Versatile coupling theories have been developed for rationalizing
unusual aggregation phenomena of multipolar chromophores. Here, diverse
excitonic couplings of a quadrupolar squaraine dye protonated by trifluoroacetic
acid could be achieved and tuned unprecedentedly in different solvation
media. Subtle changes of the solvent and ion pair influenced the aggregation
of the donor–acceptor–donor (D–A–D)-type
SQC6 and led to significant variations in optical properties. In contrast
to conventional H/J aggregates, strong spectroscopic evidence of nonfluorescent
and red-shifted hJ aggregation was obtained. Assumptions of the excitonic
interplay with variable strength stabilized by the synergic contributions
of π–π stacking and electronic interaction were
addressed. Comparative excited-state dynamics in the aggregates clarified
the distinctive excitonic coupling of adjacent quadrupolar molecules
and the nature of the excited state beyond the dimers. Meanwhile,
dominant two-photon absorption transitions could be elucidated by
a resonance-enhanced mechanism. The present unusual molecular interplay
provides a strategy to fine tune the optical properties of multipolar
aggregates.
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