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I have some doubts about the nomenclature of the lifetimes. Are singlet and fluorescence lifetimes the same? As well as the triplet and phosphorescence lifetimes?

Also, can the triplet/phosphorescence lifetimes be measured by phosphorescence decay as well as by transient absorption of the triplet state by means of laser flash photolysis?

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Yes, singlets fluoresce and triplets phosphoresce. So the singlet lifetime is the fluorescence lifetime and is the inverse of singlet excited state decay rate constant and similarly for phosphorescence.

Both singlet and triplet decay can be measured by flash photolysis more often nowadays this is called pump-probe spectroscopy or pump-probe transient absorption (or some variation of this) but it is the same technique as flash photolysis. It has kinetic (decay measured at one wavelength at a time) and spectroscopic (spectrum at sequence of times after excitation) variants.

If the excited state is, say, a quartet or quintet etc. we use the term luminescence.

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  • $\begingroup$ Thank you for your clear answer. However, it makes me feel uneasy. In some molecules the ground state is a triplet -e.g $O_2$ ($X ^3\Sigma_g^- $)- and the emission from its triplet excited states is called fluorescence taking into account that the transitions are allowed and the lifetime is short. Also the OH radical ($X ^2\Pi$) has fluorescence and the radical is frequently detected using Laser Induced Fluorescence (LIF). $\endgroup$
    – PAEP
    Sep 9, 2022 at 18:39
  • $\begingroup$ I wonder if as a general statement (without specifiyng a context) this definition maybe an oversimplification, and I wonder if it misses the importance of distinguishing between an allowed (high-probability, fluorescence) versus a forbidden transition (low-probability, fosforescence)? $\endgroup$
    – PAEP
    Sep 9, 2022 at 18:42
  • $\begingroup$ The length of an excited state emission has nothing, per se, to do with whether it is fluorescence or phosphorescence, although phosphorescence is normally longer in the same molecule. Symmetry can also be involved in determining emission probability, thus there are many molecules whose singlet is longer lived than other types of molecules triplet state. $\endgroup$
    – porphyrin
    Sep 12, 2022 at 7:33
  • $\begingroup$ Completly agree, Did not explain myself very well. I wrote a more detailed comment as an answer below. $\endgroup$
    – PAEP
    Sep 12, 2022 at 19:04
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Thanks @Achem and @porphyrin for your comments. I do not know if an Answer is the right format for this Comment but the comments do not allow enough character to develop my argument.

I think we are all in the same or close pages. My reservation is that we have to be a little nuanced when using this terminology, since otherwise it may leed to confusion.

I rewied this morning the the IUPAC's 3rd edition of Glossary of Terms Used in Photochemistry and look for the definitions of luminescence, fluorescence and phosphoresnce. I quote from the document:

Luminescence. Spontaneous emission of radiation from an electronically excited species or from a >vibrationally excited species not in thermal equilibrium with its environment.

Fluorescence Spontaneous emission of radiation (luminescence) from an excited molecular entity >with retention of spin multiplicity.

Phosphorescence Phenomenologically, term used to describe long-lived luminescence. In mechanistic >photochemistry, the term designates luminescence involving change in spin >multiplicity, typically from triplet to singlet or vice versa.

Note: For example, the luminescence from a quartet state to a doublet state is also phosphorescence.

Thus, both fluorescence and phosphorescence are examples of luminescence, independent of the spin of the states coupled in the transition.

Both triplet-triplet transitions (for example in O$_2$) and doublet-doublet transitions (for example in OH and NO) are examples of fluorescence. I am not familiar with any molecular transitions involving quartet or quintet states but that is my ignorance.

I guess that in some fields the molecular ground states are singlets and higher spin ground states and oddity, so I understand that the terms fluorescence, phosphorescence and luminescence can be used more loosely, even if it makes me unconfortable, but in that case I feel it is better to state in what context you are using the definition.

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    $\begingroup$ I think the definitions are self-consistent, a triplet to triplet in oxygen is fluorescence because the spin multiplicity is the same. I am sure there must be plenty of examples in atomic fluorescence. Like Na fluorescence is doublet to doublet. $\endgroup$
    – AChem
    Sep 13, 2022 at 0:39
  • $\begingroup$ Agreed. I meant molecular systems involving transitions between quartet or quintet states. I will edit my answer. $\endgroup$
    – PAEP
    Sep 13, 2022 at 9:09
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Regarding porphyrin's response and PAEP's reservation about oxygen emission from an excited triplet to a ground triplet being labelled as fluorescence, the issue can be addressed by defining fluorescence as the excitation and emission having the same spin multiplicity. This removes the condition of fluorescence as singlet to singlet transition and then it is not based on short lifetimes. In a similar manner, phosphorescence can be defined as emission from a different multiplicity from the ground state. IUPAC's gold book seems to silent on this topic but other IUPAC documents agree with above.

To the OP, you will have to look at the context. Singlet state does not necessarily imply fluorescence. All it means that all electrons are paired in that system. Anyway, these are man-made definitions/ labels. Luminesence to analytical chemists is a catch all term for emission of light from molecules or atoms by means of various excitations. It has no connotations associated with the spin multiplicity.

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