photoactive cascade molecules: polyether dendrimers bearing spironaphthoxazine groups on their...

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Photoactive Cascade Molecules: Polyether DendrimersBearing Spironaphthoxazine Groups on TheirPeripheries

Li Zhang, Fengwei Huo, Lixin Wu, Zhiqiang Wang, Xi Zhang*

Key Laboratory of Supramolecular Structure and Spectroscopy, Department of Chemistry, Jilin University,Changchun 130023, P. R. ChinaFax: +86-431-8923907; E-mail: [email protected]

IntroductionA relatively new type of macromolecule-dendrimer witha highly branched and monodisperse structure provides anew direction of research in supramolecular science.[1–4]

The specific structure of dendrimers leads to a number ofinteresting characteristics and features, including globu-lar, void-containing shapes and somewhat unusual physi-cal properties.[5] In the early stage of the investigation ofdendrimers, the efforts were directed toward the develop-ment of higher generation structures and new dendriticarchitectures. Lately, scientists have paid more attentionto how to take advantage of the specific structure of den-drimers and have devoted research into the synthesis offunctional dendrimers.[6] By incorporating functionalunits into the periphery, the branching units and the inter-ior core of dendrimers,[7–16] a series of functional dendri-mers have been obtained. Some of them have shownpotential applications in the fields of, for example, mole-cular biology, sensors, materials science, medicine, andcatalysis. Among them, chemically switchable entitiesare of particular interest due to their reversible responsiveproperties on input of an external influence like light.Recently, Vögtle et al.[8, 17] have reported the synthesis ofdendrimers bearing peripheral azobenzene groups, whose

reversible switching behavior and the potential use asmaterials for holographic data storage have been investi-gated. In an alternative way, Junge[18] and Jiang[19] havesynthesized a type of two-directional dendrimer with anazobenzene group in the center, the important property ofwhich is that the Z e E photoisomerization can occur byexcitation with IR radiation.[19] But as far as we know,there are few reports concerning the synthesis of dendri-mers with other switching units. In fact, spiropyrans/spirooxazines is another type of important photoactiveunit, which can isomerize reversibly between two differ-ent states upon UV and visible light irradiation: the non-ionic spiropyran/spirooxazine (SP) form and the zwitter-ionic merocyanine (MC) form.[20–27] As a derivative ofspiropyran/spirooxazine, spironaphthoxazine undergoes asimilar photoisomerization process (Figure 1) and hashigh fatigue resistance and excellent photostability.Furthermore, the colored MC form of spironaphthoxazinehas the ability to coordinate with metal ions. In the pre-vious study of spironaphthoxazine, the main researchactivity was focused on the doping or grafting of thespironaphthoxazine group to the polymer, or its connec-tion with other coordinate groups. These materials haveshown their application in the fields of sensors.

Full Paper: A type of novel spironaphthoxazine functio-nalized polyether dendrimers have been synthesized bycoupling photoactive spironaphthoxazine groups to theperipheral carboxylic acid of dendrimers. This type offunctionalized dendrimer can not only undergo photore-sponsive structural changes but also can act as carriers ofmetal ions that can be chelated and released by means of alight beam. The color of the complex of spironaphthoxa-zine dendrimer and metal ions varies with the type ofmetal used.

Macromol. Chem. Phys. 2001, 202, No. 9 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/0906–1618$17.50+.50/0

Macromol. Chem. Phys. 2001, 202, 1618–1624

Photochromic reaction between spironaphthoxazine (SP) andmerocyanine (MC).

Photoactive Cascade Molecules: ... 1619

We are interested in investigating the dendritic effectresulting from the topology and the interaction of built-inmolecular functionalities. Here we report the synthesis ofa series of new photoactive cascade molecules: polyetherdendrimers bearing spironaphthoxazine units in their per-ipheries. By coupling this type of photoactive group todendrimers, we hope to learn how the specific structureof dendrimer influences the photoactive functional units.

Experimental Part

General Methods1H NMR spectra were recorded on a Varian 400 (400 MHz)spectrometer with THF and DMSO solutions using the tetra-methylsilane (TMS) proton signal as an internal standard. IRspectra were recorded on an ISF-66 V spectrometer usingthin films on KBr plates. Molecular weights of the com-pounds were measured on a LD 11700-TOF MS mass spec-trometer. Gel permeation chromatography (GPC) was carriedout on a HSG-15H chromatograph. UV-Vis spectra were car-ried out on a Shimadzu 3100 UV-Vis-near IR recordingspectrometer. UV irradiation was carried out using 100 WXe lamp with excitation wavelength of 340 nm.

Materials

DMF was distilled from molecular sieves, acetone fromanhydrous potassium carbonate, and benzene and THF fromsodium, before use. DPTS (4-dimethylaminopyridinium saltof toluenesulfonic acid) was prepared by the method ofMoore et al.[28] The other reagents were used without furtherpurification.

Preparation of Spironaphthoxazine Dendrimer Film onQuartz Substrate

The spironaphthoxazine dendrimer film was prepared bycasting 0.1 m of spironaphthoxazine dendrimer THF solutionon quartz. We found that G1-(spironaphthoxazine)4, G2-(spironaphthoxazine)8, G3-(spironaphthoxazine)16 can allform colorless uniform films on quartz. All manipulationsand measurements were carried out with the room lightturned off.

Cation-Induced Isomerization

THF solutions containing spironaphthoxazine dendrimers(the concentration of spironaphthoxazine group is about5610–5

m) and metal ions [AgSO3 F3, ZnCl2, or Al2 (SO4)3](the concentration of metal ions is about 10–4

m) were usedin cation-induced isomerization experiments.

G1-(Spironaphthoxazine)4

To a solution of 200 mg (1.24 mmol) of spironaphthoxazine,427 mg (0.207 mmol) of polyether G1-(COOH)4, and115 mg of DPTS in 8 mL DMF, a solution of 256.2 mg ofDCC in 15 mL DMF at 0 l58C was added dropwise, andthe reaction mixture was stirred at room temperature for24 h. The excess of DCC was decomposed by stirring themixture with a few drops of dilute HCl for 4 h, the formedprecipitate filtered off, and the filtrate dried with Na2SO4.The solvent was then evaporated under vacuum, and the resi-due purified by chromatography eluting with THF/CH2Cl2

(1 :1, v/v) to give 300 mg of G1-(spironaphthoxazine)4 as agray solid. Yield 63.8%.

IR (KBr): 2960, 2869, 2812 (CH2, CH3), 1738 (C2O),1608 cm–1 (phenyl rings).

1H NMR (THF-d8, TMS): d = 8.44 (d, 4H, spironaphthox-azine rings), 7.94 (m, 16H, PhH), 7.88 (d, 4H, spiro-naphthoxazine rings), 7.82 (s, 4H, CH2N), 7.69 (d, 4H,spironaphthoxazine rings), 7.65 (m, 16H, PhH), 7.24 (d, 4H,spironaphthoxazine rings), 7.18 (m, 4H, spironaphthoxazinerings), 7.14 (m, 8H, core ArH), 7.04 (d, 4H, spironaphthoxa-zine rings), 7.02 (d, 4H, spironaphthoxazine rings), 6.94 (m,4H, spironaphthoxazine rings), 6.86 (d, 8H, ArH), 6.72 (t,4H, ArH), 6.68 (d, 4H, spironaphthoxazine rings), 5.61 and5.52 (each s, 12H, OCH2), 2.84 (s, 12H, NCH3), 1.44 (s,24H, CH3 ).


G2-(Spironaphthoxazine)8 was prepared by the reaction of48.2 mg (0.0242 mmol) of polyether G2-(COOH)8, 100 mg(0.291 mmol) of spironaphthoxazine, and 60 mg of DCC.The synthesis and purification processes are similar to thatof G1-(spironaphthoxazine)4. Yield 52%.


1H NMR (DMSO-d6, TMS): d = 8.46 (d, 8H, spiro-naphthoxazine rings), 7.97 (m, 32H, PhH), 7.87 (d, 8H, spir-onaphthoxazine rings), 7.82 (s, 8H, CH2N), 7.70 (d, 8H,spironaphthoxazine rings), 7.68 (m, 32H, PhH), 7.25 (d, 8H,spironaphthoxazine rings), 7.18 (m, 8H, spironaphthoxazinerings), 7.15 (m, 8H, core ArH), 7.04 (d, 8H, spironaphthoxa-zine rings), 7.02 (d, 8 H, spironaphthoxazine rings), 6.96 (m,8H, spironaphthoxazine rings), 6.86 and 6.76 (each d, 16H,ArH), 6.74 and 6.72 (t, 8H, ArH), 6.69 (d, 8H, spiro-naphthoxazine rings), 5.67 and 5.56 (each s, 28H, OCH2),2.86 (s, 24H, NCH3), 1.45 (s, 48H, CH3).


G3-(Spironaphthoxazine)16 was prepared by the reaction of73 mg (0.018 mmol) of polyether G3, 150 mg (0.436 mmol)

Figure 1. Photochromic reaction between spironaphthoxazine(SP) and merocyanine (MC).

1620 L. Zhang, F. Huo, L. Wu, Z. Wang, X. Zhang

of spironaphthoxazine, and 90 mg of DCC. The synthesisand purification processes are similar to that of G1-(spiro-naphthoxazine)4. Yield 40%.


1H NMR (THF-d8, TMS): d = 8.44 (d, 16H, spironaphthox-azine rings), 7.95 (m, 64H, PhH), 7.88 (d, 16H, spiro-naphthoxazine rings), 7.84 (s, 16H, CH2N), 7.70 (d, 16H,spironaphthoxazine rings), 7.65 (m, 64H, PhH), 7.25 (d,16H, spironaphthoxazine rings), 7.18 (m, 16H, spiro-naphthoxazine rings), 7.16 (m, 8H, core ArH), 7.04 (d, 16H,spironaphthoxazine rings), 7.02 (d, 16H, spironaphthoxazinerings), 6.95 (m, 16H, spironaphthoxazine rings), 6.86–6.72(m, 42H, ArH), 6.68 (d, 16H, spironaphthoxazine rings),5.65 and 5.54 (each s, 60H, OCH2), 2.86 (s, 48H, NCH3),1.46 (s, 96H, CH3).

Results and Discussion

The Synthesis of Dendrimers with PhotoactiveSpironaphthoxazine Groups on Their Peripheries

To get photoactive dendrimers, we chose FrØchet typepolyethers as dendritic “cores” due to their simple syn-thesis and the fact that they bear active peripheral car-boxylic acid groups. In addition, the spironaphthoxazinegroup is selected as the photoactive unit because of itsmore reliable and durable photochromic controlled prop-erties. Moreover, on irradiation with UV, the spiro-naphthoxazine can isomerize to zwitterionic form, andcoordinate with metal ions.[29]

The dendritic polyethers with carboxyl groups aschain-ends[30] (1 as shown in Scheme 1) and spiro-naphthoxazine[31] are prepared according to the literature.The synthesis of polyether dendrimers with peripheralspironaphthoxazine groups is summarized in Scheme 1.

The carboxyl-terminated polyether dendrimers bearmany peripheral carboxyl groups. For the attachment ofthe spironaphthoxazine moiety to the periphery of a car-boxyl-terminated dendrimer, a room temperature esterifi-cation method was adopted. Since the solubility of poly-ether in methylene dichloride is poor, whereas in DMF orTHF it is good, DMF was selected as the reaction med-ium. Reaction of spironaphthoxazine with carboxyl-ter-minated polyether and DCC in DMF with DPTS (4-(N,N-dimethylamino)pyridine/p-toluenesulfonic acid = 1:1) asthe catalyst afforded G1-(spironaphthoxazine)4, G2-(spironaphthoxazine)8, G3-(spironaphthoxazine)16 inyields of 63.8%, 52% and 40%, respectively. Thedecrease in yield with the increase in polyether size maybe the result of the steric effect. All spironaphthoxazinedendrimers are purified by chromatography on a silicacolumn. The final products of the spironaphthoxazinedendrimers were analyzed and confirmed by 400 MHz 1HNMR spectroscopy and MALDI-MS.

MALDI-TOF mass spectrometry has proved to be apowerful method for characterization of dendrimers. Fig-

Scheme 1. Synthetic route to spironaphthoxazine-terminatedpolyether.


ure 2 shows the MALDI-TOF mass spectrum of G1-(spir-onaphthoxazine)4, G2-(spironaphthoxazine)8 and G3-(spironaphthoxazine)16. The peaks at 2293.2, 4637.4 and9297.7 Dalton are attributed to M[G1-(spironaphthoxazi-ne)4] + Na+, M[G2-(spironaphthoxazine)8] + K+ and M-[G3-(spironaphthoxazine)16] + K+, respectively. There-fore, the molecular weights for G1-(spironaphthoxazine)4,G2-(spironaphthoxazine)8 and G3-(spironaphthoxazine)16

are 2270.2, 4598.3 and 9258.6, respectively, which agreewell with their theoretical data. The GPC measurements(Figure 3) show how the eluting time decreases with theincrease of spironaphthoxazine dendrimer sizes and thatthe spironaphthoxazine dendrimers have a very narrowmolecular weight distribution.

The synthesis of the desired product was further con-firmed by FT-IR spectroscopy. Taking G1-(spiro-naphthoxazine)4 as an example, compared with the IRspectra of the two reactants and product, we can see thatthe 3216 cm–1 band in spironaphthoxazine monomer,corresponding to 1OH vibration, disappeared in spiro-

naphthoxazine dendrimer, and the band at 1693 cm–1,corresponding to carboxyl C2O vibration in polyether,shifted to 1737 cm–1 after reaction, the latter correspond-ing to C2O vibration of ester in spironaphthoxazine den-drimer. G2-(spironaphthoxazine)8 and G3-(spironaphth-oxazine)16 show similar results. These IR results indicatethat an esterification reaction between 1OH of spiro-naphthoxazine and 1COOH of dendrimer did occur.

Photoisomerization of Spironaphthoxazine DendrimerCast Films

Photoisomerization behaviors of spironaphthoxazine den-drimers are observed in quartz substrate on UV-irradia-tion at 340 nm. Figure 4 shows the photo-induced isomer-ization behavior of the spironaphthoxazine dendrimerG1-(spironaphthoxazine)4 cast film in quartz. Theincrease of adsorption at 628 nm is evidence for isomeri-zation to the MC form. There is no change in the UV-Visspectrum after 5 min UV-irradiation, indicating that the

Figure 2. MS spectra of (a) G1-(spironaphthoxazine)4, (b) G2-(spironaphthoxazine)8, (c) G3-(spironaphthoxazine)16.

Figure 3. GPC spectra of G1-(spironaphthoxazine)4, G2-(spironaphthoxazine)8, G3-(spironaphthoxazine)16.

Figure 4. UV-Vis spectra of G1-(spironaphthoxazine)4 film byUV irradiation (irradiation times: 0, 0.5, 1.0, 1.5, 2.0, 3.0, 3.5,4.5, 5.0 min from bottom to top).


photoisomerization from the SP form to the MC formcompletes within 5 min. When switching off the UVlight, it was found that the spontaneous thermal reverseisomerization from MC to SP occurs very slowly, takingmore than 20 h, at room temperature. Additionally, thereverse isomerization can be accelerated by visible lightirradiation at 600 nm and will complete in 50 min. G2-(spironaphthoxazine)8 and G3-(spironaphthoxazine)16

show similar photoisomerization behavior. Due to thefast reversible MC to SP photoisomerization process insolution, it is quite hard to follow this process.

Metal Cation-Induced Photoisomerization

With the addition of metal ions into our spironaphthoxa-zine dendrimer solutions, the reversible photoisomeriza-tion process in the solution can be obtained. Figure 5ashows the photo-induced (340 nm irradiation) UV-Visspectral changes of G1-(spironaphthoxazine)4 with theaddition of Ag+. The adsorption at 478 nm increases withirradiation time, and reaches a constant (saturated) statein 40 min. In comparison, the absorption maxima of thespironaphthoxazine dendrimer's MC form without metalions occurred at 628 nm, and hence the addition of Ag+

caused a blue shift to 478 nm. As mentioned above, weknow that the photoisomerization process in a solution ofspironaphthoxazine dendrimers is too fast to be followed,but this process can easily be obtained on addition ofmetal ions. We suppose that both the shift of the absorp-tion spectra, and the retardation of the decolorization ratein the dark, result from the complexation between the sil-ver ions and spironaphthoxazine dendrimers. However nochelate is formed from the colorless form in the dark, thisindicates that complexation only occurs in the MC formof spironaphthoxazine dendrimers. From this cation-induced isomerization effect, we conjecture that in theMC form of spironaphthoxazine dendrimers, the pyranylO atom and imino N atom can coordinate with metalions, and induce different absorption shifts. It also hasbeen reported that the derivatives of spiropyrans/spiroox-azines bearing a coordinating group near the pyranyl Oatom can act as a chelating agent in the colored openform. [23, 32–36] In order to exclude the photosensitive effectof the silver ion, a control experiment is done. The THFsolution of Ag+ (10–4 M) is irradiated at 340 nm for 4 h,and no change is observed after irradiation under thesame experimental conditions. This further confirms thatthe increase of adsorption at 478 nm is due to the com-plexation between silver ions and the MC form of G1-(spironaphthoxazine)4. G2-(spironaphthoxazine)8 and G3-(spironaphthoxazine)16 show similar behavior, but thecomplex degree decreases with the increased size ofspironaphthoxazine dendrimers.

Photoinduced UV-Vis spectral changes by complexa-tion of G3-(spironaphthoxazine)16 and Ag+ are shown in

Figure 5b. We can see clearly that absorbance at the satu-rated state of G3-(spironaphthoxazine)16 is rather smallcompared to that of G1-(spironaphthoxazine)4 at the sameconcentration of spironaphthoxazine group. For the first-order complex formation processes in the dark, theobserved rate constants can be determined using ln[(Ae-At)/(Ae–A0)] = kob t,[29] where A0, At, and Ae are the absorb-ances of the complex at time 0, t and infinity, respec-tively. The rate constants obtained from Figure 5 arelisted in Table1. It is shown that the rate constantdecreases with the increased size of dendrimer. Thereshould be two possible reasons for the slower complexrate constant: isomerization from SP to MC form is hin-dered in the large dendrimer, or there is slower diffusionof metal ions in the large dendrimer due to their moredense packing. Moreover, the MC form of spironaphthox-

Figure 5. Photoinduced UV-Vis spectra changes by complexAg+ (10–4

m) with (a) G1-(spironaphthoxazine)4 (irradiationtimes: 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40 min from bottom totop) and (b) G3-(spironaphthoxazine)16 (irradiation times: 0, 4,8, 12, 16, 20, 24, 28, 32 min from bottom to top). The concentra-tion of the spironaphthoxazine group is 5610–5

m.


azine dendrimers can also complex with Zn2+ and Al3+,etc. As for the complexation with Zn2+, little shift ofabsorption spectra is observed, the color of spiro-naphthoxazine dendrimer solutions containing Zn2+ issimilar to that of spironaphthoxazine dendrimers castfilm without Zn2+ on irradiation. The color of the com-plexation with Al3+ is between that of Ag+ and Zn2+. Theabsorbance at saturated state, complex rate constant, andabsorption maxima (kmax) wavelength of the complexbetween G1-(spironaphthoxazine)4, G2-(spironaphthoxa-zine)8, G3-(spironaphthoxazine)16 and different metalions are summarized in Table1. We can see that spiro-naphthoxazine dendrimers form special colored com-plexes with the different metal ions. Furthermore, on irra-diation with visible light, the colored complex can returnto colorless again due to the release of metal ions. This isdifferent from the results of the monomer spironaphthox-azine,[29] whose MC form complex with metal ions is toostable to return its SP form even upon irradiation withvisible light. It shows that the dendrimer's frame providesa special microenvironment for spironaphthoxazinegroups that cannot form a stable complex with metalions. Therefore, the reversible chelating and releasing ofmetal ions upon light irradiation was obtained.

In summary, we have prepared a series of new photoac-tive cascade molecules: polyether dendrimers bearingspironaphthoxazine units in their peripheries. The photo-responsive property of the new dendrimer varies with thesize of the dendrimer, which is be the result of the dendri-tic effect. More importantly, in contrast to their mono-mers, spironaphthoxazine units in a dendritic structurecan act as a carries of metal ions in a reversible way.Investigation into the mechanism for the dendritic effecton the photoresponsive property of spironaphthoxazine isin progress.

Acknowledgement: The work was supported by the NationalNatural Science Foundation of China and the State EducationalMinistry of China.

Received: September 18, 2000Revised: November 27, 2000

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Table 1. The absorbance at saturated state, complex rate constant, and absorption maxima (kmax) wavelength of the complexationbetween spironaphthoxazine dendrimer and metal ions.

Metal ions G1-(spironaphthoxazine)4 G2-(spironaphthoxazine)8 G3-(spironaphthoxazine)16

kmax

nmAe kob

610ÿ3=Sÿ1

kmax

nmAe kob

610ÿ3=Sÿ1

kmax

nmAe kob

610ÿ3=Sÿ1

Ag+ 478 0.62 4.92 478 0.52 3.87 474 0.22 1.31Zn2+ 608 0.16 3.39 606 0.12 2.77 606 0.056 2.49Al3+ 551 0.034 a) 549 0.032 – 546 0.028 –

a) The rate constants of the complex of Al3+ and dendrimers can not be obtained due to their fast decoloration processes.


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