In-situ Infrared Spectroscopy of (Electro-)Catalytically Triggered Energy Release from Molecular Solar Thermals
Year: 2024
Type: dissertation
Abstract: This work focuses on the electrochemically and catalytically triggered energy release of different MOST compounds. The main tool used was photoelectrochemical/photochemical infrared reflection absorption spectroscopy (PEC-IRRAS/PC-IRRAS). This technique allows the study of photochemical and electrochemical/catalytic reactions in a single setup using an atomically defined surface as a WE or catalyst. This allows a detailed and time-resolved insight into the back-conversion of the energy-rich MOST isomer. The electrochemical back-conversion was studied for different NBD/QC derivatives. Firstly, the derivatives phenyl-ethyl-norbornadiene/quadricyclane (NBD1/QC1) and p-methoxyphenyl-ethyl-norbornadiene/quadricyclane (NBD2/QC2), which differ by one methoxy group in the push-pull system, were studied. QC1 shows a peak in the voltammogram at 0.6 Vfc. The corresponding feature for QC2 appears at 0.45 Vfc, but is much weaker for this compound. This is attributed to the electrochemical oxidation of QC1/QC2. While the formation of QC1+· radical cations leads to a partial irreversible oxidation of QC1, the formation of QC2+· triggers the back-conversion to NBD2 without major side reactions. As the latter process is a radical chain reaction, only a very small total charge is required to back-convert the QC2. The selectivity of the electrochemically initiated energy release reaction is drastically different for the two compounds. QC1 is electrochemically back-converted to NBD1 with a selectivity of only 60%. The introduction of the additional methoxy group in NBD2 significantly increases the selectivity (selectivity ≥99.3%). This effect was studied in more detail by screening the back-conversion of several NBD/QC compounds with different push-pull moieties. The onset of oxidation leading to back-conversion varies widely from -0.3 Vfc to 0.4 Vfc. There is a clear trend in the strength of the electron donating effect of the EDG. The lowest onset potentials (-0.3 Vfc and -0.2 Vfc) are observed for groups with -NMe2 groups. This is explained by the strong electron delocalization effect (+M effect). The group donates more electron density to the EDG, thus increasing the energy level of the HOMO and decreasing the energy required to oxidize the compound. The compounds with -OMe moieties show a higher selectivity (97.8% to 98.8%) than those with -NMe2 (91.7% to 96.2%). This can be explained by mesomeric effects. The -OMe moiety has a higher electronegativity than -NMe2. This leads to a destabilization of the positive charge on the tertiary carbon close to the oxygen atom and therefore to a higher localization at the QC. For -NMe2, localization of the charge close to the heteroatom is more likely. This results in a stronger overall charge distribution over the phenyl ring, resulting in more side reactions compared to -OMe. It was found that the stability window of the back-conversion varies from 0.6 V to 0.3 V. It is strongly dependent on the EDG. A higher stability window makes it easier to implement in a device as less precise potential control is required. In the next step, the reductive back-conversion of the (E)/(Z)-3-cyanophenyazothiophene isomer couple was studied. The onset potential for the reductive channel is -0.9 Vfc. In the reductive reaction channel, the rate of the back-conversion reaction can be controlled by the applied potential within (at least) two orders of magnitude between potentials of 0 Vfc and -1.5 Vfc. In tests over 100 storage cycles, we have shown that the photochemical conversion occurs quantitatively. No degradation of the photoswitch is observed after 100 conversion cycles. In the next step, the behavior of (E)/(Z)-3-cyanophenyazothiophene on Pt(111), Au(111) and graphite(0001) was investigated in terms of catalytic activity and selectivity. It was found that graphite is catalytically inactive towards the energy release reaction. The surface also does not catalyze any side reaction. This is attributed to the weak interaction of the MOST molecule with the graphite(0001) surface and the small energy difference between the (Z) and (E)-isomers of the adsorbed species. The observed inertness makes the graphite surface an ideal electrode material for electrochemically triggered energy release. Pt(111) catalyzes the energy release with a reaction rate, but with very low selectivity (0.5). The energy difference between the adsorbed (Z)- and (E)-isomers is high enough (2.03 eV) to catalyze the reaction by adsorption. However, the strong adsorption of MOST on the Pt(111) surface leads to decomposition of the MOST molecule. This limits the selectivity and adsorption of the product, and the decomposition products poison the surface and reduce its activity. Au(111) offers higher catalytic activity combined with high selectivity (>99%). To further investigate MOSTs on Au(111), the catalytic back-conversion of different QC derivates was investigated. In particular, the energy release from 2-cyano-3-(3,4-dimethoxyphenyl)-NBD/QC (NBD'/QC') using Au(111) as a catalyst was investigated. It was shown that Au(111) exhibits very high catalytic activity for the back conversion of QC' to NBD'. The highest rate constants measured for the catalytically initiated back reaction were in the order of ks = 5·10-5 m·s-1 at -0.3 Vfc. It is possible to tune the kinetics of the heterogeneously catalyzed energy release by an externally applied potential. The reaction rate of the catalytically triggered energy increases by a factor of 60 when the electrode potential is increased from -1.0 to 0.3 Vfc. After 1000 storage cycles, the rate constant for catalytic energy release decreases by a factor of 4, which means that the rate constant decreases by only 0.1% per storage cycle. No by-products or decomposition of NBD' or QC' were detected even after 1000 storage and release cycles. The investigation was repeated with PENBD to study the mechanism of the reaction in UHV combined with liquid phase. A similar activity was found. No deactivation was found either. In the last step, the energy release in the MOST system p-methoxyphenyl-ethyl ester-quadricyclane (mPEQC) to its energy poor isomer p-methoxyphenyl-ethyl ester-norbornadiene (mPENBD) was catalyzed by a monolayer of the porphyrin derivative [5-(p-carboxyphenyl)-10, 15,20-tri(p-tert-butylphenyl)porphyrinato] cobalt(II) (CoIIP) anchored on Co3O4(111) was studied. The catalyst was shown to be highly active towards the back conversion of mPEQC to mPENBD with a rate constant of ~1.0-10-7 m·s-1. It was shown that 100 cycles of conversion and back-conversion can be performed without the formation of by-products and with very little loss of activity. The catalytically active CoII was reduced to CoI which inactivates the catalyst by a factor of 100. Re-oxidation of the Co center reactivates the catalyst. This allows the noble metal free catalyst to be turned on and off at will, giving a high degree of control over the energy release in a device in a very simple way.
Author: Franz, Evanie
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W6927027666
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