Synthesis, Structural Characterization, and Optical Properties of Benzene-Fused Tetracyclic and Pentacyclic Stiboles

The expectation that antimony (Sb) compounds should display phosphorescence emissions based on the “heavy element effect” prompted our interest in the introduction of antimony to a biaryl as the bridging atom in a fused heterole system. Herein, the synthesis, molecular structures, and optical properties of novel benzene-fused heteroacenes containing antimony or arsenic atoms are described. The stiboles and arsole were prepared by the condensation of dibromo(phenyl)stibane or dichloro(phenyl)arsine with dilithium intermediates derived from the corresponding dibromo compounds. Nuclear magnetic resonance (NMR) spectroscopy and X-ray crystal analysis revealed that the linear pentacyclic stibole was highly symmetric in both the solution and crystal states. In contrast, the curved pentacyclic stibole adopted a helical structure in solution, and surprisingly, only M helical molecules were crystallized from the racemate. All synthesized compounds produced very weak or no emissions at room temperature or in the solid state. In contrast, the linear penta- and tetracyclic stiboles exhibited clear phosphorescence emissions in the CHCl3 frozen matrix at 77 K under aerobic conditions.


Introduction
The development of novel π-conjugated systems and the elucidation of their optical properties to realize desired behaviors and functionalities are of current importance in the field of organic optoelectronic materials. The introduction of a group 15 elements such as phosphorus to a biaryl, as a bridge to form a fused heterole system, is of particular interest because it can further enhance conjugation due to its electronic effects [1]. For example, five-membered phospholes and their derivatives have been extensively studied as basic units of functional materials with luminescent, semiconducting, and sensing properties [2][3][4][5][6]. Arsoles, containing the homologous element, arsenic, have been reported as luminescent substances that are less likely to be oxidized than phospholes, although toxicity is a concern [7][8][9][10][11]. Antimony, another homologue located in the fifth period of group 15 elements in the periodic table, is expected to impart phosphorescence similarly to bismuth and tellurium based on the "heavy element effect", in contrast to lighter elements such as phosphorus [12][13][14][15][16]. Furthermore, since antimony compounds such as sodium stibogluconate (Pentostam) are used as first-line drugs for leishmaniasis [17] and are less toxic than bismuth compounds [18][19][20]; they have also garnered attention in the field of biology [21]. However, five-membered heterocyclic compounds containing antimony, known as stiboles, have received little attention with respect to their synthesis and spectrochemical behavior, including luminescence. In 1985, monocyclic stibole I (Figure 1) was synthesized by Ashe III et al., but it was unstable and resinified at room temperature under inert atmosphere [22]. The addition of fused aromatic rings effectively stabilizes I, such that the isolation of bicyclic benzo [b]stibole II and tricyclic dibenzo [b,d]stibole III has been reported [23][24][25]. The functions of benzostibole derivatives, such as the optical resolution of central asymmetry on antimony [26] and anion sensing, are being clarified [27,28]. In 2012, Ohshita et al. reported the synthesis of benzothiophene-fused pentacyclic stibole IV {1-(4-methylphenyl)-2,2 -di(benzo[b]thieno)stibolein} in low yield; however, almost no luminescence was observed at room temperature [29]. Subsequently, they reported both the synthesis of pyridine-fused tricyclic stibole V (9-phenyl-stibolo [2,3c:5,4-c ]dipyridine) and its phosphorescence at low temperature. They also revealed that the copper complex derived from stibole V and Cu 2 I 2 (PPh 3 ) 3 displayed phosphorescence at room temperature [30]. We reported the synthesis and structural study of optically active 7-(p-tolyl)dinaphtho[2,1-b:1 ,2 -d]stibole (7-Tol-DNSb) and revealed the dynamic behavior of the binaphthyl skeleton based on the C 2 symmetry axis [31]. These results show that the synthesis, structural analysis, and optical properties of the parent compound, stibole, are important for the subsequent creation of functional devices. However, there are no reports of linearly extended π-conjugated tetracyclic or pentacyclic stiboles condensed with benzene rings. Here, we present the synthesis of 5-phenylbenzo (3), and 7-phenyldinaphtho[2,1-b:1 ,2 -d]stibole (5) (Scheme 1) and explore the influence of the fused benzene rings on the optical properties including the phosphorescence of polycyclic benzostiboles.

Structure Analysis
The molecular structures of compounds 2-5 were confirmed spectroscopically ( 1 Hand 13 C-NMR, and MS). In the 1 H-and 13 C-NMR spectra of linear pentacyclic stibole 3, all the corresponding aromatic protons and carbons on the two naphthalene rings were equivalent. These results show that stibole 3 has a symmetric structure in CDCl 3 solution. Pentacyclic arsole 4 also revealed a symmetrical structure in solution. In contrast, in the 13 C-NMR spectrum of stibole 5, which is an isomer of 3, all the carbon signals in the two naphthalene rings were chemically nonequivalent. This result suggests that 5 adopts a curved helical structure similar to the known Sb-tolyl derivative [31].
All the obtained heteroacenes 2-5 were colorless crystalline solids. Single crystals of stiboles 3 and 5 suitable for X-ray analysis were obtained by repeated recrystallization from benzene/hexane or Et 2 O/hexane as the solvent. Unfortunately, single crystals of 2 and 4 could not be obtained. The molecular structures of pentacyclic stiboles 3 and 5 obtained from single-crystal X-ray diffraction (SCXRD) analysis are illustrated in Figure 2, and selected bond lengths and angles are summarized in Table 1. The structure obtained for linear stibole 3 reveals naphthalene wings that are slightly bent. The angle between each naphthalene ring, defined by ten carbon atoms, is 8.05 • , and the dihedral angle of C(4)-C(5)-C(6)-C(7) is 0.0(13) • . The C-Sb bond lengths are 2.155-2.182 Å, the C(Ph)-Sb-C bond angles are 92.7(3) and 94.6(3) • , and the interior C-Sb-C angle in the stibole ring is 81.0(3) • . The sum of the bond angles around the antimony atom is 268.3 • . The antimony center is highly pyramidalized, and the Ph substituent is situated nearly perpendicular to the stibole plane. In the packing structure, π-π stacking interactions are observed along the a axis. The interplanar distances between adjacent dinaphthostiboles are 3.57 Å (Figure 2c). In contrast, in the case of curved stibole 5, the C(4)-C(5)-C(6)-C(7) dihedral angle is 36.3(3) • , and a helical structure is adopted to prevent steric strain. Surprisingly, the crystallization of racemic stibole 5 produced optically active crystals consisting of only the M helical structure, that is, spontaneous resolution occurred during crystallization. Because the homochiral association event involves a large entropy loss, it has been estimated that only 5-10% of organic racemates exist as conglomerates [33,34]. To the best of our knowledge, this is the first example of the spontaneous resolution of a stibole derivative. In contrast, 7-Tol-DNSb formed a racemic crystal containing both enantiomers in equal proportions, specifically, it did not resolve spontaneously [31]. The bond lengths and angles around the antimony atom in 5 are approximately the same as in linear stibole 3. Furthermore, similar bond lengths and angles have been observed among previously reported stiboles, including benzostiboles [27,35], dipyridostibole [30], and 7-tolyldinaphthostibole [31].  Table 1. Selected bond lengths and angles.

Optical Properties
The photophysical properties of the synthesized stiboles and related compounds were evaluated, and the corresponding data are shown in Table 2 and Figure 3. The absorption maxima (λ max ) of stiboles 2 and 3 are observed at 323 and 360 nm, respectively. Since the λ max of bicyclic benzostibole II is 313 nm, the absorption band is red-shifted as the number of fused benzenes increases, reflecting the extended π-conjugation. Conversely, dinaphtho[2,1-b:1 ,2 -d]stibole 5 has a broad absorption band at 363 nm. It is known that the λ max of [n]helicenes generally occurs at lower wavelengths than the corresponding linear-type poly(acene)s [36]. In contrast, the wavelengths of peak top positions of linear stibole 3 and curved stibole 5 are the same. Surprisingly, the absorption edge of 5 reaches over 400 nm and is observed in a longer wavelength region than 3. Arsole 4 (λ max at 358 nm) exhibits an absorption spectrum similar to that of stibole 3, which indicates that the influence of the bridging heteroatom composing the five-membered ring is small.
The fluorescence spectra of stiboles 2, 3, and 5 and arsole 4 at room temperature show weak emissions with quantum efficiencies of 3% or less. This effect can be attributed to the presence of heavy atoms that promote an effective intersystem-crossing process. In the CHCl 3 frozen matrix at 77 K under aerobic conditions, however, stiboles 2 and 3 display apparent fluorescence and phosphorescence emissions. Figure 4 shows the fluorescence spectra (a) and phosphorescence spectra (b), isolated by means of gated detection, with a delay of 27 ms, for the compounds. These spectra clearly reveal emission bands that are composed of weaker fluorescence at 350-400 nm and stronger phosphorescence at 500-650 nm. The measured phosphorescence lifetimes are of the order of 40 ms. On the other hand, arsole 4 shows both fluorescence and phosphorescence emissions under the same conditions, but the contribution from phosphorescence (530-650 nm) is less than that from fluorescence (350-420 nm). Further, its phosphorescence lifetime is much longer than those of the stiboles (205 ms). In photographs at 77 K under UV irradiation, stiboles 2 and 3 clearly exhibit green and yellow phosphorescence emissions, respectively, whereas a blue fluorescence emission is displayed for arsole 4 (Figure 4c). However, a weak emission is observed for the helical-type stibole 5, even at 77 K ( Figure S1, Supplementary Materials). Unfortunately, none of the compounds were emissive in the crystal state.
Known cyclization precursors 1b [32] and 1c [31] were prepared according to the reported procedures, and spectroscopic data are in accordance with the literature.

Conclusions
In this study, penta-and tetracyclic stiboles and a pentacyclic arsole were synthesized using a double lithiation method. The NMR spectra revealed that linear pentacyclic compounds 3 and 4 had highly symmetric structures in solution. In contrast, the two naphthalene rings in curved pentacyclic stibole 5 were chemically nonequivalent. X-ray analysis revealed that the naphthalene wings of linear stibole 3 were slightly bent and packing structure of 3 formed π-π stacking. In the case of curved stibole 5, prepared as a racemate, spontaneous resolution occurred during crystallization, and the single crystal of compound 5 contained only M helical molecules. This is the first example of spontaneous resolution for a stibole derivative. All compounds displayed very weak or no emissions at room temperature or in the solid state. In contrast, linear compounds 2-4 showed clear phosphorescence emissions in the CHCl 3 frozen matrix at 77 K under aerobic conditions. Further investigations are underway, including synthesize of other isomers by the different in a position of fused benzene ring and the evaluation of the physicochemical properties, such as fluorescence, phosphorescence and redox potential, by theoretical and electrochemical studies.