Introduction

Fluorescence resonance energy transfer (FRET) has received increasing attention by virtue of its important role in natural photosynthetic system and monitoring interactions between biomolecules1,2,3. FRET is non-radiative energy transfer that occurs by dipole-dipole coupling from the excited state of the donor (D) to the ground state of the acceptor (A)4. There is a strong correlation between the D–A distance and FRET efficiency, which makes FRET technology widely used in the following fields: light harvesting5,6,7,8,9, fluorescence sensing/imaging10,11, optoelectronic devices12,13 and determination of inter(bio)molecular interaction1,14. Nowadays, the development of FRET systems has become one of the most promising topics in the fields of analytical chemistry, chemical biology and materials science10,15,16,17. An efficient FRET process must have at least the following prerequisites: (1) since FRET efficiency is inversely proportional to the sixth power of the D-A spacing, the donor-acceptor distance should be within 10 nm; (2) the emission spectrum of the donor should have a good overlap with the absorption spectrum of the acceptor4.

It is worth noting that multi-step FRET systems containing multiple types of chromophores have stimulated a lot of interest in recent years. For example, a two-step FRET system usually has a kind of energy donor and two kinds of energy acceptors, the relay acceptor and the final acceptor. The donor chromophores act as a light collecting antenna, capturing the excitation energy and transferring it to the final acceptor via the relay acceptor. The multi-step FRET systems have several advantages over one-step FRET systems (Fig. 1)18: (1) large Stokes shifts can be achieved even without spectral overlap between donor and final acceptor; (2) long-range (>10 nm) energy transfer can be achieved; (3) the luminescent color of the system has a wider adjustment range; (4) it is a better mimicking of multi-step energy transfer in natural light-harvesting systems (LHS). On this basis, a series of multi-step FRET systems have been reported in recent decades2c)

Fig. 4: Multi-step FRET systems constructed by metal-coordination interactions.
figure 4

a Supramolecular copolymerization of 1, 2, and 3 with the sequential FRET behaviors. Reprinted with permission from ref. 103, Copyright 2022 Nature Publishing Group. b Construction of artificial LHSs based on double helicate PCP-TPy1. Reprinted with permission from ref. 104, Copyright 2023 Nature Publishing Group. c Cartoon representation of the cascade FRET system constructed by orthogonal self-assembly. Reprinted with permission from ref. 18, Copyright 2021 American Chemical Society. d Illustration of the two-step FRET system based on metallacycle H3 and guest molecule G3. Reprinted with permission from ref. 106, Copyright 2023 American Chemical Society. e Representation of the three-step FRET system. Reprinted with permission from ref. 108, Copyright 2023 Elsevier. f Schematic illustration of supramolecular coordination polymer-based LHS with sequential energy transfer. Reprinted with permission from ref. 109, Copyright 2022 American Chemical Society.

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By adopting an orthogonal self-assembly approach, Yang and co-workers prepared a two-step FRET system through platinum-based coordination interactions and pillararene-based macrocyclic host-guest interactions (Fig. 4c)18. In this system, a series of fluorophores, such as anthracene, coumarin, and BODIPY, were well arranged in the scaffold at precise distances, which enables efficient two-step energy transfer. As a result, the 1O2 generation efficiency of this sequential FRET system is 1.5 times higher and the photooxidation activity is 1.2 times higher than that of the corresponding one-step system. This work not only demonstrates an efficient multi-step FRET system through orthogonal self-assembly but also provides new ideas for creating smart multi-responsive materials. In another work, Shi and co-workers reported a two-step FRET system by using platinum metallacycle based rotaxanes as energy donor and ESY and NiR as acceptors105. Later, they introduced chromophores onto both the metallacycles and dumbbells (Fig. 4d)106. Multiple chromophores including anthracene and carbazole were easily incorporated into the metallacycles and the guest, respectively. This strategy brings the chromophores into close proximity, facilitating an efficient FRET process. The obtained two-step FRET system was further used to prepare white light-emitting diodes and used as a nanoreactor for photocatalytic reactions.

In 2021, Zhang and co-workers reported a sequential energy transfer system based on metallacycle, which was used to catalyze the alkylation of C–H bonds in aqueous solution107. In a follow-up work, they further developed a three-step FRET system based on metallacycles (Fig. 4e)108. In this system, the AIE-active metallacycles was used as energy donor/antenna, ESY and SR101 served as conveyors, and near-infrared emissive chlorin-e6 (Ce6) was used as the final energy acceptor. The donors and acceptors were co-assembled by non-covalent interactions. The metallacycle was constructed from L4 and L5 through silver coordination. β-CD was introduced by host-guest interaction to enhance the hydrophilicity of the assemblies, which ensured that the donor and acceptor were in close proximity to each other for an efficient FRET. In another work, Mukherjee and co-workers constructed TPE-based emissive Pt(II) coordination polymers toward artificial LHSs with cascade energy transfer (Fig. 4f)109. The coordination polymer exhibited significant emission enhancement in water/DMSO (v/v = 9/1) mixture as it further self-assembled into discrete spherical nanoparticles. The hydrophobic cavities can play the role of a suitable host to encapsulate organic dyes, such as ESY and NiR, to achieve efficient cascade FRET.