pubs.acs.org/JACS Article Monoradicals and Diradicals of Dibenzofluoreno[3,2‑b]fluorene Isomers: Mechanisms of Electronic Delocalization Hideki Hayashi,○ Joshua E. Barker,○ Abel Cardenaś Valdivia,○ Ryohei Kishi, Samantha N. MacMillan, Carlos J. Gomez-Garć ía, Hidenori Miyauchi, Yosuke Nakamura, Masayoshi Nakano,* Shin-ichiro Kato,* Michael M. Haley,* and Juan Casado* Cite This: J. Am. Chem. Soc. 2020, 142, 20444−20455 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: The preparation of a series of dibenzo- and tetrabenzo-fused fluoreno[3,2-b]fluorenes is disclosed, and the diradicaloid properties of these molecules are compared with those of a similar, previously reported series of anthracene-based diradicaloids. Insights on the diradical mode of delocalization tuning by constitutional isomerism of the external naphthalenes has been explored by means of the physical approach (dissection of the electronic properties in terms of electronic repulsion and transfer integral) of diradicals. This study has also been extended to the redox species of the two series of compounds and found that the radical cations have the same stabilization mode by delocalization that the neutral diradicals while the radical anions, contrarily, are stabilized by aromatization of the central core. The synthesis of the fluorenofluorene series and their characterization by electronic absorption and vibrational Raman spectroscopies, X-ray diffraction, SQUID measurements, electrochemistry, in situ UV−vis−NIR absorption spectroelectro- chemistry, and theoretical calculations are presented. This work attempts to unify the properties of different series of diradicaloids in a common argument as well as the properties of the carbocations and carbanions derived from them. ■ INTRODUCTION considered a “chemical approach”,extendingquinoidal − conjugation (Figure 1 top, e.g., A → B → C)18 25 or changing Carbon-based diradicaloids are fundamentally interesting 26− 40 molecules that have received a great deal of attention over the conjugation pattern entirely 1−4 (e.g., from para- to meta-) can yield large scale changes in the past decade. These novel conjugated polycyclic − ΔE .41 43 This often correlates with a strong increase in the hydrocarbons (CPHs) promise use in many organic electronic ST 5−7 8,9 diradical character index (y , with y → 0, a compound (OE) applications and as magnetic materials; however, an 0 0 Downloaded via UNIV OF OREGON on January 23, 2021 at 18:41:53 (UTC). becomes more closed shell and with y → 1, a compound inherent shortcoming in such open-shell constructs is 0 becomes more open-shell) resulting from a greater driving instability. Bulky groups installed at positions of high reactivity force to aromatize the central quinoidal unit and create more have enabled the isolation of kinetically stable diradicaloids, 2,26,28,29,31,38,40 See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Clar aromatic sextets. Recently our group has which have advanced the field beyond novel syntheses. With explored changing diradical character and tuning ΔE through the advent of increased stability, attention has turned to careful ST a contrasting “physical approach”, coupling synthetic modu- tuning of diradical properties with a goal toward application in 44−46 − lation with physical methods. The “two electrons in two OEs.10 16 sites” model of diradicals provides the framework for this An inherent property of diradicaloids is their ability to strategy through consideration of the effects of U (the on-site switch spins from a spin-paired singlet state to a spin-parallel − 44,45 electron electron repulsion term) and tab (the transfer triplet state. Controllable transition between these states can 46 integral, an electron delocalization term). Equation 1 be achieved through heating or cooling, and variable provides, in the two-site diradical model, the dependence of temperature (VT) measurements using SQUID magnetometry y (defined in eq 2) in terms of U and t . or ESR spectroscopy can determine the singlet−triplet energy 0 ab Δ gap ( EST). For the application of this class of molecules to be fully realized, it is essential to uncover the underlying Received: September 11, 2020 geometric/structural and theoretical parameters that can be Published: November 18, 2020 Δ modulated to optimize EST, in other words, to understand the basis of electronic diradical and spin delocalization.17 With this goal in mind, several groups have reported ff Δ di erent strategies for tuning EST. Broadly, in what could be © 2020 American Chemical Society https://dx.doi.org/10.1021/jacs.0c09588 20444 J. Am. Chem. Soc. 2020, 142, 20444−20455 Journal of the American Chemical Society pubs.acs.org/JACS Article Figure 1. (top left) Strategies for tuning diradical character showing the “chemical approach” using an expansion in the quinoidal conjugation path in indenofluorene congeners (A, B, and C for benzo-, naphtho-, and anthraceno-quinodimethanes, respectively). (top right) The arrows and boxes Δ fl fl − denote the variation that the y0/ EST points might undergo with altering the lateral Ar substituents. The B region targets uoreno uorenes 1 5 reported in this study. (bottom) Literature examples of the DBDIAn/C series with different fused Ar groups illustrating that the Ar motif exhibits fi Δ “ ” secondary control ( ne-tuning) of y0/ EST, i.e., the physical approach . In all instances, only the conjugated diradical cores are shown without the bulky alkyl and/or pendant aryl groups used to protect the radical centers. 1 around the main points such as highlighted as the expanding y =−1 0 2 arrows in the boxes also in Figure 1. As a result, the main U Δ − 1 + changes among molecules all lie along the generalized EST y ()4tab (1) curve that is dictated by the benzo-, naphtho- and anthraceno- with quinodimethane cores of A−C shown in Figure 1. The use of the “chemical approach” to reduce the quinoidal central motif from anthracenoquinodimethane to naphthoquinodimethane U 1 ff fl fl Δ=EST 1 − + 2Kab a ords uoreno[3,2-b] uorene (FF, 1). As depicted in Figure 2 − yy00(2 ) 2, inclusion of additional benzene rings to each side of the core ÅÄ ÑÉ fl fl U Å Ñ FF structure gives the representative dibenzo uoreno uorenes =+fyÅ () 2 K Ñ fl fl ÅST 0 ab Ñ (DBFFs, 2-4) and tetrabenzo uoreno uorene (TBFF, 5). 2 Å Ñ (2) Å Ñ Alternatively, these derivatives can also be viewed as fusion ÇÅ ÖÑ Δ Equation 2 further accounts for the EST in terms of y0 and along the 1,2-bond or 2,3-bond of naphthalene to a fi Kab (direct exchange integral), where a and b de ne the dicyclopenta[b,g]naphthalene (DCN, 6) core. This new series electrons in the localized natural orbitals. For most organic of compounds provides an avenue for predicting the change in fi diradicaloids, Kab is expected to be much smaller than the rst diradical index based on U/tab in molecules with larger/smaller Δ fi Δ term, and thus EST is primarily determined by the rst term. EST/y0. Given that the structural alteration consists of Δ Given that y0 is expressed as a function of U/tab by eq 1, EST changing the fusion-orientation along the 2,3-bond of DCN fi is determined from the balance of U and U/tab. Speci cally, in (6), it should be possible to connect the variation of the bond- diindenoanthracene-based diradicaloids (DIAns, Figure 1 order of the fusion bond with the value of diradical parameters Δ bottom), we have observed that for an isomeric series of to evaluate the interdependence of EST and y0 in molecules dibenzo-fused DIAn (DBDIAn) diradicaloids, the same possessing intermediate values of y0. This is in contrast to the chemical hydrocarbon composition causes the repulsion U case of diradicals with larger y0, such as the dibenzoDIAn term to be similar among them, and the differences in diradical derivatives.46 Finally, in addition to the comparative study of 46 character result from a large change in tab. neutral diradicals, both DBFF and DBDIAn are amphoteric The current study focuses on the quinoidal approach. Figure redox molecules that nicely stabilize cations or anions, which Δ 1 illustrates the generalized representation of EST versus y0 become the monoradical versions of the neutral diradicals. (blue curve, defined above in eq 2) for the quinoidal This offers the possibility of a second comparative study of diindenoacene family A−C with two assumptions: (i) lateral mono- and diradicals in the DBFF and DBDIAn series to fusion with different Ar groups in a symmetric manner and (ii) obtain insight into the similarities and differences in electron these Ar groups do not become the main stabilizing force of delocalization between isomers within each series. Surprisingly, the diradical (as would be the case of fused phenalenyls); thus, when comparing the neutral diradicals and in the radical the fused arenes always represent secondary stabilization cations, the electron delocalization is inherently the same, but motifs and impart fine-tuning of the magnetic properties in the anions some variation is seen. 20445 https://dx.doi.org/10.1021/jacs.0c09588 J. Am. Chem. Soc. 2020, 142, 20444−20455 Journal of the American Chemical Society pubs.acs.org/JACS Article qualitative discussion on the basis of the “two electrons in two sites” model (for details, see the Computational Details in the SI). Calculations predict U/tab values to vary among the studied compounds as 2.30 in 2′, 2.56 in 3′, 2.64 in 4′, and 2.81 in 5′, in good agreement with the change of diradical character at the PUHF level. The evolution of y0 can be further simplified since its change is concomitant with the decrease in the series of tab alone.