W. G. Skene

Tunable spectroscopic and electrochemical properties of conjugated push–push, push–pull and pull–pull thiopheno azomethines

Novel azomethines consisting uniquely of thiophene units were examined. The highly conjugated compounds were prepared by condensing air stable aminothiophenes with 2-thiophene aldehydes, which were substituted with various electronic groups. The resulting azomethines are highly conjugated and are both reductively and hydrolytically resistant. Various electron donating and accepting groups placed in the 2-position of 5-thiophene carboxaldehyde lead to electronically delocalized push–push, pull–pull, and push–pullazomethines. These electronic groups affect both the HOMO and the LUMO levels, which influence the absorption and emission spectra. Colors spanning the entire visible spectrum ranging from yellow to blue are possible with these nitrogen containing conjugated compounds. Excited state deactivation of the singlet excited state occurs predominately by internal conversion while only a small amount of energy is dissipated by intersystem crossing to the triplet state and by fluorescence. The ensuing fluorescence and phosphorescence of the thiopheno azomethines are similar to those of their thiophene analogues currently used in functional devices, but with the advantage of a low triplet state and tunable HOMO–LUMO energy levels extending from 3.0 to 1.9 eV. Quasi-reversible electrochemical radical cation formation is possible while the oxidation potential is dependent on the nature of the electronic group appended to the thiophene. The crystallographic data of the electronic push–push system show the azomethine bonds are planar and linear and they adopt the E isomer.

Polymer Brush Covalently Attached to OH-Functionalized Mica Surface via Surface-Initiated ATRP: Control of Grafting Density and Polymer Chain Length

The controlled grafting density of poly(tert-butyl acrylate) was studied on OH-activated mica substrates via surface-initiated atom-transfer radical polymerization (ATRP). By properly adjusting parameters such as the immobilization reaction time and the concentration of an ATRP initiator, a wide range of initiator surface coverages and hence polymer densities on mica were possible. The covalently immobilized initiator successfully promoted the polymerization of tert-butyl acrylate on mica surfaces. The resulting polymer layer thickness was measured by AFM using a step-height method. Linear relationships of the polymer thickness with respect to the molecular weight of the free polymer and with respect to the monomer conversion were observed, suggesting that ATRP is well controlled and relatively densely end-grafted layers were obtained. The polymer grafting density controlled by adjusting the initiator surface coverage was confirmed by the polymer layer swelling capacity and film thickness measurements.

Push−Pull Aminobithiophenes — Highly Fluorescent Stable Fluorophores

Stable 2-aminobithiophenes were prepared using the Gewald reaction. The resulting push-pull bithiophenes exhibited both unprecedented high fluorescence yields and stability in addition to demonstrating fluorescence on-off properties.

EDOT-containing azomethine: an easily prepared electrochromically active material with tuneable colours

An azomethine derived from EDOT segments was easily prepared and exhibited reversible oxidation. The spectroelectrochemical behaviour of this EDOT containing azomethine was observed as intense colour changes for the oxidized products relative to the neutral form. Multiple reversible colours states were observed with the anodically and hydrolytically stable azomethine upon electrochemical and chemical oxidation.

Survey of recent advances of in the field of π-conjugated heterocyclic azomethines as materials with tuneable properties

This account gives an overview of our recent work in the area of conjugated azomethines derived from 2-aminothiophenes. It will be presented that mild reaction conditions can be used to selectively prepare symmetric and unsymmetric conjugated azomethines. It further will be demonstrated that azomethines consisting of various 5-membered aryl heterocycles lead to chemically, reductively, hydrolytically, and oxidatively robust compounds. The optical and electrochemical properties of these materials can be tuned contingent on the degree of conjugation, type of aryl heterocycle, and by including various electronic groups. The end result is materials having colors spanning 250 nm across the visible spectrum. These colors further can be tuned via electrochemical or chemical doping. The resulting doped states have high color contrasts from their corresponding neutral states. The collective opto-electronic properties and the means to readily tune them, make thiophenoazomethine derivatives interesting materials for potential use in a gamut of applications.

Photophysical, crystallographic, and electrochemical characterization of novel conjugated thiopheno azomethines

New conjugated thiopheno azomethines using a stable 2,5-diamino thiophene precursor are presented. The spectroscopic and photophysical studies of these compounds confirm they dissipate their excited state energy predominately by internal conversion and not by intersystem crossing. Crystallographic data show the thermodynamically favourable E isomer is formed and that the highly conjugated azomethines are both linear and planar. Electrochemical oxidation of the azomethine compounds yields a persistent radical cation that cross couples. The resulting oxidative products undergo reversible radical cation formation and exhibit spectroscopic properties that are consistent with an increased degree of conjugation.

A fluorescent all- fluorene polyazomethine—towards soluble conjugated polymers exhibiting high fluorescence and electrochromic properties

A conjugated all-fluorene polyazomethine (6) was prepared from the condensation of 9,9′-dioctyl fluorene dialdehyde and 9,9′-dihexyl fluorene diamine. The resulting polymer was soluble in common organic solvents courtesy of the solubilizing alkyl groups in the 9,9′ positions. Determination of the polymers molecular weight was subsequently possible using standard characterization methods. Polymer formation was confirmed both by GPC and NMR with average number molecular weights ranging from 9 to 145 kg mol−1. The absolute fluorescence quantum yield (Φfl) of 6 in dichloromethane was 0.19 while in thin films the value increased to 0.40. The fluorescence could further be restored to unity at 77 K while Φfl = 0.76 was possible by protonating 6 with trifluoroacetic acid. 6 underwent stark color changes from yellow to red upon doping with TFA and FeCl3 and the resulting intermediates could be neutralized with Et3N and hydrazine hydrate, respectively. Reversible protonation/deprotonation was possible both in solution and thin films. Meanwhile, electrochemical oxidation of 6 at ca. 1.5 V resulted in a 120 nm bathochromic change in the absorbance. The intermediate could be reduced at −0.2 V to regenerate the neutral 6 that absorbed at 425 nm. The Job method confirmed that the oxidized intermediate was the radical cation.

Towards materials with reversible oxidation and tuneable colours using heterocyclic conjugated azomethines

The spectroelectrochemical behaviour of heterocyclic azomethines prepared by condensing complementary aldehydes and amines was observed as intense colour changes of the oxidized products relative to the neutral form. The tuneable colours and reversible oxidation were contingent on the heterocycles.

Photophysical, electrochemical, and crystallographic investigation of conjugated fluoreno azomethines and their precursors

The photophysical investigation of amino- and aldehyde-substituted fluorenes revealed that these compounds are not only highly fluorescent, but dissipation of their singlet excited energy occurs by a combination of nonradiative means involving intersystem crossing (ISC) and internal conversion (IC). Quantification of the triplet state formed by ISC was possible by laser-flash photolysis (LFP). The efficiency by which this manifold was populated varied between 10 and 40% depending on the fluorene substitution. Condensation of these aldehyde and amine precursors yielded conjugated thiopheno azomethines with robust covalent bonds. Fluorescence of the azomethinefluorene derivatives was reduced relative to their precursors while the degree of IC remained unchanged. Deactivation of the singlet excited state occurred predominately by ISC and the resulting triplet state was rapidly and efficiently quenched by energy transfer by the azomethine linkage. Cyclic voltammetry of the fluoreno azomethines showed both oxidation and reduction processes, and the measured redox potentials and the band-gaps are lower than a bisfluorene analogue. The fluoreno azomethine LUMO energy levels are sufficiently low, making them compatible with common cathodes, therefore eliminating the use of an electron-injection layer.

Conjugated fluorene-thiophenes prepared from azomethine connections. Part I. The effect of electronic and aryl groups on the spectroscopic and electrochemical properties.

The spectroscopic investigation of new fluoreno-thiophene azomethines revealed that these compounds are fluorescent. However, they exhibit reduced fluorescence compared to native fluorene owing to competitive deactivation of the singlet excited state by nonradiative means involving both internal conversion and intersystem crossing. The absorption and emission wavelengths can be tuned and the HOMO-LUMO energy gap modulated from 2.0 to 3.2 eV by incorporating various electronic groups, number of azomethine bonds, and the fluorene-thiophene sequence. Electrochemical investigation confirmed that both oxidation and reduction occur resulting in irreversible radical ion formation.