Contribution of local density augmentation in the vicinity of 9-cyanoanthracene and dynamic fluorescence quenching by oxygen in liquid and supercritical carbon dioxide

Abstract

The local density augmentation in the vicinity of 9-cyanoanthracene (CNA) and the dynamic fluorescence quenching of CNA by oxygen is described in order to study the contribution of local composition enhancement near the fluorophore molecule to the quenching in supercritical fluid (SCF) CO₂ at 35 °C. For comparison, the fluorescence quenching of CNA by oxygen in liquid CO₂ and n-hexane at 25 °C was also investigated. Local density augmentation determined by conventional spectral shifts in absorption and fluorescence spectra was observed at pressures below about 10 MPa in SCF CO₂ at 35 °C, in good agreement with the results reported by other workers. The pressure dependence on the quenching rate constant, k_q, showed a monotonic increase, a maximum and a minimum in liquid CO₂, n-hexane and SCF CO₂, respectively. The apparent activation volumes for k_q were evaluated to be −220 ± 10 (6.65–8.15 MPa) and −28 ± 2 (9.17–35.24 MPa) in liquid CO₂, −6.7 ± 1.4 (0.1–75 MPa) and 4.5 ± 0.2 cm³ mol⁻¹ (100–500 MPa) in n-hexane and 820 ± 50 (7.11–8.25 MPa) and −27 ± 2 (8.53–35.17 MPa) in SCF CO₂. For liquid CO₂ and n-hexane, the pressure dependence of k_q was successfully interpreted by using the quenching model we reported previously. However, for SCF CO₂, this model failed at pressures below about 10 MPa, but held in the higher-pressure region where the local density augmentation was not observed. From these results, the contribution of the local composition enhancement in the vicinity of the fluorophore molecule to the quenching was discussed.