Radiative Transitions of Singlet Oxygen: New Tools, New Techniques and New Interpretations

Abstract

The near-IR phosphorescence of singlet delta oxygen, O2(a1Ap), has provided a wealth of information since it was first observed in solution-phase systems. The techniques employed and the quality of the data obtained have improved significantly over the years that, in turn, presently makes it possible to address a wide variety of problems using both steady-state and time-resolved measurements. The development of spectroscopic methods to monitor other transitions in oxygen, specifically those that involve the singlet sigma state, 02(b1X;), and the incorporation of high-level computational methods provides access to an even broader range of fundamental issues. The expertise presently available to monitor radiative transitions in oxygen, coupled with the current understanding of the effect of solvent on these transitions as achieved through state-of-the-art theoretical modeling makes it possible to consider the next step forward: the incorporation of spatial resolution and the construction of the singlet oxygen microscope.

O2(aΔg) Absorption and O2(b1Σg+) Emission in Solution: Quantifying the a−b Stokes Shift

“…In earlier publications, we reported that the a-b Stokes shift in CS 2 was ∼10 cm -1 . 1,11,12 Although this shift is still quite small, this early determination appears to have been adversely influenced by a number of experimental factors associated with the use of a FTIR to detect emission [e.g., sample position and optics at collection port (see Experimental Section)]. A more accurate value for the a-b Stokes shift in CS 2 is the ∼3 cm -1 reported herein.…”

“…12 In all cases, both the O 2 (b 1 Σ g + ) and O 2 (a 1 ∆ g ) states of oxygen were created upon pulsed-laser irradiation of a singlet oxygen photosensitizer. The laser (Quanta-Ray GCR 230 Nd:YAG) was operated at a 10 Hz repetition rate, and the irradiation wavelength was tuned throughout the UV/vis region using (1) the second, third, and fourth harmonics of the Nd:YAG fundamental output, (2) an optical parametric oscillator whose output could also be doubled, and/or (3) a high-pressure cell of H 2 gas as a Raman shifting medium.…”

“…11,12 In this approach, O 2 (a 1 ∆ g ) is produced via pulsed-laser irradiation of a sensitizer. Most importantly, the data are obtained from common solvents at atmospheric pressure using path lengths that can be as short as ∼100 µm.…”

Oxygen‐Dependent Photochemistry and Photophysics of “MiniSOG,” a Protein‐Encased Flavin

“…The instrumentation and methods used to (1) monitor O 2 (a 1 D g ) ? O 2 (X 3 Σ g À ) phosphorescence at~1275 nm in time-resolved experiments and quantify photosensitized O 2 (a 1 D g ) quantum yields and O 2 (a 1 D g ) lifetimes (21)(22)(23), and (2) perform time-resolved triplet absorption experiments (22) are described in the references cited. For the O 2 (a 1 D g ) studies, miniSOG was irradiated with the 1 kHz output of a fs laser at 418 nm, and all O 2 (a 1 D g ) experiments were performed in 1 cm path length cuvettes under conditions where the sample absorbance at the irradiation wavelength did not exceed~0.1.…”

Singlet Sigma: The “Other” Singlet Oxygen in Solution

“…In almost all cases, it is also necessary to account for the response time of the laseddetection system. A more detailed discussion of these issues in the analysis of time-resolved O,(blZ,+) data is given in a second paper in this journal issue (15).…”