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.
Hsieh
Cheng-Chih
CC
Department of Chemistry, National Taiwan University, Da-an District, Taipei, Taiwan.
Chen
Kew-Yu
KY
Hsieh
Wan-Ting
WT
Lai
Chin-Hung
CH
Shen
Jiun-Yi
JY
Jiang
Chang-Ming
CM
Duan
Hsin-Sheng
HS
Chou
Pi-Tai
PT
D001372:Aza Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D006860:Hydrogen Bonding; D007211:Indoles / Q000138:chemical synthesis / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D009570:Nitriles / Q000138:chemical synthesis / Q000737:chemistry*; D011522:Protons; D012997:Solvents / Q000737:chemistry; D013237:Stereoisomerism
C023422:7-azaindole dimer; D001372:Aza Compounds; D007211:Indoles; D009570:Nitriles; D011522:Protons; D012997:Solvents