Date on Master's Thesis/Doctoral Dissertation

8-2021

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Liu, Jinjun

Committee Member

Wittebort, Richard

Committee Member

Baldwin, Richard

Committee Member

Sumanasekera, Gamini

Committee Member

Sunkara, Mahendra

Author's Keywords

femtosecond transient absorption; organic photocatalysis; excited state lifetime; vertical transition; adiabatic transition

Abstract

Chromophores and macromolecules containing [1,4]benzodioxino[3,2-b]oxanthrene (BDO) has attracted attention since the beginning of this century. Previous studies demonstrated their potential in wide range of applications including gas permeability, hydrogen and energy storage, sensing, and catalysis. Better understanding of the geometrical and electronic structure characteristics of BOD-based chromophores is vital to advance their employment in these applications. This work utilizes spectroscopy techniques coupled with density functional theory methods to provide this required understanding. Substituted BDOs exhibit red-shift of >1 eV in their electronic transitions compared to unsubstituted BDOs. This shift is explained by a molecular orbital reorganization induced by the substituents. Sulfonated BDOs exhibit folding in their backbone compared to the planar dicarbonitrile BDO (BDODC). The folding originates from an O···C interaction between the sulfonyl substitute and the backbone of the chromophore. Photoexcitated BDODC relaxes to its first singlet excited state (S1) in ∼3.8 ps. Fast inter-system crossing (ISC) deactivates S1 to the first triplet excited state (T1) in ∼63 ps. S1 also deactivates to the ground state (S0) through a photoluminescence (PL) process in ∼13 ns. Substituted BDOs retain their geometrical and electronic properties within the framework of the macromolecules. However, evidence suggests the presence of charge-transfer states (CTSs); rationalized by the Frenkel-Davydov exciton model. Theoretical methods indicate geometrical reorganization and charge redistribution associated with the lowest CTS. Spectroscopy methods reveal a relaxation to the S1 of the macromolecules in S1 deactivates through an ISC process in1 undergoes a fast charge-transfer to the CTS inps. Folding of sulfonated BDOs rationalizes the higher gas separation efficiency reported for their membranes. Additionally, the geometrical reorganizations of their excited states provide a new tool to fine-tune their membranes’ pore size for gas separation applications. The red-shift coupled with the fast ISC and charge-transfer processes rationalize the reported photocatalytic activity of BDO-based macromolecules. Co-polymer’s size and composition control are recommended to improve their efficiency in photocatalysis applications.

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