Design, Synthesis and Spectroscopy of Highly Absorptive Chromophores Based on the Bis(tridentate)metal-ethyne-(porphinato)metal Molecular Framework for Solar Energy Conversion

Abstract

Highly absorptive photosensitizers are in great demand for solar energy conversion applications that include dye-sensitized solar cells, photoelectrochemical cells, and photo-redox catalysis. In this dissertation, the design, synthesis and spectroscopy of a series of different highly absorptive chromophoric systems based on the bis(tridentate)metal-ethyne-(porphinato)metal molecular framework are introduced and discussed. Apart from the substantially more intense absorption in the solar spectral range comparing to traditional photosensitizers, each chromophoric system discussed here further possesses other novel properties and design characteristics, which provides interesting perspectives in resolving existing problems in the energy conversion fields as well as inspire future advancement of photosensitizers and solar energy conversion devices.Specifically, Chapter One serves as an introduction about common solar energy conversion applications and chromophores related to the discussed topics. Chapter Two describes an electron-deficient perfluoroalkyl-substituted bis(terpyridyl)Ru(II)-ethyne- (porphinato)Zn(II) chromophore that is endowed with intense panchromatic absorptivity, and long-lived, highly oxidizing singlet and triplet charge-transfer (CT) excited states. This study provides a design strategy to engineer high-potential photo- oxidants to drive challenging photo-oxidation reactions. Chapter Three reports a series of bis(terpyridyl)Fe(II)-ethyne-(porphinato)Zn(II) based Fe(II) complexes that manifest intense panchromatic light absorption, tunable potentiometric metal-to-ligand CT (MLCT) band gaps and low-lying MLCT featured highly delocalized triplet excited states. Although their 3MLCT lifetimes are short (sub-picosecond time scale) for realistic applications, this study signifies a strategy to decouple the modulation of MLCT and MC state energy levels and paves the way for realizing Fe(II) complexes with long-lived 3MLCT states. Based on the design strategy reported in Chapter Three, the prototype Fe(II) complex described in Chapter Four features a refined bis(N-heterocyclic carbene)Fe(II)-ethyne-(porphinato)Zn(II) structure, which achieves an unprecedentedly long (sub-nanosecond) phosphorescent MLCT state. It represents a new class of earth- abundant iron based photosensitizer and is expected to promote the advancement of environment-friendly and low-cost solar energy conversion devices. Chapter Four reports an asymmetric bis(terpyridyl)Ru(II)-ethyne-(porphyrin)Zn(II) donor based D−A system and its excitation-wavelength dependent photo-induced electron transfer dynamics. It shows that undesired excited-state decay channels, such as intersystem crossing, can be eliminated by designing chromophores with opposite excited-state polarizations to maximize the yields of high-energy photoproducts.