The technique of using the crossing point between absorption and emission to determine the excited state energy is only valid if the absorption and emission are from the same state, so for phosphorescence you would need to be able to detect direct absorption into the triplet state.

Another option, which doesn't require direct triplet absorption, is to draw a tangent line on the high energy side of the phosphorescence (around halfway down the curve). Where that tangent line crosses the x axis is a good estimate of your triplet energy

dr Mccusker

I do not completely follow your tangent

For instance in the link the phosphorescence of naphthalene is shown and there the 0-0 band is quite clear and also the highest peak and this is not in the center of the emission. This is the case with a large displacement in the active vibration and associated Franck-Condon factor maximising at a higher vibrational number

This is more the case for the fluorescence of the same molecule

I just notice that benzene has this feature

I've mostly used the tangent method with change transfer emission, where there's no visible vibronic progression, like the spectrum attached below. With a more structured emission you would use the blue side of highest energy (lowest wavelength) band, presumably this is the 0-0 band. This band may or may not have the highest intensity.

Dear Dr. Mccusker

Thank you very much for reply

Dr Mccusker

The examples are for Shpolskii matrices with highly resolved vibronic structure

Still, at higher temperatures the spectrum is almost structureless but the 0-0 is the strongest region and the spectrum is skewed to the higher energy and does not have the near Gaussian shape like the one you show

This is a delicate point if you do not see the vibronic structure of the phosphorescence band. What is clear is that the maximum of the phosphorescence band, as often reported in the literature, is not a good approximation.

As you correctly infer, this energy should be taken as the energy of the 0-phonon component. If the corresponding component is not seen one has to make approximations. The tangent method cited by Dr McCusker is probably a better approximation than the onset of the band or its intersection with the absorption spectrum of the singlet state. Here one could also say that the first maximum at around 22-23 kcm-1 could be taken as the 0-phonon band unless the compound has several triplet states (calculations will help here).

In any case what is very important is to precisely describe how this energy was determined and not to compare it with other triplet state energies determined in a different way...

ANother point is that people often use this energy to estimate energy transfer to a luminescent lanthanide ion. Caution should be exercised here since the TS energy will depend on the Ln ion. Moreover, energy gap is not the only criterion to assess if a compound will be highly luminescent: non-radiative de-activations in the coordination sphere have to be considered (which was not done in Latva's article).

Dear Dr. Bünzli,

Thanks for the detailed answer.

I would use excitation spectrum (if you have an effective energy transfer) the states are good visible. If the compound does not emit...it is complicated to work with absorption spectra only. And a combination of experiment + modeling looks better

I do not think that the suggested crossing of absorption and emission spectra is a valid method to determine the 0-0 energy of phosphorescence. The crossing point is determined by the positions you set for baselines of the two different experimental techniques. The tangent method is empirical and I also would not rely upon that to give a valid estimate. It is also unlikely in many cases that the excitation spectrum will show triplet state absorption because it is so weak. If a well-resolved low temperature phosphorescence emission spectrum is obtained then the 0-0 line can be deduced from the vibronic spacing and temperature dependence of intensity. Otherwise, other methods are just estimates. Calculation might give a value which is reasonable but it depends upon the sophistication and applicability of your calculation.

To get a fair estimation, we need some assumptions first:

1) Broadening is due to a single harmonic vibrational mode; 2) hbar omega >> k_B T (by a few times) ; 3) Huang_Rhys S>>1 (by a few times also).

Then w= FWHM/ hbar omega = 2 sqrt(2ln(2) S) and half of Stokes shift (delta E) = S hbar omega. This leads to detla E = w^2/(8ln2) * hbar omega.

Let me know if I made some mistakes.