In FRET, two entities play the part- one is the donor (D) molecule and the other is acceptor (A). There are certain prerequisite properties that characterizes a good donor molecule [Refer: J.R. Lakowicz, Principles of Molecular Spectroscopy, Chapter 13]. Among these, one need to look for the spectral overlap between the emission spectrum of the donor chosen and the absorption spectrum of the acceptor taken. Suppose, if D has emission maximum ~500 nm then A must have absorption maximum in between 450 to 550 nm. The donor when excited (more or less at its absorption maximum) it transfers its energy in the excited state to the acceptor. In this process, the donor looses its energy to the acceptor that is shown by the decrease in its fluorescence intensity- this is what it is commonly termed as Fluorescence quenching. Meanwhile if the acceptor is fluorescent after gaining the energy from the donor its fluorescence enhances and increase in the fluorescence intensity occurs. During the whole process, the donor is only getting excited.

Dear Sunil,

I agree withe the answer of Debanjana.

- at first please rewrite your question and give details about your rhodamine probe ( e.g. RHOD 6 G or 123 or .....) and what a kind of anthachinone used as DARK QUENCHER (e.g. kinds of substituents)

A question with details will always lead to a fast and very good answer

On other please read the pdf file joined with this answer : (Thomas Welzel et al.)

JRG

Thanks Debanjana!!

Dear Jean

I am using rhodamine B as fluorophore ( donor) and leucoquinizarin ( acceptor) as anthraquinone. I hope that will help to my question.

Many thanks

Sunil

You are welcome. Rhodamine B emits at 580 -590 nm in protic polar solvents so any acceptor you choose should have absorption around 590-620 nm

I think my acceptor has the absorption around 435- 439nm.

Dear Sunil,

Jens Spanget-Larsen worked with your acceptor (LQ) in the past. See please his paper (pdf file as link Acta Chim. Scand. 1993)).

Jens is Res. Gate member and you can contact him for hints if you want.

http://actachemscand.org/pdf/acta_vol_47_p0419-0421.pdf

On other part check please the purity of RHOD B and LQ (TLC, NMR and MS and elemental analysis ) prior investigations with FRET.

If you use toluene spectro quality for LQ UV-Vis spectra, the cut-off of toluene can be found in UV -Vis solvents suppliers sites.

JRG