Volodymyr Tsyalkovsky

Designing fluoroprobes through Förster resonance energy transfer: surface modification of nanoparticles through “click” chemistry

Aqueous-phase 83 nm poly(propargyl acrylate) (PA) nanoparticles were surface-functionalized with sparingly water soluble fluorescent moieties through a copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) (i.e., “click” transformation) to produce fluoroprobes with a large Stokes shift. For moieties which could not achieve extensive surface coverage on the particles utilizing a standard click transformation procedure, the presence of β-cyclodextrin (β-CD) during the transformation enhanced the grafting density onto the particles. Moieties containing oxadiazolyl groups exhibited an 84% increase in grafting density when the transformation was performed in the presence of the oligosaccharide, going from 1.04 oxadiazolyl groups/nm2 to 1.91 oxadiazolyl groups/nm2. Similarly, an azide-modified coumarin 6 (AD1) underwent a 17% enhancement in grafting density from 1.56 AD1 groups/nm2 to 1.82 AD1 groups/nm2 when the transformation was done in the presence of β-CD. A polyethylene glycol modified naphthalimide-based emitter (AD2) was less sensitive to the presence of β-CD due to its elevated water solubility and exhibited a 5% increase in grafting density. In contrast, a carbazolyl-containing moiety which could achieve 100% surface coverage of the particles without the use of β-CD exhibited a slight retardation in the incorporation rate (and final grafting density) onto the particle when the oligosaccharide was employed. Photoluminescence studies of the particles modified singly or with multiple moieties indicated that when oxadiazolyl and carbazolyl groups were attached to the particles, an exciplex was formed that had a peak emission at ca. 400 nm. The absorption of the surface attached naphthalimide-based dye exhibited a complete spectral overlap with the carbazole/oxadiazole exciplex emission and photoluminescence excitation studies indicated an efficient energy transfer process from the carbazolyl and oxadiazolyl groups to the dye, resulting in an emission maxima at 510 nm for the modified particles and a total Stokes shift of 180 nm. This large Stokes shift is an important determinant of the ultimate sensitivity of a fluoroprobe, where scattering and background fluorescence can interfere with the detection of low concentrations of an analyte and the ability to manipulate the separation between the excitation and emission wavelengths is a critical parameter for optimal detection.

Tuning Fluorescent Response of Nanoscale Film With Polymer Grafting

An effective method for tuning fluorescent response of an ultrathin (5 nm) polymer film, which can be used for generation of sensing arrays, is reported. This method is distinctive in that the modification of the optical response is achieved with polymer grafting of a non-fluorescent polymer to a fluorescent film. Using this approach, a number of films demonstrating different fluorescent emission when exposed to solvent vapors were synthesized.

Comparative study on the effect of thermal annealing on polymer/small molecule blend and copolymer light-emitting devices

The intrinsic deterioration in device performance of polymeric single layer OLEDs that were doped with a fluorescent emitter was studied. The specific focus was on the role that thermal aging, at sub-glass transition temperatures of the polymeric layer, has on the phase separation of the active layer. This was accomplished by the rational design of an oxadiazole-containing methylacrylate monomer that was energetically similar to the technologically important electron- transporting small molecule 2-biphenyl-4-yl-5-(4-tert-butylphenyl)-1,3,4- oxadiazole (tBu-PBD). This monomer was copolymerized with a carbazole containing hole-transporting monomer 2-(9H-carbazol-9-yl)ethyl 2-methylacrylate (CE) and the resulting copolymer was utilized as the active layer with coumarin 6. With coumarin 6, the devices exhibited a stable mean luminance of ca. 400 cd/m2 with thermal aging at temperatures ranging from 23 °C to 130 °C, while a comparable poly(9-vinyl-9H-carbazole)/tBu-PBD blend device exhibited a drop from an initial mean luminance of 2500 cd/m2 to 1.6 cd/m2. The reduction in luminance and luminance efficiency for the blend system was attributed to phase separation in the blend.