Triplet molecules

Hurley and Testa (Ref 17) exposed nitrobenzene in isopropyl alcohol, degassed and in air, to a mercury lamp at 3660A Products in the absence of air were acetone and phenyl-hydroxylamine (PHA). In air PHA was oxidized to nitro sob enzene which couples with PHA to form azoxybenzene. They hypothesized that the triplet molecule abstracted H-atoms from the solvent no effect was noted with ben zene as solvent. They also worked with nitrobenzene in isopropyl alcohol-water mixts containing HC1 with a mercury lamp at 3660A (Ref 18), and found that the quantum yields depended on pH and isopropyl alcohol content, but were independent of oxygen with acid present. Their conclusion was that the quantum yield consisted of two parts, H abstraction by the triplet, and protonation of the triplet... 

All of these reactions are exothermic but some of them violate the law of spin conservation since S2 is a triplet molecule while all species on the right side of the above equations are singlet molecules in the ground state. Therefore, a radical-chain reaction has been assumed. S3 may also be formed from S2 by picking a sulfur atom up from a di- or polysulfide in the a-layer. 

For a triplet state with a lifetime of 10 sec or longer the concentration of triplet molecules ( 10-3 mole) is sufficiently high under continuous illumination that one can obtain a UV spectrum of the excited molecules. 

The subject of delayed fluorescence was discussed in Section 5.2a. It was seen that there are two common types of delayed fluorescence, that arising from thermally activated return from the triplet state to the lowest excited singlet (E-type delayed fluorescence) and that arising from collision of two excited triplet molecules resulting in a singlet excited molecule and a ground state molecule (P-type delayed fluorescence). The P-type delayed fluorescence can be used as a convenient tool for the determination of intersystem crossing efficiencies[Pg.125]

The yields of cyclopropanes in this case are low in relation to the amount of acetophenone formed. However, similar cyclopropane product ratios are obtained when photolysis is carried out in the presence of Michler s ketone as sensitizer. Thus the carbene intermediate produced in the direct irradiation is thought to be a triplet, as suggested by the nonstereospecificity of its addition. Whether this intermediate arose from singlet diazoacetophenone (via singlet decomposition and intersystem crossing of the singlet carbene) or by decomposition of the triplet molecule was not determined. 

If et is not known, it is possible to obtain more accurate values of d>lsc than above by using a lower flash intensity such that all the molecules are not excited during the flash (70 20-100 J). For this method the intensity of the light absorbed Ia must be accurately determined from the absorption spectrum and the incident light intensity 70 determined by actinometry. The concentration of triplet molecules [A ] can be determined from A[A ] as above. Since Ia and [A ] are smaller than in the previous case, errors due to the underlying T0 -> Tx absorption are reduced. The quantum yield of triplet formation is now... 

The fact that dynamic 13C polarization is only possible through the indirect way via tire 1H spins suggests the mechanism of polarization transfer. Since the polarization transfer between the electrons and nuclei are driven by the dipolar interactions between them, and the fraction of the guest triplet molecules was small, it would be natural to assume that the polarization of the electron spins in the photo-excited triplet state is given to those H spins which happen to be close to the electron spins, and then the 1H polarization would be transported away over the whole volume of the sample by spin diffusion among the 1H spins. 

Molecules with two or more unpaired electrons may be divided into two classes by far the most common examples are molecules where the unpaired electrons are contained in a set of degenerate atomic or molecular orbitals with qualitatively similar spatial distributions, e.g., an octahedral Cr(m) (4A2g) or Ni(n) (3A2g) complex, a ground state triplet molecule like 02, or the excited triplet states of naphthalene or benzophenone. 

An external magnetic field was observed to have practically no effect on the intensity of the thianthrene (430 nm) emission, indicating that no triplet states are involved. The shorter-wavelength emission of the oxadiazole 102, however, is probably due to a triplet-triplet annihilation reaction of diphenyloxadiazole triplets. These are produced in the radical-ion reaction between 101 and 102, yielding thianthrene excited-singlet molecules and diphenyl-oxadiazole excited-triplet molecules ... 

Such compounds with conugated ir-bonds are excited by light to the triplet state, and then such a triplet molecule reacts with dioxygen with the formation of peroxide. 

The explanation for these diradical reactions can not be quite so simple as the one hinted at above however, since the quantum yield of rubrene peroxide does not appear to depend only on the concentration of photoactivated rubrene triplet molecules but also on the concentrations of rubrene itself and of oxygen.77... 

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

The correlation of the recorded IR spectrum with semiempirical calculations and the reversible cleavage into oxohexapentaenylidene (97) and carbon monoxide give a consistent picture for both, dione 96 and carbene 97. The spectrum of 97 correlates with scaled ab initio vibrational frequencies.123 C60 97 should be a triplet molecule. Indeed, Weltner et al.124 observed during the experiment concerning C4O T-88 another triplet ESR signal which they ascribed to T-97. 

Triplet-triplet annihilation (TTA), i.e., deactivation of triplet molecules as a result of their interaction, is one of the main pathways of triplet state decay in solutions, crystals and the gas-phase. TTA may become the determining pathway of triplet state deactivation under conditions of high concentrations of triplet-state molecules resulting in particular from powerful laser photoexcitation. 

Notes T) P and P2 have been calculated for one annihilation act of two triplet molecules. 2) For the P2 calculation it was assumed that probabilities of radiative S2 >... 

Here (TiTi) is the encounter complex of two triplet molecules or, in other words, a triplet pair with energy 2Ejj capable of dissociating back into two triplet molecules or which may undergo intracomplex relaxation (spin exchange, vibrational relaxation, electron transfer) creating, for instance, either singlet, or triplet, or quintet dimers (8,9), or ion-radical pairs (18). 

Q probability is linked with probability of Sj excited molecule creation at annihilation of two triplet molecules ( ) by the simple relation ... 

Experimentally the key to the method is determining the relative population of triplet molecules by the optical density of triplet-triplet absorption following flash excitation. To derive the kinetic expressions needed to determine O , the scheme is the same as that written previously with the additional proviso that the singlet interaction lead to intersystem crossing by the sensitizer. 

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