Effect of Concentration on the Formation of Rose Bengal Triplet State on Microcrystalline Cellulose: A Combined Laser-Induced Optoacoustic Spectroscopy, Diffuse Reflectance Flash Photolysis, and Luminescence Study

Abstract

Laser-induced optoacoustic spectroscopy (LIOAS), diffuse reflectance laser flash photolysis (DRLFP), and laser-induced luminescence (LIL) have been applied in conjunction to the determination of triplet state quantum yields of Rose Bengal (RB) supported on microcrystalline cellulose, a strongly light-scattering solid. Among the three used methods, the only one capable of providing absolute triplet quantum yields is LIOAS, but DRLFP and LIL aid in demonstrating that the LIOAS signal arises in fact from the triplet state and confirm the trend found with RB concentration. The coherence found for the three techniques demonstrates the usefulness of the approach. Observed triplet quantum yields are nearly constant within a limited concentration range, after which they decay strongly due to the generation of inactive dye aggregates or energy trapping centers. When quantum yields are divided by the fraction of absorbed light exciting the dye, the quotient falls off steadily with concentration, following the same trend as the observed fluorescence quantum yield. The conditions that maximize triplet formation are determined as a compromise between the rising light absorption and the decrease of quantum yield with RB concentration.

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

“…Triplet formation by dye aggregates was previously suggested for RB included in microcrystalline cellulose and confirmed for the same dye included in poly(2‐hydroxyethyl methacrylate) (pHEMA) thin films . In both cases, fluorescent dimers capable of rendering triplet states were observed, whereas higher‐order aggregates were dark and photochemically inactive.…”

“…However, our experience involving different dyes in various solid environments shows that, as dye concentration further increases, oligomeric traps are formed, leading finally to extinction of fluorescence and triplet formation. Triplet formation by dye aggregates was previously suggested for RB included in microcrystalline cellulose (26) and confirmed for the same dye included in poly(2-hydroxyethyl methacrylate) (pHEMA) thin films (4). In both cases, fluorescent dimers capable of rendering triplet states were observed, whereas higherorder aggregates were dark and photochemically inactive.…”

“…DRLFP measurements were noisier, and the number of points in the low-energy linear regime scarce, so that slopes were calculated using the fitting hyperbolic equation: signal(t = 0) = aE/(b + E) + cE, where E is the laser pulse energy. This fitting equation has no physical meaning; however, it was demonstrated in a previous work that it is a good extrapolation function (26).…”

“…For principles and details see ref. . The setup allows the detection of triplet‐triplet absorption between 600 and 700 nm and phosphorescence between 650 and 750 nm.…”

Photophysics at Unusually High Dye Concentrations

“…Photoactive traps were also found for rose bengal in microcrystalline cellulose. 31,32 Dye concentrations of about 1 M were obtained for layer-bylayer self-assembled thin films composed of rose bengal and poly[diallyldimethylammonium] chloride (PDDA), showing measurable fluorescence and singlet oxygen photogeneration even at these extreme conditions. 33 In this case, however, dyes are not randomly located, as the unfolded conformation of PDDA provides uniformly distributed charged sites for rose bengal adsorption, thus leading to some degree of molecular organization, which markedly reduces dye aggregation and energy trapping.…”