Spin Dynamics of Quintet and Triplet States Resulting from Singlet Fission in Oriented Terrylenediimide and Quaterrylenediimide Films

Singlet fission in
organic semiconductors provides an important
opportunity to study high-spin states in electronically coupled chromophores.
Photoexcitation of oriented, crystalline films of N,N-bis­(pentadec-8-yl)­terrylene-3,4:11,12-bis­(dicarboximide)
(TDI) and N,N-bis­(pentadec-8-yl)­quaterrylene-3,4:13,14-bis­(dicarboximide)
(QDI) result in the formation of the correlated triplet
pair quintet state, 5(T1T1) and its
subsequent dissociation into two triplet (T1) excitons.
Time-resolved electron paramagnetic resonance (TREPR) spectroscopy
and X-ray crystallography are used to show that the orientation dependence
of 5(T1T1) spin dynamics is retained
as it dissociates into two T1 excitons, while such information
is lost in a randomly oriented sample. In addition, the spin dynamics
depend on how the molecules are oriented relative to the external
applied magnetic field. Dissociation of 5(T1T1) to form two T1 excitons is more efficient
in TDI and leads to a longer T1 lifetime than
in QDI, making TDI a more viable candidate
for photovoltaic applications. Since QDI readily forms
a highly oriented film, and the lifetime of its 5(T1T1) state is longer than that of TDI, it may be a good candidate for quantum information science applications
that require the generation of a quantum-entangled, four-spin state.