Linear and Star‐Shaped Extended Di‐ and Tristyrylbenzenes: Synthesis, Characterization and Optical Response to Acid and Metal Ions

Abstract

Two linear 1,4‐distyrylbenzenes and five star‐shaped 1,3,5‐tristyrylbenzene derivatives (L_2a and L_2b, Y₀–Y₃ and Y_NBu) were synthesized and spectroscopically characterized. The photophysical properties, optical response to acid and metal ions were investigated. Upon backbone extension of linear distyrylbenzenes or the introduction of dibutylanilines, the electronic spectra are redshifted. Incorporation of electron‐deficient pyridyl units does not significantly affect the optical properties. Variation of the number of pyridine rings and substitution pattern tune the fluorescence response to acids and metal ions. The novel arenes discriminate Al³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Cd²⁺, Ag⁺ and Hg²⁺.

Make a star! A series of extended star‐shaped tristyrylbenzenes has been developed. Their optoelectronic properties can be tuned through the aryl terminus, which makes them useful sensors for protons or metal ions (see figure).

Introduction

Functional π‐conjugated chromophores with linear1 or star‐shaped geometry2 are relevant responsive materials.3 1,3,5‐Tristyrylbenzene4 is an excellent core for dendritic molecules.2c, 5 Peripheral and core substitution allow tuning of photoluminescence6 and mesogenic behavior.2b

Manipulation of the substituents in X‐shaped molecules leads to disjunct frontier molecular orbitals and tunable optical properties.7 If pyridines or dialkylanilines are incorporated, they become acidochromic.8 Both size and symmetry of the conjugated system affect the properties and performance in applications.5c, 9 We describe the synthesis, photophysical characterization, and acid/metal ion response of two linear 1,4‐distyryl (L_2a and L_2b) and five star‐shaped styrylbenzene derivatives (Y₀–Y₃ and Y_NBu).

Results and Discussion

Synthesis, X‐ray crystallographic analyses and liquid crystalline behavior

L_2a is a distyrylbenzene substituted with two pyridines. L_2b was designed as an analogue of L_2a with a longer effective conjugation length.10 Y₀ is a star‐shaped and C ₃‐symmetric π‐system bearing three identical arms. Based on this known skeleton (Y₀) reported by Maier et al.,5c Y_{1 –3} and Y_NBu are obtained either by variation of the number of pyridine rings or the introduction of electron‐rich dibutylaniline groups, respectively (Scheme 1).

Scheme 1.

Synthetic route to linear 1,4‐distyrylbenzenes (L_2a–L_2b) and star‐shaped 1,3,5‐tristyrylbenzene derivatives (Y₀–Y₃ and Y_NBu).

Compound 1 was reacted with 1‐bromohexane (K₂CO₃, anhydrous DMF) to afford 2, which was transformed by a Bouveault reaction to the monoaldehyde 3. Heck coupling between 3 and three arylethylenes produced 4–6, which were in turn subjected to a Horner reaction with triphosphonate 7 in the presence of sodium hydride, thus, affording Y₀ (77 %), Y₃ (33 %) and Y_NBu (68 %). As a reference system, distyrylbenzene L_2a was obtained after Heck reaction of 2 and 4‐vinylpyridine (45 %). To obtain π‐extended L_2b, 8 was subjected to an Arbuzow reaction with triethyl phosphite, followed by a Horner reaction with monoaldehyde 5. L_2b was isolated after column chromatography on silica gel (82 %). Y₁ and Y₂ were obtained via two one‐pot procedures starting from 7 with aldehydes 5 and 4. For Y₁, the first Horner reaction of 7 with monoaldehyde 5 (molar ratio: 1:1.1) afforded 10, which was subjected to a second Wittig–Horner reaction with 4 in situ (28 % over two steps). By changing the stoichiometry of 7 and 5 (1:2.2), Y₂ was isolated by a similar two‐step routine with a yield of 44 %. It should be noted that potassium tert‐butoxide as a base did not work.11 L_2a/L_2b and Y₀–Y₃ are yellow or orange solids, while Y_NBu is a yellow oil. All styrylbenzene derivatives (SBs) are well soluble in common organic solvents.

We obtained single crystals for Y₀ (from DCM/methanol), L_2b and Y₃ (from DCM/n‐hexane) and performed X‐ray analysis. Figure 1 depicts the structure and Table S1–S3 summarize the corresponding data. As shown in Figure S1, all derivatives are almost planar and their vinylic linkers display E‐configuration. However, the packing patterns of L_2b, Y₀ and Y₃ deviate significantly from each other. L_2b packs in parallel layer stacks with some displacement. Y₀ displays a 2‐dimensional wall‐like arrangement and the orientation of molecular planes in adjacent parallel walls is different. In Y₃, a pattern of parallel planes was observed, which extends in three directions.

Figure 1.

Single‐crystal structures of L_2b, Y₀ and Y₃.

Similar to Maier's star‐shaped compounds, nematic liquid crystalline phases from isotropic during cooling scans were observed for Y_{0 –2} as investigated by temperature‐dependent polarization optical micrographs (Figure S2).5c No liquid crystalline behaviour was detected for oily Y_NBu at room temperature.

Photophysical properties and theoretical calculations

The normalized absorption and emission spectra of the compounds in dilute THF are shown in Figure 2. Table 1 summarizes the photophysical data. Due to the meta‐conjugation, absorption spectra of star‐shaped series Y₀‐Y₃ are superimposable to that of L_2a, with a maximum absorption centered at around 400 nm and a shoulder peak located at 326–348 nm. It is mainly attributed to the extension of conjugation or the electron‐rich dibutylaniline groups.

Figure 2.

Normalized UV/vis absorption (a) and emission (b) spectra of SBs in THF.

Table 1.

Photophysical data (in THF) and calculated energy gaps for SBs.

Compds.	λ abs [nm][a]	λ max,em [nm]	QY[b]	τ f [ns]	λ st [nm]/ν st [cm−1][c]	Calcd energy gap [eV, nm]
L2a	329 (2.37), 400 (2.88)	461	0.64	2.00	61, 3308	3.06, 405
L2b	432 (7.31)	485	0.68	1.10	53, 2529	2.56, 484
Y0	329 (6.17), 401 (11.0)	447	0.82	1.39	46, 2566	2.97, 417
Y1	330 (5.68), 403 (9.89)	454	0.73	1.51	51, 2787	2.86, 433
Y2	331 (7.13), 405 (12.4)	456	0.72	1.52	51, 2761	2.87, 432
Y3	348 (2.29), 401 (2.86)	457	0.79	1.67	56, 3118	2.96, 418
YNBu	326 (1.91), 427 (8.64)	519	0.61	1.41	92, 4151	2.82, 440

[a] Measured in THF and extinction coefficients (ϵ _max×10⁴  m ⁻¹ ⋅cm⁻¹) are shown in parentheses. [b] QY=quantum yield. [c] ν _st=1/λ _abs−1/λ _em.

In THF, the trend in the emission maxima (Figure 2 b) follows the order Y_NBu>L_2b>L_2a>Y₃>Y₂>Y₁>Y₀. L_2b exhibits a redshifted green emission with a vibronic structure relative to that of L₂  a, owing to its longer conjugation length. Asymmetric Y₁ and Y₂ show similar emission maxima in comparison to symmetric Y₃ while their maxima are redshifted by ca. 10 nm compared to symmetric hydrocarbon Y_0. The fluorescence maximum of Y_NBu bathochromically shifted to 519 nm (Stokes shift of 4151 cm⁻¹). The large Stokes shift observed for Y_NBu is caused by its higher dipole moment in its excited state stabilized in more polar solvents.12 L_2a, L_2b and Y_{0 –3} display quantum yields (Φ _f) varying from 0.64 to 0.82, while Y_NBu exhibits a green fluorescence with a quantum yield of 0.61. Although Y_{0 –3} and Y₄ have similar conjugation skeletons, dibutylamine‐containing Y₄ shows a lower quantum yield, which might be attributed to the free intramolecular rotation or the existence of photoelectron transfer facilitated by the flexible dibutylamine groups.13

DFT calculations (B3LYP/6‐31++G**)14 provide further insight into optoelectronics. The calculated HOMO–LUMO energy gaps for L_2a/L_2b, Y₀–Y₃ and Y_NBu are in the range from 2.56 to 3.06 eV (Figure S4). The lower gaps of Y_NBu or L_2b are a result of the extended conjugation, resulting in distinct bathochromic shifts in the electronic absorption spectra. These results, as well as the trend of absorption maxima calculated by TDDFT methods (Figure S3) are consistent with the experiment.

Fluorochromicity

Figure 3 shows the emission behaviors of SBs in solvents with different polarity. Negligible changes are observed in the emission color of L_2a and Y_{1 –3} with pyridine rings. L_2b and Y_NBu emit redshifted with increasing solvent polarity.12 The effect of solvents on the emission features was evaluated by the Lippert–Mataga plot15 (Supporting Information). All of the SBs displayed linear dependence of $\Delta \nu$ on $\Delta f$ together with different slopes, which was largest for L_2b and Y_NBu (Figure S5 and Table S5). The slope of the fitting line for Y_NBu is the highest, up to 9673 cm⁻¹, comparable to that of the X‐shaped distyrylbenzenes,16 which further indicated its larger dipole moment changes between the ground and excited states (μ_e–μ_g), leading to the pronounced solvent sensitivity.17

Figure 3.

Photograph of SBs in different solvents under a hand‐held black light with illumination at 365 nm.

Optical response to protons

Pyridines or dibutylanilines are basic. All SBs (except Y₀) display an acidochromic change of their absorption and emission spectra (Figure 4). SBs with pyridine units experience a redshift, accompanied with a loss of fluorescence intensity. Upon protonation the donor‐acceptor character of these SBs increases and therefore internal charge transfer is favored. The addition of TFA to Y_NBu leads to strongly blue emissive species, in which the dibutylamino groups are protonated. The HOMO is stabilized by protonation, resulting in an increased HOMO–LUMO gap. The color change is detected by eye. As expected, Y₃ is more sensitive to protonation compared to Y₁ and Y₂, as there are three pyridine rings as interaction sites.

Figure 4.

Photograph of SBs in THF (c=4 μg mL⁻¹) with increasing content of TFA under a hand‐held black light with illumination at 365 nm (inset data: the number of equivalents of TFA).

As shown in Figure 5, when ‐log [TFA] of approx. 1.30 is reached (c=50 mm), Y₃ shows a redshifted absorption (30 nm) and a 100 nm redshift and distinct attenuation of its emission. With excess acid (c=500 mm, ‐log [TFA]<1.0), the original absorption band of Y_NBu at around 427 nm diminished and a new distinct band formed at 338 nm. A blueshift in emission of Δλ=−69 nm was observed.

Figure 5.

Normalized UV/vis absorption (left) and emission (right) spectra for the titrations of Y₃ (top) and Y_NBu (bottom) in THF with different concentration of TFA.

Optical response to metal ions

Dilute solutions of all the SBs in DCM were exposed to an excess of salts of 13 cations (Al³⁺, Zn²⁺, Cu²⁺, Cu⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cd²⁺, Ag⁺, Pb²⁺ and Hg²⁺, added as perchlorates, and CuI) (Figure 6 a). Only Hg²⁺ quenches luminescence of Y₀. Except for Zn²⁺, Cu²⁺, Ni²⁺, the addition of the remaining ten metal ions leads to either quenching or a redshift in emission of SBs with pyridine units (L_2a, L_2b and Y_{1 –3}). In the case of Y_NBu, Al³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Cd²⁺, Ag⁺, and Hg²⁺ induce a blueshift in emission, as expected for a coordination at the aniline nitrogen. Al³⁺, Mn²⁺, Cd²⁺ and Co²⁺ are quenchers for Y₁ and Y₂, but they less impact photoluminescence of Y₃. Al³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Cd²⁺, Ag⁺ and Hg²⁺ can be easily distinguished from each other through the responses of the SBs by the naked eye. It is challenging to discriminate Cd²⁺ from Zn²⁺.18 SBs with pyridine or dibutylaniline groups display fluorescence responses in the presence of Cd²⁺ but no response with Zn²⁺.

$${\displaystyle {\sigma }_{m,n}\left(r,g\right)=\sqrt{\frac{\sum _{SBs1}^{SBs7}\left(r_n-r_m\right)2+\left(g_n-g_m\right)2}{3{}^{*}7}}}$$ (1)

Figure 6.

(a) Exposure of all the SBs (10 μm) to different metal cations in DCM. Photographs were taken under a hand‐held black light (365 nm) with a digital camera; (b) Autocorrelation plot (RAW rg values) of all the SBs in DCM after exposure to metal ions.

Statistical evaluation of differences in emission colors after exposure to metal ions was performed. Figure 6 b shows the autocorrelation plot of their response. The brightness independent color coordinates rg of the RAW data of the photographs were determined and treated with MANOVA statistics (Eq. (1)).7c, 19 Zn²⁺, Cu²⁺ and Ni²⁺ are hard to discern due to their weak coordination and thus have similar color responses. All of the other investigated metal ions are distinguished.

Conclusions

We have synthesized linear 1,4‐distyryl and star‐shaped 1,3,5‐tristyrylbenzene derivatives (L_2a/L_2b, Y₀–Y₃ and Y_NBu). These are strongly fluorescent in dilute solutions. Y_NBu works as polarity sensor due to its response to different solvents. Upon protonation, all of the pyridine‐containing compounds, L_2a/L_2b and Y₀–Y₃, show a pronounced redshift, and the fluorophore with dibutylaniline groups Y_NBu displays a blueshift in emission. Ten metal ions such as Al³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Cd²⁺, Ag⁺ and Hg²⁺ were well discriminated.

Experimental Section

General procedure 1 (GP1)

Synthesis of intermediates 4–6 by Heck reaction. The reaction was performed in a heat‐gun‐dried 50 mL Schlenk tube under a nitrogen atmosphere. The brominated intermediate (1.0 equiv) and the vinyl compound (1.14 equiv) were dissolved in dry DMF. Pd(OAc)₂ (5 mol %), tris(o‐tolyl)phosphine (0.1 equiv) and dry triethylamine (0.8 mL) were added and the mixture was stirred at 110 °C for 48 h. After the reaction mixture was cooled to ambient temperature, it was poured into water to give a suspension which was extracted with DCM. The combined organic layers were washed with brine, dried over MgSO₄ and the solvents were removed under reduced pressure. The residues were purified by column chromatography.

General procedure 2 (GP2)

Synthesis of symmetric Y₀, Y₃ and Y_NBu by Wittig–Horner reaction. Under nitrogen atmosphere, the triphosphonate 7 (1.0 equiv) was dissolved in dry THF and the solution was cooled to 0 °C. NaH (15.0 equiv) was added carefully and the mixture was stirred at 0 °C for 40 min before the monoaldehyde 4–6 (4.5 equiv) was added slowly. The reaction mixture was then allowed to warm to RT and further stirred for 3 days. After removing THF on a rotary evaporator, the residues were purified on silica gel column.

4,4′‐((1E,1′E)‐(2,5‐Bis(hexyloxy)‐1,4‐phenylene)bis(ethene‐2,1‐diyl))dipyridine (L_2a)

Under nitrogen atmosphere, a solution of 2 (218 mg, 500 μmol), 4‐vinylpyridine (118 mg, 1.20 mmol), Pd(OAc)₂ (11.2 mg, 50 μmol), tris(o‐tolyl)phosphine (30.4 mg, 100 μmol) and triethylamine (1.20 mL) in dry DMF (10 mL) was stirred at 110 °C for 48 h. After the reaction mixture was cooled to ambient temperature, it was poured into water to give a suspension which was extracted with DCM. The combined organic layers were washed with brine dried over MgSO₄ and the solvents were removed under reduced pressure. The residues were purified by column chromatography (silica gel, DCM/EE=5:1, R _f=0.21) and afforded L_2a as a yellow solid (110 mg, 225 μmol, 45 %). M.p.: 135–136 °C. ¹H NMR (300 MHz, CDCl₃) δ 8.58 (d, J=6.0 Hz, 4 H), 7.70 (s, 1 H), 7.64 (s, 1 H), 7.38 (d, J=6.0 Hz, 4 H), 7.18–7.08 (m, 3 H), 7.05 (s, 1 H), 4.08 (t, J=6.5 Hz, 4 H), 1.95–1.82 (m, 4 H), 1.63–1.49 (m, 4 H), 1.47–1.33 (m, 8 H), 0.93 (t, J=7.0 Hz, 6 H). ¹³C NMR (100 MHz, CDCl₃) δ 151.7, 150.1, 149.8, 145.4, 129.7, 128.3, 128.2, 127.6, 126.9, 126.7, 126.5, 126.0, 123.6, 121.0, 114.6, 114.1, 111.2, 110.9, 69.7, 31.8, 29.5, 26.1, 22.8, 14.2. IR (cm⁻¹): 2942, 2915, 2360, 1592, 1476, 1209, 1028, 968, 850, 803, 596, 525. HRMS (MALDI): m/z [M+H]⁺ calcd for C₃₂H₄₁N₂O₂ 485.3168, found 485.372.

2,5‐Bis(hexyloxy)‐4‐(2‐(pyridin‐4‐yl)vinyl)benzaldehyde (5)

According to GP1 a solution of monoaldehyde 3 (385 mg, 1.00 mmol), 4‐vinylpyridine (214 mg, 1.14 mmol), Pd(OAc)₂ (11.2 mg, 50.0 μmol), tris(o‐tolyl)phosphine (30.4 mg, 100 μmol) and triethylamine (0.7 mL) in DMF (10 mL) was stirred at 110 °C for 48 h. Column chromatography (silica gel, PE/EA=5:1, R _f=0.20) afforded 5 as a yellow viscous oil (343 mg, 840 μmol, 84 %). ¹H NMR (300 MHz, CDCl₃) δ 10.59 (s, 1 H), 8.73 (d, J=5.2 Hz, 2 H), 7.78 (d, J=16.5 Hz, 1 H), 7.52 (d, J=5.6 Hz, 2 H), 7.47 (s, 1 H), 7.30 (t, J=8.2 Hz, 2 H), 4.24 (t, J=6.5 Hz, 2 H), 4.16 (t, J=6.5 Hz, 2 H), 2.11–1.90 (m, 4 H), 1.72–1.27 (m, 14 H), 1.05 (dd, J=6.4, 4.4 Hz, 6 H). ¹³C NMR (125 MHz, CDCl₃) δ 189.1, 156.0, 151.1, 150.2, 144.7, 132.7, 129.4, 127.6, 125.1, 121.1, 111.3, 110.3, 69.3, 69.2, 31.6, 29.2, 25.9, 22.7, 14.1. IR (cm⁻¹): 2928, 2857, 1676, 1601, 1422, 1206, 1018, 529. HRMS (DART): m/z [M+H]⁺ calcd for C₂₆H₃₆NO₃ 410.2690, found 410.2687.

4‐(4‐(Dibutylamino)styryl)‐2,5‐bis(hexyloxy)benzaldehyde (6)

According to GP1 a solution of monoaldehyde 3 (771 mg, 2.00 mmol), 4‐dibutylaminostyrene (509 mg, 2.20 mmol), Pd(OAc)₂ (22.4 mg, 100 μmol), tris(o‐tolyl)phosphine (60.9 mg, 200 μmol) and triethylamine (1.5 mL) in DMF (20 mL) was stirred at 110 °C for 48 h. Column chromatography (silica gel, PE/EA=20:1, R _f=0.50) afforded 6 as a yellow viscous oil (582 mg, 1.10 mmol, 54 %). ¹H NMR (400 MHz, CDCl₃) δ 10.42 (s, 1 H), 7.41 (d, J=8.7 Hz, 1 H), 7.32–7.27 (m, 1 H), 7.25–7.03 (m, 3 H), 6.67–6.40 (m, 3 H), 4.17–3.93 (m, 4 H), 3.39–3.20 (m, 4 H), 1.91–1.74 (m, 4 H), 1.63–1.47 (m, 8 H), 1.42–1.26 (m, 12 H), 0.92 (m, 12 H). ¹³C NMR (100 MHz, CDCl₃) δ 189.4, 189.2, 156.6, 155.8, 151.1, 150.5, 148.5, 147.7, 136.2, 135.9, 132.9, 132.8, 130.5, 128.5, 124.5, 123.9, 123.7, 123.4, 120.7, 117.6, 114.8, 111.7, 111.1, 110.2, 109.9, 109.7, 69.4, 69.3, 50.9, 31.7, 29.7, 29.4, 26.0, 22.8, 20.5, 14.2, 14.1. IR (cm⁻¹): 2955, 2928, 2870, 1682, 1612, 1519, 1466, 1366, 1214, 1186, 811. HRMS (DART): m/z [M+H]⁺ calcd for C₃₅H₅₄NO₃ 536.4098, found 536.4101.

1,3,5‐Tris((E)‐2,5‐bis(hexyloxy)‐4‐((E)‐styryl)styryl)benzene (Y₀)

According to GP2 a solution of triphosphonate 7 (42.3 mg, 80.0 μmol) in dry THF (8 mL) was treated with NaH (28.8 mg, 1.20 mmol) and monoaldehyde 4 (105 mg, 256 μmol) was added. The crude product was purified by column chromatography (silica gel, PE/EA=20:1, R _f=0.41) to yield the desired compound Y₀ as a yellow green powder (79.6 mg, 61.6 μmol, 77 %). M.p.: 110–111 °C. ¹H NMR (600 MHz, CDCl₃) δ 7.66–7.49 (m, 15 H), 7.39 (t, J=7.6 Hz, 6 H), 7.32–7.22 (m, 6 H), 7.22–7.14 (m, 9 H), 4.11 (dd, J=15.1, 6.6 Hz, 12 H), 1.99–1.85 (m, 12 H), 1.59 (td, J=13.5, 7.0 Hz, 12 H), 1.48–1.34 (m, 24 H), 0.93 (dt, J=25.7, 6.8 Hz, 18 H). ¹³C NMR (150 MHz, CDCl₃) δ 151.3, 151.2, 138.7, 138.1, 129.0, 128.9, 128.7, 127.5, 127.1, 126.6, 124.1, 123.6, 111.0, 110.8, 69.7, 31.7, 29.5, 26.1, 22.8, 14.2. IR (cm⁻¹): 2926, 2856, 2360, 1419, 1199, 958, 750, 690, 598, 506. HRMS (MALDI): m/z [M]⁺ calcd for C₉₁H₁₁₄O₆ 1290.8615, found 1290.947.

1,3,5‐Tris((E)‐2,5‐bis(hexyloxy)‐4‐((E)‐2‐(pyridin‐4‐yl)vinyl)styryl)benzene (Y₃)

According to GP2 a solution of triphosphonate 7 (42.3 mg, 80.0 μmol) in dry THF (8 mL) was treated with NaH (28.8 mg, 1.20 mmol) and monoaldehyde 5 (114.7 mg, 280 μmol) was added. The crude product was purified by column chromatography (silica gel, DCM/MeOH=50:1, R _f=0.35) to yield the desired compound Y₃ as an orange solid (34.9 mg, 26.9 μmol, 33 %). M.p.: 122–123 °C. ¹H NMR (600 MHz, CDCl₃) δ 8.58–8.50 (d, J=5.4 Hz, 6 H), 7.71–7.39 (m, 12 H), 7.23–6.63 (m, 15 H), 4.01–3.93 (m, 12 H), 1.91–1.83 (m, 12 H), 1.58 (m, 12 H), 1.39–1.27 (m, 24 H), 0.96–0.89 (m, 18 H). ¹³C NMR (150 MHz, CDCl₃) δ 151.9, 151.5, 151.4, 150.8, 150.4, 149.9, 145.6, 139.0, 138.8, 130.4, 130.3, 129.9, 128.5, 128.3, 126.3, 126.2, 125.7, 125.6, 124.6 124.3, 124.2, 121.1, 115.9, 111.3, 111.0, 109.6, 69.9, 69.7, 68.8, 32.2, 31.9, 29.9, 29.7, 26.3, 26.2, 22.9, 14.4. IR (cm⁻¹): 2925, 2856, 2320, 1591, 1498, 1416, 1204, 966, 803, 517. HRMS (MALDI): m/z [M+H]⁺ calcd for C₈₇H₁₁₂N₃O₆ 1295.8629, found 1295.8624.

4,4′,4′′‐((1E,1′E,1′′E)‐(((1E,1′E,1′′E)‐Benzene‐1,3,5‐triyltris(ethene‐2,1‐diyl))tris(2,5‐bis(hexyloxy)benzene‐4,1‐diyl))tris(ethene‐2,1‐diyl))tris(N,N‐dibutylaniline) (Y_NBu)

According to GP2 a solution of triphosphonate 7 (42.3 mg, 80.0 μmol) in dry THF (8 mL) was treated with NaH (28.8 mg, 1.20 mmol) and monoaldehyde 6 (137 mg, 256 μmol) was added. The crude product was purified by column chromatography (silica gel, PE/DCM=2:1 + 2 % triethylamine, R _f=0.45) to yield the desired compound Y_NBu as an orange viscous oil (90.9 mg, 54.3 μmol, 68 %).¹H NMR (600 MHz, CDCl₃) δ 7.61–7.37 (m, 5 H), 7.33 (d, J=8.4 Hz, 4 H), 7.26–7.03 (m, 11 H), 7.03–6.89 (m, 4 H), 6.82–6.65 (m, 1 H), 6.62–6.15 (m, 7 H), 4.16–3.60 (m, 12 H), 3.18 (m, 12 H), 1.92–1.63 (m, 12 H), 1.48 (m, 24 H), 1.41–1.26 (m, 24 H), 1.25–1.07 (m, 12 H), 0.95–0.73 (m, 36 H). ¹³C NMR (150 MHz, CDCl₃) δ 153.4, 151.4, 150.8, 147.8, 146.5, 138.9, 131.0, 130.3, 129.2, 128.6, 128.4, 128.3, 127.9, 127.7, 125.9, 125.8, 125.3, 124.3, 124.1, 118.4, 115.4, 114.0, 112.6, 112.0, 111.7, 111.2, 110.1, 69.8, 50.9, 31.8, 29.6, 27.8, 26.1, 22.8, 20.5, 16.0, 14.2. IR (cm⁻¹): 2953, 2926, 2857, 1607, 1519, 1366, 1185, 1030, 962, 805, 523. HRMS (MALDI): m/z [M+H]⁺ calcd for C₁₁₄H₁₆₅N₃O₆ 1673.2777, found 1673.293. Elemental analysis calcd (%) for C₁₁₄H₁₆₅N₃O₆: C 81.82; H 9.94; N 2.51; found: C 81.60, H 10.76, N 2.45.

1,4‐Bis((E)‐2,5‐bis(hexyloxy)‐4‐((E)‐2‐(pyridin‐4‐yl)vinyl)styryl)benzene (L_2b)

Under a nitrogen atmosphere, the bisphosphonate 9 (45.4 mg, 120 μmol) was dissolved in dry THF (10 mL) and the solution was cooled to 0 °C. NaH (28.8 mg, 1.20 mmol) was added carefully and the mixture was stirred at 0 °C for 40 min before monoaldehyde 5 (103 mg, 252 μmol) was added slowly. The reaction mixture was then allowed to warm to RT and further stirred for 2 days. After removing THF on a rotary evaporator, the residues were purified on silica gel column (silica gel, PE/EA=20:1, R _f=0.19) to yield the desired compound L_2b as an orange solid (87.9 mg, 98.9 μmol, 82 %). M.p.: 147–148 °C. ¹H NMR (600 MHz, CDCl₃) δ 8.53 (m, 4 H), 7.68 (d, J=16.4 Hz, 2 H), 7.61–7.42 (m, 6 H), 7.38 (d, J=6.0 Hz, 3 H), 7.22–6.97 (m, 9 H), 4.20–3.74 (m, 8 H), 1.99–1.73 (m, 8 H), 1.70–1.48 (m, 8 H), 1.50–1.21 (m, 16 H), 1.09–0.80 (m, 12 H). ¹³C NMR (150 MHz, CDCl₃) δ 151.7, 151.1, 150.2, 149.7, 145.4, 137.3, 129.2, 128.3, 128.2, 127.0, 126.0, 125.5, 124.1, 123.2, 120.9, 111.1, 110.5, 69.6, 31.7, 29.6, 29.5, 26.1, 22.8, 14.2. IR (cm⁻¹): 2931, 2853, 1590, 1397, 1207, 1034, 961, 847, 726, 525. HRMS (MALDI): m/z [M]⁺ calcd for C₆₀H₇₆N₂O₄ 888.5805, found 888.5835.

4‐((E)‐4‐((E)‐3,5‐Bis((E)‐2,5‐bis(hexyloxy)‐4‐((E)‐styryl)styryl)styryl)‐2,5‐bis(hexyloxy)styryl)pyridine (Y₁)

Under a nitrogen atmosphere, the triphosphonate 7 (42.3 mg, 80 μmol) was dissolved in dry THF (10 mL) and the solution was cooled to 0 °C. NaH (28.8 mg, 1.2 mmol) was added carefully and the mixture was stirred at 0 °C for 40 min before monoaldehyde 5 (35.4 mg, 86.8 μmol) was added slowly. The reaction mixture was stirred overnight at room temperature. After removing THF in vacuo, the residues containing 10 were dried and used in the next step without further purification. To a solution of 10 in dry THF (8 mL) was added NaH (28.8 mg, 1.20 mmol) at 0 °C. The mixture was stirred at 0 °C for 40 min, and then another monoaldehyde 4 (71.9 mg, 176 μmol) was added. The reaction mixture was then allowed to warm to RT and further stirred for 2 days. After removing THF on a rotary evaporator, the crude product was purified by column chromatography (silica gel, PE/EA=1:1, R _f=0.49) to yield the desired compound Y₁ as an orange solid (29.8 mg, 23.1 μmol, 28 % over two steps). M.p.: 71–72 °C. ¹H NMR (600 MHz, CDCl₃) δ 8.69 (d, J=5.8 Hz, 2 H), 7.82 (d, J=16.4 Hz, 1 H), 7.74–7.60 (m, 12 H), 7.49 (m, 6 H), 7.37 (dd, J=16.3, 7.8 Hz, 4 H), 7.34–7.23 (m, 9 H), 7.19 (d, J=16.4 Hz, 1 H), 4.20 (dt, J=10.9, 6.5 Hz, 12 H), 2.07–1.98 (m, 12 H), 1.74–1.66 (m, 12 H), 1.57–1.47 (m, 24 H), 1.08–0.99 (m, 18 H). ¹³C NMR (150 MHz, CDCl₃) δ 151.8, 151.2, 150.1, 145.5, 138.8, 138.5, 138.0, 129.8, 128.9, 128.7, 128.4, 128.31, 127.5, 127.0, 126.6, 125.9, 125.5, 124.3, 123.6, 120.9, 111.0, 110.7, 69.7, 31.7, 29.5, 26.1, 22.8, 14.2. IR (cm⁻¹): 2925, 2856, 2359, 1590, 1420, 1202, 1029, 960, 690, 507. HRMS (MALDI): m/z [M+H]⁺ calcd for C₈₉H₁₁₄NO₆ 1292.8646, found 1292.963.

4,4′‐({5‐[(E)‐2‐{2,5‐Bis(hexyloxy)‐4‐[(E)‐2‐phenylethenyl]phenyl}ethenyl]‐1,3‐phenylene}bis{[(E)ethene‐2,1‐diyl][2,5‐bis(hexyloxy)‐4,1‐phenylene](E)ethene‐2,1‐diyl})dipyridine (Y₂)

Under a nitrogen atmosphere, the triphosphonate 7 (42.3 mg, 80.0 μmol) was dissolved in dry THF (8 mL) and the solution was cooled to 0 °C. NaH (28.8 mg, 1.20 mmol) was added carefully and the mixture was stirred at 0 °C for 40 min before monoaldehyde 5 (70.8 mg, 174 μmol) was added slowly. The reaction mixture was stirred overnight at room temperature. After removing THF on a rotary evaporator, the residues containing 11 were dried and used in the next step without further purification. To a solution of 11 in dry THF (8 mL) was added NaH (28.8 mg, 1.20 mmol) at 0 °C. The mixture was stirred at 0 °C for 40 min, and then another monoaldehyde 4 (36.1 mg, 88.0 μmol) was added. The reaction mixture was then allowed to warm to RT and further stirred for 2 days. After removing THF on a rotary evaporator, the crude product was purified by column chromatography (silica gel, EE/MeOH=50:1, R _f=0.58) to yield the desired compound Y₂ as an orange solid (46.4 mg, 35.8 μmol, 44 % over two steps). M.p.: 121–122 °C. ¹H NMR (600 MHz, CDCl₃) δ 8.70 (d, J=5.8 Hz, 4 H), 7.81 (d, J=16.4 Hz, 2 H), 7.75–7.58 (m, 9 H), 7.56–7.45 (m, 6 H), 7.42–7.32 (m, 4 H), 7.27 (m, 7 H), 7.19 (d, J=16.4 Hz, 2 H), 4.21 (m, 12 H), 2.10–1.90 (m, 12 H), 1.82–1.62 (m, 12 H), 1.62–1.42 (m, 24 H), 1.04 (m, 18 H). ¹³C NMR (150 MHz, CDCl₃) δ 151.7, 151.3, 151.2, 150.2, 145.4, 138.8, 138.6, 138.0, 129.7, 128.9, 128.8, 128.3, 128.2, 127.6, 127.2, 126.9, 126.6, 126.1, 125.6, 124.3, 124.1, 123.6, 120.9, 111.1, 110.8, 69.7, 31.7, 29.6, 26.1, 22.8, 14.2. IR (cm⁻¹): 2926, 2856, 2360, 1589, 1419, 1203, 962, 690, 527. HRMS (MALDI): m/z [M+H]⁺ calcd for C₈₈H₁₁₃N₂O₆ 1293.8599, found 1293.919.

CCDC  1959608 (L_2b), 1959609 (Y₀), and 1959610 (Y₃) contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

H. Zhang, E. A. Kotlear, S. Kushida, S. Maier, F. Rominger, J. Freudenberg, U. H. F. Bunz, Chem. Eur. J. 2020, 26, 8137.