Low-energy Emission Band Research Articles

A new zero-dimensional hybrid antimony halide of (C25H46N)2SbCl5 with dual-emission and high quantum-efficiency for light-emitting application

Zero-dimensional (0D) organic-inorganic hybrid metal halides (OIHMHs) have received extensive attention due to their excellent photoelectric properties. Herein, a new 0D OIHMH single crystal, (C25H46N)2SbCl5, has been synthesized by slow antisolvent diffusion. In this crystal structure, C25H46N+ (C25H46N+ = Cetyldimethylbenzylammonium cations) cations separated SbCl52− to form a 0D square pyramid structure. (C25H46N)2SbCl5 single crystals (SCs) manifest bright orange-yellow dual-emission bands at 472 and 616 nm, while having a high photoluminescent quantum yield of 97.5 %. The lifetime of high energy (HE) emission and low energy (LE) emission at room temperature is 9.73 ns and 4.61 μs, respectively. Based on lifetime differences and temperature-dependent spectra, the HE emission band is ascribed to singlet self-trapped excitons (STEs), while the LE emission band with a broad full width at half maxima of 136 nm can be attributed to triplet STEs. (C25H46N)2SbCl5 appeared anti-thermal quenching behavior. In accordance with the luminescence characteristics of (C25H46N)2SbCl5, a warm white light-emitting diode device was fabricated with the correlated color temperature of 3410 K, the relevant color coordinate of (0.41, 0.39) and the color rendering index of 95.1, demonstrating (C25H46N)2SbCl5 a promising phosphor for solid-state light-emitting application.

Tuning the Emission of a Nonconventional Aggregation-Induced Emission Polymer via Silicon-Bridged Twisted Intramolecular Charge Transfer for Targeted Delivery and Visualized Drug Release.

The design of tunable luminescent biomaterials with large Stokes shifts is usually pursued by a twisted intramolecular charge transfer (TICT) effect with switchable emission colors in response to various external stimuli. However, such a strategy is usually realized in conjugated molecules containing benzene or its derivatives and consequently suffers from poor biocompatibility. In this work, a hyperbranched polysiloxane (HBPSi)-based non-conjugated fluorescent polymer with TICT and aggregation-induced emission (AIE) features is developed, and its luminescent properties, fluorescence mechanism, and potential applications are investigated. Initially, the non-conjugated HBPSi exhibits remarkable AIE characteristics due to the formation of through-space conjugation. With the introduction of the sulfur atom, a non-conjugated D-A type AIE material, HBPSi-Cys, that exhibits a dual-state emission with a large Stokes shift of 213 nm, is obtained. The correlation of the lower-energy emission band with solvent polarity suggests the existence of the TICT state. TICT and AIE characteristics direct different properties of HBPSi-Cys, with TICT regulating solvatochromic emission wavelengths and AIE manipulating the emission intensity with a compensation effect. Density functional theory calculations reveal that the non-conjugated D-A structure in HBPSi-Cys was formed across the silicon bridge, with auxochromic sulfhydryl groups and adjacent amide groups as acceptor units and amine and hydroxyl groups as donor units. Additionally, the AIE-active HBPSi could be utilized as a fluorescent probe for the analysis of metal ions. After grafting the AS1411 aptamer to HBPSi-Cys as the recognition motif, HBPSi-Apt possesses excellent targeted bioimaging, drug loading, pH/GSH dual-responsive drug release, and visualized drug delivery performance. This work provides a new way to design functional AIE polymers with tunable optical properties, and the synthesized HBPSi-Cys shows great potential as a smart fluorescent biomaterial.

Intramolecular dimer formation reveals anomalous solvation within fluorous solvent perfluorodecalin.

Due to the unusual properties of fluorine, a fluorous solvent obtained by replacing hydrogen atoms with fluorine atoms in a hydrocarbon solvent may afford unusual solute solvation and aggregation. A fluorescent dipyrenyl probe, 1,3-bis(1-pyrenyl)propane (BPP), was utilized to explore intramolecular aggregation within perfluorodecalin (PFD), a prototypical fluorous solvent. Steady-state fluorescence emission and excitation along with excited-state emission intensity decay of BPP were acquired in the temperature range of 293.15 to 333.15 K in fluorous solvent PFD and n-hexane-added PFD. The probe BPP is known to form an intramolecular excimer in common hydrocarbon solvents, which is characterized by a broad structureless low-energy emission band; the formation of intramolecular excimers exclusively in the excited state was confirmed via excitation scans along with the intensity decay of the monomer and excimer, respectively. While intramolecular dimerization of BPP does occur in PFD, emission wavelength-dependent excitation spectra and fits of the intensity decay data reveal that intramolecular aggregation takes place in the ground state as well. This ground-state heterogeneity is reduced as the temperature is increased. In the presence of as small as 0.1 mole fraction of n-hexane, the ground-state aggregation vanishes, and intramolecular dimerization again takes place exclusively in the excited state. The data hint at the poor solvation of BPP in PFD to be responsible for intramolecular ground-state aggregation. In PFD-rich (PFD + n-hexane) mixtures, probe pyrene (Py) was found to be preferentially solvated by n-hexane, further corroborating the above-mentioned proposition. Clear differences in solute solvation within a fluorous solvent as compared to hydrocarbon solvents leading to unexpected solubilization and aggregation in fluorous media are amply demonstrated.

Recent Advances in Organic Room-Temperature Phosphorescence of Heteroatom (B/S/P)-Containing Chromophores

Recent Advances in Organic Room-Temperature Phosphorescence of Heteroatom (B/S/P)-Containing Chromophores

Multifunctional Luminescent Sensor Based on the Pb2+ Complex Containing a Tetrazolato Ligand.

A series of luminescent Pb2+ complexes, [Pb(L1)2]n (1), [Pb(L2)2]n (2), [Pb(L3)(NO3)(H2O)2]n (3), [Pb(L3)(Br)(H2O)]n (4), [Pb(L3)(Cl)(H2O)]n (5), and [Pb(L4)(H2O)2] (6) have been synthesized by treatment of polydentate tetrazolato ligands with various hydrated Pb2+ salts (HL1 = 2-(1H-tetrazol-5-yl)pyridine, HL2 = 3-(1H-tetrazol-5-yl)isoquinoline, HL3 = 6-(1H-tetrazol-5-yl)-2,2'-bipyridine, and H2L4 = 6,6'-bis(1H-tetrazol-5-yl)-2,2'-bipyridine). These complexes have been characterized by IR, TGA, and elemental analysis. Their crystal structures have been determined by X-ray crystallography, and the phase purity of bulk samples were further confirmed by PXRD. Their luminescence properties have been investigated in detail, and their emission origin may involve ligand-centered π-π* transition, metal-centered s-p transition and charge-transfer character. It is interesting to note that 5 exhibits obviously enhanced red-shifted emission, whose photoluminescence quantum yield (PLQY = 16.5%) is much higher than the other compounds (≤2%). Most importantly, the emission property of 5 was strongly affected by temperature. When the temperature rises from 295 to 493 K, the emission maximum gradually shifts to high energy due to the loss of the aqua ligand. In contrast, when the temperature is lowered from 295 to 13 K, two emission bands were observed. The low-energy emission band exhibits a slight blue shift, while a new high-energy emission band appears at around 520 nm, which is assigned to ligand-centered phosphorescence. After removal of the coordinated aqua ligand, the emission of 5-H2O is very sensitive to the vapors of volatile primary amines and acids, although they have different response mechanisms. This result indicates that 5-H2O may be a potential multifunctional sensor for temperature, volatile amines, and acids. To decipher the emission origin, DFT calculations have also been carried out based on the structure units of these compounds.

Salen: Insight into the Crystal Structure, Hirshfeld Surface Analysis, Optical Properties, DFT, and Molecular Docking Studies

We report on a known Schiff base dye salen. The crystal structure of salen is in the enol–enol tautomer. Molecules are packed into a 3D supramolecular framework through C–H···π interactions. The absorption spectrum of salen in CH2Cl2 exhibits three bands in the UV region, while the spectrum in MeOH contains an additional band at 403 nm and a shoulder at 280 nm, corresponding to the cis-keto tautomer. The emission spectrum of salen in MeOH exhibits a band at 435 and 457 nm upon irradiation at 280 and 400 nm, respectively, arising from the enol–cis-keto* and/or cis-keto–cis-keto* tautomers. The solution of salen in CH2Cl2 showed dual emission with the bands at 349 and 462 nm upon irradiation at 290 nm with the low-energy emission band arising from the enol–cis-keto* and/or cis-keto–cis-keto* tautomers, while the high-energy band corresponds to the enol–enol* tautomer. The emission spectrum of salen in CH2Cl2 exhibits a single band at 464 nm upon irradiation at 380 nm, arising from the different conformers of the enol–cis-keto* and/or cis-keto–cis-keto* tautomers. The DFT calculations revealed that the enol–enol tautomer is the most favorable, followed by the enol–cis-keto tautomer. The global chemical reactivity descriptors were estimated from the HOMO and LUMO. The DFT calculations were also applied to probe salen as a potential corrosion inhibitor for some important metals used in implants. The enol–cis-keto and enol–trans-keto tautomers exhibit the best electron charge transfer from the molecule to the surface of all the studied metals, of which the most efficient electron charge transfer was established for Ni, Au, and Co. Molecular docking was applied to study interaction of tautomers of salen with a series of the SARS-CoV-2 proteins, of which the best binding affinity was found toward nsp14 (N7-MTase).

Novel sterically demanding Schiff base dyes: An insight from experimental and theoretical calculations

We report extensive experimental and theoretical studies on structural and photophysical properties of two new bulky Schiff base dyes 1 and 2, obtained from the reaction of cyclohexylamine with the 5-bromosalicylaldehyde or 2-hydroxy-1-naphthaldehyde. The structure of 1 exhibits the enol-imine form in the solid state and is characterized by a typical intramolecular O–H⋯N hydrogen bond. Contrarily, the structure of 2 was found to be in the cis-keto-enamine tautomer, stabilized by an intramolecular N–H⋯O hydrogen bond. Favoured intermolecular H⋯H, H⋯C, H⋯O and H⋯Br contacts in the structure of 1 and H⋯H, H⋯C and H⋯O in the structure of 2 are responsible for the overall crystal packing of the dyes. The UV–vis spectra of 1 in cyclohexane, THF, CH2Cl2 and CH3CN exhibit bands in the UV region, indicating the enol-imine form, while in MeOH the UV–vis spectrum of 1 exhibits an additional band in the visible region at 410 nm, which corresponds to the cis-keto-enamine tautomer. The absorption spectra of 2 in the same solvents, except cyclohexane, also contain a band in the visible region up to about 450 nm. This band exhibits two maxima, which can be assigned to an equilibrium between the cis-keto-enamine and zwitterion forms, of which the latter one is structurally the same as the former isomer. The UV–vis spectrum of 2 in cyclohexane reveals only traces of the cis-keto-enamine. The dye 1 was found to be emissive exclusively in MeOH, while 2 is emissive in all the studied solvents, of which the most pronounced and the most negligible emission was found for MeOH and cyclohexane, respectively. An intense emission band in the spectrum of 1 at 468 nm arises from the emission of two conformers of the cis-keto-enamine* form. The solution of 2 in MeOH showed dual emission with the bands centered at 346 and about 450 nm, of which the latter one is a superposition of two single bands. The two low-energy emission bands arise from the emission of the cis-keto-enamine* and zwitterion* forms, while the high-energy emission band most likely corresponds to the naphthalene fragment. Finally, π-resonanse stabilization (RAHB) effect is found to be crucial for tuning the tautomerization mechanism through the alteration of intramolecular O–H⋯N hydrogen bond.

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

Photoluminescent carbon nanodots (CDs), a new class of inorganic nanocarbon materials, have become a significant segments of luminescent nanomaterials in last decade. However mechanism of light emission and detailed identification of the light emitting centers is still debated. A huge number of synthesis methods were suggested, however few of them allow to monitor formation of the CDs on the early stage and development in progress. Recently we have found that zinc acetylacetonat (Zn(acac)2) in ethanol solutions tends to be hydrolysed under room temperature laboratory conditions resulting in gradual conversion of colorless and non luminescent solution to yellowish liquid accompanied by cedimentation of nonluminescent amorphous ZnO:OH. The yellow component of the solution has been identified as dispersion of carbon precipitates. The most important is that the process of hydrolysis in very slow with changing color and PL development within several months. Such a slow process allows following the details of PL development and evolution in time correlation with carbon precipitation.Ethanol solutions of Zn(acac)2 with concentrations in range of 0.1-4.0 % were prepared and their photoluminescence was monitored for time period of several months. It has been found that early stage of carbon precipitation is resulted in development of relatively narrow PL band with maximum at about 400 nm and corresponding excitation band at 360 nm. The spectrum of the emission band is excitation independent as well as excitation band is not dependent on emission wavelength. In the course of further aging and carbon precipitation the PL band is broaden and shifted to visible range with maximum intensity in range of 420-500 nm depending on excitation wavelength. Detailed emission/excitation study revealed that the visible emission band is composed of at list two components centered at about 420-450 nm and 500 nm, with the corresponding excitation bands at 365 nm and 420 nm. Time-resolved photoluminescence spectroscopy showed that the decay of low energy emission band follows the classical mono-exponential low with the characteristic time of several nanoseconds, while the behavior of high energy component can be described by the stretched exponential low with the average decay time of several nanoseconds. PL data is summarized and discussed in the terms of subnanometer molecular-like carbon emitters and core/shell structural model of amorphous carbon nanoparticles.

Wavelength-Resolved Photoluminescence and Cathodoluminescence Decay Times of LSO:Ce Scintillator CO-Doped with Lithium and Scandium

We study the luminescence decay times of Ce3+: Lu2SiO5 scintillator, co-doped with Sc3+ and Li+ ions, under the direct photoexcitation of Ce3+ ions at 337 nm and excitation by a pulsed electron beam with energies of 100 – 300 keV. The cathodoluminescence (CL) and photoluminescence (PL) decay times of studied sample are equal within the experimental error. Two emission bands are found: in the range of 370 – 480 nm (“high-energy band”) and 500 – 570 nm (“low-energy band”). The highenergy emission band has the decay times ∼40 ns. The low-energy emission band has the maximum decay time ∼60 ns at 545 nm. The observed results show that the transport stage of Li:Sc:Ce:Lu2SiO5 scintillation mechanism is not affected by a high negative space charge and high density of electronic excitations provided by the high-power electron beam.

Dipolar state assisted aggregation induced optical behavior of push-pull Salen-type Schiff base (BIHyDE) in solution

Aggregation behavior of push-pull salen-type schiff base (BIHyDE) in different solvents at ground and excited state, has been investigated. At ground state, lower energy absorption band is suppressed during concentration dependent UV-Vis study in CHCl3 solution. The broadening of lower energy emission band is observed during concentration dependent fluorescent study in CHCl3 solution. Interestingly, fluorescence life time of BIHyDE in solution is also altered during concentration dependent fluorescence study in CHCl3 solvent. Similar aggregation is also observed at ground and excited state in ACN solvent. Dipolar state and intermolecular dipolar interaction of BIHyDE molecule highly influence towards strong aggregation process and orientation of molecule in solution. Ground state and excited state optical properties are highly influenced by concentration of push-pull salen-type schiff base (BIHyDE) in different solvents.

Substituent-Mediated Transformation of Polynuclear Gold(I)-Sulfido Complexes—From Pentanuclear to Octadecanuclear Cluster-to-Cluster Transformation

Substituent-Mediated Transformation of Polynuclear Gold(I)-Sulfido Complexes—From Pentanuclear to Octadecanuclear Cluster-to-Cluster Transformation

Influence of substituents in 1,10-phenanthroline on the structural and photophysical properties of W(CO)4(1,10-phenanthroline-type) complexes

In this work, methyl and phenyl substituents were introduced in the 2,9- and 4,7-positions of the phen ligand in order to study its impact on the structural and photophysical properties of [W(CO)4(phen-type)] complexes [phen-type = 4,7-dimethyl-1,10-phenanthroline (4,7-DMPhen); 4,7-diphenyl-1,10-phenanthroline (BPhen), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)]. Crystallographic analysis of these complexes allows to investigate the correlation between the geometrical parameters with their photophysical properties measured by steady-state and time-resolved spectroscopies. The (OC)eq–W–(CO)eq angle is the most significantly influenced geometrical parameter by the substituents, showing values of 92.5°, 90.2°, and 81.2° for [W(CO)4(4,7-DMPhen)], [W(CO)4(BPhen)], and [W(CO)4(BCP)], respectively. The smallest angle observed for [W(CO)4(BCP)] is attributed to a steric repulsion exerted by the methyl groups to the equatorial carbonyls. Remarkably, a smaller OCeq–W–COeq angle favors the High-Energy (HE) emission band, as evidenced in the photoluminescence (PL) spectra of [W(CO)4(BCP)]. Conversely, a greater angle favors the Low-Energy (LE) emission band, as observed in the PL spectra of [W(CO)4(4,7-DMPhen)] and [W(CO)4(BPhen)]. The PL lifetime of the LE emission band becomes shorter when decreasing the (OC)eq–W–(CO)eq angle. Furthermore, the absorption feature was affected by the 2,9-dimethyl substituents, showing greater contribution from the high-energy shoulder on the visible absorption band. This study shows that the perturbation of the (OC)eq–W–(CO)eq angle induced by the substituents in the phen ligand influences on the photophysical properties of [W(CO)4(phen-type)] complexes.

Synthesis of luminescent rhodium(III) cyclometalated complex by sp2(C)–S bond activation: Application as catalyst in transfer hydrogenation of ketones and live cell imaging

A new fluorescent Rh(III) cyclometalated complex, [Rh(PPh3)2(L)Cl] (1) is synthesized via sp2(C)–S bond activation of a thioether containing azo-phenol ligand (L-SCH2CH3). The pseudo octahedral geometry around rhodium is confirmed by single crystal X-ray diffraction method. Cyclic voltammogram of the complex exhibits a quasi-reversible oxidation couple with E1/2 of 0.74 V (ΔE = 100 mV) along with a quasi-reversible reduction couple (E1/2 = −1.18 V, ΔE = 130 mV) in acetonitrile. The complex exhibits low energy emission band at 682 nm with emission quantum yield (ϕ = 0.103) upon excitation at 583 nm. Cytotoxicity of the complex is studied by MTT method with human breast cancer cell lines and IC50 value is found to 18.5 μM. In presence of the complex (10 μM) a bright red fluorescence image of MCF-7 cell lines is observed under fluorescence microscope. Moreover, the complex acts as effective catalyst towards transfer hydrogenation of ketones.

Carrier Transfer of Deep‐Level Localized States in Type‐II InxGa1−xAs/GaNyAs1−y Short‐Period Superlattice

Optical properties of InGaAs/GaNAs superlattice are investigated by the photoluminescence and reflectance measurements. A quantum well (QW) emission PM (centered at ≈1.2 eV) and a broad low‐energy emission (LEE) band, which can be resolved to three peaks PA′, PA, and PB (0.77, 0.83, and 0.92 eV) are observed. The peak positions of the LEE exhibit S‐shaped behavior, whereas the QW emission shows a red shift with increasing temperature. The LEE bands are attributed to N‐related deep‐level localized state. The photoluminescence (PL) peak of QW exhibits a significant blue shift and saturation effect with increasing excitation density, which is interpreted in terms of the band‐bending model in the InGaAs/GaNAs short‐period superlattice (SPSL) with a type II band alignment. Furthermore, carrier transfer among these N‐related localized states is elucidated by examining the temperature‐dependent photoluminescence. Such carrier transfer is also demonstrated by the thermal effect under high excitation density.

Neutral iridium(iii) complexes bearing BODIPY-substituted N-heterocyclic carbene (NHC) ligands: synthesis, photophysics, in vitro theranostic photodynamic therapy, and antimicrobial activity.

The synthesis, photophysics, and photobiological activities of a series of novel neutral heteroleptic cyclometalated iridium(iii) complexes incorporating boron dipyrromethene (BODIPY) substituted N-heterocyclic carbene (NHC) ligands (Ir1-Ir5) are reported. The effect of the substitution position of BODIPY on the NHC ligands, either on C4 of the phenyl ring (Ir1-Ir3) or C5 of the benzimidazole unit (Ir4 and Ir5), and its linker type (single or triple bond) on the photophysical properties was studied. Ir1-Ir5 exhibited BODIPY-localized intense 1IL (intraligand transition)/1MLCT (metal-to-ligand charge transfer) absorption at 530-543 nm and 1,3IL/1,3CT (charge transfer) emission at 582-610 nm. The nanosecond transient absorption results revealed that the lowest triplet excited states of these complexes were the BODIPY-localized 3π,π* states. Complexes Ir4 and Ir5 exhibited blue-shifted 1IL absorption and 1,3IL/1,3CT emission bands compared to the corresponding absorption and emission bands in complexes Ir1 and Ir3. However, replacing the methyl substituents on N3 of benzimidazole in complexes Ir1 and Ir4 with oligoether substituents in Ir3 and Ir5, respectively, did not impact the energies of the low-energy absorption and emission bands in the corresponding complexes. Water-soluble complexes Ir3 and Ir5 have been explored as photosensitizers for in vitro photodynamic therapy (PDT) effects toward human SKMEL28 melanoma cells. Ir3 showed no dark cytotoxicity (EC50 > 300 μM) but good photocytotoxic activity (9.66 ± 0.28 μM), whereas Ir5 exhibited a higher dark cytotoxicity (20.2 ± 1.26 μM) and excellent photocytotoxicity (0.15 ± 0.01 μM). The phototherapeutic indices with visible light (400-700 nm) activation were >31 for Ir3 and 135 for Ir5. Ir3 and Ir5 displayed 1O2 quantum yields of 38% and 22% in CH3CN, respectively, upon 450 nm excitation. Ir5 was more effective at generating reactive oxygen species (ROS) in vitro. Ir5 was also active against Staphylococcus aureus upon visible light activation, with a phototherapeutic index of >15 and EC50 value of 6.67 μM. These photobiological activities demonstrated that these neutral Ir(iii) complexes are promising in vitro PDT reagents, and substitution at C5 on the benzimidazole group of the NHC ligand was superior to C4 substitution on the phenyl ring.

Luminescent rhenium(I) carbonyl complex with redox noninnocent ONS donor azo-phenol ligand: Synthesis, X-ray structure, photophysical properties and live cell imaging

Herein, we have synthesized a new fluorescent rhenium(I) carbonyl complex 1, bearing {Re(CO)3}+ core with ONS donor thioether containing azo-phenol redox noninnocent ligand. The distorted octahedral geometry of the complex is confirmed by single crystal X-ray diffraction method. Cyclic voltammogram in acetonitrile exhibits irreversible oxidation peak (Epa = 1.36 V) along with quasi-reversible reduction peak at E1/2 = −0.92 V (ΔE = 210 mV). The complex exhibits low energy emission band at 525 nm with high emission quantum yield (ϕ = 0.115). Cytotoxicity of the complex is studied by MTT method with human breast cancer cell lines (MCF-7) and IC50 value is found to be 23.6 μM. In presence of the complex (10 μM) a bright green fluorescence image of MCF-7 cell lines is observed under fluorescence microscope.

Surface ZnSe:Ca layers with hole conductivity

The authors investigate the effect of treating n-ZnSe substrates with boiling aqueous Ca(NO3)2 suspension on their electrical and luminescent properties. Base substrates were cut from bulk pure zinc selenide crystals grown from a stoichiometric melt by the Bridgman method. It was found that the Ca-doping of the substrates causes an almost complete “quenching” of the low-energy orange emission band with a maximum near ħωmax ≈ 1,95 eV and a significant increase in the efficiency of the edge blue luminescence band.

Ab initiostudy of luminescence in Ce-dopedLu2SiO5: The role of oxygen vacancies on emission color and thermal quenching behavior

We study from first principles the luminescence of Lu$_2$SiO$_5$:Ce$^{3+}$ (LSO:Ce), a scintillator widely used in medical imaging applications, and establish the crucial role of oxygen vacancies (V$_O$) in the generated spectrum. The excitation energy, emission energy and Stokes shift of its luminescent centers are simulated through a constrained density-functional theory method coupled with a ${\Delta}$SCF analysis of total energies, and compared with experimental spectra. We show that the high-energy emission band comes from a single Ce-based luminescent center, while the large experimental spread of the low-energy emission band originates from a whole set of different Ce-V$_O$ complexes together with the other Ce-based luminescent center. Further, the luminescence thermal quenching behavior is analyzed. The $4f-5d$ crossover mechanism is found to be very unlikely, with a large crossing energy barrier (E$_{fd}$) in the one-dimensional model. The alternative mechanism usually considered, namely the electron auto-ionization, is also shown to be unlikely. In this respect, we introduce a new methodology in which the time-consuming accurate computation of the band gap for such models is bypassed. We emphasize the usually overlooked role of the differing geometry relaxation in the excited neutral electronic state Ce$^{3+,*}$ and in the ionized electronic state Ce$^{4+}$. The results indicate that such electron auto-ionization cannot explain the thermal stability difference between the high- and low-energy emission bands. Finally, a hole auto-ionization process is proposed as a plausible alternative. With the already well-established excited state characterization methodology, the approach to color center identification and thermal quenching analysis proposed here can be applied to other luminescent materials in the presence of intrinsic defects.

Role of Solvent Polarity and Hydrogen-Bonding on Excited-State Fluorescence of 3-[(E)-{4-[Dimethylamino]benzylidene}amino]-2-naphthoic Acid (DMAMN): Isomerization vs Rotomerization.

The present experimental and theoretical study on a new chromophore DMAMN of the type push-π-pull (push = dimethylaniline, π = imine, pull = 2-naphthoic acid), allows understanding of the mechanism by which the molecular conformational undergoes isomerization/rotomerization following electronic excitation. The steady-state fluorescence spectra of this compound, carried out in solvents of different polarities and proticities, showed significant changes in both the shape and peak positions. The wavelength and intensity change depend on the polarity and hydrogen-bonding environment. In highly polar solvents, the emission is weak and red-shifted compared to that for cyclohexane, but it is more red-shifted in moderate aprotic polar solvents. In hydroxyl solvents, a new weak low-energy emission band appears at ∼525 nm, attributed to the intermolecularly H-bonded open conformer. On the basis of the generated potential energy landscapes of the ground state and low-lying excited state in the gas phase and solution, we found that selective photon absorption, brings this molecule to a "bright" state, from which N═C isomerization Z → E, takes place. This isomerization in gas-phase and low-polarity solvents leads to two minima with a barrier, whereas in highly polar-protic media, it gives one minimum on the S1 surface with low ΔES1/T1 (0.17 eV), facilitating deactivation via ISC.

Broad band (UV–VIS) photoluminescence from carbonized fumed silica: Emission, excitation and kinetic properties

Carbon incorporated silica nano-powder (SiO2:C) has been fabricated by chemical treatment of a fumed silica with tetramethoxysilane and subsequent annealing at 700°C in pure nitrogen ambient. Upon thermal treatment SiO2:C powder exhibits strong photoluminescence (PL) that spans in the range from near-ultraviolet to visible. Two characteristic broad bands, with their maxima at 2.5eV (Green-band) and 3.3eV (Violet-band), contribute to the observed broadband emission spectrum from SiO2:C. PL emission and PL excitation (PLE) studies in the temperature range of 10–300K revealed a different PL behavior pertaining to the change of the peak position of the green and violet bands with respect to temperature. The low-energy emission band showed a redshift in the peak position with decreasing temperature, in contrast to the blue-shift behavior of the violet-band. The latter PL behavior suggests a different origin for the observed emission bands. Complementary time-resolved PL analysis for both emission bands was carried out and yielded PL transients of a few nanoseconds. On the basis of PL/PLE studies and structural characterization by FTIR it is suggested that the origin of the broad band emission may be attributed to radiative recombination in carbon-related surface sites/clusters.