Time-Dependent Density Functional Theoretical Investigation of Photoinduced Excited-State Intramolecular Dual Proton Transfer in Diformyl Dipyrromethanes

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• 作者：Renuka Pradhan ; Aparna Karippara Harshan ; Gowri Sreedevi Krishnavilasam Chandrika ; Alagar Srinivasan ; Upakarasamy Lourderaj
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：December 22, 2016
• 年：2016
• 卷：120
• 期：50
• 页码：9894-9906
• 全文大小：649K
• ISSN：1520-5215

文摘

In recent research [ Chem. Commun.pan class="NLM_x">p://www.w3.org/1998/Math/MathML" xmlns:ACS="http://namespace.acs.org/2008/acs" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:space="preserve"> pan>2014pan class="NLM_x">p://www.w3.org/1998/Math/MathML" xmlns:ACS="http://namespace.acs.org/2008/acs" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:space="preserve">, pan>50pan class="NLM_x">p://www.w3.org/1998/Math/MathML" xmlns:ACS="http://namespace.acs.org/2008/acs" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xml:space="preserve">, pan>8667], it was found that photoinduced enolization occurred in 1,9-diformyl-5,5-diaryldipyrromethane (DA_KK) by excited-state dual proton transfer resulting in a red-shifted absorption, a phenomena not observed in 1,9-diformyl-5,5-dimethyldipyrromethane (DM_KK) and 1,9-diformyl-5-aryldipyrromethane (MA_KK). The observation was supported by preliminary density functional theoretical (DFT) calculations. In the work reported here, a detailed and systematic study was undertaken considering four molecules, 1,9-diformyldipyrromethane (DH_KK), DM_KK, MA_KK, and DA_KK and their rotational isomers using DFT methods. Different processes, namely, cis–trans isomerization and single and double proton transfer processes, and their mechanistic details were investigated in the ground and excited states. From the simulation studies, it was seen that the presence of different substituents at the meso carbon does not affect the λ_abs values during cis → trans isomerization. However, enolization by proton transfer processes were found to be influenced by the substituents, as seen in the experiments. Enolization was observed to follow a stepwise mechanism, that is, diketo → monoenol → dienol. While monoenols showed negligible substituent effects on the λ_abs values, a large red shift in λ_abs was seen only in DA_KK, in agreement with the experimental findings. This observation can be attributed to the lowering of the keto → enol activation barrier, stabilization of DA_EE in the S₁ state, and the charge transfer nature of the transitions involved in DA_EE.