Review

Light harvesting dendrimers

Arpornrat Nantalaksakul1, D. Raghunath Reddy1, Christopher J. Bardeen2 &S. Thayumanavan1,*1Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, USA; 2Departmentof Chemistry, University of California, Riverside, California 92521-0403, USA; *Author for correspondence(e-mail: thai@chem.umass.edu )

Received 21 March 2005; accepted in revised form 2 June 2005

Key words: dendrimers, electron transfer, energy shuttling, energy transfer, light-harvesting, site-isolation

Abstract

Tree-like dendrimers with decreasing number of chromophores from periphery to core is an attractivecandidate for light-harvesting applications. Numerous dendritic designs with different kinds of light-col-lecting chromophores at periphery and an energy-sink at the core have been demonstrated with high energytransfer efficiency. These building blocks are now being developed for several applications such as light-emitting diodes, frequency converters and other photonic devices. This review outlines the efforts that arebased on both conjugated and non-conjugated dendrimers.

Introduction

In natural photosynthetic systems, a large array ofchlorophyll molecules surrounds a single reactioncenter. The intricate chlorophyll assembly acts asan efficient light harvesting antenna that capturesphotons from the sun and transfers its energy to thereaction center, where conversion of solar energyinto chemical potential energy via the formation ofa charge-separated state takes place. Interestingly,the energy of any photon absorbed anywhere inthis relatively large assembly of chromophores ispassed rapidly to the reaction center with energytransfer quantum yield that approaches unity overnanometer distances (Deisenhofer et al. 1985;Deisenhofer and Michel 1989; Lehninger et al.1993; Barber and Andersson 1994; McDermottet al. 1995; Balzani et al. 1997; Hu et al. 1998).

In the past decade, much attention has beendevoted to the design and synthesis of supramo-lecular systems that can function as artificial lightharvesting systems for the photochemical conver-sion of solar energy (Webber 1990; Fox 1992;Wasielewski 1992; Gust et al. 1993, 2001; Balzaniet al. 2003a, b). Five features of these complexes

play key roles in the efficient collection of incidentlight for conversion into chemical energy: (1) Largeabsorption cross-section of the complex due to alarge number of chromophores with high extinc-tion coefficients; (2) relative spatial orientation ofthese chromophores; (3) energy hopping of theexciton along the chromophores at the rim of thecomplex; (4) efficient and uni-directional energytransfer (ET) of the exciton from a chromophore atthe rim to the chromophore in the center of thecomplex; (5) the generation of efficient photo-induced charge separation from excited state ofperipheral chromophores and neutral state of thecore. Dendrimers, which are perfectly branchedsynthetic macromolecules having numerous chainends all emanating from core, are interesting scaf-folds for light harvesting applications. Light har-vesting is the trapping of energy via peripheralchromophores and funneling to a central point,where it can be converted back into photon energyor into chemical energy. Dendrimers possess theproperties that facilitate such a conversion.These properties include their tree-like structurethat could potentially act as an energy gradientfor the funneling process. The periphery of

Photosynthesis Research (2006) 87: 133–150 � Springer 2006

DOI: 10.1007/s11120-005-8387-3

dendrimers can be functionalized with multiplelight absorbing chromophore units that gives ahigh probability to capture light. The relativelyshort through-space distance from the peripheryto the core, due to back folding, allows for highefficiency energy transfer.

Several research groups have extensivelyworked on searching for suitable chromophoresand dendritic architectures in order to obtain thelight-harvesting systems that provide high energytransfer efficiency (Devadoss et al. 1996; Stewartand Fox 1996; Shorteed et al. 1997; Balzani et al.1998; Bar-Haim and Klafter 1998a, b; Gilat et al.1999; Adronov and Frechet 2000; Weil et al. 2002;Melinger et al. 2002; Balzani et al. 2002; Balzaniet al. 2003a, b). First, we will discuss systems inwhich the dendrimer is just a scaffold, but does notplay a photoactive role. Then, we will discusssystems in which the dendritic backbone itselfcould act as a chromophore. Next, the topics ofenergy migration within dendrimers, energy cas-cade, and site isolation using dendritic scaffoldswill be discussed. Finally, a few examples will behighlighted where photoinduced charge transferprocesses are achieved and then conclude with oursystems that contained both energy and chargetransfer in dendrimers.

Dendron as a scaffold

Non-conjugated dendrons such as the widely usedpoly(aryl ester) dendron function as just a scaffold

linking together light-harvesting chromophores atthe rim and the energy acceptor chromophore atthe core. Owing to the lack of the electroniccommunication between donor and acceptorchromophores through dendritic backbone, thesedendrimers provide the ability to independentlytune the energy level of each chromophore.Moreover, the flexibility of the backbone alsohelps increase the solubility and processability ofdendrimers.

Frechet and coworkers synthesized poly (arylether) dendrimers containing amino-functional-ized Coumarin-2 as the donor and acid-function-alized Coumarin-343 as the acceptor (1a, Figure1a) (Adronov et al. 2000). The emission spectrumof the donor overlaps well with the absorptionspectrum of the acceptor chromophore, fulfillingthe requirement for Forster energy transfer. Theabsorption spectra of dendrimers also matched thesum of the absorption spectra of model donor andacceptor implying that the dendron acts as just ascaffold holding donor and acceptor chromo-phores together and there is no electronic com-munication between donor and acceptor unitsthrough the dendritic backbone. Furthermore, theabsorbance of fully-labeled dendrimers alsoshowed that the donor absorption doubled whilethe acceptor absorption stayed relatively constantwith increasing generations. This was taken toindicate that the light-harvesting ability of thesedendrimers increases linearly with the number ofchromophores in successive generations. The

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Figure 1. G2 dendrimers containing Coumarin-2 as an energy donor and Coumarin-343 as an energy acceptor (left) and dendrimerswithout Coumarin-343 chromophore (right) for relative rate study.

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excitation of Coumarin-2 at 343 nm resulted in thefluorescence mainly at �480 nm, which repre-sented the characteristics of Coumarin-343 accep-tor emission. This result implied an efficient energytransfer within these molecules. Steady-state andtime-resolved studies revealed that the energytransfer efficiency in these dendrimers approachedunity even at higher generations. Also, an inter-esting study on the relative rate between the energytransfer and nonradiative relaxation was carriedout in this work. The model compounds of thesedendrimers containing chromophores at theperiphery, but not at the core, were also designedand used for this study, (structure 1b). The fluo-rescence spectra of G1 and G2 model dendronsshowed the quenching of the Coumarin-2 emissionin methanol upon the excitation of donors result-ing from the nonradiative relaxation initiated bythe hydrogen bonding of the solvent with the ter-tiary amine lone pair. In contrast, correspondingdendrimers with the acceptor at the core showedstrong emission exclusively from Coumarin-343core. This study revealed that the fast energytransfer can overcome the rate of nonradiativepathway. Unfortunately, higher generations inwhich conformational flexibilities become morepronounced, encountered solubility problems andthus could not be studied.

Dendrimer backbone as the chromophore

Dendrimer backbone themselves can also be con-currently used as the energy donor. Conjugated

dendrimers such as phenylacetylene chains weremainly selected for this purpose. By controllingover the conjugation length of dendritic branchesin these dendrimers, rapid and directional energytransport could be obtained resulting in efficientenergy transfer.

Efficient, unidirectional energy transfer from adendritic framework to a single core chromophorewas reported by Xu and Moore (1994) (2,Figure 2). The robust, high-yielding synthesis oftheir phenylacetylene dendrimers allowed for thepreparation of high-generation (G-n) molecules, upto G-6 (Xu and Moore 1993). These cross-conju-gated structures exhibit strong UV absorptionfeatures in the 250–350 nm range that double inabsorption with increasing generation (Devadosset al. 1996). Additionally, it was found that thesedendrimers exhibit emission in the 350–450 nmrange. By functionalizing the core of these struc-tures with the lower band gap perylene chromo-phore, an energy ‘sink’ was introduced into thesystem. Hence, the phenylacetylene monomer unitsact as the peripheral energy donors, and peryleneacts as the central energy acceptor. Excitation ofthe dendrimer backbone at 312 nm resulted inemission emanating solely from the perylene dye(450–600 nm), with nearly complete quenching ofthe dendrimer emission.

Another interesting example of phenyl acety-lene dendrimers is a new class of phenyl acetylenemonodendrons characterized by unsymmetricalbranching reported by Peng and coworkers (3,Figure 3) (Peng et al. 2000; Melinger et al. 2002).

Figure 2. Chemical structure of perylene-functionalized phenylacetylene dendrimer.

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These unsymmetrical monodendrons exhibit verybroad absorption spectra due to unsymmetricalbranching and it was found that the amount oflight absorbed approximately doubles for eachgeneration. In luminescence studies, when excitedpredominantly at phenylacetylene backbone, thefluorescence emission comes almost entirely fromperylene core due to efficient energy transfer fromphenylacetylene backbone to core. The details ofthe unsymmetrical dendrimers are a subject of areview article in this issue and therefore will not bediscussed in any further detail (Peng et al. 2006).

Mullen and co-workers have used �polyphen-ylene� building blocks to explore a variety of well-defined macromolecular architectures rangingfrom linear oligomers and complex macrocycles,to dendritic compounds. The polyphenylene den-drons, characterized by their shape-persistentstructure and out-of-plane twisted phenyl compo-nents, have been successfully attached to variousfunctions. In addition, these bulky moieties canalso absorb light energy which is then funneledefficiently to the core (Qu et al. 2003). Recently,De Schryver and co-workers have reported

polyphenylene dendrimers 4 containing a peryl-enediimide core (Figure 4) (Liu et al. 2003). Inthese systems, polyphenylene dendrimer scaffoldexhibits strong fluorescence, with quantum yieldsranging from 0.2 to 0.5 depending on the dendri-mer generation. They also noted that, highextinction coefficients of polyphenylene dendriticarms at shorter wavelength and their strong fluo-rescence intensity, together with the efficientintramolecular energy transfer, result in a strongemission from the core by indirectly exciting thepolyphenylene dendritic arms.

Energy migration

In dendrimers fully decorated with peripheralchromophores, after one of the peripheral chro-mophores were excited by incident light, it wasproved by several groups that the migration of theexcitation energy was used up before energytransfer to the core. Jiang and Aida demonstratedporphyrin dendrimers ((L5)nP, n=1–4) havingdifferent number (n) of five-layered aryl ether

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Figure 3. Unsymmetrical perylene-terminated monodendron.

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dendron subunits (L5) (5, Figure 5) (Jiang andAida 1998). The excitation of dendron subunits in(L5)4P at 280 nm in CH2Cl2 resulted in strongemission at 656 and 718 nm which is characteristicof the porphyrin core. In contrast to this result, theexcitation of partially substituted dendrimersresulted in a strong emission in the dendron regionwith only a weak emission from the porphyrincore. The energy transfer quantum yield droppeddramatically with the decreasing number of sub-stituents on the porphyrin core. (n=4, /EET

=80.3%; n=3, /EET = 31.6%; n=2, /EET =19.7%; n=1, /EET = 10.1%) For this observa-tion, the authors suggested that before energytransfer happens, the excitation energy firstmigrates among neighboring dialkyoxybenzylunits until it can find the chromophore that has asuitable orientation for energy transfer. Then, theexcitation energy is efficiently transferred to thecore. As a result, this energy migration processwould be able to enhance the energy transfer effi-ciency. The evidence for the presence of this energyshuttling was confirmed by fluorescence aniso-tropic measurements. The excitation of (L5)4P at280 nm with polarized light resulted in the depo-

larized emission whereas emission of partiallysubstituted (L5)nP (n=1–3) still exhibited polari-zation character. Also, elevated temperature couldreduce the energy transfer efficiency by inducingthe mobility of molecules and thus increasing theopportunity for nonradiative relaxation. Thissupposition was true only in the case of partiallysubstituted dendrimers. However, the energytransfer efficiency in (L5)4P was found to be sim-ilar upon increasing the temperature. This resultindicated that (L5)4P was conformationally rigidand that energy shuttling could still happen even athigh temperature.

In a more recent work, Aida and co-workerssynthesized star and conical shape multiporphyrinarray 6 containing dendritic wedges of zinc por-phyrin heptamer as the energy donor and por-phyrin free base as the energy acceptor (Figure 6)(Choi et al. 2001). The energy transfer in starshape dendrimers was more efficient than that inthe conical structure. The steady-state fluorescencedepolarization again confirmed the presence ofenergy migration among zinc porphyrins in starshape dendrimers resulting in high energy transferefficiency in these dendrimers.

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Figure 4. Molecular structure of polyphenylene dendrimer.

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In another study, Mullen and coworkers havealso reported that energy hopping takes placeamong all peryleneimide chromophores in poly-phenylene dendrimers 7 with multiple peryle-neimide donor chromophores and a singleterryleneimide acceptor, with a hopping rateconstant (khopp) of 4.6 ns)1 and 95% energytransfer (Maus et al. 2001; Gronheid et al. 2002;)(Figure 7)

Energy cascade

A versatile synthetic scheme allowed for thesynthesis of dendrimers having a directionalenergy gradient. Moore and coworkers havereported dendrimers based on phenylacetylenechains that are spatially arranged to form anenergy gradient (8, Figure 8). Interestingly, itwas found that this energy gradient dramaticallyincreases (by two orders of magnitude) theenergy transfer rate constant within the dendri-mer (Devadoss et al. 1996). Hence, the direc-tional energy transfer from periphery to coremust be greatly facilitated by the built-in energy

gradient. Indeed, theoretical work by Klafter andcoworkers afforded the same conclusion, sug-gesting that ‘random walk’ energy transfer fromperiphery to core, as in the former structures, ismuch less productive than the directed process infunnel structures (Bar-Haim et al. 1997; Bar-Haim and Klafter 1998a, b). However, themechanism of energy transfer in these systemswas difficult to ascertain. Owing to the cross-conjugated dendrimer backbone, orbital overlapcontributions to the energy transfer cannot beruled out (Gaab et al. 2003). In addition, spec-tral overlap between donor emission and accep-tor absorption is not very large in this case, andwould preclude the Forster mechanism alonefrom producing the high energy transfer effi-ciencies that were observed (Thompson et al.2004).

Dendrimers containing multichromophoricunits that can absorb light in a wide visible rangeand efficiently transfer it to the core would be idealfor light harvesting systems. Frechet andco-workers designed and synthesized poly (arylether) dendrimer containing coumarin-2 and flu-orol-7GA at the third and second branch point,

5a (L5)1P : R1 = L5, R2 = R3 = R4 = tolyl

5b (L5)2P : R1 = R2 = L5, R3 = R4 = tolyl or R1 = R3 = L5, R2 = R4 = tolyl

5c (L5)3P : R1 = R2 = R3 = L5, R4 = tolyl, 5d (L5)4P : R1 = R2 = R3 = R4 = L5

Figure 5. Structure of porphyrin dendrimers containing different numbers of dendron subunits.

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respectively as energy donors and a perylene-bis(dicarboximide) derivative at the core as theenergy acceptor (9, Figure 9) (Serin et al. 2002).The cascade energy transfer in this dendrimer wasdesigned in such a way that energy would beharvested by coumarin-2 units and transferred tofluorol- 7GA chromophores and then to perylenecore. The direct energy transfer from coumarin-2

to the perylene core was expected to be lessfavorable owing to the smaller spectral overlapbetween the emission spectra of coumarin-2 andthe absorption spectra of perylene core and thelonger interchromophore distance between thesetwo dyes. The absorption spectra of this dendrimercontained three peaks originating from all threechromophores (kmax=350 nm for coumarin-2,

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Figure 6. The star and conical-shape structures of porphyrin dendrimers containing Zn-porphyrin as the energy donor and free baseporphyrin as the energy acceptor.

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Figure 7. Polyphenylene dendrimer comprising a terrylenediimide core and four peryleneimide units at the rim.

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kmax=415 nm for fluorol -7GA and kmax

=555 nm for perylene core). However, the exci-tation of either coumarin-2 donor at 350 nm orfluorol-7GA donor at 415 nm resulted in theemission at 610 nm characteristics of perylenecore. Furthermore, the 6.9 and 3.6-fold increasesin core emission, compared to the control com-pound, were observed when Coumarin-2 andFluorol-7GA were excited. These results suggestthe presence of significant energy transfer effi-ciencies in these dendrimers. The authors showedthe evidence for a cascade energy transfer fromcoumarin-2 to fluorol-7GA and finally to perylene

core from the steady-state measurements. Theenergy transfer efficiency from coumarin-2 to flu-orol-7GA was 99% and from fluorol-7GA toperylene core was 96%. Therefore, this would be amore favorable pathway compared to a directtransfer from coumarin-2 to perylene core that wascalculated to be at the most 79%.

Another example of energy cascade wasshowed in the dendritic triad 10 where the outersphere of this macromolecule is formed bynaphthalenedicarboximonoimide chromophores,whereas perylene monoimide groups are located inthe dendritic scaffold and the terrylenediimide

Figure 8. Chemical structure of perylene-functionalized phenylacetylene dendrimer with an energy gradient.

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Figure 9. The structure of multichromophoric dendrimers containing coumarin-2 and fluorol -7GA as energy donors and perylene-bis(dicarboximide) as the energy acceptor.

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chromophores serve as a core molecule (Figure 10)(Weil et al. 2002, 2004). In this architecture, thereis a vectorial energy transduction from naphtha-lenecarboxmonoimide groups to perylenemonoi-mide units to the terrylene tetracarboxdiimidecore. Upon exciting of the high energy naphtha-lenedicarboxmonomide chromophore, an efficientvectorial energy transfer proceeds stepwise fromthe chromophores at the periphery over the func-tionalized scaffold towards the center.

While all these energy cascade schemes increasethe efficiency of energy transfer to the core, they doso at an energetic cost. The exciton loses energy atevery step down the cascade, so the energy avail-able when it reaches the core is less than what ithad when it started at the periphery. Thus whilethe efficiency of an excitation reaching the coremay be 100%, that excitation may only have 75%of the original photon energy. It is worth notingthat in nature, the light-harvesting complex con-

sists of isoenergetic chlorophylls, and that thecascade motif is not the dominant one (althoughthere is some energy gradient which directs theexcitation to the reaction center). Thus it is notimmediately clear that the cascade or energy fun-nel types of structures are necessarily the best forsolar light harvesting. High ET efficiency to thecore does not directly translate to high overallenergy efficiency of the structure.

Site isolation

It was found by Frechet and co-workers that thebulkiness of the dendritic framework causes asteric protection around the core thereby leadingto a site-isolation. The idea of using dendrons forthe site-isolation was initiated from an effort toprevent a self-quenching of lanthanide ions(Ln3+) owing to their aggregation in solid state

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Figure 10. Polyphenylene dendritic triad for vectorial energy transduction.

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(Kawa and Frechet 1998). To achieve this site-isolation, Ln3+ was encapsulated within dendriticshell of different sizes (11, Figure 11). Thephotophysical study on these molecules revealedthat upon the irradiation at 280–290 nm wheredendritic backbone absorbed, the luminescencefrom the core enhanced as the size of dendriticshell increased. This improvement of the lumi-nescence could be attributed to two complemen-tary reasons; the site isolation effect in whichdendrons help segregate each Ln3+ from oneanother thus reducing self-quenching process andan antenna effect where the energy transfer fromthe dendrons plays a role in increasing the lumi-nescence from the core.

Frechet applied the idea of using dendrons asthe site isolation scaffold to improve color tuningfor OLED applications (Freeman et al. 2000). Byjust blending the second generations ofpoly(benzyl ether) dendrimers possessing triaryl-amine (TAA) at the periphery and Coumarin-343(C343, kabs=446 nm, kem=525 nm ) or penta-thiophene (T5, kabs = 425 nm, kem = 470 nm) atthe core, achieving balanced emission from twodyes was only partially successful owing to theenergy transfer from C343 to T5 (Furuta et al.2003). This result implied that G2 dendrons arenot large enough to provide complete site isola-tion of the chromophore core. Later, larger den-drons containing flexible alkyl chain wereincorporated into these dendrimers. The attach-ment of flexible alkyl chain helps improve thesolubility of dendrimer as well as the processibilityof the film formation (12, Figure 12). As a result,

high generation dendrimers were prepared andstudied. By varying the ratio of these two den-drimers, it was found that while the fourth gen-eration was enough to give adequate site isolationfor C343 core, the fifth generation was requiredfor T5 dyes. This could be due to the morecompact nature of C343 compared to T5. Eventhough energy transfer could not be completelyprevented even with the largest dendrons, thestudy confirmed that the size of dendritic back-bone is critical to site isolation.

In order to make use of this site-isolationproperty, Aida and co-workers attached negativelycharged large dendrimeric shell onto poly(pheny-leneethynylene) conjugated backbone (13,Figure 13) (Jiang et al. 2004). The bulky dendriticskeleton was expected not merely to enhance thephotoluminescence of conjugated polymer back-bones owing to the site-isolation effect but also toincrease the solubility of the conjugated polymersin common organic solvents. The study found thatthe degree of excimer formation decreases with theincrease in the size of dendritic shell. They alsoinvestigated the photoinduced electron transferfrom conjugated backbone by attaching the large,negatively-charged dendron to methyl viologen(MV2+), a dicationic acceptor. Since MV2+ iselectrostatically attractive to the surface of den-drimers, upon the irradiation at the polymericbackbone, electron transfer is expected to takeplace, resulting in the quenching of the fluores-cence from the polymeric backbone. In fact, theirobservation corresponds to this supposition. Fur-thermore, because of the lack of electrostatic

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Figure 11. Self-assembled lanthanide-cored dendrimers for site-isolation studies.

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attraction, the addition of MV2+ to the polymerencapsulated with positively charged or nonionicdendrons did not cause any fluorescence quench-ing. Additionally, it was found that meta versionof this polymer (Figure 13), which possessesshorter conjugation length, showed much lesserelectron transfer efficiency to MV2+. This wasexplained in terms of the extended conjugationlength in para poly(phenyleneethynylene) polymerwhich facilitates hole migration along the polymerbackbone, thus reducing the possibility of chargerecombination and increasing electron transferefficiency. Therefore, it was concluded that severalcooperative factors are involved in achieving theefficient photoinduced electron transfer which arelong-range p-electronic conjugation of the poly-meric backbone, large dendrimeric envelopes, andthe suitable charge on dendritic surface. Moreover,the photochemical reduction of MV2+ caused byencapsulated poly(phenyleneethynylene) polymerwas also used to generate hydrogen from water inthe presence of colloidal PVA-Pt as a catalyst. Itwas found that, under specific conditions, hydro-gen was generated steadily for up to 5 h ofphotoirradiation and the absorbance of conju-gated backbone at 400–490 nm remained the sameduring the entire irradiation period implying thehigh photostability of this system.

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Figure 12. Dendrimers containing triarylamine at the periphery and coumarin-343 (left) or pentathiophene at the core (right).

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Figure 13. The structure of dendrimer encapsulated poly(phenyleneethynylene) polymers.

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Electron transfer

Electron transfer (ET) is a key reaction in mostnatural photosynthetic systems and it involves apair of electron-donor and acceptor entities, and itsefficiency scales exponentially with donor-acceptordistance. However, while a highly efficient FRETresults in fluorescence emitted mainly from theacceptor chromophore, a highly efficient ET usu-ally leads to a strong quenching of the fluorescenceof the emitting chromophore. Recently, Mullenand coworkers have reported perylenetetracarb-oxidimides (PDI) with peripheral triphenylamine(TPA) dendrimers (14, Figure 14) (Qu et al. 2004).Steady state and time-resolved data have revealedthat this dendrimer was capable of intramolecularelectron transfer from periphery to core and thisoccurred more efficiently in polar solvents. Therewere several important findings validating thiselectron transfer evidence. First, femtosecondtransient absorption spectroscopy results indicatedthe presence of a broad absorption band above700 nm that belongs to the radical anion of peryl-enetetracarboxidiimide. They noted that theintramolecular electron transfer becomes moresignificant with decreasing dendrimer generation.Further, they also studied dynamics of reversible

photoinduced electron transfer in perylenediimide–triphenylamine (PDI–TPA)-based dendrimer usingsingle molecule detection methods (Cotlet et al.2004a, b), and the effect of molecular oxygen onreversible photoinduced electron transfer. (Cotletet al. 2004a, b)

Guldi and co-workers have reported fullerenebased dendrimers to mimic the natural photosyn-thetic assemblies (Figure 15) (Guldi et al. 2002).These dendrimers function as rigid molecularscaffolds where dendritic spacers are end cappedwith dibutylaniline or dodecyloxynaphthalene asdonors, while the electron accepting fullerene isplaced at the focal point of the dendron, 15.Photophysical investigations showed that uponphotoexcitation there was an efficient and rapidtransfer of singlet excited state energy that controlsthe reactivity of the initially excited antenna por-tion. Spectroscopic and kinetic evidence suggeststhat photoinduced electron transfer from peripheryto core resulted in C60

.) -dendron.+ charge transferstate with quantum yields as high as 0.76 and life-times that are of the order of hundreds of nano-seconds (220–725 ns). They also found that thischarge transfer state can be modulated by varyingthe energy gap and that higher generations stabilizethis charge transfer state efficiently. Later, Guldi,

N

N OO

OO

OO

OO

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

Figure 14. Polyphenylene dendrimer with peripheral triarylamines and a central perylenetetracarboxidiimide chromophore.

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Hirsch and coworkers reported fullerene dendri-mers with Zn-Cyctochrome c where steady-stateand transient absorption spectroscopy indicates anefficient photoinduced electron transfer from theprotein to the fullerene (Braun et al. 2003).

The example of non-conjugated dendrimersthat are capable of photoinduced electron transferwas demonstrated by Aida and co-workers (Sad-amoto et al. 1996). Electron donor metalloporph-yrin having benzyl ether dendritic shell wassynthesized (16, Figure 16). In this work, methylviologen (MV2+) noncovalently-attached on theexterior surface of dendritic shell was used as anelectron acceptor. The titration of dendrimer withmethyl viologen showed no change on absorptionspectra of metalloporphyrin region implying thatdendritic shells protect metalloporphyrin core bysteric shielding and that methyl viologen has nointeraction with the metalloporphyrin core. How-ever, upon irradiation of this dendrimer in thepresence of MV2+, fluorescence from the core wasquenched and fluorescence lifetime was shortened.

This phenomenon implied the long range photo-induced electron transfer from metalloporphyrincore to methyl viologen through the dendrimerframework.

Frechet-type dendrimers containing viologen-like core and naphthalene peripheries (NBV2+) canalso enable electron transfer (17, Figure 17)(Ghaddar et al. 2002). Because of the lack of over-lapping absorption of the viologen core with emis-sion of either the naphthyl periphery or benzyl etherframework, the observed quenching of both theexcited naphthyl and the benzyl-ether groupsimplied an electron transfer from the excited donorunits (either the naphthyl or the benzyl-ether units)to the viologen acceptor. Transient absorptionmeasurements revealed that there are several for-ward and backward electron-transfer steps in thesedendrimers. The mechanism for this photoinducedelectron transfer was proposed by the authors: thefirst process was expected to be an oxidativequenching of an excited state of viologen core (V) byeither naphthyl group (N) or benzyl ether dendron

NCH3

N

N

N

N

NCH3

OC12H25

C12H25O

OC12H25

C12H25O

n n

1a: n=02a: n=1

1b n= 02b n= 1

Figure 15. First (1a, 1b) and second (2a, 2b) generations of new C60-dendron dyads.

145

(B) to form NB+V+ or N+BV+, respectively. Thesubsequent process could be either the back electrontransfer or secondary oxidative quenching. Thelatter case was finally followed by back electrontransfer. These multiple reducing systems areimportant for the improvement of effective signalamplification in optical data storage.

Bifunctional dendrimers

In order to mimic the complete photosyntheticevent, recently, we have designed dendrimers thatare capable of undergoing both energy transferand electron transfer properties. These dendrimers18 contained benzthiadiazole derivatives as theenergy and electron acceptor at the core and di-arylaminopyrene units as the energy and electrondonors at the periphery (Thomas et al. 2005). Theemission of diarylaminopyrene units overlappedwith the absorption of benzthiadiazole moietyimplying that Forster energy transfer can happenin these dendrimers. Moreover, the oxidationpotential of benzthiadiazole units obtained from

cyclic voltammogram was 595 mV which is abovethat of diarylaminopyrene units which exhibited atabout 444 mV with respect to ferrocene/ferroce-nium couple as the internal standard. This elec-trochemical data suggested that it is possible forthe excited state of the chromophore at the core tobe reduced by peripheral chromophores.

The excitation of peripheral chromophores at395 nm resulted in the fast rise of the fluorescencefrom the acceptor at 605 nm implying rapid For-ster energy transfer in these molecules. Also, theenergy transfer efficiency in these molecules washigh even in high generations (gEET � 0.89–0.97).This efficiency was found to be solvent-indepen-dent, which is common for energy transfer pro-cesses. However, we found that the fluorescence ofthe core altered with the change in the dielectricconstant of solvents. The different degree of thefluorescence quenching from the core uponchanging the solvent polarity implied the presenceof a charge transfer event in these dendrimers. Infact, this fluorescence quenching was found to befaster in high polar solvents. This would be due tothe fact that more polar solvent can better stabilize

N

N N

N

X

X

X

X X =

O

O

O

O

O

O

O

O

O

O

O

OO

O

KO2C

CO2K

CO2K

CO2K

CO2K

CO2K

CO2K

CO2K

Zn

Figure 16. Benzyl ether dendrimers having metalloporphyrin core.

146

the charged-intermediate species and therebyincrease the charge transfer rate. In addition, wefound that the long-lived (microseconds) transientabsorption spectrum closely resembled that of theradical cation spectra obtained from both chemicaland electrochemical oxidation of the peripheraldiarylaminopyrene units. This provided additionalevidence to confirm the presence of charge sepa-rated state in these dendrimers. The charge trans-fer efficiency in these dendrimers was calculated tobe as high as 70% in the high polar solvent DMF,and the overall efficiency of the photon tocharge-separated state process was calculated to beapproximately 50% (Figure 18).

Summary

Dendrimers with several light-harvesting antennassurrounding a single core are an ideal buildingblock for artificial photosynthetic systems.Recently, many designs have been explored inorder to enhance the energy transfer efficiency.For example, dendritic frameworks were usedeither as a scaffold that does not intervene the

energy transfer process or as a chromophorewhere energy level of the system can be tuned byadjusting the conjugation length of the backbone.Moreover, the effect of dendritic shells towardenergy transfer has also been extensively studied.It was found that the bulkiness of dendriticskeletons help segregate the chromophore at thecore, thus reducing the self–aggregation andenhancing the luminescence from the core chro-mophore. Also, multiple chromophores at theperiphery allow the energy migration aroundthese chromophoric arrays before the energytransfer process. Finally, a system that enablessequential energy transfer process followed bycharge separation, which more completely mimicsthe natural photosynthetic systems was also pre-sented in this article. While the entire dendrimerbackbone has been used for energy transfer pro-cesses, such an approach has not been yet takenin the charge transfer processes. We havedesigned dendrons that have such potential(Bronk and Thayumanavan 2001; 2003). Utilizingthese dendrimers for photoinduced charge trans-fer process is a focus of current study in ourlaboratories.

N N+ +

O

O

O

O

O

O

O

O

N N+ +

O

O

O

O

O

O

O

O

NB1+2

NB2+2

O

O

O

O

O

O

O

O

O

OO

O

O

O

N N

O

O

O

O

O

O

O

O

O

O

O

O

O

O+ +

NB3+2

Figure 17. Dendrimers containing viologen-like moiety at the core and naphthalene units at the periphery.

147

Acknowledgement

We thank the Department of Energy, Basic EnergySciences (DE-FG02-05 ER15747) for financialsupport.

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