 we 
 show that fret between pacific blue ( pb ) and alexa488 
 ( a488 ) covalently attached to a dna scaffold can be reversibly 
 controlled by photochromic switching of a spiropyran derivative . with 
 the spiropyran in the closed spiro isomeric form , fret occurs freely 
 between pb and a488 . 
 uv - induced isomerization 
 to the open merocyanine form shuts down the fret process by efficient 
 quenching of the pb excited state . 
 the process is reversed 
 by exposure to visible light , triggering the isomerization to the 
 spiro isomer . 
Supplementary Material