Synthesis and Computational Analysis of Rigid, Side-Strapped Phthalocyanines for Organic Photovoltaics

Abstract

Phthalocyanines (Pcs) are π-conjugated, macrocyclic molecules whose derivatives have been the subject of extensive investigations as active layer organic semiconductor (OSC) materials for organic photovoltaics (OPVs). This class of compounds are promising as the photoactive material in OPVs due to their strong near infrared (NIR) absorbance. Furthermore, Pcs can be structurally modified via the installation of metal centers and substituents to tune properties like solubility, photophysics, and condensed phase organization towards what is optimal for OPV device performance. The research presented in this thesis involves the synthesis and computational analysis of rigidly side-strapped phthalocyanines (RSS-Pcs) invented by our research group. This new class of Pc derivative includes moieties that are rigidly side-strapped via acetylene bridges allowing for extended π-conjugation and substitution away from the Pc core. This type of Pc side-strapping is promising for close chromophore packing, promoting efficient intermolecular charge transfer and high charge mobility through the bulk material. Chapter 1 provides a brief review of OPVs including an explanation of the device mechanism, the inherent limitations, and the desired properties of OSC materials. Furthermore, the computational models of charge transport in OSCs are briefly discussed. Subsequently, the characteristic properties of Pcs in the context of OPVs is discussed including structural modulation strategies. Lastly, the structure of RSS-Pc is introduced along with a discussion that highlights the interesting structural features. Chapter 2 presents the synthesis of RSS-ZnPcs with dialkyloxybenzene side-straps and the effect of phenyl ring moieties on the aliphatic side-chains on solution aggregation. It was discovered that phenyl rings that are electronically insulated from the Pc core on the side-chains of the 2-fold symmetric structure induce long-range J-aggregation in chloroform and tetrahydrofuran solutions. This is evidenced by a sharp absorbance feature that is red-shifted from that of the RSS-ZnPc Q-band in the UV-vis spectrum, which disappears in the presence of pyridine. Furthermore, this J-aggregation absorbance is seen in the thin-film UV-vis spectra and we discovered that thermal annealing promotes its development, suggesting that the thin-film polymorph is similar in structure to that of the J-aggregate. Chapter 3 discuses the importance of Pc fluorination on condensed phase morphology and presents the synthesis of RSS-ZnPcs containing fluorine atoms. Our work presented in this chapter demonstrates the synthetic challenges of maximizing fluorine atom substitution on RSS-ZnPcs due to electrophilic aromatic substitution (EAS). More specifically, EAS occurs by the nucleophilic alkoxide necessary for phthalonitrile cyclization resulting in 3,6-difluoro, 4,5-dialkyloxybenzene-strapped RSS-ZnPcs. Carbonyl functionalities on Pcs have been found to impart interesting properties such as water solubility, electron deficiency, and polymerizability when they are allylic. Chapter 4 presents the synthesis of RSS-Pcs containing carbonyl groups and the resulting properties of the like. The synthesis of a phthalimide-strapped RSS-ZnPc was attempted, but the resulting material was found to be insoluble due to either limited alkyl group substitution or dealkylation under the highly basic conditions. Phthalic acid diester-strapped RSS-H2Pc was synthesized and was found to be highly soluble in hydrogen-bonding solvents like methanol and water. Insertion of a metal center like Zn or AlCl renders the RSS-Pc insoluble possibly due to elimination of hydrogen-bond donor sites (i.e. N-H bonds). Additionally, similar properties were discovered with the synthesized dialkyloxybenzene-strapped RSS-H2Pc containing ethyl-cinnamate moieties on the side-chains. Chapter 5 presents the computational analysis of three RSS-ZnPc crystal structure dimers: dialkylbenzene-strapped, dialkyloxybenzene-straped, and thienyl-strapped. In this work, we applied Marcus theory of electron transfer rate and density functional theory (DFT) in the calculation of charge mobility of the three dimer systems. We show that the hole mobility calculated from a single electron transfer in the dimer (~3-10 cm2/Vs) is significantly higher than both calculated (~10-1 cm2/Vs) and experimentally measured (10-8 to 10-4 cm2/Vs) mobilities of OSCs commonly used in OPVs. Furthermore, our predicted RSS-ZnPc mobilities are comparable to that of rubrene, which is known for having the highest measured experimental mobility of 40 cm2/Vs in highly crystalline material. Chapter 6 summarizes the major findings presented in Chapters 2-5 and proposes future directions and paths of exploration for current research to pursue.