Solution Photoluminescence Research Articles

Evolution of UV/VIS Photoluminescence of Aged Zn(acac)2 Solutions in Correlation with Carbon Precipitation

Evolution of photoluminescence of ethanol solutions of Zn(acac) from near UV to VIS was monitored for time period of several months. It has been found that transparent solutions with time colored in light brown that is accompanied by development of strong visible photoluminescence. This phenomenon is analyzed and discussed in terms of carbon precipitation. It has been demonstrated that at short aging period the photoluminescence is observed as relatively narrow and excitation independent emission band centered at at about 400 nm. In course of aging period emission is converted from violet to blue and green with corresponding shift of excitation band from UV to VIS. Electron paramagnetic resonance spectroscopy allows to identify carbon precipitates in the solutions approving hypothesis on nanocarbon nature of the photoluminescence.

Synthesis of a Highly Fluorescent Bis(1,4-oxaborine)pentacene.

1,4-Oxaborines represent an expansion of the current array of heteroborines. Herein, we report a new ladder-type bis(1,4-oxaborine)pentacene (BO-Pen), the backbone of which can be regarded as an isoelectronic structure of pentacene. The synthesis begins with a two-fold nucleophilic aromatic substitution followed by a ring-closing reaction. The structure was confirmed by X-ray crystallographic analysis. Compound BO-Pen is stable (even after 160 h) under ambient condition and exhibits very different electronic properties as compared to its all-carbon pentacene analogue (Pen-M). Particularly, it shows bright photoluminescence (73 % relative quantum yield) in both solution and in solid state.

Bright and tunable photoluminescence from the assembly of red g-C3N4 nanosheets

It is challenging to construct g-C3N4 (normally as a photocatalyst) with tunable and stable photoluminescence (PL) in solution or in powder form. In this paper, a molten-salt-assisted route has been developed to create red superior thin carbon nitride (g-C3N4) nanosheets with redshifted absorption spectrum and narrowed bandgap of 1.91 eV. Interestingly, these g-C3N4 nanosheets exhibit excellent water soluble nature. The colloidal solution of the nanosheets reveals bright, stable, and tunable PL from blue to yellow. The PL of samples depends on the concentration of g-C3N4 nanosheets. At lower concentration, a blue-emitting band is observed, while a green to yellow emitting band is recorded at higher concentration. Namely, the powder sample reveals a broad green to yellow emitting band. A reversible PL phenomenon is observed by precipitation of g-C3N4 colloidal solution. This phenomenon is ascribed to the assembly of these g-C3N4 nanosheets. The PL efficiency of red-coloured g-C3N4 nanosheets powder is 75%. Both powder and colloidal solution g-C3N4 samples reveal excellent PL stability. The PL intensity and efficiency of the powder samples remain almost unchanged after half year. These water-soluble red g-C3N4 nanosheets thus can be effectively utilized for light emitting diodes and bioimages.

Structural Integration of Carbazole and Tetraphenylethylene: Ultrafast Excited‐State Relaxation Dynamics and Efficient Electroluminescence

The synthesis, characterization, and spectroscopic investigations of a new solution‐processable N‐styrylcarbazole‐linked tetraphenylethylene (TPE) derivative (TPE‐BCzS) are reported, exhibiting green–orange emission. While the material displays weak photoluminescence (PL) in solution with a low PL quantum yield (PLQY of 4 ± 0.2%), significant PL enhancement in neat‐film PLQY (55 ± 8%) is observed. Studies using steady‐state spectroscopy, and femtosecond and nanosecond transient absorption spectroscopy reveal details of the excited‐state dynamics, consisting of the Franck–Condon (FC) state, nonradiative conformational relaxation, and formation and decay of the triplet excited state generated via intersystem crossing (ISC) upon ultrafast photoexcitation. Solution‐processed organic light‐emitting diodes (OLEDs) based on TPE‐BCzS display maximum external quantum efficiencies of 1.8% and 1.7% for neat and blend films (20 wt% in a 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl host), approaching the theoretical efficiency limit for the determined PLQYs of the films. While TPE materials are typically associated with aggregation‐induced emission, it is reported that the enhanced PLQYs in the solid state are due to restriction of structural relaxations in the solid state and not due to the commonly misunderstood aggregation effect.

Dynamics of electron transfer in melanin-trinitrofluorenone system

The kinetics of photoluminescence (PL) and time-resolved emission spectra (TRES) of melanin solutions with an effective electron acceptor 2,4,7-trinitrofluorenone (TNF) have been studied. Based on the analysis of the kinetic decay curves and TRES, the fast and slow PL decay components can be distinguished, which characterize the processes of a relaxation of excited states and charge transfer in the melanin-TNF molecular system. In this paper, the observed dependences have been analyzed and the model to explain such behavior of PL dynamic characteristics has been proposed.

Dinuclear Triple-Stranded Helicates Composed of Tetradentate Ligands with Aluminum(III) Chromophores: Optical Resolution and Multi-color Circularly Polarized Luminescence Properties.

New chiroptical chromophores, dinuclear triple-stranded helicates, composed of tetradentate ligands with aluminum(III) ions, are described. These are synthesized in two steps using inexpensive pyrrole derivatives, hydrazine, and aluminum chloride. These molecular architectures (ALPHY) show multi-color (cyan, yellow, and orange) photoluminescence in solution and in the solid-state, which depends on the substituents of the ligands. The photoluminescence quantum yields of helicates were up to 54 %. The right-handed (P) and left-handed (M) helicates are so stable that they do not undergo racemization in some solvents and are mirror images according to circular dichroism and circularly polarized luminescence (CPL) with an absolute luminescence dissymmetry factor (glum ) of up to the 10-3 order. Mixing the different helicates produces white-light emission with CPL characters. This study offers a glimpse into the potential applications of chromophores with diverse photophysical properties.

Dinuclear Triple‐Stranded Helicates Composed of Tetradentate Ligands with Aluminum(III) Chromophores: Optical Resolution and Multi‐color Circularly Polarized Luminescence Properties

AbstractNew chiroptical chromophores, dinuclear triple‐stranded helicates, composed of tetradentate ligands with aluminum(III) ions, are described. These are synthesized in two steps using inexpensive pyrrole derivatives, hydrazine, and aluminum chloride. These molecular architectures (ALPHY) show multi‐color (cyan, yellow, and orange) photoluminescence in solution and in the solid‐state, which depends on the substituents of the ligands. The photoluminescence quantum yields of helicates were up to 54 %. The right‐handed (P) and left‐handed (M) helicates are so stable that they do not undergo racemization in some solvents and are mirror images according to circular dichroism and circularly polarized luminescence (CPL) with an absolute luminescence dissymmetry factor (glum) of up to the 10−3 order. Mixing the different helicates produces white‐light emission with CPL characters. This study offers a glimpse into the potential applications of chromophores with diverse photophysical properties.

Solution-Processed Dendrimer-Based TADF Materials for Deep-Red OLEDs

We report the first example of a thermally activated delayed fluorescence (TADF) poly(dendrimer), composed of a norbornenyl-derived polymer backbone and dendritic side-chain chromophores comprising 2,3-dicyanopyrazino as the electron acceptor and a first-generation fluorenylcarbazole derivative as the electron donor. The TADF poly(dendrimer) homopolymer, with one dendritic side chain attached to each monomer unit, emitted deep-red light. The emission of the poly(dendrimer) was found to be red-shifted relative to the nonpolymeric doubly dendronized emitter composed of the same components. The simple dendrimer was found to have a solution photoluminescence quantum yield (PLQY) of around 70%. In contrast, the poly(dendrimer) had a PLQY of 9%, which was attributed to intramolecular interchromophore interactions. An interesting feature of the poly(dendrimer) was that oxygen did not quench the TADF emission. We found that the PLQY of the simple dendrimer decreased markedly in neat films, whereas that of the poly(dendrimer) did not, with both having a solid-state PLQY of around 10%. The results suggest that intrapolymer chromophore-chromophore interactions observed in solution for the poly(dendrimer) were similar to the intermolecular chromophore-chromophore interactions of the dendrimer in the solid state. Simple two-layer organic light-emitting diodes comprising nondoped films of the materials and an electron transport layer showed red emission with CIE coordinates of (x > 0.66, y < 0.34). The dendrimer-based device had a maximum external quantum efficiency of 2.4%, which is among the best for solution-processed deep-red emissive TADF-based OLEDs but in a simpler device architecture.

Effect of protonation on the photophysical properties of 4-substituted and 4,7-disubstituted quinazoline push-pull chromophores

White-light emission from single molecular systems has attracted a great deal of attention due to their advantages over multicomponent emitters. Azaheterocyclic push-pull derivatives have been demonstrated to be white emitters by combining neutral and protonated forms in the appropriate ratio, although limited cases of white-light emission have been reported from quinazoline derivatives. Herein, we describe a series of push-pull 4-substituted and 4,7-disubstituted quinazolines that show white photoluminescence both in solution and in the solid state. All of the materials were prepared by straightforward Suzuki–Miyaura cross-coupling reactions and the compounds exhibited remarkable emission solvatochromism. In some cases the presence of acid prompted the appearance of emission bands of complementary colors. Thus, multicolor photoluminescence, including white light, could be finely tuned by the controlled protonation of the electron-deficient quinazoline ring.

Synthesis and physical property studies of cyclometalated Pt(II) and Pd(II) complexes with tridentate ligands containing pyrazole and pyridine groups

Novel substituted tridentate ligands, which contains a series of pyrazole and pyridine groups as donors, and their cyclometalated complexes have been successfully synthesized. The ligands and complexes were completely characterized by a variety of techniques, such as 1H NMR, UV–Vis, FT-IR, mass spectrometry and elemental analysis. Meanwhile, the single crystal structures of ligand 7 and complex 11 were determined by X-ray crystallography. For ligand 7, due to the effect of delocalization, the C(6)-C(7) bond length in the pyrazole ring is 0.05 Å shorter than a normal C-C bond. After coordinating with the metal centre, the configuration was limited by the coordination sphere, so the bond lengths and bond angles of complex 11 are different from those of ligand 7. Through photoluminescence in acetonitrile solution at ambient temperature, two emission bands have been found between 500 and 600 nm for complexes 9 and 10. Moreover, there are several absorbing bands respectively located at 240 to 250 nm and 300 to 310 nm for ligands 7 and 8 (π(L) → π*(L) and n → π*(L)). A red-shift was observed in the UV spectra, on comparing the complexes with electron-donating and electron-withdrawing groups on the ligand, which is due to the electronic effect of the substituent. One absorbing band is also found at 375 to 425 nm for complexes 9 and 10, and this band is attributed to a MLCT (dπ(Pt) → π*(L)).

Dependence of the performance of light-emitting diodes on the molecular weight of the electroluminescent polymer PFO-MEH-PPV

Controlled synthesis of the electroluminescent polymer PFO-MEH-PPV (poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)]) provided samples of varying molecular weight (Mw) in the range 20–360 kDa, as determined by gel-permeation chromatography and light scattering. The samples were used as the active layers in organic light-emitting diodes (OLEDs), and the performance of the devices was examined as a function of Mw. Turn-on voltages fell in the range 1.92–2.78 V, luminances varied from 231 to 5826 cd/m2, and luminous efficacies ranged from 0.06 to 0.90 lm/W. The emitted colour was found to vary from green to yellow as Mw increases. Optimal performance was attained by using PFO-MEH-PPV with Mw = 100 kDa. To help reveal how Mw determines the performance of OLEDs, relative quantum yields of photoluminescence in solutions and films were measured, and films were characterized by atomic force microscopy and transmission electron microscopy.

Synthesis, Solution and Solid‐State Fluorescence of 2‐(N‐cycloamino)cinchomeronic Dinitrile Derivatives

AbstractA facile approach to the synthesis of novel 2‐(N‐cycloamino)cinchomeronic dinitriles showing intense photoluminescence both in solution and solid state was developed. Absorption, fluorescence and solvatochromic properties were investigated. It was found that the 2‐(N‐cycloamino)cinchomeronic dinitrile derivatives containing conjugated aryl moiety show intense emission in nonpolar solvents with fluorescence quantum yield up to 0.8.

Columnar mesomorphism from a new luminescent thiazolo[5,4-d]thiazole-based core

ABSTRACT Two new mesogenic compounds having a bicyclic aromatic thiazolo[5,4-d]thiazole linked to four 1,3,4-oxadiazol heterocycles were synthesised and their liquid-crystalline properties were investigated. The compounds exhibit enantiotropic hexagonal columnar mesophase with strong homeotropic orientation induced under confinement, and a wide mesophase range. The molecular packing is preserved at room temperature, especially the π-stacking inside the columns, which is essential for good charge carrier transport. They show high thermal stability, blue photoluminescence in solution, and blue-green in thin film. The morphology of spin-coated films is smooth even after thermal annealing, where large molecular-ordered domains were induced, but preserving the photoluminescence quantum yield. All characteristics suggest that this new class of mesogenic molecules is promising for technological applications.

Visible-Light-Excited Room-Temperature Phosphorescence in Water by Cucurbit[8]uril-Mediated Supramolecular Assembly.

Solid-state materials with efficient room-temperature phosphorescence (RTP) emissions have found widespread applications in materials science, while liquid or solution-phase pure organic RTP emission systems has been rarely reported, because of the nonradiative decay and quenchers from the liquid medium. Reported here is the first example of visible-light-excited pure organic RTP in aqueous solution by using a supramolecular host-guest assembly strategy. The unique cucurbit[8]uril-mediated quaternary stacking structure allows tunable photoluminescence and visible-light excitation, enabling the fabrication of multicolor hydrogels and cell imaging. The present assembly-induced emission approach, as a proof of concept, contributes to the construction of novel metal-free RTP systems with tunable photoluminescence in aqueous solution, providing broad opportunities for further applications in biological imaging, detection, optical sensors, and so forth.

Design of a pillar-layered metal-organic framework as high-performance fluorescence sensor for nitroaromatic compounds

A ternary luminescent cadmium-triazolate-carboxylate framework, [(CH3)2NH2][Cd(1245-BTC)0.5(3S-TRZ)] (SNNU-111, 1245-BTC ​= ​1245-benzenetetra- carboxylic acid, 3S-TRZ ​= ​3-mercaptotriazole) was successfully designed in this work. X-ray single crystal diffraction results show that Cd atoms are connected by 3S-TRZ ligands to form 2D wave-like layers, which are further extended by 1245-BTC linkers to generate the 3D pillar-layered architecture of SNNU-111. Solid-state and solution photoluminescence both showed that the emissions of SNNU-111 was ascribed to the intraligand charge transfer in 1245-BTC. Benefited from this process, SNNU-111 MOF can act as excellent fluorescence sensor to quantitatively detect involved nitroaromatics including nitrobenzene (NB), 1,2-dinitrobenzene (1,2-DNB), 1,3-dinitrobenzene (1,3-DNB), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and 2,4,6-trinitrophenol (PA) through the fluorescence quenching. The corresponding quenching coefficient (Ksv) values are of 1.0 ​× ​104-1.7 ​× ​104 ​M−1, which surpass the performance of many reported sensors. Specially, the Ksv value of 1.2 ​× ​104 ​M−1 for NB is nearly the highest one among all MOF sensors up to now. On the other hand, SNNU-111 sensor also exhibits good sensitivity to nitroaromatics with the detection limits as low as 1.61–5.77 ​ppm. Overall, the definite pillar-layered structure, fast response and high sensitivity make SNNU-111 a promising fluorescent sensor to detect nitroaromatic compounds.

The effect of lead and nickel cations on AgInS2/ZnS nanocrystals photoluminescence in aqueous solution

A sensing fluorescent probe based on the photoluminescence (PL) quenching of the AgInS2 colloidal nanocrystals (NCs) with a wide bandgap ZnS shell was studied for lead ions detection. The results were compared with the work where the influence of nickel ions presence on PL was studied. Addition of lead cations was accompanied by decreasing of PL intensity of the AgInS2/ZnS NCs and redshift of maxima of PL spectra. The quenching effect was described by the Stern–Volmer relationship. The obtained results could be useful for the development of new express-methods for heavy metal detection in aqueous media.

Tuning the Photophysical Properties of Push-Pull Azaheterocyclic Chromophores by Protonation: A Brief Overview of a French-Spanish-Czech Project.

Conjugated push-pull molecules that incorporate nitrogen heterocycles as electron-withdrawing units are interesting materials because of their luminescence properties. These chromophores can be easily and reversibly protonated at the nitrogen atom of the heterocyclic ring and this can cause dramatic color changes. White and multi-color photoluminescence both in solution and in the solid state can be obtained by an accurate control of the amount of acid. Thus, with a suitable design these compounds have potential applications in the development of colorimetric pH sensors and the fabrication of OLEDs based on only one material. We provide here a brief overview of our collaborative efforts made in this area.

Synthesis, photoluminescence, and electroluminescence characterization of double tetraphenylethene‐tethered BODIPY luminogens

Three double tetraphenylethene (TPE)‐tethered 4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indance (BODIPY) fluorophores, 35TPEBODP, 88TPEBODP, and 26TPEBODP, have been synthesized and characterized. The green 35TPEBODP with deep red fluorescence shows serious thermal decomposition in the purification process of sublimation, which prohibits its test for an organic light‐emitting diode (OLED) fabricated by the vacuum–thermal evaporation process. The tethered TPE is attached to BODIPY at three different positions, resulting in different photoluminescence (emission wavelength and quantum yield) and electroluminescence (EL). Different from TPE‐tethered BODIPY fluorophores reported in literature, none of the BODIPY fluorophores studied here exhibits aggregation‐induced emission (AIE), aggregation‐induced enhanced emission (AIEE), or twisted intramolecular charge transfer (TICT) characteristics. Although solution (10−5 M THF) photoluminescence quantum yields (ϕs) are relatively high at 78%, 68%, and 86% for 35TPEBODP, 88TPEBODP, and 26TPEBODP, respectively, which are all higher than 41% of PhBODP (a non‐TPE‐tethered BODIPY), the ϕ is significantly decreased to 1–6% in 5 wt% dopant polystyrene thin film or as a solid powder, except for 13% of 26TPEBODP. Therefore, due to the low ϕ of dopant thin film or solid powder, either dopant or nondopant OLEDs exhibit inferior external quantum efficiency (EQE) and intensity of EL. The best OLED in this study is the 26TPEBODP device, and its EQE reaches 1.3%, and the highest EL intensity is approximately 1,600 cd/m2.

A PHBA-functionalized organic-inorganic hybrid polyoxometalate as a luminescent probe for selectively sensing chromium and calcium in aqueous solution

Detection of trace metal ions in drinking as well as irrigation water is crucial in understanding their roles among living humans and animals. Herein, an organic PHBA-functionalized polyoxometalate (POM) derivative [N(CH3)4]3K2Eu(PHBA)(H2O)2(α-PW11O39)]·7H2O (1) (PHBA = p-hydroxybenzoic acid) was successfully synthesized which acts as a luminescent probe for selective sensing of chromium and calcium in aqueous solution. The aqueous solution photoluminescence properties of 1 were carefully investigated, indicating PHBA ligand can sensitize the luminescence of Eu3+ ion in 1. The time-resolved emission spectroscopy was also performed to authenticate the energy transfer from π-π* transition of PHBA group to Eu3+ ion. The sensing properties of 1 were performed on Cd2+, Cr3+, Ba2+, K+, Li+, Pb2+, Zn2+, Al3+, Ag+, Hg2+, Co2+, Sr2+, Ca2+, Na+, Mg2+ ions in aqueous solution, and the photoluminescence intensity for Cr3+ ion display a quenching phenomenon, whereas, Ca2+ ion displays an enhancement signal in their photophysical behavior. The detection limits for detecting Cr3+ and Ca2+ ion were 1.423 μM and 0.676 mM, respectively. In the detection process of Cr3+ ion, the values of quenching rate constant (Kq) were varied from 4.34 × 106 L (mol s)−1 to 4.29 × 109 L (mol s)−1, which were lower than the value of the maximum scatter collision quenching constant (2.0 × 1010 L (mol s)−1). The results reveal that the quenching mechanism between 1 and Cr3+ ion can be attributed to dynamic collision quenching mechanism. Furthermore, the active sites for sensing Cr3+ and Ca2+ ions also have been investigated, determining the different interaction mechanisms exist in detection of Cr3+ and Ca2+ ions in solution.

Luminescence Tuning of Fluorinated Bistolanes via Electronic or Aggregated-Structure Control

Organic luminescent materials have a wide range of practical applications, but the understanding of the relationship between molecular structure and luminescent behavior is lacking. Herein, we synthesized fluorinated bistolanes with an electron-donating alkoxy substituent at one terminal and an electron-withdrawing substituent at the other to realize systematic control of the electron-density distribution. Evaluation of the phase transition behavior revealed that most of the fluorinated bistolanes showed liquid-crystalline (LC) behavior, with the phase transition temperature depending on the terminal substituents. Additionally, the fluorinated luminophores displayed intense photoluminescence (PL) in solution and in their crystal phases. Remarkably, the PL color shifted dramatically depending on the dipole moment (μ||) along the long molecular axis; thus, PL tuning can be achieved through electronic modulation by precise control of the μ|| of the luminophore. Interestingly, in the LC phases under thermal conditions, the maximum PL band shifted by 0.210 eV upon phase transition from the crystal to smectic A LC phases, indicating that PL tuning can also be achieved by controlling the aggregated structure. These results offer a new molecular design for easily tunable PL materials using the molecular properties or external stimuli for promising applications, including light-emitting displays and PL sensing materials.