Aggregation-induced emissive nanoarchitectures for luminescent solar concentrators

Aggregation-induced emission (AIE), the phenomenon by which selected luminophores undergo the enhancement of emission intensity upon aggregation, has demonstrated potential in materials and biomaterials science, and in particular in those branches for which spectral management in the solid state is of fundamental importance. Its development in the area of luminescent spectral conversion devices like luminescent solar concentrators (LSCs) is instead still in its infancy. This account aims at summarizing relevant contributions made in this field so far, with a special emphasis on the design of molecular and macromolecular architectures capable of extending their spectral breadth to the deep-red (DR) and the near-infrared (NIR) wavelengths. Because of the many prospective advantages characterizing these spectral regions in terms of photon flux density and human-eye perception, it is anticipated that further development in the design, synthesis and engineering of advanced molecular and macromolecular DR/NIR-active AIE luminophores will enable faster and easier integration of LSCs into the built environment as highly transparent, active elements for unobtrusive light-to-electricity conversion.

Yellow to blue switching of fluorescence by the tuning of the pentaphenylphosphole structure: phosphorus electronic state vs. ring conjugation

The interaction between diphenylacetylene and dichlorophenylphosphine under various conditions is a simple method for the preparation of pentaphenylphosphole derivatives exhibiting fluorescence properties. Depending on the electronic state of the various centers of the phospholic structure, it was possible to obtain molecules with fluorescence, as in the blue area for 1,2,3,4,5-pentaphenyl-2,5-dihydro-phosphole-1-oxide (H₂PPPO), in the yellow area for 1,2,3,4,5-pentaphenylphosphole-1-oxide (PPPO) and in the cyan area for 1,2,3,4,5-pentaphenylphosphole (PPP). The effect of the structure and π-conjugation on the optical properties of these compounds was studied using PPP derivatives as examples. Unusual changes in the optical properties of PPP derivatives in solution and in the crystalline state are explained. In the case of agglomeration of PPPO and PPP molecules, the effect of aggregation-induced emission (AIE) was observed to have weak fluorescence in solution and strong fluorescence in the aggregated state. However, for H₂PPPO, the AIE effect remains mild. With the help of experimental studies, supported by theoretical calculations, the main mechanism of the optical properties of pentaphenylphosphole derivatives has been revealed. It was observed that the intramolecular motions of PPPO and PPP are more limited in the solid state than the motions of H₂PPPO, which is associated with less conjugation of the phenyl rotors of H₂PPPO. The analysis of the structure and distribution of electron density showed why hydrogenation of the phosphole ring leads to a sharp change in the optical properties of pentaphenylphosphole derivatives, while the oxidation of phosphorus does not lead to the disappearance of the AIE effect and to a lesser extent affects the change in the fluorescence wavelength. Thus, it was shown how the regulation of various structural features of the phospholic ring helps to control the optical properties of such compounds.

Efficient deep blue emission by 4-styrylbenzonitrile derivatives in solid state: Synthesis, aggregation induced emission characteristics and crystal structures

Organic fluorescent molecules with π-conjugated system have shown great importance in numerous applications including bioimaging and optoelectronics. Planar aggregation-induced emissive (AIE) organic compounds with efficient solid-state luminescence are rarely developed and urgently needed in various applications. In this work, highly planar 4-styrylbenzonitrile derivatives have been synthesized. Most of these compounds show strong AIE properties with hundred-fold fluorescent enhancement. Moreover, these molecules are deep blue emissive in solid state, exhibiting good to excellent fluorescence quantum efficiency. The single crystal analysis shows that adjacent molecules could form special J-type aggregation. The intramolecular rotations are efficiently restricted by various noncovalent interactions. These molecular arrangements could be essential for the observed strong emission in aggregated and solid state. This work has paved a new path to efficient AIE-active organic emitters with highly planar conformations from 4-styrylbenzonitrile structure.

Peraryl Arsoles: Practical Synthesis, Electronic Structures, and Solid-State Emission Behaviors

2,3,4,5-Tetraaryl-1-phenylarsoles were synthesized by utilizing safely generated diiodophenylarsine and zirconacyclopentadienes. The obtained peraryl arsoles showed aggregation-induced emission (AIE), where intense emission was observed in the solid states (quantum yields up to 0.61), whereas the corresponding solutions were very weakly emissive. The optical and electronic properties were examined by experimental and computational methods. It was elucidated that the aryl groups at the 2,5-positions affected the frontier orbitals and the aromaticity of the arsole core. On the other hand, those at the 1,3,5-positions were perpendicular to the luminophore and effective for a restriction of aggregation-caused quenching. Because the lone pair of the arsenic atom has a sufficient coordination ability due to the low aromaticity of the arsole moiety, a gold(I) chloride complex of 1,2,3,4,5-pentaphenylarsole was synthesized. The complex formation caused a blue shift of the emission from the bare ligand. Interestingly, the complex showed luminescent mechanochromism; grinding the crystals with a blue emission (λ_em =445 nm) gave amorphous samples with a greenish-blue emission (λ_em =496 nm).

Novel enantiopure monophospholes: synthesis, spatial and electronic structure, photophysical characteristics and conjugation effects

A rational and highly efficient method to access lithium 2,3,4,5-tetraphenylphospholide directly from white phosphorus, diphenylacetylene and lithium has been developed.

Organic Electron Acceptors Comprising a Dicyanomethylene-Bridged Acridophosphine Scaffold: The Impact of the Heteroatom

Stable two-electron acceptors comprising a dicyanomethylene-bridged acridophosphine scaffold were synthesized and their reversible reduction potentials were efficiently tuned through derivatization of the phosphorus center. X-ray crystallographic analysis combined with NMR, UV/Vis, IR spectroscopic, and electrochemical studies, supported by theoretical calculations, revealed the crucial role of the phosphorus atom for the unique redox, structural, and photophysical properties of these compounds. The results identify the potential of these electron deficient scaffolds for the development of functional n-type materials and redox active chromophores upon further functionalization.

Stimuli-responsive chromism in organophosphorus chemistry

The last five years have seen a huge advance in organophosphorus chemistry with respect to compounds with strong chromic responses to external stimuli. This Frontier article briefly summarizes these trends in order to especially highlight the broad versatility of phosphole-based scaffolds in chromic materials.

Unusual Aggregation-Induced Emission of a Coumarin Derivative as a Result of the Restriction of an Intramolecular Twisting Motion

Aggregation-induced emission (AIE) is commonly observed for propeller-like luminogens with aromatic rotors and stators. Herein, we report that a coumarin derivative containing a seven-membered aliphatic ring (CD-7) but no rotors showed typical AIE characteristics, whereas its analogue with a five-membered aliphatic ring (CD-5) exhibited an opposite aggregation-caused quenching (ACQ) effect. Experimental and theoretical results revealed that a large aliphatic ring in CD-7 weakens structural rigidity and promotes out-of-plane twisting of the molecular backbone to drastically accelerate nonradiative excited-state decay, thus resulting in poor emission in solution. The restriction of twisting motion in aggregates blocks the nonradiative decay channels and enables CD-7 to fluoresce strongly. The results also show that AIE is a general phenomenon and not peculiar to propeller-like molecules. The AIE and ACQ effects can be switched readily by the modulation of molecular rigidity.

First insights into the synthesis of carbo-phospholane and carbo-phospholene oxides

Fifteen-membered ring carbo-mers of five-membered rings are considered in the heterocyclic series of the phosphole oxide and less unsaturated parents. The synthesis of the first carbo-phospholane oxides is achieved by a [14+1] two-step/one-pot macrocyclization route with 86 % yield. Reduction of the latter phosphora-[5]pericyclyne with SnCl­2 allowed consistent 1H and 31P NMR characterization of the corresponding carbo-phospholene, produced with 11 % yield. The ultimate carbo-phosphole oxide could not be isolated, but preliminary results on alternative strategies towards this 14 pz-electron Hückel carbo-aromatic are reported.

Preparation of dithienylphospholes by 1,1-carboboration

In this study the scope of the 1,1-carboboration reaction was extended to the preparation of mixed heterole-based conjugated π-systems. Two arylbis(alkynyl)phosphane starting materials 2 were synthesized bearing two thiophene isomers at the alkyne units and the bulky tipp-substituent (tipp=2,4,6-triisopropylphenyl) at the phosphorous atom. The bis(thienylethynyl)phosphanes 2 were converted into the corresponding 2,5-thienyl-substituted 3-borylphospholes 4 in a double 1,1-carboboration reaction sequence employing the strongly electrophilic B(C6 F5 )3 reagent under mild reaction conditions. Subsequent Suzuki-Miyaura type cross-coupling yielded the corresponding 3-phenylphospholes 7 in a one-pot procedure from phosphanes 2 in high yields. Phospholes 7 were converted into the respective phosphole oxides 8. A photophysical characterization of derivatives 7 and 8 was carried out. The results presented here demonstrate the suitability of the 1,1-carboboration reaction for the preparation of phosphole-/thiophene-based, light-emitting systems.

Oxaphospholes and bisphospholes from phosphinophosphonates and α,β-unsaturated ketones

The reaction of a {W(CO)5 }-stabilized phosphinophosphonate 1, (CO)5 WPH(Ph)P(O)(OEt)2 , with ethynyl- (2 a-f) and diethynylketones (7-11, 18, and 19) in the presence of lithium diisopropylamide (LDA) is examined. Lithiated 1 undergoes nucleophilic attack in the Michael position of the acetylenic ketones, as long as this position is not sterically encumbered by bulky (iPr)3 Si substituents. Reaction of all other monoacetylenic ketones with lithiated 1 results in the formation of 2,5-dihydro-1,2-oxaphospholes 3 and 4. When diacetylenic ketones are employed in the reaction, two very different product types can be isolated. If at least one (Me)3 Si or (Et)3 Si acetylene terminus is present, as in 7, 8, and 19, an anionic oxaphosphole intermediate can react further with a second equivalent of ketone to give cumulene-decorated oxaphospholes 14, 15, 24, and 25. Diacetylenic ketones 10 and 11, with two aromatic acetylene substituents, react with lithitated 1 to form exclusively ethenyl-bridged bisphospholes 16 and 17. Mechanisms that rationalize the formation of all heterocycles are presented and are supported by DFT calculations. Computational studies suggest that thermodynamic, as well as kinetic, considerations dictate the observed reactivity. The calculated reaction pathways reveal a number of almost isoenergetic intermediates that follow after ring opening of the initially formed oxadiphosphetane. Bisphosphole formation through a carbene intermediate G is greatly favored in the presence of phenyl substituents, whereas the formation of cumulene-decorated oxaphospholes is more exothermic for the trimethylsilyl-containing substrates. The pathway to the latter compounds contains a 1,3-shift of the group that stems from the acetylene terminus of the ketone substrates. For silyl substituents, the 1,3-shift proceeds along a smooth potential energy surface through a transition state that is characterized by a pentacoordinated silicon center. In contrast, a high-lying transition state TS(E'-F')R=Ph of 37 kcal mol(-1) is found when the substituent is a phenyl group, thus explaining the experimental observation that aryl-terminated diethynylketones 10 and 11 exclusively form bisphospholes 16 and 17.