Schroeder Group – Soft matter / single polymer dynamics / single-molecule electronics

The cutting edge of chemical science research lies in the ability to manipulate and control single molecules. Imagine the ability to design and engineer new soft materials with any desired functional property (electronic, mechanical, optical) by controlling chemical structure and composition at the molecular level. The Schroeder Group works to achieve this vision by understanding how form and function arise in soft materials given precise control over molecular structure and processing.

Supramolecular Assembly of Supercharged Proteins
Understanding assembly of supercharged proteins using experiments and simulations.
ACS Central Science, 2022
Automated synthesis for molecular electronics
Automated synthesis is used to prepare a library of oligomers with precisely varied composition for single-molecule electronics. Nature Communications, 2022
Expanding the Molecular Alphabet for DNA-based Data Storage

DNA data storage using an extended 'molecular alphabet' with chemically modified nucleotides.
Nano Letters, 2022
Single-molecule charge transport in host-guest complexes
Intermolecular Charge Transport in π-Stacked Pyridinium Dimers using Cucurbit[8]uril Supramolecular Complexes Intermolecular charge transport through π-conjugated molecules plays an essential role in biochemical redox processes and energy storage applications. In this work, we observe highly efficient intermolecular charge transport upon dimerization of pyridinium molecules in the cavity of a synthetic host (cucurbit[8]uril, CB[8]). Stable, homoternary complexes are formed between pyridinium molecules and CB[8] with high binding affinity, resulting in an offset stacked geometry of two pyridiniums inside the host cavity. The charge transport properties of free and dimerized pyridiniums are characterized using a scanning tunneling microscope-break junction (STM-BJ) technique. Our results show that π-stacked pyridinium dimers exhibit comparable molecular conductance to isolated, single pyridinium molecules, despite a longer transport pathway and a switch from intra- to intermolecular charge transport. Control experiments using a CB[8] homologue (cucurbit[7]uril, CB[7]) show that the synthetic host primarily serves to facilitate dimer formation and plays a minimal role on molecular conductance. Molecular modeling using density functional theory (DFT) reveals that pyridinium molecules are planarized upon dimerization inside the host cavity, which facilitates charge transport. In addition, the π-stacked pyridinium dimers possess large intermolecular LUMO–LUMO couplings, leading to enhanced intermolecular charge transport. Overall, this work demonstrates that supramolecular assembly can be used to control intermolecular charge transport in π-stacked molecules.

JACS, 2022
Resonant transport in single molecule junctions
Direct observation of a reversible transition between nonresonant and resonant charge transport in single molecule junctions. Efficient long-range charge transport is required for high-performance molecular electronic devices. Resonant transport is thought to occur in single molecule junctions when molecular frontier orbital energy levels align with electrode Fermi levels, thereby enabling efficient transport without molecular or environmental relaxation. Despite recent progress, we lack a systematic understanding of the transition between nonresonant and resonant transport for molecular junctions with different chemical compositions. In this work, we show that molecular junctions undergo a reversible transition from nonresonant tunneling to resonant transport as a function of applied bias. Transient bias-switching experiments show that the nonresonant to resonant transition is reversible with the applied bias. We determine a general quantitative relationship that describes the transition voltage as a function of the molecular frontier orbital energies and electrode Fermi levels. Overall, this work highlights the importance of frontier orbital energy alignment in achieving efficient charge transport in molecular devices.
Nano Letters, 2021
Low resistance contacts for molecular junctions
Simple and robust covalent Ag-C contacts based on terminal unprotected acetylenes Robust molecule–metal linkages are essential for developing high-performance and air-stable devices for molecular and organic electronics. In this work, we report a facile method for forming robust and covalent bonding contacts between unprotected terminal acetylenes and metal (Ag) interfaces. Using this approach, we study the charge transport properties of conjugated oligophenylenes with covalent metal–carbon contacts to silver electrodes formed from unprotected terminal acetylene anchors. We performed single molecule charge transport experiments and molecular simulations on a series of arylacetylenes using gold and silver electrodes. Our results show that molecular junctions on silver electrodes spontaneously form silver–carbynyl carbon (Ag–C) contacts, resulting in a nearly 10-fold increase in conductance compared to the same molecules on gold electrodes. Overall, this work presents a simple, new electrode–anchor pair that reliably forms molecular junctions with stable and robust contacts for molecular electronics.

Nano Letters, 2020
Charge transport in sequence-defined conjugated oligomers
Charge transport in single conjugated oligomers critically depends on monomer sequence A major challenge in synthetic polymers lies in understanding how primary monomer sequence affects materials properties. In this work, we show that charge transport in single molecule junctions of conjugated oligomers critically depends on the primary sequence of monomers. A series of sequence-defined oligomers ranging from two to seven units was synthesized by an iterative approach based on the van Leusen reaction, providing conjugated oligomers with backbones consisting of para-linked phenylenes connected to oxazole, imidazole, or nitro-substituted pyrrole. The charge transport properties of these materials were characterized using a scanning tunneling microscope-break junction (STM-BJ) technique, thereby enabling direct measurement of molecular conductance for sequence-defined dimers, trimers, pentamers, and a heptamer. Our results show that oligomers with specific monomer sequences exhibit unexpected and distinct charge transport pathways that enhance molecular conductance more than 10-fold. A systematic analysis using monomer substitution patterns established that sequence-defined pentamers containing imidazole or pyrrole groups in specific locations provide molecular attachment points on the backbone to the gold electrodes, thereby giving rise to multiple conductance pathways. These findings reveal the subtle but important role of molecular structure including steric hindrance and directionality of heterocycles in determining charge transport in these molecular junctions. This work brings new understanding for designing molecular electronic components.

JACS, 2020
Conformational dynamics of lipid vesicles in flow
Flow-phase diagram for vesicle shape and conformational transitions using a Stokes trap Lipid vesicles play a key role in fundamental biological processes. Despite recent progress, we lack a complete understanding of the non-equilibrium dynamics of vesicles due to challenges associated with long-time observation of shape fluctuations in strong flows. In this work, we present a flow-phase diagram for vesicle shape and conformational transitions in planar extensional flow using a Stokes trap, which enables control over the center-of-mass position of single or multiple vesicles in precisely defined flows [A. Shenoy, C. V. Rao and C. M. Schroeder, Proc. Natl. Acad. Sci. U. S. A., 2016, 113(15), 3976–3981]. In this way, we directly observe the non-equilibrium conformations of lipid vesicles as a function of reduced volume ν, capillary number Ca, and viscosity contrast λ. Our results show that vesicle dynamics in extensional flow are characterized by the emergence of three distinct shape transitions, including a tubular to symmetric dumbbell transition, a spheroid to asymmetric dumbbell transition, and quasi-spherical to ellipsoid transition. The experimental phase diagram is in good agreement with recent predictions from simulations [V. Narsimhan, A. P. Spann and E. S. Shaqfeh, J. Fluid Mech., 2014, 750, 144]. We further show that the phase boundary of vesicle shape transitions is independent of the viscosity contrast. Taken together, our results demonstrate the utility of the Stokes trap for the precise quantification of vesicle stretching dynamics in precisely defined flows.

Soft Matter, 2020
Single molecule charge transport in oxazole-containing oligomers
Single molecule conductance and quantum interference effects in oxazole molecular junctions.Charge transport in single molecule junctions critically depends on the chemical identity of anchor groups used to connect molecular wires to electrodes. In this work, we report the charge transport properties of conjugated oligomers with oxazole anchors, focusing on the role of the heteroatom substitution position in terminal oxazole groups. Our results show that oxazole serves as an efficient anchor group to form stable gold–molecule–gold junctions. We further observe quantum interference (QI) effects in oxazole-terminated phenylene molecular junctions, including destructive QI in meta-substituted phenyl rings and constructive QI in para-substituted phenyl rings containing terminal oxazole groups with the same chemical constitution on both termini (i.e., O₅O₅ (5-oxazolyl) or O₄O₄ (4-oxazolyl) linkages on both termini). Surprisingly, meta-substituted phenyl rings with nonequivalent constitutions (i.e., O₄O₅ oxazole terminal linkages) show unexpectedly higher conductance as compared to para-substituted analogues. These results suggest that charge transport in oxazole-terminated molecules is determined by the heteroatom substitution position of the oxazole anchor in addition to the aryl substitution pattern of the π-conjugated core. Our results further show that conjugated molecules with homogeneous oxazole linkages obey a quantum circuit rule such that G_O4-p-O4/G_O4-m-O4 = G_O5-p-O5/G_O5-m-O5, where G is molecular conductance. Overall, our work provides key insight into the development of new chemistries for molecular circuitry in the rapidly advancing field of single molecule electronics.

JACS, 2019
Threading dynamics of single ring polymers in flow
Direct observation of ring polymer threading dynamics in semi-dilute solutions of linear polymers. Understanding the dynamics of ring polymers is a particularly challenging yet interesting problem in soft materials. Despite recent progress, a complete understanding of the nonequilibrium behavior of ring polymers has not yet been achieved. In this work, we directly observe the flow dynamics of DNA-based rings in semidilute linear polymer solutions using single molecule techniques. Our results reveal strikingly large conformational fluctuations of rings in extensional flow long after the initial transient stretching process has terminated, which is observed even at extremely low concentrations (0.025 c^*) of linear polymers in the background solution. The magnitudes and characteristic timescales of ring conformational fluctuations are determined as functions of flow strength and polymer concentration. Our results suggest that ring conformational fluctuations arise due to transient threading of linear polymers through open ring chains stretching in flow.

Nature Communications, 2019