The photoprotection mechanism in the black-brown pigment eumelanin.

The natural black-brown pigment eumelanin protects humans from high-energy UV photons by absorbing and rapidly dissipating their energy before proteins and DNA are damaged. The extremely weak fluorescence of eumelanin points toward nonradiative relaxation on the timescale of picoseconds or shorter. However, the extreme chemical and physical complexity of eumelanin masks its photoprotection mechanism. We sought to determine the electronic and structural relaxation pathways in eumelanin using three complementary ultrafast optical spectroscopy methods: fluorescence, transient absorption, and stimulated Raman spectroscopies. We show that photoexcitation of chromophores across the UV-visible spectrum rapidly generates a distribution of visible excitation energies via ultrafast internal conversion among neighboring coupled chromophores, and then all these excitations relax on a timescale of ∼4 ps without transferring their energy to other chromophores. Moreover, these picosecond dynamics are shared by the monomeric building block, 5,6-dihydroxyindole-2-carboxylic acid. Through a series of solvent and pH-dependent measurements complemented by quantum chemical modeling, we show that these ultrafast dynamics are consistent with the partial excited-state proton transfer from the catechol hydroxy groups to the solvent. The use of this multispectroscopic approach allows the minimal functional unit in eumelanin and the role of exciton coupling and excited-state proton transfer to be determined, and ultimately reveals the mechanism of photoprotection in eumelanin. This knowledge has potential for use in the design of new soft optical components and organic sunscreens.

Triplets with a Twist: Ultrafast Intersystem Crossing in a Series of Electron Acceptor Materials Driven by Conformational Disorder.

Control over the populations of singlet and triplet excitons is key to organic semiconductor technologies. In different contexts, triplets can represent an energy loss pathway that must be managed (i.e., solar cells, light-emitting diodes, and lasers) or provide avenues to improve energy conversion (i.e., photon upconversion and multiplication systems). A key consideration in the interplay of singlet and triplet exciton populations in these systems is the rate of intersystem crossing (ISC). In this work, we design, measure, and model a series of new electron acceptor molecules and analyze them using a combination of ultrafast transient absorption and ultrafast broadband photoluminescence spectroscopies. We demonstrate that intramolecular triplet formation occurs within several hundred picoseconds in solution and is accelerated considerably in the solid state. Importantly, ISC occurs with sufficient rapidity to compete with charge formation in modern organic solar cells, implicating triplets in intrinsic exciton loss channels in addition to charge recombination. Density functional theory calculations reveal that ISC occurs in triplet excited states characterized by local deviations from orbital π-symmetry associated with rotationally flexible thiophene rings. In disordered films, structural distortions, therefore, result in significant increases in spin-orbit coupling, enabling rapid ISC. We demonstrate the generality of this proposal in an oligothiophene model system where ISC is symmetry-forbidden and show that conformational disorder introduced by the formation of a solvent glass accelerates ISC, outweighing the lower temperature and increased viscosity. This proposal sheds light on the factors responsible for facile ISC and provides a simple framework for molecular control over spin states.

Tuneable emission in single molecule dyads mediated by a charge transfer state.

The demand for fluorescent organic dyes across a broad range of applications has led to investigation into tuneable emission dyes. The tuneable nature of these dyes makes them desirable for applications in a variety of fields, including organic light-emitting diodes (OLEDs), optical sensing devices, and fluorescence imaging. In recent investigations, there have only been a handful of mechanisms used to tune emission. Herein, we present four novel perylene-acene dyads that undergo solvent tuneable emission, and propose a novel mechanism for this tuneability based on the presence of a charge transfer state. These dyes were shown to reach photoluminescence quantum efficiencies (PLQEs) as high as 45%, depending on the solvent, showing the ability for this mechanism to be used to access higher PLQE tuneable emission.

Free charge photogeneration in a single component high photovoltaic efficiency organic semiconductor.

Organic photovoltaics (OPVs) promise cheap and flexible solar energy. Whereas light generates free charges in silicon photovoltaics, excitons are normally formed in organic semiconductors due to their low dielectric constants, and require molecular heterojunctions to split into charges. Recent record efficiency OPVs utilise the small molecule, Y6, and its analogues, which - unlike previous organic semiconductors - have low band-gaps and high dielectric constants. We show that, in Y6 films, these factors lead to intrinsic free charge generation without a heterojunction. Intensity-dependent spectroscopy reveals that 60-90% of excitons form free charges at AM1.5 light intensity. Bimolecular recombination, and hole traps constrain single component Y6 photovoltaics to low efficiencies, but recombination is reduced by small quantities of donor. Quantum-chemical calculations reveal strong coupling between exciton and CT states, and an intermolecular polarisation pattern that drives exciton dissociation. Our results challenge how current OPVs operate, and renew the possibility of efficient single-component OPVs.

Simultaneously Enhancing Exciton/Charge Transport in Organic Solar Cells by an Organoboron Additive.

Efficient exciton diffusion and charge transport play a vital role in advancing the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, a facile strategy is presented to simultaneously enhance exciton/charge transport of the widely studied PM6:Y6-based OSCs by employing highly emissive trans-bis(dimesitylboron)stilbene (BBS) as a solid additive. BBS transforms the emissive sites from a more H-type aggregate into a more J-type aggregate, which benefits the resonance energy transfer for PM6 exciton diffusion and energy transfer from PM6 to Y6. Transient gated photoluminescence spectroscopy measurements indicate that addition of BBS improves the exciton diffusion coefficient of PM6 and the dissociation of PM6 excitons in the PM6:Y6:BBS film. Transient absorption spectroscopy measurements confirm faster charge generation in PM6:Y6:BBS. Moreover, BBS helps improve Y6 crystallization, and current-sensing atomic force microscopy characterization reveals an improved charge-carrier diffusion length in PM6:Y6:BBS. Owing to the enhanced exciton diffusion, exciton dissociation, charge generation, and charge transport, as well as reduced charge recombination and energy loss, a higher PCE of 17.6% with simultaneously improved open-circuit voltage, short-circuit current density, and fill factor is achieved for the PM6:Y6:BBS devices compared to the devices without BBS (16.2%).

Adaptive optics flood-illumination camera for high speed retinal imaging.

Current adaptive optics flood-illumination retina cameras operate at low frame rates, acquiring retinal images below seven Hz, which restricts their research and clinical utility. Here we investigate a novel bench top flood-illumination camera that achieves significantly higher frame rates using strobing fiber-coupled superluminescent and laser diodes in conjunction with a scientific-grade CCD. Source strength was sufficient to obviate frame averaging, even for exposures as short as 1/3 msec. Continuous frame rates of 10, 30, and 60 Hz were achieved for imaging 1.8,0.8, and 0.4 deg retinal patches, respectively. Short-burst imaging up to 500 Hz was also achieved by temporarily storing sequences of images on the CCD. High frame rates, short exposure durations (1 msec), and correction of the most significant aberrations of the eye were found necessary for individuating retinal blood cells and directly measuring cellular flow in capillaries. Cone videos of dark adapted eyes showed a surprisingly rapid fluctuation (~1 Hz) in the reflectance of single cones. As further demonstration of the value of the camera, we evaluated the tradeoff between exposure duration and image blur associated with retina motion.

The UMLS Knowledge Source Server: an experience in Web 2.0 technologies.

The UMLS Knowledge Source Server (UMLSKS), developed at the National Library of Medicine (NLM), makes the knowledge sources of the Unified Medical Language System (UMLS) available to the research community over the Internet. In 2003, the UMLSKS was redesigned utilizing state-of-the-art technologies available at that time. That design offered a significant improvement over the prior version but presented a set of technology-dependent issues that limited its functionality and usability. Four areas of desired improvement were identified: software interfaces, web interface content, system maintenance/deployment, and user authentication. By employing next generation web technologies, newer authentication paradigms and further refinements in modular design methods, these areas could be addressed and corrected to meet the ever increasing needs of UMLSKS developers. In this paper we detail the issues present with the existing system and describe the new system's design using new technologies considered entrants in the Web 2.0 development era.

Plug-and-play UMLS knowledge source server using web services and portlets.

The UMLS is large and complex and presents significant challenges in retrieving information in a comprehensive way. Typical users of the UMLSKS (UMLS Knowledge Source Server) run the gamut in terms of knowledge of the UMLS and retrieval needs from the UMLSKS. Even though we as developers of the UMLSKS have always strived to create a truly modular and flexible system, we felt that the available technology until recently was not mature enough to create a loosely coupled, plug-and-play UMLSKS. In the past year we became convinced that web services and portal technology had sufficiently matured to allow us to develop such a system. In this poster, we present the details of the new plug-and-play UMLSKS that has been in development for about a year.

The UMLS knowledge source server: an object model for delivering UMLS data.

The Unified Medical Language System (UMLS), a project of the National Library of Medicine (NLM), regularly distributes a set of knowledge sources to the research community. These data are made available over the Internet through the UMLS Knowledge Source Server (UMLSKS). The new version of the UMLSKS is a complete redesign of the original system using Java and the Extensible Markup Language (XML) technologies to implement a fast, reliable, flexible, and extensible UMLS data retrieval system that includes an Application Programmer's Interface (API) and an Object Model of each of the Knowledge Sources: the UMLS Metathesaurus, the Semantic Network, and the SPECIALIST Lexicon. In this paper we present the design of the new system, outline each of the system design goals, the UMLS Object Model, and statistics showing the usage of the new UMLSKS and associated data. We conclude with implications for future work.