Supermolecular-Chromophore-Sensitized Near-Infrared-to-Visible Photon Upconversion

Selective near-IR (NIR) excitation (780 nm) of the conjugated supermolecule ruthenium(II) [15-(4?-ethynyl-(2,2?;6?,2??-terpyridinyl))-bis[(5,5?,-10,20-di(2?,6?-bis(3,3-dimethylbutoxy)phenyl)porphinato)zinc(II)]ethyne][4?-pyrrolidin-1-yl-2,2?;6?,2??-terpyridine] bis(hexafluorophosphate) (Pyr1RuPZn2) in solutions containing N,N-bis(ethylpropyl)perylene-3,4,9,10-tetracarboxylicdiimide (PDI) or tetracene gives rise to a substantial anti-Stokes energy gain (PDI, 0.70 eV; tetracene, 0.86 eV). Experimental data clearly demonstrate that this upconverted fluorescence signal is produced via Pyr1RuPZn2-sensitized triplet?triplet annihilation (TTA) photochemistry. The TTA process was confirmed by the quadratic dependence of the integrated 1PDI* emission centered at 541 nm derived from 780 nm laser excitation. The T1?Tn excited state absorption decay of Pyr1RuPZn2, monitored at 900 nm as a function of PDI concentration, revealed Stern?Volmer and bimolecular quenching constants of 10?048 M?1 and 5.9 ? 108 M?1 s?1, respectively, for the PDI triplet sensitization process. The T1?Tn PDI extinction coefficient at 560 nm (εT = 6.6 ? 104 M?1 cm?1) was determined through the triplet energy transfer method utilizing anthracene as the donor chromophore. 3PDI* transient triplet absorption dynamics observed as a function of 485 nm incident nanosecond pump laser fluence demonstrate a bimolecular 3PDI*?3PDI* TTA rate constant (kTT = 1.0 ± 0.2 ? 109 M?1 s?1). The maximum quantum yield of the supermolecule-sensitized PDI upconverted emission (ΦUC = 0.0075 ± 0.0002) was determined relative to [Os(phen)3][PF6]2 at an incident laser power of 22 mW at 780 nm. This study successfully demonstrates NIR-to-visible photon upconversion and achieves a new record anti-Stokes shift of 0.86 eV for sensitized TTA, using the supermolecular Pyr1RuPZn2sensitizer. The stability of the Pyr1RuPZn2/PDI chromophore combination is readily apparent as continuous irradiation at 780 nm produces 541 nm centered fluorescence with no significant decrease in intensity measured over time domains exceeding several hours. The molecular components of these NIR-to-vis upconverting compositions illustrate that substantial anti-Stokes energy gains via a TTA process can be effortlessly realized

Self-Assembly of 9,10-Bis(phenylethynyl) anthracene (BPEA) Derivatives: Influence of pi-pi and Hydrogen Bonding Interactions on Aggregate Morphology and Self-Assembly Mechanism

9,10-Bis(phenylethynyl)anthracenes (BPEAs) are an important class of dyes with various applications including chemiluminescence emitters, materials for photon upconversion and for optoelectronic devices. Some of these applications require control over the packing modes of the active molecules within the active layer, which can be effected by bottom-up self-assembly. Studies aimed at controlling the molecular organization of BPEAs have primarily focused on bulk or liquid crystal materials, while in-depth investigations of BPEA-based assemblies in solution remain elusive. In this article, we report the self-assembly of two new BPEA derivatives with hydrophobic side chains, one of them featuring amide functional groups (2) and the other one lacking them (1). Comparison of the self-assembly behaviour in solution of both systems via spectroscopic (UV/Vis, fluorescence and NMR), microscopic (AFM) and theoretical (PM6) studies reveals the crucial role of the amide groups in controlling the self-assembly. While for both systems the formation of H-type face-to-face -stacks is proposed, the interplay of -stacking and H-bonding is responsible of driving the formation of 1D stacks and increasing the binding constant two-to-three orders of magnitude. Our findings show that H-bonding is a prerequisite to create ordered BPEA assemblies in solution

Strongly exchange-coupled triplet pairs in an organic semiconductor

From biological complexes to devices based on organic semiconductors, spin interactions play a key role in the function of molecular systems. For instance, triplet-pair reactions impact operation of organic light-emitting diodes as well as photovoltaic devices. Conventional models for triplet pairs assume they interact only weakly. Here, using electron spin resonance, we observe long-lived, strongly-interacting triplet pairs in an organic semiconductor, generated via singlet fission. Using coherent spin-manipulation of these two-triplet states, we identify exchange-coupled (spin-2) quintet complexes co-existing with weakly coupled (spin-1) triplets. We measure strongly coupled pairs with a lifetime approaching 3 µs and a spin coherence time approaching 1 µs, at 10 K. Our results pave the way for the utilization of high-spin systems in organic semiconductors.Gates-Cambridge Trust, Winton Programme for the Physics of Sustainability, Freie Universität Berlin within the Excellence Initiative of the German Research Foundation, Engineering and Physical Sciences Research Council (Grant ID: EP/G060738/1)This is the author accepted manuscript. The final version is available from Nature Publishing Group at http://dx.doi.org/10.1038/nphys3908

Manipulating molecules with strong coupling: harvesting triplet excitons in organic exciton microcavities

Exciton-polaritons are quasiparticles with mixed photon and exciton character that demonstrate rich quantum phenomena, novel optoelectronic devices and the potential to modify chemical properties of materials. Organic semiconductors are of current interest for their room-temperature polariton formation. However, within organic optoelectronic devices, it is often the 'dark' spin-1 triplet excitons that dominate operation. These triplets have been largely ignored in treatments of polariton physics. Here we demonstrate polariton population from the triplet manifold via triplet-triplet annihilation, leading to polariton emission that is longer-lived (>microseconds) even than exciton emission in bare films. This enhancement arises from spin-2 triplet-pair states, formed by singlet fission or triplet-triplet annihilation, feeding the polariton. This is possible due to state mixing, which -in the strong coupling regime- leads to sharing of photonic character with states that are formally non-emissive. Such 'photonic sharing' offers the enticing possibility of harvesting or manipulating even states that are formally dark

Towards Efficient Spectral Converters through Materials Design for Luminescent Solar Devices.

Single-junction photovoltaic devices exhibit a bottleneck in their efficiency due to incomplete or inefficient harvesting of photons in the low- or high-energy regions of the solar spectrum. Spectral converters can be used to convert solar photons into energies that are more effectively captured by the photovoltaic device through a photoluminescence process. Here, recent advances in the fields of luminescent solar concentration, luminescent downshifting, and upconversion are discussed. The focus is specifically on the role that materials science has to play in overcoming barriers in the optical performance in all spectral converters and on their successful integration with both established (e.g., c-Si, GaAs) and emerging (perovskite, organic, dye-sensitized) cell types. Current challenges and emerging research directions, which need to be addressed for the development of next-generation luminescent solar devices, are also discussed.This work was supported by the Science Foundation Ireland under Grant No. 12/IP/1608