High-contrast multicolour photoswitching based on dithienylethene Schiff base with a hydrazinylquinoline moiety

A new asymmetrical photochromic diarylethene DTE-HQo composed of a 2-hydrazinoquinoline moiety as the binding unit for ions and dithenylethene as a photoswitching trigger was reported. DTE-HQo displayed favourable photochromism upon irradiation with UV/vis light. Its fluorescent behaviour could be efficiently modulated by light, Zn2+, Cd2+ and HSO4−. The binding of Zn2+ induced a strong fluorescence peak at 510 nm in DTE-HQo due to the formation of a 1:2 complex [Zn2+ + 2DTE-HQo], resulting in a notable colour change from dark to intense white emission. Triggered by Cd2+, DTE-HQo formed a 1:1 complex [Cd2+ + DTE-HQo], leading to an enhanced emission intensity by 21-fold with an emission peak red-shifted from 461 nm to 514 nm. Unexpectedly, [Zn2+ + 2DTE-HQo] underwent hydrolysis when stimulated with water, generating a yellow-emitting complex [Zn2+ + DTE-HQo]. This color change easily distinguishes it from Cd2+ complex. Additionally, DTE-HQo showed high selectivity towards HSO4− and exhibited distinct “turn-on” fluorescence with a colour change from dark to bright blue upon stimulation. Moreover, the strong emission complexes of DTE-HQo with Zn2+, Cd2+ and HSO4− could be effectively quenched during the photocyclization process. Therefore, DTE-HQo can serve as an unimolecular multicolour photoswitching chemosensor, offering potential applications as a multifunctional probe for detecting Zn2+, Cd2+ and HSO4−.

Photoluminescent iridium(III) complexes with phenylvinylpyridine-base cyclometalating ligands: Synthesis, photophysical properties, and photocyclization

Photoresponsive materials have recently garnered significant research interest and have been designed using various molecules. A viable approach to develop photoresponsive materials is by integrating photoisomeric structures into the organic ligands of phosphorescent transition metal complexes. In this study, we designed two photoresponsive cyclometalating ligands, 2-phenyl-3-vinylpyridine (HL1) and trans-4-methyl-2-phenyl-3-styrylpyridine (HL2), via the replacement of a phenyl group with a pyridyl group in vinylbiphenyl. Upon photoirradiation under an argon atmosphere, both ligands underwent photocyclization and demonstrated hypochromism and bathochromic shifts in absorption. Notably, HL2 underwent additional oxidative dehydrogenation upon photoirradiation in the presence of oxygen, resulting in an elongated conjugation structure that exhibited significant fluorescence intensity increase at 363 nm. Additionally, three photoresponsive iridium(III) complexes cyclometalated with HL1 or HL2 were synthesized and characterized. The photocyclization process of the complexes was much faster than that of the free ligands because the phenylpyridine moiety became coplanar upon cyclometalation. Upon photoirradiation under an argon atmosphere, these complexes exhibited significant luminescence enhancement by about 10–20 folds because of photocyclization resulting in a rigid closed structure that limited nonradiative decay of the excited complexes. These molecules present a unique strategy to construct photocyclization complexes and provide important insights into the development of photoresponsive materials.

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.

Investigating the Properties of Double Triangle Terthiophene Configured Dumbbell-Like Photochromic Dye with Ethyne and 1,3-Butadiene Bridge.

Dumbbell-like photochromic dyes were constructed by incorporation of double triangle terthiophene with ethyne or 1,3-butadiene bridge. Regular photochromic behavior was investigated with alternated UV (365nm) and Visible light (˃ 400nm) irradiation. However, the different bridge group leads to distinct difference in their photochromic wavelength. For the ethyne bridged triangle terthiophene (DT1), the photochromic wavelength was observed around 500-700nm (peak value: 605nm) and the solution turned to red with 365nm light irradiation. However, the photochromic wavelength was blue shift to 418-550nm and the solution was turned to light yellow for 1,3-butadiene bridged dye (DT2). Both of the colored solution can be bleached via visible light irradiation. Additionally, the two dyes in THF were emissive with absolute quantum yield (QY) of 0.36/0.40. Along with the photo-induced photocyclization process, the emissive solution can be effectively quenched at photo-stationary sate (Φ = 0.05/0.04). And emission "on-off" cycle could be established based on the UV/visible light irradiation cycle.

Emerging Designs of Aggregation-Induced Emission Agents for Enhanced Phototherapy Applications

Emerging Designs of Aggregation-Induced Emission Agents for Enhanced Phototherapy Applications

Unidirectional electron injection and accelerated proton transport in bacteriorhodopsin based Bio-p-n junctions

Hampered by the absence of evidence and theoretical model of biological semiconductors, the unidirectional electron transport via the p-n junction between functional proteins and abiotic materials remains a challenge for bioelectronics. Bacteriorhodopsin (bR), a representative transmembrane protein, has demonstrated exceptional optoelectronic effects in bR/semiconductor hybrid materials and offers a possible pathway for addressing this challenge. In the present work, for the first time, bR is proved to be an n-type semiconductor with an indirect electron transition. Through the photo-electrochemical method used for studying the p-n junction effect in the bR and p-type semiconductor combined electrodes, we reached several important conclusions: The self-corrosion of bR integrated Cu2O electrodes is delayed for about 36 times; The photocurrent of bR integrated CuSCN electrodes is enhanced by about 400%, which is attributed to the directional migration of electrons via the p-n junction. Furthermore, the ultrafast kinetics we have explored, shows that the injection of electrons shortens the lifetime of the intermediate state O640 from 37.3 μs to 20.1μs, what means that the protons transport rate accompanying the bR photocycle process is accelerated. Therefore, we believe that the concept of the bio-p-n junction and the mechanism of electron coupled proton transport, which are discussed herein, will promote useful research on bioelectronic applications for bR and its homologs.

Cyclization Dynamics and Competitive Processes of Photochromic Perfluorocyclopentene Dithienylethylene in Solution.

Time-resolved absorption spectroscopy measurements were performed to study the dynamics of photochromic 1,2-Bis(2,4-dimethylthiophene-3-yl)perfluoro-cyclopentene (DMTPF) in chloroform, including antiparallel conformer ring-closure reaction and parallel conformer photophysics. All characteristic times are given, discussed and compared to a previous publication concerning the close molecule substituted with phenyl rings. (Hamdi et al., PCCP, 2016). Apart from the expected photocyclization process, condensed ring by-product formation is observed and hypotheses concerning the origin of this by-product are presented.

Synthesis and photorearrangement of furanone diarylethenes with an additional π-system

In recent years, the synthetic potential of the photorearrangement of diarylethenes leading to naphthalene and phenanthrene derivatives via a cascade process of photocyclization/[1,9]-H shift has been demonstrated. In this work, the influence of various substituents on the efficiency of the photorearrangement of diarylethenes (DAEs) of the furanone series containing an additional π-system has been explored. It was found that electron-withdrawing substituents contribute only to the photocyclization process, while electron-donating groups (methoxy or dialkylamino groups) lead to E/Z-isomerization. However, in the case of dialkylamino substituted diarylethenes, the addition of a strong Brønsted acid promotes the photorearrangement, but prevents E/Z isomerization.

Regiodivergent Photocyclization of Dearomatized Acylphloroglucinols: Asymmetric Syntheses of (-)-Nemorosone and (-)-6-epi-Garcimultiflorone A.

Regiodivergent photocyclization of dearomatized acylphloroglucinol substrates has been developed to produce type A polycyclic polyprenylated acylphloroglucinol (PPAP) derivatives using an excited-state intramolecular proton transfer (ESIPT) process. Using this strategy, we achieved the enantioselective total syntheses of the type A PPAPs (-)-nemorosone and (-)-6-epi-garcimultiflorone A. Diverse photocyclization substrates have been investigated leading to divergent photocyclization processes as a function of tether length. Photophysical studies were performed, and photocyclization mechanisms were proposed based on investigation of various substrates as well as deuterium-labeling experiments.

Amplified spontaneous emission based on an excited-state intramolecular-proton-transfer molecule with solid-state-induced emission enhancement

In this work, we developed a simple molecule with the effective generation of intense green emission in the amorphous thin-film. The synthesized novel molecule with excited-state intramolecular proton transfer (ESIPT), 3-Dimethylamino-1-(2-hydroxy-phenyl)-propenone (DHP), shows significantly enhanced green fluorescence in solid-state (fluorescence quantum yield Φf = 0.53), compared with the faint emission in the solution (Φf = 0.037). DHP molecules in amorphous thin-film effectively reinforce the radiative decay pathway, evidenced by enlarged Φf, shortened excited state lifetime τ. Taking advantage of the solid-state-induced emission enhancement characteristics, four-level ESIPT photocycle process and large Stokes shift, an efficient amplified spontaneous emission at 559.14 nm with a threshold of 12.41 mJ/cm2 is observed from the solid-state thin-film.

Hydrogen Bonding Environments in the Photocycle Process around the Flavin Chromophore of the AppA-BLUF domain

Three kinds of photochemical reactions are known in flavins as chromophores of photosensor proteins, reflecting the various catalytic reactions of the flavin in flavoenzymes. Sensor of blue light using the flavin FAD (BLUF) domains exhibit a unique photoreaction compared with other flavin-binding photoreceptors in that the chromophore does not change its chemical structure between unphotolyzed and intermediate states. Rather, the hydrogen bonding environment is altered, whereby the conserved Gln and Tyr residues near FAD play a crucial role. One proposal for this behavior is that the conserved Gln changes its chemical structure from a keto to an enol. We applied light-induced difference Fourier transform infrared (FTIR) spectroscopy to AppA-BLUF. The spectra of AppA-BLUF exhibited a different feature upon 15N-Gln labeling compared with the previously reported spectra from BlrB, a different BLUF domain. The FTIR signals were interpreted from quantum mechanics/molecular mechanics (QM/MM) calculation as the keto-enol tautomerization and rotation of the Gln63 side chain in the AppA-BLUF domain. The former was consistent with the result from BlrB, but the latter was not uniquely determined by the previous study. QM/MM calculation also indicated that the infrared signal shape is influenced depending on whether a Trp side chain forms a hydrogen bond with the Gln side chain. FTIR spectra and QM/MM simulations concluded that Trp104 does not flip out but is maintained in the intermediate state. In contrast, our data revealed that the Trp residue at the corresponding position in BlrB faces outward in both states.

Photoinduced Proton Transfer between Photoacid and pH‐Sensitive Dyes: Influence Factors and Application for Visible‐Light‐Responsive Rewritable Paper

AbstractInk‐free printing based on rewritable paper is an efficient and environmental friendly way to reuse paper, protect resources, and save energy for sustainable development of human society. Among various kinds of rewritable media, light responsive rewritable paper (LRP) is one of the most popular research areas due to its clean and favorable noncontact writing. Visible light is more suitable for LRP for its superior penetration and much less damages to organic molecules than UV light. However, visible‐light‐responsive rewritable paper (VLRP) has only limited successes so far. Herein, a VLRP is newly designed and fabricated based on photoinduced proton transfer (PPT) between photoacid and pH‐sensitive dyes. Success of it is highly benefited from systematical investigation and in‐depth understanding on the key influence factors, such as concentration‐induced undesired isomerization, temperature, humidity, and light intensity, on the PPT and its inverse process. As‐prepared VLRP shows long‐awaited properties, such as, high color contrast and resolution, appropriate legible time of prints, excellent reversibility (>100 cycles), easiness to achieve multicolor prints, and agreeing well with environmental concept of green printing. In addition, study of influence factors on PPT in this work, to some extent, may also help people understand complex photocycle process in biosystem.

Crystallization‐Induced Emission Enhancement and Amplified Spontaneous Emission from a CF3‐Containing Excited‐State Intramolecular‐Proton‐Transfer Molecule

In this work, a simple but effective molecular design strategy is developed for the generation of intense blue emission in the solid state including single crystals through the multiple secondary intermolecular interactions such as CF…HC hydrogen bonding. The synthesized novel imidazole‐based excited‐state intramolecular proton transfer (ESIPT) molecule, 2‐(1‐(3,5‐bis(trifluoromethyl)phenyl)‐4,5‐diphenyl‐1H‐imidazol‐2‐yl)phenol (HPI‐CF3), shows significantly enhanced blue fluorescence in single crystal (ΦF = 0.67) compared to faint emission in solution (ΦF < 0.05). It is considered that tight but slipped stacking structure of HPI‐CF3 molecules in the single crystal not only effectively suppresses the nonradiative decay pathways such as twist intramolecular charge transfer but also induces highly allowed transition character. Taking advantage of the crystallization‐induced emission enhancement characteristics and four‐level ESIPT photocycle process of HPI‐CF3, an efficient amplified spontaneous emission at 475 nm with a threshold of 13.1 mJ cm−2 is observed from the single crystal by picosecond laser optical pumping.

A highly selective fluorescent sensor for Cd2+ based on a new diarylethene with a 1,8-naphthyridine unit

A new asymmetrical photochromic diarylethene has been synthesized by using 1,8-naphthyridine as a functional group and perfluorodiarylethene as photoswitching unit via a peptide bond linkage, and its molecular structure was characterized by single crystal X-ray diffraction analysis. The compound exhibited favorable photochromism upon irradiation with UV/vis light, and its fluorescent behaviour could be efficiently modulated by light, base/acid, and metal ion in THF. The deprotonated derivative of the diarylethene also showed good photochromism, and displayed remarkable “turn-on” fluorescence response during photocyclization process. Furthermore, the diarylethene was highly selective toward Cd2+ with an obvious fluorescent color change from dark to bright green in THF. Finally, a logic circuit was fabricated with four inputs of the combinational stimuli of UV/vis and Cd2+/EDTA, and one output of fluorescence intensity.

Deactivating effect of the pyridine n,π* states on the photoreactivity of 5-[2-(pyrid-n-yl)ethenyl]oxazole (n= 2, 3 and 4)

Abstract This paper describes the results obtained in the study of the photobehaviour of heteroanalogs of stilbene bearing oxazole and pyridine rings at the opposite sides of the ethene moiety. The effect of the positional isomerism of the n -pyridyl group ( n = 2, 3 and 4) on the competitive relaxation processes of the excited states (fluorescence, isomerization and cyclization) was investigated and compared with the behaviour previously reported for the 5-styryloxazole analogue. The group in Zagreb prepared the compounds and followed their photochemistry in preparative conditions while the group in Perugia studied the spectral properties of the trans/cis isomers and the cyclization products and measured the quantum yields of the competitive processes in mild conditions with the main aim to clarify the mechanism of the primary stages after excitation. The photobehaviour revealed an important deactivation effect of the n,π* states introduced by the nitrogen atom. This effect reflects the one reported for styrylpyridines, the analogous compounds with a phenyl instead of an oxazolyl ring. Quantum-mechanical Hyperchem calculations proved to be useful to describe the conformational equilibria and the role of conformers on photoreactivity while more refined DFT calculations on the Z isomers allowed the structure dependent competition between their isomerization/cyclization processes and the possible role of intramolecular H-bonds on the deactivation pathways to be explained. For the compound with n = 4, side processes of hydrogen shift in the primary dihydrophenanthrene-like intermediate and of solvent addition accompanying the photocyclization process were evidenced.

Diastereoselective Strategies towards Thia[n]helicenes.

In the present study, we have investigated different strategies for diastereoselective synthesis of thia[n]helicenes. We describe the introduction of different chiral auxiliaries at various positions and investigated their effect in the photocyclization reaction. Different chiral groups were placed at the sterically hindered position of the helical core and their interactions with various solvents and metals like copper were investigated. The use of Cu(I) salts has led to high diastereoselectivity in the photocyclization process and we were successful in obtaining the thia[5]helicene in enantiomerically pure form in good yield. The single diastereomer obtained was characterized by X-ray crystallography. From the study of the barrier of racemization of these thia[5]helicenes, the stability was found to be comparable to unsubstituted tetrathia[7]helicenes and substituted diazadithia[7]helicenes. This approach provides an easy access to enantiopure helicenes.

Heterogeneous bacteriorhodopsin/gold nanoparticle stacks as a photovoltaic system

To mimic the stack structure of granum, for the first time we demonstrate bacteriorhodopsin (bR)/gold nanoparticles (AuNPs) heterogeneous multilayers to perform as a novel photovoltaic stack system with improved photoelectric performance. Upon a cooperative control of the diameter of AuNPs and the stacking layers, the photocurrent can be effectively regulated to reach about 350nAcm−2. The possible mechanism for enhanced photocurrent recorded in our system is revealed by analyzing the flash kinetics of photocycle intermediates M412, O640, and J600 of bR layers, which are accelerated under proper AuNPs in this stacking structure. A bypass photocycle model of bR photocycle from M412 to B570 state is proposed and supported by tested accumulation of M412 and O640 intermediate during the bR photocycle process. Interestingly, with this stack system, the input flickering light can be effectively modulated into regular photoelectric signals that are well controlled under different flickering frequency and proton concentration gradient. Based on above results, this system potentially serves as solar energy converter to power nano-devices.

Heterohelicenes with Embedded P‐Chiral 1H‐Phosphindole or Dibenzophosphole Units: Diastereoselective Photochemical Synthesis and Structural Characterization

The oxidative photocyclization reactions of olefins that contain 1H-phosphindole or dibenzophosphole substituents have been applied to the synthesis of P/N-bi-heterosubstituted dimeric helicenes, as well as of new [6]- and [8]phosphahelicenes. In these photocyclization processes, the configuration of the stereogenic phosphorus center dictates the sense of helical chirality. Thus, by starting from enantiomerically pure P-menthylphosphole-oxide units, this method affords enantiopure helical compounds. The helical phosphine oxides were characterized by X-ray diffraction. After reduction of the phosphine-oxides, the corresponding helical phosphines have been used as ligands in transition-metal complexes. The X-ray crystal structure of a gold chloride complex of a [6]helicene is reported.