Excited State Emission Research Articles

Tunable Multicolor Phosphorescence of Crystalline Polymeric Complex Salts with Metallophilic Backbones.

A total of 35 [Au(NHC)2 ][MX2 ] (NHC=N-heterocyclic carbene; M=Au or Cu; X=halide, cyanide or arylacetylide) complex salts were synthesized by co-precipitation of [Au(NHC)2 ]+ cations and [MX2 ]- anions. These salts contain crystallographically determined polymeric Au⋅⋅⋅Au or Au⋅⋅⋅Cu interactions and are highly phosphorescent with quantum yields up to unity and emission color tunable in the entire visible regions. The nature of the emissive excited states is generally assigned to ligand (anion)-to-ligand (cation) charge-transfer transitions assisted by d10 ⋅⋅⋅d10 metallophilicity. The emission properties can be further tuned by controlled triple-component co-crystallization or by epitaxial growth. Correct recipes for white light-emitting phosphors with quantum yields higher than 70 % have been achieved by screening the combinatorial pool.

Tunable Multicolor Phosphorescence of Crystalline Polymeric Complex Salts with Metallophilic Backbones

AbstractA total of 35 [Au(NHC)2][MX2] (NHC=N‐heterocyclic carbene; M=Au or Cu; X=halide, cyanide or arylacetylide) complex salts were synthesized by co‐precipitation of [Au(NHC)2]+ cations and [MX2]− anions. These salts contain crystallographically determined polymeric Au⋅⋅⋅Au or Au⋅⋅⋅Cu interactions and are highly phosphorescent with quantum yields up to unity and emission color tunable in the entire visible regions. The nature of the emissive excited states is generally assigned to ligand (anion)‐to‐ligand (cation) charge‐transfer transitions assisted by d10⋅⋅⋅d10 metallophilicity. The emission properties can be further tuned by controlled triple‐component co‐crystallization or by epitaxial growth. Correct recipes for white light‐emitting phosphors with quantum yields higher than 70 % have been achieved by screening the combinatorial pool.

Probing Triplet Excited States and Managing Blue Light Emission of Neutral Tetradentate Platinum(II) Complexes.

The structural and photophysical properties of tetradentate Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy), and Pt(czpyOczpy) have been experimentally and theoretically explored. Single-crystal diffraction measurements provided accurate structural information. Electrochemical and photophysical characterizations revealed internal electronic energy levels in ground and excited states. (Time-dependent) Density functional theory calculation revealed electron distributions in transition processes of S0 → S1 and S1 → T1 → S0. Electronic transition study indicated that Pt(ppzOppz) demonstrated mixed MLCT/LC states and Pt(czpyOczpy) showed MLCT-dominated states in S1 and T1. Both Pt(ppzOpopy) and Pt(ppzOczpy) presented strong delocalized spin transition (DST) during intersystem crossing. Upon frame modification of Pt(ppzOczpy), we found that their S1 and T1 can be independently manipulated. These blue emitters showed a tunable and narrow emission band (the narrowest fwhm was 19 nm) with luminescence efficiency as high as 86%. The findings of the DST transition mode in the neutral Pt(II) complexes provide guidance for rational design of novel phosphorescent materials.

Excited-state absorption and fluorescence dynamics of Er3+:KY3F10

We report here on a complete investigation of the excited-state absorption and fluorescence dynamics of Er3+ doped KY3F10 single crystals versus dopant concentrations and optical excitation conditions. Radiative and effective (including non-radiative relaxations) emission lifetimes and branching ratios are determined from a Judd-Ofelt analysis of the absorption spectra and via specific fluorescence experiments using wavelength selective laser excitations. Excited-state absorption and emission spectra are registered within seven spectral domains, i.e. 560 nm, 650 nm, 710 nm, 810 nm, 970 nm, 1550 nm and 2750 nm. A maximum gain cross-section of 0.93 × 10−21 cm2 is determined at the potential laser wavelength of 2.801 μm for a population ratio of 0.48. Saturation of fluorescence intensities and variations of population ratios versus pumping rates are registered and confronted with a rate equation model to derive the rates of the most important up-conversion and cross-relaxation energy transfers occurring at high dopant concentrations.

Quantum Yields, Surface Quenching, and Passivation Efficiency for Ultrasmall Core/Shell Upconverting Nanoparticles.

We synthesized and characterized a set of ultrasmall hexagonal-phase NaGdF4: 20% Yb3+, 2% Er3+ upconversion nanoparticles with core diameters of 3.7 ± 0.5 nm. In order to assess passivation effects and the influence of possible core-shell intermixing and to identify optimum particle structures for combined imaging in the visible and near-infrared (vis-NIR: 410-850 nm) and short-wave infrared (SWIR: 1520 nm), NaYF4 shells of varying thicknesses (monolayer to 10 nm) were introduced and the influence of this parameter on the upconversion and downshifting photoluminescence of these particles was studied at different excitation power densities. This included excitation power-dependent emission spectra, slope factors, quantum yields, and excited state decay kinetics. These measurements revealed enhancement factors of the upconversion quantum yield of >10 000 in the low power region and an excitation power density-independent quantum yield of the downshifted emission at 1520 nm between 0.1 and 14%. The optimized shell thickness for combined vis and SWIR imaging was identified as 5 nm. Moreover, lifetimes and quantum yields can be continuously tuned by shell thickness which can be exploited for lifetime multiplexing and encoding. The fact that we did not observe a saturation of the upconversion quantum yield or the excited state decay kinetics with increasing shell thickness is ascribed to a strong intermixing of the active core with the inert shell during the shelling procedure. This indicates the potential of spectroscopic tools to detect cation intermixing.

Circular Polarized Luminescence of Hydrogen-Bonded Molecular Assemblies of Chiral Pyrene Derivatives

The absorption and fluorescence spectra of chiral alkylamide-substituted pyrene derivatives (R-1 and S-1) in the solution phase were consistent with the formation of N–H···O═ hydrogen-bonded helical π-stacked one-dimensional (1D) supramolecules (R-1)n and (S-1)n in methylcyclohexane (MCH), toluene, chloroform (CHCl3), and tetrahydrofuran (THF); the π-stacked structures and aggregation number (n) were governed by the concentration (c) and solvent polarity. The aggregation of (R-1)n and (S-1)n in MCH and toluene was much greater than that in THF and CHCl3, and excimer emission of excited-state (R-1)n* and (S-1)n* supramolecules with n ≥ 2 was observed in all solvents. Interestingly, the circular polarized luminescence (CPL) spectra of (R-1)n* and (S-1)n* revealed a glum value of 0.03 in MCH at c > 1 × 10–6 M. Circular dichroism and CPL spectra revealed the formation of hydrogen-bonded helical 1D supramolecular assemblies in the ground and excited states. The helicities of the 1D supramolecular assemblies of...

Pyrenylpyrazole-based donor/acceptor fluorescent dyes: Synthesis and photophysical properties

The reaction of 1-acetylpyrene phenylhydrazone with Vilsmeier reagent (DMF-POCl3) afforded 1-phenyl-3-(pyren-1-yl)-1H-pyrazole-4-carbaldehyde in good yield. Condensation of this compound with a series of active methylene compounds (methyl ethyl ketone, diethyl ketone, malononitrile, 1,3-indandione and N,N-diethythiolbarbituric acid) gave a series of donor/acceptor dyes exhibiting fluorescence in the almost whole range of the visible spectrum. The electronic structure of the dyes was studied theoretically by the (TD) DFT method. Calculations elucidated the nature of intramolecular charge transfer (ICT) processes leading to emissive excited states of the synthesised compounds. It turned out that the electron acceptor ability influences the direction of ICT. For compounds with relatively weak acceptor groups, ICT from the pyrene moiety toward the remaining part of the molecule prevailed, whereas for those having stronger acceptor groups the main ICT pathway became that from the phenylpyrazole group to the acceptor group.

The Extraordinary Outburst in the Massive Protostellar System NGC 6334I-MM1: Emergence of Strong 6.7 GHz Methanol Masers

Abstract We report the first sub-arcsecond VLA imaging of 6 GHz continuum, methanol maser, and excited-state hydroxyl maser emission toward the massive protostellar cluster NGC 6334I following the recent 2015 outburst in (sub)millimeter continuum toward MM1, the strongest (sub)millimeter source in the protocluster. In addition to detections toward the previously known 6.7 GHz Class II methanol maser sites in the hot core MM2 and the UCHII region MM3 (NGC 6334F), we find new maser features toward several components of MM1, along with weaker features ∼1″ north, west, and southwest of MM1, and toward the nonthermal radio continuum source CM2. None of these areas have heretofore exhibited Class II methanol maser emission in three decades of observations. The strongest MM1 masers trace a dust cavity, while no masers are seen toward the strongest dust sources MM1A, 1B, and 1D. The locations of the masers are consistent with a combination of increased radiative pumping due to elevated dust grain temperature following the outburst, the presence of infrared photon propagation cavities, and the presence of high methanol column densities as indicated by ALMA images of thermal transitions. The nonthermal radio emission source CM2 (2″ north of MM1) also exhibits new maser emission from the excited 6.035 and 6.030 GHz OH lines. Using the Zeeman effect, we measure a line-of-sight magnetic field of +0.5 to +3.7 mG toward CM2. In agreement with previous studies, we also detect numerous methanol and excited OH maser spots toward the UCHII region MM3, with predominantly negative line-of-sight magnetic field strengths of −2 to −5 mG and an intriguing south–north field reversal.

Engineering steady Knill-Laflamme-Milburn state of Rydberg atoms by dissipation.

The Knill-Laflamme-Milburn (KLM) states have been proved to be a useful resource for quantum information processing [Nature409, 46 (2001)]. For atomic KLM states, several schemes have been put forward based on the time-dependent unitary dynamics, but the dissipative generation of these states has not been reported. This work discusses the possibility for creating different forms of bipartite KLM states in neutral atom system, where the spontaneous emission of excited Rydberg states, combined with the Rydberg antiblockade mechanism, is actively exploited to engineer a steady KLM state from an arbitrary initial state. The numerical simulation of the master equation signifies that a fidelity above 99% is available with the current experimental parameters.

Effectual binding of gallic acid with p-sulfonatocalix[4]arene: An experimental and theoretical interpretation

The host-guest interaction of gallic acid (GA) with p-sulfonatocalix[4]arene (p-SC4) is studied using emission and excited state lifetime techniques. The quenching effect on the emission intensity and excited state lifetime is observed upon binding. The impact of oxidation potential upon binding is studied using cyclic voltammetric technique. The structural features and the mode of binding of GA with p-SC4 is examined using 1H NMR and rotating frame overhauser effect spectroscopy (ROESY) techniques. The binding of GA with p-SC4 has also been examined by means of density functional theory simulations. The calculated interaction energy of GA with p-SC4 (22.15kcal/mol) indicates the strong binding nature.

The 6-GHz Multibeam Maser Survey – III. Comparison between the MMB and HOPS

We have compared the occurrence of 6.7-GHz and 12.2-GHz methanol masers with 22-GHz water masers and 6035-MHz excited-state OH masers in the 100 square degree region of the southern Galactic plane common to the Methanol Multibeam (MMB) and H2O southern Galactic Plane surveys (HOPS). We find the most populous star formation species to be 6.7-GHz methanol, followed by water, then 12.2-GHz and, finally, excited-state OH masers. We present association statistics, flux density (and luminosity where appropriate) and velocity range distributions across the largest, fully surveyed portion of the Galactic plane for four of the most common types of masers found in the vicinity of star formation regions. Comparison of the occurrence of the four maser types with far-infrared dust temperatures shows that sources exhibiting excited-state OH maser emission are warmer than sources showing any of the other three maser types. We further find that sources exhibiting both 6.7-GHz and 12.2-GHz methanol masers are warmer than sources exhibiting just 6.7-GHz methanol maser emission. These findings are consistent with previously made suggestions that both OH and 12.2-GHz methanol masers generally trace a later stage of star formation compared to other common maser types.

Electrochemiluminescence of nitrogen- and sulfur-doped graphene quantum dots

Nitrogen- and sulfur-doped graphene quantum dots (NS-GQDs) have been synthesized via one-pot pyrolysis approach using citric acid and l-Cysteine as precursors. The obtained NS-GQDs have a size around 2.8 ± 1.0 nm. The fluorescence quantum yield of NS-GQDs was calculated to be 84 times that of Ru(bpy)3Cl2, which is 9.3-fold higher than that of undoped GQDs. Electrochemiluminescence (ECL) of NS-GQDs was found to be robust with K2S2O8 as a coreactant and an ECL efficiency of 32% relative to the Ru(bpy)3Cl2/K2S2O8 was reached, which is also 5.8-fold enhanced when compared with the same system of undoped GQDs. By means of spooling ECL spectroscopy, a potential-dependent ECL emission was discovered in the ECL processes of both NS-GQDs and GQDs. Scanning the applied potential to different values changed actually the driving force, and thus various emissive excited states were produced, leading to variable emission wavelengths. Importantly, because much more surface capturing centers of excitons for NS-GQDs than GQDs created by nitrogen and sulfur doping effect, the NS-GQDs can capture more holes from SO4− and thereby give higher ECL efficiency. The ECL and electron transfer mechanisms proposed here provide insight into development of new GQD for light emitting devices and bioanalytical applications.

Synthesis and photophysical properties of bis(phenylpyridine) iridium(III) dicyanide complexes

Our research group is focusing on luminescence of coordination compounds which based on cyanide bridging interactions. Recently, four cyanometallate complexes based on Ir(III) complex [Ir(ppy)2(CN)2]-, which is one of an extensive family of Ir(III) complexes based on cyclometallating phenylpyridine ligands that have achieved prominence for their desirable photophysical properties, were successfully synthesised. The present study showed that, the introduction of fluorine atoms to the phenyl rings could enhance the triplet excited state energy and thus caused a blue-shift phosphorescence peak (λem = 453 nm) as shown by (PPN)[Ir(2,4-F2ppy)2(CN)2] (1) compared to that of complex without fluorine atom (PPN)[Ir(ppy)2(CN)2] (4) (λem=474 nm). However, (PPN)[Ir(3,5-F2ppy)2(CN)2] (2) shows a negligible hypsochromic shift (λem = 473 nm) in the emission. Conversely, (PPN)[Ir(4-CF3ppy)2(CN)2] (3) shows a red-shift (λem = 479 nm) compared to complex (4). These complexes, like [Ru(bipy)(CN)4]2−, have a strong emissive and high energy excited state and thus, are potentially good energy donors; and are anionic with cyanide groups, and so can form cyanide-bridged complexes with other metal and lanthanide cations.

High-efficiency dimer fluorescence system based on π-π interaction between anthracenes: Recognition of excimer

Excimer is a class of dimeric excited-state species, which is often formed by association of an electronically excited molecule with the same one in the ground state. In the excited state, the intermolecular interactions are attractive while they are repulsive in the ground state. Therefore, as typical spectral characteristics, excimer usually exhibits red-shifted, structureless and broadened emission spectrum relative to that of monomer. Since Forster and Kasper firstly found the pyrene excimer fluorescence in hexane solution in 1954, aromatic excimer formation has received tremendous attention because of its many potential applications in organic light-emitting diodes (OLEDs), chemo-sensors, bio-probes, high-resolution bio-imaging, organic lasers and so on. Although the spectral characteristics of excimer in rigid media (crystals, glassy matrixes, supramolecular capsules, etc.) are similar to those of excimer in fluid or gas media, the definition of excimer in rigid media is not supported by that in fluid or gas media. Therefore, we prefer to name them as dimer emission as a result of the pre-associated π-π dimer of ground state in solid state. Generally, in the solid state of planar and rigid aromatic molecules, the strong intermolecular π-π interactions tend to quench the solid-state luminescence, which commonly causes very low efficiency of solids. As a solution, the chemical modification with bulky substituents or physical doping was usually adopted to suppress the intermolecular π-π stacking and achieve the high-efficiency single-molecule luminescence. Interestingly, we have found a class of dimer fluorescence in crystal with high efficiency and long lifetime, which is based on intermolecular π-π interaction between two anthracenes in ground state. The emission spectra of these crystals are characterized by those of excimer. Experimental evidences and theoretical calculations revealed the essences for high-efficiency luminescence: on the one hand, the constringent interplanar π-π distance of dimer excited state reinforces the geometric rigidity and significantly decreases the non- radiative energy decay; on the other hand, the monodisperse dimeric π-π stacks produce the pure and uniform dimer emissive excited state to avoid the formation of energy-trapping dark state, which effectively suppresses the non-radiative energy transfer. The explanation of high-efficiency dimer fluorescence has been also evidenced by the further experimental data, i.e., the more anthracene dimers in solids, the higher fluorescence efficiency. Furthermore, based on the relation between molecular conformation and stacked structure in crystal, molecular design strategy of high- efficiency dimer fluorescence is put forward to successfully construct the monodisperse dimeric π-π stacks in solids. The molecular structure should contain two moieties, i.e., π-conjugated group and substituent group, and the spatial position of substituent group leans to the one side of π plane. When two π-conjugated units stack in dimeric form, the substituent groups play a role in isolating every dimer which makes the dimer monodisperse in crystal structure. Intermolecular dimer belongs to the simplest supermolecular system, and is significant for the study on supermolecular luminescence materials. Besides, this review concludes the latest progresses of dimer fluorescence, existent issues and application prospects of supermolecular luminescence materials. Expectedly, new-generation high-efficiency dimer and supermolecular luminescence materials could be developed on the basis of new concepts, new principles and new aplications, as well as the structure-property relationship could be established among molecular (stacking) structure, excited state property and luminous performance.

Elucidating the Excited State of Mechanoluminescence in Organic Luminogens with Room-Temperature Phosphorescence.

Two stable, purely organic luminogens exhibit both mechano- (ML) and photoluminescence (PL) with dual fluorescence-phosphorescence emissions at room temperature. Careful analysis of the crystal structures, coupled with theoretical calculations, demonstrate that room-temperature phosphorescence and ML properties are strongly related to molecular packing. In particular, the formation and fracture of molecular dimers with intermolecular charge-transfer properties has a significant effect on intersystem crossing, as well as excited triplet state emissions, in both PL and ML processes.

Gold Corroles as Near-IR Phosphors for Oxygen Sensing.

The triplet state of gold(III) corroles is exploited for optical oxygen sensing. We report intense phosphorescence for gold(III) corroles in the near-IR, an optical window that is ideal for tissue transparency. Moreover, the triplet excited-state emission exhibits significant changes in intensity and lifetime over the 0-160 Torr O2 pressure range. This renders these compounds sensitive at biologically relevant pressures and overcomes the spectral limitations of palladium and platinum porphyrins for oxygen sensing in biology.

Fluorescent 1-Arylidene-1,3-dihydroisobenzofuran: Ligand-Free Palladium Nanoparticles, Catalyzed Domino Synthesis and Photophysical Studies

The present work describes synthesis and photophysical studies of a class of fluorophores containing biologically relevant 1,3-dihydroisobenzofuran scaffold. Ligand-free palladium nanoparticles have been utilized for domino synthesis of 1-arylidene-1,3-dihydroisobenzofurans from 2-iodobenzyl alcohol and terminal arylacetylenes under mild reaction conditions. Reaction of terminal alkylacetylenes with 2-iodobenzyl alcohol however, provided only the Sonogashira products (2-(alk-1-ynyl)phenyl)methanols. The 1-arylidene-1,3-dihydroisobenzofuran derivatives showed structured fluorescence spectra in non-polar cyclohexane, originated from a locally excited state and structureless emission in polar acetonitrile involving intramolecular charge transfer process. The vibronic structures in the locally excited emission were confined to the ring C=C stretching of the dyes. A detailed structure-property relationship of the 1-arylidene-1,3-dihydroisobenzofuran dyes is presented in this study.

Direct Observation of a Triplet-State Absorption-Emission Conversion in a Fullerene-Functionalized Pt(II) Metallacycle

An interesting triplet excited-state absorption-emission conversion of a fullerene-functionalized Pt(II) metallacycle (C 60 –Pt) caused by a concentration effect was directly observed by nanosecond transient absorption (ns TA) spectroscopy. In dilute solution, the triplet excited-state absorption (TESA) band was observed at about 750 nm with a lifetime of ca. 10.7 μs. However, with increasing the concentration, the absorption band converted to a triplet excited-state emission (TESE) band with a longer lifetime of ca. 15.4 μs. Femtosecond transient absorption experiments and quantum chemistry calculations were performed to reveal the excited-state decay pathways of C 60 –Pt in concentrated solution. This conversion was ascribed to the formation of a triplet excimer, which forms at localized 3C60 * states. This work demonstrates that radiative excimers with longer-lived triplet excited states can exist in concentrated solution, and this finding will provide useful information for applications of fullerene c...

A combined spectroscopic and TDDFT investigation of the solute-solvent interactions of two coumarin derivatives.

The UV/Vis absorption and fluorescence characteristics of 3-cyano-7-hydroxycoumarin [CHC] and 7-amino-4-methyl-3-coumarinylacetic acid [AMCA-H] were studied at room temperature in several neat solvents and binary solvent mixtures of 1,4-dioxane/acetonitrile. The effects of solvent on the spectral properties are analyzed using single and multi-parameter solvent polarity scales. Both general solute/solvent interactions and hydrogen bond interactions are operative in these systems. The solvation of CHC and AMCA-H dyes in 1,4-dioxane/acetonitrile solvent mixtures has been studied. The solutes CHC and AMCA-H are preferentially solvated by acetonitrile and a synergistic effect is observed for both molecules in dioxane/acetonitrile solvent mixtures. In addition, using the solvatochromic method the ground- and the excited-state dipole moments of both the dyes were calculated. The ground- and excited-state dipole moments, absorption and emission maxima and HOMO-LUMO gap were also estimated theoretically using B3LYP/6-311+ G (d,p) level of theory in the gaseous phase, dioxane and acetonitrile solvents. Furthermore, changes in dipole moment values were also calculated using the variation of Stokes shift with the molecular-microscopic empirical solvent polarity parameter ( ETN). The observed excited-state dipole moments are larger than their ground-state counterparts, indicating a substantial redistribution of the electron densities in a more dipolar excited state for both coumarins investigated.

Effects of spontaneous emission excited state lifetime on the output performance of quantum well lasers

In this paper, we have investigated the effect of spontaneous emission’s excited state lifetime on the output performance of the quantum well laser using quantum well transmission line modelling technique. It has been found that as the spontaneous emission lifetime increases the turn-on transient response of the quantum well laser is delayed, exhibiting the larger peak and steady state arrival times and the laser steady output has larger noise. Meanwhile, the increase in spontaneous emission lifetime leads to the larger peak and steady state arrival times of the carrier density variation during the turn-on process. Also, due to the increase of the spontaneous emission lifetime, the bandwidth and magnitude of the laser steady output spectra increase while the side lobe suppression ratio decreases and there exists a blue shift in the peak wavelength. These studies confirm that the quantum well laser with less spontaneous emission lifetime has more superior output performance (fast transient response, less noise and narrow bandwidth); they also provide an effective method to compare and predict the spontaneous emission lifetimes for different semiconductor optical materials.