Easy but Efficient: Facile Approach to Molecule with Theoretically Justified Donor-Acceptor Structure for Effective Photothermal Conversion and Intravenous Photothermal Therapy

Abstract

To accelerate the pace in the field of photothermal therapy (PTT), it is urged to develop easily accessible photothermal agents (PTAs) showing high photothermal conversion efficiency (PCE). As a proof-of-concept, hereby a conventional strategy is presented to prepare donor-acceptor (D-A) structured PTAs through cycloaddition-retroelectrocyclization (CA-RE) reaction, and the resultant PTAs give high PCE upon near-infrared (NIR) irradiation. By joint experimental-theoretical study, these PTAs exhibit prominent D-A structure with strong intramolecular charge transfer (ICT) characteristics and significantly twisting between D and A units which account for the high PCEs. Among them, the DMA-TCNQ exhibits the strongest absorption in NIR range as well as the highest PCE of 91.3% upon irradiation by 760-nm LED lamp (1.2 W cm^-2). In vitro and in vivo experimental results revealed that DMA-TCNQ exhibits low dark toxicity and high phototoxicity after IR irradiation along with nude mice tumor inhibition up to 81.0% through intravenous therapy. The findings demonstrate CA-RE reaction as a convenient approach to obtain twisted D-A structured PTAs for effective PTT and probably promote the progress of cancer therapies.

Keywords: CA–RE reaction; D–A junction; antitumor; photothermal therapy; twisted molecules.

Scheme 1

a) Simplified Jablonski energy diagram illustrating the excitation of an electron and subsequent energy relaxation way, b) mechanism of CA–RE reaction between electron‐donating substituents on the alkyne reactivity with TCNE or TCNQ, c) schematic illustration of DMA‐TCNQ self‐assembly into nanoparticles, and subsequent imaging‐guided PTT in vivo, d) the D–A structured molecules studied in this work.

Figure 1

The optimized structures a), the HOMO b) and LUMO c) distributions of DMA‐TCNE, DPA‐TCNE, DMA‐TCNQ, and DPA‐TCNQ (the orange region corresponds to a positive value, while the blue region represents a negative value with isovalue of 0.05 au.), the electron‐hole distribution d) of DMA‐TCNE, DPA‐TCNE, DMA‐TCNQ, and DPA‐TCNQ (the orange part depicts hole while green part refers to electron with isovalue of 0.002 au.).

Figure 2

a) The UV–vis–NIR spectra of the D–A structured molecules in DCM (20 µm) and b) in 1% DMSO PBS solution (80 µm); the size distribution of c) DMA‐TCNE, d) DPA‐TCNE, e) DMA‐TCNQ, and f) DPA‐TCNQ in 1% DMSO PBS solution measured by DLS, g) heating and cooling curves, and h) linear time data versus −lnθ from the cooling period of D–A structured molecules (80 µm in 1% DMSO PBS solution) under 760‐nm LED lamp irradiation (1.2 W cm⁻²), i) temperature elevation of D–A structured molecules (80 µm in 1% DMSO PBS solution) under 760‐nm LED lamp irradiation (1.2 W cm⁻²) for 5 irradiation–cooling cycles.

Figure 3

a) Schematic representation of the reorganization energy for four‐point calculations, b) summary of reorganization energy, radiative transition rate, and nonradiative transition rate for D–A structured molecules through the theoretical calculations, calculated total reorganization energy versus normal mode frequencies for c) DMA‐TCNE, d) DPA‐TCNE, e) DMA‐TCNQ, and f) DPA‐TCNQ, insets are contributions to the total reorganization energy from dihedral angle, bond length, and bond angle, minimum energy geometries calculated for the S₀ (gray) and S₁ (orange) for g) DMA‐TCNE, h) DPA‐TCNE, i) DMA‐TCNQ, and j) DPA‐TCNQ.

Figure 4

Cell viability of NCl‐H460 cells with different concentrations of DMA‐TCNQ. a) After 0.5 h of incubation, cells were irradiated with a 760‐nm LED lamp (30, 60, and 90 mW cm⁻²) irradiation for 20 min, b) after 0.5 h of incubation, cells were irradiated for different time durations (0, 5, 10, and 15 min) with a 760‐nm LED lamp (90 mW cm⁻²), Error bars, mean ± SD (n = 4). * p < 0.05, ** p < 0.01, *** p < 0.001, c,d) fluorescence images of living and dead NCI‐H460 cells (in both 2D monolayers and 3D tumorspheres) with calcein AM/PI staining and after different treatments, scale bar = 100 µm, e–f) western blot analysis and quantification of HSP70 and HSP90 proteins after a 24‐h treatment (separated into four groups, including “control”, “0 µm + L”, “15 µm + L”, and “30 µm + L”, Light: 760‐nm LED lamp, 90 mW cm⁻², 20 min) Error bars, mean ± SD (n = 3), * p < 0.05, ** p < 0.01, *** p < 0.001, g) flow cytometry analysis of NCI‐H460 cells using Annexin V‐FITC and PI staining following different treatments (cells were incubated under various conditions and separated into four groups, including “control”, “0 µm + L”, “15 µm + L”, and “30 µm + L”).

Figure 5

a) Schematic illustration of the PTT treatment, b, c) temperature changes at the tumor site irradiated with a 760‐nm LED lamp (1.2 W cm⁻²) for 10 min from infrared images of nude mice on (b) day 0 and (c) day 8, d) body weight curves, e) tumor volume curves, f) final tumor weight of mice, g) photograph of final tumor nodules of the vehicle group and the photothermal therapy of DMA‐TCNQ groups. Error bars, mean ± SD (n = 6), statistical significance was determined at * p < 0.05, ** p < 0.01, *** p < 0.001, representing significant difference compared with the “Vehicle + L”), red arrows indicate time points of intravenous injection treatments, h) IR images of NCI‐H460 lung tumor‐bearing BALB/c nude mice at different time after intravenous injection of DMA‐TCNQ (irradiation by 760‐nm LED lamp, 4 mg kg⁻¹ DMA‐TCNQ), i) The final temperature at the tumor site after 10 min of light exposure. Error bars, mean ± SD (n = 3).