Understanding and Controlling Short- and Long-Range Electron/Charge-Transfer Processes in Electron Donor–Acceptor Conjugates

Abstract

We probed a series of multicomponent electron donor2–donor1–acceptor1 conjugates both experimentally and computationally. The conjugates are based on the light harvester and primary electron-donor zinc-porphyrin (ZnP, donor1) to whose β positions a secondary electron-donor ferrocene (Fc, donor2) and the primary electron-acceptor C60-fullerene (C60, acceptor1) are attached. Linking all of them via p-phenylene-acetylene/acetylene bridges of different lengths to gain full control over shuttling electrons and holes between C60, ZnP, and Fc is novel. Different charge-separation, charge-transfer, and charge-recombination routes have been demonstrated, both by transient absorption spectroscopy measurements on the femto, pico-, nano-, and microsecond time scales and by multiwavelength and target analyses. The molecular wire-like nature of the p-phenylene-acetylene bridges as a function of C60–ZnP and ZnP–Fc distances is decisive in the context of generating distant and long-lived C60 •––ZnP–Fc•+ charge-separated states. For the first time, we confirm the presence of two adjacent charge-transfer states, a C60–ZnP•––Fc•+ intermediate in addition to C60 •––ZnP•+–Fc, en route to the distant C60 •––ZnP–Fc•+ charge-separated state. Our studies demonstrate how the interplay of changes in the reorganization energy and the damping factor of the molecular bridges, in addition to variation in the solvent polarity, affect the outcome of the charge-transfer and corresponding rate constants. The different regions of the Marcus parabola are highly relevant in this matter: The charge recombination of, for example, the adjacent C60 •––ZnP•+–Fc charge-separated state is located in the inverted region, while that of the distant C60 •––ZnP–Fc•+ charge-separated state lies in the normal region. Here, the larger reorganization energy of Fc relative to ZnP makes the difference.

Designing Cascades of Electron Transfer Processes in Multicomponent Graphene Conjugates

“…At first glance, the fluorescence spectra of 5 a‐GNP , 5 b‐GNP , and 5 c‐GNP resemble that seen for ZnTPP . However, a notable red‐shift of ≈20 nm confirms the more extended π‐conjugation [32, 34, 44] . A closer look reveals a distance‐dependent trend when comparing the ZnP fluorescence as a function of separation between the electron donating Fc and ZnP (Figure 7).…”

“…Various references were used in the following sections: commercially available zinc‐tetraphenylporphyrin( ZnTPP ), Fc‐ZnP based conjugates ( HZnPa , HZnPb , (Figure S1)), [52] a β‐substituted zinc porphyrin‐based reference ( β‐ZnP‐ref ) [32] (Figure S2), and 5‐GNP (Figure S3).…”

“…Upon excitation, the 1* ZnP singlet excited state with its characteristic maxima and minima in the visible region is directly formed, along with the GNP phonon bleaching in the NIR region was observed. Considering the redox gradient, which we corroborated in the electrochemical studies, we adopted a kinetic model from the previous work on electron accepting C 60 linked to ZnP and Fc (Figures S34 and S35) [32] . As a matter of fact, the model gave rise to a 100 % convergence at a maximum of 40 iterations to fit the data qualitatively.…”

“…In terms of molecular building blocks, we decided to explore (metallo)porphyrins and ferrocenes conjugates owing to their different strength of electron‐donation. This approach is similar to that we explored in fullerene‐based electron donor‐acceptor conjugates, which exhibit long‐lived charge‐separated states as the product of a cascade of several charge‐transfer events [32] . In particular, we focused on the synthesis of β‐modified porphyrins, which feature electron‐accepting C 60 [32–34] and/or electron‐donating ferrocenes at the β‐, meso ‐, or 4‐phenyl positions [35–41] .…”

Distance‐Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine‐Fullerene Conjugates

“…[24] That is why donor-acceptor arrangements based on porphyrin/phthalocyanine and C 60 -fullerene, represent an outstanding majority. [25][26][27][28][29][30][31][32][33] In the context of donor-acceptor design, there are few examples of covalent multimodular artificial photosynthetic systems, where electron-acceptor units are arranged in the axial positions of a suitable porphyrinoid macrocycle, thus providing molecular systems that undergo photo-induced electron transfer processes in a very efficient way. [19,[34][35][36][37][38][39][40][41][42][43][44][45] One of the most versatile building-blocks is silicon phthalocyanine (SiPc), which can be easily functionalized at axial positions through a variety of functionalities.…”