Date of Degree

9-2020

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Dorthe M. Eisele

Committee Members

John R. Lombardi

Seogjoo J. Jang

Vinod Menon

Subject Categories

Chemistry | Materials Chemistry

Abstract

Delocalized Frenkel excitons-coherently-shared excitations among chromophores-are responsible for the remarkable effciency of supramolecular light-harvesting aggregates within photosynthetic organisms. Despite tremendous progress towards a deeper understanding of these photosynthetic complexes, as well as progress in the development of organic solar cells, the possibility of using bio-inspired molecular nanostructures for large-scale solar energy conversion has not been realized yet. The persistent limitations in translating natures design principles for applications in optoelectronic devices have been the supramolecular structures fragility, and the Frenkel excitons delicate nature. The structural instability, both dis-assembly and spontaneous re-assembly of aggregates. Analysis of the absorption spectrum of a model molecular aggregate system-light-harvesting nanotubes (LHNTs) of cyanine dye derivative C8S3-revealed the supramolecular structure is inherently unstable under mild changes to native solvent environment, only retaining the intrinsic double-walled structure within limited range of dye concentration (0.40-0.12 mM C8S3), solvent polarity (11-16% by volume methanol), solvent pH (4.0-7.0), and solvent temperature (23-45 degrees C).

The extent of the aggregates' structural instability is further complicated upon isolation from solution and deposition onto solid substrates. Near-field scanning optical microscopy revealed nanoscale emission of substrate-isolated aggregates was substantially altered after several hours of deposition, suggesting the stability of substrate-immobilized molecular aggregates is dependent on deposition time. Surface-enhanced Raman spectroscopy investigations revealed the charge-transport dynamics of these aggregates are structure-dependent with highly efficient charge-injection observed in an hierarchical structures consisting of multiple, densely packed LHNTs, and no charge-injection observed in the individual LHNT. These results highlight the importance of stabilizing molecular aggregates, both in-solution and on-substrates, as their desirable optical properties are structure-dependent.

Through in situ cage-like scaffolding of individual supramolecular LHNTs, the intrinsic barriers towards utilization of self-assembled molecular aggregates can be overcome. Highly stable organic-inorganic supramolecular nanocomposites composed of self-assembled dye nanotubes and cross-linked silica scaffolds were self-assembled in aqueous methanol solution. High-resolution cryogenic electron microscopy revealed the dimensions of scaffolded LHNTs were discretely tunable (~4.3-4.9 nm) and highly uniform (±0.3 nm). A comprehensive microscopic and spectroscopic characterization, including linear spectroscopy, scanning electron microscopy, broadband femtosecond transient absorption spectroscopy, and near-field scanning optical microscopy revealed supramolecular excitons within cage-like scaffolds are robust, even under extreme heat-stress, and precise control over nanocomposite dimensions is maintained on solid substrates. Our bio-inspired nanocomposites provide a general framework for the development of next-generation organic devices made from stable supramolecular materials.

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