Dark States in Ionic Oligothiophene Bioprobes—Evidence from Fluorescence Correlation Spectroscopy and Dynamic Light Scattering

Abstract

ABSTRACT. Luminescent conjugated polyelectrolytes (LCPs) can upon interaction with biological macromolecules change their luminescent properties, and thereby serve as conformation-and interaction-sensitive biomolecular probes. However, to exploit this in a more quantitative manner, there is a need to better understand the photophysical processes involved. We report studies of the conjugated pentameric oligothiophene derivative p-FTAA, which changes optical properties with different p-FTAA concentrations in aqueous buffers, and in a pH and oxygen saturation dependent manner. Using dynamic light scattering, luminescence spectroscopy and fluorescence correlation spectroscopy, we find evidence for a monomer-dimer equilibrium, for the formation of large clusters of p-FTAA in aqueous environment, and can couple aggregation to changed emission properties of oligothiophenes.In addition, we observe the presence of at least two dark transient states, one presumably being a triplet state. Oxygen was found to statically quench the p-FTAA fluorescence, but also to promote molecular fluorescence by quenching dark transient states of the p-FTAA molecules. Taken together, this study provides knowledge of fluorescence and photophysical features essential for applying p-FTAA and other oligothiophene derivatives for diagnostic purposes, including detection and staining of amyloid aggregates.

Tyrosine Side‐Chain Functionalities at Distinct Positions Determine the Chirooptical Properties and Supramolecular Structures of Pentameric Oligothiophenes

“…In 20 mM NaOH, p‐HTA‐L‐Tyr and p‐HTA‐D‐Tyr, displayed similar absorption spectra with a maximum at 400 nm, as well as comparable emission maxima around 520 nm (Figure 2A). These photophysical characteristics are similar to ones reported for other anionic pentameric oligothiophenes in alkaline conditions, suggesting that the absorption‐ and emission spectra from the ligands in 20 mM NaOH originates from fully dissolved single oligothiophene chains or tiny clusters of chains [29–33] . Both dyes also showed analogous optical characteristic in 20 mM HCl, but the absorption spectra showed a red‐shift, as well as two additional peaks around 330 nm and 275 nm, compared to the spectra in alkaline conditions.…”

“…Next the photo‐physical characteristics of the tyrosine‐functionalized oligothiophenes dissolved in 20 mM HCl or 20 mM NaOH (pH 12.3) were evaluated (Figure 2). These acidic and basic conditions were selected, since earlier studies have shown that pH‐induced conformational changes of the thiophene backbone can be afforded for both anionic oligo‐ and polythiophenes [29–33] . In addition, in 20 mM HCl (pH 1.7) all the carboxyl groups of the oligothiophenes should predominantly be protonated, whereas in 20 mM NaOH (pH 12.3), the carboxyl groups, as well as the phenolic hydroxy groups should be deprotonated.…”

“…These photophysical characteristics are similar to ones reported for other anionic pentameric oligothiophenes in alkaline conditions, suggesting that the absorptionand emission spectra from the ligands in 20 mM NaOH originates from fully dissolved single oligothiophene chains or tiny clusters of chains. [29][30][31][32][33] Both dyes also showed analogous optical characteristic in 20 mM HCl, but the absorption spectra showed a red-shift, as well as two additional peaks around 330 nm and 275 nm, compared to the spectra in alkaline conditions. A red-shift of the spectra were also observed for the emission and is most likely associated with a planarization of the backbone that are induced upon protonation of the carboxyl groups.…”

“…These acidic and basic conditions were selected, since earlier studies have shown that pH-induced conformational changes of the thiophene backbone can be afforded for both anionic oligo-and polythiophenes. [29][30][31][32][33] In addition, in 20 mM HCl (pH 1.7) all the carboxyl groups of the oligothiophenes should predominantly be protonated, whereas in 20 mM NaOH (pH 12.3), the carboxyl groups, as well as the phenolic hydroxy groups should be deprotonated. In 20 mM NaOH, p-HTA-L-Tyr and p-HTA-D-Tyr, displayed similar absorption spectra with a maximum at 400 nm, as well as comparable emission maxima around 520 nm (Figure 2A).…”

Assembly of Heterometallic Rigid-Rod Complexes and Coordination Oligomers from Gold(I) Metalloligands

“…XXXX, XXX, XXX−XXX However, considering the smaller areas of the broad peaks, and the effect of the particle size on the scattered intensity (see above), these large particles should represent a very small fraction of the total weight of the solute. 89,93 In fact, they are neither detected in the number particle size distributions (Figure 7, bottom) nor in the NMR spectra of the measured samples (Supporting Information). The size distribution of both Fe complex and oligomers could not be determined because they scattered very poorly in these conditions.…”