Near-infrared fluorescent and columnar liquid crystal: synthesis, and photophysical and mesomorphic properties of triphenylene-Bodipy-triphenylene triad

Over the past decades, columnar liquid crystals have attracted considerable academic and commercial interest based on their unique p–p stacking structures. They exhibited various potential applications for organic eld-effect transistors, organic light-emitting diodes, organic photovoltaic cells, gas sensors, etc. Recently, the columnar liquid crystals with high uorescence, such as the perylene liquid crystals, have received signicant attention due to their broad application in novel liquid crystalline materials with photophysical properties. However almost all of the known uorescent liquid crystals emitted light in the visible spectral region. Lately, the nearinfrared materials displayed potential application in a number of research elds, such as telecommunications, thermal imaging, and biological imaging. Thus, the nearinfrared-emitting liquid crystals were also prepared, which exhibited interesting near-infrared photophysical properties. They were usually obtained as organic–inorganic hybrid materials by combining organic liquid crystals with C60, graphene and rare earth metal ions, etc. However, this type of a hybrid material was difficult to construct as a columnar liquid crystal. As for the near-infrared organic columnar liquid crystal, only one example derived from benzo[1,2-c:4,5-c0]bis([1,2,5] thiadiazole) has been presented with good near-infrared uorescence and hexagonal columnar mesophase to date. Studies on the near-infrared columnar liquid crystal have rarely been reported. On the other hand, 4,4-diuoro-4-borata-3a,4a-diaza-s-indacene (Bodipy) is a class of famous uorescent dyes with intense uorescence, good photochemical stability and energy/electron-

Over the past decades, columnar liquid crystals have attracted considerable academic and commercial interest based on their unique p-p stacking structures. They exhibited various potential applications for organic eld-effect transistors, organic light-emitting diodes, organic photovoltaic cells, gas sensors, etc. [1][2][3][4][5] Recently, the columnar liquid crystals with high uorescence, [6][7][8][9][10][11] such as the perylene liquid crystals, [12][13][14][15][16] have received signicant attention due to their broad application in novel liquid crystalline materials with photophysical properties. However almost all of the known uorescent liquid crystals emitted light in the visible spectral region. 10 Lately, the nearinfrared materials displayed potential application in a number of research elds, such as telecommunications, thermal imaging, and biological imaging. 17,18 Thus, the nearinfrared-emitting liquid crystals were also prepared, which exhibited interesting near-infrared photophysical properties. [19][20][21][22][23] They were usually obtained as organic-inorganic hybrid materials by combining organic liquid crystals with C 60 , graphene and rare earth metal ions, etc. However, this type of a hybrid material was difficult to construct as a columnar liquid crystal. As for the near-infrared organic columnar liquid crystal, only one example derived from benzo[1,2-c:4,5-c 0 ]bis( [1,2,5] thiadiazole) has been presented with good near-infrared uorescence and hexagonal columnar mesophase to date. 10 Studies on the near-infrared columnar liquid crystal have rarely been reported.
On the other hand, 4,4-diuoro-4-borata-3a,4a-diaza-s-indacene (Bodipy) is a class of famous uorescent dyes with intense uorescence, good photochemical stability and energy/electron-transfer capabilities. [24][25][26][27][28] Some uorescent Bodipy liquid crystals were reported by introducing long alkyl chains on the Bodipy skeleton. [29][30][31][32][33][34] Recently, triphenylene-Bodipy dyads were also prepared, which exhibited good columnar liquid crystals based on the effective p-p stacking of triphenylene units. 13 On the other hand, Bodipy had been applied as a good platform to construct near-infrared materials by extending its aromatic conjugated system. [35][36][37][38][39][40] Therefore, it can be question whether it is possible to design and synthesize the columnar Bodipy liquid crystal with near-infrared uorescence. This type of nearinfrared columnar liquid crystal has not thus far been reported. Based on this consideration, in this study, the rst example of near-infrared uorescent and columnar liquid crystal was designed and synthesized using Bodipy as the building platform. The inuences of the triphenylene unit and the alkenyl groups on the mesomorphic and uorescent properties were investigated. The experimental results showed that the near-infrared uorescent and columnar liquid crystal were difficult to achieve for the triphenylene-Bodipy dyad, whereas the triphenylene-Bodipy-triphenylene triad with 3,5-alkenyl groups was favourable for good near-infrared uorescence and hexagonal columnar mesophase. The multiple triphenylene units effectively induced columnar mesophase and the 3,5alkenyl groups resulted in the near-infrared uorescence.
As the triphenylene-Bodipy dyad exhibited columnar liquid crystalline behaviour 13 and the near-infrared Bodipy could be prepared by the Knoevenagel condensation reaction on 3,5methyl of Bodipy, 17 we initially tried to design the synthetic route for near-infrared uorescent and columnar liquid crystal by the Knoevenagel condensation of the triphenylene-Bodipy dyad (Scheme 1). According to the published procedure, 41 the triphenylene triazo derivative 3 was prepared by the ether-ication of triphenylene 1 with 1,6-dibromohexane, with subsequent nucleophilic substitution with NaN 3 , resulting in a yield of 76%. Moreover, according to the typical procedure for preparing Bodipy, 13 the Bodipy derivative containing alkynyl group 5 was obtained by treating 4-(prop-2-ynyloxy) benzaldehyde 4 with 2,4-dimethylpyrrole via sequential condensation, oxidation, and complexation reactions, resulting in the moderate yield of 22% aer columnar chromatography. Furthermore, the click reaction of compounds 3 and 5 was carried out smoothly by the catalysis of CuSO 4 and sodium ascorbate in DMF solution. Triphenylene-Bodipy dyad 6 was collected in a yield of 66% aer columnar chromatography. Finally, by Knoevenagel condensation of compound 6 with pdimethylaminobenzaldehyde under the catalysis of glacial acetic acid and piperidine, the triphenylene-Bodipy dyad 7 with mono-alkenyl group was prepared in a yield of 20%. As compounds 6 and 7 were prepared, their mesomorphic and photophysical properties were investigated (the detailed experimental data are discussed later). The results implied that compound 6 exhibited a mesophase, but compound 7 showed no liquid crystalline behavior. These phenomena might suggest that one triphenylene unit was not enough to induce the liquid crystalline behavior of compound 7 containing the Bodipy unit with alkenyl group because of its larger rigid structure compared to the normal Bodipy unit. Based on this analysis, it could be deduced that compound A (6 with 3,5-alkenyl groups, as shown in Scheme 1) had no liquid crystalline behavior due to the larger rigid structure of compound A compared to that of compound 7. Moreover, compound A (using MS, this was detected as a by-product formed when preparing compound 7) was difficult to be puried due to similar polarity with compound 7. Thus, the preparation of compound A was abandoned and a means to devise a new strategy to design and synthesize a liquid crystalline molecule containing the Bodipy unit with 3,5-alkenyl groups was sought.
In order to overcome the negative inuence of the large rigid structure of the Bodipy unit with alkenyl groups on the mesomorphic properties, increasing the number of triphenylene units may be an effective strategy. More triphenylene units produce stronger induction for the mesophase. The new synthetic route for the triphenylene-Bodipy-triphenylene triad with 3,5-alkenyl groups is presented as Scheme 2. First, the Bodipy derivative, containing two alkynyl groups 9, was synthesized by reacting 4-(prop-2-ynyloxy)benzaldehyde 8 with 2,4-dimethylpyrrole via a similar procedure for the preparation of compound 5. Compound 9 was obtained in the moderate yield of 24% aer columnar chromatography. Second, the triphenylene-Bodipy-triphenylene triad 10 was further synthesized in a yield of 64% by reacting compound 9 and 3 via click chemistry. Finally, triphenylene-Bodipy-triphenylene triad 11 with 3,5-alkenyl groups was prepared by the condensation of compound 10 with p-dimethylaminobenzaldehyde under the catalysis of glacial acetic acid and piperidine. The yield was 21% aer columnar chromatography. One can see that in the structure of compound 11, the two triphenylene units were favourable for enhancing the inductive effect for mesophase, and the two 3,5-alkenyl groups on the Bodipy unit greatly extended the aromatic conjugated system, resulting in uorescence in the near-infrared region.
The target compounds were fully characterized by 1 H NMR, 13 C NMR, HR-MS spectra and elemental analysis. The MS spectra showed corresponding molecular ion peaks (M + , MH + or MNa + , see ESI †), indicating condensation. In their 1 H NMR spectra, all peaks were well assigned to the corresponding structures (see ESI †). For example, one and two singlets for the Bodipy skeleton of compounds 11 and 7 certainly indicated the bis-substituted and mono-substituted alkenyl groups, respectively. The 13 C NMR spectra also supported the corresponding structures of the target compounds.
The liquid crystalline properties of new compounds 6, 7, 10 and 11 were investigated preliminarily by differential scanning calorimeter (DSC). Their DSC curves for second heating and cooling are illustrated in Fig. 1. The corresponding phase Scheme 1 The synthetic route of triphenylene-Bodipy dyad with mono-alkenyl group 7.
Scheme 2 The synthetic route of the triphenylene-Bodipy-triphenylene triad with 3,5-alkenyl groups 11. transition temperatures and enthalpy changes are summarized in Table 1. One can see that triphenylene-Bodipy dyad 6 showed two phase transition peaks on cooling at 100.4 C and 37.8 C and upon second heating at 38.2 C and 104.9 C. These data might indicate the mesophase of compound 6, which was conrmed further by POM and XRD analysis. However, compound 7 with alkenyl group derived from compound 6 exhibited only one peak upon cooling and second upon heating, suggesting the crystalline phase-isotropic phase transition without mesophase. These differences in mesomorphic properties of compounds 6 and 7 could be explained by the fact that the alkenyl group on the Bodipy skeleton of compound 7 enlarged the rigid aromatic structure and destroyed the mesomorphic property. These results may suggest that one triphenylene unit was not enough to induce liquid crystalline behavior in the Bodipy skeleton with an alkenyl group. On the contrary, both triphenylene-Bodipy-triphenylene triad 10 and its condensation derivative 11 presented two phase transition peaks on cooling (114.8 C and 23.7 C for 10, and 109.2 C and 10.2 C for 11) and second heating (24.2 C and 118.0 C for 10, and 19.6 C and 112.5 C for 11). These data implied that both compounds 10 and 11 had good reversible phase transition behaviors with mesophase. Moreover, the results suggested that although two alkenyl groups were introduced onto the Bodipy skeleton of compound 11, the latter still possessed liquid crystalline behaviour due to the strong mesomorphic-induced effect of two triphenylene units. Thus, it could be concluded that compounds 6, 10, and 11 were liquid crystalline molecules, but compound 7 presented no mesophase. More triphenylene units on Bodipy resulted in a strong mesomorphic-induced effect.
Moreover, the mesophase of compounds 6, 7, 10 and 11 were studied by polarizing optical microscopy (POM). Although only Cr-Iso phase transition was observed for compound 7, the Cr-Col and Col-Iso phase transition of compounds 6, 10 and 11 were observed at the approximate temperatures of DSC curves. Their liquid crystalline textures clearly appeared aer gradual cooling from the isotropic phase. Fig. 2 illustrates the mesomorphic textures at 60 C. All textures were pseudo-confocal conic types, suggesting that columnar liquid crystal structures exist for compounds 6, 10 and 11.
As the liquid crystalline behaviors of compounds 6, 10 and 11 were conrmed by DSC and POM, the X-ray diffraction (XRD) was further employed to investigate the columnar stacking behaviors of mesophase. The XRD traces at 60 C are illustrated in Fig. 3. In the small-angle region, the strong peaks are observed at 5.41 , 5.25 , 5.28 for compounds 6, 10 and 11, respectively. The corresponding d-spacings calculated using the formula d ¼ l/(2 sin q) were 16.32Å, 16.81Å and 16.72Å for compounds 6, 10 and 11, respectively. These data were in agreement with the [100] reections of the column phase. Thus, the lattice parameter a for compounds 6, 10 and 11 could be calculated as 18.85Å, 19.41Å and 19.30Å, respectively. In the wide-angle region, the broad halos between 15 and 25 suggest Fig. 1 The DSC traces of compounds 6, 7, 10 and 11 on second heating and cooling (scan rate 10 C min À1 ).    showed not only a strong and narrow peak at 5.28 , but also two new peaks at 9.15 and 10.58 . These results suggested that compounds 6 and 10 might possess a mixed columnar phase or a disordered columnar phase, but compound 11 had an ordered columnar phase. Based on the reections at 5.28 , 9.15 and 10.58 of compound 11, the d-spacings following the calculations were found to be 16.72Å, 9.65Å and 8.36Å, which were in accordance with the ratio of 1 : 1/O3 : 1/O4 for (100), (110) and (200) reections. These results supported the assumption that compound 11 had the typical reection mode for the hexagonal columnar liquid crystal. The possible molecular stacking mode of hexagonal columns of compounds 11 was proposed, as shown in Fig. 4. The XRD analyses implied that compounds 6, 10 and 11 were columnar liquid crystals. In particular, compound 11 exhibited ordered hexagonal columnar mesophase.
The photophysical properties of compounds 6, 7, 10 and 11 were studied by absorption spectra and uorescent spectra. Their corresponding spectra in different organic solvents (hexane, toluene, CH 2 Cl 2 and DMF) are illustrated in Fig. 5-8. It can be seen that compounds 6 and 10 without alkenyl group showed similar absorption spectra with strong peaks observed at 501 nm. These results could be attributed to the same structure of their Bodipy units. The different solvents had little inuence on the absorption spectra. However, compound 7 with the mono-alkenyl group exhibited a clear red shi from 501 nm to 607 nm, and compound 11 with the 3,5-alkenyl groups presented a very large red shi from 501 nm to 622 nm. These phenomena could be explained by the larger aromatic conjugated structures of Bodipy units with alkenyl groups. In relation to the uorescent spectra, compounds 6 and 10 without alkenyl groups possessed narrow uorescent emission. The strongest uorescence intensity appeared at 515 nm in the DMF solution. Compounds 7 and 11 with alkenyl groups had broad emissions with large red shis, compared with compounds 6 and 10. The emission peaks of compound 7 with a mono-alkenyl group were between 600-700 nm in different solvents. It is noteworthy that compound 11 with 3,5-alkenyl groups displayed the near-infrared emission of 650-800 nm, as was expected. The solvents also greatly inuenced the emission of compounds 7 and 11. Compounds 7 and 11 exhibited the strongest emissions in toluene, but the emissions became signicantly weaker and were seen to red shi in the DMF solution. The low uorescences and further red shis in DMF    solution could be attributed to polar-solvent effect and potential aggregation. These results implied that by comparing with compounds 6 and 10, compounds 7 and 11 with the larger aromatic conjugated structures of Bodipy units exhibited red shis of uorescence and stronger p-p stacking for aggregation.
Furthermore, the stokes shis and the uorescent quantum yields (F F ) of compounds 6, 7, 10 and 11 were calculated, and the results are listed in Table 2. It can be seen that compounds 7 and 11 with alkenyl groups show a larger stokes shi compared with compounds 6 and 10. The stokes shis of compound 11 were as large as 114 nm and 129 nm in CH 2 Cl 2 and DMF, respectively. On the other hand, the F F of compounds 6, 7, 10 and 11 uctuated between 0.02 and 0.79 in different solvents. The F F of compounds 7 and 11 with alkenyl groups were lower than that of compounds 6 and 10. These phenomena also could be ascribed to the larger aromatic conjugated structures of Bodipy units in compounds 7 and 11, which showed a stronger p-p stacking for potential aggregation resulting in uorescent quenching. Nevertheless, compound 11, possessing the nearinfrared uorescence, had a moderate uorescence quantum yield of 19% in CH 2 Cl 2 and 31% in toluene. On the other hand, the absorption and uorescence emission spectra of compounds 6, 7, 10 and 11 were investigated in solid lms. The results are shown in Fig. 9. The uorescence quantum yields were 0.10, 0.12, 0.02 and 0.03 for the solid lms of compounds 6, 7, 10 and 11, respectively, which were lower than those in solutions. On comparing with corresponding spectra in solutions, clear red shis were observed in solid lms. These results suggested that strong aggregations existed in solid lms, resulting in the red shis of spectra and a decrease in the uorescence quantum yields. The emission spectra of compounds 6, 7, 10, and 11 at different temperatures (25 C and 50 C) were also studied. The results are exhibited in Fig. S23 and S24. † One can see that the uorescence maintained stable as a whole, with some decrease in the uorescent intensities, which may be attributed to the inuence of the lower solvent viscosity at higher temperatures. Based on the above analysis, it could be concluded that all of these Bodipy derivatives 6, 7, 10 and 11 showed good photophysical properties with moderate uorescence quantum yields. Particularly, triphenylene-Bodipytriphenylene triad 11 with 3,5-alkenyl groups was found to emit uorescence in the near-infrared region with a reasonable uorescence quantum yield of 31% in toluene.

Conclusions
In summary, triphenylene-Bodipy dyad 6 and its derivative 7 with mono-alkenyl group, and triphenylene-Bodipytriphenylene triad 10 and its derivative 11 with 3,5-alkenyl groups were designed and synthesized via the click chemistry reaction and Knoevenagel condensation. Compound 6 exhibited mesophase but compound 7 showed no liquid crystalline behavior. Both compound 10 and its derivative 11 exhibited mesomorphic properties based on the strong mesomorphicinduced effect of two triphenylene units. The investigation of photophysical properties suggested that all compounds emitted uorescence with moderate uorescent quantum yields.   Compound 11 displayed the near-infrared emission of 650-800 nm. These results implied that the triphenylene-Bodipytriphenylene triad 11 with 3,5-alkenyl groups was the rst example of a columnar Bodipy liquid crystal with near-infrared uorescence. The multiple triphenylene units were favourable for columnar mesophase and the 3,5-alkenyl groups on Bodipy led to near-infrared uorescence. This study presents a model on how to design and synthesis of novel columnar liquid crystal with near-infrared uorescence.