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Solid-State Photosensitizer for Singlet Oxygen Generation

A film-based photosensitizer made of perylene bisimide (PBI) modified with long alkyl chains and spirofluorene units has been developed for production of singlet oxygen.

Singlet oxygen, one of the excited electronic states of molecular oxygen O2, is a double-edged sword. On one hand, singlet oxygen can damage cells via oxidation of proteins and cell materials; on the other hand, it is the basis of photodynamic therapy for cancer treatment, which has proven to be very effective with tolerable side effects.

Although singlet oxygen can be produced in a variety of ways, energy transfer from organic molecules under light illumination is one of the most efficient and controllable methods, where the organics are called photosensitizers. Therefore, the performance of photosensitizers plays a vital role in the production and application of singlet oxygen.

To date, coordination compounds based on polypyrrole are the most commonly used and studied photosensitizers. These compounds, however, suffer from limitations such as low photochemical stability and low absorption efficiency of visible light. In addition, they are generally used in solution or suspension, which further restricts their practicality. Therefore, the development of high performance, solid-state photosensitizers of molecular oxygen remains a challenge.

In a recent development, a joint team from Shaanxi Normal University and Xi’an Jiaotong University fabricated a unique film-based, non-polypyrrole photosensitizer, where the active layer was perylene bisimide (PBI) modified with long alkyl chains and spirofluorene units. The authors explained that PBI was chosen because it possesses both high photochemical stability and high absorption efficiency in the visible light region. The alkyl chains were introduced to ameliorate the inherent poor solubility of PBI, while spirofluorene was employed to avoid the aggregation-cased quenching (ACQ) effect as well as to create voids and channels when the compound is deposited in the film, which is believed to be beneficial to the entry of molecular oxygen and exit of singlet oxygen.

Photophysical studies demonstrated that the compound showed good solubility in common organic solvents, great photochemical stability, and high absorption efficiency in the visible light region. Its ability as an effective photosensitizer in the generation of singlet oxygen was also verified by its reaction with uric acid. Further studies revealed that the effective photoproduction of singlet oxygen could be realized via the utilization of a tiny and low-priced LED lamp as a light source and a film support.

The researchers, who published their findings in Science China Chemistry, believe the film-based photosensitiser possess a number of advantages over conventional ones used in the solution or suspension state, such as reusability, being contamination-free, and allowing for device-making. They next plan to focus on the application of the conceptual device as a medical instrument for photodynamic therapy. [APBN]

Source: Xu, W. et al. (2020). A new spirofluorene-based nonplanar PBI-dyad and its utilization in the film-based photo-production of singlet oxygen. Science China Chemistry63(4), 526-533.