Kinetically blocked stable heptazethrene and octazethrene: closed-shell or open-shell in the ground state?
Polycyclic aromatic hydrocarbons with an open-shell singlet biradical ground state are of fundamental interest and have potential applications in materials science. However, the inherent high reactivity makes their synthesis and characterization very challenging. In this work, a convenient synthetic route was developed to synthesize two kinetically blocked heptazethrene (HZ-TIPS) and octazethrene (OZ-TIPS) compounds with good stability. Their ground-state electronic structures were systematically investigated by a combination of different experimental methods, including steady-state and transient absorption spectroscopy, variable temperature NMR, electron spin resonance (ESR), superconducting quantum interfering device (SQUID), FT Raman, and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. All these demonstrated that the heptazethrene derivative HZ-TIPS has a closed-shell ground state while its octazethrene analogue OZ-TIPS with a smaller energy gap exists as an open-shell singlet biradical with a large measured biradical character (y = 0.56). Large two-photon absorption (TPA) cross sections (σ((2))) were determined for HZ-TIPS (σ((2))(max) = 920 GM at 1250 nm) and OZ-TIPS (σ((2))(max) = 1200 GM at 1250 nm). In addition, HZ-TIPS and OZ-TIPS show a closely stacked 1D polymer chain in single crystals.
Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore.
Yuan
Y
Li
Wee-Kuan
WK
Heng
Byung Sun
BS
Lee
Naoki
N
Aratani
José L
JL
Zafra
Nina
N
Bao
Richmond
R
Lee
Young Mo
YM
Sung
Zhe
Z
Sun
Kuo-Wei
KW
Huang
Richard D
RD
Webster
Juan T
JT
López Navarrete
Dongho
D
Kim
Atsuhiro
A
Osuka
Juan
J
Casado
Jun
J
Ding
Jishan
J
Wu
D018360:Crystallography, X-Ray; D007700:Kinetics; D008958:Models, Molecular; D015394:Molecular Structure; D011084:Polycyclic Aromatic Hydrocarbons / Q000737:chemistry*; D011789:Quantum Theory
D011084:Polycyclic Aromatic Hydrocarbons; C577909:heptazethrene; C577910:octazethrene