Dk. Hahn et Pr. Callis, LOWEST TRIPLET-STATE OF INDOLE - AN AB-INITIO STUDY, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(14), 1997, pp. 2686-2691
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 perma
nent 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 m
edia, whereas the fluorescence from (1)L(a) is broad and nearly featur
eless, 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 interf
erence terms involving minor configurations that have considerable lev
erage on the permanent dipole because of large transition dipoles betw
een the minor and major configurations. Fluorescence and phosphorescen
ce vibronic band shapes were calculated using these wave functions and
are in good qualitative agreement with experimental results.