Reactivity of Molecular Oxygen in Ground and Excited State: Why is Singlet Oxygen Super-Reactive?

#chemistry #photochemistry #molecular #education #oxygen #anticancer #quantumphysics #quantumtheory #molecularorbitaltheory
This is an edited video-lecture for chemists that are interested in molecular oxygen, photochemistry and singlet oxygen.
The triplet ground state of molecular oxygen and the excited state, singlet oxygen, are discussed.
(A similar lecture has been presented for BSc students, for MSc students, for PhD students as well as for Professors at an institutional talk).
For more information on PDT and singlet oxygen, see also
• The Photochemistry of ...
(and text below video, for references)
Learning objectives:
1
Molecular oxygen has a triplet ground state, responsible for many of its special properties.
It has "bi-radial character". We need the Lewis structure with the double bond as well as the Lewis structure with the two unpaired electrons (with the same spin) to describe it, in a simple way.
2
Singlet oxygen, the delta form, can react similar to ozone, it can result in similar reaction products. We can use the zwitter-ionic resonance form of singlet oxygen to describe some of its reactivity.
3
Lewis structures of excited states can be very useful, and give insight, in a different way. We have to use quantum mechanics to describe the system fully, and as good as we can. It can help to than translate the full quantum chemical results into "simplified Lewis structure of excited states".
We do that on a regular basis for charge transfer states as well as for push-pull chromophores.
doi.org/10.3906/kim-0811-33
(Figure 4)
doi.org/10.1039/B818371F
(Figure 10)
doi.org/10.1002/cphc.200500127
(computations)
doi.org/10.1142/9789814327824...
(Figure 7)
doi.org/10.1039/c2pp05378k
(Figure 6)
doi.org/10.1039/c9pp00399a
(Figure 13, 14, 15, in an abstract way)
doi.org/10.1002/chem.202001494
(scheme 1)
www.researchgate.net/profile/...
Some related work on molecular oxygen:
The most important information presented in this video can be found on pages 583-586 of
Modern Molecular Photochemisry, by Nicolas J. Turro, 1991.
Hoffman
doi.org/10.1021/jacs.7b04232
Greer (Ene reaction)
doi.org/10.1021/ja027225p
Turro & Murthy 1976:
doi.org/10.1021/ja00437a088
(picture with zwitterionic form!!)
"Thus, the reaction of 1 and singlet oxygen produces an intermediate whose decomposition results in formation of excited states of 2. This intermediate is stable for days at -90 °C. Reaction of
1 with the phosphorus ozonides 39a and 49b also results in chemiluminescence at temperatures below 0 °C."
DFT
doi.org/10.1063/1.2217733
Ogilby papers:
doi.org/10.1039/B926014P
Wilkinson papers
doi.org/10.1063/1.555965
doi.org/10.1063/1.555934
CZcams lectures on singlet oxygen reactivity:
• Photoreduction and sin...
(note mistake in MO scheme of oxygen!!).
• Mod-01 Lec-20 Singlet ...
(at ~9:10 zwitter-ionic form of singlet oxygen is used!!)
ADDITION:
20-04-2023
see also Figure 3 in:
Journal of Porphyrins and Phthalocyanines J. Porphyrins Phthalocyanines 2001; 5: 105-129
REVIEW
Basic principles of photodynamic therapy
IAN J. MACDONALD* and THOMAS J. DOUGHERTY
doi.org/10.1002/jpp.328
oxygen v2 28 01 2023

[@eisvogel.1481]

Thank you very much for your explanations!😍

[@oguzcangokce7636]

Thank you for your detailed presentation!!

[@renem.williams9257]

Glad you enjoyed it!

[@panananananana]

i thought the direct transition from triplet oxygen to singlet oxygen was spin forbidden?

[@renem.williams9257]

Yes indeed! The absorptive and emissive transitions of molecular oxygen are spin-forbidden!! Forbidden transitions still occur! But with a low probability. This implies that the molar absorption coefficient is very low (for the absorption) and the emission is very long lived. The latter is especially influenced by vibronic coupling to the solvent. See: czcams.com/video/gbSQkrs49ss/video.html at ~44 to 46 minutes. The absorptive transitions of pyrene to S1 are also forbidden: czcams.com/video/OPRLcTzjc_8/video.html