Knowledge Bank

The phosphorescence spectra and lifetimes of naphthalene, quinoline, isoquinoline, quinoxaline, 1,8-naphthyridine, and a number of their methylated derivatives have been measured in glassy solution at $77\circ K$. The observed phosphorescence lifetimes of the methyl naphthalenes were generally found to decrease slightly or to remain essentially unchanged relative to naphthalene. With the exception of isoquinoline, the phosphorescence lifetimes of the methyl azanaphthalenes are increased by as much as a factor of two over those of the parent molecule. The increases upon the addition of one or two methyl groups to quinoxaline are significantly greater than that which occurs upon complete deuteration. An analysis of the effect of position and number of methyl groups in each series of compounds on phosphorescence lifetime, energy of $T1\to S0$ transition, and changes in vibronic intensity of the phosphorescence spectrum, has been made. This analysis suggests that, unlike deuterium substitution which reduces the radiationless transition rate, methylation introduces several conflicting factors into both radiative and radiationless transition probabilities. An assessment of the importance of these factors is made, and it is concluded that methyl substitution is a far more complex perturbation than was previously thought.