Aggregation of Surfactant Squaraine Dyes in Aqueous Solution and Microheterogeneous Media:  Correlation of Aggregation Behavior with Molecular Structure

Abstract

The synthesis of several amphiphilic squaraine dyes and a study of their aggregation behavior and photophysics are reported. The several different squaraines are found to give spectrally blue-shifted aggregates in aqueous and mixed aqueous-organic solution and in microheterogeneous media (bilayer vesicles). While in some cases an intermediate dimer can be detected in the monomer to aggregate conversion process, in others direct conversion of monomer to aggregate is observed. The aggregation number can be determined together with the equilibrium constant and thermodynamic parameters for some of the squaraines in different environments. In several cases the aggregation number is found to be ca. 4. The finding of a strong induced circular dichroism signal when the aggregate (but not dimer or monomer) is generated in the presence of a chiral host (or counterion) suggests that the aggregate is chiral. From these results and molecular simulations indicating that an extended monolayer of some of the squaraines adopts a glide or herringbone lattice we propose a chiral "pinwheel" structure for the unit aggregate and suggest that extended aggregate structures or crystals may be a mosaic of these unit aggregates. In contrast to the monomers, which are strongly fluorescent, the squaraine dimers and aggregates are nonfluorescent and have extremely short exciton lifetimes, as indicated by transient spectroscopy.

Molecular aspects of squaraine dyes aggregation and its influence on spectroscopic properties

“…The obtained computational effects of aggregation on the absorption spectrum comply well with the aggregation manifestation in the experiment: the signal shift and broadening [11,[18][19][20].…”

“…A stepwise mechanism of some squaraines aggregation has been observed by Chen et al [20] where first the dimers and later on higher aggregates were formed. Another amphiphilic squaraines have disclosed a tendency to form a stable tetrameric unit with a pinwheel structure suggested by Chen et al [20].…”

“…Another amphiphilic squaraines have disclosed a tendency to form a stable tetrameric unit with a pinwheel structure suggested by Chen et al [20].…”

Squaraine‐Doped Functional Nanoprobes: Lipophilically Protected Near‐Infrared Fluorescence for Bioimaging

“…It is known that squaraines are apt to self-aggregate, which causes absorption shift and broadening as well as fl uorescence quenching. [ 12 ] In the case of Sq-X /PMAO nanoparticles, we see no such manifestation: the absorption spectra are still sharp with a full-width-at-half-maximum (FWHM) of 38 nm, comparable to that in THF solution (27 nm). Importantly, hydrophobic Sq-X retained their capability of fl uorescence emission in water on encapsulation in PMAO nanoparticles (the Φ f of as-prepared Sq-St/PMAO nanoparticles is 0.25).…”

Photophysical Characterization and BSA Interaction of Direct Ring Carboxy Functionalized Symmetrical squaraine Dyes

“…The squaraine dyes belongs to the interesting organic chromophoric system, where oxocyclobutenolate core is linked by aromatic or heterocyclic components at both ends leading to donor-acceptor-donor molecular motif. Sensitizing dyes belonging to squaraine family are of considerable interest as fluorescent labels/probes for biological and pharmaceutical research owing to their tunable absorption/emission from visible to NIR wavelength region, high molar absorptivity and reasonably good quantum yields [3][4]. By logical molecular design, the absorption and emission behaviour of these dyes can be tuned from visible to IR wavelength making them suitable for fluorescence imaging since they are exterior to the self-absorption and auto-fluorescence regions of biological matrices [5].…”