Tianjun Yu

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.

Locked planarity: a strategy for tailoring ladder-type π-conjugated anilido-pyridine boron difluorides.

A novel series of syn- and anti-ladder-type anilido-pyridine boron difluorides (APBDs) were synthesized by stepwise incorporation of boron into laddered ligands. The boron coordination-locked strategy endows the ladder-type APBDs with a stiff conformation, which results in a substantial bathochromic shift of their absorption spectra and a narrowed HOMO-LUMO energy gap, reinforcing the compounds for potential applications in organic electronics.

Intramolecular triplet–triplet energy transfer enhanced triplet–triplet annihilation upconversion with a short-lived triplet state platinum(II) terpyridyl acetylide photosensitizer

A model of a dendritic compound (Pt–DPA) with two Pt-complex photosensitizer chromophores and two 9,10-diphenylanthracene (DPA) acceptor groups covalently attached to the periphery and the core of the poly(aryl ether) dendrimer of generation 1 was prepared. A triplet–triplet annihilation upconversion (TTA-UC) system (Pt–DPA/DPA–OH) was constructed in deaerated DMF by combining Pt–DPA with a dissociative acceptor (DPA–OH). Although the lifetime of the triplet state of the Pt-complex is only 52 ns, the upconversion fluorescence from DPA (400–460 nm) in the Pt–DPA/DPA–OH system was observed with a quantum yield of 0.22% upon selective excitation of the Pt-complex with a 473 nm laser, which is due to the efficient intramolecular triplet–triplet energy transfer (ΦTTET > 0.81) from the Pt-complex photosensitizer to the DPA acceptor within Pt–DPA. The acceptor covalently linked with the photosensitizer acts as an energy-relay to transfer the harvested energy to the dissociative acceptor which further undergoes the TTA process. The efficient intramolecular triplet–triplet energy transfer process between the photosensitizer and the acceptor plays an important role in the TTA-UC system building with a short-lived triplet state photosensitizer, which facilitates the production of the triplet state of the acceptor, thus advancing the TTA-UC process. This work presents a new strategy for construction of efficient TTA-UC systems utilizing short-lived triplet state photosensitizers.

Pd–Porphyrin Oligomers Sensitized for Green‐to‐Blue Photon Upconversion: The More the Better?

A series of directly meso-meso-linked Pd-porphyrin oligomers (PdDTP-M, PdDTP-D, and PdDTP-T) have been prepared. The absorption region and the light-harvesting ability of the Pd-porphyrin oligomers are broadened and enhanced by increasing the number of Pd-porphyrin units. Triplet-triplet annihilation upconversion (TTA-UC) systems were constructed by utilizing the Pd-porphyrin oligomers as the sensitizer and 9,10-diphenylanthracene (DPA) as the acceptor in deaerated toluene and green-to-blue photon upconversion was observed upon excitation with a 532 nm laser. The triplet-triplet annihilation upconversion quantum efficiencies were found to be 6.2 %, 10.5 %, and 1.6 % for the [PdDTP-M]/DPA, [PdDTP-D]/DPA, and [PdDTP-T]/DPA systems, respectively, under an excitation power density of 500 mW cm(-2) . The photophysical processes of the TTA-UC systems have been investigated in detail. The higher triplet-triplet annihilation upconversion quantum efficiency observed in the [PdDTP-D]/DPA system can be rationalized by the enhanced light-harvesting ability of PdDTP-D at 532 nm. Under the same experimental conditions, the [PdDTP-D]/DPA system produces more (3) DPA* than the other two TTA-UC systems, benefiting the triplet-triplet annihilation process. This work provides a useful way to develop efficient TTA-UC systems with broad spectral response by using Pd-porphyrin oligomers as sensitizers.

A colorimetric and ratiometric fluorescence sensor for sensitive detection of fluoride ions in water and toothpaste

Fluoride is a well-known anion that plays a significant physiological role. Sensitive and quantitative sensing of fluoride is of great importance to public health investigation. A colorimetric and ratiometric fluorescence sensor for fluoride ions based on silyl capped hydroxylpyrenealdehyde is designed and synthesized. The sensor detects fluoride ions through the desilylation mediated by fluoride ions and the consequent spectral change of the pyrene derivative. A significant absorption change from 420 to 523 nm in the visual region and a fluorescence shift from 492 to 603 nm can be observed upon addition of fluoride ions of tens of ppb, enabling direct observation with the bare eye. Based on the ratiometric fluorescence with up to 255-fold enhancement, the sensor can rapidly and selectively detect F− in water with a limit as low as 2.7 ppb, and furthermore, the sensor is successfully applied for determining the levels of F− in commercially available toothpaste.

Artificial photosynthesis dendrimers integrating light-harvesting, electron delivery and hydrogen production

A series of artificial photosynthesis complexes, Gn–Ir–Hy (n = 1–4), were constructed by attaching iridium complexes and [Fe–Fe]-hydrogenase mimic to the periphery and core, respectively, of poly(aryl ether) dendrimers of different generations. The iridium complexes act as the light-harvesting antennae and the hydrogenase mimic core is the catalytic center. Light-harvesting, photoinduced electron-transfer, and hydrogen photochemical production were hierarchically realized within the dendritic photosynthesis mimics using triethylamine as the sacrificial electron donor. The catalytic activity is enhanced as the generation of dendritic catalysts increases, and the turnover number for generation 4 is nearly 4 times more than that of generation 1. The peripheral antennae not only harvest photons but also act as electronic energy reservoirs for the photochemical production, which facilitates the catalytic process together with the increased light-harvesting and protection effects of dendritic frameworks. Therefore, efficient artificial photosynthesis systems with complete light-harvesting and catalytic functions can be advanced with such rational design.

A “breathing” dendritic molecule—conformational fluctuation induced by external stimuli

A series of PAMAM dendrimers peripherally modified with tetraphenylethylene (TPE), a typical chromophore with emission enhancement in aggregate, were synthesized (D0–D4). The dendrimers are molecularly dissolved in toluene, a selective solvent. The intramolecular rotation of TPE is depressed by the restrained conformation caused by the congested packing periphery in higher generation dendrimers (D2–D4), which results in a remarkable emission enhancement. The fluorescence quantum yield increases more than 10-fold in D3 and D4 dendrimers compared with a model compound. Furthermore, the emission intensity of dendrimers can be tuned by changing the temperature or solvent conditions, which alters the conformational confinement. Unimolecular spheres with multiple chromophores and conformation-controlled emission are reminiscent of proteins with modulated sizes and photofunctions. Developing these dendritic systems will promote the research of multi-functionalized chromophores and lead to their potential applications in biomimics and sensors.

Tetrathiafulvalene terminal-decorated PAMAM Dendrimers for triggered release synergistically stimulated by redox and CB[7].

A series of polyamidoamine (PAMAM) dendrimers with tetrathiafulvalene (TTF) at the periphery (Gn-PAMAM-TTF), generation 0-2, were synthesized. These functionalized dendrimers exist as nanospheres with diameters around 80-100 nm in aqueous phase, which can encapsulate hydrophobic molecules. The terminal TTF groups can go through a reversible redox process upon addition of the oxidizing and reducing agents. Each terminal TTF(+•) group of the oxidized Gn-PAMAM-TTF assembled with cucurbit[7]uril (CB[7]) forming a 1:1 inclusion complex with association constants of (3.14 ± 0.36) × 10(5), (1.29 ± 0.12) × 10(6), and (1.79 ± 0.24) × 10(6) M(-1) for generation 0-2, respectively, even at the aggregate state. The formation of the inclusion complex loosened the structure of the nanospheres and initiated the release of cargo, and the release mechanism was validated by dynamic light scattering (DLS), cryo-transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR) experiments. This study provides a potential strategy for the development of drug delivery systems synergistically triggered by redox and supramolecular assembly.

Efficient photochemical production of hydrogen in aqueous solution by simply incorporating a water-insoluble hydrogenase mimic into a hydrogel

A composite catalyst Hy-pyr/PVP constructed by incorporating a hydrophobic [Fe–Fe]-hydrogenase mimic (Hy-pyr) into the self-crosslinked polyvinylpyrrolidone (PVP) hydrogel has been successfully applied to photocatalytic production of hydrogen in aqueous solution with high catalytic activity and efficiency. The present study provides a general, simple and efficient strategy for the application of water-insoluble catalysts in aqueous solution by using the PVP hydrogel.

Dendrimers-merging biomimics and photoenergy conversion

Dendrimers are well-defined tree-like macromolecules possessing numerous chain ends emanating from a single core, which makes them attractive candidates for mimicking light-harvesting systems and hydrogenases. Photoinduced electron and energy transfers are main processes involved in light-harvesting and photocatalysis. In this article, the general concepts of design strategies and recent developments of photofunctional dendrimers in biomimics of light-harvesting systems and hydrogenases are discussed. The energy transfer and electron transfer processes in light-harvesting dendrimers and the effect of dendritic structures in photochemical hydrogen production are illustrated.