Restriction Of Intramolecular Rotation Research Articles

Theoretical study of radiative and non-radiative decay processes in pyrazine derivatives

Aggregation-induced emission (AIE) phenomenon has attracted much attention in recent years due to its potential applications in optoelectronic devices, fluorescence sensors, and biological probes. Restriction of intramolecular rotation has been proposed as the cause of this unusual phenomenon. Rational design of AIE luminogens requires quantitative descriptions of its mechanism. 2,3-dicyano-5,6-diphenylpyrazine (DCDPP) with "free" phenyl rings is an AIE active compound, whereas 2,3-dicyanopyrazino [5,6-9,10] phenanthrene (DCPP) with "locked" phenyl rings is not. Quantum chemistry calculations coupled with our thermal vibration correlation function formalism for the radiative and non-radiative decay rates reveal that the radiative decay rates for both DCPP and DCDPP are close to each other for all the temperatures, but the non-radiative decay processes are very different. For DCDPP, the low-frequency modes originated from the phenyl ring twisting motions are strongly coupled with the electronic excited state, which dissipate the electronic excitation energy through mode-mixing (Duschinsky rotation effect), and the non-radiative decay rate strongly increases with temperature. For DCPP, however, such mode-mixing effect is weak and the non-radiative decay rate is insensitive to temperature. These findings rationalize the fact that DCDPP is AIE active but DCPP is not, and are instructive to further development of AIE luminogens.

Synthesis of novel diketopyrrolopyrrole-based luminophores showing crystallization-induced emission enhancement properties

A series of diketopyrrolopyrrole-based luminophores were synthesized and characterized. Their photoluminescent properties were affected significantly by the change of π-conjugated units and alkyl groups linked to the diketopyrrolopyrrole-ring. 2,5-Dialkyl-3,6-bis(4-((10-oxoanthracen-9( 10H)-ylidene)methyl)phenyl)pyrrolo[3,4-c]pyrrole-1,4-dione showed aggregate-induced emission enhancement and crystallization-induced emission enhancement properties while 3,6-bis(4-[1,3]dioxolan-2-yl-phenyl)-2,5-dialkylpyrrolo[3,4-c]pyrrole-1,4-dione, 2,5-dialkyl-3,6-bis(4-formylphenyl)pyrrolo[3,4-c]pyrrole-1,4-dione, and 2,5-dioctyl-3,6-bis{4-[2-(4-bromophenyl)-2-cyanovinyl]-phenyl}pyrrolo[3,4-c]pyrrole-1,4-dione did not exhibit these properties. The results showed that emission enhancements of the luminophores depended on the competition between the restriction of intramolecular rotation and π–π aggregation with the former causing emission enhancement and the latter inducing fluorescence quenching.

Construction of efficient solid emitters with conventional and AIE luminogens for blue organic light-emitting diodes

9,9-Bis(9-heptyl-3-carbazolyl)fluorenes (BPyBCF, BAnBCF, BTPABCF, and BTPEBCF, where B = Bis, Py = pyrene, C = carbazole, F = fluorene, An = anthracene, TPA = triphenylamine, and TPE = tetraphenylethene) with different chromophoric units at the 2,7-positions are synthesized in moderate to high yields (52–89%) by Suzuki coupling reactions of 9,9-bis(9-heptyl-3-carbazolyl)-2,7-dibromofluorene with the corresponding arylboronic acid and utilized as active layers for the construction of blue organic light-emitting diodes (OLEDs). BPyBCF, BAnBCF and BTPABCF emit intense blue light with high fluorescence quantum yields (ΦF = 75–94%) in solution. However, they exhibit much lower ΦF values (30–61%) in the film state, revealing that aggregate formation has quenched their light emission. On the contrary, BTPEBCF is weakly emissive in solution (ΦF = 0.3%) but becomes a strong emitter (ΦF = 100%) when fabricated into solid film, demonstrating a phenomenon of aggregation-induced emission (AIE). Restriction of intramolecular rotation and suppression of intermolecular interactions due to the propeller-like tetraphenylethene unit are responsible for the AIE phenomenon. All the luminogens are thermally and morphologically stable, showing high glass-transition (Tg = 109–147 °C) and thermal-degradation temperatures (Td = 396–478 °C). Non-doped OLEDs using BPyBCF, BAnBCF, and BTPABCF as light-emitting layers are constructed, which give blue electroluminescence with maximum current (ηC,max) and external quantum (ηext,max) efficiencies of 4.8 cd A−1 and 2.3%. With the same device configuration, BTPEBCF shows higher ηC,max and ηext,max values of 7.9 cd A−1 and 2.9%, respectively, thanks to its AIE feature.

Creation of cationic iridium(iii) complexes with aggregation-induced phosphorescent emission (AIPE) properties by increasing rotation groups on carbazole peripheries

Three cationic iridium complexes containing 4,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-1,10-phenanthroline (L(1)) and 4,7-bis(3',6'-di-tert-butyl-6-(3,6-di-tert-butyl-9H-carbazol-9-yl)-3,9'-bi(9H-carbazol)-9-yl)-1,10-phenanthroline (L(2)) as the ancillary ligands, namely, [Ir(ppy)(2)(L(1))]PF(6) (1), [Ir(ppy)(2)(L(2))]PF(6) (2) and [Ir(oxd)(2)(L(2))]PF(6) (3) (ppy is 2-phenylpyridine, oxd is 2,5-diphenyl-1,3,4-oxadiazole), have been designed and prepared. With more intramolecular rotational units on the ancillary ligand (L(2)), 2 and 3 possess a unique aggregation-induced phosphorescent emission (AIPE) property. This phenomenon was unprecedentedly observed in the cationic iridium(III) complexes. In order to investigate the underlying mechanism of this AIPE behavior, their photophysical, temperature-dependent aggregation properties as well as theoretical calculations, were performed. The results suggest that restricted intramolecular rotation is responsible for the AIPE of cationic complexes. Moreover, photoluminescent quantum yields in the neat film, thermal stabilities and off/on luminescence switching of 2 were investigated, revealing its potential application as a candidate for LECs and organic vapor sensing.

Highly fluorescent organic nanoparticles of thiacyanine dye: A synergetic effect of intermolecular H-aggregation and restricted intramolecular rotation

Highly fluorescent organic nanoparticles are synthesized via ion-association between 3,3′-diethylthiacyanine (TC) cation and tetrakis(4-fluorophenyl)borate anion in the presence of a neutral stabilizing polymer in water. The fluorescence shows a significant increase with a decrease in the nanoparticle size, resulting in a high fluorescence quantum yield of about 0.8–0.9. The observed fluorescence enhancement is due to a synergetic effect of both restriction of intramolecular rotation of TC and intermolecular H-aggregation in an inclined manner.

Efficient crystallization induced emissive materials based on a simple push–pull molecular structure

Solid state luminescent materials are the subject of ever growing interest both from a scientific and a technological point of view. Aggregation caused quenching (ACQ) processes however represent an obstacle to the development of most luminogens in the condensed phase. This is why particularly fascinating are those materials showing higher emission intensity in the solid state than in solution. Here we report on three 4-dialkylamino-2-benzylidene malonic acid dialkyl esters, very simple push-pull molecules, which are hardly emissive in solution and in the amorphous phase but become good emitters in the crystalline phase according to what has been indicated as crystallization induced emission (CIE). Thanks to combined emission and NMR spectroscopies at different temperatures on the prototype compound 4-dimethylamino-2-benzylidene malonic acid dimethyl ester in solution, we give full evidence that a restricted intramolecular rotation (RIR) phenomenon, in particular the hindered rotation around the aryl main axis of the compound, is at the origin of this behaviour. In addition, solid state photophysical and X-ray diffraction structural characterization allow us to identify J-dimeric interactions as responsible for the particularly intense emission of two of the three compounds. Moreover, by exploiting the compounds' acidochromic properties, applications in sensors and optoelectronics are envisaged.

Aggregation-Induced Emission Enhancement of Aryl-Substituted Pyrrole Derivatives

The relationship between the structures and light emission properties of five aryl-substituted pyrrole derivatives was studied during aggregation in THF-water mixtures. Only pentaphenylpyrrole clearly shows, however, an aggregation-induced emission enhancement (AIEE) phenomenon. On comparison of the optical properties and single-crystal structures of these pyrrole derivatives, it is suggested that the more twisted configuration which prevented parallel orientation of conjugated chromophores combined with the restricted intramolecular rotation (RIR) effect was the main cause of the AIEE phenomenon.

Aggregation-Induced Emission of Tetraarylethene Luminogens

It has been a textbook knowledge that chromophore aggregation generally quenches light emission. While in 2001, we discovered a novel phenomenon of aggregation-induced emission (AIE): a series of propeller-shaped molecules were found nonluminescent in the solution state but emissive in the aggregate and solid states. Attracted by the intriguing phenomenon and its fascinating perspectives, we launched a research program on developing new AIE luminogens and exploring their high-tech applications. In this review, we present our recent works on developing a novel AIE system based on tetraarylethenes. We show that restriction of intramolecular rotation is the main cause for the AIE effect. Utilizing such treasured AIE characteristics, the tetraarylethenes can find an array of applications, including fluorescence sensors, biological probes, and active materials in the fabrication of organic light-emitting diodes. Keywords: Chromophore, aggregation, tetraarylethene, light emission, light-emitting diode, excimer, sensor, biological probe

Aggregation-Induced Emission in Tetraphenylthiophene-Derived Organic Molecules and Vinyl Polymer

In this study, organic molecules of tetraphenylthiophene (TP) and the derived model compound of TP-Qu and vinyl polymer of PS-Qu with the pendant group of TP-Qu were prepared and characterized to identify their photoluminescent (PL) responses toward the effect of aggregation-induced emission (AIE). During aggregate formation by adding the nonsolvent water to the THF solvent, the corresponding TP solutions greatly gained the emission intensity. In contrast, TP-Qu and PS-Qu in THF/water solution mixtures emitted strongly with nearly the same emission intensity despite the composition differences on the applied THF/water mixtures. Restricted intramolecular rotation (RIR) is the key factor deciding the AIE effect in different media. With four small phenyl rotors around the central thiophene stator, the RIR of the TP molecules in dilute solution is low but increases upon aggregate formations. In contrast, the bulky C-2 quinoline rotor of the TP-Qu molecule enhances the RIR effect and its PL emissions in the aggregate and in the solution states are essentially the same (irrespective of the aggregate formation by adding nonsolvent water). With the inherent TP-Qu pendant groups, the vinyl polymer PS-Qu behaves similarly to the TP-Qu molecule; that is, solutions of PS-Qu have similar emission spectra in different solution mixtures. Several methods including PL excitation (PLE), low-temperature PL emission study, and computer simulation were used in this study to estimate the relative extents of RIR and therefore, the resulting AIE effect in all the TP-derived materials.

Aggregation Emission Properties of Oligomers Based on Tetraphenylethylene

A series of eight derivatives based on tetraphenylethylene were prepared, and two of these, i.e., 1,1-bis(4-phenylcarbonyl)-2,2-diphenylethylene (2), 1,1,2,2-tetrakis(4-phenylcarbonyl)phenylethylene (4), were characterized crystallographically. Because the rigidity and steric hindrance in the molecular structure enhanced regularly from sample 5 to 8, UV-visible absorption and PL spectra of 5-8 show the transition from aggregation-induced emission (AIE) to aggregation-induced emission enhancement (AIEE) behavior. Solid fluorescence lifetime characterization shows that samples with less steric hindrance and more interaction in or between molecules will result in a short fluorescence lifetime. All samples 5-8 become more emissive when their chains are induced to aggregate by adding water into their acetonitrile solutions. Cyclic voltammetry measurements taken give the band gap of sample 5-8 as 2.88, 2.70, 2.56, and 2.43 eV, and theoretical calculations also support these bad gap results. Conformational simulations also suggest that the origin of transition from AIE to AIEE behavior is due to the restricted intramolecular rotations of the aromatic rings in samples.

Aggregation-induced Emission of Silole Molecules and Polymers: Fundamental and Applications

Aggregation generally quenches the light emissions of chromophoric molecules. In this review, we demonstrate that 1,1-disubstituted 2,3,4,5-tetraphenyl siloles and 2,5-difunctionalized siloles as well as their polymers exhibit the opposite behaviors. Instead of quenching, aggregation has greatly boosted their photoluminescence quantum yields by up to two orders of magnitude, turning them from faint fluorophores into strong emitters. Such “abnormal” phenomenon of “aggregation-induced emission (AIE)” is attributed to restricted intramolecular rotations of the peripheral phenyl rings against the central silole core, which block the nonradiative channel via the rotational energy relaxation processes and effectively populate the radiative decay of the excitons. Utilizing such a novel effect, siloles and their polymers find an array of applications as: sensors for chemicals, explosives, pH, and biomacromolecules (proteins, DNAs and RNAs), indicators for determining CMC and monitoring layer-by-layer self-assembling, biocompatible fluorogens for cell imaging, visualizing agent for DNA gel electrophoresis, biolabels for immunoassay, stimuli-responsive organic nanomaterials, magnetic fluorescent nanoparticles for potential bio-imaging and -separation, and outstanding materials for efficient OLEDs and PV cells.

Label‐Free Fluorescent Probing of G‐Quadruplex Formation and Real‐Time Monitoring of DNA Folding by a Quaternized Tetraphenylethene Salt with Aggregation‐Induced Emission Characteristics

Biosensing processes such as molecular beacons require non-trivial effort to covalently label or mark biomolecules. We report here a label-free DNA assay system with a simple dye with aggregation-induced emission (AIE) characteristics as the fluorescent bioprobe. 1,1,2,2-Tetrakis[4-(2-bromoethoxy)phenyl]ethene is nonemissive in solution but becomes highly emissive when aggregated. This AIE effect is caused by restriction of intramolecular rotation, as verified by a large increase in the emission intensity by increasing viscosity and decreasing temperature of the aqueous buffer solution of 1,1,2,2-tetrakis[4-(2-triethylammonioethoxy)phenyl]ethene tetrabromide (TTAPE). When TTAPE is bound to a guanine-rich DNA strand (G1) via electrostatic attraction, its intramolecular rotation is restricted and its emission is turned on. When a competitive cation is added to the G1 solution, TTAPE is detached and its emission is turned off. TTAPE works as a sensitive poststaining agent for poly(acrylamide) gel electrophoresis (PAGE) visualization of G1. The dye is highly affinitive to a secondary structure of G1 called the G-quadruplex. The bathochromic shift involved in the G1 folding process allows spectral discrimination of the G-quadruplex from other DNA structures. The strong affinity of TTAPE dye to the G-quadruplex structure is associated with a geometric fit aided by the electrostatic attraction. The distinct AIE feature of TTAPE enables real-time monitoring of folding process of G1 in the absence of any pre-attached fluorogenic labels on the DNA strand. TTAPE can be used as a K+ ion biosensor because of its specificity to K+-induced and -stabilized quadruplex structure.

Fluorescence enhancements of benzene-cored luminophors by restricted intramolecular rotations: AIE and AIEE effects

Photoluminescence of simple arylbenzenes with ready synthetic accessibility is enhanced by two orders of magnitude through aggregate formation; viscosity and temperature effects indicate that the emission enhancement is due to the restriction of their intramolecular rotations in the solid state.

Enhanced Photoluminescence from Group 14 Metalloles in Aggregated and Solid Solutions

The unusual photoluminescence characteristics of a series of six group 14 metalloles (1,1-dimethyl-2,3,4,5-tetraphenylmetalloles and 1,1-diphenyl-2,3,4,5-tetraphenylmetalloles) containing silicon, germanium, or tin have been investigated. Although the compounds are weakly luminescent in dilute fluid solution at room temperature, they undergo a substantial enhancement of photoluminescence when forced to aggregate, as in mixed solvent systems. The compounds also exhibit considerable emission when incorporated into rigid room-temperature glasses of sucrose octaacetate. Absorption and emission characteristics of the compounds, including luminescence quantum yields, in fluid solution, solution-phase aggregates, and room-temperature glasses are reported. Quantum yields increase by as much as 2 orders of magnitude in the aggregates and glasses, compared to fluid solution. Experimental evidence supports the conclusion that the aggregation-induced enhancement of luminescence results from restricted intramolecular rotations in the packed metalloles. The unusual aggregation-induced enhancement of these compounds makes them potentially useful for the fabrication of a variety of electrooptical devices and sensors. In addition, the X-ray crystal structure of hexaphenylgermole is reported.

Aggregation-Induced Emission of cis,cis-1,2,3,4-Tetraphenylbutadiene from Restricted Intramolecular Rotation

cis,cis-1,2,3,4-Tetraphenylbutadiene (TPBD) exhibits aggregation-induced emission (AIE) in the UV-blue band: the photoluminescence (PL) quantum yield of TPBD aggregates can differ from that of molecularly dissolved species by 2 orders of magnitude (>200). When the isolated molecules in solutions are cooled to extremely low temperature, they also emit intense light comparable to that in the solid state. TPBD thin layer shows on−off fluorescence switching behavior that can be utilized for the sensing of organic vapors. The phenyl substituents in TPBD are twisted in the solid state, and excimer formation is greatly prohibited. The cooling-enhanced emission of the TPBD solution and the fluorescence switching behavior suggest that the aggregation-induced emission is caused by restricted intramolecular rotation of the phenyl groups. The intramolecular phenyl rotations of TPBD can be regarded as rotational relaxations around their equilibrium positions, from which mean relaxation time is defined based on an Arrhenius equation. All the PL behaviors of TPBD can be well explained qualitatively by the magnitude of the relaxation time.

Synthesis, Thermal Stability, and Light-Emitting Properties of Hyperbranched Poly(phenylenegermolene)s

A germole-containing diacetylene, 1,1-diethynyl-2,3,4,5-tetraphenylgermole (2), was synthesized. Homopolymerization of 2 and its copolycyclotrimerization with 1-octyne were effected by TaCl5–Ph4Sn in toluene, producing completely soluble polymers (1) in moderate yields. The molecular structure of 1 was characterized by spectroscopic methods. The thermal stability of 1 was evaluated by thermogravimetric analysis, which detected virtually no weight loss when the polymers were heated to ∼350°C. These hyperbranched polymers were electronically conjugated as suggested by their strong absorption in the visible spectral region (λ max∼540 nm). Photoluminescence of the polymers was dramatically enhanced by cooling their solutions to low temperatures. Restricted intramolecular rotations of the phenyl rings upon the axes of the single bonds linked to the germole cores at the cryogenic temperatures might be responsible for this unique phenomenon of cooling-enhanced emission.

Silole‐containing linear and hyperbranched polymers: synthesis, thermal stability, light emission, nano‐dimensional aggregation, and optical power limiting

AbstractLinear polyacetylenes and hyperbranched polyphenylenes carrying 1,2,3,4,5‐pentaphenylsilolyl (PS) pendants are designed and synthesized. Homo‐polymerization of HC≡CPS, HC≡C(CH2)9OPS, and C6H5C≡C(CH2)9OPS and (co)polycyclotrimerization of (HC≡C)2PS with 1‐octyne are effected by NbCl5‐, WCl6‐, MoCl5‐, and TaCl5‐based catalysts. High molecular weight linear (1‐3) and hyperbranched polymers (6) are obtained in high yields (Mw up to ∼70 × 103 and yield up to 85%). All the polymers are thermally stable, losing little weight when heated to 350°C. Whereas all the polymers emit faint light when molecularly dissolved, polymers 2, 3, and 6 become emissive when aggregated in poor solvents or when cooled to low temperatures. Restricted intramolecular rotations of the phenyl rings upon the axes of the single bonds linked to the silole cores may be responsible for the aggregation‐ or cooling‐induced emission. A multilayer electroluminescent device using 3 as an active layer emits a blue light of 496 nm with maximum brightness, current efficiency, and external quantum yield of 1118 cd/m2, 1.45 cd/A, and 0.55%, respectively. Polymers 6 are non‐linear optically active and strongly attenuate the optical power of intense laser pulses, whose optical limiting performances are superior to that of C60, a best‐known optical limiter.

Hyperbranched Poly(phenylenesilolene)s: Synthesis, Thermal Stability, Electronic Conjugation, Optical Power Limiting, and Cooling-Enhanced Light Emission

Silole-containing hyperbranched polyphenylenes (1) are synthesized, which exhibit high thermal stability, extended electronic conjugation, excellent optical power limiting performance, and novel cooling-enhanced photoluminescence. The homopolycyclotrimerization of 1,1-diethynyl-2,3,4,5-tetraphenylsilole (2) and its copolycyclotrimerizations with 1-octyne catalyzed by TaCl5−Ph4Sn proceed smoothly at room temperature and produce completely soluble polymers in high yields (up to ∼85%). The molecular structures of 1 are characterized by spectroscopic analyses. The thermal stability of 1 is evaluated by thermogravimetric analyses, which detect virtually no weight losses when the polymers are heated to ∼300 °C. The hyperbranched polyphenylenes are electronically conjugated, as suggested by their strong absorption in the visible spectral region (λmax ∼ 520 nm). Because of this extended electronic conjugation, polymers 1 are nonlinear optically active and strongly attenuate the optical power of intense laser pulses, whose optical limiting performances are superior to that of C60, the best-known optical limiter. The photoluminescence of the polymers is dramatically enhanced by cooling their solutions to low temperatures. This unique phenomenon of cooling-enhanced emission is probably caused by the restricted intramolecular rotations of the phenyl rings upon the axes of the single bonds linked to the silole cores at the cryogenic temperatures.

Synthesis, Light Emission, Nanoaggregation, and Restricted Intramolecular Rotation of 1,1-Substituted 2,3,4,5-Tetraphenylsiloles

A series of ten 2,3,4,5-tetraphenylsiloles with different 1,1-substituents [XYSi(CPh)4] were prepared, and three of these, i.e., 1,1,2,3,4,5-hexaphenylsilole [X = Y = Ph (3)], 1-ethynyl-1,2,3,4,5-pentaphenylsilole [X = Ph, Y = C⋮CH (15)], and 1,1-bis(phenylethynyl)-2,3,4,5-tetraphenylsilole [X = Y = C⋮CPh (18)], were characterized crystallographically. The ground- and excited-states of the siloles were influenced by the inductive effect of the 1,1-substituents: with an increase in their electronegativity, the absorption and emission spectra of the siloles bathochromically shifted. A simple and reliable TLC-based method was developed for measurement of the solid-state luminescence spectra of the siloles. When molecularly dissolved in common solvents at room temperature, all the siloles were practically nonemissive (“off”). When poor solvents were added, the silole molecules clustered into nanoaggregates, which turned the emission “on” and boosted the photoluminescence quantum yields by up to 2 orders of m...

Silole-Containing Polyacetylenes. Synthesis, Thermal Stability, Light Emission, Nanodimensional Aggregation, and Restricted Intramolecular Rotation

We synthesized three substituted polyacetylenes carrying 1,2,3,4,5-pentaphenylsilolyl (PS) pendants, i.e., −[HCC(PS)]n− (1), −{HCC[(CH2)9OPS]}n− (2) and −{(C6H5)CC[(CH2)9OPS]}n− (3), and succeeded in turning polymers 2 and 3 from weak luminophors into strong emitters by external stimuli of aggregation and cooling. The silolylacetylene monomers HC⋮CPS (10), HC⋮C(CH2)9OPS (11), and C6H5C⋮C(CH2)9OPS (12) were polymerized by NbCl5− and WCl6−Ph4Sn catalysts, which gave high molecular weight polymers in high yields (Mw up to ∼70 × 103 Da and yield up to ∼80%). The structures and properties of the polymers were characterized and evaluated by IR, UV, NMR, DSC, TGA, PL, EL, and nanoparticle size analyses. The polymers were thermally stable and lost little weights when heated to ∼350 °C. Whereas all the polymers were practically nonluminescent when molecularly dissolved, polymers 2 and 3 became emissive when aggregated in poor solvents or when cooled to low temperatures. Restricted intramolecular rotation or twisting of the silole chromophores in the solid nanoaggregates or at the low temperatures may be responsible for the aggregation- or cooling-induced emission. A multilayer electroluminescence device using 3 as an active layer emitted a blue light of 496 nm with maximum brightness, current efficiency, and external quantum yield of 1118 cd/m2, 1.45 cd/A, and 0.55%, respectively.