Direct Spectroscopic Evidence of Photosensitized O₂ 765 nm (¹Σ⁺_g → ³Σ^-_g) and O₂ Dimol 634 and 703 nm ((¹Δ_g)₂ → (³Σ^-_g)₂) Vibronic Emission in Solution

Photosensitized Generation of Singlet Oxygen

“…This is at first sight a quite surprising finding, as the processes leading to formation of S 0 and O 2 ( 1 Σ g + ), O 2 (1Δ g ), or O 2 ( 3 Σ g − ) are spin‐allowed and exothermic. However, the spectral overlap integrals between the normalized spectra of the broad triplet sensitizer emission and the very narrow O 2 ( 1 Σ g + ) ← O 2 ( 3 Σ g − ) and O 2 (1Δ g ) ← O 2 ( 3 Σ g − ) absorptions with respective half widths of only 10 nm at 765 nm (8,10) and 15 nm at 1275 nm (11,12) are much smaller than in the common case of exothermic energy transfer to polyatomic molecules. Thus, correspondingly smaller rate constants result for exchange energy transfer to O 2 according to Dexter (13).…”

“…Quenching of the lowest electronically excited singlet (S 1 ) and triplet (T 1 ) states of most substances by O 2 produces electronically excited oxygen. Both lowest excited singlet states 1 Σ g + and 1Δ g of O 2 are formed besides the 3 Σ g − triplet ground state, if the energy transferred exceeds the excitation energy E Σ = 157 kJ mol −1 of the upper excited singlet oxygen species O 2 ( 1 Σ g + ) (1–8). Electronical to vibrational energy transfer deactivates O 2 ( 1 Σ g + ) very rapidly and completely to the metastable and highly reactive O 2 (1Δ g ) with excitation energy E Δ = 94 kJ mol −1 (6,9), which is the active species in many important photoprocesses and is commonly called singlet oxygen.…”

Singlet Sigma: The “Other” Singlet Oxygen in Solution

“…The first real time-resolved experiment was performed by using a faster visible detector to monitor the b + X transition at -765 nm (16). Unlike the IR experiments, however, these experiments at -765 nm have the added complexity that one must correct for luminescence from other sources, including impurities in the sample (16,17). Nevertheless, they have provided the bulk of the experimental data thus far.…”

“…Of specific interest has been the possibility that the 02(a1Ag) dimol could transfer its energy of excitation to a dissolved organic molecule M and thus sensitize the fluorescence of M (29,9698). It is well known that the O,(alA,) dimols can also couple to the radiation field and emit light at a wavelength that is exactly one-half (-635 nm) that for the monomer a + X emission (-1270 nm) (17,(99)(100)(101).…”

Singlet Sigma: The “Other” Singlet Oxygen in Solution

“…Of specific interest has been the possibility that the 02(a1Ag) dimol could transfer its energy of excitation to a dissolved organic molecule M and thus sensitize the fluorescence of M (29,9698). It is well known that the O,(alA,) dimols can also couple to the radiation field and emit light at a wavelength that is exactly one-half (-635 nm) that for the monomer a + X emission (-1270 nm) (17,(99)(100)(101).…”