Tuning the Photophysical Properties of Photostable Benzo[b

Apr 11, 2017 - Preface for the Advances in Main-Group Inorganic Chemistry Forum. Philip P. Power and Ching-Wen Chiu. Inorganic Chemistry 2017 56 (15),...
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Tuning the Photophysical Properties of Photostable Benzo[b]phosphole P‑Oxide-Based Fluorophores Raúl A. Adler,† Chenguang Wang,† Aiko Fukazawa,*,‡ and Shigehiro Yamaguchi*,†,‡ †

Institute of Transformative Bio-Molecules (WPI-ITbM) and ‡Department of Chemistry, Graduate School of Science, and Integrated Research Consortium on Chemical Sciences (IRCCS), Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan S Supporting Information *

ABSTRACT: We previously reported that constrained 2-phenylbenzo[b]phosphole P-oxides bearing a diphenylamino group show high photostability and thus are promising dyes for fluorescence imaging. Herein we investigated the impact of the bridging moieties on their photophysical properties. A series of benzo[b]phosphole P-oxides constrained with various carbon or silicon bridges were synthesized. All of these compounds showed significant solvatochromism in fluorescence due to the intramolecular charge-transfer character in the excited state. The dipole moments in the excited state for the carbon-bridged derivatives are slightly larger than the silicon-bridged counterparts. Nevertheless, the latter compounds showed orange-red fluorescence in polar solvents with ca. 30 nm red-shifted maxima compared to the carbon analogues. Most importantly, the assessment of their photobleaching resistance revealed that the photostability of this compound series highly relies on the steric bulkiness of the bridging moiety, and even the silicon-bridged derivative can show outstanding photostability, as far as the silicon-bridging moiety has sufficient bulkiness.

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INTRODUCTION Fluorescence imaging is an indispensable tool in biological research.1−4 Its progress relies on the advancement not only in microscopy technologies but also in the development of fluorescent dyes.5−10 For the latter purpose, while a prevailing strategy is to modify known fluorophore skeletons so as to gain the required properties and functions, it is also essential to produce a new fluorophore skeleton itself. Among the fluorescent probes, red-emissive compounds are especially useful because they can avoid emission color interference from biological background fluorescence. High photobleaching resistance is another crucial property required for fluorescent probes. As exemplified by stimulated emission depletion (STED) microscopy, most advanced super resolution microscopies need strong laser irradiation. Under such conditions, rapid photobleaching of the fluorescent dyes is always a serious issue, which limits the utility on a practical level.11−14 In this context, we have recently developed a series of promising fluorophore skeletons exploiting a phosphine oxide as a key building unit.15 One of the compounds is Ph-BPhox (or LipiDye; Figure 1), which consists of electron-accepting © 2017 American Chemical Society

benzo[b]phosphole P-oxide and electron-donating triphenylamine moieties.16 This donor−π−acceptor-type (D−π−A) fluorophore showed environmental polarity-responsive fluorescence properties and high practical utility for selective staining of lipid droplets in adipocytes. Its performance was superior to conventional dyes, such as Nile red, in terms of the signal-to-noise ratio of images by suppressing background fluorescence. However, its photostability still needed to be improved. We tackled this problem and have recently succeeded in improving its photostability by constraining the π-conjugated skeleton with a diphenylmethylene bridge in a five-membered ring fashion.17 Compound 1a thus produced showed outstanding photostability. Its naphthalene analogue CNaphox (Figure 1) was also synthesized in order to increase the molar absorption coefficient as well as the fluorescence quantum yield. C-Naphox was stable even under the irradiation of strong lasers and thereby enabled us to conduct repeated Special Issue: Advances in Main-Group Inorganic Chemistry Received: March 13, 2017 Published: April 11, 2017 8718

DOI: 10.1021/acs.inorgchem.7b00658 Inorg. Chem. 2017, 56, 8718−8725

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workers have recently synthesized silicon-bridged benzo[b]phosphole P-oxide derivatives for application to organic lightemitting diodes, their synthesis was limited to derivatives without amino groups at the terminal position.18,19 Our synthesis should provide an alternative route to silicon-bridged benzophosphole P-oxides. Our important finding in this work is the impact of the bridging moieties on the photostability. Even a silicon-bridged compound exhibits high photostability under the measurement conditions, as far as the silicon bridge has sufficient steric bulkiness, while attaining a red-shifted emission compared to the carbon-bridged counterparts. The details of the structure−property relationship will be discussed.

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RESULTS AND DISCUSSION Synthesis. A series of bridged benzophosphole P-oxides 1 and 2 were synthesized from the 3-brominated key precursor 3 (Scheme 1), which was readily prepared from (2bromophenyl)[4-(diphenylamino)phenyl]acetylene by an intramolecular trans-halophosphanylation method that we previously reported.20 The dimethylmethylene-bridged derivative 1b was obtained in three steps from 3. Lithiation of 3 with 2 equiv of tert-butyllithium (t-BuLi) followed by treatment with ethyl chloroformate gave the ethyl ester derivative 4. For this step, reduction of phosphine oxide in 3 to the corresponding phosphine prior to lithiation was essential to efficiently conduct subsequent lithiation. Compound 4 was again reduced in situ with HSiCl3 and then reacted successively with MeMgCl and H2O2 to afford alcohol 5b, which was finally treated with Sc(OTf)3 to give 1b in 35% yield. For the last Friedel−Crafts cyclization step, we first tried to use BF3·OEt2 as a Lewis acid, but it produced an inseparable byproduct, although conversion to the target cyclized product itself was high (around 75%).

Figure 1. Constrained benzo[b]phosphole P-oxides with diphenylamino groups investigated in this study.

STED imaging. Its performance was much superior to other commercially available imaging probes, such as Alexa Fluor 488 and ATTO 488. These results suggest a promising utility of this compound class as a benchmark dye in fluorescence imaging technologies. To establish a design principle for more photostable fluorophores with precise tuning of the excitation and emission wavelengths, elucidation of the impact of the bridging moieties in this compound series is essential. Herein we report the synthesis of a series of constrained benzo[b]phosphole Poxides, including not only carbon-bridged derivatives 1 but also silicon-bridged counterparts 2 (Figure 1) and an in-depth study of their photophysical properties. While Mathey and co-

Scheme 1. Synthesis of Constrained Benzo[b]phosphole P-Oxides

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DOI: 10.1021/acs.inorgchem.7b00658 Inorg. Chem. 2017, 56, 8718−8725

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Inorganic Chemistry A similar procedure allowed us to obtain the silicon-bridged derivatives 2a and 2b. Namely, after in situ reduction and lithiation of 3 with t-BuLi, treatment with the corresponding chlorosilanes followed by oxidation with H2O2 afforded 6a and 6b. The next cyclization was accomplished by a rhodiumcatalyzed intramolecular cyclization via C−H and Si−H bond activation, which was originally reported by Takai and coworkers.21 When an excess amount of neohexene as a hydrogen scavenger was employed, target compounds 2a and 2b were successfully obtained. The bulkier diphenylsilyl derivatives 6a and 2a tend to be obtained in better yields compared to the dimethylsilyl counterparts. All products were unequivocally characterized by NMR spectroscopies and mass spectrometry (MS). Photophysical Properties. The photophysical properties of the thus-prepared compounds 1 and 2 were first studied to elucidate the impact of the bridging moieties. The absorption and emission spectra in methanol (MeOH) are shown in Figure 2 as representative examples. In this solvent, the

Table 1. Photophysical Properties of Compounds 1a, 1b, 2a, and 2b in Various Solvents compd

solvent

λabs /nm

ε/104 M−1 cm−1

λem/nm

ΦFa

1a

toluene MeCN MeOH toluene MeCN MeOH toluene MeCN MeOH toluene MeCN MeOH

431 427 438 418 412 424 459 456 466 446 440 452

1.82 1.70 1.60 1.51 1.40 1.33 1.62 1.47 1.48 1.39 1.30 1.35

522 594 608 511 580 598 558 629 640 544 611 624

0.95 0.81 0.40 0.76 0.65 0.44 0.86 0.52 0.17 0.94 0.65 0.27

1b

2a

2b

a

Absolute quantum yields were determined with a calibrated integrating-sphere system (error < 3%).

Figure 3. Absorption (solid line) and fluorescence (dashed line) spectra of compound 2a in various solvents. Figure 2. Absorption (solid line) and fluorescence (dashed line) spectra of compounds 1a, 1b, 2a, and 2b in MeOH.

solvatochromism in fluorescence due to the ICT character in the excited state. For example, Figure 3 shows the spectra of compound 2a, where the emission maximum wavelength (λem) shifts from 542 nm in hexane to 629 nm in acetonitrile (MeCN) to 640 nm in MeOH. The silicon-bridged compounds tend to have longer λem compared to the carbon-bridged counterparts. To quantify the degree of the solvent effect on fluorescence, Lippert−Mataga plots were made for these compounds, from which the dipole moments (μe) in the excited state were determined to be 21.4, 21.3, 20.3, and 19.5 D for 1a, 1b, 2a, and 2b, respectively (for more details, see the Supporting Information).22,23 The carbon-bridged derivatives tend to have slightly larger dipole moments in the excited state compared to the silicon-bridged counterparts. Another intriguing property of this compound series is their high fluorescence quantum yields. Irrespective of the bridging elements, their fluorescence quantum yields (ΦF) are high in nonpolar solvents like hexane and toluene (e.g., ΦF = 0.95 and 0.86 for 1a and 2a, respectively, in toluene). They retain high quantum yields even in the polar solvent MeCN (e.g., ΦF = 0.81 and 0.52 for 1a and 2a, respectively), although they decrease to some extent in protic solvents. Nevertheless, the carbon-bridged compounds 1a and 1b still maintain high quantum yields of 0.40 and 0.44 in MeOH, respectively. Density Functional Theory (DFT) Calculations and Cyclic Voltammetry (CV). To elucidate the effect of the

absorption maximum wavelength (λabs) for 1b at 424 nm was the shortest among the derivatives. The introduction of two phenyl groups at the bridging position in 1a resulted in a red shift of 14 nm, shifting λabs to 438 nm. A similar effect was also observed for the silicon-bridged series, where the absorption of 2a is 14 nm longer than that in 2b. These silicon-bridged compounds have longer absorption maxima by ca. 30 nm than their carbon-bridged counterparts (466 nm for 2a and 452 nm for 2b). The molar absorption coefficients (ε) are similar for all compounds of the series 1 and 2 and range from 16000 M−1 cm−1 in 1a to 13300 M−1 cm−1 in 1b. Notably, despite their constrained rigid structures, these compounds show significantly large Stokes shifts, which can be attributed in part to the intramolecular-charge-transfer (ICT) character in the excited state (supported by the large change in the dipole moments in the excited states; see below). However, the difference in the degree of the Stokes shift is small between the carbon- and silicon-bridged compounds. The Stokes shifts observed for 1a and 2a are 157 nm (6380 cm−1) and 171 nm (5830 cm−1), respectively, and thereby both compounds show red-orange and red emissions, respectively, in MeOH. The fluorescence properties of compounds 1a, 1b, 2a, and 2b in various solvents are summarized in Table 1. As is typical for D−π−A-type compounds, these compounds show large 8720

DOI: 10.1021/acs.inorgchem.7b00658 Inorg. Chem. 2017, 56, 8718−8725

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Photostability. Finally, we assessed the effect of the bridging moieties on the photostability. Considering the solubility of the compounds, MeCN was used as the solvent. The concentration of each solution was adjusted to be similar to each other [ca. (2−3) × 10−5 M]. The solutions were irradiated with a xenon lamp equipped with a 350 nm shortwavelength cutoff filter. The power of the irradiance was set to 1780 W m−2, and the temperature was kept constant at 25 °C. The decrease in the relative absorbance was monitored in air every 20 min for 1 h (Figure 6). As representative dyes, BODIPY 493/503 and Coumarin 545T were also evaluated for comparison.

bridging moieties on the photophysical properties, DFT calculations were carried out for 1a and 2a at the B3LYP/631G(d) level of theory (Figure 4). In both compounds, the

Figure 4. Kohn−Sham molecular orbitals, excitation energies, and oscillator strengths of 1a (left) and 2a (right) calculated at the B3LYP/6-31G(d) level of theory.

highest occupied molecular orbital (HOMO) is mostly localized at the electron-donating triphenylamine moiety, while the lowest unoccupied molecular orbital (LUMO) is mainly delocalized on the benzophosphole oxide moiety. A major difference is that the silicon-bridging moiety participates in the LUMO through σ*−π* conjugation,24 resulting in a decrease in the LUMO level for 2a in comparison to 1a. The difference in the LUMO energies amounts to 0.16 eV, which is reflected in the smaller transition energy for 2a according to the time-dependent DFT calculations at the same level of theory, consistent with the experimental result, while the oscillator strengths (f) are comparable to each other. The differences in the frontier orbital energies are reflected in the redox potentials of the compounds. The electrochemical properties of 1a and 2a were evaluated by CV in MeCN with [Bu4N][PF6] as an electrolyte (Figure 5). Both compounds

Figure 6. Relative absorbance changes for benzo[b]phosphole P-oxide based dyes upon irradiation of light with a xenon lamp (>350 nm cutoff filter; 1780 W m−2) in MeCN in air.

We previously reported that the carbon-bridged compound 1a has outstanding photostability.17 We have now found that the substituents on the carbon atom have a large impact on the stability. Namely, the dimethylmethylene-bridged derivative 1b has inferior photostability compared to the diphenylmethylenebridged 1a. Aiming at ultimately high stability, we also synthesized the spirocyclic derivative bearing the fluorene moiety 1c essentially in the same manner as was previously described for 1a (Scheme 1). Surprisingly, this compound shows lower photostability than that of 1a, despite its rigid structure. Interestingly, the diphenylsilylene-bridged derivative 2a shows remarkable photostability, which is comparable to that of 1a, whereas the bulkiness of the silylene moiety is again crucial to the photostability and the dimethylsilylene-bridged derivative 2b shows much lower photostability. These results reveal that even the silicon-bridged compounds can attain high photostability, as far as the steric bulkiness of the silicon moiety is high enough. The protection of the CC bond might be a key factor for achieving high photostability for this compound series. Importantly, the photostabilities of all of these compounds have higher values than that of Coumarin 545T, and the photostability of 2a is even higher than BODIPY493/ 503, demonstrating the potential utility of these compounds as a scaffold for photostable fluorescent probes.

Figure 5. Cyclic voltamogramms of 1a (top) and 2a (bottom) in MeCN (sample concentration = 1 mM; 0.1 M [Bu4N][PF6] as the supporting electrolyte; scan rate = 100 mV s−1; Fc = ferrocene).

showed reversible redox waves for oxidation and reduction. The half-wave potentials for oxidation are comparable to each other (Eox,1/2 = 0.49 and 0.51 V for 1a and 2a, respectively, vs ferrocene/ferrocenium). In contrast, the reduction half-wave potential of 2a is more positive than that of 1a by 0.18 V (Ered,1/2 = −1.87 and −2.05 V for 2a and 1a, respectively). This difference should be understood as the effect of σ*−π* conjugation in the silicon-bridged 2a.

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CONCLUSION We have synthesized a series of constrained benzo[b]phosphole P-oxides bearing an electron-donating triphenylamine moiety and a carbon- or silicon-bridged moiety. Their fluorescence 8721

DOI: 10.1021/acs.inorgchem.7b00658 Inorg. Chem. 2017, 56, 8718−8725

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HPLC to give a product (141 mg, 0.261 mmol, 48%) as a yellow solid. Mp: 87.0−89.0 °C. 1H NMR (400 MHz, CDCl3, 293 K): δ 7.79−7.74 (m, 2H), 7.70−7.65 (m, 1H), 7.56−7.52 (m, 3H), 7.46−7.34 (m, 5H), 7.30−7.26 (m, 4H), 7.11−7.05 (m, 2H), 7.11−7.05 (m, 4H), 6.96− 6.94 (m, 2H), 4.39 (q, J = 7.2 Hz, 3H), 1.30 (t, J = 7.2 Hz, 2H). 13 C{1H} NMR (68 MHz, CDCl3, 293 K): δ 166.3 (d, J = 19.9 Hz), 149.0, 147.1, 140.3 (d, J = 26.5 Hz), 138.9 (d, J = 91.0 Hz), 137.8 (d, J = 22.5 Hz), 133.6 (d, J = 2.3 Hz), 132.6 (d, J = 3.2 Hz), 131.2 (d, J = 107.5 Hz), 131.2 (d, J = 10.8 Hz), 129.8 (d, J = 60.6 Hz), 129.6, 129.32, 129.30, 129.2, 129.1, 125.4, 124.9 (d, J = 9.0 Hz), 124.0, 123.6 (d, J = 10.2 Hz), 121.8, 61.8, 14.0. 31P{1H} NMR (162 MHz, CDCl3, 293 K): δ 39.8. HRMS (ESI). Calcd for [C35H28NO3P] + H+: m/z 542.1880. Obsd: m/z 542.1881. 3-(2-Hydroxypropan-2-yl)-2-[4-(N,N-diphenylamino)phenyl]-1phenylbenzo[b]phosphole P-Oxide (5b). To a toluene (2 mL) solution of 3 (108 mg, 0.197 mmol) was added HSiCl3 (0.060 mL, 0.60 mmol), and the mixture was stirred for 30 min at room temperature. All volatiles were removed under reduced pressure, and the resulting mixture was dissolved in 2 mL of toluene, followed by filtration through dried silica gel under an Ar atmosphere. After the solvent was removed under reduced pressure, the obtained solid was dissolved in 4 mL of Et2O and cooled to 0 °C. A 3.0 M solution of MeMgCl in tetrahydrofuran (THF; 0.20 mL, 0.60 mmol) was added dropwise, and the reaction mixture was stirred for 2 h at room temperature. The reaction mixture was quenched with a 5% NH4Cl aqueous solution. After the aqueous layer was extracted with Et2O three times, the combined organic layer was dried over MgSO4 and concentrated under reduced pressure. The mixture was purified by PTLC (2/1 EtOAc/hexane; Rf = 0.36) to give a product as a paleyellow solid (26.1 mg, 0.0495 mmol, 25%). Mp: 258.0−262.0 °C (dec). 1H NMR (400 MHz, CDCl3, 293 K): δ 8.26−8.23 (m, 1H), 7.65−7.60 (m, 1H), 7.55−7.45 (m, 4H), 7.36−7.28 (m, 3H), 7.27− 7.22 (m, 4H) 7.05−7.02 (m, 4H), 7.05−7.00 (m, 2H), 6.86−6.84 (m, 2H), 6.69 (br s, 2H), 1.58 (s, 3H, CH3), 1.49 (s, 3H, CH3). 13C{1H} NMR (100 MHz, CDCl3, 293 K): δ 155.1 (d, J = 18.8 Hz), 147.6, 143.0 (d, J = 28.2 Hz), 134.7, 133.0, 132.4, 131.5 (d, J = 10.5 Hz), 129.6 (d, J = 4.0 Hz), 129.5, 128.8 (d, J = 12.3 Hz), 128.7, 127.3, 127.2 (d, J = 7.6 Hz), 124.9, 123.4, 122.9, 74.5 (d, J = 14.4 Hz), 31.2, 31.2. 31 1 P{ H} NMR (162 MHz, CDCl3, 293 K): δ 40.4. HRMS (ESI). Calcd for [C35H30NO2P] + H+: m/z 528.2087. Obsd: m/z 528.2085. Dimethylmethylene-Bridged Phenylbenzo[b]phosphole P-Oxide 1b. Sc(OTf)3 (16.7 mg, 0.0339 mmol) was heated with a heat gun to ca. 150 °C for 10 min prior to use and added to a solution of compound 5b (9.0 mg, 0.017 mmol) in CH2Cl2 (1 mL) at room temperature followed by stirring for 3 h. After the addition of water, the organic layer was diluted with CH2Cl2 and separated. The organic layer was washed with water, dried over MgSO4, and concentrated under reduced pressure. The resulting mixture was purified by PTLC (2/1 EtOAc/hexane; Rf = 0.24). Further purification by HPLC (2/1 EtOAc/hexane) afforded compound 1b as a yellow solid (3.0 mg, 0.0059 mmol, 35%). Mp: 111.5−116.0 °C (dec). 1H NMR (400 MHz, CDCl3, 293 K): δ 7.82−7.77 (m, 2H), 7.68−7.63 (m, 1H), 7.54−7.42 (m, 5H), 7.33−7.30 (m, 1H), 7.28−7.23 (m, 5H), 7.15 (s, 1H), 7.11− 7.09 (m, 4H), 7.04−7.00 (m, 2H), 6.90−6.89 (m, 1H), 1.55 (s, 3H), 1.52 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3, 293 K): δ 168.7 (d, J = 19.7 Hz), 158.7 (d, J = 10.2 Hz), 148.0, 147.1, 138.5 (d, J = 20.4 Hz), 132.9 (d, J = 1.9 Hz), 132.5 (d, J = 2.9 Hz), 131.1 (d, J = 11.1 Hz), 130.0 (d, J = 10.0 Hz), 129.5, 129.2 (d, J = 12.5 Hz), 128.6 (d, J = 11.5 Hz), 124.6, 123.2, 123.0, 122.6, 121.8 (d, J = 9.0 Hz), 117.7, 48.9 (d, 3JP−C = 10.8 Hz), 24.6, 24.3. 31P{1H} NMR (162 MHz, CDCl3, 293 K): δ 27.3. HRMS (ESI). Calcd for [C35H28NOP] + H+: m/z 510.1981. Obsd: m/z 510.1981. 3-(9-Hydroxyfluoren-9-yl)-2-[4-(N,N-diphenylamino)phenyl]-1phenylbenzo[b]phosphole P-Oxide (5c). To a suspension of 3 (1.65 g, 3.00 mmol) in toluene (3 mL) was added HSiCl3 (0.91 mL, 9.0 mmol). The mixture was stirred for 30 min at room temperature, and then all volatiles were removed under reduced pressure. The mixture was dissolved in toluene (10 mL) and filtered under a N2 atmosphere through dried silica gel. After the solvent was removed under reduced pressure, the obtained solid was dissolved in THF (15 mL) and cooled

properties have been comprehensively studied, which demonstrates that the replacement of the bridging carbon atom with the silicon bridges results in a 30 nm red shift while maintaining the absorption coefficients and fluorescence quantum yields. The photostability of the silicon-bridged dyes remains high, especially for 2a, whose performance is higher than the representative dyes Coumarin 545T and BODIPY 493/593. The observation that the dyes with sterically demanding groups at the bridging atom show better photostability will help us to optimize the molecular design for the next generation of super photostable dyes. Further studies to demonstrate the utility of these new kinds of dyes in fluorescence microscopy are in progress in our laboratory.

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EXPERIMENTAL SECTION

General Experimental Remarks. When specified, reactions were performed under a dry N2 atmosphere or in a glovebox Unilab (M. Braun) under an Ar atmosphere. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded with an AL-400/NRY (JEOL) spectrometer (400 MHz for 1H, 100 MHz for 13C, and 162 MHz for 31P), an A-600 (JEOL) spectrometer (600 MHz for 1 H and 150 MHz for 13C), or an EX-JNM270 (JEOL) spectrometer (270 MHz for 1H and 68 MHz for 13 C) in CDCl3. The chemical shifts (δ) in the 1H and 13C{1H} NMR spectra are reported in parts per million using the solvent peaks as an internal standard (CHCl3 = 7.26 ppm in 1H NMR; CDCl3 = 77.16 ppm in 13C{1H} NMR). The chemical shifts in 31P{1H} NMR spectra are reported using H3PO4 (δ = 0.00 ppm) as an external standard. Melting points (mp) or decomposition temperatures were determined with an MP-S3 instrument (Yanaco). MS spectra were measured on an Exactive mass spectrometer (Thermo Fisher Scientific) by electrospray ionization (ESI) using MeOH solutions of the samples. Thin-layer chromatography (TLC) was performed on glass plates coated with a 0.25 mm thickness of silica gel 60F254 (Merck). Preparative TLC (PTLC) was performed on glass plates coated with 1.0 mm thickness of silica gel EMD Millipore 1.13895.0001 (Merck). Column chromatography was performed using neutral silica gel PSQ100B or PSQ60B (Fuji Silysia Chemicals). Recycling preparative high-performance liquid chromatography (HPLC) was performed using a YMC LC-forte/R equipped with a silica gel column (YMC-Actus SIL). Recycling preparative gel permeation chromatography (GPC) was performed using a YMC LC-forte/R equipped with polystyrene gel columns (YMC-GPC T2000 and T4000) and CHCl3 as the eluent. 3Bromo-2-[4-(N,N-diphenylamino)phenyl]-1-phenylbenzo[b]phosphole P-oxide (3) and compound 1a were synthesized according to the literature.16,17 Synthesis and Characterization of the Compounds. 3(Ethoxycarbonyl)-2-[4-(N,N-diphenylamino)phenyl]-1phenylbenzo[b]phosphole P-Oxide (4). To a suspension of 3 (300 mg, 0.547 mmol) in toluene (2 mL) was added HSiCl3 (0.20 mL, 2.0 mmol), and the mixture was stirred for 30 min at room temperature. All volatiles were removed under reduced pressure, and the mixture was dissolved in 2 mL of toluene. The mixture was subsequently filtered under a N2 atmosphere through dry silica gel (dried in vacuum while heating with a heat gun for 10 min) and rinsed with toluene. After the solvent was removed under reduced pressure, the obtained solid was dissolved in diethyl ether (Et2O; 8 mL) and cooled to −78 °C. A solution of t-BuLi (0.85 mL, 1.60 M in pentane, 1.37 mmol) was added, and the dark-brown solution was stirred for 2 h at the same temperature. Ethyl chloroformate (0.060 mL, 0.63 mmol) was added dropwise, and then the mixture was allowed to warm to room temperature. The mixture was quenched with water (2 mL), and then H2O2 (5%, 5 mL) was added at 0 °C in one portion. After stirring for 30 min at room temperature, the mixture was cooled again to 0 °C and quenched with a saturated aqueous solution of Na2SO3. The mixture was extracted with Et2O, and the combined organic layer was washed with water, dried over MgSO4, filtered, and concentrated under reduced pressure. The mixture was purified by silica gel column chromatography (1/1 EtOAc/hexane; Rf = 0.41) and subsequently by 8722

DOI: 10.1021/acs.inorgchem.7b00658 Inorg. Chem. 2017, 56, 8718−8725

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Inorganic Chemistry to −78 °C. A solution of t-BuLi (5.50 mL, 1.60 M in pentane, 8.97 mmol) was added, and the dark-brown solution was stirred for 2 h at the same temperature. Fluorenone (901 mg, 4.60 mmol) was added in one portion, and the mixture was allowed to warm to room temperature. After quenching with an aqueous 5% solution of NH4Cl at 0 °C, H2O2 (30%, 2 mL) was added to the mixture in one portion. The mixture was cooled again to 0 °C and quenched with a saturated aqueous solution of Na2SO3. The mixture was extracted with Et2O three times, and the combined organic layer was washed with water, dried over MgSO4, filtered, and concentrated under reduced pressure. The mixture was purified by silica gel column chromatography (CHCl3 to 17/3 CHCl3/EtOAc; Rf = 0.27) and subsequently recrystallized from toluene to give a product (472 mg, 0.726 mmol, 24%) as a pale-yellow solid. Mp: 274.0−277.0 °C. 1H NMR (400 MHz, CDCl3, 293 K): δ 7.93−7.83 (m, 4H), 7.60−7.48 (m, 4H), 7.43−7.37 (m, 3H), 7.24−7.16 (m, 3H), 7.13−7.07 (m, 6H), 7.02−6.98 (m, 1H), 6.91−6.86 (m, 8H), 6.53 (s, 1H), 6.09−6.06 (m, 1H). The hydroxyl proton was not observed. 31P{1H} NMR (162 MHz, CDCl3, 293 K): δ 26.8. 13C{1H} NMR was not obtained because of the low solubility. HRMS (ESI). Calcd for [C45H32NO2P] + H+: m/z 650.2243. Obsd: m/z 650.2242. Fluorenyl-Bridged 2-Phenylbenzo[b]phosphole P-Oxide 1c. To a suspension of 5c (373 mg, 0.575 mmol) in CH2Cl2 (40 mL) was added BF3·OEt2 slowly at room temperature (0.145 mL, 1.14 mmol). After stirring for 30 min, 1 mL of EtOH was added. The mixture was washed with water, and the organic layer was concentrated under reduced pressure. The mixture was purified by silica gel column chromatography (CH2Cl2 to 1/1 CH2Cl2/EtOAc; Rf = 0.60) and by GPC. Further recrystallization from EtOH gave a product (345 mg, 0.546 mmol, 95%) as a yellow solid. Mp: 98.5−103.5 °C. 1H NMR (400 MHz, CDCl3, 293 K): δ 8.01−7.90 (m, 4H), 7.65−7.54 (m, 4H), 7.48−7.44 (m, 3H), 7.31−7.19 (m, 4H), 7.17−7.13 (m, 4H), 7.06− 7.03 (m, 1H), 6.98−6.93 (m, 8H), 6.62 (s, 1H), 6.17−6.14 (m, 1H). 13 C{1H} NMR (100 MHz, CDCl3, 293 K): δ 164.0 (d, J = 21.5 Hz), 153.4 (d, J = 9.9 Hz), 147.5, 147.4, 144.4, 144.1, 142.3, 141.9, 140.8 (d, J = 104.1 Hz), 137.8, 137.6, 136.9 (d, J = 109.5 Hz), 133.5 (d, J = 11.5 Hz), 132.9, 132.6 (d, J = 2.6 Hz), 131.0 (d, J = 11.3 Hz), 130.3 (d, J = 103.7 Hz), 129.4 (d, J = 10.2 Hz), 129.3 (d, J = 12.5 Hz), 129.2, 128.7, 128.6, 128.5 (d, J = 12.7 Hz), 128.4, 128.1, 124.4, 124.2, 123.6 (d, J = 1.5 Hz), 123.0, 122.5, 121.2 (d, J = 9.1 Hz), 120.9, 120.7, 119.2, 65.9 (d, J = 11.3 Hz). 31P{1H} NMR (162 MHz, CDCl3, 293 K): δ 26.9. HRMS (ESI). Calcd for [C45H30NOP] + H+: m/z 632.2138. Obsd: m/z 632.2134. 3-(Diphenylhydrosilyl)-2-[4-(N,N-diphenylamino)phenyl]-1phenylbenzo[b]phosphole P-Oxide (6a). To a suspension of 3 (300 mg, 0.547 mmol) in toluene (2 mL) was added HSiCl3 (0.20 mL, 2.0 mmol), and the mixture was stirred for 30 min at room temperature. After all volatiles were removed under reduced pressure, the mixture was dissolved in toluene (2 mL) and filtered under a N2 atmosphere through dried silica gel. After the solvent was removed under reduced pressure, the solid was dissolved in Et2O (8 mL) and cooled to −78 °C. A solution of t-BuLi (0.85 mL, 1.60 M in pentane, 1.37 mmol) was added, and the dark-brown solution was stirred for 2 h at the same temperature. ClSiHPh2 (0.21 mL, 1.1 mmol) was added dropwise to the mixture, followed by stirring for 1 h at −78 °C, 1 h at −30 °C, and 2 h at room temperature. H2O2 (5%, 5 mL) was added in one portion at 0 °C. After stirring for 30 min at room temperature, a saturated aqueous solution of Na2SO3 was added. The mixture was extracted with Et2O, and the combined organic layer was washed with water, dried over MgSO4, filtered, and concentrated under reduced pressure. The mixture was purified by silica gel column chromatography (2/1 EtOAc/hexane; Rf = 0.57) to give a product (198 mg, 0.304 mmol, 55%) as a yellow solid. Mp: 107.0−110.0 °C. 1H NMR (400 MHz, CDCl3, 293 K): δ 7.74−7.69 (m, 2H), 7.66−7.59 (m, 5H), 7.53−7.35 (m, 10H), 7.31−7.20 (m, 7H), 7.08−7.06 (m, 2H), 7.03−7.00 (m, 5H), 6.78−6.76 (m, 2H), 5.43 (s, 1H). 13C{1H} NMR (100 MHz, CDCl3, 293 K): δ 154.2 (d, J = 83.7 Hz), 148.4, 147.4, 145.9 (d, J = 34.5 Hz), 144.9 (d, J = 12.0 Hz), 135.9, 135.9, 132.4 (d, J = 2.9 Hz), 132.0, 131.6, 131.5 (d, J = 109.7 Hz), 131.2 (d, J = 10.7 Hz), 130.4 (d, J = 2.7 Hz), 129.9 (d, J = 5.0 Hz), 129.4, 129.3 (d, J = 10.8 Hz), 129.1

(d, J = 12.2 Hz), 128.6 (d, J = 2.9 Hz), 128.4, 127.6 (d, J = 11.9 Hz), 126.5 (d, J = 9.8 Hz), 125.2, 123.6, 121.9. 31P{1H} NMR (162 MHz, CDCl3, 293 K): δ 40.7. HRMS (ESI). Calcd for [C44H34NOPSi] + Na+: m/z 674.2039. Obsd: m/z 674.2036. Diphenylsilylene-Bridged Phenylbenzo[b]phosphole P-Oxide 2a. Compound 6a (50.0 mg, 0.0767 mmol) and tris(triphenylphosphine)rhodium(I) chloride (7.8 mg, 0.084 mmol) were placed in an autoclave, and then dry 1,4-dioxane (4 mL) was added under a N2 atmosphere, followed by 3,3-dimethyl-1-butene (0.050 mL, 0.39 mmol). The mixture was heated at 140 °C for 18 h, and then the mixture was diluted with Et2O, filtered, and concentrated under reduced pressure. The mixture was separated by silica gel column chromatography (2/1 EtOAc/hexane; Rf = 0.45) and subsequently by HPLC (2/1 EtOAc/hexane) to give a product (27.1 mg, 0.0417 mmol, 54%) as an orange solid. Mp: 148.0−150.5 °C. 1H NMR (400 MHz, CDCl3, 293 K): δ 7.86−7.80 (m, 2H), 7.69−7.64 (m, 5H), 7.53−7.38 (m, 12H), 7.29−7.21 (m, 6H), 7.10−7.08 (m, 4H), 7.04−7.00 (m, 2H), 6.97−6.95 (m, 1H). 13C{1H} NMR (68 MHz, CDCl3, 293 K): δ 156.0 (d, J = 87.4 Hz), 153.4 (d, J = 10.1 Hz), 148.4, 147.5, 143.2 (d, J = 28.7 Hz), 139.3 (d, J = 8.4 Hz), 137.7 (d, J = 14.5 Hz), 135.9, 135.7, 134.9 (d, J = 113.7 Hz), 133.5 (d, J = 2.0 Hz), 132.5 (d, J = 2.9 Hz), 131.1 (d, J = 11.1 Hz), 131.0, 130.9, 130.4 (d, J = 0.9 Hz), 130.0 (d, J = 97.8 Hz), 129.7 (d, J = 11.7 Hz), 129.5, 129.2 (d, J = 12.3 Hz), 129.0, 128.73, 128.71, 128.5 (d, J = 10.7 Hz), 125.5 (d, J = 8.5 Hz), 125.0, 124.8, 123.5. 31P{1H} NMR (162 MHz, CDCl3, 293 K): δ 35.1. HRMS (ESI). Calcd for [C44H32NOPSi] + H+: m/z 650.2064. Obsd: m/z 650.2059. 3-(Dimethylhydrosilyl)-2-[4-(N,N-diphenylamino)phenyl]-1phenylbenzo[b]phosphole P-Oxide (6b). This compound was synthesized from 3 (145 mg, 0.264 mmol) essentially in the same manner as that described for 6a using ClSiHMe2 (0.10 mL, 0.99 mmol). The mixture was purified by silica gel column chromatography (1/1 EtOAc/hexane; Rf = 0.65) to give a product (45.0 mg, 0.0853 mmol, 32%) as a yellow-green solid. Mp: 98.5−103.5 °C. 1H NMR (400 MHz, CDCl3, 293 K): δ 7.67−7.58 (m, 4H), 7.53−7.46 (m, 2H), 7.40−7.36 (m, 2H), 7.33−7.29 (m, 1H), 7.25−7.21 (m, 4H), 7.07− 7.05 (m, 4H) 7.05−6.98 (m, 4H), 6.94−6.92 (m, 2H), 4.49 (sep, 1H, 3 JHH = 4 Hz), 0.31 (d, 3JHH = 4 Hz, 3H), 0.27 (d, 3JHH = 4 Hz, 3H). 13 C{1H} NMR (100 MHz, CDCl3, 293 K): δ 151.6, 150.2 (d, J = 11.5 Hz), 148.3, 147.5, 146.2, 145.8, 133.4, 132.31, 132.29, 132.1, 131.2 (d, J = 10.7 Hz), 130.1, 129.7 (d, J = 4.6 Hz), 129.6, 129.5, 129.0 (d, J = 12.0 Hz), 128.6 (d, J = 10.4 Hz), 128.2 (d, J = 12.1 Hz), 125.2 (d, J = 10.7 Hz), 125.0, 123.5, 122.4, −3.26, −3.32. 31P{1H} NMR (162 MHz, CDCl 3 , 293 K): δ 40. 9. HRMS (ESI). Calcd for [C34H30NOPSi] + Na+: m/z 550.1726. Obsd: m/z 550.1727. Dimethylsilylene-Bridged Phenylbenzo[b]phosphole P-Oxide 2b. This compound was synthesized from 6b (65.0 mg, 0.123 mmol) essentially in the same manner as that described for 2a. The mixture was purified by silica gel column chromatography (1/1 EtOAc/ hexane; Rf = 0.27) and then by HPLC (1/1 EtOAc/hexane) to give a product (14.0 mg, 0.0266 mmol, 22%) as an orange solid. Mp: 126.0− 126.5 °C. 1H NMR (400 MHz, CDCl3, 293 K): δ 7.84−7.79 (m, 2H), 7.66−7.62 (m 1H), 7.57−7.44 (m, 4H), 7.32−7.26 (m, 8H), 7.12− 7.10 (m, 4H), 7.08−7.04 (m, 2H), 6.97−6.95 (m, 1H), 0.52 (s, 6H). 13 C{1H} NMR (100 MHz, CDCl3, 293 K): δ 156.5 (d, J = 9.7 Hz), 153.6 (d, J = 88.3 Hz), 148.0, 147.7, 143.5 (d, J = 29.4 Hz), 141.9 (d, J = 8.8 Hz), 137.3 (d, J = 14.5 Hz), 134.9 (d, J = 113.8 Hz), 133.3 (d, J = 1.8 Hz), 132.4 (d, J = 2.6 Hz), 131.0 (d, J = 11.1 Hz), 130.2 (d, J = 96.9 Hz), 129.7 (d, J = 11.6 Hz), 129.5, 129.1 (d, J = 12.2 Hz), 128.3 (d, J = 10.7 Hz), 127.9, 125.1, 125.0, 124.8, 124.7, 123.3, −3.7, −3.8. 31 1 P{ H} NMR (162 MHz, CDCl3, 293 K): δ 35.4. HRMS (ESI). Calcd for [C34H28NOPSi] + H+: m/z 526.1761. Obsd: m/z 526.1749. Photophysical Properties. UV−vis absorption spectra were measured with a UV-3150 spectrometer (Shimadzu) with a resolution of 1 nm using dilute sample solutions in spectrophotometric-grade solvents in a 1 cm square quartz cuvette. Emission spectra were measured with a F-4500 spectrometer (Hitachi) with a resolution of 1 nm. Absolute fluorescence quantum yields were determined with a C9920-02 calibrated integrating-sphere system (Hamamatsu Photonics) equipped with a PMA-11 multichannel spectrometer. Time8723

DOI: 10.1021/acs.inorgchem.7b00658 Inorg. Chem. 2017, 56, 8718−8725

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Inorganic Chemistry resolved fluorescence spectra were measured with a C11200 Picosecond Fluorescence Measurement System (Hamamatsu Photonics) equipped with a PLP-10 picosecond light pulser (excitation wavelength of 464 or 403 nm with a repetition rate of 10 Hz). All decay profiles were fitted reasonably well with a single-exponential function. Evaluation of the Photostability. The photostability of the fluorophores was measured based on the absorption variation of each sample at the absorption maximum (λabs) after a certain irradiation period. For a comparison of the photostabilities, the samples were dissolved in MeCN and transferred to a 1 cm quartz cuvette with a stirring magnet. The setup consisted of a Max-302 xenon lamp (Asahi Spectra Co.) equipped with a Max-P 300−600 T-011 300−600 nm mirror module (Asahi Spectra Co.) and a 350 nm short-wavelength cutoff filter. The light intensity was measured with a HD2302.01 LightMeter (Delta Ohm). The xenon lamp was coupled to a UV spectrometer 8453 (Agilent) equipped with a stirrer and a Unispeks temperature controller (Unisoku Scientific Instruments). The solutions were adjusted to have similar concentrations [(2−3) × 10−5 M]. The absorption was measured and compared after the designated time intervals. Electrochemical Properties. CV was performed on a Chi-617A (ALS) electrochemical analyzer. The CV cell consisted of a glassy carbon electrode, a platinum-wire counter electrode, and an Ag/ AgNO3 reference electrode. Measurements were carried out in a glovebox under an Ar atmosphere using an MeCN solution of the sample with a concentration of 1.0 mM and 0.1 M [Bu4N][PF6] as the supporting electrolyte. The reduction and oxidation potentials were calibrated with a ferrocene/ferrocenium ion couple.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00658. Absorption and fluorecence spectra, summary of the photophysical properties, DFT calculation results, and NMR spectra for new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.F.). *E-mail: [email protected] (S.Y.). ORCID

Shigehiro Yamaguchi: 0000-0003-0072-8969 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by JSPS KAKENHI Grant 16K13949 (to S.Y.). Financial support from the Nagase Science and Technology Foundation and the Naito Foundation to S.Y. is acknowledged. ITbM is supported by the World Premier International Research Center Initiative, Japan.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00658 Inorg. Chem. 2017, 56, 8718−8725