e.g. one attached Fig. 1 (broad) vs. Fig. 2 (slightly sharp) does it tell anything about nature of triplet state? it was measured by using Laser Flash Photolysis nanosecond transient absorption (LP 920, Edinburgh instruments)
I agree with Gert that the widths don't seem to have a large difference. However, in the presence of oxygen on the 1 microsecond timescale there is clear evidence that photobleaching is occuring, likely due to the formation of singlet oxygen via triplet energy transfer from the excited chromophore.
I don't think there is such a big difference between the widths. First of all you should plot them on the same scale (not one from 400 to 540 nm and the other from 250-700 nm). Secondly the width should be expressed in energy (wavenumbers) and not in wavelengths. On a wavelength scale lines appear narrower at shorter wavelengths. Converting to wavenumbers (107/nm gives you cm-1) shows that the widths are very similar.
I agree with Gert that the widths don't seem to have a large difference. However, in the presence of oxygen on the 1 microsecond timescale there is clear evidence that photobleaching is occuring, likely due to the formation of singlet oxygen via triplet energy transfer from the excited chromophore.
Joseph is right of course, on that time scale at atmospheric concentrations of oxygen you expect that, plenty of time for the triplet to meet an oxygen. That is also why you don't see phosphorescence unless you deoxygenate your sample. But I don't think that changes the line width (if they even are different) for the shorter times, which will be mainly due to inhomogeneous broadening.
Two experts on the field gave answers to this question: this website is awesome!!! (greetings, Joe).
Just wanted to add that the difference between broad vs. sharp absorption signals (plotted in energy/wavenumbers scale), coupled with signal intensity, is usually evidence of how well vibronic transitions happen between states (vibronic transitions are those happening simultaneously between electronic and vibrational levels). This is the heart of the Franck-Condon principle, which states "...electronic transitions occur most favorably when the nuclear structure of the initial and final states are most similar." (N. Turro). The broader the signal, the more access to many vibrational levels on the upper electronic state at the expense of signal intensity. The sharper the signal, the more "selective" the transition is and is usually accompanied by large intensity.
Dear All,
Thanks for your feedback. It really make sense to change the x-axis scale to have better sense of this data. I will really appreciate your feedback on the below linked questions, of similar nature.
https://www.researchgate.net/post/What_does_the_sharp_versus_broad_peaks_in_electronic_absorption_spectrum_tells_about_nature_of_electronic_transition
All excellent answers. I feel as though I'm too late to add anything meaningful to the question posed. I will, however, add that you can probe the triplet excited state with redox active compounds. For instance, in my own work I was investigating the photochemistry of 4-benzoylpyridine N-oxide and used ABTS. Since you have a laser flash photolysis unit you can get a feel for the reduction potential of your excited state. Just be sure that you choose a redox active compound that won't be excited at your laser wavelength (or at the very least that its absorbance at that wavelength is much lower than the compound that you want to excite). There are many examples of this technique in the literature.
Phys. Chem. Chem. Phys., 2014,16, 19429-19436
DOI: 10.1039/C4CP02751E
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