Tuning the concentration of dye loaded polymer films for maximum photosensitization efficiency: phloxine B in poly(2-hydroxyethyl methacrylate)

Abstract

Fluorescence and singlet molecular oxygen ((1)O2) quantum yields for phloxine B loaded poly(2-hydroxyethyl methacrylate) thin films are determined at dye concentrations from 0.015 to 22 wt%. Fluorescence self-quenching and the fall off of the (1)O2 quantum yield observed above 0.1 wt% are attributed to very weakly interacting close-lying dye molecules acting as energy traps arising from molecular confinement. The maximum singlet oxygen generation efficiency (quantum yield × absorption factor) lies at concentrations around 2 wt%, where fluorescence self quenching amounts to more than 80%. Data are fitted quantitatively by using a quenching radius model involving energy migration and trapping with rQ = 1.2 nm. The present results constitute a proof of concept for the rational design of heterogeneous photosensitizers in general and, particularly, for applications in which the antimicrobial activity of singlet oxygen is central.

Photophysics of Xanthene Dyes at High Concentrations in Solid Environments: Charge Transfer Assisted Triplet Formation

“…Parameters found are Φ F0 = 0.70 and c = 1.43 lmol g À1 for EoY and Φ F0 = 0.82 and c = 1.05 lmol g À1 for PhB. The values of Φ F found in basic ethanol, where the dyes remain dianionic, are 0.68 for EoY (30) and 0.76 for PhB (3). For xanthene dyes, Φ F values expected in cellulose have to be somewhat larger than those found in alcohols (23).…”

“…In other words, no distinct fluorescence features are observed at the highest concentrations. Absorption and corrected fluorescence spectra for both dyes in cellulose, aside from a small red‐shift, resemble those of the dianionic form of the dyes in basic ethanol .…”

“…Thus, changes in the supporting material may have strong influence on the photophysical behavior. Recently, we studied the photophysics of PhB included in pHEMA thin films as a function of dye concentration . Fluorescence and singlet oxygen quantum yields follow the same trend on increasing dye concentration.…”

“…Charge recombination in 3 (M + ÀM À ) leads to the pair triplet state 3 (MÀM)* under appropriate energetic conditions. Additionally, 3 (MÀM)* may be formed directly from 1 (MÀM)* (regular triplet state formation, Scheme 2). As excitonic interactions depend on the square of the transition moment, negligible for triplet dye pairs, the pair behaves as an isolated, monomeric triplet, decaying therefore with the same lifetime as 3 M.…”

“…Under appropriate conditions, geminate radical anionÀradical cation recombination may lead to the formation of dye triplet states, as it has been found in the reaction center of photosystem II. The 1 (P680 + Ph À ) (Ph: pheophytin) radical pair formed by excitation of P680 and subsequent electron transfer to Ph reverts to 3 (P680 + Ph À ), decaying finally to the P680 triplet state. Energy transfer from the triplet radical pair to molecular oxygen has been recognized as an important source of 1 O 2 in photosynthesis (13).…”