Abstract
Abstract : pi-Conjugated molecular systems show attractive semiconducting and optical properties, which makes them highly interesting materials with a great potential for applications in organic electronic devices and molecular electronics. This report covers a thorough investigation into the photophysical properties of various functional pi-electronic molecular systems (e.g., expanded and Mobius porphyrins; pi-pi stacked molecular assemblies) using combined integrated time- and space-resolved spectroscopic methodologies. This study focused on examining pi-conjugation pathways and pi-pi stacking interactions by using highly designed molecules to relate topology with the number of pi-electrons of pi-conjugated molecules and thus provide a detailed understanding of the relationship between structure and photophysical properties. Control of conformational flexibility was critical to determining the pi-conjugation pathway and thus important to understand structure/property relationships in view of three-dimensional aromaticity. Experimentally, an array of analytical approaches was used in this study, including: 1) Steady-state (UV-Vis-NIR Absorption & Emission; NIR Photoluminescence); 2) time-resolved laser (Time-Correlated Single Photon Counting (TCSPC) Method Nanosecond Transient Absorption; Femtosecond Transient Absorption; Femtosecond Broadband Transient Absorption Femtosecond Up-conversion; Femtosecond Broadband Up-conversion; Femtosecond Impulsive Stimulated Raman); and 3) non-linear (Femtosecond Z-scan Method) spectroscopies. Exciton dynamics and excited-species formation processes in columnar, helical stacked H-aggregates of planar perylene bisimides (PBI) were examined by time correlated single-photon counting (TCSPC) and femtosecond pump-probe measurements with anisotropy changes. The photogenerated exciton experiences complicated relaxation processes contributed by excited-state interactions such as exciton delocalization and excimer formation.
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