Excited-State Intramolecular Proton-Transfer Process of Crystalline 6-Cyano-2-(2′-hydroxyphenyl)imidazo[1,2a]pyridine, as Revealed by Femtosecond Pump–Probe Microspectroscopy
2019
Ishibashi, Yukihide | Murakami, Mako | Araki, K. (Koji) | Mutai, Toshiki | Asahi, Tsuyoshi
We have investigated the excited-state dynamics of 6-cyano-2-(2′-hydroxyphenyl)imidazo[1,]pyridine (6-CN HPIP) in tetrahydrofuran (THF) solution and crystalline phases by means of femtosecond transient absorption spectroscopy. In the THF solution, the excited-state intramolecular proton transfer (ESIPT) of the hydrogen-bonded enol form (E form) occurred with a time constant of 0.6 ps, followed by the formation of the excited state of a twisted keto form. On the other hand, in the crystalline phase, heteroexcimer formation between the excited keto form with coplanar conformation and the ground-state E form was observed after ESIPT of the E form in a 1 ps time scale. 6-CN HPIP has three crystalline polymorphs: 6-CN(O), 6-CN(Y), and 6-CN(R), which emit fluorescence having different spectral shapes and lifetimes. We compared the ESIPT and heteroexcimer formation dynamics for three polymorphs. It was found that the time constant of the ESIPT process was in the order 6-CN(Y) (0.30 ± 0.1 ps) < 6-CN(R) (0.50 ± 0.2 ps) < 6-CN(O) (0.80 ± 0.2 ps). This order agrees with the difference in the O–N distance of the ground-state E form in each polymorph, i.e., faster ESIPT takes place in a shorter O–N distance. Furthermore, the heteroexcimer formation, which occurred in a few tens of picoseconds, demonstrated polymorph dependence. We discussed the difference from the viewpoint of the small dihedral angle between the two aromatic rings of the ground-state E form and the longitudinal slippage (the overlap) of the π–π stacked dimeric unit.
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