Design and Interrogation of Photophysics in (Porphinato)zinc(II)-based Chromophores

Abstract

Developing organic, light-weight materials that possess electrical conductivitieswhich rival those seen in inorganic semiconductors would revolutionize solar cells. As the market for indoor and building-integrated photovoltaics expands, the versatility of organic conducting materials becomes promising due to the abundance and flexibility of small-molecule and polymer-based systems. One key technological hurdle is the need to evolve organic photovoltaic components in which photogenerated excitons readily dissociate into free charge carriers, as this process plays a central role in photovoltaic devices. Towards these goals my dissertation research focuses on developing highly conjugated porphyrin materials that enable facile exciton dissociation and probing how electronic structure impacts: i) the spatial distribution of electronically excited singlet and triplet states, ii) exciton migration, iii) electron-hole pair separation, and iv) electron transfer dynamics. My doctoral research has focused on engineering chromophores with highly tunable excited states from molecular design, spectroscopic, and excited-state dynamical standpoints. Specifically, Chapter One serves as an introduction to (porphinato)zinc(II) chromophores as an ideal benchmark from which to engineer highly conjugated supermolecules to study the fundamental photophysics that drive contemporary technological advances in solar energy conversion. Chapter Two describes the synthesis v and photophysical characterization of electron-deficient (porphinato)zinc(II) chromophores (RfPZn) which feature incredible oxidative stability, long-lived singlet lifetimes, and high quantum yields. Chapter Three discusses how, through the careful synthesis of RfPZn monomers, we can outline a framework for modulation of singlettriplet energy gaps via reducing the exchange and coulomb interactions. Chapter Four reports the photophysical and computational analysis of three new families of proquinoidal-integrated PZn arrays which facilitate high quantum yield extending through the NIR. In-depth solvent-dependent ultrafast transient dynamical studies are described in Chapter Five, which facilitate the development of design principles for augmenting radiative rate constants while selectively minimizing the non-radiative internal conversion and intersystem crossing rates in highly conjugated fluorophores based on proquinoidal connection motif. Chapter Six presents the design of a novel donor-acceptor porphyrin motif in which electron rich and electron poor (porphinato)zinc(II) arrays are covalently linked, acting as a single molecule p-n junction. These molecules represent a rare example of ultrafast photoinduced charge separation in a highly conjugated array yielding highly delocalized electrons and holes residing on opposite ends of the system. Chapter Seven details progress towards synthesizing (porphinato)zinc(II)-iron(II) supermolecules. This chapter highlights our strategy of pairing highly electron deficient PZn macrocycles with N-heterocycle carbene coordinated Fe(II) complexes towards the goal of stabilizing the triplet metal-tovi ligand charge transfer excited state and destabilizing the triplet and quintet metalcentered states that dominate the decay pathways of conventional polypyridylsubstituted Fe(II) transition metal complexes. Finally, Chapter Eight extends the ideas of molecular design in electron transfer to the development of (porphinato)zinc(II)-based systems for spin information transfer. Herein I discuss the synthesis and photophysics of covalently linked PZn and nitroxide stable radical chromophores.