Ultrafast charge separation and charge stabilization in axially linked ‘tetrathiafulvalene–aluminum(iii) porphyrin–gold(iii) porphyrin’ reaction center mimics

Abstract

The axial bonding ability of aluminum(III) porphyrin (AlPor) has been exploited to synthesize the vertically linked dyad 'aluminum(III) porphyrin-gold(III) porphyrin' (AlPor-Ph-AuPor(+)) and the two corresponding self-assembled triads 'tetrathiafulvalene-aluminum(III) porphyrin-gold(III) porphyrin' (TTF-py→AlPor-Ph-AuPor(+) and TTF-Ph-py→AlPor-Ph-AuPor(+)). The unique topology of these triads provides an excellent opportunity to investigate the sequential electron transfer in the perpendicular direction to the AlPor plane where the AlPor acts as a photosensitizer and primary electron donor while the AuPor and TTF serve as an electron acceptor and donor, respectively. The ground state properties of the dyad and triad suggest that there are no direct intramolecular interactions between the oppositely disposed AuPor and TTF units of the triad. However, the NMR and UV-visible absorption studies of the dyad reveal intermolecular interactions in non-coordinating solvents due to the coordination of counterion PF6(-) to the Al center of AlPor. Steady-state and femtosecond transient absorption studies of the dyad show that the lowest excited singlet state of AlPor ((1)AlPor*) is strongly quenched by ultrafast electron transfer to AuPor(+) with a time constant of 3.16 ps. The resulting charge separated state (AlPor(+)˙-AuPor˙) decays to ground state biexponentially with time constants of 27.26 and 2557 ps. Analogously, upon photo-excitation the triads also produce the same primary radical pair (AlPor(+)˙-AuPor˙). However, the formed radical pair is further involved in a rapid hole transfer from AlPor(+)˙ to TTF to form a stable final radical pair TTF(+)˙-AlPor-AuPor˙. The lifetime of the charge separated state exhibits an increase from 27.26 ps in AlPor-Ph-AuPor to 1393 ps in TTF-py→AlPor-Ph-AuPor(+) and 1484 ps in TTF-Ph-py→AlPor-Ph-AuPor(+). These results reveal successful charge stabilization in the self-assembled supramolecular reaction center mimics constructed via the axial linkage strategy.

High-Energy Charge-Separated States by Reductive Electron Transfer Followed by Electron Shift in the Tetraphenylethylene–Aluminum(III) Porphyrin–Fullerene Triad

“…39−51 These properties make Al(III) porphyrin an excellent candidate for preparation of "axial-bonding"type multicomponent D−A systems where the photoinduced processes (energy and electron transfer) are perpendicular to the porphyrin plane, a crucial factor that maximizes the spatial separation of the charges produced by the electron transfer. [46][47][48]50,51 In addition, the π* orbital of the porphyrin resides in the same region of space as the axial ligands, increasing the electronic coupling between the excited state of the porphyrin and the acceptor and thus promoting electron transfer. The TPE derivative has been selected as a primary electron donor due to its moderate oxidation potentials as well as due to the fact that its first oxidation potential is close to that of the AlPor, it is expected to generate high-energy chargeseparated states if it is coupled with AlPor through the reductive electron transfer process.…”

Design and photochemical study of supramolecular donor–acceptor systems assembled via metal–ligand axial coordination

“…Recently, a triad consisting of Al(III)Por covalently connected to Au(III)Por via ester linkage on one side and axially coordinated with TTF functionalized with imidazole on the other side of Al porphyrin was designed and studied as an light energy harvesting system, as shown in Fig. 43[129] The perpendicular arrangement of the self-assembled system provided an opportunity to study a sequential energy and electron transfer processes where the Al(III)Por acted as a primary electron donor, while the Au(III)Por and TTF served as an electron acceptor and hole shift agents, respectively. The main reason to select Au(III)Por in this study was the minimum spectral overlap between AlPor and AuPor absorption, positive shift in redox potential, and an increase in the singlet excited state energy suitable for photoinduced electron transfer events.The UV/Visible absorption spectrum of the dyad AlPor-AuPor + was recorded in DCM which revealed peaks at 406 and 520 nm corresponding to AuPor, and peaks at 415 and 550 nm corresponding to AlPor.…”

Gold(II) Porphyrins in Photoinduced Electron Transfer Reactions

“…Obviously, the 400 nm pulse excites both the zinc and the gold site as discussed above [S 2 (Zn), S 1 (Zn), S 2 (Au), S 1 (Au)] and excited states from both sites are likely observable. Indeed, the excited‐state absorption (ESA) at 575 nm can be assigned to the T 1 (Zn) state, whereas the broad ESA between 590–650 nm is a feature of the T 1 (Au) state . With this assignment, the local triplet states T 1 (Zn) and T 1 (Au) are populated in the sub‐picosecond time scale.…”

“…Indeed, the excited-state absorption (ESA) at 575 nm can be assigned to the T 1 (Zn) state, [28] whereas the broad ESA between 590-650 nm is af eature of the T 1 (Au) state. [29] With this assignment, the local triplet states T 1 (Zn) and T 1 (Au) are populated in the sub-picosecondt ime scale. After 1ns, features of al ong-lived state (t = 3.0 ns) reminiscent of those of the radical cation [Zn(TPP · + )] + remain.…”