Molecular crystals have applications in nonlinear optics, organic electronics, and particularly in pharmaceuticals, as most drugs are marketed in the form of crystals of the pharmaceutically active ingredient. Molecular crystals are bound by dispersion (van der Waals) interactions, whose weak nature generates potential energy landscapes with many local minima that may be extremely close in energy. This often results in polymorphism, the crystallization of the same molecule in several different structures. Crystal structure may profoundly influence the physical and chemical properties, including the electronic and optical properties relevant for device applications.

We perform large scale quantum mechanical simulations to predict the structure of molecular crystals and investigate the effect of crystal packing on their electronic and optical properties. The massively parallel genetic algorithm (GA) package, GAtor, relies on the evolutionary principle of survival of the fittest to find low-energy crystal structures of a given molecule. Dispersion-inclusive density functional theory (DFT) is used for structural relaxation and accurate energy evaluations. Evolutionary niching is performed by using machine learning to perform clustering on the fly. The structure generation package, Genarris, performs fast screening of randomly generated structures with a Harris approximation, whereby the molecular crystal density is constructed by replicating the single molecule density, which is calculated only once. Many-body perturbation theory, within the GW approximation and the Bethe-Salpeter equation (BSE), is then employed to describe properties derived from charged and neutral excitations.

An emerging application of molecular crystals is singlet fission (SF), the down-conversion of one photogenerated singlet exciton into two triplet excitons. SF has the potential to significantly increase the efficiency of organic photovoltaics beyond the Shockley-Queisser limit by harvesting two charge carriers from one photon. However, the realization of SF-based solar cells is hindered by the dearth of suitable materials. We aim to discover new SF materials and optimize the crystal packing of known materials to enhance SF efficiency. We predict that crystalline quaterrylene and a lesser known monoclinic crystal structure of rubrene may exhibit high singlet fission efficiency, possibly rivaling that of the quintessential SF material, pentacene. Quaterrylene has the additional advantages of high stability, a narrow band gap, and a triplet energy in the optimal range to maximize photoconversion efficiency.

 

Linking molecular structure with excited-state photochemical dynamics is crucial for engineering efficient organic solar cells and related photochemical applications. Two molecular dye systems are currently under investigation in our lab: carboalkoxyphenylporphyrins in solution-cast thin films and a new family of highly fluorescent thiazolothiazole viologen dyes. The singlet exciton diffusion lengths of solution-cast porphyrin thin films with various alkyl chain lengths have been examined. Modifications of peripheral solubilizing groups help orientate porphyrins in solution processed thin films and influence molecular orientation. The photoluminescent singlet decay lifetime of pristine porphyrin films and films lightly doped with [6,6]-phenyl-C61-butryic acid methyl ester (PCBM) were used in a 3D exciton diffusion Monte Carlo simulation to extract the exciton diffusion parameters and the nanocomposition. Longer alkyl chain derivatives yielded increased PL decay lifetimes and lengthened exciton diffusion lengths (L_D) for octyl and hexyl containing porphyrin derivatives. GIWAXS and XRD data indicates that molecular organization is strongly dependent upon the peripheral carboalkoxy substituent, and that nematic molecular organization resulted in an increase in the exciton diffusion length. Our findings are an important step toward a deeper understanding of the exciton diffusivity and molecular packing relationship. We have also synthesized a class of extended viologen dye structures that incorporate a thiazolo[5,4-d]thiazole backbone. The dyes exhibit both reversible yellow to dark blue electrochromism and high fluorescence quantum efficiency that is deactivated upon electrochemical reduction. The fused bicyclic thiazolothiazole heterocycle allows the alkylated pyridinium groups to remain planar, strongly affecting their electrochemical properties. The electrochromic and strongly fluorescent properties make these materials attractive for molecular electronics, biomolecular sensing, and other photochemical applications.

 

Although “ordered” organic π-conjugated assemblies outperform “disordered” ones in many optoelectronic device applications, we are far from being able to port the well-understood supramolecular recipes of π-systems from solution to solid-state device environments. For the past several years we have been exploring hydrogen bond (H-bond) directed self-assembly of π-systems along these lines, for example, to enhance their absorption and charge transport properties for organic photovoltaic (OPV) applications. Various examples of oligothiophenes outfitted with heterocycles capable of forming H-bonded “rosettes” will be discussed in this context. The second part of the talk will introduce new monomers derived from [2.2]paracyclophane (pCp) that are capable of robust H-bond directed self-assembly into one-dimensional nanostructures in solution and the solid state. The design introduces transannular (intramolecular) H-bonds between pairs of pseudo-ortho-positioned amides as a way to preorganize the molecules for intermolecular H-bonding with two neighbors. The result is formation of homochiral, one-dimensional pCp stacks that show supramolecular polymer signatures in solution.

Abstract: The philosophy of science studies how individual scientific concepts, models, theories, and experiments all influence the development of scientific knowledge. My research in the philosophy of nanoscience applies methods from philosophy of science to understanding how problems raised by nanoscience have changed our understanding of concepts, theories, and models from physics, materials science, and inorganic chemistry. For instance, studying the synthesis, simulation, and characterization of mixed-metal nanoclusters raises questions about whether these objects count as alloys. In this talk, I examine some keys questions from philosophy of science for chemistry and nanoscience and highlight some results from my approach to answering these questions. Bio: Julia Bursten is a second-year assistant professor of philosophy at the University of Kentucky specializing in philosophy of the physical sciences. Her research studies how theories and models work in nanoscience, chemistry, and materials science, and why theories in these sciences often work differently than theories in areas like quantum physics and biology.

This presentation will highlight two platforms recently developed in the Sagle group which combine lipids and plasmonic nanoparticles.  The first platform involves sandwiching a liposome between a planar gold surface and a gold colloid to generate a biocompatible, highly enhancing surface enhanced Raman spectroscopy (SERS) substrate.  Our initial characterization of these novel substrates investigates substrate stability, temperature inside the liposome component, SERS activity inside the liposome, SERS mechanism and reproducibility.  The substrates are shown to be stable to laser irradiation and exhibit a temperature increase of only 20 degrees Celsius inside the liposome component.  The SERS enhancement of dye residing in the liposome component was found to be 8 x 10⁶, higher than expected considering the dye molecules are at least 4 nm from either gold surface.  Finite Difference Time Domain (FDTD) calculations reveal that the field enhancements inside the liposome are uniform with the major contributing factor being long range coupling between the gold nanoparticle and the mirror.  Lastly, these substrates show greater reproducibility than typical SERS substrates in which dye is sandwiched between two metallic surfaces, and are expected to allow for the non-perturbative measurement of biological molecules in their native state, freely diffusing in solution.  The second platform involves interfacing a gold nanodisc array with solid supported lipid bilayers for label-free biosensing of membrane-associated proteins.  This platform is shown to have superior sensitivity due to elongated gold nanodics (exhibiting greater sensitivity than typical nanoparticle arrays) and an ultrathin silica layer above the nanodiscs, enabling the lipid bilayer to reside close to the nanoparticle surface.  Further studies currently underway are using this platform with silver nanodiscs to carry out label-free SERS measurements of lipid components in the freely diffusing bilayer.

 

Abstract: Recently, dye-sensitized solar cells (DSCs) were shown to be the highest power conversion efficiency technology of any solar cell technology when using photons from the beginning of the solar spectrum until 700 nm. Two key directions are apparent in further elevating this technology: (1) broadening the spectral window used, and (2) efficiently subdividing the spectrum further for multijunction devices which can be used in combination with many solar cell technologies. Progress toward designing optimal panchromatic organic sensitizers to use NIR photons based on physical organic concepts such as proaromaticity and cross conjugation will be discussed. Additionally, the design and realization of a series sequential multijunction dye sensitized solar cell (SSM-DSC) system for effective photon management will be discussed. Ongoing research to optimize this system based on transition metal redox shuttle design and high voltage organic dye design will be analyzed. The SSM-DSC system coupled with electrocatalysts as solar-to-fuel systems has been shown to power water splitting and CO₂ reduction coupled with water oxidation from a single illuminated area without external bias.

Jared Delcamp

Assistant Professor 

University of Mississippi

Department of Chemistry & Biochemistry