Redox-Triggered Activation of Heavy-Atom-Free Photosensitizer and Implications in Targeted Photodynamic Therapy

Activatable Heavy-Atom-Free Photosensitizer with Large Stokes Shift and a NIR-II Emission Harnessing Rhodamine Ring-Opening Strategy

Activatable photosensitizers (PSs) generating 1O2 only under specific conditions can minimize concomitant injury to normal tissues. Heavy-atom-free PSs hold the merits of low dark toxicity, long triplet-state lifetimes, good photostability, and relatively low cost. PSs with emission in the second near-infrared (NIR-II) window are highly valuable for deep-tissue, high-contrast imaging. Herein, we have designed and synthesized a series of heavy-atom-free PSs by a one-step reaction between an easily accessible rhodamine derivative and commercially available thiophene aldehydes. One of the as-prepared PSs, 2b-3T, exhibits emission maxima at 810 nm and tails to the NIR-II region at 1140 nm, together with large Stokes shift (178 nm). Importantly, the newly developed PSs, featuring functional carboxylic acid groups, present promising opportunities as versatile platforms for creating activatable PSs. To validate our concept, we developed Cu2+/pH-activatable PSs using the spirocyclization mechanism of rhodamine. Ultimately, we showcased the effectiveness of these innovative PSs in photodynamic therapy through in vitro experiments.

Thiocarbonyl-Bridged N-Heterotriangulenes for Energy Efficient Triplet Photosensitization: A Theoretical Perspective

Structurally-rigid metal-free organic molecules are of high demand for various triplet harvesting applications. However, inefficient intersystem crossing (ISC) due to large singlet-triplet gap (Δ E S - T ${\Delta {E}_{S-T}}$ ) and small spin-orbit coupling (SOC) between lowest excited singlet and triplet often limits their efficiency. Excited electronic states, fluorescence and ISC rates in several thiocarbonyl-bridged N-heterotriangulene ( m ${m}$ S-HTG) with systematically increased thione content (m = ${m=}$ 0-3) are investigated implementing polarization consistent time-dependent optimally-tuned range-separated hybrid. All m ${m}$ S-HTGs are dynamically stable and also thermodynamically feasible to synthesize. Relative energies of several low-lying singlets (S n ${{S}_{n}}$ ) and triplets (T n ${{T}_{n}}$ ), and their excitation nature (i. e.,n π * ${n{\pi }^{^{\ast}}}$ orπ π * ${\pi {\pi }^{^{\ast}}}$ ) and SOC are determined for these m ${m}$ S-HTGs in dichloromethane. Low-energy optical peak displays gradual red-shift with increasing thione content due to relatively smaller electronic gap resulted from greater degree of orbital delocalization. Significantly large SOC due to different orbital-symmetry and heavy-atom effect produces remarkably high ISC rates (k I S C ${{k}_{ISC}}$ ~1012 s-1) for enthalpically favouredS 1 n π * → T 2 ${{S}_{1}\left(n{\pi }^{^{\ast}}\right)\to {T}_{2}}$ (π π * ${\pi {\pi }^{^{\ast}}}$ ) channel in these m ${m}$ S-HTGs, which outcompete radiative fluorescence rates (~108 s-1) even directly from higher lying optically brightπ π * ${\pi {\pi }^{^{\ast}}}$ singlets. Importantly, high energy triplet excitons of ~1.7 eV resulting from such significantly large ISC rates from non-fluorescentS 1 n π * ${{S}_{1}\left(n{\pi }^{^{\ast}}\right)}$ make these thiocarbonylated HTGs ideal candidates for energy efficient triplet harvest including triplet-photosensitization.

TME-Triggered Degradable Phototheranostic Nanoplatform for NIR-II Fluorescence Bioimaging-Guided Phototherapies and Immune Activation

The low therapeutic efficacy and potential long-term toxicity of antitumor treatments seriously limit the clinical application of phototherapies. Herein, we develop a degradable phototheranostic nanoplatform for NIR-II fluorescence bioimaging-guided synergistic photothermal (PTT) and photodynamic therapies (PDT) and immune activation to inhibit tumor growth. The phototheranostic nanoplatform (CX@PSS) consists of multidisulfide-containing polyurethane loaded with a photosensitizer CX, which can be specifically degraded in the GSH overexpressed tumor microenvironment (TME) and exhibits good NIR-II fluorescence, photodynamic, and photothermal properties. Under 808 nm light irradiation, CX@PSS exhibits efficient photothermal conversion and ROS generation, which further induces immunogenic cell death (ICD), releasing tumor-associated antigens and activating the immune response. In vitro and in vivo studies confirm the potential of CX@PSS in NIR II FL imaging-guided tumor treatments by synergistic PTT, PDT, and immune activation. This work is expected to provide a new pathway for clinical applications of imaging-guided tumor diagnosis and treatments.

First-principles design of heavy-atom-free singlet oxygen photosensitizers for photodynamic therapy

In photodynamic therapy (PDT) treatment, heavy-atom-free photosensitizers (PSs) are a great source of singlet oxygen photosensitizer. Reactive oxygen species (ROS) are produced by an energy transfer from the lowest energy triplet excited state to the molecular oxygen of cancer cells. To clarify the photophysical characteristics in the excited states of a few experimentally identified thionated (>C=S) molecules and their oxygenated congeners (>C=O), a quantum chemical study is conducted. This study illustrates the properties of the excited states in oxygen congeners that render them unsuitable for PDT treatment. Concurrently, a hierarchy is presented based on the utility of the lowest-energy triplet excitons of thionated compounds. Their non-radiative decay rates are calculated for reverse-ISC and inter-system crossover (ISC) processes. In addition, the vibronic importance of C=O and C=S bonds is clarified by the computation of the Huang-Rhys factor, effective vibrational mode, and reorganization energy inside the Marcus-Levich-Jörtner system. ROS generation in thionated PSs exceeds their oxygen congeners as kf ≪ kISC, where radiative decay rate is designated as kf. As a result, the current work offers a calculated strategy for analyzing the effectiveness of thionated photosensitizers in PDT.

Photocleavable Visible Light-Triggered Anthraquinone-Derived Water-Soluble Block Copolymer for Peroxynitrite Generation in Cancer Therapy

We report a facile stimuli-responsive strategy to generate reactive oxygen and nitrogen species (ROS and RNS) in the biological milieu from a photocleavable water-soluble block copolymer under visible light irradiation (427 nm, 2.25 mW/cm2). An anthraquinone-based water-soluble polymeric nitric oxide (NO) donor (BCPx-NO) is synthesized, which exhibits NO release in the range of 40-65 μM within 10 h of photoirradiation with a half-life of 30-103 min. Additionally, BCPx-NO produces peroxynitrite (ONOO-) and singlet oxygen (1O2) under photoirradiation. To understand the mechanism of NO release and photolysis of the functional group under blue light, we prepared a small-molecule anthraquinone-based N-nitrosamine (NOD). The cellular investigation of the effect of spatiotemporally controlled ONOO- and 1O2 generation from the NO donor polymeric nanoparticles in a triple negative breast adenocarcinoma (MDA-MB-231) under visible light irradiation (white light, 5.83 mW/cm2; total dose 31.5 J/cm2) showed an IC50 of 0.6 mg/mL. The stimuli-responsive strategy using a photolabile water-soluble block copolymer employed to generate ROS and RNS in a biological setting widens the horizon for their potential in cancer therapy.