Aggregation behavior of α-substituted dibenzoylmethanatoboron difluoride complex with various alkyl groups in water/acetone mixtures

Abstract

Dibenzoylmethanatoboron difluoride complexes (BF₂DBM) exhibit aggregation-induced emission (AIE) properties by introducing a methyl group at the α-position of the dioxaboroline ring. In this study, we synthesized α-substituted BF₂DBM derivatives with alkyl groups and evaluated their photophysical and AIE properties as a function of water fraction (f_w) in water/acetone mixtures. The aggregation behavior of the BF₂DBMs differed depending on f_w, although they exhibited AIE independently of the alkyl groups. The f_w dependence suggested that the alkyl groups exhibit hierarchical aggregation behavior, indicating that the alkyl groups affect the intermolecular stacking pattern based on the X-ray crystallography, leading to the difference in the fluorescence properties of the crystals.

Graphical Abstract

aggregation-induced emission, α-substitution, dibenzoylmethanatoboron difluoride

Luminescent organic solids have potential applications in luminescence solar concentrators and organic light-emitting diodes. However, it is generally known that the emission efficiency of fluorophores decreases in a concentrated state due to various intermolecular interactions by the dense packing, such as van der Waals interactions, π–π interactions, and hydrogen bonding interactions between molecules, leading to the fluorescence quenching (so-called aggregation-caused quenching). For this reason, organic molecules with aggregation-induced emission (AIE) properties, which emit light in the solid state but hardly in solution, have attracted much attention to the viewpoint of device applications.^1–4

The design guideline of AIE compounds is the possessing sites or parts that enable free rotational or vibrational motion within the molecule. Tetraphenylethylene, hexaphenylsilole, and cyclooctatetrathiophene have been reported as typical AIE molecule.^1,5 These molecules show very weak fluorescence in low-viscosity solutions, which is attributed to the enhancement of non-radiative deactivation due to the conformational changes by rotational or vibrational motions in the excited state, or reaching the conical intersection (CI) through the downhill potential surface associated with conformational changes.⁶ In the solid state, on the other hand, the molecules are spatially dense, suppressing the conformational changes (restriction of the intramolecular motion) or restricting access to a CI. Therefore, it is important to design a molecular structure that can induce conformational changes in the excited state in a fluid for AIE molecules.

Dibenzoylmethanatoboron difluoride complex (BF₂DBM) exhibits high fluorescence quantum yields both in solution and the solid state.^7–11 Fraser et al. reported substituent effects at the α-position of the dioxaborine ring on 1,3-bis-(4-methoxyphenyl)methanatoboron difluoride (2aBF₂).¹² Substituting the H-atom to the methyl group (1,3-bis-(4-methoxyphenyl)-2-methyl methanatoboron difluoride; 2amBF₂),¹² weak fluorescence is observed in solution, but the crystals show strong emission.¹³ To clarify the difference in the fluorescence property, we investigated the AIE of 2amBF₂ using time-resolved absorption spectroscopy and quantum chemical calculations.¹⁴ Time-resolved visible absorption measurements revealed peak shifts and broadening of the stimulated emission in the solution of 2amBF₂ owing to the rapid change in the molecular geometry. With the temporal evolution of the time-resolved IR absorption signals and DFT calculations of these systems, it was deduced that 2amBF₂ has a bending geometry in the S₁ state, leading to rapid deactivation to the S₀ state.

Based on the above reports, it is expected that the introduction of an alkyl group at the α-position can impart AIE properties to BF₂DBM derivatives (BF₂DBMs). In this study, we synthesized BF₂DBMs with various alkyl groups at the α-position (ethyl group: EtBF₂, n-propyl group: n-PrBF₂, n-butyl group: n-BuBF₂, n-pentyl group: n-PeBF₂, i-pentyl group: i-PeBF₂; Fig. 1) for the first time. We evaluated the photophysical properties in solution, in the crystalline state, and in dispersed polymer thin films to confirm the AIE properties. To verify the molecular aggregation behavior, we also measured the fluorescence spectrum and intensity as a function of the water fraction (f_w).

Fig. 1.

Synthetic route and molecular structures of BF₂DBMs substituted by the alkyl groups at α-position.

The synthesis of α-substituted BF₂DBMs involved a 2-step process (Fig. 1). Initially, an SN₂ reaction was conducted between 1,3-bis-(4-methoxyphenyl)-1,3-propadione and alkyl iodide in tetrahydrofuran. Subsequently, boration was performed using BF₃·OEt₂ in dichloromethane (DCM). Synthesis details and ¹H-NMR, ¹³C-NMR, and MS spectroscopies data are given in Supplementary Supporting information. Details of the sample preparation and measurements are also described in Supplementary Supporting information.

To verify the AIE properties of α-substituted BF₂DBMs, the fluorescence spectra were measured in DCM dilute solutions, poly(methyl methacrylate) (PMMA) polymer films, and crystalline states.

Weak emission was observed in all α-substituted BF₂DBM solutions, probably originating from its bending geometry similar to 2amBF₂ reported previously.¹⁴ As shown in Fig. 2, blue fluorescence was observed in the PMMA films and green fluorescence was observed in the crystalline states. AIE molecules can emit not only in molecular aggregates, such as crystals, but also in rigid media when intramolecular motion is restricted.^1,13,15 We have verified the generality of the AIE properties of BF₂DBMs for which the α-position is substituted with the alkyl groups.^12–14 The fluorescence spectra of all α-substituted BF₂DBMs in polymer films showed a peak around 450 nm. In the crystalline states, however, only n-BuBF₂ showed a peak around 490 nm, while the other derivatives showed a peak around 510 nm.

Fig. 2.

Fluorescence images and spectra of α-substituted BF₂DBMs in DCM solutions (hashed lines), PMMA polymer films, and crystals (solid lines).

To investigate the aggregation phenomena in addition to AIE properties of α-substituted BF₂DBMs, the photophysical properties of water/acetone mixtures as a function of f_w were investigated. Figure 3 shows the fluorescence photographs in the water/acetone mixtures as a function of f_w. All molecules showed almost no fluorescence below f_w = 50%. EtBF₂ and n-PrBF₂ showed green fluorescence from f_w = 60% to 90%. On the other hand, n-BuBF₂, n-PeBF₂, and i-PeBF₂ exhibit cyan fluorescence at f_w = 60% and turn to green fluorescence over 70%. The photograph of the solution in n-BuBF₂ displays weak fluorescence with suspension that differs from others.

Fig. 3.

Fluorescence images of α-substituted BF₂DBMs in water/acetone solution.

Figure 4 shows the fluorescence spectra and intensity changes of α-substituted BF₂DBMs in the water/acetone mixture. All derivatives exhibited significant fluorescence with f_w >60%. In the range of f_w = 0% to 60%, the absorption bands originating from the monomer decreased with increasing f_w (shown in Supplementary Fig. S2), suggesting that the monomer forms aggregates with increasing f_w. Therefore, the abrupt increase in fluorescence intensity above f_w = 60% is attributed to the formation of aggregates, strongly supporting the AIE properties of α-substituted BF₂DBMs. The fluorescence spectrum of EtBF₂ exhibited a peak around 480 nm with a shoulder around 450 nm at f_w = 60% (indicated in cyan line). As increasing the f_w, the peak position was turned to 450 nm, especially n-BuBF₂ displays the most clearly. The band around 450 nm is identical to the fluorescence peak observed in the PMMA film, indicating the restriction of the bending motion of the dioxaborine ring.¹⁴ It is suggested that highly crowded environments or loosely packed aggregation structures may be formed in the initial stage of aggregate formation based on the previous work reported by our group¹⁶ and Tsuji et al.¹⁷ Notably, n-BuBF₂ demonstrated a red shift to around 500 nm from f_w = 60% to 90%. The shifts in fluorescence maxima were 10, 30, 50, 20, and 40 nm for EtBF₂, n-PrBF₂, n-BuBF₂, n-PeBF₂, and i-PeBF₂, respectively (the normalized fluorescence spectra are presented in Supplementary Fig. S3).

Fig. 4.

Fluorescence spectra and intensity ratio (I/I₀) of α-substituted BF₂DBMs in water/acetone solution. I/I₀ indicates the relative intensity to the intensity at f_w = 0%.

These observations suggest the existence of the sequential aggregation process at α-substituted BF₂DBMs. The decrease in emission intensity at f_w = 90% observed in all systems is probably due to fluorescence quenching associated with the formation of tighter aggregates.¹⁷ We performed the single-crystal X-ray structure analysis to investigate the effect of the alkyl groups on the stacking pattern. Figure 5 shows the stacking structure of the 2 molecules extracted from the crystal structure data (details of the crystal structures are shown in Supplementary Figs. S4 to S8 and Table S1). The percentages of the overlap area were estimated by Image-J based on the adjacent molecular stacking. EtBF₂, n-PrBF₂, n-PeBF₂, and i-PeBF₂ have face-to-face stacked dimeric structures with a large overlap area between the phenyl rings and the dioxaborine rings, the values of which are 44%, 55%, 53%, and 69%, respectively. On the other hand, n-BuBF₂ showed a large slip in the long-axis direction of the molecule, and the overlap between molecules was small (2%). Intermolecular interactions between atoms were evaluated using Crystal Explorer.¹⁸ The results are shown in Supplementary Figs. S9 and S10. In n-BuBF₂, the C–C ratio was significantly smaller than the C–H ratio. As shown in Fig. 5c, n-BuBF₂ in the crystal has π electron-conjugated planes and butyl groups that are orthogonal to each other, and therefore the enhancement of the interaction between the alkyl groups results in the suppression of the π–π interaction. This is supported by the fact that the melting point of n-BuBF₂ crystals is lower than that of the other derivatives, as shown in Supplementary Table S2. This monomer-like stacking structure may be related to the monomer-like fluorescence bands observed in n-BuBF₂ at f_w = 60%. We have previously reported a polymorphic transition process in re-precipitated solutions,¹⁹ thus the f_w-dependent peak changes observed in n-BuBF₂ after fluorescence enhancement probably suggest the presence of polymorphs.

Fig. 5.

Dimeric stacked structure in the crystalline state obtained by the X-ray structure analysis. a) EtBF₂, b) n-PrBF₂, c) n-BuBF₂, d) n-PeBF₂, e) i-PeBF₂. The filled areas of the capped stick styles indicate overlapping parts of the π conjugation.

In conclusion, we evaluated the AIE properties of BF₂DBMs with an alkyl group substituted at the α-position. It was found that the aggregation behavior of the BF₂DBMs differed depending on f_w, although they exhibited AIE independently of the chain length. In particular, α-substitution of the alkyl groups exhibits hierarchical aggregation behavior. In addition, we found that the alkyl groups affect the intermolecular stacking pattern, leading to the difference in the fluorescence properties of the crystals.

Acknowledgments

We acknowledge Prof. Hiroyuki Suga, Prof. Yasunori Toda, and Prof. Hidehumi Makabe (Shinshu University) for their assistance with the NMR and TOF-MS measurements.

Supplementary data

Supplementary material is available at Chemistry Letters online.

Funding

This work was partly supported by Japan Society for the Promotion of Science (JSPS) KAKENHI grant numbers supported by 24K08350, 24KJ1208, 21KK0092, 19H02686, 23K26619, 24K01458, and 24K21794.

Author notes

Conflict of interest statement. None declared.