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We report the absorption spectra and photophysical properties of homo and hetero‐aggregate assemblies of a strongly emissive N‐annulated perylene dye (P) and of a dyad made of P and a methyl viologen derivative (P‐MV), in ethanol‐water solutions. In homo‐aggregate assemblies of P, the π‐π* fluorescence of the isolated chromophore is replaced by excimer emission at lower energy, with a lifetime of 900 ps, due to excimer formation from the initially prepared excitons. In homo‐aggregate assemblies of P‐MV, photoinduced charge separation, with formation of P+‐MV− species, occurs in 3 ps with a charge recombination of 20 ps. In hetero‐aggregate P/P‐MV systems, the light energy absorbed by the P components delocalizes over various P subunits, and when a P‐MV unit is reached, charge separation occurs; however, excimer emission is present for P/P‐MV ratio larger than 3 : 1, indicating that delocalized excitons within the hetero‐aggregate systems extend over a limited number of P chromophores. [ABSTRACT FROM AUTHOR]

Ce+3 doped CsLaSiS 4 single crystals were synthesized using high-temperature flux synthesis. XRD analysis showed that the products are isostructural to CsLaSiS 4 and crystallize in orthorhombic space group Pnma. Absorption spectra were studied at room temperature, and photoluminescence properties were examined in the temperature range of 5–310 K. The energy of interband transitions E g = 3.75 eV was determined at room temperature in the Tauc model. At room temperature, only a non-elementary d → f emission band of Ce+3 ions was observed in the 520 nm region. The luminescence kinetics upon excitation by a pulsed cathode beam or X-ray synchrotron radiation exhibited a dominant nanosecond component. Decay time, as well as build-up time, decreased with increasing concentration of Ce+3 ions. Concentration quenching of Ce+3 emission was not observed up to 11.9 mol% of Ce+3 ions. At a low temperature of 5 K, new wide emission bands at 422 and 688 nm appeared in the photoluminescence spectrum. It is shown that the 422 nm band in the photoluminescence spectrum corresponds to host emission, specifically the luminescence of self-trapped excitons (STE). The 688 nm emission band corresponds to defect-related luminescence. STE emission is quenched according to the Mott law at temperatures above 26 K with an activation energy of 20 meV. An efficient energy transfer channel from the STE to the Ce+3 ion via the radiative resonance mechanism is observed. The emission of Ce+3 ions and defect-related luminescence can be excited by the intracenter way, due to electron-hole recombination, or by the creation of defect bound excitons. Based on the obtained spectroscopic data, a band scheme illustrating the processes of relaxation of electronic excitations at T = 5 K in Ce+3 doped CsLaSiS 4 crystals is proposed. [Display omitted] • Ce+3 doped CsLaSiS 4 single crystals were obtained through high-temperature flux synthesis. • Inter configurational d-f transitions in Ce3+ ions were studied upon both UV- and X-ray excitation. • Luminescence kinetic studies were conducted using a pulsed cathode beam and X-ray synchrotron radiation. • Self-trapped excitons, defect-bound excitons, and impurity luminescence were observed at T = 5 K. [ABSTRACT FROM AUTHOR]

This paper reports on the defect correlated self-quenching and spectroscopic investigation of calcium titanate (CaTiO3) phosphors. A series of CaTiO3 phosphors doped with trivalent europium (Eu3+) and codoped with potassium (K+) ions were prepared by the solid state reaction method. The X-ray diffraction results revealed that the obtained powder phosphors consisted out of a single-phase orthorhombic structure and it also indicated that the incorporation of the dopants/co-dopants did not affect the crystal structure. The scanning electron microscopy images revealed the irregular morphology of the prepared phosphors consisting out of lm sized diameter particles. The Eu3+ doped phosphors illuminated with ultraviolet light showed the characteristic red luminescence corresponding to the 5D0→7Fj transitions of Eu3+. As a charge compensator, K+ ions were incorporated into the CaTiO3:Eu3+ phosphors, which enhanced the photoluminescence (PL) intensities depending on the doping concentration of K+. The concentration quenching of Eu3+ in this host is discussed in the light of ion-ion interaction, electron phonon coupling, and defect to ion energy transfer. The spectral characteristics and the Eu-O ligand behaviour were determined using the Judd-Ofelt theory from the PL spectra instead of the absorption spectra. The CIE (International Commission on Illumination) parameters were calculated using spectral energy distribution functions and McCamy's empirical formula. Photometric characterization indicated the suitability of K+ compensated the CaTiO3:Eu3+ phosphor for pure red emission in light-emitting diode applications. [ABSTRACT FROM AUTHOR]

In this work, we present measurements of the dynamics of photoexcited carriers in GaInN/GaN quantum wells (QWs) grown on ammonothermal GaN, especially thermalization and recombination rates. Emission properties were measured by time-resolved photoluminescence (PL) and electroluminescence spectroscopy. Due to the use of high quality homoepitaxial material, we were able to obtain very valuable data on carrier thermalization. The temperature dependence of the QW energy observed in PL shows characteristic S-shape with a step of about 10 meV. Such a behavior (related to thermalization and localization at potential fluctuations) is often reported for QWs; but in our samples, the effect is smaller than in heteroepitaxial InGaN/GaN QWs due to lower potential fluctuation in our material. Absorption properties were studied by photocurrent spectroscopy measurements. A comparison of emission and absorption spectra revealed a shift in energy of about 60 meV. Contrary to PL, the QW energy observed in absorption decreases monotonically with temperature, which can be described by a Bose-like dependence E(T) = E(0) - λ/(exp(θ/T) - 1), with parameters λ = (0.11 ± 0.01) eV, θ = (355 ± 20) K, or by a Varshni dependence with coefficients α = (10 ± 3) × 10-4 eV/K and β = (1500 ± 500) K. Taking into account absorption and emission, the fluctuation amplitude (according to Eliseev theory) was σ = 14 meV. The time resolved PL revealed that in a short period (<1 ns) after excitation, the PL peaks were broadened because of the thermal distribution of carriers. We interpreted this distribution in terms of quasi-temperature (Tq) of the carriers. The initial Tq was of the order of 500 K. The thermalization led to a fast decrease of Tq. The obtained cooling time in the QW was τC = 0.3 ns, which was faster than the observed recombination time τR = 2.2 ns (at 4 K). [ABSTRACT FROM AUTHOR]

The observed Dy3+ ion upconversion luminescence in LiNbO3 crystal heavily codoped with ZnO was spectrally and temporally analyzed by microsecond time-resolved spectrum under 806 nm intense femtosecond laser excitation at room temperature. Absorption spectrum and modified Judd-Ofelt approach were used to investigate its spectroscopic properties. The bright blue and intense yellow emissions are assigned to transitions 4F9/2→6H15/2 and 4F9/2→6H13/2, respectively. It is concluded that the main upconversion mechanism is excited state absorption by pump power dependence in combination with luminescence intensity temporal evolvements. [ABSTRACT FROM AUTHOR]

We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J-aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J-aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J-aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = ϵLR/2b, wherein ϵLR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J-aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J-aggregates can have both a weak (g < 1) and a strong (g > 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J-aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J-aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J-aggregates by varying the values of g, which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J-aggregates, which has thus far been explained by the strongly localized exciton model. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

Particular features and quenching mechanisms of exciton luminescence of water-soluble nanocomposites that are formed as a result of the interaction of surface charged semiconductor quantum dots (QDs) CdSe/ZnS ( d = 2.8 nm) and cationic porphyrins (HTMPyrP and ZnTMPyrP) have been studied theoretically and experimentally. It has been found that, in CdSe/ZnS⊕Porphyrin conjugates, there occurs long-range inductive resonance electronic excitation energy transfer from surface modified (with thioglycolic or mercaptoundecanoic acid) QDs to porphyrins, which is accompanied by quenching of the exciton luminescence of QDs and an increase in the fluorescence intensity of porphyrin. It has been shown that, when mercaptoundecanoic acid is used as a QD shell, the QD luminescence quenching efficiency by porphyrins follows the Förster-Galanin theory and depends on the overlap integral between the CdSe/ZnS luminescence band and the absorption spectra of free-base porphyrin HTMPyrP and its metal complex ZnTMPyrP. It has been revealed that, as the QDs ↔ Zn-porphyrin intercenter distance decreases from 39.1 (mercaptoundecanoic acid) to 30.1), a considerable QD luminescence quenching is observed; however, the energy transfer efficiency substantially decreases, from 55% in the former case to 23% in the latter one. Based on the spectral-luminescent data and quantum-chemical calculations, it has been found that the chemical change of HTMPyrP in the structure of the complex with CdSe/ZnS QDs passivated by thioglycolic or mercaptoundecanoic acid is caused by the formation of a metal complex ZnTMPyrP. Based on calculations of the redox-potentials, it has been concluded that the low luminescence quantum yield of CdSe/ZnS QDs passivated by residues of mercaptocarboxylic acids S(CH)COO and its dependence on the number of CH groups are related to the possibility of photoinduced electron transfer from the HOMO of passivating molecules to QDs (QD* ⇐ S(CH) nCOO hole transfer). It has been shown that the quenching of the exciton luminescence of QDs in heterogeneous structures CdSe/ZnS(thioglycolic acid)⊕ZnTMPyrP, which is complementary to the energy transfer, can be caused by the photoinduced electron transfer that involves the participation of the LUMO of the ZnTMPyrP molecule (QD* ⇒ ZnTMPyrP). [ABSTRACT FROM AUTHOR]

It has been demonstrated on the basis of absorption spectra measurements that the activation of glassy lithium tetraborate by terbium results in the formation of band of localized electronic states in the pseudo-band gap, which correspond to the energy levels of rare earth element ions. As follows from the analysis of the X-ray luminescence spectra, the activator ion plays the most important role in the emergence of luminescent properties of glassy lithium tetraborate. It has been established that X-ray luminescence in glassy lithium tetraborate activated by terbium is mostly realized due to transitions from excited D- and D-states to the levels of spin-orbital splitting of the main F state of Tb ion. [ABSTRACT FROM AUTHOR]

Three new tripyridyl tripodal ligands appended with either fullerene or pyromellitdiimide moieties, named C60-s-Tripod, C60-l-Tripod, and PI-Tripod, were synthesized and introduced into a porphyrin macroring N-(1-Zn)3 (where 1-Zn = trisporphyrinatozinc(II)). From UV-vis absorption and fluorescence titration data, the binding constants of C60-s-Tripod, C60-l-Tripod, and PI-Tripod with N-(1-Zn)3 in benzonitrile were estimated to be 3 × 108, 1 × 107, and 2 × 107 M-1, respectively. These large binding constants denote multiple interactions of the ligands to N-(1-Zn)3. The binding constants of the longer ligand (C60-l-Tripod) and the pyromellitdiimide ligand (PI-Tripod) are almost the same as those without the fullerene or pyromellitdiimide groups, indicating that they interact via three pyridyl groups to the porphyrinatozinc(II) coordination. In contrast, the larger binding constants and the almost complete fluorescence quenching in the case of the shorter ligand (C60-s-Tripod) indicate that the interaction with N-(1-Zn)3 is via two pyridyl groups to the porphyrinatozinc(II) coordination and a π-π interaction of the fullerene to the porphyrin(s). The fluorescence of N-(1-Zn)3 was quenched by up to 80% by the interaction of C60-l-Tripod. The nanosecond transient absorption spectra showed only the excited triplet peak of the fullerene on selective excitation of the macrocyclic porphyrins, indicating that energy transfer from the excited N-(1-Zn)3 group to the fullerenyl moiety occurs in the C60-l-Tripod/N-(1-Zn)3 composite. In the case of PI-Tripod, the fluorescence of N-(1-Zn)3 was quenched by 45%. It seems that the fluorescence quenching probably originates from electron transfer from the excited N-(1-Zn)3 group to the pyromellitdiimide moiety. [ABSTRACT FROM AUTHOR]

Abstract: Homogeneous monolith of AlPO4 gel doped with Rhodamine 6G (Rh6G) in different dye concentration is prepared by one step process with sol–gel method using the precursors Al(lact)3 and H3PO4. The optical properties of AlPO4 gel doped with Rh6G have been characterized by UV–vis absorption spectra and fluorescence spectra. Rh6G molecular J-dimers and H-dimers even multimers are analyzed by excitation spectra based on Exciton theory. The structure of AlPO4 gel doped with Rh6G is investigated by 27Al and 31P Magic Angle Solid Nuclear Magnetic Resonance (MAS NMR), 27Al {31P} Rotational Echo Double Resonance (REDOR) NMR and 27Al Triple Quantum Magic Angle Spinning (TQ-MAS) NMR. Based on the results of optical spectra and the structural analysis by NMR techniques, The AlPO4 gel doped with Rh6G dye with molar ratio of Rh6G/Al(lact)3 of 1×10−4 displays excellent optical properties. [Copyright &y& Elsevier]

Abstract: The structure of the radioluminescent (RL) spectrum of Li2B4O7:Cu crystal is complex, having no normal Gauss peaks. The RL spectrum recorded showed one sharp and strong peak near 365nm. For the purpose of explaining this phenomenon, we used the self-trapped exciton (STE) model to analyze experimental data of absorption spectrum, emission spectrum, radioluminescence spectrum and decay-time curves. We found a short lifetime component (of the order of ns) from phosphorous signals of Li2B4O7:Cu. We realized that the short lifetime component related to inherent signal of host lattice and the d–p transitions of Cu+. This result is applicable in the detector system with high time resolution. [ABSTRACT FROM AUTHOR]

Abstract: Thin films of microcrystalline (C n H2n +1NH3)2PbBr4 (n =4, 5, 7 and 12) have been prepared by a modified spin-coating method, and the effect of the number of carbon atoms of the alkyl chain length (n) on optical properties has been investigated. Absorption spectra reveal that (C n H2n +1NH3)2PbBr4 films show stable excitons with a binding energy of a few hundred meV. The excitonic structure of (C n H2n +1NH3)2PbBr4 varies with the number of carbon atoms. The lowest-energy exciton splits into a few fine-structure levels at low temperature. (C n H2n +1NH3)2PbBr4 films (n =5, 7 and 12) show not only singlet excitons but also triplet excitons at low temperature, while (C4H9NH3)2PbBr4 films show only singlet excitons. The intersystem crossing from excited singlet state to triplet state plays an important role in the relaxation process of excitons. [Copyright &y& Elsevier]

Luminescence behaviors of Ce3+ ions in GeS2–Ga2S3–CsCl chalcohalide glasses are reported for the first time. The dependence of Ce3+ concentration and host composition on absorption and photoluminescence spectra are investigated. Results show that these glasses exhibit broadband emission from 470 to 600 nm under a blue excitation. Luminescence intensity is enhanced with the increasing of Ce3+ ions and reaches the maximum at 0.08 mol%. The decrease of CsCl content results in the red shift of absorption cut-off edge and emission band, and decreased luminescence intensity. The mechanism for these phenomena is discussed with the theory of optical basicity. [ABSTRACT FROM AUTHOR]

Abstract: Properties of two luminescence centers, which are observed in anthracene-doped fluorene crystal as concentration increases, have been investigated to clarify their origin. Excitation spectra and concentration dependence of absorption spectra were measured and two types of luminescence are attributed to anthracene molecule pair centers with weak and strong interaction. Dipole–dipole interactions between two anthracence molecules of various configurations are estimated and possible configuration for two types of luminescence is proposed. [Copyright &y& Elsevier]

A series of new mixed-metal Ru(II)-Ir(III) trinuclear complexes have been prepared and characterized, together with their mononuclear parents and a series of closely related dinuclear and trinuclear homometallic Ru(II) and Ir(III) species, and their absorption spectra and luminescence properties (both at 77 K in rigid matrix and at room- temperature in fluid solution) have been studied. The absorption spectra and luminescence properties of the Ru(II) species and subunits are dominated by metal-to-ligand charge-transfer (MLCT) transitions and excited states, whereas ligand centered (LC) transitions and excited states govern the spectroscopic and photophysical properties of most of the Ir(III) species here studied, with MLCT states playing an important role when cyclometalated Ir(III) subunits are present. Each metal-based subunit retains in the multinuclear arrays its own spectroscopic properties, but in the case of the mixed Ru-Ir species an efficient, additional decay channel is opened for the excited states involving the Ir- centered subunits, that is, photoinduced energy transfer to the lower-lying MLCT state(s) involving the Ru centers. [ABSTRACT FROM AUTHOR]

Two-photon absorption (TPA) spectra of a series of highly symmetric, strong phosphorescence AuI−CuIcomplexes 1−7bearing alkynyl−phosphine ligands in solution were obtained over the biological windows (760−840 nm) by the two-photon-induced emission technique under femtosecond laser excitation. The resulting TPA cross sections are further confirmed by the open-aperture Z-scan method, which offers the advantage of reducing the excited-state reabsorption for these long-lived phosphorescence dyes. The results of these two methods are mutually in good agreement, showing that complexes 1−7exhibit moderately strong TPA cross section, for example, ∼710 GM at 760 nm for 1in CH2Cl2. Owing to the highly centrosymmetric property, the resulting trend in TPA values for 1−7can be rationalized by the unit of X-group−alkynyl−heterometallic-core−alkynyl−X-group fragment, categorized as A−π−D−π−A or D−π−A−π−D structural motif, which renders large changes in polarization upon excitation and provides significant near-IR TPA strength. [ABSTRACT FROM AUTHOR]

Two types of melanin films: self-arranged ordered films and amorphous films have been studied in the present paper. The structure, absorption spectra, spectra of photovoltage, and spectra of time-resolved photoluminescence have been investigated for both types of melanin films. Time-resolved photoluminescence was measured at room temperature at different delay times. The reason for the difference in the electronic properties of structurized and amorphous films of melanin is attributed to the structural features of the films related to ordered dendrite-like structures and amorphous disordered ones, respectively. [ABSTRACT FROM AUTHOR]

Chlorophyll is suggested as a suitable medium for realizing optical Boolean logic at the molecular level in view of its wavelength-selective property and polarization sensitivity in the visible region. Spectrophotometric studies are made with solutions of total chlorophyll and chromatographically isolated components, viz. chlorophyll a and b and carotenoids extracted from pumpkin leaves of different maturity stages. The absorption features of matured chlorophyll with two characteristic absorption peaks and one transmission band are molecular properties and independent of concentration. A qualitative explanation of such an absorption property is presented in terms of a ‘particle in a box’ model and the property is employed to simulate two-input optical logic operations. If both of the inputs are either red or blue, absorption is high. If either one is absent and replaced by a wavelength of the transmission band, e.g. green, absorption is low. Assigning these values as 0 s or 1 s, AND and OR operations can be performed. A NOT operation can be simulated with the transmittance instead of the absorbance. Also, the shift in absorbance values for two different polarizations of the same monochromatic light can simulate two logical states with a single wavelength. Cyclic change in absorbance is noted over a rotation of 360° for both red and blue peaks, although the difference is not very large. Red monochromatic light with polarizations apart by 90°, corresponding to maximum and minimum absorption, respectively, may be assigned as the two logical states. The fluorescence emissions for different pigment components are measured at different excitation wavelengths and the effect of fluorescence on the red absorbance is concluded to be negligible. [ABSTRACT FROM AUTHOR]

The luminescence spectra of Cd0.75Mn0.25Te/Cd1 − y Mg y Te superlattices with narrow-band-gap Cd0.75Mn0.25Te nanolayers having nominal thicknesses of 0.5, 1.5, and 3.0 monolayers (MLs) exhibit exciton emission bands of Cd1 − y Mg y Te barriers and nanolayers, as well as intracenter luminescence of Mn2+ ions. For all samples, the luminescence spectra of Cd0.75Mn0.25Te nanolayers consist of two bands. From analyzing the dependences of the luminescence intensity on the nanolayer thickness and temperature, these bands are assigned to excitons localized at two-and zero-dimensional potentials. The intracenter emission of Mn2+ in 3.0-ML-thick Cd0.75Mn0.25Te clearly reveals excitation migration. [ABSTRACT FROM AUTHOR]

In this paper, the absorption and fluorescence spectra of xanthene, 9-trimethylsilyl xanthene (1) and 9-methyl-9-trimethylsilyl xanthene (2) are presented and discussed with the help of CNDO/S and ab initio calculations. Introduction of the trimethylsilyl group at the 9-position of the xanthene molecule considerably shifts the short-wavelength band (246 nm) to the red and reduces the fluorescence quantum yields (Φfl = 0.03 for xanthene, 0.008 for 1 and <10−4 for 2 in MeCN) and the fluorescence lifetimes (τfl = 7.4 ns for xanthene, 220 ps for 1 and <100 ps for 2 in MeCN), while it does not affect seriously the long-wavelength band and the singlet excited state energies (ES = 97.7 kcal/mol for xanthene, 94.5 kcal/mol for 1 and 97.4 kcal/mol for 2 in MeCN). Ab initio calculations predict a ‘roof-like’ structure for 1 with folding angles 30° for the S0 and 20° for the S1 state. Laser (248, 266 and 308 nm) and lamp photolysis (254 nm) of 1 and 2 (MeCN or cyclohexane) results in [1,3]-trimethylsilyl rearrangement into the ortho-position of the xanthene moiety in the sense of a photo-Fries type reaction. The corresponding photo-Fries intermediates (exocyclic cyclohexatrienes: trimethyl-(1H-xanthen-1-yl)-silane, 1CHT and trimethyl-(9-methyl-1H-xanthen-1-yl)-silane, 2CHT) are formed within the 20 ns laser pulse and show absorption spectra peaking up at 410 and 403 nm, respectively. Additionally, small amounts of the corresponding 9-xanthyl radicals were detected as a result of the C-Si bond rupture. Using ps-laser flash photolysis (266 nm laser, MeCN) we observed a broad absorption spectrum peaking up at 960 nm and decaying monoexponentially with a lifetime of 130 ps, close to the measured fluorescence lifetime. We assigned therefore this transient to the singlet excited state of 1 (S1 → Sn absorption). We assume the S1 state as the origin of the photo-Fries rearrangement, giving via C-Si bond dissociation a singlet geminal radical pair (9-xanthyl radical + SiMe3). In the next step, the radical pair undergoes predominantly in-cage recombination to the persistent photo-Fries intermediates with high quantum yields (Φ1CHT = 0.70 and Φ2CHT = 0.50), while to a lesser extent it escapes the solvent cage (Φ = 0.30 for 1) and undergoes typical free radical reactions (e.g., scavenging with O2). The estimation of the above quantum yields was possible only after determination of the absorption coefficients (ɛ) of the photo-Fries intermediates [ɛ1CHT(410 nm) = 16,800 M−1 cm−1 and ɛ2CHT(403 nm) = 15,900 M−1 cm−1] using three independent methods; this represents the first example in the literature. [Copyright &y& Elsevier]

Abstract: Er3+/Yb3+ co-doped TeO2–B2O3–Nb2O5–ZnO (TBN) glasses were prepared. The absorption spectra and upconversion luminescence spectra of TBN glasses were measured and analyzed. The upconversion emission bands centered at 530, 546 and 658nm were observed under the excitation at 975nm, corresponding to the transitions of 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 respectively. The ratio of red emission to green emission increases with an increasing of Yb3+ ions concentration. According to the quadratic dependence on excitation power, the possible upconversion mechanisms and processes were discussed. [Copyright &y& Elsevier]

The absorption spectra at room temperature and the spectra, the quantum yields, and the decay times of the luminescence at 77 K of binuclear complexes [X(bpy)2Ru(BL)Ru(bpy)2Cl]2+ (bpy = 2,2′-bipyridyl; X = Cl, BL = pyrazine, 4,4′-bipyridyl, trans-1,2-bis(4-pyridyl)ethylene, and trans-1,2-bis(4-pyridyl)ethane and X = NO2, BL = 4,4′-bipyridyl) in alcoholic (4 : 1 EtOH–MeOH) solutions are studied. It is shown that the interaction between the metal centers (MCs) of the complexes affects the characteristics of the electronically excited states (EESs) of each of them and facilitates increasing the transition dipole moment Ru(dπ)→BL(π*). The deactivation rate constants of the lowest electronically excited metal-to-ligand charge transfer (3MLCT) state of the complexes are determined. In an asymmetric binuclear complex, the energy transfer from MC(NO2) to MC(Cl) is revealed, with the rate constant of this transfer being not smaller than 3.2×1010s–1. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

Abstract: Five Schiff bases derived from 4-aminoantipyrine and benzaldehyde derivatives (I) are prepared and their UV–vis, IR, 1H NMR and fluorescence spectra are investigated and discussed. The electronic absorption spectra of the hydroxy 4-aminoantipyrine Schiff bases Ib and Ie as well as the fluorescence spectra of Ie are studied in the organic solvents of different polarity. The UV–vis absorption spectra of 4-aminoantipyrine Schiff bases Ib, Id and Ie are investigated in aqueous buffer solutions of varying pH and utilized for the determination of pK a and ΔG of the ionization process. The reactions of the hydroxy compounds Ib and Ie with Ni(II) and Cu(II) ions are also studied. The results of spectral studies are supported by some molecular orbital calculations using an atom superposition and electron delocalization molecular orbital theory for a compound Ib. [Copyright &y& Elsevier]

Abstract: Absorption and emission properties of some phenoxy derivatives of diphenyloxazole (PPO) are presented and discussed. The photophysical properties reflect a dependence on the substituted 2 or 5 position of the oxazole ring. The experimental data were correlated with molecular parameters such as molecular flexibility, electronic structure and the singlet–triplet gap. The substitution with heteroatomic-bridged phenyls maintains the same frontier orbitals (m, m +1) as in the parent compound, but introduces a new molecular orbital, m −1, located on the terminal phenyl. The presence of the so-called band B in the absorption spectra of some diaryloxazoles was attributed to the m −1→ m +1 transition participating to the second excited state wave function. The other main component of this state, the m → m +2 transition, was found to have a forbidden character, explaining the lack or the low intensity of band B. The decrease of the fluorescence quantum yield subsequent to substitution of PPO with phenoxy fragments was found to be due to the enhanced molecular flexibility comparing to PPO. The differences between the 2- or 5-substituted derivatives were rationalized in terms of a smaller S1–T2 gap for the former, thus increasing the rate of the overall nonradiative intersystem crossing processes. [Copyright &y& Elsevier]

Abstract: Erbium chelates including tris(acetylacetonato) erbium(III) monohydrate, tris(acetylacetonato)(1,10-phenanthroline) erbium(III) and tris(trifluoroacetylacetonato)(1,10-phenanthroline) erbium(III) are synthesized. Judd–Ofelt theory is employed on basis of the UV–Vis–NIR absorption spectra of erbium chelates dissolved in methanol. Judd–Ofelt parameters of erbium chelates are determined by a least square fitting and dealt with the chemical structure of erbium chelates. Photoluminescence characteristics of erbium chelates are investigated upon excitation at 488nm by an Ar+ laser. The qualitative correlation of Judd–Ofelt parameters with photoluminescence properties for erbium chelates is also discussed. It is found that larger Ω 6 value for erbium chelate is and larger photoluminescence intensity at 1.54μm is, and Ω 2 value should contribute to the photoluminescence full width at half maximum (FWHM) at 1.54μm. The changes of Judd–Ofelt parameters result from the introduction of the second ligand phenathroline or the substitution of electron-drawing group CF3 in β-diketone for erbium chelates. [Copyright &y& Elsevier]

Abstract: Lithium cesium mixed alkali borate glasses of the composition 67B2O3·xLi2O·(32−x)Cs2O (where x=8, 12, 16, 20 and 24) containing 1mol% Nd2O3 were prepared by melt quenching. The absorption spectra of Nd3+ were studied from the experimental oscillator strengths and the Judd–Ofelt intensity parameters were obtained. The intensity parameters are used to determine the radiative decay rates (emission probabilities of transitions) (A T), branching ratios (β) and integrated absorption cross-sections (Σ) of the Nd3+ transitions from the excited state J manifolds to the lower lying J′ manifolds. Radiative lifetimes (τ R) are estimated for certain excited states of Nd3+ in these mixed alkali borate glasses. Luminescence spectra were measured and the emission cross-sections (σ p) were evaluated for the three emission transitions. The variation of luminescence intensity with x was recorded for the three transitions at different excitation power to see the effect of mixed alkalies in these borate glasses. [Copyright &y& Elsevier]

Directly meso--meso linked porphyrin rings CZ4, CZ6, and CZ8 that respectively comprise four, six, and eight porphyrins have been synthesized in a stepwise manner from a 5,10-diaryl zinc(II) porphyrin building block. Symmetric cyclic structures have been indicated by their very simple ¹H NMR spectra that exhibit only a single set of porphyrin and their absorption spectra that display a characteristic broad nonsplit Soret band around 460 nm. Energy minimized structures calculated at the B3LYP/6-31 G* level indicate that a dihedral angle between neighboring porphyrins decreases in order of CZ6 > CZ8> CZ4 which is consistent with the ¹H NMR data. Photophysical properties of these molecules have been examined by the steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related with the excitation energy migration processes within the porphyrin rings, where the exciton--exciton annihilation time and the polarization anisotropy rise time are well described in terms of the Förster-type incoherent energy hopping model. Consequently, the excitation energy hopping rates have been estimated for CZ4 (119 ± 2 fs)-1, CZ6 (342 ± 59 fs)-1, and CZ8 (236 ± 31 fs)-1, which reflect the magnitude of the electronic coupling between the neighboring porphyrins. Overall, these porphyrin rings serve as a well-defined wheel-shaped light, harvesting antenna model in light of very efficient excitation energy hopping along the ring. [ABSTRACT FROM AUTHOR]

Planar structures consisting of oxidized copper granules obtained by laser electrodispersion are studied. The samples have different packing densities of granules and different amounts of their chains and aggregates. Each granule 5.5 ± 0.5 nm in size consists of a copper core with an amorphous structure and an oxide shell of about 0.7 nm thick. Some granules are randomly charged. The spectra of coherent transmission, diffusion transmission, and reflection of the samples are measured. Using the experimental data, the absorption spectra and the effective absorption, extinction, and scattering coefficients of monolayers are calculated and the luminescence spectra are estimated. A long-wavelength shift of the plasmon resonance of the copper granules with oxide shells as compared to that of the unoxidized granules is observed. The shift depends on the thickness of the oxide layer. A similar shift of the plasmon resonance is observed for the chains of copper granules. The spectra are compared with the spectra calculated theoretically taking into account some parameters of the planar structures and the size dependence of the optical constants of copper. The luminescence observed in some cases is associated with specifics of oxidation of copper granules. © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

Abstract: The fluorescence and fluorescence excitation spectra (conjugated spectra) of new synthesized substituted arylpolyene (C6H5–[CH=CH]2–C6H4–NH2) in solid n-octane at T=4.2K were studied. Spectra have feebly marked vibrational structure. The method of finding vibronic interaction parameters from conjugated spectra was developed. The method is based on the modeling of spectra in the form of set of vibronic bands. Each vibronic band is represented as the sum of narrow zero-phonon line and broad phonon wing. It allows carrying out the theoretical calculation of the conjugated spectra with poor resolved vibrational structure, which can be compared with the measured spectra. The vibronic interaction parameters which determine shape of the spectra can be found from comparative analysis of experimental and model spectra. This analysis allowed us to explain the deviation from the mirror symmetry between measured conjugated spectra as a joint manifestation of Franck-Condon and Herzberg-Teller interactions. The parameters of both interactions for main normal vibrations of molecule were found. [Copyright &y& Elsevier]

Abstract: Triplet state properties including transient triplet absorption spectrum, intersystem crossing yields in solution at room temperature and phosphorescence spectra, quantum yields and lifetimes at low temperature as well as singlet oxygen yields were obtained for poly(N-vinylcarbazole) (PVK) in 2-methyl-tetrahydrofuran (2-MeTHF), cyclohexane or benzene. The results allow the determination of the energy value for the lowest lying triplet state and also show that triplet formation and deactivation is a minor route for relaxation of the lowest excited singlet state of PVK. In addition, they show the triplet state is at higher energy than reported heavy metal dopants used for electrophosphorescent devices, such that if this is used as a host it will not quench their luminescence. [Copyright &y& Elsevier]

This work describes a study of Ru(ll) and Os(Il) polypyridyl complexes of the symmetrical, fused-aromatic bridging ligand dibenzoeilatin (1). The synthesis, purification, and structural characterization by NMR of the mononuclear complexes [Ru(bpy)2(dbneil)]2+ (2), [Ru(tmbpy)2(dbneil)]2+ (3), and [Os(bpy)2(dbneil)]2+ (4), the homodinuclear complexes [{Ru(bpy)2}2{µ-dbneil}]4+ (5), [{Ru(tmbpy)2}2{µ-dbneil}]4+(6), and [{Os(bpy)2}2{µ-dbneil}]4+ (7), and the heterodinuclear complex [{Ru(bpy)2}{µ-dbneil}{Os(bpy)2}]4+ (8) are described, along with the crystal structures of 4, 6, and 7. Absorption spectra of the mononuclear complexes feature a low-lying MLCT band around 600 nm. The coordination of a second metal fragment results in a dramatic red shift of the MLCT band to beyond 700 nm. Cyclic and square wave voltammograms of the mononuclear complexes exhibit one reversible metal-based oxidation, as well as several ligand-based reduction waves. The first two reductions, attributed to reduction of the dibenzoeilatin ligand, are substantially anodically shifted compared to [M(bpy)3]2+ (M = Ru, Os), consistent with the low-lying π* orbital of dibenzoeilatin. The dinuclear complexes exhibit two reversible, well-resolved, metal-centered oxidation waves, despite the chemical equivalence of the two metal centers, indicating a significant metal-metal interaction mediated by the conjugated dibenzoeilatin ligand. Luminescence spectra, quantum yield, and lifetime measurements at room temperature in argon-purged acetonitrile have shown that the complexes exhibit ³MLCT emission, which occurs in the IR-region between 950 and 1300 nm. The heterodinuclear complex 8 exhibits luminescence only from the Ru-based fragment, the intensity of which is less than 1% of that observed in the corresponding homodinuclear complex 5; no emission from the Os-based unit is observed, and an intramolecular quenching constant of kq ≥ 3 × 109 s-1 is evaluated. The nature of the quenching process is briefly discussed. [ABSTRACT FROM AUTHOR]

The 2 P state energy of the valence exciton in SrF 2 has been obtained to be 11.1 eV at 298 K through two-photon absorption (TPA) spectroscopy. The luminescence of self-trapped excitons was monitored as the signal of TPA. The exciton binding energy is estimated to be 0.9 eV. This value of binding energy is comparable not only to those obtained through TPA in other alkaline-earth fluorides of CaF 2 and BaF 2 , but also to those of alkali fluorides. The TPA coefficient of SrF 2 was about 5×10 -9  cm/W for 5.55 eV-photons. The value is in the same order of magnitude as those measured at 2 P state of the exciton in CaF 2 and BaF 2 crystals. [ABSTRACT FROM AUTHOR]

Measuring the spectra of absorption, luminescence and excitation, and the decay times of the luminescence, at various temperatures, we investigate the scintillation mechanism of Ce[sup 3+] doped Gd[sub 2]SiO[sub 5] (GSO:Ce). We conclude that the excitation energy created by gamma-ray irradiation relaxes to the core exciton states formed by the 4f-4f transitions of Gd[sup 3+], and that the core exciton migrates to collide with Ce[sup 3+], and transfers the energy to Ce[sup 3+] to emit the luminescence. We also conclude that the decay time of the Ce[sup 3+] luminescence in GSO:Ce (70 ns for the case of 0.5 tool% of Ce concentration) is decided by the diffusion time of the core exciton. A very small value for the diffusion coefficient of the core exciton is estimated to be 7 × 10[sup -7]cm²/s. [ABSTRACT FROM AUTHOR]

Abstract: The luminescence properties of Ba3Tb0.9Eu0.1(PO4)3 and Ba3Gd0.9Eu0.1(PO4)3 phosphors were studied for excitation over the 120–300nm wavelength range. It is found that Tb3+, which exhibits a strong vacuum-ultraviolet (VUV) absorption band, provides sensitisation of Eu3+ emission in this host. This effect can be used to develop phosphors with enhanced conversion efficiency of the VUV radiation into visible light. [Copyright &y& Elsevier]

Materials emitting light in the deep ultraviolet region around 200 nanometers are essential in a wide- range of applications, such as information storage technology, environmental protection, and medical treatment. Hexagonal boron nitride (hBN), which was recently found to be a promising deep ultraviolet light emitter, has traditionally been synthesized under high pressure and at high temperature. We successfully synthesized high-purity hBN crystals at atmospheric pressure by using a nickel- molybdenum solvent. The obtained hBN crystals emitted intense 215-nanometer luminescence at room temperature. This study demonstrates an easier way to grow high-quality hBN crystals, through their liquid-phase deposition on a substrate at atmospheric pressure. [ABSTRACT FROM AUTHOR]

The energy band gap (Eg) of colloidal InGaP/ZnS core/shell nanocrystals is investigated by using the optical absorption and the photoluminescence techniques. While the photoluminescence spectra exhibit a strong emission, the optical absorption spectra show a very weak excitonic absorption. From Eg values estimated from the optical absorption spectra, the In composition is calculated to be on the order of 45%-48%. The estimated In mole fraction suggests that InGaP is still a direct band gap semiconductor. The energy band gap is recorded as a function of temperature from which the Debye temperature and the average phonon energy were extracted. [ABSTRACT FROM AUTHOR]

Self-assembled ZnO quantum dots (QDs) were achieved by a vapor phase transport process. ZnO nanodots were naturally formed on solid substrates in the Volmer-Weber growth mode. Size control of nanodots could be readily realized by varying the growth time. The as-prepared ZnO QDs are of high quality and very stable after formation. The blueshift of band gap energies derived from quantum confinement effects was confirmed by optical absorption spectra. Photoluminescence spectra revealed the tunable behavior of ultraviolet luminescence due to exciton localization. The realization of size-tuned color from ZnO QDs makes them more promising for practical applications. [ABSTRACT FROM AUTHOR]