Room-temperature phosphorescence ZnPc(COOH)₄/Ch/MgO system

Abstract

In this study, we conjugated ZnPc(COOH)₄ with MgO nanoparticles (NPs) coated with chitosan through the free –OH and –NH₂ groups to enhance the photosensitizing properties. MgO NPs were synthesized by the co-precipitation method using sodium dodecyl sulfate (SDS), a well-known surfactant that improves the surface properties of nanoparticles, and aloe vera extract (AVE), which acts as an eco-friendly reducing agent and a non-hazardous gelling agent for stabilizing the nanostructures. On the other hand, the peripheral carboxy ( – COO⁻) substituted ZnPc soluble in aqueous media can forms homogeneous solutions. The UV-Vis absorption spectrum of ZnPc(COOH)₄/Ch/MgO in water exhibits an intense "shoulder" peak (extinction coefficient > 200,000) at approximately 380 nm (the Soret band), followed by a Q-band extending from 600 nm to 760 nm, with two maxima of subbands located around 647–649 nm and 705–709 nm, respectively. The fluorescence spectrum occurs at longer wavelengths than the absorption spectrum. The ZnPc(COOH)₄/Ch/MgO solutions, when excited at 360 nm show an emission band at 479 nm, and an additional, weaker band appears at 708 nm. When excited at 640 nm, the ZnPc(COOH)₄/Ch/MgO solutions exhibit the same emission bands but of a higher intensity at 708 nm and 782 nm. The fluorescence intensity of ZnPc(COOH)₄/Ch/MgO increases significantly due to intramolecular proton transfer between the ZnPc(COO⁻)₄ and the NH₃⁺ groups of the MgO/chitosan nanoparticles. The absolute quantum yield of fluorescence of ZnPc(COOH)₄/Ch/MgO system equals approximately 4%. Also, ZnPc(COOH)₄/MgO/chitosan system exhibits phosphorescence at room temperature. The triplet excited state lifetime of the system under 355 nm excitation, displays a bi-exponential decay. The corresponding lifetime values are reached values of 1.16 μs and 8.89 μs. The phosphorescence quantum yield of this system reaches 10.63%. To summarize, the present study facilitates the development of new sustainable room-temperature phosphorescent materials. This result indicates that the interaction between ZnPc(COOH)₄ and MgO/Ch NPs was strong, which was beneficial for restricting the molecular vibrations of MgO/Ch and promoting room-temperature phosphorescence emission. The extended lifetime of the triplet excited state facilitates the development of relatively inexpensive detection systems based on decay time measurements.