Suppression of Dexter transfer by covalent encapsulation for efficient matrix-free narrowband deep blue hyperfluorescent OLEDs

Hyperfluorescence shows great promise for the next generation of commercially feasible blue organic light-emitting diodes, for which eliminating the Dexter transfer to terminal emitter triplet states is key to efficiency and stability. Current devices rely on high-gap matrices to prevent Dexter transfer, which unfortunately leads to overly complex devices from a fabrication standpoint. Here we introduce a molecular design where ultranarrowband blue emitters are covalently encapsulated by insulating alkylene straps. Organic light-emitting diodes with simple emissive layers consisting of pristine thermally activated delayed fluorescence hosts doped with encapsulated terminal emitters exhibit negligible external quantum efficiency drops compared with non-doped devices, enabling a maximum external quantum efficiency of 21.5%. To explain the high efficiency in the absence of high-gap matrices, we turn to transient absorption spectroscopy. It is directly observed that Dexter transfer from a pristine thermally activated delayed fluorescence sensitizer host can be substantially reduced by an encapsulated terminal emitter, opening the door to highly efficient 'matrix-free' blue hyperfluorescence.

Synthesis and intramolecular singlet fission properties of ortho-phenylene linked oligomers of diphenylhexatriene

In molecular dimers that undergo intramolecular singlet fission (iSF), efficient iSF is typically accompanied by triplet pair annihilation at rates which prohibit effective triplet harvesting. Collisional triplet pair separation and intramolecular separation by hopping to additional sites in extended oligomers are both strategies that have been reported to be effective for acene based iSF materials in the literature. Herein, a family of highly soluble diphenylhexatriene (DPH) oligomers were synthesized and investigated using transient absorption spectroscopy to determine whether these approaches can be applied to the non-acene singlet fission chromophore, DPH. While iSF proceeds rapidly for all three new materials, neither concentration nor oligomer size result in significantly enhanced triplet pair lifetime relative to the dilute dimer case. These null results indicate the fallibility of the collisional separation and oligomerisation strategies.

Soluble Diphenylhexatriene Dimers for Intramolecular Singlet Fission with High Triplet Energy

Intramolecular singlet fission (iSF) facilitates single-molecule exciton multiplication, converting an excited singlet state to a pair of triplet states within a single molecule. A critical parameter in determining the feasibility of SF-enhanced photovoltaic designs is the triplet energy; many existing iSF materials have triplet energies too low for efficient transfer to silicon via a photon multiplier scheme. In this work, a series of six novel dimers based upon the high-triplet-energy, SF-active chromophore, 1,6-diphenyl-1,3,5-hexatriene (DPH) [E(T₁) ∼ 1.5 eV], were designed, synthesized, and characterized. Transient absorption spectroscopy and fluorescence lifetime studies reveal that five of the dimers display iSF activity, with time constants for singlet fission varying between 7 ± 2 ps and 2.2 ± 0.2 ns and a high triplet yield of 163 ± 63% in the best-performing dimer. A strong dependence of the rate of fission on the coupling geometry is demonstrated. For optimized iSF behavior, close spatial proximity and minimal through-bond communication are found to be crucial for balancing the rate of SF against the reverse recombination process.

Quantitative Singlet Fission in Solution-Processable Dithienohexatrienes

Singlet fission (SF) is a promising strategy to overcome thermalization losses and enhance the efficiency of single junction photovoltaics (PVs). The development of this field has been strongly material-limited, with a paucity of materials able to undergo SF. Rarer still are examples that can produce excitons of sufficient energy to be coupled to silicon PVs (>1.1 eV). Herein, we examine a series of a short-chain polyene, dithienohexatriene (DTH), with tailored material properties and triplet (T1) energy levels greater than 1.1 eV. We find that these highly soluble materials can be easily spin-cast to create thin films of high crystallinity that exhibit ultrafast singlet fission with near perfect triplet yields of up to 192%. We believe that these materials are the first solution-processable singlet fission materials with quantitative triplet formation and energy levels appropriate for use in conjunction with silicon PVs.

Mast Cells Contribute to Porphyromonas gingivalis-induced Bone Loss

Periodontitis is a chronic inflammatory and bone-destructive disease. Development of periodontitis is associated with dysbiosis of the microbial community, which may be caused by periodontal bacteria, such as Porphyromonas gingivalis Mast cells are sentinels at mucosal surfaces and are a potent source of inflammatory mediators, including tumor necrosis factors (TNF), although their role in the pathogenesis of periodontitis remains to be elucidated. This study sought to determine the contribution of mast cells to local bone destruction following oral infection with P. gingivalis Mast cell-deficient mice (Kit(W-sh/W-sh)) were protected from P. gingivalis-induced alveolar bone loss, with a reduction in anti-P. gingivalis serum antibody titers compared with wild-type infected controls. Furthermore, mast cell-deficient mice had reduced expression of Tnf, Il6, and Il1b mRNA in gingival tissues compared with wild-type mice. Mast cell-engrafted Kit(W-sh/W-sh) mice infected with P. gingivalis demonstrated alveolar bone loss and serum anti-P. gingivalis antibody titers equivalent to wild-type infected mice. The expression of Tnf mRNA in gingival tissues of Kit(W-sh/W-sh) mice was elevated following the engraftment of mast cells, indicating that mast cells contributed to the Tnf transcript in gingival tissues. In vitro, mast cells degranulated and released significant TNF in response to oral bacteria, and neutralizing TNF in vivo abrogated alveolar bone loss following P. gingivalis infection. These data indicate that mast cells and TNF contribute to the immunopathogenesis of periodontitis and may offer therapeutic targets.

Lymphocytes Protect Cortical Neurons Against Excitotoxicity Mediated by Kainic Acid, an in vitro Model for Neurodegeneration

Background Neurodegenerative disease is a progressive loss of neurons from the central nervous system (CNS). Various conditions have been implicated for such conditions including ageing, inflammation, stress and genetic predisposition. Recently, studies have linked neurodegeneration with inflammation. Some studies have suggested the harmful effect of immune response while others have argued its neuroprotective role in neurodegeneration of the CNS. However, the precise role of inflammation and immune cells in such condition is still not clear. Objective To investigate the role of lymphocytes in neurodegeneration of the CNS and determine the underlying mechanism. Method We have used 4-7 days old mouse pups (C57Bl6) to prepare organotypic slice cultures which were cultured for 13-15 days prior to experiment. To induced cell death kainic acid was used and considered as an in vitro model for neurodegeneration. Lymphocytes were obtained from peripheral lymph nodes of 5-10 weeks old adult mouse which were used in the current study. Propidium iodide was used as a fluorescent dye to determine cell death in brain slice cultures. Result Lymphocytes do not induce cell death in slice cultures in the absence of any toxic insult whereas, after applying toxic insult to the slice cultures using kainic acid, lymphocytes show neuroprotection against such insult. Similarly, purified non-activated and purified activated T cells along with T cells depleted lymphocyte preparation also exhibit neuroprotection against kainic acid-induced cell death. We further, have demonstrated that the observed neuroprotection is contact-independent and soluble mediators released from lymphocytes are responsible for the observed neuroprotection. Moreover, our study has revealed that soluble mediators exhibiting neuroprotection act via astrocytes. Conclusion Lymphocyte preparations are neuroprotective and the observed neuroprotection is contact-independent. Soluble mediators released from lymphocytes are responsible for the observed neuroprotection. DOI: http://dx.doi.org/10.3126/kumj.v11i2.12488 Kathmandu University Medical Journal Vol.11(2) 2013: 132-138

Is central nervous system an immune-privileged site?

The central nervous system (CNS) was once considered to be an immune-privileged area. However, increasing evidence shows that the central nervous system is not an immune-privileged but is an active surveillance site. There is a bi-directional communication between the central nervous system and immune system. Normally, immune cells migrate into the central nervous system microenvironment through choroid plexus and interact with the central nervous system resident cells through either through neuromediators or immunomediators. This finding has led to a significant interest in neuroimmunological interactions and investigation onto the role of the immune system in the pathology of various neurological disorders and examine whether it can be targeted to produce novel therapeutic strategies.