Pseudorotaxane strategies for guiding self-assembly and the application of molecular machinery in photoelectrochemical devices

Over the years, chemists have become masters of covalent bond formation, demonstrated by the ability to synthesize complicated natural products and their derivatives. Conversely, control of structures beyond the molecule has not yet reached the same level of sophistication. Supramolecular organization is ubiquitous in biology and non-covalent interactions are extensively applied to (pre-)organize the individual compositional elements to achieve optimal operation as emergent functionality. Understanding (supra)molecular organization could enable the creation of novel types chemical systems with new functions. This thesis aims to explore the use of supramolecular organization by using pseudorotaxane strategies to create functional chemical systems. As both fundamental understanding and application are investigated in this thesis, the work is divided into two parts. Part A demonstrates two examples of pathway engineering for non-covalent synthesis by employing a pseudorotaxane strategy. In one example the ring is used as a catalyst to guide self-assembly, while in the second example the ring binding impedes on the possible outcomes of multi-ligand architectures, and thereby organizing the self-assembled structures. In part B the central topic is supramolecular organization to improve charge separation in artificial photosynthesis. Photoelectrochemical devices benefit from implementing supramolecular organization promoting the forward electron propagation, eventually leading to enhanced device performance. Using this bio-inspired approach, three different types of molecular machines were designed and applied to address electron-hole recombination issues found in p-DSSCs. In both parts of the thesis, the power of the non-covalent bond is demonstrated, gaining insight in possible applications and underlying concepts of chemistry beyond the molecule

Reionization and feedback in overdense regions at high redshift

Observations of galaxy luminosity function at high redshifts typically focus on fields of view of limited sizes preferentially containing bright sources. These regions possibly are overdense and hence biased with respect to the globally averaged regions. Using a semi-analytic model based on Choudhury & Ferrara (2006) which is calibrated to match a wide range of observations, we study the reionization and thermal history of the universe in overdense regions. The main results of our calculation are: (i) Reionization and thermal histories in the biased regions are markedly different from the average ones because of enhanced number of sources and higher radiative feedback. (ii) The galaxy luminosity function for biased regions is markedly different from those corresponding to average ones. In particular, the effect of radiative feedback arising from cosmic reionization is visible at much brighter luminosities. (iii) Because of the enhanced radiative feedback within overdense locations, the luminosity function in such regions is more sensitive to reionization history than in average regions. The effect of feedback is visible for absolute AB magnitude $M_{AB} \gtrsim -17$ at $z=8$, almost within the reach of present day observations and surely to be probed by JWST. This could possibly serve as an additional probe of radiative feedback and hence reionization at high redshifts.Comment: 10 pages, 5 figures. Added Section 2.5, Figure 5, and an Appendix. This version accepted for publication in MNRA

The Taiwan ECDFS Near-Infrared Survey: Very Bright End of the Luminosity Function at z>7

The primary goal of the Taiwan ECDFS Near-Infrared Survey (TENIS) is to find well screened galaxy candidates at z>7 (z' dropout) in the Extended Chandra Deep Field-South (ECDFS). To this end, TENIS provides relatively deep J and Ks data (~25.3 ABmag, 5-sigma) for an area of 0.5*0.5 degree. Leveraged with existing data at mid-infrared to optical wavelengths, this allows us to screen for the most luminous high-z objects, which are rare and thus require a survey over a large field to be found. We introduce new color selection criteria to select a z>7 sample with minimal contaminations from low-z galaxies and Galactic cool stars; to reduce confusion in the relatively low angular resolution IRAC images, we introduce a novel deconvolution method to measure the IRAC fluxes of individual sources. Illustrating perhaps the effectiveness at which we screen out interlopers, we find only one z>7 candidate, TENIS-ZD1. The candidate has a weighted z_phot of 7.8, and its colors and luminosity indicate a young (45M years old) starburst galaxy with a stellar mass of 3.2*10^10 M_sun. The result matches with the observational luminosity function analysis and the semi-analytic simulation result based on the Millennium Simulations, which may over predict the volume density for high-z massive galaxies. The existence of TENIS-ZD1, if confirmed spectroscopically to be at z>7, therefore poses a challenge to current theoretical models for how so much mass can accumulate in a galaxy at such a high redshift.Comment: 14 pages, 11 figures, ApJ accepte

Newly Discovered Bright z~9-10 Galaxies and Improved Constraints on Their Prevalence Using the Full CANDELS Area

We report the results of an expanded search for z~9-10 candidates over the ~883 arcmin^2 CANDELS+ERS fields. This study adds 147 arcmin^2 to the search area we consider over the CANDELS COSMOS, UDS, and EGS fields, while expanding our selection to include sources with bluer J_{125}-H_{160} colors than our previous J_{125}-H_{160}>0.5 mag selection. In searching for new z~9-10 candidates, we make full use of all available HST, Spitzer/IRAC, and ground-based imaging data. As a result of our expanded search and use of broader color criteria, 3 new candidate z~9-10 galaxies are identified. We also find again the z=8.683 source previously confirmed by Zitrin+2015. This brings our sample of probable z~9-11 galaxy candidates over the CANDELS+ERS fields to 19 sources in total, equivalent to 1 candidate per 47 arcmin^2 (1 per 10 WFC3/IR fields). To be comprehensive, we also discuss 28 mostly lower likelihood z~9-10 candidates, including some sources that seem to be reliably at z>8 using the HST+IRAC data alone, but which the ground-based data show are much more likely at z<4. One case example is a bright z~9.4 candidate COS910-8 which seems instead to be at z~2. Based on this expanded sample, we obtain a more robust LF at z~9 and improved constraints on the volume density of bright z~9 and z~10 galaxies. Our improved z~9-10 results again reinforce previous findings for strong evolution in the UV LF at z>8, with a factor of ~10 evolution seen in the luminosity density from z~10 to z~8.Comment: 22 pages, 12 figures, 6 tables, accepted for publication in the Astrophysical Journa

The Average Physical Properties and Star Formation Histories of the UV-Brightest Star-Forming Galaxies at z~3.7

[Abridged] We investigate the average physical properties and star formation histories of the most UV-luminous star-forming galaxies at z~3.7. Our results are derived from analyses of the average spectral energy distributions (SEDs), constructed from stacked optical to infrared photometry, of a sample of the 1,902 most UV-luminous star-forming galaxies found in 5.3 square degrees of the NOAO Deep Wide-Field Survey. We bin the sample according to UV luminosity, and find that the shape of the average SED in the rest-frame optical and infrared is fairly constant with UV luminosity: i.e., more UV luminous galaxies are, on average, also more luminous at longer wavelengths. In the rest-UV, however, the spectral slope (measured at 0.13-0.28 um) rises steeply with the median UV luminosity from -1.8 at L L* to -1.2 in the brightest bin (L~4-5L*). We use population synthesis analyses to derive the average physical properties of these galaxies and find that: (1) L_UV, and thus star formation rates (SFRs), scale closely with stellar mass such that more UV-luminous galaxies are also more massive; (2) The median ages indicate that the stellar populations are relatively young (200-400 Myr) and show little correlation with UV luminosity; and (3) More UV-luminous galaxies are dustier than their less-luminous counterparts, such that L~4-5L* galaxies are extincted up to A(1600)=2 mag while L L* galaxies have A(1600)=0.7-1.5 mag. Based on these observations, we argue that the average star formation histories of UV-luminous galaxies are better described by models in which SFR increases with time in order to simultaneously reproduce the tight correlation between the observed SFR and stellar mass, and the universally young ages of these galaxies. We demonstrate the potential of measurements of the SFR-M* relation at multiple redshifts to discriminate between simple models of star formation histories.Comment: 14 pages, 7 figures. Accepted for publication in Astrophysical Journa

Free-form lens model and mass estimation of the high redshift galaxy cluster ACT-CL J0102-4915, "El Gordo"

We examine the massive colliding cluster El Gordo, one of the most massive clusters at high redshift. We use a free-form lensing reconstruction method that avoids making assumptions about the mass distribution. We use data from the RELICS program and identify new multiply lensed system candidates. The new set of constraints and free-form method provides a new independent mass estimate of this intriguing colliding cluster. Our results are found to be consistent with earlier parametric models, indirectly confirming the assumptions made in earlier work. By fitting a double gNFW profile to the lens model, and extrapolating to the virial radius, we infer a total mass for the cluster of $M_{200c}=(1.08^{+0.65}_{-0.12})\times10^{15}$M$_{\odot}$. We estimate the uncertainty in the mass due to errors in the photometric redshifts, and discuss the uncertainty in the inferred virial mass due to the extrapolation from the lens model. We also find in our lens map a mass overdensity corresponding to the large cometary tail of hot gas, reinforcing its interpretation as a large tidal feature predicted by hydrodynamical simulations that mimic El Gordo. Finally, we discuss the observed relation between the plasma and the mass map, finding that the peak in the projected mass map may be associated with a large concentration of colder gas, exhibiting possible star formation. El Gordo is one of the first clusters that will be observed with JWST, which is expected to unveil new high redshift lensed galaxies around this interesting cluster, and provide a more accurate estimation of its mass.Comment: 19 pages, 10 figures. Updated figure

Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing Galaxies Survey: A Candidate Protocluster at Redshift z~8

Theoretical and numerical modeling of dark-matter halo assembly predicts that the most luminous galaxies at high redshift are surrounded by overdensities of fainter companions. We test this prediction with HST observations acquired by our Brightest of Reionizing Galaxies (BoRG) survey, which identified four very bright z~8 candidates as Y-dropout sources in four of the 23 non-contiguous WFC3 fields observed. We extend here the search for Y-dropouts to fainter luminosities (M_* galaxies with M_AB\sim-20), with detections at >5sigma confidence (compared to >8sigma confidence adopted earlier) identifying 17 new candidates. We demonstrate that there is a correlation between number counts of faint and bright Y-dropouts at >99.84% confidence. Field BoRG58, which contains the best bright z\sim8 candidate (M_AB=-21.3), has the most significant overdensity of faint Y-dropouts. Four new sources are located within 70arcsec (corresponding to 3.1 comoving Mpc at z=8) from the previously known brighter z\sim8 candidate. The overdensity of Y-dropouts in this field has a physical origin to high confidence (p>99.975%), independent of completeness and contamination rate of the Y-dropout selection. We modeled the overdensity by means of cosmological simulations and estimate that the principal dark matter halo has mass M_h\sim(4-7)x10^11Msun (\sim5sigma density peak) and is surrounded by several M_h\sim10^11Msun halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M_h>2x10^14Msun galaxy cluster by z=0. Follow-up observations with ground and space based telescopes are required to secure the z\sim8 nature of the overdensity, discover new members, and measure their precise redshift.Comment: Minor revision: ApJ accepted [17 pages (emulateapj style), 7 figures, 2 tables