  Diketopyrrolopyrroles are a popular class of electron-withdrawing unit in
optoelectronic materials. When combined with electron donating side-chain
functional groups such as thiophenes, they form a very broad class of
donor-acceptor molecules: thiophene-diketopyrrolopyrroles (TDPPs). Despite
their widescale use in biosensors and photovoltaic materials, studies have yet
to establish the important link between the electronic structure of the
specific TDPP and the critical optical properties. To bridge this gap,
ultrafast transient absorption with 22 fs time resolution has been used to
explore the photophysics of three prototypical TDPP molecules: a monomer, dimer
and polymer in solution. These studies show that the photophysics of these
molecular prototypes under visible photoexcitation are determined by just two
excited electronic states, having very different electronic characters (one is
optically bright, the other dark), their relative energetic ordering and the
timescales for internal conversion from one to the other and/or to the ground
state. The underlying difference in electronic structure alters the branching
between these excited states and their associated dynamics. In turn, these
factors dictate the fluorescence quantum yields, which are shown to vary by
~1-2 orders of magnitude across the TDPP prototypes investigated here. The fast
non-radiative transfer of molecules from the bright to dark states is mediated
by conical intersections. Remarkably, wavepacket signals in the measured
transient absorption data carry signatures of the nuclear motions that enable
mixing of the electronic-nuclear wavefunction and facilitate non-adiabatic
coupling between the bright and dark states.
