Phosphorescence Bands Research Articles

Trinuclear Gold(I) Triazolates: A New Class of Wide-Band Phosphors and Sensors

A new cyclic gold(I) triazolate trimer, [Au(3,5-i-Pr2Tz)]3 (1), exhibits fully overlapping aurophilically bonded dimer-of-trimer units that lead to multiple phosphorescence bands in both the solid state and solution. The conformation of the hexanuclear unit exhibits reversible interconversion between C2 and D3 effective symmetries, depending on the crystal temperature or solution concentration, the variation of which leads to isoemissive and isosbestic points. Solutions of 1 exhibit remarkable quenching properties that demonstrate molecular recognition with high selectivity and hypersensitivity for some reagents, as influenced by protonation via Brønsted acids, pi intercalation, and/or energy transfer. The quenched phosphorescence of 1 by acetic acids can be regenerated by NEt3.

Synthesis and luminescent properties of lanthanide homoleptic mercaptothi(ox)azolate complexes: Molecular structure of Ln(mbt) 3 (Ln = Eu, Er)

The thiolate complexes of rare earth metals Ln(SR) 3 (La, HSR = 2-mercaptothiazoline ( 1); La, HSR = 2-mercaptobenzoxazole ( 2); Y, La, Sm, Eu, Tb, Gd, Er, Tm, HSR = 2-mercaptobenzothiazole ( 3)) were synthesized in 84–97% yield by the reactions of silylamides Ln[N(SiMe 3) 2] 3 with respective thiols. The products were characterized by elemental analysis, IR and UV/Vis spectroscopy. The structures of 3(Eu) and 3(Er) were determined by single-crystal X-ray diffraction. All obtained compounds revealed efficient luminescence in the region 400–550 nm at 293 K assigned to the ligands emission. Besides, the luminescent spectra of thiolates 3 at 77 K displayed the phosphorescent band of the ligand at 550 nm and in the cases of 3(Eu) and 3(Tb) the sets of emissions bands characteristic for Eu 3+ and Tb 3+ ions.

Triplet–triplet energy transfer from naphthalene to biacetyl in the vapor phase

In principle the optical energy absorbed by a complex molecule raises that molecule to one of its excited states, and afterwards this excitation energy decays through the different relaxation channels. Initially, electronically excited naphthalene emits photons in the phosphorescence band of naphthalene and these emitted photons are absorbed by the acceptor molecule biacetyl, then excited biacetyl phosphoresces. In this investigation, sensitized phosphorescence decay times in different conditions were measured for naphthalene–biacetyl system in the vapor phase. The ultraviolet–visible spectrum of biacetyl vapor at room temperature conditions was broad and structureless.

Estimated IR and phosphorescence emission fluxes for specific polycyclic aromatic hydrocarbons in the Red Rectangle

Following the tentative identification of the blue luminescence in the Red Rectangle by Vijh et al. (2005), we compute absolute fluxes for the vibrational IR emission and phosphorescence bands of three small polycyclic aromatic hydrocarbons. The calculated IR spectra are compared with available ISO observations. A subset of the emission bands are predicted to be observable using presently available facilities, and can be used for an immediate, independent, discriminating test on their alleged presence in this well-known astronomical object.

Assignments of Lowest Triplet State in Ir Complexes by Observation of Phosphorescence Excitation Spectra at 6 K

We tried the assignment of the origin of phosphorescent bands in Ir complexes. It is important to elucidate the luminescent mechanism in order to design organic light-emitting devices (OLEDs) besed on new materials. The Stokes shift between the phosphorescence and phosphorescence excitation spectra of Ir complexes such as fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3], fac-tris(2-(2-thienyl)pyridine) iridium(III) [Ir(thpy)3], bis(2-phenylpyridine) iridium(III)benzo[h]quinoline [Ir(ppy)2bzq], fac-tris(benzo[h]quinoline) iridium(III) [Ir(bzq)3] and bis[(4,6-difluorophenyl)pyridinato](picolinato) iridium(III) [FIrpic] was measured in a solution and (phenyl)4Sn at 6 K. The amount of Stokes shift corresponds to the nature of the lowest triplet state. We discovered that the amount of Stokes shift clearly differs depending on whether the lowest triplet state of each Ir complex is triplet metal-to-ligand charge transfer (3MLCT) or 3π–π*. Namely, the case of 3MLCT shows a large shift, while the case of 3π–π* shows a small shift. We also present the resolved phosphorescence and phosphorescence excitation spectra of Ir complexes in (phenyl)4Sn. The sharp bands were assigned to the 3π–π* transition, and the broad bands were assigned to the 3MLCT state. The nature of the lowest triplet state is also discussed on the basis of resolved spectra.

Synthesis, structures and photoluminescence behavior of some group 10 metal alkynyl complexes derived from 3-( N-carbazolyl)-1-propyne

A new class of luminescent and thermally stable mononuclear group 10 platinum(II) and palladium(II) acetylides trans-[Pt(PR 3) 2(L) 2] (R = Bu, Et) and trans-[Pd(PBu 3) 2(L) 2] ( LH = 3-( N-carbazolyl)-1-propyne) have been successfully synthesized and characterized. The structural properties of these discrete metal complexes have been studied by X-ray crystallography. We report their optical absorption and photoluminescence spectra and interpret the results in terms of the nature of the metal center and the type of phosphines used. Our investigations indicate that they display heavy metal-enhanced phosphorescence bands at 77 K and we find that the platinum complexes afford more intense triplet emission than that for the palladium congener, consistent with the stronger heavy-atom effect of the third row element than the second row neighbor of the same group.

Metal Effect on the Supramolecular Structure, Photophysics, and Acid−Base Character of Trinuclear Pyrazolato Coinage Metal Complexes

Varying the coinage metal in cyclic trinuclear pyrazolate complexes is found to significantly affect the solid-state packing, photophysics, and acid-base properties. The three isoleptic compounds used in this study are [[3,5-(CF3)2Pz]M]3 with M = Cu, Ag, and Au (i.e., Cu3, Ag3, and Au3, respectively). They form isomorphous crystals and exist as trimers featuring nine-membered M3N6 rings with linear two-coordinate metal sites. On the basis of the M-N distances, the covalent radii of two-coordinate Cu(I), Ag(I), and Au(I) were estimated as 1.11, 1.34, and 1.25 angstroms, respectively. The cyclic [[3,5-(CF3)2Pz]M]3 complexes pack as infinite chains of trimers with a greater number of pairwise intertrimer M...M interactions upon proceeding to heavier coinage metals. However, the intertrimer distances are conspicuously short in Ag3 (3.204 angstroms) versus Au3 (3.885 angstroms) or Cu3 (3.813 angstroms) despite the significantly larger covalent radius of Ag(I). Remarkable luminescence properties are found for the three M3 complexes, as manifested by the appearance of multiple unstructured phosphorescence bands whose colors and lifetimes change qualitatively upon varying the coinage metal and temperature. The multiple emissions are assigned to different phosphorescent excimeric states that exhibit enhanced M...M bonding relative to the ground state. The startling luminescence thermochromic changes in crystals of each compound are related to relaxation between the different phosphorescent excimers. The trend in the lowest energy phosphorescence band follows the relative triplet energy of the three M(I) atomic ions. DFT calculations indicate that [[3,5-(R)2Pz]M]3 trimers with R = H or Me are bases with the relative basicity order Ag << Cu < Au while fluorination (R = CF3) renders even the Au trimer acidic. These predictions were substantiated experimentally by the isolation of the first acid-base adduct, [[Au3]2:toluene]infinity, in which a trinuclear Au(I) complex acts as an acid.

Yellow and Red Electrophosphors Based on Linkage Isomers of Phenylisoquinolinyliridium Complexes: Distinct Differences in Photophysical and Electroluminescence Properties

We report the synthesis and organic light-emitting devices (OLEDs) made from a series of 1-phenyl- and 3-phenylisoquinolinyliridium complexes in which the phenyl group is linked to the C1 and C3 carbons of isoquinoline, respectively. These linkage isomers show distinct differences in their photophysical and electroluminescence (EL) properties, including the magnitude of phosphorescent lifetimes and photoluminescence (PL) and EL emission wavelengths, as well as the phenomenon of triplet–triplet (T–T) annihilation. Complexes of these two families show a strong absorption band in the region 440–490 nm assignable to spin-forbidden 3MLCT (metal–ligand charge-transfer) bands. The extinction coefficients of these bands are similar to those of spin-allowed 1MLCT bands, indicative of an anomalously strong spin–orbital coupling. Upon excitation, 1-phenylisoquinolinyliridium complexes exhibit a single phosphorescent emission band in the red region (595–631 nm). All of these red phosphors show outstanding EL performance with negligible T–T annihilation because of short phosphorescent lifetimes (1.04–2.46 μs in CH2Cl2) and good emission quantum yields. One representative, [Ir(5-f-1piq)2(acac)] (acac = acetylacetonate) (3) (5-f-1piqH = 5-fluoro-1-phenylisoquinoline), is not only the brightest at low voltages (1883 cd m–2 at 7.1 V; 8320 cd m–2 at 9.0 V) but also shows a ηext value of. 6.50 % at high current (J = 400 mA cm–2). The maximum brightness is 38 218 cd m–2 (x = 0.68, y = 0.31) with the full width at half maximum (FWHM) only 50 nm at 8 V. In contrast, 3-phenylisoquinolinyliridium complexes show phosphorescent emissions in the yellow region (534–562 nm) but with a long phosphorescent lifetime (3.90–15.6 μs in CH2Cl2). Most of these yellow phosphors suffer T–T annihilation in the EL performance. The exception is [Ir(3-piq)2(acac)] (5) (3-piqH = 3-phenylisoquinoline), which has a relatively short lifetime 3.90 μs in CH2Cl2. Complex 5 achieves an external efficiency (ηext) value of 5.27 % at J = 20 mA cm–2 and maintains a ηext value of 3.58 at J = 400 mA cm–2 with a maximum brightness of 65 448 cd m–2 (x = 0.49, y = 0.51).

Electronic Structure of Mercury Oligomers and Exciplexes: Models for Long-Range/Multicenter Bonding in Phosphorescent Transition-Metal Compounds

Spectroscopic and bonding properties of Hg(n) oligomers and *Hg(n) exciplexes are determined by rigorous theoretical treatments. Reliable values that agree well with experimental data have been computed for the luminescence energies and other molecular spectroscopic parameters by making a careful selection of theoretical methods and basis sets. The calculations clarified the assignments for several phosphorescence bands in the mercury vapor based on calculated energies and other parameters that quantify the large excited-state distortion in the emitting states. Both the weak ground-state mercurophilic bonding and the stronger covalent bonding in the triplet and quintet excited states studied are found to be cooperative, which is important for fundamental and applied research for luminescent and magnetic materials that have spectral behavior similar to that of Hg(n) systems.

Synthesis, electrochemical, photovoltaic, and photo-physical properties on the lanthanide(III) complexes of acetylacetonate and meso-tetraalkyltetrabenzoporphyrin with electric-field induction

Lanthanide(III) complexes with acetylacetonate and meso-tetraalkyltetrabenzoporphyrin (TATBP) having the general formula Ln ( TATBP )acac (where Ln = Tb , Dy , Ho , Er , Tm , Yb ; A = C 12 H 25; Hacac = acetylacetone) are reported. These compexes have been studied by elemental analyses, ultraviolet visible spectra, infrared spectra, molar conductance, 1 H NMR spectra, cyclic voltammetry, surface photovoltage spectroscopy (SPS), and luminescence spectroscopy. The infrared spectral bands of the ligand and complexes were assigned. In dimethylformamide (DMF), 0.1 M tetrabutylammonium perchlorate (TBAP), the synthesized TATBP exhibit two one-electron reversible redox reactions, and Ln(TATBP)acac shows three redox reactions respectively, within the accessible potential window of the solvent. The absorption bands of the complexes appear in the range 431-433 (Soret band), 578-580 (Q band) and 627-631 (Q band) nm. The photovoltaic properties and charge transfer process of these compounds were investigated by surface photovoltage spectroscopy (SPS) and electric-field-induced surface photovoltage spectroscopy (EFISPS) techniques, which reveal that the ligand TATBP and the complexes Er(TATBP)acac are p-type semiconductors. The spectral bands of TATBP correspond to π → π* transitions. Quantum yields of the S 1 → S 0 fluorescence are in the region 0.25-0.27 and the fluorescence lifetimes are in the region 0.014-0.022 ms at room temperature. The phosphorescence bands of the complex at 77 K appears 714 nm.

Synthesis and photoelectronic properties on a series of lanthanide dysprosium(III) complexes with acetylacetonate and meso-tetraalkyltetrabenzoporphyrin

Lanthanide dysprosium(III) complexes with acetylacetonate and meso-tetraalkyltetrabenzoporphyrin (TATBP) having the general formula Dy(TATBP)acac (where A=alkyl=C 6H 13, C 8H 17, C 10H 21, C 12H 25, C 14H 29, C 16H 33 and C 18H 37; Hacac=acetylacetone) are reported. (The structures of ligand and complexes were shown in Fig. 1(a) tetraalkyltetrabenzoporphyrin, TATBP, n=5, 7, 9, 11, 13, 15, 17 correspond to 6L, 8L, 10L, 12L, 14L, 16L and 18L, respectively. Fig. 1(b) lanthanide(III) complexes with acetylacetonate and meso-tetraalkyltetrabenzoporphyrin, Dy(TATBP)acac, n=5, 7, 9, 11, 13, 15, 17 correspond to 6Dy, 8Dy, 10Dy, 12Dy, 14Dy, 16Dy and 18Dy, respectively.) These compexes have been studied by elemental analyses, UV–vis spectra, infrared spectra, molar conductance, 1H NMR spectra, cyclic voltammetry, surface photovoltage spectroscopy (SPS) and luminescenc spectroscopy. The infrared spectral bands of the ligand and complexes were assigned. In dimethylformamide (DMF), 0.1 M tetrabutylammonium perchlorate (TBAP), the synthesized TATBP exhibit two one-electron reversible redox reactions and Dy(TATBP)acac show three redox reactions, respectively, within the accessible potential window of the solvent and the measured electrochemical redox potentials. The absorption bands of the complexes appear in the range 431–434 (Soret band), 578–581 (Q band) and 629–630 nm (Q band). The photovoltaic properties and charge transfer process of these compounds were investigated by surface photovoltage spectroscopy (SPS), which reveal that these ligands and complexes are all p-type semiconductor. Quantum yields of the S 1→ S 0 fluorescence are in the region 0.26–0.28 and the fluorescence lifetimes are in the region 0.013–0.019 ms at room temperature. The phosphorescence bands of the complex at 77 K appears 714 nm. It is found that the stronger the fluorescence intensity is, the weaker the surface photovoltage intensity is.

Surface Photovoltage, Luminescence, and Cyclic Voltammetry on the First Series of Lanthanide(III) Monobenzoporphyrin Compound Liquid Crystals and Relative Transition Metal Benzoporphyrin Compound Liquid Crystals

The first series of lanthanide(III) monobenzoporphyrin liquid crystalline compounds, meso-tetraalkyltetrabenzoporphyrin (TATBP) ytterbium hydroxy, and relative transition metal benzoporphyrin liquid crystalline compounds (4 series, 24 kinds) are reported that display a hexagonal columnar discotic columnar (Colh, previously designated Dh) phase. These liquid crystalline compounds have been studied by cyclic voltammetry, luminescence, and surface photovoltage spectroscopies. In a dimethylformamide (DMF) solution of 0.1 M tetrabutylammonium perchlorate (TBAP), the synthesized TATBP and TATBPZn exhibit two one-electron reversible redox reactions, and TATBPYbOH and TATBPCoCl show three redox reactions, respectively, within the accessible potential window of the solvent and the measured electrochemical redox potentials. The absorption bands of the porphyrins appear in the ranges 425−438 (Soret band), 572−581 (Q-band), and 625−631 (Q-band) nm. The photovoltaic properties and charge-transfer process of the liquid crystalline compounds were investigated by surface photovoltage spectroscopy (SPS) and electric-field-induced surface photovoltage spectroscopy (EFISPS) techniques, which reveal that all compounds are P-type semiconductors. The spectral bands of TATBP and metal TATBP correspond to the π → π* transition, and the metal ion corresponds to the π* orbital (d → eg(π*)) transition, respectively. The electron (or hole) can be trapped on the liquid crystal porphyrin film by applying both light and a negative (or positive) electric field. Quantum yields of the S1 → S0 fluorescence are in the region 0.20−0.28, and the fluorescence lifetimes are in the region 0.015−0.028 ms at room temperature. The phosphorescence bands of the complexes at 77 K appear in the range 715−725 nm. It is found that the stronger the fluorescence intensity is, the weaker the surface photovoltage intensity is. The TATBPCo(III)Cl are 1:1 type electrolytes. The study contributes to further choice and applications of the liquid crystals.

Long-lasting phosphorescent pigments of the type SrAl2O4:Eu2+, R3+(R = Dy, Nd) synthesized by the sol-gel method

Phosphorescent pigments have been investigated for many years, being the SrAl 2 O 4 :Eu 2+ ,R 3+ with a structure of the stuffed trydimite type the one which exhibits better luminescent properties. The ceramic synthesis of this pigment requires, however, high temperatures during the firing process, reason why it is necessary to use mineralizers that give rise to problems not only in the firing process but also from an environmental point of view. Besides, a mixture of N 2 -H 2 is needed to reduce the Eu (III) to Eu (II) during the heat treatment. This paper reports the synthesis of the compound by means of an alternative method to the ceramic route. In this case the used method is a variant of the Pechini's one because ethilenglycol is not utilized and a resin, difficult to manipulate, is not formed. The use of gaseous currents is not required, since the organic matter of the gel itself generates the reducing atmosphere. Once mixed the different solutions, the gel formation takes place by the addition of citric acid and further elimination of the solvent. The gel is dried, fired and analyzed by X-ray diffraction. Luminescence related to Eu 3+ 4f crystal field transitions shows a drastic reduction of trivalent Eu concentration when codoping with Dy or Nd. The characteristic broad phosphorescence band centered at 520 nm is observed under ultraviolet illumination and is enhanced in Dy codoped pigments.

Synthesis and properties of transition metal benzoporphyrin compound liquid crystalsElectronic supplementary information (ESI) available: CHN data, absorption spectral maxima (nm), and infrared spectral data of the ligands and complexes. See http://www.rsc.org/suppdata/jm/b3/b302308g/

meso-Tetraalkyltetrabenzoporphyrin (TATBP) and related transition metal benzoporphyrin liquid crystalline compounds, TATBPMnCl, TATBPNi, and TATBPCu (4 series, 28 kinds) are reported that display a hexagonal columnar discotic columnar (Colh) phase. These liquid crystalline compounds have been studied by cyclic voltammetry, surface photovoltage spectroscopy (SPS), and luminescence spectroscopy. In dimethylformamide (DMF), 0.1 M tetrabutylammonium perchlorate (TBAP), these synthesized compounds exhibit two one-electron reversible redox reactions, within the accessible potential window of the solvent and the measured electrochemical redox potentials. The absorption bands of the complexes appear in the range 425–431 (Soret band), 576–579 (Q band) and 627–631 (Q band ) nm. The photovoltaic properties and charge transfer process of the liquid crystalline compounds were investigated by surface photovoltage spectroscopy (SPS) and electric-field-induced surface photovoltage spectroscopy (EFISPS) techniques, which reveal that all the compounds are p-type semiconductors. The spectral bands of TATBP and metal TATBP correspond to π → π* transitions. Electrons (or holes) can be trapped on the liquid crystal porphyrin film by applying both light and negative (or positive) electric field. Quantum yields of the S1 → S0 fluorescence are in the region 0.19–0.29 and the fluorescence lifetimes are in the region 0.015–0.017 ms at room temperature. The phosphorescence bands of the complexes at 77 K appear at 725 nm. It is found that the stronger the fluorescence intensity is, the weaker the surface photovoltage intensity is. The TATBPMn(III)Cl are 1 ∶ 1 type electrolytes. The study contributes to further choice and applications of the liquid crystals.

Blue Luminescent Organosilicon Compounds Based on 2,2‘-Dipyridylaminophenyl and 2,2‘-Dipyridylaminobiphenyl

Five new luminescent organosilicon compounds, bis[p-(2,2‘-dipyridylamino)phenyl]dimethylsilane (1), bis[p-(2,2‘-dipyridylamino)-4,4‘-biphenyl]dimethylsilane (2), p-(2,2‘-dipyridylamino)phenyldimethylhydroxysilane (3), p-(2,2‘-dipyridylamino)-4,4‘-biphenyldimethylhydroxysilane (4), and 1,1,3,3-tetramethyl-1,3-bis[p-(2,2‘-dipyridylamino)phenyl]disiloxane (5), are synthesized. The structures of compounds 1 and 3−5 are determined by single-crystal X-ray diffraction. All five compounds have fluorescent emissions at 379−383 nm at room temperature. At 77 K, compounds 1−5 have phosphorescent emission bands with λmax ranging from 407 to 518 nm. Increasing the degree of conjugation of the ligand has no significant effect on the fluorescent emission energy of the complexes, but causes a substantial red shift of the phosphorescent emission energy. Both fluorescent and phosphorescent emission of these compounds originates from π* → π transitions centered on the p-(2,2‘-dipyridylamino)phenyl and p-(2,2‘-dipyridylamino)-4,4‘-biphenyl ligands.

1,8-Naphthalimides in phosphorescent organic LEDs: the interplay between dopant, exciplex, and host emission.

Four different 1,8-naphthalimide derivatives were examined in phosphorescent organic light emitting diodes (OLEDs), i.e., 1,8-naphthalimide, N-phenyl-1,8-naphthalimide, N-2,6-dibromophenyl-1,8-naphthalimide (niBr), and bis-N,N-1,8-naphthalimide. Photoluminescence from all four naphthalimides have violet-blue fluorescence and phosphorescent bands between 550 and 650 nm (visible at 77 K). While all four compounds gave good glassy films when doped with a phosphorescent dopant, only the niBr films remained glassy for extended periods. OLED studies focused on niBr, with two different architectures. One OLED structure (type 1) had the niBr layer as a doped luminescent layer and an undoped niBr layer to act as a hole-blocking layer. The alternate structure (type 2) utilizes a doped CBP layer as the luminescent layer and the niBr layer is used as a hole-blocking layer only (CBP = 4,4'-N,N'-dicarbazolylbiphenyl). Type 1 and 2 OLEDs were prepared with green, yellow, and red emissive phosphorescent dopants (Irppy, btIr, and btpIr, respectively). The dopants were organometallic Ir complexes, previously shown to give highly efficient OLEDs. Of the three dopants, the btpIr-based OLEDs showed the best device performance in both structures (peak efficiencies for type 2: 3.2% and 2.3 lum/W at 6.3 V; type 1: 1.7% and 1.3 lm/W at 6.1 V). The green and yellow dopants gave very similar performance in both type 1 and 2 devices (peak efficiencies are 0.2-0.3%), which were significantly poorer than the btpIr-based OLEDs. The emission spectrum of the btIr- and btpIr-based devices (type 1 and 2) are the same as the solution photoluminescence spectrum of the dopant alone, while the Irppy device gives a broad red emission line (lambda(max) = 640 nm). The red Irppy.niBr emission line is assigned to an Irppy.niBr exciplex. The type 2 Irppy-based device gave a voltage-dependent spectrum, with the red emission observed at low bias (4-8 V), switching over to strong green emission as the bias was raised. All other devices showed bias-independent spectra. Estimates of HOMO, LUMO, and excited-state energies (dopant, niBr, and exciplex) were used to explain the observed spectral properties of these devices. btpIr-based devices emit efficiently from isolated dopant states (external efficiencies = 3.2 %, 2.3 lum/W). Irppy-based devices emit only from exciplex states, with low efficiency (external efficiency = 0.3%). btIr.niBr films have very similar energies for the dopant, exciplex, and niBr triplet states, such that relaxation can go through any of these states, leading to low device efficiency (external efficiency = 0.4%). High device efficiency is achieved only when dopant emission is the dominant pathway for relaxation, since exciplex and niBr triplet states give either weak or no electroluminescence.

Rotamerisation and intramolecular proton transfer of 2-(2′-hydroxyphenyl)oxazole, 2-(2′-hydroxyphenyl)imidazole and 2-(2′-hydroxyphenyl)thiazole: a theoretical study

Semi-empirical (AM1–SCI) calculations have been performed on 2-(2′-hydroxyphenyl)oxazole (HPO), 2-(2′-hydroxyphenyl)imidazole (HPI) and 2-(2′-hydroxyphenyl)thiazole (HPT) to rationalise the photophysical behaviour of the compounds exhibiting intramolecular rotation as well as excited state intramolecular proton transfer (ESIPT). The calculations reveal that there is a gradual variation in the properties from HPO to HPT through HPI so far as the existence of the rotational isomers in the ground state is concerned. While HPO gives rise to two stable rotamers ( I and II) in all the common solvents, there is only one stable species for HPT in the S 0 state. For HPI, rotamer II is possible only in the isolated state and/or in solvents of low polarity, but in high polar solvents it gives rise to the normal form ( I) only. For all the molecules in the series, however, intramolecular proton transfer (IPT) takes place in the lowest excited singlet ( S 1) and the triplet ( T 1) states. Combination of the rotamerism and ESIPT gives rise to multiple fluorescence bands for the fluorophores. Theoretical assignments have been made for the excitation, fluorescence and phosphorescence bands. Simulated potential energy curves (PEC) in different electronic states reveal that the IPT process is feasible in either of the S 1 and T 1 states but not in the ground state. The ESIPT reaction has been found to be favoured both thermodynamically and kinetically in these electronic states compared to the ground state. However, quantum mechanical tunnelling has been proposed for the prototropic reaction to proceed in the excited states.

Improvement of persistent phosphorescence of Ca0.9Sr0.1S : Bi3+ by codoping Tm3+

Persistent phosphors Ca0.9Sr0.1S:0.01Bi3+ and Ca0.9Sr0.1S:0.01Bi3+,0.01Tm3+ were prepared via solid chemical reactions at 1200°C. Luminescence, excitation spectra, decays of afterglow and thermoluminescence were investigated. Blue persistent phosphorescent band of 3P1–1S0 transition of Bi3+ was observed at 453nm with Stoke’s shift of 45nm. The influence of codoping of Tm3+ on phosphorescence was studied. It was found that adding Tm3+ to the mixed sulfide host created a new trapping center, and the brightness and the persistent time of the phosphor were improved by more than 35%.

Investigations on nonradiative transitions in different environments using excited (or ground state) 1,2,3,4-tetrahydroquinoline (THQ) as donor and ground state (or excited) 9-fluorenone (9FL) or 2-nitro-9-fluorenone (2N9FL) as acceptors

By using steady state and time resolved spectroscopic techniques, investigations were made on the nature and mechanisms of different nonradiative processes involved within the excited donor (or acceptor) and ground state acceptor (or donor) in nonpolar n-heptane (NH), polar protic ethanol (EtOH), and polar aprotic acetonitrile (ACN) solvents at the ambient temperature as well as in EtOH rigid glassy matrix at 77 K. The bicyclic molecule 1,2,3,4-tetrahydroquinoline (THQ) was used as a donor (D) in the present investigation whereas 9-fluorenone (9FL) and 2-nitro-9-fluorenone (2N9FL) were chosen as electron acceptors (A). When the donor chromophore was excited in presence of an acceptor, highly exothermic electron transfer (ET) reactions seemed to occur from observed large negative driving energy (deltaG0) values, measured by electrochemical techniques, Förster's type energy transfer, static quenching, etc. An attempt was made to estimate separately the contributions of static and dynamic quenching modes in the overall quenching mechanism of donor fluorescence by using a model of modified Stern-Volmer (SV) relation. From this relation it seemingly indicated that the major contribution in quenching originated from the static mode. When excited acceptors react with the ground state donor THQ it is primarily ET reactions that seem to occur. Observations of the transient absorption spectra, by laser flash photolysis techniques, of contact ion-pairs of the present D-A systems along with the triplet absorption spectra of the monomeric acceptor (9FL and 2N9Fl) corroborate our views regarding the occurrence of photoinduced ET reactions within the present reacting systems. At 77 K the combined effect of Förster type energy transfer and static quenching seems to be responsible for observed lowering of donor fluorescence emission intensity in presence of acceptors (9FL or 2N9FL). However, the phosphorescence lifetime measurements reveal that the triplet donor might be involved in photoinduced ET reaction with the unexcited acceptor in rigid glassy matrix at 77 K and this possibly causes the reduction in the phosphorescence band intensity of the donor THQ in presence of the latter one. Key words: electron transfer, static quenching, fluorescence quenching, phosphorescence, energy transfer.

Photogeneration Action Spectroscopy of Neutral and Charged Excitations in Films of a Ladder-Type Poly(Para-Phenylene)

The photogeneration quantum efficiency action spectra of long-lived neutral and charged excitations in films of a ladder-type poly(para-phenylene) were measured. We found that both triplet and polaron action spectra show, in addition to a step function increase at the optical gap, a monotonic rise at higher energies. For triplets this rise is explained by singlet exciton fission into triplet pairs from which the triplet exciton energy in the gap was obtained; this energy was also confirmed by measuring the weak phosphorescence band. For polarons the photogeneration increase at high energies is modeled by a novel hot electron interchain tunneling process.