# Are there examples of “loss-less” Forster Resonance Energy Transfer (FRET)?

When I think of a Forster Resonance Energy Transfer (FRET) process, I typically imagine the initial excitation of an absorbing chromophore with a photon followed by the subsequent emission of a lower frequency photon from a secondary emitting chromophore, where this process occurs as a result of dipole-dipole coupling between electronic states of the two chromophores allowing for an InterSystem Crossing (ISC) event.

However, isn't this just because Kasha's rule (http://en.wikipedia.org/wiki/Kasha's_rule) or some variant of this rule, tells us that ISC events are most likely to occur from the lowest excited eigenstate of the initially excited chromophore, where we shed energy non-radiatively to get to this state?

Can't we use a laser and a band-pass filter to directly excite to absorbing chromophore to the lower regime of the first excited singlet state where ISC can occur with high probability (provided the right chromophore system)? Are there examples of "loss-less" FRET energy transfer where the excitation and emission wavelengths are identical or near-identical (e.g. where the wavelengths are less than a nanometer apart)?

• "Can't we use a laser and a band-pass filter to directly excite to absorbing chromophore to the lower regime of the first excited singlet state where ISC can occur with high probability (provided the right chromophore system)?" Possibly, but remember that by the Kasha-Vavilov rule, the quantum efficiency is independent of excitation wavelength provided the energy is sufficiently high (because internal conversion is usually pretty efficient), and so you're not really gaining any quantum efficiency by tuning the incident radiation to the $S_0\rightarrow S_1$ gap of the absorbing chromophore. – DumpsterDoofus May 16 '14 at 0:19
• And depending on the absorbing chromophore's absorption spectrum, you may actually be hurting the process as far as quantum efficiency of the incident light is concerned, since the minimum $S_0\rightarrow S_1$ gap might not be the most intense absorption, and shorter wavelengths may actually do better as far as quantum efficiency is concerned (although you technically do waste energy from internal conversion by using excessively short wavelengths, but the laboratory electric bill is the least of your concerns). – DumpsterDoofus May 16 '14 at 0:23
• As for your last question, there might be cases of systems where the incident radiation and fluorescence of the first chromophore are similar, but I don't really have experience with FRET systems, so I'll await the replies of people who know more than I do. – DumpsterDoofus May 16 '14 at 0:24
• @DumpsterDoofus "Possibly, but remember that by the Kasha-Vavilov rule, the quantum efficiency is independent of excitation wavelength provided the energy is sufficiently high..." I'm not disagreeing with this, but the idea was that pushing down the excitation wavelength would make it more likely to be able to have an emitted fluorophore with approximately the same energy. Yes, this could also hurt quantum efficiency. – RGrey May 16 '14 at 1:27
• @DumpsterDoofus In any case, this might be a stupid question because, once one has an ISC event, IC will give the emitted photon a fairly decent spread for any chromophore FRET system I'm aware of. – RGrey May 16 '14 at 1:27