Description of excitation energy transfer (EET) in large
multichromophoric
systems of nanoscale size is often inconsistent with the classical
Förster model. In this work, abnormal enhancement of emission
in a heterogeneous mixture of J-aggregates of donor and acceptor thiamonomethinecyanine
dyes is reported, which surpasses the sum of emissions of the neat
donor and acceptor J-aggregate solutions. Despite the tight overlap
of the photoluminescence emission band of the donor J-aggregate and
the absorption band of the acceptor J-aggregate, a direct dipole–dipole
resonant energy transfer was found to have only a marginal contribution
to the overall enhancement, with efficiency less than 10%, whereas
the major contribution came from the non-Förster mechanism,
which was interpreted as exciplex formation in the heterogeneous system.
The enhanced emission was preceded by the exciton migration within
the J-aggregate to specific sites, where a competition between formation
of exciplex and excimer took place due to interaction with the foreign
or own dimers in the ground state, respectively, due to which the
excitation lifetime exceeded that of a free exciton in the neat J-aggregate.