  Charge transfer states in organic semiconductors play crucial roles in
processes such as singlet fission and exciton dissociation at donor/acceptor
interfaces. Recently, a time-resolved spectroscopy study of
dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene (DNTT) thin films provided
evidence for the formation of mixed Frenkel and charge-transfer excitons after
the photoexcitation. Here we investigate optical properties and excitation
dynamics of the DNTT thin films by combining ab initio calculations and a
stochastic Schrodinger equation. Our theory predicts that the low-energy
Frenkel exciton band consists of 8 to 47% CT character. The quantum dynamics
simulations show coherent dynamics of Frenkel and CT states in 50 fs after the
optical excitation. We demonstrate the role of charge delocalization and
localization in the mixing of CT states with Frenkel excitons as well as the
role of their decoherence.
