Salt-induced thermochromism of a conjugated polyelectrolyte

Abstract

We report here the photophysical properties of a water-soluble conjugated polythiophene with cationic side-chains. When dissolved in aqueous buffer solution (PBS, phosphate buffered saline), there is ordering of the polymer chains due to the presence of the salts, in contrast to pure water, where a random-coil conformation is adopted at room temperature. The ordering leads to a pronounced colour change of the solution (the absorption maximum shifts from 400 nm to 525 nm). Combining resonance Raman spectroscopy with density functional theory computations, we show a significant backbone planarization in the ordered phase. Moreover, the ratio of ordered phase to random-coil phase in PBS solution, as well as the extent of intermolecular interactions in the ordered phase, can be tuned by varying the temperature. Femtosecond transient absorption spectroscopy reveals that the excited-state behaviour of the polyelectrolyte is strongly affected by the degree of ordering. While triplet state formation is favoured in the random-coil chains, the ordered chains show a weak yield of polarons, related to interchain interactions. The investigated polyelectrolyte has been previously used as a biological DNA sensor, based on optical transduction when the conformation of the polyelectrolyte changes during assembly with the biomolecule. Therefore, our results, by correlating the photophysical properties of the polyelectrolyte to backbone and intermolecular conformation in a biologically relevant buffer, provide a significant step forward in understanding the mechanism of the biological sensing.

Femtosecond Dynamics of Photoexcited C60 Films

“…Intermolecular delocalization leads to formation of narrow energy bands, which increases the number of available end states and homogeneously broadens transition for charges and excitons . This effect has been observed a number of times in organic semiconductors, − and it should not be confused with inhomogeneous broadening due to disorder . We note that electron delocalization into fullerene aggregates plays an important role in the functioning of organic solar cells, ,, highlighting the importance of evaluating this parameter via the shape of the C 60 anion band.…”

Complexation of a Conjugated Polyelectrolyte and Impact on Optoelectronic Properties

“…Attempts to design a better aqueous formulation have thus far focused on tailoring the polyelectrolyte scaffold, through variation of the charge moieties, chain stiffness, and concentration of the polyelectrolyte, ,− which have shown that the nature of the scaffold proves crucial not just for the resulting PEDOT:PSS structure, but also in the level of doping and stabilization of the PEDOT. ,,, An alternative to the dispersion polymerization approach has relied on making the electronically or optically active component intrinsically water-soluble by the use of conjugated polyelectrolytes (CPEs), which combine a π-conjugated polymer backbone with pendant ionic groups . This approach leads to better tunability over the solution structures, − film morphologies, and resulting optoelectronic properties by using a single-component material …”

Tailoring optical properties and stimulated emission in nanostructured polythiophene

“…These differences in the steady-state spectra of polymeric assemblies are commonly analyzed within an HJ-aggregate model 32–34 . Owing to the comparable intrachain and interchain couplings, subtle changes in the morphology of the crystalline phase of P3HT have been shown to be able to induce alternatively H-like or J-like dominant character of the spectra 33 .…”

“…As detailed in previous studies 36,39 , in TF samples any SE contribution is usually mostly or completely overcome by the PA signal, especially for excitation with short-wavelength/high-photon-energy pump pulses, and residual SE has been mainly attributed to singlet exciton emission from the amorphous regions 49 . A note should be done for the 0-0 SE transition (at around 670 nm) which decreases in the TF sample (H-like) not just because of the higher PA signal but also due to the decreased oscillator strength of the transition 34 .…”