DE GRUYTER Physical Sciences Reviews. 2017; 20160109 Soumyajit Das / Jishan Wu Polycyclic Hydrocarbons with an Open-Shell Ground State DOI: 10.1515/psr-2016-0109 1 Introduction π-Conjugated polycyclic hydrocarbons (PHs) can either accommodate π-electrons in the bonding orbitals to form a closed-shell ground state or show open-shell ground state due to the existence of one or more un- paired electrons [1]. Monoradical PH is comprised of an unpaired electron in neutral molecule in the ground state [2]. Existence of two unpaired electrons (radicals) in a molecule can be classified by either diradical or biradical [3]. In the spin-restricted (R) molecular orbital (MO) representation [4], diradical character tends to increase as the highest occupied molecular orbital (HOMO)– lowest unoccupied molecular orbital (LUMO) gap gets reduced, causing a variation in bond nature from the stable bond regime to the bond dissociation regime through increasing the weight of doubly excited configuration from HOMO to LUMO. In a stable bond limit, i.e., closed-shell, no diradical character exists while in the bond dissociation limit, the perfect biradical or a pure open-shell configuration emerges. Any intermediate state is referred toas “diradicaloid” (diradical-like). Diradicals are even-electron molecules that have one bond less than the number permitted by the standard rule of valence and the two electrons occupy two near-degenerate MOs. The spin multiplicity (2S + 1) of mono- radicals is doublet, whereas for diradicals, two spins can orient in parallel or antiparallel fashion, producing triplet biradical or singlet diradical species. Singlet diradical, as a molecular species, has all the electrons paired but a pair of these electrons, being weakly coupled through anti-ferromagnetic interaction, occupies different parts of the space with a small shared region. PHs with moderate to strong singlet diradical character gener- ally show a broad electron spin resonance (ESR) signal due to the reduced singlet–triplet energy gap, which is believed to increase the population of the magnetically active triplet species. However, the ground state of such species is still singlet as at least one Kekulé structural formula can be drawn (e.g., p-quinodimethane (p- QDM)- and o-quinodimethane (o-QDM)-based PHs, Figure 1(a)). On the other hand, a pure biradical PH (e.g., m-quinodimethane (m-QDM)-based PHs, Figure 1(a)) is a molecular species with two electrons occupying two degenerate or nearly degenerate MOs, and there exists no Kekulé structural formula for this type of system [3]a]. from River Valley Technologies Ltd ProofCheck Jishan Wu is the corresponding author. © 2017 Walter de Gruyter GmbH, Berlin/Boston. Automatically generated rough PDF by This content is free. 1 Das and Wu DE GRUYTER Figure 1: (a) Structures of p-QDM, o-QDM and their singlet diradical resonance forms (Kekulé form); m-QDM in its triplet biradical form. (b) The concept of proaromaticity is shown as an example of quinoidal oligothiophene. Aromaticity is a key concept for chemistry in the electronic ground state, and reactions in which aromaticity is gained are normally highly favorable. Antiaromatic molecules like parent cyclobutadiene can be categorized as delocalized diradicals according to their MO diagram as two electrons occupy two degenerate non-bonding MOs which are “spin-up” unpaired, making them strongly reactive species and thus their synthesis is highly elusive. In fact, later it was found that cyclobutadiene derivatives prefer a rectangular shape in its singlet ground state instead of delocalized square geometry caused by the Jahn–Teller effect [5]. In the proaromatic concept (Figure 1(b)) for the formation of diradicals [5]b], the driving force for such structure is the recovery of aromatic- ity from consecutive non-aromatic rings (quinoidal structure). A net energy minimization that can eventually surpass the energy required to break a C–C double bond is the key in generation of an open-shell diradical ground state. The smaller proaromatic tetracyano-quinoidal-thiophene oligomers are closed shell, whereas the ESR spectra of the 5QT and 6QT constitute the first reports on the unquestionable diradical character due tothe population of thermally (room temperature (rt)) excited magnetically active species as a result of aromatization of the thiophene rings [6]. Acenes can be regarded as one-dimensional fragments of graphene and belong to a class of PHs consisting of linearly fused benzene rings. Bendikov et al. showed that the RB3LYP wave function becomes unstable for oligoacenes as small as hexacene and all higher oligoacenes, implying that the calculated energies for singlet states are unbelievably high. Re-optimization by using the broken-symmetry (BS) UB3LYP/6-31G* method, it was shown that acenes, larger than hexacene, possess a nonzero bandgap with a large amount of diradical character in a singlet open-shell ground state and the singly occupied molecular orbital (SOMO) is largely pop- ulated on the zigzag edges [7]. Later, Hachmann et al. also showed that the ground state of linear polyacenes is singlet for all chain lengths from naphthalene to dodecacene, and acenes larger than dodecacene were found to exhibit singlet polyradical character in their ground state [8]. Interestingly, experimental observations already established the closed-shell ground states (as evident from clear and sharp 1H nuclear magnetic resonance (NMR) signal) of higher-order acenes except for the first isolated crystalline nonacene derivative [9] that appar- ently showed low HOMO–LUMO gap of 1.2 eV which is a criterion for the origin of diradical character in PH. Absence of NMR signal and the presence of ESR signal with ge = 2.0060 for nonacene derivative were reported, although no clear conclusion regarding the ground state electronic structure was given in the report as the ESR signal may also come from some monoradical impurities. Theoretically, diradical character is estimated by a complete active space self-consistent field (SCF) calcula- tion using the restricted Hartree–Fock and two-configuration SCF calculations. A BS approach [10], later mod- ified with spin-projection technique [11] to discard spin contamination in BS approach, using the unrestricted Hartree–Fock wave function [12] can define the occupation number of the lowest unoccupied natural orbital (LUNO) as the extent of diradical character. As simplified by Nakano [4], for a two-electron two-orbital model the ground state closed-shell molecule has the natural orbital (NO) occupation numbers 2 (occupied) or 0 (un- occupied). So the LUNO is 0 for closed shell, whereas in the open-shell singlet molecules, the NO occupation number can be an intermediate value between 0 and 2, depending on the diradical character; therefore, the occupation number of the LUNO should increase with the increase in the diradical character and eventually approaches 1 for a pure diradical. This implies that the diradical character, which is denoted by y (0 ≤ y ≤ 1 where y = 0 is closed shell and y = 1 is pure open-shell biradical), can be defined by the occupation number of the LUNO. Experimentally, diradical character can also be determined using the following equation [13]: √ √ 퐸 − 퐸 2 √ ⎛ 푆1푢,푆1푔 푇1푢,푆1푔 ⎞ from River Valley Technologies Ltd 푦 = 1 − 1 − ⎜ ⎟ 퐸 ⎷ ⎝ 푆2푔,푆1푔 ⎠ , where ES1u,S1g and ES2g,S1g correspond to the energy of the lowest energy peaks in the one- and two-photon ProofCheck absorption (TPA)spectra, and ET1u,S1g corresponds to the energy gap between the triplet and the singlet ground state. 2 Quinodimethane-Based Open-Shell Polycyclic Hydrocarbons The most basic, yet powerful, approach to generate a diradicaloid PH is to embed an o-QDM or p-QDM subunit into a π-conjugated framework. This approach has been a topic of intense interest due to their inherent diradical character arising from the recovery of the aromaticity of the central benzenoid ring of the respective quinoidal subunits, in the ground state. Comparatively, o-QDM derivatives such as pleiadene [14] are extremely reactive Automatically generated rough PDF by compared to p-QDM derivatives. Unlike o- and p-QDM, the m-QDM (i.e., m-xylylene biradical) cannot have a 2 DE GRUYTER Das and Wu closed-shell quinoidal structure and thus is classified into non-Kekulé PH. Embedding ofan m-xylylene birad- ical subunit into a planarized polycyclic benzenoid hydrocarbon generates open-shell π-conjugated triplet PH systems such as triangulenes (vide infra), with large spin densities at the edge sites. 2.1 o-QDM-Embedded Diradicaloids Tobe et al. reported [15] the synthesis and crystallographic structure of an indeno-[2,1-a]fluorene (1) derivative 5 which could show diradical resonance contribution due to recovery of one additional Clar’s sextet (highlighted in gray color) in the ground state, with radical sites mainly located on the five-membered rings (Figure 2). Mesityl groups were introduced to kinetically protect the reactive sites, by adding mesitylmagnesium bromide to the diketone 3 to afford the diol 4 which on treatment with tin(II)chloride gave 5 as a stable, purple-colored solid. The unaffected 1H NMR spectra of 5 at higher temperature and a significant bond-length alternation in o-QDM core, as per crystal structure analysis, indicate a large singlet–triplet energy gap with small diradical contribution, which was also supported by theory. The singlet diradical character of 1 and 5 estimated by the Yamaguchi scheme was 0.33 and 0.21, respectively. Nucleus-independent chemical shift calculation further supports that 5 is weakly antiaromatic as a result of the as-indacene moiety. The vertical π-extension of o-QDM can lead to Kekulé structure 2 which can also have two diradical resonance forms and the spin density may distribute to the inner naphthalene ring. A similar synthetic approach was conducted to prepare the vertically extended o-QDM derivative 8 which was confirmed by X-ray crystallographic analysis [16].