Though surface modifications of organic thin films dramatically improve optoelectronic device performance, chemistry at organic surfaces presents new challenges that are not seen in conventional inorganic surfaces. This work demonstrates that the subsurface of pentacene remains highly accessible, even to large adsorbates, and that three distinct reaction regimes (surface, subsurface, and bulk) are accessed within the narrow thermal range of 30–75 °C. Progression of this transition is quantitatively measured via polarization modulation infrared reflection absorption spectroscopy, and atomic force microscopy is used to measure the thin-film morphology. Together, they reveal the close relationship between the extent of the reaction and the morphology changes. Finally, the reaction kinetics of the pentacene thin film is measured with a series of adsorbates that have different reactivity and diffusivity in the thin film. The results suggest that reaction kinetics in the thin film is controlled by both the reactivity and the adsorbate diffusivity in the thin-film lattice, which is very different than the traditional solution kinetics that is dominated by the chemical activation barriers. Combined, these experiments guide efforts toward rationally functionalizing the surfaces of organic semiconductors to enable the next generation of flexible devices.

Thin-films of three dihydroindolizine molecular switches were monitored via polarization modulation infrared reflection absorption spectroscopy to quantify solid state and surface-based inhibition of switching as a function of irradiation time. In the solid state, the molecular switches diverged dramatically with flexible alkyl substituents resulting in switching rates up to three times that of switches containing rigid analogs. For thin-films, decreasing film thickness from 30 nm to approximately 4 molecules thick resulted in an increase in inhibition. This was found to be consistent across all molecules regardless of structure. Increased inhibition is isolated as a metal/molecule interaction, and its consistency across structure is suggestive of energy transfer to the surface. These molecular switches highlight the interplay between molecular design, electronic structure, and switching efficiency.

To eliminate many of the traditional weaknesses of thin-film organic semiconductor materials, chemistry has been developed which reacts with the surface of these materials in a manner reminiscent of monolayers on traditional substrates. In the described approach, vapor phase small molecules react with the surface of tetracene and pentacene substrates to form an adlayer via classical Diels–Alder chemistry. The bonding is confirmed via measurement of several coupled vibrations via polarization modulation infrared reflection absorption spectroscopy, which importantly allows for differentiation from physisorbed materials. These films are then used to tune the materials' interaction with overlayers, as measured via a change in the contact angle the surface generates with water.

In an attempt to understand which factors influence constitutional isomer control of 6′- and 8′-substituted dihydroindolizines (DHIs), a series of asymmetric pyridines was condensed with dimethyl spiro[cycloprop[2]ene-1,9′-fluorene]-2,3-dicarboxylate. The substituents on the pyridial derivatives ranged from donating to withdrawing and demonstrated control over the isomeric ratios for all DHIs. Substituent control proved to be selective for the highly donating amino, which exclusively formed the 8′ isomer. The same ratios were reproduced via photolytic experiments, which suggested that the condensation reaction is dominated by the product’s thermodynamic stability. The electronic influences of the substituents extends beyond isomer control, as it impacts the DHIs’ optical properties and electrocyclization (switching) rates to the spiro conformers. Our results allow us to predict the syntheses and properties of future 6′- or 8′-substituted DHIs, molecules that will be applied in understanding the role of the dipole vector orientation to work function switching.

A surface adlayer is generated on organic single crystals (tetracene and rubrene) using the site specific Diels–Alder reaction and a series of vapor phase dienophiles. X-ray photoelectron spectroscopy (XPS) confirms adsorption on the surfaces of tetracene and rubrene and mass spectrometry demonstrates the reaction’s applicability to a range of dienophiles.

Taking advantage of surfaces’ response to interfacial dipoles, a class of photochromophores (dihydroindolizine) is demonstrated to alter the work function of the underlying substrate (∼170 meV). This same molecule also provides spectroscopic signatures for correlating the change in molecular structure to the induced change in the surfaces’ electronic properties. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) allows analysis of the characteristic dihydroindolizine C═C (1559 cm–1) and pyridinium (1643 cm–1) stretch as a function of photoexcitation. Structural assignments of this photochromophore are corroborated to density function theory calculations. Conformational changes in the monolayers appear in parallel with work function changes and are consistent with both its rate and magnitude.

Seeking to immobilize photochromophores on metallic surfaces, we have synthesized four molecules which contain both a photoresponsive dihydroindolizine (DHI) core and a sulfur containing moiety, which allow for their assembly onto gold substrates. Sonogashira, Suzuki, or Ullmann couplings are employed to generate pyridines with pendant thioacetates (or disulfides). The pyridines are condensed with spiro[2-cyclopropene-1,9′-[9H]fluorene]-2,3-dimethyl ester affording the targeted DHIs.

The assembly mechanism by which hundreds of thousands of two-segment gold-polypyrrole nanorods are assembled into kinetically controlled shape-directed superstructures is examined to predict the range of nanoparticle sizes and materials that can be utilized in their formation. Four processes are responsible for assembly: templating, capillary force assembly, adhesion, and polymer hydration. It is shown that templating, where rods are prepositioned for assembly, is scale invariant and that the energy-minimized state after this step is highly disordered. In addition, we predict that superstructures can be made independently from patterns of rods separated by a distance as small as six times the inter-rod spacing. Both modeling and experiment show that adhesion and polymer dehydration, which induces curvature in the superstructures, are applicable to other materials. However, the high surface energy and low elastic modulus of polypyrrole are advantageous toward generating three-dimensional structures, inducing curvature at gold/polypyrrole length ratios as large as 7:1.Keywords: adhesion; capillary force assembly; nanorods; self-assembly; templating

A surface adlayer is generated on organic single crystals (tetracene and rubrene) using the site specific Diels–Alder reaction and a series of vapor phase dienophiles. X-ray photoelectron spectroscopy (XPS) confirms adsorption on the surfaces of tetracene and rubrene and mass spectrometry demonstrates the reaction’s applicability to a range of dienophiles.