• Chinese Journal of Chemical Physics
  • Vol. 33, Issue 5, 05000635 (2020)
Long Jing1, Ye Zhao1, Du Yong2, Zheng Xu-ming1, and Xue Jia-dan1、*
Author Affiliations
  • 1Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China
  • 2Centre for THz Research, China Jiliang University, Hangzhou 310018, China
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    Abstract

    Photo-induced proton coupled electron transfer (PCET) is essential in the biological, photosynthesis, catalysis and solar energy conversion processes. Recently, $ p $-nitrophenylphenol (HO-Bp-NO2) has been used as a model compound to study the photo-induced PCET mechanism by using ultrafast spectroscopy. In transient absorption spectra both singlet and triplet states were observed to exhibit PCET behavior upon laser excitation of HO-Bp-NO2. When we focused on the PCET in the triplet state, a new sharp band attracted us. This band was recorded upon excitation of HO-Bp-NO2 in aprotic polar solvents, and has not been observed for $ p $-nitrobiphenyl which is without hydroxyl substitution. In order to find out what the new band represents, acidic solutions were used as an additional proton donor considering the acidity of HO-Bp-NO2. With the help of results in strong ($ \sim $10$ ^{-1} $ mol/L) and weak ($ \sim $10$ ^{-4} $ mol/L) acidic solutions, the new band is identified as open shell singlet O-Bp-NO2H, which is generated through protonation of nitro O in $ ^3 $HO-Bp-NO2 followed by deprotonation of hydroxyl. Kinetics analysis indicates that the formation of radical $ \cdot $O-Bp-NO2 competes with O-Bp-NO2H in the way of concerted electron-proton transfer and/or proton followed electron transfers and is responsible for the low yield of O-Bp-NO2H. The results in the present work will make it clear how the $ ^3 $HO-Bp-NO2 deactivates in aprotic polar solvents and provide a solid benchmark for the deeply studying the PCET mechanism in triplets of analogous aromatic nitro compounds.

    Ⅰ INTRODUCTION

    Photo-induced proton coupled electron transfer (PCET) has received many attentions [1, 2] in the past decades since it plays an important role in the biological [3-6], photosynthesis [7, 8], catalysis [9, 10], and solar energy conversion [11-13] processes. In general, PCET can take place in three ways: stepwise starting with either electron or proton transfer and concerted electron-proton transfer (EPT) [1]. Understanding the PCET principle allows us to tune and control the reaction process, thereby designing and synthesizing new functional molecules. Recently, $ p $-nitrophenylphenol (HO-Bp-NO2, Scheme 1) was used as a model compound to study the photo-induced PCET mechanism by using ultrafast spectroscopy [14], which provided a direct evidence for the presence of EPT process. In the system of HO-Bp-NO2 and amine where intermolecular hydrogen bond would form, two distinct states were observed upon laser excitation, and they were interpreted as conventional intramolecular charge transfer (ICT) state and ICT-EPT photoproduct [14]. The ICT state remains the proton associated with the donor and sequentially transfers the proton to the acceptor. Hammers-Schiffer et al. [13, 15] have well explained the configurations [16] of these two states and given a quantitative description on how the population decayed from the ICT state to the EPT state in the singlet manifold by quantum chemistry computation method [17].

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    Jing Long, Zhao Ye, Yong Du, Xu-ming Zheng, Jia-dan Xue. Direct Observation of Transient Species Generated from Protonation and Deprotonation of the Lowest Triplet of p-Nitrophenylphenol[J]. Chinese Journal of Chemical Physics, 2020, 33(5): 635
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    Received: Jun. 22, 2020
    Accepted: Jul. 13, 2020
    Published Online: Apr. 21, 2021
    The Author Email: Xue Jia-dan (jenniexue@zstu.edu.cn)