Red-shifted Fluorescence Research Articles

Solid-state carbon dots with orange phosphorescence and tunable fluorescence via in-situ growth in phthalimide crystal matrix

Solid-state fluorescent and phosphorescent carbon dots (CDs) have widespread applications, with difficulties in obtainment. In this study, solid-state carbon dots (SCDs) emitting green, yellow, and orange fluorescence were fabricated using urea and phthalic acid as precursors. The band gap size was systematically controlled by varying the ratio of raw material nitrogen. By changing the temperature during synthesis, the phosphorescent properties of SCDs were adjusted, and the longest lifetime possessed 156.8 ms of orange phosphorescence. CDs were attached to the crystals of phthalimide, allowing a certain spacing between the particles and suppressing the phenomenon of aggregation-induced quenching (ACQ), thus obtaining solid-state fluorescence. Detailed characterization and density flooding theory (DFT) calculations further indicate that the reduction of the conjugation domain leads to the redshift of the solid-state fluorescence. The phosphorescence appearance is attributed to the presence of crystals that inhibit the nonradiative relaxation channels, and N elements that promote spin-orbit coupling interactions. Based on their special luminescent properties, they have been successfully applied in the fabrication of (0.38,0.54), (0.44,0.54) and (0.51, 0.47) light-emitting diodes, as well as in the production of secure anti-counterfeit cryptographic inks and their successful application in digital encryption.

TDDFT study on the ESPT and ICT mechanism of a bifunctional fluorescent probe for detecting fluoride and sulphite

TDDFT method was applied to study the excited-state mechanism of a reported fluorescent probe for fluoride and sulfite. Theoretical results showed the excitation of the probe molecule involved an intramolecular charge transfer process. Based on potential energy scanning and frontier orbital analysis, the excited-state proton-transfer process of the fluoride-probe product resulted in the significant red-shifted fluorescence. In contrast, the intramolecular charge transfer process was inhibited by sulfite anions. Due to the curtailing of the conjugated system, a blue-shifted emission was observed in the fluorescence spectrum. By exploiting the difference in fluorescence wavelengths,fluoride and sulfite anions could be recognized.

Remarkable Piezofluorochromism of an Organoboron Complex Containing [2.2]Paracyclophane

Piezofluorochromism (PFC) of crystals of a tert-butyl and [2.2]paracyclophane-containing diaroylmethanatoboron difluoride (pCP-tBu), which is expected to possess both intermolecular and intramolecular π–π interactions, was investigated. pCP-tBu crystals were found to exhibit remarkable PFC associated with an over 150-nm redshift of fluorescence (FL) wavelength occurring under increasing isotropic pressure applied by using a diamond anvil cell. X-ray crystallographic analysis showed that pCP-tBu molecules in the crystal at atmospheric pressure exist in the form of an intermolecular π-stacking dimer through back-to-back π–π interactions. Moreover, the crystallographic data under high pressure showed that increasing isotropic pressure causes a reduction in both the intermolecular π-stacking distance of the dimer (DINTER) and the intramolecular π-stacking distance between two benzene rings in the [2.2]paracyclophane moiety (DINTRA). Density functional theory calculations, using the Cartesian coordinates obtained by X-ray analysis, suggest that the origin of PFC of pCP-tBu crystals is the reduction of the LUMO energy of the π-stacking dimer and that the intermolecular π–π interaction is still the important factor leading to PFC of pCP-tBu crystals.

Real-time visualization of lysosomal pH fluctuations in living cells with a ratiometric fluorescent probe

Abnormal variations of lysosomal pH values are closely associated with various diseases, thus it is of great necessity to develop some reliable lysosome-targeted fluorescent probes for real-time monitoring pH changes. Herein, a pyridine-decorated ratiometric fluorescent probe Lyso-CMCP based on camphor-coupled coumarin fluorophore was developed for monitoring pH fluctuations in lysosome of living cells. Lyso-CMCP could sensitively and selectively respond to acidic pH in the range of 7.54–3.19 with a significantly red-shifted fluorescence emission from 455 nm to 516 nm. Besides, this probe also possessed some favorable features, such as good photostability, fine reversibility, excellent anti-interference ability, and low cytotoxicity. The pKa value of 5.14 endowed Lyso-CMCP with the potential to accumulate in lysosomes. With this probe in hand, the lysosomal pH fluctuations were successfully monitored and quantified by ratiometric fluorescence imaging.

A C 54 B 2 Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity

A C ₅₄ B ₂ Polycyclic π-System with Bilayer Assembly and Multi-Redox Activity

Synthesis and optical properties of novel key electron donors-based pinacol boronate ester derived from phenothiazine, phenoxazine and carbazole

Pinacol boronate esters D1-D3 derived from phenothiazine (PTZ), phenoxazine (POZ), and carbazole (CZ), having a tolyl moiety as an auxiliary donor and 2-ethylhexyl as a branched alkyl chain to prevent the molecular aggregation have been synthesized and characterized. Thus, the N-alkylation of PTZ, POZ, and CZ followed by bromination afforded the corresponding products in good yield. These compounds were CC coupled via the Suzuki coupling reaction with tolylboronic acid and then underwent the Miyaura borylation reaction to get the key electron donors D1-D3. The chemical structures of all compounds were confirmed by spectral data (IR, 1H NMR, 13C NMR, 11B-NMR, and HRMS). The UV–vis absorption and fluorescence characteristics of D1-D3 compared with their cores PTZ, POZ and CZ in THF solution have been investigated. The results indicated a red-shift in both absorption and fluorescence of D1-D3, indicating that the tolyl moiety increased the conjugation of the donor cores. A huge Stokes shift ranging from 15,000 to 17,000 cm−1 was observed for D1-D3 intermediates. The newly synthesized boronate key electron donors have the potential to construct optoelectronic materials.

Spontaneous Coassembly of the Protein Terthiophene into Fluorescent Electroactive Microfibers in 2D and 3D Cell Cultures

Protein-based microfibers are biomaterials of paramount importance in materials science, nanotechnology, and medicine. Here we describe the spontaneous in situ formation and secretion of nanostructured protein microfibers in 2D and 3D cell cultures of 3T3 fibroblasts and B104 neuroblastoma cells upon treatment with a micromolar solution of either unmodified terthiophene or terthiophene modified by mono-oxygenation (thiophene → thiophene S-oxide) or dioxygenation (thiophene → thiophene S,S-dioxide) of the inner ring. We demonstrate via metabolic cytotoxicity tests that modification to the S-oxide leads to a severe drop in cell viability. By contrast, unmodified terthiophene and the respective S,S-dioxide cause no harm to the cells and lead to the formation and secretion of fluorescent and electroactive protein–fluorophore coassembled microfibers with a large aspect ratio, a micrometer-sized length and width, and a nanometer-sized thickness, as monitored in real-time by laser scanning confocal microscopy (LSCM). With respect to the microfibers formed by unmodified terthiophene, those formed by the S,S-dioxide display markedly red-shifted fluorescence and an increased n-type character of the material, as shown by macroscopic Kelvin probe in agreement with cyclovoltammetry data. Electrophoretic analyses and Q-TOF mass spectrometry of the isolated microfibers indicate that in all cases the prevalent proteins present are vimentin and histone H4, thus revealing the capability of these fluorophores to selectively coassemble with these proteins. Finally, DFT calculations help to illuminate the fluorophore–fluorophore intermolecular interactions contributing to the formation of the microfibers.

Ratiometric Monitoring of Biogenic Amines by a Simple Ammonia-Response Aiegen.

Herein, we developed a paper-based smart sensing chip for the real-time, visual, and non-destructive monitoring of food freshness using a ratiometric aggregation-induced emission (AIE) luminogen (i.e., H+MQ, protonated 4-(triphenylamine)styryl)quinoxalin-2(1H)-one) as pH sensitive indicators. Upon exposure to amine vapors, the deprotonation of H+MQ occurs and triggers its color change from blue to yellow, with the fluorescence redshift from blue to amaranth. Consequently, we successfully achieved the sensitive detection of ammonia vapors by recording the bimodal color and fluorescence changes. Given the high sensitivity of H+MQ to ammonia vapor, a paper-based smart sensor chip was prepared by depositing H+MQ on the commercial qualitative filter paper through a physical deposition strategy. After being placed inside the sealed containers, the developed H+MQ-loaded paper chip was applied to the real-time monitoring of biogenic amine contents according to its color difference and ratio fluorescence change. The detection results were further compared with those obtained by the high-performance liquid chromatography method, which verified the feasibility of the designed paper chip for the food spoilage degree evaluation. Briefly, this work indicates that the designed H+MQ-loaded paper chip could be a promising approach for improving food freshness monitoring.

Photophysical properties of BODIPYs with sterically-hindered nitrophenyls in meso-position

During recent years, the BODIPY core became a popular scaffold for designing of dyes with desirable properties. In this paper, we extend the library of possible modification of BODIPYs, presenting experimental data for a new family meso-substituted with nitrophenyl groups. We also present quantum chemical calculations, which complement the experimental data and give additional insight in the underlying photochemical and photophysical processes. Optical properties of novel compounds are characterized. It is shown that the fluorescence quantum yield and the efficiency of singlet oxygen generation differs by two orders of magnitude across the family. In aqueous solution, red-shifted fluorescence emission band is detected, corresponding to aggregates. This band has shorter decay time and wide excitation spectrum. We show that photolysis is not accompanied by the release of nitric oxide (NO) which was observed earlier for some dyes with nitrophenyl substituents. The experimental data presented in this paper may be useful for further designing of BODIPY dyes and NO photodonors.

Sequence-specific destabilization of azurin by tetramethylguanidinium-dipeptide ionic liquids

The thermal unfolding of the copper redox protein azurin was studied in the presence of four different dipeptide-based ionic liquids (ILs) utilizing tetramethylguanidinium as the cation. The four dipeptides have different sequences including the amino acids Ser and Asp: TMG-AspAsp, TMG-SerSer, TMG-SerAsp, and TMG-AspSer. Thermal unfolding curves generated from temperature-dependent fluorescence spectroscopy experiments showed that TMG-AspAsp and TMG-SerSer have minor destabilizing effects on the protein while TMG-AspSer and TMG-SerAsp strongly destabilize azurin. Red-shifted fluorescence signatures in the 25 °C correlate with the observed protein destabilization in the solutions with TMG-AspSer and TMG-SerAsp. These signals could correspond to interactions between the Asp residue in the dipeptide and the azurin Trp residue in the unfolded state. These results, supported by appropriate control experiments, suggest that dipeptide sequence-specific interactions lead to selective protein destabilization and motivate further studies of TMG-dipeptide ILs.

Novel strategy of highly efficient solar-driven water evaporation using MWCNTs-ZrO2-Ni@CQDs composites as photothermal materials

Solar-driven water evaporation is a promising technology for high efficiency water purification by absorbing solar energy and converted into localized heat. Herein, we report a low-cost approach for developing a novel photothermal materials based on combination of ZrO2 nanoparticles (NPs) with Ni doped carbon quantum dots (Ni@CQDs) and multi-walled carbon nanotube (MWCNT) coated on melamine foam (MF) surface. The formation of Ni@CQDs was confirmed by red-shift absorbance and fluorescence. Moreover, the shift in XRD diffraction angle indicated the Ni@CQDs incorporated into the ZrO2 and carbon framework of MWCNT. The composite absorber demonstrates excellent solar-water evaporation rate of 1.95 kg m−2 h−1 and high evaporation efficiency of 89.5%, which is ten-fold improvement as compared to pure water (0.19 kg m−2 h−1) under 1-Sun irradiation. The enhancement in evaporation rate is mainly attributed to the strong light absorption of 94% and extremely low thermal conductivity of MWCNT/ZrO2/Ni@CQDs (0.038 W m−1 K−1). The composite absorber capable of continuous operation of 6 cycles under 1-Sun irradiation, indicates high durability and completely reduced organic dyes (99.8%). Accordingly, the novel design of MWCNT/ZrO2/Ni@CQDs as composite absorber as high performance solar-driven water evaporation and wastewater purification.

Restriction of intramolecular torsion induces abnormal blue-shifted fluorescence in the aggregate state

The aggregation-induced emission (AIE) effect and corresponding restriction of intramolecular motions strategy are important mechanisms for regulating the performance of novel organic light-emitting diode materials and constituent molecules, such as emission intensity and peak position. Nevertheless, molecules in the aggregate state usually yield a red-shifted fluorescence due to the π-π interaction enhancement. Here, we report a blue-shifted fluorescence mechanism induced by the restriction of intramolecular torsion in the aggregate state for an interesting excited-state intramolecular proton transfer fluorophore. A combined experimental and theoretical method is performed to investigate the excited state dynamics of the molecules in solvent and the aggregate state respectively. Multiple regression analysis reveals that the changes of reorganization energy caused by AIE effect induce the blue-shifted fluorescence. These findings contribute in several ways to our understanding of the AIE effect, and provide a new strategy to regulate the emission performance of AIE materials.

Stable bifunctional ZnII-based sensor toward acetylacetone and l-histidine via a fluorescence red shift and turn-on effect

A ZnII-based coordination polymer was synthesized by a mixed-ligand strategy, and it could be viewed as the first bifunctional fluorescent sensor with good stability for acac and l-His via a fluorescence red shift and enhancement.

A Two-Birds-With-One-Stone Synthesis of Hydrophilic and Hydrophobic Fluorescent Carbon Nanodots from Dunaliella Salina Biomass as 4-Nitrophenol Nanoprobes Based on Inner Filter Effect and First Derivative Fluorescence Redshift

A Two-Birds-With-One-Stone Synthesis of Hydrophilic and Hydrophobic Fluorescent Carbon Nanodots from Dunaliella Salina Biomass as 4-Nitrophenol Nanoprobes Based on Inner Filter Effect and First Derivative Fluorescence Redshift

Simple Strategy for Scalable Preparation Carbon Dots: RTP, Time-Dependent Fluorescence, and NIR Behaviors.

Transforming carbon dots (CDs) fluorescent materials into smart materials with complex functions is a topic of great interest to nanoscience. However, designing CDs with regulating fluorescence/phosphorescence that can be visually monitored with the environment changes in real‐time remains a challenge. Here, a very simple strategy, one‐step solvent‐free catalytic assistant strategy, which is low cost, facile, environment‐friendly, and high throughput, is put forward. Hydrogen bond is used to manipulate nanostructure of CDs, and the obtained carbon dots (M‐CDs) show a series of attractive properties including matrix‐free room‐temperature phosphorescence, time‐dependent fluorescence, and near‐infrared emissive characteristics. Different from the traditional aggregation caused quenching or aggregation‐induced emission fluorescent materials, M‐CDs exhibit unprecedented and unique dispersion induced redshift fluorescence phenomenon, promoting the studies of fluorescence from static to dynamic. The causes of this phenomenon are further analyzed in detail. As a kind of intelligent fluorescent materials, this new designed CDs greatly enrich the basic recognition of CDs by illustrating the relationship between redshift fluorescence behaviors and the dispersion states, and may provide with an opportunity for solid‐state fluorescent materials, anti‐counterfeiting, cellular imaging, and hopefully many others.

Solvent-Dependent Excited-State Evolution of Prodan Dyes.

Excited-state character and dynamics of two 6-(dimethylamino)-2-acylnaphthalene dyes (Prodan and Badan-SCH2CH2OH) were studied by picosecond time-resolved IR spectroscopy (TRIR) in solvents of different polarity and relaxation times: hexane, CD3OD, and glycerol-d8. In all these solvents, near-UV excitation initially produced the same S1(ππ*) excited state characterized by a broad TRIR signal. A very fast decay (3, ∼100 ps) followed in hexane, whereas conversion to a distinct IR spectrum with a ν(C═O) band downshifted by 76 cm-1 occurred in polar/H-bonding solvents, slowing down on going from CD3OD (1, 23 ps) to glycerol-d8 (5.5, 51, 330 ps). The final relaxed excited state was assigned as planar Me2N → C═O intramolecular charge transfer S1(ICT) by comparing experimental and TDDFT-calculated spectra. TRIR conversion kinetics are comparable to those of early stages of multiexponential fluorescence decay and dynamic fluorescence red-shift. This work presents a strong evidence that Prodan-type dyes undergo solvation-driven charge separation in their S1 state, which is responsible for the dynamic fluorescence Stokes shift observed in polar/H-bonding solvents. The time evolution of the optically prepared S1(ππ*) state to the S1(ICT) final state reflects environment relaxation and solvation dynamics. This finding rationalizes the widespread use of Prodan-type dyes as probes of environment dynamics and polarity.

Aggregation-induced emission with large redshift in 2,7-diphenyl-fluorenone: Reality or artifact?

The luminescence property of 2,7-diphenyl-fluorenone (DPFO) was previously reported to be very unusual with a large aggregation-induced effect associated with a fluorescence redshift of 150 nm. The phenomenon is reexamined in this work. It is found that the abnormal observations are caused by the presence of a trace amount of impurity 2,7-diphenyl-fluorene (DPF) in the as-synthesized DPFO. The pure DPFO molecule does have an intense fluorescence (FL) in solid (528 nm), about 4–5 times larger than in its dilute dichloromethane solutions (542 nm), but with a blueshifted rather than redshifted FL wavelength in solid. The enormous FL enhancement and redshifted FL wavelength of the as-synthesized DPFO solid are due to the presence of impurity DPF. The FL of DPF is much stronger than that of DPFO in dilute solutions and it also has shorter FL wave-lengths. In a dilute solution of DPFO with a trace amount of DPF (~1%), the dominant FL peaks are from DPF. Because the electronic absorption peaks of DPF overlaps with DPFO, the electronic energy of DPF can transfer to DPFO. The energy transfer is faster with the increase of concentration because DPF and surrounding DPFO molecules become closer, which quenches the FL of DPF (356 and 372 nm) and enhances the FL of DPFO (542 nm in solution and 528 nm in solid). Therefore, at high concentrations or in solids, only peak at about 542 or 528 nm shows up, and peaks at 356 and 372 nm disappear.

Luminescent Metal–Organic Framework-Based Fluorescence Turn-On and Red-Shift Sensor toward Al3+ and Ga3+: Experimental Study and DFT Calculation

A dual-functional sensor based on a two-dimensional metal–organic framework (MOF), namely, {[Cd(BBZB)(2,6-NDC)]·CH3OH}n (JXUST-6, BBZB = 4,7-bis(1H-benzimidazole-1-yl)-2,1,3-benzothiadiazole and 2,6-NDC = 2,6-naphthalenedicarboxylate), has been designed and synthesized under solvothermal conditions. JXUST-6 exhibits good stability in aqueous solution, boiling water, and N,N-dimethylformamide (DMF) solution. Luminesecence studies of JXUST-6 reveal the existence of fluorescence red shift toward Al3+ and Ga3+ accompanied by turn-on fluorescence response. After gradually adding Al3+ solution to JXUST-6 suspended solution, the emission peak moved from 505 to 529 nm and triggered the color conversion from yellow to yellow-green under 365 UV light irradiation. Similarly, the addition of Ga3+ to JXUST-6 suspended solution caused the enhancement of fluorescence intensity as well as the red-shift from 505 to 530 nm. The detection limits of JXUST-6 toward Al3+ and Ga3+ ions are 0.081 and 0.047 ppm, respectively. Actually, the test paper can be used to sense Al3+ and Ga3+ ions, thereby expanding the potential application scope of JXUST-6. Importantly, JXUST-6 represents the rare turn-on MOF toward Ga3+. The red-shift effect of JXUST-6 toward Al3+ and Ga3+ could account for the formation of a strong exciplex between JXUST-6 and Al3+ and Ga3+. The theoretical caculation suggests that the occurrence of strong intramolecular charge transfer leads to turn-on effect toward Al3+ and Ga3+ ions.

Discerning the multi-color fluorescence in donor-π-acceptor molecules by femtosecond transient absorption spectroscopy

Time-resolved femtosecond transient absorption (fs-TA) spectroscopy was used to measure the multi-color fluorescence of two donor-π-acceptor (D-π-A) models based on (2-(4-vinylbenzylidene)malononitrile (VBM) derivatives. 2-[[4-[2-(2-benzothiazolyl)ethenyl]phenyl]methylene]-Propanedinitrile (α-VBM) in DMSO presents a sequential red-shift fluorescence (80 nm) changing from 460 nm (blue) to 540 nm (green) within 669.2 fs, corresponding the twisted→planar conformational torsion. The delay prolongs to 1.406 ps and 2.26 ps respectively with glycerin fractions at 30% and 50% in the glycerin/DMSO mixed solutions. Theoretical emission spectra attribute the shifts to a rotation of the hole-electron acceptor axis of A-Ph-A-C(CN)2 during charge transfer, and represents the characteristics of a twist in orientation of the more distal acceptor A-C(CN)2 (along t3 in Fig. 1) in preference to the rotation of the two more proximal D-π-A axes (along t1 and t2 in Fig. 1). This interpretation is supported by calculations showing t3's favorable energy barrier and lower Laplacian bond orders (t3 (1.477) < t1 (1.536) < t2 (1.549)). The exchange of the nitrile groups by the weaker electron deficiency of the aldehyde group in 4-[2-(2-benzothiazolyl)ethenyl]-benzaldehyde (collectively β-VBM) produces a smaller fluorescence emission shift (30 nm) compared to VBM derivatives. In contrast to previously reported D-π-A rotation axes, this work stresses the major contribution of the more distal A-Ph-Asubstituent on multi-color fluorescence.

Emissive and reactive excimers in a covalently-linked supramolecular multi-chromophoric system with a balanced rigid-flexible structure

A novel multi-chromophoric system, triad, in which two styrylbenzoquinoline (SBQ) photochromes are connected by a balanced rigid-flexible linker comprising 2,3-naphthylene framework (a residue of 3-oxy-2-naphthoic acid) and tetramethylene groups, was designed and synthesized to study an excimer formation in the excited state. The 1H NMR data testified that triad exists in solution as folded conformers with asymmetric parallel-displaced SBQ units. Under light irradiation, in the triad, competitive photoisomerization and [2 + 2] photocycloaddition reactions were observed, both reactions being reversible. The photocycloaddition resulted in a tetrasubstituted cyclobutane. The red-shifted fluorescence spectrum and the appearance of a long-lived component in the triad fluorescence decay indicated formation of an 'emissive' excimer. The photocycloaddition is assumed to occur in a 'reactive' excimer, in which the ethylene groups of the SBQ photochromes are located at a distance sufficient for the formation of the σ-bonds between them. Quantum-chemical density functional theory (DFT) calculations at M06-2X/6-31G* level predicted the existence of the triad conformers with π-stacking interaction of SBQ photochromes, the structure of which is pre-organized for the excimer formation and photocycloaddition. For the first time, both emissive and reactive excimers were experimentally observed in the multi-chromophoric system with two diarylethylene photochromes undergoing [2 + 2] photocycloaddition.