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NIR Emission and Acid-Induced Intramolecular Electron Transfer Derived from a SOMO–HOMO Converted Non-Aufbau Electronic Structure
journal contribution
posted on 2019-02-08, 20:33 authored by Akira Tanushi, Shun Kimura, Tetsuro Kusamoto, Moe Tominaga, Yasutaka Kitagawa, Masayoshi Nakano, Hiroshi NishiharaSome organic radicals violate the
Aufbau principle and have unique
electronic structures in which the energy level of the singly occupied
molecular orbital (SOMO) is formally lower than that of the highest
occupied molecular orbital (HOMO). We synthesized a novel organic
radical with a SOMO–HOMO converted electronic structure, TPA-R•, a novel donor–acceptor hybrid of triphenylamine
(an electron donor), and a stable polychlorinated diphenyl(4-pyridyl)methyl
radical (an electron acceptor). TPA-R• exhibited
fluorescence in the near-infrared region (λmax =
910 nm) in cyclohexane from a polar intramolecular charge-transfer
excited state. Cyclic voltammetry, absorption spectroscopy, and density
functional theory calculation revealed the inversion of the SOMO and
HOMO levels in the electronic structure of TPA-R•. Addition of trifluoromethanesulfonic acid to TPA-R• caused a two-step change. Protonation initially occurred on the
diphenylpyridylmethyl radical moiety to form TPA-[RH]•+. This protonation induced large modulation in the frontier orbitals,
while the SOMO–HOMO converted electronic structure was conserved.
Further addition of the acid caused unprecedented intramolecular electron
transfer from the triphenylamine moiety to the protonated radical
moiety, generating [TPA]•+-RH. TPA-[RH]•+ and [TPA]•+-RH could be switched by changing the
acidity of the solution. These results constitute the first example
of the multistep switching behavior stimulated by a single external
stimulus in the SOMO–HOMO converted non-Aufbau electronic structure
and demonstrate its great potential for realizing unique molecular
photonic and electronic functions.
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Cyclic voltammetryenergy levelAufbau principletrifluoromethanesulfonic acidintramolecular charge-transfertheory calculationTPAtriphenylamine moietySOMOabsorption spectroscopyTPA-Relectron acceptorHOMO levelsfrontier orbitals910 nmRHAcid-Induced Intramolecular Electron Transfer DerivedNIR Emissionintramolecular electron transfer
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