Fluorescence lifetime and quantum yield

Two months ago I saw a tweet noting the linear relationship between quantum yield and fluorescence lifetime in fluorescent proteins. I hadn’t seen this before, so I wanted to see if it held on a wider range of fluorescent proteins, so I added the ability to plot lifetimes on my fluorescent protein visualization and added lifetimes for all the proteins I could find (37 in total).

Quantum yield vs. fluorescent lifetime (ns) for 37 fluorescent proteins, colored by emission wavelength and brightness. Click for full size image.

As you can see, the correlation is pretty good, although there is clearly some curvature to the plot, fitting a straight line gives lifetime = 3.67 * QY + 0.85 with an R2 of 0.789. This suggests that short lifetimes are mainly caused by non-radiative de-excitation of the fluorophore, the long fluorescence lifetimes and high quantum yields both result from little coupling into other de-excitation modes and are governed mainly by the intrinisic radiative lifetime of the protein. This further suggests that the radiative lifetime of all fluorescent proteins is around 3 Р5 ns.  What sets this? Interestingly, Alexa dyes show a similar relationship of QY and lifetime, with maximum lifetimes around 4 ns.

Any comments by someone who knows more than I do would be most welcome.

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