Energy and Electron Transfer in Bifunctional Non-Conjugated Dendrimers

Total synthesis of (6R,12R)-6,12-dimethylpentadecan-2-one, the sex pheromone of Diabrotica balteata LeConte

Diabrotica balteata LeConte is one of the most important polyphagous agricultural pests. The sex pheromone of this pest was synthesized using Evans asymmetric alkylation, ring-opening reaction of (R)-2-methyloxirane, SN 2 alkylation of secondary tosylate, and coupling of chiral tosylate with Grignard reagent as central strategies. The sex pheromone prepared herein would be useful to control D. balteata.

Cascade Energy Transfer in Biopolymer-Templated Multi-Chromophoric Assemblies: Ratiometric Temperature Sensing

Natural light-harvesting complexes collect energy from sunlight and transfer it to the reaction center through a cascade of energy and electron transfer steps. Artificial light-harvesting systems functioning in aqueous media mimic natural photosynthetic systems. However, their design remains a challenging task as closely packed antenna chromophores often undergo severe self-quenching. Herein, we report luminescent co-assemblies between cationic pyrene-appended imidazolium amphiphiles and two anionic biopolymeric scaffolds, heparin and DNA in aqueous media. These co-assemblies served as excellent platforms for constructing artificial light-harvesting systems as upon co-embedding of multiple external dyes, highly efficient single-step and cascade energy transfer was observed from the pyrene donors to the acceptor dyes. Most notably, the efficiency of the energy transfer process was possible to modulate by employing multiple stimuli such as pH and temperature, and this resulted in the generation of multi-color luminescent materials in solution and film states, and they were also exploited in ratiometric temperature sensing. Their stimuli-responsive luminescence in the solid state was found to be advantageous for encryption studies.

Specific Bifunctionalization on the Surface of Phosphorus Dendrimers Syntheses and Properties

Dendrimers are highly branched macromolecules possessing, in most cases, identical terminal functions. However, it is sometimes desirable to have two types of surface functions in order to fulfil specific properties. The stochastic functionalization is frequently used for such purposes, but the presence of an uncontrolled number of each type of terminal function, albeit acceptable for research purposes, has no practical use. Thus, it is highly desirable to find strategies suitable for the precise grafting of two different functional groups on the surface of dendrimers. The easiest way, and the most widely used, consists in using a bifunctional monomer to be grafted to all of the surface functions of the dendrimers. Two other strategies are known but are rarely used: the modification of an existing function, to generate two functions, and the sequential grafting of one function then of a second function. The three methods are illustrated in this review with polyphosphorhydrazone (PPH) dendrimers, together with their properties as catalysts, for materials, and as biological tools.

Multi-stimuli programmable FRET based RGB absorbing antennae towards ratiometric temperature, pH and multiple metal ion sensing

A red-green-blue (RGB) multichromophoric antenna 1 consisting of energy donors naphthalimides and perylenediimides and a central aza-BODIPY energy acceptor along with two subchromophoric red-blue (RB 6) and green-blue (GB 12) antennae was designed that showed efficient cascade Förster resonance energy transfer (FRET). RGB antenna 1 showed pronounced temperature-dependent emission behaviour where emission intensities in green and red channels could be tuned in opposite directions by temperature giving rise to unique ratiometric sensing with a temperature sensitivity of 0.4% °C. RGB antenna 1 showed reversible absorption modulation selectively in the blue region (RGB ↔ RG) upon acid/base addition giving rise to pH sensing behaviour. Furthermore, RGB antenna 1 was utilized to selectively sense metal ions such as Co2+ and Fe3+ through a FRET turn-off mechanism induced by a redox process at the aza-BODIPY site that resulted in the selective spectral modulation of the red band (i.e., RGB → GB). Model antenna RB 6 showed white light emission with chromaticity coordinates (0.32, 0.33) on acid addition. Antennae 1, 6 and 12 also exhibited solution state electrochromic switching characterized by distinct colour changes upon changing the potential. Finally, antennae 1, 6 and 12 served as reversible fluorescent inks in PMMA/antenna blends whereby the emission colours could be switched or tuned using different stimuli such as acid vapour, temperature and metal ions.

Chemical Tuning of Exciton versus Charge-Transfer Excited States in Conformationally Restricted Arylene Cages

Covalent assemblies of conjugated organic chromophores provide the opportunity to engineer new excited states with novel properties. In this work, a newly developed triple-stranded cage architecture, in which meta-substituted aromatic caps serve as covalent linking groups that attach to both top and bottom of the conjugated molecule walls, is used to tune the properties of thiophene oligomer assemblies. Benzene-capped and triazine-capped 5,5'-(2,2-bithiophene)-containing arylene cages are synthesized and characterized using steady-state and time-resolved spectroscopic methods. The conformational freedom and electronic states are analyzed using time-dependent density functional theory. The benzene cap acts as a passive spacer whose electronic states do not mix with those of the chromophore walls. The excited state properties are dominated by through-space interactions between the chromophore subunits, generating a neutral Frenkel H-type exciton state. This excitonic state undergoes intersystem crossing on a 200 ps time scale while the fluorescence output is suppressed by a factor of 2 due to a decreased radiative rate. Switching to a triazine cap enables electron transfer from the chromophore-linker after the initial excitation to the exciton state, leading to the formation of a charge-transfer state within 10 ps. This state can avoid intersystem crossing and exhibits red-shifted fluorescence with enhanced quantum yield. The ability to interchange structural modules with different electronic properties while retaining the overall cage morphology provides a new approach for tuning the properties of discrete chromophore assemblies.

Ruthenium-Benzothiadiazole Building Block Derived Dynamic Heterometallic Ru-Ag Coordination Polymer and Its Enhanced Water-Splitting Feature

This article deals with the development of the unprecedented redox-mediated heterometallic coordination polymer {[Ru^III(acac)₂(μ-bis-η¹-N,η¹-N-BTD)₂Ag^I(ClO₄)]ClO₄}_n (3) via the oxidation of the monomeric building block cis-[Ru^II(acac)₂(η¹-N-BTD)₂] (1) by AgClO₄ (BTD = exodentate 2,1,3-benzothiadiazole, acac = acetylacetonate). Monomeric cis-[Ru^II(acac)₂(η¹-N-BTD)₂] (1) and [Ru^II(acac)₂(η¹-N-BTD)(CH₃CN)] (2) were simultaneously obtained from the electron-deficient BTD heterocycle and the electron-rich metal precursor Ru^II(acac)₂(CH₃CN)₂ in refluxing CH₃CN. Molecular identities of 1-3 were authenticated by their single-crystal X-ray structures as well as by solution spectral features. These results also reflected the elusive trigonal-planar geometry of the Ag ion in Ru-Ag-derived polymeric 3. Ru(III) (S = 1/2)-derived 3 displayed metal-based anisotropic EPR with ⟨g⟩/Δg = 2.12/0.56 and paramagnetically shifted ¹H NMR. Spectroelectrochemistry in combination with DFT/TD-DFT calculations of 1ⁿ and 2ⁿ (n = 1+, 0, 1-) determined a metal-based (Ru^II/Ru^III) oxidation and BTD-based reduction (BTD/BTD^•-). The drastic decrease in the emission intensity and quantum yield but insignificant change in the lifetime of 3 with respect to 1 could be addressed in terms of static quenching and/or a paramagnetism-induced phenomenon. A homogeneously dispersed dumbbell-shaped morphology and the particle diameter of 3 were established by microscopic (TEM-EDX/SEM) and DLS analysis, respectively. Moreover, the dynamic nature of polymeric 3 was highlighted by its degradation to the η¹-N-BTD coordinated monomeric fragment 1, which could also be followed spectrophotometrically in polar protic EtOH. Interestingly, both monomeric 1 and polymeric 3 exhibited efficient electrocatalytic activity toward water oxidation processes (OER, HER) on immobilization on an FTO support, which also divulged the better intrinsic water oxidation activity of 3 in comparison to 1.

Synthesis and Photophysical Properties of Light-Harvesting Gold Nanoclusters Fully Functionalized with Antenna Chromophores

The development of efficient light-harvesting systems is important to understand the key aspects of solar-energy conversion processes and to utilize them in various photonic applications. Here, atomically well-defined gold nanoclusters are reported as a new platform to fabricate artificial light-harvesting systems. An efficient amide coupling method is developed to synthesize water-soluble Au₂₂ clusters fully protected with pyrene chromophores by taking advantage of their facile phase-transfer reaction. The synthesized Au₂₂ clusters with densely packed 18 pyrene chromophores (Au₂₂ -PyB₁₈ ) exhibit triple-emission in blue, green, and red wavelength regions arising respectively from pyrene monomer, pyrene excimer, and Au₂₂ emission, producing bright white light emission together. The photoluminescence of Au₂₂ is enhanced by more than tenfold, demonstrating that pyrenes at the periphery efficiently channel the absorbed energy to the luminescent Au₂₂ at the center. A combination of femtosecond transient absorption and anisotropy measurements of Au₂₂ -PyB₁₈ explicitly reveals three main decay components of 220 fs, 3.5 ps, and 160 ps that can be assigned to energy migration between pyrenes and energy transfer processes from pyrene monomer and excimer to the central Au₂₂ , respectively.

Synthesis of a bead-like multicyclic polymer by UV-induced coupling of an anthracene-telechelic monocyclic precursor and its reversible topological conversion

In this study, a bead-like multicyclic polymer was synthesized by the UV-induced coupling reaction of an anthracene-telechelic monocyclic precursor and the reversible topological transformation between the monocyclic polymer and the multicyclic polymer was realized.

Direct Measure of the Local Concentration of Pyrenyl Groups in Pyrene-Labeled Dendrons Derived from the Rate of Fluorescence Collisional Quenching

The model-free analysis (MFA) was applied to measure the average rate constant (<k>) for pyrene excimer formation (PEF) in a series of pyrene-labeled dendrons referred to as Py_x-G(N), where x (= 2^N) is the number of pyrenyl labels born by a dendron of generation N ranging from 1 to 6. <k> was measured in four different solvents, namely tetrahydrofuran (THF), toluene, N,N-dimethylformamide (DMF), and dimethylsulfoxide (DMSO). <k> was found to increase linearly with increasing local pyrene concentration ([Py]_loc), where [Py]_loc had been determined mathematically for the Py_x-G(N) dendrons. The slope of each straight line changed with the nature of the solvent and represented k_diff, the bimolecular rate constant for PEF. k_diff depended on the solvent viscosity (η) and the probability (p) for PEF upon encounter between an excited and a ground-state pyrene. In a same solvent, k_diff for the Py_x-G(N) dendrons was about 360 ± 30 times smaller than k_diff obtained for ethyl 4-(1-pyrene)butyrate (PyBE), a pyrene model compound similar to the pyrene derivative used to label the dendrons. The massive decrease in k_diff observed for the Py_x-G(N) samples reflected the massive loss in mobility experienced by the pyrenyl labels after being covalently attached onto a macromolecule compared to freely diffusing PyBE. Interestingly, the k_diff values obtained for the Py_x-G(N) dendrons and the PyBE model compound followed similar trends as a function of solvent, indicating that the difference in behavior between the k_diff values obtained in different solvents were merely due to the changes in the η and p values between the solvents. Normalizing the <k> values obtained with the Py_x-G(N) dendrons by the k_diff values obtained for PyBE in the same solvents accounted for changes in η and p, resulting in a master curve upon plotting <k>/(f_diff × k_diff) as a function of [Py]_loc, where f_diff was introduced to account for some pyrene aggregation in the higher generation dendron (Py₆₄-G(6)). This result demonstrates that <k> represents a direct measure of [Py]_loc in pyrene-labeled macromolecules.

Water-soluble oligofluorenes bearing N1-alkylcytosine side chains as turn-on and turn-off materials in telomere DNA length sensing

Water-soluble cationic and anionic oligofluorenes bearing N1-alkylcytosine side chains, namely OF-1 and OF-2, were synthesized. The photoluminescence (PL) intensity of an aqueous solution of OF-1 decreased on the addition of (TTAGGG)m as telomere DNA models. In contrast, the PL intensity of OF-2 increased on the addition of DNA.

Self-assembly of an alkynylpyrene derivative for multi-responsive fluorescence behavior and photoswitching performance

Highly emissive fluorophores based on polyaromatic hydrocarbons with tunable emission properties and aggregated structures play a very important role in relevant functional studies. In this study, a novel alkynylpyrene derivative 1 was synthesized, which exhibits unimolecular to excimer emission in methanol with an increasing concentration accompanied by the formation of nanovesicles via the π-π stacking, hydrogen bond and hydrophobic interaction. The self-assembly behavior as well as emission properties of 1 in aprotic polar solvents (ACN, acetone, DMF and DMSO) can also be adjusted by the volume fraction of the poor solvent H2O, which can induce 1 self-assembly to excimer state and could be applied in information transfer. Moreover, upon visible light irradiation, photoswitchable performance of nanovesicles of 1 was observed in which the emission markedly changes from yellow to blue; this is attributed to the cycloaddition reaction of alkynyl groups and singlet oxygen, which can be generated without the addition of external photosensitizers. The multi-responsive and fluorescence behavior of the alkynylpyrene derivative show that the self-assembly can be used to expand the development of this type of fluorophores, and the novel photoinduced tunability of the fluorescence emission provides an effective strategy to obtain high-performance transmitting and sensing materials.

Morpholinyl dendrimer phthalocyanine: synthesis, photophysical properties and photoinduced intramolecular electron transfer

The photophysical properties of a novel series of morpholinyl dendrimer phthalocyanines exhibited dependence on the number of morpholinyl groups and the central ion. The photoinduced electron transfer from the morpholinyl units to phthalocyanine ring was evidenced.

Time-resolved spectroscopic analysis of the light-energy harvesting mechanism in carbazole-dendrimers with a blue-phosphorescent Ir-complex core

In order to understand the light energy-harvesting mechanism, singular value decomposition analysis was performed to classify the temporal and spectral species in transient absorption spectra.

Use of Bromine and Bromo-Organic Compounds in Organic Synthesis

Bromination is one of the most important transformations in organic synthesis and can be carried out using bromine and many other bromo compounds. Use of molecular bromine in organic synthesis is well-known. However, due to the hazardous nature of bromine, enormous growth has been witnessed in the past several decades for the development of solid bromine carriers. This review outlines the use of bromine and different bromo-organic compounds in organic synthesis. The applications of bromine, a total of 107 bromo-organic compounds, 11 other brominating agents, and a few natural bromine sources were incorporated. The scope of these reagents for various organic transformations such as bromination, cohalogenation, oxidation, cyclization, ring-opening reactions, substitution, rearrangement, hydrolysis, catalysis, etc. has been described briefly to highlight important aspects of the bromo-organic compounds in organic synthesis.

Self-assembled light-harvesting supercomplexes from fluorescent surface-cross-linked micelles

Fluorescent nanoparticles made of cross-linked dansylated surfactants allowed efficient donor-donor energy migration within and beyond the nanoparticles when the nanoparticles aggregated in the presence of oppositely charged energy acceptors. The light-harvesting system enabled a single acceptor to quench the emission of over 500 donor chromophores.

Deep-blue emitting pyrene–benzimidazole conjugates for solution processed organic light-emitting diodes

Pyrene–benzimidazole hybrids have been demonstrated as deep blue emitters for organic light-emitting diodes.

Facile synthesis of self-host functional iridium dendrimers up to the fourth generation with N-phenylcarbazole-based polyether dendrons for non-doped phosphorescent organic light-emitting diodes

Self-host functional iridium dendrimers with N-phenylcarbazole-based polyether dendrons.

A detailed investigation of light-harvesting efficiency of blue color emitting divergent iridium dendrimers with peripheral phenylcarbazole units

The increase in phosphorescence efficiency was estimated by the energy transfer mechanism for divergent iridium dendrimers with peripheral phenylcarbazole units. A series of Ir-core/phenylcarbazole-end dendrons of the type, Ir(dfppy)2(pic-Czn) (Gn, n = 0, 1, 2, and 3), was synthesized, where dfppy, pic, and Czn (n = 2, 4, and 8) are 2-(4,6-difluorophenyl)pyridine, picolinate substituted with Czn at the 3-position, and 4-(9-carbazolyl)phenyldendrons connected with 3,5-di(methyleneoxy)benzyloxy branches, respectively. Selective excitation of the Czn units of G1–G3 resulted in >90% quenching of the Cz fluorescence accompanied by the growth of phosphorescence from the Ir(dfppy)2(pic) core as a consequence of energy transfer from the excited-singlet Czn chromophore to the core. The rate constants of energy transfer were determined by steady-state and transient spectroscopic measurements to be 4.32 × 10(9) s(−1) (G1), 2.37 × 10(9) s(−1) (G2), and 1.46 × 10(9) s(−1) (G3), which were in good agreement with those calculated using the Förster model. The phosphorescence enhancements were 157 (G1), 213 (G2), and 264% (G3) when compared to the phosphorescence of the core Ir(dfppy)2(pic-Ph2) (G0), in which pic-Ph2 is 3-(3,5-dibenzyloxybenzyl)picolinate.

Advances in Photofunctional Dendrimers for Solar Energy Conversion

Dendrimers are regularly and hierarchically branched synthetic macromolecules with numerous chain ends all emanating from a single core, which makes them attractive candidates for energy conversion applications. During photosynthesis and photocatalysis, photoinduced electron transfer and energy transfer are the main processes involved. Studies on these processes in dendritic systems are critical for the future applications of dendrimers in photochemical energy conversion and other optoelectronic devices. In this Perspective, the recent advances of photofunctional dendrimers in energy conversion based on light-harvesting systems, solar cells, and photochemical production of hydrogen will be discussed. The electron-transfer and energy-transfer characteristics in light-harvesting photofunctional dendrimers and the regulation of the electron-transfer process and the stabilization of the charge separation state in hydrogen photoproduction are emphasized.

Bottom-up synthesis of nanoscale conjugation-interrupted frameworks and their electrical properties

Soluble covalent organic nanoframeworks up to generation 2.5 (G2.5) are synthesized with self-similar H-shaped conformations by using a bottom-up approach including iterative C-H bond functionalization. The electrical characteristics of nanoscale thin-film semiconductors of the conjugation-interrupted frameworks can be tuned by post-modification with diazonium salt.

Unusual emission from norbornene derived phosphonate molecule--a sensor for Fe(III) in aqueous environment

This communication describes the synthesis and characterization of norbornene derived phosphonates (NDP1 to NDP3). The unusual fluorescence of NDP1 is attributed to the difference between the positions of HOMO and LUMO wave functions that lead to the prevention of non-radiative relaxation pathways. Due to the significant non-radiative relaxation pathway, the control molecule NDP2 shows weak fluorescence whereas NDP3 shows no fluorescence at all. Similar experiments are carried out on NDPH1, the homopolymer of NDP1. Our hypothesis is further strengthened as there is no emission from NDPH1. The unusual fluorescence is utilized as a new fluorometric sensor that selectively detects Fe(III) ions.

Benzothiadiazole-based D-π-A-π-D organic dyes with tunable band gap: synthesis and photophysical properties

A series of symmetrical D-π-A-π-D molecules based on benzothiadiazole and oligo(thienylenevinylene)s were facilely developed. The investigation of their photophysical and electrochemical properties demonstrated that these compounds exhibited broad absorption covering the whole visible range with appropriate energy levels for light-harvesting donors.