Tuning the Emission of Bis-ethylenedioxythiophene-thiophenes upon Aggregation

Abstract

The ability of small lipophilic molecules to penetrate the blood-brain barrier through transmembrane diffusion has enabled researchers to explore new diagnostics and therapies for brain disorders. Until now, therapies targeting the brain have mainly relied on biochemical mechanisms, while electrical treatments such as deep brain stimulation often require invasive procedures. An alternative to implanting deep brain stimulation probes could involve administering small molecule precursors intravenously, capable of crossing the blood-brain barrier, and initiating the formation of conductive polymer networks in the brain through in vivo polymerization. This study examines the aggregation behavior of five water-soluble conducting polymer precursors sharing the same conjugate core but differing in side chains, using spectroscopy and various computational chemistry tools. Our findings highlight the significant impact of side chain composition on both aggregation and spectroscopic response.

Figure 1

Functionalized ETE (“EDOT–thiophene–EDOT”) monomers and precursor 2-(2,5-dibromothiophen-3- yl)ethanol molecular structures. Anionically charged ETE-COO and ETE-S, cationically charged ETE-TMEA (short alkyl chain) and ETE-TMA (long alkyl chain), and zwitterion/neutral ETE-PC.

Figure 2

Experimental absorption and emission spectra for ETE-S (A, F, K), ETE-COO (B, G, L), ETE-PC (C, H, M), ETE-TMEA (D, I, N), and ETE-TMA (E, J, O) at different concentrations in water. The insets in Figure 2A–E are enlargements of the absorption spectra in the 400–550 nm range. Emission spectra were measured at excitation wavelengths of 350 nm (F–J) and 460 nm (K–O), and the corresponding excitation spectra at 432 and 550 nm are shown in the insets.

Figure 3

Absorption and emission in stacked ETE depending on the number of monomers in the crystallite. (A) Simulated geometry and absorption properties in monomer and dimer ETE-COO. (B) Absorption peaks were convolved with the peak smearing half-width half-maximum 0.33 eV. (C) Natural transition orbitals simulated for the excited state S₁ of the monomer and dimer ETE-COO. Emission mechanisms for both systems schematically show the details of the S₁ → S₀ emissive transition. (D) Absorption of all five ETE-based compounds in both monomer and dimer geometries.

Figure 4

Molecular dynamics simulations of five different ETE compounds. Predicted aggregation of monomers into clusters of aggregates after equilibration and 100 ns of a production run. (A) Calculated S–S radial distribution functions of the sulfur atom in the central thiophene ring of ETE-based monomers (B).