Insights into the unusual dual fluorescence of the ortho-amino analogue of green fluorescent protein chromophore

Abstract

Even though both p-aminobenzonitrile (p-ABN) and o-ABDI [o-amino analogue of green fluorescent protein (GFP)] have the same electron-donating amino group, o-ABDI displays dual fluorescence while p-ABN exhibits single fluorescence. Even though both o-AABDI (o-acetamido analogue of GFP) and o-ABDI have the similar intramolecular hydrogen bonding, o-ABDI displays dual fluorescence while o-AABDI shows single fluorescence. To explore the unusual phenomenon of dual fluorescence for o-ABDI, we used the time-dependent density functional theory (TD-DFT) method with the polarizable continuum model (PCM) to study the S1 excited state of o-ABDI in acetonitrile. What we found is that o-ABDI in acetonitrile has two isomeric conformations (minima) in the S1 excited state, the twisted intramolecular charge transfer (TICT) and the localized excited (LE) states. The TICT state involves a large dipole moment change during fluorescence emission, while the LE state with a flat structure involves only a slight dipole moment change without a charge transfer during fluorescence emission. Even though both o-DMABDI (o-dimethylamino analogue of GFP) and o-ABDI have the TICT excited state, o-DMABDI does not have the stabilized LE S1 state while o-ABDI does. It is because the intramolecular hydrogen bonding of o-ABDI makes its LE S1 state highly stabilized, and that leads its LE S1 state to become a global minimum. That is why o-ABDI displays dual fluorescence.