Modulating the ground state, stability and charge transport in OFETs of biradicaloid Hexahydro-diindenopyrene derivatives and a proposed method to estimate the biradical character
Biradicaloid compounds with an open-shell ground state have been the subject of intense research in the past decade. Although diindenoacenes are one of the most developed families, only few examples have been reported as active layer in organic field-effect transistors (OFETs) with charge mobility of around 10−3 cm2 V−1 s−1 due to a steric disadvantage of the mesityl group to kinetically stabilize compounds. Herein, we disclose our efforts to improve the charge transport of the diindenoacene family based on hexahydro-diindenopyrene (HDIP) derivatives with different annelation modes for which the most reactive position has been functionalized with (triisopropylsilyl)ethynyl (TIPS) groups. All the HDIP derivatives show remarkably higher stability than that of TIPS-pentacene, enduring for 2 days to more than 30 days, which depends on the oxidation potential, the contribution of the singlet biradical form in the ground state and the annelation mode. The annelation mode affects not only the band gap and the biradical character (y0) but also the value of singlet-triplet energy gap (ΔES-T) that does not follow the reverse trend of the y0. A method based on comparison between experimental and theoretical bond lengths has been disclosed to estimate the y0 and shows that the y0 computed at projected unrestricted Hartree-Fock (PUHF) is most relevant among all other methods. Thanks to the high stability, thin-film OFETs were successfully fabricated. Well balanced ambipolar transport was obtained in the order of 10−3 cm2 V−1 s−1 in the bottom-gate/top-contact configuration, and unipolar transport in top-gate/bottom-contact configuration was obtained in the order of 10−1 cm2 V−1 s−1 which is the highest value obtained for biradical compounds with a diindenoacene skeleton.