Date of Award

Fall 12-2015

Degree Type

Dissertation

Degree Name

PhD. Chemistry

Department

Chemistry and Biochemistry

Advisor

Stephen P. Kelty, Ph.D

Committee Member

Wyatt R. Murphy, Ph.D

Committee Member

Sergiu M. Gorun, Ph.D

Committee Member

Nicholas H. Snow, Ph.D

Keywords

Phthalocyanine, Density Functional Theory, Molecular Dynamics, Dye Densitized Solar Cell, Charge Transport, Electronic Structure, DFT, MD, CHARMM, Forcefield

Abstract

Phthalocyanines (Pc) have gained intense research attention in many diverse application areas due to their highly tunable electronic and structural properties through modification of the molecular periphery and metal center. Throughout this work a series of novel perfluoro-isopropyl substituted MPc have been investigated through theoretical methods. First, the synthetic mechanisms of these Pcs will be explored to gain insight into the experimentally observed Pc product distribution. By examining the electronic structure and formation energies of the various Pc precursors, we explain the product distribution as well as propose the formation of additional Pcs, which were not currently believed to form.

The effect of metal center and peripheral modification on the Pc structural and electronic properties is also determined through a systematic investigation of several Pcs with varying degree of peripheral modification as well as several different metal centers. Increased modification of the Pc periphery with strongly electron withdrawing groups lowers the energy of the molecular frontier orbitals; increasing the chemical stability of the Pc. Open d-shell metal centers also introduce several partially occupied states near the top of the Pc valence band, which have electron density localized on the metal center.

The bulky groups on the periphery of the Pc also act to mitigate molecular aggregation. To access the degree of aggregation as a function of peripheral modification, a molecular dynamics forcefield within the CHARMM parameterization model was developed specific to these Pcs. This also allows for the simulation of bulk and thin film properties important to various application areas. Finally, we propose a completely solid state dye sensitized solar cell (DSSC) design in which these chemically robust modified Pcs are sandwiched between n-TiO2 and p-NiO, acting as both photosensitizer and electron shuttle. Through analysis of the electronic structure of the Pc|semiconductor systems, the free energy associated with hole injection into the valence band of NiO upon photoexcitation of the sensitizer and electron injection into the conduction band of TiO2 from the reduced form of the Pc are calculated. Significant molecular orbital coupling between the Pc and semiconductors results in estimated charge transfer lifetimes on the femtosecond time scale on both NiO and TiO2. Additionally, the calculated excited state lifetimes of the Pc is found to be on the nanoseconds time scale, allowing ample time for charge transfer prior to the spontaneous relaxation of the Pc excited state.

In the absence of a liquid electrolyte solution, the Pc molecule will need to also act as electron shuttle in our cell design. The charge transfer properties within the Marcus-Hush electron transfer theoretical framework are calculated. Results indicate that intermediate modification of the Pc periphery leads to high hole and electron mobilities. This is a promising result for our proposed DSSC design, but also makes these Pcs a viable semiconducting material in other application areas, such as light emitting diodes (LEDs) or organic field effect transistors (OFETs).

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