Polymer Journal, Vol. 39, No. 9, pp. 889–922 (2007) #2007 The Society of Polymer Science, Japan AWARD ACCOUNTS: SPSJ WILLY AWARD (2006)

Dendritic Architectures for Design of Photo– and Spin–Functional Nanomaterials

y Zheng HE,1 Tomoya ISHIZUKA,1 and Donglin JIANG1;2;

1Department of Materials Molecular Science, Institute for Molecular Science, National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan 2Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan

(Received April 6, 2007; Accepted April 24, 2007; Published June 12, 2007)

ABSTRACT: Unlike ordinary linear polymers, dendritic architecture is unique in the terms of its elaborative ca- pability for total control over molecular design parameters at the single molecular level, i.e., molecular size, branching pattern, structure, and morphology, thereby provides a new platform for the creation of functional materials with nano- meter–scale precision. This review mainly concerns recent works on the development of dendritic nanomaterials with a focus on photo– and spin–related functionalities. Strategy for the incorporation of chromophores to build up light–har- vesting antennae is presented with an emphasis on morphology and size effects. Dendritic macromolecules for photo- induced electron transfer are categorized based on chromophores that serve as the active center to absorb light and trig- ger photochemical process. In this context, dendrimers bearing porphyrin, conjugated polymer, and fullerene for realization of long–lived charge–separation state and light energy conversion are highlighted. On the other hand, design of dendritic macromolecules for spin–functional materials is focused on dendronized organic radicals and dendritic co- ordination polymers. Especially, spin–functional soft materials with an aim for spin manipulation and novel magnetic– optical switches are emphasized. [doi:10.1295/polymj.PJ2007006] KEY WORDS Dendrimer / Light Harvesting / Photoinduced Energy Transfer / Photoinduced Electron Transfer / Magnetic Soft Materials / Spin Transition / Radicals /

Dendritic architecture, characterized by its itera- natural processes such as light harvesting, energy tively branched structure, is one of the most pervasive transduction and conversion stimulated chemists in topology, which can be easily found at a variety of the design of various functional molecules and their dimensional scales such as in abiotic phenomena in- assemblies, which lead to many significant scientific cluding snow crystals and lightning patterns, and in and technological advances over the past decades. biological systems, for example, tree branches and In relation to this, dendritic architecture, due to its neuron networks. From a synthetic point of view, such well–defined morphology, provides a new platform architecture will enable chemists to construct molecu- for exploring novel materials with nanometer scale les with their components linked in a radial fashion, precision. Recent studies on dendrimers have extend- which is hardly achieved via ordinary synthetic meth- ed the scope of research from synthesis to applications odology. With the development of two main strat- for catalysts, photoactive and electronic materials, egies, i.e. divergent and convergent approaches,1–3 in- medicinal and biomedical materials, and other func- numerable dendritic structures have been reported up tional materials.4,5 In this review, we focused on re- to date. ‘Dendrimer’ termed by Tomalia in 1985 has cent works on functional dendrimers, for light-har- been recognized as the fourth major class of macro- vesting and photoinduced energy transduction, for molecular architectures. Benefiting from the well–es- photoinduced electron transfer to achieve long–term tablished synthetic strategies, chemists are now able charge–separation state and trigger hydrogen evolu- to precisely control the size and shape of the dendri- tion from water, for controlled self-assembly to fabri- mer frameworks as well as the numbers and positions cate photofunctional nanomaterials with well–defined of the functional groups. This molecular design flexi- structure, and for spin manipulation to control spin bility is one of the most unique characters, which dis- state of dendritic radicals and polynuclear metal tinguished dendrimers from all other linear, hyper- chains, with an emphasis on correlation between branched, and star–shaped macromolecules. structures and properties of these dendritic macro- On the other hand, the myriad of sophisticated molecules.

yTo whom correspondence should be addressed (Tel: +81-564-59-5520, Fax: +81-564-59-5520, E-mail: [email protected]).

889 Z. HE,T.ISHIZUKA, and D. JIANG

1: R1 = R2 = R3 = R4 = G3 Me Me O Me Me R2 1 2 3 4 O Me 2: R = R = R = R = G4 O O 1 2 3 4 O NH N 3: R = R = R = G4, R = tolyl Me Me R1 3 O R 1 3 2 4 Me O N HN 4: R = R = G4, R = R = tolyl O 1 2 3 4 O R4 5: R = R = G4, R = R = tolyl Me O O Me O O 1 2 3 4 O Me 6: R = G4, R = R = R = tolyl O O O Me O Me O Me O Me O O Me O O O O Me O O O Me Me O O O Me O O O O Me O O O Me O Me O O Me O

O O O Me O O O Me O O O O O Me O Me O O O Me O O O O O O O Me O Me O O O Me O Me O O O O O O Me O O Me O O O Me O O Me O O Me O Me Me O Me Me O O Me O Me O Me O Me Me Me G3 G4

dritic architecture for constructing artificial light–har- DENDRITIC ARCHITECTURES FOR vesting antennae. One way is to employ chromophore PHOTOINDUCED ENERGY TRANSFER units as building blocks for a dendrimer and thereby the whole framework serves as an antenna. An alter- Photosynthesis that transforms carbon dioxide and native strategy is to anchor dye units on the exterior water to glucose and oxygen discovered in plants surface of the dendrimer framework. In this case, and photosynthetic bacteria, is triggered by a highly the dendrimer framework functions as a scaffold to efficient photoinduced energy transfer from light–har- connect the exterior donor moieties with a focal ac- vesting antennae to photosynthetic reaction center, ceptor unit and intervenes the intramolecular energy where photoinduced electron transfer takes place to transfer event. So far, both convergent and divergent initiate the conversion of solar energy into chemical methodologies have been applied for site–specific energy.6,7 To trap low–density solar photons, natural placement of dye components for the construction of photosynthetic systems have developed multi–chloro- light–harvesting antennae. phyll based wheel–like arrays, which enable ultrafast intra– and inter–wheel energy transfer and channel Antennae for Ultraviolet Light Harvesting excitation energy to special pair at 100% quantum Poly(benzyl ether) dendrimers with porphyrin func- yield.8 Such an elaborate energy transduction system tionality have been synthesized by convergent method inspired chemists to develop artificial light–harvesting via coupling reactions of dendron subunits with a por- antenna systems, with an aim for realizing efficient phyrin core.9,10 Owing to the fact that meso–phenyl and vectorial photoinduced energy transfer. groups are nearly perpendicular to the porphyrin Due to their capability for accommodating a large plane, the fully substituted dendrimer porphyrins 1 number of chromophores in their nano–sized frame- and 2 adopt a spherical three–dimensional morpholo- works together with a well–defined three–dimensional gy, whereas those partially–substituted homologues structure, dendrimers are attractive motif for creation (3–6) resemble a corn–like structure. Upon excitation of artificial light–harvesting antennae. Up to date, a at 280 nm, which is an absorption band due to the variety of photofunctional dendrimers have been syn- poly(benzyl ether) dendrimer framework, 1 and 2 emit thesized upon incorporating chromophore units at the mainly a fluorescence from the porphyrin core at 656 focal core, in the interior blocks, and on the exterior and 718 nm. Since the dendrimer framework itself surface. From a viewpoint of molecular design, main- emits at 310 nm, the emission from porphyrin core ob- ly two approaches have been developed to utilize den- served for 1 and 2 is clearly the result of an intramo-

890 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

MeO OMe OMe MeO OMe MeO MeO OMe MeO O O OMe OMe OMe O MeO MeO MeO OMe O O OMe OMe OMe MeO OMe O O O MeO MeO O O O OMe O O OMe O MeO O O O O O O MeO O O MeO

O O O

O m O m O m OMe

O OMe O O O O O O O OMe MeO O O O O MeO O O O OMe MeO OMe O OMe O

MeO MeO MeO O 7 OMe O OMe OMe MeO OMe MeO O O OMe MeO O MeO 8 OMe MeO OMe OMe MeO MeO OMe

9 lecular energy transfer from the dendrimer framework rigidity, conjugated polymers have limited solubility to the focal porphyrin unit. In comparison with excita- and are difficult to process. On the other hand, from tion fluorescence spectrum, the efficiency of energy a photochemical point of view, their high tendency transfer of 1 and 2 has been evaluated to be 80%. In to form aggregates also leads to collisional energy dis- sharp contrast, partially substituted 3, although bear- sipation and eventually spoils their utility as light ing three large dendron subunits in its framework, ex- emitter. When a conjugate poly(phenyleneethynylene) hibits a quite low energy transfer efficiency to be only backbone is appended with flexible poly(benzyl ether) 30%. This decreasing tendency in energy transfer effi- dendritic wedges, the resultant rod–like dendritic ciency is more explicit when the number of the den- conjugated polymers (7–9) become highly soluble dritic wedges becomes less, where one–dendron sub- in common organic solvents such as THF, benzene, stituted 6 shows an efficiency as low as 10%. These dichloromethane, and chloroform. In addition, simple results demonstrate that the intramolecular energy casting of the polymer solutions gives a homogenous transfer event is highly dependent on the morphology thin film. Upon direct excitation of the polymer back- of the dendritic macromolecules. bone in THF, dendrimer rod 9 bearing the largest Fluorescence anisotropy measurements reveal that dendritic envelope emits a strong blue fluorescence excitation energy is not localized but is able to migrate centered at 454 nm with the quantum yield (ÈFL) efficiently over the continuous dendrimeric array of around 100%, which remains constant over a wide the chromophoric building units surrounding the ener- range of concentration. However, 7 and 8 with smaller gy trap. Consequently, for the spherical dendrimer, the dendritic wedges exhibit a significant concentration– probability of energy transfer to the porphyrin core is dependent fluorescence activity. Although 8 shows greatly enhanced. On the other hand, non–spherical a high ÈFL value around 100% in dilute solution dendrimers have a much looser and discontinuous (Abs431 nm ¼ 0:01), this value drops to 67% when the dendrimeric array of the aromatic units, where the en- solution is concentrated to give Abs431 nm of 0.1. This ergy migration should be less efficient. As the result, phenomenon is much explicit for the lowest–genera- the most excitation energy is lost by radiation before tion 7, whose ÈFL value is only 56% in dilute condition transferred to the energy trap. This finding is interest- and further decreases upon concentration. The large ing with respect to the energy transduction events ob- dendrimeric framework is likely to encapsulate the served for the wheel–like arrays of chromophores in conjugated backbone as an envelope and prevents the natural photosynthetic system, where the efficient co- photoexcited state from collisional quenching. Such operation of chromophores allows rapid excitation en- a dendrimer size–dependent luminescence activity of ergy migration along the wheels, followed by transfer conjugated backbone has been observed again for to the focal special pair to initiate the photosynthesis. dendritic conjugated polymers with discrete lengths.15 In this sense, this work offers a new strategy for the The intramolecular singlet energy transfer from the molecular design of light–harvesting materials. dendrimer framework to the focal conjugated back- In relation to above results, poly(benzyl ether) den- bone has been investigated upon excitation of the den- dron, capable of UV light harvesting, has been attach- dritic wedge at 278 nm by the use of fluorescence ed to conjugated polymer chains to afford dendritic spectroscopy. As a result, all these dendritic rods emit polymeric wires.11–14 Due to their conformational blue light without any luminescence from the dendrit-

Polym. J., Vol. 39, No. 9, 2007 891 Z. HE,T.ISHIZUKA, and D. JIANG

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O - O CO2

O O O O O O 3+ O O Ln O O O O O O O O O O CO - - O 2 O2C O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O

10 ic wedges, where the energy transfer efficiency value bearing benzoic acid unit at the focal core, for self– has been evaluated to be 100%. Owing to the promi- assembly with lanthanide ions such as Eu3þ,Tb3þ, nent light–harvesting function together with efficient and Er3þ, to synthesize dendritic lanthanide complex site–isolation effect of the large dendritic envelope, (10).17–19 Excitation on the dendrimer framework at the luminescence activity of the conjugated backbone 280 nm induces predominant luminescence from the is significantly enhanced. Notably, the luminescence focal Tb3þ and Eu3þ ions at 545 and 615 nm, respec- of 9 upon excitation of dendritic wedges is 12 folds tively, indicating intramolecular energy transfer to the more intense than that of 7 under identical conditions. focal lanthanide core. As a result of suppressed self– Therefore, dendritic conjugated polymer 9 with a quenching upon encapsulation within the large dendri- large dendrimer framework severs as an excellent or- mer framework, the luminescence efficiency is greatly ganic light emitter, which can efficiently collect pho- enhanced with the generation number. To clarify site– tons of a rather wide wavelength range from ultravio- isolation effect of dendritic wedges, Kim and cowork- let to visible, and convert them to blue luminescence ers have employed poly(benzyl ether) dendrons bear- at a quantum yield near 100%. ing a focal anthracene carboxylic acid unit to coordi- Artificial light–harvesting antennae coupled with nate with Ln3þ in the presence of terpyridine as a co– lanthanide complexes, owing to their potential pho- ligand.20 A film of Er3þ complex 11, upon excitation tonic utility as optical signal amplifiers, light–emitting of the anthracene unit at 357 nm, emits at 1530 nm, diodes, and luminescent probes, have attracted much whose intensity is 4.8 times stronger than that upon attention in recent years. Compared with other emit- excitation of poly(benzyl ether) dendron at 290 nm. ters, lanthanide complexes are characterized by a rel- As the generation number of the dendrons increases atively long lifetime of the excitation state along with from 0 to 3, the emission intensity is enhanced by a narrow emission band.16 However, the excitation more than 100 folds upon excitation at the anthracene energy of lanthanide complex is easily dissipated unit. This demonstrates an efficient site–isolation of through collision with solvent molecules and/or vi- the Er3þ core with the large poly(benzyl ether) den- brational escape to neighboring units with high vibra- drons that prevents its excitation state from collisional tional modes. For these reasons, one important issue self–quenching. in designing luminescent lanthanide complex is to In addition to the carboxylic unit as a coordination spatially isolate lanthanide center from environment. group, amide groups have been utilized for the syn- By taking advantages of the coordination chemistry thesis of lanthanide–dendrimer complexes. Polylysin of carboxylic acid with lanthanide metal ions, Kawa dendrimer 12 with 24 dansyl groups as light-absorbing and coworkers have used poly(benzyl ether) ligand units on the exterior surface and 21 amide units in the

892 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

of light–harvesting antennae due to their large absorp- N tion cross–section and high luminescence efficiency. Den O ErIII N Fre´chet and coworkers have developed a series of O 3 N laser dye–labeled dendrimers. Coumarin 2 has been 11 selected as the donor to anchor on the surface of poly-

Den = (benzyl ether) dendrons 13–16 containing a focal Coumarin 343 unit as an energy trap.23,24 In this case, O poly(benzyl ether) dendrimer framework is photo- O chemically silent, serving as a scaffold to link donor O O and acceptor units and mediating the energy transfer process. 13–16 exhibit an efficient energy transfer O O O from Coumarin 2 units on the exterior surface to the focal Coumarin 343 core, as evidenced by a strong O O O emission band at 470 nm from Coumarin 343 upon ex- O O citation of Coumarin 2 moieties. The energy transfer O through the poly(benzyl ether) framework is domi- O nated by Fo¨rster–type mechanism and this event is extremely fast with a rate constant higher than 3:3 Â 10À10 sÀ1. Considering the fact that the energy transfer G3 efficiency of 13–16 is similar to one another, their light–harvesting activity is in proportion with the number of Coumarin 2 units on the exterior surface interior, has been found to host Ln3þ ions.21,22 Upon of the dendrimer framework. excitation of the dansyl groups at 343 nm, 12 itself In general, Fo¨rster–type energy transfer is highly emits at 514 nm with a ÈFL value of 28%. Addition dependent on the spectral overlap and transition di- of Ln3þ to 12 results in a significant quenching of pole moment.25 This is the same case for the intramo- fluorescence at 514 nm, along with a new emission lecular energy transfer observed for 16. A comparison band of Ln3þ in the near–IR region, suggesting that study has been carried out on compounds 17 and 18 the excitation energy absorbed by the dansyl units with oligothiophene as the focal energy trap in place can be channeled to the focal Ln3þ core. of Coumarin 343.26 As oligothiophene has a larger Laser dyes are attractive motifs for the construction spectral overlap with Coumarin 2 and a higher transi-

NMe2

Me2N

NMe2 NMe2 O S O2S 2 NH NH Me2N NMe O2S 2 O2S NMe HN NH 2 O S 2 N O H HN O H N HN S O2 O SO2 N O H NMe2 Me2N NH O N H HN NH NH S SO2 O O 2 NMe2 SO NH N Me2N 2 O2S HN O O HN NMe H O 2 O NH O N 2 NH S H O O O N N NH N HN H SO2 N O N O2 H S NH O HN N SO O2 H NMe2 2 HN O2S Me N S 2 NH H HN O N Me N O 2 HN NMe2 Me2N O H N O O S HN 2 NH H O2 SO N S 2 O NH O NMe2 NH NH O2S Me N Me2N 2 SO2 O2 HN S HN NMe2 SO2 NMe2

Me2N 12 Me2N

Polym. J., Vol. 39, No. 9, 2007 893 Z. HE,T.ISHIZUKA, and D. JIANG

O O

O O O O O O O O O O N O N N O O O N N O O N O

N O N O N O O O N N O O N N 13 O O O O O O O O O O O O O O O N O O O O O N O O O N N N N O O O O O O O O O O N N N O O O N R O O O O O N O O O N N O O N O 15 O O O O O 14 O S O O O O O N S O N N O O C H N H 16: R = O O 8 17 O S Coumarin 2 O N

O n-Bu N O O S OH 18: R = O O N O O S N O O O C H S S S 8 17 N O 17: R = O S S S O O Coumarin 343 O C8H17 O S

O

O

O O N N N O O N O O N O O O N O N O N O O N O O O O O O O O O O O O N O O N O N O N O O O N O O N O O N N N O O

O

O 19

O O O

N N N O O N NH H O O O Coumarin 2 Fluorol 7GA Perylene Core tion dipole moment than those of Coumarin 343, the Coumarin 2 and fluorol 7GA moieties as donors in efficiency of energy transfer from Coumarin 2 to the the outer layers and the perylene unit as an acceptor oligothiophene unit is nearly 100%. On the basis of at the focal core, have been incorporated for the syn- the above results, three different chromophores, i.e. thesis of a dendritic triad 19.27 Because there is no

894 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

O N O

O

N

O

O O

N N

O O O O

N N

O O O N O

O O

O O

O N O O O

N O O N

O N N O

O O

O N O

O

N

O O N O

O N O

NMI O N O O N O PMI

TDI 20 spectral overlap between Coumarin 2 and the perylene terrylene tetracarboxydiimide (TDI) as acceptor, units, it is expected that the energy transfer in 19,if perylene dicarboxymonoimide (PMI) and naphthalene any occurs upon excitation of Coumarin 2 unit, should dicarboxymonoimide (NMI) as donors at the center, be in a cascade way involving the mediation of fluorol in the interior, and on the periphery, respectively.29 7GA. Indeed, excitation of the Coumarin 2 unit at 342 Upon excitation of the exterior NMI moiety at 370 nm gives an emission at 610 nm due to the focal per- nm, 20 displays only weak emissions centered at ylene unit with intensity 6.9 folds that of direct exci- 431 and 555 nm due to NMI and PMI, respectively. tation at 555 nm. This indicates that the excitation en- Instead, a strong emission is observed at 700 nm from ergy of the Coumarin 2 on the dendrimer surface can the focal TDI unit, suggesting a two–step energy be funneled to the focal perylene trap through a vecto- transfer from the NMI moiety to the focal TDI, rial two–step process mediated by fluorol 7GA. The through interior PMI chromophores. overall energy transfer efficiency from the exterior From an application point of view, an important as- Coumarin 2 moiety to the focal perylene unit is higher pect is to integrate the light–harvesting antenna in an than 95%, and each step with an efficiency of 98%. intermolecular energy transfer system. This concept In contrast to conformationally flexible dendrimer has been demonstrated with a self–assembled mono- frameworks, rigid scaffolds, c.f. polyphenylene den- layer of Coumarin 2–modified poly(aryl ether) den- drimer has been utilized for the construction of dron as light–harvesting segment in the presence of light–harvesting antenna.28 Weil, Mu¨llen, and co- Coumarin 343 unit as energy acceptor unit.30 Although workers have reported a stepwise energy transfer the two components are randomly anchored on silicon process in a shape–persistent polyphenylene dendri- wafer, intermolecular energy transfer from the den- mer 20 that bears three layered chromophores, with dron to Coumarin 343 unit takes place, where the ef-

Polym. J., Vol. 39, No. 9, 2007 895 Z. HE,T.ISHIZUKA, and D. JIANG

Me Me Me Me Me O Me Me Me O O Me Me O O Me O O O O O Me Me O O Me O Me O Me O O Me O O O O O O O Me O Me O Me O O Me O Me O O O O Me O O O O Me O Me O O O O Me O N N Me N N O O O Zn Zn O Me Me N N O O O N N O Me O O N O Me N O O N N Zn Me N Zn N O Me Me N O O O N Me Me Me O O Me O O O O Me Me O O Me Me O O O O N N O N N O Zn Zn O N N Me Me O O N N O Me Me O O O O O O O Me Me O O O O O Me Me O O O N

N Zn N N N Me Me Zn N O O N N N O Me Me O N O O Zn N N O Me Me O O O O O O O O O Me Me N N N N N O Zn O O Zn N O O N N N O N O Zn N O Me Me O N N Zn N N O Me Me O O O N

O N N O O N NH Me Me O Zn N N O O N N Zn NH O Me Me N O O N N O

N O O O Me Me O N N Zn N N O Me Me O N O N N Zn N O O O N N Zn O O Zn O N N N N N Me Me O O O O O O O O O Me Me O N N Zn O O N O Me Me O N N N O O N Zn Me Me N Zn N N N N O O Me Me O O O O O O Me Me O O O O O O O Me O Me N N O O Me N N O Me Zn Zn N N O O N N O O O O Me Me O O Me Me O O O O Me Me Me O O Me N O O O N Me Me O N Zn Me N Zn N N O O N Me O N O O Me O N O O N Me O O N N Me O Zn Zn O O N N Me N N O Me O O O O O Me Me O O O O O Me O O Me O O O O Me Me O Me O O O O Me O O O O Me O O Me O Me O Me O O Me Me O O Me O O O O O O Me Me O O Me Me Me Me Me Me Me Me 21

ficiency is highly dependent on the donor–acceptor compound 22, a conical analogue of 21, emits mainly molar ratio and the generation number of the dendron. from the zinc porphyrin units with only a weak emis- sion from the focal free–base porphyrin core under Antennae for Visible and Infrared Light Harvesting identical conditions. The energy transfer in 22 is much As one of the strategies for the preparation of visi- less efficient as evidenced by its smaller ÈENT and 9 À1 ble light–harvesting systems, multiporphyrin dendritic lower kENT values of 19% and 0:10 Â 10 s , respec- arrays31–33 are among the most promising candidates tively. Such a significant difference indicates that the to mimic the natural light–harvesting complexes be- morphology of the chromophore array in the dendri- cause of their structural similarity. Multiporphyrin mer framework plays an important role in intramo- dendrimer 21, with a spherical-shaped morphology, lecular energy transfer, as observed for the UV consists of a focal free–base porphyrin core as the ac- light–harvesting dendrimer porphyrins 1–6. Excita- ceptor surrounded by four dendritic wedges of zinc tion of spherical array 21 at 544 nm with polarized porphyrin heptamer as energy–donating units.34 21 light gives a highly depolarized fluorescence spectral upon excitation at 544 nm due to the zinc porphyrin profile with an anisotropy of only 0.03. By contrast, units, emits a predominant fluorescence at 658 and conical analogue 22 shows a larger anisotropy value 723 nm from the focal free–base porphyrin units, sug- of 0.10. This indicates that the zinc porphyrin units gesting the occurrence of an efficient energy transfer in 21 can cooperate with one another within the four from zinc porphyrin units to free–base porphyrin core. dendritic wedges, which facilitates the energy transfer The energy transfer efficiency (ÈENT) and the energy to the focal free–base porphyrin core. transfer rate constant (kENT) have been evaluated to Detailed investigations on the light–harvesting be 71% and 1:04 Â 109 sÀ1, respectively. In contrast, properties of the morphologically different spherical

896 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

Me Me Me Me Me O Me Me Me O O Me Me O O Me O O O O O Me Me O O Me O Me O Me O O Me O O O O O O O Me O Me O Me O O Me O Me O O O O Me O O O O Me O Me O O O O Me O N N Me N N O O O Zn Zn O Me Me N N O O O N N O Me O O N O N Me O O N N Zn Me N Zn N O N O O N

O O

N N N Zn Zn N N N N N

O O

N N Zn N N

O

N NH MeO OMe NH N

OMe 22 and conical dendrimers reveal that the two series mophore. One–branched 27 exhibits a single anisotro- of compounds display different generation–number py decay with a rotational diffusion time of 320 Æ dependency. As the generation number increases, the 10 ps, while 30 shows an excitation energy transfer spherical series show only a slight decreasing in ÈENT time constant of 542 Æ 1 ps. AM1 Hamiltonian calcu- value from 87% (25) to 80% (23), and to 71% (21).35 lation based on the molecular geometry suggests that On the other hand, the ÈENT value of the conical ser- the six zinc porphyrin moieties in 30 are located ies significantly decreases from 86% (26) to 66% (24), in–plane with a two–dimensional circular geometry. and then to 19% (22) with increasing the generation Two–branched dendron 28 shows a fast excitation en- number. The light–harvesting activities of spherical ergy transfer time constant of 316 Æ 2 ps, which is re- series, if termed as the multiple of absorption cross– lated to the conformer with short inter–chromophore section with the energy transfer efficiency, considera- distance. In agreement with its biexponential decay, bly increase with the generation number, whereas two–branched dendrimer 31 with 12 zinc porphyrin those of the conical series are much lower and less units on the exterior surface, as optimized with dependent on the generation number.36 AM1 Hamiltonian, adopts dual hexagonal wheels in Along this line, in order to elucidate the energy a staggered conformation. Since the shortest distance transfer process, Li and coworkers have synthesized between two chromophores located in different hexaarylbenzene–anchored zinc(II) porphyrin dendri- wheels is smaller than that within a same wheel, the mers with 6 (30), 12 (31), 18 (32), 24 (33), and 36 fast anisotropy decay is attributable to inter–wheel (34) zinc porphyrin units on the exterior surface, as and the slow component to intra–wheel excitation en- well as their constituent one– (27), two– (28), and ergy transfer processes. Accordingly, the experimental three–branched (29) dendrons.37,38 Steady–state ab- inter– and intra–wheel excitation energy transfer sorption and fluorescence spectroscopy show that times are estimated as 24 Æ 1 and 623 Æ 15 ps, respec- each zinc porphyrin moiety acts as an individual chro- tively. Larger dendrimer 33 bearing 24 zinc porphyrin

Polym. J., Vol. 39, No. 9, 2007 897 Z. HE,T.ISHIZUKA, and D. JIANG

Me Me Me Me O O Me Me Me O O O O Me Me O O Me O O

Me O O O Me O O O Me Me O O O O

Me O O Me O O O O O O

N N N Zn N N N Zn N N

Me O Me O O O Me O Me Me O O Me O Me O N N O O Me Zn O O Me O N N Me O O O O Me O O Me O O N O O Me N Zn Me O O N O N O N N O N Zn O Me N Me O O N N O NH N Zn N N O O N N Zn N HN O N N O O N Me Me O N O N N Zn N O O O Zn N O Me N Me O O N O O Me O Me O O O O O Me N N O Me O O Zn Me O O N N Me O O O Me O Me Me O Me O O O Me O Me

N N N N Zn Zn N N N N

O O O O O O Me O O Me

O O O O Me Me O O O Me O O O Me

O O Me O O Me O Me O O O Me Me O Me O Me Me Me Me

23

units on the exterior surface also shows biexponential The slow depolarization times of 900 and 740 ps ob- anisotropy decay, which, however, is faster than that served for 32 and 34 are assignable to the inter–cluster of 31 with 12 chromophores. Together with the fact excitation energy transfer process. Therefore, the fast of a similar rotation diffusion time of 33 to that of excitation energy transfer process takes place in clus- 31, 33 with doubled numbers of zinc porphyrin units ters and the slow one occurs between clusters. All is likely to be a highly congested wheel with tightly these observations indicate that the excitation energy packed zinc porphyrin units. is not localized on a specific chromophore but can mi- Compared to two–branched dendron 28, the three– grate over multiporphyrin arrays on the dendritic sur- branched dendron 29 with a compact three–dimen- face, where the energy transfer is highly dependent on sional triangular structure, displays biexponential an- the arrangement of chromophores. isotropy decay and dual energy migration paths. The In addition to above multiporphyrin dendrimers, fast and slow components can be assigned to the exci- dendrimer 35 bearing free–base porphyrin core, cova- tation energy transfer times between neighboring and lently linked with eight zinc porphyrin units, has been next nearest neighboring zinc porphyrin units, respec- reported.39 Excitation of zinc porphyrin units leads to tively. Three–branched dendrimers 32 and 34 exhibit an emission from the focal free–base porphyrin core, biexponential anisotropy decays with similar time where a slow zinc porphyrin–to–free–base porphyrin 8 À1 constants to those of dendron 29. These features sug- energy transfer (kENT ¼ 9 Â 10 s ) takes place with gest that three–branched dendron 29 acts as a cluster ÈENT of 60%. A much larger multiporphyrin array 36, in the excitation energy transfer processes because which contains a free–base porphyrin at the focal core the zinc porphyrin units are already in close contact. and 20 zinc porphyrin units in the branches, displays

898 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

Me Me Me Me O O Me Me Me O O O O Me Me O O Me O O Me Me Me O O Me Me Me Me O O O O O Me O Me O O O Me O Me Me O O O O O O O O Me O O Me O O O O O O O O O O N N N Zn Zn N N N N N N N Zn N N

O O

N N O Zn N N

N HN MeO OMe NH N O

OMe NH N MeO OMe 26 N HN

OMe 24 Me Me O O Me Me Me Me O O O Me Me O O O O O O O

O O

N N Zn N N

O Me

Me O

O O Me O Me Me O O O

Me O O O Me N N O O O N HN N Zn O N O Me Me O N O Zn N NH N O O O Me N N O O O Me O O Me Me O O Me O O

O Me

Me O

N N Zn N N

O O

O O O O O O Me Me O O O O Me Me Me O O Me Me Me 25

40 highly efficient energy transfer with ÈENT of 92%. Another aspect of equal importance is the utilization As described above, single molecular approach al- of dendritic macromolecule as a building block to fab- lows for the synthesis of light–harvesting dendrimers ricate light–harvesting supramolecular arrays.41 Li, capable of intramolecular vectorial energy transfer. Wasielewski, and coworkers have synthesized a rigid

Polym. J., Vol. 39, No. 9, 2007 899 Z. HE,T.ISHIZUKA, and D. JIANG

R N

N Zn N N N N R O Zn O N N O O O O N N O N N O O Zn R Zn R Cl3CH2C O N N H3C O N N Cl3CH2C O O O O N N N O Zn N N R N Zn N N R 27 28 29

R R R N N R N R Zn N N N Zn N N N Zn N N R N N N N R Zn N Zn N N R N N N N N N Zn N N R Zn N R N Zn N N N N R N N O Zn O O N N N N O Zn O N N O N N Zn O O O N O O N O O O O O O O O O O R N O N O O O O N R N O O Zn N Zn O N O N O O N O O O O N O O O O O O O O N O N Zn N N N N R N O O N Zn O Zn R Zn N N O N R N O N N O O N O R O O O O O N N N O O N Zn O Zn N N R O O N N N N R Zn O O N N O O O O O O O O N N O N O Zn N R Zn O O N N O O O R N O N R O O 30 O O O O O N O O N R R O O O Zn N N O N Zn O N N N N O N N N R N N Zn Zn N N Zn R N N N Zn N N R N N N N N N N N R Zn N Zn R N N N Zn N N N R N N N Zn N Zn N N N N Zn N N R N R N O O O O O R R O O N R N R N O O O Zn 33 N N O Zn O N N N O O O O O O R O R R O R O O O O N N N N N R N N N R N Zn N O Zn N Zn O N Zn N R Zn N N N N R O O O N R N N N N Zn N N N N Zn N Zn R N O O R N N N N N N R N Zn N O N Zn O O N N N O O N N N N Zn O Zn N N N Zn N N O N N R O O N O O N O R N Zn N N O N N O O O R Zn O O N N Zn N N O O R R N O R N N N N N N N O O O O O N Zn N Zn Zn N Zn O N O N N N O N N N O O O O O O R N O O N R N O O N Zn R R N Zn O O N N O N O O O O O O 31 O O O O O N N N N R Zn O Zn O N N R N N O O O O O R O O N N N O N R Zn O O Zn R N N N N O O O O R N N R O O Zn O O O O N N O N N N R N N O O O N N Zn O N Zn N Zn Zn R N O O O R N N N N N O R N N O O O O N O O N N Zn N N O N Zn N O O N N N Zn O O O Zn N N O N N R N O O N R O N R N N Zn N O O O N R O N O O Zn N N O O Zn N N O O O O R O N N N O O N Zn O N O O R O N O N N N O O N Zn O Zn N N N O N N N O O O Zn Zn N N N R N N N N O O O O Zn R N N N N R N N O N N Zn Zn O O N N N N R Zn O N N N N R N N N N Zn R N N O O Zn Zn Zn N N R N N N R N N N O O N N O Zn R R O O R R N N O N N R R Zn O O O O N N O O O O 34 O O O N N O N Zn N O O N Zn O O N N O R N N O R N Me Zn O N N N Zn N N N N O Me N N N R O Zn Zn N N R N N R = N N Zn O O Me R N N R

O R Me 32 planar ‘baby’ dendrimer 37 having a zinc phthalocya- ZnPc, respectively, both largely blue–shifted in rela- nine (ZnPc) core tethered with four perylenediimide tive to those of corresponding monomers, suggesting (PDI) units.42–44 Electronic spectroscopy of 37 shows the formation of H–aggregates. Small–angle X–ray two broad bands at 500 and 645 nm due to PDI and scattering measurement confirms that seven coaxially

900 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

R R

N N N N R Zn R R Zn R N N N N

R R

R R R R

N N N N Zn Zn N N N N

O O R R O O R R R R

O O NH N

N HN O O

R R R R O O R R O O

N N N N Zn Zn N N N N

R R R R

R R

N N N N R Zn R R Zn R N N N N

R R

35

N N Zn N N

N N N N N N Zn Zn Zn N N N N N N

N N N N Zn N N Zn N N Zn N N N N

N N N N N N Zn N N NH N Zn Zn N N N N N N Zn N N N N Zn Zn N N N NH N N N N

N N N N Zn N N Zn N N Zn N N N N

N N N N N N Zn Zn Zn N N N N N N

N N Zn N N 36 stacked molecules self–assemble into a stable cylin- hardly transfer to a designated site. A poly(benzyl drical aggregate (37)7mer with an overall dimension ether) azodendrimer 38 carrying photoresponsive azo- of 6 Â 6 Â 3 nm. Selective excitation of the peripheral benzene core have been synthesized.45,46 Of interest, (PDI)7mer in (37)7mer at 500 nm results in the formation cis form of 38 in CHCl3 isomerizes to the trans form, À1 of single excitation state of (PDI)7mer, which under- when exposed to an IR light of 1597 cm , corre- goes quantitative singlet–singlet energy transfer to sponding to the vibrational band of the aromatic rings the focal (ZnPc)7mer column, followed by the forma- in the dendrimer framework, whereas no acceleration tion of singlet excitation state of (ZnPc)7mer with a occurs in low–generation dendrimers or mono–den- lifetime of 1.3 ps. dritic analogues. The large dendritic shell not only Unlike UV and visible light, low energy photons in serves as an insulator to prevent collisional energy the IR region is easy to dissipate to surroundings and dissipation, but also functions as an IR light–harvest-

Polym. J., Vol. 39, No. 9, 2007 901 Z. HE,T.ISHIZUKA, and D. JIANG

increase with increasing the generation number and

O O C H IR intensity. For high–generation dendrimers, ÁT355 C8H17 8 17 N N O of Fe–Cl stretching is always larger than ÁT of O O O 390 porphyrin in–plane mode. These results demonstrate

O O O that the IR energy captured by the 8 mode of the aryl O N N dendritic antenna is most likely transferred to the axial O O O O N N Fe–Cl bond of the Fe–porphyrin core and then relaxed O O N to the porphyrin macrocycle.

N N N Zn N N N Antennae for Two–Photon Absorption N Molecular design of two–photon absorption (TPA) O O

N N systems has attracted much attention due to their po- O O O O tential utilities in diverse fields such as photodynamic N N O O therapy of cancer, fluorescence microscopy, and opti- O O cal power limiting.48–51 One attractive way to enhance

O TPA cross–section is to design a polymer containing a O O O N N C8H17 large number of TPA units without causing any aggre- C8H17 O O gation of chromophores. In this sense, dendritic mac- 37 romolecules emerge as promising candidates, since their unique three–dimensional scaffolds can accom- modate many TPA chromophores and have a chance ing antenna. Such an intriguing IR effect has been in- to increase significantly the molecular absorption vestigated by Mo and coworkers with Raman spec- cross–section. troscopy of a dendritic iron porphyrin complex (1).47 Since 39 is a well–established TPA chromophore Upon IR irradiation of the dendrimers in dioxane at with a large TPA cross–section of 6,700 (Æ15%) À1 À50 4 1597 cm due to aryl 8 mode of the aryl ether den- GM (1 GM = 10 cm s) at 800 nm, Adronov and drons, the Boltzmann temperature of metalloporphyr- Fre´chet have employed 39 derivative for the synthesis in vibrational modes including axial Fe–Cl stretching of a series of TPA–dendrimers 40–42 with different À1 52 mode at 355 cm (T355) and porphyrin in–plane generation numbers. As the generation number in- À1 mode at 390 cm (T390) is higher than that of the sol- creases, the TPA cross–section of dendrimers increas- vent (T835). While T835 of neat solvent remains unaf- es proportionally to show a linear correlation with the fected by IR irradiation, the differential temperatures, number of the chromophores. Based on this observa- ÁT355 defined as T355–T835 and ÁT390 as T390–T835, tion, nile red chromophore has been anchored at the

Me Me Me Me Me Me O O O Me Me O Me Me O Me Me O Me O O O O O Me Me O Me Me Me O O O O O O Me O O O O Me Me Me O O O O O O O Me O O Me Me O O O O O Me O O O O O O Me O O O O O O O Me O Me O O O O O Me O O O O O Me

O N Me O O Me N O O Me O O Me O O O O O O O Me O Me O O O O O O Me O O O O O O Me O O O O O Me O O O Me Me O O O Me O O O O Me O O O O Me O Me O O Me O O O Me O Me Me O Me O O Me O Me O O Me O Me O O Me Me Me Me O Me O Me Me Me Me 38

902 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

N N O N O N N O N O N N N N N N N O N O O N N N N N N N N O HO N O O N O O 39 N O O O N O N N O O N

O O O O N N O O O O O O N N N N N N O O O O HO N N O O HO N O O N N O O O N N O O O O N O N O N N O O HO N O O O N O O O N N O O O N N O N N O N O O N N N O N N N N N O N N 40 N 41 O N N O N N O N O N N

42 focal core of a series of dendrons bearing 39 units on cence from the focal porphyrin core with an intensity the exterior surface to give dendritic nile red (43–45). 17 folds that of the simple porphyrin reference 47, in- Two–photon excitation of 43, 44, and 45 with 815–nm dicating that AF 343 units can efficiently harvest and laser leads to an emission at 595 nm from the nile red transfer the two–photon excitation energy to the focal core, whose intensity is increased by 8, 20, and 34 porphyrin core. Two–photon excitation of 46 in oxy- folds compared to that upon excitation of nile red gen–saturated solution at 780 nm affords a fluores- itself at the same wavelength, respectively.53,54 The cence at 1,270 nm characteristic of singlet oxygen, TPA cross–sections of 43–45 double from 6,700 whereas 47 does not show any emission under identi- (Æ15%), to 11,900 (Æ15%), and then to 24,100 cal conditions. Therefore, the near IR energy harvest- (Æ15%) GM, while the number of TPA moieties in- ed by AF 343 units on the exterior surface of the den- creases from one, to two, and then to four. drimer framework is channeled to the focal porphyrin Porphyrin derivatives are well–known as photosen- core and can be utilized in the generation of singlet sitizers for activation of oxygen to generate singlet oxygen. In fact, a water–soluble version, 48, tethered oxygen with high quantum yield, which is attrac- with tri(ethylene glycol) monomethyl ether groups on tive as potential agents for photodynamic therapy the surface, allows the generation of singlet oxygen (PDT).55–57 However, ordinary porphyrins show ab- in aqueous solution upon two–photon excitation at sorption for UV and visible light, which have only 780 nm.59 Such a water–soluble dendrimer capable limited transmission through skin. From a PDT point of near IR excitation is highly interesting from an ap- of view, porphyrin derivatives capable of absorbing plication point of view. A water–soluble poly(arylgly- near–IR such as 750–1,000 nm lights, which reach cine) dendritic platinum porphyrin 49 bearing Cou- much deeper tissues, is highly required. To challenge marin–type TPA chromophores has been developed this subject, a promising way is to combine a TPA as a phosphorescent probe for the imaging of tissue antenna with a dendritic porphyrin photosensitizer. oxygen, based on a similar photoinduced energy trans- Fre´chet and coworkers have designed and synthesized fer reaction.60,61 a dendritic porphyrin 46 containing eight TPA units, i.e., AF 343 moieties on the surface.58 AF 343 has DENDRITIC ARCHITECTURES FOR been chosen as an energy–donating unit because of PHOTOINDUCED ELECTRON its high TPA cross–section and large emission spectral AND HOLE TRANSFER overlap with porphyrin absorption band. Upon two– photon excitation of AF 343 units with a 780–nm Unlike traditional linear, star–shaped, and branched fs–mode locked Ti:Sapphire laser, 46 emits a fluores- polymers, dendritic architecture is characterized by its

Polym. J., Vol. 39, No. 9, 2007 903 Z. HE,T.ISHIZUKA, and D. JIANG

N N N N O O

N

O

N

N O O 43

N N O O N N

N O O N

O O N N N N O O O

O

N

N O O 44

N N N O O N

N N N N

O N N O

O O O O

O O O O O O N N N O O N

O N N

N

N O O

N N O O N N 45

OH

N

N O O

nile red derivative elaborative molecular design capability, especially for the construction of photoinduced electron–transferring site–specific placement of electron donor and acceptor (PET) systems based on dendritic architectures. In components in the three–dimensional scaffold, thus both cases, the dendrimer framework serves as a me- provides a well–defined molecular system for the pho- dium for regulating the photoinduced electron transfer toinduced electron and hole transfer. Covalent and reaction and the stability of resultant charge–separa- non–covalent approaches have been developed for tion (CS) state.

904 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

S S N N

N N

O O S N N

O O O S Br S N N N N AF-343

O O O O NH N

N HN O O O O

O O N N N S N S O O

O O NH N O O N HN N N O O

O O 47 N N S S

46

Porphyrin–Based Dendritic PET systems transfer. The rate constant of the photoinduced elec- In the natural photosynthetic reaction center, multi– tron transfer has been estimated as 2:6 Â 109 sÀ1. step electron transfer reactions, mediated by porphyrin Therefore, the pre–organized electron donor–acceptor derivatives, play a crucial role and the resulting array, which is spatially separated by the large dendri- charge–separation state between the special pair and mer framework, shows an efficient photoinduced elec- end quinone group generate chemical potentials from tron transfer from the focal core to MV2þ through the photo–energy. To accomplish an artificial photosyn- 2–nm thick dendrimer framework. The large dendri- thesis, a huge number of multi–porphyrin systems mer framework prevents the photoactive core from bound to an electron acceptor have been synthesized aggregation, and thus greatly lowers the possibility and the photoinduced charge–separation states have of collisional dissipation of the photoexcited state. been well investigated.62 In particular, dendrimers Fre´chet and coworkers have synthesized a series of consisting of porphyrin moieties as electron–donating poly(benzyl ether) dendrimer zinc porphyrins to show groups coupled with various electron acceptors have the site–isolation effect on the focal zinc porphyrin been synthesized for the investigation of photoin- unit.64 High–generation dendrimer 52 exhibits a lower duced electron transfer event with an aim to achieve activity in electron transport from/to electrode, as evi- long–lived charge–separation state. denced by a larger redox potential. However, the large By encapsulating a zinc porphyrin unit at the core dendrimer framework does not restrict the access by of a carboxyl–terminated poly(benzyl ether) dendri- small molecules. For example, upon addition of ben- mer framework, Sadamoto and Aida have demonstrat- zylviologen as an electron acceptor, 52 shows even ed the photoinduced electron transfer from porphyrin an enhancement by 33% in the photoinduced electron core to a positively charged electron acceptor, i.e. transfer rate. methyl viologen (MV2þ), which is electrostatically In contrast to the self–assembled donor–acceptor self–assembled on the surface of the dendrimer frame- array, Modarelli and coworkers have developed a co- work.63 The photoinduced electron transfer event is valently linked system, by appending electron accep- highly dependent on the generation number of the tor, i.e. anthraquinone to the exterior surface of dendrimer framework. In contrast to the 1st–genera- polyamide dendrimer porphyrins.65 In this case, both tion dendrimer 50, the 3rd–generation dendrimer 51 fluorescence quenching degree and photoinduced in a phosphate buffer, upon titration with MV2þ, ex- electron transfer rate retain at similar level, irrespec- hibits higher degree in fluorescence quenching, indi- tive of the generation number. Although the largest cating a much more efficient photoinduced electron dendrimer 53 bearing 108 anthraquinone groups on

Polym. J., Vol. 39, No. 9, 2007 905 Z. HE,T.ISHIZUKA, and D. JIANG

S N O O N S O O O O

O O O O O O

O O Ph O O O O Ph O N N O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O S O O O O O O S O O N O O N O O O O O O O O N O O N Ph Ph O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O NH N O O O O O O O O O O O O O O O N HN O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O Ph Ph N O O N O O O O O O O O N O O N O O S O O O O O O S O O

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O N N O Ph O O O O Ph O O

O O O O O O

O O O O S N O O N S

48

RO2C CO2R RO2C CO2R

O O O O O O O O RO2C CO2R CO2R O N N O O N N O H H H H O O RO2C O O HN O O NH RO2C CO2R CO R RO2C HN O 2 O NH O O N O O O O N N N H H N N H H O N O

N Pt N

O N O H H N N H H N N N O O O O N O O HN O O NH CO2R RO2C RO2C CO2R HN O O NH O O O O CO2R H H H H O N N O O N N O

RO2C O O O O RO2C O O O O CO2R

RO2C CO2R RO2C CO2R

49 R = tBu or H

906 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

X X X X X X

X X O O O O O O X X O O X X X O O X O O O O

OO O O O O O O X O O X X X

O O O O O O N N X O N N O X Zn Zn N N X O N N O X O O O O O O

X X X O O X OO O O OO O O

O O O O X X O O X X O O 50 X X O O O O – O O X = CO2 X X

X X X X X X 51 – X = CO2

O O O O O O O O O O O O O O

O O O O O O O O Den O O O O O O O O N N Zn N N O O O O O O O Den O

O O O O

O O O O

O O O O O O O O O O O O O O

52 the exterior surface, it exhibits a fluorescence quench- as a result of the decrement of –electron acceptabil- ing degree of only 60%. The low fluorescence quench- ity for substituted fullerene. ing efficiency is likely owed to the distance between The integration of dendritic light–harvesting anten- donor and acceptor as long as 2.6 nm, which is close na in electron–transfer relay systems is of interest due to the spatial limit for a through–space electron trans- to the high probability for triggering vectorial energy fer reaction. flow and electron–transfer process. Choi, Aida, and Porphyrin–fullerene dyads have attracted much at- coworkers have reported the pioneering work on the tention as novel photoinduced electron transfer units, synthesis of a series of fullerene–terminated dendritic in which the charge–separation state can take advant- zinc porphyrin arrays, 54–56, consisting of one, three, age of the low reorganization energy of fullerene. In and seven zinc porphyrin units as light–harvesting 1999, Hirsch and coworkers have reported fullerene– moieties.67 Average fluorescence–decay rate constant porphyrin dyads with different numbers of poly(ben- (kCS) becomes smaller by a factor of four as the zinc zyl ether) dendritic wedges anchored on the fullerene porphyrin array becomes larger from 54 (1:55 Â 66 9 À1 9 À1 moiety. As the number of dendritic wedges increas- 10 s )to55 (0:40 Â 10 s ). 56 shows a kCS value es, the fluorescence quenching becomes less efficient, of 0:43 Â 109 sÀ1, which is comparable to that of 55,

Polym. J., Vol. 39, No. 9, 2007 907 Z. HE,T.ISHIZUKA, and D. JIANG

CO2R CO2R RO2C RO2C RO2C CO2R

RO2C CO2R RO2C CO2R O O O O RO2C O O O O O O CO2R CO R RO C O 2 2 O O NH HN O RO2C O HN NH O CO2R O O O NH O O HN O O O RO2C CO2R O O O O O O CO2R RO2C

RO2C Den CO2R NH HN O O O O O RO2C O CO2R O O O O N N O O RO C O Zn O CO R 2 N O O N 2 O O H O NH N N HN O H O N N O O H H O RO C H O O H CO R 2 O N O O N O 2 O O Den O O HN NH O O RO2C O NH HN O CO2R O O RO2C O O O O CO2R O O O O O O RO2C CO2R CO R 2 O O O O CO2R RO2C NH O O HN CO2R O HN NH O NH NH O CO R RO C O O O 2 2 O O RO2C CO2R O O O O O O CO2R RO2C O O O O RO2C CO2R RO2C CO2R CO2R CO2R RO2C CO R CO2R 2 RO C 53 2

O

R =

O although 56 bears a larger number of zinc porphyrin tron microscopy and TEM reveal the formation of units located away from the fullerene terminus. Simi- core–shell spherical particles with a shell thickness larly, the quantum efficiency for charge separation of 25–30 nm, indicating that they consist of a multila- (ÈCS)of56 has been estimated as 51%, which is mellar membrane. smaller than that of 54 (80%) but comparable to that A long tailing in the near–IR region (650–1,000 of the lower–generation 55 (49%). Interestingly, the nm) in the electronic absorption spectrum is character- largest, 56, shows a smaller charge–recombination istic of charge–transfer –electronic interactions be- 6 À1 rate constant (kCR ¼ 1:5 Â 10 s ) than those of 54 tween zinc porphyrins and fullerene. This suggests (2:9 Â 106 sÀ1) and 55 (2:4 Â 106 sÀ1). Thus, the zinc that the multilamellar membrane has an interdigitated porphyrin cationic radical species, which is initially structure, in which the fullerene moieties are sand- generated at the focal point, can move away from wiched by the zinc porphyrin units. The fluorescence the fullerene anionic radical terminus towards the is completely quenched by fullerene units when 57 is periphery by intramolecular hole hopping. The large interdigitated in the vesicles, since electron transfer dendritic antenna not only harvests visible light for takes place intermolecularly as well as intramolecular- electron transfer to the fullerene terminus, but also re- ly in the vesicular membrane. tards the back electron–transfer process, thus indicat- Li and coworkers have recently reported multipor- ing a new potential of dendritic macromolecules for phyrin–multifullerene arrays self–assembled from conversion of light energy into chemical potentials. multiporphyrin dendrimers (30, 31, and 33) and fuller- Self–assembled biological structures, such as cell ene–tethered bipyridine derivatives (58–60).69 Time– membranes formed from amphiphilic lipids and pro- resolved fluorescence spectroscopy of 33 gives a de- teins, are of great interest in relation to their dynamic cay profile at 590 nm with a lifetime of 300 ps upon transportation of ions and molecules, and the transfer addition of fullerene–free bipyridine and a much fast- of energy across the membranes. Inspired by this elab- er decay with a lifetime of 150 ps in the presence of orate natural system, the development of novel vesic- 60. Of interest, as the generation number increases, ular self–assemblies, especially in the creation of new the kCS values of 30, 31, and 33 in combination with functional artificial systems, has evolved as an impor- 60, increase from 0:57 Â 1010 to 1:5 Â 1010, and to tant subject. Charvet and coworkers have reported that 2:3 Â 1010 sÀ1. This generation–dependent increasing a zinc porphyrin–fullerene amphiphile 57 self–assem- tendency is also observed for the combinations with 68 bles to form photoactive vesicles. Dynamic light– 58 and 59. On the other hand, the kCR values are sim- scattering analysis of 57 in a mixture of THF and ilar to one another around 6:7 Â 106 sÀ1. Therefore, water shows that the vesicles are uniformly–sized the kCS=kCR values, corresponding to the lifetime of with 100 nm in diameter on average. Scanning elec- the charge–separation state, increase with increasing

908 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

OMe OMe OMe OMe

MeO OMe MeO OMe MeO OMe MeO OMe O O O O

O O OMe O O OMe

O O MeO OMe N N O O MeO O O OMe N N O O Zn Zn N N OMe OMe O N N O

O O N N MeO OMe MeO OMe Zn N N

N N Zn N N

O

O

N O

O 54 N

55

MeO OMe MeO OMe MeO OMe MeO OMe

O O O O

MeO OMe MeO OMe O O O O O O O O

OMe OMe OMe OMe

N N N N Zn Zn N N N N

O O MeO OMe MeO OMe

O N N O N N N Zn Zn N O N N O N Zn N N N N N Zn MeO O N N O OMe O O O O

OMe O O OMe

N N O O O O Zn MeO OMe MeO OMe N N

OMe OMe OMe OMe

O

O

N

56

the generation number of the multiporphyrin dendri- spacer for spatial separation of porphyrin units from mers as well as the number of fullerene units. The SWNT and prevents direct – interactions between largest multiporphyrin dendrimer 33 in conjunction porphyrin and SWNT. Thus, the photoinduced elec- with the largest multifullerene 59 shows the largest tron transfer reaction takes place from porphyrin units kCS=kCR value as 3,400 among the family. The increas- to SWNT and thereby leads to the generation of ing tendency of kCS=kCR values suggests that excita- charge–separation state. tion energy may migrate over the multiporphyrin array –Helical oligopeptide chains, each containing one and facilitates the photoinduced electron transfer. histidine unit as an axial ligand, have been introduced Instead of fullerene derivatives, single–walled car- to the surface of polyamidoamine dendrimers and uti- bon nanotube (SWNT) has been used as an electron lized for the assembly with zinc porphyrin to form su- acceptor by Campidelli, Prato, and coworkers for the pramolecular multiporphyrin arrays on the surface.71 construction of a hybrid 61 bearing multiporphyrin Due to the presence of positively charged arginine poly(amidoamine) dendrons as an electron–donating units in the oligopeptide chains, negatively charged group.70 The dendrimer framework serves as a bulky acceptor such as naphthalene sulfonate induces a

Polym. J., Vol. 39, No. 9, 2007 909 Z. HE,T.ISHIZUKA, and D. JIANG

MeO OMe

MeO OMe O O

O O O O O O

O O O O

O O

O O O O O O O O MeO O O OMe

O O

O O N N Zn O O N N MeO OMe

O

O

N

57 higher fluorescence quenching degree than that of attracted much attention and has been considerably in- positive one, i.e. MV2þ. As the generation number vestigated in relation to solar energy conversion, for increases, the fluorescence quenching degree of which organic dyes have been utilized as photosensi- multiporphyrin arrays increases. Based on this result, tizers. Conjugated polymers with extended –elec- Mihara and coworkers have fabricated a four–compo- tronic conjugation are promising as photosensitizers, nent hydrogenase–mimicking system with MV2þ as as they have large absorption cross–sections, show an electron acceptor, triethanolamine as a sacrificed tunable light–absorbing properties, and allow exciton electron donor, for the photoinduced hydrogen evolu- and hole migration along the backbone. However, tion form water in the presence of hydrogenase as the their photosensitization activity remains unknown, redox catalyst.71b Based on a similar dendrimer, uti- owing to their insolubility in water and the strong lization of iron(III) porphyrin in couple with hydrogen tendency towards self–quenching of the photoexcited peroxide as an oxidant allows for the construction of states. In order to explore the photosensitization effect an artificial system mimicking the activity of peroxi- of –conjugated polymers and their activity in photo- dase.71c induced hydrogen evolution from water, Jiang, Aida, Due to the possibility for mediating hole and/or and coworkers have synthesized a series of water– electron migration, DNA and its intercalation deriva- soluble conjugated polymers, 62–69, wrapped with tives with chromophores such as porphyrin have at- poly(benzyl ether) dendrimer frameworks bearing tracted much attention as a motif for fabrication of charged exterior surfaces.73 photofunctional devices. Ikeda and coworkers have Upon excitation at 420 nm of the conjugated back- reported a photocurrent generator assembled on ITO bone, 64 in an aqueous Tris–HCl buffer (pH 7.4; electrode, by using DNA/porphyrin composite as 5 mM) emits an intense blue fluorescence centered electron donor, polyamidoamine dendrimer as sta- at 461 nm with a ÈFL value of 57%. In contrast, the tionary phase, and poly(vinyl sulfonate) or poly(allyl smallest homologue 62, emits at a longer wavelength 72 amine) as a supporting membrane. As the generation of 521 nm with a ÈFL value of only 7%. This is also number increases, the photocurrent increases to give the case for one–generation higher 63, which forms an overall quantum yield of about 1% for the 4th–gen- excimer (521 nm) to give a low quantum yield of eration poly(amidoamine) dendrimer. To enhance 29%. Therefore, the large dendrimer framework (2– photocurrent and quantum yield is yet a key point nm thick) of 62 is effective in encapsulating the con- for further challenge. jugated backbone even in water and preventing the loss of its excitation energy from the excimer forma- Dendritic Conjugated Polymers for PET tion. The addition of a cationic acceptor, i.e. MV2þ Photoinduced hydrogen evolution from water has results in a significant decrease in the fluorescence in-

910 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

À1 2þ 15 N (none)) )/[MV ], is evaluated as 1:2 Â 10 À1 À1 O M s , which is considerably larger than ordinary O 9 O O diffusion–controlled quenching rate constants (10 – O O O O 10 À1 À1 O O 10 M s ). These observations indicate that the

N electron transfer takes place within the pre–organized supramolecular complex between 64 and MV2þ.A control experiment with the meta–linked 69, which 58 has a molecular weight almost identical to that of 6 À1 64, shows a KSV value of only 2:0 Â 10 M , which is one order of magnitude smaller than that of 64. O Therefore, the –electronic conjugation along the O O

O backbone plays an important role in the electron trans- N 2þ O fer to MV . O O O O Fluorescence–quenching profiles are highly de- O pendent on the generation number and the surface of O O

O the dendrimer framework. For example, the lowest– N O O generation 62 displays a low fluorescence–quenching O O O activity, with a maximum quenching degree of only one–seventh that of 64. In contrast, positively charged compound 65 does not exhibit fluorescence quench- ing, even at high [MV2þ], indicating that MV2þ can- 59 not be trapped on the dendrimer surface due to elec- trostatic repulsion. On the other hand, 67, a nonionic derivative of 64 bearing tetraethylene glycol chains on the dendrimer surface, does not show quenching þ O 2 O at all, even in the presence of large excess MV . O Therefore, several structural factors are cooperative O

N to achieve the efficient photoinduced electron transfer O O in the above system. The inherent photoactivity of O O O O O the poly(phenyleneethynylene) backbone, because of O O O O O O O O a long–range –conjugation, is essential for the high O photosensitivity of the system. Of equal importance N O is that, the large dendrimeric shell helps isolate the O O O conjugated backbone and maintain the inherently high

O photoactivity even in water. Finally, the negative charges on the dendrimer surface are also essential for the acceptor to be trapped within an electron–

60 transfer distance from the conjugated backbone. Continuous exposure of a mixture of the Tris–HCl buffer of 64 and MV2þ, in the presence of triethanol- amine (TEOA) as a sacrificial donor to a 150–W tensity of 64, even at a low [MV2þ]of2:6 Â 10À8 M, Xenon arc light results in catalytic photoreduction of indicating that photoinduced electron transfer to MV2þ to MVþ, with an overall quantum efficiency MV2þ takes place efficiently from the conjugated (È = number of MVþ/number of photons absorbed) backbone, although it is embedded in the large dendri- of 29%. In contrast, systems with one–generation meric shell. smaller 63 and lowest–generation 62 show lower cat- Time–resolved fluorescence spectroscopy shows alytic photoreduction activity, whereas positively that the lifetime of the fluorescence of 64 (880 ps) is charged 65 does not respond to light irradiation at considerably shortened to 96 ps in the presence of all. Of much interest, –electronic conjugation of MV2þ (7.8 mM), where the rate constant for the elec- the backbone also significantly affects the photoreduc- 2þ tron transfer (kET) from 64 to the trapped MV mole- tion. For example, In contrast to the heptamer 66 that 9 À1 cule is estimated to be 9:3 Â 10 s . This kET value shows relatively high efficiency (30%), both short– is sufficiently large, assuming a 2–nm thickness for chain 68 and meta–linked 69 exhibit only low efficien- the dendrimeric shell of 64. The quenching–rate cy of 3 and 5%, respectively. In the terms of efficiency, À1 constant (kq), as given by ((ave (acceptor)) À (ave 64 is also supreme to representative sensitizers, such

Polym. J., Vol. 39, No. 9, 2007 911 Z. HE,T.ISHIZUKA, and D. JIANG

NHR

HN RHN O NH

O N NHR O NH NH N N O N R = Zn HN NH O N N N O O NHR

O

O

N

N

N O NHR O O HN O N O O O H O O NHR N N HN RHN N N H NH N O N H O HN O N RHN N NH H O

N O NHR HN O O

NH HN NHR

RHN NHR

61

2þ as Ru(bpy)3Cl2, 10–methyl–acridine orange, and zinc MV in the bulk solution, due to the lower affinity of tetrakis(1–methylpyridinium–4–yl)porphyrin, which the former towards the negatively charged dendrimer show È values of only 1, 3 and 6%, respectively. surface. Consequently, the energy of visible photons Based on these results, the above photoreduction sys- harvested by the conjugated wire chain flows steadily tem has been further coupled with PVA–Pt colloidal into MV2þ and is stored as MVþ, which triggers hy- as a catalyst for hydrogen evolution. The 64/MV2þ/ drogen evolution from water. Dendritic architectures TEOA/PVA–Pt system shows a steady generation of are unique in that they possess a unique mechanical hydrogen without a decrease in activity during 5–h stability unlike micellar aggregates, whilst maintain- irradiation and is tolerant to photo–bleaching. The ing appropriate conformational change dynamics. overall quantum efficiency for hydrogen evolution These properties cast a sharp contrast to other nano- (number of H2 molecules generated/number of pho- scopic architectures, such as inorganic multilayers tons absorbed) has been evaluated as 13%, which is and zeolite pores, which have previously been inves- more than one order of magnitude higher than those tigated as intervening media for electron–transfer re- of previously reported systems. actions. The built–in dendrimeric core–shell strategy re- veals the great potential of conjugated polymers as Other Dendritic PET Systems highly efficient photosensitizers for hydrogen evolu- As mentioned above, fullerene is an attractive elec- tion from water. Three–dimensional wrapping with a tron acceptor to afford long–lived charge–separation surface–charged large dendrimeric shell allows the states because of small reorganization energy. Other suppress of self–quenching of the photoexcited conju- than porphyrin derivatives as electron–donating com- gated backbone, while MV2þ is trapped on the nega- ponents, a variety of dendritic systems for photoin- tively charged dendrimer surface to form a spatially duced electron transfer reaction have been reported separated donor–acceptor supramolecular complex. using amine, cytochrome c, and phenanthlorine Together with this unique donor–acceptor geometry, copper(I) complex as electron donors.74–79 Utilization hole migration along the conjugated backbone might of the dendrimer framework to encapsulate acceptor lower the relative rate of charge recombination. This moieties c.f. viologen80,81 and norbornadiene,82 or is also suppressed by a rapid exchange of MVþ with donor units such as ruthenium bipyridine complex83

912 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

X X

X X

–O C – 2 CO2 O O X X – O O O2C – – – CO2 O2C CO2 O O O O O O O O O O X X O O O O O O O O

O O O

O m O m O m

O O O O O O X O O X O O O O O O O O O O –O C – 2 CO2 – O2C – CO2 O O 62 X X O O –O C – 2 CO2 X X 63

X X

– 64: X = CO2 + 65: X = CONH(CH2)2N Me3 – 66: X = CO2 ; m = 7 67: X = CO2(CH2O)4Me

– – – – O2C CO2 O2C CO2

– – – – O2C CO2 O2C CO2

O O O O – – – – O2C CO2 O2C CO2 O O O O

O O O O O O O O

O O O O –O C CO – –O C CO – 2 O O O O 2 2 O O O O 2

O O

O O m

– O O O O – – O O O O – O2C CO2 O2C CO2 O O O O

O O O O O O O O

O O O O – – – – O2C CO2 O2C CO2 O O O O

– – – – O2C CO2 O2C CO2

– – – – O2C CO2 O2C CO2

68 69 and anthracene84 have also been reported for the con- ing motif for the investigation of photoinduced elec- struction of electron transfer systems. A further exten- tron transfer event at the single molecular level. Re- sion of using dendrimers for triggering photoinduced cently, De Schryver and coworkers have explored electron transfer is to employ photoactive units as this possibility by using rigid polyaryl dendrimers the building blocks in the framework. In this sense, 70 and 71 with peryleneimide core as electron ac- poly(phenyl acetylene) dendrimer is unique since the ceptor and arylamine units on the surface as electron whole dendrimer framework can serve as electron–do- donor, upon embedded in a polystyrene matrix.87 In nating or electron–accepting components, highly de- general, photoinduced electron transfer process is pending on the external electron–active molecules uti- hardly to be monitored with single–molecule spectros- lized.85 In relation to this, recently, a series of poly- copy, because the fluorescence is usually quenched (viologen) dendrimers with different numbers of viol- and no signals can be detected. However, the ogen units have been synthesized by Marchioni and charge–separation states of 70 and 71 decay to form coworkers to investigate the stepwise photoreduction again locally excited state through a reverse electron of viologen arrays in the bulky dendrimer frame- transfer process. Therefore, the delayed fluorescence work.86 with a high quantum yield enables the monitor of pho- A well–defined three–dimensional structure along toinduced electron transfer event with single–mole- with a precise location of functional groups in its cule spectroscopy. Dynamic fluctuations of the fluo- framework makes dendritic macromolecule a promis- rescence decay times have been observed for 70.A

Polym. J., Vol. 39, No. 9, 2007 913 Z. HE,T.ISHIZUKA, and D. JIANG

N N

N N

O N O

O O O O

O N O

N N

N N

70

N N

N N

N N

O N O N N

O O O O

N N O N O

N N

N N

N N

71 detailed study shows that the conformational changes ported by Hara, Majima, and coworkers in a poly(ben- of the dendrimers (70 and 71) and reorientation of the zyl ether) dendrimer 72 with stilbene focal core.88 matrix polystyrene chains induce the fluctuations of Two–photon excitation of the dendrimer framework the fluorescence decay times. The torsional motion at 266 nm generates a hole in the dendrimer frame- of the adjacent phenyl rings next to the donor in the work. The hole thus produced can further transfer to dendrimer is attributable to the small fluctuations in the focal stilbene core to result in the formation of stil- decay times, whereas polystyrene motion causes much bene cationic radical. As the generation number of the dynamic conformational change of the dendrimer and dendrimer framework increases, the lifetime of stil- thereby leads to the large fluctuations.87e bene cationic radical increases, as a result of site iso- lation. Although the quantum yield of two–photon Dendrimers for Photoinduced Hole Transfer ionization is low as 6.9%, the hole transfer efficiency As described above, the dendrimer framework form the dendrimer framework to the focal stilbene serves as light–harvesting antenna and mediates pho- core is quantitative. In a three–component system toinduced electron transfer. In relation to these func- using biphenyl (BP) as a hole transporter, 9,10–dicya- tions, to explore dendrimer for hole transfer is a noanthracene (DCAN) as a photosensitizer, and 72 as subject of importance, due to its high possibility to a hole trap, the same group have reported that photo- develop hole–transporting materials for organic excitation of DCAN at 355 nm results in a consecutive light–emitting diodes (OLEDs). Photoinduced hole electron and hole transfer, to afford stilbene cationic transfer within the dendrimer framework has been re- radical and DCAN anionic radical. As the generation

914 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O

72 number increases from 1 to 4, the rate constant of in that they allow for designable ‘engineering’ of the intermolecular hole transfer from BP cationic radical spin–active components and the materials thus ob- to dendrimers remains unchanged (about 2  109 tained can be easily processed. Therefore, by integrat- MÀ1 sÀ1), but that of intramolecular hole transfer from ing suitable organic groups and/or segments, c.f. those exterior surface to the focal stilbene unit decreases responsive to external stimuli, one may have the from 4:2  106 to 1:6  106 sÀ1. The large dendrimer chance to create novel spin–active soft materials. framework because of its steric hindrance, not only in- hibits the direct hole–transfer from BP cationic radical Dendritic Organic Radicals to stilbene core but retards charge recombination be- Organic radical compounds have been a central tween stilbene cationic radical and DCAN anionic concern in relation to the development of magnetic radical as well. soft materials with a recent focus on –conjugated Besides the above photo–generated hole transporta- polymers, because of their possibility to achieve tion event in the dendrimer framework, electrically strong exchange interactions between spin sites triggered hole transfer have also been investigated. through –conjugation network. From a viewpoint Moore and coworkers have reported that an anthra- of the molecular design, a system with a high density cene–cored poly(phenylene acetylene) dendrimer with of cross links and alternating connectivity of radical triaryl amine units on the surface, can transport hole in sites is a structural prerequisite for achieving large OLEDs.89 More recently, Yamamoto and coworkers values of spin–quantum number S.91 Conjugated den- have reported that poly(phenylazomethine) dendri- drimers have a high potential to fulfill these require- mers upon complexation with Sn2þ show an activity ments and are promising motif to mediate spin–spin in hole transport.90 interactions, due to the possibility for ‘tailor–made’ placement of spin sites in the three–dimensional DENDRITIC ARCHITECTURES FOR framework. SPIN–FUNCTIONAL NANOMATERIALS Veciana and coworkers have reported a dendritic polyradicals 73 with four radical sites based on poly- As described above, dendritic macromolecules are (triphenyl methylene) dendrimer upon treatment with versatile in controlling electron charge to perform tetrabutyl ammonium hydroxide followed by oxida- vectorial excitation energy transduction and efficient tion with p–chloranil.92 EPR measurement shows that photoinduced electron transfer. On the other hand, to 73 displays a profile due to the forbidden transition control electron spin with dendrimers has attracted with ÁmS ¼Æ2, but without any signs from transi- much attention in recent years, with an even increas- tions with ÁmS ¼Æ3 and ÁmS ¼Æ4. Magnetic–field ing interest due to their high probability for regulating dependency of the susceptibility indicates that the spin–spin interactions. In contrast to traditional inor- dendritic polyradicals 73 is mainly at a triplet state ganic crystalline frameworks, dendrimers are unique with an S value of 1. The fact that magnetic ground

Polym. J., Vol. 39, No. 9, 2007 915 Z. HE,T.ISHIZUKA, and D. JIANG

Cl Cl Cl Cl ClCl N

Cl Cl Cl Cl Cl Cl Cl ClCl ClCl Cl N Cl Cl Cl Cl Cl Cl R N Cl ClCl ClCl Cl N N Cl ClCl Cl Cl Cl N N Cl Cl Cl N N Cl Cl Cl R N 73 R R n 75 state of 73 is triplet other than quintet is likely due to the non–planarity of sp2–central carbon induced by distortion of the neighboring large aromatic rings. place between radical sites. On the other hand, the The bulky dendrimer framework enhances the stabili- dendritic trityl–anionic radicals show a ground state ty of high–spin species but simultaneously results in with an S value of 1, derived from biradicals that undesired spatial congestions, which may block ferro- are bridged by Kþ (introduced at the reduction step). magenetic coupling between the spin sites. A dendrimer–like aromatic poly(aminium cationic Mu¨llen and co–workers have synthesized poly- radicals) 75 has been synthesized by Nishide and co- phenylene dendrimers bearing eight trityl–radicals or workers. 75 forms a 2–D polyradicals network with trityl–anionic radicals in the frameworks.93 In com- an S value of 8.4 at 5 K.94 This high–spin ground state parison with simple trityl–radical reference, dendritic indicates a ferromagnetic coupling between eight or trityl–radicals 74 exhibits a 70 nm red–shifted absorp- nine unpaired electrons at low temperature in the den- tion band, suggesting an efficient delocalization of drimer–like poly(aminium cationic radicals). spin density over the dendrimer framework. The TTF is an attractive motif to build up molecular ground state of this dendritic trityl–radicals is S ¼ conductors due to the formation of cationic radical 1=2, suggesting no ferromagnetic interactions take in the presence of magnetic counter ions. Up to date,

Ar

Ar Ar

Ar

Ar

Ar Ar

Ar

74

Ar = or

916 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

R R R S R S S S S R S S S

R R R S S O R S O O S S R R O O S O S R O O S S S R O P S S P R O N O R R R S N O S N S N S O S S O R R S O N O O O O P P O R S O O R S N N O P N P O S S O N R N OO P R S N O S S S O S O N N N R S N P S O P O O O

O O O O O O O O R

S S R R S S S S R S S S S S S R R R S S S R S R R R R

76 Spacer O a variety of dendrimers containing TTF building Upon anchoring on the poly(amide amine) dendrimer blocks have been synthesized with an aim to achieve surface, Brechbiel and coworkers have reported that conductive and/or magnetic active materials.95 Bryce the TEMPO units are robust for EPR imaging to retain and coworkers have synthesized a family of multi– spin activity for a much longer period even in an TTF dendrimers.96 Electrochemical oxidation of the aqueous solution.102 In relation to this, Francese and multi–TTF dendrimers affords polycationic radical coworkers have reported a water–soluble poly(amide species. On the other hand, controlled chemical oxida- amine) dendrimer with nitroxyl nitroxide radicals on 103 tion of a multi–TTF dendrimer 76 with PhI(OAc)2 in the surface as a contrast agent for MRI. the presence of CF3SO3H gives mixed–valence cat- ionic radical salts, which show electric conductivity.97 Spin–Transition Soft Materials Based on Dendritic The conductivity is originated from electron transfer Coordination Polymers between partially oxidized dendrimers as well as in- Metal complexes with d4–d7 first–row transition termolecular charge transfer. metal ions in octahedral environment exhibit a spin Nitroxide radicals, owing to their broad applica- crossover (SCO) between low–spin (LS) and high– tions as initiator, spin tag, and magnetic contrast spin (HS) states under certain conditions. Such a bist- agent, have attracted great attention in recent years.98 ability phenomenon has attracted intense attentions Dendritic macromolecules containing nitroxide radi- not only from the basic scientific viewpoint but also cals have been synthesized mainly based on poly- for its potential applications in molecular electronics. amine framework. Tomalia and coworkers immobi- By employing chemical approaches such as crystal lized TEMPO moiety to the exterior surface of engineering, much effort has been made for the prep- polyamine dendrimer as a spin probe to gain structural aration of SCO crystalline solids, which suffer from information and to investigate interactions between the low processability.104 Hence, it is yet a challeng- dendrimers with mesoporous surface, vesicles, and ing subject to fabricate SCO soft materials that not liposomes.99,100 Poly(propylene imine) dendrimers only have tunable spin states by external stimuli but bearing multi–nitroxyl groups on the surface display also have improved processability.105–107 strong exchange interaction between radical sites.101 Fujigaya and coworkers have reported SCO phe-

Polym. J., Vol. 39, No. 9, 2007 917 Z. HE,T.ISHIZUKA, and D. JIANG

12 – 77: R = G0 , X = CH3SO3 12 – 78: R = G1 , X = CH3SO3 12 – R R 79: R = G2 , X = CH3SO3 N N X- X- X- R R N N R N N 8 – FeX , Vc 80: R = G0 , X = ClO4 N 2 N N 12 – FeN N Fe N N Fe 81: R = G0 , X = ClO4 NN 16 – X- N N X- N N X- 82: R = G0 , X = ClO4 N N R R 12 – 83: R = G0 , X = C8H17SO3 12 – 84: R = G0 , X = C12H25SO3 12 – 85: R = G0 , X = C16H33SO3

R = C12H25 O O C H 8 17 C H O 12 25 O NH C H O C H O C H 12 25 12 25 8 17 O O 8 G C12H25 0 O O O C12H25 O C12H25 O O O O O

NH NH NH O C12H25 O O 12 O C12H25 G0 O O C12H25 O O O C H 16 33 C H O C H O 12 25 12 25 12 G O NH 1 C H O C H 12 25 16 33 O 16 C H G0 12 25 12 G2 nomenon of dendritic Fe(II) triazole complexes 77–79 (XRD) pattern among the series, which is indicative self–assembled from Fe(II) ions and dendritic of a 2D hexagonal ordering with a d–spacing of triazole ligands with poly(benzyl ether) dendrons.108 37.4 A˚ . Considering the Fe–Fe distance of 3.7 A˚ as These complexes are colored violet at room tempera- evaluated by EXAFS, the unit cell has been calculated ture due to a d–d electronic transition of the LS Fe(II) as 43.2 A˚ in diameter and 3.7 A˚ in height (V ¼ 5:42 species, and decoloration occurs upon heating to give nm3, W ¼ 5:79 Â 10À21 g). Since the density of the off–white solids, as a result of spin transition to the material, measured using a gradient density column, HS state. Such a thermally induced spin transition is is 1.07 g cmÀ3, the unit cell can accommodate 2.8 den- reversible and can be repeated for many times without dron subunits. This number is very close to the requi- any deterioration. Of interest, the spin transition is site stoichiometry of 3:1 for complexation between highly dependent on the generation number. SQUID triazole and Fe(II). Thus, the dendritic wedges in 78 measurement of 78 shows a complete spin transition are densely packed around the oligomeric Fe(II)–tria- in a temperature range of only 10 K, which is in good zole backbone and perfectly fit the surrounding space. agreement with an abrupt coloration–decoloration re- In comparison to 78, the unit cells of 77 and 79 are sponse to the temperature change. In contrast, 77 and estimated to accommodate 4.1 and 1.6 units of den- 79 require a much broader temperature range of 30 dron subunits, respectively, indicating that the den- and 50 K, respectively. Besides, the spin–transition dritic wedge of 77 is not large enough to fill the unit temperature decreases from 335 to 315 to 300 K when cell, while the unit cell of 79 is not large enough to the generation number increases from 0 to 1 to 2. satisfy the ideal 3:1 complexation. From these results, DSC measurement shows that the ÁH and ÁS it is most likely that self–assembled 77 and 79 are values for the spin transition of 77 are 15.7 kJ molÀ1 structurally defective and frustrative, so that the Fe(II) and 46.7 J molÀ1 KÀ1, respectively, which are typical sites are hardly cooperative with one another for the of those reported for spin transition of Fe(II)–triazole spin transition. This study shows that dendrimers are complexes.109 Contrastingly, 78 exhibits much higher useful components for design ‘‘cooperativity,’’ which ÁH and ÁS values of 73.1 kJ molÀ1 and 230.0 J molÀ1 can enhance or amplify certain functions in self– KÀ1, respectively. In regard to the abrupt spin transi- organized states. tion event together with the extraordinary high ÁH and ÁS values, the Fe(II) sites in self–assembled 78 Spin–Transition Soft Materials Switchable with Phase are strongly correlated with one another and highly Transitions cooperate in the spin transition. In relation to this fea- By appending the triazole unit with long alkyl ture, 78 shows the most distinct X–ray diffraction chains of different lengths, i.e. C8, C12, and C16,

918 Polym. J., Vol. 39, No. 9, 2007 Photo– and Spin–Functional Dendrimers

Fujigaya and coworkers have synthesized dendrimers phase exists at 25 C. Upon heating to induce the gel– 80–82 in which each Fe(II) is bridged by three biden- to–sol phase transition, the liquid crystalline texture tate triazole ligands to afford one–dimensional rod.110 disappears completely at 62 C. Rheological study at Of interest, the length of the alkyl chains strongly 25 C shows that the system consists of a long–range affects their SCO profiles. Compound 82 with the alignment of rod–like 84. Temperature–dependent longest alkyl chains displays the spin transition at rheological properties of 84 show that sol–gel transi- 310 K with a hysteresis width of 5 K. Compound tion occurs at a temperature same as that for the spin 81 exhibits a spin–crossover profile similar to that of transition, indicating that the spin–transition and 82, whereas 80 with the shortest alkyl chains hardly phase–transition events are perfectly synchronous to shows a clear spin transition. each other. XRD pattern of 82 shows a d spacing of 36.5 A˚ , The spin–transition events of the gels are much suggesting an interdigitation of the long alkyl chains quicker than those in solution. Both the transitions to give parallel–aligned Fe(II)–triazole backbones, from the LS to HS and from HS to LS states are es- whose center–to–center separation is estimated to be tablished only within a very short period of time 36 A˚ . 81 shows a similar XRD pattern but with a (< 2 min). In sharp contrast, a m-xylene solution of smaller d spacing of 30.2 A˚ . Thus, the Fe(II) centers 84 (0.6 wt %) shows an extremely slow recovery are fastened to one another not only by ligation with (> 300 min) of the LS state on rapid cooling from the bidentate triazole ligands but also through interdi- 50 to 20 C. VT FT–IR measurement shows that a hy- gitation with the long alkyl chains. drogen–bonding network plays an important role in Variable–temperature IR spectroscopy of 82 shows stabilization of the LS state. Therefore, mixing the that the alkyl chains in LS state of 82 are crystallized gel of 84 with 1-octanol, which is a scavenger of with a stretched conformation, whereas those in HS the hydrogen bonding, results in spin transition and state adopt a shrunk conformation. DSC measure- gel–to–sol phase transition simultaneously. In fact, ments of 81 and 82 reveal that both show single endo- on addition of different amounts of 1–octanol to the thermic and exothermic peaks, the transition tempera- gel, the spin–transition temperature can be changed tures of which are consistent with their spin–transition over a wide range from 65 to 20 C. temperatures. Therefore, the phase transition of the al- The spin–crossover gels are characterized by their kyl chains triggers the SCO of 81 and 82. At low tem- narrower temperature range for the spin transition peratures, the crystalline alkyl chains, upon interdigi- than the solid samples, synchronous to the sol–gel tation, lock the Fe–N bond distance of the LS Fe(II) phase transition, and their quick response to tempera- complexes. On heating, this lock is released by melt- ture changes both on heating and cooling. Since gels ing of the alkyl chains, thereby permitting the elonga- are generally more sensitive than solids to external tion of the Fe–N bond that is necessary for spin tran- perturbations, the spin–crossover gels may have a sition to the HS state. This supramolecular self– large potential in, e.g., sensory applications. assembly approach allows mesoscopic cooperativity among the magnetic species through interdigitation SUMMARY of the alkyl chains, and thereby enables a ‘lock– and–release’ feature of the spin state. This article deals with two aspects of molecular Of interest, compound 83–85 having alkyl sulfonate functionality based on dendritic architectures; one is as counter ion forms a purple–colored transparent related to photoinduced energy and electron transfer physical gel in paraffins even at a low concentration and the other concerns spin state regulation. One such as 0.6 wt %.111 Upon heating to 80 C, the purple may notice that cooperativity is one of the most unique gel collapsed to give a pale yellow clear solution. characteristics of a dendritic macromolecule, which Variable–temperature absorption spectroscopy on heat- enables the enhancement of certain functions and/or ing from 15 to 80 C shows the appearance of a broad creation of new phenomena. The role of the dendrimer 5 5 band centered at 850 nm, due to a T2g– Eg d–d tran- framework is multiple and can be synthetically design- sition of the HS state, at the expense of the LS absorp- able. In this sense, the well–defined and nano–sized tion band at 537 nm. On cooling to 15 C, the hot solu- dendritic macromolecules will continue to grow to tion immediately turns to a purple–colored transparent be a central player in the materials science and serve gel and recovers the LS absorption band at 537 nm. as an interesting and fascinating motif in nanoscience, Such a thermal–induced discoloration–coloration cy- nanotechnology, and other interdisciplinary fields. cle can be repeated without any sign of deterioration. Polarized optical microscopy shows that self–as- Acknowledgment. This work was partially sup- sembly of 84 plays a major role in spin transition. A ported by JSPS Asian CORE Program on Frontiers of characteristic texture due to a liquid crystalline meso- Materials, Photo, and Theoretical Molecular Sciences.

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Zheng He received a B.S. degree from Peking University in 2000 and obtained a Ph.D degree in inorganic chem- istry from the same university in 2005 under the supervision of Professor Chun–hua Yan. She started her postdoc- toral work with Professor Donglin Jiang at Institute for Molecular Science, National Institutes of Natural Sciences from 2006. Her research interest is photo– and spin–functional dendritic macromolecules.

Tomoya Ishizuka received a B.S. degree in science from Kyoto University in 1999 and obtained a Ph.D. in chem- istry from Graduate School of Science, Kyoto University in 2004 under the guidance by Professor Atsuhiro Osuka and Professor Hiroyuki Furuta (Kyushu University). Then, he became a postdoctoral fellow in University of Pennsylvania, working with Professor Michael J. Therien and received a JSPS Postdoctoral Fellowship for Research Abroad from April 2005. He joined as a research associate in the group of Professor Donglin Jiang at Institute for Molecular Science, National Institutes of Natural Sciences from February 2006 and became an assistant professor from April 2007 in the same group due to the change of the research faculty organization at IMS. His research interest is synthesis and new aspects of the physical properties of electronic and spin–functional materials.

Donglin Jiang received a B.S. degree in chemistry from Zhejiang University in 1989 and obtained a Ph.D. in poly- mer chemistry from The University of Tokyo in 1998 under the direction of Professor Takuzo Aida. He then became an academic carrier at The University of Tokyo and had been involved until 2000 in the development of functional polymers based on dendritic architecture. In 2000, he was appointed as a group leader of the JST ERATO project on ‘‘Nanospace.’’ In 2005, he moved to Institute for Molecular Science, National Institutes of Natural Sciences as associate professor. His research interests include (1) synthesis and functions of metal polymers and (2) photo– and spin–functional dendritic macromolecules. In 2005, he was selected as a researcher of the JST PRESTO project on ‘‘Structure Control and Function.’’ He received SPSJ Wiley Award 2006.

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