My fluorescence titration spectra show a blue shift in emission maximum of my compound during the quenching process. I guess there is an ultrafast ET process which is even faster than the solvation dynamics. If I choose the intensity at the emission maximum of compound in the absence of quencher, Stern-Volmer plot has an upward curvature. If I choose the intensity of the emission maximum in each emission spectrum, Stern-Volmer plot is linear. In this case, is Stern-Volmer plot still suitable for the evaluation of bimolecular rate constant? If not, which quenching model should be applied?
I'm not sure what you mean by a blue shift during the quenching process, or an et process faster than solvation. Why would it still emit if it had transferred an electron? Are we talking about the possibility that the emitter has formed charge transfer complex with the quencher, and blue shifted its spectrum? If so, all bets are off on attempting stern-volmer. Out of curiosity, does the absorption spectrum of the quencher overlap the emission spectrum of the emitter?
Hi,William! If I use a very amount of quencher, the emission intensity of my compound can be totally quenched. The possible charge transfer complex can emit light at a longer wavelenth.This band can be always observed .The absorption of quencher overlaps the emission of emitter.But it cannot emit light.Because I tried to excite quencher using emisssion wavelength of emitter, no emitted can be detected. Hence, the band at longer wavelength comes from charge transfer complex. Could you suggest some other quenching model which can be dealt with this kind of quen ching?
Hi
I wonder if the blue shift in emission is not due to any complex of transfer process, but is simply that the quencher is reabsorbing the emitted light; this can easily happen if the absorption of the quencher at the emitted wavelength is above .1 or so. This can perturb any quantitative measurement, but if you really have to you can correct the measured spectra for the absorption by assuming the measured light propagated on average through half the cuvette. This is rough, of course, but might work well enough.
You mentioned there is a redder emission from the likely CT complex. Does this also get quenched? Does it also overlap the absorption spectra of the quencher?
Or does it increase at the same rate the normal emisison decreases?
About the use of Stern-Vollmer, though, I would think it should be completely invalid in the presence of a significant amount of complex formation. I don't know what you're trying to learn, but would something like the Benesi-Hildebrand equation be useful? It uses the optical data to deduce the equilibration constant of complex formation and the extinction coefficient for the complex. It would tell you how much of your emitter is free and able to emit, and how much is bound up in CT complexes.
http://en.wikipedia.org/wiki/Benesi%E2%80%93Hildebrand_method
Wow, that PL "peak" to the red would have thrown me off the trail, too. As it is, I guess there's no reason to worry about complex formation, etc. Maybe you can rescue a Stern-Volmer analysis by correcting the emission spectra for the re-absorption.
Good luck
From the comments written there are some unclear aspects. However I wish to say, in a general sense, that pigments in solution usually have a large heterogeneous broadeing due to to a variety of pigment/solvent interactions. In other words there is a distrubution of absorptions, and this may cover 5-10nm in some cases. It may be that the quencher interacts preferentially with molecules on the low energy side of the distribution. In this case of "selectivity" the Sternj Volmer plot is without use.
The most interesting thing is that if I increase the concentration of my fluorophore by around 20 times, the emission intensities are almost same. Just there is little change in the emission spectrum shape. Then I measured the excitation spectrum. The electronic transition energy shifted to longer wavelength. My collegue who synthesized it told me that the formation of ground-state dimer is impossible for our fluorophore due to the lack of hydrogen bond. Anybody could give any suggestion to this strange behavior?
^ The results above can be attributed to light re-absorption. Use a narrower cuvette and the spectral changes will be less prominent (at the same dye concentration). You should work with absorbances of about 0.1.
Comparing to the last plot, I think this latest one is the excitation curve? Re-absorption effects and other optical-density problems can actually be rather subtle in excitation spectra, and I agree with Sergio that it might cause what you're seeing. That said, I don't see why you can't have ground state dimer formation. The concentrations on the last plot are more than high enough for such things, and the relatively large shift in excitation is a classic symptom. Is it also seen in absorption? These things are more easily believed in absorption spectra because the optical densities encountered with such concentrations cause fewer problems. In absorption, a dimer can appear as a new peak to the red, or to the blue, can masquerade as part of the Franck-Condon progression, and sometimes doesn't affect a low-resolution spectra any noticeable amount but can be spotted as a Beer's Law anomaly while comparing different concentrations.
Without knowing more structural info, I don't see why a hydrogen bond should be necessary for dimer formation. Napthalocyanines, for example, are well known to form dimers even at micromolar concentrations, and there are certainly no h-bonds present. In some sense, the intermolecular forces that cause dimer formation are the same ones that allow a sample to form a solid phase - the only way a dimer would be "impossible" would be if your molecule were a gas, even at absolute zero. But maybe I'm being overly literal with your synthetic colleague.
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