Toluidine blue O demethylated photoproducts as type II photosensitizers

Area of Science:

• Photochemistry
• Photodynamic Therapy
• Photosensitizers

Background:

• Toluidine blue O (TBO) is a photosensitizer with demonstrated efficacy in antimicrobial and anticancer photodynamic therapy.
• The complex photochemistry of TBO, leading to multiple photoproducts, has limited its application.
• Previous work identified demethylation as a key TBO photochemical pathway, yielding demethylated-TBO (d-TBO) and double-demethylated-TBO (dd-TBO).

Purpose of the Study:

• To investigate the photophysical properties of TBO photoproducts (d-TBO and dd-TBO).
• To understand how demethylation affects the photochemistry and singlet oxygen generation of TBO.
• To evaluate the potential of TBO photoproducts for photodynamic therapy.

Main Methods:

• Spectroscopic analysis of TBO, d-TBO, and dd-TBO in acetonitrile and phosphate buffer.
• Measurement of absorption and fluorescence emission spectra.
• Determination of fluorescence quantum yields and lifetimes.
• Evaluation of triplet excited state lifetimes and singlet oxygen quantum yields.

Main Results:

• Demethylation of TBO caused a hypsochromic shift in absorption and fluorescence emission.
• Fluorescence quantum yields and lifetimes increased slightly for d-TBO and dd-TBO.
• Singlet oxygen quantum yield significantly increased upon demethylation, reaching unity for dd-TBO.
• Demethylation is the primary deactivation pathway for TBO, while singlet oxygen generation dominates for dd-TBO.

Conclusions:

• TBO photoproducts, d-TBO and dd-TBO, exhibit altered photophysical properties compared to the parent compound.
• The dd-TBO photoproduct demonstrates highly efficient singlet oxygen generation, a critical factor in photodynamic therapy.
• These findings highlight the potential of TBO demethylated photoproducts as improved photosensitizers for photodynamic therapy.