Intersystem crossing in pyrene-h(10) and pyrene-d(10)

The phosphorrescence intensity, phosphorescence lifetime, and fluorescence lifetime of pyrene-h(10) and pyrene-d(10) were measured as a function of temperature from 80 degrees K to 300 degrees K. Using this this data we determine the temperature dependence of the rate constant fro intersystem crossing. We find that there are two different routes for intersystem crossing from the lowest excited singlet states S-1 to the lowest triplert state T-0. In addition to a thermally activated transition from S-1 to the first excited triplet state T-1, intersystem crossing occurs directly from S-1 to T-0. Below similar to 100 degrees K only the direct transition T-0-S-1 can compete with the radiative decay of S-1.