Structured Emission Band Research Articles

Luminescent biscyclometalated arylpyridine iridium(iii) complexes with 4,4′-bi-1,2,3-triazolyl ancillary ligands

The synthesis, characterization and photophysical investigation of complexes of the form [Ir(R-ppy)2(btz)]PF6 (1 to 3) are reported (btz = 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl, R-ppy = 4-(pyrid-2-yl)benzaldehyde (1), 2-phenylpyridine (2) and 2-(2,4-difluorophenyl)pyridine (3)). Complexes 1, 2 and 3 are luminescent and exhibit structured emission bands with vibronic progressions at 532 & 568 nm (ϕ 0.28%), 476 & 508 nm (ϕ 0.82%) and 454 & 483 nm (ϕ 4.3%) respectively. The structuring of these emission bands is indicative of cyclometalated ligand centred emissive states and is further corroborated by the nearly identical emission spectra for 2 and 3 to previously reported analogous complexes with 4-(pyrid-2-yl)-1,2,3-triazole based ancillary ligands. Computational density functional theory calculations on these complexes show that the LUMOs of 2 and 3 are largely btz-centred but with some cyclometalated pyridine π* character. The LUMO of 1 on the other hand is localized primarily on the cyclometalated ligands. Spin population analysis of the lowest lying triplet excited states for these complexes indicate significant spin population over the iridium centres and the aryl and pyridyl moieties in these complexes with virtually no localization of unpaired electrons over the btz ancillary ligands. This is therefore in agreement with the assignment of the emissive state having largely cyclometalated (3)LC character and being independent of the ancillary ligand.

Excited-State Dynamics and Self-Organization of Poly(3-hexylthiophene) (P3HT) in Solution and Thin Films

The fluorescence decays of a stereoregular head-to-tail RR-HT poly(3-hexylthiophene), P3HT, in methylcyclohexane (MCH) are described by sums of three or four exponential terms, respectively above and below -10 °C. In the high-temperature region, the polymer lifetime (ca. 500 ps) is accompanied by two shorter decay times (ca. 20 and 120 ps), which are assigned to intrachain energy transfer from high to lower energy excitons on the basis of temperature and wavelength dependence of the fluorescence decays. The absence of conformational (torsional) relaxation is attributed to the small dihedral angle between monomers that is predicted for the stereoregular polymer in the ground state. Below -10 °C, the polymer forms excimer-like aggregates, showing vibrational structured absorption and emission bands similar to those observed in thin films. The vibrational structure is attributed to a deep minimum in the ground-state energy surface of the dimer or aggregate. Below -40 °C, the fluorescence measured at the aggregate emission wavelength (670 nm) basically results from direct excitation of the aggregate and decays with a sum of three exponential terms (decay times of ca. 0.14, 0.6, and 1.5 ns, with similar weights). Because the spectral similarities between film and aggregates indicate similar electronic first singlet excited states (and oscillator strengths), the much shorter decay times (0.05, 0.15, and 0.43 ns) and lower fluorescence quantum yield of P3HT in films are assigned to efficient exciton dissociation and/or phonon-induced internal conversion competing with radiative decay (>1 ns).

Long-Lived Room Temperature Deep-Red/Near-IR Emissive Intraligand Triplet Excited State (3IL) of Naphthalimide in Cyclometalated Platinum(II) Complexes and Its Application in Upconversion

[C(^)NPt(acac)] (C(^)N = cyclometalating ligand; acac = acetylacetonato) complexes in which the naphthalimide (NI) moiety is directly cyclometalated (NI as the C donor of the C-Pt bond) were synthesized. With 4-pyrazolylnaphthalimide, isomers with five-membered (Pt-2) and six-membered (Pt-3) chelate rings were obtained. With 4-pyridinylnaphthalimide, only the complex with a five-membered chelate ring (Pt-4) was isolated. A model complex with 1-phenylpyrazole as the C(^)N ligand was prepared (Pt-1). Strong absorption of visible light (ε = 21,900 M(-1) cm(-1) at 443 nm for Pt-3) and room temperature (RT) phosphorescence at 630 nm (Pt-2 and Pt-3) or 674 nm (Pt-4) were observed. Long-lived phosphorescences were observed for Pt-2 (τ(P) = 12.8 μs) and Pt-3 (τ(P) = 61.9 μs). Pt-1 is nonphosphorescent at RT in solution because of the acac-localized T(1) excited state [based on density functional theory (DFT) calculations and spin density analysis], but a structured emission band centered at 415 nm was observed at 77 K. Time-resolved transient absorption spectra and spin density analysis indicated a NI-localized intraligand triplet excited state ((3)IL) for complexes Pt-2, Pt-3, and Pt-4. DFT calculations on the transient absorption spectra (T(1) → T(n) transitions, n > 1) also support the (3)IL assignment of the T(1) excited states of Pt-2, Pt-3, and Pt-4. The complexes were used as triplet sensitizers for triplet-triplet-annihilation (TTA) based upconversion, and the results show that Pt-3 is an efficient sensitizer with an upconversion quantum yield of up to 14.1%, despite its low phosphorescence quantum yield of 5.2%. Thus, we propose that the sensitizer molecules at the triplet excited state that are otherwise nonphosphorescent were involved in the TTA upconversion process, indicating that weakly phosphorescent or nonphosphorescent transition-metal complexes can be used as triplet sensitizers for TTA upconversion.

Kinetics and optical properties of the silver nanoparticles in aqueous L64 block copolymer solutions

Silver nanoparticles were prepared by the chemical reduction of silver nitrate, AgNO 3, with excess of sodium borohydride, NaBH 4, in dilute aqueous solutions containing triblock copolymers poly(ethylene oxide) (PEO)–poly(propylene oxide) (PPO)–poly(ethylene oxide) (PEO) Pluronic L64. The synthesis was completed in ∼24 h and produced highly stable particle dispersion. The growth of nanoparticles was assessed by combining Mie theory, UV–vis spectroscopy and dynamic light scattering, and the average particle size was determined from transmission electron microscopy. The silver nanoparticles were sterically stabilised as a result of the block copolymer adsorption onto the nanoparticle surface. Besides the stabilizing character, the copolymer affected the kinetics of particle growth, the size and optical properties of as-synthesized Ag nanoparticles. It was revealed that the particle formation took place in two distinct stages. The first step was dominated by Ag ion reduction and growth driven by accretion, while in the second stage coagulated clusters of nanoparticles were obtained obtained. The as-synthesized particles were polydisperse, the individual ones had a size of ∼7.6 nm as given by transmission electron microscopy, and the hydrodynamic radius of clusters amounted to 60 nm. The silver nanoparticles exhibited broad and structured emission bands centered at 300 and 330 nm for excitation wavelengths in two different ranges, 220–240 nm and 260–290 nm, respectively. This process may be assigned to two emission components corresponding to two different excitation wavelength domains.

New Palladium(II) and Platinum(II) Complexes with 9-Aminoacridine: Structures, Luminiscence, Theoretical Calculations, and Antitumor Activity

The new complexes [Pd(dmba)( N10-9AA)(PPh 3)]ClO 4 ( 1), [Pt(dmba)( N9-9AA)(PPh 3)]ClO 4 ( 2), [Pd(dmba)( N10-9AA)Cl] ( 3), and [Pd(C 6F 5)( N10-9AA)(PPh 3)Cl] ( 4) (9-AA = 9-aminoacridine; dmba = N,C-chelating 2-(dimethylaminomethyl)phenyl) have been prepared. The crystal structures have been established by X-ray diffraction. In complex 2, an anagostic C-H...Pt interaction is observed. All complexes are luminescent in the solid state at room temperature, showing important differences between the palladium and platinum complexes. Complex 2 shows two structured emission bands at high and low energies in the solid state, and the lifetimes are in agreement with excited states of triplet parentage. Density functional theory and time-dependent density functional theory calculations for complex 2 have been done. Values of IC 50 were also calculated for the new complexes 1- 4 against the tumor cell line HL-60. All of the new complexes were more active than cisplatin (up to 30-fold in some cases). The DNA adduct formation of the new complexes synthesized was followed by circular dichroism and electrophoretic mobility. Atomic force microscopy images of the modifications caused by the complexes on plasmid DNA pB R322 were also obtained.

Waveguiding, absorption and emission properties of dye‐impregnated oxidized porous silicon

AbstractThe waveguiding, absorption and emission properties of oxidised porous silicon waveguides when impregnated with Nile Blue have been studied. We present m‐line measurements before and after the impregnation showing that the effective indices of the modes remain the same. When performing guided luminescence experiments, a structured emission band is measured. Using the refractive index profile extracted from m‐line measurements it has been possible to simulate the emission lineshape assuming the observation of an interference pattern formed across the waveguide. We demonstrate that these oscillations appear because in the first hundreds of nanometers the dye concentration is several orders of magnitude higher than in the rest of the sample. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polymerization and Conformational Transition of Poly(methacrylic Acid) Probed by Electronic Spectroscopy of Aminoacridines

9-Aminoacridinium vinyl monomers with intramolecular charge transfer (ICT) state were studied as molecular probes for copolymerization with methacrylic acid. The fading of the dye color after its addition to the poly(methacrylic acid) (PMA) chain leads to sweeping spectral changes from a broad absorption band in the red region before the polymerization to structured absorption and emission bands in the blue region after labeling. Also, the dyes bound lost their multiexponential decay behavior due to the ICT process and have only a single lifetime of the locally excited state of the aminoacridinium fluorophore. The conformational transition of PMA is sharply determined at pH = 5.2 by time-resolved fluorescence anisotropy. This probe technique also indicates strong polymer segments interactions prior to the critical transition of the chain opening with increases of the pH of the aqueous PMA solution.

Synthesis, Structure, and Photophysical Properties of Luminescent Platinum(II) Complexes Containing Cyclometalated 4-Styryl-Functionalized 2-Phenylpyridine Ligands

A series of new luminescent cyclometalated platinum(II) complexes functionalized with various substituted styryl groups on the cyclometallating ligand [Pt(C/\N-ppy-4-styryl-R)(O/\O-(O)CCR'CHCR'C(O))] (ppy-4-styryl-R = E-4(4-(R)styryl-2-phenylpyridine) (3, R' = Me (acac); 4, R' = (t)Bu (dpm); R = H, OMe, NEt2, NO2) have been prepared. All complexes undergo an E-Z photoisomerization process in CH2Cl2 solution under sunlight, as monitored by 1H NMR. The solid-state structures of 3-OMe, 3-NEt2, 3-NO2, and 4-OMe have been determined by X-ray diffraction studies and compare well with optimized geometries obtained by density functional theory (DFT) calculations. The orbital pictures of 3-H, 3-OMe, and 3-NO 2 are very similar, the highest occupied molecular orbital (HOMO) being highly Pt(5d) metal-based. For 3-NMe2, an additional contribution from the amino-styryl fragment leads to a decreased metal parentage of the HOMO, suggesting a predominantly ILCT character transition. Complexes 3-H, 3-OMe, and 3-NO2 show a low-energy band (350-400 nm) assigned to predominantly charge-transfer transitions. The amino derivative 3-NEt2 displays a very strong absorption band at 432 nm, tentatively assigned to a mixture of ILCT (Et2N --> CH=CH) and metal-to-ligand charge-transfer (MLCT) (dpi(Pt) --> pi) transitions. Complexes 3 are weakly luminescent in CH2Cl2 solution at room temperature; the low intensity may be due to a competitive quenching through the E-Z photoisomerization process. All complexes exhibit similar structured emission bands under these conditions (around 520 nm), independent of the nature of the styryl-R group. In a frozen EPA glass (77 K), the spectrum of the representative complex 3-H exhibits two sets of vibronically structured bands (460-560, 570-800 nm; lambda(max) = 596 nm), due to the presence of two emitting species, the E and Z isomers, which have significantly different triplet excited-state energies. The other three complexes show similar behavior to 3-H at 77 K, but the lower-energy emission bands are progressively red-shifted in the order H < OMe < NO2 < NEt2 (e.g., for 3-NEt2, lambda(max)(em) = 658 nm; tau = 26 micros). The very large red-shift compared to related unsubstituted complexes (e.g., to [Pt(C/\N-ppy)(O/\O-acac)]) is the result of the extension of the pi-conjugated system and the electronic effects of substituent R.

Synthesis, Structures, and Luminescence Behavior of Mixed Metal Alkynyl Platinum−Cadmium Complexes

This article describes the preparation, characterization, and optical properties of new bi- and trinuclear tweezer-type platinum cadmium derivatives, stabilized by η2-alkyne−cadmium bonding. Treatment of Q2[cis-Pt(C6F5)2(C⋮CR)2] (1a−1c) with CdCl2·2,5 H2O (1:1) produces dinuclear Pt−Cd adducts Q2[{cis-Pt(C6F5)2(μ-κCα:η2-C⋮CR)2}(CdCl2)] (Q = NBu4, R = t-Bu 2a; Q = PMePh3, R = Ph 2b; R = Tol 2c). However, reaction of (NBu4)2[cis-Pt(C6F5)2(C⋮CR)2] with Cd(NO3)2·4H2O produces trinuclear Pt2Cd dianionic complexes (NBu4)2[{cis-Pt(C6F5)2(μ-κCα:η2-C⋮CR)2}2Cd] (3a−3c). The analogous (PMePh3)2[{cis-Pt(C6F5)2(μ-κCα:η2-C⋮CR)2}2Cd] (R = Ph 3b‘; Tol 3c‘) were prepared similarly starting from (PMePh3)2[cis-Pt(C6F5)2(C⋮CR)2]. The crystal structures of 2c and 3a,b show that the cadmium center is well embedded by the cis-bis(alkyne)platinate entities, leading to planar PtC4Cd cores. In the heterometallic species, the low-energy absorption, which is ascribed to an admixture of π → π*(C⋮CR) IL/dπ(Pt) → π*(C⋮CR) MLCT, exhibits a clear hypsochromic shift compared to its precursors, probably due to the existence of a lesser delocalization on the alkynyl fragments upon the η2-complexation. The arylalkynyl derivatives (b, c) display intense structured emission bands, arising from 3ππ*(C⋮CR) (IL) and/or mixed 3ππ*/Pt(dπ)(C⋮CR) → π*(C⋮CR) (3MLCT) manifolds with a predominant IL character.

Unique fluorescence spectra of a disubstituted pyrene surfactant in Langmuir–Blodgett films

We first observed a unique pyrene (Py) emission from 10-(1-(6-hexyl)pyrenyl)decanoic acid (1,6-HPDA) Langmuir–Blodgett (LB) films. In the fluorescence spectra of the 1,6-HPDA LB films, the normal bands of monomer and excimer emission of Py were absent but the new structured emission band with peaks at 422, 448, and 473 nm was observed. Besides the intensity, the shape of the characteristic structured emission band at shorter wavelengths than normal excimers did not changed essentially on lowering the Py concentration in the LB monolayer from 100 to 1 mol%. The spectroscopic studies on HPDA LB films of a variety of Py concentrations suggested that a novel excimer responsible for the structured new emission would be efficiently formed by photo-excitation of a Py–Py pre-associated dimer in different manners than those of normal excimer formations.

Anthrylmethylamine functionalised mesoporous silica-based materials as hybrid fluorescent chemosensors for ATP

A number of functionalised mesoporous solids containing anchored anthrylmethylamine groups have been prepared using different co-hydrolysis or grafting synthetic routes. The solids have been characterised using standard solid-state techniques. Solids with a low loading of the anthrylmethylamine probe show typical well defined and structured emission bands centred at ca. 415 nm. Addition of ATP to suspensions of these solids at pH 2.8 resulted in a quenching of the anthracene emission. These solids showed a cooperative effect that resulted in quite a remarkable improvement in ATP response with respect to the free anthrylmethylamine probe in solution. Certain prepared solids showed a remarkable detection limit for ATP detection in the micromolar range. An equation based on a Langmuir-type analysis has been developed and used to obtain binding constants (adsorption constants) for the interaction of the solids with ATP and with some related anions. Some other solids containing larger amounts of anthrylmethylamine groups have also been prepared. In these cases a dual monomer–excimer fluorescence emission was observed. ATP addition to these solids resulted in an enhancement of the excimer emission band.

Synthesis, characterization, and photochemical and computational investigations of Ru(II) heterocyclic complexes containing 2,6-dimethylphenylisocyanide (CNx) ligand.

The isocyanide ligand forms complexes with ruthenium(II) bis-bipyridine of the type [Ru(bpy)(2)(CNx)Cl](CF(3)SO(3)) (1), [Ru(bpy)(2)(CNx)(py)](PF(6))(2) (2), and [Ru(bpy)(2)(CNx)(2)](PF(6))(2) (3) (bpy = 2,2'-bipyridine, py = pyridine, and CNx = 2,6-dimethylphenylisocyanide). The redox potentials shift positively as the number of CNx ligands increases. The metal-to-ligand charge-transfer (MLCT) bands of the complexes are located at higher energy than 450 nm and blue shift in proportion to the number of CNx ligands. The complexes are not emissive at room temperature but exhibit intense structured emission bands at 77 K with emission lifetimes as high as 25 micros. Geometry optimization of the complexes in the singlet ground and lowest-lying triplet states performed using density functional theory (DFT) provides information about the orbital heritage and correlates with X-ray and electrochemical results. The lowest-lying triplet-state energies correlate well with the 77 K emission energies for the three complexes. Singlet excited states calculated in ethanol using time-dependent density functional theory (TDDFT) and the conductor-like polarizable continuum model (CPCM) provide information that correlates favorably with the experimental absorption spectra in ethanol.

Photoluminescence of iodine-doped ZnTe homoepitaxial layer grown by metalorganic vapor phase epitaxy

Photoluminescence (PL) properties of I-doped ZnTe homoepitaxial layers grown by atmospheric pressure metalorganic vapor phase epitaxy have been investigated as a function of n-butyliodide (n-BuI) transport rate. The PL spectrum changed drastically even when a low dopant transport rate was used. A structured emission band at 2.22 eV was detected predominantly at low n-BuI transport rates, whereas a structureless broad emission band at around 2.17 eV or less dominates the PL spectrum at high n-BuI transport rates. PL spectra of I-doped ZnTe layers were analyzed at varying excitation power and temperature. These luminescence bands are due to donor–acceptor pair recombination, and may be ascribed to complexes consisting of Zn vacancies and I on Te sites close to donors.

Excimer and Aggregate Formations in Poly(fluorene)s

This paper investigates the absorption and emission changes in poly(di-n-hexylfluorene)s. We prepared the poly(di-n-hexylfluorene)s end capped with 2-bromofluorene, 2-bromo-9,9-di-n-hexylfluorene, and 9-bromoanthracene through Ni (0) mediated polymerization. In addition, we also synthesized a structurally distorted copolymer of 2,7-dibromo-9,9-di-n-hexylfluorene and 9,9-bis(4-bromophenyl) fluorene end capped with 2-bromofluorene through the same polymerization method. The absorption and emission changes of these polymers between before and after thermal annealing in a nitrogen atmosphere clarify the role of aggregate/excimer formation in poly(fluorene)s. The large absorption changes must be attributed to aggregate formation (ground state interaction), which causes only a slight red shift of the vibronically structured emission bands. We assign the additional long wavelength emission as an excimer band (excited state interaction), which is preferably formed at chain ends.

Syntheses, electronic absorption, emission, and ion-binding studies of platinum(III) C empty set N empty set C and terpyridyl complexes containing crown ether pendants.

A series of platinum (II) C empty set N empty set C complexes, [Pt(C empty set N empty set C) (L)] (HC empty set N empty set CH=2,6-diphenylpyridine (dppy); L=Ph(2)PB15C5 (1, B15C5=benzo[15]crown-5), Ph(2)PDMP (2, DMP=3,4-dimethoxyphenyl), pyCOA15C5 (3, A15C5=aza[15]crown-5), pyCON(CH(2)CH(2)OCH(3))(2) (4), pyC[triple bond]CB15C5 (5), pyC[triple bond]CDMP (6)) and terpyridyl complexes, [Pt(trpy)(L)](X)(2) (trpy=2,2':6',2''-terpyridine; L=Ph(2)PB15C5, X=OTf (7 a), PF(6) (7 b); X=PF(6), L=Ph(2)PDMP (8), pyC[triple bond]CB15C5 (9), and pyC[triple bond]CDMP (10)) have been successfully synthesized and characterized. The structures of 1, 3, and 7 a have been determined by X-ray crystallography. Excitation of complexes 1-6 in EtOH/MeOH (4:1 v/v) glass gave high-energy structured emission bands, assigned as derived from states of metal-perturbed intraligand (IL) origin. At higher concentrations, complexes 3-6 each displayed an additional, structureless emission band at 600-615 nm, with complexes 5 and 6 showing an obvious increase in the intensity of this emission band when the concentration was increased further. In dichloromethane at room temperature, complexes 3-6 showed, in addition to the high-energy emission at 490-505 nm, an extra, broad emission band at 620-625 nm when the concentration was increased. The emission origins of the low-energy band in glass and in fluid solutions are suggested to be derived from the ground-state oligomerization or aggregation process of the complexes. In the solid state at room temperature, complexes 1-6 each showed a broad, unstructured emission band at 560-600 nm, which was shifted to lower energy upon cooling to 77 K. On the other hand, the terpyridyl analogues 7-10 displayed intense vibronic-structured intraligand (IL) emissions at 460-472 nm in butyronitrile glass at 77 K. Solid-state samples of 9 and 10 displayed strong phosphorescence upon photoexcitation at 298 K and 77 K, tentatively assigned as derived from states of Pt(d pi)-->pi*(trpy) (3)MLCT origin(MLCT=metal-to-ligand charge transfer). The ion-binding properties of complexes 5 and 9 for Na(+), Ba(2+), and K(+) ions have been studied by UV/Vis spectrophotometric methods, and confirmed by ESI mass spectrometric studies. The ion-binding properties for Na(+) ions have also been probed by (1)H NMR experiments. For the same crown ether-containing ligand and the same metal ions, the neutral cyclometalated complexes gave larger binding constants than the positively charged terpyridyl analogues.

Characterization and Fluorescence of Macrocyclic Polystyrene by Anionic End to End Coupling. Role of Coupling Reagents

Macrocyclic polystyrene (PS) was synthesized by initiation of styrene by naphthalide anion or benzylic dianions in the presence of Li or K ions followed by reaction of the resulting PS dianions with dibromomethane (DBM) or 1,4-bis(bromomethylbenzene) (DBX) in THF at −78 °C under high dilution conditions. MALDI and NMR studies of the DBM cycles showed the presence of 1,2-diphenyl linkages. Fluorescence studies on the DBM cycles show strongly enhanced and anomalous structured emission bands between 300 and 320 nm attributable to linear chains containing styrenic chain end impurities. This indicates rapid metal bromine exchange to give chain end benzyl bromides that eliminate with polystyryl anion to give small (<1.0%) fractions of styrene type chain end structures. In comparison, the cycles obtained with DBX show only the PS monomer and excimer bands at 285 and 330 nm, respectively, and appear to be substantially free of such side reactions. These DBX coupled cycles show up to a 2-fold enhancement in the mo...

Concentration effect on the up-conversion luminescence of neodymium activated calcium gallium germanium garnet crystal

In this work, we report on the infrared (pump) to visible luminescence of Nd 3+ ion in Ca 3Ga 2Ge 3O 12 for different Neodymium concentrations. Three visible structured emission bands centred at 541, 601 and 677 nm are observed, which are related to emissions mainly from the excited state 4G 7/2. The relative intensity between the red band (677 nm) and the other bands changes with both concentration and excitation intensity for Nd 3+ contents >2 at.%. In addition, a new up-conversion emission band in the blue region (centred about 460 nm) was detected in these samples. These results can be explained because of a pump induced thermalisation of the 4G 11/2 excited state from which additional emissions appear.

Fluorescence study of Eu-Ba-Cu-O type compounds

A study of the excitation and fluorescence spectra of the high temperature superconductor EuBa 2Cu 3O 7- x , the green phase Eu 2BaCuO 5, and the insulating phase EuCuO 2 has been performed at room temperature as well as at low temperatures. The spectra of the trivalent europium ion in these compounds show a similar fluorescence structure when the 7F 0→ 5D 2 transition is excited. On the other hand, these compounds also present a structured emission band characteristic of the Cu + ion in low symmetry sites. From a comparison with the corresponding spectra of the EuCuO 2 compound it can be concluded that this is present as an impurity phase in the superconductor as well as in the green phase, giving rise to the observed spectra.

Assignment of 522 nm InHg emission band

Selective dye laser excitation of atomic indium to the In(6 2S 1 2 ) state in the presence of mercury vapour has shown that the structured emission band at ≈522 nm, observed in excited InHg mixtures, can be assigned to emission arising from the bound excimer state of InHg correlating with the In(6 2S 1 2 )+Hg(6 1S 0) asymptote.

Selective excitation of localised excitons in ZnSe1-xTexsolid solutions

Employing selective monochromatic laser excitation, the photoluminescence in ZnSe1-xTe/sub /x alloys with Te concentrations 1%<or=x<or=2.3% is investigated. The structured emission band is interpreted as being due to recombination of excitons localised at different types of small Te clusters. The energy spectrum of these excitons is inhomogeneously broadened. The details of exciton-lattice interaction revealed under resonant excitation of these exciton states strongly favour the model of large-radii excitons tightly bound through the hole. For excitation above the exciton gap, resonant Raman scattering by longitudinal optical (LO) phonons is observed. The measured degree of polarisation is used to discriminate between this process and the narrow-line LO resonance luminescence found under selective excitation of the localised states. The smooth transformation from Raman scattering to resonance fluorescence indicates that, owing to strong interaction between the cluster and band states, no energy gap exists between the free and localised exciton states.