Showing papers on "Triplet state published in 1996"
TL;DR: In this article, a large basis set of α-oligothiophenes with two to seven rings (α2−α7), also including thiophene, α1, have been investigated in five solvents regarding absorption, fluorescence and phosphorescence.
Abstract: A large basis set of α-oligothiophenes with two to seven rings (α2−α7), also including thiophene, α1, have been investigated in five solvents regarding absorption, fluorescence and phosphorescence, quantum yields of fluorescence (φF) and triplet formation (φT), lifetimes of fluorescence and the triplet state, quantum yields of singlet oxygen production (φΔ), all rate constants kF, kIC, kISC, and several of the foregoing as a function of temperature. Ten different theoretical calculations across several levels including three levels of ab initio have been carried out regarding which conformer is lowest in energy and the ΔH's among all conformers of α2, α3 and α5, as well as calculations of transitions energies of the α-oligothiophenes. We have shown that the (l) 1Bu state is the lowest singlet state for all α2−α7 in any solvent, in contradiction to previous predictions for the higher members. Based on absorption and fluorescence data and calculations of atomic charges in S0 and S1, the ground state is twis...
495 citations
TL;DR: In this article, the porphyrin first excited singlet states decay in about 20 ps by singlet−singlet energy transfer to the fullerene, and the first-excited singlet state is not quenched and undergoes intersystem crossing to the triplet.
Abstract: Time-resolved fluorescence and absorption techniques have been used to investigate energy and photoinduced electron transfer in a covalently linked free-base porphyrin−fullerene dyad and its zinc analog. In toluene, the porphyrin first excited singlet states decay in about 20 ps by singlet−singlet energy transfer to the fullerene. The fullerene first excited singlet state is not quenched and undergoes intersystem crossing to the triplet, which exists in equilibrium with the porphyrin triplet state. In benzonitrile, photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene competes with energy transfer. The fullerene excited singlet state is also quenched by electron transfer from the porphyrin. Overall, the charge-separated state is produced with a quantum yield approaching unity. This state lives for 290 ps in the free-base dyad and 50 ps in the zinc analog. These long lifetimes suggest that such dyads may be useful as components of more complex light-harvesting systems.
314 citations
TL;DR: In this paper, the authors calculate the vertical excitation energies from the singlet ground state S0 to the first one-photon allowed singlet excited state S1 as well as the energy difference between the ground state and the lowest triplet state T1.
Abstract: On the basis of configuration interaction calculations, we first describe the nature of the lowest singlet and triplet excited states in oligothiophenes ranging in size from two to six rings. We calculate the vertical excitation energies from the singlet ground state S0 to the first one-photon allowed singlet excited state S1 as well as the energy difference between the ground state and the lowest triplet state T1. The computed transition energies are in very good agreement with the measured values and indicate a strong confinement of the lowest triplet. We also uncover the nature of the higher-lying triplet excited state Tn that is coupled via a large oscillator strength to T1. The evolution with chain length of the T1−Tn excitation energies compares well with the experimental evolution based on photoinduced absorption data. Next, we investigate the geometry relaxation phenomena occurring in the S1 and T1 states; more pronounced and localized bond-length deformations are calculated in the triplet state t...
223 citations
TL;DR: In this paper, the energy barriers governing proton transfer in the ground and lowest two excited electronic states of malonaldehyde have been investigated by using density functional and post-Hartree-Fock methods.
Abstract: Intramolecular proton transfer in the ground and the lowest two excited electronic states of malonaldehyde has been investigated by using density functional and post‐Hartree–Fock methods. Our best estimates of the energy barriers governing proton transfer in the ground and lowest triplet state are quite low (4.3 and 6.6 kcal/mol, respectively), whereas a significantly higher barrier (12.0 kcal/mol) is obtained for the second triplet state. The coupled cluster approach provides reliable results already with relatively compact basis sets, its only drawback being the very unfavorable scaling with the number of active electrons. Among the cheaper methods, those based on the many‐body perturbative approach provide good results for the ground electronic state, but their performances strongly deteriorate for excited states. The overestimation of correlation energy by conventional density functional methods produces an excessive degree of conjugation in the backbone of malonaldehyde with the consequent underestimation of energy barriers governing proton transfer. A more coherent picture is offered by a hybrid density functional/Hartree–Fock approach, which couples good structural predictions with a reduced, although still not negligible, underestimation of energy barriers. Furthermore, different electronic states are described with comparable accuracy.
207 citations
TL;DR: In this paper, the photobleaching properties of fluorophores in solution were investigated by using fluorescence correlation spectroscopy (FCS) to calculate the photodestruction quantum yield and relate the photostability of a fluorescent compound in a certain environment to the photodynamical behaviour of the singlet-triplet transitions.
Abstract: Fluorescence correlation spectroscopy (FCS) can be used to investigate the photobleaching properties of fluorophores in solution. The advantage with this method is that in addition to the photobleaching rate the formation and decay rates of the triplet state can be measured. In this way, it is possible to calculate the photodestruction quantum yield and relate the photostability of a fluorescent compound in a certain environment to the photodynamical behaviour of the singlet-triplet transitions. This is likely to contribute to a better understanding of the mechanisms of photobleaching given the central importance of dye triplet states in photobleaching processes. The approach was applied to the measurement and characterization of the photobleaching of Rh6G in aqueous solution and FITC in 1 mM sodium carbonate buffer (pH 9). The photobleaching yields measured are discussed in view of the simultaneous triplet properties at different excitation intensities, oxygen concentrations as well as in the presence or absence of quencher molecules. This study suggests that FCS is likely to provide a valuable tool for the elucidation of the mechanisms of photobleaching, which are far from understood in all their details.
191 citations
TL;DR: In this paper, laser kinetic spectroscopy was used to determine the photophysical parameters (fluorescence and triplet-triplet (T-T) spectra, lifetimes of the triplet and singlet states, bimolecular constant for oxygen quenching, fluorescence quantum yield and quantum yield of formation of the triplet state.
Abstract: Laser kinetic spectroscopy was used to determine the photophysical parameters (fluorescence and triplet-triplet (T-T) spectra, lifetimes of the triplet and singlet states, bimolecular constant for oxygen quenching, fluorescence quantum yield and quantum yield of formation of the triplet state) of the water-soluble complexes of Ag(II), Cd(II), Co(II), Cu(II), Mg(II), Ni(II), Pd(II), Pt(II) and Zn(II) with the tetrasodium salt of meso-tetrakis(4-sulphonatophenyl)porphyrin (H 2 TPPS 4 ). The complexes of metals with d 0 and d 10 electron configurations (Cd(II), Mg(II) and Zn(II)) are characterized by high triplet quantum yields and strong fluorescence in the red spectral region and are thus more efficient photosensitizers than the ligand alone.
170 citations
TL;DR: In this article, the spin-orbit coupling matrix elements between singlet and triplet states are evaluated as residues of (multi-configuration) linear response functions, and are therefore automatically determined between orthogonal and non-interacting states.
Abstract: We review response theory and calculations of molecular properties involving spin-orbit interactions. The spin-orbit coupling is evaluated for reference states described by single- or multi-configuration self-consistent field wave functions. The calculations of spin-orbit related properties rest on the formalism of linear and quadratic response functions for singlet and triplet perturbations when no permutational symmetry in the two-electron operators is assumed and from which various triplet as well as singlet response properties are derived. The spin-orbit coupling matrix elements between singlet and triplet states are evaluated as residues of (multi-configuration) linear response functions, and are therefore automatically determined between orthogonal and non-interacting states. Spin-forbidden radiative transition intensities and lifetimes are determined from the spin-orbit coupling induced dipole transitions between two electronic states of different multiplicity and are obtained as residues of quadratic response functions. The potential of the theory and its range of applications is illustrated by a selection of recent investigations covering different molecular phenomena. The applications include second-order energy contributions, intensity rearrangement in electron spectra, calculation of predissociative lifetimes of dicationic states, assignment of triplet bands in absorption spectra, intersystem crossings and reactivity, external heavy atom effects on S-T transitions, phosphorescence spectra and radiative lifetimes of triplet states. We give an outlook on spin-orbit interaction induced phenomena in extended systems and on applications to general spin catalysis phenomena.
138 citations
TL;DR: In this paper, it was shown that the acridinium-localized excited (LE) triplet state is formed from the CSH singlet state by spin−orbit coupling.
Abstract: The 10-methylacridinium ion displays an emission associated with a charge-shifted (CSH) species when substituted in the 9-position with a substituent having a relatively low ionization potential (naphthyl, biphenyl). Flash photolysis and time-resolved EPR (TREPR) measurements show that photoexcitation of these donor−acceptor systems generates an acridinium-localized excited (LE) triplet state. While values of zero-field splitting parameters are virtually unaffected by the nature of the substituent, spin polarization patterns observed in the TREPR spectra display a striking dependence on substituent as well as orientation of donor ring system. Flash photolysis and TREPR data show that the LE triplet state is formed from the CSH singlet state. In these directly linked donor−acceptor molecules, in which the aromatic rings are near perpendicular because of steric hindrance, CSH singlet → LE triplet intersystem crossing (isc) is driven by spin−orbit coupling. This mechanism generates a unique dependence of isc...
133 citations
TL;DR: In this paper, the authors investigated the mechanism of O-atom loss from carbonyl oxides, as well as the competition between the cyclization to dioxirane and the tautomerization to hydroperoxide in methyl-substituted carbonyls.
Abstract: The lowest singlet and triplet potential energy surfaces of formaldehyde carbonyl oxide (1) and acetaldehyde carbonyl oxide (2) have been investigated in the regions concerning the most relevant unimolecular reactions by means of CASSCF and MRDCI ab initio quantum-chemical calculations. The questions related to the mechanism of O-atom loss from carbonyl oxides, as well as the competition between the cyclization to dioxirane and the tautomerization to hydroperoxide in methyl-substituted carbonyl oxides are addressed in this investigation. The theoretical predictions are consistent with experimental findings obtained from stopped-flow studies of the gas-phase ozonation of both trans-butene and tetramethylethylene. An unexpected result is that the most reasonable pathway for O-atom loss from “hot” singlet carbonyl oxides 1 and 2 involves internal rotation about the CO bond axis, followed by intersystem crossing to the lowest triplet state and subsequent scission of the OO bond.
133 citations
TL;DR: In this article, the effects of substituents on the N-phenyl group on solvent polarity and viscosity have been studied, and a short-wavelength (SW) fluorescence, similar to the luminescence emitted by the n-alkyl derivatives, and/or a considerably red shifted long-wvelength (LW) luminecence are observed, and the ratio of the SW and LW fluorescence components is found to depend on substitution and on solvent properties.
Abstract: Absorption and fluorescence spectra, fluorescence decay times, and quantum yields of fluorescence and triplet state formation have been determined for N-phenyl and substituted N-phenyl derivatives of 1,2-, 2,3-, and 1,8-naphthalimides, using stationary irradiation and laser flash excitation methods. The effects of substituents on the N-phenyl group on solvent polarity and viscosity have been studied. A short-wavelength (SW) fluorescence, similar to the luminescence emitted by the N-alkyl derivatives, and/or a considerably red shifted long-wavelength (LW) luminescence are observed, and the ratio of the SW and LW fluorescence components is found to depend on substitution and on solvent properties. A striking characteristic of the N-phenylnaphthalimides (in contrast to the N-alkyl derivatives) is the very efficient internal conversion which results in short fluorescence decay times and in low fluorescence and triplet yields. On the basis of the experimental results, it is suggested that solvent and geometric...
124 citations
TL;DR: In all cases, the triplet states of the sensitizers were efficiently quenched by the nucleotides, although different reaction mechanisms were observed depending on the reaction pa...
Abstract: The study of triplet excited state behavior of nucleic acids and component mononucleotides is hampered by the very small yields produced by direct photolysis. We have used high energy triplet sensitizers to generate these species in high yield, thus facilitating the study of their photophysical and photochemical behavior. Acetone-sensitized triplet formation of all triplet state nucleotides allowed nucleotide triplet−triplet absorption spectra to be measured. Triplet−triplet absorption coefficients were determined using comparative actinometry. Self-quenching of the nucleotide triplet states was found to occur efficiently with rate constants, ksq > 107 M-1 s-1. The interaction of a variety of ketone triplet sensitizers with mononucleotides has been studied as a function of the relative energies of the sensitizer−nucleotide pair. In all cases, the triplet states of the sensitizers were efficiently quenched by the nucleotides, although different reaction mechanisms were observed depending on the reaction pa...
TL;DR: The electron spin dynamics associated with intramolecular electron transfer in a photosynthetic model system, which consists of a linear structure of the type A-B-C, is described in this article.
Abstract: The electron spin dynamics associated with intramolecular electron transfer in a photosynthetic model system, which consists of a linear structure of the type A-B-C, is described. In this structure, donor A is a p-methoxyaniline, chromophore B is a 4-amino-1,8-naphthalimide, and acceptor C is a 1,4:5,8-naphthalenediimide. This supramolecular electron donor−acceptor array was isotropically oriented in toluene, and anisotropically oriented in liquid crystal matrices, and studied by time-resolved electron paramagnetic resonance spectroscopy. Photoexcitation of B results in a two-step electron transfer to yield the radical ion pair, A•+-B-C•-. Charge recombination within A•+-B-C•- produces a molecular triplet state, A-B-3*C, which exhibits the unique spin-polarized electron paramagnetic resonance signal that has been observed only in photosynthetic reaction-center proteins.
TL;DR: In this paper, the global potential energy surfaces (PESs) of the [H,C,N,O] system in singlet and triplet states have been investigated using the hybrid density functional B3LYP/6−311G(d,p) method.
Abstract: Global potential energy surfaces (PESs) of the [H,C,N,O] system in singlet and triplet states have been investigated using the hybrid density functional B3LYP/6−311G(d,p) method. Isocyanic acid, HNCO 1, has been found to be the most stable isomer for both multiplicities. The adiabatic singlet–triplet splitting for 1 is 82.6 kcal/mol. In the singlet state, HNCO is energetically followed by cyanic acid, HOCN 2, 28.7 kcal/mol higher than 1, fulminic acid, HCNO 3 (67.9 kcal/mol), and isofulminic acid, HONC 4 (87.1 kcal/mol). In the triplet state, the branched NC(H)O isomer 37 is 0.3 kcal/mol higher than 31, followed by HOCN 32 (27.9 kcal/mol relative to triplet HNCO) and HCNO 33 (40.6 kcal/mol). The barriers for intramolecular rearrangements within singlet and triplet [H,C,N,O] system have been calculated to be high, and the isomerization processes in most cases are not expected to compete with fragmentations. Several minima on the singlet–triplet seam of crossing, relevant to the singlet [H,C,N,O] decomposit...
TL;DR: In this paper, three potential photosensitizers for photodynamic therapy: tetra-t-butyl phthalocyaninato Zn(II), tetrakis-(1,1-dimethyl-2-phthalimido)ethylphthalocalinato (Zn)-Zn(III), and tetra -tbutyl polybutyl (PTP)-Phthalocynato (PhCynoZn), have been studied in homogeneous organic media and the photo-physical characterization of the triplet state was performed.
Abstract: Three potential photosensitizers for photodynamic therapy: tetra-t-butyl phthalocyaninato Zn(II), tetrakis-(1,1-dimethyl-2-phthalimido)ethylphthalocyaninato Zn(II), and tetrakis-(1,1-dimethyl-2-amino)ethylphthalocyaninato Zn(II), have been studied in homogeneous organic media. Dimerization constants and monomer and dimer spectra have been determined. Fluorescence, triplet and singlet oxygen quantum yields were measured and the photo-physical characterization of the triplet state was performed for the three dyes. These parameters were evaluated in connection with aggregation results.
TL;DR: In this article, the authors developed a model which will yield steady state vibrational level populations for both the singlet and triplet valence states of N2, which are responsible for auroral emissions from the UV (Vegard-Kaplan (VK), second positive (2PG)) through the visible to the infrared (first positive (1PG), infrared afterglow (IRA), Wu-Benesch (WB)).
Abstract: Previous model results have shown that the N2 triplet vibrational level populations in the aurora are strongly affected by cascade and quenching by atomic and molecular oxygen. As the aurora penetrates to lower altitudes (less than 100 km) the role of quenching by atomic oxygen becomes less important and processes involving N2 collisions begin to play a more prominent part. We are developing a model which will yield steady state vibrational level populations for both the singlet and triplet valence states of N2. The model currently provides results for the seven low-lying N2 triplet states (A 3Σu+, B 3IIg, W3Δu, B′ 3Σu−, C3IIu, D3Σu+, and E3Σg+). These states are responsible for auroral emissions from the UV (Vegard-Kaplan (VK), second positive (2PG)) through the visible to the infrared (first positive (1PG), infrared afterglow (IRA), Wu-Benesch (WB)). We have included two additional collisional processes in the current model which were not treated previously. These are the intersystem collisional transfer of excitation (ICT) between the B state and the A, W, and B′ states and vibrational redistribution within the A state vibrational manifold, both due to collisions with ground state N2. The present work compares our current model results with those of a previous model, as well as ground, airborne, and rocket observations. The comparison between N2(A) (VK) and N2(B) (1PG) vibrational level populations predicted by our model and a number of auroral observations indicate that the current model achieves a significant improvement in the fit between calculation and observation. In addition, the current model predicts a shift in the band intensity distribution of the 1PG Δν = 3 sequence from the infrared into the visible red at the lower altitudes (less than 90 km) as well as an overall enhancement in the entire 1PG system. Consequently, this provides a possible explanation of a dominate feature of type b aurora, the auroral red lower border.
TL;DR: In this paper, the fluorescence properties of two macrocyclic ligands incorporating bis-(β-naphthyl amide) groups, CH2CON(CH2C10H7)2, as fluorophores, have been studied in water and acetonitrile.
Abstract: The fluorescence properties of two macrocyclic ligands incorporating bis-(β-naphthyl amide) groups, –CH2CON(CH2C10H7)2, as fluorophores, have been studied in water and acetonitrile. The ligands exhibit distinctive luminescence behaviour in the presence of quenching (e.g. Pb2+, Ni2+, Cu2+ and Eu3+) and non-quenching ions (e.g. Zn2+ and Cd2+). Excimer-to-monomer intensity ratios are affected to differing extents, as are the total integrated fluorescence intensities. The behaviour is contrasted to that reported previously for related ligands containing ‘isolated’ naphthyl units. The luminescence properties of the terbium complex of one of these earlier ligands, containing just one naphthalene group, have been studied in detail. Naphthyl-sensitised metal emission is observed but a nanosecond laser flash photolysis study reveals that back energy transfer to the naphthyl triplet state provides a facile deactivation mechanism for the metal excited state.
TL;DR: In this article, the triplet-mediated reactions between nucleic acid bases in solution, at room temperature, were investigated and the absolute value for TMP triplet energy was estimated as 310 kJ mol-1, based on triplet transfer studies involving acetophenone and 3-methoxyacetophenone.
Abstract: This study aims to provide information on the fate of triplet energy in DNA by considering the triplet-mediated reactions which occur between nucleic acid bases in solution, at room temperature. Following sensitization of base triplet states by acetone, the subsequent photophysics and photochemistry are highly dependent on the nature of the nucleotide pair under study. By establishing the direction of triplet energy transfer between pairs of mononucleotides, the relative triplet energy ordering was determined under physiologically relevant conditions, i.e. aqueous solution at room temperature. This order (C > U > G > A > T) is in agreement with that previously reported which was obtained at 77 K in rigid media. The absolute value for TMP triplet energy has been estimated as 310 kJ mol-1, based on triplet energy transfer studies involving acetophenone and 3-methoxyacetophenone. Determination of the triplet energy gaps between all mononucleotides, to within 1 kJ mol-1, has allowed an estimation of the absol...
TL;DR: In this paper, the authors measured absolute VUV optical absorption cross sections for ozone between 325 and 110 nm (3.0 - 11.3 eV) using a synchrotron radiation source.
Abstract: Absolute VUV optical absorption cross sections for ozone have been measured between 325 and 110 nm (3.0 - 11.3 eV) using a synchrotron radiation source. Vibrational fine structure is resolved in Rydberg bands and comparison of this with the limiting bands in the photoelectron spectrum confirms that the order (increasing ionization energy) of the three lowest ionization bands is . Near-threshold electron energy-loss spectra have also been recorded. In these, in addition to the known triplet states between 1 and 2 eV, a low-lying triplet state has been located around 3.4 eV and several others between 6 and 9 eV. Characterization of the valence states (both optically allowed and forbidden) are discussed in relation to the results of early theoretical computations which seem to give a good account of the ozone spectrum.
TL;DR: In this paper, the authors examined activation barriers for the penetration through an intact cage and for the insertion through the two possible one-bond windows and six different two bond windows both in the lowest singlet triplet state.
Abstract: Reaction mechanisms for helium incorporation into C{sub 60} have been examined by semiempirical (MNDO), ab initio (HF/3-21G), and density functional (BLYP/3-21G, BP86/3-21G) calculations. Effective activation barriers for the penetration through an intact cage and for the insertion through the two possible one-bond windows and six different two-bond windows are reported both in the lowest singlet triplet state. The computed barriers are very high and exceed 200 kcal/mol for each of the 21 pathways considered. These results require a new mechanistic interpretation of the high-pressure experiments where helium incorporation into fullerenes is observed under thermal conditions. Mechanistic alternatives are discussed including the possible role of imperfect C{sub 60} isomers and other intermediates. 57 refs., 10 figs., 5 tabs.
TL;DR: In this paper, cyclic voltammetry (cv) and DPV studies of C120O reveal that this bis(fullerene) undergoes six sequential, one-electron reductions to form the ions [C120O]n- where n is −1 to −6.
Abstract: Cyclic voltammetry (cv) and differential pulse voltammetry (dpv) studies of C120O reveal that this bis(fullerene) undergoes six sequential, one-electron reductions to form the ions [C120O]n- where n is −1 to −6. The first two reductions of C120O occur at potentials that are close to that for the reduction of C60 to [C60]- and it appears that the two electrons are added successively to each of the two fullerene cages. Similarly, the second two reductions of C120O occur at potentials that are close to the reduction potential for the conversion of [C60]- to [C60]2-. Electron paramagnetic resonance spectra have been observed for both [C120O]2- and [C120O]4-. For [C120O]2- the EPR spectrum in o-dichlorobenzene at 77 K consists of a doublet-like feature (g = 2.0016) that is surrounded by features characteristic of a triplet state (g = 2.0030). It is concluded that for [C120O]2- the spectrum indicates that there is significant interaction between the two electrons that have been added to the fullerene and that t...
TL;DR: From a practical viewpoint, amino and phosphino derivatives and derivatives bearing large alkyl groups (R) are the most plausible and feasible targets for preparing phosphinidenes possessing a closed-shell singlet ground state.
Abstract: We have examined singlet−triplet energy separations in different phosphinidenes (RP) substituted by first- and second-row elements, making use of ab initio molecular orbital theory. Our main purpose is to find out the substituents that particularly favor the singlet electronic state. The QCISD(T)/6-311++G(3df,2p) + ZPE level has been applied to small molecules and the CISD(Q) and QCISD(T) with the 6-311G(d,p) basis set for all species considered. We have identified few factors that come into play rendering the singlet phosphinidene more stable than the triplet. The parent phosphinidene, PH, has a triplet ground state lying 28 kcal/mol below the closed-shell singlet excited state. The triplet ground state is mainly favored when negative hyperconjugation is involved. In the boryl-, alkyl-, and silyl-substituted phosphinidenes, the triplet state remains by far the ground state. When the substituents have π-type lone pair electrons (i.e., −NX2, −PX2, −OX, −SX), the singlet state becomes stabilized by such an ...
TL;DR: In this article, the authors investigated the kinetics of the fluoride-induced decomposition of thermally stable silyloxyaryl-substituted spiroadamantyl dioxetanes 1a,b and the excited state formation of this chemically initiated electron exchange luminescence (CIEEL) have been investigated.
Abstract: — The kinetics of the fluoride-induced decomposition of the thermally stable silyloxyaryl-substituted spiroadamantyl dioxetanes 1a,b and the excited state formation of this chemically initiated electron exchange luminescence (CIEEL) have been investigated. Two limiting kinetic regimes flash and glow have been identified, which depend on the fluoride concentration, the first at high, the second at low [F-] triggering, whose detailed kinetic analysis affords the rate constants for the deprotected dioxetanes 2a,b cleavage in acetonitrile and dimethyl sulfoxide and chemiluminescence measurements the CIEEL and phen-olate 4 (CIEEL emitter) excitation yields. Chloro-substi-tution in the spiroadamantyl dioxetane does not affect the deprotection step k2 but leads to a ca five-fold faster cleavage of the deprotected dioxetane 2, while the chemiexcitation yield is the same for both dioxetanes. The energies of the first excited singlet and triplet states of the emitting phenolate 4 were estimated by AM1 configuration interaction calculations with explicit consideration of acetonitrile as solvent (self-consistent reaction field approach). The first excited singlet and triplet state of the CIEEL emitter phenolate 4 possess π,π* character, as suggested by the π-type molecular orbitals and the large singlet-triplet energy gap. The chemiexcitation of both singlet and triplet states of the excited phenolate 4 is feasible during the dioxetanes 1a,b cleavage, but the experimentally determined high singlet excitation yields suggest that preferentially the phenolate 4 singlet state is populated in the fluoride ion-triggered CIEEL process.
TL;DR: In this paper, electron delocalization over an extended π*-orbital in the π-radical anions was investigated for a series of mono-and binuclear ruthenium(II) and osmium (II) 2,2‘-bipyridyl complexes that contain an ethynyl-bridged ditopic ligand.
Abstract: Photophysical and electrochemical properties have been recorded for a series of mono- and binuclear ruthenium(II) and osmium(II) 2,2‘-bipyridyl complexes that contain an ethynyl-bridged ditopic ligand. In particular, the electrochemical properties are indicative of electron delocalization over an extended π*-orbital in the π-radical anions. The site of attachment of the ethynyl substituent to the 2,2‘-bipyridyl ring affects the various properties, especially absorption and emission spectral maxima. In most cases, the rates of nonradiative deactivation of the lowest-energy triplet excited states are slower than expected for a corresponding complex not possessing a conjugated substituent. This effect is rationalized in terms of electron delocalization over part of the ditopic ligand within the triplet state and its significance depends markedly on the triplet energy of the complex in question. The lowest-energy triplet mixes to some extent with an upper-lying triplet that is more strongly coupled to the gro...
TL;DR: In this article, the importance of Coulombic energy transfer in intramolecular triplet energy transfer was discussed, and it was considered that the PtII center decreases the magnitude of the electronic coupling matrix element (HDA < 0.2 cm-1) for electron exchange, in part because of the high energy of its LUMO and HOMO states, relative to the butadiynyl bridge.
Abstract: Intramolecular triplet energy transfer occurs from a ruthenium(II) centre to an osmium(II) centre in heteropolynuclear polypyridine complexes at room temperature in acetonitrile solution. When the metal complexes are linked together by way of a butadiynyl bridge (polypyridine = 2,2′: 6′,6″-terpyridine), energy transfer is extremely fast and takes place via Dexter-type (through-bond) electron exchange. This process is facilitated by selective electron donation to the bridging ligand in the MLCT triplet state of the RuII complex, the electron residing in an extended π*-orbital that partially encompasses the bridge. In marked contrast, insertion of a PtII centre into the butadiynyl bridge (polypyridine = 2,2′-bipyridine) serves to inhibit through-bond electron exchange so that triplet energy transfer occurs slowly. It is considered that the PtII centre decreases the magnitude of the electronic coupling matrix element (HDA < 0.2 cm–1) for electron exchange, in part because of the high energy of its LUMO and HOMO states, relative to the butadiynyl bridge (HDA≈ 12 cm–1). The PtII centre, which donates charge to the bridging ligand, also induces localisation of the promoted electron at an unsubstituted 2,2′-bipyridyl ligand in the triplet excited state. The corresponding cis isomer (with respect to the PtII centre) gives a slightly slower rate of intramolecular triplet energy transfer. The importance of Coulombic energy transfer in these systems is discussed.
TL;DR: In this article, the authors investigated the Singlet and triplet energy transfer processes in [Ru(bipy) 2 (4-methyl-4′-(2-arylethyl)-2,2′-bipyridine)] 2+ have been investigated.
TL;DR: The first nanosecond time-resolved resonance Raman study of carotenoid radical cations is reported in this article for the polyenes septapreno-β-carotene and 7,7‘-dihydro-β-, and for the ground and triplet states of these molecules.
Abstract: The first nanosecond time-resolved resonance Raman study of carotenoid radical cations is reported for the polyenes septapreno-β-carotene and 7,7‘-dihydro-β-carotene. In addition, previously unreported resonance Raman spectra of the ground and triplet states of these molecules are reported. The radical cations were generated following electron transfer quenching of triplet 1-nitronaphthalene in methanol and Triton X-100 micelles. The quenching of triplet 1-nitronaphthalene by these carotenoids involves solvent-dependent competition between energy transfer and electron transfer, and for both carotenoids, estimates are given for the efficiencies of these two processes in methanol and hexane. The resonance Raman spectra of septapreno-β-carotene ground and triplet states are consistent with spectra reported previously for other carotenoids. However, the resonance Raman spectrum of the triplet state of 7,7‘-dihydro-β-carotene displays an intensity profile not found in the triplet spectra of other carotenoids. ...
TL;DR: The sensitizer in its excited triplet state can undergo many collisions with other molecules during its lifetime and, as a result, can mediate photosensitized reactions with high efficiency.
TL;DR: In this article, the collisional deactivation of pyrazine (C4N2H4) in the electronic ground state by 19 collider gases was studied using the time-resolved infrared fluorescence (IRF) technique.
Abstract: The collisional deactivation of vibrationally excited pyrazine (C4N2H4) in the electronic ground state by 19 collider gases was studied using the time‐resolved infrared fluorescence (IRF) technique. The pyrazine was photoexcited with a 308 nm laser and its vibrational deactivation was monitored following rapid radiationless transitions to produce vibrationally excited molecules in the electronic ground state. The IRF data were analyzed by a simple approximate inversion method, as well as with full collisional master equation simulations. The average energies transferred in deactivating collisions (〈ΔE〉d) exhibit a near‐linear dependence on vibrational energy at lower energies and less dependence at higher energies. The deactivation of ground state pyrazine was found to be similar to that of ground state benzene [J. R. Barker and B. M. Toselli, Int. Rev. Phys. Chem. 12, 305 (1990)], but it is strikingly different from the deactivation of triplet state pyrazine [T. J. Bevilacqua and R. B. Weisman, J. Chem. ...
TL;DR: In this article, the properties of two indoline spirobenzopyrans, one containing a 6-nitro and an 8-methoxy group in the benzopyran portion and the other no nitro group (2 ), were studied by nanosecond laser photolysis in solution at room temperature.
TL;DR: In this paper, the excitation and emission spectra of CaMoO4, MgWO4 and PbWO 4 were analyzed by a configurational coordination model, and it was concluded that the potential curve at the excited state becomes shallower than that at the ground state for these systems.
Abstract: The excitation and emission spectra of CaWO4, MgWO4, PbWO4, ZnWO4, and CaMoO4 were analyzed by a configurational coordination model. The spectral line shapes simulated by a configurational coordination model with a singlet ground state and an excited triplet state well explained the line shape of the luminescence spectra and the line shape at the rising part of the excitation spectra of these salts at room temperature. The energy level of the excited triplet state and the reorganization energies at the ground sate and the triplet state were determined. It was concluded that the potential curve at the excited state becomes shallower than that at the ground state for these systems.