  Studying optical properties of organic materials is important due to the
rapid development of organic light-emitting diodes, solar cells, and photon
detectors. Here for the first time we have performed tight-binding calculations
for singlet excitons, in combination with first-principles calculations of the
excited states in molecular dimers, to describe the optical properties of a
zinc-phthalocyanine one-dimensional molecular chain. We have included the
intra-molecule and charge-transfer excitations and the coupling between them.
Our calculations have successfully interpreted a body of experimental UV-Vis
optical spectra of transition-metal phthalocyanines. Compared with the previous
ab initio calculations for a molecular dimer, the optical absorptions at the
split peaks of the Q-bands can be comparable, which indicates the importance of
the coupling between the intra-molecular and charge-transfer excitons.
