Using
the density functional theory method, the crystalline packing
effect on the singlet fission (SF) rate of oligorylenes, some of which
are found to exhibit SF in crystal forms, is revealed by evaluating
the effective electronic coupling (|Veff|), the square of which is proportional to the SF rate. The |Veff| values for terrylene and quaterrylene dimer
models are investigated for a variety of slip-stacked forms. It is
found that these values show similar dependences on the intermolecular
packing as a function of lateral and longitudinal displacements of
monomer frameworks, and that they are maximized in several configurations
of one monomer slipped from another along the longitudinal axis. The
present estimation method of the SF rate is also found to qualitatively
explain the experimental SF rate difference between terrylene derivatives
with different packing forms. Furthermore, by analyzing the effect
of electronic couplings on the adiabatic electronic states related
to SF, we predict several favorable molecular packings leading to
a fast SF with a high triplet yield.