The exciton physics of organic semiconductors is exotic. It is a domain in which singlet and triplet kinetics both play an important role in determining the performance of various optoelectronic devices. Since triplet excitons are non-emissive, it brings further challenges in the understanding of triplet kinetics. In this work, we have studied the effect of polymer chain packing on triplet diffusion in the polyfluorene based polymeric system, which is known to give efficient organic light emitting diode (OLED) efficiency for display devices. Furthermore, this polyfluorene system exhibits an efficient triplet–triplet fusion process, which provides singlet excitons as delayed fluorescence and becomes a tool to study triplet exciton kinetics. We have developed a unique method to trace the position of the triplet exciton in the emissive layer of OLEDs by analyzing angle-resolved delayed electroluminescence emission patterns as a function of time. This study could provide exciton transport kinetics in the transverse direction from the substrate plane. Furthermore, direct visualization of the delayed photoluminescence imaging technique could provide lateral transport kinetics of triplet excitons. Results suggest that the diffusion is significantly anisotropic in thinner films. As the thickness of the film increases, anisotropy reduces in triplet transport. Moreover, we noticed that in thicker polymeric semiconductor films, diffusivity approaches close to ultrahigh 10−3 cm2 s−1, which is similar to the values that are reported for acene-based molecular crystalline thin films. Our results also provide important insight into efficient electroluminescence in unusually thick (1.2 μm) polyfluorene-based emissive layers of OLEDs.

Porous materials that synergistically combine high reversible mechanical elasticity with tunable in situ reaction kinetics can find several biological and industrial applications. However, such materials remain elusive in the literature. Herein, we show that by utilizing the intrinsic elastic property of a 3D macroporous material/scaffold comprising polymer-coated biomimetic ceria nanoparticles, the rate of in situ dephosphorylation reactions can be enhanced efficiently. The elasticity of the scaffold is dynamically controlled by employing compression−decompression, [C−D], cycles.Varying the [C−D] frequency from 0 to 4 increases the formation of dephosphorylation reaction products, such as molecular p-nitrophenol or the in situ self-assembled Fmoc-L-tyrosine-based network. Further, we employ a numerical method improvised on an existing stochastic reaction diffusion approach to explain the unusual increase in product formation as a function of the frequency of the [C−D] cycles. The proposed computational methodology predicts rational design of the enzyme-mimicking material by analyzing the efficiency of the product formation under various absolute amounts of the catalyst and substrate levels, suggesting possible applications of such materials. Finally, as a proof of concept, we demonstrate the use of these materials for culturing mammalian cells, which suggests their potential biological applications after implementing appropriate postsynthetic chemical modifications.

Polyethyleneimine (PEI), a cationic polyelectrolyte, finds great utility as a nonviral gene transfection vector. The mechanism of gene delivery process across the cell membrane through PEI-based polyplexes is still much debated; however, a general consensus is that the proton binding-based PEI conformational changes occur as the pH alters during the process. Taking a step back, even the understanding of wide pH range (from 1 to 10) dependent conformational changes for neat PEI in explicit water remains elusive. In pursuit of this objective, using a combination of optical and electron microscopy, we observed that a dilute aqueous solution of linear or branched PEI (M w ranging from 0.8 to 750 kDa) at room temperature and having pH in the range 2.5–4 undergoes a completely novel and highly unprecedented slow self-assembly process to form a micrometer-sized thick fibrillar network. These self-assembled structures are highly robust, irreversible, and interestingly, generic for PEI and form over 24–72 h in a dilute aqueous solution irrespective of the molecular weight and configuration of PEI. A combination of turbidity and acid titration experiments on PEI aqueous solutions having different pH values reveal that the mechanism of this hierarchical self-assembly (between pH 2.5-4) can be explained by the unique protonation behavior of PEI chains and their ability to undergo conformational/morphological transitions.