Title: Investigations into the Optical and Electronic Properties of Perylene Diimide-Based Organic Materials as a Function of Molecular Aggregation in Solution and in Thin Films
Description: In Chapter 1, evidence is presented to correlate the vibronic progression in steady-state optical absorption spectra of a dimeric, organic material to its performance in field-effect transistor devices. The organic material, hPDI2, is fitted with solubilizing side chains of varying structure and length to investigate the effects that side chains have on both the optical and electronic properties of hPDI2. In solution, these side chains influence the character of aggregation and in thin films, the side chains influence film morphology. The character of aggregation in solution is determined by the change in relative peak intensities in optical absorption spectra with increasing concentration in solution. The change in relative peak intensity with increasing concentration in solution is a result of intermolecular electronic coupling, which alters the transitional symmetry of optical excitations. The character of aggregation in solution and the morphology of an organic material in thin films are akin to one another.

In thin films, the intermolecular electronic coupling can facilitate the charge-transfer characteristics of an organic material in field-effect transistors. It is concluded that the structure and length of molecular side chains do indeed influence the optical and electronic properties of organic materials as a function of aggregation in solution and morphology in thin films. However, more evidence is necessary to elucidate a convincing correlation between the relative peak intensities in optical absorption spectra to the performance of the organic material in field-effect transistors. In Chapter 2, the fundamental electronic and chiroptical properties of a helical, polyaromatic molecule are demonstrated. Structurally, the organic material, NP3H, is a helix of helicenes, which generates intense circular dichroism. The circular dichroism is measured in spin-cast thin films. Electronic transfer characteristics are also presented for enantiopure NP3H as well as the racemic mixture. Upon fabricating field-effect transistors using spin-cast thin films of NP3H, the racemic mixture exhibits a marginally superior electron mobility over the enantiopure material.

However, single crystals of enantiopure NP3H were grown and exhibited a two-fold increase in electron mobility when fabricated into a field-effect transistor device in comparison to its amorphous, spin-cast counterpart. It is concluded that enantiopure NP3H exhibits the necessary physical prerequisites to be useful in chiral device applications such as electron spin-filters and chiral light detectors. In Chapter 3, hPDI2 and NP3H are investigated for their ability to aggregate and form ordered films at the air-water interface of a Langmuir-Blodgett trough. Isotherms are presented and compared for each side chain derivative of hPDI2 as well as enantiopure and racemic NP3H. Additionally, an enhancement in circular dichroism is observed when a system of ordered layers of enantiopure NP3H are deposited from the Langmuir-Blodgett trough in comparison to its amorphous, spin-cast counterpart. Furthermore, ordered layers of enantiopure NP3H exhibit an enhancement in electron mobility when fabricated into field-effect transistor devices.

The electron mobility is also demonstrated to enhance as the number of ordered layers that increases up to five layers. When ten ordered layers are deposited, a slight decrease is observed. Lastly, single crystals of hPDI2 were grown by solvent annealing a system of ordered layers deposited from the Langmuir-Blodgett trough, which is significant because, to the best of the author’s knowledge, a similar technique for single crystal growth of an organic material from ordered layers of Langmuir-Blodgett films has not yet been published in peer-reviewed scientific literature. It is concluded that the increased order that is induced by the Langmuir-Blodgett technique does indeed enhance the optical and electronic properties of organic materials in comparison to amorphous, spin-cast films and that this enhancement could be advantageous in device applications.