Synthesis of a highly luminescent three-dimensional pyrene dye based on the spirobifluorene skeleton.

We have synthesized a highly luminescent (log e > 5.0, F > 0.9) pyrene dye based on a spirobifluorene skeleton [2,2',7,7'-tetrakis(7-tert-butyl-1-pyrenyl)-9,9'-spirobi[9H-fluorene; 4-PySBF]. The use of spirobifluorene prevents fluorescence quenching by intramolecular energy transfer and/or electron transfer among the chromophores in the excited state. The emission spectra of 4-PySBF exhibited a red shift of 20 nm in comparison to a model compound [9,9'-dioctyl-2,7-bis(7-tert-butyl-1-pyrenyl)-9H-fluorene; 2-PyF], but its UV-Vis spectrum remained unchanged.

Introduction

Recently, blue emitting dyes with high efficiency (Φ ≈ 1.0, log ε > 4.5) [1] have been attracting considerable attention because of their applicability to molecular electronic materials such as organic field effect transistors (OFETs) and organic light-emitting diodes (OLEDs). Many studies have been conducted on strategies for synthesizing a highly efficient dye. For example, rod- [1,2] and star-shaped [3,4,5,6] hydrocarbons that expand π-conjugation to two dimensions can be used to control the emission color of a dye with high efficiency.

Spirobifluorene derivatives are considered the most promising candidates for organic optoelectronics [7,8,9,10,11]. The rigidity of spiro-compounds affords them high thermal stability. In addition, their solubility is higher than that of corresponding compounds without a spiro moiety, because their perpendicular conformations that are based on the spiro-linkage efficiently suppress intermolecular interactions between π-systems. Recently, several studies were conducted on introducing dyes into a molecule for π-conjugation in three dimensions. Examples of such studies include those on the introduction of dyes into the spirobifluorene skeleton [12,13,14,15,16,17,18,19]. Because of the high thermal stability and unique characteristics of the spirobifluorene skeleton, it has been adopted in various materials [20,21,22].

In this paper, we report the synthesis and characterization of a new pyrene dye that is based on the spirobifluorene skeleton. Pyrene derivatives are highly absorptive [23,24,25,26]; further, the introduction of pyrene substituents into spirobifluorene derivatives is expected to improve the fluorescence quantum yield and thermal stability of the derivatives because the substituents are highly emissive, bulky, and rigid.

Results and Discussion

A spirobifluorene dye, 2,2′,7,7′-tetrakis(7-tert-butyl-1-pyrenyl)-9,9′-spirobi [9H-fluorene] (4-PySBF), and a model compound, 9,9′-dioctyl-2,7-bis(7-tert-butyl-1-pyrenyl)-9H-fluorene (2-PyF), were synthesized in 37% and 24% yields, respectively, by palladium-catalyzed Suzuki-Miyaura coupling reactions (Scheme 1). The precursors, 7-tert-butylpyrene-1-boronic acid pinacol ester (1) and 1-bromo-7-tert-butylpyrene (3), were prepared from pyrene according to a previously reported method [27]. The chemical structures of 4-PySBF and 2-PyF were confirmed by 1H-NMR, 13C-NMR, and FT-IR spectroscopy. Each compound was found to be soluble in various organic solvents such as chloroform, tetrahydrofuran (THF), dichloromethane, and toluene. It should be noted that the tert-butyl group is necessary for the successful synthesis of 4-PySBF; we found that without tert-butyl groups, the solubility is quite poor, making purification and structural analysis by techniques such as 1H-NMR and 13C-NMR rather difficult.

Scheme 1

Synthesis of spirobifluorene dye and model compound.

We measured the UV-Vis spectra, fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes, and lifetimes of 4-PySBF and 2-PyF in CH2Cl2 solution. The UV-Vis spectra (Figure 2, left side) show that the absorption maxima (λmax) of 4-PySBF and 2-PyF are at 360 nm and 359 nm, with molar absorption (λmax) coefficients ε of 131,000 and 71,000 M−1 cm−1, respectively. The finding that the ε value of 4-PySBF is about twice that of 2-PyF is in accordance with the fact that 4-PySBF has two chromophores per molecule whereas 2-PyF has but one.

Figure 2

Emission spectra of 2-PyF and 4-PySBF in CH2Cl2; Abs = 0.1, λex = 360 nm.

The observed fluorescence spectra (Figure 2, right side) show that 4-PySBF and 2-PyF exhibit intense blue emissions (λem) at 441 nm (Φ = 0.92) and 421 nm (Φ = 0.90), respectively, in CH2Cl2 solution (λex = 360 nm; c = 1.0 × 10−6 M). The fluorescence spectra also reveal that the fluorescence intensity of 4-PySBF is about twice that of 2-PyF, again because 4-PySBF has two chromophores per molecule. A 20-nm red-shift between 4-PySBF and 2-PyF was observed. It is noted that we observed a spiroconjugation (a large red shift in the fluorescence spectrum and no red shift in the absorption spectrum in this 4-PySBF and 2-PyF pair). We hypothesize that this observed red-shift could be due to conjugation of 4-PySBF via the spiro carbon [28,29,30,31,32] in the excited state [33]. Another possibility is a through-space interaction between upper and lower pyrene chromophores.

Absorption and emission spectra of 2-PyF and 4-PySBF in CH2Cl2; c = 10 μM, λex = 360 nm.

Measurement of the photophysical properties of both compounds at the same concentration confirm that 4-PySBF had twice the number of chromophores in solution as compared to 2-PyF. Next, we measured the fluorescence spectra of both compounds under the same absorption conditions. We then found that the solutions of 4-PySBF and 2-PyF had the same number of chromophores. The fluorescence spectra (Figure 3) revealed that both 4-PySBF and 2-PyF had approximately the same fluorescence intensity. This result shows that the spirobifluorene derivatization doubled the performance of the chromophores by preventing intramolecular energy transfer and/or electron transfer, although the pyrene chromophores are placed close together. The compound 4-PySBF was found to have ideal luminescence properties (log ε > 5.0, Φ > 0.9).

Fluorescence quantum yield can be expressed as Φf = kfτs, or Φf = kf/(kf + kisc + knr + kq[O2]), where kf, kisc, knr, and kq[O2] denote the rate constants for fluorescence radiation, intersystem crossing, nonradiative decay, and fluorescence quenching by oxygen, respectively, and τs denotes the lifetime (s = singlet) [14]. Table 1 summarizes the values of kf and kisc + knr for 4-PySBF and 2-PyF, calculated from the experimental values of Φf and τs under degassed conditions, that is, under the assumption that kq[O2] = 0. As knr for aromatic hydrocarbons is known to be negligible [34], kisc should be the dominant component in kisc+knr [35,36,37,38]. Under aerated conditions, kq[O2] is estimated to be nearly diffusion controlled (up to 3.89 × 1010 m−1 s−1 in hexane) [34,39], and [O2] is less than 2.0 × 10−3 M in usual solvents. These data imply that the maximum possible value of kq[O2] is 8.0 × 107 s−1. As can be seen from Table 1, the kf values for 4-PySBF and 2-PyF are much larger than the kq[O2] values. This situation is most probably responsible for the observed large values of Φf for both compounds, even in the presence of oxygen.

Table 1

Photochemical data for compounds 4-PySBF and 6.

dye	λab[nm]	ε (×105) [l/mol･cm]	log ε	λem[nm]	Φ	τ [ns]	kf (×108) [S-1]	kisc + knr (×107) [S-1]
4-PySBF	360	1.31	5.12	441	0.92	1.52	6.1	5.3
2-PyF	359	0.71	4.85	421	0.90	1.88	4.8	5.3

Conclusions

In summary, we have synthesized a highly luminescent spirobifluorene dye (log ε > 5.0, Φ > 0.9) with tetrapyrene chromophores (4-PySBF) and measured its photophysical properties. In comparison to a model compound, 2-PyF, the emission spectrum of 4-PySBF exhibited a red shift of 20 nm, but its UV-Vis spectrum remained unchanged. It is not easy to find a satisfactory explanation for this observed red-shift at the present stage. However, in our next study, we intend to calculate the electron states of these compounds in the ground and excited states by the molecular orbital (MO) method and examine this conjugation system in greater detail. Further, we intend to apply 4-PySBF in a previously developed cholesteric liquid crystal laser (distributed feedback laser) as the laser dye in the future.

Experimental

Instruments

All the 1H- and 13C-NMR spectra were recorded on a 400 MHz JEOL LMN-EX400 instrument with tetramethylsilane (TMS) as the internal standard. FT-IR spectra were recorded on a JASCO FT-IR 469 plus spectrometer. Melting points were obtained by a Stuart Scientific Melting Point Apparatus SMP3. MS spectra (FAB) were obtained by JEOL JMS700 mass spectrometer. UV-Vis spectra were recorded with a Beckman Coulter DU800 UV-Vis Spectrophotometer. Fluorescence spectra were recorded on a JASCO FP-6500 Spectrofluorometer. Quantum Yields were measured by a Hamamatsu Photonics C9920-02 Absolute PL Quantum Yield Measurement system. Fluorescence lifetimes were measured using a Hamamatsu Photonics OB 920 Fluorescence Lifetime Spectrometer.

Materials

Unless otherwise noted, all reagents, chemicals and solvents were obtained from commercial sources and used without further purification. Pyrene, 2-chloro-2-methylpropane, Pd(PPh3)4, 4,4,5,5-tetramethyl-1,3,2-dioxaborolane were obtained from TCI. 9,9-Dioctyl-fluorene-2,7-bis(trimethylene-borate) (4) and PdCl2(PPh3)2, were obtained from Aldrich. 2,2′,7,7′-Tetrabromospirobifluorene (2) was a gift of JFE Chemical Coporation (Japan).

2,2′,7,7′-tetrakis(7-tert-Butyl-1-pyrenyl)-9,9′-spirobi [9H-fluorene] (3, 4-PySBF)

Under an argon atomosphere, 2,2′,7,7′-tetrabromospirobifluorene (0.25 g, 0.4 mmol), 7-tert-butylpyrene-1-boronic acid pinacol ester (0.77 g, 2.0 mmol) and cesium carbonate (5.21 g, 16 mmol) were mixed together with Pd(PPh3)4 (100 mg, 0.1 mmol) and degassed THF (30 mL). The mixture was refluxed for 24 h. After cooling to room temperature, the resulting mixture was extracted with chloroform. The organic extract was washed sequentially with water and brine and then dried over MgSO4. After removal of the solvent, the residue was purified by column chromatography (chloroform:hexane = 1:5) and preparative HPLC (CHCl3). After removal of the solvent, the product was recrystallized from cyclohexane to afford 4-PySBF as a yellow powder in 37% yield. 1H-NMR (400 MHz, THF-d8): δ 8.23–7.36 (m, Ar-H, 44H), 1.53 (s, tert-butyl-H, 36H) ppm.; 13C-NMR (100 MHz, CDCl3): δ 149.3, 149.1, 140.9, 140.7, 137.4, 131.3, 130.7, 130.5, 130.3, 128.4, 128.2, 128.1, 127.7, 127.5, 127.3, 127.2, 126.5, 125.0, 124.9, 124.4, 123.1, 122.4, 122.0, 120.2 (aromatic C), 66.3 (spiro-C), 35.2 (Ar-C (CH3)3), 31.9 (Ar-C (CH3)3) ppm; FT-IR (KBr): 3042, 2961, 2901, 2867, 1594, 1457, 1227 cm−1; mp 286–288 °C; HR-MS (FAB+) Calcd. for C105H80 [M] 1341.6294, found [M+] 1341.6290.

9,9′-Dioctyl-2,7-bis (7-tert-butyl-1-pyrenyl)-9H-fluorene (6, 2-PyF)

Under an argon atomosphere, 2-bromo-7-tert-butylpyrene (0.8 g, 2.4 mmol) and 9,9-dioctyl-fluorene-2,7-bis(trimethyleneborate) (0.28 g 0.5 mmol) were mixed together with Pd(PPh3)4 (60 mg, 0.05 mmol), degassed toluene (20 mL), one drop of Aliquat 336, and 1M aqueous sodium carbonate solution (2 mL). The resulting mixture was refluxed for 24 h. After cooling to room temperature, the mixture was extracted with chloroform. The organic extract was washed sequentially with water and brine and then dried over MgSO4. After removal of the solvent, the residue was purified by column chromatography using chloroform/hexane (5:1, v/v) following HPLC (CHCl3). After subsequent recrystallization in ethanol, 2-PyF was obtained as a yellow powder in 24% yield. 1H-NMR (400 MHz, CDCl3): δ 8.27–7.97 (m, Ar-H, 18H), 7.69–7.67 (m, Ar-H, 4H), 2.12–2.08 (s, Ar-CH-Ar, 2H), 1.60 (s, tert-butyl-H, 18H), 1.25–1.19 (m, alkyl-H, 20H), 0.96 (m, alkyl-H, 4H), 0.82 (t, J = 6.84, alkyl-H, 6H) ppm; 13C-NMR (100 MHz, CDCl3): δ 151.2, 149.1, 140.1, 140.0, 138.1, 131.4, 130.9, 130.4, 129.4, 128.4, 127.6, 127.5, 127.3, 125.4, 125.3, 125.0, 124.5, 123.2, 122.4, 122.0, 119.8 (aromatic C), 55.3 (spiro-C), 40.4 (alkyl-C), 35.2 (Ar-C (CH3)3), 31.9 (Ar-C (CH3)3), 30.1, 29.7, 29.4, 29.3, 24.2, 22.7, 14.2 (alkyl-C) ppm; FT-IR (KBr): 3050, 2953, 2925, 2853, 1608, 1481, 1457, 1310 cm−1; mp 206–208 °C; HR-MS (FAB+) calcd. for C69H74 [M] 902.5791, found [M+] 902.5786.