On the origin of the heterogeneous emission from pyrene sequestered within tetramethylorthosilicate-based xerogels: A decay-associated spectra and O₂ quenching study

Steady-state and time-resolved fluorescence spectroscopy are used to determine the local microheterogeneity surrounding pyrene molecules sequestered within tetramethylorthosilicate-derived xerogels. After compensation for the intrinsic background emission from the xerogel, we find that the pyrene intensity decay kinetics are best described by a two-term rate law. This is consistent with the pyrene molecules distributing primarily into two microenvironments. Under ambient conditions, the individual pyrene microenvironments exhibit excited-state fluorescence lifetimes that differ by approximately 100 ns. However, the pyrene I₁ to I₃ band ratios that are associated with each microenvironment are statistically equivalent to one another. These results show that the local dipolarity surrounding these pyrene microenvironments are similar, but the decay rates associated with each microenvironment are very different. The longer-lived pyrene species (Environment #1) constitutes approximately 1/2 of the total fluorescence and it exhibits an O₂ quenching sensitivity (K_sv1) of (5.19±0.52)×10^-3 %O₂^-1 and a bimolecular quenching constant (k_q1) of (2.30±0.23)×10⁴ %O₂^-1 s^-1. Environment #2, associated with the shorter-lived pyrene species, exhibits an O₂ quenching sensitivity (K_sv2) of (2.31±0.16)×10^-2 %O₂^-1 and a bimolecular quenching constant (k_q2) of (2.11±0.23)×10⁵ %O₂^-1 s^-1. These results are interpreted as follows: Environment #1 consists of pyrene molecules sequestered within a relatively rigid siloxane network wherein non-radiative decay pathways are lessened, but these pyrene molecules are not quenched readily by O₂. Environment #2 consists of pyrene molecules adsorbed onto surface silanols within the xerogel. These pyrene molecules are quenched by the silanols and they are simultaneously more accessible to O₂ compared to Environment #1.