Fluorescence and Intersystem Crossing of Benzene-h6, -d6, and -1,4-d2

Abstract

Fluorescence produced by absorption of the 2537‐Å radiation in benzene‐‐h₆, benzene‐d₆, and benzene‐1,4,‐d₂ was studied down to 10⁻² torr. 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 benzene‐h₆ at 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. Benzene‐1,4‐d₂ at low pressures showed no spectroscopic evidence of isomerization (to ‐1,3‐d₂) even on very prolonged exposure to 2537‐Å radiation. Isomerization of cis‐ into trans‐butene by benzene‐h₆ was studied at lower pressures. In the range of 0.5–0.1 torr a mechanism involving first‐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 2735‐Å radiation, proceeding with quantum yields well above unity, complicates the interpretation of the benzene‐sensitized reaction data. It is concluded that the “singlet‐triplet intersystem crossing” in benzene is an intramolecular process. A model involving three vibronic levels of the ${}^1{B}_{\text{2u}}$ state, 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 benzene‐h₆. The phosphorescence intensity of the triplet state of pyrazine C₄H₄N₂ upon excitation into the first singlet state by 3110‐Å radiation was studied at pressures from 5 to 0.05 torr. The pressure dependence observed is such that a mechanism involving only first‐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 singlet–triplet intersystem crossing in pyrazine may to some extent be a collision‐induced process.