Cyano Analogues of 7‐Azaindole: Probing Excited‐State Charge‐Coupled Proton Transfer Reactions in Protic Solvents

The interplay between excited‐state charge and proton transfer reactions in protic solvents is investigated in a series of 7‐azaindole (7AI) derivatives: 3‐cyano‐7‐azaindole (3CNAI), 5‐cyano‐7‐azaindole (5CNAI), 3,5‐dicyano‐7‐azaindole (3,5CNAI) and dicyanoethenyl‐7‐azaindole (DiCNAI). Similar to 7AI, 3CNAI and 3,5CNAI undergo methanol catalyzed excited‐state double proton transfer (ESDPT), resulting in dual (normal and proton transfer) emission. Conversely, ESDPT is prohibited for 5CNAI and DiCNAI in methanol, as supported by a unique normal emission with high quantum efficiency. Instead, the normal emission undergoes prominent solvatochromism. Detailed relaxation dynamics and temperature dependent studies are carried out. The results conclude that significant excited‐state charge transfer (ESCT) takes place for both 5CNAI and DiCNAI. The charge‐transfer specie possesses a different dipole moment from that of the proton‐transfer tautomer species. Upon reaching the equilibrium polarization, there exists a solvent‐polarity induced barrier during the proton‐transfer tautomerization, and ESDPT is prohibited for 5CNAI and DiCNAI during the excited‐state lifespan. The result is remarkably different from 7AI, which is also unique among most excited‐state charge/proton transfer coupled systems studied to date.     

Pressure‐Induced Emission Enhancement of a Series of Dicyanovinyl‐Substituted Aromatics: Pressure Tuning of the Molecular Population with Different Conformations

A series of dicyanovinyl‐substituted aromatic compounds (Ar‐DCV; Ar=9‐anthracenyl, 1‐naphthyl, 1‐pyrenyl) with dual fluorescence are prepared, and their emission properties—when molecularly dispersed in a polymer medium—are investigated under pressure perturbation. The total emission intensity is enhanced drastically from ambient pressure up to 70 kbar. Emission 30–107 times more intense than that at ambient pressure is observed at higher pressure. In dual emission, the enhancement of the local excited state (LE state) is significantly different from that of the intramolecular charge‐transfer state (ICT state). The intensity of the ICT emission increases faster (30–370 times) than that of the LE emission (less than 20 times). In accordance with spectroscopic data, emission dynamics at different pressures, and computational studies on the molecular conformations of these compounds, a kinetic model is proposed to explain the effect of pressure on the emissive properties of the Ar‐DCV compounds from the point of view of pressure‐dependent populations of the species in the ground state.     

Piezofluorochromic and Aggregation‐Induced‐Emission Compounds Containing Triphenylethylene and Tetraphenylethylene Moieties

New fluorescent compounds containing triphenylethylene and tetraphenylethylene moieties were synthesized, and their piezofluorochromic and aggregation‐induced emission behaviors were investigated. The results show that all compounds exhibit aggregation‐induced emission characteristics and only the crystalline compound possesses piezofluorochromic properties. The color, emission spectra, and morphological structures of the one piezofluorochromic compound exhibit reversibility upon grinding and annealing (or fuming) treatments. The piezofluorochromic behaviors are caused by a change between different modes of solid state molecular packing under external pressure. The single crystal X‐ray diffraction analysis reveals that the twisted conformation of the aggregation‐induced emission compound leads to the formation of metastable crystal lattice with cavity which is readily destroyed under external pressure. A possible mechanism of piezofluorochromic phenomenon has been proposed.     

Tuning the Excited—state Properties of Cyclometalated Platinum（Ⅱ）Complexes of 6—Pheny1—2,2’bipyridine by Ancillary Acetylide Ligand

A series of luminescent cyclometalated platinum(Ⅱ）complexes,(C^N^N)Pt(C≡CR)[HC^N^N=4-(4-tolyl)-6-phenyl-2,2’-bipyridine;R=4-chlorophenyl(1),phenyl(2) and 4-tolyl(3)],were synthesized,and their spectroscopic properties have been examined.These complexes are brightly emissive both in fluid solution and in the solid state,attributed to triplet metal-to-ligand charge transfer(^3MLCT)state.The excited state energy can be tuned by ancillary acetylide ligands.The emission lifetimes in dichloromethand solution at room temperature were up to 1.64 μs and the emission quantum yields were in the range of 0.03-0.15.     

Correlation of architecture with excimer emission in 100% pyrene‐labeled self‐assembled polymers

Pyrene was incorporated as pendant unit to side‐chain urethane methacrylate polymers having a short ethyleneoxy or a long polyethyleneoxy spacer segment. The short‐spacer pyrene urethane methacrylate was also incorporated either as block or random copolymer (1:9) along with polystyrene. The excimer emission was observed to be different for different polymers with the random copolymer exhibiting the lowest efficiency. But, the total quantum yield was highest (? = 0.58) for random copolymer due to the high emission coefficient of monomer compared to that of excimer. The polymer dynamics were compared by steady state emission and fluorescence decay in THF or THF/water (9:1) solvent mixture and films. The solid state decay profile showed decay without a rise time indicating presence of ground state aggregates. In THF/water (9:1), the decay profile at the excimer emission (500 nm) showed a rise time indicating dynamic excimers. The evolution of excimeric emission centred ～430 or ～480 nm as a function of temperature was also studied in THF/water (9:1). The I_E/I_M ratio for the λ_{343 nm} excitation exhibited steady increase with temperature with the block copolymer PS‐b‐PIHP exhibiting the highest ratio and highest rate of increase; whereas, the random copolymer PS‐r‐PIHP had the lowest I_E/I_M ratios. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Sub‐cycle Dynamics of High Harmonic Generation of Ne Atoms Excited by Attosecond Pulses and Driven by Near‐infrared Laser Fields: A Self‐Interaction‐Free TDDFT Approach

In the framework of the self‐interaction‐free time‐dependent density‐functional theory (TDDFT), we have performed three‐dimensional ab initio calculations of Ne atoms in near‐infrared (NIR) laser fields subject to excitation by a single extreme ultraviolet (XUV) attosecond pulse (SAP). The TDDFT equations are solved accurately and efficiently by means of the time‐dependent generalized pseudo spectral (TDGPS) method. We have explored the transient dynamical behavior of the sub‐cycle high harmonic generation (HHG) for transitions from the excited states to the ground state and found oscillation structures with respect to the time delay between the SAP and NIR fields. We investigate the harmonic emission spectrum from singly excited state 2p3s, 2p4s, 2p3d, 2p5s, 2p4d and 2p6s, 2p5d and the virtual states 2p3p‐, 2p4f‐ and 2p4p+ as the function of time delay. We explore the sub‐cycle Stark shift phenomenon in NIR fields and its influence on the photon emission process. Our analysis reveals several novel features of the sub‐cycle transient HHG dynamics and spectra, the quantum interference pattern between different multiphoton excitation pathways, and we identify the mechanisms responsible for the observed peak splitting in the photon emission spectra.     

N‐Boc‐Indolylbenzothiadiazole Derivatives: Efficient Full‐Color Solid‐State Fluorescence and Self‐Recovering Mechanochromic Luminescence

Herein, the solid‐state emission with good fluorescence quantum yields of N‐Boc‐indolylbenzothiadiazoles as a new class of fluorophores is described. Their solid‐state emission covers the wide range of the visible spectrum and the emission color can be tuned easily by changing the substituents on the two heteroaromatic rings. Among these, 3‐methylindolyl derivatives exhibit moreover autonomously self‐recovering mechanochromic luminescence, whereby the original solid‐state emission could be recovered spontaneously at room temperature after exposure to a mechanical stimulus. The emission color, as well as the recovery time for the color change could be tuned via the introduction of different substituents on the benzothiadiazole ring. We propose that the mechanism of the autonomously self‐recovering mechanochromic luminescence of 3‐methylindolylbenzothiadiazoles is based on a partial amorphization of the crystals upon exposure to the mechanical stimulus, followed by autonomous recovering in the form of recrystallization.     

Stacking‐Induced Diamagnetic/Paramagnetic Conversion of Imidazo[1,2‐a]pyridin‐2(3 H)‐one Derivatives: Near‐Infrared Absorption and Magnetic Properties in the Solid State

Herein, we present three imidazo[1,2‐a]pyridin‐2(3 H)‐one derivatives that are diamagnetic in solution, but paramagnetic in the solid state, possibly owing to a stacking‐induced formation of phenoxide‐type radicals. Notably, a larger bathochromic shift of the absorption (even up to the near‐ infrared region) of these three compounds was observed in the solid state than in solution, which was attributable to the ordered columnar stacking arrangements or their single‐electron character as radicals in the solid state. Interestingly, compared to that in solution, (E)‐3‐(pyridin‐4′‐ylmethylene)imidazo[1,2‐a]pyridine 2(3 H)‐one displayed a largely red‐shifted emission (centered at 660 nm, with tailing above 800 nm) in the solid state. A larger bathochromic shift (260 nm) of the emission is an indication of better order and tight stacking in the solid state, which is brought about by the rigid and polar acceptor. These three compounds also reveal different magnetic susceptibilities at 300 K, thus implying that they possess various columnar stacking structures. Most interestingly, these three radicals exhibit unusual ferromagnetic‐to‐antiferromagnetic phase transitions, which can be attributed to anisotropic contraction and non‐uniform slippage of the columnar stacking chains.     

Electron‐Deficient Tetrabenzo‐Fused Pyracylene and Conversions into Curved and Planar π‐Systems Having Distinct Emission Behaviors

Polycyclic aromatic compounds containing fully unsaturated five‐membered ring(s) have been intensively studied because of their unique properties, which include high electron affinity and reactivity. Reported herein is an efficient route for the synthesis of tetrabenzo‐fused pyracylene, which comprises pyracylene and tetracene segments, using intramolecular oxidative C? H coupling. It was shown to possess high electron affinity and was found to undergo addition reactions with n‐butyllithium or benzyne. These reactions led to either a 1,4‐addition compound or triptycene‐type adduct with a curved or planar π‐system, respectively. Although these compounds exhibited similar sky‐blue emissions in a dilute solution, the emission band of the 1,4‐addition compound was significantly red‐shifted in the solid state and exhibited intense yellow emission attributable to the excimer, while the triptycene‐type adduct retained the intense blue color emission in the solid state.     

π‐Stacking Modulates the Luminescence of [(dppz)Ni(Mes)Br] (dppz = dipyrido[3,2‐a:2′,3′‐c]phenazine,Mes = 2,4,6‐trimethylphenyl)

The red colour of the novel organonickel complex [(dppz)Ni(Mes)Br] (dppz = dipyrido[3,2‐a:2′,3′‐c]phenazine, Mes = 2,4,6‐trimethylphenyl) originates from long‐wavelength MLCT/L′LCT charge transfer bands. However, luminescence in dilute solution comes presumably from the ³π‐π* (phenazine) excited state. The red‐shifted emission exhibited in concentrated solutions is assigned to dimers. In the solid state emission is quenched. The crystal structure reveals two different types of π‐π stacking along the crystallographic a axis.     

3,14‐Bis(p‐nitrophenyl)‐17,17‐dipentyltetrabenzo[a,c,g,i]‐fluorene: A New Fluorophore Displaying Both Remarkable Solvatochromism and Crystalline‐Induced Emission

A series of 17,17‐dialkyl‐3,14‐diaryltetrabenzofluorenes were efficiently prepared by using Suzuki–Miyaura cross‐coupling reactions of the corresponding 3,14‐dibromo derivatives. Studies of the unique fluorescence properties of these compounds showed that they display intense blue to yellow fluorescence with high quantum yields in the solution state and blue to orange fluorescence with moderate quantum yields in the solid state. In addition, the fluorescence wavelength of the bis(p‐nitrophenyl) derivative is remarkably solvent‐dependent in a manner that correlates with the solvent polarity parameter E_T(30). The results of density function theory calculations suggest that the intramolecular charge‐transfer character of the HOMO–LUMO transition is responsible for the large solvent effect. Moreover, addition of water to a tetrahydrofuran (THF) solution of this compound leads to quenching of the yellow fluorescence owing to an increase in the solvent polarity. However, when the amount of water fraction exceeds 70 %, a new fluorescence band appears at the same orange‐red emission wavelength as that of the solid‐state fluorescence. This observation suggests the occurrence of a crystallization‐induced emission (CIE) phenomenon in highly aqueous THF.     

Spiroconjugated Intramolecular Charge‐Transfer Emission in Non‐Typical Spiroconjugated Molecules: The Effect of Molecular Structure upon the Excited‐State Configuration

A set of terfluorenes and terfluorene‐like molecules with different pendant substitutions or side groups were designed and synthesized, their photophysical properties and the excited‐state geometries were studied. Dual fluorescence emissions were observed in compounds with rigid pendant groups bearing electron‐donating N atoms. According to our earlier studies, in this set of terfluorenes, the blue emission is from the local π–π* transition, while the long‐wavelength emission is attributed to a spiroconjugation‐like through‐space charge‐transfer process. Herein, we probe further into how the molecular structures (referring to the side groups, the type of linkage between central fluorene and the 2,2′‐azanediyldiethanol units, and—most importantly—the amount of pendant groups), as well as the excited‐state geometries, affect the charge‐transfer process of these terfluorenes or terfluorene‐like compounds. 9‐(9,9,9′′,9′′‐tetrahexyl‐9H,9′H,9′′H‐[2,2′:7′,2′′‐terfluoren]‐9′‐yl)‐1,2,3,5,6,7‐hexahydropyrido[3,2,1‐ij]quinolone (TFPJH), with only one julolidine pendant group, was particularly synthesized, which exhibits complete “perpendicular” conformation between julolidine and the central fluorene unit in the excited state, thus typical spiroconjugation could be achieved. Notably, its photophysical behaviors resemble those of TFPJ with two pendant julolidines. This study proves that spiroconjugation does happen in these terfluorene derivatives, although their structures are not in line with the typical orthogonal π fragments. The spiroconjugation charge‐transfer emission closely relates to the electron‐donating N atoms on the pendant groups, and to the rigid connection between the central fluorene and the N atoms, whereas the amount of pendant groups and the nature of the side chromophores have little effect. These findings may shed light on the understanding of the through‐space charge‐transfer properties and the emission color tuning of fluorene derivatives.     

Synthesis and Spectral Properties of 4‐(2,5‐Dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO)

An interesting flourophore, 4‐(2,5‐dimethoxyphenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) was synthesized by mixing an equivalent molar quantity of hippuric acid and 2,5‐dimethoxybenzaldehyde in acetic anhydride in the presence of anhydrous sodium acetate. The absorption and fluorescence characteristics of 4‐(2,5‐dimethoxy‐phenylmethelene)‐2‐phenyl‐5‐oxazolone (DMPO) were investigated in different solvents. DMPO dye exhibits red shift in both absorption and emission spectra as solvent polarity increases, indicating change in the dipole moment of molecules upon excitation due to an intramolecular charge transfer interaction. The fluorescence quantum yield depends strongly on the properties of the solvents, which was attributed to positive and negative solvatokinetic effects. A crystalline solid of DMPO gave strong excimer like emission at 630 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B‐type class of Steven's Classification. DMPO displayed fluorescence quenching by triethylamine via nonemissive exciplex formation.     

Self‐Assembly and Aggregate‐Induced Enhanced Emission of Amphiphilic Fluorescence Dyes in Water and in the Solid State

1‐Cyano‐1,2‐bis(biphenyl)ethene (CNBE) derivatives with a hexa(ethylene glycol) group as an amphiphilic side chain were synthesized and the self‐assembling character and fluorescence behavior were investigated. The amphiphilic derivatives showed aggregate‐induced enhanced emission (AIEE) in water and in the solid state. The fluorescence quantum yield increased as the rigidity of the aggregates increased (i.e., in ethyl acetate<in water<in the solid state). As determined from measurements of fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes, a key factor for the enhanced emission is suppression of the nonradiative decay process arising from restricted molecular motion. Additionally, the difference in the emission rate constant is not negligible and can be used to interpret the difference in fluorescence quantum yield in water and in the solid state.     

Phenothiazine‐S,S‐dioxide‐ and fluorene‐based light‐emitting polymers: Introduction of e−‐deficient S,S‐dioxide into e−‐rich phenothiazine

A novel series of poly(10‐hexyl‐phenothiazine‐S,S‐dioxide‐3,7‐diyl) and poly(9,9′‐dioctyl‐fluorene‐2,7‐diyl‐alt‐10‐hexyl‐3,7‐phenothiazine‐S,S‐dioxide) (PFPTZ‐SS) compounds were synthesized through Ni(0)‐mediated Yamamoto polymerization and Pd(II)‐catalyzed Suzuki polymerization. The synthesized polymers were characterized by ¹H NMR spectroscopy and elemental analysis and showed higher glass transition temperatures than that of pristine polyfluorene. In terms of photoluminescence (PL), the PFPTZ‐SS compounds were highly fluorescent with bright blue emissions in the solid state. Light‐emitting devices were fabricated with these polymers in an indium tin oxide/poly(3,4‐ethylene dioxythiophene):poly(styrene sulfonate)/polymer/Ca/Al configuration. The electroluminescence (EL) of the copolymers differed from the PL characteristics: the EL device exhibited a redshifted greenish‐blue emission in contrast to the blue emission observed in the PL. Additionally, this unique phenothiazine‐S,S‐dioxide property, triggered by the introduction of an electron‐deficient SO₂ unit into the electron‐rich phenothiazine, gave rise to improvements in the brightness, maximum luminescence intensity, and quantum efficiency of the EL devices fabricated with PFPTZ‐SS. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1236–1246, 2007     

One‐pot synthesis of carbon dots@ZrO2 nanoparticles with tunable solid‐state fluorescence

In recent years, fluorescent carbon dots (CDs) have been developed and showed potential applications in biomedical imaging and light‐emitting diodes (LEDs) for their excellent fluorescent properties. However, it still remains a challenge to incorporate fluorescent CDs into the host matrix in situ to overcome their serious self‐quenching. Herein, a one‐pot hydrothermal method is used to prepare nano‐zirconia with CDs (CDs@ZrO₂) nanoparticles. During the reaction, CDs and nano‐zirconia are generated simultaneously and connected with silane coupling agent. The CDs@ZrO₂ nanoparticles exhibit tunable emission wavelength from 450 to 535 nm emission by regulating the content of citric acid in the feed. The quantum yield of the CDs@ZrO₂ is up to 23.8%. Furthermore, the CDs@ZrO₂ nanoparticles with regulable fluorescence emission can be used for the fluorescent material to prepare white LEDs. The prepared LED has significant white light emission with color coordinates of (0.30, 0.37) and its color rendering index (CRI) is 67.1. In summary, we have developed the solid‐state CDs@ZrO₂ nanoparticles with tunable emission by a valuable strategy, that is, one‐pot method, for white LEDs.     

Solvent‐Dependent Truxene‐Based Nanostructures

Three truxene derivatives functionalized with alkyl chains, either attached directly or distanced by linking phenyl or ethynyl groups, self‐assemble in solution and induce the gelation of different solvents in spite of not being endowed with groups able to establish strong directional interactions. A ¹H NMR study points to face‐to‐face alternating π‐stacked motifs at the origin of nucleation. Solvents play an important role in modulating the aggregation of these derivatives giving rise to fibrous or spherical superstructures. Analysis of the influence of different solvents on the morphology of the aggregates provides a better understanding of the various stages of the hierarchical self‐assembly. The way in which alkyl chains are attached to the central core also strongly affects the self‐assembling properties and gelation ability of this series. Phenyl spacers present the highest association constants in solution and give rise to gelation in a broader range of solvents. This behavior has been rationalized by means of ¹H NMR spectroscopy, X‐ray powder diffraction, SEM, and photophysical measurements. Interestingly, it was found that these compounds in the gel state exhibit unusual emission properties most likely arising from the formation of excimers, which evidences that π–π interactions also occur in the excited state.     

A New Series of C‐6 Unsubstituted Tetrahydropyrimidines: Convenient One‐Pot Chemoselective Synthesis,Aggregation‐Induced and Size‐Independent Emission Characteristics

A new series of C‐6 unsubstituted tetrahydropyrimidines 6 have been directly synthesized via a convenient urea‐catalyzed chemoselective five‐component reaction (5CR) under mild conditions. Compounds 6 show typical aggregation‐induced emission enhancement (AIEE) characteristics because they are practically no emissive in solution but emit blue or green fluorescence in aggregates with fluorescence yield up to 93 %. One of the 5CR products, 6 aa , exhibits blue‐ and green‐fluorescence aggregates (bf‐ and gf‐aggregates). The bf‐ and gf‐aggregates are prepared under different conditions and proved to result from different J‐aggregations by single‐crystal X‐ray analysis. In addition, the bf‐ and gf‐aggregates of 6 aa show unusual size‐independent emission (SIE) characteristics because their maximum emission wavelengths in different sizes (suspension particles, film, powder and crystals) are the same, 434 and 484 nm, respectively. Based on the obtained experimental results, the 5CR mechanism, the origins of AIEE and SIE characteristics are discussed.     

8‐Hydroxy Quinoline Derivatives as Auxiliary Ligands for Red‐Emitting Cyclic‐Platinum Phosphorescent Complexes: Synthesis and Properties

Heavy metal complexes exhibit high phosphorescent efficiency and have been used extensively for electrophosphorescent emitters in the past 16 years. In 2006, we initially reported the use of the popular ligand, 8‐hydroxyquinoline (Q) to coordinate with the heavy metal ions and obtained the red‐infrared phosphorescent emission. In this paper, 8‐hydroxyquinoline has been modified at the 5‐position by electron‐donating and attracting groups and platinum complexes based on 2‐phenylpyridine and 8‐hydroxyquinoline derivatives were synthesized. The electron‐withdrawing group CF ₃ and NO ₂ lowers the HOMO level of the Q ligand and results in a N^O centered enhanced red‐infrared phosphorescence emission. The complex with CF ₃ modification exhibits the highest phosphorescence quantum yield in solid state with a life time of 1.17 μs.     

Structures and properties of liquid‐crystalline polymers based on laterally attached oligo p‐phenylenes

The structures and properties of liquid‐crystalline polymers containing laterally attached p‐terphenyl and p‐pentaphenyl have been studied. In contrast to their mesogenic groups, that is, p‐terphenyl and p‐pentaphenyl, the polymers have much lower crystallinity and also lower nematic‐to‐isotropic transition temperatures. The significant depression in crystallinity can be attributed to flexible chain segments laterally attached to the oligo p‐phenylene rods, which prevent close packing of the rods and thus disrupt the crystallization. The destabilization of the liquid‐crystalline phase is due to the diluting effect of the flexible polymer backbones; that is, the concentration of the mesogenic groups is reduced. The polymer containing p‐pentaphenyl can still exhibit good solubility in common solvents and emit light at about 402 nm in the solvent tetrahydrofuran. In the solid state, the emission redshifts to 418 nm, which is fairly close to the blue‐light emission. An interdigitated packing structure of mesogenic groups has been proposed to represent the structure of the polymer in the oriented state. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3394–3402, 2005