Fluorescence and Intersystem Crossing of Benzenehyphen;h6, hyphen;d6, and hyphen;1,4hyphen;d2

摘要

Fluorescence produced by absorption of the 2537hyphen;Aring; radiation in benzenehyphen;hyphen;h6, benzenehyphen;d6, and benzenehyphen;1,4,hyphen;d2was studied down to 10minus;2torr. At pressures below about 0.4 torr the spectral intensity distribution and quantum yield of the resonance fluorescence of each benzene are independent of pressure. The quantum yields of the three benzenes, determined by comparison of the work of others with benzenehyphen;h6at high pressures, were found to be 0.39, 0.53, and 0.38. At higher pressures the yields are lower and the spectral intensity distribution is different. Benzenehyphen;1,4hyphen;d2at low pressures showed no spectroscopic evidence of isomerization (to hyphen;1,3hyphen;d2) even on very prolonged exposure to 2537hyphen;Aring; radiation. Isomerization ofcishyphen; intotranshyphen;butene by benzenehyphen;h6was studied at lower pressures. In the range of 0.5ndash;0.1 torr a mechanism involving firsthyphen;order rates of formation and destruction of the triplet state of benzene, the latter process competing with the energy transfer to butene molecules, describes the data well. At still lower pressures a direct photochemical isomerization of butene by the 2735hyphen;Aring; radiation, proceeding with quantum yields well above unity, complicates the interpretation of the benzenehyphen;sensitized reaction data. It is concluded that the ldquo;singlethyphen;triplet intersystem crossingrdquo; in benzene is an intramolecular process. A model involving three vibronic levels of the1B2ustate, each of which fluoresces and also suffers intersystem crossing and the two upper levels of which also lose vibrational energy on collisions, describes quantitatively the dependence of the fluorescence yield on pressure for benzenehyphen;h6. The phosphorescence intensity of the triplet state of pyrazine C4H4N2upon excitation into the first singlet state by 3110hyphen;Aring; radiation was studied at pressures from 5 to 0.05 torr. The pressure dependence observed is such that a mechanism involving only firsthyphen;order (i.e., intramolecular) processes could not be devised to describe it. It is suggested that because of a smaller number of vibrational degrees of freedom and lower excitation energy above the ground level of the triplet state, the singletndash;triplet intersystem crossing in pyrazine may to some extent be a collisionhyphen;induced process.