Recently, ultralong phosphorescence lifetime has been observed in 4,6-diethoxy-2-carbazolyl-1,3,5-triazine, and H-aggregation induced stabilization of the T-1, state was suggested as its source. The response theory calculations demonstrate that the Davydov stabilization of the T-1, state of the dimer is marginal with respect to the monomer and the corresponding transition moments are virtually the same. Moreover, the calculated radiative rate constant is far from the experimental value, indicating that the ultralong lifetime is not likely to be of electronic origin only. Our calculations reveal that the dual peak emission from the T-1, state is due to strong vibronic coupling between the T-1, and S-0, states along selected normal modes. Interestingly, the calculated vibronic radiative rate constant of the dieter(2.33 X 10(-3) s(-1)) is comparable to the experimental value(4.7 X 10(-3) s(-1))supporting the notion that vibronic contributions to the transition moment are responsible for the ultralong lifetime observed in the bulk system.