Document Type

Dissertation - Open Access

Award Date

2016

Degree Name

Doctor of Philosophy (PhD)

Department / School

Chemistry and Biochemistry

First Advisor

Cheng Zhang

Keywords

photo-oxidation, photostability, radical, singlet oxygen

Abstract

Organic Conjugated polymers (CPs) are a subject of intense research for their application in organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), solid-state dye lasing, biological imaging and sensing, chemical sensing and remote sensing. CPs are key materials in the quest for more sustainable forms of renewable energy, making electronics more versatile and light weight, and increasing the functionality of everyday materials. For these applications and others that use CPs as the photoactive material, one of their main drawbacks is their susceptibility to photodegradation. Photodegradation occurs when the material is exposed to light leading to irreversible changes in the materials, most often resulting from photoxidation. These irreversible changes cause loss of mechanical, electronic and photophysical characteristics. For practical applications of CP devices, lifetime is as important as device efficiency. The following research is focused on studying the photodegradation mechanisms in various CPs to better understand the relationship between structure and stability, which may lead to the design of CPs which are more intrinsically photostable. To study how dependent photostability is on a polymer’s chemical structure and frontier orbital energies, two series of CPs were studied. The first series contained two dicyano-substituted polyphenylenevinylene polymers with different side chains: poly(2,5-dioctyl-1,4-phenylene-1,2-dicyanovinylene) (C8-diCN-PPV) and poly(2,5-bis(decyloxy)- 1,4-phenylene-1,2-dicyanovinylene) (RO-diCN-PPV). The second series included a wellknown polymer, poly(3-hexylthiophene) (P3HT), and a newly synthesized CP, Poly(3,5- didodecyl-cyclopenta[2,1-b;3,4-b']dithiophen-4-one) (C6-CPDTO). The photodegradation mechanisms were studied through a combination of UV-Vis, PL, FTIR and NMR spectroscopy as well as gel permeation chromatography. There are two main degradation mechanisms that lead to photodegradation of CPs, the radical mechanism is initiate by generation of superoxide radical anion and the other mechanism is initiated by singlet oxygen. C8-diCN-PPV has a lower LUMO level compared to RO-diCN-PPV, yet photodegrades more quickly. RO-diCN-PPV and C8-diCN-PPV both photodegrade through a singlet oxygen mechanism. If DABCO, a singlet oxygen quencher, is introduced into the diCN-PPV polymer films, DABCO induces a radical degradation in RO-diCN-PPV and quenches the singlet oxygen degradation in C8-diCN-PPV. C6-CPDTO contains a fused ring monomer with an electron withdrawing ketone unit that lowers the LUMO level compared to P3HT. C6-CPDTO also contains half the number of side-chains, compared to P3HT, per structural unit; side-chains are a known weakness of a polymer’s photostability. It was observed that P3HT and C6-CPDTO degrade by radical degradation, while P3HT also degrades somewhat by singlet oxygen. The findings show that the mechanism by which a particular CP photodegrades is determined by a combination of the electronic and photophysical characteristics of a polymer.

Library of Congress Subject Headings

Conjugated polymers.

Photodegradation.

Structural stability.

Active oxygen.

Description

Includes bibliographical references (pages 75-82)

Format

application/pdf

Number of Pages

97

Publisher

South Dakota State University

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Rights Statement

In Copyright