Fluorescence In Dilute Solution Research Articles

Color‐Tunable Carbon Dots with Aggregation‐Induced Emission Constructed by FRET between Surface Luminescence Centers

AbstractAs a class of new fluorescent materials, carbon dots (CDs) have drawn widespread attention by virtue of their tunable photoluminescence (PL), excellent biocompatibility, and novel physicochemical properties. Unfortunately, in solid state, most CDs experience serious aggregation‐caused quenching (ACQ). In this work, color‐tunable CDs with aggregation‐induced emission (AIE) are prepared by simply changing the filling ratio of the autoclave. The obtained CDs show dual emission (green/red) in solid state, and the fluorescent color can be tuned from green to red. These CDs all exhibit blue fluorescence in dilute solution, and aggregation‐induced redshift behaviors are observed when the CDs form aggregates. Structural and optical characterizations prove that blue fluorescence is contributed by the carbon cores, whereas the green/red emission is attributed to the surface groups. Multicolor fluorescence is caused by increased fluorescence resonance energy transfer (FRET) efficiency among the dual emission (green/red) peaks from g‐CDs to r‐CDs. Surprisingly, delayed fluorescence (DF) of CDs are brightened by inserting the CDs into poly(vinyl alcohol) (PVA) films, which can be easily recognized by the naked eyes with a lifetime up to ms. Finally, owing to the unique AIE and DF properties, the four CDs can be applied to multimodal advanced anti‐counterfeiting and information encryption.

Rational design strategies for nonconventional luminogens with efficient and tunable emission in dilute solution

The design and synthesis of organic nonconventional luminescent (NL) materials with efficient emission have attracted significant attention. However, these materials usually suffer from the fact of low photoluminescent quantum efficiency (Φ), poor tunability of emission wavelength, and non-luminescence in dilute solution, limiting their further application. Herein, rational molecular design strategies for NL compounds with efficient and tunable emission in dilute solution were presented via theoretical calculation and practical verification. Firstly, increasing the ratio of π → π* transition in the excited state to suppress the n → π* transition, inhibit intersystem crossing, and avoid the generation of triplet excitons which can be easily quenched by water or oxygen. Secondly, introducing strong intramolecular hydrogen bonds to suppress intramolecular rotation and inhibit non-radiative transitions. Besides, D-π-A structure can effectively adjust the luminescence color. Based on the proposed design strategies, two novel compounds named LJ-1 and LJ-2 were synthesized, both compounds can emit bright green fluorescence in dilute solution with Φ of up to 59.8 %, and their photophysical properties are pH-dependent. The results showed that pH had a great impact on the strength of intramolecular hydrogen bonds and the degree of conjugation. For example, the emission wavelength of LJ-1 exhibited red-shift as the increase of pH value. Furthermore, LJ-1 can be used for visual detection of Ag+ in water solution in naked eye and the precise localization of lysosomes. We believe that our work will greatly broaden the application range of nonconventional luminogenic compounds in future.

Bioinspired fluorescent molecules realize super bright blue luminescence under sunlight

In recent years, non-aromatic fluorescent materials have attracted widespread attention, but achieving a high quantum yield (QY) in their dilute solutions still remains a challenge. In this work, inspired by the Clustering-triggered emission (CTE) phenomenon of non-aromatic amino acids and poly(amino acids), we successfully designed a bioinspired molecule with a dipeptide-like structure and active silanol groups. The as-prepared bioinspired molecule can be hydrolyzed by simple heating to obtain the hydrolysate h-PDs, which can emit strong blue fluorescence in dilute solution. And this was the first time to realize the emission behavior of non-aromatic fluorescent materials under sunlight. The QY of h-PDs in aqueous solution and solid state were as high as 98.2% and 96.3%, respectively, which were attributed to the condensation of active silanol groups at the end of the molecule, and the existence of hydrogen bonds and N → Si coordination bonds that made small molecules cross-linked and aggregated into dense clusters. Finally, the application of h-PDs in the detection of intracellular and exogenous Hg2+ and Cys and luminescent films were also explored. Significantly, this work provides a design strategy for synthesizing high-fluorescence QY non-aromatic fluorescent materials, and verifies their potential application value in the fields of environment, biology and light-emitting devices.

Efficient Aggregation-Induced Delayed Fluorescence Luminogens for Solution-Processed OLEDs With Small Efficiency Roll-Off.

Purely organic small molecules with thermally-activated delayed fluorescence have a high potential for application in organic light-emitting diodes (OLEDs), but overcoming severe efficiency roll-off at high voltages still remains challenging. In this work, we design and synthesize two new emitters consisting of electron-withdrawing benzoyl and electron-donating phenoxazine and 9,9-dihexylfluorene. Their electronic structures, thermal stability, electrochemical behaviors, photoluminescence property, and electroluminescence performance are thoroughly investigated. These new emitters show weak fluorescence in dilute solution, but they can emit strongly with prominent delayed fluorescence in the aggregated state, indicating the aggregation-induced delayed fluorescence (AIDF) character. The solution-processed OLEDs based on the two emitters show high external quantum efficiency of 14.69%, and the vacuum-deposited OLEDs can also provide comparable external quantum efficiency of 14.86%. Significantly, roll-offs of the external quantum efficiencies are very small (down to 0.2% at 1,000 cd m−2) for these devices, demonstrating the evidently advanced efficiency stability. These results prove that the purely organic emitters with AIDF properties can be promising to fabricate high-performance solution-processed OLEDs.

Synthesis, characterization, antibacterial studies and quantum-chemical investigation of the new fluorescent Cr(III) complexes

Coordination of the ligands derived from benzimidazole with Cr(III) led to the formation of new fluorescent Cr (III) complexes. The structures of the new complexes were established by spectral, analytical data and Job’s method and an octahedral geometry was proposed for the complexes. Also, the DFT methods were employed to gain a deeper insight into geometry and spectral properties of the new Cr (III) complexes. The DFT-calculated vibrational modes of Cr(III) complexes are in good agreement with the experimental values, confirming suitability of the optimized geometries for the complexes. Fluorescent ligands and chromium complexes were spectrally characterized by UV–Vis and fluorescence spectroscopy. Results revealed that Cr(III) complexes generate fluorescence in dilute solution of DMSO. Calculated electronic absorption spectra were also provided by time-dependent density functional theory (TD-DFT) method. The new complexes exhibited potent antibacterial activity against a panel of strains of Gram negative bacterial and Gram positive species and their MIC was also determined. Two strains of Gram positive and two strains of Gram negative bacteria.

New soluble anthracene-based polymer for opto-electronic applications

A new anthracene-based polymer containing the isosorbide group in the side chain (PPVIs-An) has been synthesized via Wittig reaction. The polymer is soluble in common organic solvents and shows good film-forming abilities. The macromolecular structure was characterized by NMR and FT-IR spectroscopies. This organic material was investigated by differential scanning calorimetry and thermogravimetric analysis and showed an amorphous behavior with a T g of 48 °C. The surface property of the PPVIs-An film was analyzed by contact angle and by atomic force microscopy. The optical properties of this organic semi-conductor were investigated by UV–Visible absorption and photoluminescence spectroscopies. The polymer exhibits a blue fluorescence in dilute solution and a green emission in thin film. The introduction of the polar isosorbide groups shown to improve the PL intensity, and a quantum yield of 72 % was obtained. The absence of π–π interaction between the chromophore units in polymer thin layer at fundamental electronic state was observed, due to the presence of steric repulsions between anthracene and isosorbide groups. The HOMO–LUMO energy levels were determined by cyclic voltammetry. An ITO/PPVIs-An/Al single-layer device was elaborated and investigated using current–voltage measurements which show a turn-on voltage of 5.3 V. The conduction mechanism seems to be a space-charge-limited current with exponential trap distribution at high applied bias voltage. The electrical transport properties of this device were studied as a function of frequency (100 Hz–10 MHz) and applied bias in impedance spectroscopy analyses.

Design of turn-on fluorescent probe for effective detection of Hg2+ by combination of AIEE-active fluorophore and binding site

By combination of the AIEE-active fluorophore and coordination binding group, a new selective and sensitive probe (1) for Hg2+ detection has been synthesized. 1 exhibits weak fluorescence in dilute solution, but exhibits dramatically enhanced emission in aggregation. In the solid state, a remarkable red-shifted emission and longer PL lifetime of 1 are present, which is caused by the formation of excimers due to the π–π stacking interactions of the coumarin groups. This AIEE effect of 1 is utilized to design probe for the detection of Hg2+ in a luminescence “turn-on” mode. The functionalization of the AIEE-active molecule with chelating group provides the potential binding sites for Hg2+. The selective coordination-induced aggregation bursts the emission. 1 has been successfully applied to detect Hg2+ in aqueous and living cells.

Intermolecular hydrogen bonding-assisted high contrast fluorescent switch in the solid state

We describe a convenient approach to achieve fluorescence modulation based on self-assembled complexes between photochromic diarylethenes and solid-emissive Boron–Fluorine dye. Photochromism of the self-assemblies results in slight modulation of the Boron–Fluorine dye's fluorescence in dilute solution, but high-contrast ON/OFF fluorescence switching in nanoparticles and in the solid state. During this process, intermolecular hydrogen bonds between the carboxyl units on diarylethenes and the imidazole ring on Boron–Fluorine dye play important role in the efficient fluorescence resonance energy transfer from Boron–Fluorine dye to closed isomer of diarylethenes. The excitation wavelength (405 nm) of the self-assembled complexes hardly induces photochromism of diarylethene molecules, providing non-destructive readout ability. This system represents a simple and efficient alternative to the covalent system to achieve fluorescent switch in the solid state, and it may be potentially applied in rewritable high-density optical storage.

Host–guest complexation induced emission: a pillar[6]arene-based complex with intense fluorescence in dilute solution

A host-guest inclusion complex was constructed from a water-soluble pillar[6]arene and a tetraphenylethene derivative in water and it exhibited strong fluorescence in dilute solution.

New poly(p-phenylenevinylene) derivatives containing isosorbide unit in the side-chain

New conjugated PPV derivatives containing the chiral isosorbide group (P1-3) have been synthesized via the Gilch reaction. The polymers are optically active, soluble in common organic solvents and show good film-forming abili- ties. High number-average molecular weights were determined by size exclusion chromatography (SEC) (16·10 3 - 21·10 3 g·mol -1 ). The molecular structures of the polymers were confirmed by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies. Thermogravimetric analysis of the polymers showed good thermal sta- bility up to 320°C. The optical properties of these !-conjugated materials were investigated by UV-vis absorption and pho- toluminescence (PL) spectroscopies. The polymers show a yellow fluorescence in dilute solution, and an orange emission is observed in thin films. The introduction of the polar isosorbide groups improved the PL intensity, and quantum yields between 50 and 73% were obtained. The HOMO-LUMO energy levels were estimated by cyclic voltammetry, and the elec- trochemical gaps were 1.81, 1.83 and 2.48 eV for P1, P2 and P3, respectively. Single-layer diode devices were fabricated and show relatively low turn-on voltages between 3.1 and 3.4 V.

Crystal structures and solid-state fluorescence of BODIPY dyes based on Λ-shaped Tröger's base

Two Λ-shaped BODIPY dyes based on Tröger's base, 2,8-(6H,12H-5,11-methanodibenzo[b,f]diazocineylene)-di(4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene) (DFTMB) and 2,8-(6H,12H-5,11-methanodibenzo[b,f]diazocineylene)-di(4,4-difluoro-2,6-diethyl-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene) (DFDEB), were designed and synthesized. Both compounds exhibit intense fluorescence in dilute solution and polycrystalline powder form. From the crystal structure, the impact of the substitutions at the BODIPY core on the crystal packing mode, intermolecular π–π interactions and optical behavior is compared and discussed. The results indicate that the close π–π stacking in the crystalline state has been effectively inhibited by the twisted Λ-shaped Tröger's base.

Confocal Fluctuation Spectroscopy and Imaging

Currently, work with subnanomolar concentrations is routine while femtomolar and even single-molecule studies are possible with some efforts getting high on single-molecule biophysics and biochemistry. Methodological breakthroughs, such as reducing the background light contribution in single-molecule studies, which has plagued many studies of molecular fluorescence in dilute solution, and particularly in live cells, have recently described by us. We first demonstrated how optimized time-gating of the fluorescence signal, together with time-correlated, single-photon counting, can be used to substantially boost the experimental signal-to-noise ratio about 140-fold, making it possible to measure analyte concentrations that are as low as 15 pM. By detection of femtomolar bulk concentrations, confocal microsopy has the potential to address the observation of one and the same molecule in dilute solution without immobilization or hydrodynamic/electrokinetic focusing at longer observation times than currently available. We present relevant physics. The equations are derived using Einstein's approach showing how it fits with Fick's law and the autocorrelation function. An improved technology is being developed at ISS for femtomolar microscopy. The general concepts and provided experimental examples should help to compare our approach to those used in conventional confocal microscopy.

Synthesis and properties of polyacetylenes having pendent phenylethynylcarbazolyl groups

Novel acetylene monomers substituted with phenylethynylcarbazolyl groups, 3-[(4-octylphenyl)ethynyl]-9-propargylcarbazole (1), 3,6-bis[(4-octylphenyl)ethynyl]-9-propargylcarbazole (2), 9-(4-ethynylphenyl)-3-[(4-octylphenyl)ethynyl]carbazole (3), and 9-(4-ethynylphenyl)-3,6-bis[(4-octylphenyl)ethynyl]carbazole (4) were synthesized, and polymerized with Rh+(nbd)[η6–C6H5B−(C6H5)3] and WCl6–n-Bu4Sn catalysts. The corresponding polyacetylenes with number-average molecular weights ranging from 9200 to 94 000 were obtained in 20–98% yields. The IR spectra of the polymers revealed that acetylene polymerization took place at the terminal ethynyl group, while the ethynylene group remained intact. The UV–vis absorption band edge wavelengths of W-based poly(3) and poly(4) were longer than those of the other polymers. W-Based poly(4) emitted fluorescence with the highest quantum yield (41%). Poly(1) exhibited excimer-based fluorescence in dilute solution.

Conjugated and partially conjugated 2,5‐diphenylthiophene‐containing light‐emitting copolymers in polymeric light‐emitting diode (PLED) device fabrications

AbstractTo study the effect of nonconjugation on polymeric and photophysical properties of thiophene‐containing polymers, new light‐emitting copolymers comprising either alternate 2,5‐diphenylthiophene and vinylene or alternate 2,5‐diphenylthiophene and aliphatic ether segments were synthesized. Both copolymers contained 2,5‐diphenylthiophene as the major chromophore and emitted a sky bluish fluorescence in dilute solution (10−2 mg/mL). With a rigid and planarity structure and the concomitant crystallinity, the former copolymer (fully conjugated) possessed a higher quantum efficiency, a higher glass‐transition temperature, and a better thermal stability. In contrast, the latter copolymer (conjugated–nonconjugated) had better solubility and provided enhanced photophysical properties for the fabricated polymeric light‐emitting diode (PLED) device: at 15 V, the maximum current and brightness were 110 mA/cm2 and 4289 cd/m2, respectively, and the electroluminescence efficiency remained constant at approximately 4.9 cd/A in a voltage range of 8 to 14 V. The existence of intramolecular/intermolecular aggregates in the latter copolymer was corroborated from the the UV–vis and photoluminescence spectra of its solutions. With an increase in solution concentration, the shape and λmax of the photoluminescence spectrum were redshifted. In a solution with a concentration as high as 10 mg/mL, the redshift was so drastic that the photoluminescence spectrum was nearly identical to that of a solid‐film. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6061–6070, 2004

Photoluminescence Properties of Conjugated Phenylacetylene Monodendrons in Thin Films

We report the absorption, photoluminescence (PL), and time-dependent PL of thin films of conjugated phenylacetylene monodendrons at both room temperature and at cryogenic temperature. We find that the PL properties of the monodendron thin films are significantly different from their fluorescence in dilute solution due to the presence of interactions between monodendrons in the thin film. These interactions lead to aggregate species in the thin films, which result in broader PL spectra and lower PL quantum yields than for monodendrons in dilute solution. Evidence for excimer-like aggregates in the monodendron thin films is found from time-resolved PL spectra.

Synthesis and properties of poly[2,6-( p -phenylvinyl)-4-(4'-octyloxybiphenyl-4-yl)quinoline

A novel polyquinoline (PQDBP8) containing the pendent 4'-octyloxybiphenyl group in the 4-position of the quinoline ring was prepared by the acid-catalyzed polymerization (Friedlander quinoline synthesis) of 1-(4-{2-[4-amino-3-(4'-octyloxybiphenyl-4-carbonyl)-phenyl]vinyl}phenyl)ethanone (6). PQDBP8 showed highly thermal stability (T d =384°C, T g = 183 °C). PQDBP8 showed blue fluorescence in dilute solution (λ max = 449 nm) and green fluorescence in solid state (λ max = 494, 540 nm) due to excimer formation. EL spectrum of PQDBP8 lies in the green region (λ max = 572 nm) and PQDBP8/PVK blend film lies in the blue region (λ max = 446 nm).

Intramolecular formation of excimers in model compounds for polyesters obtained from 2,6‐naphthalene dicarboxylic acid and cyclohexanediols

AbstractThe fluorescence in dilute solution has been measured as a function of solvent viscosity for four bichromophoric models for polyesters with naphthalene in the rigid aromatic unit and diols derived from cyclohexane as the flexible spacer. The spacers are 1,2‐cis‐cyclohexanediol, 1,2‐trans‐cyclohexanediol, a 1:2 mixture of 1,3‐cis‐ and 1,3‐trans‐cyclohexanediols, and a 1:2 mixture of 1,4‐cis‐ and 1,4‐trans‐cyclohexanediols. The shape of the emission spectra for the molecules in this series is less sensitive to the viscosity of the medium than was the case for an analogous series in which a methylene or oxyethylene spacer replaces the cyclohexanediol spacer. The dependence of the excimer emission on the type of spacer is different also in the series in which the rigid units contain naphthalene or benzene. When the rigid units contain naphthalene, excimer formation is maximal if the spacer contains 1,2‐trans‐cyclohexanediol, but this spacer produces a molecule with a very small tendency for excimer formation in its polymers with terephthalate. A conformational analysis correctly concludes that the spacer most conducive to excimer formation should be 1,2‐trans‐cyclohexanediol, but it does not identify the correct order of the remaining three bichromophoric model compounds. The problem may reside in the method for taking into account the finite width of the torsional well associated with each rotational isomer. © 1994 John Wiley & Sons, Inc.

A Photophysical Study of Tri-chromophoric Systems Containing Two Naphthalene Moieties: Intramolecular Excimer Fluorescence, Energy Transfer, Phosphorescence, and Delayed Fluorescence

Abstract Photophysical properties of 9,10-di[(l-naphthyl)-methyl]-9,10-dihydro-9,10-phenanthrenediol (1), 9,10-di(l-naphthyl)phenanthrene (2), 9,10-di(2-naphthyl) phenanthrene (3), 10,10-di(1 -naphthyI)- 9,10-dihydro-9-phenanthrone (5 a) and 10,10-di(2-naphthyl)-9,10-dihydro-9-phenanthrone (5 b) have been studied. In contrast to l,4-di(l-naphthyl)butane compound 1 shows strong (intramolecular) excimer fluorescence in dilute solution at room temperature. As a result of intramolecular triplettriplet energy transfer the phosphorescence of 2, 3, 5 a and 5 b stems from the naphthalene moieties. The increase (compared to naphthalene) of the rate constant of the radiative deactivation of the lowest triplet state is probably due in the case of 2 and 3 to the non-planarity of the molecules, which is expected to increase spin-orbit coupling, while in compounds 5 a and 5 b intramolecular chargetransfer interaction between the carbonyl group and the naphthalene moieties may be responsible for this effect. Delayed fluorescence (from the phenanthrene moiety) of 2 has been observed in fluid solution at room temperature, and an excitation mechanism is proposed

Intramolecular excimer formation in polyesters from terephthalic acid and cyclohexanediols with different stereochemical compositions

The fluorescence in dilute solution has been measured for five polyesters with terephthalate as the rigid aromatic unit and diols derived from cyclohexane as the flexible spacer. The spacers are 1,2- cis-cyclohexanediol, 1,2- trans-cyclohexanediol, a 1:2 mixture of 1,3- cis- and 1,3- trans-cyclohexanediols, a 1:2 mixture of 1,4- cis- and 1,4- trans-cyclohexanediols, and a 1:3 mixture of 1,4- cis- and 1,4- trans-cyclohexanedimethanol. The two polymers with the largest ratio of the intensities of excimer and monomer emission are those that contain either the 1:2 mixture of 1,3- cis- and 1,3- trans-cyclohexanediol or the 1:3 mixture of 1,4- cis- and 1,4- trans-cyclohexanedimethanol. A conformational analysis concludes that the spacers most conducive to excimer formation are the 1,3- cis-cyclohexanediol and 1,4- cis-cyclohexanedimethanol. This result from calculations is compatible with experimental results.

Die Abhängigkeit der Fluoreszenzquantenausbeute von der Anregungswellenlänge bei Carbonylverbindungen des Pyrens

Abstract Pyrene-3-aldehyde, 3-acetylpyrene and 3-benzoylpyrene show almost no fluorescence in dilute solution in non-polar solvents. In more concentrated solutions, however, a long wave fluorescence is observed, showing the existence of weak intermolecular forces, even in the ground state. In polar solvents, pyrene-3-aldehyde and 3-acetylpyrene exhibit short wave monomer fluorescence whilst 3-benzoylpyrene does not fluoresce. The intensity for both long and short wave fluorescence is dependent upon the excitation wave length and is a maximum for excitation in the weak long wave tail of the first absorption band. Molecules are thought to have a greater absorption in this region if they interact strongly with their polar or polarizable surroundings. Due to the very short singlet lifetime of isolated monomers (τ<C10-10 sec), such molecules have a greater probability of forming a fluorescence state than more weakly interacting ones.