Crystal engineering is a rapidly developing area of research with goals aimed at designing functional molecular solids using reliable intermolecular interactions. By designing these intermolecular interactions using principles of supramolecular chemistry, favorable molecular arrangements can be achieved, which is manifested in desirable properties. We have applied crystal engineering strategies to the synthesis of unique materials for advanced applications including a metal-organic semiconductor, photochromic co-crystals, and a co-crystalline thin film for photolithography. We designed a metal-organic complex based on Ag(I) that exhibits π-π stacking interactions in the organic ligands, which is favorable for electrical conductivity in organic-based semiconductors. The nanocrystalline complex exhibits remarkable electrical conductivity and is also designed to undergo a [2+2] cycloaddition reaction, resulting in over a 70% increase in electrical conductivity. The increase in conductivity is supported by an increased contribution of Ag(I) ions to the top edge of the valence band, as well as new Ag···C(phenyl) interactions that can provide a charge transport pathway. Co-crystallization strategies were used to switch a non-photochromic compound photochromic upon incorporation into a series of co-crystals. Previously in our group, a co-crystalline thin film was applied for photolithography, and here, the crystal structure of the co-crystalline film is elucidated.
We have also applied principles of crystal engineering to the synthesis of materials that are candidates for electrical property characterization measurements. First, we utilize Ag(I) to synthesize 0D and 1D metal-organic complexes. These complexes are also designed to undergo [2+2] photocycloaddition reactions and upon reaction, an increase in dimensionality by at least one order (i.e. 0D to 1D) is achieved. In one complex, photodimerization resulted in a 3D metal-organic framework (MOF), and we successfully applied a ‘green’ synthetic method to the synthesis of the 3D MOF via vortex grinding. We also report the X-ray crystal structure and solid-state packing of an organic molecule involving tetrathiafulvalene, a classic organic semiconductor. The molecule is susceptible to solvent uptake/loss and exhibits π-π stacking arrangements that are not ideal for favorable electrical properties. Through co-crystallization strategies, we achieve a unique ‘lock-arm’ motif that results in infinite stacking in the tetrathiafulvalene core, an ideal property for semiconductivity.
This thesis will also focus on the solid-state [2+2] photodimerization reactions of styrylthiophenes, molecules that rarely undergo the reaction in either the solution or solid state. There have been very few efforts to attain regiocontrol of the products and high yielding photodimerizations of thiophenes are rare. We utilized a ditopic resorcinol template to afford the head-to-head photodimerization product, and by using a dicarboxylic acid-based co-crystal former, we were able to synthesize the head-to-tail photodimer. Both products were achieved in quantitative yield. We have also expanded our approach by employing silver templates, which have previously been successfully applied to photodimerizations of olefins substituted with six-membered rings (i.e. phenyl). We examined photoreactivity in Ag(I) coordination complexes with both α- and β-substituted thiophenes. Both head-to-head and head-to-tail products can be achieved and, in some complexes, both products are produced.
In our studies examining thiophene photoreactivity with dicarboxylic acid templates, we discovered a unique co-crystal wherein two strong supramolecular synthons contribute equally to the solid-state packing. Due to this rare observation, we performed a survey of the Cambridge Crystallographic Database for co-crystals dominated equally by the same two strong supramolecular synthons. We found that co-crystals including both of these synthons are quite rare, and our co-crystal was the first to include a monopyridine. We discuss differences in pKas between the hydrogen-bond donor and acceptor to understand situations where these interactions do not form. We also highlight optimization of crystal symmetry and favorable secondary interactions, such as weak hydrogen bonding and π-π stacking, which may lead to and support the unique synthon formation.
Lastly, we utilize co-crystallization strategies to modify the degree of dynamic molecular motion in the azo functional group, a group that is known to exhibit pedal motion in the solid state. The molecular motion is related to the thermal expansion behavior of the crystals and only upon co-crystallization with a ditopic receptor is the molecular motion capability of the azo group unlocked and ‘colossal’ thermal expansion properties achieved. By systematically modifying the non-azo component, we achieve thermal expansion ranging from ‘colossal’ to nearly zero, as well as rare negative thermal expansion.