Investigation of the Photodynamics of Potentially Bi-stable ESIPT Switches by Femtosecond Time-Resolved Spectroscopy

Aim of this Thesis was to obtain a broad understanding of the structurefunction relationship of such organic ESIPT chromophores. For that, three paradigmatic ESIPT switches exploiting the frequently encountered twist-based deactivation mechanism were characterized using a combination of femtosecond time-resolved spectroscopies and quantum chemical calculations. Furthermore, a basic understanding of five potential additional compounds is elaborated. The three systems studied in detail are 7-hydroxyquinoline-8-carbaldehyde (HQC), N-2pyridinyl-2-pyridinecarboxamide (N2PPCA) and N-isoquinolin-3-yl-pyridine-2- carboxamide (N3IPCA). For these systems, an ultrafast ESIPT following photoexcitation to the lowest ¹ππ^∗ state was identified. Additionally, the formation of a photoproduct was spectroscopically confirmed. Spectroscopic evidence also points towards the existence of intermediate species with out-of-plane-rotated transporter units in the electronic ground state, which then either repopulate the initial ground state or evolve further to form the photoproduct. All observed photoproducts were considered stable towards the longest accessible time delay (∼1.5 ns) of the ultrafast time-resolved experiments. Moreover, in case of HQC, the stability of the photoproduct was confirmed on the microsecond time domain by a newly implemented nanosecond transient electronic absorption spectrometer (nsTEAS). The two carboxamides were observed to possess similar relaxation pathways, which likely result from highly similar structures and electronic properties. However, the dynamics of N3IPCA were significantly slower, allowing for precise assessments of the individual reaction steps.