Formation and Decay Behaviors of Laser-Induced Transient Species from Pyrene Derivatives 1. Spectral Discrimination and Decay Mechanisms in Aqueous Solution

The ultraviolet and visible absorption spectra of laser-induced transient species from the following pyrene derivatives were measured in aqueous or aqueous ethanol solution: sodium pyrenesulfonate (NaPS), tetrasodium pyrenetetrasulfonate (Na 4 PS 4 ), pyrenecarboxylic acid (HPC), and pyrenebutyric acid (HPB). The major transient species contributing to the intense absorption peaks were revealed on the basis of the quenching experiments. The absorption spectra of triplets, as well as cation radicals, were separable from the other transients by selecting the experimental conditions (atmosphere, coexisting quencher, and delay time). The cation radicals produced from PS -, PB - and PC - anions showed an absorption maximum at ∼460 nm, close to that of pyrene cation radical. Their decay behaviors in the absence of any additional quenchers were dominated by bimolecular reaction kinetics with each parent molecule, of which the rate constants were very similar. This result is consistent with the previous proposal that these cation radicals exist as zwitterions such as P •+ S - . The cation radical from Na 4 PS 4 showed a strong absorption peak at 505 nm and exhibited different decay behaviors, suggesting that this cation radical appears not to be a simple zwitterion. Three specific quenchers, I -, OH -, and SO 3 2-, were found to strongly accelerate the decay rates of these cation radicals. CH 3 SO 3 - anion, mimicking the headgroup of tentative anionic surfactants, and several inorganic anions such as ClO 4 - were poor quenchers for the cation radicals even at the highest concentration. We also discussed the origin of two additional peaks at 375 and 395 nm observed in the transient absorption spectra of NaPS and HPB. On the triplet−triplet absorption, the molar extinction coefficients of Na 4 PS 4 and NaPS could be determined using the ground-state depletion method. These results are discussed in terms of applications to probe the micellar microenvironment.