au!b2gand b1u!b3goptical transitions have nearly the same
energy as the allowed b1u!b2gand au!b3gtransitions, the elec-
tronic states undergo configuration interaction and split, giving
rise to a pair of low energy and low intensity (Qxand Qy) trans-
itions and another of high energy (Bxand By) transitions. The
pair Bx and Byis often observed a single intense band called
Soret band. Most of the intensity of the transitions is carried by
the B bands because they result from the addition of the transi-
tion dipoles, whereas the weakness of the Q bands comes from
the near cancelation of the transition dipoles. The skeletal
distortions in the free-base due to the presence of the NH bonds
are responsible for lifting the b2g–b3gorbital degeneracy and for
the observed splitting of the Q bands. The lowest energy transi-
tion, b1u!b2g, is polarized along the proton axis and is labeled
FIG. 2. Fundamental porphyrin systems with numbering scheme,
pictorial representation of the HOMOs and LUMOs, and contributions
to the absorption bands of bacteriochlorins.
DESIGN OF PORPHYRIN-BASED PHOTOSENSITIZERS 191