and Dicationic P‑Bridged Ladder Stilbenes - ACS Publications

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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P‑Protected Diphosphadibenzo[a,e]pentalenes and Their Mono- and Dicationic P‑Bridged Ladder Stilbenes Patrick Federmann,† Hannah K. Wagner,† Patrick W. Antoni,† Jean-Marc Mörsdorf,‡ Jose ́ Luis Peŕ ez Lustres,‡ Hubert Wadepohl,† Marcus Motzkus,*,‡ and Joachim Ballmann*,† †

Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 276, D-69120 Heidelberg, Germany Physikalisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany

‡

Org. Lett. Downloaded from pubs.acs.org by ALBRIGHT COLG on 03/21/19. For personal use only.

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ABSTRACT: The previously elusive diphosphadibenzo[a,e]pentalene core skeleton was assembled via a surprisingly straightforward cyclization pathway starting from R2P-substituted 2,2′-diphosphinotolanes (R = Ph, iPr). The resulting P-protected diylidic compounds 4 (R = Ph, iPr) were converted to the corresponding P-bridged ladder stilbenes via two consecutive oxidation steps: upon selective one-electron oxidation, the persistent radical monocations 5 (R = Ph, iPr) were obtained and further oxidized to afford the respective fluorescent and airstable dications 6 (R = Ph, iPr).

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hroughout the decades, pentalenes1 and dibenzopentalenes2 have been studied intensively,3 in particular with respect to their differently pronounced Hückel antiaromaticity. Pentalene itself (1a, see Figure 1), which is known for its

optoelectronic properties,2c,3b,c it is not surprising that polycyclic dibenzo[a,e]pentalene-based frameworks emerged as a key subject in recent research on conjugated materials. The presence of the partially antiaromatic pentalene subunit and its propensity to be either reduced or oxidized lead to a decreased HOMO−LUMO gap, thus rendering these species promising candidates for semiconducting materials.3d,f,h Incorporating heteroatoms into the parent pentalene core or into dibenzopentalenes is considered a prosperous methodology to tune the electronic properties of the resulting materials, but mastering this approach remains a synthetic challenge for pentalene-based systems, such as 1b5 and 1c.6 In the case of heteroatom-functionalized dibenzo[a,e]pentalenes, only the nitrogen-bridged derivative 2b has been prepared recently,7 but numerous synthetic routes to the closely related 2e−-oxidized heteroatom-bridged ladder stilbenes 3a−3i have been reported (cf. Figure 1).8 The dichalcogenophenes 3f and 3g, for example, have been applied in organic field effect transistors,9 while equally promising electronic and photophysical properties were uncovered for the B- (3a,b),10 C(3c),11 Si- (3d),12 N- (3e),13 and P-bridged (3h−i)14 ladder stilbenes. In contrast to the 16 π-electron-containing dibenzo[a,e]pentalene derivatives 2a and 2b, all the aforementioned ladder stilbenes 3 comprise either 18 (3e−h) or 14 (3a−d, 3i) πelectrons, while 16 π-electron P-bridged species akin to 2 have not been explored yet, although distinct optoelectronic properties are to be expected for these skeletons.16 Herein, we report on the synthesis and the comprehensive character-

Figure 1. E-containing pentalenes and dibenzo[a,e]pentalenes together with the related E-bridged ladder stilbenes. The P-bridged compounds shown in the red insets are reported herein (R = Ph [X− = PF6−], iPr [X− = Cl−]). Note that ladder stilbenes comprising two different heteroatoms E have been reported as well.15

antiaromaticity,4 has not been isolated but only detected in a hydrocarbon matrix at −196 °C,1b due to its highly reactive nature. In contrast, a significantly diminished antiaromaticity4a,b has been recognized for dibenzo[a,e]pentalene 2a and its derivatives, which are held responsible for the enhanced stability of these readily accessible molecules.2i Given that πextended pentalene skeletons were shown to exhibit desirable © XXXX American Chemical Society

Received: January 14, 2019

A

DOI: 10.1021/acs.orglett.9b00161 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters ization of such a system, i.e., on the P-diylidic framework 4 (see Figure 1), and its oxidation to the dicationic ladder stilbenes 6 (R = Ph, iPr), which were isolated as air-stable highly fluorescent materials. Along the way, the radical monocations 5 were isolated and fully characterized as well. For the synthesis of 4a (R = Ph) and 4b (R = iPr), a novel cyclization pathway starting from the respective 2,2′diphosphinotolanes 7 (R = Ph, iPr, prepared from 2,2′dibromotolane, n-BuLi and ClPR2, see SI) was exploited. Upon heating, the latter tolanes were found to undergo intramolecular cyclization reactions17 to selectively afford the airand moisture-sensitive diylides 4a and 4b in high yields (see Scheme 1).18 Due to the intensively green and blue colors of Scheme 1. Synthesis of 4 via 2-Fold Cyclization of 7 and Oxidation of 4 to the Mono- and Dications 5 and 6

Figure 2. ORTEP plot of the molecular structure of 4b (displacement ellipsoids drawn at 50% probability; hydrogen atoms were omitted for clarity; and only one of the two independent molecules present in the asymmetric unit is shown, see SI for details) together with the calculated molecular structure (B3LYP/def2TZVPP, hydrogen atoms omitted for clarity) and representative resonance structures.

(E = CH) and 4 (see Figure 3) only allow for a rough estimate of the global ring currents; i.e., the aromaticity of

Figure 3. Comparison of NICS(1)oop values for the five- and sixmembered rings in 2a (E = CH) and 4 (nearly identical values have been obtained for 4a and 4b, see SI for details). The corresponding values for benzene, 1a (E = CH), and 1c (E = PPh2, PiPr2) are shown for comparison.

4a and 4b, both reactions may be followed qualitatively by the naked eye, but monitoring the reaction progress quantitatively by 31P NMR spectroscopy (δ = −9.3 and −3.4 ppm for 7a and 4a, δ = 4.9 and 24.2 ppm for 7b and 4b) or UV−vis spectroscopy (4a: λmax = 687 nm with ε = 3.30 × 103 L·mol−1· cm−1; 4b: λmax = 633 nm with ε = 4.25 × 103 L·mol−1·cm−1) is certainly more reliable (see SI for UV−vis spectra). The single-crystal X-ray structure analysis of 4b (see Figure 2) revealed the presence of two centrosymmetrical molecules with endocyclic P−C bond lengths of P1−C1 1.7210(10) Å [1.7210(11)] and P1−C2 1.7742(10) Å [1.7760(11)], respectively19 (average P = C bond lengths in R3P = CR2 ylides: 1.74 ± 0.04 Å).20 Further insights into the electronic structures of 4a and 4b were gained by DFT calculations: At the B3LYP/def2TZVPP level of theory (dispersion corrected),21 the crystallographically determined bond lengths and angles in 4b were well reproduced by the corresponding calculated parameters as illustrated in Figure 2. According to NBO and NRT analysis (NRT = natural resonance theory),21 compounds 4 are best described as doubly zwitterionic species with delocalization patterns reminiscent to the ones in allylic or benzylic carbanions. Consistently, the central C1−C1′ bond in 4b (experimental: 1.486(2) Å [1.484(2)],20 DFT: 1.491 Å) is best represented as a C(sp2)−C(sp2) single bond (see Figure 2).The NICS(1)oop values (oop = out-of-plane component of the chemical shift value 1 Å above the respective ring centers)21 indicated that the paratropic ring currents in 4 are reduced in comparison to 2a, while the diatropic ring currents are nearly unaffected.22 Note that the NICS(1)oop values of 2a

polycondensed π-systems cannot be deduced by analyzing fragmented molecules22 (cf. NICS(1)oop values of the building blocks matching to 2a and 4 shown in Figure 3). Compounds 4a and 4b readily degrade in air. Selective oxidation with [Ph3C][PF6] or Ph3CCl under argon led to the radical cations 5a and 5b (see Scheme 1),23 which were isolated as dark blue powders (5a: λmax = 598 nm with ε = 5.10 × 103 L·mol−1·cm−1; 5b: λmax = 571 nm with ε = 6.75 × 103 L· mol−1·cm−1). A single-crystal X-ray structure determination of a dichloroethane solvate of 5b (see Figure 4) showed significantly shortened C1−C1′ and elongated C1(′)−P1(′) bond lengths, indicating that the P-diylidic binding motif is weakened in 5b compared to 4b. In the X-band EPR spectrum of 5b, a triplet signal (g = 2.002, A0 = 21 G) was detected in CH2Cl2 at rt (see Figure 4), while a broad isotropic singlet (g = 2.002) was found for 5a under identical experimental conditions; i.e., no coupling to the 31P nuclei was observed in the latter case.24 In the cyclic voltammograms (CH2Cl2, recorded against Fc/Fc+ using NBu4PF6 as electrolyte), two redox waves were found for each radical cation (irreversible for 5a with Epc = −0.61 V for 5a/6a and −1.31 V for 4a/5a; quasireversible for 5b with E1/2 = −0.76 V for 5b/6b and −1.50 V for 4b/5b), suggesting that both compounds are readily oxidized to the respective dications. On a preparative scale, the diamagnetic dications 6a and 6b were indeed easily B

DOI: 10.1021/acs.orglett.9b00161 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

C1′ bond lengths within the typical range for conjugated double bonds were found (1.359(2) Å in 6a and 1.364(2) Å in 6b), which agrees well with the corresponding Cinner−Cinner bond lengths in other ladder stilbenes.14a,15a,b In comparison to 4b and 5b, less elongated C2−C7(′) bond distances were noticed in 6b, indicating that the benzannulated rings in 6 are well-described as delocalized Clar sextets. This is in line with the more negative NICS(1)oop values of the six-membered rings in 6 (approximately −23 ppm, cf. −11 ± 2 ppm for 4) and with the more positive NICS(1)oop values of the fivemembered rings (approximately +9 ppm in 6 vs 0 ± 2 ppm in 4).21 For the phosphorus(V)-bridged ladder stilbene trans-3i, similar values were found for the corresponding hexa- (−23 ppm) and pentagonal (+7 ppm) rings, indicative of an overall comparable aromaticity (see SI for details). In the solid state, both compounds 6 were found to exhibit a bright greenishyellow fluorescence, which is retained upon dissolution in MeCN (6a) or water (6b). From the maxima of the absorption and emission spectra (see Table 1 and Figure 6), Stokes shifts

Figure 4. ORTEP plot of the molecular structure of 5b (only one of the two independent molecules is shown; displacement ellipsoids are drawn at 50% probability; and hydrogen atoms and cocrystallized solvent molecules are omitted, see SI) together with the X-band EPR spectrum of 5b (recorded in CH2Cl2 at rt).

obtained via reaction of 5a and 5b with ferrocenium hexafluorophosphate and iodobenzene dichloride, respectively.23 The resulting air-stable, bright yellow P-bridged ladder stilbenes precipitated from solution and were isolated in 69% yield each. In the 31P{1H} NMR spectra, sharp singlets were detected at 58.5 ppm (6b, D2O) and at 25.8 ppm (6a, MeCN-d3) in addition to a septet for the PF6 counterion in 6a (δ = −144.8 ppm). The 13C{1H, 31P} NMR resonances at 148.2 ppm (6a) and 146.6 ppm (6b) were assigned to the central sp2 carbons of the stilbene cores, and the assignment was further corroborated by two-dimensional NMR experiments and DFT calculations (GIAO method).21 Single-crystal structure analyses were carried out for 6a and a water solvate of 6b (see Figure 5). In 6a, the molecules were found to assemble in alternating pillars, while the overall packing in 6b is best described as herringbone motif along the a axis (cf. Figure 5). In both compounds, relatively short C1−

Table 1. Photophysical Data for Compounds 6 compounda 6a 6b compounda 6a 6b

λabsb(nm) 417 399 Φ fe 0.24 0.74

εc (M−1 cm−1)

Δνd (cm−1)

488 518 τe (ns)

5.17 × 10 8.09 × 103 krf (ns−1)

3490 5780 knrf (ns−1)

15.2 14.6

0.016 0.051

0.050 0.018

λemb (nm)

3

a

Data for 6a were collected in MeCN, and data for 6b were collected in H2O. bMaximum in the absorption and emission spectra. c Extinction coefficient at λabs. dStokes shift. eFluorescence quantum yield and fluorescence lifetime. fRadiative and nonradiative decay constants.

Figure 6. UV/vis absorption (solid lines) and corrected fluorescence spectra (dashed lines, normalized to the corresponding absorption band of lowest energy) of 6a (MeCN, red) and 6b (H2O, blue) together with the fluorescence decay curve of 6b (H2O) monitored at 481 nm (λex = 375 nm). A monoexponential response function was convoluted with the instrument response function (IRF, gray) and fitted (red, weighted residuals for the fit within ±3%, see SI).

of 3490 cm−1 (6a) and 5780 cm−1 (6b) were determined, indicating that the molecular geometries in the ground state and the first excited state are significantly different for both molecules. For 6b, a considerably higher fluorescence quantum yield (74% in water) was determined in comparison to 6a (24% in MeCN),25 while nearly identical fluorescence lifetimes were found for both compounds (see Table 1). Rate constants for the radiative (and nonradiative) fluorescence decay were calculated according to kr = Φf/τ (and knr = (1 − Φf)/τ) and found to compare well with the corresponding values determined for phosphoranyl-bridged

Figure 5. ORTEP plots of the molecular structures and packing diagrams of 6a and 6b (displacement ellipsoids drawn at 50% probability; all hydrogen atoms and cocrystallized solvent molecules are omitted; and bond lengths labeled in red are significantly different in comparison to 4b and 5b, cf. corresponding bond lengths in Figures 2 and 4). C

DOI: 10.1021/acs.orglett.9b00161 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters ladder stilbenes (see Table 1, cf. kr(trans-3i) = 0.062 ns−1).14a The larger kr-value of 6b (in comparison to 6a) essentially mirrors the higher fluorescence quantum yield of 6b, given that the oscillator strength for the lowest-energy absorption of 6a (f = 0.19) compares well to the corresponding oscillator strength of 6b (f = 0.24).21 In view of these emission properties, we posed the question whether compounds 6 may be promising for an application in organic semiconducting devices.26 To estimate the ease of electron (or hole) injection into 6, the MO energies of the frontier orbitals were inspected by means of DFT analysis (6b: see Figure 7; 6a: see SI). As expected for

Accession Codes

CCDC 1884989−1884992 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Joachim Ballmann: 0000-0001-6431-4197 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from the Bernthsen foundation (University of Heidelberg) and the DFG (BA 4859/2) is gratefully acknowledged. The authors acknowledge support by the state of Baden-Württemberg through bwHPC (bwUniCluster) and the German Research Foundation (DFG) through grant no INST 40/467-1 FUGG (JUSTUS cluster). We thank Dr. M. Kerscher and D. Gutruf for recording low-temperature EPR spectra and cyclic voltammograms, respectively. J.B. thanks Prof. L. H. Gade for generous support over the past years and for his enduring interest in our work.

Figure 7. MO energy diagram of 6b (B3LYP-D3/def2TZVPP, PCM solvent-corrected for CH2Cl2) together with isodensity plots of the Kohn−Sham HOMO and LUMO of 6b.

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dicationic species, low-lying LUMOs were found (DFTELUMO(6a) = −4.22 eV; DFTELUMO(6b) = −4.17 eV) in silico, which agrees well with the corresponding cyclic voltammograms (CVELUMO(6a) = −4.19 eV; CVELUMO(6b) = −4.04 eV; EHOMO(Fc) set to −4.80 eV). Thus, electron injection into the highly electron-accepting π-systems of 6 is thermodynamically favored by approximately 1 eV in comparison to uncharged P-bridged ladder stilbenes (cf. CV ELUMO(3i) ≈ − 3.2 eV),14 suggesting that compounds 6 may be employed as n-type semiconductors. However, future studies will be required to evaluate whether this is indeed the case. In conclusion, the R2P-protected diphosphadibenzo[a,e]pentalene skeletons 4a (R = Ph) and 4b (R = iPr) have been prepared via a simple cyclization reaction starting from the respective 2,2′-diphosphinotolanes. The electron-rich Pdiylides 4 were oxidized to afford the persistent radical monocations 5 and the fluorescent P-bridged ladder stilbenes 6. The promising photophysical properties and the low-lying LUMO energies of 6 warrant further studies aiming toward an application of these compounds.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00161. Detailed experimental procedures, product identification data, NMR and EPR spectra, photophysical measurements, computational methods, additional ORTEP drawings, and crystallographic data (PDF) D

DOI: 10.1021/acs.orglett.9b00161 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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Letter

Organic Letters (stepwise or concerted nucleophilic attack of the phosphines on the alkyne or a radical initiated mechanism) are considered equally plausible. (19) Values in square brackets correspond to the second independent molecule present in the asymmetric unit. (20) Values based on a CSD search (version 5.39 update Aug 2018). (21) For details and references on DFT calculations (functionals, basis sets, program packages, etc.) see SI. (22) (a) Báez-Grez, R.; Ruiz, L.; Pino-Rios, R.; Tiznado, W. RSC Adv. 2018, 8, 13446−13453. (b) Fowler, P. W.; Steiner, E.; Havenith, R. W. A.; Jenneskens, L. W. Magn. Reson. Chem. 2004, 42, S68−S78. (c) Stanger, A. J. Org. Chem. 2006, 71, 883−893. (d) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; van Eikema Hommes, N. J. R. J. Am. Chem. Soc. 1996, 118, 6317−6318. (23) The oxidants were chosen on the basis of CV measurements vs Fc/Fc+ (Epc = −1.31 V for 4a/5a and −0.61 V for 5a/6a; E1/2 = −1.50 V for 4b/5b and −0.76 V for 5b/6b; see: Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877−910. Several other oxidants (e.g., AgPF6, DDQ, or I2) failed to react cleanly. (24) EPR spectra recorded at 6 K were found to closely resemble the ones recorded at room temperature. In silico (see SI), lower Mulliken spin densities were found on the phosphorus atoms of 5a in comparison to 5b, which is in line with the observed singlet (5a) and triplet (5b) signals. However, other factors, e.g., spin polarization and different orientations in (frozen) solution due to different counterions, presumably play a role as well. For persistent phosphorus radicals and their EPR spectra, see for example: (a) Leca, D.; Fensterbank, L.; Lacôte, E.; Malacria, M. Chem. Soc. Rev. 2005, 34, 858−865. (b) Witwicki, M. ChemPhysChem 2015, 16, 1912−1925. (c) Giffin, N. A.; Hendsbee, A. D.; Masuda, J. D. Dalton Trans 2016, 45, 12636−12638. (d) Pan, X.; Wang, X.; Zhang, Z.; Wang, X. Dalton Trans 2015, 44, 15099−15102. (e) Su, Y.; Zheng, X.; Wang, X.; Zhang, X.; Sui, Y.; Wang, X. J. Am. Chem. Soc. 2014, 136, 6251−6254. (25) Care should be taken upon comparing fluorescence quantum yields in different solvents. However, 6a was found to be insoluble in water, while 6b was found to be nearly insoluble in acetonitrile; i.e., the use of different solvents was inevitable in the present case. (26) (a) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Brédas, J.-L. Chem. Rev. 2007, 107, 926−952. (b) Klauk, H. Chem. Soc. Rev. 2010, 39, 2643−2666.

F

DOI: 10.1021/acs.orglett.9b00161 Org. Lett. XXXX, XXX, XXX−XXX