Fluorescent Heteroacenes with Multiply-Bonded Phosphorus

Article pubs.acs.org/Organometallics

Fluorescent Heteroacenes with Multiply-Bonded Phosphorus Feng Li Laughlin,† Nihal Deligonul,† Arnold L. Rheingold,‡ James A. Golen,§ Brynna J. Laughlin,∥ Rhett C. Smith,∥ and John D. Protasiewicz*,† †

Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States § Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth, North Dartmouth, Massachusetts 02747, United States ∥ Department of Chemistry and Center for Optical Materials Science and Engineering Technologies (COMSET), Clemson University, Clemson, South Carolina 29634, United States ‡

S Supporting Information *

ABSTRACT: An air-stable primary phosphine, 2,6-diphosphinonaphthalene-1,5-diol (4), has been synthesized and structurally characterized. A series of π-conjugated heteroacenes containing two phosphaalkene (PC) units, 2,7-R2-naphtho[1,2-d:5,6-d′]bi(soxaphosphole)s [R2-NBOP, R = tBu (5a), Ad (5b), and Ph (5c)], have been synthesized from reactions of 4 and benzimidoyl chlorides. These novel fluorescent analogues of organic acenes were characterized by multinuclear NMR, UV−vis, and fluorescence spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction experiments.

P

direct conjugation with carbocyclic ring systems should be accessible. Chart 1 highlights representative examples of PCcontaining materials that have at least three annulated conjugated rings. Phosphaanthracenes I and phosphaphenanthrenes II, III, and IV can be isolated as stable compounds,

entacene and related acenes have been intensely investigated for uses as organic semiconductors, for they show high mobilities and on/off ratios in thin-film transistors for p-type semiconductors.1 The electronic properties of pentacenes arise from the conjugated nature of the fused ring system that constitute these structures. While the electronic properties of acenes can be altered by the addition of various substituents, the integration of heteroatoms into carbocyclic frameworks is an interesting route to influence HOMO and LUMO orbital energetics and HOMO−LUMO gaps, and to modulate UV−vis absorption and fluorescence emission.2,3 Replacement of carbon atoms in pentacenes and other acenes by π-bonded heteroatoms provides an even greater opportunity to directly influence the inherent nature of acene π-systems. Changes in the nature of the π-system could also affect the arrangement of molecules (e.g., π−π stacking) in the solid state. Heteroacenes have thus been attracting the attention of synthetic chemists. N-Heteroacenes, for example, are now an established class of compounds with their own attractive properties.4 There are now a growing number of heteroacenes incorporating phosphorus atoms.5,6 Most of these materials feature 1H-phospholes having three-coordinate phosphorus centers. Phosphorus-containing acenes that possess phosphaalkene units (two-coordinate doubly bonded phosphorus centers) within the π-conjugated extended acene framework are much rarer, however. Because of the similarities of PC and CC bonds, analogues of acenes having PC bonds in © XXXX American Chemical Society

Chart 1

Received: August 20, 2013

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dx.doi.org/10.1021/om400838g | Organometallics XXXX, XXX, XXX−XXX

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especially when substituted with additional phenyl groups.7−15 Compound V is an unusual example of a formally antiaromatic compound.16,17 Compounds VI−IX feature additional threecoordinate N and P atoms.18−21 We have recently been exploring the chemistry and photochemistry of 1,3-benzoxaphospholes and 2,6-benzobis(oxaphosphole)s (R-BOPs and R2-BBOPs, respectively; Chart 2).22−24 Unlike other materials featuring phosphaalkene groups,

solid 2,6-diphosphinonaphthalene-1,5-diol (4) in 95.6% yield. Compounds 2−4 were readily identified by 1H, 13C, and 31P NMR spectroscopy. The 31P NMR shift for 4 (δ −156.3) is comparable to that we reported previously for 3-phosphino-2naphthol (δ −148.3). Notably, compound 4 displays significant air stability, paralleling the air stability of 3-phosphino-2naphthol.25 By contrast, 2-phosphinonaphthalene slowly degrades in CDCl3 in the presence of air.30 The solid-state packing within crystals of 4 was revealed by an X-ray diffraction study (Figure 1). Intermolecular O···HO

Chart 2

Figure 1. Structural diagrams showing the solid-state hydrogenbonding network for 4.

however, these types of compounds are unique in that they are highly fluorescent and can have quite high quantum yields. These initial discoveries have been expanded, and we have found that 2-R-naphtho[2,3-d]oxaphospholes (R-NOPs; Chart 2) are also fluorescent and represent phosphaacenes.25 In this work, we have achieved the synthesis and characterization of fluorescent 2,7-R2-naphthobis(oxaphosphole)s (R2-NBOPs; Chart 2) as new examples of extended heteroacenes bearing multiple PC units. The synthetic route to R2-NBOPs begins with commercially available naphthalene-1,5-diol (1). The reaction of 1 with diethyl chlorophosphate in the presence of triethylamine in THF produces brown solid tetraethyl naphthalene-1,5-diylbis(phosphate) (2) in 84.4% isolated yield (Scheme 1). Compound 2 in the presence of LDA undergoes a double anionic phospho-Fries rearrangement26−29 to give tetraethyl 1,5-dihydroxynaphthalene-2,6-diylbis(phosphonate) (3), which was isolated in 89.0% yield. Reduction of 3 with lithium aluminum hydride in THF at room temperature afforded white

bonding is indicated by short O···O contacts (2.74 Å), which are represented by dashed lines in Figure 1. The two independent molecules of 4 form columns (3.43 Å) bridged by the hydrogen bonds within the lattice. No evidence for intramolecular hydrogen bonding involving phosphorus atoms is noted. The reaction of compound 4 with several benzimidoyl chlorides in refluxing THF produced 2,7-R2-naphtho[1,2-d:5,6d′]bis(oxaphosphole)s in good to modest yields (Scheme 2). Scheme 2

Scheme 1

The R2-NBOPs were isolated as white (5a, 5b) or yellow (5c) solids. Compounds 5a−c were characterized by 31P, 1H, and 13 C NMR spectroscopy. The phosphorus NMR chemical shifts are consistent with the presence of the PC functionality and are 6.7−7.9 ppm downfield from those of similarly substituted R-NOPs. The 13C{1H} chemical shifts for the PC units of 5 are doublets (JPC = 55.3−63.6 Hz) located 3.9−5.5 ppm upfield of those for comparably substituted R-NOPs. The new R2NBOPs displayed significant air stability, even in solution. For example, in a solution of 5a in CDCl3 left open to the air for 60 days (which required addition of further CDCl3 as it evaporated), approximately 80% of the 5a remained, as judged B


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by 31P{1H} NMR spectroscopy. By contrast, tBu2-BBOP and t Bu-NOP were completely decomposed in CDCl3 after 5 and 28 days, respectively. Single crystals of 5a and 5c suitable for an X-ray diffraction study were obtained by slow evaporation of diethyl ether and CH2Cl2 solutions, respectively. The results are portrayed in Figure 2. The PC bond lengths of 1.705(1) Å for 5a and

Figure 4. (top) UV−vis absorption and (bottom) fluorescence emission spectra for 5a−5c in CH2Cl2 (excitation wavelength = 387 nm).

Figure 2. Structural diagrams for compounds 5a (top) and 5c (bottom).

1.729(3) Å for 5c are consistent with such distances found in pClC6H4-BOP [1.712(7) Å] and tBu2-BBOP [1.694(1) Å].24,31 The structure of 5c is essentially planar, and analysis of the crystal packing revealed that the molecules of 5c are π-stacked along the a axis with only 3.49 Å separating the molecules (Figure 3). A packing diagram for 5a is available in Figure S1 in the Supporting Information.

Table 1. Absorption and Emission Data for 5a−c in CH2Cl2 5a 5b 5c

λmax (nm)

ε (M−1 cm−1)

λF,max (nm)

ΦF

τ (ns)

316 318 387

24600 24300 44800

386 387 422

0.07 0.08 0.63

0.82(5) 1.50(8) 1.04(1)

at varying concentrations are nearly identical to spectra recorded in CH2Cl2. The emission spectra, however, show a small additional band at 394 nm (see the Supporting Information) in addition to a broad absorption around 413 nm that is concentration independent. These latter observations do not suggest that excimer emission is important. This emission band of 5c in CH2Cl2 is also blue-shifted by around 40 nm compared with that for previously reported PhNOP (λF,max = 461 nm).24 The quantum yield of emission from 5c (Table 1) is about 8 times higher than those of the two alkyl-NBOPs, paralleling the observed differences between alkyl- and aryl-substituted BOP derivatives.24 Electrochemical experiments were performed on 5a and 5c to probe their reduction potentials in THF. Compound 5a did not show evidence for reduction within the electrochemical window examined (to −2.5 V vs SCE) and thus behaves similarly to alkyl-substituted R-BOPs and R-NOPs.23,25 By contrast, 5c features an irreversible reduction wave (Epc at −1.90 V vs SCE). The reduction of 5c occurs more readily than for Ph-BOP (Epc = −2.02 V) and is close to that observed for Ph-NOP (Epc = −1.92 V). Single-electron reductions of many Ar-BOPs are reversible. The reasons for the irreversible reduction of 5c, however, are unclear at this time. The easier reduction of 5c relative to 5a or 5b is consistent with a lower

Figure 3. Packing diagram for compound 5c.

As shown in Figure 4, the nature of the absorption and emission spectra for the alkyl- and aryl-substituted NBOPs in CH2Cl2 are very different. In particular, the absorption maximum for 5c is red-shifted by almost 70 nm relative to λmax for 5a and 5b. Key absorption and photoluminescence data are compiled in Table 1. All three compounds are notably fluorescent under UV light, appearing blue. The fluorescence emission spectra of 5a and 5b exhibit three emission peaks around 352, 369, and 387 nm. In contrast, 5c displays only one broad peak at 422 nm with high intensity. This shift is presumably due to the extended conjugation of 5c relative to 5a and 5b. The nature of the emission of 5c in CH2Cl2 does not change upon dilution. The absorption spectra of 5c in hexanes C


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prior to use for UV−vis and fluorescence measurements. All of the substituted benzimidoyl chlorides were prepared by reported procedures.32 NMR spectra (1H and 31P{1H}) were recorded in CDCl3 on a Varian Inova AS-400 spectrometer operating at 399.7 and 161.8 MHz, respectively, and 31P{1H} NMR spectra were referenced to 85% H3PO4. 13C{1H} NMR spectra were recorded in CDCl3 on a Varian Inova AS-600 spectrometer operating at 150.9 MHz. UV−vis and fluorescence spectra were recorded using a Cary 5G UV−vis−NIR spectrophotometer and a Cary Eclipse spectrometer, respectively. Anthracene in ethanol was used as the quantum yield standard (conc. 5.0 × 10−6 M). The excitation slit width for all of the measurements was kept at the default setting (5 nm). Melting points were measured on a Thomas-Hoover capillary melting-point apparatus. Elemental analyses were performed by Robertson Microlit Laboratories (Ledgewood, New Jersey). High-resolution mass spectrometry was performed by the University of Michigan Mass Spectrometry facility using a VG (Micromass) 70-250-S magnetic sector spectrometer with the EI technique at 70 eV. Single-crystal X-ray data were collected with a Bruker AXS SMART APEX II CCD diffractometer using monochromatic Mo Kα radiation with the ω scan technique. Tetraethyl Naphthalene-1,5-diylbis(phosphate) (2). Triethylamine (26.3 g, 187 mmol) was added dropwise to a mixture containing naphthalene-1,5-diol (10.0 g, 62.4 mmol) and diethyl chlorophosphate (22.6 mL, 156 mmol) in 250 mL of THF. The solution was stirred for 12 h. Aqueous HCl (1.2 M, 100 mL) was added, and the mixture was stirred for 30 min. Diethyl ether (250 mL) was added, and the organic layer was separated and washed successively with degassed aqueous HCl (1.2 M, 100 mL), aqueous NaOH (1 M, 250 mL), and distilled water (500 mL). The solution was then dried over anhydrous sodium sulfate and filtered, and the solvent was removed by rotary evaporation to yield 2 as a brown oil (22.8 g, 84.4%). 1H NMR: δ 8.00 (d, 2H, 3JPH = 8.0 Hz), 7.54 (m, 2H), 7.47 (m, 2H), 4.26 (m, 8H), 1.35 (t, 12H, 3 JHH = 7.2 Hz). 31P{1H} NMR: δ −5.3 (s). 13C{1H} NMR: δ 146.8 (d, JPC = 6.9 Hz), 128.0 (d, JPC = 6.9 Hz), 126.2, 118.7, 115.8 (d, JPC = 2.3 Hz), 65.0 (d, JPC = 6.1 Hz), 16.3 (d, JPC = 6.6 Hz). Mp: 73−74 °C. Anal. Calcd for C18H26O8P2: C 50.01%, H 6.06%. Found: C 50.13%, H 6.01%. Tetraethyl 1,5-Dihydroxynaphthalene-2,6-diylbis(phosphonate) (3). To a solution of diisopropylamine (37.6 mL, 266 mmol) in 100 mL of THF at −78 °C was added nBuLi (2.5 M in hexane, 102 mL, 266 mmol) by cannula. The mixture was stirred for 15 min, generating a white slurry of lithium diisopropylamide (LDA). Tetraethyl naphthalene-1,5-diylbis(phosphate) (2) (23.0 g, 53.2 mmol) was dissolved in 100 mL of THF, and this solution was transferred to the LDA solution by cannula. The resulting solution was stirred for 1 h at −78 °C and then warmed to RT and stirred for an additional 2 h. The reaction mixture was poured into an aqueous saturated ammonium chloride solution (250 mL) and stirred until the precipitate was dissolved. The mixture was extracted with three 200 mL portions of methylene chloride. The combined organic layers were separated and washed with three 300 mL portions of distilled water. The solution was then dried over anhydrous sodium sulfate and filtered. The solvent was removed by rotary evaporation to yield 3 as a brown solid (20.5 g, 89.0%). 1H NMR: δ 11.2 (s, 1H, OH), 7.87 (m, 1H), 7.36 (m, 1H), 4.11 (m, 4H), 1.33 (t, 6H, 3JHH = 7.2 Hz). 31P{1H} NMR: δ 23.6 (s). 13 C{1H} NMR: δ 160.4 (d, JPC = 7.0 Hz), 128.5 (dd, JPC = 13.2 Hz, JPC = 7.7 Hz), 126.0 (d, JPC = 7.0 Hz), 114.7 (d, JPC = 14 Hz), 104.2 (d, JPC = 178 Hz), 63.0 (d, JPC = 4.6 Hz), 16.4 (d, JPC = 6.7 Hz). Mp: 141.0−141.5 °C. Anal. Calcd for C18H26O8P2: C 50.01%, H 6.06%. Found: C 50.12%, H 6.02%. 2,6-Diphosphinonaphthalene-1,5-diol (4). Tetraethyl 1,5-dihydroxynaphthalene-2,6-diylbis(phosphonate) (3) (10.0 g, 23.1 mmol) was slowly added to a solution of LiAlH4 (5.27 g, 139 mmol) in 150 mL of THF. The solution was stirred for 2 h. An aqueous solution of saturated ammonium chloride was added dropwise to the reaction mixture, and the mixture was extracted with chloroform (300 mL). The organic layer was separated and filtered through Celite. The solvent was removed by rotary evaporation to yield 4 as a white solid (2.41 g, 95.6%). 1H NMR: δ 7.70 (d, 1H, 3JPH = 8.4 Hz), 7.52 (t, 1H, 3 JPH = 8.4 Hz), 5.98 (d, 1H, OH, 4JPH = 4.0 Hz), 3.84 (d, 2H, JPH =

π* orbital. This hypothesis was further corroborated by theoretical DFT calculations (B3LYP/6-31G*) performed on 5c. The HOMO and LUMO showing the delocalized nature of the π-system are presented in Figure 5 (see the Supporting

Figure 5. Computed LUMO (top) and HOMO (bottom) for compound 5c.

Information for other orbitals). The minimized structure also shows PC bond lengths of 1.737 Å, in agreement with those determined experimentally [1.729(3) Å]. For comparison, calculations were undertaken for Me2-NBOP to model compounds 5a and 5b. The LUMO for 5c (−1.97 eV) is raised to −1.28 eV upon replacement of the phenyl groups with methyl substituents. We previously established a working model to predict the electrochemical activity of benzoxaphospholes23 and found that those compounds with a computed LUMO energy higher than −1.4 eV would not be easily reduced in THF. The lack of reduction of compounds 5a and 5b thus conform to this model. The oxidation chemistry of 5a− c was not examined because analyses of simpler analogues showed no reversible electrochemistry and provided few insights.23

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CONCLUSION The air-stable primary phosphine 2,6-diphosphinonaphthalene1,5-diol has been synthesized and fully characterized. This phosphine allowed ready access to a series of heteroacenes, R2NBOPs, that possess two phosphaalkene units. These conjugated compounds possess significant air and water stability and exhibit significant blue fluorescence. Comparisons of the optical and electrochemical properties to related benzoxaphospholes and naphthoxaphospholes show that the R2-NBOPs behave more like benzoxaphospholes than naphthoxaphospholes. Efforts are now underway to prepare systems with a greater degree of π-conjugation.

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EXPERIMENTAL SECTION

General. Experimental procedures for air- and water-sensitive reactions were conducted under nitrogen using either Schlenk line techniques or an MBraun drybox. Tetrahydrofuran, toluene, and diethyl ether were dried by distillation from sodium and benzophenone ketyl. Hexanes were dried by distillation from sodium and benzophenone in the presence of tetra(ethylene glycol)dimethyl ether. Methylene chloride was dried by distillation from calcium hydride. Methylene chloride (99.99%) and methanol were degassed D


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208 Hz). 31P{1H} NMR: δ −156.3 (s, 1P). 13C{1H} NMR: δ 154.9 (d, JPC = 7.9 Hz), 133.2 (d, JPC = 19.3 Hz), 125.7 (d, JPC = 57.4 Hz), 114.2 (d, JPC = 7.7 Hz), 106.0 (d, JPC = 7.4 Hz). Mp: 140−145 °C. Anal. Calcd for C10H10O2P2: C 53.59%, H 4.50%. Found: C 53.86%, H 4.53%. 2,7-Di-tert-butylnaphtho[1,2-d:5,6-d′]bis(oxaphosphole) (5a). 2,6-Diphosphinonaphthalene-1,5-diol (4) (0.50 g, 2.2 mmol) and Nphenylpivalimidoyl chloride (2.6 g, 13 mmol) were dissolved in 20 mL of THF within a 100 mL round-bottom flask with a stir bar. The flask was outfitted with a reflux condenser, and then the solution was refluxed under nitrogen for 16 h. The reaction mixture was filtered using a glass-fritted filter funnel. The solid was extracted two times with THF (5 mL), and the combined filtrates were evaporated under vacuum to yield a brown solid. Purification by column chromatography (1:1 CH2Cl2/hexanes, Rf = 0.9) led to the isolation of white solid 5a (0.21 g, 27%). 1H NMR: δ 8.24 (dd, 1H, 3JPH = 8.4 Hz, 3JHH = 2.0 Hz), 8.03 (dd, 1H, 3JPH = 8.4 Hz, 3JHH = 2.0 Hz), 1.56 (d, 9H, 4JHH = 1.2 Hz). 31P{1H} NMR: δ 81.8 (s). 13C{1H} NMR: δ 212.4 (d, JPC = 63.6 Hz), 156.3 (d, JPC = 3.2 Hz), 132.2 (d, JPC = 40.0 Hz), 126.0 (d, JPC = 14.4 Hz), 121.0 (d, JPC = 1.5 Hz), 116.0 (d, JPC = 9.1 Hz), 38.1 (d, JPC = 11.3 Hz), 30.1 (d, JPC = 8.6 Hz). UV (CH2Cl2): λmax/nm (ε/ M−1 cm−1) 316 (24550). Fluorescence (CH2Cl2, 5 × 10−6 M): λem/ nm (int.) 368 (690). Quantum yield Φ (CH2Cl2): 0.067. Mp: 208− 212 °C. Anal. Calcd for C20H22O2P2: C 67.41%, H 6.22%. Found: C 67.40%, H 6.21%. 2,7-Diadamantylnaphtho[1,2-d:5,6-d′]bis(oxaphosphole) (5b). 2,6-Diphosphinonaphthalene-1,5-diol (4) (0.50 g, 2.2 mmol) and Nadamantylbenzimidoyl chloride (3.1 g, 11 mmol) were dissolved in 25 mL of THF in a 100 mL round-bottom flask with a stir bar. The flask was outfitted with a reflux condenser, and the solution was refluxed under nitrogen for 3 h. The reaction mixture was filtered using a glassfritted filter funnel. The solid was extracted two times with THF (5 mL), and the combined filtrates were evaporated under vacuum yield a brown solid. Purification by column chromatography (1:1 CH2Cl2/ hexanes, Rf = 0.9) led to isolation of white solid 5b (0.54 g, 47%). 1H NMR: δ 8.25 (dd, 1H, 3JPH = 8.4 Hz, 3JHH = 1.6 Hz), 8.04 (dd, 1H, 3 JPH = 8.4 Hz, 3JHH = 2.0 Hz), 2.17 (m, 9H), 1.86 (s, 6H). 31P{1H} NMR: δ 80.5 (s). 13C{1H} NMR: δ 212.8 (d, JPC = 62.7 Hz), 156.1 (d, JPC = 3.0 Hz), 132.0 (d, JPC = 40.0 Hz), 126.1 (d, JPC = 21.0 Hz), 121.0, 116.1 (d, JPC = 9.2 Hz), 42.4 (d, JPC = 8.9 Hz), 40.1 (d, JPC = 9.2 Hz), 36.7, 28.4. UV (CH2Cl2): λmax/nm (ε/M−1 cm−1) 318 (24336). Fluorescence (CH2Cl2, 5 × 10−6 M): λem/nm (int.) 369 (786). Quantum yield Φ (CH2Cl2): 0.080. Mp: 358−362 °C. Anal. Calcd for C32H34O2P2: C 74.99%, H 6.69%. Found: C 74.00%, H 6.63%. 2,7-Diphenylnaphtho[1,2-d:5,6-d′]bis(oxaphosphole) (5c). 2,6Diphosphinonaphthalene-1,5-diol (4) (1.0 g, 4.5 mmol) and Nphenylbenzimidoyl chloride (4.8 g, 22 mmol) were dissolved in 50 mL of THF in a 250 mL round-bottom flask with a stir bar. The flask was outfitted with a reflux condenser, and the solution was refluxed under nitrogen for 21 h. The reaction mixture was filtered using a glass-fritted filter funnel. The solid residue was collected and purified by flash column chromatography (1:4 CH2Cl2/hexanes), leading to the isolation of yellow solid 5c (0.19 g, 11%). 1H NMR: δ 8.40 (d, 1H, 3 JPH = 8.4 Hz), 8.15 (d, 3H, 3JPH = 8.0), 7.52−7.43 (m, 3H). 31P{1H} NMR: δ 91.6 (s). 13C{1H} NMR: δ 196.0 (d, JPC = 55.7 Hz), 156.2, 134.7 (d, JPC = 13.0 Hz), 134.0, 129.8 (d, JPC = 4.5 Hz), 129.0, 126.3 (d, JPC = 21.4 Hz), 124.8 (d, JPC = 13.9 Hz), 121.5 (d, JPC = 2.9 Hz), 116.9 (d, JPC = 10.1 Hz). UV (CH2Cl2): λmax/nm (ε/M−1 cm−1) 387 (44834). Fluorescence (CH2Cl2, 5 × 10−7 M): λex/nm (int.) 422 (960). Quantum yield Φ (CH2Cl2): 0.626. Mp: 265−270 °C. HRMS m/z: 396.0478 (calcd 396.0469).

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Article

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation for support (CHE1150721).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Details for the UV−vis absorption and fluorescence emission spectra, electrochemical measurements, DFT calculations, crystallographic studies, and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org. E


Organometallics

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