The lineshape function for the So + TI absorption in 1,4dlbrornonJpilthalcnc (DBN) is analyzed in term of exchange theory. It is shown that the dominant optical &phasing nwchanism for the ciectric dipole transition to the k = 0 state in the band results from the absorption and emission of a low energy optic phonon. 'I his procc% dcphases the optical ab$orption because of frequency differences of the phonon in the ground and excited state. In addition, it 1s shown how to extract the energy of the phonon rcsponslble for dcphasing, the phonon absorption rate, and the lifetunc in the phonon promoted state from the data. The analysis of the data for DDN shows that very little dephaeing of the optical Lrdnsition occurs beforc = 15 K but from 15 K to = 40 K the singlet--triplet transitions to site 1 (20192 cm-') and site II (20245 cm-') arc dephased by absorption and emission of an = 38 cm-' and 45 cm-' phonon respectively. The phonon absorption rates by the k = 0 state in the exciton band arc similar for both sites being 5 X IO6 S-' and 3 X 10' S--I at 4 K and 7 X IO" S-l and 4 X 10" s-l at 30 K for site I and II respectively. Pinaily, the lifetimes in the phonon promoted state for sites 1 and II arc 0.23 and 0.28 ps over the range 15-40 K.