Dye-loaded photoactive thin films: towards maximum singlet oxygen generation efficiency

SERGIO D. EZQUERRA RIEGA; HERNÁN B. RODRÍGUEZ; ENRIQUE SAN ROMÁN

Santiago de Chile

Encuentro; 25th IAPS Meeting; 2016

Inter-American Photochemical Society (IAPS)

Photoactive dye-loaded polymer films capable of yielding singlet molecular oxygen (1O2) upon irradiation have potential applications, among others, in the development of wound dressings and coatings for medical and cosmetic use. In a previous work, Phloxine B (PhB) loaded poly(2-hydroxyethyl methacrylate) (pHEMA) thin films (~245 nm thick) were studied as a function of dye-loading. The 1O2 generation efficiency (absorption factor X ΦΔ) at the surface of the film achieved its maximum at high dye concentrations (~0.02 M).1 The apparent 1O2 quantum yield, ΦΔ, was rather low even at low PhB concentrations.In the present work Rose Bengal (RB) loaded pHEMA thin films were studied as a function of dye loading and conditions for the retrieval of maximum 1O2 generation were established. Thin films (~200 nm thick) were prepared by spin coating on glass substrates from ethanol solutions. Dye concentrations spanning more than three orders of magnitude were explored. Films were characterized by absorption and fluorescence spectroscopy, singlet oxygen was quantified using DPBF in CH2Cl2 as chemical monitor and the triplet decay was determined by laser flash-photolysis. Though absorption spectra are similar to those in ethanol solution, slight changes at dye concentrations in excess of 0.05 M point to weak dye to dye interactions in the ground-state. Fluorescence quantum yields start from ΦF = 0.050 ± 0.006 at low dye concentrations but self quenching occurs after 0.01 M (see figure). Apparent 1O2 quantum yields follow the same trend, starting from ΦΔ = 0.03 ± 0.01. Results are interpreted in the context of a quenching radius model considering a random distribution of dye molecules. Calculation of energy migration and trapping rates are based on Loring, Andersen and Fayer (LAF) theory.2 Best fits are obtained in the whole concentration range with a quenching radius rQ = 1.5 nm, in the order of molecular dimensions, only if photoactive dimeric traps are considered. The observed fluorescence quantum yield ratio, Φtrap/Φmonomer ~ 0.4, is in line with previous reports on RB fluorescent dimers in cellulose.3 The occurrence of bright traps, capable of yielding triplet states and 1O2, increase the concentration of maximum 1O2 generation efficiency to about 0.1 M. Its value reaches a maximum of ~0.2%, 7 times higher than in PhB / pHEMA films. Results show that maximum efficiency can be achieved at very high dye concentrations even for random dye distributions and provide relevant information for the design of ordered dye arrays with photosensitization activity.1) Y. Litman, H. B. Rodríguez, E. San Román, Photochem. Photobiol. Sci., 2016, 15, 80.2) L. Kulak, C. Bojarski, Chem. Phys., 1995, 191, 67.3) H. B. Rodríguez, M. G. Lagorio, E. San Román, Photochem. Photobiol. Sci., 2004, 3, 674.