Authors: Kim A. Winterfeld
Source: OPUS Repository (Kooperativer Bibliotheksverbund Berlin-Brandenburg) (article, 2020-01-01)
Field: Materials Science — Materials Chemistry

Electronic Interplay between Porphyrinoids and Push-Pull Chromophores The superordinate goal of this thesis was to develop a detailed and fundamental understanding of the electronic interactions in novel electron donor-acceptor ensembles that mimic the key processes in natural photosynthetic reaction centers using sunlight to provide chemical energy. To this end, on one hand, nonplanar push-pull chromophores were integrated into photoactive subporphyrinoid-based conjugates and investigated in projects P1−P3 in view of photoinduced charge transfer processes. On the other hand, a multichromophoric porphyrinoid ensemble was designed and studied in project P4 with regard to its ability to power a cascade of energy and/or electron transfer events. To get insights into the physicochemical properties of the respective compounds, a wide range of experimental techniques including steady-state absorption and fluorescence spectroscopy, electrochemistry (cyclic voltammetry, square wave voltammetry, differential pulse voltammetry, and spectroelectrochemistry), as well as time-resolved absorption and fluorescence spectroscopy were applied. In publication P1, subphthalocyanines (SubPcs) axially substituted with a nonplanar 1,1,4,4-tetracyanobuta-1,3-diene (TCBD)−aniline moiety were probed with respect to their intramolecular charge transfer interactions. Despite their similar structures, the different periphery, i.e. hydrogenation (H12SubPc) vs fluorination (F12SubPc), of the two SubPcs has a dramatic influence on their electronic properties: While hydrogenation renders the SubPc electron-donating allowing for strong electronic communications in the ground state between H12SubPc and TCBD, fluorination lowers the SubPc’s reduction potential to such an extent that a relatively long-lived intramolecular exciplex in the excited state between the electron-rich aniline and the electron-withdrawing F12SubPc through-space is formed. In report P2, the focus lies on the impact of peripheral vs axial functionalization with TCBD. In detail, two novel SubPcs decorated with three TCBD−aniline moieties in the periphery and four TCBD−aniline units at the peripheral and axial positions, respectively, were explored. In both conjugates, photophysical and electrochemical studies pointed to a significant degree of electronic communications between the electron-rich SubPc and the surrounding electron-withdrawing TCBDs in the ground and excited state. As concerns the excited state dynamics, photoexcitation affords polarized charge transfer (CT) species between the electron-donating SubPc and the electron-accepting TCBD fragments in both conjugates. However, the fourth axial TCBD unit has a decisive effect on the excited state dynamics in that it enables the formation of a fully charge-separated state out of the polarized CT state prior to deactivation to the ground state. Publication P3 reports on a new class of subporphyrinoids, that is, subporphyrins (SubPs) connected to a TCBD unit in their meso or axial position. Physicochemical investigations revealed that the position of the TCBD moiety has a significant impact on the optoelectronic behavior of the respective conjugate. For example, the strength of the ground state CT interactions between SubP and TCBD is directly affected, whereby axial substitution is less effective than peripheral functionalization. As regards the excited state dynamics, photoexcitation enables an emissive CT state between the electron-rich SubP and the electron-accepting TCBD, followed by an indirect charge separation in case of peripheral substitution. In contrast, axial functionalization enforces a radiative exciplex between the electron-donating aniline and the electron-withdrawing SubP. En route towards artificial photosynthetic reaction center mimics, we investigated in project P4 a covalent multichromophoric electron donor-acceptor ensemble comprising a zinc(II)porphyrin (ZnP) decorated with three bis(biphenyl)phenylamine (BBPA) units, a central zinc(II)phthalocyanine (ZnPc), and a terminal perfluorinated SubPc as ultimate electron accepting unit. This panchromatic tetrad turned out to undergo a cascade of electron transfer events along the individual components upon photoexcitation to finally afford a (BBPA)3•+–ZnP–ZnPc–SubPc•− radical ion pair state with a lifetime in the range of tens of nanoseconds. Hence, we could present a new covalent porphyrinoid-based artificial reaction center mimic, which triggers a cascade of light-harvesting across the entire visible range of the solar spectrum and mediates a sequence of electron transfer reactions along the individual building blocks. Chromophore ; Photochemistry ; Chemistry ; Intramolecular force ; Electron transfer ; Absorption spectroscopy ; Electron acceptor ; Cyclic voltammetry ; Electrochemistry ; Physical chemistry ; Organic chemistry Porphyrin and Phthalocyanine Chemistry