RFG1 – Photoreactions and Their Mechanisms:
Project 1: Photochemical release of aldehydes and ketones
Mentor: Prof. Igor Alabugin
Cycloaromatization reactions defy common chemical logic by creating diradical species from closed shell reactants without external radical initiators. In these unusual but very useful processes, one chemical bond is always created at the expense of two sacrificed chemical bonds. The explosion of interest in cycloaromatization reactions dates back to the discovery of natural enediyne antibiotics, the closed-shell molecules that are transformed by the Bergman cycloaromatization into reactive diradical species capable of targeting and damaging cellular DNA with astounding efficiency via rapid and irreversible H atom abstraction from the sugar phosphate backbone. In 2015, we discovered the last member of the cycloaromatization reaction family, an efficient C1-C5 cyclization of enynes, by employing an innovative solution in order to overcome the possible problems associated with trapping the diradical product of the thus far elusive process. By utilizing a “self-terminating” reaction strategy, the cyclization is terminated by a stereoelectronically promoted and thermodynamically favorable carbon-carbon bond fragmentation that produced formaldehyde. Considering the biological importance of many aldehydes and ketones, we plan to expand this chemistry from release of CH2O to the photochemical release (uncaging) of a variety of more complex aldehydes and ketones. Participants in this project will use the tools of organic synthesis to prepare the starting material and then test their photochemical reactivity.
Project 2: Make it or Break it?: Using light to study the synthesis or decomposition of plastics
Mentor: Prof. Justin Kennemur
The last decade has seen large advancements towards the use of modern light sources, such as LEDs, as a means to photo-initiate chain reactions that ultimately lead to macromolecules (polymers). Certain chromophores, such as dithiocarbonates and trithiocarbonates, effectively absorb light within the visible spectrum. With the right chemistry, these species may fragment to form an active radical species that can initiate the radical polymerization of olefins in a controlled fashion. The advantage of using light over other methods, such as thermally induced initiation, is the ability to perform the polymerizations at colder temperatures where the equilibrium of polymerization favors the formation of macromolecules at high conversion. Students on this project will study the phenomenology of LED-light driven photopolymerization at cold temperatures using monomers that are stubborn to polymerize under the same conditions at higher temperatures. The research will involve the construction of an LED photo-reactor, the basic synthesis of monomers and polymers, and characterization methods needed to understand the progress of the polymerization. The latter includes NMR spectroscopy, UV-Vis spectroscopy, and size exclusion chromatography. Further strategies can also be explored to depolymerize the resulting materials back to monomeric form through the increase in temperature.
Project 3: Dissecting ultrafast chemical reactions in transition metal photocatalysts
Mentor: Prof. Bryan Kudisch
Photocatalysis has permeated its way into the methods used to synthesize fine chemicals, ranging from drugs to perfumes, due to it being a powerful entryway into organic radical reactivity combined with its relatively benign reaction conditions. While most photocatalysts employ heavy or precious metals with favorable and well-studied photochemical reactivity, the Kudisch lab uses a variety of steady-state and time-resolved spectroscopic methods to characterize a new generation of earth-abundant transition metal photocatalysts for chemical synthesis. These new photocatalysts boast surprising and potentially highly impactful photoreactivity, but with unexplained and lacking per-photon reaction efficiency (quantum yield, Ф). An understanding of this low quantum yield on a fundamental level could open the door to faster, more energy-efficient synthetic schemes employing photocatalysis, while also unveiling the fundamental nature of this interesting class of light absorbers. An undergraduate in our lab will investigate the excited-state behavior of one of these transition metal photocatalysts under photocatalytic conditions, learning skills centered on understanding photochemical dynamics on the timescale of picoseconds and how it connects to impactful photochemical reactions that can be performed on the benchtop using techniques ranging from NMR spectroscopy to ultrafast transient absorption spectroscopy.
Project 4: Trapping conical intersections in excited state reactions
Mentor: Prof. Jack Saltiel
The Photoaddition of Methanol to α-Methylstilbene. Stilbene (1,2-diphenylethene) photoisomerization is the prototypical reaction that has been used as a model for trans-cis photoisomerization in biological systems such as rhodopsin and bacteriorhodopsin. In solution the reaction proceeds in the lowest excited singlet state via torsional relaxation to a twisted intermediate. We initially proposed its structure to consist roughly of two orthogonal benzyl groups. However, recent theoretical calculations predicted that this excited intermediate returns to the ground state by approaching a ground state geometry that is pyramidal (anionic) at one of the two central carbon atoms and planar (cationic) at the other. At that geometry the ground state and excited state potential energy surfaces touch at what is known as a conical intersection (CI). Decay through the CI is expected to be very fast explaining its observed lifetime of 0.23 ps. Despite this short lifetime, we have recently shown that this intermediate can be trapped by methanol to give a methoxy ether by carbene and direct addition pathways. It was thus established that the identical CI is accessed from the cis and trans isomers of stilbene, a matter of considerable theoretical interest. In further exploring the chemistry of this CI we propose to study the photoaddition of methanol to α-methylstilbene. α-Methyl substitution forces the molecule out of planarity, breaks the symmetry of the stilbene molecule and speeds up the formation of the twisted intermediate. We propose to trap that intermediate in ether formation. In contrast to stilbene, different ethers may form by the carbene and direct pathways. The two pathways 1,2-addition vs. 1,1-addition of methanol, will be distinguished by use of deuterated methanol and gas phase/mass spectroscopic analysis.
RFG2 – Solar Energy Conversion:
Project 5: Heavy atom free sensitizers for photon upconversion via triplet-triplet annilhilation
Mentor: Prof. Kenneth Hanson
Photon upconversion—combining two or more low energy photons to generate a higher energy excited state—is an intriguing strategy for increasing the maximum theoretical solar cell efficiencies from 31% to above 43%. The Hanson research group recently introduced self-assembled bilayers as an effective architecture to facilitate photon upconversion and photocurrent generation via triplet-triplet annihilation (TTA-UC). While the devices have the highest efficiency yet achieved by directly harnessing TTA-UC they rely on a porphyrin sensitizer containing a scarce and expensive platinum heavy atom. The next step in our research progression is to develop metal ion/heavy atom free sensitizer molecules. In this project students will synthesize new purely organic sensitizer molecules with phosphonate/carboxylate surface binding groups. Once synthesized students will characterize the compound using UV-Vis spectroscopy, steady-state and time-resolved emission, and cyclic voltammetry. The dyes will then be incorporated into a self-assembled bilayer and we will measure upconverted emission (on ZrO2) and photocurrent generation (on TiO2) from the bilayer film. Through this experience REU students in the Hanson research group will become acquainted with organic synthesis as well as spectroscopic and electrochemical techniques.
Project 6: Synthesis of metal sulfides in sulfur/iodine melts
Mentor: Prof. Susan Latturner
Metal sulfides such as CuInS2, CdS, ZnS, and BiSI are of interest as solar-cell absorbers, catalysts, and photoconductor materials. The REU students carrying out research in the Latturner group will investigate the synthesis of sulfides in sulfur/iodine melts. The addition of a small amount of I2 lowers the melting point of sulfur and makes it much less viscous than the pure element. This flux can be used from 100 to 500 °C, acting as a reactive solvent toward most metals. After synthesizing parent sulfides (e.g., BiSI or CuInS2), the students will carry out additional flux reactions to make systematically substituted analogs (e.g., reacting Bi and Sb in S/I2 to make Bi1-xSbxSI and reacting Cu, In, and Ga in S/I2 to make CuIn1-xGaxS2). They will use powder X-ray diffraction to identify the products and determine unit cell parameters. The students will record optical spectra to establish the band gap of the materials. This project will give them experience in synthesis and characterization of inorganic solids. They will learn how the structure of a crystalline material can be modified to tailor its band gap/optical absorption for solar cell applications.
Project 7: Interconversion of photon wavelengths (upconversion or downconversion) is a promising strategy to increase solar cell efficiencies
Mentor: Prof. Lea Nienhaus
Photon upconversion bears promise in sub-bandgap sensitization of solar cells by combining two or more low energy photons to one high energy photon. Photon downconversion: the splitting of one high energy photon into two lower energy photons bears potential to reduce thermal losses in solar cells, and hence increase the device efficiency. The Nienhaus lab seeks to investigate the energy transfer processes relevant to photon up- and downconversion between nanostructured semiconductors such as conventional quantum dots or perovskite-based materials and organic semiconductors by both steady-state and time-resolved optical spectroscopy, as well as scanning probe techniques, such as AFM or STM.
Project 8: Assembly of quantum dots onto carbon nanotubes for hybrid photovoltaics
Mentor: Prof. Geoffrey Strouse
The Strouse group will develop inorganic-organic hybrid technology that harnesses the advances of nanoscience to allow efficient solar-energy conversion. As a step to achieving this goal, the REU students will investigate binding of metal chalcogenide quantum dots (QDs) that absorb strongly into the near-IR to multi-walled carbon nanotubes (CNTs) to act as ballistic conductors of electrons to an indium tin oxide (ITO) surface. The students will carry out controlled attachment chemistry to append QDs to CNT surfaces, using either covalent or electrostatic interactions (3 weeks). The students will characterize these hybrid systems via transmission electron microscopy and UV-Vis spectroscopy (3 weeks). They will analyze how the nature of QDs and the mode of their attachment to CNTs affect light-harvesting properties (3 weeks). If the time permits, the students will also learn to assemble solar cells with these nanocomposites and perform the current-voltage (I-V) and incident photon to current efficiency (IPCE) measurements (1 week).
RFG3 – Photoresponsive Materials:
Project 9: Modeling plasmons and plasmon-mediated excitations in molecular systems
Mentor: Prof. Eugene DePrince
The DePrince Group uses ab initio time-dependent electronic structure methods to model plasmon-mediated excitation processes in small molecules in the vicinity of plasmonic materials. The REU students will learn several popular electronic structure packages, including the Gaussian and Psi4. Cavity quantum electrodynamics (CQED) simulations of molecule-nanoparticle or nanoparticle-nanoparticle assemblies provide a qualitative description of interesting phenomena, e.g., the emergence and “reversal” of Fano-like resonances or entangled plasmon states in pairs of nanoparticles. We have developed CQED methods in Psi4 that treat the molecular component of such systems using either a simple model Hamiltonian or in a fully ab initio way. Using time-dependent Hartree-Fock methods in Gaussian or Psi4, an REU student will identify low-energy electronic excitations in atomic (e.g. sodium) chains or other simple molecules that overlap well with the plasmon resonance energy in gold or silver nanoparticles. The student will then use our ab initio CQED methods to explore the ultrafast dynamics of the coupled system as a function of system geometry and external electric field (laser) parameters. The student will analyze the time-evolution of the electric dipole moment to generate scattering and absorption cross sections for the coupled system. The student will learn how system parameters influence the coupling between the plasmonic and molecular system components, the plasmon lifetime, and nanoparticle-molecule energy transfer.
Project 10: Charge-transfer interactions in higher-order donor-acceptor complexes
Mentor: Prof. Edwin Hilinski
Charge-transfer (CT) interactions are involved in a wide range of supramolecular chemistry associated with important phenomena in biological systems and materials science. The ground- and excited-state properties of CT complexes involving one donor (D) and one acceptor (A) are generally well understood. The roles and consequences of more than one donor or more than one acceptor are less clear. The Hilinski group is studying systems that allow for investigation of ground- and excited-state properties of homogenous higher-order CT complexes—such as DAD, DDA, ADA, or AAD—along with heterogeneous higher-order CT complexes—such as DAD′, D′DA, DD′A, ADA, A′AD, or AA′D. The REU students working on this project will start by synthesizing various D and A components (5 weeks), followed by the synthesis of covalently linked D/A moieties and characterization of their structures (3 weeks). Finally, they will study the influence of externally added D′ or A′ on the structure of the covalently linked D/A complexes (2 weeks). The characterization methods include NMR and UV-Vis spectroscopy, electrochemistry and X-ray crystallography (in collaboration with the Shatruk group), and measurements of the association equilibrium constants. All these methods have successfully been performed in the past by undergraduates with appropriate supervision in the PI’s labs.
Project 11: X-ray scintillators based on low dimensional perovskites
Mentor: Prof. Biwu Ma
Developing technologies for the detection of X-ray radiation is an important area of research for their wide applications in security and heath areas. Current technologies for X-ray detection rely on scintillation from expensive inorganic crystals prepared at high-temperature. Recently, solution processed metal halide perovskite single crystals have been used to construct scintillators for X-ray, thanks to their efficient X-ray absorption and excellent optical properties. However, the photoluminescence quantum efficiency (PLQE) of bulk perovskite single crystals is low, due to defects, self- absorption and quenching, and many other nonradiative decay channels. The Ma group has pioneered research in molecular level low dimensional organometal halide perovskites, which can be prepared in large single crystals by low cost wet-chemistry approaches to exhibit superior luminescent properties with PLQEs of up to 100%. This new class of organic-inorganic hybrid materials with exceptional structural and property tenability have potential applications in luminescence based optical sensors, including X-ray scintillators. The REU students will be working with graduate students and postdoctoral fellows in the Ma group to develop and study new low dimensional perovskite materials for X-ray detection. The students will synthesize 2-3 zero-dimensional organometal halide perovskites (4 weeks), characterize their structural and optical properties, using single crystal crystallography and optical spectroscopies (3 weeks), test their X-ray detection capability (2 weeks), write a report and prepare oral and poster presentations (1 week).
Project 12: Photomagnetic transitions of organic radical dimers
Mentor: Prof Michael Shatruk
Light-induced switching of the magnetic state of transition metal ions is a well-established phenomenon that leads to the effective conversion of the thermodynamically stable low-spin (LS) state to the metastable high-spin (HS) state. The HS state remains stable for hours or days provided that the temperature is sufficiently low to prevent thermally activated relaxation to the LS state. The persistence of the photoinduced HS state allows measurement of its properties by conventional characterization methods. In contrast, studies of photoinduced singlet-triplet conversion in organic systems have been limited to transient species. Recently, the Shatruk group has discovered photoinduced conversion of a diamagnetic hypervalent σ-dimer to a pair of paramagnetic radicals. The light-induced paramagnetic state shows unprecedented stability, converting to the diamagnetic state above 242 K. This value is substantially higher than record relaxation temperatures reported for the photoinduced HS states in metal complexes. REU students will investigate the possibility of photoinduced magnetic switching in other organic systems that are known to exhibit temperature-driven conversion between diamagnetic and paramagnetic states. The students will synthesize 2-3 such molecules. The synthetic efforts will follow literature procedures (~4 weeks). The students then will test the possibility to cleave the diamagnetic dimers of radicals into paramagnetic radical pairs under action of temperature, pressure, or light. They will use UV-Vis spectroscopy, X-ray crystallography, and magnetometry to follow the conversion between the states with different optical, structural, and magnetic properties (~6 weeks).
Project 13: The effect of proton transfer on absorption and emission of tautomerizable molecules
Proton transfer chemistry such as acid/base equilibrium and tautomerization is a difficult subject in undergraduate organic chemistry. To establish the conceptual clarity on proton transfer chemistry is of paramount importance for students to embark on future graduate research in a variety of areas including catalysis and spectroscopy. The Zhu group is interested in the molecular photophysics of tautomerizable molecules, such as derivatives of 2-(2′-hydroxyphenyl)benzoxazole (HBO). These molecules, assuming starting from the enol structure in the center, may undergo intramolecular proton transfer to form the keto tautomer (right), or intermolecular proton transfer to solvent molecules (S) or other bases to form the enolate anion (left), in either the ground state or the excited state. The REU students will study the enol form and its proton transferred products, the keto form and the enolate, by absorption and fluorescence spectroscopies in different solvents. The HBO derivatives will be synthesized via a short sequence of multistep synthesis, which has been developed in the Zhu laboratory. The students will learn to acquire the absorption and emission spectra in different solvents and in the presence of various acids or bases. For each HBO-derivative, the experimental sequence will last no longer than 6 weeks. The last 4 weeks will be spent on analyzing the data, writing up the report, and running control experiments that might become necessary during the course of data analysis. The students will gain technical skills in organic synthesis and optical spectroscopies.