Fluorobenzene triplet state

When irradiated at 2420 A or above, the sum of the fluorescent and triplet-state yields of fluorobenzene is unity or slightly greater than unity. Use of the correct branching ratio is impossible because it has not been determined for fluorobenzene.30,31 This has been substantiated recently, using the Cundall technique, to measure triplet-state yields rather than the biacetyl sensitization technique. The... 

The Cundall (see p. 64) method shows triplet molecules to be formed when fluorobenzene is irradiated at wavelengths from 2537 to 2420 A. Biacetyl also shows triplet-state fluorobenzene molecules to be formed at all wavelengths from 2640 to 2420 A. There is, however, a sharp decrease at 2350 A. Thus, fluorobenzene contrasts with benzene which shows no apparent triplet formation at 2420 and at 2350 A. 

Plots of Qs vs. pressure of biacetyl pass through maxima both with benzene and with fluorobenzene. With benzene, this maximum occurs at about 0.1 mm. pressure of biacetyl. Moreover, the ratio QsVQs , i-e., the ratio of sensitized emission efficiency from the triplet state to that of the singlet state, is greater than 170. These facts indicate that emis-... 

Another fact must be considered for fluorobenzene, viz., if the Almy and Gillette value for the emission yield from the triplet state of biacetyl is 0.15, there is some discrepancy in the energy balance. However, recent work seems to have removed this difficulty. ... 

Thus it can be concluded that fluorobenzene gives triplet-state molecules to the extent that it does not fluoresce but there is a relatively large uncertainty in this statement. The fluorescence yield decreases and the crossover yield apparently increases as the frequency of the incident light increases. 

Since QsVQs is much larger than it is in pure biacetyl, one can infer that the main excitation is from triplet fluorobenzene to give B . Some energy transfer from the singlet state may lead to emission by biacetyl but the amount must be groall. One more piece of evidence that fluoro-... 

Pulsed-radiolysis of gas-phase systems containing C02 or N20 results in the production of oxygen atoms, which are in the triplet ground state. In the presence of small amounts of benzene, for example, an optical absorption is found to be formed after the pulse, with a maximum at 275 m and a shoulder in the region of 300 m/. Similar absorption spectra are found for other compounds. Rate constants for the process 0(3P) -f- X -> products are, where the compound X is given in parentheses and the rate constant is in units of 108 liter mole 1 sec. 1, 0.36 (benzene), 1.4 (toluene), 3.2 (ethylbenzene), 6.7 (o-xylene), 7.7 (m-xylene), 4.5 (p-xylene), 3.1 (chlorobenzene), 0.27 (fluorobenzene), and 1.0 (pyridine). The estimated limits of error are about 25%. 

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