This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 6861--6863 6861

Cite this: Chem. Commun.,2013,49, 6861

Dynamic control of dendrimer–fullerene association byaxial coordination to the core†

Ken Albrecht, Yuto Kasai, Yasunori Kuramoto and Kimihisa Yamamoto*

The effect of axial coordination of pyridine derivatives to the core

porphyrin on the fullerene encapsulation of the 4th generation

carbazole–phenylazomethine dendrimer (ZnPG2-2) was investi-

gated. The axial coordination of large (bulky) pyridine derivatives

affects the cavity in an allosteric manner, and the size-selectivity of

the fullerene association could be controlled.

Molecular recognition plays an important role in natural pro-teins,1 and especially, dynamic (allosteric) molecular recognitionis attracting many scientists. Naturally, several trials to constructartificial hosts2 and studies on allosterism have been com-pleted.3 For definite molecular recognition, several macrocyclicor double-decker type molecules are widely used, because thesemolecules can determine the size of the binding site (cavity).There are also several polymeric molecular recognition systems.These systems introduce low molecular weight hosts into thepolymer chain4 or imprint guest structure into the crosslinkedpolymer network,5 and are used for chemosensing applications.6

However, a methodology to develop an artificial macromolecularsystem with allosterism using the protein-like cavity that isprovided by the polymer chain is not yet established.

Dendrimers7 are often compared with proteins because oftheir monodispersity, distinct interior space, and relatively stableconformation. Actually, encapsulation,8 catalytic reactions,7a

molecular recognition,9,10 and imprinting11 have been studied.However, many reports have shown that dendrons simply act as asteric hindrance in these applications. The fourth-generationcarbazole–phenylazomethine double layer-type dendrimer with azinc porphyrin core has a p-electron rich surface with a rigid(carbazole12) and a semi-rigid (phenylazomethine13) backbone(ZnPG2-2, Fig. 1).14 According to this structure, this dendrimerhas an ca. 1 nm diameter cavity on top of the porphyrin plane andcan form a 1 : 1 complex with fullerenes.15 This association with

fullerenes has size-selectivity (C60 o C70 o C84) and this resultdemonstrates that high-generation dendrons do not only act as asteric hindrance. ZnPG2-2 has two cavities at the axial position ofthe porphyrin and only one is occupied when fullerenes bind.Therefore, it should be possible to bind another guest such aspyridine16 at the same time and control the fullerene association(Fig. 2). Now we report allosteric fullerene recognition of ZnPG2-2that is controlled by pyridine complexation to the core porphyrin.

To confirm the concept of the allosteric control of thefullerene association (Fig. 2), the UV-vis titration of a fullerene

Fig. 1 Structure of the compounds (dendrimer and pyridine) that are used inthis study.

Fig. 2 Co-binding of pyridines and fullerenes to the ZnPG2-2 dendrimer.

Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta

Midori-ku, Yokohama 226-8503, Japan. E-mail: yamamoto@res.titech.ac.jp;

Fax: +81-45-924-5260; Tel: +81-45-924-5260

† Electronic supplementary information (ESI) available: Experimental section,synthesis, titrations, and thermodynamic parameters. See DOI: 10.1039/c3cc43249a

Received 2nd May 2013,Accepted 7th June 2013

DOI: 10.1039/c3cc43249a

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6862 Chem. Commun., 2013, 49, 6861--6863 This journal is c The Royal Society of Chemistry 2013

(C60) in an excess amount of 4-phenylpyridine (1000 eq.) wasperformed (under these conditions, all ZnPG2-2 are bound with4-phenylpyridine). Surprisingly, the binding constant of C60

increased ca. 1.5 times (Fig. S1, ESI,† 9.4 � 1.7 � 103 M�1 to1.5 � 0.2 � 104 M�1). For further studies, similar titrations wereperformed in the presence of bulkier phenylazomethine den-dron substituted pyridine derivatives (DPA pyridines (GnPy,n = generation), Fig. 1). Intriguingly, when the generation ofthe DPA pyridines increased, the binding constants of C60 andC70 increased. On the other hand, the binding constants of C84

were almost the same or slightly decreased with a peak at G1Py(Fig. 3). Within the experimental error it is difficult to predict,but notably the binding constant of C70 is higher than that ofC84 in the presence of G3Py. To confirm this result, MALDI-TOF-MS measurements were performed (Fig. 4). As reportedpreviously,15 when ZnPG2-2, C70 (10 eq.), and C84 (10 eq.) aremixed, the MALDI-TOF-MS spectra show only the m/z peak thatcorresponds to [ZnPG2-2 + C84]� due to the thermodynamicstability of ZnPG2-2–C84 in the solid state. Upon addition ofG3Py (1 eq.) to this mixture, the peak of the ZnPG2-2–C84

complex decreased and the peak that corresponds to theZnPG2-2–C70 complex appeared. Upon further addition ofG3Py (5 eq.), the peak of the ZnPG2-2–C84 complex disappeared.These binding constants and MALDI-TOF-MS results clearly

show that the fullerene binding can be controlled by co-bindingof pyridine derivatives to the core of ZnPG2-2. This can beexplained by (1) the steric change in the second binding site, (2)the electric change in the second binding site, and (3) thesolvent polarity change upon the addition of an excess amountof pyridine (solvent polarity strongly affects the binding15).

To exclude the third reason, a free base porphyrin coreddouble layer-type dendrimer (H2PG2-2) was synthesized andthe titration with C60 with and without 4-phenylpyridine wasperformed. Without pyridine, the binding constant of H2PG2-2and C60 was nearly the same as that of ZnPG2-2. Under thecondition of 1000 eq. of 4-phenylpyridine, the binding constantof H2PG2-2 and the fullerene decreased slightly. This slightdecrease might be explained by the weak pyridine–fullereneinteraction17 and is in contrast to the behaviour of ZnPG2-2(Fig. S2, ESI†). H2PG2-2 and 4-phenylpyridine do not form acomplex, therefore, this result clearly indicates that the addi-tion of an excess amount of 4-phenylpyridine did not affectthe polarity of the solvent. This means that the increase in thebinding constant of ZnPG2-2 and C60 is derived from theelectronic or steric change upon the complexation of pyridineto the porphyrin.

To study the electronic effect of the complexation of pyridineto the porphyrin, the complexation constants were examined(UV-vis titration Fig. 5, and ITC (isothermal titration calorime-try, Fig. S3 and Table S1, ESI†)). Interestingly, the bindingconstants between the DPA pyridines and ZnPG2-2 increasedwhen the generation of DPA increased up to the third genera-tion. However, G4Py did not show any interaction with thedendrimer. On the other hand, the binding constants of thesimple zinc tetraphenylporphyrin (ZnTPP) and DPA pyridineswere almost constant (G1 to G4). This means that the electronicstates of the pyridine (nitrogen) of each DPA pyridine aresimilar and the electronic effect after complexation should alsobe similar. Usually, dendritic hosts have a negative steric effectdue to the bulky dendron that covers the core binding site.9 Theunusual behaviour of DPA pyridines (larger binding constantwith larger guests) was also observed in the case of fullerenes(the order of binding constants was C60 o C70 o C84),15 anddefinitely proves the existence of a stable cavity. The size-selective

Fig. 3 Binding constants of ZnPG2-2 with fullerenes in the presence of 1000 eq.DPA pyridines. The binding constants were determined by UV-vis titrations intoluene : acetonitrile = 2 : 1 at 20 1C.

Fig. 4 MALDI-TOF-MS spectra of ZnPG2-2 with (a) 10 eq. of C70, and C84, (b)10 eq. of C70 and C84, and 1 eq. of DPAG3py, and (c) 10 eq. of C70 and C84, and5 eq. of DPAG3py.

Fig. 5 Binding constants of ZnTPP and ZnPG2-2 with DPA pyridines. Thebinding constants were determined by UV-vis titrations in toluene : acetonitrile =2 : 1 at 20 1C.

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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 6861--6863 6863

complexation of pyridines to ZnPG2-2 indicates that G3Py wellmatches the cavity size of ZnPG2-2 and has a positive interactionwith the dendron (probably p–p interaction) and the solventexclusion effect.

From the discussion above, the behaviour of the bindingconstants of fullerenes in the presence of pyridine derivativescannot be explained by electronic change or the solvent effect.However, it can be understood from the steric effect, i.e., thesize of the second cavity for the fullerene becomes narrowerwhen large pyridines bind to the first site, and for C60 and C70

the cavity size becomes more comfortable, but for C84, thecavity size gets slightly too small when in DPAG3Py complexes.

For further thermodynamic study, the DH–DS plot18 ofZnPG2-2 with several guests was created (Fig. S4, ESI†). Theslope of 311 and the intercept of �5.13 kcal mol�1 wereobtained from the plot and this relatively large slope andintercept suggest that this dendritic host is more similar toprotein type hosts than to synthetic macrocyclic hosts, i.e., thestructure change and structural fixing after binding are large,and the elimination of the solvent in the cavity plays animportant role. This result is consistent with the result thatwas described earlier, i.e., the co-binding behaviour of ZnPG2-2suggests that bulky pyridine complexation results in largeconformational change of the second cavity. The structuralchange effect could also be determined using the thermo-dynamic parameter of the C70 binding with and without G3Py(determined using van’t Hoff equation, Fig. S5, ESI†). The DH0

of �21 � 3 kcal mol�1 and DS0 of �54 � 5 cal mol�1 K�1

without G3Py increased to �4 � 4 kcal mol�1 and +9 �12 cal mol�1 K�1, respectively, upon complexation of G3Py tothe dendrimer. The large increase in DS0 indicates that thesecond cavity changes the conformation and decreases thevibration after the initial complexation with G3Py.

In conclusion, it was found that the size-selective fullereneassociation of the fourth-generation carbazole–phenylazomethinedendrimer can be controlled by axial coordination to the coreporphyrin. The thermodynamic studies and experiments usingbulky pyridine derivatives revealed that this modulation happensmainly in a steric (allosteric) manner. As a consequence, theselectivity of the fullerene could be changed by coordination ofG3Py (C70 o C84 to C70 > C84). This is one of the first examples ofan allosteric macromolecular host, and further studies on thehigher allosteric effect, higher selectivity, and applications tofullerene isolation are underway.

This work was supported in part by the CREST program ofthe Japan Science and Technology (JST) Agency, Grant-in-Aidsfor Scientific Research on Innovative Areas ‘‘CoordinationProgramming’’ (area 2107, no. 21108009) and by a Grant-in-Aidfor Encouragement of Young Scientists (B) (no. 24750099) fromthe Japan Society for the Promotion of Science (JSPS).

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