Synthesis of Perylenediimide Derivatives for Multi-Chromophoric Surface Bound Molecular Heterojunctions

Abstract

Perylenediimide (PDI) derivatives are lightfast red dyes that exhibit excellent optical and electronic properties for modern organic semiconductor applications. In this dissertation, several multi-chromophoric donor-acceptor systems containing PDI moieties are synthesized and assembled as electroactive monolayers covalently attached to transparent conducting oxides (TCOs). These surface assemblies provide platforms to study of interfacial charge injection rates that have a major influence on overall organic photovoltaic (OPV) efficiency. Chapter 1 provides a brief introduction to OPVs and physical processes involved. Developments and multiple synthetic strategies of PDI derivatives are also reviewed. Examples of photoinduced energy and charge transfer studies with multi-chromophoric dyads and triads in the literature are analyzed for rational molecular design. Surface-bound molecular level heterojunction models are introduced with the effort of understanding interfacial charge injection. Chapter 2 discusses the synthesis of multi-chromophoric Pc-PDI ensembles by Cu(I) catalyzed azide-alkyne cycloaddition. With the tool of click chemistry, a small library of compounds was built upon an alkyne-terminated asymmetric phthalocyanine (Pc) core and functionalized azides. Click reaction with PDI-azide was tracked by spectroscopic methods to optimize the reaction conditions including time, choice of catalyst, and stoichiometry. A similar ensemble was achieved on ITO surfaces as well, with reaction progress monitored by attenuated total reflectance (ATR) UV-vis spectroscopy. Chapter 3 demonstrates the synthesis of a series of Pc-PDI dyads with varying PDI bay-substituent groups that alter the electronic properties of the PDI component. Fluorescence quenching was observed as preliminary evidence of charge separation in solution. Phosphonic acid pendant dyads were able to form a robust monolayer on ITO and TiO2 for studies of electron injections. Surface orientation of each chromophore was calculated by NEXAFS and polarized ATR spectra. Potential dependent spectra provide useful information of the radical species absorption profiles for subsequent surface transient spectral measurements and the calculations of charge separated state lifetime. Chapter 4 discusses the synthesis of a pair of ZnP-PDI dyads that are able to bind to TCOs. Fluorescence of the PDI component was quenched in all dyads in solution. Carboxylic acid bearing dyads were able to adsorb to a TiO2 surface and correlations of surface coverage and orientation are discussed. The orientations of ZnP and PDI chromophores observed by polarized ATR spectra are also explained in details. Preliminary spectroelectrochemical results are included at the end of the chapter. Chapter 5 briefly reviews the chapters 2-4 and discusses future research directions based on current progress. Preliminary results of this research are also included.