Design,synthesis,and photoelectric properties of V-shaped organic fluorescent compounds with a 1,3,4-oxadiazole moiety

Abstract

A series of symmetric, silicon-linked organic fluorescent compounds with two electron-deficient 1,3,4-oxadiazole units were synthesized and characterized. The compounds possessed a V-shaped structure with a silicon atom, which weakened π–π stacking, promoting aggregation-induced emission. The compounds were fluorescent in both solution and solid-state thin films. The efficient fluorescent behavior of the materials was confirmed through optical and electrochemical measurements. The compounds displayed excellent thermal stability, with decomposition temperatures exceeding 400 °C. Amorphous films of the compounds possessed high morphological stability. These results indicate that the compounds may be promising emissive and electron-transporting materials.

Introduction

Organic fluorescent materials have been the subject of great interest because of their potential applications in many aspects of modern life, such as lighting, displays, bioprobes, and chemosensors.^1–5 Generally, the luminescence properties of organic molecules in the solid state are determined mainly by their chemical structures. Most fluorescent organic molecules with highly conjugated planar structures emit reasonably intense fluorescence in dilute solutions but emit weak or nonexistent emission in the solid state because of aggregation-induced quenching.^6–9 However, some groups have recently reported the unique phenomenon of aggregation-induced emission (AIE).^10,11 Propeller-shaped molecules such as tetraphenylethene, hexaphenylsilole, and quinoline–malononitrile, along with their derivatives, are known to display AIE.^12–17 Molecular design optimization can be used to increase molecular luminous efficiency and stability, leading to organic light-emitting diodes (OLEDs) with improved optoelectronic properties.^18–21

Considering all organic luminescent materials, silicon (Si)-based compounds have been reported to perform well in optical applications because of their high brightness, thermal stability, and amorphous film-forming capability.^22–24 Si-based tetrahedral organic molecules and polymers have been intensively investigated as electroluminescent materials. For example, Partee et al. ²⁵ and Zhen et al. ²⁶ recently reported a series of tetrahedral luminescent materials with SiAr₄ cores. Meanwhile, 1,3,4-oxadiazoles are classic heterocyclic compounds that have also attracted substantial interest for use in OLEDs because of their favorable electronic transmission properties. Many kinds of oxadiazole-based materials have been synthesized and used in different applications, and some are considered to be potential blue emitters for OLEDs.^27,28

Results and discussion

Continuing our interest in V-shaped luminescent materials,^29,30 in this article, we report the development of a series of V-shaped fluorescent molecules (1a–d) containing a naphthalene-linked 1,3,4-oxadiazole and an Si core atom, which weakens π–π stacking interactions. The length of the alkoxy chain attached to naphthalene was changed to study the effect on the photochemical and electrochemical properties of the materials in solution and in the solid state.

Starting compounds 2–5 were synthesized according to the reported methods.^31,32 The synthetic procedure followed to obtain target compound 1a–d is summarized in Scheme 1, and the details are given in the section “Experimental.” An important intermediate (5) with a silylbenzohydrazide moiety was prepared quantitatively by the reaction of 4 with excess hydrazine monohydrate under reflux. The reaction of 5 and 2-naphthoic acid 6a or 6-alkoxy-2-naphthoic acid 6b–d in phosphorus oxychloride under reflux gives compound 1a–d as white crystals in yields of 32%–39%. The structures of 1a–d were characterized by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and elemental analysis.

Scheme 1.

Synthetic route of 1a–d.

The ultraviolet (UV)-Vis absorption properties of compounds 1a–d in CHCl₃ solution are presented in Figure 1. The absorption spectra contain a wide peak in the region of 315–330 nm originating from the π–π^* absorption of the conjugated naphthalene-phenyl 1,3,4-oxadizole moiety. The absorption spectra of compounds 1b–d possessing alkoxy groups displayed bathochromic shifts of 15 nm compared with that of compound 1a without a substituent. These results illustrate that the alkoxy chain can lower the energy of the π–π^* transition of the molecules. The optical band gaps $(E_{\mathrm{g}}^{\mathrm{opt}})$ of the series were calculated from the relevant absorption onsets and found to be 3.88, 3.76, 3.76, and 3.76 eV, respectively. ³³

Figure 1.

UV-Vis absorption spectra of compounds 1a–d in CHCl₃ solution.

The photoluminescence (PL) spectra of compounds 1a–d in CHCl₃ solution and in solid-state films obtained by spin-coating a CHCl₃ solution are shown in Figure 2. The PL spectrum of compound 1a in CHCl₃ contains two emission peaks at around 340 and 400 nm (Figure 2(a)). The emission spectra of compounds 1b–d in CHCl₃ also contain one similar broad peak. Interestingly, the emission maxima of compounds 1b–d are red-shifted to 382 nm compared with that of 1a because of the alkoxy chain substituents. The fluorescence quantum yields (QYs) of 1a–d measured at room temperature in dilute CHCl₃ were moderate with the values of 0.71, 0.72, 0.72, and 0.72, respectively, compared with the PL intensity of a standard solution of quinine sulfate (QY = 0.55). The emission spectra of the films were quite different to those for the solutions with regard to the two main emission peaks, as demonstrated in Figure 2(b). The emission maxima of compounds 1b–d red-shifted with increasing length of the alkyl chain. The intensity of one of the emission peaks increased, while that of the other decreased as the alkyl chain lengthened. This phenomenon may originate from the dihedral angle of the molecules increasing with the length of the alkoxy chain.

Figure 2.

Normalized PL spectra of compounds 1a–d in (a) CHCl₃ and (b) as films.

The electrochemical properties of 1a–d in dimethylformamide (DMF) were investigated by cyclic voltammetry (CV) as summarized in Figure 3 and Table 1. Figure 3 displays representative CV traces for the oxidation and reduction of 1c. The CV curves reveal that 1c undergoes a quasi-reversible reduction at the cathodic potential associated with the reduction of an electron-deficient oxadiazole moiety to form an anion radical. Compound 1c also exhibits an irreversible oxidation process corresponding to the removal of an electron from the peripheral phenyl group to give a radical cation.

Figure 3.

CV of 1c (1 × 10⁻³ M) in 0.1 M tetrabutylammonium hexafluorophosphate in CH₂Cl₂ at a scan rate of 0.1 V s⁻¹ (inset: differential pulse voltammogram of 1c under the same conditions).

Table 1.

Photophysical, physical, and electrochemical data.

Compound	a Amax,Abs (nm)	a λmax,em (nm)	ΦF	Ered/peak (V)	$E_1^{\mathrm{ox/peak}}$ (V)	$E_{\mathrm{g}}^{\mathrm{opt}}$	$E_{\mathrm{g}}^{\mathrm{Cal}}$	T d	T g
1a	316	366	0.712	−0.65	1.36	3.98	4.43	416	87.2
1b	329	382	0.715	−0.88	1.53	3.83	4.15	409	99.1
1c	330	382	0.716	−0.89	1.53	3.82	4.15	406	95.8
1d	330	382	0.716	−0.89	1.51	3.82	4.16	402	92.6

Φ_F is the fluorescence quantum yield, which was measured in CHCl₃ using quinine sulfate as a standard (Φ_F = 0.55); ³⁴ $E_{\mathrm{g}}^{\mathrm{opt}}$ is the optical band gap, which was calculated using E_g = 1240/λ_absonset; $E_{\mathrm{g}}^{\mathrm{Cal}}$ is the calculated energy gap; T_d represents the temperature at which 5% weight loss occurred; T_g represents the glass transition temperature.

a

Measured in CHCl₃.

Calculations of 1a–d were carried out using the Gaussian 09 (G09) package. ³⁵ GaussView 5.0.8 was used for the visualization of the structures and orbital manipulation. The calculated highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and energy gap for each of 1a–d are displayed in Figure 4. The calculated energy gaps agreed well with the experimental photophysical data, suggesting that density functional theory (DFT) calculations can be used to predict the photophysical behavior of these complexes to some extent. The electron density of the HOMO of 1a–d is predominantly localized on the naphthalene donor moiety and diffusively distributed over the π-conjugation onto the 1,3,4-oxadiazole ring.

Figure 4.

Contour plots of the HOMO and LUMO for complexes 1a–d.

The thermal stability of a material is important for device longevity. Figure 5 and Table 1 show the results of thermogravimetric analysis (TGA) of compounds 1a–d. The TGA analysis revealed that the compounds possessed relatively high thermal stability with an initial weight loss (5%) temperature of 400–434 °C. The glass transition temperature (T_g) values of 1a–d were in the range of 87–100 °C. The thermal stabilities of the compounds progressively decreased as the substituent alkoxy chain lengthened. The high thermal stability of 1a–d would benefit the formation of amorphous films.

Figure 5.

Thermal gravimetric analysis and differential scanning calorimetry thermograms of 1b.

In summary, we developed a facile and effective method to synthesize V-shaped organic fluorescent compounds. These synthesized compounds do not aggregate in the condensed state as shown by their similar properties either in solution or in film states. These emitters are solution processable and exhibit good film-forming ability as well as high thermal stability, making them attractive for use in OLEDs.

Experimental

NMR spectra were recorded in CDCl₃ with a Bruker AV-400 spectrometer. Chemical shifts were referenced relative to tetramethylsilane (δ_H/δ_C = 0). Infrared (IR) spectra were recorded on a Perkin Elmer 2400 spectrophotometer (using KBr pressed disks). Mass spectra were collected on an Autoflex Speed^™ matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) spectrometer. Elemental analyses were performed on an Elementar Vario EL CHN elemental analyzer. UV-Vis spectra were recorded on a Lambda 25 spectrophotometer in CHCl₃. Fluorescence spectra were obtained from a Shimadzu RF-5301PC fluorescence spectrophotometer in CHCl₃. CV was conducted using a CHI 852C instrument with 10⁻³ M solutions using CH₂Cl₂ as the solvent (10⁻³) and containing 0.1 M Bu₄NPF₆ as the supporting electrolyte. Counter and working electrodes consisted of a Pt wire and a Pt disk, respectively, and the reference electrode was Ag/AgCl. The thermal stability of the target compounds was characterized using a Diamond TG/DTA thermogravimetric analyzer. Starting compounds 2–5 were synthesized according to the methods described in the literature.^31,32 The commercially available 6-alkoxy-2-naphthoic acids 6b–d were purchased from J&K Chemical Company.

General procedure for the preparation of compounds 1a–d

A mixture of the appropriate 2-naphthoic acid 6a–d (2 mmol), 4,4′-(dimethylsilanediyl)di(benzohydrazide) (1 mmol), and phosphorus oxychloride (10 mL) was heated under reflux for 16 h under N₂, then cooled to room temperature, and poured onto crushed ice. The resulting dark green solid precipitate was filtered and then washed thoroughly with distilled ice water. The solid was recrystallized from DMF/H₂O to obtain the pure final compound.

Dimethylbis{4-[5-(naphthalen-2-yl)-1,3,4-oxadiazol-2-yl]phenyl}silane (1a): White solid (Yield: 32%), m.p. 249.60 °C (by differential scanning calorimetry (DSC)); ¹H NMR (400 MHz, CDCl₃): δ 0.69 (s, 6H), 7.55–7.64 (m, 4H), 7.74 (d, J = 8.0 Hz, 4H), 7.88–7.94 (m, 2H), 8.00 (d, J = 8.0 Hz, 4H), 8.15–8.25 (m, 6H), 8.64 (s, 2H); ¹³C NMR (100 MHz, CDCl₃): δ −2.50, 121.38, 123.45, 124.92, 126.39, 127.31, 127.56, 128.18, 129.03, 129.26, 133.09, 134.95, 134.98, 142.52, 164.85, 165.09; IR (KBr), ν_max/cm⁻¹: 2968, 1626, 1497, 1408, 1346, 1292, 1250, 1118, 1079, 830, 779; MALDI-TOF MS m/z (%) = 601.1372 (M⁺ + 1, 100); anal. calcd for C₃₈H₂₈N₄O₂Si: C, 75.97; H, 4.70; found: C, 75.62; H, 4.76%.

Bis{4-[5-(6-ethoxynaphthalen-2-yl)-1,3,4-oxadiazol-2-yl]phenyl}dimethylsilane (1b): White solid (Yield: 35%), m.p. 227.21 °C (by DSC); ¹H NMR (400 MHz, CDCl₃): δ 0.67 (s, 6H), 1.50 (t, J = 6.8 Hz, 6H), 4.18 (q, J = 6.8 Hz, 4H), 7.16 (d, J = 2.0 Hz, 2H), 7.21 (dd, J = 8.9, 2.3 Hz, 2H), 7.71 (d, J = 8.0 Hz, 4H), 7.85 (t, J = 9.8 Hz, 4H), 8.11–8.19 (m, 6H), 8.53 (s, 2H); ¹³C NMR (100 MHz, CDCl₃): δ −2.50, 14.89, 63.86, 106.90, 119.01, 120.49, 123.99, 124.99, 126.31, 127.34, 127.88, 128.40, 130.52, 134.95, 136.55, 142.37, 158.90, 164.57, 165.27; IR (KBr), ν_max/cm⁻¹: 2981, 1629, 1555, 1543, 1504, 1399, 1259, 1209, 1115, 1045, 811, 779, 731; MALDI-TOF MS m/z (%) = 689.1630 (M⁺ + 1, 100); anal. calcd for C₄₂H₃₆N₄O₄Si: C, 73.23; H, 5.27; found: C, 73.19; H, 5.32%.

Bis{4-[5-(6-butoxynaphthalen-2-yl)-1,3,4-oxadiazol-2-yl]phenyl}dimethylsilane (1c): White solid (Yield: 39%), m.p. 214.16 °C (by DSC); ¹H NMR (400 MHz, CDCl₃): δ 0.68 (t, 6H), 1.02 (t, J = 7.2 Hz, 6H), 1.49–1.62 (m, 4H), 1.81–1.92 (m, 4H), 4.12 (t, J = 6.8 Hz, 4H), 7.18 (d, J = 1.6 Hz, 2H), 7.23 (dd, J = 8.8 Hz, 2.4 Hz, 2H), 7.72 (d, J = 8.0 Hz, 4H), 7.86 (t, J = 9.8 Hz, 4H), 8.13–8.21 (m, 6H), 8.55 (s, 2H); ¹³C NMR (100 MHz, CDCl₃): δ −2.49, 13.99, 19.46, 31.40, 68.11, 106.92, 119.00, 120.52, 124.00, 125.02, 126.32, 127.34, 127.87, 128.39, 130.49, 134.95, 136.58, 142.36, 159.13, 164.57, 165.29; IR (KBr), ν_max/cm⁻¹: 2962, 1635, 1559, 1500, 1394, 1275, 1210, 1005, 810, 736; MALDI-TOF MS m/z (%) = 745.2318 (M⁺ + 1, 100); anal. calcd for C₄₆H₄₄N₄O₄Si: C, 74.17; H, 5.95; found: C, 74.13; H, 5.98%.

Bis{4-[5-(6-hexyloxynaphthalen-2-yl)-1,3,4-oxadiazol-2-yl]phenyl}dimethylsilane (1d): White solid (Yield: 39%), m.p. 199.65 °C (by DSC); ¹H NMR (400 MHz, CDCl₃): δ 0.68 (s, 6H), 0.92 (t, J = 7.2 Hz, 6H), 1.32–1.42 (m, 8H), 1.45–1.53(m, 4H), 1.79–1.91 (m, 4H), 4.10 (t, J = 6.8 Hz, 4H), 7.16 (d, J = 2.4 Hz, 2H), 7.22 (dd, J = 9.0, 2.4 Hz, 2H), 7.71 (d, J = 8.0 Hz, 4H), 7.85 (t, J = 9.6 Hz, 4H), 8.15–8.19 (m, 6H); 8.53(s, 2H); ¹³C NMR (100 MHz, CDCl₃): δ −2.49, 14.17, 22.76, 25.93, 29.31, 31.75, 68.43, 106.93, 119.00, 120.53, 124.00, 125.02, 126.32, 127.34, 127.87, 128.39, 130.49, 134.95, 136.58, 142.36, 159.12, 164.57, 165.29; IR (KBr), ν_max/cm⁻¹: 2967, 1629, 1554, 1499, 1389, 1265, 1210, 1067, 1025, 815, 736; MALDI-TOF MS m/z (%) = 801.3242 (M⁺ + 1, 100); anal. calcd for C₅₀H₅₂N₄O₄Si: C, 74.97; H, 6.54; found: C, 74.92; H, 5.71%.

The ¹H and ¹³C NMR spectra, TOF-MS data, cyclic voltammograms, and TGA data for 1a -d in detail can be found in Supplemental material.

Supplemental Material

Supplementary_info – Supplemental material for Design, synthesis, and photoelectric properties of V-shaped organic fluorescent compounds with a 1,3,4-oxadiazole moiety

Supplemental material, Supplementary_info for Design, synthesis, and photoelectric properties of V-shaped organic fluorescent compounds with a 1,3,4-oxadiazole moiety by Rui-Bin Hou, Ji-Ying Su, Ling-Ling Zhang, Dong-Feng Li and Yan Xia in Journal of Chemical Research