Cascading Electron Transfer In D-π-A Graphene Nanoconjugates

Background Introduction

In 2004, Geim and Novoselov et al. first isolated a monolayer honeycomb network of graphene based on sp²-hybridized carbon atoms from graphite using mechanical exfoliation. Since then, due to its unprecedented physicochemical properties, such as high specific surface area (the theoretical specific surface area of monolayer graphene is 2630 m² g^-1), excellent thermal and electrical conductivity, impermeability, strong mechanical strength, and high electron transport capability (electron and hole mobility can reach 15,000-25,000 cm² V^-1 s^-1 at room temperature), graphene has gradually attracted widespread attention from researchers in various fields. However, as an ideal material with zero bandgap, graphene is still limited by existing conditions in terms of preparation, making mass production difficult and hindering further practical applications. Among the reported methods to date, using wet chemical methods for exfoliation is one of the best ways to produce graphene-related materials on a large scale.

The wet chemical exfoliation method often employs suitable chromophores to effectively chemically functionalize graphene in solvents, suppressing the aggregation of graphene layers, thus making graphene more stable in the solvent and easier to process and disperse. This method also utilizes the strategy of bandgap design, primarily selecting high-conjugation organic materials such as polymers and dyes for the chromophores, resulting in hybrid materials that usually possess a donor-acceptor (D-A) structure. Therefore, these materials also have good application prospects in biosensing, photovoltaics, photocatalysis, and nonlinear optics. Among them, functionalizing graphene with π-electron highly conjugated porphyrins or phthalocyanines can induce effective electron/charge transfer within the system, thereby enhancing the application potential of graphene hybrid materials in nonlinear optics. To date, many graphene-porphyrin/phthalocyanine nano-hybrid materials have been reported, where the graphene component generally acts as an electron acceptor, while porphyrins or phthalocyanines often serve as electron donors. Under light induction, electrons flow from porphyrins/phthalocyanines to graphene, effectively increasing the ratio of the excited state absorption cross-section to the ground state absorption cross-section, thus promoting the material’s reverse saturation absorption and optical limiting performance. On this basis, introducing a D-π-A ternary conjugated system to extend the lifetime of charge-separated states in the multi-component system may further increase the ratio of the excited state absorption cross-section to the ground state absorption cross-section, potentially providing a novel effective strategy to enhance the material’s optical limiting performance.

Results Overview

This article reports a D-π-A type porphyrin-phthalocyanine covalently bifunctionalized graphene nonlinear nano-conjugate material. Phthalocyanine with strong electron-withdrawing groups and tetra-phenyl porphyrin were sequentially linked to the surface of graphene through radical addition reactions. In the formed ternary hybrid system, graphene acts as a bridging agent, extending the charge-separated state of the hybrid system and enhancing the third-order nonlinear properties of this organic-inorganic nano-conjugate material. In this novel structure, six strong electron-withdrawing sulfonyl groups substituted asymmetric amino phthalocyanine (Pc) act as electron acceptors, while 5,10,15,20-tetraphenylporphyrin (Por) plays the role of electron donor, and reduced graphene oxide (RGO) uniquely serves as an electron bridge for the first time. To date, the simultaneous introduction of electron-donating porphyrins and electron-accepting phthalocyanines on the surface of graphene in a ternary system has not been explored. Transient absorption spectroscopy indicates that in Por-RGO-Pc, the D-π-A gradient structure leads to cascading charge transfer, extending the lifetime of charge separation and improving charge transfer efficiency, consistent with the highly quenched (99.5%) fluorescence results. Z-scan results show that the significant nonlinear absorption of the ternary system is markedly enhanced, with a nonlinear absorption coefficient reaching up to 8.7ⅹ10² cm/GW on the nanosecond timescale. In this work, the design strategy of the D-π-A ternary system provides a new example for the next generation of graphene-based optoelectronic devices, offering significant insights for designing better-performing optical biomimetic devices, solar cells, photocatalysis, and particularly optical limiting devices.

Illustrated Guide

Cascading Electron Transfer In D-π-A Graphene Nanoconjugates

Figure 1. Schematic diagram of the synthesis of D-π-A type porphyrin-phthalocyanine bifunctionalized graphene nano-conjugate material Por-RGO-Pc.

To prepare Por-RGO-Pc, we first need to synthesize a phthalocyanine Pc with electron-withdrawing ability and active functional groups (amino). The first step involves reducing nitro-ortho-phenylenediamine to obtain amino-ortho-phenylenediamine, which is then protected through acylation to obtain acylated ortho-phenylenediamine, preventing the amino group from undergoing side reactions in subsequent condensation reactions that would hinder the radical addition reaction. The second step involves introducing long-chain thiols on the basis of dichloro-ortho-phenylenediamine to prepare long-chain ortho-phenylenediamine, which is further oxidized by an oxidant to obtain ortho-phenylenediamine substituted with strong electron-withdrawing sulfonyl groups. The third step involves stoichiometric condensation of the two different ortho-phenylenediamines in the presence of zinc salts to obtain the amide asymmetric phthalocyanine Pc1. The fourth step involves deprotection of Pc1 to obtain the asymmetric electron-withdrawing phthalocyanine with active amino groups. Next, meso-substituted phenylamine porphyrin was prepared: tetra-phenyl porphyrin was nitrated on one of the meso-positioned phenyl rings using acidified NaNO₃, and then the nitro group was reduced to an amino group using stannous chloride. The resulting amino porphyrin was then complexed with zinc ions at the center position, yielding the mono-amino substituted zinc porphyrin. RGO was prepared by reducing Hummers method oxidized graphene with hydrazine hydrate, and the obtained equivalent amino phthalocyanine and porphyrin were sequentially modified onto the surface of graphene through azo salt reactions, successfully preparing the D-π-A type porphyrin-phthalocyanine bifunctionalized graphene nano-conjugate material Por-RGO-Pc.

Cascading Electron Transfer In D-π-A Graphene Nanoconjugates

Figure 2. a) Transient absorption spectrum of the D-π-A type porphyrin-phthalocyanine bifunctionalized graphene nano-conjugate material Por-RGO-Pc; b) Lifetime profile of Por^*+ (inset: Pc^*-); c) Schematic representation of the relative energy levels of HOMO-LUMO for porphyrin, electron-withdrawing phthalocyanine Pc, electron-donating phthalocyanine ED-Pc, and reduced graphene oxide RGO; d) Schematic diagram of the cascading electron transfer in Por-RGO-Pc.

To further explore the internal cascading electron transfer process of the D-π-A type porphyrin-phthalocyanine bifunctionalized graphene nano-conjugate material Por-RGO-Pc, transient absorption spectroscopy analysis was conducted, with results shown in Figures 2a and 2b. The Por RGO-Pc nano-conjugate material was excited with a 100 fs laser pulse at 400 nm, revealing the presence of Por^*+ and Pc^*- at different time points. The peak at 673 nm corresponds to the maximum absorption of Por^*+, while the peak at 596 nm corresponds to the maximum absorption of Pc^*-. Within a timescale of 3000 ps, the lifetimes of Por^*+ and Pc^*- in the Por-RGO-Pc nano-conjugate material were found to be longer than those in RGO-Por and RGO-Pc, consistent with the hypothesis that charge separation and recombination in the cascading electron transfer of the ternary system may require a longer time than in binary nano-hybrid systems. The HOMO-LUMO energy level distribution can also be used to predict electron transfer between molecules in nano-conjugate materials; thus, the electrochemical behavior of Por-RGO-Pc nano-conjugate material was further studied through cyclic voltammetry. As shown in Figure 2c, the phthalocyanine with strong electron-withdrawing groups is easier to reduce and harder to oxidize compared to the electron-donating ED-Pc (phthalocyanine with six thioether alkyl chains and one amino group). The EHOMO of the phthalocyanine decreases from -4.5 eV for the electron-donating ED-Pc to -5.0 eV for the electron-withdrawing phthalocyanine Pc, facilitating electron flow from graphene to phthalocyanine, which is consistent with our hypothesis and transient absorption results. Therefore, in the Por-RGO-Pc nano-conjugate material, phthalocyanine not only serves as one of the reverse saturation absorbers but also uniquely plays the role of electron acceptor, successfully constructing a D-π-A type ternary system. This is contrary to the previously reported results where porphyrins/phthalocyanines acted as electron donors in carbon-based materials, and it fully validates that the cascading electron transfer occurs from porphyrin to graphene to phthalocyanine.

Cascading Electron Transfer In D-π-A Graphene Nanoconjugates

Figure 3. a) Nonlinear optical absorption spectra of D-π-A type porphyrin-phthalocyanine bifunctionalized graphene nano-conjugate material Por-RGO-Pc, precursor materials, and reference hybrid materials under 532 nm, 12 ns laser; b) Optical limiting curve of Por-RGO-Pc under 532 nm, 12 ns laser conditions.

We performed open-aperture Z-scan tests on dispersions of RGO, Por, Pc, and Por-RGO-Pc nano-conjugate materials as well as the corresponding reference materials RGO-Por, RGO-Pc, and ED-Pc-RGO-Por in N’N’-dimethylformamide to explore the third-order nonlinear properties under 532 nm nanosecond laser. At the focus position (Z=0), the transmittance (T) of RGO, Por, RGO-Por, Pc, RGO-Por, ED-Pc-RGO-Por, and Por-RGO-Pc nano-conjugate materials decreased from the normalized initial 100% to 87%, 89%, 70%, 92%, 80%, 59%, and 31%, respectively, under 0.45 J cm^-2 laser irradiation, as shown in Figure 3. The nonlinear absorption coefficient increased from 14.35 (RGO), 11.20 (Por 1), 46.35 (RGO-Por), 9.55 (Pc), and 22.35 (RGO-Pc) to 101.30 (ED-Pc-RGO-Por) and 827.44 (Por-RGO-Pc) cm/GW. Among all tested samples, the Por-RGO-Pc nano-conjugate material exhibited the lowest normalized transmittance value (31%) and the highest nonlinear absorption coefficient value (827.44 cm/GW) at the beam focus, with its absorption coefficient nearly an order of magnitude higher than that of the conventional binary systems RGO-Por and RGO-Pc. Additionally, the Por-RGO-Pc nano-conjugate material demonstrated the smallest optical limiting threshold (0.44 J cm^-2, T = 50%), indicating that the D-π-A type porphyrin-phthalocyanine bifunctionalized graphene nano-conjugate material Por-RGO-Pc exhibits significantly stronger optical limiting effects than its parent materials RGO, Por, Pc, and the binary reference materials RGO-Por and RGO-Pc, as well as being superior to the reference material ED-Pc-RGO-Por, fully demonstrating the feasibility of introducing a ternary system and cascading electron transfer to enhance the nanosecond third-order nonlinear optical performance.

Author Introduction

Professor Zhang Chi is currently the Dean of the School of Chemical Science and Engineering at Tongji University, Director of the China-Australia International Joint Research Center for Functional Molecular Materials, Director of the National International Joint Research Center for Light Responsive Functional Materials under the Ministry of Science and Technology, Member of the Materials Division of the Science and Technology Committee of the Ministry of Education, Member of the Expert Committee of the China Overseas Chinese Federation, and Fellow of RSC, IMMM, IET, and RACI. Professor Zhang Chi’s research group has long been engaged in the nonlinear optical and optoelectronic functional research of functional materials, proposing new methods for optimizing functional cluster frameworks and enhancing material optical limiting performance through secondary molecular modification, revealing the modulation rules of functional organic conjugated units on the optical nonlinearity of cluster materials, and developing theories on optical saturation/reverse saturation absorption that integrate conjugated unit modification and modulation of the optical activity of two-dimensional carbon-based materials. He has been evaluated as representing the highest research level in the field of optical functional materials spectroscopy and diffraction method experimental research, and the organic-inorganic conjugated materials created have great potential to become photonic or optoelectronic devices, pushing forward the academic frontier at the nanoscale. To date, he has published over 330 papers in internationally renowned academic journals, with over 8000 citations and 32 authorized invention patents. He leads the Ministry of Education’s innovation team and the key innovation team in key areas of the Ministry of Science and Technology, and has presided over over 20 national and international research projects, including the National Natural Science Foundation’s outstanding youth, key, and general projects, key international cooperation projects from the Ministry of Science and Technology, and ARC Discovery Project, DIISR Intl. Sci. Linkages.

Article Information

Fu L, Li H, Fang Y, et al. Cascading electron transfer and photophysics in a donor-π-acceptor graphene nanoconjugate. Nano Research, 2022, https://doi.org/10.1007/s12274-022-5167-8.

Cascading Electron Transfer In D-π-A Graphene Nanoconjugates

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