I think that we should be careful when saying that stacking can quench fluorescence, when the actual mechanism is photoinduced electron transfer. Stacking interactions just bring both the donor and the acceptor into close contact and make this process easier.

A very nice reference on this topic is:

S. Doose, H. Neuweiler, M. Sauer, Fluorescence Quenching by Photoinduced Electron Transfer: A Reporter for Conformational Dynamics of Macromolecules. ChemPhysChem 2009, 10, 1389–1398.

Thanks Peter. Could you provide any relevant reference in support of the fact, then it will be of immense help.

I think that we should be careful when saying that stacking can quench fluorescence, when the actual mechanism is photoinduced electron transfer. Stacking interactions just bring both the donor and the acceptor into close contact and make this process easier.

A very nice reference on this topic is:

S. Doose, H. Neuweiler, M. Sauer, Fluorescence Quenching by Photoinduced Electron Transfer: A Reporter for Conformational Dynamics of Macromolecules. ChemPhysChem 2009, 10, 1389–1398.

I think due to the pi-pi stacking between the host and gust molecules open the way to external conversion and quenching the fluorescence.

Thanks Badr.. Kindly suggest me some reference as I am interested to know more regarding external conversion

Dear Peter, I know I was being kind of anal, and many times the term "stacking" is used loosely as a very general description, just as you did ;-) but I think the distinction is relevant, because there could stacking without quenching (e.g., exciplexes) and the other way around as well quenching without stacking.... I wanted to clarify that mainly for Pritam just in case. 

thanks Eugenio and Peter..

let me more elaborate the finding:

one Schiff base compound was taken having pyridine group at edge.

now picric acid was added gradually and monitored the emission intensity.. It get quenched. I performed the NMR and it shows downfield shifting.. Now I perform DFT with two different configuration.. One case is interaction of pyridine with picric acid..another is pi-pi interaction.. DFT suggest that pi-pi is the stable configuration (although the energy difference is very small,just 0.0012 Hartree)..

now I am concern that ...ok..may be pi-pi is a reason for the quenching along with RET..

here I need some reference in support of the finding..help me in this regard..

thanks once again.

regards,

PG

thanks Peter for ur queries.

1. SV plot is non linear.

2. Spectral overlap is there (between host PL and guest Absorption) which suggest RET (Resonance energy transfer) is a responsible mechanism.

3. As there is a spectral shift that's why ICT is also taken into consideration.

4. Gradual addition of total 50microL guest of 10-5 M has been taken place.. NOT AT ALL ANY DILUTION EFFECT!

5. As the titration has been carried out in normal room temperature thus no question of temperature effect arises. In addition, host guest mixing doesn't rise temperature inside the systems..

6. O2 has been removed by Argon purging. No chance of this type of quenching.

7. The quenching of emission intensity is from 450 unit to 25 unit.

I would rather suggest that don't worry about this basic level points.. These all are already taken care..

now let's concern about the pi-pi stacking.

I think all the issues raised by Peter are relevant and need clarification.

Moreover, you should also carefully monitor the absorption spectral behavior of the interaction and fl. excitation spectral change. You should also carry out time-resolved fl. measurements in parity with steady-state observations and look into the results whether they're tallying or contradicting steady-state results.

Moreover, you talked about RET! Make sure that you have enough experimental data to conclude about RET. Unfortunately a plenty of papers are published on RET every year, which are highly mistaken and misleading. You must be very careful about RET and design specific expts. to justify any possibility of it.

But the answer is still pending regarding PL quenching due to pi -pi stacking!!!

Dear Pritam:

Please note that it is difficult to make definitive conclusions on your queries with such limited information. First you have to establish pi-pi stacking in your system and then only the question arises of modulation (quenching) of fl. inten. with pi-pi stacking.

I think you may look into calculation of the fluo. QY before and after the stacking operation and also measurement of fluo. decay under identical exptl. conditions. If you can successfully do these expts., your data will clearly tell you the solution. However, the possibilities raised by Peter must also be clarified in this context.

[Note that it is common that pi-pi stacking may lead to fluo. quenching (enhancement of fluo. is also not rare though); but you have to perform conclusive expts to this end and provide necessary data in order to get some conclusive discussions!]

Dear Dr Paul:

I am extremely thankful for your suggestions. 

Can it happen reverse way,

Pi-stacking interaction can increase fluorescence?