X³Σ_g^– → b¹Σ_g⁺ Absorption Spectra of Molecular Oxygen in Liquid Organic Solvents at Atmospheric Pressure

Abstract

Spectra and absorption coefficients of the forbidden 765 nm X 3 Σ g − → b 1 Σ g + transition of molecular oxygen dissolved in organic solvents at atmospheric pressure were recorded over a 5 m path length using a liquid waveguide capillary cell. The results show that it is possible to investigate this weak near-infrared absorption transition in a common liquid hydrocarbon solvent without the need for a potentially dangerous high oxygen pressure. Proof-ofprinciple data from benzene, toluene, chlorobenzene, bromobenzene, and iodobenzene reveal a pronounced heavy atom effect on this spin-forbidden transition. For example, the absorption coefficient at the band maximum in iodobenzene, (28.9 ± 3.3) × 10 −3 M −1 cm −1 , is approximately 21 times larger than that in benzene, (1.4 ± 0.1) × 10 −3 M −1 cm −1 . These absorption measurements corroborate results obtained from O 2 (X 3 Σ g, which depended on data from a plethora of convoluted experiments. Spectroscopic studies of molecular oxygen in liquid solvents can help evaluate aspects of the seminal Strickler−Berg approach to treat the effect of solvent on Einstein's A and B coefficients for radiative transitions. In particular, our present results are a key step toward using the O 2 (X 3 Σ g − ) → O 2 (b 1 Σ g + ) transition to evaluate the speculated limiting condition of applying the Strickler−Berg treatment to a highly forbidden process. This latter issue is but one example of how an arguably simple homonuclear diatomic molecule continues to aid the scientific community by providing fundamental physical insight.

Additional Insights into the Design of Cr(III) Phosphorescent Emitters Using 6-Membered Chelate Ring Bis(imidazolyl) Didentate Ligands

“…The latter aligns with the quenching mechanisms elucidated for the [Cr III (bipy) 3 ] 3+ complex operating through an energy-transfer process from the Cr( 2 E) excited state to the ground-state triplet 3 O 2 . This finding may apply to other Cr(III) complexes, as depicted in Figure S45 (and Table S10), showing an increasing dioxygen insensitivity as their emission maxima deviate from the absorption maximum of dioxygen (setting at 760 nm). Finally, one should not lose sight that the dioxygen sensitivity in the general case of Cr(III) complexes depends on multiple parameters whose main contribution(s) may be difficult to elucidate or to define absolute trends because they appear to be complex-dependent.…”

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

“…We recently reported that absorbance measurements can be performed at ambient pressures using a waveguide capillary cell that has a path length of 5 m and does not require a large volume of the solvent. 770 However, due to limitations in the transmission spectrum of the waveguide, we were only able to obtain data for the 765…”

“…9,13 Although eq 35 was derived for an allowed transition, 774 the evidence obtained thus far indicates that it is nevertheless also applicable to the weak forbidden transitions of dissolved oxygen. 69,132,768,770 In this way, transition probabilities that may be easier to quantify in emission can still be correlated to molar absorption coefficients, with the caveat that assumptions may have to be made regarding the dependence of ε lower→upper on ν (i.e., the integral in eq 35). For oxygen, however, errors that derive from the latter may be small due to the narrow spectral bandwidths of the transitions.…”

“…Although work in this regard has focused on allowed transitions (i.e., fluorescence), we have nevertheless assumed that the error in using n 2 , instead of some other function of n, 897 will be small when applied to the forbidden transitions in oxygen; an assumption that has gained credence through recent singlet oxygen experiments. 770 For the solvents used in the studies of oxygen's radiative transitions, n 2 does not exceed a value of ∼2.5. As such, experimentally observed solvent-dependent changes as large as a factor of ∼25 for k r aX and as large as a factor of ∼160 for k r bX point to a phenomenon other than what is expected from the fundamental radiation laws.…”

Endogenous Photosensitizers in Human Skin

“…Electronic energy transfer (ET) from excited triplet photosensitizers to molecular oxygen is the most common way to produce singlet oxygen using light. Direct photoexcitation of oxygen is limited due to spin conservation restrictions and a very small molar absorption coefficient (ε 765nm in benzene = (1.4 ± 0.1) × 10 –3 L mol –1 cm –1 ) (Figure d). ,− Singlet oxygen is relatively long-lived in liquid water (lifetime, τ Δ , ∼3.5 μs) and its diffusion coefficient ( D = 2 × 10 –5 cm 2 s –1 ) corresponds to an average diffusion length [= (2 Dτ Δ ) 1/2 ] of approximately 170 nm within two lifetimes in water . Because the rheological properties of the cytoplasm differ from those of water, great effort has been made to measure the lifetime of 1 O 2 in living cells through time-resolved phosphorescence measurements.…”