Singlet Sigma: The “Other” Singlet Oxygen in Solution

Abstract

The lowest excited electronic state of molecular oxygen, 02(a1Ag), is often called simply singlet oxygen. This singlet delta state is an acknowledged and well-studied intermediate in many solution-phase photosystems. However, the second excited electronic state of oxygen, O,(b'X,+), is also a singlet. It has recently become possible to monitor this singlet sigma state in solution, which, in combination with studies of the singlet delta state, contributes to a better understanding of a variety of general problems in chemistry.

Time-resolved luminescence and singlet oxygen formation under illumination of hypericin in acetone

“…Our results suggest that Hyp is found in the monodeprotonated state at room temperature. The observed maximum for singlet oxygen luminescence (1274 nm) has the expected value and is in accordance with previously published data [9].…”

Direct 1O2 optical excitation: A tool for redox biology

“…4B) represent the simplest excitation mechanism: a single 3 O 2 molecule absorbs one photon and gets electronically excited, either to 1 O 2 or 1 O 2 (b) states. If it is the case that 1 O 2 (b) is the generated species, it undergoes very fast conversion to 1 O 2 (as mentioned in Section 2 [37]), which is the biologically relevant molecule. Monomol absorption bands are very weak: the corresponding molar absorption coefficients are ~10 −3 – 10 −4  M −1  cm −1 [45], [46], [105], [106], [107] or even lower.…”

“…This band presents the peculiarity that the produced singlet oxygen species is the second excited state ( 3 O 2 v = 0 → 1 O 2 (b) v = 0) [35], [36], [37]. The transition energy is 1.63 eV and the bandwidth is fairly narrow (±10 nm) [9], [46], [113].…”

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

“…[15][16][17] Most of this work, however, focused on the a f X transition. To complement b-a experimental studies, therefore, it will be necessary to extend our theoretical effort to include more work on O 2 (b 1 Σ g + ).…”