Details

Autor(en) / Beteiligte

• Chen, Yi-An
• Meng, Fan-Yi
• Hsu, Yen-Hao
• Hung, Cheng-Hsien
• Chen, Chi-Lin
• Chung, Kun-You
• Tang, Wei-Feng
• Hung, Wen-Yi
• Chou, Pi-Tai

Titel

N−H-Type Excited-State Proton Transfer in Compounds Possessing a Seven-Membered-Ring Intramolecular Hydrogen Bond

Ist Teil von

• Chemistry : a European journal, 2016-10, Vol.22 (41), p.14688-14695

Ort / Verlag

Germany: Blackwell Publishing Ltd

Erscheinungsjahr

2016

Link zum Volltext

Quelle

Wiley Online Library Journals Frontfile Complete

Beschreibungen/Notizen

• A series of compounds containing 5‐(2‐aminobenzylidene)‐2,3‐dimethyl‐3,5‐dihydro‐4H‐imidazol‐4‐one (o‐ABDI) as the core chromophore with a seven‐membered‐ring N−H‐type intramolecular hydrogen bond have been synthesized and characterized. The acidity of the N−H proton and thus the hydrogen‐bond strength can be fine‐tuned by replacing one of the amino hydrogen atoms by a substituent R, the acidity increasing with increasing electron‐withdrawing strength of R, that is, in the order H<COCH3<COPh<Tosyl<COCF3. The tosyl and trifluoroacetyl derivatives undergo ultrafast, irreversible excited‐state intramolecular proton transfer (ESIPT) that results in proton‐transfer emission solely in the red region. Reversible ESIPT, and hence dual emission, involving the normal and proton‐transfer tautomers was resolved for the acetyl‐ and benzyl‐substituted counterparts. For o‐ABDI, which has the weakest acidity, ESIPT is prohibited due to its highly endergonic reaction. The results clearly demonstrate the harnessing of ESIPT by modifying the proton acidity and hydrogen‐bonding strength in a seven‐membered‐ring intramolecular hydrogen‐bonding system. For all the compounds studied, the emission quantum yields are weak (ca. 10−3) in dichloromethane, but strong in the solid form, ranging from 3.2 to 47.4 %. Enhancing proton transfer: Upon increasing the electron‐withdrawing strength of the R substituent in the amino moiety of a series of amino‐imidazole derivatives, the rate of NR−H excited‐state intramolecular proton transfer (ESIPT) increases and the reaction thermodynamics change from the endergonic [R=H (4)] to the equilibrium [R=COCH3 (5 a) and COPh (5 b)] and then to the highly exergonic [R=Tosyl (5 c) and COCF3 (5 d)], with the corresponding tautomer emission spanning from 580 to 640 nm (see figure).