Contribution of Diffusion to the Quenching by Oxygen of the Lowest Electronically Excited Singlet and Triplet States of Aromatic Molecules in Liquid Solution under High Pressure

Abstract

The quenching by oxygen of the lowest electronically excited singlet (S1) and triplet (T1) states of five aromatic molecules in methylcyclohexane (MCH) at pressures up to 400 MPa was investigated. The apparent activation volume for the S1 state, Δ , at 0.1 MPa fell in the range from 14 to 16 cm3/mol, which is significantly smaller than Δ (25 cm3/mol) determined from the pressure dependence of the solvent viscosity, η, whereas that for the T1 state, Δ , at 0.1 MPa changed from +6.1 (anthracene) to −15.2 cm3/mol (triphenylene) and correlated approximately linearly with the triplet energy, E T. However, Δ at 350 MPa was positive and almost independent of E T (3−6 cm3/mol). The pressure dependence of was interpreted in the framework reported previously by us, and that of was attributed to the contribution of diffusion to the quenching in which the encounter complex pair with singlet, triplet, and quintet spin multiplicities formed between the oxygen and T1 state molecules is involved.

Solvent and Pressure Effects on Quenching by Oxygen of 9,10-Dimethylanthracene Fluorescence in Liquidn-Alkanes: A Study on Solvent Cage Effects

“…The fluorescence quenching by oxygen of 9,10-dimethylanthracene (DMEA) in liquid ethane and propane has been investigated. From the solvent and pressure dependence of the quenching rate constant, k q , together with the results in n-alkanes (C 2 -C 7 ) and methylcyclohexane (MCH) that were previously reported, it was revealed clearly that the quenching is not fully but partially diffusion-controlled in agreement with our previous conclusion 6,17,[19][20][21][22]24 and was found that the contribution of diffusion was successfully analyzed by eq 16. Finally, k diff /k -diff in n-alkanes (C 2 -C 7 ) and MCH was evaluated by assuming that k S is independent of the solvents examined in this work (see eq 21), and it was found that k diff /k -diff increases with increasing solvent density (Figure 7).…”

“…We have previously studied the fluorescence quenching of DMEA by oxygen (DMEA/O 2 ) at high pressure in mainly normal solvents and showed phenomenologically and also from the analysis by eq 5 that the quenching is not fully but nearly diffusion-controlled. ,,− , In this work, all of the data including the results in n -alkanes (C 4 −C 7 ) and methylcyclohexane (MCH) studied previously are analyzed by eq 6.…”

“…In eq 3, R ex is a constant determined experimentally. 16,17,[20][21][22]24 From eqs 2 and 3, we have eq 4.…”

“…23 Thus, the pressure dependence of k q revealed that the quenching of the S 1 state is not fully but nearly diffusion-controlled for various quenching systems. [15][16][17][18][19][20][21][22][23][24][25] In this work, we investigated the fluorescence quenching by oxygen of 9,10-dimethylanthracene (DMEA) in various solvents including liquefied gases such as ethane, propane, and n-butane in order to characterize experimentally the oxygen quenching in the solvent cage since the bimolecular quenching in the solvent cage is supposed to depend significantly on the solvent properties such as size, density, and viscosity. For this purpose, we should introduce g(σ) Pref common to the solvents examined since k bim 0 in eq 5 depends on g(σ) Pref .…”

“…In fact, eq 5 was successfully applied for various quenching systems in supercritical fluids 17,19 as well as in liquids, − ,− except for the fluorescence quenching of 9-cyanoanthracene by oxygen in supercritical carbon dioxide . Thus, the pressure dependence of k q revealed that the quenching of the S 1 state is not fully but nearly diffusion-controlled for various quenching systems. − …”