Showing papers on "Triplet state published in 1986"

Triplet-triplet absorption spectra of organic molecules in condensed phases

Abstract: We present a compilation of spectral parameters associated with triplet–triplet absorption of organic molecules in condensed media. The wavelengths of maximum absorbance and the corresponding extinction coefficients, where known, have been critically evaluated. Other data, for example, lifetimes, energies and energy transfer rates, relevant to the triplet states of these molecules are included by way of comments but have not been subjected to a similar scrutiny. Work in the gas phase has been omitted, as have theoretical studies. We provide an introduction to triplet state processes in solution and solids, developing the conceptual background and offering an historical perspective on the detection and measurement of triplet state absorption. Techniques employed to populate the triplet state are reviewed and the various approaches to the estimation of the extinction coefficient of triplet–triplet absorption are critically discussed. A statistical analysis of the available data is presented and recommendations for a hierarchical choice of extinction coefficients are made. Data collection is expected to be complete through the end of 1984. Compound name, molecular formula and author indexes are appended.

Phase diagrams and correlation exponents for quantum spin chains of arbitrary spin quantum number.

Abstract: The low-temperature properties of the spin-S quantum spin chain are studied representing a spin-S operator as the sum of 2S spin-1/2 operators. The resulting system of 2S-coupled spin-1/2 systems is studied in the weak-coupling limit, using a continuum representation. It is shown that under scaling, the coupling becomes strong. Under the additional hypothesis (which is shown to be true for S=1) that strong coupling represents correctly the properties of the spin-S system, the following results are obtained: (i) there are, in general, two types of planar (massless) phases XY1 and XY2, separated by an Ising-like transition; (ii) for half-odd-integer S the exponent \ensuremath{\eta} governing the asymptotic power-law decay of transverse spin correlations takes the universal value \ensuremath{\eta}=1 at the boundary between XY1 and the adjacent uniaxial antiferromagnetic phase; (iii) for integer S there is an additional singlet phase between XY1 and the antiferromagnetic state, with \ensuremath{\eta}=(1/4) at the limit of XY1; (iv) a spin-Peierls instability only exists for half-odd-integer S; (v) a magnetic field along the z direction may lead to a transition from the singlet or antiferromagnetic state to a planar phase. Universal scaling relations between exponents for transverse and longitudinal correlations in the phases XY1 and XY2 and an explicit asymptotic expression for correlation functions are derived. Finally, implications of the present results for some generalizations of the spin chain problem are briefly discussed. The above points (ii) and (iii) confirm predictions by Haldane, which were derived using quite different methods.

Relation between singlet-triplet gaps and bond energies

Abstract: shortened (strengthened). The parenthetical equivalents are justified because of the well-established correlation between bond length and bond strength. Indeed, the lower interaction energy for the H 2 0 proton acceptor case (11) can be rationalized by the shortened Si0 bond. Figure 2 shows deformation electron density maps for the optimized silanol-H20 structures displayed in Figure 1. The peak in the electron density of the S i 0 bond in silanol is shifted toward the oxygen and toward the inside of the SiOH angle, joining the charge density of the OH bond to form a continuum of density around the inside of the SiOH angle. This feature is also present in silanol with adsorbed H20 molecules. Although the bond length changes induced upon adsorption are significant, the corresponding changes in electron density are not as marked. The second positive contour in the electron density region within the SiOH angle of the deformation density maps best shows the change in electron density. The contour joins between OH and S i 0 when the S i 0 bond shortens and separates when the S i 0 bond lengthens. To examine the structure and energetics of two water molecules simultaneously adsorbed on an isolated silanol group, we have optimized the geometry (within C, symmetry) of a H,Si(OH). 2 H 2 0 complex with the waters in the proton donor and acceptor positions, using the 6-31G* basis. Figure 1D summarizes the final optimized geometry for H3Si(OH)-2H20. Whereas for H,Si(O-H).0H2 (11) the plane of the water molecule is perpendicular to the mirror plane of the whole complex, for H,Si(OH).2H20 the proton acceptor H,O is rotated into the mirror plane. Relative to the single water complexes, the water molecules are closer together, and the 0-H distance between the water molecules is consistent with a long hydrogen bond. This water-water interaction enhances the total energy of the whole complex, which explains the larger interaction energies for the second water molecule addition compared with the single adsorption. In exploring the bond length variations upon adsorption of a second water molecule, comparisons can be made with the single water complexes and with silanol without water. For both comparisons (see Figure l), the bond length variations follow the empirical rules stated previously. The net bond length changes for the double water adsorption are approximately the combined changes for the two single water complexes considered individually. The Si0 bond is slightly shortened …

Observation of the triplet excited state of a conjugated-polymer crystal.

Abstract: By studying a single crystal of polydiacetylene-toluene-sulfonate, we have made the first identification of the triplet excited state of a conjugated polymer. The basis for this identification is the magnetic field dependence of the excited-state lifetime, which is measured by time-resolved optical absorption. Additionally, we have detected the Am = ±1 transitions between triplet sublevels by measuring microwave-induced changes in the triplet-state lifetime.

Energy transfer and fine structure axes determination in a hybrid porphyrin dimer oriented in a liquid crystal. Time resolved triplet EPR spectroscopy

Abstract: A study of laser photoexcitation‐EPR diode detection of the transient triplet state of the hybrid covalenty linked porphyrin dimer [ZnP–H2P] in a uniaxial liquid crystal and a glass matrix at 100 and 10 K, is reported. This time resolved technique enables monitoring the transient behavior of the magnetization until thermal relaxation sets in. Selective singlet excitation of the dimer components reveals a singlet energy transfer process [ZnP*−H2P]–––ket→ZnP–H2P*, whose rate at 10 K is calculated to be [1.0±0.2]×109 s−1. Triplet line shape analysis of the subunit monomer triplet and the dimer yields: (a) the location of the fine structure tensor axes in the molecular frame (X, Y along the pyrrole nitrogens and Z perpendicular to the porphyrin plane); consequently, the signs of the parameters D and E are found to be positive for the monomeric free base and the dimer; (b) the ratio of the sublevel population rates; and (c) the spin‐lattice relaxation rate.

Laser‐induced phosphorescence spectroscopy in supersonic jets. The lowest triplet states of glyoxal, methylglyoxal, and biacetyl

Abstract: We report the first structural study of the lowest triplet states of three α‐dicarbonyls (glyoxal, methylglyoxal, and biacetyl) using the technique of laser‐induced phosphorescence (LIP) spectroscopy in supersonic jets. At the level of vibrational resolution, 3Au glyoxal appears to have a geometry very similar to that of the ground state. But the T1←S0 transitions of methylglyoxal and biacetyl each exhibit strong progressions in the torsional vibrations of the methyl groups, showing that these molecules undergo a conformational change on excitation to the lowest triplet state. A Franck–Condon analysis of the methylglyoxal spectrum, with proper consideration for nuclear spin statistics, yields a methyl barrier of V3=115±5 cm−1 in this state. This value has been confirmed by a direct measurement of the tunneling splitting of A and E torsional levels. The hindering potential in the lowest triplet state of methylglyoxal is substantially different from those in the ground (V3=269 cm−1) and first excited single...

Phosphorescence and zero-field optically detected magnetic resonance studies of the lowest excited triplet states of organometallic diimine complexes. 1. Rhodium bipyridine and rhodium phenanthroline complexes [Rh(bpy)3]3+ and [Rh(phen)3]3+

Abstract: The authors have made spectroscopic and optically detected magnetic resonance studies on /sup 3/..pi pi..* (Rh(bpy)/sub 3/)/sup 3 +/ and (Rh(phen)/sub 3/)/sup 3 +/ at liquid helium temperature. They have observed a well-resolved phosphorescence spectrum of (Rh(phen)/sub 3/)(BF/sub 4/)/sub 3/ that resembles that of 1,10-phenanthroline except for changes in some vibrational frequencies. The triplet lifetimes of the Rh complexes were compared with those of the Cd and Ir complexes in the light of the effect of d..pi..* states. Zero-field (zf) ODMR signals of the /sup 3/..pi pi..*Rh complexes were observed, and triplet sublevel properties, such as zf splittings (zfs) and radiative and nonradiative decay rate constants, were obtained. Very interestingly, small zfs (3-5 GHz) were observed, contrary to previous estimates. Rationalization of this observation is given. On the basis of the obtained data they discuss the structures, zf schemes, and radiative and nonradiative mechanisms in the /sup 3/..pi pi..* Rh complexes.

Halocarbenes CHF, CHCl, and CHBr: geometries, singlet-triplet separations, and vibrational frequencies

Abstract: The fluorocarbene, chlorocarbene, and bromocarbene molecules in their lowest singlet and triplet electronic states have been studied via ab initio quantum mechanical methods. Basis sets of triple-delta-plus double polarization quality were used in conjunction with self-consistent-field (SCF), two-configuration (TC) SCF, and configuration interaction (CI) methods. All three molecules have singlet ground states, with the singlet-triplet separations predicted to be 13.2 kcal (CHF), 5.4 kcal (CHCl), and 4.1 kcal (CHBr). Vibrational frequencies, infrared intensities, molecular structures, and dipole moments are predicted for the lowest singlet and triplet electronic states of all three molecules.

Rate-determining step for decay of triplet biradicals: intersystem crossing vs. chain dynamics

Abstract: The temperature and viscosity dependence of the lifetimes of triplet biradicals derived from 2-phenylcycloalkanones have been studied by means of nanosecond transient UV absorption. Arrhenius plots for the temperature range 20 to -90/sup 0/C are strongly curved. In the high-temperature range (ca. -10 to +20/sup 0/C) log A (S/sup -1/) and E/sub a/ (kcal/mol) are ca. 8.0 and 1.0, respectively. In the low-temperature range (-50 to -90/sup 0/C) the values are ca. 10-11 and 3-4, respectively, typical of chain cyclization processes. The biradical lifetimes are strongly affected by solvent viscosity at low temperature but are nearly insensitive to viscosity at room temperature. The data suggest a transition in the rate-determining step for biradical decay from intersystem crossing control at room temperature to chain dynamics control at low temperature. At room temperature and above, the observable biradicals are almost exclusively triplets, but at low temperature one observes the decay of an approximately equilibrium mixture of singlets and triplets.

Excited-state proton-transfer reactions in 2-(2'-hydroxyphenyl)benzoxazole. Role of triplet states.

Abstract: The fluorescence of 2-(2'-hydroxyphenyl)benzoxazole (HBO) is strongly Stokes-shifted due to intramolecular proton transfer in the excited singlet state by which the enol form ('E*) of HBO is converted into the keto form ('K*). At room temperature the decay of 'K* is dominated by a thermally activated radiationless transition. At lower temperatures fluorescence and intersystem crossing are the main deactivation processes. In the triplet state of the keto tautomer proton back-transfer to the enol triplet state, 3E*, takes place. At high triplet-state concentrations the transient decay is mainly governed by triplet-triplet annihilation. This process was used to determine the extinction coefficients of the triplet-triplet absorption. In contrast to other work the singlet ground-state absorption of the keto tautomer, 'K, was not observed.

Memory of spin polarization in triplet-doublet systems

Abstract: The interaction between triplet molecules and nitroxide radicals is studied in solution by the time-resolved ESR technique. Spin polarization induced in the radical reflects that of the triplet molecule which is an encounter partner. The spin-polarized ESR signals observed in nitroxide radicals are interpreted in terms of electron and/or spin exchange mechanisms.

State-selective photochemistry of singlet oxygen precursors: kinetics and wavelength dependence of the photodissociation of anthracene endoperoxides

Abstract: The photochemical behavior of the two isomeric endoperoxides (9,10-PMO2 and 1 ,4-PMO2) of 1,4-dimethyl-9,lO-diphenylanthracene was found to differ in their kinetics and reaction efficiencies. Consistent with the work of Rigaudy et al. and Brauer et al. on a related endoperoxide we find that the generation of IO2 is wavelength dependent, occurring from upper excited singlet states of the endoperoxide, whereas bond cleavage of the 0-0 endoperoxide bond occurs principally from the lowest excited singlet and triplet states. Results of picosecond kinetics and absolute quantum yield measurements are discussed in terms of various concerted and nonconcerted mechanisms for the formation of IO2 and the anthracene fragment.

Photochemical sensitization by azathioprine and its metabolites--I. 6-Mercaptopurine.

Abstract: — Azathioprine is used as an immunosuppressant for renal transplant recipients, but is frequently associated with severe skin cancer as a side effect. 6-Mercaptopurine, the primary metabolite of azathioprine. absorbs strongly in the UVA region and displays substantial photochemical reactivity. The primary photochemical processes in aqueous solution are triplet state formation and photoionization, as shown by flash photolysis. In oxygenated solution, singlet oxygen and superoxide are produced, and ground state 6MP appears to react with these species. Glutathione inhibits this reaction and is itself oxidized. In deoxygenated solution, reactions implicating the thiyl radical and hydrogen atom and electron transfer occur as shown by reaction with histidine and p-nitrosodimethylaniline. Nitro Blue Tetrazolium or acrylamide.

The photo-excited triplet state of CaMoO4

Abstract: An E.P.R. study of the luminescent state in the CaMoO4 crystal at 1·2 K with a 60–80 GHz spectrometer is reported. The spectra can be fitted with a spin hamiltonian for an orbitally nondegenerate triplet state with D = - 94·358 GHz and E = 32·697 GHz. The directions of the principal axes of the ZFS tensor indicate that the MoO2- 4 ion, having a D2d ground state symmetry, is distorted to an approximate C 3v symmetry on excitation. An analysis of the spin-orbit coupling is given that accounts for the decay characteristics of the sublevels.

Photoreactivity of .alpha.-fluorinated phenyl alkyl ketones.

Abstract: The photoreactivities of the mono-, di-, and tri-..cap alpha..-fluorinated acetophenones have been compared to that of acetophenone itself. All four ketones have similar triplet excitation energies; the three fluorinated ketones have reduction potentials 0.5-0.7 eV lower than that of acetophenone. Triplet reactivity toward alkylbenzenes keeps increasing with fluorine substitution, since the rate-determining step becomes charge-transfer complexation as the ketone reduction potential decreases. The primary/tertiary C-H selectivity toward p-cymene increases with the number of fluorines. Triplet reactivity toward cyclopentane also is increased by fluorination but peaks at two fluorines, since the lowest triplet switches from n,..pi..* to ..pi..,..pi..* with two or three fluorines and ..pi..,..pi..* triplets are unreactive in simple hydrogen atom abstraction. In contrast, ..cap alpha..-fluorination of valerophenone does not significantly increase the rate of triplet ..gamma..-hydrogen abstraction. The inductive effect on reactivity apparently is offset by a conformational effect. The ..cap alpha..-fluorinated phenones give predominantly cyclobutanols instead of Norrish type II elimination. ..cap alpha..-Fluoroacetophenone forms predominantly acetophenone and HF when irradiated with 2-propanol, in what appears to be a short chain process involving electron transfer to ketone followed by fluoride ion loss. Finally, the radical coupling products in these reactions are formed in varying yields, depending on solvent andmore » additives.« less

The unimolecular reaction of t‐BuNO on singlet and triplet surfaces: Spectroscopy, real‐time rate measurements, and NO energy distributions

Abstract: The predissociation of jet‐cooled (CH3)3CNO (t‐BuNO) following laser excitation in the A 1A‘←X 1A’ system has been studied in both the energy (frequency) and time domains. Unlike the smaller nitroso compounds whose predissociation has been already examined, unimolecular reaction is the rate determining step for predissociation near threshold. Consequently, it has been possible to separately measure radiationless transition rates and unimolecular reaction rates in real time. Dissociation on both the ground state (S0) and the first triplet state (T1) has been identified. At threshold, dissociation proceeds only on S0, with lifetimes >3.5 μs, but for E°≥650 cm−1, fast (<10 ns) predissociation via T1 becomes progressively the dominant dissociative route. Nascent NO photofragments have been characterized in detail using one‐photon LIF. The rotational and spin‐orbit distributions of NO following dissociation on S0 are statistical, depending only on E°. The NO derived from dissociation on the T1 surface is not...