The K2 Mission: Characterization and Early results

The K2 mission will make use of the Kepler spacecraft and its assets to expand upon Kepler's groundbreaking discoveries in the fields of exoplanets and astrophysics through new and exciting observations. K2 will use an innovative way of operating the spacecraft to observe target fields along the ecliptic for the next 2-3 years. Early science commissioning observations have shown an estimated photometric precision near 400 ppm in a single 30 minute observation, and a 6-hour photometric precision of 80 ppm (both at V=12). The K2 mission offers long-term, simultaneous optical observation of thousands of objects at a precision far better than is achievable from ground-based telescopes. Ecliptic fields will be observed for approximately 75-days enabling a unique exoplanet survey which fills the gaps in duration and sensitivity between the Kepler and TESS missions, and offers pre-launch exoplanet target identification for JWST transit spectroscopy. Astrophysics observations with K2 will include studies of young open clusters, bright stars, galaxies, supernovae, and asteroseismology.Comment: 25 pages, 11 figures, Accepted to PAS

Characterizing Mode Anharmonicity and Huang–Rhys Factors Using Models of Femtosecond Coherence Spectra

Femtosecond laser pulses readily produce coherent quantum beats in transient–absorption spectra. These oscillatory signals often arise from molecular vibrations and therefore may contain information about the excited-state potential energy surface near the Franck–Condon region. Here, by fitting the measured spectra of two laser dyes to microscopic models of femtosecond coherence spectra (FCS) arising from molecular vibrations, we classify coherent quantum-beat signals as fundamentals or overtones and quantify their Huang–Rhys factors and anharmonicity values. We discuss the extracted Huang–Rhys factors in the context of quantum-chemical computations. This work solidifies the use of FCS for analysis of coherent quantum beats arising from molecular vibrations, which will aid studies of molecular aggregates and photosynthetic proteins

Symmetry Breaking Charge Transfer in DNA-Templated Perylene Dimer Aggregates

Molecular aggregates are of interest to a broad range of fields including light harvesting, organic optoelectronics, and nanoscale computing. In molecular aggregates, nonradiative decay pathways may emerge that were not present in the constituent molecules. Such nonradiative decay pathways may include singlet fission, excimer relaxation, and symmetry-breaking charge transfer. Singlet fission, sometimes referred to as excitation multiplication, is of great interest to the fields of energy conversion and quantum information. For example, endothermic singlet fission, which avoids energy loss, has been observed in covalently bound, linear perylene trimers and tetramers. In this work, the electronic structure and excited-state dynamics of dimers of a perylene derivative templated using DNA were investigated. Specifically, DNA Holliday junctions were used to template the aggregation of two perylene molecules covalently linked to a modified uracil nucleobase through an ethynyl group. The perylenes were templated in the form of monomer, transverse dimer, and adjacent dimer configurations. The electronic structure of the perylene monomers and dimers were characterized via steady-state absorption and fluorescence spectroscopy. Initial insights into their excited-state dynamics were gleaned from relative fluorescence intensity measurements, which indicated that a new nonradiative decay pathway emerges in the dimers. Femtosecond visible transient absorption spectroscopy was subsequently used to elucidate the excited-state dynamics. A new excited-state absorption feature grows in on the tens of picosecond timescale in the dimers, which is attributed to the formation of perylene anions and cations resulting from symmetry-breaking charge transfer. Given the close proximity required for symmetry-breaking charge transfer, the results shed promising light on the prospect of singlet fission in DNA-templated molecular aggregates

Probing DNA Structural Heterogeneity by Identifying Conformational Subensembles of a Bicovalently Bound Cyanine Dye

DNA is a re-configurable, biological information-storage unit, and much remains to be learned about its heterogeneous structural dynamics. For example, while it is known that molecular dyes templated onto DNA exhibit increased photostability, the mechanism by which the structural dynamics of DNA affect the dye photophysics remains unknown. Here, we use femtosecond, two-dimensional electronic spectroscopy measurements of a cyanine dye, Cy5, to probe local conformations in samples of single-stranded DNA (ssDNA–Cy5), double-stranded DNA (dsDNA–Cy5), and Holliday junction DNA (HJ–DNA–Cy5). A line shape analysis of the 2D spectra reveals a strong excitation–emission correlation present in only the dsDNA–Cy5 complex, which is a signature of inhomogeneous broadening. Molecular dynamics simulations support the conclusion that this inhomogeneous broadening arises from a nearly degenerate conformer found only in the dsDNA–Cy5 complex. These insights will support future studies on DNA’s structural heterogeneity

Tuning Between Quenching and Energy Transfer in DNA-Templated Heterodimer Aggregates

Molecular excitons, which propagate spatially via electronic energy transfer, are central to numerous applications including light harvesting, organic optoelectronics, and nanoscale computing; they may also benefit applications such as photothermal therapy and photoacoustic imaging through the local generation of heat via rapid excited-state quenching. Here we show how to tune between energy transfer and quenching for heterodimers of the same pair of cyanine dyes by altering their spatial configuration on a DNA template. We assemble “transverse” and “adjacent” heterodimers of Cy5 and Cy5.5 using DNA Holliday junctions. We find that the transverse heterodimers exhibit optical properties consistent with excitonically interacting dyes and fluorescence quenching, while the adjacent heterodimers exhibit optical properties consistent with nonexcitonically interacting dyes and disproportionately large Cy5.5 emission, suggestive of energy transfer between dyes. We use transient absorption spectroscopy to show that quenching in the transverse heterodimer occurs via rapid nonradiative decay to the ground state (∼31 ps) and that in the adjacent heterodimer rapid energy transfer from Cy5 to Cy5.5 (∼420 fs) is followed by Cy5.5 excited-state relaxation (∼700 ps). Accessing such drastically different photophysics, which may be tuned on demand for different target applications, highlights the utility of DNA as a template for dye aggregation

Excited-State Lifetimes of DNA-Templated Cyanine Dimer, Trimer, and Tetramer Aggregates: The Role of Exciton Delocalization, Dye Separation, and DNA Heterogeneity

DNA-templated molecular (dye) aggregates are a novel class of materials that have garnered attention in a broad range of areas including light harvesting, sensing, and computing. Using DNA to template dye aggregation is attractive due to the relative ease with which DNA nanostructures can be assembled in solution, the diverse array of nanostructures that can be assembled, and the ability to precisely position dyes to within a few Angstroms of one another. These factors, combined with the programmability of DNA, raise the prospect of designer materials custom tailored for specific applications. Although considerable progress has been made in characterizing the optical properties and associated electronic structures of these materials, less is known about their excited-state dynamics. For example, little is known about how the excited-state lifetime, a parameter essential to many applications, is influenced by structural factors, such as the number of dyes within the aggregate and their spatial arrangement. In this work, we use a combination of transient absorption spectroscopy and global target analysis to measure excited-state lifetimes in a series of DNA-templated cyanine dye aggregates. Specifically, we investigate six distinct dimer, trimer, and tetramer aggregates—based on the ubiquitous cyanine dye Cy5—templated using both duplex and Holliday junction DNA nanostructures. We find that these DNA-templated Cy5 aggregates all exhibit significantly reduced excited-state lifetimes, some by more than 2 orders of magnitude, and observe considerable variation among the lifetimes. We attribute the reduced excited-state lifetimes to enhanced nonradiative decay and proceed to discuss various structural factors, including exciton delocalization, dye separation, and DNA heterogeneity, that may contribute to the observed reduction and variability of excited-state lifetimes. Guided by insights from structural modeling, we find that the reduced lifetimes and enhanced nonradiative decay are most strongly correlated with the distance between the dyes. These results inform potential tradeoffs between dye separation, excitonic coupling strength, and excited-state lifetime that motivate deeper mechanistic understanding, potentially via further dye and dye template design

Architecture of Kepler's Multi-transiting Systems: II. New investigations with twice as many candidates

We report on the orbital architectures of Kepler systems having multiple planet candidates identified in the analysis of data from the first six quarters of Kepler data and reported by Batalha et al. (2013). These data show 899 transiting planet candidates in 365 multiple-planet systems and provide a powerful means to study the statistical properties of planetary systems. Using a generic mass-radius relationship, we find that only two pairs of planets in these candidate systems (out of 761 pairs total) appear to be on Hill-unstable orbits, indicating ~96% of the candidate planetary systems are correctly interpreted as true systems. We find that planet pairs show little statistical preference to be near mean-motion resonances. We identify an asymmetry in the distribution of period ratios near first-order resonances (e.g., 2:1, 3:2), with an excess of planet pairs lying wide of resonance and relatively few lying narrow of resonance. Finally, based upon the transit duration ratios of adjacent planets in each system, we find that the interior planet tends to have a smaller transit impact parameter than the exterior planet does. This finding suggests that the mode of the mutual inclinations of planetary orbital planes is in the range 1.0-2.2 degrees, for the packed systems of small planets probed by these observations.Comment: Accepted to Ap

Validation of Kepler's Multiple Planet Candidates. III: Light Curve Analysis & Announcement of Hundreds of New Multi-planet Systems

The Kepler mission has discovered over 2500 exoplanet candidates in the first two years of spacecraft data, with approximately 40% of them in candidate multi-planet systems. The high rate of multiplicity combined with the low rate of identified false-positives indicates that the multiplanet systems contain very few false-positive signals due to other systems not gravitationally bound to the target star (Lissauer, J. J., et al., 2012, ApJ 750, 131). False positives in the multi- planet systems are identified and removed, leaving behind a residual population of candidate multi-planet transiting systems expected to have a false-positive rate less than 1%. We present a sample of 340 planetary systems that contain 851 planets that are validated to substantially better than the 99% confidence level; the vast majority of these have not been previously verified as planets. We expect ~2 unidentified false-positives making our sample of planet very reliable. We present fundamental planetary properties of our sample based on a comprehensive analysis of Kepler light curves and ground-based spectroscopy and high-resolution imaging. Since we do not require spectroscopy or high-resolution imaging for validation, some of our derived parameters for a planetary system may be systematically incorrect due to dilution from light due to additional stars in the photometric aperture. None the less, our result nearly doubles the number of verified exoplanets.Comment: 138 pages, 8 Figures, 5 Tables. Accepted for publications in the Astrophysical Journa

Oblique Packing and Tunable Excitonic Coupling in DNA-Templated Squaraine Rotaxane Dimer Aggregates

When molecules are aggregated such that their excited states form delocalized excitons, their spatial arrangement, or packing, can be coarsely controlled by templating and finely controlled by chemical substitution; however, challenges remain in controlling their packing on intermediate length scales. Here, we use an approach based on mechanically interlocked molecules to promote an elusive oblique packing arrangement in a series of three squaraine rotaxane dimers. We template the squaraine rotaxane dimers using DNA and observe two excitonically split bands of near-equal intensity in their absorption spectra – a distinct signature of oblique packing, validated by theoretical modeling of the experimental results. Additional fine control of packing is demonstrated by fluorinating the macrocycle of the rotaxane, which promotes denser packing and stronger excitonic interactions