2 research outputs found
Discrete π‑Stacks of Perylene Bisimide Dyes within Folda-Dimers: Insight into Long- and Short-Range Exciton Coupling
Four well-defined
π-stacks of perylene bisimide (PBI) dyes
were obtained in solution by covalent linkage of two chromophores
with spacer units of different length and sterical demand. Structural
elucidation of the folda-dimers by in-depth nuclear magnetic resonance
studies and geometry optimization at the level of density functional
theory suggest different, but highly defined molecular arrangements
of the two chromophores in the folded state enforced by the various
spacer moieties. Remarkably, the dye stacks exhibit considerably different
optical properties as investigated by UV/vis absorption and fluorescence
spectroscopy, despite only slightly different chromophore arrangements.
The distinct absorption properties can be rationalized by an interplay
of long- and short-range exciton coupling resulting in optical signatures
ranging from conventional H-type to monomer like absorption features
with low and appreciably high fluorescence quantum yields, respectively.
To the best of our knowledge, we present the first experimental proof
of a PBI-based “null-aggregate”, in which long- and
short-range exciton coupling fully compensate each other, giving rise
to monomer-like absorption features for a stack of two PBI chromophores.
Hence, our insights pinpoint the importance of charge-transfer mediated
short-range coupling that can significantly influence the optical
properties of PBI π-stacks
Exciton Coupling of Merocyanine Dyes from H- to J‑type in the Solid State by Crystal Engineering
A key
issue for the application of π-conjugated organic molecules
as thin film solid-state materials is the packing structure, which
drastically affects optical and electronic properties due to intermolecular
coupling. In this regard, merocyanine dyes usually pack in H-coupled
antiparallel arrangements while structures with more interesting J-type
coupling have been rarely reported. Here we show that for three highly
dipolar merocyanine dyes, which exhibit the same π-scaffold
and accordingly equal properties as monomers in solution, the solid-state
packing can be changed by a simple variation of aliphatic substituents
to afford narrow and intense absorption bands with huge hypsochromic
(H) or bathochromic (J) shifts for their thin films and nanocrystals.
Time-dependent density functional theory calculations show that the
energetic offset of almost 1 eV magnitude results from distinct packing
motifs within the crystal structures that comply with the archetype
H- or J-aggregate structures as described by Kasha’s exciton
theory