Emission properties of cyclodextrin dimers linked with perylene diimide—effect of cyclodextrin tumbling

Abstract

Perylene diimide (PDI) derivatives with cyclodextrins (PDI-CD₂s) exhibit specific emission properties, which depend on the type of CDs in an aqueous solution. Herein we successfully create an emission film-kneaded PDI-CD₂ derivatives via effective tumbling of the altropyranose unit. PDI-6CD₂s are crosslinked with PDI between 6-amino-CDs. Although the emission intensities of PDI-6CD₂s in dimethyl sulfoxide are similar regardless of the type of CD, PDI-6γCD₂ has a relatively high emission intensity in aqueous solutions. In contrast, for PDIC₇-3CD₂s, which are linked with N,N’-bis(6-carboxylhexyl)perylene-3,4,9,10-tetracarboxyl diimide (BisC₇-PDI) between 3-amino-CDs, the emission intensity of PDIC₇-3βCD₂ is stronger than those of PDIC₇-3αCD₂, PDIC₇-3γCD₂, and PDI-6CD₂s in aqueous solutions. The selective emission behavior of PDIC₇-3CD₂s is due to the formation of the pseudo[1]rotaxane dimer through tumbling of the altropyranose unit in an aqueous solution. PDIC₇-3βCD₂ in the solid state does not demonstrate a distinctive emission due to self-quenching, whereas PDIC₇-3βCD₂ kneaded into the polyvinyl alcohol (PVA) film exhibits a bright yellow emission. The order of the emission intensities of PDIC₇-3CD₂s kneaded into PVA films is similar to those in aqueous solutions.

Introduction

Recently, supramolecular assemblies with extended aromatic compounds have attracted much attention.^{1, 2, 3, 4} Perylene diimides (PDIs) derivatives effectively form supramolecular assemblies through π–π stacking interaction.^{5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21} In particular, supramolecular assemblies based on calixarenePDI conjugates show efficient energy and electron transfer properties due to the well-defined rigid and electron-rich scaffolds of calixarenes.^{22, 23, 24, 25, 26, 27} We have prepared PDI derivatives with a high emission property in aqueous solutions.

Generally, PDI derivatives have low solubilities in aqueous solutions. Even if PDI derivatives are dissolved in aqueous solutions by introducing a hydrophilic group, the emission of PDI derivatives exhibit self-quenching in aqueous solutions due to the formation of supramolecular assemblies and folders. To create water-soluble and effective emission sensor materials with PDI derivatives, we have introduced CDs into the PDI motif. The CDs are a family of macrocyclic oligosaccharides; the most common are composed of 6 (α), 7 (β), or 8 (γ) α-1,4-linked D-glucopyranose units.^{28, 29, 30, 31, 32, 33, 34} The introduction of CDs should influence the molecular recognition property and increase the emission intensity. Although supramolecular assemblies based on permethylated CDPDI conjugates have been previously reported,^{35, 36, 37, 38} the formation of assemblies cannot prevent the decrease in the monomer emission intensity of the PDI units due to self-quenching. In addition, the affinity of permethylated CDs with guest molecules significantly decreases compared with native CDs.³⁹

Herein we report the selective emission properties of PDI-CD₂ derivatives through the tumbling of the altropyranose unit in an aqueous solution. Previously, we have reported the formation of pseudo[1]rotaxane dimer from the altro-α-CD (altro-α-CD) dimer via tumbling of altro-α-CD,^{40, 41} which consists of one altropyranose unit and five glucopyranose units. Although some research groups have reported the tumbling of permethylated glucopyranose type CD,^{42, 43, 44, 45, 46, 47} tumbling of the altropyranose unit in altro-CDs has yet to be reported. We have successfully observed the selective emission of PDI-CD₂s in aqueous solutions and prepared emission films based on PDI-CD₂s kneaded into polyvinyl alcohol (PVA) films. The emission depends on the cavity size of the altro-CDs formed through altro-CD tumbling.

Experimental procedure

Preparation of PDI-6αCD₂

6-NH₂-α-CD (100 mg, 0.103 mmol) and 3,4,9,10-perylene tetracarboxylic dianhydride (20.0 mg, 0.0511 mmol) were dissolved in anhydrous dimethyl formamide (DMF; 10 ml) and stirred at 120 °C for 24 h under Ar atmosphere. Reaction mixture was dried under vacuum and dissolved in 5 ml of water. The solution was poured into acetone (100 ml) and participate was filtered. This precipitate was absorbed on Celite and wash with acetone and chloroform. After washing, crude product was eluted from Celite with water and purified by DIAION HP-20 reverse phase column (Mitsubishi Chemical Co., Tokyo, Japan, eluent: water to water/methanol=70: 30). Fraction of water/methanol=70:30 was collected and dried to obtain the product as a red solid in the yield of 43.3 mg (36.8%). ¹H nuclear magnetic resonance (NMR) and ¹³C NMR spectra are shown in Supplementary Figures S1 and S2. Contour plot of fluorescence intensity versus excitation and emission wavelengths in DMF or aqueous solution are shown in Supplementary Figures S3 and S4.

Preparation of PDI-6βCD₂

PDI-6βCD₂ was synthesized in the same manner as PDI-αCD₂, using 6-NH₂-β-CD (113 mg, 0.103 mmol), 3,4,9,10-perylene tetracarboxylic dianhydride (20.0 mg, 0.0511 mmol) and DMF (10 ml). Crude product was purified by DIAION HP-20 reverse phase column (eluent: water to water/methanol=70:30). The product was obtained as a red solid in the yield of 66.0 mg (49.3%). ¹H NMR and ¹³C NMR spectra are shown in Supplementary Figures S5 and S6. Contour plot of fluorescence intensity versus excitation and emission wavelengths in DMF or aqueous solution are shown in Supplementary Figures S7 and S8.

Preparation of PDI-6γCD₂

PDI-γCD₂ was synthesized in same manner as PDI-αCD₂, using 6-NH₂-γ-CD (259 mg, 0.206 mmol), 3,4,9,10-perylene tetracarboxylic dianhydride (40.0 mg, 0.102 mmol) and DMF (20 ml). Crude product was purified by DIAION HP-20 reverse phase column (eluent: water to water/methanol=70:30). The product was obtained as a red solid in the yield of 53.4 mg (18.1%). ¹H NMR and ¹³C NMR spectra are shown in Supplementary Figures S9 and S10. Contour plot of fluorescence intensity versus excitation and emission wavelengths in DMF or aqueous solution are shown in Supplementary Figures S11 and S12.

Preparation of PDIC₇-3αCD₂

N,N’-bis(6-carboxylhexyl)perylene-3,4,9,10-tetracarboxyl diimide (32.3 mg, 0.050 mmol), 3-NH₂-α-CD (97.2 mg, 0.100 mmol), PyBOP (57.2 mg, 0.110 mmol) and a drop of triethylamine were dissolved in DMF and stirred at room temperature for 48 h. DMF was evaporated and residue was dissolved in a small amount of water. Aqueous solution was poured in acetone (50 ml) and precipitate was collected (this manipulation was repeated three times). Crude product was purified by DIAION HP-20 reverse phase column (eluent: water to water/methanol=50:50). Fraction of water/methanol=50:50 was collected and dried to obtain the product as a red solid in the yield of 25.6 mg (20.0%). ¹H NMR and ¹³C NMR spectra are shown in Supplementary Figures S14 and S15. Contour plot of fluorescence intensity versus excitation and emission wavelengths in DMF or aqueous solution are shown in Supplementary Figures S17 and S18.

Preparation of PDIC₇-3βCD₂

PDIC₇-3βCD₂ was synthesized in same manner as PDIC₇-αCD₂, using 3-NH₂-β-CD (113 mg, 0.100 mmol). Crude product was purified by DIAION HP-20 reverse phase column (eluent: water to water/methanol=50:50). Fraction of water/methanol=60:40 was collected and dried to obtain the product as a red solid in the yield of 43.6 mg (30.3%). ¹H NMR and ¹³C NMR spectra are shown in Supplementary Figures S19 and S20. Contour plot of fluorescence intensity versus excitation and emission wavelengths in DMF or aqueous solution are shown in Supplementary Figures S22 and S23.

Preparation of PDIC₇-3γCD₂

PDIC₇-3γCD₂ was synthesized in same manner as PDIC₇-3αCD₂, using 3-NH₂-γ-CD (130 mg, 0.100 mmol). Crude product was purified by DIAION HP-20 reverse phase column (eluent: water to water/methanol=50: 50). Fraction of water/methanol=70:30 was collected and dried to obtain the product as a red solid in the yield of 33.0 mg (20.6%). ¹H NMR and ¹³C NMR spectra are shown in Supplementary Figures S23 and S24. Contour plot of fluorescence intensity versus excitation and emission wavelengths in DMF or aqueous solution are shown in Supplementary Figures S27 and S28.

Preparation of PVA films with PDI-CD₂s

The 300 μl of aqueous solution of PDI-CD₂s (1 mM) was dropped on the Teflon petri. The 8 g of poly(vinyl alcohol) (M_n=2000) aqueous solution (10 wt%) was added to the PDI-CD₂s aq. The PVA films with PDI-CD₂s were obtained by evaporating the water at 75 °C in a thermostatic chamber.

Results

Preparation and chemical structures of PDI-6CD₂s and PDIC₇-3CD₂s

Figure 1 shows the six different PDI-CDs. PDI-6CD₂s (PDI-6αCD₂, PDI-6βCD₂ and PDI-6γCD₂) are directly crosslinked between CDs with PDI. PDI-6CD₂s were prepared by the reaction of 6-amino-CDs with perylene dicarboxylic acid dihydride. PDIC₇-3CD₂s (PDIC₇-3αCD₂, PDIC₇-3βCD₂ and PDIC₇-3γCD₂) were prepared by the reaction of 3-amino-CDs with N,N’-bis(6-carboxylhexyl)perylene-3,4,9,10-tetracarboxyl diimide (BisC₇-PDI, see in Supplementary information in Supplementary Figure S13.) using 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM). The photophysical properties of PDI-6CD₂ and PDIC₇-3CD₂ were characterized by absorption and fluorescence spectroscopies. The supramolecular structures of PDI–CD derivatives were characterized by ¹H NMR spectroscopic methods.

Figure 1

Chemical structures of PDI-6CD₂s and PDIC₇-3CD₂s.

Absorption and fluorescence properties of PDI-6CD₂s

Figures 2 and 3 show the absorption and fluorescence spectra of PDI-6CD₂s in DMF and aqueous solutions, respectively. The absorption and fluorescence spectra of PDI-6CD₂s in DMF are similar regardless of the type of CD. Table 1 summarizes the quantum yields (Φ_em) of PDI-6CD₂s and PDIC₇-3CD₂s in DMF and water. The Φ_em values of PDI-6αCD₂ and PDI-6βCD₂ are slightly higher than that of PDI-6γCD₂ in DMF. On the other hand, the fluorescence intensities of PDI-6CD₂s in aqueous solutions significantly decrease due to self-quenching. The difference in the fluorescence intensities and Φ_em of PDI-6CD₂s are due to self-aggregation. Φ_em of PDI-6γCD₂ is higher than those of PDI-6αCD₂ and PDI-6βCD₂, and this higher value is related to the inhibition of self-aggregation through π–π stacking interactions due to the bulkiness of γ-CD in aqueous solutions.

Figure 2

Absorption (a) and fluorescence (b) spectra of PDI-6CD₂s in DMF. Concentrations are adjusted to 15 μM. Samples in fluorescence measurements are excited at λ_ex=489 nm.

Figure 3

Absorption (a) and fluorescence (b) spectra of PDI-6CD₂s in water. Concentrations are adjusted to 15 μM. Samples in fluorescence measurements are excited at λ_ex=495 nm.

Table 1 Photophysical properties of PDI-6CD₂s and PDIC₇-3CD₂s^a

Absorption and fluorescence properties of PDIC₇-3CD₂

PDIC₇-3CD₂s exhibit unpredictable specific emission behaviors in aqueous solutions. First, we investigated the dependence of the photophysical properties of PDIC₇-3CD₂s on the solvent. Figure 4, which shows the absorption and fluorescence spectra of PDIC₇-3CD₂s in DMF, demonstrates that the absorption and fluorescence spectra of PDIC₇-3CD₂s are similar for all the CDs. However, in aqueous solutions, the absorption band of PDIC₇-3CD₂s depends on the CD cavity (Figure 5). The absorption band of PDIC₇-3βCD₂ exhibits distinctive vibrational-electronic coupling and vibronic transitions on top of the allowed π–π^* electronic transition can be resolved, whereas PDIC₇-3αCD₂ and PDIC₇-3γCD₂ display broad absorption bands. Φ_em of PDIC₇-3βCD₂ is significantly higher than those of PDIC₇-3αCD₂ and PDIC₇-3γCD₂ (Table 1). Φ_em of PDIC₇-3βCD₂ in an aqueous solution is close to that of PDIC₇-3βCD₂ in DMF.

Figure 4

Absorption (a) and fluorescence (b) spectra of PDIC₇-3CD₂ in DMF. Concentrations are adjusted to 15 μM. Samples in fluorescence measurements are excited at λ_ex=489 nm.

Figure 5

Absorption (a) and fluorescence (b) spectra of PDIC₇-3CD₂s in water. Concentrations are adjusted to 15 μM. Samples in fluorescence measurements are excited at λ_ex=494 nm.

Due to π-stacking interactions, large aromatic ring derivatives can easily form supramolecular assemblies. Hence, the strong intermolecular vibrational-electronic coupling in PDI derivatives has been well studied.^{6, 16, 17, 18, 19, 20, 21} The formation of PDI assemblies leads to broad vibrational-electronic coupling due to fused aromatic rings whose molecular orbital overlap with adjacent neighbors. In contrast, free PDI derivatives provide clear vibrational-electronic coupling. The Huang−Rhys factor, S=A^{0 → 1}/A^{0 → 0} (A^{0 → 0}, absorption intensity of 0 ^→0 band; A^{0 → 1}, absorption intensity of 0 → 1 band), is an indicator of the formation of supramolecular assemblies.^{6, 16, 17, 18, 19, 20, 21} The intensities of 0 → 0 band (A^{0 → 0}) and 0 → 1 band (A^{0 → 1}) in DMF do not have a measurable difference between PDIC₇-3CD₂s (Figure 4), whereas the intensities in aqueous solutions depend on the CD cavity (Figure 5). The S ratio of PDIC₇-3βCD₂ in an aqueous solution is similar to that in DMF (S=0.74 (aq), S=0.64 (DMF)). The S ratios of PDIC₇-3γCD₂ in aqueous solutions is 1.25, which is the value intermediate between PDIC₇-3αCD₂ and PDIC₇-3βCD₂ in aqueous solutions. The S ratios of PDIC₇-3αCD₂ in aqueous solutions is 2.05, which is close to those of PDI-6CD₂s in aqueous solutions.

Although the association behavior of PDIC₇-3αCD₂ is similar to the values for known PDI derivatives,^{5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21} PDIC₇-3βCD₂ shows a definite emission, suggesting that PDIC₇-3βCD₂ is relatively dispersed even in aqueous solutions. PDIC₇-3αCD₂ and PDIC₇-3γCD₂ do not show distinctive quantum yields (Φ_em=0.90 and 11%). However, Φ_em of PDIC₇-3βCD₂ is 43%, which is close to that in DMF. These results indicate that PDIC₇-3CD₂s are effectively dispersed and form monomers in DMF, but PDIC₇-3αCD₂ and PDIC₇-3γCD₂ form supramolecular assemblies in aqueous solutions. However, PDIC₇-3βCD₂ is effectively dispersed even in an aqueous solution.

Supramolecular structure of PDIC₇-3CD₂s

We hypothesized that the emission differences are due to the formation of supramolecular complexes. The two-dimensional rotating-frame overhauser spectroscopy spectrum of PDIC₇-3αCD₂ indicates the C7 alkyl unit and the inner protons of altro-α-CD end groups are correlated, but the PDI unit and inner protons in aqueous solutions are not (see Supporting information in Supplementary Figure S16). The two-dimensional rotating-frame overhauser spectroscopy spectrum of PDIC₇-3βCD₂ demonstrates that the inner protons of altro-β-CD are correlated to the protons of PDI and C7 alkyl units (Figure 6). The two-dimensional rotating-frame overhauser spectroscopy spectrum of PDIC₇-3γCD₂ shows the altro-γ-CD inner protons are correlated to the protons of PDI and C7 alkyl units (Supplementary Figure S25). These results show that the altro-CD unit includes the C₇PDI axis molecule in aqueous solutions.

Figure 6

Two-dimensional nuclear overhauser effect spectroscopy NMR spectrum of PDIC₇-3βCD₂ in D₂O (1 mM) at 30 °C (mixing time=150 ms) and proposed structure of the pseudo[1]rotaxane dimer from PDIC₇-3βCD₂ in water.

How is the altro-CD unit included the C₇PDI axis molecule in the cavity? PDIC₇-3CD₂s are regarded to have a dumbbell shape. Other CD molecules cannot physically slip through the end of the altro-CDs of PDIC₇-3CD₂s to form pseudo[2]rotaxane. Previously, we have reported that an alkyl altro-α-CD dimer is converted into a pseudo[1]rotaxane dimer through tumbling of the altropyranose unit of altro-α-CD in D₂O.^{40, 41} On the basis of these results, PDIC₇-3βCD₂ forms a pseudo[1]rotaxane dimer through tumbling of the altropyranose unit of altro-β-CD in aqueous solutions (Scheme 1).

PDIC₇-3αCD₂ forms a pseudo[1]rotaxane dimer, which includes the C7 alkyl unit in the altro-α-CD cavity (Scheme 2). The cavity size of altro-α-CD is too small to include the PDI unit. Altro-β-CD and altro-γ-CD possess a sufficient cavity size to include the PDI unit. PDIC₇-3βCD₂ and PDIC₇-3γCD₂ form pseudo[1]rotaxane dimmers, where the PDI unit is covered by the altro-CDs end groups. Coverage of the PDI unit inhibits the π–π stacking interaction between PDI units, leading to self-quenching. In contrast, PDIC₇-3αCD₂ cannot inhibit self-quenching due to the defective coverage of the PDI unit. Actually, we have investigated the inhibition of altro-β-CD tumbling using a competitive guest, adamantane carboxylic acid sodium salt, which is strongly included in the cavity of β-CD.^{39, 48, 49, 50, 51, 52} Mixing 100 equivalents of adamantane carboxylic acid sodium salt guest molecules with a PDIC₇-3βCD₂ aqueous solution decreases the emission intensities of PDIC₇-3βCD₂ (Supplementary Figure S29). The addition of an excess of adamantane carboxylic acid sodium salt into a PDIC₇-3βCD₂ aqueous solution causes a marked decrease in the emission intensity, which is 61% of the initial intensity. Hence, adamantane carboxylic acid sodium salt included in the altro-β-CD unit of PDIC₇-3βCD₂ inhibits tumbling of the altropyranose unit.

Association constants of CDs with BisC₇-PDI

To investigate the differences in the correlation peaks of the two-dimensional nuclear overhauser effect spectroscopy NMR spectra and the emission intensities for the various CDs, the stoichiometric proportion and the association constants were determined by UV titration measurements (see Supporting Information, Supplementary Figures S33–S35). Job’s plots suggest that each CD forms a 2:1 complex for BisC₇-PDI in aqueous solutions. (see Supporting information, Supplementary Figures S30–S32) The K₁ values of α-CD and γ-CD with BisC₇-PDI are 6.2 × 10⁴ M⁻¹ and 1.5 × 10⁵ M⁻¹, respectively, whereas that of β-CD with BisC₇-PDI is larger (4.0 × 10⁵ M⁻¹; Table 2) These results indicate that the emission intensity of PDIC₇-3βCD₂ is selectively higher than those of PDIC₇-3αCD₂ and PDIC₇-3γCD₂ due to the suppression of self-aggregation.

Table 2 Association constants of CDs with BisC₇-PDI in aqueous solutions

Emission properties of PDIC₇-3CD₂ films

Figure 7 shows photographs of PDIC₇-3CD₂s in aqueous solutions. The emission intensities do not differ for PDIC₇-3CD₂s in DMF solutions, whereas in aqueous solutions, only PDIC₇-3βCD₂ shows a distinct green−yellow emission.

Figure 7

Emission properties of PDIC₇-3αCD₂ (a), PDIC₇-3βCD₂ (b) and PDIC₇-3γCD₂ (c) in aqueous solutions under UV light (λ_ex=365 nm).

Using the emission properties in aqueous solutions, we prepared a PVA film with PDIC₇-3CD₂s. An aqueous solution of PDIC₇-3CD₂s (0.3 μmol) was mixed with an aqueous solution of PVA (0.80 g), and subsequently dried at 75 °C (see Supporting information, Supplementary Figure S36). Although the resulting PVA films with PDIC₇-3CD₂s are slightly red, they are the same under visible light (Figure 8). Only the film with PDIC₇-3βCD₂ shows a bright yellow emission under UV light. PVA films with PDIC₇-3αCD₂ and PDIC₇-3γCD₂ do not display distinct emissions; actually, the PDIC₇-3αCD₂ and PDIC₇-3γCD₂ films do not show distinct quantum yields (Φ_em=14 and 24%), but Φ_em of a film of PDIC₇-3βCD₂ is 54%. The PVA film with PDIC₇-3βCD₂ can be prepared from the dimethyl sulfoxide solutions, but this method is unsuited for a homogeneous, large-area film with a flat surface (Figure 8d). Dropping a PDIC₇-3βCD₂ aqueous solution onto a quartz plate and subsequent drying in air produce a PDIC₇-3βCD₂ solid that exhibits a weak red light instead of a bright yellow emission under UV light (λ_ex=365 nm; see Supporting information, Supplementary Figure S37).

Figure 8

Emission properties of PVA films dissolved PDIC₇-3αCD₂ (a), PDIC₇-3βCD₂ (b), and PDIC₇-3γCD₂ (c), under visible light and UV light (λ_ex=365 nm). PVA film (d) is prepared via a dimethyl sulfoxide solution of PDIC₇-3βCD₂.

PDIC₇-3βCD₂ is self-quenching in the solid state due to self-aggregation through a π–π stacking interaction. First, we speculated that the emission intensity differences between PDIC₇-3CD₂s would not be observed in PVA films, because the structure of the pseudo[1]rotaxane dimer from PDIC₇-3βCD₂ would decompose in the PVA matrix. However, the supramolecular structure of PDIC₇-3βCD₂ remains in the PVA films. Only the PDIC₇-3βCD₂ film shows a bright green−yellow emission under UV light.

Discussion

We prepared CD-PDI derivatives where the emission properties depend on the type of CD. Although PDI derivatives usually have low solubilities in aqueous solutions, the introduction of the CD units improves the solubility. PDI-6CD₂s, in which the CD unit is directly introduced into the PDI unit without spaces, dissolves in water, but does not show an emission difference with the type of CD unit. In contrast to PDI-6CD₂s, PDIC₇-3βCD₂ displays a bright yellow emission and PDIC₇-3γCD₂ has a weak emission. The emission properties of PDIC₇-3CD₂s are due to the tumbling of the altropyranose unit, which prevents self-aggregation and self-quenching in aqueous solutions.

To utilize the selective emission properties, PVA films woven with PDIC₇-3CD₂s were prepared. Even in PVA films, PDIC₇-3CD₂s show selective emission behaviors, which depend on the CD. We initially speculated that the pseudo[1]rotaxane dimer would decompose to the original dimer in the film, and the emission differences according to the type of CD would not be observed. However, the opposite results are observed, indicating PDIC₇-3βCD₂ forms pseudo[1]rotaxane dimers even in PVA films. The PVA films with PDIC₇-3βCD₂ should function as emission and quenching sensor films for chemical compounds based on the molecular recognition property of CDs.

Formation of the pseudo[1]rotaxane dimer from PDIC₇-3βCD₂ via tumbling of the altropyranose unit.

Schematic illustration of the solvent polarity dependent formation of the pseudo[1]rotaxane dimer from PDIC₇-3αCD₂ (a), PDIC₇-3βCD₂ (b), and PDIC₇-3γCD₂ (c).