Radiative Decay Constants Research Articles

Characterization of exciplexes generated by excited tris-diimine—ruthenium(II) complexes and silver(I) ion of ground state

Luminescent bimolecular and termolecular exciplexes of *[Ru(bpy) 3] 2− and *[Ru(phen) 3] 2+ (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline) formed by silver ions have been characterized and compared. Formation constants, luminescence lifetime, radiative and non-radiative decay constants were determined at room temperature. Oxidative quenching measurements in the [Ru(bpy) 3] 2−—Ag + system using MV 2+ (MV 2+ = 1,1'-dimethyl-4,4'-bipyridinium) as an electron acceptor resulted in quenching rate constants, (3.1 ± 0.2) × 10 9, (3.1 ± 0.3) × 10 9, (1.6 ± 0.3) × 10 9 and cage escape efficiencies, 0.11 ± 0.01, 0.05 ± 0.01, 0.06 ± 0.01, for *[Ru(bpy) 3] 2+, *[Ru(bpy) 3]Ag 3+ and *[Ru(bpy) 3]Ag 2 4+, respectively. Temperature-dependent luminescence lifetime measurements in aqueous solutions revealed that the exciplex formation causes a considerable change in the deactivation pathways of the excited species. Exciplex formation results in a decrease in the efficiency of the thermally activated decay through MLCT å dd transition and in an increase of the efficiency of deactivation to the ground state via other non radiative processes. The thermodynamic parameters of the exciplex formation have been estimated.

Solvatochromic effects in the absorption and fluorescence spectra of indazole and its amino derivatives

Absorption and fluorescence spectra of Indazole (In) in five solvents and those of 5-amino, 6-amino, and 7-aminoindazoles (5-AI 6-AI, 7-Al) in thirteen solvents have been studied. Using the multiparametric approach of Taft et al., spectral characteristics of these molecules have been analysed on the basis of electrostatic effects, hydrogen bond donor ability and hydrogen bond accepting ability of the solvents. In the excited singlet state, all three effects are nearly equally prominent, whereas in the ground state 5-AI acts as a better hydrogen bond acceptor than the other amines. Stokes shifts and the difference between the radiative decay constants determined experimentally and using Strickler and Berg's equation indicate different geometries of the amines in the S 0 and S 1 states. A decrease in the non-radiative decay constants of 5-AI and 6-AI with an increase in the polarity of the solvents predict better planar geometry in the S 1 state in comparison to that in the S 0 state. Semi-empirical quantum mechanical calculations have been used to find the nature of transitions, total atomic charges at the basic centres and dipole moments of all the aromatic amines in the S 0 and S 1 states. Results so obtained are compared and discussed.

Solution photophysics of ketocyanine dyes in neat and mixed binary solvents

The effect of the solvent on the excited state photophysics of two ketocyanine dyes was examined by studying the lifetime () of the S 1 state in the picosecond domain. The overall luminescence decay constant (1/Gt) shows a linear correlation with the polarity parameter E T(30) in neat solvents. The radiative decay constant is practically independent of solvation, whereas the radiationless deactivation constant is sensitive to solvation; this has been attributed to hydrogen bond interaction of the solute carbonyl oxygen with the solvent. Preferential solvation by the protic component occurs in protic-aprotic binary mixed solvents. The influence of the solute-solvent interaction parameters and solvent non-ideality in determining the preferential solvation characteristics has been examined.

Photophysics of van der Waals dimers of Cl 2CS in perfluoroalkane solvents

The photophysics of thiophosgene in perfluoro-1,3-dimethylcyclohexane solutions of widely varying concentrations, C, has been investigated. For 10 −3 M < C < 10 −1 M, the dominant absorbing species is the van der Waals dimer, (Cl 2CS) 2, the S 2→S 0 fluorescence quantum yield and S 2 lifetime of which have been determined as a function of C and excitation wavelength. The data have been analyzed to yield values of the radiative and nonradiative decay constants and concentration quenching rate constant of the excited dimer. Excitation in the visible produces both S 1→S 0 fluorescence of the monomer and dimer and T 1→S 0 phosphorescence of the monomer. The concentration dependence of the fluorescence and phosphorescence lifetimes and quantum yields have been analyzed in order to extract the radiative and nonradiative decay parameters of the dimer in its S 1 state. At C> 10 −1 M, the spectroscopy and photophysics of the system are dominated by higher oligomers of thiophosgene.

Comparative triplet-state properties of the three tryptophan residues in bacteriophage T4 lysozyme and in the enzyme complex with methylmercury(II).

Triplet-state energies, zero-field splittings (ZFS), and total decay rate constants of the individual triplet-state sublevels of the tryptophan (Trp) residues located at positions 126, 138, and 158 in bacteriophage T4 lysozyme have been determined by using low-temperature phosphorescence and optical detection of magnetic resonance spectroscopy in zero applied magnetic field. An investigation of spectral and kinetic properties of individual Trp residues was facilitated by measurements on point-mutated proteins containing two Trp----Tyr substitutions. We find that the phosphorescence lifetime of the buried Trp-138 is considerably shorter than those of the solvent-exposed Trp residues. CH3HgII binding to cysteine residues in T4 lysozyme selectively perturbs the triplet state of Trp-158 by means of an external heavy-atom effect. In contrast with the previous observation of selective x-sublevel perturbation in the Trp-CH3Hg complex, the radiative character of the z sublevel (z is the out-of-plane axis) is selectively enhanced due to the heavy-atom perturbation of Trp-158. The observed pattern of radiative and total sublevel decay constants of the perturbed Trp is attributed to a special orientation of the Hg atom with respect to the indole plane.

Radiative and nonradiative decay constants of jet-cooled molecular exciplexes

Mesure des durees de vie et des rendements quantiques de l'emission de complexes moleculaires d'anthracene et de perylene avec l'aniline et les N-alkylanilines. Discussion des vitesses de declins radiatifs et non radiatifs en relation avec le couplage interetats et avec les processus de desactivation des etats de transfert de charge par transfert de proton

Fluorescence at a surface

Fluorescence emitted by atoms near a metal surface is used to study the coupling of the surface plasmon field of a dielectric substrate with a molecular electron transition. Explicit expressions are derived for the atomic and molecular lifetimes. It is shown that the classical phase shift of a plane wave accompanying reflection from the surface is responsible for changes in the atomic lifetime. Moreover, it is demonstrated that the dipole direction of an atom can be established by exposing the system to polarized radiation. The contribution of the surface plasmons and the surface roughness to the radiative decay constant is determined for molecular transitions. A comparison between image theory and energy transfer theory showed that the first may be extremely inaccurate for the description of molecules far from the surface or for a highly conducting substrate.

T 1–T 2 inversion in aromatic thiones

The emission spectra and phosphorescence lifetimes and quantum yields, of xanthione (XT), 4H-pyran-4-thione (PT) and 4H-1-benzopyran-4-thione (BPT) have been measured in five solvents of varying polarities at room temperature. A dramatic change in the T 1→S 0 phosphorescence spectrum is observed, and this is interpreted in terms of inversion of the 3(n, π*) and 3(π,π*) states with increasing solvent polarity. Only modest variations in the radiative and non-radiative decay constants for the triplet are observed, and no evidence of bi-exponential decay is found. Strong coupling between T 1-T 2 and between the singlet and triplet manifolds is indicated. Comparisons are drawn with ketones of similar structure.

Fluorescence of some dipolar N,N-disubstituted 4-(dichloro-1,3,5-triazinyl)anilines. Part 4. Internal and molecular rotations in homogeneous and inhomogeneous media

Radiative and non-radiative decay constants have been obtained for the N,N-diethyl and N,N-diphenyl derivatives of the title compounds (DETA and DPTA, respectively). DETA fluorescence occurs from a planar conjugated excited singlet state which decays by an internal rotation about the anilino nitrogen–carbon bond at a rate close to the polar solvent dielectric relaxation rate By contrast, the detectable emission from DPTA is from a twisted species, and its decay is an order of magnitude slower than that of the DETA emission.The molecular tumbling of the DPTA molecules in homogenous solvents is deduced to be isotropic. Preliminary studies on the applicablity of the triazine dyes as fluorescence probes are reported. In aqueous neutral or alkaline media the dyes undergo hydrolysis but the product fluorescence can be utilised Hydrolysed DPTA probe rotation in egg lecithin unilamellar vesicles is probably isotropic and has an activiation barrier of 26 kJ mol–1 in the liquid crystalline phase.DPTA dye is adsorbed strongly onto porous Vycor glass but the fluorescence emission is complex.

Electronic spectroscopy of aromatic Schiff bases. V. Specific interactions involved between some typical benzylideneaniline molecules and small polar molecules

Fluorescence quantum yields [Formula: see text], theoretical first-singlet radiative decay constants (kF(t)), and first-singlet non-radiative decay constants (knr) of some benzylideneanilines (BA) dissolved in methylcyclohexane have been obtained. High values of knr are interpreted by the twisting of the molecules around the azomethine bond (C=N) in the first singlet excited state. The absorption spectra of some BA's have also been obtained in methylcyclohexane with various added amounts of polar molecules. BA's with strong electron-donor and/or strong electron-acceptor substituents in the para positions give rise to the formation of ground state complexes with small polar molecules. The stoichiometry of the complex is one BA molecule for two polar solvent molecules; when ethanol is used as a polar solvent, it is further involved in hydrogen bonding formation. The fluorescence spectra of 4-dimethylaminobenzylidene-4′-nitroaniline have been obtained in methylcyclohexane with various added amounts of polar species. The results are interpreted by the formation of exciplexes with polar compound. The Lippert–Mataga theory and the more generalized Kawski theory failed to explain the solute-solvent interactions disussed in this paper.

Decay of the first triplet Ne2 excimer in the presence of O2

Measurements of the rate of decay of the first triplet state of the Ne2 molecule in Ne/O2 mixtures have been performed by fast spectrophotometric monitoring of this species at 814 nm. The rate constant for electronic quenching of vibrationally relaxed Ne2 (3Σu+) by O2 has been found to be equal to 3.67 (±0.14)10−10 cm3 molecule−1 s−1 at 298 K. A new upper limit to the value of the spontaneous radiative decay constant is equal to 1.28(±0.02) 105 s−1. Dependence of the excimer yield on O2 concentraion appears to indicate that electronic three-body quenching of the atomic precursor(s) predominates markedly over vibrational relaxation of Ne2(3Σu+, v). The excimer yield is, accordingly, extremely sensitive to the presence of impurities at high neon pressures.

Kinetic Processes of the Scintillation in Helium Excited by Alpha Particles

The time dependence of the scintillations in pure helium excited by alpha particles has been measured in the pressure range of 100 to 1200 Torr by using a single photon counting technique. The time dependence of helium scintillation in the vacuum-ultraviolet region consists of a slow component and a fast one. The slow component emission appears to arise from two-body Frank-Condon collisions involving a 2 1 S atomic state and from the deexcitation of the A 1 Σ + u molecular state created from three-body conversion of a 2 1 S state. The fast component emission arises from the deexcitation of the D 1 Σ + u molecualar state formed by three-body collisions involving a 2 1 P atomic state. Radiative decay constant for the D 1 Σ + u molecules is (2.2±0.2)×10 8 sec -1 .

Laser excitation study of the second excited singlet state photophysics of ClFCS

A tunable dye laser has been used to excite selected single vibronic levels of the ? (1A′) state of ClFCS in the gas phase. Fluorescence (?→?) excitation spectra, quantum yields, and excited state lifetimes have been measured. Radiative and nonradiative decay constants and bimolecular self-quenching constants have been calculated for each selected state. A model in which a single dissociative vibrational mode provides the sole nonradiative decay mechanism is proposed.

Fluorescence decay of the O+u and 1u states of Xe2

Synchrotron radiation from the Stanford storage ring (SPEAR) was used to measure the radiative decay constants of the lowest-lying excited electronic states of Xe2. Both a monochromator and interference filters were used in the fluorescence channel. The results suggest that at low pressures (PXe&amp;lt;125 Torr) there is substantial deviation from the Stern–Volmer behavior predicted by the three-state models used previously to describe this lasing system. A new theoretical analysis was developed that includes participation in the kinetic scheme of intermediate vibrational levels of the O+u and 1u manifolds.

Optical properties of 3(nπ*) benzophenones in 4, 4′-dibromodiphenylether host crystals

A variety of optical and zero-field optically-detected magnetic resonance (zf ODMR) experiments have been performed on the lowest (nπ*) triplet states of benzophenone (BP-h 10), perdeuterobenzophenone (BP-d 10), and 4,4′-dibromobenzophenone (4,4′-DBBP) in single crystals of 4,4′-dibromodiphenylether (DDE) at temperatures of 1·4 K and above. The data obtained include high resolution phosphorescence, phosphorescence excitation, zf ODMR, amplitude-modulated zf ODMR phosphorescence excitation, and single-sublevel phosphorescence spectra as well as the guest sublevel phosphorescence lifetimes, relative radiative and non-radiative decay constants, intersystem crossing rates, and steady-state populations. At short wavelengths, the guest triplets are also populated via two energy transfer routes (S 0 H → T 1 H ↝ T 1 G and S 0 H → S 1 H ↝ T 1 H ↝ T 1 G); hence, the relative radiative rate constants and intersystem crossing rates of the host 3(ππ*) state sublevels have also been measured. The results are analysed to determine the principal spin-orbit (SO) and spin-orbit plus vibronic (SOV) mechanisms responsible for the population and depopulation of the spin sublevels of the parent molecule and its derivatives. A localized model suffices for triplet BP-h 10 in which the z sublevel is most active. The S 1 ↝ T 1 intersystem crossing process is governed primarily by SOV terms involving 3(ππ*) [and possibly 1(ππ*)] states and vibrations of a 2 symmetry, the T 1 → S 0 radiative process by SO coupling with 1(ππ*) states, and the T 1 ↝ S 0 non-radiative process by SO coupling with the ground state. Deuterium substitution has only a small effect on the radiative and non-radiative properties of 3(nπ*) BP. In contrast, bromine substitution has a large (and selective) effect; this is a consequence of n and π spin delocalization. All ISC rates and decay constants of 4,4′-DBBP are larger than those of the parent molecule. This is attributed to the increased importance of SO coupling with 1(σπ*) and 1(nσ*) states having significant heavy-atom character. The reasons for the preferential enhancement of Kx and Ky, kx r, and ky nr and kz nr are discussed. New vibronic coupling routes influencing the radiative decay are also introduced by the substitution of bromine atoms; these involve totally symmetric vibrations, affect only the Tx and Ty sublevels, and produce a decrease in the relative intensity of the carbonyl-stretching bands in the emission spectra of these sublevels. Finally, it is shown that correlations exist between the optical and magnetic properties of 3(nπ*) benzophenone and between the changes in these properties which are produced by heavy-atom substitution.

Radiative decay constants of the H2 Fulcher bands

The radiative decay constants corresponding to the Fulcher bands of the H2 molecular spectrum are obtained using pulsed-electron beam techniques. Data in the range ∼1.8 to 2.8×107 s−1 are obtained at 25 eV incident electron energy for the d 3πu →a 3Σ+g (v′, v″) Q (K) and k 3πu →a 3Σ+g (v′,v″) Q (K) rotational lines with K=1,2,3 and v′=0,1,2,3,→v″=0,1,2,3. Additional measurements of the d 3πu→a 3Σg+(0,0) Q1 rotational line intensity indicate the presence of a two-mode decay over the energy range 50–300 eV. The long-lived decay component, identified as radiative cascade from upper levels, contributes ∼15% to the total d 3πu(v′=0) population.

Boltzmann population and spin—lattice relaxation effects on phosphorescing benzene and the spin sublevel origin of radiative and radiationless decays

Precise lifetime and relative intensity measurements have been made for C 6H 6 and C 6D 6 in several glassy and crystalline media ≈2–30 K. For rapid electron spin—lattice relaxation (SLR), the increase in the observed phosphorescence decay time (τ), as the system is cooled below 10 K, can be essentially accounted for by changes in the Boltzmann population of the spin sublevels. The data require that nearly all of both the radiative and radiationless decays originate from the nearly degenerate, upper pair of spin sublevels. In stable cyclohexane, a slower SLR increases τ more and introduces a transient decay, which represents a redistribution of the spin sublevel populations. Model calculations and the transient's intensity yield the fraction (0.88) of intersystem crossing to the upper spin sublevels. The transient decay time provides the average SLR rate between the lowest and upper spin sublevels. These are equal for C 6H 6 and C 6D 6 and vary as aT + bT 7. The Boltzmann and SLR effects account entirely for the increase in τ with cooling. No temperature dependence of either radiative or radiationless decay constants is indicated at these temperatures.

Effect of methyl and halogen substituents on the decay of singlet excited benzenes

Radiative and nonradiative decay constants for singlet excited states of halogen- and methyl-substituted benzenes were determined in solution. The effect of halogen substitution on the nonradiative decay is much greater than that of methyl substitution, which is obscured by effects of the solvent.

Infrared radiative decay constants for the vibrational levels of CO (a 3Π)

The decay of the CO (a 3Π,v′=0–3) levels have been studied in a flowing-afterglow apparatus. The vibrational distribution was monitored by observing the CO (a 3Π→X 1Σ+) emission intensity. The results were interpreted to obtain 52.5, 77, and 178 sec−1 for the infrared radiative decay constants for the v′=1, 2, and 3 vibrational levels of CO (a 3Π). The data also establish that the upper limit to the rate constant for vibrational relaxation of CO (a) by collision with He is ?6×10−17 cm3 molecule−1 sec−1, corresponding to a collision probability of ?1.5×10−7.

External heavy-atom effect on the triplet states of quinoxaline and 2,3-dichloroquinoxaline

Mechanism of external heavy-atom effect is discussed for quinoxaline and 2,3-dichloroquinoxaline on the basis of the high resolution phosphorescence spectra, total decay constants, and relative radiative decay constants obtained for all the triplet sublevels. Experimental results are interpreted in terms of a theory which includes intermolecular coulombic interaction and intramolecular spin-orbit interaction. Finally, it is pointed out that the appearance of a forbidden band considered at a specific point group could be interpreted without assuming molecular distortion.