In situ Cyclization of Aromatic Thioethers in Emissive Materials to Generate Phosphorescent Dibenzothiophenes.

In this contribution, we explored the photocyclization of thioethers to highly substituted dibenzothiophenes (DBT) using solely UV-light without any need for additives. This cost-effective, robust and environmentally friendly approach yielded phosphorescent compounds, which were characterized through X-ray crystallography and state-of-the-art photophysical methods. The resulting DBTs feature ultralong photoluminescence lifetimes and quantum yields close to unity in frozen glassy matrices. The reaction mechanism was investigated in detail through a combination of quantum chemical calculations and experimental results, providing evidence that triplet states are involved in the cyclization process. Additionally, the photoreaction can also be induced within materials. For this purpose, the precursors can be integrated into polymer films or polymer resins suitable for 3D printing. Irradiation of these polymeric objects allows motifs with ultralong phosphorescence to be irreversibly inscribed through the proceeding photocyclization. The in situ photogeneration of DBTs from aromatic thioethers overcomes the observed incompatibilities regarding solubility in polymer resins for 3D printing.

Photoinduced in situ generation of DNA-targeting ligands: DNA-binding and DNA-photodamaging properties of benzo[c]quinolizinium ions.

The photoreactions of selected styrylpyridine derivatives to the corresponding benzo[c]quinolizinium ions are described. It is shown that these reactions are more efficient in aqueous solution (97-44%) than in organic solvents (78-20% in MeCN). The quinolizinium derivatives bind to DNA by intercalation with binding constants of 6-11 × 104 M-1, as shown by photometric and fluorimetric titrations as well as by CD- and LD-spectroscopic analyses. These ligand-DNA complexes can also be established in situ upon irradiation of the styrylpyridines and formation of the intercalator directly in the presence of DNA. In addition to the DNA-binding properties, the tested benzo[c]quinolizinium derivatives also operate as photosensitizers, which induce DNA damage at relative low concentrations and short irradiation times, even under anaerobic conditions. Investigations of the mechanism of the DNA damage revealed the involvement of intermediate hydroxyl radicals and C-centered radicals. Under aerobic conditions, singlet oxygen only contributes to marginal extent to the DNA damage.

Thermoreversible helical fibers from photoreactive triphenylene-derived liquid crystals in liquid paraffin.

Liquid crystalline triphenylene derivatives, TPC1p-n (n = 6, 12, 14, 16) were prepared using p-alkoxycinnamate as the [2+2] photo-cyclization site. TPC1p-n (n = 12, 14, 16) showed Colr phase and gave crescent-shaped or helical fibers after UV-irradiated in liquid paraffin solutions at 90 and 110 °C in the Colr temperature range. The apparent photoreaction products were shown to be thermally reversible, i.e. they dissolved in liquid paraffin at high temperatures and reappeared on cooling, indicating that they were aggregates of oligomerized TPC1p-n. The reaction mechanism was discussed in terms of the structure of the liquid crystalline phase.

Free Radical-Mediated Photocyclization of Triphenylphosphindole Oxides for Photoactivated and Self-Reported Lipid Peroxidation.

Photocyclization is demonstrated as a powerful tool for building complicated polycyclic molecules. And efficient photocyclization is competent as an artful strategy to develop photo-responsive smart materials. Herein, an efficient free radical-mediated photocyclization for triphenylphosphindole oxide (TPPIO) derivatives to generate tribenzophosphindole oxide (TBPIO) derivatives at ambient condition is reported. The reaction mechanism and substituent effect on photocyclization efficiency are thoroughly investigated. Additionally, photophysical and photochemical properties of TPPIO and TBPIO derivatives are measured for comparison and deeply deciphered by theoretical calculation. TPPIO derivatives own typical aggregation-induced emission feature but barely generate reactive oxygen species (ROS), while TBPIO derivatives experience aggregation-caused quenching but show efficient Type I ROS generation capacity. Further, in vitro experiments demonstrate that this photo-conversion can efficiently occur in situ in living cells to activate photodynamic therapy (PDT) effect to trigger lipid peroxidation with selective fluorescence "light up" in lipid droplet area under continuous irradiation. This work extends the optoelectronically and biologically interesting phosphindole oxide-containing π-conjugated systems through an efficient synthetic strategy, provides in-depth mechanistic descriptions in the aspects of reaction and property, and further presents their great potentials for photoactivated and self-reported PDT.

Synthesis and Properties of Boron-Containing Heteromerous Bistricyclic Aromatic Enes: Structural Effects on Thermodynamic Stability and Photoreactivity.

Overcrowded bistricyclic aromatic enes (BAEs) have several conformations such as twisted and anti-folded conformers, and their stereochemistry and chromism have been studied in earnest. In this study, boron-containing heteromerous BAEs having various tricyclic structures were synthesized and their photophysical properties investigated. Single-crystal X-ray analysis revealed that the introduction of a rigid fluorene unit resulted in a twisted conformer, whereas the introduction of flexible units such as thioxanthene and 9,9-dimethyl-9,10-dihydroanthracene units resulted in an anti-folded conformer. The absorption spectra of the heteromerous BAEs were dependent on the introduced tricyclic structures, suggesting the immense impact of the tricyclic structures on the electronic structures of BAEs. DFT calculations revealed the large effect of the flexibility of the tricyclic structures on the thermodynamic stability of the conformers. In addition, the boron-containing heteromerous BAEs underwent photocyclization reactions, indicating their potential application as precursors of polyaromatic hydrocarbons and helical aromatic materials.

Synthesis of substituted 8H-benzo[h]pyrano[2,3-f]quinazolin-8-ones via photochemical 6π-electrocyclization of pyrimidines containing an allomaltol fragment.

For the first time, we elaborated a method for the synthesis of pyrimidines containing an allomaltol unit. The suggested approach is based on the reaction of 2-(1-(dimethylamino)-3-oxo-3-arylprop-1-en-2-yl)-3-hydroxy-6-methyl-4H-pyran-4-ones with cyanamide. The photochemical behavior of the obtained pyrimidines was investigated. It was shown that for the hydroxy derivatives the main pathway of phototransformation is a 6π-electrocyclization of the 1,3,5-hexatriene system and subsequent [1,9]-H sigmatropic shift leading to dihydrobenzo[h]pyrano[2,3-f]quinazolines. At the same time, for methylated analogues the photoreaction proceeds in two directions resulting in the formation of a mixture of the corresponding dihydrobenzo[h]pyrano[2,3-f]quinazolines and polyaromatic products. The obtained dihydro derivatives are stable compounds and do not undergo aromatization upon further UV irradiation. The structures of two of the dihydrobenzo[h]pyrano[2,3-f]quinazolines were confirmed by X-ray diffraction analysis. Based on the performed studies, a two-stage telescopic method for the synthesis of polyaromatic benzo[h]pyrano[2,3-f]quinazolines including the initial photocyclization of the starting pyrimidines and the final dehydration was proposed.

Perturbing π-clouds with Substituents to Study the Effects on Reaction Dynamics of gauche-1,3-Butadiene to Bicyclobutane Electrocyclization.

The conical intersection (CI) governs the ultra-fast relaxation of excited states in a radiationless manner and are observed mainly in photochemical processes. In the current work, we investigated the effects of substituents on the reaction dynamics for the conversion of gauche-1,3-butadiene to bicyclobutane via photochemical electrocyclization. We incorporated both electron withdrawing (-F) and donating (-CH3 ) groups in the conjugated system. In our study, we optimized the minimum energy conical intersection (MECI) geometries using the multi-configurational state-averaged CASSCF approach, whereas, to study the ground state reaction pathways for the substituted derivatives, dispersion corrected, B3LYP-D3 functional was used. The non-adiabatic surface hopping molecular dynamics simulations were performed to observe the behaviour of electronic states involved throughout the photoconversion process. The results obtained from the multi-reference second-order perturbation correction of energy at the XMS-CASPT2 level of theory, topography analysis, and non-adiabatic dynamics suggest that the -CH3 substituted derivatives can undergo faster thermal conversion to the product in the ground state with a smaller activation energy barrier compared to -F substituted derivative. Our study also reveals that the GBUT to BIBUT conversion follows both conrotatory and disrotatory pathways, whereas, on substitution with -F or -CH3 , the conversion proceeds via the conrotatory pathway.

A Platinum(II)-Based Molecular Cage with Aggregation-Induced Emission for Enzymatic Photocyclization of Alkynylaniline.

Enzymes facilitate chemical conversions through the collective activity of aggregated components, but the marriage of aggregation-induced emission (AIE) with molecular containers to emulate enzymatic conversion remains challenging. Herein, we report a new approach to construct a PtII -based octahedral cage with AIE characteristics for the photocyclization of alkynylaniline by restricting the rotation of the pendant phenyl rings peripheral to the PtII corner. With the presence of water, the C-H⋅⋅⋅π interactions involving the triphenylphosphine fragments resulted in aggregation of the molecular cages into spherical particles and significantly enhanced the PtII -based luminescence. The kinetically inert Pt-NP chelator, with highly differentiated redox potentials in the ground and excited states, and the efficient coordination activation of the platinum corner facilitated excellent catalysis of the photocyclization of alkynylaniline. The enzymatic kinetics and the advantages of binding and activating substrates in an aqueous medium provide a new avenue to develop mimics for efficient photosynthesis.

Facile Route to Novel Diazaphosphinine-Based Polycyclic Aromatic Hydrocarbons.

Doping main-group elements has become a unique strategy to fine-tune optoelectronic properties of organic π-conjugated molecules. Herein, we report the first example of diazaphosphinine based polycyclic hydrocarbons (DP-PAHs) via facile synthetic protocols. Synthesis of diazaphosphinine heterocycles harnesses mild and efficient P-N chemistry. The heterocycles readily undergo photocyclizations by using only oxidants. The photocyclization protocol is applicable to the heterocycles with various P-centers. We successfully extended the protocols in extended DP-PAHs with 13-fused aromatic rings. Further adjusting P-chemistry allows to efficiently fine-tune the singlet and triplet emission characteristics. More importantly, single-molecule white-light-emitting character was achieved via the balanced singlet and triplet emission of DP-PAH. Therefore, this work opens up new horizons in the area of organic π-conjugated materials.

Turn-On Mode Circularly Polarized Luminescence in Self-Organized Cholesteric Superstructure for Active Photonic Applications.

Developing circularly polarized luminescence (CPL)-active materials with a large luminescence dissymmetry factor (glum) or stimulus responses has evoked a lot of interest in the past few years; however, the light-controllable "on/off" CPL still remains a challenge. Here, a novel diarylethene-based chiral fluorescent photoswitch featuring "turn-on" CPL characteristic is developed, designated as (S,S)-switch 6, which can undergo reversible photocyclization/cycloreversion upon irradiation with UV and visible light. (S,S)-Switch 6 shows completely reversible "off-on-off"-responsive CPL behavior in solution. By doping (S,S)-switch 6 into nematic liquid crystals (LCs), the consequent luminescent cholesteric LCs (CLCs) exhibit a larger glum value enhanced 2 orders of magnitude when irradiated with UV light, which can be attributed to the highly ordered helical arrangement of CLCs. The potentials of this turn-on type CPL material for anticounterfeiting and information encryption are illustrated. Furthermore, the visualization of circularly polarized (CP) fluorescent patterns can be successfully achieved by constructing the double-layer CPL system consisting of a CP luminescent layer and a polymer cholesteric reflective layer. The proposed concept establishes a light-controlled off-on-off CPL platform that is of tremendous potential for applications in multi-informational data storage and encryption devices.

UVA Light-promoted Catalyst-free Cyclization of Vinyl Selenides: Green and Efficient Synthesis of C3-Unsubstituted 2-Selanyl Benzochalcogenophenes.

A metal- and catalyst-free photo-promoted cyclization of properly substituted vinyl selenides was developed using UVA irradiation. A total of eighteen new C3-unsubstituted 2-selanyl benzochalcogenophenes (benzofurans, benzothiophenes and benzoselenophenes) were prepared in 30-86% yield after irradiation with UVA at room temperature. The usefulness of the title compounds was demonstrated in the easy functionalization of the remaining free C-H bond of the benzochalcogenophenes to form new C-Se and C-Br bonds by simple procedures. Furthermore, the reaction can be performed under natural sunlight irradiation and the solvent is easily reused further in several subsequent runs.

Nanographene with Multiple Embedded Heptagons: Cascade Radical Photocyclization.

Although heptagons are widely found in graphenic materials, the precise synthesis of nanocarbons containing heptagons remains a challenge, especially for the nanocarbons containing multiple-heptagons. Herein, we show that photo-induced radical cyclization (PIRC) can be used to synthesize multi-heptagon-embedded nanocarbons. Notably, a nanographene containing six heptagons (1) was obtained via a six-fold cascade PIRC reaction. The structure of 1 was clearly validated and showed a Monkey-saddle-shaped conformation. Experimental bond analysis and theoretical calculations indicated that the heptagons in 1 were non-aromatic, whereas the peripheral rings were highly aromatic. Compared to planar nanographene with the same number of π electrons, 1 had a similar optical gap due to a compromise between the decreased conjugation in the wrapped structure and enhanced electronic delocalization at the rim. Electrochemical studies showed that 1 had low-lying oxidation potentials, which was attributed to the nitrogen-doping.

Synthesis, Photochemistry, Computational Study and Potential Application of New Styryl-Thiophene and Naphtho-Thiophene Benzylamines.

In this research, the synthesis, photochemistry, and computational study of new cis- and trans-isomers of amino-thienostilbenes is performed to test the efficiency of their production and acid resistance, and to investigate their electronic structure, photoreactivity, photophysical characteristics, and potential biological activity. The electronic structure and conformations of synthesized thienostilbene amines and their photocyclization products are examined computationally, along with molecular modeling of amines possessing two thiophene rings that showed inhibitory potential toward cholinesterases. New amino-styryl thiophenes, with favorable photophysical properties and proven acid resistance, represent model compounds for their water-soluble ammonium salts as potential styryl optical dyes. The comparison with organic dyes possessing a trans-aminostilbene subunit as the scaffold shows that the newly synthesized trans-aminostilbenes have very similar absorbance wavelengths. Furthermore, their functionalized cis-isomers and photocyclization products are good candidates for cholinesterase inhibitors because of the structural similarity of the molecular skeleton to some already proven bioactive derivatives.

Facile Azabenz-Annulations through UV-induced Photocyclization: A Promising Method for Perylenediimide-Based Organic Semiconductors.

The derivatization of perylenediimides (PDIs) by bay decoration is essential for the development of PDI-based semiconductors owing to their excellent photoelectric properties. Herein, four bis-azabenz-annulated PDIs (bis-AzaBPDIs) are concisely synthesized in high yields through ultraviolet-induced photocyclization, where the reaction processes including aldimine condensation, cyclization, and oxidative re-aromatization are investigated. The optical characterizations and theoretical simulation reveal that the unique properties of the four bis-AzaBPDIs are comparable to their parent PDI. Organic field effect transistors with compounds 2, 3, or 4 as active layers indicated that all compounds showed unipolar electron transport properties with the mobilities of 1.1×10-3 , 5.8×10-4 , and 8.5×10-6  cm2  V-1 s-1 , respectively. These results suggest the great potential of bis-AzaBPDIs as organic semiconductors. The easy preparation approach reported in this work would renew research interest in developing bis-AzaBPDI-based optoelectronic molecules.

Alkylated Benzodithienoquinolizinium Salts as Possible Non-Fullerene Organic N-Type Semiconductors: An Experimental and Theoretical Study.

Three photobicyclized benzodithienoquinolizinium tetrafluoroborates (BPDTQBF4) were prepared and evaluated by UV-Vis and fluorescence spectral, electrochemical analysis, and by theoretical calculations as possible organic n-type semiconductors. Evaluation and comparison of their LUMO levels, HOMO-LUMO energy gaps as monomeric and π-stacked dimers with those of other materials, suggest their potential as organic n-type semiconductors. Calculations of their relative charge carrier mobilities confirmed this potential for one derivative with a long (C-14) alkyl chain appended to the polycyclic planar π-system.

Photochemical transformation of a cellulose biosynthesis inhibitor into phytoene desaturase inhibitors.

Mode of action studies showed that 5-methyl-N,N-bis[6-(trifluoromethyl)pyridin-3-yl]pyridin-2-amine (4), a representative from a new class of herbicidal tris-pyridyl amines, is an inhibitor of cellulose biosynthesis (CB). The compound undergoes an oxidative photocyclization, when exposed to UV-B light (300-340 nm) in the presence of oxygen, to give a new class of herbicidal pyrrolodipyridines. These compounds are potent inhibitors of the herbicide target enzyme phytoene desaturase and no longer inhibit CB.

Contorted Heteroannulated Tetraareno[a,d,j,m]coronenes.

Fused polycyclic aromatic compounds are interesting materials for organic electronics applications. To fine-tune photophysical or electrochemical properties, either various substituents can be attached or heteroatoms (such as N or S) can be incorporated into the fused aromatic backbone. Coronenes and heterocoronenes are promising compounds in this respect. Up until now, the possibilities for varying the attached fused heteroaromatics at the coronene core were quite limited, and realizing both electron-withdrawing and -donating rings at the same time was very difficult. Here, a series of pyridine, anisole and thiophene annulated tetraareno[a,d,j,m]coronenes has been synthesized by a facile two-step route that is a combination of Suzuki-Miyaura cross-coupling and a following cyclization step, starting from three different diarenoperylene dibromides. The contorted molecular π-planes of the obtained cata-condensed tetraarenocoronenes were analyzed by single-crystal X-ray crystallography, and the photophysical and electrochemical properties were systematically investigated by UV/Vis spectroscopy and cyclovoltammetry.

Controlled Fabrication and Characterization of Coronene Diimide-Based Insoluble Thin Films Produced by Photoinduced Cyclization.

An insoluble thin film of a coronene diimide (CDI) derivative was fabricated from a soluble precursor of perylene diimide (PDI) by photoirradiation. We prepared a 1,7-diarylated PDI (TP-PDI) that can be converted into a coronene diimide (TP-CDI) derivative via a Scholl-type photocyclization reaction. This reaction was accompanied by structural changes from a twisted structure to a π-extended planar molecule. It was found that this photoconversion reaction occurs for both solution-based and thin-film-based reactants investigated by the changes of UV-vis absorption spectra and 1 H NMR spectra. The photocyclization reactions were found to proceed smoothly in polar solvents. In the thin-film state, the solvent vapor annealing method is a key process for achieving photoconversion reaction. Additionally, the fabrication of multi-layered thin films was achieved without undesirable dissolution of the underlying layers because of different solubilities of TP-PDI and TP-CDI.

Oxidative Photocyclization of Aromatic Schiff Bases in Synthesis of Phenanthridines and Other Aza-PAHs.

The oxidative photocyclization of aromatic Schiff bases was investigated as a potential method for synthesis of phenanthridine derivatives, biologically active compounds with medical applications. Although it is possible to prepare the desired phenanthridines using such an approach, the reaction has to be performed in the presence of acid and TEMPO to increase reaction rate and yield. The reaction kinetics was studied on a series of substituted imines covering the range from electron-withdrawing to electron-donating substituents. It was found that imines with electron-withdrawing substituents react one order of magnitude faster than imines bearing electron-donating groups. The 1H NMR monitoring of the reaction course showed that a significant part of the Z isomer in the reaction is transformed into E isomer which is more prone to photocyclization. The portion of the Z isomer transformed showed a linear correlation to the Hammett substituent constants. The reaction scope was expanded towards synthesis of larger aromatic systems, namely to the synthesis of strained aromatic systems, e.g., helicenes. In this respect, it was found that the scope of oxidative photocyclization of aromatic imines is limited to the formation of no more than five ortho-fused aromatic rings.

A simple and easy to perform synthetic route to functionalized thienyl bicyclo[3.2.1]octadienes.

In order to prepare novel polycyclic derivatives of bicyclo[3.2.1]octadiene systems fused with a thiophene ring, photochemical cyclization and aldol condensation reactions were carried out. The starting substrates were easily obtained by a Vilsmeier-Haack reaction of bicyclo[3.2.1]octadiene thiophene derivatives with dimethylformamide. From the obtained carbaldehydes, novel methyl, methoxy, and cyano-substituted styryl thienobenzobicyclo[3.2.1]octadiene derivatives were synthesized through Wittig reactions and subjected to photochemical cyclization, in terms of obtaining the new annulated structures. As part of this study, the aldol reaction of the starting 2-substituted carbaldehyde with acetone was also performed, which produced the thieno-fused benzobicyclo[3.2.1]octadiene compound with an extended conjugation.