Ultrafast excited state relaxation of a model green fluorescent protein chromophore: Femtosecond fluorescence and transient absorption study

The intrinsic excited state properties of green fluorescent protein (GFP) chromophore have inspired syntheses of its structural analogues with a motivation to replicate or improve its excited state properties for biological imaging. In this work, we report torsional-motion mediated multi-coordinate relaxation pathway to explain the excited state dynamics of a GFP chromophore analogue, (4Z)-4-(4-N,N-Dimethylaminobenzylidene)-1,2-diphenyl-1,4-dihydro-5H-imidazolin-5-one (DPI), using femtosecond fluorescence up-conversion, femtosecond transient absorption and quantum chemical calculations. The excited state dynamics of this chromophore involves the formation of an N,N-disubstituted amine twisting mediated intramolecular charge transfer state on a sub-picosecond timescale. This intramolecular charge transfer event prepares an intermediate state that undergoes a “non-volume conserving” twist motion along an exocyclic double bond. The quantum chemical computations further predict that the twist motion along the exocyclic double bond is activated and leads to the formation of a conical intersection between S₁ and S₀ states of DPI, which is the main nonradiative channel of deactivation. This study confirms that for a structural analogue of GFP chromophore to exhibit excited state properties like the GFP, would require preventing such nonradiative pathways, which can be achieved by restricting the twisting degree of freedom about the exocyclic double bond.