Water-Catalyzed Excited-State Proton-Transfer Reactions in 7‑Azaindole and Its Analogues

The mechanism of the water-catalyzed excited-state proton-transfer (ESPT) reaction for 7-azaindole (<b>7AI</b>) has long been investigated, but there are some controversial viewpoints. Recently, owing to the superiority of sensing biowaters in proteins by a <b>7AI</b> analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed ESPT in <b>7AI</b> and its analogues in an attempt to unify the mechanism. Herein, a series of <b>7AI</b> analogues and their methylated derivatives were synthesized to carry out a systematic study on p<i>K</i>_a, p<i>K</i>_a*, and the associated fluorescence spectroscopy and dynamics. The results conclude that all <b>7AI</b> derivatives undergo water-catalyzed ESPT in neutral water. However, for those derivatives with −H (<b>7AI</b>) and a electron-donating substituent at C(3), they follow water-catalyzed ESPT to form an excited N(7)–H proton-transfer tautomer, T*. T* is rapidly protonated to generate an excited cationic (TC*) species. TC* then undergoes a fast deactivation to the N(1)–H normal species in the ground state. Conversely, protonation in T* is prohibited for those derivatives with an electron-withdrawing group at the C(2) or C(3) or with the C(2) atom replaced by an electron-withdrawing nitrogen atom (N(2) in, e.g., 2,7-diazatryptophan), giving a prominent green T* emission. Additional support is given by the synthesis of the corresponding N(7)–CH₃ tautomer species, for which p<i>K</i>_a* of the cationic form, that is, the N(7)–CH₃N­(1)–H⁺ species, is measured to be much greater than 7.0 for those with electron-donating C(3) substituents, whereas it is lower than 7.0 upon anchoring electron-withdrawing groups. For <b>7AI</b>, the previously missing T* emission is clearly resolved with a peak wavelength at 530 nm in the pH interval of 13.0–14.3 (<i>H</i>_– 14.2)

Excited-State Conformational/Electronic Responses of Saddle-Shaped <i>N</i>,<i>N</i>′‑Disubstituted-Dihydrodibenzo[<i>a</i>,<i>c</i>]phenazines: Wide-Tuning Emission from Red to Deep Blue and White Light Combination

A tailored strategy is utilized to modify 5,10-dimethylphenazine (<b>DMP</b>) to donor–acceptor type <i>N</i>,<i>N</i>′-disubstituted-dihydrodibenzo­[<i>a</i>,<i>c</i>]­phenazines. The representative compounds <b>DMAC</b> (<i>N</i>,<i>N</i>′-dimethyl), <b>DPAC</b> (<i>N</i>,<i>N</i>′-diphenyl), and <b>FlPAC</b> (<i>N</i>-phenyl-<i>N</i>′-fluorenyl) reveal significant nonplanar distortions (i.e., a saddle shape) and remarkably large Stokes-shifted emission independent of the solvent polarity. For <b>DPAC</b> and <b>FlPAC</b> with higher steric hindrance on the <i>N</i>,<i>N</i>′-substituents, normal Stokes-shifted emission also appears, for which the peak wavelength reveals solvent-polarity dependence. These unique photophysical behaviors are rationalized by electronic configuration coupled conformation changes en route to the geometry planarization in the excited state. This proposed mechanism is different from the symmetry rule imposed to explain the anomalously long-wavelength emission for <b>DMP</b> and is firmly supported by polarity-, viscosity-, and temperature-dependent steady-state and nanosecond time-resolved spectroscopy. Together with femtosecond early dynamics and computational simulation of the reaction energy surfaces, the results lead us to establish a sequential, three-step kinetics. Upon electronic excitation of <i>N</i>,<i>N</i>′-disubstituted-dihydrodibenzo­[<i>a</i>,<i>c</i>]­phenazines, intramolecular charge-transfer takes place, followed by the combination of polarization stabilization and skeletal motion toward the planarization, i.e., elongation of the π-delocalization over the benzo­[<i>a</i>,<i>c</i>]­phenazines moiety. Along the planarization, <b>DPAC</b> and <b>FlPAC</b> encounter steric hindrance raised by the <i>N</i>,<i>N</i>′-disubstitutes, resulting in a local minimum state, i.e., the intermediate. The combination of initial charge-transfer state, intermediate, and the final planarization state renders the full spectrum of interest and significance in their anomalous photophysics. Depending on rigidity, the <i>N</i>,<i>N</i>′-disubstituted-dihydrodibenzo­[<i>a</i>,<i>c</i>]­phenazines exhibit multiple emissions, which can be widely tuned from red to deep blue and even to white light generation upon optimization of the surrounding media