Phenylcarboxyl-decorated tetraphenylethene with diverse molecular RIM-induced emission from host–guest inclusion and aggregation formation

Abstract

A novel synthesized 4-carboxylphenoxy-decorated tetraphenylethene and α-cyclodextrin could form a host–guest inclusion complex with molecular RIM-induced Emission. On the basis of this, the TPE-COOH⊂α-CD inclusion complex exhibits a reversible fluorescence intensity change response to the photo-isomerization of 4-(phenylazo)benzoic acid (PBA) under alternate UV- and visible-light irradiation.

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Luminogenic molecules with aggregation-induced emission (AIE) nature have been attracting increasing research interest in the past decade owing to their various potential applications in optoelectronic devices, such as photoelectric conversion, biological probes, and organic light-emitting diodes (OLEDs).^1,2 In contrast to the fluorophores showing notorious aggregation-caused quenching (ACQ) characteristics in the solid state, the AIE luminogens (AIEgens) are able to emit efficiently in the concentrated state due to aggregate formation.³ The fluorescence enhancement for AIEgens is mainly attributed to the restriction of intramolecular motions (RIM) in the aggregate state, which can block the radiationless relaxation channel and open the radiative decay pathway, switching on fluorescence emission of AIEgens. This RIM-induced emission mechanism for rotor-carrying AIEgens has been approved by manipulating external experiment conditions (like increasing solvent viscosity, decreasing solution temperature, or pressurizing solid film) or by modulating such internal control experiments as introducing sterically bulky groups and cross-locking with tethering units.^1,4–6 Additionally, the intermolecular network structures which can confine the intramolecular motions of AIEgen such as microporous polymers⁷ and metal–organic framework⁸ were also employed to reach this goal.

Cyclodextrin (CD) are well-known for its unique inclusion property toward a wide range of hydrophobic guest species.^9–11 The inclusion of hydrophobic guest species inside the cavity of CD may restrict the free rotation of corresponding guest species such as AIE-active tetraphenylethene (TPE) molecule. As a consequence, a few of CD-consisting host–guest complexes with TPE derivatives were fabricated to investigate the RIM effect of AIE-active guest molecules. Tang and coworkers reported a series of α-, β-, or γ-CD-decorating TPEs with high emission efficiency in dimethylsulfoxide (DMSO) solution due to the intramolecular host–guest inclusion.^12a They revealed that the diboronic acid-containing TPE can be utilized to recognize β-CD from α- or γ-CD based on host–guest interaction-aided boronic acid/diol-binding.^12b Zheng et al. found that oligo ethylene glycol-modified TPE exhibit an enhanced monomer emission and a decreased aggregate emission when they were included in the cavity of γ-CD.¹³ It is noteworthy that despite a few reports on RIM process of CD-consisting host–guest complex with TPE moiety, systemic exploration on RIM-induced emission from host–guest inclusion and the difference from aggregation induced emission has not yet been reported out thus far.

In the present paper, a 4-carboxylphenoxy-decorated TPE (TPE-COOH) luminogen was designed and synthesized. This compound exhibited typical AIE characteristics in the mixed solvent of DMSO (or THF) and water. Meanwhile, addition of the α-CD into TPE-COOH in the basic DMSO/H₂O (1/1, v/v) solution switches on its molecular fluorescence emission centered at 455 nm due to the RIM process associated with host–guest inclusion, which is slightly different from its red-shifted emission (466 nm) attributed to the AIE process in DMSO/H₂O (1/9, v/v). Particularly, on the basis of the RIM-induced emission from host–guest inclusion, TPE-COOH⊂α-CD complex exhibits a reversible fluorescence intensity response to the azobenzene derivatives under alternate UV- and visible-light irradiation.

The synthesis of 4-carboxylphenoxy substituted TPE derivative (TPE-COOH) was shown Scheme 1. 1-(4-(Bromomethylphenyl))-1,2,2-triphenylethylene was prepared according to the precious report with some modification.¹⁴ This compound was then treated with methyl 4-hydroxybenzoate in the presence of K₂CO₃ in DMF, affording compound 3. Further hydrolysis was performed in saturated NaOH solution of THF and H₂O to get TPE-COOH 4. All the compounds were characterized by ¹H NMR, ¹³C NMR, 2D COSY, and ESI-MS spectroscopies (Fig. S1–12, ESI†).

Scheme 1 Synthesis of compound TPE-COOH(4).

To investigate whether the synthesized TPE-COOH is endowed with the same AIE characteristics as those exhibited by other TPE derivatives,¹ the fluorescence emission behavior of this compound in the THF–H₂O mixture with different H₂O fraction (f_w, the volume percentage of H₂O in the THF or DMSO–H₂O mixture) was studied. As shown in Fig. S13 (ESI†), when the water fraction f_w is below 75%, TPE-COOH is almost non-emissive. However, along with the increase of f_w from 75 to 90%, weak fluorescence emission of this compound is swiftly enhanced due to the aggregation formation, which restricts the free motions of TPE moiety and in turn switches on its fluorescence emission, revealing the AIE nature of TPE-COOH at high water fraction.¹

Since host–guest inclusion can restrict the intramolecular motions (RIM) of guest inside the cavity of host, to study the spectroscopic properties of RIM process associated with host–guest inclusion with AIE-active guest, the fluorescence emission spectrum of TPE-COOH upon addition of CD was recorded with the excitation of 330 nm. It is well known that the solvent choice plays a very important role in studying RIM-induced emission of AIE-active host–guest complex. In general, the aqueous media is preferable to form the host–guest inclusion owing to the favorable hydrophobic interaction between guest molecule and CD.¹² Meanwhile molecular aggregation is very common for AIE-active luminogen in high water fraction. As a result, it seems very puzzling to exactly attribute the fluorescence enhancement of AIE-active host–guest complex to the AIE effect or the host–guest inclusion in high water media. Similarly, the organic solvent was rarely utilized because the strong solvated effect can make the guest molecule hardly get into the cavity of host to form host–guest complex.^11,12 As a consequence, it is essential to determine an appropriate ratio of organic/water mixture, whose solvation effect can just endow TPE-COOH with weakly non-aggregated emission but synchronously is sufficient to facilitate the formation of TPE-COOH-based host–guest complex with RIM-induced emission after adding CD.

Since the alkaline media can enhance the amphiphilicity of guest with carboxylic acid group which will be beneficial to the formation of host–guest complex,¹¹ NaOH (1.1 equiv, relative to TPE-COOH) was added into the experimental solution (the apparent pH value is 7.94). In the preliminary experiments, we tried to use the basic THF–H₂O mixture with different water fraction as solvent. As shown in Fig. S14 (ESI†), the fluorescence emission behavior of TPE-COOH upon addition of α-CD (2 equiv.) takes little change relative to that without addition of α-CD in basic THF–H₂O mixture, suggesting that the investigation on RIM-induced emission of TPE-COOH-based host–guest complex in this media is not suitable for THF's strong solvated effect. Taking it into account that DMSO could be miscible with water and sometime is utilized as experimental media in the formation of host–guest complex, the fluorescence emission property of TPE-COOH upon addition of α-CD was probed in the basic DMSO–water mixture. Similar to that in basic THF–H₂O mixture, TPE-COOH also exhibits an obvious aggregation-induced emission effect when the f_w of basic DMSO–H₂O mixture is above 50% (Fig. 1). In other words, TPE-COOH in this media is weak emissive. Notably, the weak emission of TPE-COOH in this medium is found to be obviously increased upon addition of α-CD (2 equiv.), despite little optical change occurring in other ratio of basic DMSO–H₂O mixture (Fig. 2). Nevertheless, in neutral DMSO–H₂O mixture with different f_w being from 0 to 90%, addition of α-CD (2 equiv.) into TPE-COOH induces little optical change relative to that without addition of α-CD (Fig. S15 and 16, ESI†). These results show that the basic DMSO–H₂O mixture (f_w = 50%) can be employed as experimental medium to investigate the RIM effect of TPE-COOH-based host–guest complex with α-CD. The fluorescence increase of TPE-COOH upon addition of α-CD (2 equiv.) in this medium is attributed to the formation of TPE-COOH-based host–guest complex with α-CD, which can restrict the phenyl motions of TPE moiety in sufficiently and in turn lead to the fluorescence emission of TPE-COOH.

Fig. 1 (A) Fluorescence spectra and (B) plots of the fluorescence intensity at 455 nm of TPE-COOH in DMSO–H₂O mixtures with different water fractions. Concentration: 30 μM (contain 1.1 equiv. NaOH); λ_ex: 330 nm (10 nm, 10 nm); 283 K.

Fig. 2 Change in fluorescence intensity of TPE-COOH at 455 nm upon addition of 2 equiv. α-CD in different water fraction DMSO–H₂O mixtures. Concentration: 30 μM (contain 1.1 equiv. NaOH); λ_ex: 330 nm (10 nm, 10 nm); 283 K.

To explore the binding stoichiometry between TPE-COOH and α-CD in TPE-COOH-based host–guest complex with α-CD, the quantitative fluorescent titration experiments of TPE-COOH (30 μM) with increasing amount of α-CD (0–1.5 equiv.) were carried out in a basic H₂O–DMSO mixture (f_w = 50%).¹⁵ As shown in Fig. 3, the weak fluorescence emission of TPE-COOH at 455 nm is gradually enhanced (60-fold) along with the increasing amount of α-CD from 0 to 1 equiv. Then further increase in the α-CD amount added even to 1.5 equiv. leads to almost no change in the emission spectrum. Moreover, the fluorescence intensity of this compound increases linearly based on Benesi–Hildebrand plot of 1/(F − F₀) against 1/[α-CD] (Fig. S17, ESI†), implying the possible 1 : 1 binding stoichiometry between TPE-COOH and α-CD in TPE-COOH–based host–guest complex, with the association constant (K_a) being close to 1.81 × 105 M⁻¹.¹⁶ This is further confirmed by fluorescent Job's plot (Fig. S18, ESI†). By plotting the intensity of the fluorescence peak at 455 nm versus the mole fraction of TPE-COOH (total concentration = 50 μM), the curve clearly showed a maximum at 0.5. In addition, ESI-MS spectrum for the mixture of TPE-COOH and α-CD displays an obvious molecular ion peak at 1452.75 designated to TPE-COOH–α-CD complex (calcd 1453.50 [M − H]⁻), (Fig. S19, ESI†), again approving the 1 : 1 chelating model between TPE-COOH and α-CD in TPE-COOH-based host–guest complex with α-CD.

Fig. 3 (A) Change in fluorescence spectrum of TPE-COOH upon titration with α-CD in DMSO–H₂O (f_w = 50%) and (B) plots of the fluorescence intensity at 455 nm against [α-CD]/[TPE-COOH]. Concentration: 30 μM (contain 1.1 equiv. NaOH); λ_ex: 330 nm (10 nm, 10 nm); 283 K.

It is noteworthy that the maximum emission of TPE-COOH at 455 keeps almost unchanged along with increasing the amount of α-CD added from 0 to 1.5 equiv. in basic DMSO–H₂O mixture (f_w = 50%) despite the obvious enhancement in its fluorescence intensity (Fig. 3A). This is also true for the maximum absorption of this compound at ca. 300 nm (Fig. S20, ESI†). In contrast, the aggregate-induced fluorescence emission for TPE-COOH not only is increased but also takes slight red-shift from 455 to 466 nm (11 nm) along with increasing the f_w of H₂O–DMSO from 50 to 90% (Fig. 1A),meanwhile a slight red-shift (8 nm) is also observed in its maximum absorption (Fig. S21, ESI†), due probably to the strong intermolecular interactions in the aggregate state of this compound.¹⁷ These results clearly indicate that the fluorescence emission of TPE-COOH ascribed to its host–guest complex with α-CD is obviously different from the one due to the AIE process of this compound in H₂O–DMSO mixture.

It is well known that adamantane enters preferentially into the cavity of CD in comparison with phenyl derivatives due to its strong and specific interaction with CD.¹¹ As a consequence, to further reveal the host–guest inclusion between AIE-active TPE-COOH and α-CD, the fluorescence emission behaviors of TPE-COOH⊂α-CD (1 : 1) system after adding 1-adamantanecarboxylate (AD-COOH) was investigated in basic DMSO–H₂O mixture (f_w = 50%). As expected, the fluorescence emission of TPE-COOH–α-CD complex at 455 nm is weakened to be almost quenched along with the increasing amount of AD-COOH added from 0 to 1.5 equiv. (Fig. S22, ESI†), probably because the phenyl rings of TPE moiety in TPE-COOH were driven out from the cavity of α-CD owing to the strong and specific interaction of AD-COOH with α-CD. In addition, the effect of cavity size of the CD on the RIM process of host–guest complex was also investigated under the same experimental condition as mentioned above. As shown in Fig. 4, different from that of α-CD, the weak fluorescence emission of TPE-COOH at 455 nm upon addition of β- or γ-CD keeps almost unchanged, revealing that the small cavity of α-CD can more efficiently restrict the phenyl motions of TPE-COOH than the larger-sized one in β- or γ-CD. These results give an additional support for the formation of TPE-COOH⊂α-CD complex upon addition of α-CD into TPE-COOH, which in turn leads to RIM-induced fluorescence emission in basic DMSO–H₂O mixture (f_w = 50%).

Fig. 4 Change in fluorescence intensity of TPE-COOH upon addition of 2 equiv. α-CD, β-CD, and γ-CD in DMSO–H₂O mixture (f_w = 50%). Concentration: 30 μM (contain 1.1 equiv. NaOH); λ_ex: 330 nm (10 nm, 10 nm); 283 K.

To investigate the potential application of RIM-induced fluorescence emission from TPE-COOH–α-CD inclusion complex, 4-(phenylazo)benzoic acid (PBA) with the reversible photo-isomerization effect between trans and cis isomers under alternate UV- and visible-light irradiation was employed as the recognition guest in basic DMSO–H₂O mixture (f_w = 50%), Fig. 5.^18–20 As shown in Fig. S23 (ESI†), the fluorescence emission of TPE-COOH⊂α-CD complex is obviously decreased upon addition of trans-PBA (1.5 equiv.) in basic DMSO–H₂O (f_w = 50%), suggesting more favorably chelating affinity of the α-CD with trans-PBA than with TPE-COOH, which in turn induces the dissociation of TPE-COOH from the cavity of α-CD in their inclusion complex. It is well known that PBA exhibits the reversible photo-isomerization effect between trans and cis isomers under alternate UV- and visible-light irradiation (Fig. S24, ESI†). Based on the above-mentioned results, one TPE-COOH/α-CD/PBA (1 : 1 : 2) ternary system is fabricated to investigate the photoresponsive effect on RIM-induced emission of host–guest inclusion in basic DMSO–H₂O (f_w = 50%). As shown in Fig. S25 (ESI†), upon irradiation with UV light (365 nm) for 120 s, the quenched fluorescence emission for TPE-COOH–α-CD complex is restored probably due to the formation of cis-PBA associated with its isomerization from trans- to cis-isomer, which is hard to be included into the cavity of α-CD owing to the mismatch interaction between them, inducing the reformation of TPE-COOH–α-CD complex.

Fig. 5 Schematic illustration of the preparation of the TPE-COOH/α-CD/PBA composite and its photoreversible behavior with UV and visible light.

In contrast, the fluorescence emission of TPE-COOH⊂α-CD complex is obviously decreased after adding trans-PBA for 120 s under visible-light (435 nm) irradiation, as a result of the formation of α-CD-trans-PBA inclusion as just mentioned (Fig. S26, ESI†). Then upon further irradiation with UV light, the PBA returns to the cis state and TPE-COOH is included into the cavity of α-CD instead. Moreover, this fluorescence responsive of TPE-COOH⊂α-CD complex to PBA can be recycled many complex with RIM-induced emission as reversible photo-times under alternate UV- and visible-light irradiation, Fig. 6, suggesting the potential application of the TPE-COOH⊂α-CD responsive fluorescence sensor.

Fig. 6 Reversible switching of fluorescence intensity at 455 nm under alternate UV (365 nm) and visible (>435 nm) light irradiation.

Conclusions

A 4-carboxylphenoxy-decorating TPE derivative (TPE-COOH) with obvious AIE characteristics has been designed and synthesized. In basic DMSO–H₂O (f_w = 50%) mixture, the inclusion of TPE-COOH inside the cavity of α-CD switches on the non-aggregated emission of TPE-COOH at 455 nm owing to restricting phenyl motions of AIE-active TPE moiety, which is different from the aggregate-induced emission red-shifted to 466 nm in H₂O–DMSO (f_w = 90%) mixture. The clarification of this slight difference provides the basis for the application of TPE derivatives in host–guest inclusion induced emission system, such as the utilization of fluorescence titration to distinguish the change of the emission is difficult and changing the molecular state to achieve the obvious emission wavelength shift is hopeless. Additionally, based on the RIM-induced fluorescence emission mechanism of host–guest inclusion, TPE-COOH⊂α-CD complex can be utilized as a reversible fluorescence intensity responsive sensor for the azobenzene derivatives under alternate UV- and visible-light irradiation.

Notes and references

1. (a) J. Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang and B. Z. Tang, Adv. Mater., 2014, 26, 5429; (b) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2011, 40, 5361; (c) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun., 2009, 4332.
2. D. Ding, K. Li, B. Liu and B. Z. Tang, Acc. Chem. Res., 2013, 46, 2441.
3. Z. Zhao, J. W. Y. Lam and B. Z. Tang, J. Mater. Chem., 2012, 22, 23726.
4. (a) J. Q. Shi, N. Chang, C. H. Li, J. Mei, C. M. Deng, X. L. Luo, Z. P. Liu, Z. S. Bo, Y. Q. Dong and B. Z. Tang, Chem. Commun., 2012, 48, 10675; (b) H. Tong, Y. Dong, Y. Hong, M. Haussler, J. W. Y. Lam, H. H.-Y. Sung, X. Yu, J. Sun, I. D. Williams, H. S. Kwok and B. Z. Tang, J. Phys. Chem. C, 2007, 111, 2287; (c) C. Zhang, Z. Wang, L. Tan, T.-L. Zhai, S. Wang, B. Tan, Y.-S. Zheng, X.-L. Yang and H.-B. Xu, Angew. Chem., Int. Ed., 2015, 54, 9244.
5. T. Noguchi, B. Roy, D. Yoshihara, Y. Tsuchiya, T. Yamamoto and S. Shinkai, Chem.–Eur. J., 2014, 20, 381.
6. J. Zhao, D. Yang, Y. Zhao, X.-J. Yang, Y.-Y. Wang and B. Wu, Angew. Chem., Int. Ed., 2014, 53, 6632.
7. (a) Y. H. Xu, L. Chen, Z. Q. Guo, A. Nagai and D. L. Jiang, J. Am. Chem. Soc., 2011, 133, 17622; (b) E. Preis, W. Dong, G. Brunklaus and U. Scherf, J. Mater. Chem. C, 2015, 3, 1582.
8. (a) N. B. Shustova, B. D. McCarthy and M. Dinca, J. Am. Chem. Soc., 2011, 133, 20126; (b) N. B. Shustova, T. C. Ong, A. F. Cozzolino, V. K. Michaelis, R. G. Griffin and M. Dinca, J. Am. Chem. Soc., 2012, 134, 15061; (c) Z. Hu, G. Huang, W. P. Lustig, F. Wang, H. Wang, S. J. Teat, D. Banerjee, D. Zhang and J. Li, Chem. Commun., 2015, 51, 3045.
9. S. Li and W. C. Purdy, Chem. Rev., 1992, 92, 1457.
10. V. T. D'Souza and K. B. Lipkowitz, Cyclodextrins: Introduction, Chem. Rev., 1998, 98, 1741.
11. V. Mikhail and Y. I. Rekharsky, Chem. Rev., 1998, 98, 1875.
12. (a) G. Liang, J. W. Y. Lam, W. Qin, J. Li, N. Xie and B. Z. Tang, Chem. Commun., 2014, 50, 1725; (b) Y. Liu, A. Qin, X. Chen, X. Y. Shen, L. Tong, R. Hu, J. Z. Sun and B. Z. Tang, Chem.–Eur. J., 2011, 17, 14736.
13. S. Song, H.-F. Zheng, D.-M. Li, J.-H. Wang, H.-T. Feng, Z.-H. Zhu, Y.-C. Chen and Y.-S. Zheng, Org. Lett., 2014, 16, 2170.
14. Y. Liu, A. Qin, X. Chen, X. Y. Shen, L. Tong, R. Hu, J. Z. Sun and B. Z. Tang, Chem.–Eur. J., 2011, 17, 14736.
15. K. J. Hirose, J. Inclusion Phenom. Macrocyclic Chem., 2001, 39, 193.
16. (a) H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949, 71, 2703; (b) A. D. Bani-Yaseen, Spectrochim. Acta, Part A, 2015, 148, 93.
17. B. Z. Tang, Y. Geng, J. W. Y. Lam, B. Li, X. Jing, X. Wang, F. Wang, A. Pakhomov and X. X. Zhang, Chem. Mater., 1999, 11, 1581.
18. B. Pietro and M. Sandra, J. Phys. Chem., 1987, 91, 5046.
19. D.-H. Qu, Q.-C. Wang, Q.-W. Zhang, X. Ma and H. Tian, Chem. Rev., 2015, 115, 7543.
20. Z.-Q. Dong, Y. Cao, X.-J. Han, M.-M. Fan, Q.-J. Yuan, Y.-F. Wang, B.-J. Li and S. Zhang, Langmuir, 2013, 29, 3188.

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Footnote

† Electronic supplementary information (ESI) available: Materials and instruments, synthesis and characterization of the compounds 1–4, additional spectra and experimental details. See DOI: 10.1039/c5ra27292k

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