The absorption spectrum of polythiophene and its derivative poly͑3-hexylthiophene͒ ͑P3HT͒ is usually described in terms of an intrachain exciton coupled to a single phonon mode. We show that this model is too simplistic for highly ordered, regioregular P3HT and that, analogous to the case of charged polarons in this material, interchain interactions must be taken into account to correctly describe the absorption spectrum. We show that the lowest energy feature in the-* region of the absorption spectrum is associated with an interchain absorption, the intensity of which is correlated with the degree of order in the polymer. Correspondingly, we show that the emission from P3HT also exhibits contributions from both interchain and intrachain states, in a manner similar to that recently shown for poly͑phenylenevinylene͒. Having reinterpreted the physical origin of the features in the absorption and emission spectra of P3HT, we then model these spectra and show how they evolve as the degree of order in the polymer is changed by varying several physical parameters including temperature and regioregularity of the polymer.

The energy gap law established for aromatic hydrocarbons and rare earth ions relates the nonradiative decay rate to the energy gap of a transition through a multiphonon emission process. We show that this energy gap law can be applied to the phosphoresce of a series of conjugated polymers and monomers for which the radiative decay rate has been enhanced through incorporation of a heavy metal. We find that the nonradiative decay rate from the triplet state T(1) increases exponentially with decreasing T(1)-S(0) gap for the polymers and monomers at 300 and 20 K. Comparison of the nonradiative decay of polymers with that of their corresponding monomers highlights the role of electron-lattice coupling.

The efficiency of light-emitting diodes (LEDs) made from organic semiconductors is determined by the fraction of injected electrons and holes that recombine to form emissive spin-singlet states rather than non-emissive spin-triplet states. If the process by which these states form is spin-independent, the maximum efficiency of organic LEDs will be limited to 25 per cent. But recent reports have indicated fractions of emissive singlet states ranging from 22 to 63 per cent, and the reason for this variation remains unclear. Here we determine the absolute fraction of singlet states generated in a platinum-containing conjugated polymer and its corresponding monomer. The spin-orbit coupling introduced by the platinum atom allows triplet-state emission, so optically and electrically generated luminescence from both singlet and triplet states can be compared directly. We find an average singlet generation fraction of 22 +/- 1 per cent for the monomer, but 57 +/- 4 per cent for the polymer. This suggests that recombination is spin-independent for the monomer, but that a spin-dependent process, favouring singlet formation, is effective in the polymer. We suggest that this process is a consequence of the exchange interaction, which will operate on overlapping electron and hole wavefunctions on the same polymer chain at their capture radius.

Control of intermolecular interactions is crucial to the exploitation of molecular semiconductors for both organic electronics and the viable manipulation and incorporation of single molecules into nano-engineered devices. Here we explore the properties of a class of materials that are engineered at a supramolecular level by threading a conjugated macromolecule, such as poly(para-phenylene), poly(4,4'-diphenylene vinylene) or polyfluorene through alpha- or beta-cyclodextrin rings, so as to reduce intermolecular interactions and solid-state packing effects that red-shift and partially quench the luminescence. Our approach preserves the fundamental semiconducting properties of the conjugated wires, and is effective at both increasing the photoluminescence efficiency and blue-shifting the emission of the conjugated cores, in the solid state, while still allowing charge-transport. We used the polymers to prepare single-layer light-emitting diodes with Ca and Al cathodes, and observed blue and green emission. The reduced tendency for polymer chains to aggregate allows solution-processing of individual polyrotaxane wires onto substrates, as revealed by scanning force microscopy.

Progress reports are a new type of article in Advanced Materials, dealing with the hottest current topics, and providing readers with a critically selected overview of important progress in these fields. It is not intended that the articles be comprehensive, but rather insightful, selective, critical, opinionated, and even visionary. We have approached scientists we believe are at the very forefront of these fields to contribute the articles, which will appear on an annual basis. The article below describes the latest advances in fluorescence and phosphorescence in organic materials.

Articles you may be interested inDistinguishing triplet energy transfer and trap-assisted recombination in multi-color organic light-emitting diode with an ultrathin phosphorescent emissive layer Wave function instabilities in the cis-trans isomerization and singlet-triplet energy gaps in a push-pull compound J. Chem. Phys. 119, 4112 (2003); 10.1063/1.1592492 Long-range energy transfer of singlet and triplet excitations in dye-doped tris(phenylquinoxaline)We have studied the evolution of the T 1 triplet excited state in an extensive series of phenylene ethynylene polymers and monomers with platinum atoms in the polymer backbone and in an analogous series of all-organic polymers with the platinum͑II͒ tributylphosphonium complex replaced by phenylene. The inclusion of platinum increases spin-orbit coupling so T 1 state emission ͑phosphorescence͒ is easier to detect. For both, the platinum-containing polymer series and for the all-organic polymers, we find the T 1 state to be at a constant separation of 0.7Ϯ0.1 eV below the singlet S 1 state. It is not possible to change this singlet-triplet splitting by altering the size or the charge-transfer character of the polymer repeat unit or by changing the electron delocalization along the polymer backbone. The S 1 -T 1 gap can be increased by confining the S 1 state in oligomers and monomers.

By use of optical steady state and time resolved spectroscopy, we studied the evolution of the triplet excited state in a series of six ethynylenic polymers of the structure ͓-Pt(PBu 3 n) 2-CwC-R-CwC-͔ n where the spacer unit R is systematically varied to give optical gaps from 1.7-3.0 eV. The inclusion of platinum in the polymer backbone induces a strong spin-orbit coupling such that triplet state emission ͑phosphorescence͒ associated with the conjugated system can be detected. Throughout the series we find the S 1-T 1 ͑singlet-triplet͒ energy splitting to be independent of the spacer R, such that the T 1 state is always 0.7Ϯ0.1 eV below the S 1 state. With decreasing optical gap, the intensity and lifetime of the triplet state emission were seen to reduce in accordance with the energy gap law.

Pharmacologically active compounds with preferential cytotoxic activity for senescent cells, known as senolytics, can ameliorate or even revert pathological manifestations of senescence in numerous preclinical mouse disease models, including cancer models. However, translation of senolytic therapies to human disease is hampered by their suboptimal specificity for senescent cells and important toxicities that narrow their therapeutic windows. We have previously shown that the high levels of senescence-associated lysosomal β-galactosidase (SA-β-gal) found within senescent cells can be exploited to specifically release tracers and cytotoxic cargoes from galactose-encapsulated nanoparticles within these cells. Here, we show that galacto-conjugation of the BCL-2 family inhibitor Navitoclax results in a potent senolytic prodrug (Nav-Gal), that can be preferentially activated by SA-β-gal activity in a wide range of cell types. Nav-Gal selectively induces senescent cell apoptosis and has a higher senolytic index than Navitoclax (through reduced activation in nonsenescent cells).Nav-Gal enhances the cytotoxicity of standard senescence-inducing chemotherapy (cisplatin) in human A549 lung cancer cells. Concomitant treatment with cisplatin and Nav-Gal in vivo results in the eradication of senescent lung cancer cells and significantly reduces tumour growth. Importantly, galacto-conjugation reduces Navitoclaxinduced platelet apoptosis in human and murine blood samples treated ex vivo, and thrombocytopenia at therapeutically effective concentrations in murine lung cancer models. Taken together, we provide a potentially versatile strategy for generating effective senolytic prodrugs with reduced toxicities.