Lowest triplet state of indole: An ab initio study

Ab initio electronic structure calculations at the CIS/3-21G and CIS/6-31G(p,d) levels on the (3)L(a) State of indole confirm that its permanent dipole is much smaller than that of its singlet counterpart, (1)L(a) - despite having nearly the same configuration content - and they predict that it is even smaller than that of the ground state because the electron density shift is opposite that for the (1)L(a) transition. This not only explains why the phosphorescence of indole derivatives, including tryptophan, has well-defined vibronic structure in polar media, whereas the fluorescence from (1)L(a) is broad and nearly featureless, but also suggests that the tryptophan phosphorescence spectrum maximum for proteins at low temperature may be anticorrelated with the fluorescence maximum. Reasons for the difference are traced to interference terms involving minor configurations that have considerable leverage on the permanent dipole because of large transition dipoles between the minor and major configurations. Fluorescence and phosphorescence vibronic band shapes were calculated using these wave functions and are in good qualitative agreement with experimental results.