Synthesis of a Highly Luminescent Three-Dimensional Pyrene Dye Based on the Spirobifluorene Skeleton

We have synthesized a highly luminescent (log ε > 5.0, Φ > 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,

OPEN ACCESS
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.
We measured the UV-Vis spectra, fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes, and lifetimes of 4-PySBF and 2-PyF in CH 2 Cl 2 solution. The UV-Vis spectra ( Figure 2, left side) show that the absorption maxima (λ max ) of 4-PySBF and 2-PyF are at 360 nm and 359 nm, with molar absorption (λ max ) 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.
The observed fluorescence spectra ( Figure 2, right side) show that 4-PySBF and 2-PyF exhibit intense blue emissions (λ em ) at 441 nm (Φ = 0.92) and 421 nm (Φ = 0.90), respectively, in CH 2 Cl 2 solution (λ ex = 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.   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 , where k f , k isc , k nr , and k q [O 2 ] denote the rate constants for fluorescence radiation, intersystem crossing, nonradiative decay, and fluorescence quenching by oxygen, respectively, and τ s denotes the lifetime (s = singlet) [14]. Table 1 summarizes the values of k f and k isc + k nr for 4-PySBF and 2-PyF, calculated from the experimental values of Φ f and τ s under degassed conditions, that is, under the assumption that k q [O 2 ] = 0. As k nr for aromatic hydrocarbons is known to be negligible [34], k isc should be the dominant component in k isc +k nr [35][36][37][38]. Under aerated conditions, k q [O 2 ] is estimated to be nearly diffusion controlled (up to 3.89 × 10 10 m −1 s −1 in hexane) [34,39], and [O 2 ] is less than 2.0 × 10 −3 M in usual solvents. These data imply that the maximum possible value of k q [O 2 ] is 8.0 × 10 7 s −1 . As can be seen from Table 1, the k f values for 4-PySBF and 2-PyF are much larger than the k q [O 2 ] values. This situation is most probably responsible for the observed large values of Φ f for both compounds, even in the presence of oxygen.

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.

Instruments
All the 1 H-and 13 C-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.