Results and dicussion (continued)
The values of the fluorescence lifetimes are quite large and almost constant (42.0-43.4 ns) over the vibrational progressions examined. Based on arguments used for anthracene derivatives, which have a 14π-electron-system as DPBBTD does, the nonradiative relaxations from the S1 state of DPBBTD are supposed to be unavailable as for 9,10-dichloroanthracene (DCA) or 9,10-dicyanoanthracene (DCNA). That is, the fluorescence quantum yield of jet-cooled DPBBTD is expected to be unity. This prediction can be verified by a check as to whether there is a heavy-atom effect on the fluorescence lifetime, i.e., shortening in the fluorescence lifetime due to the acceleration of the intersystem crossing (ISC) rate, for the DPBBTD-Xe vdW complexes.
The fluorescence decay curve measured for the vdW complexes is single-exponential and its decay rate is definitely slower than the fluorescence decay rate of the bare molecule. The decay curve for the vdW complexes does not have a significant component of the fluorescence decay indicating the lifetime of 42.0-43.4 ns. Moreover, there is no component of the decay curve indicating the heavy-atom effect, which is the acceleration of the fluorescence decay due to ISC, at all. It seems that ISC does not occur, even if the spin-orbit coupling is enhanced by the heavy-atom effect. Therefore the fluorescence quantum yields should be unity.
