Kinetically blocked stable heptazethrene and octazethrene: Closed-shell or open-shell in the ground state?
Zafra, José Luis
Webster, Richard D.
Lõpez Navarrete, Juan Teodomiro
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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AbstractPolycyclic 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. © 2012 American Chemical Society.
SponsorsJ.W. acknowledges the financial support from the BMRC-NMRC grant (no. 10/1/21/19/642), MOE Tier 2 grant (MOE2011-T2-2-130), and A*STAR 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 the financial support from KAUST. We thank Ji'En Wu and Yanhui Han for their kind assistance on the NMR analysis and Prof. Yuan-Chung Cheng for useful discussion.
PublisherAmerican Chemical Society (ACS)
Is Supplemented ByLi, Y., Heng, W.-K., Lee, B. S., Aratani, N., Zafra, J. L., Bao, N., … Wu, J. (2013). CCDC 875408: Experimental Crystal Structure Determination [Data set]. Cambridge Crystallographic Data Centre. https://doi.org/10.5517/ccycxz5
Li, Y., Heng, W.-K., Lee, B. S., Aratani, N., Zafra, J. L., Bao, N., … Wu, J. (2013). CCDC 875409: Experimental Crystal Structure Determination [Data set]. Cambridge Crystallographic Data Centre. https://doi.org/10.5517/ccycy07
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