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제124회 대한화학회 학술발표회, 총회 및 기기전시회 안내 Excitation Wavelength Selective Control of the Reactivity of Photoacids

2019년 8월 29일 15시 41분 45초
PHYS.P-212 이곳을 클릭하시면 발표코드에 대한 설명을 보실 수 있습니다.
10월 17일 (목요일) 11:00~12:30
Physical Chemistry
저자 및
Changmin Lee, Hayoung Song, Iseul Jang, Taiha Joo*
Department of Chemistry, Pohang University of Science and Technology, Korea
Proton transfer has been widely investigated because of its biological and chemical importance. Excited-state proton transfer (ESPT) may provide valuable information about dynamical aspects of proton transfer reaction via real-time observation. Photoacids, which have acidic proton in the excited state, are representative examples showing ESPT [1]. As the ESPT of photoacids is a bimolecular reaction, the rate of reaction is at best several picoseconds in general because of the diffusion rate. Recently, a strong photoacid named as QCy9 is reported to have an ultrafast ESPT [2]. Furthermore, it has two absorption bands which show different photophysical properties. We have investigated the origin of two different absorption bands via various time-resolved fluorescence (TF) techniques.
The absorption spectrum with respect to the pH value shows that it is already weak acid in the ground state, with pKa≈4.3. In pH 1, with acidic form, there are two absorption bands in 400 nm and in 340 nm. Two different bands have same basic form fluorescence band but different acidic form fluorescence bands. When we excite the molecule with 400 nm, the ESPT rate is 100 fs. When the excitation wavelength is converted to 320 nm, the reaction is slower and shows a long lifetime even in the acidic band of fluorescence. The origin of two different proton transfer kinetics is the structural difference in the ground state. Using the DFT calculation, we found four different conformations in the ground state with similar energy. In conclusion, we found the photoacid which shows different reactivity with respect to the excitation wavelength.

[1] N. Agmon, J. Phys. Chem. A 109, 13 (2005).
[2] R. Simkovitch, N. Karton-Lifshin et al., J. Phys. Chem. A 117, 3405 (2013).