title: Time-Dependent Density Functional Theoretical Investigation of Photoinduced Excited-State Intramolecular Dual Proton Transfer in Diformyl Dipyrromethanes.
abstract: In recent research [ Chem. Commun. 2014 , 50 , 8667 ], it was found that photoinduced enolization occurred in 1,9-diformyl-5,5-diaryldipyrromethane (DAKK) by excited-state dual proton transfer resulting in a red-shifted absorption, a phenomena not observed in 1,9-diformyl-5,5-dimethyldipyrromethane (DMKK) and 1,9-diformyl-5-aryldipyrromethane (MAKK). 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 (DHKK), DMKK, MAKK, and DAKKand 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 λabsvalues 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 λabsvalues, a large red shift in λabswas seen only in DAKK, in agreement with the experimental findings. This observation can be attributed to the lowering of the keto → enol activation barrier, stabilization of DAEEin the S1state, and the charge transfer nature of the transitions involved in DAEE.
authors.affiliation: School of Chemical Sciences, National Institute of Science Education and Research (NISER) Bhubaneswar, HBNI , P. O. Jatni, Khurda, 752050 Odisha, India.
authors.forename: Renuka
authors.initials: R
authors.lastname: Pradhan
authors.affiliation: School of Chemical Sciences, National Institute of Science Education and Research (NISER) Bhubaneswar, HBNI , P. O. Jatni, Khurda, 752050 Odisha, India.
authors.forename: Aparna Karippara
authors.initials: AK
authors.lastname: Harshan
authors.affiliation: School of Chemical Sciences, National Institute of Science Education and Research (NISER) Bhubaneswar, HBNI , P. O. Jatni, Khurda, 752050 Odisha, India.
authors.forename: Gowri Sreedevi
authors.initials: GS
authors.lastname: Krishnavilasam Chandrika
authors.affiliation: School of Chemical Sciences, National Institute of Science Education and Research (NISER) Bhubaneswar, HBNI , P. O. Jatni, Khurda, 752050 Odisha, India.
authors.forename: Alagar
authors.initials: A
authors.lastname: Srinivasan
authors.affiliation: School of Chemical Sciences, National Institute of Science Education and Research (NISER) Bhubaneswar, HBNI , P. O. Jatni, Khurda, 752050 Odisha, India.
authors.forename: Upakarasamy
authors.initials: U
authors.lastname: Lourderaj