Kinetics of Excimer Electrogenerated Chemiluminescence of Pyrene and 1-Pyrenebutyricacid 2-Ethylhexylester in Acetonitrile and an Ionic Liquid, Triethylpentylphosphonium Bis(trifluoromethanesulfonyl)imide

Ryoichi Ishimatsu; Shuya Tashiro; Takashi Kasahara; Juro Oshima; Jun Mizuno; Koji Nakano; Chihaya Adachi; Toshihiko Imato

doi:10.1021/acs.jpcb.9b08813

Kinetics of Excimer Electrogenerated Chemiluminescence of Pyrene and 1-Pyrenebutyricacid 2-Ethylhexylester in Acetonitrile and an Ionic Liquid, Triethylpentylphosphonium Bis(trifluoromethanesulfonyl)imide

J Phys Chem B. 2019 Dec 19;123(50):10825-10836. doi: 10.1021/acs.jpcb.9b08813. Epub 2019 Dec 5.

Authors

Ryoichi Ishimatsu¹, Shuya Tashiro¹, Takashi Kasahara², Juro Oshima³, Jun Mizuno⁴, Koji Nakano¹, Chihaya Adachi¹, Toshihiko Imato¹

Affiliations

¹ Department of Applied Chemistry, Graduate School of Engineering , Kyushu University , 744 Motooka , Nishi-ku, Fukuoka 819-0395 , Japan.
² Department of Electrical and Electronic Engineering, Faculty of Science and Engineering , Hosei University , 3-7-2 Kajino-cho , Koganei , Tokyo 184-8584 , Japan.
³ Frontier Materials Research Department, Materials Research Laboratories , Nissan Chemical Corporation , 488-6 Suzumi-cho , Funabashi , Chiba 274-0052 , Japan.
⁴ Research Organization for Nano and Life Innovation , Waseda University , 513 Wasedatsurumaki-cho , Shinjuku-ku, Tokyo 162-0041 , Japan.

PMID: 31804083
DOI: 10.1021/acs.jpcb.9b08813

Abstract

We describe the kinetics of excimer electrogenerated chemiluminescence (ECL) of a liquid pyrene derivative, 1-pyrenebutyricacid 2-ethylhexylester (PLQ) dissolved in a molecular solvent, acetonitrile (MeCN), and an ionic liquid, triethylpentylphosphonium bis(trifluoromethanesulfonyl)imide ([P₂₂₂₅][TFSI]). Pyrene was also used for comparison. To discuss the kinetics of the excimer ECLs, the photophysical and electrochemical properties and electronic states of PLQ and pyrene were revealed. The photoluminescence (PL) spectra, rate constants for the radiative transitions, and redox potentials of PLQ and pyrene dissolved in MeCN and [P₂₂₂₅][TFSI] suggest that as a solvent, [P₂₂₂₅][TFSI] behaves more polar than MeCN. By analyzing the PL decay curves, the rate constants to form the excimer were determined to be on the order of 10⁹ and 10⁷ M^-1 s^-1 in MeCN and [P₂₂₂₅][TFSI], respectively, which were limited by the diffusion. For neat PLQ (1.6 M), a delay of 0.3-0.4 ns for the excimer emission compared to the monomer emission was observed. It is likely that the delay corresponds to the timescale for arranging the conformation to form the excimer. The ECL of PLQ was generated by applying a square wave voltage to produce the radical anion and cation, and on the ECL spectra, the excimer emission was more prevailed compared to the PL spectra. Kinetic analysis for the electron transfer reaction between the radical ions based on Marcus theory indicates that the electron transfer is limited by the diffusion of the radical ions. Moreover, the electron transfer distance (d_et) between the radical cation and anion to generate excited states was calculated with a framework of the theory. Kinetically, the electron transfer can take place at d_et < ∼11 Å in MeCN and d_et < ∼12 Å in [P₂₂₂₅][TFSI]. The density functional theory (DFT) and time-dependent DFT calculations show that the potential energy curve of the excimer against the distance between the pyrene rings reaches a minimum at 3.50 Å. This suggests that through the electron transfer, the process of the direct formation of the monomer S₁ state followed by the excimer formation is more prevailed than that of the direct excimer formation.