Nonradiative Decay Processes Research Articles

Comparison of Photophysical Properties of the Hemicyanine Dyes in Ionic and Nonionic Solvents

The fluorescence properties of 4-[4-(dimethylamino)styryl]-1-n-alkylpyridinium bromide (hemicyanine) dissolved in solvents of different polarities and viscosities (methanol, ethylene glycol, tetra-ethylene glycol, glycerol, benzyl alcohol, pyridine, and two ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM]BF4, and 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM]PF6) were investigated. Significant increase in the fluorescence quantum yield and the fluorescence decay lifetime was observed with the increase in the viscosity of the solvent medium. It is because the intramolecular rotational motion of the molecule becomes more difficult in viscous liquid, which leads to a decrease in the nonradiative decay processes. The fluorescence quantum yields for all of the solutions followed a semiempirical law that depends only on the solvent viscosity. The correlation function C(t) was obtained for each solution by joining fluorescence decay curves measured at different wavelengths. From the fitted results of C(t), we observed the distinctive feature unique to the ionic liquids, in which the correlation functions for ionic liquid solutions are fitted to be biphasic, while they are monophasic for other solvents. The fluorescence maximum of hemicyanine dissolved in these ionic liquids red-shifted following the increase in the excitation wavelength.

Spontaneous decay process of a two-level atom embedded in a one-dimensional structure containing left-handed material

The spontaneous decay process of a two-level atom embedded in a one-dimensional structure containing left-handed material (LHM) is investigated. We first consider the case of the atom being in front of a LHM layer mounted on a mirror, where it has been claimed that the atomic decay would be inhibited completely. Here we introduce dispersion and dissipation in the LHM, and find that there are two factors that weaken the inhibition of spontaneous decay. One is atomic nonradiative decay at shorter distances and the other is the instantaneous radiative decay process for larger distances. Then we discuss the decay process of an atom embedded in a one-dimensional photonic crystal composed of alternate LHM and ordinary material [right-handed material (RHM)] layers (LHM-RHM PC). Because of two special characteristics of the LHM, phase compensation and the fact that a frequency with negative index is usual near the material resonance frequency, both the radiative and nonradiative decay processes in the LHM-RHM PC are much different from those in a RHM PC. Under appropriate condition, the population evolution of atoms in a LHM-RHM PC displays oscillatory behavior.

Optical properties of structurally sorted single‐wall carbon nanotube ensembles

AbstractUsing density gradient ultracentrifugation and isopycnic fractionation we structurally sort traditionally prepared single‐wall carbon nanotube suspensions, and show that fractions with photoluminescence (PL) quantum yields of over 1% can be isolated. Moreover, we find that traditionally prepared suspensions appear to be composed of a significant fraction of small nanotube aggregates which reduce the quantum yield of the ensembles by a factor of 5 with respect to the most photoluminescent fractions. In order to better understand the nature of the nonradiative decay processes that lead to lower ensemble PL quantum yields, nanotube ropes of diameter enriched tubes are made and compared with the aforementioned ensemble measurements. Changes in the optical properties are discussed in terms of energy transfer to metallic tubes, carbonaceous particles, and smaller band gap tubes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of the structure, the optical properties, and the photophysics of some indolocarbazoles having terminal aromatic rings

Structures, optical properties, and photophysics of ladder indolo[3,2- b]carbazoles substituted symmetrically by phenylene and thiophene rings have been investigated theoretically and experimentally. The ground state optimized structures were obtained using the density functional theory (DFT) as approximated by the B3LYP functional and employing the 6-31G ∗ basis set. All derivatives were found nonplanar in their electronic ground states. The character and the energy of the singlet–singlet electronic transitions have been investigated by applying the time-dependent density functional theory (TDDFT) to the correspondingly optimized-ground-state geometries. The ab initio restricted configuration interaction (singles) method (RCIS/6-31G ∗) was adopted to obtain the first singlet excited-state structures (S 1) of the molecule. TDDFT calculations performed on the S 1 optimized geometries was used to obtain emission energies. UV–vis and fluorescence spectroscopies were analyzed in conjunction with theoretical calculations. The computed excitation and emission energies were found in reasonable agreement with the experimental absorption and fluorescence spectra. Finally, the photophysical behavior of the indolocarbazoles have been studied by means of steady state and time resolved fluorescence. The overall data have allowed the determination of the rate constants for the radiative and nonradiative decay processes. Both theoretical and experimental data show that the replacement of phenylene rings by thiophene units induces a red shift in the absorption and fluorescence spectra. This behavior is interpreted in terms of the electron donor properties of the thiophene ring. On the other hand, the change of the substitutional pattern, from 2,8 to 3,9, causes a significant hypsochromic shift of the absorption and fluorescence bands.

Growth and spectroscopic characterization of Er3+:CaWO4

In this work we present the growth and spectroscopic investigation of Er3+ in CaWO4 single crystals. Er3+ polarized absorption has been measured at room temperature from UV to 1700nm, and the fluorescence from the most important emitting states from the visible to the infrared has been recorded at 10 and 300K in the two possible polarizations. The lifetimes of the lower lying excited states have been measured as a function of the temperature to evaluate the contribution of nonradiative decay processes. The absorption spectra enabled us to determine radiative lifetimes of the emitting states by means of the the Judd-Ofelt approach. The stimulated emission cross section has been estimated for the 1.5μm transition. A comparison of the theoretical and experimental results permitted to obtain an estimate of the actual Er3+ doping level. A calculation of the effective gain cross section for the 1.5μm transition is also presented. These experimental results are discussed in order to evaluate the potentialities of Er3+ in this material as active medium in solid state laser devices.

Nonlocal optical effects on the fluorescence and decay rates for admolecules at a metallic nanoparticle

A phenomenological model is implemented to study the decay rates of fluorescing molecules in the vicinity of a metallic nanoparticle, wherein the nonlocal optical response of the particle is accounted for via the hydrodynamic model for the description of the free electrons in the metal. These nonlocal effects are examined for each of the radiative rate and the nonradiative rate of the admolecule, respectively. In addition, the overall fluorescence rate which includes the enhancement ratio for the driving field intensity is also studied. It is found that for particles of very small sizes (<10 nm), the nonlocal effects, in general, lead to significantly greater fluorescence rates and smaller nonradiative decay rates for the admolecules, with the effects on radiative rates depending crucially on the orientation of the molecules. Furthermore, the effects are mostly noticeable for molecules close to the metal particle and in processes where higher multipolar interactions are significant such as those in nonradiative decay processes. Above all, these nonlocal effects can still be observable in the presence of large surface damping imposed on the metallic electrons due to the ultrasmall sizes of these nanoparticles. The relevance of these effects to some of the latest experiments is discussed.

Synthesis of p-type conjugated dendrimers bearing phenothiazine moiety at the periphery and their light-emitting device characterization

New dendrimers bearing a phenothiazine heterocyclic building block have been successfully prepared and their absorption and emission properties have been investigated in terms of photoluminescence (PL) and electroluminescence (EL). Particularly, the EL has been studied precisely in the heterostructural devices that are fabricated with two dendrimers. The spectrum and the intensity of EL and PL are varied with the number of generation. Significant difference of the I– V– L characteristics and the external quantum efficiency in the dendrimer devices reveals that the EL performance of the higher-generation dendrimers is higher than that of the lower-generation ones. These results suggest that the site isolation between the chromophores suppresses the non-radiative decay process and reduces the interchain molecular excimer emission from the aggregated dendrimers.

Relaxation properties of rare-earth ions in sulfide glasses: Experiment and theory

Both radiative and nonradiative relaxation rates for a series of rare-earth ions doped $20\mathrm{Ge}\text{\ensuremath{-}}5\mathrm{Ga}\text{\ensuremath{-}}10\mathrm{Sb}\text{\ensuremath{-}}65\mathrm{S}$ $(\mathrm{GeGaSbS})$ sulfide (chalcogenide) glasses have been determined. Temperature-dependent lifetimes were carried out for various excited levels of the sample. Radiative decay rates were derived by using the Judd-Ofelt approach. Nonradiative decay rates are evaluated by comparing the inversion of measured lifetimes with the calculated radiative decay rates. We have found that the multiphonon relaxation rates should be a predominant decay mechanism among the excited states if the energy gap to the next lower level is smaller than $2500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, and the decay mechanism can be determined using the semiempirical ``energy-gap law.'' For an energy gap larger than $2500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, additional nonradiative decay processes become dominant over the multiphonon decay. Additional nonradiative decay processes have been quantitatively identified with the diffusion-limited relaxation calculations.

Dual phosphorescence and excited state dynamics of p-bromotoluene

Excited state dynamics of dual phosphorescent p-bromotoluene (PBT) was investigated in low temperature matrix between 77 K and 91 K. Dual phosphorescence was ascribed to two low-lying triplet states, 3ππ * and 3πσ *. The phosphorescence decay rate constants were divided into temperature dependent and independent terms. The temperature independent decay rate constants of 3ππ * state was 180 times larger than that of p-chlorotoluene (PCT). The decay rate constant of chlorobenzene (CB) and bromobenzene (BB) were determined in our previous study, and the ratio of the decay rate constants were 13, which has been successfully explained by increase of spin–orbit interaction due to internal heavy atom effect. In the case of p-bromotoluene, the large decay rate constant is partly explained by taking into account of non-radiative decay process due to spin–vibronic interaction through promoting vibrational modes possible even at 77 K. Moreover, homolysis reaction to produce bromine atom was also suggested to induce non-radiative process within triplet manifold.

The Role of πσ* Excited States in the Photodissociation of Heteroaromatic Molecules

High-resolution measurements of the kinetic energies of hydrogen atom fragments formed during ultraviolet photolysis of imidazole, pyrrole, and phenol in the gas phase confirm that N(O)-H bond fission is an important nonradiative decay process from their respective 1pisigma* excited states. The measurements also reveal that the respective cofragments (imidazolyl, pyrrolyl, and phenoxyl) are formed in very limited subsets of their available vibrational states. Identification of these product states yields uniquely detailed insights into the vibronic couplings involved in the photoinduced evolution from parent molecule to ultimate fragments.

Self‐Assembled, Kinetically Locked, RuII‐Based Metallomacrocycles: Physical, Structural, and Modeling Studies

By using a "complex as ligand approach," the metal-ion-templated self-assembly of heterometallic tetranuclear metallomacrocycles containing kinetically locked Ru(II) centers is described. Depending on the metal-ion template employed in the self-assembly process, the final macrocycle can be kinetically labile or inert. Electrochemical studies reveal that the kinetically inert macrocycles display reversible Ru(III/II) oxidation couples. The crystal structure of a kinetically inert Ru2Re2 macrocycles reveals a structurally complex palmate anion-binding pocket. Host-guest studies carried out with the same macrocyle in organic solvents reveals that the complex functions as a luminescent sensor for anions and that binding affinity and luminescent modulation is dependent on the structural nature and charge of the guest anion. Computational density functional theory (DFT) studies support the hypothesis that the luminescence of the macrocycle is from a 3MLCT state and further suggests that the observed guest-induced luminescence changes are most likely due to modulation of nonradiative decay processes.

Interatomic Electronic Decay in Endohedral Fullerenes

Ionization of an atom in an endohedral fullerene complex can lead to a wealth of nonradiative decay processes. These interatomic processes occur due to the correlation existing between the atomic and the fullerene electrons and do not take place in the free species . Considering as an example, we calculate the rates of the interatomic decay processes and show that the interatomic decay in is ultrafast. Moreover, our analysis suggests that interatomic decay in an endohedral fullerene does not necessarily lead to the destruction of the complex.

Bridged diiridium complexes for electrophosphorescent OLEDs: synthesis, X-ray crystal structures, photophysics, and devices

Results are presented which challenge the accepted view that dinuclear transition metal–ligand complexes are unsuitable for organic light-emitting device (OLED) applications due to their low luminescence quantum efficiencies. We establish for the first time that halo- and pseudo-halo-bridged diiridium(III) species are viable electrophosphorescent dopants in OLEDs. New cyclometalated chloro- and isocyanate-bridged diiridium(III) complexes, viz. tetrakis[9,9-dihexyl-2-(pyridin-2-yl)fluorene-C2,N′]-bis(μ-chloro)diiridium(III) [Ir(flpy)2Cl]2 (complex 3) and tetrakis[9,9-dihexyl-2-(pyridin-2-yl)fluorene-C2,N′]-bis(μ-isocyanate)diiridim(III) [Ir(flpy)2NCO]2 (complex 4) were obtained in high yield from the 9,9-dihexyl-2-(pyridin-2-yl)fluorene (flpyH) ligand 1. The X-ray crystal structures are described for 3 and the monomeric complex Ir(flpy)2NCO(DMSO) (5) which was obtained from 4. The solution-state photophysical properties of complexes 3 and 4 are characterised by emission from mixed 3π–π*/3MLCT states at ∼545–550 nm. Complex 4 displays a solution-state photoluminescence quantum yield which is five times that of the dichloro-bridged analogue 3. This is ascribed to an increase in the ligand-LUMO/metal eg gap which reduces the probability of non-radiative decay processes. Spin-coated organic light emitting devices (OLEDs) made from the host polymer poly(9,9-bis-2-ethylhexylfluorene-2,7-diyl) (PF2/6) end-capped with bis-(4-methylphenyl)phenylamine (PF2/6am4) doped with 12.5 wt% of the complexes 3 and 4 show good stability: turn-on voltages are low (<4 V) with maximum EL intensities of ∼1300 and 13 000 cd m−2, and peak external quantum efficiencies (EQE) of 0.1 and 0.8%, at ca. 400 and 60 mA cm−2, respectively.

The Fluorosolvatochromism of Brooker's Merocyanine in Pure and in Mixed Solvents

The fluorescence-based solvatochromism (fluorosolvatochromism) of 4-[(1-methyl-4(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (Brooker's merocyanine) was studied. The results revealed that the fluorescence emission band of the dye was dependent on the medium (lambda(fl)(max)= 573 nm in water and lambda(fl)(max)=622 nm in DMF). The fluorescence quantum yields (phi (f)) were calculated for the dye in the solvents investigated. Low phi (f) values ( < 10%) were obtained for the dye and in order to better comprehend the radiative and nonradiative decay processes of this dye, its fluorescence lifetime in methanol was measured and was found to be very short (230 ps). The results suggest that the dye in the excited state decays rapidly through nonradiative processes. The behavior of the probe in binary mixtures including a hydrogen-bond accepting solvent (acetonitrile, N,N-dimethylformamide, and dimethylsulfoxide) and a hydroxylic solvent (water, methanol, ethanol, propan-2-ol, and butan-1-ol) was also investigated. All data were successfully fitted to a model based on solvent exchange equilibria, which allowed the separation of the different contributions of the solvent species in the solvation shell of the dye. The data obtained for the mixed solvents were explained based on solute-solvent and solvent-solvent interactions.

Ab initiocalculation of interatomic decay rates by a combination of the Fano ansatz, Green’s-function methods, and the Stieltjes imaging technique

A new computational technique is introduced for the ab initio calculation of the rates of interatomic and intermolecular nonradiative decay processes occurring due to electronic correlation. These recently discovered phenomena are described theoretically using the configuration-interaction formalism first introduced by Fano [Phys. Rev. 124, 1866 (1961)] and later adapted to an Auger decay by Howat et al. [J. Phys. B 11, 1575 (1978)]. The boundlike and the continuumlike components of the wave function of the decaying state are constructed using a Green's-function method known as algebraic diagrammatic construction. A combination of atomic and distributed Gaussian basis sets is shown to provide an adequate description of both boundlike and continuumlike wave-function components. The problem of the normalization of the continuum (final state) wave function is addressed using the Stieltjes imaging technique. The new method is applied to the calculation of the rates of interatomic decay in alkaline-earth-rare-gas clusters. The obtained results help to verify our earlier conclusions [Phys. Rev. Lett. 93, 263002 (2004)] regarding the validity of the virtual-photon transfer model for the interatomic Coulombic decay. In addition, we demonstrate that the process of electron-transfer-mediated decay is responsible for the finite lifetimes of the outer valence vacancies in alkaline-earth-rare-gas clusters.

Ultrafast exciton dynamics in isolated single-walled nanotubes

Ultrafast relaxation dynamics of photoexcitations in semiconducting single-walled carbon nanotubes (S-NT) isolated in D 2O solution have been investigated using polarized pump–probe photomodulation (with 150 fs time resolution), and cw polarized photoluminescence (PL). Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands were observed, which also showed photoinduced dichroism. After taking into account the PB spectral shift, then the PA and PB bands decay together in time following a power law of the form ( t) − a with a in the range of 0.7–1. The PL emission shows polarization degree that agrees with that of the transient photoinduced dichroism. We therefore conclude that the primary photoexcitations in S-NT are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime in isolated NTs in solution to be of the order of 500 ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the minute PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation.

Photophysical Properties of Ru(II) Bipyridyl Complexes Containing Hemilabile Phosphine−Ether Ligands

Emission and absorbance spectra, along with low-temperature excited-state lifetimes, were obtained for the hemilabile complexes, [Ru(bpy)2L](PF6)2 [L = (2-methoxyphenyl)diphenylphosphine (RuPOMe) (1) and (2-ethoxyphenyl)diphenylphosphine (RuPOEt) (2)] in solid 4:1 ethanol/methanol solution. Spectral data were evaluated with ground-state reduction potentials using Lever parameters. Lifetime data for these complexes were collected from 77 to 160 K, and the rate constant for the combined radiative and nonradiative decay process, k, the thermally activated process prefactor, k'(0), the rate constant for the MLCT --> d-d transition, k', and the activation energy, DeltaE', were calculated from a plot of ln(1/tau) versus 1/T for both (1) and (2). The low-temperature luminescence lifetimes of (1) were observed to decrease with increases in water concentration. The photophysical and kinetic data of (1) and (2) are compared to literature data for [Ru(bpy)3](PF6)2. The emission maxima of (1) and (2) are blue-shifted relative to [Ru(bpy)3](PF6)2 due to the presence of the strong-field phosphine ligand, which enhances pi back-bonding to the bipyridyl ligands. The thermal activation energy, DeltaE', is significantly larger for [Ru(bpy)3](PF6)2 than for (1) and (2) resulting in a faster MLCT --> d-d transition for (1) and (2). These results are discussed in the context of radiationless decay through thermally activated ligand-field states on the metal complex.

Structure Property Relationships in Conjugated Organic Systems

A series of π conjugated oligomers were studied by absorption and photoluminescence spectroscopy. A linear relationship between the positioning of the absorption and photoluminescence maxima plotted against inverse conjugation length is observed. The relationships are in good agreement with the simple particle in a box method, one of the earliest descriptions of the properties of one-dimensional organic molecules. In addition to the electronic transition energies, it was observed that the Stokes shift also exhibited a well-defined relationship with increasing conjugation length, implying a correlation between the electron-vibrational coupling and chain length. This correlation is further examined using Raman spectroscopy, whereby the integrated Raman scattering is seen to behave superlinearly with chain length. There is a clear indication that the vibrational activity and thus nonradiative decay processes are controllable through molecular structure. The correlations between the Stokes energies and the vibrational structure are also observed in a selection of PPV based polymers and a clear trend of increasing luminescence efficiency with decreasing vibrational activity and Stokes shift is observable. The implications of such structure property relationships in terms of materials design are discussed.

Excited State Relaxation Dynamics of the Zinc(II) Tetraphenylporphine Cation Radical

A new technique employed to study the photophysical properties of the zinc(II) tetraphenylporphine cation radical is reported. It employs a combination of controlled potential coulometry and femtosecond absorption spectrometry. The fast transient lifetime of 17 ps of the pi-cation species originates in very efficient mixing of the a(2u) HOMO cation orbital that places electronic density mainly on pyrrolic nitrogens and metal d-orbitals. That explains the lack of any emission of the cationic species. This nonradiative decay process might elucidate the processes taking place in photosynthetic systems when an electron is removed from the porphyrinic moiety and the hole is produced.

Organic Electroluminescence Devices Based on Mixture of Aluminum–Hydroxyquinoline Complex and Compound with Low Dielectric Constant

The quantum efficiency and lifetime of fluorescence of an aluminum–hydroxyquinoline (Alq3) complex in solution increased with decreasing polarity of the solvent. Similarly, the fluorescence quantum efficiency and lifetime of Alq3 films were increased by mixing the complex with a compound with a low dielectric constant. These results suggested that the nonradiative decay process of Alq3 is retarded in less polar media. The quantum efficiency and lifetime of fluorescence of Alq3 films were 20±2% and 17 ns, respectively. When Alq3 was mixed with a less polar substance in the film state, the quantum efficiency and lifetime were increased to 25±2% and 20 ns, respectively. These results indicate that the nonradiative decay process in the film state is also retarded as the polarity decreases. Using a film consisting of Alq3 and a compound with a low dielectric constant as the light emitting layer of organic electroluminescence devices, the luminance efficiency was increased by about 20%.