Supramolecular Structure of TTBC J‑Aggregates
in Solution and on Surface
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Abstract
The aggregation behavior of cationic
5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidacarbocyanine
with chloride (TTBC-Cl) or iodide counterions (TTBC-I) in aqueous
solution is investigated by absorption, linear dichroism, and fluorescence
spectroscopies, as well as cryogenic transmission electron microscopy
(cryo-TEM) and atomic force microscopy (AFM). TTBC-Cl is found to
form J-aggregates with a classical Davydov-split absorption band (type
I spectrum) even under different preparation conditions. These aggregates
remain stable for months. Unlike the chloride salt, the iodide salt
TTBC-I forms two different types of J-aggregates depending on the
pH of the aqueous solution. The TTBC-I aggregates prepared in pure
water (pH = 6) are characterized by a single redshifted absorption
band (type III spectrum), whereas those prepared in alkaline solution
at pH = 13 show a typical Davydov-split (type I) absorption band.
Despite differences in counterions, preparation method, stability,
and spectroscopic behavior, cryo-TEM reveals an identical tubular
architecture for all these J-aggregates. Among the new structure models
discussed here is a cylindrical brickwork layer of dye molecules for
single-banded J-aggregates (type III). For Davydov-split aggregates
(type I), a molecular herringbone-like pattern is proposed instead.
Moreover, absorption spectra have revealed an additional single redshifted
absorption band (type II spectrum) that is assigned to a surface aggregate
and is induced by a specific interaction of the dye cation with the
negatively charged cuvette wall. AFM measurements of analogous preparations
on negatively charged mica surfaces have supported this interpretation
and revealed the formation of monolayered sheet structures