Synthesis of Di‐ and Trixanthones that Display High Stability and a Visual Fluorescence Response to Strong Acid

Three dixanthones ( 1 – 3 ) and an unprecedented C_3h‐symmetric trixanthone ( 4 ) were synthesized through a three‐step approach in overall yields above 63 %. These compounds possessed a planar π‐conjugated system and formed tight face‐to‐face columnar stacks, as confirmed by single‐crystal structural analysis. In comparison with xanthone, the fluorescence emissions of compounds 1 – 4 showed significant red‐shifts, with improved quantum yields. Moreover, the fluorescence emissions of compounds 1 – 4 could be modulated in a strongly acidic environment without decomposition, which led to a further red‐shift of the emissions, as well as enhancement of the emission intensities. These compounds have potential applications as optoelectronic materials and/or chemosensors.     

Preparation and Acid‐Responsive Photophysical Properties of T‐Shaped π‐Conjugated Molecules Containing a Benzimidazole Junction

T‐shaped π‐conjugated molecules with an N‐methyl‐benzimidazole junction have been synthesized and their acid‐responsive photophysical properties owing to the change in the π‐conjugation system are discussed. T‐shaped π‐conjugated molecules consist of two orthogonal π‐conjugated systems including a phenyl thiophene extended from the 2‐position and alkyl phenylenes connected through various π‐spacers from the 4,7‐positions of the N‐methyl‐benzimidazole junction. The π‐spacers, such as thiophene, ethyne, and ethane, have an effect on the acid response of photophysical properties in terms of changes in conformation, excited‐state energy and charge‐transfer (CT) characteristics. In particular, the π‐conjugated molecule with ethynyl spacers exhibited a marked redshift in the fluorescence spectrum with a large Stokes shift upon the addition of acid, whereas the other molecules showed substantial quenching. The redshift in emission was studied in detail by temperature‐dependent fluorescence measurements, which indicated the transition to a CT state over the finite activation energy at the excited state. The change in the frontier molecular orbitals upon acid addition was further discussed by means of DFT calculations.     

Through‐Space Conjugated Molecular Wire Comprising Three π‐Electron Systems

A [2.2]paracyclophane‐based through‐space conjugated oligomer comprising three π‐electron systems was designed and synthesized. The arrangement of three π‐conjugated systems in an appropriate order according to the energy band gap resulted in efficient unidirectional photoexcited energy transfer by the Förster mechanism. The energy transfer efficiency and rate constants were estimated to be >0.999 and >10¹² s^?1, respectively. The key point for the efficient energy transfer is the orientation of the transition dipole moments. The time‐dependent density functional theory (TD‐DFT) studies revealed the transition dipole moments of each stacked π‐electron system; each dipole moment was located on the long axis of each stacked π‐electron system. This alignment of the dipole moments is favorable for fluorescence resonance energy transfer (FRET).     

Synthesis and characterization of novel π‐conjugated polymers with phosphole ring derivatives

Novel π‐conjugated polymers ( 8 – 10 ) were prepared by the palladium‐catalyzed Sonogashira coupling reaction of three kinds of phosphole‐ring‐containing monomers with 2,5‐dihexyloxyl‐1,4‐diethynylbenzene. The obtained polymers ( 8 – 10 ) were regioregulated with the 2,5‐substituted phosphole ring in the polymer main chain and characterized with ¹H, ¹³C, and ³¹P NMR and FTIR. Polymers 8 – 10 were found to have an extended π‐conjugated system according to the results of UV–vis absorption spectra. In the fluorescence emission spectra of 8 – 10 , moderate emission peaks were observed in the visible blue‐to‐green region. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2867–2875, 2007     

From A Fluorescent Chromophore in Solution to An Efficient Emitter in the Solid State

Polycyclic aromatic hydrocarbons (PAHs) normally exhibit efficient fluorescence in dilute solutions, whereas their emission is significantly quenched in the aggregated state due to the formation of π‐aggregates/excimers. The rigid and planar structure of PAHs plays a positive role in terms of fluorescence in solution but a negative one in the aggregated state. To bestow PAHs a luminescent ability in the solid state, we constructed a non‐coplanar PAH‐substituted ethene. By using the planar PAH fluoranthene as a building block, a highly efficient solid‐state emitter with a fluorescence quantum efficiency of unity in the aggregated state was obtained. OLEDs with contain the molecule as an emitter reach a luminance up to 20 520 cd m^?2 and an efficiency of 10 cd A^?1.     

Crystal Engineering of Room Temperature Phosphorescence in Organic Solids

We report a series of highly emissive azatriangulenetrione (TANGO) solids in which the luminescent properties are controlled by engineering the molecular packing by adjusting the steric size of substituents. The co‐alignment of “phosphorogenic” carbonyl groups within the π‐stacks results in an almost pure triplet emission in HTANGO, TCTANGO, TBTANGO and TITANGO, while their rotation by ≈60° in the sterically hindered tBuTANGO leads to an almost pure singlet emission. Despite strong π‐interactions, aggregation‐induced quenching and triplet–triplet annihilation are avoided in HTANGO and TCTANGO which display efficient phosphorescence in the solid state. To our knowledge, HTANGO with the solid‐state phosphorescence quantum yield of 42 % at room temperature is the most efficient phosphor composed of the 1^st/2^nd raw elements only.     

Solvent Control over Supramolecular Gel Formation and Fluorescence for a Highly Crystalline π‐Conjugated Polymer

In π‐conjugated polymers (πCPs), crystallinity and fluorescence typically exhibit a trade‐off relationship. Here, we have synthesized a highly crystalline and fluorescent π‐conjugated polymer with a simple alternating structure of 1,2,4,5‐tetrafluorophenylene and 3,3′‐dihexyl‐2,2′‐bithiophene units. In film, the polymer exhibited efficient red‐colored fluorescence, an improved quantum yield (Φ_sol=13 %→Φ_film=23 %) and a crystalline structure. Interestingly, supramolecular gel formation occurred in appropriate solvents, and the macrostructure and fluorescence properties of the gel could be directly controlled by the choice of the solvent. The polymer self‐assembled into a spherical form that exhibited red fluorescence in non‐aromatic solvent (1,2‐dichloroethane) and into a fibrous form that exhibited yellow fluorescence in aromatic solvent (mesitylene).     

High‐Nuclear Organometallic Copper(I)–Alkynide Clusters: Thermochromic Near‐Infrared Luminescence and Solution Stability

Cu(CF₃COO)₂ reacts with tert‐butylacetylene (tBuC≡CH) in methanol in the presence of metallic copper powder to give two air‐stable clusters, [Cu^I₁₅(tBuC≡C)₁₀(CF₃COO)₅]?tBuC≡CH ( 1 ) and [Cu^I₁₆(tBuC≡C)₁₂(CF₃COO)₄(CH₃OH)₂] ( 2 ). The assembly process involves in situ comproportionation reaction between Cu²⁺ and Cu⁰ and the formation of two different clusters is controlled by reactants concentration. The clusters consist of Cu₁₅ and Cu₁₆ cores co‐stabilized by strong by σ‐ and π‐bonded tert‐butylethynide and CF₃COO^? (together with methanol molecule in 2 ). Their stabilities in solution were confirmed using electrospray ionization mass spectrometry in which the cluster core remains intact for 1 in chloroform and acetone, and for 2 in acetonitrile. Strong thermochromic luminescence in the near infrared (NIR) region was observed in the solid‐state. Of particular interest, the emission maximum of 1 is red‐shifted from 710 nm at 298 K to 793 nm at 93 K, along with a 17‐fold fluorescence enhancement. In contrast, 2 exhibits red shift from 298 to 123 K followed by blue shift from 123 to 93 K. The emission wavelength was correlated with the structural parameters using variable‐temperature X‐ray single‐crystal analyses. The rich cuprophilic interaction plays a significant role in the formation of ³LMCT (tBuC≡C→Cu_x) excited state mixed with cluster‐centered (³CC) characters, which can be considerably influenced by temperature, leading to thermochromic luminescence. The present work provides 1) a new synthetic protocol for the high‐nuclear Cu^I–alkynyl clusters; 2) a comprehensive insight into the mechanism of thermochromic luminescence; 3) unusual emissive materials with the characters of NIR and thermochromic luminescence simultaneously.     

Far‐Red/Near‐Infrared Emissive (1,3‐Dimethyl)barbituric Acid‐Based AIEgens for High‐Contrast Detection of Metastatic Tumors in the Lung

Despite of the enthusiastic research in aggregation‐induced emission luminogens (AIEgens) in recent years, the ones that can be smoothly used for sophisticated biomedical applications such as in vivo bioimaging of pulmonary metastatic tumors during surgery are still limited. Herein, we report the design and synthesis of a new series of far‐red/near‐infrared (FR/NIR) fluorescent AIEgens that consist of methoxy‐substituted tetraphenylethene (TPE) as the electron‐donating moiety, (1,3‐dimethyl)barbituric acid as the electron‐withdrawing moiety, and different π‐bridge units. As compared to benzene or 3,4‐ethylenedioxythiophene, using thiophene as the π‐conjugation unit between the donor and acceptor results in a relatively higher absolute fluorescence quantum yield (14.5 %) in water when formulating the corresponding AIEgens into nanoparticles (AIE dots) with an amphiphilic co‐polymer as the doping matrix. The highly FR/NIR‐emissive thiophene‐based AIE dots are demonstrated to be potent for intraoperative detection of pulmonary metastatic tumors, particularly the micro‐sized ones, with excellent signal‐to‐background ratio.     

Regulation of π‐Stacked Anthracene Arrangement for Fluorescence Modulation of Organic Solid from Monomer to Excited Oligomer Emission

The construction and precise control of the face‐to‐face π‐stacked arrangements of anthracene fluorophores in the crystalline state led to a remarkable red shift in the fluorescence spectrum due to unprecedented excited oligomer formation. The arrangements were regulated by using organic salts including anthracene‐1,5‐disulfonic acid (1,5‐ADS) and a variety of aliphatic amines. Because of the smaller number of hydrogen atoms at the edge positions and the steric effect of the sulfonate groups, 1,5‐ADS should prefer face‐to‐face π‐stacked arrangements over the usual edge‐to‐face herringbone arrangement. Indeed, as the alkyl substituents were lengthened, the organic salts altered their anthracene arrangement to give two‐dimensional (2D) edge‐to‐face and end‐to‐face herringbone arrangements, one‐dimensional (1D) face‐to‐face zigzag and slipped stacking arrangements, a lateral 1D face‐to‐face arrangement like part of a brick wall, and a discrete monomer arrangement. The monomer arrangement behaved as a dilute solution even in the close‐packed solid state to emit deep blue light. The 1D face‐to‐face zigzag and slipped stacking of the anthracene fluorophores caused a red shift of 30–40 nm in the fluorescence emission with respect to the discrete arrangement, probably owing to ground‐state associations. On the other hand, the 2D end‐to‐face stacking induced a larger red shift of 60 nm, which is attributed to the excimer fluorescence. Surprisingly, the brick‐like lateral face‐to‐face arrangement afforded a remarkable red shift of 150 nm to give yellow fluorescence. This anomalous red shift is probably due to excited oligomer formation in such a lateral 1D arrangement according to the long fluorescence lifetime and little shift in the excitation spectrum. The regulation of the π‐stacked arrangement of anthracene fluorophores enabled the wide modulation of the fluorescence and a detailed investigation of the relationships between the photophysical properties and the arrangements.     

Hybridization of a Flexible Cyclooctatetraene Core and Rigid Aceneimide Wings for Multiluminescent Flapping π Systems

The hybridization of flexible and rigid π‐conjugated frameworks is a potent concept for producing new functional materials. In this article, a series of multifluorescent flapping π systems that combine a flexible cyclooctatetraene (COT) core and rigid aceneimide wings with various π‐conjugation lengths has been designed and synthesized, and their structure/properties relationships have been investigated. Whereas these molecules have a V‐shaped bent conformation in the ground state, the bent structure changes to a planar conformation in the lowest excited singlet (S₁) state irrespective of the lengths of the aceneimide wings. However, the fluorescence behavior in solution is distinct between the naphthaleneimide system and the anthraceneimide system. The former has a nonemissive S₁ state owing to the significant contribution of the antiaromatic character of the planar COT frontier molecular orbitals, thereby resulting in complete fluorescence quenching in solution. In contrast, the latter anthraceneimide system shows an intense emission, which is ascribed to the planar but distorted S₁ state that shows the allowed transition between the π‐molecular orbitals delocalized over the COT core and the acene wings. The other characteristic of these π systems is the significantly redshifted fluorescence in the crystalline state relative to their monomer fluorescence. The relationship between the packing structures and the fluorescence properties was investigated by preparing a series of hybrid π systems with different sizes of substituents on the imide moieties, which revealed the effect of the twofold π‐stacked structure of the V‐shaped molecules on the large bathochromic shift in emission.     

Near‐Infrared Quadrupolar Chromophores Combining Three‐Coordinate Boron‐Based Superdonor and Superacceptor Units

Herein, two new quadrupolar acceptor‐π‐donor‐π‐acceptor (A‐π‐D‐π‐A) chromophores have been prepared featuring a strongly electron‐donating diborene core and strongly electron‐accepting dimesitylboryl (BMes₂) and bis(2,4,6‐tris(trifluoromethyl)phenyl)boryl (B^FMes₂) end groups. Analysis of the compounds by NMR spectroscopy, X‐ray crystallography, cyclic voltammetry, and UV/Vis‐NIR absorption and emission spectroscopy indicated that the compounds have extended conjugated π‐systems spanning their B₄C₈ cores. The combination of exceptionally potent π‐donor (diborene) and π‐acceptor (diarylboryl) groups, both based on trigonal boron, leads to very small HOMO–LUMO gaps, resulting in strong absorption in the near‐IR region with maxima in THF at 840 and 1092 nm and very high extinction coefficients of ca. 120 000 m ^?1 cm^?1. Both molecules also display weak near‐IR fluorescence with small Stokes shifts.     

Perylene Bisimide and Naphthyl‐Based Molecular Dyads: Hydrogen Bonds Driving Co‐planarization and Anomalous Temperature‐Response Fluorescence

The origin of the positive temperature effect in fluorescence emission of a newly designed perylene bisimide (PBI) derivative with two naphthyl units containing ortho‐methoxy group (NM) at its bay positions (PBI‐2NM) was elucidated. A key point is the finding of a weak hydrogen bond (<5.0 kcal mol^?1) between the methoxy group of the NM unit and a nearby hydrogen atom of the PBI core. It is the bonding that drives co‐planarization of the different aromatic units, resulting in delocalization of the π‐electrons of the compound as synthesized, inducing fluorescence quenching via intramolecular charge transfer (ICT). With increasing temperature, the co‐planar structure could be distorted in part, resulting in a decreased degree of ICT, and hence leading to enhanced fluorescence emission. The unique positive temperature effect in emission induced by H‐bond‐driven co‐planarization may pave a new avenue in designing functional molecular systems complementary to conventional methods.     

Luminescent Pincer Platinum(II) Complexes with Emission Quantum Yields up to Almost Unity: Photophysics,Photoreductive C?C Bond Formation,and Materials Applications

Luminescent pincer‐type Pt^II complexes supported by C‐deprotonated π‐extended tridentate R C^N^N R′ ligands and pentafluorophenylacetylide ligands show emission quantum yields up to almost unity. Femtosecond time‐resolved fluorescence measurements and time‐dependent DFT calculations together reveal the dependence of excited‐state structural distortions of [Pt(R C^N^N R′)(CC‐C₆F₅)] on the positional isomers of the tridentate ligand. Pt complexes [Pt(R‐C^N^N R′)(CC‐Ar)] are efficient photocatalysts for visible‐light‐induced reductive C C bond formation. The [Pt(R‐C^N^N R′)(CC‐C₆F₅)] complexes perform strongly as phosphorescent dopants for green‐ and red‐emitting organic light‐emitting diodes (OLEDs) with external quantum efficiency values over 22.1 %. These complexes are also applied in two‐photon cellular imaging when incorporated into mesoporous silica nanoparticles (MSNs).     

Luminescent Pincer Platinum(II) Complexes with Emission Quantum Yields up to Almost Unity: Photophysics,Photoreductive CC Bond Formation,and Materials Applications

Luminescent pincer‐type Pt^II complexes supported by C‐deprotonated π‐extended tridentate R? C^N^N? R′ ligands and pentafluorophenylacetylide ligands show emission quantum yields up to almost unity. Femtosecond time‐resolved fluorescence measurements and time‐dependent DFT calculations together reveal the dependence of excited‐state structural distortions of [Pt(R? C^N^N? R′)(C?C‐C₆F₅)] on the positional isomers of the tridentate ligand. Pt complexes [Pt(R‐C^N^N? R′)(C?C‐Ar)] are efficient photocatalysts for visible‐light‐induced reductive C? C bond formation. The [Pt(R‐C^N^N? R′)(C?C‐C₆F₅)] complexes perform strongly as phosphorescent dopants for green‐ and red‐emitting organic light‐emitting diodes (OLEDs) with external quantum efficiency values over 22.1 %. These complexes are also applied in two‐photon cellular imaging when incorporated into mesoporous silica nanoparticles (MSNs).     

Fluorescence Study of Energy Transfer in PMMA Polymers with Pendant Oligo‐Phenylene‐Ethynylenes

A spectroscopic characterization of polymers containing rigid π‐conjugated oligo(phenyleneethynylene) chromophores as well as oligo(phenyleneethynylene) and methyl methacrylate is presented. The polymers exhibit molar masses of up to 15 000 g mol^?1 and a degree of polymerization between 22 and 80. Emission measurements of the monomeric and polymeric species show that radiative as well as nonradiative rates are influenced by the degree of polymerization due to intramolecular interactions of chromophores pendant to the polymer backbone. Time‐resolved emission anisotropy measurements suggest that energy migrates within the polymers. Steady‐state emission anisotropy measurements also point to energy migration. Additionally, two oligo(phenyleneethynylene)s with different sizes of the conjugated system are copolymerized in order to enable energy trapping due to energy transfer. The shortened energy‐donor fluorescence lifetime within the donor–acceptor copolymers suggest energy transfer. Depending on the degree of polymerization, dispersion of the donor fluorescence lifetime is observed.     

Carbo‐biphenyls and Carbo‐terphenyls: Oligo(phenylene ethynylene) Ring Carbo‐mers

Ring carbo‐mers of oligo(phenylene ethynylene)s (OPEn, n=0–2), made of C₂‐catenated C₁₈ carbo‐benzene rings, have been synthesized and characterized by NMR and UV‐vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT‐optimized structures show that the C₁₈ rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). Carbo‐terphenyls (n=2) are reversibly reduced at ca. ?0.42 V/SCE, i.e. 0.41 V more readily than the corresponding carbo‐benzene (?0.83 V/SCE), thus revealing efficient inter‐ring π‐conjugation. An accurate linear fit of E_1/2^red1 vs. the DFT LUMO energy suggests a notably higher value (?0.30 V/SCE) for a carbo‐quaterphenyl congener (n=3). Increase with n of the effective π‐conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT‐calculated excited states, one of them restricting to a HOMO→LUMO main one‐electron transition.     

Synthesis,crystal structure and photophysical properties of 1,4‐bis(1,3‐diazaazulen‐2‐yl)benzene: a new π building block

A dimerized 1,3‐diazaazulene derivative, namely 1,4‐bis(1,3‐diazaazulen‐2‐yl)benzene [or 2,2′‐(1,4‐phenylene)bis(1,3‐diazaazulene)], C₂₂H₁₄N₄, (I), has been synthesized successfully through the condensation reaction between 2‐methoxytropone and benzene‐1,4‐dicarboximidamide hydrochloride, and was characterized by ¹H NMR and ¹³C NMR spectroscopies, and ESI–MS. X‐ray diffraction analysis reveals that (I) has a nearly planar structure with good π‐electron delocalization, indicating that it might serve as a π building block. The crystal belongs to the monoclinic system. One‐dimensional chains were formed along the a axis through π–π interactions and adjacent chains are stabilized by C—H…N interactions, forming a three‐dimensional architecture. The solid emission of (I) in the crystalline form exhibited a 170 nm red shift compared with that in the solution state. The observed optical bandgap for (I) is 3.22 eV and a cyclic voltammetry experiment confirmed the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The calculated bandgap for (I) is 3.37 eV, which is very close to the experimental result. In addition, the polarizability and hyperpolarizability of (I) were appraised for its further application in second‐order nonlinear optical materials.     

Crystallization‐Induced Reversal from Dark to Bright Excited States for Construction of Solid‐Emission‐Tunable Squaraines

Squaraines (SQs) with tunable emission in the solid state is of great importance for various demands; however a remaining challenge is emission quenching upon aggregation. Herein, a unique SQ, named as CIEE‐SQ, is designed to exhibit strong emission in crystal, undergoing crystallization‐induced reverse from dark ¹(n+σ,π*) to bright ¹(π,π*) excited states. Such an excited state of CIEE‐SQ can be subtly tuned by molecular conformation changes during the unexpected temperature‐triggered single‐crystal to single‐crystal (SCSC) reversible transformation. Furthermore, co‐crystallization between CIEE‐SQ and chloroform largely stabilize the ¹(π,π*) state, enhancing the transition dipole moment and decreasing the reorganization energy to boost the fluorescence, which is promising in data encryption and decryption.     

A Molecular Nanotube with Three‐Dimensional π‐Conjugation

A π‐conjugated twelve‐porphyrin tube is synthesized in 32 % yield by a template‐directed coupling reaction that joins together six porphyrin dimers, forming twelve new C? C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm³. Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six‐porphyrin ring, but are red‐shifted by approximately 300 cm^?1, reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited‐state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.