A bioinspired sequential energy transfer system constructed via supramolecular copolymerization

Abstract

Sequential energy transfer is ubiquitous in natural light harvesting systems to make full use of solar energy. Although various artificial systems have been developed with the biomimetic sequential energy transfer character, most of them exhibit the overall energy transfer efficiency lower than 70% due to the disordered organization of donor/acceptor chromophores. Herein a sequential energy transfer system is constructed via supramolecular copolymerization of σ-platinated (hetero)acenes, by taking inspiration from the natural light harvesting of green photosynthetic bacteria. The absorption and emission transitions of the three designed σ-platinated (hetero)acenes range from visible to NIR region through structural variation. Structural similarity of these monomers faciliates supramolecular copolymerization in apolar media via the nucleation-elongation mechanism. The resulting supramolecular copolymers display long diffusion length of excitation energy (> 200 donor units) and high exciton migration rates (~10¹⁴ L mol^-1 s^-1), leading to an overall sequential energy transfer efficiency of 87.4% for the ternary copolymers. The superior properties originate from the dense packing of σ-platinated (hetero)acene monomers in supramolecular copolymers, mimicking the aggregation mode of bacteriochlorophyll pigments in green photosynthetic bacteria. Overall, directional supramolecular copolymerization of donor/acceptor chromophores with high energy transfer efficiency would provide new avenues toward artificial photosynthesis applications.

Fig. 1. Natural and artificial sequential energy transfer systems.

a Direct supramolecular aggregation of bacteriochlorophyll c (BChl c) into a light-harvesting antenna, together with the sequential energy transfer process in green photosynthetic bacteria. b Supramolecular copolymerization of 1, 2, and 3 (cartoon symbols with green, pink, and purple colors in the middle parts, respectively) with the sequential energy transfer behaviors. The supramolecular homopolymers of 1 emit green light centered at 507 nm. The binary supramolecular copolymers 1/2 display orange emission (λ_max = 599 nm) with a one-step energy transfer character. For the ternary supramolecular copolymers 1/2/3, sequential energy transfer takes place from 1 via 2 to 3, giving rise to the emission enhancement in the near-infrared region (λ_max = 775 nm).

Fig. 2. Spectroscopic characterizations of the designed compounds 1–3.

a, b Absorption and emission spectra of 1–3 (c: 1.0 × 10⁻⁵ mol L^–1 for 1–2 in dichloromethane and for 3 in 1,2-dichloroethane), together with the corresponding photographs taken under the same concentration and solvent conditions. The inset photographs in Fig. 2b were taken under a hand-held ultraviolet lamp with an excitation wavelength of 365 nm. c Energy level diagram of 1–3 based on DFT computations. HOMO and LUMO represent the highest occupied molecular orbital and the lowest unoccupied molecular orbital, respectively.

Fig. 3. Supramolecular polymerization behaviors.

a CD spectra (1 mm cuvette, 298 K) for 1–3 in MCH (c: 2 × 10⁻⁴ mol L⁻¹ for 1 or 2, and 8 × 10⁻⁵ mol L⁻¹ for 3). The lower concentration of 3 is ascribed to its poor solubility in MCH. b Non-sigmoidal heating curves acquired via monitoring CD intensity changes of 1–2 (λ: 486 nm for 1 and 579 nm for 2), together with the absorption intensity changes of 3 (λ: 709 nm) (c: 1.4 × 10⁻⁴ mol L⁻¹ for 1 or 2, and 8 × 10⁻⁵ mol L⁻¹ for 3 in MCH). In panels, the curves are shown with a 0.1 offset. Data and fit are represented as colored and black lines, respectively. All melting curves were fitted by the mass balance model developed by Markvoort and ten Eikelder. c, d Optimized geometries of trimeric species 2₃ and 1/2/1. For both optimized geometries, non-metallic elements were described by a 6-31 G basis set, while Lanl2dz effective core potential developed by Los Alamos National Laboratory was employed to describe Pt(II) ions. Dispersion-corrected exchange functional ωb97xd was employed to optimize geometries of the trimeric species.

Fig. 4. Supramolecular copolymerization and energy transfer behaviors of the binary species 1/2.

a CD melting curves of supramolecular homopolymers 1 (c: 8.0 × 10⁻⁵ mol L⁻¹ in MCH, red line) and supramolecular copolymers 1/2 (c: 8.0 × 10⁻⁵ mol L⁻¹ for 1 and 1.6 × 10⁻⁵ mol L⁻¹ for 2 in MCH, blue line) by tracking the CD intensities at 486 nm. In panels, the curves are shown with a 0.2 offset. Data and fit are represented as colored and black lines, respectively. Both melting curves were fitted by the mass balance model developed by Markvoort and ten Eikelder. b Steady-state fluorescent emission changes upon increasing the acceptor molar ratios. The concentration of 1 is kept at 8.0 × 10⁻⁵ mol L⁻¹. Filling colors are defined based on the CIE coordinates of the emission spectra. c Ratiometric plot of 2 upon direct excitation of 1 (c: 8.0 × 10⁻⁵ mol L⁻¹ in MCH). The linear fitting of the ratiometric plot is obtained by plotting the value of I₅₉₉/I₅₀₇ versus the molar ratio of 2. I₅₉₉ and I₅₀₇ denote the emission intensity of 2 at 599 nm and 1 at 507 nm, respectively. d Fluorescence lifetime decay of supramolecular homopolymers 1 (c: 8.0 × 10⁻⁵ mol L⁻¹ in MCH, red line) and supramolecular copolymers 1/2 (c: 8.0 × 10⁻⁵ mol L⁻¹ for 1 and 1.6 × 10⁻⁵ mol L⁻¹ for 2 in MCH, blue line).

Fig. 5. Sequential energy transfer behaviors of the ternary supramolecular copolymers 1/2/3.

a Steady-state fluorescence emission changes upon titrating 3 into supramolecular copolymers 1/2 (100: 20 mol%) (c: 8.0 × 10⁻⁵ mol L⁻¹ for 1 and 1.6 × 10⁻⁵ mol L⁻¹ for 2 in MCH). The filling colors of emission profiles are defined according to the Commission Internationale de l’Eclairage (CIE) coordinates. b Fluorescence lifetime decay curves of supramolecular copolymers 1/2 (100: 20 mol%) and 1/2/3 (100: 20: 10 mol%) at 599 nm (c: 8.0 × 10⁻⁵ mol L⁻¹ for 1, 1.6 × 10⁻⁵ mol L⁻¹ for 2, and 8.0 × 10⁻⁶ mol L⁻¹ for 3 in MCH). c Two-dimensional excitation spectrum of the binary supramolecular copolymers 1/2. d Two-dimensional excitation spectrum of the ternary supramolecular copolymers 1/2/3.

Fig. 6. Sequential energy transfer parameters and control experiment.

a Energy level diagram for energy transfer in supramolecular copolymers 1/2/3. It is assumed that S₀ → S₁ electronic transition exclusively takes place for 1 upon 450 nm excitation. b Steady-state fluorescence emission changes upon titrating 5 and 5/6 into the supramolecular polymers of 1 (c: 8.0 × 10⁻⁵ mol L⁻¹ for 1 in MCH, 1.6 × 10⁻⁵ mol L⁻¹ for 5, and 8.0 × 10⁻⁶ mol L⁻¹ for 6 in MCH). The k_ET refers to the energy transfer rate between donor and acceptor.

Fig. 7. Emission color tuning of the ternary supramolecular copolymers 1/2/3.

a Fluorescent emission spectral changes of supramolecular copolymers 1/2/3 without light irradiation (red line) and with 460 nm light irradiation for 36.5 min (green line). Inset: changes of CIE (x, y) coordinates versus the 460 nm irradiation time (t). b CIE coordinate changes upon 460 nm irradiation. The measured concentrations are 8.0 × 10⁻⁵ mol L⁻¹ for 1, 1.6 × 10⁻⁵ mol L⁻¹ for 2, and 8.0 × 10⁻⁶ mol L⁻¹ for 3 in MCH.