Triplet state formation

Consequently, if the reaction enthalpy is unknown for a given process, the quantum yield must be determined from other measurements. Conversely, if the reaction enthalpy is known, then the quantum yield for the photochemical reaction can be measured. PAC has been used to obtain quantum yields for excited state processes, such as fluorescence, triplet state formation, and ion pair formation and separation. In systems in which competitive reactions occur, care must be taken to accurately account for the partitioning. For example, if a reactive intermediate yields two products, then the measured heat of reaction is the sum of the two individual heats of reaction multiplied by their respective yields. Consequently, there are three unknowns, the partitioning and the individual heats of reaction. Two of them must be known to properly evaluate the third. 

It has been suggested that this increase in yield is due to the heteroatom which reduces the energy difference between the singlet and triplet states of the common, biphenyl nucleus. - - The lifetime and quantum yields of triplet state formation have been measured at 25° and interpreted in the context of spin-orbital coupling. -... 

Compounds 1,2,3,5,10,11,12,13,14 were dissolved in EPIP (diethyl ether, petroleum ether, isopropanol 5 5 2)whereas compounds 4,6,7,8,9,15 were dissolved in THF-DE (tetrahydrofurane, diethyl ether 1 1). These solvent mixtures can be frozen as glassy samples at 77 K. The absorption spectra were recorded on a standard spectrophotometer SF-10 or Beckman-5270. The measurements of fluorescence excitation and emission spectra were made with the aid of a spectrofluorometer SLM-4800 with automatic correction of spectral response. Fluorescence lifetimes were measured with the aid of a pulse fluorometer PRA-3000. Magnetic circular dichroism (MCD) measurements were carried out in a 8 kG magnetic field using a JASCO J-20 circular dichrometer. Triplet state formation was observed for investigated compounds at the experimental set up, whose detailed description can be found in our paper (27). The optical experiments were carried out with a porphyrin concentration of 4.10- - 4.10 mol.l". In NMR investigations (Bruker WM-360) we used higher concentrations ( 5.10" raol.l ) and dried solvents (CDCl, C 2 and toluene-d0). 

Daniels352 has shown that the excitation spectra for triplet state formation and fluorescence emission from uracil and thymine in neutral aqueous solution at room temperature were anomalous when compared with the UV absorption spectra of these pyrimidines. These experimental facts have been critically examined with respect to three molecular models, of which the model based on tautomerism of uracil in aqueous solution is, in the opinion of Daniels, the best. The data suggested that the fluorescing tautomer contained an enol group, and the UV data favored the 4-hydroxy structure, i.e., form 28. The second tautomer, from which the triplet originated, was expected to be the predominant diketo form (32). 

The apparent anomaly in Porter s results probably reflected the fact that he was oxidizing triplet CF3COCF3 rather than CF3 radicals. That Porter failed to find CO is consistent with this explanation. To check this hypothesis Heicklen71 photolyzed a mixture of 86 torr of CF3COCF3 and 591 torr of 02 with radiation mainly at 3130 A. These conditions favor triplet-state formation.30 As products Heicklen found CFaO with a quantum yield of 0.030, CF3CFO with a quantum yield of about 9 x 10 ... 

Observation of a light emission points to the fact that the energetic criterion necessary for the triplet state formation is evidently fulfilled... 

TT,TT )min is ultimately reached. The initial steps of the photochemical mechanism after absorption of a photon involve the decay of its lowest singlet excited state S in times shorter than the experimental time resolution to an intermediate related to Tn. The population transfer occurs before the S -minimum is encountered, which is consistent to the fact that the fluorescence quantum yield of the flavins compounds is lower than the intersystem crossing quantum yield and triplet state formation [85], Polar solvents seem to increase ISC and decrease fluorescence quantum yields in... 

It is clear that a number of questions need to be answered. Why, in the condensed phase, is the intersystem crossing between two nn states so efficient What is the explanation of the conflict between the linewidth studies of Dym and Hochstrasser and the lifetime studies of Rentzepis and Busch, with respect to the vibrationally excited levels It was in an attempt to provide some answers to these questions that Hochstrasser, Lutz and Scott 43 carried out picosecond experiments on the dynamics of triplet state formation. In benzene solution the build up of the triplet state had a lifetime of 30 5 psec, but this could only be considered as a lower limit of the intersystem crossing rate since vibrational relaxation also contributed to the radiationless transition to the triplet state. The rate of triplet state build-up was found to be solvent-dependent. 

Also, the quantum yield of triplet-state formation (<()isc) is depressed according to Eq. (8). 

Transient absorption spectroscopy of pyridine vapour 305 following excitation to S,. Triplet state formation and quenching... 

Such chemical evidence as exists, plus a strong wavelength dependence of fluorescence yield from benzene and the low yield of triplet state formation at 2400 A. all point to a competing process whose importance at 2537 A. cannot for the moment be estimated. One must state, therefore, the evidence for the effect of colhsions on the rate of crossover of benzene from excited singlet to excited triplet states is conflicting. In solid matrix, vibrations definitely play an important role in intersystem crossover for benzene. ... 

Damoiseau X, Tfibel F, Hoebeke M, Fontaine-Aupart MP. Effect of aggregation on bacteriochlorin a triplet-state formation a laser flash photolysis study. Photochem Photobiol 2002 76(5) 480-5. 

With polymethylmethacrylate, absorption is observed between 350 and 500 nm. The emitted fluorescence is weak and decays exponentially with a half-life of 40—50 ns. In the presence of biphenyl, T — T2 absorption of Ci2H10 at 370 nm is observed the anion of biphenyl absorbs at 410 and 590 nm. A third group of bands at 370—390 nm is assigned to a reaction product of diphenyl and polymethylmethacrylate. As with polystyrene, no correlation could be found between the decay of the excited singlet molecule and the intensity of the Tj—T2 absorption. Triplet state formation was thus also assigned to triplet energy transfer from the polymer host. 

An apparatus for measuring the time dependence in the xs range of lightscattering intensity has been used to investigate the degradation of poly(phenyl vinyl ketone) (PVK) in solution.205-250 The PVK was irradiated with 25 ns pulses at 347.1 nm, and butyrophenone was also investigated in the same way. In both cases a transient absorption, identified by sensitization measurements as the PVK and butyrophenone triplet state, was shown to have a first-order decay rate-constant of 1.0 0.2 x 107 s-1. The quantum yield of triplet state formation in PVK was estimated to be between 0.1 and 0.3, whereas the quantum yield of main-chain scission was found to be 0.4—0.6, insensitive to the presence of Oa. 

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