PhD Oral Exam - Zujhar Singh, Chemistry

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

To-date, most molecular photosensitizers used in dye-sensitized photoelectrochemical cells are ruthenium-based. The incorporation of copper(I) complexes into such systems has been limited because of their shorter excited-state lifetimes. Furthermore, molecular donor-chromophore-acceptor systems offer vectorization of charge movement within the excited state to yield spatially separated cation and anion radicals at opposite ends of the molecule to slow charge recombination.

First, this thesis reports a copper(I)-based donor-chromophore-acceptor triad bearing a 1,8-napthalenemonoimide electron accepting moiety and a carbazole electron donating moiety serving to generate a charge separated state under visible light irradiation. This molecular assembly was integrated onto a zinc oxide nanowire surface on a conductive glass slide. Upon photoexcitation the generated oxidizing equivalents are transferred to a copper(II) water oxidation catalyst in aqueous solution oxidizing water with a Faradaic efficiency of 76%. As an improvement on this initial system, this thesis also reports a copper(I)-based donor-chromophore-acceptor triad that bears a triphenylamine electron donor and a phenazine electron acceptor. This triad was also surface grafted onto zinc oxide nanowires, and the as-constructed photoelectrodes were used for photodriven alcohol oxidation. The final evolution of these systems is reported in the form of a copper(I)-based donor-chromophore-acceptor triad containing 1,8-napthalenemonoimide as the electron acceptor moiety and triphenylamine as the electron donor. The final charge-separated state formed on photoexcitation of this triad has a long (18 ns) excited state lifetime in acetonitrile, which is one of the longest reported to-date for copper(I)-based donor-chromophore-acceptor systems and exceeds the first two systems reported in this work.

As the photocathode complement to the previous systems that were used as photoanodes, three copper(I) complexes with progressively extended ligand conjugation through terminal phenazine moieties have been synthesized. Acting as dyads, these have been surface grafted onto nickel oxide and their transient photocurrent properties have been investigated. It was found that even though their electronic properties can be modulated in solution through ligand variations, all three complexes generate the same photocurrent. The results provide evidence of hole injection to nickel oxide and spectroscopic investigations confirm that this is happening in the femtosecond time regime even before the formation of final excited state.

In all systems, the generation of photocurrent under white light illumination has validated the utility of these molecular architectures as photosensitizers for dye-sensitized photoelectrochemical cells.