Two
stacking manners, that is, π- and lamellar stacking,
are generally found for organic semiconductors, in which the π-stacking
occurs between conjugated groups and the lamellar stacking refers
to the separation of the conjugated and aliphatic moieties. The stacking
principles are yet not well-defined. In this work, extended transition
state–natural orbitals for chemical valence (ETS–NOCV),
an energy decomposition analysis, is utilized to examine the π-
and lamellar stacking for a series of naphthalenetetracarboxylic diimide
(R-NDI) crystals. The crucial role of dispersion is validated. The
perception that π-stacking is merely determined by the conjugated
moiety is challenged. The stacking principles are associated with
the closest packing model. Nanoscopic phase separation of conjugated
and aliphatic moieties and the formation of lamellar and herringbone
motifs in the R-NDIs can thus be clarified. Moreover, the interactions
between NDI and the alkyl chain are investigated, revealing that the
interactions can be significant, being contradictory to the conventional
point of view. Along with R-NDIs, additional organic crystals consisting
of various conjugated functionalities and substituents are also investigated
by ETS–NOCV. The sampling scope is up to 108 conjugated molecules.
The dominant role of dispersion force irrespective of the variation
in the conjugated moieties and substituents is further confirmed.
It is envisaged that the established principles are applicable to
other organic semiconductors. The perspective toward the π-
and lamellar stacking might be modified, paving the way for ultimate
morphological control.