Absence of dual fluorescence with 4-(dimethylamino) phenylacetylene. A comparison between experimental results and theoretical predictions

From ab initio electronic structure calculations it has been predicted that 4-(dimethylamino)phenylacetylene (DACET) will undergo an exothermic intramolecular charge transfer (ICT) reaction with dual fluorescence even under isolated-molecule conditions. Photostationary and time-resolved fluorescence measurements reveal however that an ICT reaction does not occur with DACET under any condition of solvent polarity or temperature studied. DACET is similar to 4-(methyl)amino-benzonitrile, which shows only fluorescence from a locally excited state, but is clearly different from the dual fluorescent 4-(dimethyl)aminobenzonitrile (DMABN). The energy gap Δ E(S1,S2) of DACET is larger than that of DMABN, supporting the importance of this gap in these ICT reactions.