Showing papers on "Triplet state published in 2004"
TL;DR: The effects of substituents and solvents on the photophysical and photochemical parameters of zinc(II) phthalocyanines are reported in this paper, where it was found that the presence of peripheral substituent on the macrocycle enhances the yield of the triplet state.
Abstract: The effects of substituents and solvents on the photophysical and photochemical parameters of zinc(II) phthalocyanines are reported. The complexes studied are zinc phthalocyanine (ZnPc), zinc tetra(tert-butylphenoxy)phthalocyanine [ZnPc(TBPh)4], zinc octa(methylphenoxy)phthalocyanine [ZnPc(MPh)8], zinc tetranitrophthalocyanine [ZnPc(NO2)4], zinc octachlorophthalocyanine (ZnPcCl8), zinc tetrasulfophthalocyanine [ZnPc(SO3−)4], a mixture of zinc mono-, di-, tri- and tetrasulfophthalocyanine [ZnPc(SO3−)mix] and zinc naphthalocyanine (ZnNPc). It was found that the presence of peripheral substituents on the macrocycle enhances the yield of the triplet state. Among the different substituents, the sulfonated derivative, ZnPc(SO3−)mix, has the longest triplet lifetime (τT) and the highest singlet oxygen quantum yield (ϕΔ). The near infra-red absorptions of the solvents reveal that solvents that absorb around 1100 nm (triplet energy level) and around 1270 nm (singlet oxygen energy level), quench the triplet state of the ZnPc derivative as well as singlet oxygen. Although water has a high singlet oxygen quenching effect, the ϕΔ value for ZnPc(SO3−)mix in water is still reasonably high at 0.48, which may provide an explanation for the efficient photosensitization by this molecule in photodynamic studies.
662 citations
Journal Article•
TL;DR: In this article, the electron-hole recombination should preferentially occur on the triplet emitter itself, rather than on matrix molecules with subsequent energy transfer to the emitter.
Abstract: Triplet emitter materials present attractive possibilities for optimizations of organic/organometallic light emitting diodes (OLEDs) This is due to the significantly higher efficiencies obtainable with these compounds ascompared to organic emitters In this contribution, first a schematic introduction is given, how an OLED device is built-up and why multi-layer structures are preferred Then a basic model is presented, how electron-hole recombination, ie the exciton formation process, can be visualized and how the singlet and triplet states of the (doped) emitter compounds are populated This takes place by specific singlet and triplet paths The occurrence of such paths is explained by taking into account that the dynamical process of exciton trapping involves dopant-to-matrix charge transfer states ( 1 3 DMCT states) It is also explained, why the excitation energy is harvested in the lowest triplet state of organo-transition-metal complexes Due to spin statistics, one can in principle obtain an efficiency of a factor of four higher than using organic singlet emitter molecules Simple comparisons suggest that electron-hole recombination should preferentially occur on the triplet emitter itself, rather than on matrix molecules with subsequent energy transfer to the emitter Further, it is pointed out that essential photophysical properties of organometallic triplet emitters depend systematically on the metal participation in the triplet state and on the effective spin-orbit coupling These factors control the amount of zero-field splitting (ZFS) of the triplet state into substates Increase of ZFS corresponds to higher metal character in the triplet state Higher metal character reduces the energy difference between excited singlet and triplet states, enhances the singlet-triplet intersystem crossing rate, lowers the emission decay time, changes the vibrational satellite structure, decreases the excited state reorganization energy, etc These effects are discussed by referring to well characterized compounds Based on a new ordering scheme presented for triplet emitter materials, a controlled development of compounds with pre-defined photophysical properties becomes possible
377 citations
TL;DR: In this paper, the triplet exciton of Ir(ppy)3 was investigated in hole-transport layers, and the authors compared energy-dissipative processes of the three-way exciton in 4,4′-bis, 1,1-bis and TAPC hosts.
Abstract: To understand confinement of the triplet exciton of Ir(ppy)3 by hole-transport layers, we compared energy-dissipative processes of the triplet exciton of Ir(ppy)3 which is doped into 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), 4,4′-bis [N-(p-tolyl)-N- phenyl-amino]biphenyl (TPD), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), and 4,4′-N,N′-dicarbazole-biphenyl hosts. Significant energy transfer from Ir(ppy)3 into the triplet levels of α-NPD was observed. In the case of the TPD host, however, partial confinement of the Ir(ppy)3 triplet exciton was observed. This result suggests both forward and backward energy transfer from Ir(ppy)3 to the TPD triplet levels. Furthermore, employing TAPC as a hole-transport layer achieved strong confinement of the Ir(ppy)3 triplet exciton. One conclusion from these results is that electrophosphorescence efficiency is well correlated with the triplet energy level of the hole-transport layer host materials.
297 citations
TL;DR: In this paper, the gauge-including magnetically induced current method for calculating the components of the current density tensor using atomic orbitals has been extended to treating open-shell molecules and its applicability is demonstrated by calculations of first-order induced current densities on cyclobutadiene, Al(3), and B(3) at correlated ab initio levels of theory.
Abstract: The gauge-including magnetically induced current method for calculating the components of the current-density tensor using gauge-including atomic orbitals has been extended to treating open-shell molecules. The applicability of the method is demonstrated by calculations of first-order induced current densities on cyclobutadiene, Al(3), and B(3) at correlated ab initio levels of theory. For comparison, current-density calculations were also performed on the lowest closed-shell singlet state of cyclobutadiene as well on the closed-shell Al(3)(-) and B(3)(-) anions. The ring-current susceptibilities of the open-shell species are computed at the Hartree-Fock self-consistent-field, second-order Moller-Plesset perturbation theory, and coupled-cluster singles and doubles levels, whereas for the closed-shell systems also density functional theory calculations were employed. Explicit values for the current strengths caused by α and β electrons as well as the difference, representing the spin current, were obtained by numerical integration of the current-density contributions passing a plane perpendicular to the molecular ring. Comparisons of the present results to those recently obtained for the lowest triplet state of biphenyl emphasize that electron correlation effects must be considered for obtaining an accurate description of spin-current densities.
289 citations
TL;DR: These covalently bound materials show improvements in efficiency over simple blends and will form the basis of future investigations into energy-transfer processes occurring in light-emitting diodes.
Abstract: We report the synthesis and photophysical study of a series of solution-processible phosphorescent iridium complexes. These comprise bis-cyclometalated iridium units [Ir(ppy)(2)(acac)] or [Ir(btp)(2)(acac)] where ppy is 2-phenylpyridinato, btp is 2-(2'-benzo[b]thienyl)pyridinato, and acac is acetylacetonate. The iridium units are covalently attached to and in conjugation with oligo(9,9-dioctylfluorenyl-2,7-diyl) [(FO)(n)] to form complexes [Ir(ppy-(FO)(n))(2)(acac)] or [Ir(btp-(FO)(n))(2)(acac)], where the number of fluorene units, n, is 1, 2, 3, approximately 10, approximately 20, approximately 30, or approximately 40. All the complexes exhibit emission from a mixed triplet state in both photoluminescence and electroluminescence, with efficient quenching of the fluorene singlet emission. Short-chain complexes, 11-13, [Ir(ppy-(FO)(n)-FH)(2)(acac)] where n = 0, 1, or 2, show green light emission, red-shifted through the FO attachment by about 70 meV, but for longer chains there is quenching because of the lower energy triplet state associated with polyfluorene. In contrast, polymer complexes 18-21 [Ir(btp-(FO)(n))(2)(acac)] where n is 5-40 have better triplet energy level matching and can be used to provide efficient red phosphorescent polymer light-emitting diodes, with a red shift due to the fluorene attachment of about 50 meV. We contrast this small (50-70 meV) and short-range modification of the triplet energies through extended conjugation, with the much more substantial evolution of the pi-pi* singlet transitions, which saturate at about n = 10. These covalently bound materials show improvements in efficiency over simple blends and will form the basis of future investigations into energy-transfer processes occurring in light-emitting diodes.
286 citations
TL;DR: Efficient photocyclization from a low-lying triplet state is reported for a photochromic dithienylperfluorocyclopentene with Ru(bpy)3 units attached via a phenylene linker to the thiophene rings.
Abstract: Efficient photocyclization from a low-lying triplet state is reported for a photochromic dithienylperfluorocyclopentene with Ru(bpy)3 units attached via a phenylene linker to the thiophene rings. The ring-closure reaction in the nanosecond domain is sensitized by the metal complexes. Upon photoexcitation into the lowest Ru-to-bpy 1MLCT state followed by intersystem crossing to emitting 3MLCT states, photoreactive 3IL states are populated by an efficient energy-transfer process. The involvement of these 3IL states explains the quantum yield of the photocyclization, which is independent of the excitation wavelength but decreases strongly in the presence of dioxygen. This behavior differs substantially from the photocyclization of the nonemissive dithienylperfluorocyclopentene free ligand, which occurs from the lowest 1IL state on a picosecond time scale and is insensitive to oxygen quenching. Cyclic voltammetric studies have also been performed to gain further insight into the energetics of the system. The ...
213 citations
TL;DR: In agreement with quantum Monte Carlo numerical simulations, a distinct lambda anomaly in the specific heat together with a maximum in the magnetic susceptibility upon cooling down to liquid helium temperatures is observed.
Abstract: Besides being an ancient pigment, BaCuSi2O6 is a quasi-2D magnetic insulator with a gapped spin dimer ground state. The application of strong magnetic fields closes this gap, creating a gas of bosonic spin triplet excitations. The topology of the spin lattice makes BaCuSi2O6 an ideal candidate for studying the Bose-Einstein condensation of triplet excitations as a function of the external magnetic field, which acts as a chemical potential. In agreement with quantum Monte Carlo numerical simulations, we observe a distinct lambda anomaly in the specific heat together with a maximum in the magnetic susceptibility upon cooling down to liquid helium temperatures.
202 citations
TL;DR: Using time-resolved and continuous wave (cw) photoluminescence (PL) spectroscopy of Si nanocrystals embedded in SiO 2 with varying content of the nanocrystalline silicon phase, this article was able to distinguish between microscopic characteristics of the PL decay that are associated with quantum confinement effects, and macroscopic properties of the decay which are affected by the environment of the nocrystals.
Abstract: Using time-resolved and continuous wave (cw) photoluminescence (PL) spectroscopy of Si nanocrystals embedded in ${\mathrm{SiO}}_{2}$ with varying content of the nanocrystalline silicon phase, we were able to distinguish between microscopic characteristics of the PL decay that are associated with quantum confinement effects, and macroscopic characteristics of the decay that are affected by the environment of the nanocrystals. The PL decay is characterized by a stretched exponential function and two PL lifetimes associated with an upper (allowed) singlet excitonic state and a lower (forbidden) triplet state. In particular, we have found that while the upper radiative lifetime of the singlet state and the singlet-triplet energy splitting originate from quantum confinement, the lower state lifetime and the dispersive nature of the PL decay are connected with characteristics of the crystallites' environment. In addition, we have found that the oscillator strength for radiative transitions is significantly weaker compared to that of direct gap semiconductors and therefore, we concluded that the efficient PL from these nanocrystals should be assigned to the exclusion of nonradiative channels in the medium.
157 citations
TL;DR: Few-electron quantum dots are investigated in the regime of strong tunneling to the leads and Cotunneling allows orbital correlations and parameters characterizing entanglement of the two-Electron singlet ground state to be extracted from dc transport.
Abstract: Few-electron quantum dots are investigated in the regime of strong tunneling to the leads. Inelastic cotunneling is used to measure the two-electron singlet-triplet splitting above and below a magnetic field driven singlet-triplet transition. Evidence for a nonequilibrium two-electron singlet-triplet Kondo effect is presented. Cotunneling allows orbital correlations and parameters characterizing entanglement of the two-electron singlet ground state to be extracted from dc transport.
143 citations
TL;DR: Results are an indication that this type of modelling can lead to discriminate in terms of the position of the lowest ligand triplet energy level the best antenna among a family of chromophoric compounds.
Abstract: In this paper, we evaluate the potential use of theoretical calculations to obtain an energy scale of the lowest ligand-centred triplet excited state in luminescent terbium(III) complexes. In these complexes, non-radiative deactivation of the terbium emitting state via a back-energy transfer process (T1
← Tb(5D4)) is a common quenching process. Consequently the prediction of the energy gap between these two excited states should be useful for programming highly luminescent TbIII systems. We report on a strategy based upon experimental and theoretical investigations of the excited state properties of a series of four simple aromatic hydroxamate ligands coordinated to TbIII and GdIII ions. By using previously reported crystallographic data, the structural and energies properties of these systems were investigated in the ground and first excited triplet states at the density functional theory (DFT) level of calculations. Our theoretical results are consistent with a triplet excited state T1 which is localised on one ligand only and whose the energy level is independent of the lanthanide ion nature (TbIII, GdIII). A good agreement between the calculated adiabatic transition energies and experimental data derived from emission spectra is obtained when a corrective term is considered. These satisfactory results are an indication that this type of modelling can lead to discriminate in terms of the position of the lowest ligand triplet energy level the best antenna among a family of chromophoric compounds. In addition this theoretical approach has provided indications that the difference between the adiabatic transition energies of all the investigated complexes can be mainly explained by metal–ligand electrostatic interactions. The influence of the number of antennae on the quantum yield and the luminescence lifetime is discussed.
142 citations
TL;DR: Computations on the singlet and triplet potential energy surfaces of several processes of this type are reported and it is shown that the topology of the individual surfaces, as well as of the crossing regions between them, can be used to rationalize the observed reactivity in all cases.
Abstract: Spin changes occur often in organometallic chemistry, and their effect on kinetics is not well understood. We report computations on the singlet and triplet potential energy surfaces of several processes of this type and show that the topology of the individual surfaces, as well as of the crossing regions between them, can be used to rationalize the observed reactivity in all cases. In particular, the slow addition of dihydrogen to W[N(CH2CH2NSiMe3)3]H (Schrock, R. R.; Shih, K. Y.; Dobbs, D. A.; Davis, W. M. J. Am. Chem. Soc. 1995, 117, 6609) is shown to be a “spin-blocked” reaction with a high barrier due to the crossing between reactant triplet and product singlet surfaces. In contrast, addition of CO to TpCo(CO) (Detrich, J. L.; Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 1995, 117, 11745) is fast because the triplet and singlet surfaces cross at low energy. Particular care is taken to use DFT methods which are in adequate agreement with experimental and high-level computational...
TL;DR: Photosensitization experiments on porphyra-334 support the participation of the triplet state in the photodecomposition mechanism yielding a more precise value of [capital Phi](T), and photoacoustic calorimetry experiments allowed the first direct quantification of the nonradiative relaxation pathways of the excited MAAs in solution.
Abstract: In vitro studies on the structurally related mycosporine-like amino acids (MAAs) porphyra-334 and shinorine in aqueous solutions were carried out aiming at their full photochemical and photophysical characterization and expanding the evidence on the assigned UV-photoprotective role of the molecules in vivo. The experiments on shinorine confirmed a high photostability and a poor fluorescence quantum yield, in concordance with previous results on porphyra-334. The estimation of triplet production quantum yields for both MAAs was achieved by laser-flash photolysis measurements. In particular, photosensitization experiments on porphyra-334 support the participation of the triplet state in the photodecomposition mechanism yielding a more precise value of ΦT. As well, photoacoustic calorimetry experiments allowed the first direct quantification of the nonradiative relaxation pathways of the excited MAAs in solution, corroborating that the vast majority (ca. 97%) of the absorbed energy is promptly delivered to the surroundings as heat, consistently with the low photodecomposition and emission yields observed.
TL;DR: In this article, it was shown that square-planar (LCoR)-R-I complexes of a diiminopyridine ligand are best regarded as containing low-spin Co-II antiferromagnetically coupled to a ligand radical anion.
Abstract: DFT calculations show that square-planar (LCoR)-R-I complexes of a diiminopyridine ligand are best regarded as containing low-spin Co-II antiferromagnetically coupled to a ligand radical anion. The lowest triplet state, corresponding to a 3d(z)(2)-->pi* excitation, is calculated to be only a few kcal/mol above the ground state, and is thermally accessible. The anomalous H-1 NMR chemical shifts of the LCoR complexes are suggested to be due to thermal population of the triplet state at room temperature. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.
TL;DR: Benko et al. as mentioned in this paper showed that the interligand electron transfer time is on the picosecond time scale, depending on the relative energies of the two ligands, and controls the electron injection from the excited triplet state of the sensitizer.
Abstract: Electron injection from the transition metal complex Ru(dcbpy)(2)(NCS)(2) (dcbpy = 2,2'-bipyridine-4,4'-dicarboxylate) into a titanium dioxide nanoparticle film occurs along two pathways. The dominating part of the electron injection proceeds from the initially excited singlet state of the sensitizer into the conduction band of the semiconductor on the sub-hundred-femtosecond time scale. The slower part of the injection occurs from the thermalized triplet excited state on the picosecond time scale in a nonexponential fashion, as was shown in a previous study (Benko, G.; et al. J. Am. Chem. Soc. 2002, 124, 489). Here we show that the slower channel of injection is the result of the excited state being localized on a ligand of the sensitizer that is not attached to the semiconductor; hence, the electron cannot be injected directly from such an excited state into the semiconductor. Before being injected, it has to be transferred from the non-surface-attached ligand to the attached one. The results show that the interligand electron-transfer time is on the picosecond time scale, depends on the relative energies of the two ligands, and controls the electron injection from the excited triplet state of the sensitizer. The findings provide information relevant to the design of molecular-based assemblies and devices. (Less)
TL;DR: In this paper, the authors studied the charge transport properties of a diffusive normal metal triplet superconductor (DN∕TS) junction based on the Keldysh-Nambu quasiclassical Green's function formalism.
Abstract: Charge transport properties of a diffusive normal metal∕triplet superconductor (DN∕TS) junction are studied based on the Keldysh-Nambu quasiclassical Green's function formalism. Contrary to the unconventional singlet superconductor junction case, the mid-gap Andreev resonant state at the interface of the TS is shown to enhance the proximity effect in the DN. The total resistance of the DN∕TS junction is drastically reduced and is completely independent of the resistance of the DN in the extreme case. Such anomalous transport accompanies a giant zero-bias peak in the conductance spectra and a zero-energy peak of the local density of states in the DN region. These striking features manifest the presence of novel proximity effect peculiar to triplet superconductor junctions.
TL;DR: In this article, the thermal, structural and photophysical properties of different polycrystalline phases of the organic semiconductor Alq3 are described, in particular the new blue luminescent δ-phase is shown to contain the facial isomer.
Abstract: This review describes the thermal, structural and photophysical properties of different polycrystalline phases of the organic semiconductor Alq3. In particular the new blue luminescent δ-phase is shown to contain the facial isomer. The results obtained by using differential scanning calorimetry, X-ray diffraction, infrared spectroscopy, transient and delayed photoluminescence measurements clearly demonstrate the existence of this isomer. From the results presented it is now possible to obtain the pure facial isomer of Alq3 in large quantities, providing the basis for further investigations to determine its effects on the performance of organic light-emitting diodes. Furthermore, recent results on the properties of the triplet states in Alq are presented. This includes the population of the electronic excited triplet state due to intersystem crossing and the spectrum of the phosphorescence. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: The origin for the abnormal solvent- and temperature-dependent emission measured in pi-extended Ru complexes is revisited and the electronic and optical properties of the free polyazaaromatic ligands and their corresponding ruthenium(II) complexes are determined on the basis of correlated Hartree-Fock semiempirical approaches.
Abstract: Quantum-chemical methods are applied to study the nature of the excited states relevant in the photophysical processes (absorption and emission) of a series of polyazaaromatic-ligand-based ruthenium(II) complexes The electronic and optical properties of the free polyazaaromatic ligands and their corresponding ruthenium(II) complexes are determined on the basis of correlated Hartree-Fock semiempirical approaches While the emission of complexes containing small-size ligands, such as 1,10-phenanthroline or 2,2'-bipyridine, arises from a manifold of metal-to-ligand charge-transfer triplet states ((3)MLCTs), an additional ligand-centered triplet state ((3)L) is identified in the triplet manifold of complexes containing a pi-extended ligand such as dipyrido[3,2-a:2',3'-c]phenazine, tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, and 1,10-phenanthrolino[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene Recent experimental data are interpreted in light of these theoretical results; namely, the origin for the abnormal solvent- and temperature-dependent emission measured in pi-extended Ru complexes is revisited
TL;DR: A new pyridine-containing ligand, N,N'-bis(6-carboxy-2-pyridylmethyl)ethylenediamine-N,N-N'-diacetic acid (H(4)L), has been designed for the complexation of lanthanide ions and is characterized by theoretical calculations both in vacuo and in aqueous solution (PCM model) at the HF level.
Abstract: A new pyridine-containing ligand, N,N'-bis(6-carboxy-2-pyridylmethyl)ethylenediamine-N,N'-diacetic acid (H(4)L), has been designed for the complexation of lanthanide ions. (1)H and (13)C NMR studies in D(2)O solutions show octadentate binding of the ligand to the Ln(III) ions through the nitrogen atoms of two amine groups, the oxygen atoms of four carboxylates, and the two nitrogen atoms of the pyridine rings. Luminescence measurements demonstrate that both Eu(III) and Tb(III) complexes are nine-coordinate, whereby a water molecule completes the Ln(III) coordination sphere. Ligand L can sensitize both the Eu(III) and Tb(III) luminescence; however, the quantum yields of the Eu(III)- and Tb(III)-centered luminescence remain modest. This is explained in terms of energy differences between the singlet and triplet states on the one hand, and between the 0-phonon transition of the triplet state and the excited metal ion states on the other. The anionic [Ln(L)(H2O)]- complexes (Ln=La, Pr, and Gd) were also characterized by theoretical calculations both in vacuo and in aqueous solution (PCM model) at the HF level by means of the 3-21G* basis set for the ligand atoms and a 46+4 f(n) effective core potential for the lanthanides. The structures obtained from these theoretical calculations are in very good agreement with the experimental solution structures, as demonstrated by paramagnetic NMR measurements (lanthanide-induced shifts and relaxation-rate enhancements). Data sets obtained from variable-temperature (17)O NMR at 7.05 T and variable-temperature (1)H nuclear magnetic relaxation dispersion (NMRD) on the Gd(III) complex were fitted simultaneously to give insight into the parameters that govern the water (1)H relaxivity. The water exchange rate (k(298)(ex)=5.0 x 10(6) s(-1)) is slightly faster than in [Gd(dota)(H2O)]- (DOTA=1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane). Fast rotation limits the relaxivity under the usual MRI conditions.
TL;DR: In this paper, a photo-induced electron transfer from the carotene to the porphyrin radical cation gave a final C·+−P−C60·− state with an overall yield of 0.95.
TL;DR: Time-resolved measurements using laser flash photolysis reveal the presence of two components with different lifetimes in triplet decay, which are ascribed to the CP/HSA complexes in site I, where stereodifferentiation is more important.
Abstract: A remarkable stereodifferentiation has been observed in the interaction between the excited triplet state of carprofen (CP) and human serum albumin (HSA). Time-resolved measurements using laser flash photolysis reveal the presence of two components with different lifetimes in triplet decay. This is explained by complexation of CP to the two possible HSA binding sites. The shorter-lived components are ascribed to the CP/HSA complexes in site I, where stereodifferentiation is more important (tauR/tauS ca. 4). This is correlated with formation of a dehalogenated photoproduct upon steady-state photolysis.
TL;DR: In this paper, the photochemical [small alpha]-cleavage of acetone is analyzed in view of recent results obtained for the isolated molecule in supersonic jets, showing that the fluorescence decay time of this isolated molecule spans a range of more than six orders of magnitude, from approximately 10(-6) s near the origin of the S(0)-S(1) transition to less than 10(-12) s at about 20 kcal x mol(-1) excess energy.
Abstract: The photochemical [small alpha]-cleavage of acetone is analyzed in view of recent results obtained for the isolated molecule in supersonic jets. The fluorescence decay time of the isolated molecule spans a range of more than six orders of magnitude, from approximately 10(-6) s near the origin of the S(0)-S(1) transition to less than 10(-12) s at about 20 kcal x mol(-1) excess energy. In contrast, the decay time of the excited singlet (S(1), (1)n pi) in the bulk is around 10(-9) s and independent of excitation wavelength. Initial excitation to the (1)npi state is followed by internal conversion (IC) to the ground state and intersystem crossing to the lowest-lying triplet. The rate constants of these processes are comparable to the radiative decay rate constant for excess energy up to 7 kcal x mol(-1) above the origin of the S(0)-S(1) transition. Beyond that energy, the triplet state becomes dissociative and the ISC rate becomes much larger than other processes depleting S(1). The primary reaction on the triplet surface is a barrier-controlled alpha-cleavage to form the triplet radical pair CH(3)(*)+ CH(3)CO(*). Direct reaction from the S(1) is negligible, and the non-quenchable reaction (by triplet quenchers) observed in the bulk gas phase is due to hot triplet molecules that dissociate on the timescale of 10(-12) s or less. The singlet-state decay time measured in the bulk (approximately 1-2 ns) arises from collision-induced processes that populate low-lying levels of S(1). The analysis is aided by detailed state-resolved studies on related molecules (in particular formaldehyde and acetaldehyde) whose photophysics and photochemistry parallel those of acetone.
TL;DR: In this paper, correlated quantum-chemical calculations that account for both the electronic couplings and energetics of the charge-recombination process from a pair of positive and negative polarons into singlet and triplet excitons were performed.
Abstract: The operation and efficiencies of molecular or polymer organic light-emitting diodes depend on the nature of the excited species that are formed. The lowest singlet and triplet excitons display different characteristics that impact on the quantum yields achievable in the devices. Here, by performing correlated quantum-chemical calculations that account for both the electronic couplings and energetics of the charge-recombination process from a pair of positive and negative polarons into singlet and triplet excitons, we show that the formation rates for singlet over triplet excitons vary with chain length and favor singlet excitons in longer chains. Thus, in polymer devices, the resulting singlet/triplet fraction can significantly exceed the spin-statistical limit.
TL;DR: Stable radicals are encapsulated by the cage to become a radical pair in triplet state in both solid and solution states, although the radicals themselves show doublet character without particular intermolecular interaction in solution.
Abstract: We present a new approach for manipulating spin−spin interaction by the self-assembled M6L4-coordination cage. Stable radicals are encapsulated by the cage to become a radical pair in triplet state in both solid and solution states, although the radicals themselves show doublet character without particular intermolecular interaction in solution. These results were confirmed by ESR spectroscopy and X-ray crystallography.
TL;DR: In this article, the triplet properties under direct and benzophenone-sensitized excitation conditions are outlined, whereas any singlet state reactions towards photodimers are unlikely.
Abstract: The photoprocesses of parent coumarin (1) and a series of 6-alkyl substituted coumarins (2–11, alkyl chain lengths between 1 and 16 C-atoms) in solution were studied by time-resolved UV-vis absorption spectroscopy The triplet properties under direct and benzophenone-sensitized excitation conditions are outlined, whereas any singlet state reactions towards photodimers are unlikely The quantum yield of intersystem crossing of 1–11 is small, Φisc = 002–005, in benzene, acetonitrile or methanol and much larger, Φisc = 01–05, in water and 2,2,2-trifluoroethanol The rate constant for intrinsic decay was found to be below 1 × 105 s−1 in most cases The observed triplet state reacts with coumarins; the rate constant for this self-quenching step is largest in aqueous solution, 3 × 109 M−1 s−1, and smallest, 3 × 107 M−1 s−1, in benzene The results for the derivatives are very similar and the main difference is a lower Φisc for 1 in aqueous solution The published product yields and patterns from preparative irradiations as a function of solvent polarity are discussed in view of the triplet properties
TL;DR: The reaction between the triplet excited state of riboflavin and amino acids, peptides, and bovine whey proteins was investigated in aqueous solution using nanosecond laser flash photolysis and H-atom abstraction seems to operate at low pH, which with rising pH gradually is replaced by electron transfer from the thiol anion.
Abstract: The reaction between the triplet excited state of riboflavin and amino acids, peptides, and bovine whey proteins was investigated in aqueous solution in the pH range from 4 to 9 at 24 degrees C using nanosecond laser flash photolysis. Only tyrosine and tryptophan (and their peptides) were found to compete with oxygen in quenching the triplet state of riboflavin in aqueous solution, with second-order rate constants close to the diffusion limit, 1.75 x 10(9) and 1.40 x 10(9) L mol(-1) s(-1) for tyrosine and tryptophan, respectively, with beta-lactoglobulin and bovine serum albumin having comparable rate constants of 3.62 x 10(8) and 2.25 x 10(8) L mol(-1) s(-1), respectively. Tyrosine, tryptophan, and their peptides react with the photoexcited triplet state of riboflavin by electron transfer from the tyrosine and tryptophan moieties followed by a fast protonation of the resulting riboflavin anion rather than by direct H-atom abstraction, which could be monitored by time-resolved transient absorption spectroscopy as a decay of triplet riboflavin followed by a rise in riboflavin anion radical absorption. For cysteine- and thiol-containing peptides, second-order rate constants depend strongly on pH, for cysteine corresponding to pKaRSH = 8.35. H-atom abstraction seems to operate at low pH, which with rising pH gradually is replaced by electron transfer from the thiol anion. From the pH dependence of the second-order rate constant, the respective values for the H-atom abstraction (k = 1.64 x 10(6) L mol(-1) s(-1)) and for the electron transfer (k = 1.20 x 10(9) L mol(-1) s(-1)) were determined.
TL;DR: In this article, triplet state properties including transient triplet absorption spectrum, intersystem crossing yields in solution at room temperature and phosphorescence spectra, quantum yields and lifetimes at low temperature as well as singlet oxygen yields were obtained for poly(N-vinylcarbazole) (PVK) in 2-methyl-tetrahydrofuran (2-MeTHF), cyclohexane or benzene.
TL;DR: The bimolecular reactions of acetylnitrene and methoxycarbonylnitrene with propane, ethylene, and methanol were calculated and found to have enthalpic barriers that are near zero and free energy barriers that is controlled by entropy.
Abstract: Density functional theory (DFT), CCSD(T), and CBS-QB3 calculations were performed to understand the chemical and reactivity differences between acetylnitrene (CH(3)C(=O)N) and methoxycarbonylnitrene (CH(3)OC(=O)N) and related compounds. CBS-QB3 theory alone correctly predicts that acetylnitrene has a singlet ground state. We agree with previous studies that there is a substantial N-O interaction in singlet acetylnitrene and find a corresponding but weaker interaction in methoxycarbonylnitrene. Methoxycarbonylnitrene has a triplet ground state because the oxygen atom stabilizes the triplet state of the carbonyl nitrene more than the corresponding singlet state. The oxygen atom also stabilizes the transition state of the Curtius rearrangement and accelerates the isomerization of methoxycarbonylnitrene relative to acetylnitrene. Acetyl azide is calculated to decompose by concerted migration of the methyl group along with nitrogen extrusion; the free energy of activation for this concerted process is only 27 kcal/mol, and a free nitrene is not produced upon pyrolysis of acetyl azide. Methoxycarbonyl azide, on the other hand, does have a preference for stepwise Curtius rearrangement via the free nitrene. The bimolecular reactions of acetylnitrene and methoxycarbonylnitrene with propane, ethylene, and methanol were calculated and found to have enthalpic barriers that are near zero and free energy barriers that are controlled by entropy. These predictions were tested by laser flash photolysis studies of benzoyl azide. The absolute bimolecular reaction rate constants of benzoylnitrene were measured with the following substrates: acetonitrile (k = 3.4 x 10(5) M(-1) (s-1)), methanol (6.5 x 10(6) M(-1) s(-1)), water (4.0 x 10(6) M(-1) s(-1)), cyclohexane (1.8 x 10(5) M(-1) s(-1)), and several representative alkenes. The activation energy for the reaction of benzoylnitrene with 1-hexene is -0.06 +/- 0.001 kcal/mol. The activation energy for the decay of benzoylnitrene in pentane is -3.20 +/- 0.02 kcal/mol. The latter results indicate that the rates of reactions of benzoylnitrene are controlled by entropic factors in a manner reminiscent of singlet carbene processes.
TL;DR: Time-resolved spectroscopic experiments confirmed the formation of the excited triplet state of bixin and its deactivation by ground state bixIn and molecular oxygen quenching processes, and the consequences for food color are discussed.
Abstract: The photosensitized isomerization reaction of the natural cis carotenoid bixin (methyl hydrogen 9'-cis-6, 6'-diapocarotene-6, 6'-dioate) with rose bengal or methylene blue as the sensitizer in acetonitrile/methanol (1:1) solution was studied using UV-vis spectroscopy, high-performance liquid chromatography (HPLC), and time-resolved spectroscopic techniques, such as laser-flash photolysis and singlet oxygen phosphorescence detection. In both N(2)- and air-saturated solutions, the main product formed was all-trans-bixin. The observed isomerization rate constants, k(obs), decreased in the presence of air or with increase in the bixin concentration, suggesting the participation of the excited triplet state of bixin, (3)Bix, as precursor of the cis--> trans process. On the other hand, bixin solutions in the absence of sensitizer and/or light did not degrade, indicating that the ground state of bixin is stable to thermal isomerization at room temperature. Time-resolved spectroscopic experiments confirmed the formation of the excited triplet state of bixin and its deactivation by ground state bixin and molecular oxygen quenching processes. The primary isomerization products only degraded in the presence of air and under prolonged illumination conditions, probably due to the formation of oxidation products by reaction with singlet molecular oxygen. An energy-transfer mechanism was used to explain the observed results for the bixin transformations, and the consequences for food color are discussed.
TL;DR: In this article, the spin states of a few-electron quantum dot defined in a two-dimensional electron gas, by applying a large in-plane magnetic field, were studied, and it was shown that higher-order tunnel processes and spin-orbit interaction have negligible effect on the polarization.
Abstract: We study the spin states of a few-electron quantum dot defined in a two-dimensional electron gas, by applying a large in-plane magnetic field. We observe the Zeeman splitting of the two-electron spin triplet states. Also, the one-electron Zeeman splitting is clearly resolved at both the zero-to-one and the one-to-two electron transition. Since the spin of the electrons transmitted through the dot is opposite at these two transitions, this device can be employed as an electrically tunable, bipolar spin filter. Calculations and measurements show that higher-order tunnel processes and spin-orbit interaction have a negligible effect on the polarization.
TL;DR: Two rotaxanes tethering [60]fullerene (C60) and triphenylamine (TPA) moieties were synthesized in good yields by the urethane end-capping method using a crown ether−secondary amine motif as mentioned in this paper.
Abstract: Two rotaxanes tethering [60]fullerene (C60) and triphenylamine (TPA) moieties were synthesized in good yields by the urethane end-capping method using a crown ether−secondary amine motif. In these rotaxanes, the C60 group serving as electron acceptor is attached to the crown ether wheel through which the axle with a TPA group acting as electron donor on its terminal penetrates. One rotaxane has an ammonium moiety, whereas the other has a neutral amide moiety in the center of the axle. The corresponding reference compounds without rotaxane structures were also prepared. The intra-rotaxane photoinduced electron-transfer processes of C60 and TPA have been investigated by time-resolved transient absorption and fluorescence measurements with changing solvent polarity and temperature. Nanosecond transient absorption measurements of these rotaxanes demonstrated that the long-lived charge-separated state (C60•-,TPA•+)rotaxane is formed via the excited triplet state of C60 (3C60*) in polar solvents. The rate const...