The strategy to improve thermal and optical properties of diphenylfluoranthene based on silicon-cored derivatives

Abstract

A series of novel silicon-cored diphenylfluoranthene derivatives was synthesized in this paper to realize efficient solid-state emission. These silicone-cored diphenylfluoranthene derivatives show better fluorescent properties in the solid state than diphenylfluoranthene because these silicone-cored derivatives exhibit weaker π–π interactions among molecules. What is more, these silicone-cored derivatives exhibit very high thermal stabilities and exceptionally high glass transition temperatures. Interestingly, one silicon-cored diphenylfluoranthene derivative exhibited similar fluorescence emission spectra in both solution and solid state, and may be the most obvious candidate for an efficient solid-state emitter.

---

Introduction

Luminescent materials have attracted a surge of interest recently because of their great potential applications in many aspect of modern life, such as flat panel and full color range displays.^1–3 Flat aromatic molecules and linear π-conjugated systems are highly fluorescent and exhibit other impressive properties such as thermal stability. However, an enfeebling and long-standing problem with such systems is the molecular aggregation via π–π stacking. This kind of aggregation usually leads to the formation of π-aggregates/excimers and causes bathochromic shifts and low solid-state fluorescence quantum yields.⁴ Many publications and patents have been reported to produce an efficient, cheap, and robust blue-light emitter.⁵ It is, of course, not an easy task to find a small molecule that possesses not only a very large band gap but also thermal and chemical stabilities and a very large quantum yield in the solid state.⁶ Nonplanarity is an important solution to overcome the aggregation, and nonplanar configuration can be easily achieved with the use of a silicon-cored molecular structure.^7,8 In all organic luminescent materials, silicon-cored compounds have been reported to exhibit more advantages than other materials in optical applications because of their high brightness, thermal stability, amorphous film-forming capability, and so on.^6,9–17 Surprisingly, no systematic structural analysis and comparison of properties between silicon-cored materials and common structural materials have been reported.

The chemistry of fluoranthene and studies of the optical properties developed rapidly after the elucidation of its structure at the turn of the 20th century.^18,19 Fluoranthene and many of its derivatives have been reported to exhibit excellent optical properties and thermal properties.^20–28 However, the utility of fluoranthene and its derivatives as emitting materials has been greatly restricted due to aggregation between the planar fluorophore, like other planar molecules.^29,30 To meet the practical demand for highly efficient solid-state emitters in optical fields, much effort has been devoted to eliminate the undesirable effect of aggregation-caused quenching of planar molecules.³¹ The fluoranthene derivatives laterally substituted with phenyl groups have been reported to reduce facial contacts that lead to excimer quenching and bathochromic shifts.²¹ However, because every chromophore has an inherent tendency to aggregate in the solid phase, the resistance against the intrinsic aggregation nature is not good enough to make use of it. Recently, aggregation-induced emission has been reported to modify the fluoranthene molecule and obtain an efficient emitter in the solid state.^25,28 However, although the fluorescence in the solid state of this derivative was strong, the maximum emission wavelength also changed to 526 nm from the 451 nm of fluoranthene. For the application of devices, the optical band gap, energy level of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of luminescent materials should correlate with those of the host molecules for efficient energy or charge transfer processes. Therefore, a new method to weaken the intermolecular interactions and keep the fluorescent properties of fluoranthene molecules was necessary in these applications. From this viewpoint, the characteristics of silicon-cored structures may serve the role.

In this paper, we report the synthesis of diphenylfluoranthene and new silicon-cored diphenylfluoranthene derivatives. Systematic comparison of the properties and structural analysis of common structural materials and silicon-cored materials are discussed in particular. By introducing silicon-cored structures into fluoranthene, we hope to take advantage of the rigid and planar structure of this compound and turn it into an efficient solid-state emitter. In contrast to diphenylfluoranthene, the silicon-cored compounds exhibit high thermal stability and exceptionally high glass transition temperatures. Particularly, due to the steric hindrance of the silicon-cored structure, all the silicon-cored derivatives exhibit weaker π–π molecular interactions and less bathochromic shift of fluorescence emission in the solid state than does diphenylfluoranthene. Moreover, other groups at the Si atom exhibit a great influence on the molecular structures and result in different optical properties. We hope the study in this paper will provide some guidance on the molecular design of silicon-cored planar materials in optical application.

Experimental section

Material

Acenaphthenequinone and 1,3-diphenylacetone were obtained from the Shanghai Chemical Reagent Company. Chlorosilanes were obtained from the Zibo Yuxing Chemical Engineering Company and used after fractionating.

Synthesis

The silicon-cored fluoranthene derivatives were synthesized via the Knoevenagel/Diels–Alder method from commercial starting materials, such as acenaphthenequinone and diphenylacetone, using only ethanol (EtOH) and (optionally) xylenes for solvents; all are inexpensive and readily available (Scheme 1).

Scheme 1 The synthesis of diphenylfluoranthene and the silicon-cored derivatives.

7,9-Diphenyl-cyclopenta[a]acenaphthylen-8-one (1). 5.45 g acenaphthenequinone (0.03 mol) and 6.30 g 1,3-diphenylacetone (0.03 mol) were dissolved in 60 mL ethanol, and the mixture was heated to 78 °C. A solution of 0.8 g KOH in 8 mL ethanol was added slowly to the mixture. The reaction mixture was stirred at 78 °C for 15 min, and then cooled to room temperature. Solid product was filtrated and washed three times with ethanol. After drying, a bright black solid was obtained. The total yield was 90% (mp. 218 °C).

(7,10-Diphenylfluoranthen-8-yl)trimethylsilane (2). 0.74 g (7.5 mmol) ethynyltrimethylsilane and 1.71 g (5 mmol) 1 were dissolved in 30 mL xylene in a sealed stainless steel reactor, and the reaction was run for 20 h at 230 °C under a nitrogen atmosphere. Then xylene was removed under reduced pressure. The product was precipitated in methanol and purified with silica gel column chromatography. A yellow solid powder was obtained with yield 57%. ¹H NMR (DMSO, 400 MHz, ppm): δ 0.02 (s, 9H), 6.30 (d, 1H), 7.35 (t, 1H), 7.46–7.68 (m, 13H), 7.82–7.89 (m, 2H). ¹³C NMR (CDCl₃, 100 MHz, ppm): δ 1.07 (Si–CH₃), 123.19, 123.31, 127.30, 127.76, 128.28, 128.51, 128.65, 129.13, 129.27, 129.35, 129.94, 130.06, 132.427, 135.53, 135.63, 136.37, 136.64, 137.03, 137.24, 138.72, 140.83, 141.35.

7,10-Diphenylfluoranthene (3). 1.00 g 2 (2.3 mmol) and 2 mL concentrated hydrochloric acid was dissolved in 30 mL THF. The mixture was refluxed for 10 h with stirring, and then the solvent was distilled off. The product was precipitated in methanol and purified with silica gel column chromatography. A yellow crystal was obtained. The yield was 91%. ¹H NMR (CDCl₃, 400 MHz, ppm): δ 7.24 (d, 2H), 7.29 (s, 2H), 7.37 (t, 2H), 7.52–7.60 (m, 6H), 7.67 (m, 4H), 7.78 (d, 2H). ¹³C NMR (CDCl₃, 100 MHz, ppm): δ 123.0, 126.7, 127.6, 127.8, 128.7, 129.1, 129.8, 132.8, 136.3, 136.8, 138.0, 141.0. Anal. calcd for C₂₈H₁₈: C 94.88%, H 5.12%; found: C 94.26%, H 5.07%.

Diethynylsilanes (4–6). Diethynyldimethylsilane (4), diethynylmethylphenylsilane (5) and diethynyldiphenylsilane (6) were prepared according to the published method.^32,33

4.8 g (0.2 mol) magnesium powder and 50 mL dry THF were mixed into a 250 mL three-necked, round-bottomed flask under an argon atmosphere with stirring. 22.0 g (0.2 mol) bromoethane in 50 mL dry THF were slowly dropped into the mixture. After the magnesium powder disappeared, the mixture was kept boiling for 2 h. After cooling to room temperature, the bromoethane Grignard reagent was obtained.

Acetylene, which was cooled at −78 °C, was bubbled into the dry 150 mL THF for 1 h to obtain the acetylene saturated solution. Then the bromoethane Grignard reagent was dropwise added into the acetylene saturated solution. In this process the acetylene was always kept bubbling into the mixture. The reaction was kept for 3 h after the addition of the bromoethane Grignard reagent was completed, and then the ethynylmagnesium chloride was obtained.

A 1 L three-necked, round-bottomed flask was equipped with a reflux condenser and a Teflon-covered magnetic stirring bar. The flask was charged with the ethynylmagnesium chloride obtained at the last step under an argon atmosphere. Dimethyldichlorosilane (7.80 g, 60 mmol) and THF (50 mL) were placed into the addition funnel and added dropwise to the well-stirred reaction mixture over 1 h. It was quenched by addition of saturated aqueous ammonium chloride (10 mL), and THF (30 mL) was added. The organic layer was washed with water, dried with anhydrous MgSO₄, filtered, and concentrated at reduced pressure. The residue was then fractionally distilled to give 4 (colorless liquid with yield 34%); bp 86.5 °C; ¹H NMR (CDCl₃, 400 MHz, ppm): δ 2.48 (s, 2H, –C≡CH), 0.35 (s, 6H, –SiCH₃).

5 was prepared in a similar manner to that for the synthesis of 4. The methylphenyldichlorosilane (11.5 g, 60.0 mmol) was used to obtain colorless liquid with yield 70%. ¹H NMR (400 MHz, CDCl₃, ppm): δ 7.76 (m, 2H, ArH), 7.28–7.46 (m, 3H, ArH), 2.75 (s, 2H, –C≡CH), 0.64 (s, 3H, –SiCH₃).

6 was prepared in a similar manner to that used for the synthesis of 4. The diphenyldichlorosilane (15.2 g, 60.0 mmol) was used to obtain the straw-yellow crystal with yield 70%. ¹H NMR (400 MHz, CDCl₃, ppm): δ 7.75 (m, 4H, ArH), 7.26–7.50 (m, 6H, ArH), 2.75 (s, 2H, –C≡CH).

Bis(7,10-diphenylfluoranthene)dimethylsilane (7). 7.12 g 1 (0.02 mol) and 1.08 g diethynyldimethylsilane (0.01 mol) were dissolved in 30 mL xylene in a sealed stainless steel reactor, and the reaction was run for 20 h at 230 °C under a nitrogen atmosphere. Then xylene was removed under reduced pressure. The product was purified with silica gel column chromatography. The yield was 46%. ¹H NMR (DMSO, 400 MHz, ppm): δ 0.08 (s, 6H), 6.17 (s, 2H), 7.19 (t, 6H), 7.26 (t, 2H), 7.37 (t, 2H), 7.46 (t, 6H), 7.56–7.63 (m, 10H), 7.81–7.88 (m, 6H). ¹³C NMR (CDCl₃, 100 MHz, ppm): δ −1.10 (Si–CH₃), 120.99, 121.07, 124.64, 125.17, 125.76, 125.84, 125.99, 126.29, 126.85, 126.96, 127.65, 127.90, 130.44, 133.72, 133.94, 134.40, 134.69, 135.03, 135.66, 138.56, 138.83. ²⁹Si NMR (CDCl₃, 80 MHz, ppm): δ −7.56. Anal. calcd for C₅₈H₄₀Si: C 91.06%, H 5.27%; found: C 90.47%, H 5.05%.

Bis(7,10-diphenylfluoranthene)methylphenylsilane (8). This compound was prepared by a procedure similar to that for 7. The yield was 41%. ¹H NMR (DMSO, 400 MHz, ppm): δ 0.35 (s, 3H), 6.34 (d, 2H), 7.17 (t, 4H), 7.23–7.32 (m, 12H), 7.38–7.49 (m, 13H), 7.58 (d, 4H), 7.70 (d, 2H), 7.77 (d, 2H). ¹³C NMR (CDCl₃, 100 MHz, ppm): δ −0.85 (Si–CH₃), 123.19, 123.29, 126.33, 126.88, 127.49, 127.55, 127.57, 127.61, 127.68, 128.16, 128.18, 128.53, 128.81, 129.19, 129.71, 129.98, 130.17, 132.89, 135.43, 136.14, 136.19, 136.80, 137.42, 137.60, 137.98, 138.62, 141.11, 143.77. ²⁹Si NMR (CDCl₃, 80 MHz, ppm): δ −8.42. Anal. calcd for C₆₃H₄₂Si: C 91.49%, H 5.12%; found: C 91.36%, H 5.18%.

Bis(7,10-diphenylfluoranthene)diphenylsilane (9). The compound was synthesized using a similar process. The yield was 35%. ¹H NMR (DMSO, 400 MHz, ppm): δ 6.95 (d, 2H), 6.94 (d, 4H), 7.03 (t, 4H), 7.16–7.26 (m, 16H), 7.41 (s, 2H), 7.47–7.60 (m, 12H), 7.80 (d, 2H), 7.89 (d, 2H). ¹³C NMR (CDCl₃, 100 MHz, ppm): δ 123.19, 123.42, 126.27, 126.96, 127.14, 127.30, 127.41, 127.61, 127.69, 127.96, 128.28, 128.56, 129.26, 129.66, 130.06, 132.87, 133.88, 135.96, 135.99, 136.45, 136.57, 136.85, 137.55, 138.39, 139.00, 140.56, 141.13, 144.43. ²⁹Si NMR (CDCl₃, 80 MHz, ppm): δ −14.71. Anal. calcd for C₆₈H₄₄Si: C 91.85%, H 4.99%; found: C 91.28%, H 4.83%.

Results and discussion

Thermal properties of diphenylfluoranthene and its silicon-cored derivatives

The thermal properties of diphenylfluoranthene (3) and its silicon-cored derivatives (7, 8 and 9) were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen conditions.

TGA was measured with a heating rate of 10 °C min⁻¹, and is displayed in Fig. 1 and Table 1. Corresponding to 5% weight loss, decomposition temperatures (T_d) are 298 °C, 390 °C, 430 °C and 444 °C for 3, 7, 8 and 9, respectively. All these compounds possess excellent thermal stability. Moreover, all the silicon-cored diphenylfluoranthene derivatives exhibited higher thermal stability than diphenylfluoranthene. The reason could be attributed to the activity change of the H in the C8 position. This H in molecules of 3 was reactive. After the H was replaced by the Si atom, the thermal stabilities of the compounds were enhanced. Furthermore, the T_d of the silicon-cored compounds increased in proportion to the size of substitution groups at the Si atom. The data of DSC are listed in Table 1. The melting temperatures (T_m) were identified for 3, 7, 8 and 9 at 165 °C, 260 °C, 286 °C and 350 °C. The glass transition temperatures (T_g) were obtained for 3, 7, 8 and 9 at 60 °C, 150 °C, 220 °C and 250 °C, respectively. Similar to that of TGA, all the silicon-cored diphenylfluoranthene derivatives exhibited higher glass transition temperatures than diphenylfluoranthene. Compared to the blue luminescent materials, such as the fluorene and anthracene derivatives, the silicon-cored compounds in this paper showed higher thermal stabilities.^6,34 The high glass transition temperatures of the compounds were expected to give a high operational lifetime and stability for optical devices.

Fig. 1 TGA thermograms of compounds 3, 7, 8 and 9.

Table 1 Thermal properties of diphenylfluoranthene and silicon-cored derivatives. T_g: glass transition; T_m: melting temperature; T_d: decomposition temperature

Compound	Tg/°C	Tm/°C	Td/°C
3	60	165	298
7	150	260	390
8	220	286	420
9	250	350	444

---

Above all, the T_d, T_m and T_g of the aromatic chromophores could be enhanced after they were transformed to silicon-cored derivatives. Moreover, these silicon-based compounds have great potential for optical devices.

Density functional theory calculations of diphenylfluoranthene and its silicon-cored derivatives

Density functional theory (DFT) calculations have also been performed to characterize the three-dimensional geometries and the frontier molecular orbital energy levels of 3, 7, 8 and 9 at the B3LYP/6-31G* level by using the Gaussian 03 program. The calculated geometries of 3, 7, 8 and 9 are shown in Fig. 2. The diphenylfluoranthene showed two phenyl groups twisting out of the plane of the aromatic π-system and do not extend the conjugation of the core dramatically. For the silicon-cored derivatives, the two 7,10-diphenylfluoranthene units are significantly twisted against each other because of the tetrahedral environment of the central silicon core, resulting in a non-coplanar structure in each molecule. These geometrical characteristics can effectively prevent intermolecular interactions between π-systems and suppress molecular recrystallization, which improves the morphological stability of thin films of these molecules. Calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) density maps of 3, 7, 8 and 9 are also included in Fig. 2. The electron densities of the HOMOs and LUMOs are mostly localized on the diphenylfluoranthene unit, implying that the absorption and emission processes can only be attributed to the π–π* transition centered at the diphenylfluoranthene unit. As a result, the molecular orbital analysis clearly indicates that the groups at the silicon atom do not influence the blue emission nature of the diphenylfluoranthene unit at all.

Fig. 2 Optimized geometries and calculated HOMO and LUMO density maps, respectively, of (a–c) 3, (d–f) 7, (g–i) 8 and (j–l) 9.

Optical properties of diphenylfluoranthene and its silicon-cored derivatives in solution

Fig. 3(a) shows the absorption spectra of diphenylfluoranthene (3) and its silicon-cored derivatives (7, 8 and 9) in THF solution. A summary of photophysical data of the four compounds is also given in Table 2. In solution, diphenylfluoranthene exhibited three absorption peaks centered at 284, 326 and 371 nm, respectively, originating from the π–π* transitions. It is interesting to note that the data are not significantly different from those observed for the basic unit fluoranthene (289 and 360 nm). This suggests that the outer phenyl rings do not extend the conjugation of the core dramatically. This is possibly due to the twisting of the phenyl groups out of the plane of the aromatic π-system which inhibits an effective π-conjugation with the fluoranthene core. All the silicon-cored diphenylfluoranthene derivatives exhibited similar peak shapes to that of diphenylfluoranthene with a very slight red shift. This could be attributed to a very weak dπ–pπ effect of the Si atom and diphenylfluoranthene. It indicated that the silicon-cored derivatives did not extend the conjugation of diphenylfluoranthene. Meanwhile, the three silicon-cored compounds exhibited the same absorption spectra in solution. This indicated that besides the two diphenylfluoranthene groups, the other substitution groups at the Si atom did not have influence on the conjugation of the molecular structure.

Fig. 3 Absorption and fluorescence spectra of diphenylfluoranthene (3) and the silicon-cored derivatives (7, 8 and 9) in solution.

Table 2 Optical properties of diphenylfluoranthene and silicon-cored derivatives

Compound	λmax(abs)/nm	λmax(em)/nm (solution)	λmax(em)/nm (solid)
3	371, 326, 284, 250	443	509
7	375, 328, 288, 251	445	480
8	376, 328, 287, 251	445	453
9	377, 328, 289, 251	446	479

---

The fluorescence emission spectra of diphenylfluoranthene (3) and its silicon-cored derivatives (7, 8 and 9) in THF solution are shown in Fig. 3(b). The emission peak of diphenylfluoranthene in solution appears at 443 nm. The emission spectra of the silicon-cored derivatives exhibited similar phenomena to the absorption spectra. All the silicon-cored derivatives exhibited similar fluorescence emission peaks to that of diphenylfluoranthene. A slight red shift was also observed from all the silicon-cored compounds to diphenylfluoranthene. Similarly, all the three silicon-cored compounds exhibited the same fluorescence emission in solution. These data also proved the existence of a weak dπ–pπ effect.

The fluorescence emission spectra of diphenylfluoranthene (3) and its silicon-cored derivatives (7, 8 and 9) in different solvents are shown in Fig. 4. As the polarities of solvent increased, the maximum emission wavelength of diphenylfluoranthene (3) has no obvious variation and the emission intensity increased. The silicon derivatives exhibited similar properties. This indicated that the introduction of a silicon core could retain the optical properties of diphenylfluoranthene in dilute solution.

Fig. 4 The fluorescence emission spectra of 3 (a), 7 (b), 8 (c) and 9 (d) in different solvents.

The electrochemical behaviors of diphenylfluoranthene (3) and its silicon-cored derivatives (7, 8 and 9) were studied by cyclic voltammetry (CV). The HOMO and LUMO levels of four compounds were obtained according to the method in the literature.²² The HOMO energy levels (E_HOMO) are calculated from the oxidation potential (E_ox) using ferrocene as a standard reference. The HOMO levels of 3, 7, 8 and 9 are 5.58, 5.59, 5.60 and 5.59 eV below the vacuum level, respectively. The LUMO levels are estimated from the optical band-gap energy, which is obtained from the onset of the absorption edge. The optical band-gap energies of 3, 7, 8 and 9 are 3.85, 3.88, 3.90 and 3.87 eV, respectively. The LUMO levels of 3, 7, 8 and 9 are 1.73, 1.71, 1.70 and 1.72 eV, respectively. The HOMO and LUMO energy levels were nearly same as those of diphenylfluoranthene (3) and its silicon-cored derivatives (7, 8 and 9). Therefore, the silicon-cored structure derivatives retained the optical band-gap energy level of HOMO and LUMO of diphenylfluoranthene.

The fluorescence emission spectra of diphenylfluoranthene (3) and its silicon-cored derivatives (7, 8 and 9) in CH₂Cl₂ (DCM) solution with different concentrations were shown in Fig. 5. For 3, as the concentration increased, the emission raised at first. When the concentration was higher than 2 × 10⁻³ mol L⁻¹, the emission intensity began to fail. When the concentration was 2 × 10⁻² mol L⁻¹, the emission spectra showed a red shift. The similar silicon-cored derivatives 7 and 9 exhibited the similar phenomenon. However, the silicon-cored derivative 8 showed no obvious red shift at 10⁻² mol L⁻¹. The difference could be attributed to the distinction of the aggregation states. A detailed explanation could be found in the discussion of the optical properties of diphenylfluoranthene and its silicon-cored derivatives in solid state. Specifically, when the concentration of diphenylfluoranthene groups were all the same at 2 × 10⁻² mol L⁻¹ for 3, 7, 8 and 9, the emission intensities of the silicon-cored derivatives were all higher than that for diphenylfluoranthene.

Fig. 5 The fluorescence emission spectra of 3 (a), 7 (b), 8 (c) and 9 (d) in CH₂Cl₂ solutions with different concentrations.

In summary, compared to the diphenylfluoranthene aromatic compound, the absorption and emission spectra of its silicon-cored derivatives exhibited no significant difference from those observed for the diphenylfluoranthene aromatic compounds. Besides the two diphenylfluoranthene aromatic groups, the other substitution groups at Si atom did not have influence in the conjugation of the molecular structure.

Optical properties of diphenylfluoranthene and its silicon-cored derivatives in solid state

The fluorescence emission spectra of diphenylfluoranthene (3) and its silicon-cored derivatives (7, 8 and 9) in the solid state were also measured, and are shown in Fig. 6. The emission spectra of diphenylfluoranthene in the solid state exhibited a significant difference from that observed in solution. The emission peak in the solid state shifted to 509 nm. Compared to that at 443 nm in solution, the emission spectra in the solid state showed a significantly red-shifted and broad emission band. This could be attributed to the extensive excimer formation caused by the close-packing and intermolecular interaction of the molecules.

Fig. 6 Fluorescence spectra of diphenylfluoranthene (3) and the silicon-cored derivatives (7, 8 and 9) in the solid state.

The PL spectra of the silicon-cored derivatives in the solid state also exhibited a red-shift compared with those in solution owing to the solid-state effect. Differently, the PL spectra of all the silicon-cored derivatives showed shorter wavelengths of red shift than that of diphenylfluoranthene. This indicated that the silicon-cored compounds had weaker intermolecular interactions than the aromatic compounds. The tetrahedron structure of silicon-based compounds could effectively prevent the close-packing of constituent molecules and thus effectively shield the intermolecular interactions.

For the silicon-cored derivatives with different substituting groups at the Si atom, the emission peaks of 7 and 9 shifted to 480 nm and 479 nm. Compared with the PL spectra observed in solution, the fluorescence emission spectra showed an obvious red shift of about 30 nm in solid state. Interestingly, the PL spectra of 8 exhibited no obvious red shift between the solid state and the solution. The emission peak appeared at 453 nm, which was very similar to that at 445 nm in solution. It could be found that the substituting groups at the Si atom had a great influence in the fluorescence emission spectra. These interesting properties could be attributed to the influence of the different substituting groups at the Si atom. This could be further investigated according to the single crystal structures of diphenylfluoranthene and its silicon-cored derivatives.

Crystal structures of diphenylfluoranthene and its silicon-cored derivatives

The crystal structures of diphenylfluoranthene and its silicon-cored derivatives were further investigated to explain their optical properties.

As shown in Fig. 7(a), in contrast to fluoranthene molecules, 3 has two peripheral phenyl groups with torsional angles (50.2° and 58.0°) to the main plane of the molecule, in which the strong intermolecular π–π interaction generally observed in the solid state of such planar molecules could be slightly weakened. The π–π distance between two central planes of two adjacent molecules was 4.0 Å and the C–H⋯π distance of two adjacent molecules was 2.67–3.19 Å. The molecular cohesion in crystals of 3 is dominated by special C–H⋯π interactions and weak π–π interactions involving the central aromatic ring. In this packing mode, the planes of adjacent molecules were parallel. As a result, the fluorescence of 3 in the solid state has a large red shift in contrast to that in solution.

Fig. 7 The single crystal structures of diphenylfluoranthene and the silicon-cored derivatives (3, 7, 8 and 9).

The structures of silicon-cored diphenylfluoranthene derivatives were further examined. Such silicon-cored compounds of 7, 8 and 9 have characteristic nonplanar toroidal topologies which were quite different from the diphenylfluoranthene molecules. First, the torsional angles between the central plane and peripheral phenyl groups become larger in silicone-cored compounds. Two phenyl groups were almost perpendicular to the main plane of the molecules. Therefore, the π–π distance between two parallel central planes of two adjacent molecules was much larger than 4.0 Å. This indicated that the tetrahedron structure of silicon-cored compounds could effectively prevent the π–π interactions which were generally observed in the solid state. Secondly, the molecular cohesions in crystals of 7, 8 and 9 are dominated by weak van der Waals forces and special C–H⋯π interactions involving the central aromatic ring. Therefore, the fluorescence of all silicon-cored derivatives in the solid state has a smaller red shift in contrast to that of diphenylfluoranthene.

In three silicon-cored diphenylfluoranthene derivatives, the different substituent groups which were attached to Si atom also have great influence on the crystal structures. The molecular structures of the silicon-cored diphenylfluoranthene derivatives in the crystal were extremely different from those of theoretical calculations. Because the other two substituent groups attached to the Si atom were same, crystals of 7 and 9 exhibited symmetrical structures. As shown in Fig. 7(b) and (d), the two fluoranthene planes at the same Si atom showed a “V” shape with large angles, 99° for 7 and 83° for 9. The torsional angles of the central plane and peripheral phenyl groups become larger in silicone-cored compounds. Interestingly, crystals of 8 exhibited a great difference to those of 7 and 9. The other two substituent groups attached to the Si atom were different and the molecular structure showed a twisty “V” shape in contrast to that of 7 and 9. As shown in Fig. 7(c), the diphenylfluoranthene group was approximately perpendicular to the other diphenylfluoranthene group in one molecule of 8. The reason for the twisty structure formation could be attributed to the intramolecular CH⋯π interactions. Because of this unique structure, the intermolecular π–π interactions of 8 molecules in the solid state were greatly weakened. Therefore, the photoluminescence spectra of 8 exhibited no obvious red shift from the solid state to solution.

Conclusion

Diphenylfluoranthene and new silicon-cored diphenylfluoranthene derivatives were synthesized using novel and convenient ways. Systematic structural analysis and comparison of properties of silicon-cored materials and common structural materials were discussed. The silicon-cored compounds exhibited higher thermal stabilities and glass transition temperatures than did diphenylfluoranthene. In solution, diphenylfluoranthene and silicon-cored derivatives showed similar optical properties, which indicated that the silicon-cored structure could retain the nature of the properties of diphenylfluoranthene. Particularly, due to the steric hindrance of the silicon-cored structure, all the silicon-cored derivatives exhibited weaker π–π molecular interactions and lesser bathochromic shifts of fluorescence emission in the solid state than in diphenylfluoranthene. Moreover, the substituting groups at the Si atom exhibited great influence on the molecular structures and resulted in different optical properties. The most obvious candidate for an efficient solid-state emitter was the methylphenyl silicon-cored diphenylfluoranthene derivative, which exhibits similar fluorescence emission spectra in solution and in the solid state. We hope the study in this paper can provide some guidance on the molecular design of silicon-cored planar materials in optical applications.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21274080 and 21204043), the Key Natural Science Foundation of Shandong Province of China (no. ZR2011BZ001 and ZR2009BZ006) and the Doctor Station Foundation of Chinese Education Department (no. 200603086).

Notes and references

1. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
2. K. S. Yook and J. Y. Lee, Adv. Mater., 2012, 24, 3169.
3. M. Zhu and C. Yang, Chem. Soc. Rev., 2013, 42, 4963.
4. F. Liu, C. Tang, Q.-Q. Chen, F.-F. Shi, H.-B. Wu, L.-H. Xie, B. Peng, W. Wei, Y. Cao and W. Huang, J. Phys. Chem. C, 2009, 113, 4641.
5. D. Kumar, K. R. J. Thomas, C.-C. Lin and J.-H. Jou, Chem. – Asian J., 2013, 8, 2111.
6. Y.-Y. Lyu, J. Kwak, O. Kwon, S.-H. Lee, D. Kim, C. Lee and K. Char, Adv. Mater., 2008, 20, 2720.
7. W. J. Oldham, R. J. Lachicotte and G. C. Bazan, J. Am. Chem. Soc., 1998, 120, 2987.
8. D. Kumar, K. R. J. Thomas, Y.-L. Chen, Y.-C. Jou and J.-H. Jou, Tetrahedron, 2013, 69, 2594.
9. F. May, M. Al-Helwi, B. Baumeier, W. Kowalsky, E. Fuchs, C. Lennartz and D. Andrienko, J. Am. Chem. Soc., 2012, 134, 13818.
10. S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma, Adv. Funct. Mater., 2011, 21, 1168.
11. W. Wei, P. I. Djurovich and M. E. Thompson, Chem. Mater., 2010, 22, 1724.
12. S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin and D. Ma, Adv. Mater., 2010, 22, 5370.
13. D.-R. Bai, X.-Y. Liu and S. Wang, Chem. – Eur. J., 2007, 13, 5713.
14. L.-H. Chan, R.-H. Lee, C.-F. Hsieh, H.-C. Yeh and C.-T. Chen, J. Am. Chem. Soc., 2002, 124, 6469.
15. L. H. Chan, H. C. Yeh and C. T. Chen, Adv. Mater., 2001, 13, 1637.
16. H. Wang, H. Xie, Y. Liang, L. Feng, X. Cheng, H. Lu and S. Feng, J. Mater. Chem. C, 2013, 1, 5367.
17. H. Wang, Y. Liang, L. Feng, H. Xie, J. Zhang, X. Cheng, H. Lu and S. Feng, Dyes Pigm., 2013, 99, 284.
18. S. H. Tucker and M. Whalley, Chem. Rev., 1952, 50, 483.
19. I. B. Berlman, H. O. Wirth and O. J. Steingraber, J. Am. Chem. Soc., 1968, 90, 566.
20. Y. Zhou, L. Ding, K. Shi, Y.-Z. Dai, N. Ai, J. Wang and J. Pei, Adv. Mater., 2012, 24, 957.
21. R. C. Chiechi, R. J. Tseng, F. Marchioni, Y. Yang and F. Wudl, Adv. Mater., 2006, 18, 325.
22. Y.-H. Lee, T.-C. Wu, C.-W. Liaw, T.-C. Wen, T.-F. Guo and Y.-T. Wu, J. Mater. Chem., 2012, 22, 11032.
23. J. Xu, J. Hou, S. Zhang, Q. Xiao, R. Zhang, S. Pu and Q. Wei, J. Phys. Chem. B, 2006, 110, 2643.
24. Q. Yan, Y. Zhou, B.-B. Ni, Y. Ma, J. Wang, J. Pei and Y. Cao, J. Org. Chem., 2008, 73, 5328.
25. N. Kapoor and K. J. Thomas, New J. Chem., 2010, 34, 2739.
26. T. Murahashi, N. Kato, T. Uemura and H. Kurosawa, Angew. Chem., Int. Ed., 2007, 119, 3579.
27. X. Ma, W. Wu, Q. Zhang, F. Guo, F. Meng and J. Hua, Dyes Pigm., 2009, 82, 353.
28. Y. Liu, Y. Lv, X. Zhang, S. Chen, J. W. Y. Lam, P. Lu, R. T. K. Kwok, H. S. Kwok, X. Tao and B. Z. Tang, Chem. – Asian J., 2012, 7, 2424.
29. A. Goel, V. Kumar, S. Chaurasia, M. Rawat, R. Prasad and R. S. Anand, J. Org. Chem., 2010, 75, 3656.
30. Z.-Y. Xia, J.-H. Su, H.-H. Fan, K.-W. Cheah, H. Tian and C. H. Chen, J. Phys. Chem. C, 2010, 114, 11602.
31. T. M. Figueira-Duarte, P. G. D. Rosso, R. Trattnig, S. Sax, E. J. W. List and K. Müllen, Adv. Mater., 2010, 22, 990.
32. U. Krüerke, J. Organomet. Chem., 1970, 21, 83.
33. A. B. Holmes and C. N. Sporikou, Org. Synth., 1987, 65, 61.
34. K.-T. Wong, Y.-Y. Chien, R.-T. Chen, C.-F. Wang, Y.-T. Lin, H.-H. Chiang, P.-Y. Hsieh, C.-C. Wu, C. H. Chou, Y. O. Su, G.-H. Lee and S.-M. Peng, J. Am. Chem. Soc., 2002, 124, 11576.

---

Footnote

† Electronic supplementary information (ESI) available: The ¹H NMR, ¹³C NMR and ²⁹Si NMR spectra of 3, 7, 8 and 9. CCDC 971315–971318 contain the supplementary crystallographic data for compounds 3, 7, 8 and 9. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ra47751g

---