Stable tetrabenzo-Chichibabin's hydrocarbons: Tunable ground state and unusual transition between their closed-shell and open-shell resonance forms
Zafra, José Luis
Lõpez Navarrete, Juan Teodomiro
Webster, Richard D.
KAUST DepartmentBiological and Environmental Sciences and Engineering (BESE) Division
KAUST Catalysis Center (KCC)
Chemical Science Program
Physical Sciences and Engineering (PSE) Division
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AbstractStable open-shell polycyclic aromatic hydrocarbons (PAHs) are of fundamental interest due to their unique electronic, optical, and magnetic properties and promising applications in materials sciences. Chichibabin's hydrocarbon as a classical open-shell PAH has been investigated for a long time. However, most of the studies are complicated by their inherent high reactivity. In this work, two new stable benzannulated Chichibabin's hydrocarbons 1-CS and 2-OS were prepared, and their electronic structure and geometry in the ground state were studied by various experiments (steady-state and transient absorption spectra, NMR, electron spin resonance (ESR), superconducting quantum interference device (SQUID), FT Raman, X-ray crystallographic etc.) and density function theory (DFT) calculations. 1-CS and 2-OS exhibited tunable ground states, with a closed-shell quinoidal structure for 1-CS and an open-shell biradical form for 2-OS. Their corresponding excited-state forms 1-OS and 2-CS were also chemically approached and showed different decay processes. The biradical 1-OS displayed an unusually slow decay to the ground state (1-CS) due to a large energy barrier (95 ± 2.5 kJ/mol) arising from severe steric hindrance during the transition from an orthogonal biradical form to a butterfly-like quinoidal form. The quick transition from the quinoidal 2-CS (excited state) to the orthogonal biradicaloid 2-OS (ground state) happened during the attempted synthesis of 2-CS. Compounds 1-CS and 2-OS can be oxidized into stable dications by FeCl 3 and/or concentrated H 2SO 4. The open-shell 2-OS also exhibited a large two-photon absorption (TPA) cross section (760 GM at 1200 nm). © 2012 American Chemical Society.
SponsorsJ.W. acknowledges financial support from the BMRC-NMRC grant (no. 10/1/21/19/642), MOE Tier 2 grant (MOE2011-T2-2-130), and IMRE Core funding (IMRE/10-1P0509). The work at Yonsei University was supported by WCU (World Class University) programs (R32-2010-10217-0) and an AFSOR/APARD grant (no. FA2386-09-1-4092). K.-W.H. acknowledges financial support from KAUST. The work at the University of Malaga was supported by the Ministerio de Educacion y Ciencia (MEC) of Spain and by FEDER funds (project CTQ2009-10098 and to the Junta de Andalucia for the research project PO9-4708). We thank Dr. Tan Geok-Kheng for crystallographic analysis.
PublisherAmerican Chemical Society (ACS)
Is Supplemented ByZeng, Z., Sung, Y. M., Bao, N., Tan, D., Lee, R., Zafra, J. L., … Wu, J. (2013). CCDC 945080: Experimental Crystal Structure Determination [Data set]. Cambridge Crystallographic Data Centre. https://doi.org/10.5517/cc10qfgm
Zeng, Z., Sung, Y. M., Bao, N., Tan, D., Lee, R., Zafra, J. L., … Wu, J. (2013). CCDC 945081: Experimental Crystal Structure Determination [Data set]. Cambridge Crystallographic Data Centre. https://doi.org/10.5517/cc10qfhn