It is non radiative - that's right - but it is also resonant. That is why the emission dipole of the donor, and the absorption dipole of the acceptor must be at the same frequency (wavelength) in order to be in the resonance. If this is somehwat not self-explanatory imagine these dipoles as tuning-forks or strings of a guitar.

If I'm understanding your question correctly it is at the heart of FRET by definition. I'll just give a very simple definition of what a fluorescent molecule is to begin with. A fluorescent molecule is any molecule that emits its own source of light when excited with some other external source of light (either from a device or another molecule).  When you excite a molecule with light you are pushing electrons from the ground state (see electron configurations) to higher energy orbits. This excitation occurs due to the electron getting an 'energy boost' from this incoming light. So, the electron must now return from higher to lower energy orbit, and once this occurs a loss of energy occurs in which light is liberated. This liberated light is what you see as fluorescence. 

Now why must they overlap? Unlike observing fluorescence using a 'normal' fluorescence microscope where you simply use an external UV lamp to excite your fluorophore, in FRET excitation of the "acceptor" molecule comes from the "donor" molecule itself. For this to happen the donor’s emission band MUST overlap with the acceptor’s excitation/adsorption band. That’s why if the two molecules are different (and with overlapping emission and adsorption spectrums) we can observe FRET by fluorescence quenching of the donor (since emission from donor is absorbed by acceptor) or by fluorescence appearance of the acceptor (since emission from donor excites acceptor). This cannot happen if there is no overlap in the two spectrums you inquire about.

Actually, FRET refers to "Foerster Resonanace Energy Transfer" after the man who described the phenomenon. There is, in fact, no fluorescence by the donor; transfer itself is a radiationless process. It is a dipole-dipole interaction between donor and acceptor molecules.

EJ Bowen and R Livingston  ( J Am Chem Soc 76, 6300 (1954) observed efficient ET from  donor 1-Chloroanthracene to acceptor perylene despite the low fluorescence yield of the donor.  

Thank you all, for explaining the phenomena. 

But if emission band of donor will overlap absorption band of acceptor (for resonance) the acceptor should absorb the fluorescence energy of donor (by radiative energy transfer) as energy (emitting from donor and required for acceptor to excite) are the same.   

why donor-acceptor pair will wait for FRET to happen.

Is FRET is faster than radiative transition. 

Is FRET occur first and if molecules are far ( more than 10 nm) then only raditive transfer occur.

Please reply.

Hi Sunita. I started my first answer off in debating rather or not if I was understanding your question correctly, It seems that I have missed the target. To me it seemed that you already had an understanding that  "Forster Resonance Energy Transfer"  is a non-radiative pathway since it was clearly put in your initial question. I will try not to repeat that again here, but that left me with two options, explaining the process of FRET or having to go into a bit of quantum mechanics. I have to admit that I'm not an expert in quantum mechanics, but I'll give it my best shot and tell you what I know about FRET and it's relationship with quantum mechanics.

So, I've already mentioned that the donor molecule in a FRET experiment will be excited by some outside source of UV lamp. Once excited the electron has two fates (1) it can return to the ground state and emit light or (2) go through the non-radiative decay process which requires an "acceptor" molecule to be in close proximity. FYI, this is why I mentioned that in a FRET experiment you can infer fret by observing a decrease in quantum yield of the donor. If the fluorophores are not in close proximity then guess what? You'll observe fluorescence from the donor which brings me to point #1 above in this paragraph. OK. Moving ahead.  

I think your questions will be answered in this paragraph. You cannot think of FRET as a process in which radiative transfer  occurs, even if the molecules are far apart. And I forgot the name of the theory here so I had to look it up, but as a matter of fact, complete quantum electrodynamical  analysis was used a long time ago to draw this distinction between radiative and nonradiative energy transfer  between donor and acceptor, and the use of the latter has been verified and used now for decades now. Lets take a look into why you can't use the simultaneous emission of light from the donor followed by adsorption of this light by acceptor and acceptor emission. In theory this should be able to be done, but these photons are so short lived that none their properties can be measured. There is a special name for this type of light, "virtual light". Certainly since you cannot assume complete energy conservation due to the time-of-flight of the photon (from donor to acceptor), and we cannot use this form of radiative transfer because of the time-energy uncertainty principle. And I reiterate once again, that these photons are short lived and their properties cannot be measured, therefore we use non-radiative transfer as described using quantum mechanics. So, we assume energy transfer by ONLY one of these mechanism, non-radiative. 

Now, paraphrasing you here, but to your first point you ask "Why does donor-acceptor pair wait for FRET and not radiative transfer"? Well  I think I just answered that above. It's not so much as to waiting for it, it's just that we don't consider the latter. But if you don't know about the Forster Radius (Ro) I think you should look into that. It too is derived from quantum mechanics, and in essence it states that when donor and acceptor are within Ro then FRET will be the major decay mechanism (non-radiative decay of the donor), but when donor and acceptor are outside of Ro then decay will occur mostly due to spontaneous decay in which light is liberated from the donor as a result of the donor going back to ground state from an excited state. I made these two points in my 2nd paragraph. Lastly, I guess you don't have to worry about your other two questions if you understand now that radiative transtion is not a mode in which we observe fluorescence from the acceptor or quenching of the donor. 

Hope this helps.  

Hi Christopher,

Thank you for your explanation on FRET.

From emission spectrum of a donor- acceptor pair,

how one can distinguish whether there was radiative energy transfer or non-radiative energy transfer or it is mandatory to measure quantum yield of donor in presence and absence of acceptor.  

I mean, the emission spectrum will have two peaks corresponding to donor and acceptor, if  emission intensity of acceptor dominates donor emission (donor emission intensity decreased significantly comparison to emission intensity in absence of acceptor)   then can I say there was FRET (if conditions for FRET satisfied).  

All answers are correct. I refer you to my review article in Chemical Reviews of 1996

Dear Prof. Speiser,   Thank you for your suggestion. I got the paper I will go through it and come back to you.

Sunita,

You ask how to distinguish between simple radiative energy transfer and FRET. For the trivial radiative case the luminescence lifetime of the donor is not affected by the presence of the acceptor (it simple emits as normal and the acceptor  absorbs that photon). For FRET, the energy transfer is a driven processes mediated by a virtual photon. In this case the luminescence lifetime of the donor is decreased by the presence of the acceptor, thus to confirm that you have FRET, measure the donor lifetime as a function of acceptor concentration, if it is effectively quenched by the acceptor you have FRET. At very high concentrations FRET could turn into Dexter energy transfer (where wave function overlap between donor and acceptor exists and the energy transfer occurs within a couple of picoseconds or faster). FRET is a special case of Dexter transfer.

Look at the Förster equation thich states that the rate of energy transfer is directly proportional to the value of the J-integral (the spectral overlap between the donor emission and the acceptor absoprtion).  If there is no overlap the rate will become zero meening that there is no energy transfer.  

And yes you are right it is not a radiative process but the spectra do give youtyhe energy (frequency) of the "emission" which must match eachother in order for the donor and acceptor to enter into resonance with eachother. 

Interesting discussion! I would like to add to the part on how to distinguish between radiative and non-radiative energy transfer. The first one involves the actual (visual) photons and the interaction length is infinity; however, the later is driven by virtual photons. The non-radiative interactions depends on the interaction length (inverse sixth power in case of dipole-dipole interactions) as well as time (short time interactions). As proposed by Andy Monkman, the donor decay lifetime can suitably differentiate between the two. In a direct energy transfer case, the donor decay profile exhibits non-linear nature at short time in presence of acceptor. You can go one step forward by analyzing the decay profile by certain equations (IH, YT, hopping) to understand the multipole interactions involved therein (Refer R. C. Powell's book on energy transfer in solids).

As a rule, for any kind of energy transfer, the spectral overlap is essential (resonance condition in standard case, no phonon assistance), and therefore there is always possibility for both the mechanisms to occur simultaneously. The non-radiative interactions are superstrong at short distances but vanishes at longer distance. Therefore in a moderately codoped media, the energy transfer is attributed to the non-radiative multipole interactions (the radiative part is too weak, neglected). Even in rarely codoped media, the non-radiative interactions compete with the radiative interactions (random statistical distribution), and therefore it becomes difficult to differentiate the mechanism. Here, you can use the donor emission profile. Normalize the donor emission profile for singly and codoped samples and compare there shape. If the energy transfer is non-radiative, no significant change will occur; however, the profile will change (longer wavelength part diminishes) in case of radiative transfer mechanism. I hope this will work.

Regards,

Atul

Great discussion.

I would like to add that it is exactly because of its nonradiative nature that it is not Fluorescence Resonance Energy Transfer, as is commonly used, but rather Forster Resonance Energy Transfer to distinguish it both from radiative transfers as well as other nonradiative energy transfers such as Dexter mechanism for distances shorter than those affected in FRET.

Another note is that the term Resonance means that the energy transfer can resonate between two states, donor excited & acceptor ground states and donor ground & acceptor excited states, but also vice versa. Most of the reverse resonant mode is "unseen" due to competition of it with internal conversion from high vibronic state. It can happen more occasionally in FRET between same fluorophores (homoFRET). If resonance was solely unidirectional, still Donor excited & Acceptor ground states and Donor ground & Acceptor excited states can be coupled through the coulombinc interaction operator, and the J overlap integral gives you a density of energy levels for these couplings and their relative probabilities.

1 to 10 nm

1 to 10nm is more like a rule of thumb. It is more like:

0.5R0

Thank you all for such a nice discussion.

Please let me know,

Other than  FRET and Dexter, is there other mechanisms by which non-radiative energy transfer is possible ?

Types of excited-state electron transfer, which have exponential distance dependencies.

Dear Dr. Eitan, I am curious to know more about the excited-state electron transfer mechanism you just mentioned above. Are you referring to electron exchange interactions or it is electron transfer such as IVCT mechanism? I observed the later mechanism in one of my current study and I am interested if such interactions follow exponential distance dependence. 

Dear Dr Sontakke

I am sorry but I mostly work with organic dyes and IVCT mostly deals with charge transfer between redox states of metal centers.

Just out of reading I can just tell in this case the distance dependence of charge transfer depends on:

1. whether it is a ground-state or light-induced transfer

2. whether it is metal-to-metal or metal-to-ligand transfer

3. Geometry considerations of the bridging molecule

4. spin-orbit dependencies

5. whether the bridging molecule is a full pi-conjugated system, or whether it is interrupted in the middle?

The distance dependence should basically be reported as in :

http://www.ias.ac.in/chemsci/Pdf-Aug2002/Pc3191.pdf

http://www.ias.ac.in/chemsci/Pdf-Aug2002/Pc3191.pdf

Dear Dr. Eitan, thanks for the useful details and attached pdf file. I agree with you that our field are quite different. However, I believe that the charge transfer may behave in same way no matter the host or active centers are. I would like to discuss more with you, in case we get something interesting in coming days.

With regards,

Atul

If we wish to observe resonance energy transfer mechanism, it is essential to have spectral overlap as it is the region where donor molecule underwent emission and acceptor molecule have good absorption. Therefore, this overlap is crucial to have energy transfer. 

Found this discussion on FRET very interesting, and I have my own queries regarding the basics of it all. How best would you explain R0 and the way the K2 (factor describing relative orientation), J (overlap integral) relate to R0 value in FRET? What exactly is K2?