Spectroscopy and Femtosecond Dynamics of Excited-State Proton / Charge Transfer Reaction
Date Issued
2007
Date
2007
Author(s)
Hsu, Chia-Jung
DOI
en-US
Abstract
Studies of excited state intramolecular proton transfer (ESIPT) coupled with charge transfer (ESICT) are presented here. As for a strategic design, we synthesized a series of derivatives based on the famous ESIPT molecules 2-(2’-hydroxy) benzoxazole (HBO) or 2-(2’-hydroxy) benzothiazole (HBT). In HBO and HBT, the lone pair electrons at the nitrogen atom have been incorporated into the resonance, i.e. the construction of aromaticity, such that its electron donating property, comparing with those of alkyl amines, is negligibly small. Upon Franck-Condon excitation, one can thus perceive its lack of ESICT. Instead, ESIPT takes place, forming a proton transfer tautomer, in which the central nitrogen atom acts as the secondary alkyl amine. If the para-position of the nitrogen is anchored by a strong electron withdrawing group, ESICT should proceed simultaneously at every moment ESIPT takes place. In other words, one can envisage such types of reaction as the photon induced ESICT associated with a nuclear (proton) motion, rendering an ideal system to study the role of solvent polarity for the ESICT/ESIPT coupled reaction, and hence a paradigm to test the “Marcus theory (modified by Hynes et al.) based on the proton transfer reaction.
Although di-CN group is an electron acceptor anchored on the para position of the center nitrogen atom in diCN-HBO and diCN-HBT, the distance between di-CN group and hydroxy group is long enough to prevent affection in the ground state. Started by Franck-Condon excitation, the ESIPT takes place first to create a chance for electron delocalization. As electron motion is faster than nuclear motion, ESICT is initiated and finished during ESIPT. For ESIPT and ESICT is correlated, the combination rate is solvent polarity dependent. The enol form decay rate of diCN-HBO decreases with increasing solvent polarity. The phenomena provides us an ideal model different from our preceding proton transfer compounds such as 4’-(Dialkylamino)-3-hydroxyflavone, p-N,N-Ditolylaminosalicylaldehydes,and HABT, etc.
Subjects
質子/ 電子轉移
飛秒雷射動力學
Proton / Charge transfer
Spectroscopy and Femtosecond Dynamics
Type
thesis