Influence of Chain Length and Derivatization on the Lowest Singlet and Triplet States and Intersystem Crossing in Oligothiophenes

On the basis of configuration interaction calculations, we first
describe the nature of the lowest singlet
and triplet excited states in oligothiophenes ranging in size from two
to six rings. We calculate the vertical excitation
energies from the singlet ground state S0 to the first
one-photon allowed singlet excited state S1 as well as the
energy
difference between the ground state and the lowest triplet state
T1. The computed transition energies are in
very
good agreement with the measured values and indicate a strong
confinement of the lowest triplet. We also uncover
the nature of the higher-lying triplet excited state
Tn that is coupled via a large oscillator
strength to T1. The evolution
with chain length of the T1−Tn
excitation energies compares well with the experimental evolution based
on
photoinduced absorption data. Next, we investigate the geometry
relaxation phenomena occurring in the S1 and
T1
states; more pronounced and localized bond-length deformations are
calculated in the triplet state than in the singlet,
confirming the more localized character of T1. We also
analyze the influence on the lowest excited states of
grafting
electroactive end-groups on the conjugated path of terthiophene.
Finally, the various mechanisms involved in the
nonradiative decay of the singlet excitations are discussed, and
results are presented as a guide toward the optimization
of light emission efficiency in conjugated systems.