Direct Evidence of Torsional Motion in an Aggregation-Induced Emissive Chromophore

The aggregation induced emission (AIE) behavior shown by organic chromophores is very interesting for the development of efficient solid state devices. The restriction of intramolecular rotation is by far the most frequently assumed mechanism to explain this behavior; by blocking or reducing this rotation, upon rigidification of the environment, molecular luminescence is restored. By means of ultrafast pump-probe spectroscopy combined with density functional theory (DFT) and time-dependent DFT calculations, we show direct evidence of intramolecular rotation in a simple push-pull organic chromophore,4-diethylamino-2 benzylidene malonic acid dimethyl ester, possessing AIE properties. The spectral evolution of the stimulated emission band of the chromophore in the first 45 ps after photoexcitation is fully consistent with the presence of a torsional relaxation toward the equilibrium geometry of the excited state, taking place on time scales that depend on the solvent viscosity. The structural features of the excited state fully account for the different photoluminescence efficiencies observed in solvents with different viscosities.