Significant Spectral Red-shift Research Articles

Stereo effects for efficient synthesis of orange-red multiple resonance emitters centered on a pyridine ring.

Despite theoretical difficulties, we herein demonstrate an effective strategy for the inaugural synthesis of an orange-red multiple resonance (MR) emitter centered on a pyridine ring via stereo effects. Compared to conventional benzene-centered materials, the pyridine moiety in the novel MR material acts as a co-acceptor. This results in a significant spectral redshift and a narrower spectrum, as well as an improved photoluminescence quantum yield (PLQY) due to the formation of intramolecular hydrogen bonds. As envisioned, the proof-of-concept emitter Py-Cz-BN exhibits bright orange-red emission peaking at 586 nm with a small full width at half maximum (FWHM) of 0.14 eV (40 nm), exceeding both the wavelength and FWHM achieved with benzene-centered BBCz-Y. Benefiting from high PLQYs (>92%) and suppressed chromophore interactions, the optimized organic light-emitting diodes achieved high maximum external quantum efficiencies of 25.3-29.6%, identical small FWHMs of 0.18 eV (54 nm), and long lifetimes over a wide range of dopant concentrations (1-15 wt%). The performance described above demonstrates the effectiveness of this molecular design and synthesis strategy in constructing high performance long-wavelength MR emitters.

Ce/Mn/Cr: (Re,Y)3Al5O12 Phosphor Ceramics (Re = Gd, Tb and Lu) for White LED Lighting with Significant Spectral Redshift and Improved Color-Rendering Index.

In order to attain phosphor ceramics with a high Color-Rendering Index (CRI), samples with the composition of Y0.997-xRexCe0.003)3(Al0.9748 Mn2+0.024Cr3+0.0012)5O12(Rex = 0, Gd0.333, Gd0.666, Gd0.997, Tb0.333, Tb0.666, Tb0.997 and Lu0.997 were prepared by solid-state reaction and vacuum sintering, and exhibited potential for high-quality, solid-state lighting. Doping with Cr3+ and Mn2+ effectively enhanced the red component of Ce3+ spectra through the intense energy transfer from Ce3+ ions to Mn2+/Cr3+ ions. The crystal field splitting of [GdO8] and [TbO8] was more extensive than that of [YO8], causing a massive redshift in the Ce3+ emission peaks from 542 to 561 and 595 nm, while [LuO8] had an opposite effect and caused a blueshift with a peak position at 512 nm. White LED devices incorporating Ce/Mn/Cr: (Gd0.333Y0.664)3Al5O12 phosphor ceramic exhibited a high CRI of 83.97, highlighting the potential for enhancing the red-light component of white LED lighting.

Structure and tunable broadband near-infrared luminescence of Cr3+ in borophosphate glass

In this work, a series of Cr3+ ion-doped borophosphate glasses were prepared by the melt-quenching method. The structure, optical absorption, and near-infrared luminescence properties of the borophosphate glasses were investigated in detail. The results showed that Cr2O3 plays a modifier role in borophosphate glass consisting of borate and phosphate groups. The indirect optical bandgap and Urbach energy were found to vary in the range of 3.293–3.344 eV and 0.290–0.394 eV, respectively. Meanwhile, the Cr3+ ions located in a weak crystal field environment were confirmed by a Dq/B value of less than 2.3. Under the excitation of 440 nm, the as-prepared glasses exhibit broadband near-infrared (NIR) luminescence with an FWHM of about 210 nm and demonstrate a significant spectral red-shift from 858 nm to 894 nm. When the Cr2O3 content is greater than 0.30 mol%, the fluorescence intensity appears obvious concentration quenching. This work illustrates the potential application prospects of Cr3+ ion-doped glass materials as broadband NIR fluorescent devices.

Ultrasensitive detection of lead (II) ion by dark-field spectroscopy and glutathione modified gold nanoparticles

Exposure to trace amounts of lead is known to harm human health, especially for children, which results in irreversible detriment to brain and nerve development. However, most of the sensor platforms using the colorimetric method to detect lead (II) ions (Pb2+) are difficult to reach a decent limit of detection (LOD) and a large detection range. This work presents a practical method to detect Pb2+ in aqueous-medium with the help of single-particle dark field spectroscopy, which allows us to obtain good LOD and wide detection range. Our results illustrate that the glutathione functionalized AuNPs are able to capture free AuNPs in the presence of Pb2+ to form multipliers. Such a process results in a significant spectral redshift of scattering spectra due to plasmonic coupling, which allows us to reach a good LOD of 3.5 pM, and a large linear quantifiable detection range from 10 pM to 10 μM. The performance of our sensor platform provides an alternative way to detect traces of Pb2+ in real-life water samples.

Dynamics of Preferential Solvation of 5-Aminoquinoline in Hexane-Alcohol Solvent Mixtures.

Dynamics of preferential solvation of 5-aminoquinoline (5AQ) has been studied in hexane-alcohol solvent mixtures. Significant spectral red shift and fluorescence quenching are observed, even for miniscule amounts of alcohols. A nonlinear variation of these parameters, with alcohol mole fraction, indicates dipolar enrichment around 5AQ in its excited state. A nanosecond rise at the red end of emission spectra is ascribed to translational diffusion of alcohol molecules and consequent preferential solvation of 5AQ. Electrostatic stabilization and hydrogen bonding between the dipolar excited state of the fluorophore and polar alcohol molecules are major driving factors here. Time evolution of emission spectra, from those in a predominantly hexane-like environment to those in a predominantly alcohol-like environment, takes place in nanosecond time scale and depends on the alcohol concentration in the mixture. This in agreement with calculations using an existing model. Hence, 5AQ is found to be a suitable fluorescent probe for the study of dynamics in complex systems.

Donor Effect on the Photoinduced Interfacial Charge Transfer Dynamics of D−π–A Diketopyrrolopyrrole Dye Sensitizers Adsorbed on Titanium Dioxide

In this report, we investigate the effects of a strong electron-donating group on the electron injection and recombination processes of diketopyrrolopyrrole (DPP)-sensitized TiO2 films by comparing the dynamics of triphenylamine donor-installed DPP_A and donor-absent DPP_R dyes. We show that the electron-donating group enhances the intramolecular charge transfer (ICT) characteristics of DPP_A, as demonstrated by a significant spectral redshift in the steady-state measurement of DPP_A compared to that of DPP_R. By comparing the ultrafast transient behaviors of the dye-coated films through femtosecond transient absorption (fs-TA) measurements, we provide insight into the high efficiency of DPP_A: most of the electrons from DPP_A are injected into the conduction band of TiO2 within the time resolution of TA (<150 fs), while DPP_R displays a biphasic electron injection process of ca. 340 ps (including slow decay). Additionally, due to the existence of lateral intermolecular interactions on TiO2, a phenomenon ...

Exciton Localization in Extended π-Electron Systems: Comparison of Linear and Cyclic Structures.

We employ five π-conjugated model materials of different molecular shape-oligomers and cyclic structures-to investigate the extent of exciton self-trapping and torsional motion of the molecular framework following optical excitation. Our studies combine steady state and transient fluorescence spectroscopy in the ensemble with measurements of polarization anisotropy on single molecules, supported by Monte Carlo simulations. The dimer exhibits a significant spectral red shift within ∼100 ps after photoexcitation which is attributed to torsional relaxation. This relaxation mechanism is inhibited in the structurally rigid macrocyclic analogue. However, both systems show a high degree of exciton localization but with very different consequences: while, in the macrocycle, the exciton localizes randomly on different parts of the ring, scrambling polarization memory, in the dimer, localization leads to a deterministic exciton position with luminescence characteristics of a dipole. Monte Carlo simulations allow us to quantify the structural difference between the emitting and absorbing units of the π-conjugated system in terms of disorder parameters.

New organic sensitizers using 4-(cyanomethyl)benzoic acid as an acceptor group for dye-sensitized solar cell applications

Two design elements for donor-acceptor organic dyes used in dye-sensitized solar cells (DSSCs) are presented. 4-(Cyanomethyl)benzoic acid was used as an acceptor group and was compared to the commonly used cyanoacrylic acid acceptor group in a pair of otherwise structurally equivalent sensitizers. The 4-(cyanomethyl)benzoic acid based dyes showed significantly improved photovoltaic performance when tested in DSSCs. Furthermore, it was shown that the replacement of the bridging para-phenyl group with a thiophene group in triarylamino-oligothiophene-based sensitizers resulted in a significant spectral red-shift and improved device performance when tested in DSSCs. DSSCs that were constructed using a sensitizer embodying both the design features show an increase in electron lifetime when compared with DSSCs fabricated using conventional sensitizer. The most promising of the novel dyes incorporating both the design features displayed a power conversion efficiency of 6.37%, short circuit photocurrent density of 11.63 mA cm−2, open-circuit voltage of 0.754 V and fill factor of 0.73.

The effect of direct amine substituted push–pull oligothiophene chromophores on dye-sensitized and bulk heterojunction solar cells performance

The effect of direct amine substitution on the photovoltaic performance of donor–acceptor oligothiophenes in dye-sensitized and bulk heterojunction (BHJ) solar cells was investigated. These dyes have a common donor (di-p-tolylamino), π-bridge (terthiophene), and tunable acceptors to provide new materials for evaluation in dye-sensitized solar cells (DSCs) and BHJ solar cells. They were tested with liquid, ionic liquid, and metal-complex based electrolytes for DSCs and PC61BM for solution processable BHJ solar cells. Direct amine substitution of the oligothiophene resulted in a significant spectral red-shift, reduction in band-gaps and improved photovoltaic performance in DSCs but not for BHJ devices.

Pi-pi stacking of the aromatic groups in lignosulfonates

p–p stacking of the aromatic groups in SL. Results show that the charge-free aromatic groups of SL tend to form oriented p–p stacking with the spectroscopic characteristics of J–aggregates. The formation of J–aggregates in SL are recognized by a significant spectral red shift in fluorescent excitation spectra. The other effects that may cause spectral shift, such as the SL species, solvent effect, and the impurities, are investigated to confirm that the formation of J-aggregates is the only viable explanation for the significant spectral redshift of SL. causes molecular shrinkage of SL polyelectrolytes, but has no influence on J–aggregates of the aromatic groups as detected by lack of spectral shift, indicating that the aromatic groups are charge-free. This suggests that not all the aromatic groups but only the charge-free aromatic groups can form p–p stacking. This work demonstrates the presence of J–aggregation in aqueous SL solutions for the first time, which gives an insight in understanding the preferred orientation of the aromatic groups in lignin-based biopolymers.

Reversible Unfolding of a Thermophilic Membrane Protein in Phospholipid/Detergent Mixed Micelles

Folding mechanisms and stability of membrane proteins are poorly understood because of the known difficulties in finding experimental conditions under which reversible denaturation could be possible. In this work, we describe the equilibrium unfolding of Archaeoglobus fulgidus CopA, an 804-residue α-helical membrane protein that is involved in transporting Cu+ throughout biological membranes. The incubation of CopA reconstituted in phospholipid/detergent mixed micelles with high concentrations of guanidinium hydrochloride induced a reversible decrease in fluorescence quantum yield, far-UV ellipticity, and loss of ATPase and phosphatase activities. Refolding of CopA from this unfolded state led to recovery of full biological activity and all the structural features of the native enzyme. CopA unfolding showed typical characteristics of a two-state process, with ΔGw°=12.9 kJ mol−1, m=4.1 kJ mol−1 M−1, Cm=3 M, and ΔCpw°=0.93 kJ mol−1 K−1. These results point out to a fine-tuning mechanism for improving protein stability. Circular dichroism spectroscopic analysis of the unfolded state shows that most of the secondary and tertiary structures were disrupted. The fraction of Trp fluorescence accessible to soluble quenchers shifted from 0.52 in the native state to 0.96 in the unfolded state, with a significant spectral redshift. Also, hydrophobic patches in CopA, mainly located in the transmembrane region, were disrupted as indicated by 1-anilino-naphtalene-8-sulfonate fluorescence. Nevertheless, the unfolded state had a small but detectable amount of residual structure, which might play a key role in both CopA folding and adaptation for working at high temperatures.

Effects of substituent and solvent on the structure and spectral properties of maleimide derivatives

The maximum absorption wavelength ( λ abs max ) , emission wavelength ( λ em) and the related oscillator strength ( f) of the maleimides in the ground and first excited states were calculated by using the DFT, CIS and the time-dependent density functional theory (TD-DFT) methods, where the molecular structures were optimized by DFT/B3LYP/6-31G ∗ calculation. Solvent effects on the maleimides were examined using the PCM simulation at DFT/B3LYP level with the 6-31G ∗ basis set. For N-substituted maleimide, the substituent gives only a slight influence on the maleimide chromophore, while planar conformation of PhMLH leads to the improvement in π-delocalization from substituent to maleimide unit. For 3,4-substituted maleimide, the steric repulsion between substituent and maleimide chromophore influences the extent of π-delocalization and the molecular conformation. The calculated ( λ abs max ) and λ em of maleimides are in good agreement with the experimental data. In the gas phase, both absorption and emission peaks are red-shift as compared to the non-substituted maleimide. Under solvent environment, the more planar conformation of PhMLH shows a blue-shift in the calculated ( λ abs max ) and λ em as compared with other N-substituted maleimides. For 3,4-substituted maleimides, the effect of substitution produces the most significant spectral red-shift as compared to other maleimides.

Function of extracellular loop 2 in rhodopsin: glutamic acid 181 modulates stability and absorption wavelength of metarhodopsin II.

The second extracellular loop of rhodopsin folds back into the membrane-embedded domain of the receptor to form part of the binding pocket for the 11-cis-retinylidene chromophore. A carboxylic acid side chain from this loop, Glu181, points toward the center of the retinal polyene chain. We studied the role of Glu181 in bovine rhodopsin by characterizing a set of site-directed mutants. Sixteen of the 19 single-site mutants expressed and bound 11-cis-retinal to form pigments. The lambda(max) value of mutant pigment E181Q showed a significant spectral red shift to 508 nm only in the absence of NaCl. Other substitutions did not significantly affect the spectral features of the mutant pigments in the dark. Thus, Glu181 does not contribute significantly to spectral tuning of the ground state of rhodopsin. The most likely interpretation of these data is that Glu181 is protonated and uncharged in the dark state of rhodopsin. The Glu181 mutants displayed significantly increased reactivity toward hydroxylamine in the dark. The mutants formed metarhodopsin II-like photoproducts upon illumination but many of the photoproducts displayed shifted lambda(max) values. In addition, the metarhodopsin II-like photoproducts of the mutant pigments had significant alterations in their decay rates. The increased reactivity of the mutants to hydroxylamine supports the notion that the second extracellular loop prevents solvent access to the chromophore-binding pocket. In addition, Glu181 strongly affects the environment of the retinylidene Schiff base in the active metarhodopsin II photoproduct.