DOI: 10.1002/asia.201200780

Beyond Perylene Diimides—Diazaperopyrenium Dications as ChameleonicNanoscale Building Blocks

Ashish N. Basuray, Henri-Pierre Jacquot de Rouville, Karel J. Hartlieb,Albert C. Fahrenbach, and J. Fraser Stoddart*[a]

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FOCUS REVIEW

Introduction

One of the most sought after goals in chemistry is the abilityto manipulate complex systems with relative ease. The pros-pect of scaling precise architectures from molecules to self-assembled[1] extended systems beyond the molecule withprecision is a challenging concept. Moreover, these extendedframeworks should function to serve global demands ina cost-effective manner. Organic electronics are oftenviewed as a holy grail when in search of cost-effective re-placements for present technologies related to photovolta-ics,[2] capacitors,[3] transistors,[4] and light-emitting electro-chemical systems[5] within the next century. Although allo-tropes of carbon, such as graphene, have shown[6] considera-ble promise in relation to these applications, manipulatinglarge sheets of carbon effectively is still out of our reach.More tangible is our ability to functionalize and developtechnologies using graphene-mimetic materials, such aspyrene and perylene. As chemists develop their level of ex-pertise with these graphene analogues, tangential pathwaysarise allowing for new synthetic methodologies to facilitatethe development of organic electronics. This Focus Reviewaddresses the synthesis and applications of nonconventionalperylene derivatives, namely, reduced perylene diimides, andthe corresponding aromatized diazaperopyrenium deriva-tives.

1. Background to Perylene Derivatives

Rigid polyaromatic hydrocarbons have been investigated forwell over a century.[7] During the past several decades, how-ever, the utilization of functional perylenes has evolved withincreasing complexity and applicability. Perylene-3,4,9,10-tetracarboxylic acid diimide (PDI) derivatives have been

[a] A. N. Basuray, Dr. H.-P. J. de Rouville, Dr. K. J. Hartlieb,A. C. Fahrenbach, Prof. J. F. StoddartDepartment of ChemistryNorthwestern University2145 Sheridan Road, Evanston, IL 60208 (USA)Fax: (+1) 847-491-1009E-mail : stoddart@northwestern.eduHomepage: http://stoddart.northwestern.edu

Ashish N. Basuray holds a BSc inChemistry from Carnegie-Mellon Univer-sity, graduating in 2002. He worked at theNational Institutes of Health, NationalCancer Institute, until he joined theUnited States Peace Corps in early 2003.He served in the US Peace Corps Nepal,working to develop a scientific curriculumand construct scientific laboratories forunderprivileged districts. Upon his returnto the US, he worked as a ManagementConsultant until his return to academia in2007 to study for his PhD degree, underthe tutelage of Professor Fraser Stoddart.Ashish�s research focuses on host–guestchemistry, mechanically interlocked mole-cules, and applications resulting from theresearch.

Henri-Pierre Jacquot de Rouville gradu-ated from the University Paul Sabatier ofToulouse. In 2007, he joined ChristianJoachim�s Group and obtained a PhDdegree for his synthesis of technomimeticmolecules for nanomechanical applica-tions under the supervision of Prof. Gw�-na�l Rapenne in the Nanosciences Group,CEMES-CNRS, Toulouse. In 2010, hejoined Professor Fraser Stoddart�s Groupas a postdoctoral fellow where he investi-gated the chemistry of mechanically inter-locked molecules.

Karel J. Hartlieb is a graduate from theUniversity of Western Australia (UWA)where he was awarded Bachelor of Sci-ence and Bachelor of Engineering de-grees, both with first class honors, andsubsequently a PhD in Chemistry. Duringhis time as a graduate student, under thesupervision of Professor Colin Rastonand Professor Martin Saunders, Karelwas awarded a Hackett Scholarship fromUWA and a Fulbright Scholarship, whichallowed him to carry out part of his grad-uate research at Clarkson Universityunder the supervision of Professor

Roshan Jachuck. Karel is currently a postdoctoral scholar with ProfessorFraser Stoddart and is working on the chemistry of mechanically inter-locked molecules, utilizing different recognition motifs in their template-di-rected syntheses.

Albert C. Fahrenbach was born andraised in the United States in rural Indi-ana. He entered Indiana University ofBloomington in 2003 and received anHonors BSc Degree there in 2008 inChemistry, while conducting chemicalsynthesis under the tutelage of ProfessorAmar Flood. In the same year, Albertmoved to Northwestern University tostudy for his Ph.D. Degree with ProfessorFraser Stoddart. The joy associated withthe design, synthesis, and investigation ofmolecular switches and machines is whatkeeps Albert coming into laboratory earlyevery morning. Enjoying the company ofhis fellow students is what keeps him inthe laboratory late into the night.

Fraser Stoddart received all (B.Sc.,Ph.D., D.Sc.) of his degrees from the Uni-versity of Edinburgh, United Kingdom.Presently, he holds a Board of TrusteesProfessorship in the Department ofChemistry at Northwestern University,where he is also the Director of theCentre for the Chemistry of Integral Sys-tems. His research has opened up a newmaterials world of mechanically inter-locked molecules and, in doing so, hasproduced a blueprint for the subsequentgrowth of functional molecular nanotech-nology.

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used historically as industrial pigments (Figure 1) with per-haps the most well-known application being car paint. Morerecent levels of sophistication include multifunctionalizedperylene mono- and diimides sculpted into organic photo-voltaics,[8] nanofibers,[9] optical sensors,[10] thin-film organicfield effect transistors[11] (OFETs), and molecular electron-ics.

Developments in thin-film OFETs promise transparent,flexible electronics—an area of research that has attractedattention from both academia and industry. Although a vari-ety of polymeric and dendritic conductive molecules havedemonstrated high electron mobilities, PDIs have beenshown to produce comparable mobilities at a fraction of thecost. Marder and co-workers[12] provide examples of synthet-ically facile PDIs that have electron carrier coefficients of1.7 cm2 V�1 s�1 under vacuum. Recently, Bao and co-work-ers[13] reported oxygen-stable, room temperature PDI deriv-atives, which have mobilities of m= 0.5 cm2 V�1 s�1, that is,more than two orders of magnitude better than those previ-ously reported[14] for room temperature PDI OFETs.

2. Synthesis

2.1. Perylene Diimides—The Foundation of PeryleneFunctionalization

Functionalization of PDIs has been all but perfected overthe past 20 years, to the point where thousands of PDI de-rivatives have been described[15] in the literature, each withtheir own unique electronic, optical, and bulk characteristics.The popularity of PDIs has persisted partly on account oftheir high yielding, straightforward synthesis and purifica-tion. The common precursor, perylene-3,4,9,10-tetracarbox-ylic dianhydride (PTCDA) undergoes efficient condensa-tions with primary amines or aniline derivatives to yield(Scheme 1) the corresponding mono- or diimide derivative,generally in high yield, following simple purification.

When designing nanoscale architectures for molecularelectronic applications, PDI derivatives are one of the mostuseful building blocks because of their functional versatility,yet they are plagued with one main drawback—namely, sol-

ubility that is often related to aggregation.[16] Extended p–p

systems provide an attractive surface for intermolecular vander Waals interactions, leading to aggregation limiting theirsolubility to a narrow range of intercalating solvents. Thediimide derivatives share these solubility issues, unless theyare functionalized with suitable solubilizing groups at theirN-terminal positions, such as aromatic or alkyl appendages,thereby preventing p–p stacking from occurring in solu-tion.[17] Thus, imide functionalization is usually the first stepfrom PTDCA to increase solubility and generate functionalhandles. In an alternative approach, recently reported[18] byScherman and co-workers, deaggregation of water-solublePDI derivatives was facilitated by host–guest chemistry afteraddition of cucurbit[8]uril (CB8) in water, simultaneouslyenhancing the fluorescence quantum yield (f) from approxi-mately 0.03 to 0.90�0.10.

Marder and co-workers[12] have reviewed PDI functionali-zation and the different classes of PDI derivatives exhaus-tively. Scheme 2 illustrates a small fraction of PDI deriva-tives reported to date, where the N-terminal functionaliza-tion includes n-alkyl chains, “sparrow-tail” alkyl chains,polyethylene glycols, and almost any conceivable combina-tion of aryl, benzyl, or heteroaryl functionalization. Halidefunctionalizations of the “bay” region of PDIs are noted fortheir effect on the HOMO/LUMO levels, leading to elec-tronic and optical tunability,[19] while also serving as synthet-ic handles for further modification.

In recent times, PDI derivatives have been employed ascomponents in the construction of mechanically interlockedmolecules (MIMs). A notable example of a PDI-basedMIM, recently reported by Champness and co-workers,[20] isthat of a [2]rotaxane (Figure 2 a) formed by the slippage ofa polyether macrocycle onto a PDI-based dumbbell compo-nent. A single crystal sufficient for X-ray analysis (Fig-ure 2 b) was grown by layering methanol onto a solution ofrotaxane in toluene.

Although the incremental synthetic sophistication of PDIshas increased our knowledge of their photophysical, opticaland electrochemical properties, the reduced diazatetrahy-dro- and diazaperopyrenium derivatives have only been re-searched in a cursory manner by comparison, leaving a wide

Figure 1. Pigment Red 179—a simple, yet effective PDI derivative, whichhas been used as an industrial pigment. Sun Chemical produces this pery-lene derivative under the trade name of Perrindo Maroon 179.

Scheme 1. The facile syntheses of perylene diimides have been widelyused in the form of a quantitative condensation, wherein a primaryamine or aniline can produce the mono- or difunctionalized perylene de-rivative.

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swath of chemistry still to be explored. In the following sec-tions, we will discuss the evolution into these compounds,and the potential that these reduced perylene derivativeshold for future developments.

2.2. Synthetic Routes to Perylene Derivatives

For photophysical chemists, the bay region of the perylenecore is the most interesting one for functionalization as theHOMO/LUMO symmetries are positioned along the lengthof the molecule. Bay functionalization therefore affects theoptical and photophysical properties significantly. However,because the imide nitrogens are located in-line with theHOMO/LUMO nodal plane, functionalization of the imide

nitrogen results in limitedchanges in the photophysicalproperties: for a more in-depthdiscussion, refer to Marderand co-workers.[12] Conversely,for supramolecular complexesand MIMs, in which the pery-lene core serves as a recogni-tion unit, the peripheries arethe coveted functional posi-tions. While the imide nitrogenis an excellent functionalhandle, imides lock the confor-mational freedom of substitu-ents on tertiary amines. Asthey also continue to pose sol-ubility constraints, moving toa reduced form allows forgreater processability, whilemaintaining similar optical andphotophysical properties.

Stang and co-workers[21]

have reported the reduction ofthe N,N’-dimethyl-PDI thatemploys lithium aluminumhydride, using Soxhlet extrac-tion and recrystallization to

arrive at the reduced 1,3,8,10-tetrahydro-2,9-dimethyl-2,9-di-azadibenzo ACHTUNGTRENNUNG[cd, lm]perylene in 54 % yield. This reduction wasfollowed by a high temperature palladium-assisted demethy-lation and aromatization to furnish the 2,9-diazaperopyrene,which has been utilized as a ditopic ligand in the self-assem-bly of a molecular square, thus making use of palladiummetal centers as corners.

Recently, we reported[22] the oxidation of 1,3,8,10-tetrahy-dro-2,9-dimethyl-2,9-diazadibenzoACHTUNGTRENNUNG[cd, lm]perylene by 2,3-di-chloro-5,6-dicyano-1,4-benzoquinone (DDQ), leading to thedicationic N,N’-dimethyl-2,9-diazaperopyrenium bis(hexa-fluorophosphate) (MP·2PF6) in 74 % yield (Scheme 3). Analternative synthetic route to neutral 1,3,8,10-tetrahydro-2,9-diaza-dibenzo ACHTUNGTRENNUNG[cd, lm]perylenes (Scheme 4), wherein the

Scheme 2. The PDI core has been extensively manipulated into libraries of compounds, each library with a subset of varying functional handles to aidand abet in solubility, synthesis and physical and/or optical properties. Halide functionalization of the bay region and terminal imide substitution providessynthetic handles for further modification.

Figure 2. Champness and co-workers[20] utilized a thermodynamic approach recently to form the [2]rotaxaneemploying slippage of the macrocyclic polyether DN38C10 onto the perylene dumbbell. The crystal structureillustrates the macrocycle�s aromatic p-systems overlapping with the perylene core.

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PDTCA ring is hydrolyzed with a strong base, esterified,and reduced to the tetraol, has been explored by Takahashiand co-workers.[23] This method of reducing the carbonylgroup of the dianhydride is more elegant than previously re-ported routes, as reducing the diimide directly tends to below-yielding and the products are difficult to purify. Afteresterification, the reduced perylene can be purified and thenconverted nearly quantitatively to the tetrachloro-perylenederivative. When this compound is treated with an excess ofan amine or aniline derivative, the generation of 1,3,8,10-tet-rahydro-2,9-diazadibenzo ACHTUNGTRENNUNG[cd,lm]perylenes proceeds effi-ciently in high yields. A modified ring-opening approachwas recently employed[24] by Scherf and co-workers, whereinPBr3 was used to generate a tertrabromide directly after the

diisobutylaluminium hydride (DIBAL-H) reduction. Thesemethods allow for a variety of amines and anilines to be at-tached at the peripheries, thus promoting higher solubilitiesby greater conformational freedom of the tertiary aminecompared with its PDI counterparts. Similarly, the aromat-ized diazaperopyrenium dication, relative to the neutral ter-tiary amine, exhibits greater solubility, mainly due to thecharges, allowing the compound to become soluble ina larger range of solvents, including water, depending on thenature of the counterion. This enhanced solubility is criticalto achieve a good understanding of the physical and supra-molecular interactions these molecules possess.

3. Physical Chemistry

The physical properties of the MP2+ dication—namely, thephotophysical and electrochemical characteristics—are simi-lar to those of the perylene diimides, and, as such, are im-portant physical parameters to consider when anticipatingthe design of solid-state devices, such as organic batteries,photovoltaic cells, and light-emitting diodes. The propertiesof MP2+ beyond the molecule, that is, the intermolecularnoncovalent bonding interactions, which lead to its self-as-sembly, forming both the hetero- and homodimers as well ashost–guest complexes. In addition, the formation of MIMs iscrucial to understand these recognition processes in the fab-rication of solid-state devices. In this section, we describethe photophysical and electrochemical properties of theMP2+ dication, in addition to an analysis of its supramolec-ular behavior, and solid state (i.e., X-ray crystal superstruc-tures and macroscopic) morphologies.

The photophysical properties of the MP2+ dication, spe-cifically its absorption and emission characteristics in aque-ous solution, were investigated back in 1989 by Lehn andco-workers.[25] Intense absorbance bands are observed atlmax =293, 390, 412, 430, 465, and 501 nm. The emissionspectrum displays three bands at 508, 546, and 590 nm, withintensities that roughly mirror those of the correspondingabsorption bands. The fluorescence quantum yield of theMP2+ dication was found to be 0.53�0.03. The decay of theexcited state was observed to be monoexponential, charac-terized by a time constant (t) of 27.35 ns in an aqueous solu-tion degassed under N2.

Shortly thereafter, in 1991, Harriman and Brun[26] fol-lowed Lehn�s investigation with a study of the electrochemi-cal properties and the transient absorption behavior of theexcited states that occur after irradiation with 355 nm light.The quantum yield and fluorescence lifetimes of the MP2+

dication were investigated in pH neutral aqueous solutions,and were found to be 0.79 and 24.4 ns, respectively, valueswhich correlate well with those reported[25] by Lehn and co-workers under similar conditions. Cyclic voltammetry re-vealed that the MP2+ dication undergoes two sequentialone-electron reductions observed at peak potentials of�0.34 V and �0.47 V versus normal hydrogen electrode(NHE). The authors[25] noted, however, that in aqueous

Scheme 3. The novel synthesis of the MP2+ dication was achieved usingDDQ as an oxidant to convert the tetrahydrodiamine to the diazapero-pyrenium dication in respectable yields, after counterion exchange(74 %).

Scheme 4. Takahashi�s ring-opening synthetic strategy[23] of the perylenedianhydride to the tetrachloro derivative 5 constitutes a unique approachto functionalized tetrahydro-diazadibenzo ACHTUNGTRENNUNG[cd,lm]perylenes. Based on thissynthetic protocol, compound 6 has been obtained in high yields withsimple purification, dependent on the primary amine/aniline used.

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media, the reduction processes are not reversible, mostlikely as a consequence of precipitation onto the surface ofthe electrode. The absorption spectrum of the MPC+ radicalcation has been recorded[27] by using a pulse radiolysis tech-nique. The absorption bands around 420 nm have been ob-served to decrease as an intense absorption band emergesaround 650 nm.

X-Ray Crystallography

Recently, we have obtained[22] the solid-state superstructureof the MP2+ dication by single crystal X-ray crystallographyof its PF6

� salt (Figure 3). Single crystals of the MP·2 PF6

were grown by slow vapor diffusion of iPr2O into a MeCNsolution of the salt at 23 8C. The X-ray analysis reveals analmost nearly completely planar molecular structure, an in-dication of the rigidity of the dication. Further insight re-garding the rigidity of the MP2+ dication�s molecular struc-ture can be gained by drawing comparisons with the struc-ture and electrochemical properties of a related dication,[28]

N,N’-dimethyl-bis(p-pyridinium)phenylene (P-BIPY2+). Theability of P-BIPY2+ to adopt torsional angles, which mini-mize proton–proton steric interactions, leads to a decreasein the overlap of the p-clouds associated with each aromaticring. This decreased overlap results in a significant loss of“communication” between the two pyridinium redox cen-ters, as evidenced by the cyclic voltammogram, which dis-

plays two overlapping one-electron reduction processes oc-curring at nearly the same potential of approximately�0.9 V (Ag/AgCl). In comparison, the p-clouds of the MP2+

dication are all forced into positions that maximize theiroverlap by the rigidity of the underlying covalent structure,thus increasing communication between the pyridiniumredox centers as becomes evident from two separated one-electron reduction processes occurring at less negative po-tentials in comparison to the less rigid P-BIPY2+ . The su-perstructure of the MP2+ dication reveals the continuousone-dimensional p-stack of the MP2+ dication. Analysis ofthe observed superstructural details will be discussed inmore depth in the following section related to the dication�srecognition properties.

4. Supramolecular Chemistry

In a recent communication,[22] we rationalized the uniquemolecular recognition processes that are known to occurwith the MP2+ dication as being both heterophilic and ho-mophilic. Heterophilic molecular recognition was observedwith a variety of p-electron-rich donors in addition to theformation of a homodimer, an inherently less obvious elec-tron-deficient molecular pairing, at least in aqueous environ-ments. The homophilic recognition process leads to the for-mation of homodimers, whereas the heterophilic process re-sults in heterodimers. Lehn and co-workers[25] first observedthe formation of the homodimer of the MP2+ dication inaqueous solutions expressed by the following equilibrium, asshown in Equation (1):

2 MP2þ Ð ðMP2þÞ2 ð1Þ

The dimerization association constant was found to be ashigh as 2 �105

m�1 (0.25m NaCl, pH 7.5) despite the Coulom-

bic repulsion that must result upon bringing the MP2+ dicat-ions within p–p stacking distances. The association constantwas found to vary with the ionic strength of the solution,suggesting the mediating role of the Coulombic repulsionenergy in the homophilic dimerization process. We hypothe-sized that the large value of the dimerization constant inaqueous media is largely the result of hydrophobic interac-tions, which act to minimize the amount of nonpolar pery-lene core exposed to the hydrophilic environment. Insightsinto the superstructure of this dimer in solution can begleaned from the solid-state superstructure (Figure 3). Wefound that the MP2+ dications stack with each other in thesolid state in such a way that maximizes p-overlap at theperylene core, whilst minimizing Coulombic repulsions aris-ing from the charged pyridinium units at the peripheries ofthe molecular structure (Figure 3 b).

Lehn and co-workers[25a] also reported on the heterophilicmolecular recognition properties of the MP2+ dication withvarious electron-rich dianions. In particular, their investiga-tions uncovered the nature of heterodimerization of theMP2+ dication with the p-electron-rich 2,6-dicarboxynaph-

Figure 3. a) A space-filling representation of the supramolecular homo-philic recognition of MP2+ dication. The distances shown represent cent-roid-to-centroid spacing between the perylene cores. b) The moleculesare rotated approximately 608 to maximize the distance between the for-mally charged nitrogen atoms on the peripheries.

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thalene dianion. Changes in the emission intensity resultingfrom the heterodimer formation, and subsequent red shiftsin the observed bands at high concentrations suggest a 2:1complex. Although the superstructure of the heterodimer ishypothesized to have a face-to-face geometry, there is nosolid-state superstructure to confirm this hypothesis as yet.

The MP2+ dication can express both the heterophilic andhomophilic recognition properties under identical condi-tions, as a consequence of its large, planar aromatic surface.This fact led us to suggest the analogy to the chameleon as,both possess a special ability to adapt effortlessly and func-tion efficiently with their surrounding environment. No ex-ternal stimuli are needed to toggle between hetero- and ho-mophilic recognition, which makes this molecule relativelyunique as a potential building block for self-assembling de-vices.

Host-Guest Chemistry

Although the hetero- and homodimers of the MP2+ dicationmost likely result from a tendency to minimize hydrophobicinteractions, in organic solvents, alternative noncovalentbonding interactions contribute to the observed behavior.The formation of the homodimers is suppressed and elec-tron-rich macrocyclic polyethers can serve as hosts for theMP2+ dications. Harriman and co-workers[29] investigatedthe formation of the [2]- and [3]pseudorotaxanes involvingencirclement of bisparaphenylene[34]crown-10 (BPP34C10)around the MP2+ dication. The binding constants governingthe strength of the [2]- and [3]pseudorotaxanes were foundto be K1 = 510 m

�1 and K2 =430 m�1 in MeCN.

The co-conformation of the [2]pseudorotaxane most likelyplaces the BPP34C10 rings towards the peripheries, therebymaximizing overlap with the most electron-deficient regionsof the planar aromatic surface, rather than with the perylenecore. The superstructure of the [3]pseudorotaxane mostlikely involves another BPP34C10 ring encircled around theremaining periphery of the MP2+ dication. This hypothesisis consistent with the experimental observation[29] of a broadband in the visible region of the spectrum, which can be at-tributed to a charge transfer (CT) band, after addition ofBPP34C10 to a solution of the MP2+ dication in MeCN. Inconjunction with this CT band is a partial quenching of thefluorescence emissions of the MP2+ dication.

Harriman and co-workers have also investigated theeffect on the transient absorption spectrum by irradiatingwith 554 nm light a solution of mostly [3]pseudorotaxane.After irradiation with a sub-picosecond laser pulse, a sharpabsorption band centered around 650 nm is observed togrow in with a time constant of t= 3 ps, characteristic of theMPC+ radical cation. Shortly thereafter, the absorption spec-trum returns to that of the MP2+ dication with a time con-stant of t= 25 ps. The observation of the MPC+ radicalcation is the result of electron-transfer from the phenyleneunits of the BPP34C10 ring to the MP2+ dication, followedby charge recombination. On the timescale of these experi-ments (ca. 100 ps), there is no evidence for dissociation of

the radical-ion pair, that is, the BPP34C10C+ from the MPC+

radical cation guest. Recently, we have shown[22] that the p-electron-rich 1,5-dinaphtho[38]crown-10 (DN38C10) ringserves as a suitable host for the MP2+ dication in MeCN.Compared with the BPP34C10 ring, the greater aromaticbulk of the naphthalene units in DN38C10 sterically hinderthe stability of the [3]pseudorotaxane. At concentrations ofapproximately 1 mm, only the [2]pseudorotaxane is ob-served, while isothermal titration calorimetry (ITC) meas-urements reveal a strong association constant (Ka) of 1.05 �104

m�1. The formation of the [2]pseudorotaxane is enthalpi-

cally driven by DH =�8.60 kcal mol�1 and mediated by anentropic cost of DS=�10.5 cal mol�1 K�1. The 1:1 stoichiom-etry of the binding event was further confirmed by a Jobplot calculated from chemical shift data obtained froma 1H NMR spectroscopic titration. The favorable bindingconstant of MP2+ dication is most likely due to co-confor-mation brought about by the rigid, planar, dicationic natureof the guest molecule when involved in heterophilic recogni-tion.

We have also reported[22] the synthesis of a family of ro-taxanes, using a threading-followed-by-stoppering approach,incorporating a MP2+ dicationic subunit as a large and rigidrecognition unit in the dumbbell component. Reaction ofthe perylene tetrachloro derivative 5 with 2-azidoethana-mine and aromatization with DDQ allowed us to install(Scheme 5) convenient handles for a copper-assisted azide–alkyne cycloaddition[30–32] (CuAAC) stoppering reaction,using [Cu ACHTUNGTRENNUNG(MeCN)4]PF6 and tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA) as the catalytic system. BothDN38C10 and BPP34C10 were used as templates in the syn-thesis of the [2]rotaxanes in 22 and 14 % yields, respectively.We were also able to isolate the [3]rotaxane in 4 % yieldfrom the reaction mixture containing BPP34C10 as a tem-plate, thus confirming the formation of a 1:2 complex, MP2+

�ACHTUNGTRENNUNG(BPP34C10)2 in solution. This rare example of a singlerecognition site, accommodating two macrocycles, is akin toother rigid scaffolds for p–p supramolecular recognition.[33]

5. Applications

The MP2+ dication has been investigated[26a,34] for its poten-tial use as a ubiquitous DNA intercalator, as well as a selec-tive DNA cleavage agent.[25] Heterodimeric and trimericcomplexes of the MP2+ dication have been observed withadenosine, thymidine, adenosine triphosphate (ATP), andmore. In the case of ATP, binding constants of K1 =1.3 �104

m�1 and K2 =250 m

�1 were measured. In the case of thy-midine, only a single complexation event was observed, thatis, a 1:1 heterodimer characterized by a binding constant ofK=15 m

�1. Further investigations revealed that the MP2+ di-cation can bind single-stranded polynucleotides, and after ir-radiation with visible light, cleavage products were ob-served, mostly at guanine sites. This finding provides evi-dence for the MP2+ dication being utilized as a sequence-specific artificial nuclease.

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Conclusions and Perspectives

PDI derivatives have proven to be useful in numerous appli-cation-oriented fields on account of their ease of functionali-zation and use in many devices with greater demands beingimposed on organic materials that are cheap, easily tunea-ble, and processable. The myriad of research being under-taken with PDI derivatives has contributed a great deal ofknowledge regarding the unique supramolecular interactionsof these molecules, which serves as an inspiration for thedesign of new architectures. Although largely overlookedfor over two decades, possibly because of the synthetic chal-lenges, diazaperopyrenium-based compounds possess manyof the inherent properties of PDI derivatives, as well asa rare chameleonic host–guest nature, allowing them to dis-play both homo- and heterophilic affinities. With theirunique supramolecular interactions, diazaperopyrenium-

based materials have a promising future in device applica-tions, including semiconductors, sensors, biochemical pro-cesses, as well as a host of other applications that will relyupon this chameleon of chemistry.

Acknowledgements

This work was supported by the Non-Equilibrium Energy ResearchCenter (NERC) at Northwestern University, funded by the US Depart-ment of Energy (DOE), Office of Basic Energy Sciences under awardnumber DE-SC0000989. The authors wish to thank and acknowledge thesupport from the Air Force Office of Scientific Research (AFOSR)under the Multidisciplinary Research Program of the University Re-search Initiative (MURI), award number FA9550-07-1-0534 entitled“Bioinspired Supramolecular Enzymatic Systems.” A.C.F. acknowledgesthe award of a Graduate Research Fellowship from the National ScienceFoundation.

Scheme 5. Utilizing the tetrachloro perylene derivative 5, both the [2]- and [3]rotaxanes were produced by aromatizing the diazido-tetrahydrodiazadibenzo ACHTUNGTRENNUNG[cd,lm]perylene, followed by threading of two BPP34C10 macrocycles and then stoppered using CuAAC.

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Received: August 21, 2012Published online: November 23, 2012

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www.chemasianj.org J. Fraser Stoddart et al.