Fluorescence decay of aromatic vapours. IV. Lifetime measurements on three further azabenzenes

Abstract

Lifetime studies of the three azabenzenes s-triazine, pyridazine and pyrazine are reported. They show quite different behaviour. Lifetimes of four single vibronic levels of s-triazine were measured and show that an additional decay channel is operating. The fluorescence from pyridazine is so fast that it was indistinguishable from the exciting lamp pulse. Lifetimes of six single vibronic levels of pyrazine were measured and were shown to be pressure-dependent. These results fit quite well into the current understanding of the behaviour of the azabenzenes.

Magnetic Field Effects on Fluorescence in Isolated Molecules with the Intermediate Level Structure of Singlet−Triplet Mixed States

“…Rovibrational State Dependence of Fluorescence of Azaaromatic Molecules at Zero Field. Pyrazine, pyrimidine, and s -triazine vapors show the intermediate case behavior in fluorescence properties as a result of strong spin−orbit interaction between singlet and triplet states. − The significance of the S−T interaction in these azaaromatic molecules is known from the presence of phosphorescence following photoexcitation into a singlet state. − The presence of a nonzero magnetic dipole moment at the photoexcited singlet state in pyrazine or pyrimidine also shows the presence of effective S−T interaction. ,, Fluorescence of these azaaromatic molecules in the vapor phase is affected by H , depending on the rovibronic level excited. − All the results of the magnetic field effects on fluorescence are strongly related to the fluorescence property at zero field, and so the fluorescence property at zero field is briefly described in this section with particular regard to rotational and vibrational state dependence of the yield and lifetime for each of fast and slowly decaying portions of fluorescence. τ f at around the S 1 origin was evaluated to be ∼100 ps, ∼3 ns, and ∼300 ps in pyrazine, − pyrimidine, , and s -triazine, , respectively, and τ f varies with the vibronic level excited.…”

“…Special attention has been paid to azaaromatic molecules which exhibit the intermediate case behavior of fluorescence characterized by a biexponential decay composed of fast and slowly decaying portions. Those decays are interpreted in terms of strong interaction between a zero-order singlet state carrying the absorption intensity and a number of isoenergetic zero-order triplet states. − Molecular rotation as well as molecular vibration plays a significant role in intramolecular ISC of these molecules, as confirmed by the excited level dependence of fluorescence property. − Fluorescence of these molecules is efficiently affected by H , though the fluorescence is considered to be emitted from the diamagnetic singlet state, and a number of interesting problems concerning the magnetic field effect on level structure of the S−T mixed states and its relation to ISC have been elucidated on the basis of the measurements of the field dependence of intensity and decay of fluorescence following excitation into the individual rovibronic levels of S 1 . − Especially, magnetic quenching of fluorescence was confirmed to depend on the following: (1) molecular rotation; (2) molecular vibration; (3) methyl internal rotation; (4) deuterium substitution. These dependences are systematically interpreted in terms of the level density of the triplet state coupled to a singlet state.…”

Effects of Molecular Vibration andRotation on Intramolecular Dynamics in S1 of s‐Triazine Vapor

“…The component II of the broad fluorescence for these excitations is reasonably attributed to the reverse ISC from Ta to U, whereas the componerit III may be attributed to the reverse ISC to U from the triplet levels other than Ta, i.e., Tb. Thus, the fluorescence properties at low pressures on these excitations can be classified as the intermediate between Case (21) and Case (3). In fact, S levels reached by these excitations have moderate AE.…”

“…ks kr + k R + ksv kr= k + k R+ krv kv k + k R + kvr (21) lterpretation of the Emission Properties According to Eq. (7), the fast component of the IPL fluorescent exhibits an exponential decay due to exp (-kst).…”