Coupling molecular rigidity and flexibility on fused backbones for NIR-II photothermal conversion

Abstract

Great attention is being increasingly paid to photothermal conversion in the near-infrared (NIR)-II window (1000-1350 nm), where deeper tissue penetration is favored. To date, only a limited number of organic photothermal polymers and relevant theory have been exploited to direct the molecular design of polymers with highly efficient photothermal conversion, specifically in the NIR-II window. This work proposes a fused backbone structure locked via an intramolecular hydrogen bonding interaction and double bond, which favors molecular planarity and rigidity in the ground state and molecular flexibility in the excited state. Following this proposal, a particular class of NIR-II photothermal polymers are prepared. Their remarkable photothermal conversion efficiency is in good agreement with our strategy of coupling polymeric rigidity and flexibility, which accounts for the improved light absorption on going from the ground state to the excited state and nonradiative emission on going from the excited state to the ground state. It is envisioned that such a concept of coupling polymeric rigidity and flexibility will offer great inspiration for developing NIR-II photothermal polymers with the use of other chromophores.

Fig. 1. Ideal molecular design of organic NIR-II photothermal materials, which require (a) molecular rigidity and planarity in the ground state and (b) molecular flexibility and rotatability in the excited state. (c) Design of NIR-II photothermal polymers and schematic illustration of the related photothermal process. Images of the three polymer solutions (50 μg ml⁻¹ in chlorobenzene) and their specific molecular structures, from left to right, (d) Y1, (e) Y2, and (f) Y3, respectively.

Scheme 1. Synthetic route for the Y2 and Y3 polymers.

Fig. 2. Absorption spectra of Y1, Y2, and Y3 (from top to bottom) in chlorobenzene solutions. The absorption curves of each polymer at different concentrations and temperatures are provided in the boxes, respectively. The curves of the same color, from top to bottom for each polymer, represent the absorption spectra of samples with a concentration of 400 μg ml⁻¹ (Y1-1, Y2-1, and Y3-1), 200 μg ml⁻¹ (Y1-2, Y2-2, and Y3-2), 100 μg ml⁻¹ (Y1-3, Y2-3, and Y3-3), 50 μg ml⁻¹ (Y1-4, Y2-4, and Y3-4), and 25 μg ml⁻¹ (Y1-5, Y2-5, and Y3-5). The black curve is the background absorption of the pure chlorobenzene solution. The different colors of the curves represent different temperatures, ranging from 20 to 70 °C at intervals of 10 °C.

Fig. 3. Structure–property analysis of light absorption. (a) Schematic energy diagram of the transition corresponding to the maximum absorption peak. (b) Schematic illustration of electron changes in the transition, in which the structure of the light red box is where the hole of the ground state is, and that of the light green box is where the electron excited state is. During the transition, the electron goes from the light red box to the light green box. The detailed changes are provided in (c), and the dark red and green parts indicate the hole and electron clouds of all of the molecular orbitals in the ground and excited states related to the transition. The isovalue is 0.002.

Fig. 4. Photothermal performance of the NIR-II photothermal polymers. (a) Temperature changes of their solutions irradiated using different light powers for 2 min, in which the control group is pure chlorobenzene. The wavelength of the laser is 1064 nm, and the absorbance of the polymer solutions is around 0.5, with an optical path length of 0.1 mm. (b) The on–off curve of the polymer solution irradiated using a 1064 nm laser of different power, and ΔT is the difference between the average temperature of the irradiated area and the room temperature (20 °C). (c) Temperature distributions of the polymer solution irradiated by a 0.9 W laser for 2 min.

Fig. 5. Molecular vibration analysis of the NIR-II photothermal polymers. (a) Schematic diagram of the photothermal polymers from the excited state to the ground state via internal conversion. (b) Parameters included in the molecular vibration. The red numbers are used to specify atoms involved in the theoretical calculation. (c) Differential curve from the relative energy–dihedral angle curve. The intersection of the curve and the line dRE/dDA = 0 is the lowest energy point, and the corresponding relative energy is 0 eV. k is the slope of the linear variation region of the above curve from the intersection point to the largest dihedral angle. k₁, k₂, and k₃ correspond to the slopes of the curves of the Y1-trimer-B, Y2-trimer-B and Y3-trimer-B, respectively. (d) The dihedral angles and the corresponding hydrogen and double bond lengths of the three trimers in their optimal conformations in the ground and excited states.