Triplet Species Research Articles

Reaction Pathways for the Additions of Triplet Carbene, Silylene, Germylene and Nitrene onto the Diamond (100) Surface: A Theoretical Investigation

The additions of triplet carbene (CH2), silylene (SiH2), germylene (GeH2), and nitrene (NH) onto the diamond (100) surface have been investigated by means of density functional theory in conjunction with effective cluster models. The calculated reaction energies and reaction pathways have demonstrated that the reaction profiles for the additions of triplet species onto the diamond (100) surface is clearly different from those for singlet ones. Namely, triplet CH2, SiH2, GeH2, and NH are inclined to form the one-end-sided species by attaching to only one carbon of the surface dimer of diamond (100). Hence, the other carbon of the surface dimer will be open, which can be potentially used for anchoring other functional groups. This is in distinct contrast to the profile for the additions of singlet ones onto diamond (100) in which the as-formed surface species feature a three-membered ring structure. The results illustrate that the addition profiles of CH2s, SiH2s, GeH2s, and NHs onto the diamond (100) surface could be appropriately tuned depending on their spin states.

Manipulating the Through‐Space Spin–Spin Interaction of Organic Radicals in the Confined Cavity of a Self‐Assembled Cage

We show a new approach to manipulating the through-space spin-spin interaction by utilizing the confined cavity of a self-assembled M6L4 coordination cage. The coordination cage readily encapsulates stable organic radicals in solution, which brings the spin centers of the radicals closer to each other. In sharp contrast to the fact that the radical in solution in the absence of the cage is in a doublet state, in the presence of the cage through-space spin-spin interaction is induced through cage-encapsulation effects in solution as well as in the solid state, resulting in the triplet state of the complex. These results were confirmed by ESR spectroscopy and X-ray crystallography. The quantity of triplet species generated by encapsulation in the cage increases with increasing affinity of the radicals to the cage. We estimated the affinity between several types of guests and the cage in solution by cyclic voltammetry. We also demonstrate that the through-space interaction of organic radicals within the self-assembled coordination cage can be controlled by external stimuli such as heat or pH.

White‐Light Emission from a Single Polymer with Singlet and Triplet Chromophores on the Backbone

AbstractSummary: A strategy to generate an efficient white‐light emission has been developed by mixing fluorescence and phosphorescence emission from a single polymer. Fluorene is used as the blue‐emissive component, benzothiadiazole (BT) and the iridium complex [(btp)2Ir(tmd)] are incorporated into a polyfluorene backbone, respectively, as green‐ and red‐emissive chromophores by Suzuki polycondensation. By changing the contents of BT and [(btp)2Ir(tmd)] in the polymer, the electroluminescence spectrum from a single polymer can be adjusted to achieve white‐light emission. A white polymeric light‐emitting diode (WPLED) with a structure of ITO/PEDOT:PSS/PVK/PFIrR1G03/CsF/Al shows a maximum external quantum efficiency of 3.7% and the maximum luminous efficiency of 3.9 cd · A−1 at the current density of 1.6 mA · cm−2 with the CIE coordinates of (0.33, 0.34). The maximum luminance of 4 180 cd · m−2 is achieved at the current density of 268 mA · cm−2 with the CIE coordinates of (0.31, 0.32). The white‐light emissions from such polymers are stable in the white‐light region at all applied voltages, and the electroluminescence efficiencies decline slightly with the increasing current density, thus indicating that the approach of incorporating singlet and triplet species into the polymer backbone is promising for WPLEDs.Structure of PFIrR1G04 and the EL spectra of its devices under various voltages. Device structure: ITO/PEDOT:PSS/PVK/polymer/CsF/Al.magnified imageStructure of PFIrR1G04 and the EL spectra of its devices under various voltages. Device structure: ITO/PEDOT:PSS/PVK/polymer/CsF/Al.

Theoretical study of the global potential energy surface of the [CH 3,N,C,S] system in singlet and triplet states

The global potential energy surface (PES) of the [CH 3,N,C,S] system in singlet and triplet states, involving 16 isomers and 15 transition structures, is studied at DFT(B3LYP), MP2 and QCISD levels. It is shown that the chainlike singlet isomer CH 3NCS is the most stable species among all the isomers and the branched-C(CH 3)NS has the lowest energy among the triplet species. The stability of these isomers, their isomerizations and dissociations are discussed and theoretical results are consistent with the available experimental ones.

Theoretical investigation on potential energy surface of CSiNP molecule

The structural, spectroscopic and kinetic properties of small molecules composed of second-row elements such as Si and P are vital to understanding the growing process of relevant semiconductors and functional films as well as to predicting new species to be detected in space. In this paper, a detailed computational study is performed on the singlet and triplet [C,Si,N,P] potential energy surfaces at the DFT/B3LYP, QCISD and CCSD(T) (single-point) levels. A total of 38 (15 for singlet and 23 for triplet) minimum isomers and 37 (19 for singlet and 18 for triplet) interconversion transition states are located. Generally, the triplet species lie energetically higher than the singlet ones. At the CCSD(T)/6-311G(2df)//B3LYP/6-311G(d) + ZPVE level, the lowest-lying isomer is SiNCP( 1Σ) 1 1 (0.0 kcal/mol) followed by NCPSi( 1A′) 1 2 (8.9), NCSiP( 1Σ) 1 3 (19.7), PCNSi( 1A′) 1 4 (19.8), CNSiP( 1Σ) 1 5 (26.5), SiCNP 1 6( 1Σ) (31.1) and CNPSi( 1A′) 1 7 (32.2). Except for the isomer 1 4 with very low isomerization barrier, the remaining six ones have reasonable kinetic stabilities. In addition, although the linear isomers NSiCP 1 8 (63.8) and SiCPN 1 10 (73.5) are energetically very high-lying, they contain very large isomerization barriers to allow for existence either in laboratory or in space. By means of structural and bonding analysis, the interesting Si- or P-related triple-bonding are revealed among species like 1 1 (C P), 1 3 (Si P), 1 5 (Si P), 1 6 (Si C, N P). Since up to now, stable typical Si P and Si N triply-bonded species are still experimentally unknown, isomers 1 1, 1 3, 1 5 and 1 6 may represent such good examples. Moreover, the intramolecular donor–acceptor mechanism from the exocyclic π-bonding to the vacant orbital of one ring-atom is proposed to account for the unique three-membered ring-like structure of the chainlike isomers 1 2, 1 4 and 1 7. Finally, the similarities and discrepancies of CSiNP molecule are compared to the analogous 18-valence-electrons species AA′BB′ (A, A′ are group IV elements and B, B′ are group V elements) such as C 2N 2, C 2NP, CSiN 2, Si 2N 2, Si 2NP, and Si 2P 2.

α-Carbonyl Substituent Effect on the Lifetimes of Triplet 1,4-Biradicals from Norrish-Type-II Reactions

Triplet 1,4-biradicals were generated by Norrish-Type-II hydrogen transfer from alpha-heteroatom-substituted beta-branched butyrophenones 1-6 and detected by laser flash absorption measurements. For three oxy-substituted compounds 2-4 (R(alpha)=OH, OCOMe, OCOOEt) comparable lifetimes were determined in acetonitrile (roughly 1.5 micros). In benzene, divergent trends were observed: for the hydroxy compound 2 a lower lifetime of 790 ns was determined, whereas for 3 and 4 the lifetimes increased to 4.9 micros. Photolyses of the alpha-amino-substituted compounds 1 and 6 resulted in transient species with significant lower lifetimes (for 1 160 ns in benzene and 450 ns in acetonitrile; for 6 <100 ns in both solvents). The mesyloxy substrate 5 undergoes rapid C-O bond cleavage upon photolysis and no transient triplet species were detected. Computational (UB3 LYP/6-31G* and natural don orbital (NBO) analyses) results supported the assumption of a negative hyperconjugative interaction strongly stabilizing alpha-oxy-substituted over alpha-amino-substituted radicals.

Monosilicon‐substituted cyanoacetylene: A computational study

A detailed theoretical investigation of the [H,Si,C(2),N] potential energy surfaces including 28 minimum isomers and 65 interconversion transition states is reported at the Gaussian-3//B3LYP/6-31G(d) level. Generally, the triplet species lie energetically higher than the singlet ones. The former three low-lying isomers are linear HCCNSi 1 (0.00 kcal/mol), branched SiC(H)CN 12 (7.09 kcal/mol), and bent HNCCSi 7 (14.22 kcal/mol), which are separated by rather high barriers from each other and are kinetically very stable with the least conversion barriers of 32.6-70.5 kcal/mol. Two energetically high-lying isomers HCNCSi 3 (42.99 kcal/mol) and SiC(H)NC 13 (36.05 kcal/mol) are also kinetically stable with a barrier of 49.19 and 21.42 kcal/mol, respectively. Additionally, five high-lying isomers, that is, three chainlike isomers, HCCSiN 2 (55.17), HCSiNC 6 (47.80), HSiNCC 11 (78.83), and one three-membered ring isomer HN-cSiCC 19 (51.21), and one four-membered ring isomer cSiCN(H)C 27 (50.6 kcal/mol), are predicted to each have lower conversion barriers of 12-18 kcal/mol and can be considered as meta-stable species. All of the predicted 10 isomers could exist as stable or meta-stable intermediates under suitable conditions. Finally, the structural and bonding analysis indicate that the [H,Si,C(2),N] molecule contains various properties that are of chemical interest (e.g., silylene, SiC triple bonding, and conjugate SiN triple bonding and CC triple bonding, charge-transfer specie, planar aromatic specie, cumulate double bonding). This is the first detailed theoretical study on the potential energy surfaces of the series of hydrogenated Si,C,C,N-containing molecules. The knowledge of the present monohydrogenated SiC(2)N isomerism could provide useful information for more highly hydrogenated or larger Si,C(2),N-containing species.

Diastereoselectivity and Site Dependency in the Photochemistry of Ketoprofen in the Bovine Serum Albumin Matrix†

The photodegradation of the S(+)- and R(-)-ketoprofen (KP) enantiomers in the bovine serum albumin matrix was studied by steady-state photolysis with the use of lambda(irr) > 320 nm and transient absorption spectroscopy with lambda(exc) = 355 nm, at 1/1 and 2/1 KP/BSA molar ratios. R(-)-KP was found to be more labile than S(+). Triplet ketoprofen species were evidenced with lifetimes of 400 ns for S(+) and 600 ns for R(-)-KP. Further longer-lived transients with lifetimes of 2.6 and 6.0 mus for S(+) and R(-), respectively, were detected. On the basis of the binding constants of the drug enantiomers to the two main binding sites of the protein, obtained from circular dichroism experiments, the individual disappearance quantum yields of the 1:1 and 2:1 diastereomeric KP:BSA complexes could be estimated. The photoreactivity in the BSA matrix was rationalized on the basis of diastereoselective photodecarboxylation in the two main protein sites.

Artificial Photosynthetic Reaction Centers: Mimicking Sequential Electron and Triplet‐Energy Transfer

An artificial photosynthetic reaction center consisting of a carotenoid (C), a dimesitylporphyrin (P), and a bis(heptafluoropropyl)porphyrin (P(F)), C-P-P(F) , and the related triad in which the central porphyrin has been metalated to give C-P(Zn)-P(F) have been synthesized and characterized by transient spectroscopy. These triads are models for amphipathic triads having a carboxylate group attached to the P(F) moiety; they are designed to carry out redox processes across lipid bilayers. Triad C-P-P(F) undergoes rapid singlet-singlet energy transfer between the porphyrin moieties, so that their excited states are in equilibrium. In benzonitrile, photoinduced electron transfer from the first excited singlet state of P and hole transfer from the first excited singlet state of P(F) yield the initial charge-separated state C-P(.) (+)-P(F) (.) (-). Subsequent hole transfer to the carotenoid moiety generates the final charge-separated state C(.) (+)-P-P(F) (.) (-), which has a lifetime of 1.1 mus and is formed with a quantum yield of 0.24. In triad C-P(Zn)-P(F) energy transfer from the P(Zn) excited singlet to the P(F) moiety yields C-P(Zn)-(1)P(F) . A series of electron-transfer reactions analogous to those observed in C-P-P(F) generates C(.) (+)-P(Zn)-P(F) (.) (-), which has a lifetime of 750 ns and is formed with a quantum yield of 0.25. Flash photolysis experiments in liposomes containing an amphipathic version of C-P(Zn)-P(F) demonstrate that the added driving force for photoinduced electron transfer in the metalated triad is useful for promoting electron transfer in the low-dielectric environment of artificial biological membranes. In argon-saturated toluene solutions of C-P-P(F) and C-P(Zn)-P(F) , charge separation is not observed and a considerable yield of triplet species is generated upon excitation of the porphyrin moieties. In both triads triplet energy localized in the P(F) moiety is channeled to the carotenoid chromophore by a triplet energy-transfer relay mechanism. Certain photophysical characteristics of these triads, including the sequential electron transfer and the triplet energy-transfer relay mechanism, are reminiscent of those observed in natural reaction centers of photosynthetic bacteria.

Time-Resolved Resonance Raman and Density Functional Theory Study of the Deprotonation Reaction of the Triplet State of p-Hydroxyacetophenone in Water Solution

[reaction; see text] Picosecond and nanosecond time-resolved resonance Raman (TR(3)) spectroscopy was employed to investigate the deprotonation/ionization reaction of p-hydroxyacetophenone (HA) after ultraviolet photolysis in water solution. The TR(3) spectra in conjunction with density functional theory (DFT) calculations were used to characterize the structure and dynamics of the excited-state HA deprotonation to form HA anions in near neutral water solvent. DFT calculations based on a solute-solvent intermolecular H-bonded complex model containing up to three water molecules were used to evaluate the H-bond interactions and their influence on the deprotonation reaction and the structures of the intermediates. The deprotonation reaction was found to occur on the triplet manifold with a planar H-bonded HA triplet complex as the precursor species. The HA triplet species is generated within several picoseconds and then decays with a approximately 10 ns time constant to produce the HA triplet anion species after 267 nm photolysis of HA in water solution. The triplet anion species was observed to decay with a time constant of about 90 ns into the ground-state anion species that was found to have a lifetime of about 200 ns. The DFT calculations on the H-bonded complexes of the anion triplet and ground-states species suggest that these anion species are H-bonded complexes with planar quinonoidal structures containing two water molecules H-bonded, respectively, with oxygen lone pairs of the carbonyl and deprotonated hydroxyl moieties. A deactivation scheme of the photoexcited HA in regard to the deprotonation reaction in neutral water solutions was proposed. With the above dynamic and structural information available, we briefly discuss the possible implications of the model HA photochemistry in water solutions for the photodeprotection reactions of related p-HP phototrigger compounds in aqueous solutions.

Pd-Mediated Activation of Molecular Oxygen in a Nonpolar Medium

The mechanism for direct insertion of O(2) in a toluene-solvated palladium-hydride bond (avoiding palladium zero) has been elucidated using quantum mechanics (B3LYP/LACVP** with the PBF polarizable continuum solvent model) for Pd(II)(-)-sparteine)(Cl)(H) and the model compound Pd(II)(bipyridine)(Cl)(H). We find that the process involves (1) the abstraction of the hydrogen atom by triplet oxygen, (2) the formation of a stable L(2)XPd(I)OOH triplet species, (3) a spin transition resulting in a stable L(2)XPd(II)OOH singlet species, and (4) the loss of H(2)O(2)and completion of the catalytic cycle upon the addition of HX. The limitations involved in the spin transition, the formation of the triplet Pd(I)-OOH species and the stability of that triplet species are all dependent on the presence of an H-bond acceptor cis to the hydride and the electronic characteristics of the other ligands which may or may not stabilize the Pd(I) species. Without this cis H-bond acceptor and/or electron-withdrawing ligands that can stabilize Pd(I), the reaction will not proceed via the palladium hydride insertion mechanism in a nonpolar environment.

Time-resolved spectroscopic studies of the photochemistry of some diphenylgermylene (Ph2Ge:) precursors

The photochemistry of diphenylbis(trimethylsilyl)germane (2a) and 1,4-dihydro-5-methyl-1,2,3,4,9,9-hexaphenyl-1,4-germanonaphthalene (11) has been studied in solution by steady-state and laser flash photolysis methods with a view to detecting the transient germylene derivative diphenylgermylene (Ph2Ge), which has previously been shown to be the major product of photolysis of 2a and a closely related derivative of 11. Steady-state trapping experiments confirm the formation of Ph2Ge as the major germanium containing primary product in both cases; with 2a, the results indicate that other transient species are also formed in minor yields, including phenyl(trimethylsilyl)germylene (Ph(TMS)Ge, ca. 6%) and diphenyl(trimethylsilyl)germyl radicals (Ph2(TMS)Ge, ≥15%). Laser flash photolysis of 2a in deoxy genated hexane solution yields a complex mixture of overlapping transient absorptions, which is shown to be comprised of Ph2Ge, tetraphenyldigermene (15) and its oligomerization products, and another species with spectral characteristics similar to the Ph2(TMS)Ge radical. The latter has been independently generated by hydrogen abstraction from diphenyl(trimethylsilyl)germane by tert-butoxyl radicals. Compound 11 extrudes Ph2Ge more cleanly and efficiently upon photolysis in solution, yet laser flash photolysis affords excited triplet and triplet-derived species as the only detectable transient products; interpretation of the results for this compound is made difficult by its slow thermal decomposition to 5-methyl-1,2,3,4-tetraphenylnaphthalene. It is concluded that in spite of the fact that both 2a and 11 afford Ph2Ge in high yield upon photolysis, they are poor precursors for study of the species in solution by time-resolved UV–vis methods, owing to the formation of other, more strongly absorbing transient products than Ph2Ge, whose lowest energy absorption is characteristically weak.Key words: germylene, germyl radical, flash photolysis, disilylgermane, photochemistry.

DFT investigation of nitrenium ions derived from metabolism of antitumor 2-(4-aminophenyl)benzothiazoles

Ab initio molecular orbital computation has been performed to study the electronic structure, substituent and solvent effects on the singlet–triplet gaps of a series of nitrenium ions. DFT calculations at the B3LYP/6-311++G** level of theory predict that, the nitrenium ions derived from the metabolism of the antitumor 2-(4-aminophenyl)benzothiazoles exist in the ground singlet state. The preferential stabilization of the singlet state relative to the triplet is due to the transfer of charge density from the phenyl ring to formally electron deficient nitrogen atom. Calculations in solution were carried out using the PCM model. Solvent–solute interaction energies were estimated and reflect a considerable net stabilization of the triplet species at the PCM model. Fourier transform analysis of the internal rotation potential energy function, around the exocyclic bond (C 4–N), has been performed for niternium ions studied. The magnitudes and origin of the potential energy barriers have been discussed.

Spectroscopy and Photophysics of Self-Organized Zinc Porphyrin Nanolayers. 3. Fluorescence Detected Magnetic Resonance of Triplet States

Fluorescence detected magnetic resonance (FDMR) has been applied to approximately 25-nm-thick porphyrin films, containing ordered domains of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides. Illuminating the films at 1.4 K with 457.9-nm light from a continuous wave Ar(+) laser produces at least two different, Jahn-Teller-distorted, ZnTOPP triplet species, labeled i and ii. Microwave-induced magnetic resonance of i and ii in the absence or presence of an externally applied magnetic field affects the fluorescence intensity of ZnTOPP, thus allowing FDMR. For triplet species i, formed in films spin-coated from toluene solution, the zero-field splitting (ZFS) parameters were determined as |D| = (316.9 +/- 0.1) x 10(-4) cm(-1) and |E| = (32.0 +/- 0.5) x 10(-4) cm(-1). By exposure of the spin-coated films to chloroform vapor at room temperature, triplet i is converted into species ii, with |D| = (295 +/- 3) x 10(-4) cm(-1) and |E| = (121 +/- 3) x 10(-4) cm(-1). For the excited triplet state of ZnTOPP in a toluene glass, ZFS parameters with values of |D| = (295 +/- 1) x 10(-4) cm(-1) and |E| = (91 +/- 1) x 10(-4) cm(-1) are found. From a combined study of the FDMR- and microwave-induced fluorescence spectra, i and ii are identified as unligated and ligated ZnTOPP triplet species, respectively. From the asymmetrically shaped zero-field FDMR signals of i, we conclude that the local crystal field perturbations of the stacked molecules are anisotropic. The FDMR results of the ZnTOPP films are compared with those for a film of zinc tetraphenylporphyrin (ZnTPP), which lacks the octyl substituents, and therefore is nonordered. Upon illumination, the ZnTPP films contain only a single, ligated, triplet species with ZFS parameters very similar to those of ligated ZnTOPP. At approximately 5 K, the lifetime of triplet i is considerably shortened compared to that of ZnTOPP in a glass at the same temperature.

Modulative magnetic interaction in π-diradicals of directly meso-linked porphyrin dimers

Abstract A series of directly meso-linked Zn(II) porphyrin dimers were prepared, and properties of their bis(π-radical cation)s were investigated. The redox potentials of the dimers bearing various meso-substituents are distributed in wide range although the potential difference (ΔE) between the first and the second oxidations were similar values. Among these dimers, di-tert-butylphenyl derivative and octyloxyphenyl derivative were chemically oxidized by the treatment with NaAuCl4 in chloroform to afford mono(π-radical cation)s and bis(π-radical cation)s. In the mono(π-radical cation), the π-radical resides one of the porphyrin rings in view of the ESR hyperfine structure. The bis(π-radical cation)s of these dimers showed fine-structured ESR spectra due to triplet species, indicating that the orthogonal alignment of π-radicals leads a stable triplet state. The plots of the triplet ESR signal intensity of the bis(π-radical cation)s versus temperature showed an anomalous feature, in which slight change at lower temperatures and drastic change at higher temperatures were observed. The spectral behavior at lower temperatures was elucidated to be derived from the existence of several kinds of frozen dimers with various dihedral angles between two porphyrin rings. An abrupt increase of triplet signal intensity around 130–160 K can be rationalized by molecular motion around the meso–meso linkage. Such phenomena are comparable with spin-crossover. It also proved that the magnetic interaction in the π-diradical is modulated by the electron-donating ability of the meso-aryl substituents.

Photochemical Reaction Mechanism of 2,2′-Pyridil in Alcoholic Media

Abstract Time-resolved transient absorption spectra of photo-irradiated 2,2′-pyridil were measured in PVA films. The broad bands at 552 and ∼455 nm, appearing just after photo-excitation of 2,2′-pyridil, are assigned to the skewed triplet species. The triplet species changes into 1-hydroxy-2-oxo-1,2-di(2-pyridyl)ethyl (pyridoil) radical via the abstraction of a hydrogen atom from the solvent. The radical produced by the photo-irradiation of 2,2′-pyridil is confirmed to be the pyridoil radical from the measurement of the polarized absorption spectrum, with the aid of the stretched polymer film technique and MO calculations. The time-resolved transient absorption spectra of 2,2′-pyridil irradiated by a xenon flash lamp in a 1:1 (v/v) mixed solvent of methanol and ethanol show that the pyridoil radical further abstracts a hydrogen atom from the solvent to afford cis-enediol (cis-1,2-di(2-pyridyl)-1,2-ethenediol) and that cis-enediol is converted into trans-enediol (trans-1,2-di(2-pyridyl)-1,2-ethenediol). 2,2′-Pyridil shows a structured Tn ← T1 absorption spectrum in a 1:1 (v/v) mixed solvent of methylcyclohexane and 2-methylbutane, indicating that 2,2′-pyridil takes a planar structure in the lowest triplet excited state in a nonpolar medium.

Biradicaloid character of phenalenyl-based aromatic compounds with a small HOMO–LUMO gap

Abstract Biradicaloid character of three Kekule aromatic compounds containing two phenalenyl moieties is discussed on the basis of the theoretical and experimental results. DFT calculation of the compounds reveals a small HOMO–LUMO gap with a large spatial overlap between them, leading to a singlet biradical character in a ground state and an excited triplet biradical state with a small ΔES–T. Singlet biradical character for 1 (see Fig. 1 ) is indicated by the X-ray crystallographic analysis, which shows dimeric pairs with substantially short non-bonding contacts of ∼3.1 A. The ESR measurements for 1 and 3 give typical spectra for triplet species and the temperature dependence of the half-field signal indicates the thermal excitation to the triplet states.

A theoretical study of the SiCNH isomers

We have studied the lowest-lying species of the SiCNH system, which is of astrophysical and general chemical interest. We have characterized 12 singlet and triplet species, established an energy ordering and computed the enthalpies of formation using accurate ab initio methods that include basis set and wavefunction extrapolations. The lowest-lying form is NCSiH( 1A′). CNSiH( 1A′) and c-SiNCH( 1A′) are only 2.2 and 3.8 kcal/mol higher in energy, whereas SiNCH( 1A′), SiCNH( 1A′) and c-SiCNH( 1A′) lie at 29.3, 30.3 and 33.1 kcal/mol, respectively. The most stable triplets are NCSiH( 3A″) (26.5 kcal/mol), SiCNH( 3A″) (29.1 kcal/mol) and SiNCH( 3A″) (29.3 kcal/mol); the last two species are very close in energy to the corresponding singlets, SiCNH( 1A′) and SiNCH( 1A′). The electronic structure has been rationalized through the natural bond orbital method. The vertical electronic spectrum has been computed by multi-reference configuration interaction methods. All the singlet species present 1 1A″ as the lowest-lying excited singlet. The corresponding transition from the ground state has a moderate intensity in the case of NCSiH( 1A′) and CNSiH( 1A′). There is, at least, one intense valence transition in each of the singlet forms, although they tend to be of relatively high energy. SiNCH( 1A′) and SiCNH( 1A′) have very small 1 1A″–X 1A′ vertical gaps (15.7 and 9.22 kcal/mol), respectively. Among the triplet species, SiNCH( 3A″) and SiCNH( 3A″) present rather intense valence transitions.

Triplet state interaction of Safranine T with inorganic cations in different solvents

The triplet–triplet absorbance of Safranine T (3,7-Diamino-2,8-dimethyl-5-phenylphenaznium chloride) has been studied in a series of polar and non-polar solvents. When non-aqueous solvents are gradually added to the aqueous solution of Safranine T (ST) the lifetime of the triplet state decreases. It has been established that the dye forms weak complex with the solvent molecules. The quenching of triplet state of ST by Fe 2+, Co 2+ and Mn 2+ has been studied in different solvents. The double protonated long-lived triplet species of ST; DH 2 + 2 3 absorbs at 580 nm. The decay of the triplets follows first order kinetics. The quencher generates reduced semidye from 3ST by electron transfer mechanism. The reduction of 3ST by Mn 2+ and Fe 2+ is thermodynamically allowed but electron transfer from Co 2+ to ST is endoergic. The mechanism of quenching by Co 2+ can be explained as compensation by charge transfer mechanism between 3ST and Co 2+.

Intra- and Intermicellar Triplet−Triplet Annihilation of Pyrenetetrasulfonate in an AOT Reverse Micellar Solution: Relation to the Electric Percolation Transition

Decay behaviors of triplets were investigated as a function of temperature in ternary (water/AOT/oil) and quaternary (water/AOT/cosurfactant/oil) reverse micellar solutions. The triplet species generated by the laser irradiation of tetrasodium pyrenetetrasulfonate dissolved in a reverse micelle were detected by monitoring the triplet-triplet absorption at 430 nm. The observed decay curves were analyzed with the kinetic model combining the intermicellar triplet-triplet annihilation (TTA) with the intramicellar TTA. The kinetic model for the intermicellar TTA process was proposed on the basis of the equilibrium between the monomer and dimer micelles. The obtained kinetic parameters and thermodynamic quantities are discussed in relation to electric percolation phenomena in these micellar systems. A fraction of the dimer micelles was found to be almost constant (0.27 ′ 0.03) at the percolation temperature for all of the reverse micellar systems that were investigated. Furthermore, an average number of triplets per micelle and the Gibbs free energy change for dimer micelle formation seem to be closely related to the percolation threshold. These results suggest that the laser-induced transient absorption spectroscopy of a probe is a useful method for studying dynamic behavior in micellar systems.