  Important questions concern the existence of excitonic strings in organic
compounds and their signatures in the photophysics of these systems. A model in
terms of Hard Core Bosons is proposed to study this problem in one dimension.
Mainly the cases with two and three particles are studied for finite and
infinite lattices, where analytical results are accessible. It is shown that if
bi-excitonic states exist, three-excitonic and even, n-excitonic strings, at
least in a certain range of parameters, will exist. Moreover, the behaviour of
the transitions from one exciton to the biexciton is fully clarified. The
results are in agreement with exact finite cluster diagonalizations of several
model Hamiltonians.
