Cyano Analogues of 7-Azaindole: Probing Excited-State Charge-Coupled Proton Transfer Reactions in Protic Solvents

The interplay between excited‐state charge and proton transfer reactions in protic solvents is investigated in a series of 7‐azaindole (7AI) derivatives: 3‐cyano‐7‐azaindole (3CNAI), 5‐cyano‐7‐azaindole (5CNAI), 3,5‐dicyano‐7‐azaindole (3,5CNAI) and dicyanoethenyl‐7‐azaindole (DiCNAI). Similar to 7AI, 3CNAI and 3,5CNAI undergo methanol catalyzed excited‐state double proton transfer (ESDPT), resulting in dual (normal and proton transfer) emission. Conversely, ESDPT is prohibited for 5CNAI and DiCNAI in methanol, as supported by a unique normal emission with high quantum efficiency. Instead, the normal emission undergoes prominent solvatochromism. Detailed relaxation dynamics and temperature dependent studies are carried out. The results conclude that significant excited‐state charge transfer (ESCT) takes place for both 5CNAI and DiCNAI. The charge‐transfer specie possesses a different dipole moment from that of the proton‐transfer tautomer species. Upon reaching the equilibrium polarization, there exists a solvent‐polarity induced barrier during the proton‐transfer tautomerization, and ESDPT is prohibited for 5CNAI and DiCNAI during the excited‐state lifespan. The result is remarkably different from 7AI, which is also unique among most excited‐state charge/proton transfer coupled systems studied to date. Systematic study: A series of new 7‐azaindole derivatives are designed and synthesized to study the excited‐state charge‐coupled proton transfer reactions in protic solvents (see figure). The results conclude that solvent‐polarity induced barrier plays a key role for the excited state proton transfer of 7‐azaindoles in methanol.