A series of linear π conjugated arylene oligomers based on 1,5-naphthalene subunit: synthesis, characterization, and properties

A series of novel alkoxy-substituted arylene-ethynylene oligomers based on 1,5-naphthalene subunit have been successfully synthesized by palladium-catalyzed cross-coupling reaction. The solubility, thermal, optical, and electrochemical properties have been investigated in detail. The title compounds are soluble in common organic solvents and emit a blue color both in solution and in the solid state, indicating that these oligomers might serve as potential materials in optoelectronic devices.


Introduction
The synthesis and characterization of nanometer-sized conjugated molecules of precise length and constitution are of widespread interest, due to their inherent synthetic flexibility which permits the design of molecular architectures with important properties. [1,2] Molecules with π-extended conjugation exhibit high thermal stability and can present electroconductive, magnetic, and optical properties. [3][4][5] Arylene derivatives with extended π-conjugated systems, such as π-conjugated arylene oligomers, have triggered intense investigations due to their potential applications in photonics and optoelectronics, such as organic light-emitting diodes (OLEDs), solar cells, and field-effect transistors (FETs). [6][7][8][9][10] Those materials offer the possibility of tuning both the characteristics of the emitted light and the efficiency of the devices by means of simple chemical modifications of their structures. The three most common strategies for chemical modification are as follow: (a) the introduction of lateral chains that can improve the solubility of the compounds and effect the conjugation of the system by steric hindrance, [11] (b) the introduction of electron-withdrawing substituents which increase the electron affinity of the molecules, [12,13] and (c) the replacement of the phenylene ring by other aromatic structures. [14][15][16] Naphthalene-containing conjugated oligomers are a unique class of electrically active materials. [17] In particular, conjugated polymers incorporating the naphthalene units through the 1,5-positions have shown electroluminescence properties. [18] As is well known, stable blue luminescent compounds are rare and very much in demand, due to their extensive applications in electroluminescent devices. Our previous study demonstrated that linear molecules based on α-naphthal ethynylphenyl subunit exhibited blue luminescence with high solution emission efficiency. [19] However, there were no further investigations concerning corresponding oligomers.
Herein, we synthesized a set of novel oligomers based on 1,5-naphthalene subunit, and systematically studied their solubility property, thermal property, optical property, and electrochemical property.

Instruments and measurement
Melting points were determined on XRC melting point apparatus and uncorrected. FTIR spectra were recorded on a PK1600 FTIR-type spectrophotometer using KBr pellets. 1 H NMR spectra were recorded on Bruker instruments (400 MHz). The chemical shifts were recorded in ppm relative to tetramethylsilane and with the solvent resonance as the internal standard. Data were reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet), coupling constants (Hz), and integration. 13 C NMR data were collected on Bruker instruments (100 MHz) with complete proton decoupling. Chemical shifts were reported in ppm from the tetramethylsilane with the solvent resonance as internal standard. MS and elemental analysis were recorded on Q-TOF-Premier and Euro EA 3000 apparatus, respectively. The UV-Vis absorption spectra were measured on Hitachi U-4100 UV-Vis-NIR scanning spectrophotometer. The PL emission spectra were recorded on a Perkin Elmer LS55 fluorescence spectrophotometer at 298 K. Differential scanning calorimetry (DSC) analysis was carried out using a TA Instrument DSC Q100 under nitrogen atmosphere (sample purge flow 50 mL/min). Thermogravimetric analysis (TGA) was performed on a TA instrument Q50 series analyzer system under nitrogen atmosphere (sample purge flow 60 mL/min). Cyclic voltammetry (CV) measurement was carried out on a PARSTAT 2273 electrochemical workstation at 298 K. 0.1 M tetrabutylammonium perchlorate (TBAClO 4 ) dissolved in acetonitrile was used as electrolyte solution at a scanning rate of 50 mV s −1 . The CV system was constructed using a platinum plate, a Ag/AgCl (0.1 M in acetonitrile) electrode, and a platinum wire as the working electrode, quasi-reference electrode, and counter electrode, respectively.
The fluorescence quantum yields (Φ) were determined by using quinine sulfate in 0.5 M H 2 SO 4 as the reference according to the literature method. [20,21] Quantum yields were corrected as follows: where the s and r indices designate the sample and reference samples, respectively, A is the absorbance, η is the average refractive index of appropriate solution, and D is the integrated area under the corrected emission spectrum.

Preparation of intermediates via base-promoted deprotection
To a solution of diol (1.0 mmol) in the mixture solvents of toluene (30 mL) and THF (10 mL) was added potassium hydroxide (6.0 eq.). After refluxed for around 10 h (monitored by TLC), the mixture was poured into water. The organic layer was successively washed with H 2 O, diluted HCl, and H 2 O, respectively. The solvent was removed under reduced pressure to give the crude product.

Synthesis and characterization
A variety of methods have been reported for the synthesis of π-conjugated arylene derivatives. [3] Cross-coupling reactions, such as Sonogashira coupling reactions, are the most versatile tools for construction of π-conjugated molecules containing two different units connected via acetylene linkers. [25] Therefore, we employ the Sonogashira coupling reactions and base-promoted deprotections to synthesize two types of oligomers with various molecular sizes (trimers 6 and dimers 10). The synthetic routes are illustrated in Schemes 1 and 2, respectively.
To obtain oligomers with various sizes, we employed two different diyne (1 and 7) as starting materials, which were prepared according to literatures. [26,27] In the synthesis of 6, Pd/Cu-catalyzed coupling of 1 and 4-(5iodonaphthalen-1-yl)-2-methylbut-3-yn-2-ol provided 2 with relative excellent yields, which was followed by a base-promoted deprotection to afford 3. Similar procedures were performed step by step from 3 to 4, which was then transformed into 5. Treatment of 5 with 1-iodonaphthalene in the presence of Pd(PPh 3 ) 2 Cl 2 , PPh 3 , CuI, Et 3 N, and THF afforded 6 in moderate yields after column chromatography. Similarly, dimer 10 was synthesized through Sonogashira coupling reactions and basepromoted deprotections in moderate yields. The title oligomers and their intermediates were characterized by IR, 1 H NMR, 13 C NMR, MS, and elemental analysis, respectively (vide Supplementary information). In addition, we have also tried to synthesize unsubstituted oligomers as comparative investigation, but the low solubility together with the low yields of corresponding intermediates makes it extremely difficult to synthesize these parent molecules.

Solubility properties
The solubility of the title oligomers is related to their film-forming ability. Harmless, low-cost, and power-dissolving solvent might lead to excellent organic film. [28] Therefore, the solubilities of title oligomers were investigated in common solvents. Ten milligrams of samples of title compounds were added to 1 mL various solvents and the results are summarized in Table 1.
As reported in our previous work, 1,4-di(1-naphthalethynyl)benzene was not soluble in most organic solvents except for chloroform. However, their derivatives with alkoxy lateral chains were soluble in most organic solvents. [19] Therefore, we synthesize the oligomers with various alkoxy side chains in their backbones. Afterwards, the title oligomers are not only soluble in lowpolar solvents, such as cyclohexane, chloroform, and THF, but also have some solubility in high-polar or nonpolar solvent, such as hexane, dioxane, ethyl acetate, acetone, and DMF. The solubilities of the title oligomers are improved with the increasing number of carbon atoms in lateral chains. In general, the introduction of alkoxy side chains is beneficial for the title oligomers to dissolve in common organic solvents.

Thermal properties
The thermal stabilities of the title oligomers were examined by TGA under nitrogen atmosphere. As shown in Figure S2, the decomposition temperatures (T d ) with 5% loss of initial weight were all above 200°C, indicating good thermal stability of the title oligomers. As compared with trimers (6a and 6b, Table 2), dimers (10a and 10b, Table 2) have higher decomposition temperatures, due to the higher thermostability of dimers than trimers. [29] The phase transitions of the oligomers were studied by DSC. Figure S3 shows the second heating and the first cooling DSC curves of the oligomers. In the second heating curves, all the oligomers were found to have only one endothermal peak, which was detected to be 169, 60, 172, and 135°C for 6a, 6b, 10a, and 10b, respectively (as shown in Table 2). A similar phenomenon could be observed on the first cooling scan, where the exothermal peaks were found to be 134, 57, 155, and 133°C for 6a, 6b, 10a, and 10b, respectively. No glass transition temperature was observed according to the DSC curves for all the title oligomers. It is clear that the phase transition temperature is related not only to the backbone of the molecules but also to the chain length of the alkoxy group. The higher phase transition temperature was found for larger backbone of the oligomers, which was in line with the results observed in TGA. In addition, the oligomers with longer alkoxy chains displayed lower melting point, which was consistent with the literatures. [19,30] The suitable phase transition temperature and relatively high thermal stability of these oligomers make them possible candidates for applications in OLEDs.

Optical properties
The spectroscopic property is one of the most important aspects in applications of novel materials. The photophysical properties of oligomers were examined by UV-Vis and photoluminescence spectroscopy in dilute CHCl 3 solutions and the results are listed in Table 3. Figure 1 shows the absorption spectra of oligomers in dilute chloroform solutions at concentrations of 10 μM. The maximum absorption peaks of 6a, 6b, 10a, and 10b are located at 422, 414, 400, and 392 nm respectively, which could be ascribed to the π-π* transition of the molecular backbone. [31] The shoulder peaks at 402 nm for 6a and 396 nm for 6b also dominate the  spectra, which can be ascribed to the presence of the supermolecular entities formed in chloroform, based on self-assembly. [32] It is worth noting that upon lengthening of the conjugation backbone, the π-π* absorption bands of the trimers in CHCl 3 solutions show a bathochromic shift by 22 nm as compared with the dimer analogs. The red-shifted absorption maximum for increasing the conjugation backbone is consistent with greater πconjugation upon lengthening of the arylene-ethynylene chain. [33] In addition, upon increasing of the side alkoxy chain, the oligomers with n-octyloxy chains have a blueshift by 8 nm as compared with the ones with n-butoxy chains. This may be ascribed to the fact that introducing larger side chains reduced the conjugation of the oligomers by steric hindrance. The title oligomers emit a blue color when irradiated by UV in the solid state or in solution. The naphthalene moiety in the molecules is believed to be responsible for the blue luminescence based on previous studies. [16,19] As shown in Figure 2(a), the emission features in solution of these oligomers with two major emission peaks are all similar to each other. One peaks at 435-440 nm, and the other peaks at 454-456 nm. The two emission peaks are all red-shifted as compared with analogous compounds (λ max = 370-420 nm) we previously reported, due to the larger conjugation backbones of the oligomers. [19] As shown in Figure 2(b), the emission spectra of oligomers in solid state display obvious changes as compared with those in dilute solutions: (a) the outlines of the spectra have changed with wider emission peaks and shown a significant bathochromic shift, which might be related to the better π-π interaction in the solid, (b) only one emission peak have been observed, possibly due to the polarity of the solvent, and (c) the major emission peaks of trimers show a significant bathochromic shift compared with those of dimers, which is in line   with the results in dilute solutions. The quantum yields of title oligomers are determined to be 0.26, 0.25, 0.33, and 0.24, respectively (relative to quinine sulfate in 0.5 M H 2 SO 4 ), indicating that these molecules are fairly efficient blue emitters and can be used in OLEDs.

Electrochemical properties
The electrochemical behaviors of these oligomers are investigated using CV at room temperature, and the results are listed in Table 3. The outlines of CV curves of these oligomers are very similar to each other. As shown in Figure S4, only one reversible oxidative peak is observed and the half-wave oxidation potentials (E ox 1=2 ) are in the range of 0.49-0.58 eV. Thus, the HOMO levels of these compounds can be estimated from the onsets of oxidation waves calculated according to empirical formula, E HOMO = -(E ox 1=2 þ 4:4) eV. In the cathodic scan, unfortunately, no reduction peak is detected for these compounds after several trials. The band gaps (E g ) are obtained from the absorption spectra (absorption edge) of these oligomers. The LUMO energy level is calculated from the values of band gap and HOMO energy. It is worth noting that HOMO levels of these compounds are in the range of -4.89~-4.97 eV, which are slightly above that of ITO (-5.1 eV), and thus might be beneficial for the hole injection. However, the LUMO energy is calculated in range of -1.31~-2.05 eV, which is higher than the work function of commonly used cathode materials (2.9 eV for Ca, 4.3 eV for Al and 4.5 eV for Cr), indicating that electron-transporting layer may be added between the emitting layer (the oligomers) and the cathode in the device.

Conclusion
In summary, we have successfully synthesized and characterized a set of novel ethynylene-containing oligomers based on 1,5-naphthalene subunit by palladium-catalyzed cross-coupling reaction. These oligomers are soluble in common organic solvents. The absorption and emission spectra of these compounds in dilute chloroform solution have been analyzed. The results indicate that these oligomers are fairly efficient blue emitters and could be used in OLEDs. Detailed investigations of electrochemical properties show that these compounds possessed relatively highest occupied molecular orbital levels, which indicates that the hole injection capabilities of these compounds are relatively excellent. Further study will be focused on their applications in devices.