Dear Mikhail,

please have a look here:

http://pubs.acs.org/doi/abs/10.1021/ja00349a007

http://pubs.acs.org/doi/abs/10.1021/ja00371a065

There are many more papers about that...

Regards,

Michal

Thank you Michal. These papers are very informative.

An important fact is that the lifetime of singlet oxygen greatly increases (up to 20 times) when the solvent is deuterated.

Dear Mikhyl,

It is a challenging therapy to generate singlet oxygen in biological tissue especially in cancer tissues. It is also crucial to maximize its stability. With the addition of Michal you can also read the following link of Columbia university:

http://turroserver.chem.columbia.edu/courses/MMP_Chapter_Updates/Chapter%2014%20current.pdf

Dear Mikhail,

as you may already have found out, singlet oxygen exhibits very long lifetimes (several milliseconds) in CCl4 or C6F6. These solvents have been often used especially in older times when the detectors were not yet good enough to detect the very weak singlet oxygen phosphorescence in other solvents. As you say, deuterated solvents work also well.

Marek

Thank you Marek!

By the way, what is the reason for an increase of 1O2 lifetime in deuterated solvents or  solvents containg no hydrogen atoms?

In the above mentioned papers there are only some basic considerations devoted to this effect of solvent-induced energy transfer. Are there some follow-up research devoted to isotope effects in singlet oxygen chemistry?

There is something like an "electronic-to-vibrational" energy transfer from singlet oxygen to vibrations of O-H and C-H bonds (and also other bonds). O-H bond in water is an exceptionally good deactivator of singlet oxygen. Vibrational energy of O-D bond is different, which makes it much less efficient in singlet oxygen deactivation. Please refer to the classical review by Schweitzer and Schmidt: http://pubs.acs.org/doi/abs/10.1021/cr010371d  (page no. 1700 radiationless deactivation)

Amazing. Vibrational-to-electronic ET from solvent to O2 is also known to produce singlet oxyfgen: Singh, A.; Koroll, G.W.; Antonsen, S.A., J. Photochem. 25: 99-104 (1984).

However, I found example of V->E ET only for small molecules, like O2, CO, N2.

Is there anything like this for organic dyes, or, probably, the  difference in energies between electronic and vibrational level is too high to allow for such ET?

This is getting a bit far from my expertise and I may not understand what you mean exactly, but I will try to tell you my opinion:

The 'molar' rate constants of e-v ET deactivation of singlet oxygen (1Delta-g) are generally by many orders of magnitude smaller than the molar rates of deactivation by typical 1O2 quenchers such as sodium azide (Schweizer p. 1748) (e-v deactivation molar rates are also very small compared to rates of 1O2 generation from triplet states of organic dyes)  Therefore, e-v deactivation is of practical importance for solvents, but not that much for solutes at small concentrations. Regarding the reversed process - generation of singlet oxygen from vibrationally excited bonds: I've never heard about this. It may be theoretically possible but my first opinion would be that this process would be extremely inefficient. I think that the excited vibrations would dissipate very quickly before they have a chance to meet oxygen. (The electronic excitation energy of singlet oxygen (94 kJ/mol, 7860 cm-1) is so much above k_B T at room temperature (2.5 kJ/mol) -- therefore, 94 kJ/mol would correspond to thermodynamic temperature of ~10000K.). Unfortunately, I do not have the access to the Singh1984 paper to check it out.