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).
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 – 5 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.