Hypervalent Phosphorus(V) Porphyrins with meso-Methoxyphenyl Substituents: Significance of the Number and Position of Methoxy Groups in Promoting Intramolecular Charge Transfer

Abstract

To study the photophysical and redox properties as a function of meso-aryl units, a series of hypervalent phosphorus­(V) porphyrins, PP­(OMe)2·PF6, PMP­(OMe)2·PF6, PDMP­(OMe)2·PF6, P345TMP­(OMe)2·PF6, and P246TMP­(OMe)2·PF6, with phenyl (P), 4-methoxyphenyl (MP), 3,5-dimethoxyphenyl (DMP), 3,4,5-trimethoxyphenyl (345TMP), and 2,4,6-trimethoxyphenyl (246TMP) units, respectively, have been synthesized. The P­(+5) in the cavity makes the porphyrin ring electron-poor, whereas the methoxy groups make the meso-phenyl rings electron-rich. The presence of electron-rich and electron-poor portions within the porphyrin molecule promoted an intramolecular charge transfer (ICT). Also, the study suggests that the ICT depends on the number and position of the methoxy groups. The ICT is more prominent in m-methoxy-substituted phosphorus­(V) porphyrins (PDMP­(OMe)2.PF6, P345TMP­(OMe)2·PF6) and almost no ICT was found in no-methoxy, o-methoxy, and/or p-methoxy phosphorus­(V) porphyrins (PP­(OMe)2·PF6, PMP­(OMe)2·PF6, P246TMP­(OMe)2·PF6). Transient absorption studies indicate that the ICT takes place on the picosecond time scale. The most striking results come from P246TMP­(OMe)2·PF6, where each phenyl ring carries three methoxy units, like the P345TMP­(OMe)2·PF6, but it failed to induce the ICT process. Electrochemical studies and time-dependent density functional theory (TD-DFT) calculations were used to support the experimental results. This study extensively explores why and how slight variations in meso-aryl substitutions lead to intricate changes in the photophysical and redox properties of phosphorus­(V) porphyrins.

Fluorinated Antimony(V) Tetraarylporphyrins as High-Valent Electron Acceptors with Unparalleled Reduction Potentials

“…Among the main-group porphyrins, group 15 porphyrins intrinsically possess very positive potentials. 10,11 Moreover, such high-potential porphyrins have found increasing interest in their use in the construction of photoelectrodes for artificial photosynthetic systems for solar energy conversion and storage applications. 18,32 Combining the high potentials with semiconductor materials necessitates the use of high-potential porphyrins to absorb light and oxidize nearby water oxidation catalysts.…”

“…Main-group porphyrins are one of the most versatile and fascinating molecules in the porphyrin family as they are highly photoactive, redox-rich, and possess flexible structural properties. − Notably, these properties can be tuned easily by incorporating a variety of main-group elements in the porphyrin cavity . Additionally, the photophysical and redox properties can be adjusted by functionalizing the porphyrin peripherals. − Axial bonding is another active mode of functionalizing that is distinctive of the main-group porphyrins. ,,, These adaptable properties make them potential candidates for a range of applications which include artificial photosynthesis, ,,− dye-sensitized solar cells, , molecular catalysis, ,, molecular electronics and photonics, − photodynamic therapy, − etc. Among the main-group porphyrins, group 15 porphyrins intrinsically possess very positive potentials. , Moreover, such high-potential porphyrins have found increasing interest in their use in the construction of photoelectrodes for artificial photosynthetic systems for solar energy conversion and storage applications. , Combining the high potentials with semiconductor materials necessitates the use of high-potential porphyrins to absorb light and oxidize nearby water oxidation catalysts.…”

“…Additionally, the photophysical and redox properties can be adjusted by functionalizing the porphyrin peripherals. − Axial bonding is another active mode of functionalizing that is distinctive of the main-group porphyrins. ,,, These adaptable properties make them potential candidates for a range of applications which include artificial photosynthesis, ,,− dye-sensitized solar cells, , molecular catalysis, ,, molecular electronics and photonics, − photodynamic therapy, − etc. Among the main-group porphyrins, group 15 porphyrins intrinsically possess very positive potentials. , Moreover, such high-potential porphyrins have found increasing interest in their use in the construction of photoelectrodes for artificial photosynthetic systems for solar energy conversion and storage applications. , Combining the high potentials with semiconductor materials necessitates the use of high-potential porphyrins to absorb light and oxidize nearby water oxidation catalysts. As the catalysts need to be oxidized multiple times, a sufficiently high oxidation potential is required. − By further increasing the redox potentials of these porphyrins, it is possible to maximize the driving force for electron transfer, thus also increasing the kinetics of the electron transfer. , …”

“…A less common way that can be used to attain high potential in porphyrins is to insert high-valent elements into the porphyrin cavity. Porphyrin complexes of hypervalent group 15 elements, such as phosphorus­(V) and antimony­(V), show very high oxidation potentials and low reduction potentials. ,, Specifically, antimony­(V) porphyrins have been shown to have the highest oxidation potentials among the group 15 porphyrins. These high-potential porphyrins have found use in the design of photocatalysts, ,,,− water oxidation systems, , photocatalyst for solar fuels, donor–acceptor systems, − and dyes for use in artificial photosynthetic applications. , …”

Advances and opportunities in Group 15 porphyrin chemistry

“…Remarkably, such high potentials are achieved by mere insertion of the P( v ) ion in the porphyrin cavity without modifying the structure or compromising the optical properties. For example, TPP has potentials of 1.01, 1.26, −1.17, and −1.49 V, and the corresponding [PT(P)P(OMe) 2 ] + has 1.65, −0.53, and −0.98 V, where the potentials are positively shifted by approximately 0.64 V. 36 Furthermore, the resulting phosphorus( v ) porphyrin acquires two new axial functional modes. These impressive properties are exploited in the construction of several donor–acceptor systems for artificial photosynthesis and molecular photonics.…”