Theses and Dissertations

Issuing Body

Mississippi State University

Advisor

Foster, C. Stephen

Committee Member

Henry, P. William

Committee Member

Zhang, Dongmao

Committee Member

Gwaltneym R. Steven

Committee Member

Wipf, O. David

Date of Degree

8-7-2010

Document Type

Dissertation - Open Access

Major

Chemistry

Degree Name

Doctor of Philosophy

College

College of Arts and Sciences

Department

Department of Chemistry

Abstract

High-energy light was generated from lower-energy photons through an upconversion process using a mixture of a photosensitizer and an emitter. Factors that influence efficiency of the process were studied. Several ruthenium(II) complexes coordinated with bi- and polypyridyl ligands were prepared and used as photosensitizers. Anthracene and its derivatives were used as emitters. In each experiment, the upconversion sample was irradiated with a laser and the emission was monitored. The emission spectra exhibited upconversion (415-513 nm), scattering laser light (514 or 632.8 nm), and phosphorescence (>550 nm). The laser beam was positioned close to the edge of the sample cuvette to avoid a reduction in the upconversion emission caused by self absorption. Increases in laser power, photosensitizer concentration, or emitter concentration increased the upconversion intensity (Iu). Dissolved oxygen caused a minor decrease in Iu. Different photosensitizer and emitter derivatives were tested. Homoleptic ruthenium complexes were more effective photosensitizers with DPA as emitter than their heteroleptic analogues. Upconversion was detected in the [Ru(deab)3](PF6)2 (deab = 4,4'-bis(N,N-diethylamino)-2,2'-bipyridine) and DPA system using helium-neon (632.8 nm) and argon ion (514 nm) lasers, indicating the same process can occur whenever the photosensitizer absorbs the incident radiation. A detailed mechanism is proposed in which an excitation photon is absorbed by a sensitizer to produce an excited triplet state. Energy is transferred from sensitizer to emitter by collision, generating triplet excited emitter. Two emitter triplets annihilate to produce one highly excited singlet. This singlet emits the upconversion photon. The steady-state approximation is used to explore the upconversion and phosphorescence (Ip) intensities. Ip has a first order dependence on laser power, while Iu varies between first and second order. The variable power dependence of Iu occurs because of the competition between triplet-triplet annihilation and other decay pathways. Finally, (Iu/Ip2) is proportional to the second order of DPA concentration. These results generate a better understanding of the upconversion process and they will help to direct the work of others to enhance the efficiency of photonic devices. Practical applications of upconversion, such as the development of better photovoltaic cells, will be aided by the work described herein.

URI

https://hdl.handle.net/11668/15397

Comments

Upconversion||Ruthenium complex||Photosensitizer||Anthracene||Emitter

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