Enhancement of charge-assisted hydrogen bond capabilities due to O-alkylation proximity in alkoxy cationic polythiophenes: solution- and solid-state evidence via EPR, AFM and surface free energy.

Despite the array of applications for cationic polythiophenes (CPTs), there is still a need for structure-function guidelines and mechanistic understanding of their solution- and solid-state properties. This work presents a solution- and solid-state investigation of the effect of O-alkylation proximity on the hydrogen bonding (H-bonding) capabilities of alkoxy-CPTs, based on comparing an imidazolium alkoxy CPT with strong cation-pi, pi+ and positive charge-assisted hydrogen bonding (+CAHB) capabilities (PIMa), with two isothiouronium alkoxy CPTs with two-point +CAHB capabilities (PT1 & PT2), which have short and long alkoxy side chains, respectively. Our results show that a closer proximity of O-alkylation strengthens the +CAHB capabilities of PT1: in aqueous solutions, PT2 aggregates have a stronger interaction with cationic EPR spin probes than aggregates of PIMa and PT1, which in turn show a similar extent of repulsion towards the cationic spin probes. In solid-state, atomic force microscopy (AFM) shows that PIMa generates dendritic structures onto mica, with features of diffusion-limited aggregation (DLA), indicating strong interactions with the anionic substrate due to a high configurational entropy during spreading, regardless of being drop-casted from water or 1,4-dioxane-water (W-DI), despite the latter disturbing H-bonding due to selective solvation. PT1 is also capable of generating dendritic structures resembling ballistic aggregation (BA). However, this occurs only when casting from water, since W-DI generates island-like aggregates resembling attachment limited aggregation (ALA), which is the morphology generated by PT2 regardless of the solvent. Finally, spin-coated films of PIMa and PT1 show similar dispersivity of the surface free energy (SFE), which in turn is larger than that in PT2 films, which are also more affected when casted from W-DI, presenting much larger decreases of dispersivity. These results constitute a novel empirical structure-function guideline that could be useful for optimal design and/or processing of alkoxy CPTs. For example, dendritic patterns have recently gained attention since the colloidal droplet drying is related to engineering applications including inkjet printing, biosensing, and functional material design, while the SFE is relevant for opto- and bio-electronic applications of conjugated polyelectrolytes (CPEs). This information could also be useful when analyzing previous results obtained from alkoxy CPTs with different side chain lengths.

Cationic polythiophene-anionic fullerene pair in water and water-dioxane: studies on hydrogen bonding capabilities, kinetic and thermodynamic properties.

Despite the vast array of solution- and solid-state bio-analytical, bioelectronic and optoelectronic applications of cationic polythiophenes (CPTs), the number of studies focused on the role of hydrogen bonding (H-bonding) between these and other molecules is scarce, regardless of whether H-bonding is expected to play an important role in several such applications. Also, despite the advantages of using cosolvents to systematically examine the molecular interactions, there are no such studies for CPTs to our knowledge. This work presents a steady-state UV-vis/fluorescence spectroscopic, kinetic and thermodynamic study on the H-bonding interactions between a water-soluble, cationic-anionic (isothiouronium-tetraphosphonate), polythiophene-fullerene donor-acceptor pair with two-point, charge-assisted H-bonding (CAHB) capabilities, tuned using water or a 1,4-dioxane-water mixture (W-DI). Both solvents generate photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), spontaneous binding, H-bonding, ground-state complexing via multiple site binding, formation of micelle-like aggregates and equivalence points at a similar concentration of the quencher. However, in comparison with water, W-DI promotes less-ordered, less packed micellar aggregates, due to hydrophobic desolvation of the H-bond and larger solvent displacement during the PT1-4Fo complexation. This would decrease the extent of charge-transfer and the size of the sphere-of-quenching, mainly by displacements or rotations of the H-bonds, instead of elongations, together with a possible larger extent of diffusion-controlled static quenching. At [4Fo] larger than the equivalence point the micelles formed in water do not have available binding sites due to a tighter aggregation, causing a decrease in the quenching efficiency, while the micelles formed in W-DI start showing larger quenching efficiencies, possibly due to an increase in entropy that overcomes the desolvation of the H-bonding. These results could be useful when analyzing outputs from systems including CPTs with H-bonding capabilities, operating in (or casted from) solvents with clear differences in polarity and/or H-bonding capacity.

J-like aggregation of a cationic polythiophene with hydrogen-bonding capabilities due to 1,4-dioxane: Solution excitation spectra and fluorescence, morphology and surface free energy of films.

This work presents solution- and solid-state evidence of the enhancement of J-like aggregation of a cationic polythiophene (CPT) with isothiouronium functionalities (PT1), caused by a decrease in the polarity and hydrogen-bonding (H-bonding) capacity of the solvent, generated by using a 50:50 v/v 1,4-dioxane-water mixture (W-DI) instead of water. In solution, the presence of 1,4-dioxane (DI) seems to generate selective solvation, tuning the energy transfer within PT1 from inter-chain into intra-chain, enhancing J-like aggregation. On the other hand, during the casting process, the presence of DI directs the interaction with solid-substrates, generating an increase in the solid-state fluorescence, modifying the morphology from one similar to ballistic-aggregation (BA) into one similar to attachment limited aggregation (ALA), DI also modifies the SFE by increasing slightly its polar contribution (γSp) and decreasing the dispersive one (γSd). These results can be explained to be caused by a "coating" effect in presence of DI (as proposed before experimentally and computationally). Our results show a clear correlation between the solution- and solid-state properties of PT1 in each solvent, further validating the use of the fluorescence excitation spectra to trace J-like aggregation of water-soluble conjugated polymeric fluorophores in solution. This information could be useful for predicting and designing specific mesoscopic architectures of CPTs (and conjugated polyelectrolytes in general), which are molecules lacking of clear structure-function guidelines for designing high-performance polythiophene-based interlayer materials, especially for CPTs (and conjugated polyelectrolytes (CPEs) in general), particularly those with H-bonding capabilities. To the best of our knowledge the use of solution-state fluorescence excitation spectra to identify J-like aggregation of water-soluble conjugated polymers (CPs) has been scarcely used/discussed in literature and no correlation with solid-state properties was reported previously.

Cationic Imidazolium Polythiophenes: Effects of Imidazolium-Methylation on Solution Concentration-Driven Aggregation and Surface Free Energy of Films Processed from Solvents with Different Polarity.

Cationic imidazolium-functionalized polythiophenes with single- or double-methylation of the imidazolium ring were used to study the impact of imidazolium-methylation on (i) the solution concentration-driven aggregation in the presence of paramagnetic probes with different ionic and hydrophobic constituents and (ii) their surface free energy (SFE) as spin-coated films deposited on plasma-activated glass. Electron paramagnetic resonance spectroscopy shows that the differences in film structuration between the polymers with different methylations originate from the early stages of aggregation. In the solid state, higher degree of imidazolium-methylation generates smaller values of total SFE, γS, (by around 2 mN/m), which could be relevant in optoelectronic applications. Methylation also causes a decrease in the polar contribution of γS (γSp), suggesting that methylation decreases the polar nature of the imidazolium ring, probably due to the blocking of its H-bonding capabilities. The values of γS obtained in the present work are similar to the values obtained for doped films of neutral conjugated polymers, such as polyaniline, poly(3-hexylthiophene), and polypyrrole. However, imidazolium-polythiophenes generate films with a larger predominance of the dispersive component of γS (γSd), probably due to the motion restriction in the ionic functionalities in a conjugated polyelectrolyte, in comparison to regular dopants. The presence of 1,4-dioxane increases γSp, especially, in the polymer with larger imidazolium-methylation (and therefore unable to interact through H-bonding), probably by a decrease of the imidazolium-glass interactions. Singly-methylated imidazolium polythiophenes have been applied as electrode selective ("buffer") interlayers in conventional and inverted organic solar cells, improving their performance. However, clear structure-function guidelines are still needed for designing high-performance polythiophene-based interlayer materials. Therefore, the information reported in this work could be useful for such applications.

Multidentate europium chelates as luminoionophores for anion recognition: impact of ligand design on sensitivity and selectivity, and applicability to enzymatic assays.

The design of photoluminescent molecular probes for the selective recognition of anions is a major challenge for the development of optical chemical sensors. The reversible binding of anions to lanthanide centers is one promising option for the realization of anion sensors, because it leads in some cases to a strong luminescence increase by the replacement of quenching water molecules. Yet, it is an open problem to gain control of the sensitivity and selectivity of the luminescence response. Primarily, the selective detection of (poly)phosphate species such as nucleotides has emerged as a demanding task, because they are involved in many biological processes and enzymatic reactions. We designed a series of pyridyl-based multidentate europium complexes (seven-, six-, and five-dentate) including sensitizing chromophores and studied their luminescence intensity and lifetime responses to different (poly)phosphates (adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), cyclic adenosine monophosphate (cAMP), pyrophosphate, and phosphate anions), and carboxyanions (citrate, malate, oxalacetate, succinate, α-ketoglutarate, pyruvate, oxalate, carbonate). The results reveal that the number of free coordination sites has a significant impact on the sensitivity and selectivity of the response. Because of its reversibility, the lanthanide probes can be applied to monitor the activity of ATP-consuming enzymes such ATPases and apyrases, which is demonstrated by means of the five-dentate complex.

Aqueous dispersion, surface thiolation, and direct self-assembly of carbon nanotubes on gold.

We report the efficient aqueous dispersion of pristine HiPco single-walled carbon nanotubes (SWNTs) with ionic liquid (IL)-based surfactants 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), the thiolation of nanotube sidewalls with 2, and the controlled self-assembly of positively charged SWNT-1,2 composites on gold. Optical absorption spectra and resonance Raman (RR) data of obtained aqueous SWNT-1,2 dispersions are consistent with debundled and noncovalently functionalized nanotubes whose electronic properties have not been disturbed. Additionally, the dispersion of pristine nanotube material with surfactants 1 and 2 leads to a high degree of purification from carbonaceous particles. The chiralities of the 14 smallest semiconducting HiPco SWNTs in resonance with Raman excitation at 1064 nm (1.165 eV) were determined in SWNT-2 aqueous dispersion using UV-vis-NIR and RR spectra. X-ray photoelectron spectroscopy (XPS) and surface-enhanced resonance Raman scattering (SERRS) spectroscopy of SWNT-2 submonolayers on gold verified the encapsulation of individualized SWNTs with IL surfactants, the cleavage of S-S disulfide bonds formed in aqueous SWNT-2 suspensions, and the direct chemisorption of the SWNT-2 composite on bare gold via the Au-S bond. Aqueous dispersions of SWNTs with IL-based surfactants add biofunctionality to carbon nanotubes by imparting the positive surface charge necessary for interactions with cell membranes. Our technique, which purifies pristine nanotube material and produces water-soluble, positively charged nanotubes with pendent surface-active thiol groups, may also be translated to other carbon nanotubes and carbon nanostructures. Self-assembled, positively charged submonolayers of SWNTs can be further used for applications in cell biology and sensor technology.

Structure of self-assembled multilayers prepared from water-soluble polythiophenes.

We have studied the structure and morphology of self-assembled polyelectrolyte multilayers prepared using poly(styrenesulfonate) (PSS) and four different cationic poly(alkoxythiophene) derivatives bearing methylimidazolium-terminated ionic side chain at the 3-position of the thiophene ring: poly(1-methyl-3-[3-[3-thienyloxy]-propyl]-1H-imidazolium) (P3TOPIM), poly(1-methyl-3-[6-[3-thienyloxy]-hexyl]-1H-imidazolium) (P3TOHIM), poly(1-methyl-3-[2-[(4-methyl-3-thienyl)oxy]-ethyl]-1H-imidazolium ) (P4Me-3TOEIM), and poly(1-methyl-3-[6-[(4-methyl-3-thienyl)oxy]-hexyl]-1H-imidazolium ) (P4Me-3TOHIM). All the multilayers exhibited regular growth. The thickness of the multilayers was measured with ellipsometry, their layer-by-layer growth was followed by polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) and ellipsometry, and the morphology of the films was studied by atomic force microscopy (AFM). The length of the methylimidazolium-terminated side chain (C(n), n = 2, 3, 6) and the substituent (H or Me) at the 4-position of the thiophene ring were varied. All multilayers were inhomogeneous in the sub-micrometer scale and contained aggregates of two kinds. The large ones with a low and constant surface number density were attributed to PSS, whereas the small aggregates were polythiophene-based. The surface density of these organic semiconducting nanoparticles greatly depended on the structure of polythiophene, being favored by polymer regioregularity and the length of the side chain. The side chains remained disordered in all the multilayers, but with polythiophenes having hexyl chains both the imidazolium and thiophene rings tended to orient themselves more perpendicular to the surface than in films containing shorter chains (C2 or C3). The relative water content of the multilayers (at 7.1% relative humidity) did not depend on the film thickness and was the lowest for P4Me-3TOHIM. As the number of bilayers increased the methylimidazolium-sulfonate ion pairs gradually weakened and became more individually hydrated.

Layer-by-layer electrostatic self-assembly of single-wall carbon nanotube polyelectrolytes.

We have used anionic and cationic single-wall carbon nanotube polyelectrolytes (SWNT-PEs), prepared by the noncovalent adsorption of ionic naphthalene or pyrene derivatives on nanotube sidewalls, for the layer-by-layer self-assembly to prepare multilayers from carbon nanotubes with polycations, such as poly(diallyldimethylammonium) or poly(allylamine hydrochloride) (PDADMA or PAH, respectively), and polyanions (poly(styrenesulfonate), PSS). This is a general and powerful technique for the fabrication of thin carbon nanotube films of arbitrary composition and architecture and allows also an easy preparation of all-SWNT (SWNT/SWNT) multilayers. The multilayers were characterized with vis-near-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), surface plasmon resonance (SPR) measurements, atomic force microscopy (AFM), and imaging ellipsometry. The charge compensation in multilayers is mainly intrinsic, which shows the electrostatic nature of the self-assembly process. The multilayer growth is linear after the initial layers, and in SWNT/polyelectrolyte films it can be greatly accelerated by increasing the ionic strength in the SWNT solution. However, SWNT/SWNT multilayers are much more inert to the effect of added electrolyte. In SWNT/SWNT multilayers, the adsorption results in the deposition of 1-3 theoretical nanotube monolayers per adsorbed layer, whereas the nominal SWNT layer thickness is 2-3 times higher in SWNT/polyelectrolyte films prepared with added electrolyte. AFM images show that the multilayers contain a random network of nanotube bundles lying on the surface. Flexible polyelectrolytes (e.g., PDADMA, PSS) probably surround the nanotubes and bind them together. On macroscopic scale, the surface roughness of the multilayers depends on the components and increases with the film thickness.

Water-soluble full-length single-wall carbon nanotube polyelectrolytes: preparation and characterization.

HiPco single-wall carbon nanotubes (SWNTs) have been noncovalently modified with ionic pyrene and naphthalene derivatives to prepare water-soluble SWNT polyelectrolytes (SWNT-PEs), which are analogous to polyanions and polycations. The modified nanotubes have been characterized with UV-vis-NIR, fluorescence, Raman and X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The nanotube-adsorbate interactions consist of pi-pi stacking interactions between the aromatic core of the adsorbate and the nanotube surface and specific contributions because of the substituents. The interaction between nanotubes and adsorbates also involves charge transfer from adsorbates to SWNTs, and with naphthalene sulfonates the role of a free amino group was important. The ionic surface charge density of the modified SWNTs is constant and probably controlled by electrostatic repulsion between like charges. The linear ionic charge density of the modified SWNTs is similar to that of common highly charged polyelectrolytes.