Poly(acrylic acid) (PAA) brushes covalently linked to mica were prepared using the graft from approach in a two-step process: (i) poly(tert-butyl acrylate) (PtBA) brushes were first synthesized by atom transfer radical polymerization directly from an activated mica substrate (ii) followed by hydrolysis to generate PAA brushes. The hydrolysis reaction was confirmed by water contact angle measurements, polymer thickness measurements, and FTIR. The swelling behavior of the brushes in aqueous solutions was measured by examining the change in brush thickness (L), using atomic force microscopy (AFM), as a function of polymer grafting density (σ), pH, and salt (NaCl) concentration (C s). A sharp transition from collapsed to stretched conformation was found at pH 7.5. For pH ≤ 7, the acrylic acid groups are not dissociated, and no swelling of the polymer layer was observed relative to the dry state, regardless of grafting density and salt concentration. For pH ≥ 7.5, the brushes behaved as charged polymer brushes exhibiting Pincus and salted-brush regimes that were dependent on the salt concentration. In salt-free solution, the equilibrium thickness scales with surface grafting density according to L ∝ σ0.87, and at high salt concentrations, the brushes collapse according to L ∝ σ1.09 C s −0.17. The swelling behavior of PAA brushes was reversible with changes in pH and salt concentration under the studied experimental conditions.

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Dimaleimide fluorogens are being developed for application to fluorescent protein labeling. In this method, fluorophores bearing two maleimide quenching groups do not fluoresce until both maleimide groups have undergone thiol addition reactions with the Cys residues of the target protein sequence [J. Am. Chem. Soc. 2005, 127, 559-566]. In this work, a new convergent synthetic route was developed that would allow any fluorophore to be attached via a linker to a dimaleimide moiety in a modular fashion. Series of dimaleimide and dansyl derivatives were thus prepared conveniently and used to elucidate the mechanism of maleimide quenching. Intersystem crossing was ruled out as a potential quenching pathway, based on the absence of a detectable triplet intermediate by laser flash photolysis. Stern-Volmer rate constants were measured with exogenous dimaleimide quenchers and found to be close to the diffusion-controlled limits, consistent with electron transfer being thermodynamically favorable. The thermodynamic feasibility of the photoinduced electron transfer (PET) quenching mechanism was verified by cyclic voltammetry. The redox potentials measured for dansyl and maleimide confirm that electron transfer from the dansyl excited state to a pendant maleimide group is exergonic and is responsible for fluorescence quenching of the fluorogens studied herein. Taking this PET quenching mechanism into account, future fluorogenic protein labeling agents will be designed with spacers of variable length and rigidity to probe the structure-property PET efficiency relationship.

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-pull azomethines. 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.