Synthesis, optical and electrochemical properties of 2-[(9H-fluoren-2-yl)aryl]-1H-benz[d]imidazole and 2,7-bis[(1H-benz[d]imidazol-2-yl)aryl]- 9H-fluorene derivatives

Introduction

Fluorescent heterocyclic compounds are of interest as emitters for electroluminescence devices, molecular probes for biochemical research, dyes for textiles and polymers, fluorescent whitening agents and photo-conducting materials [1–9] among other things. In particular, imidazole derivatives including phenylbenzo[d]imidazoles [10–21] and 1,3,5-tris(1-phenylbenzo[d]imidazol-2-yl)benzene (TPBI) have found applications in electrical and optical materials [22–24].

Due to their rigid oligophenyl unit within the molecular backbone and the possibility of facile functionalization at the methylene bridge, fluorene-based derivatives, including polymers and oligomers, are important as luminescent materials [19, 25–28], as well as carrier transport materials for field effect transistors (FETs) [29–31]. Their fluorescent characteristics rely largely on molecular structure and molecular assembly. Changes in the substitution pattern, conjugation, and molecular electronic structure can bring about very different optical and physical properties for such materials. There is presently a great interest to increase the structural or spatial dimensions of π conjugated molecules in order to acquire more favorable physical properties. In continuation of our efforts in synthesizing various fluorescent small molecules [32–35], in this paper we describe molecular combinations of fluorene and benzo[d]imidazole linked by π-conjugated moiety. We report the structure-optical behavior characteristics and further details of the electronic properties from cyclic voltammetry measurements and theoretical calculations of novel 2-[(9H-fluoren-2-yl)aryl]-1H-benzo[d]imidazole derivatives 11a–c, 12a,b, 13a,b and 2,7-bis[(1H-benzo[d]imidazol-2-yl)aryl]-9H-fluorene derivatives 14a–c, 15a,b, 16a,b. Extended conjugation is believed to result in intermolecular stacking interactions which are detrimental to the emission characteristics. Therefore, two n-hexyl or two methyl groups were introduced to the C-9 position of the fluorene moiety to increase solubility as well as to hinder intermolecular π-π stacking.

Results and discussion

The key aldehyde intermediates 3–5 and 7–9 were prepared as shown in Scheme 1. The bromo precursors 2 and 6 were conveniently converted to the arylated aldehyde derivatives 3–5 and 7-9 in 70–80% yields by treatment with the corresponding boronic acids in the presence of PdCl₂(PPh₃)₂ and K₂CO₃ under the Suzuki reaction conditions [36]. Condensation of 3–5 or 7–9 with the diaminobenzene derivatives 10 in dioxane at 110°C for 2-5 h afforded the target compounds 11a–13b and 14a–16b in 60–80% yields (Scheme 2). All target compounds were purified by column chromatography on silica gel and their molecular structures were confirmed by ¹H NMR, ¹³C NMR, and HR-MS.

Scheme 1

Synthesis of the aldehyde intermediates.

Scheme 2

Synthesis of 11a–16b.

The ORTEP view of the structure of 11b in Figure 1A indicates that the moieties of fluorene, phenylene and 2-phenyl-1H-benzo[d]imidazole are non-coplanar with dihedral angles of 7.829^o and 33.613°, respectively. In compound 13b, as shown in Figure 2A, the fluorene moiety forms torsion angle of 13.100° with the adjacent thiophene ring ii, and the thiophene ring ii forms a torsion angle of 18.325° with the imidazole ring iii. The crystal packings of 11b and 13b are shown in Figures 1B and 2B. The molecules are stacked in the overlapping manner and face-to-face π-π intermolecular interactions between molecules can be regarded as the stacking force.

Figure 1

Molecular structures of compound 11b.

The H atoms are omitted for clarity: (A) ORTEP drawing, (B) a view of the molecule packing structure.

Figure 2

Molecular structures of compound 13b.

The H atoms are omitted for clarity: (A) ORTEP drawing, (B) a view of the molecule packing structure.

The photophysical properties of compounds 11a–13b were examined using UV-vis absorption and fluorescence spectra in dilute solutions and in solid films. The absorption peak wavelengths (λ_abs) and the emission wavelengths (λ_em) are listed in Experimental. The emission spectra in the solid state were obtained for films spin-coated onto quartz from chloroform solution. All compounds show similar absorption profiles, principally owing to π-π* transitions of the molecular backbone. Compounds 11a, 12a, 13a, 14a, 15a and 16a exhibit the absorption maxima at 337 nm, 368 nm, 378 nm, 360 nm, 394 nm, and 400 nm in diluted CH₂Cl₂ solutions, respectively. The absorption maxima of 11a–16b follow the order: 11a–c (benzene) <12a,b (furan) <13a,b (thiophene) (Figure 3), 14a–c (benzene) <15a,b (furan) <16a,b (thiophene). This result demonstrates that the heterocyclic ring plays an important role in the UV-vis absorption spectra due to the lone pair electrons which can cause n-π* transitions with lower energy in the heteroatom (O, S). Compounds 14a–c, 15a,b, and 16a,b have longer absorption wavelengths than 11a–c, 12a,b, and 13a,b due to the elongation of conjugation. Compounds 15a,b and 16a,b exhibit the longest absorption wavelengths. Interestingly, compounds 11b,c, 12b, 13b, 14b,c, 15b and 16b with the phenyl or para-MeO substituted phenyl group in N-1 positions have almost the same absorption maxima as those of N-H compounds 11a, 12a,13a, 14a, 15a and 16a, indicating that arylation of benz[d]imidazole has a limited effect on the energy levels of these compounds.

Figure 3

Normalized absorption spectra of 11a–13b (1×10^-5 M) recorded in CH₂Cl₂.

Theoretically, the absorption spectra caused by π-π* transitions exhibit red shifts with the increase in solvent polarity. However, compounds 11a–16b show blue-shifted absorption maxima by 5–26 nm in diluted MeOH solutions in comparison with the dilute CH₂Cl₂ solutions. The blue-shifted emission in MeOH may be ascribed to hydrogen-bonding interactions, which probably inhibits the reorientation of the benzo[d]imidazole in the excited state by hindering the rotation of the benzene, furan and thiophene subunits.

All compounds 11a–16b display blue or blue-green emissions in dilute CH₂Cl₂ solutions (Figure 4). Compounds 11a, 12a, 13a, 14a, 15a and 16a show emission peaks at 452 nm, 456 nm, 461 nm, 457 nm, 463 nm and 496 nm, respectively. In contrast to the UV-vis absorption properties, the heteroatom (O, S) and the length of conjugation have less significant effects on the emission spectra of these compounds. A slight red-shift is observed for compounds 11c and 14c compared with their corresponding N-H analogues 11a and 14a. This result attests the partial involvement of the N-substituted chromophores in the electronically excited state. By comparison with the dilute CH₂Cl₂ solutions, the emission spectra of all compounds in MeOH solutions show shifts to shorter wavelengths by 37–68 nm, probably due to the hydrogen-bonding effects in MeOH solutions [37].

Figure 4

Normalized emission spectra of 11a–13b (1×10^-5 M) recorded in CH₂Cl₂ solutions; excitation at λ_ex at room temperature.

In comparison with the dilute CH₂Cl₂ solution, the emission spectra of compounds 11b,c, 12b, 13b, 14b,c and 15a,b in solid state are shifted to shorter wavelengths by 10–53 nm. These compounds probably form H-aggregates, which decreases the π-conjugation significantly, in turn, resulting in the observed blue shift [38]. However, compounds 11a, 12a and 14a show shift to longer wavelengths by 5–8 nm. The red shift of the emission observed in the solid state is probably due to the strong intermolecular forces, which drive the molecules to pack at high density to form J-aggregates with restricted molecular rotation and an increased π-conjugation, in turn, resulting in the red shift [39].

Most compounds show high fluorescence in CH₂Cl₂ solution with fluorescence quantum yields φ_FL in the range of 0.77–0.99 (Table 1). The relatively low quantum yields observed for compounds 13a,b and 15a,b with heteroatoms suggest a fluorescence quenching caused by the heteroatom.

The redox potentials of all compounds were determined by cyclic voltammetry (CV) measurements which were carried out in a three-electrode cell setup with 0.1 M tetrabutylammonium perchlorate (Bu₄NClO₄) as a supporting electrolyte in anhydrous CH₂Cl₂ to probe the electrochemical behavior of the materials. The HOMO values were calculated as follows: HOMO = [E_onset]_red/ox (vs SCE) + 4.6, E_g = 1240/UV (onset) [40, 41]. As can be seen from Table 1, the band gaps (E_g) of about 2.51–2.92 eV were estimated from the optical edge. Compounds 12a–15b exhibit relatively smaller band gaps by comparison with compounds 11a–c. The HOMO energies of these compounds were calculated by using the ferrocene/ferrocenium redox couple as a reference (4.6 eV) and were within the range of 5.43–5.84 eV. The LUMO energies were calculated from their corresponding HOMO energies and the optical band gap as estimated from the intersection of the absorption and emission peaks, which ranged from 2.59 eV to 3.36 eV. Compounds 14a–c show relatively higher HOMO and LUMO energies than 11a–c, probably due to the extended length of π-conjugation. However, in the case of 15a,b, the HOMO and LUMO are lower in energy than those of 12a,b; the ability of the heterocyclic ring (furan) to donate electrons is a possible explanation [42].

Density functional theory (DFT) calculations were also used to characterize the three-dimensional geometries and the frontier molecular orbital energy levels of the representative compounds 11b, 12b, 13b and 14b, 15b, 16b at the B3LYP/6-31G* level by using the Gaussian 03 program [40, 41]. The orbital plots of the HOMO-LUMO are illustrated in Figure 5. In the case of compounds 11b, 12b, 14b, 15b and 16b, the HOMO and LUMO are mainly localized on the fluorene and benzene (11b, 14b), furan (12b, 15b), and thiophene (16b) subunits, indicating that the absorption and emission processes are mostly attributed to the π-π* transitions centered at the fluorene and aromatic moieties. Interestingly, compound 13b spreads HOMO and LUMO through the thiophene subunit, which can explain the relatively high energy level. The LUMO of all compounds are not localized on the benzo[d]imidazole, which can explain the shorter-wavelength absorptions in polar solutions. The trends in the experimentally observed longer-wavelength absorptions for compounds 14b and 15b matches well the computed low-energy excitations.

Figure 5

HOMO and LUMO electronic density distributions of 11b, 12b, 13b and 14b, 15b, 16b.

Conclusions

2-[(9H-Fluoren-2-yl)aryl]-1H-benzo[d]imidazole and 2,7-bis [(1H-benzo[d]imidazol-2-yl)aryl]-9H-fluorene derivatives containing central aromatic or heteroaromatic subunits were conveniently synthesized in good yields. These compounds were thoroughly characterized by spectral and computational methods. Most compounds display emission in a blue region with φ_FL values in the range of 0.31–0.99. The HOMO energy levels of about 5.43–5.84 eV and the LUMO energy levels of about 2.59–3.36 eV were calculated. Further research regarding application of these systems as organic light-emitting diodes and as fluorescent probes is under way in our laboratory.

Experimental

Commercially available reagents were purchased and used without further purification. ¹H NMR (500 MHz) and ¹³C NMR (125 MHz) spectra were recorded on a Bruker Avance spectrometer in CDCl₃ or DMSO-d₆. Chemical shifts in CDCl₃ were referenced to CHCl₃ (7.26 ppm) for ¹H NMR, and to CDCl₃ (77.16 ppm) for ¹³C NMR. The center line of the multiplet of DMSO-d₆ was taken as δ 2.50 for ¹H NMR. Silica gel plates GF₂₅₄ were used for thin layer chromatography (TLC), and silica gel H or 300–400 mesh was used for flash column chromatography. Melting points were measured on a digital melting point apparatus without correction. The absorption and fluorescence spectra were recorded using a UV-2501Pc spectrophotometer and a RF-5301 fluorescence spectrophotometer, respectively. High-resolution mass spectra were recorded on a Bruker APEX III 7.0 instrument. The redox potentials of compounds were determined with cyclic voltammetry (CV) using a Base 2000 CV system. The HOMO values were recalculated from the reported literature values as HOMO = [E_onset]_red/ox (vs SCE) + 4.6; E_g = 1240/UV (onset).