Comprehensive Studies on Dual Excitation Behavior of Double Proton versus Charge Transfer in 4-(N-Substituted amino)-1H-pyrrolo[2,3-b]pyridines

Comprehensive spectroscopic and dynamical studies on the dual excitation behavior of proton vs charge transfer for 4-(dimethylamino)-1H-pyrrolo[2,3- b ]pyridine (DPP) and its related derivatives are reported. In cyclohexane, DPP dimer and/or dual hydrogen-bonded complex are formed with association constants K a as high as ∼4.2 × 10 3 and 5.2 × 10 4 M -1 (e.g., the DPP/acetic acid complex) at 298 K, respectively, which upon electronic excitation undergo ultrafast rate (≫6.7 × 10 10 s -1 ) of double-proton transfer, resulting in a unique tautomer emission. Dual fluorescence was observed in polar, aprotic solvents, in which the large Stokes shifted emission band originates from the charge-transfer species incorporating a dimethylamine and pyridine ring as electron donor and acceptor, respectively. Detailed solvent−polarity and temperature-dependent studies in combination with theoretical approaches have been performed to determine the excited-state charge-transfer properties such as dipole moment, orbital configuration, etc. Supplementary support for the dual charge/proton-transfer behavior was provided by the comparative spectroscopy and dynamics of various DPP-related derivatives. Further time-resolved measurements conclude that dual emissions share a common Franck−Condon excited state but undergo two independent relaxation channels. In protic solvents, such as ethanol, following fast solvent relaxation dynamics, the excited charge-transfer state further undergoes a solvent (i.e. alcohol) assisted proton-transfer reaction. The charge versus proton-transfer emission can be distinguished via the temporal spectral evolution. The results demonstrate DPP to be a unique model among 7-azaindole analogues in which the interplay between charge and proton-transfer reactions is operative in the excited state.