2,6-Naphthoquinodimethane (2,6-NQDM)- and 1,5-naphthoquinodimethane (1,5-NQDM)-bridged porphyrin dimers, 2,6-P2 and 1,5-P2, were synthesized as relatively stable compounds. Both exhibit open-shell singlet ground state according to variable-temperature (VT) NMR and magnetic measurements, as well as restricted active space spin-flip (RAS-SF) calculations. The 1,5-P2 isomer has a larger diradical character (y0 = 0.772, based on the RAS-SF calculations) and smaller singlet-triplet energy gap (ΔES-T = -2.6 kcal/mol, based on SQUID measurements) compared to the 2,6-P2 isomer (y0 = 0.501, ΔES-T = -3.1 kcal/mol). In addition, 2,6-P2 shows intense one-photon absorption (OPA) (λmax = 976 nm, = 72200 M-1 cm-1) and a large two-photon absorption (TPA) cross-section (σmax(2) = 3500 GM at 1400 nm) in the near-infrared region, while 1,5-P2 with larger diradical character displays red-shifted but weaker OPA (λmax = 1147 nm, = 18030 M-1 cm-1) and a smaller TPA cross-section (σmax(2) = 1750 GM at 1600 nm). Both compounds show four reversible redox waves and 1,5-P2 has a smaller electrochemical energy gap (1.06 eV vs.1.16 eV for 2,6-P2). Therefore, the bridge structure has a significant impact on the diradical character, electronic properties, and magnetic behaviors of the obtained porphyrin-based diradicaloids.
Bibliographical noteFunding Information:
We acknowledge financial support from MOE Tier 3 programme (MOE2014-T3-1-004) and NRF Investigatorship (NRF-NRFI05-2019-0005). The work at Yonsei University was supported by Strategic Research (NRF2016R1E1A1A01943379) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science.
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