Timo Ääritalo

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.

ESR/UV-Vis-NIR spectroelectrochemical study and electrochromic contrast enhancement of a polythiophene derivative bearing a pendant viologen

The importance of viologens in the field of electrochromic materials is well recognized due to their intensely colored radical cation formation. In this study, 1-[6-[(4-methyl-3-thienyl)oxy]hexyl]-4,4′-bipyridium hexafluorophosphate (Th-V) was synthesized and electropolymerized in a solvent mixture comprising water and acetonitrile (v/v; 50 : 50) with 0.1 M lithium perchlorate (LiClO4) as an electrolyte salt, yielding a viologen bearing polythiophene (PTh-V) film on an electrode surface. The resulting polymer shows electrochemical activity from both the redox active viologen and the conjugated polythiophene moieties. The redox behavior of the polymer was studied by multi in situ spectroelectrochemical technique by means of simultaneous recording of electron spin resonance and UV-Vis-near infrared (ESR/UV-Vis-NIR) spectra. The results indicate that only polaron charge carriers are created during both n- and p-doping of the PTh-V film. The polymer film shows enhanced electrochromic contrast due to the introduction of a pendant viologen group into the thiophene unit. The film switched reversibly between dark violet (at −0.6 V) and almost transparent (at 1.0 V) showing good optical contrast with a coloration efficiency of ca. 305 cm2 C−1 at 610 nm. The switching transmittance kinetics demonstrate fast response times to attain a bleached state and excellent operational stability with repeatable voltage switching between colored/bleached states for 1000 cycles. The polythiophene backbone was found to strengthen the thermal stability of the conjugated PTh-V redox polymer. The excellent optical contrast with sharp color changes and high color efficiency combined with adequate thermal behavior suggests the potential of PTh-V in the electrochromic device (ECD) application.

Study of the electrochemical and optical properties of fullerene and methano[60]fullerenediphosphonate derivatives in solution and as self-assembled structures

Four methanofullerenes with phosphonate groups attached to a C60 core were synthesized to probe their electrochemical and optical properties both in solution and as self-assembled monolayer structures (SAMs). As the methano[60]fullerenediphosphonate is a water soluble derivative of the fullerene it has high potential as an imaging molecule in biological applications and in optoelectronics. For the processing of fullerene based SAMs different electrode substrates (ITO, Au and Si) with specific anchoring groups (zirconium, cysteamine and amino-silane) were used. The formation of the C60-SAMs to the surfaces were investigated by atomic force microscopy (AFM), infrared spectroscopy, contact angle measurements and cyclic voltammetry. Using cyclic voltammetry it was shown that the reduction potentials of substituted methanofullerenes, both in solution and as SAMs, were slightly higher as compared with formal potentials of the redox reactions of C60. The AFM results show that the fullerene molecules produce surface features with an apparent height of ∼2 nm. The self-assembly strategy aims towards fabrication of electronic devices with improved interfacial contact, a prerequisite in order to obtain enhanced electron transfer between acceptor–donor materials.

Effect of alkoxy-spacer length and solvent on diluted solutions of cationic isothiouronium polythiophenes

This contribution presents a study on two cationic isothiouronium polythiophenes (poly-3-(N,N-diethyl-S-iso-thiouronium)alkoxy-4-methyl thiophene), with alkoxy chains of different length in the 3-position of the thiophene ring, results of steady-state absorption and fluorescence spectroscopies in four solvents of different polarities: protic water, protic–protic water–isopropanol, protic–aprotic water–tetrahydrofuran and protic–non polar water–1,4-dioxane (all mixtures 0.5 v/v) are presented. Molar absorption coefficient (e), quantum yield (ϕf), spectral maxima (λmax, λem) and Stokes shift (Δν) are presented, and the effect of solvents on these parameters is discussed. Empirical optical and electrochemical energy levels were estimated using drop cast films on glass and on glassy carbon electrode, respectively. Density functional theory was used to gain supporting structural, energy and electronic information of different oligomers, simulating solvents of different polarity.

Polyviologen synthesis by self-assembly assisted grafting

In this work a new method for electrochemical polymerization of a viologen derivative is presented. A precursor, having an isothiocyanate group as a “head”-group and a cyanopyridine group as a “tail”-group (SCNCP), suitable for self-assembly was designed and synthesized. An adsorbed layer of SCNCP was achieved by assembling isothiocyanate “head”-groups onto gold electrodes or onto gold with an amino-terminated monolayer of cysteamine (SNH2). The purpose of the cyanopyridine “tail”-group is to act as a hook and linker to cyanopyridine based monomers. In this work dimerization between two cyanopyridines, the “tail”-group and a synthesized linear cyanopyridine monomer (LCP), were induced by electrochemical reduction. The SAMs and films were characterized by electrochemical and spectroscopic techniques (UV-vis and FTIR), and show characteristc FTIR spectra and electrochemical redox responses of viologens. AFM and SEM images show wires extending out from the electrode substrate when viologen films were formed on SNH2–SCNCP modified gold surfaces. The new viologen derivative not only broadens the field of applications of viologens in various electronics, but also enables structural regularity and formation at different surfaces.

Conformational changes of a self-assembled polyalkoxythiophene during electrochemical doping: an in situ SERRS study

Abstract The changes in charge carrier generation, structure, and conjugation length during the oxidative doping of a self-assembled poly-3-(3′-thienyloxy)-1-propanesulfonate (P3TOPS) monolayer on a gold surface modified by 2-mercaptoethylamine (MEA) were studied by electrochemical in situ Surface Enhanced Resonance Raman spectroscopy (SERRS) using laser excitation at 1064 nm. The oxidative doping of P3TOPS has been carried out in the potential range from −0.3 to +0.7 V vs Ag/AgCl. The potential dependence of the integrated normalized SERRS intensities of the inter-ring vibrations exhibits a bell-shaped curve with a maximum at 0 V. The highest effective conjugation was observed between −0.1 to +0.1 V. During the oxidation polaronic charge carriers appear first, followed by the generation of bipolarons at higher oxidation levels. Our results imply that the highest expected p -conductivity of P3TOPS might be a result of joint action of polarons and bipolarons. The alkoxy side chain experiences trans – gauche conformational changes at potentials above 0 V, resulting in the orientation more normal to the surface. Distortions in the CSC bonds with further deplanarization of the thiophene rings were observed at potentials positive of +0.3 V.

Oxidation induced variation in polyelectrolyte multilayers prepared from sulfonated self-dopable poly(alkoxythiophene)

Polyelectrolyte multilayers have been prepared using both the neutral and oxidised forms of a poly(alkoxythiophene) derivative with pendant sulfonate groups and it is shown that the oxidation state of the polymer affects multilayer formation.

Effect of Spacer Length and Solvent on the Concentration-Driven Aggregation of Cationic Hydrogen-Bonding Donor Polythiophenes

Aggregation of cationic isothiouronium polythiophenes with alkoxy-spacers of different lengths at the 3-position of the thiophene ring was studied in solvents of different polarities. Hydrogen-bonding capacity was assessed by steady-state absorption and fluorescence spectroscopy, whereas the aggregation in aqueous solutions was studied by electron paramagnetic resonance spectroscopy, using paramagnetic probes of different polarities. The two polymers displayed similar features in respect to conformation, effect of cosolvents on aggregation, unstructured absorption–fluorescence spectra, Stokes shifts when aggregated, solvatochromic effect, and self-quenching concentration. However, these polymers also showed different specific interactions with water, Stokes shifts in water, effect of the solvent on the extent of dominant state of the S1 level, and also different inner cavities and hydrophobic–hydrophilic surface area in aqueous solution aggregates. Water maximized the difference between the polymers concerning the effect of specific increases in concentration, whereas the presence of 1,4-dioxane generated almost identical effects on both polymers.