Synthesis, structures and optical properties of 3,3′-disubstituted biphenyl compounds

2.1 Instruments and reagents

IR spectra were recorded in the range 450–4,000 cm⁻¹ on a Perkin Elmer Tensor II spectrometer using KBr pellets. ¹H and ¹³C NMR analyses were performed using a Bruker Avance III 600 MHz spectrometer. As internal references for ¹H and ¹³C NMR spectroscopy the signals of CDCl₃ were used and calculated relative to tetramethylsilane (TMS). Melting points were measured with a SGW X-4 apparatus and were not corrected. The high resolution mass spectra were measured with a Thermo Fisher Scientific LTQ FTICR-MS instrument (DART positive ion mode or ESI positive, or AP MALDI ion mode). UV/Vis spectra were recorded using a UV-9000S Plus spectrometer. Fluorescence was measured on Edinburgh FLS920 and FLS1000 spectrometers and absolute quantum yield was tested using a barium sulfate integrating sphere. GC-MS was measured on a GCMS-QP2020 coupled with SH-RXi-5SilMS capillary column. The solvents were commercially available and used without further purification.

2.2 Synthesis of bromobiphenyl compounds

The starting material 6,6′-difluoro-[1,1′-biphenyl]-2,2′-dicarbonitrile (compound A) was synthesized according to the literature methods. ^{9 , 11}

3.1 Bromination reaction of compound A

The reactions of compound A with elemental bromine in mixed acids are shown in Scheme 1. In this process, nitric acid acts as an oxidizing agent with sulfuric acid to generate an intermediate and interacts with Br₂ to form the Br⁺ HSO₄ ⁻ complex ¹² to react with compound A. In the reaction using 1 equivalent of bromine, the reaction occurs first at the position next to the cyano group (i.e. 3,3′-position) to produce the dibromobiphenyl 1, which is in agreement with the results of Mulliken’s charge distribution computed at the B3LYP/6-311 + G(d,p) level of theory by using the density-functional theory (DFT) method, ²⁵ but not consistent with the results at the B3LYP/6-31G(d,p) level (Figure 3), which suggests the basis set dependence in such calculations. In addition, the reaction produces 3 with a nitro group on the other benzene ring, which results from the competing reactions of bromination and nitration under these conditions. ^{12 , 18} The total yields of the main products 1 and 2 reach 70 %.

Scheme 1:

Bromination of biphenyl compound A with Br₂ (isolated yields are in parentheses).

Figure 3:

Mulliken charges of some ring carbon atoms of compound A (left: B3LYP/6-311 + G(d,p) level; right: B3LYP/6-31G(d,p) level).

When the amount of bromine is increased to 1.5 equivalents, the yield of product 1 is lower. When the amount of bromine is increased to two equivalents, the yield of the dibromo product 1 is only 20 %, but the nitration product 2 is not separated, and a small amount of the tetrabromo product 3 is obtained. When the amount of bromine is further increased to three equivalents, no product 1 was obtained, and product 3 is produced in a yield of 10 %, and a very small amount of the polybrominated products 4 and 5 is formed, and the latter two products are difficult to separate, and insoluble in common organic solvents.

The above results of the bromination of compound A with elemental bromine show that the optimal amount of bromine is 1 equiv., and the main products are the dibromo compound 1 and bromo/nitro product 2 with functionization at the 3,3′-positions. With more than two equiv. of bromine, the bromination occurs at the 4- or 4,4′-position(s) with the corresponding products in much lower yields.

3.2 Synthesis of carbazolyl biphenyls from compound 1

When compound 1 is reacted with carbazole in the presence of a base such as NaH, the biscarbazolyl product 3,3′-dibromo-6,6′-di(9H-carbazol-9-yl)-[1,1′-biphenyl]-2,2′-dicarbonitrile (6) is obtained in 45 % yield, together with a small amount of the monocarbazolyl product 7 and triscarbazolyl compound 8 (Scheme 2). Compounds 7 and 8 are difficult to separate and were characterized by high resolution mass spectrometry. The substitution of a bromine atom by a carbazolyl group in the formation of compound 8, via an aromatic nuleophilic substitution (S_NAr) reaction, is due to the obvious electron deficiency of the benzene ring in the starting compound 1, and is a relatively rare example of metal catalyst-free C–N coupling reaction. ²⁶

Scheme 2:

Reaction of 1 with carbazole (isolated yields are in parentheses).

3.3 Crystal structures of compounds 1–3 and 6

The major brominated biphenyl products 1–3 and the biscarbazolyl compound 6 were structurally characterized by infrared, ¹H NMR, ¹³C NMR spectroscopy, and high-resolution mass spectrometry. Single crystals of compounds 1–3 and 6 could be obtained and their crystal structures were determined by X-ray diffraction, the details of which can be found in the Supporting Information available online.

Single crystals of compound 1 were obtained by slow evaporation of an ethyl acetate solution. Compound 1 crystallizes in the monoclinic space group C2/c with Z = 4. Selected bond lengths, bond angles and torsion angles are summarized in the Supporting information. The C–C bond length between the two benzene rings is 1.488(5) Å, similar to those in the above-mentioned biphenyls (1.488(5)−1.502(5) Å). ^{5 , 7 , 9 , 10 , 11} The C–Br and C–F bond lengths are 1.881(3) Å and 1.341(3) Å, respectively. The dihedral angle of the two benzene rings of the biphenyl framework is 63.48° (Figure 4). Intermolecular C–H⋯F (H(4)⋯F(1) 2.572 Å) and C–H⋯N (H(5)⋯N(1) 2.717 Å) hydrogen bonding interactions are present in crystals of compound 1 (Figure 5).

Figure 4:

Molecular structure of compound 1 (left; displacement ellipsoids are drawn at the 30 % probability level, H atoms as spheres with arbitrary radii) and dihedral angle of the biphenyl (right). Note that the entire molecule has crystallographic 2 (C ₂) symmetry, the two-fold axis bisecting the C(7)–C(7) bond and lying approximately in the paper plane.

Figure 5:

Weak intermolecular interactions in crystals of compound 1.

Single crystals of compound 2 were grown from an ethyl acetate solution. The compound crystallizes in the monoclinic space group P2₁/c with Z = 4. Due to the problem of the crystal quality, a discrepancy factor R1 of 0.1489 was obtained in the refinement. Therefore, the bond lengths and angles will not be discussed here. The molecular structure is shown in Figure 6 (left). The dihedral angle of its biphenyl framework is 113.25° (Figure 6, right). Weak C–H⋯O (H(5)⋯O(2) 2.684 Å) hydrogen bonding and C–F⋯π halogen bonding ^{16 , 27} (F···centroid distance of 3.688 Å) interactions are present in the crystals of compound 2 (Figure 7).

Figure 6:

Molecular structure of compound 2 (left; displacement ellipsoids are drawn at the 30 % probability level, H atoms as spheres with arbitrary radii) and dihedral angle of the biphenyl (right).

Figure 7:

Weak intermolecular interactions in crystals of compound 2.

Single crystals of compound 3 were also obtained by the same method as that for 1. Compound 3 crystallizes in the monoclinic space group P2₁/c with Z = 8. In the crystal structure (Figure 8), there are two crystallographically independent molecules 3A and 3B. The C–C bond lengths between the benzene rings of the two molecules are 1.499(10) Å (C7–C8) and 1.500(10) Å (C21–C22), respectively, which are essentially the same as that in compound 1, indicating that the number of bromine atoms has no significant effect on the C–C bond length of the biphenyl. In molecule 3A, the C–Br bond lengths are 1.859(8) Å–1.891(8) Å and the C–F bond lengths are 1.340(9) Å (C1–F1) and 1.338(8) Å (C9–F2), respectively. In molecule 3B, the corresponding bond lengths are 1.864(8) Å–1.895(8) Å (C–Br), and 1.339(9) Å (C20–F3) and 1.318(9) Å (C23–F4), respectively. The halogen-related bond lengths are essentially the same in the two molecules, and those in compound 1. Quite interestingly, in the biphenyl framework, the dihedral angles of the biphenyl are 61.23° (Cg1 (C16–C17–C18)/Cg2 (C22–C23–C24) and 116.03° (Cg3 (C1–C2–C3)/Cg4 (C8–C9–C10), respectively, for the two independent molecules (Figure 8, right). Note that the noticeably different dihedral angles imply grossly different F⋯F and (CN)⋯(CN) intramolecular distances or, in other words, they are not just the complementary dihedral angles which in one molecule add to 180°.

Figure 8:

Molecular structure of compound 3 (left; two independent molecules 3A and 3B; displacement ellipsoids are drawn at the 30 % probability level, H atoms as spheres with arbitrary radii) and dihedral angles of the biphenyls (right).

In the crystals of compound 3, there are intermolecular C–H⋯N (H(2)⋯N(2) 2.535 Å) hydrogen bonding and a number of C–Br⋯π halogen bonding ^{28 , 29} (Br⋯centroids distances of 3.501–3.843 Å) interactions (Figure 9).

Figure 9:

Weak intermolecular interactions in crystals of compound 3.

Single crystals of compound 6 were grown from a chloroform solution. The compound crystallizes in the monoclinic space group I2 with Z = 4. There are two crystallographically independent molecules 6A and 6B, each of which has crystallographic 2 (C ₂) symmetry, the two-fold axes bisecting the central biphenyl C–C bonds (Figure 10). The C–C bond lengths between the benzene rings of the two molecules are 1.508(9) Å (C19–C19) and 1.479(11) Å (C38–C38), respectively, similar to that of 1.502(5) Å in compound D, ⁵ indicating that the different bromine positions have little effect on the length of this C–C bonds. The C–Br bond lengths in compound 6 are 1.906(5) Å (C16–Br1) and 1.916(7) Å (C35–Br2), respectively, similar to those in the above-mentioned structures. The dihedral angles of its biphenyl framework are 64.08° and 70.63° for the two independent molecules. The corresponding dihedral angles between the planes of the carbazolyl and the biphenyl are 71.3° and 61.38°, respectively (Figure 10). In addition, weak intermolecular C–H⋯π (C(14)–H(14)⋯Cg1/C(15)–H(15)⋯Cg2) and C–H⋯N (C(4)–H(4)⋯N(2)/C(9)–H(9)⋯N(2)) hydrogen bonding interactions and the intramolecular π⋯π stacking of the carbazolyl rings are present in crystals of compound 6 (Figure 11).

Figure 10:

Molecular structure of compound 6 (left; two independent molecules 6A and 6B; displacement ellipsoids are drawn at the 30 % probability level, H atoms as spheres with arbitrary radii) and dihedral angles of the biphenyl (right).

Figure 11:

Weak intra- and intermolecular interactions in crystals of compound 6.

3.4 Electronic structures of compounds

DFT methods ²⁵ have been utilized to study the electronic structures of compounds A and 1–3, and their optimized structures and frontier molecular orbital composition/energy levels are shown in Figures 12 and 13, respectively. The HOMO of compound 1 is mainly concentrated on the benzene rings, the bromine and fluorine atoms, and the cyano group is much less involved, while the LUMO mainly resides on the cyano group and biphenyl skeleton. Similar to that in 1, the HOMO of compound 3 is mainly concentrated on the biphenyl skeleton, bromine and fluorine atoms, and the LUMO is mainly distributed on the biphenyl skeleton and cyan group, with trace on the bromine atoms; and the HOMO of compound 2 is mainly distributed on the nitro group and one benzene ring and fluorine atom, with a small amount of distribution on the other benzene ring, and LUMO is mainly on the nitro group and one benzene ring and cyan group. Compared with compound A, the HOMO energy level of compound 1 is essentially unchanged, but that of compound 3 is slightly decreased and that of compound 2 is significantly increased. The LUMO energy levels of compounds 1–3 decreased to different extent when compared to that of compound A. The HOMO-LUMO energy gaps of compounds 1–3 are all significantly lower than that of compound A. The above results show that the different substitutions lead to the differences in the electronic structures.

Figure 12:

Optimized structures of compounds 1–3 at the B3LYP/6-31G (d,p) level of theory.

Figure 13:

Frontier molecular orbitals and energy levels of compounds A and 1–3 (B3LYP/6-31G (d,p) level).

As to compound 6 (Figure 14), the HOMO mainly resides on the two carbazolyl rings, and the LUMO mainly on the two cyano groups and the biphenyl framework, as expected. The corresponding singlet-triplet energy gap (ΔE _ST) is calculated to be 0.09 eV, which is well within the range (< 0.3 eV) expected for a system with TADF property. ³

Figure 14:

Frontier molecular orbitals and energy levels, and the singlet-triplet energy gap of compound 6 (B3LYP/6-31G (d,p) level).

3.5 Optical properties

The UV absorption spectra of compounds 1–3 were measured in THF solutions at room temperature (Figure 15). All three compounds show two absorption peaks, with a stronger one at 225–228 nm (compound 1: 225 nm, compound 2: 227 nm; compound 3: 228 nm) and a weaker one at 302–314 nm (compound 1: 303 nm, 2: 302 nm, 3: 314 nm), respectively. In this case, the short wavelength absorption peak is attributed to the intramolecular π→π* electron transition and the longer wavelength absorption peak is assigned to the intramolecular charge transfer. ^{6 , 8 , 9}

Figure 15:

UV absorption spectra of compounds 1–3 (THF, c = 10⁻⁵ mol L⁻¹).

The UV absorption spectrum of compound 6 in toluene was measured at room temperature (Figure 16). It shows a strong absorption band at 287 nm attributable to the intramolecular π→π* transition of the aromatic rings, and a weak absorption band at 330 nm due to the intramolecular charge transfer transition. The absorption bands of compound 6 are red-shifted compared to those of 1–3, but similar to those of the analogous compounds B (in toluene) ⁹ and D (in 2-MeTHF). ⁵

Figure 16:

UV absorption spectrum of compound 6 (toluene, c = 10⁻⁵ mol L⁻¹).

The emission spectra of compound 6 were measured in THF-H₂O system at room temperature with different water fractions (Figure 17). The compound is hardly emissive in dilute THF solution. As the water fraction increases to 90 %, the emission is significantly enhanced. At a water fraction of 98 %, the emission is the strongest. This may be due to the formation of molecular aggregates, which increases the rigidity of the molecules and hinders the internal rotations, resulting in enhanced emission and exhibiting typical Aggregation-Induced Emission (AIE) properties. ³⁰ For a sample with fw = 98 % after 30 min, the emission intensity further increases indicating that the emission of the aggregates is time-dependent. Under these conditions, the emissions are first red-shifted (518–543 nm) and then blue-shifted (543–504 nm). These changes are mainly caused by the differences in the aggregates formed. For comparison, compound B also displays AIE phenomenon, ⁹ whereas compound D is emissive in both solution and film states, indicating that different bromination position has significant effect on the emission property in this system.

Figure 17:

Fluorescence spectra of compound 6 in THF-H₂O (c = 1 × 10⁻⁵ mol L⁻¹, λ _ex = 330 nm) with different water fractions (fw) and the one with fw = 98 % after 30 min (left), and the corresponding changes in intensity and wavelength (insets: photos of the systems under UV light at 365 nm).

The solid-state fluorescence spectrum of compound 6 was measured at room temperature (Figure 18), with an emission maximum at 503 nm, very similar to those of compound B (in methanol) ⁹ and D (in 2-MeTHF), ⁵ indicating that the bromination (position) and the state of the compound have little effect on the emission wavelength. The powder of compound 6 exhibits strong emission with an absolute quantum yield of 72 %, at the higher end in the reported PLQYs of AIDF materials. ³¹ The photoluminescence decay curve of the doped film (in 9,9′-(1,3-phenylene)bis-9H-carbazole, mCP) displays a transient fluorescence lifetime of 0.64 ns and a delayed fluorescence lifetime of 5.1 μs (Figure 19). The former lifetime can be attributed to the fluorescence of the S₁ to S₀ transition; Due to the small ΔE_ST of compound 6, the latter lifetime comes from the efficient reverse intersystem crossing (RISC) process of T₁ upconversion to S₁ followed by radiative decay. These results indicate that compound 6 is TADF-active. Combining the AIE and TADF properties, compound 6 exhibits AIDF characteristics.

Figure 18:

Fluorescence spectrum of compound 6 in powder (λ _ex = 400 nm) (insets: photos of the powder under ambient (top) and UV light at 365 nm (bottom)).

Figure 19:

Delayed fluorescence decay curve of compound 6 in powder (5 wt% in mCP) (inset: the corresponding transient decay curve).