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View Article OnlineView Journal | View Issue

aHunter College of the City University of Ne

York, 10065 USA. E-mail: cdrain@hunter.cubLoyola Marymount University, LMU Driv

E-mail: alessandro.varotto@gmail.comcThe Rockefeller University, 1230 York Aven

† Electronic supplementary informationsynthesis and characterization of the dyedata. See DOI: 10.1039/c3ra43795g

Cite this: RSC Adv., 2013, 3, 21360

Received 19th July 2013Accepted 9th September 2013

DOI: 10.1039/c3ra43795g

www.rsc.org/advances

21360 | RSC Adv., 2013, 3, 21360–21

Self-organized nanostructured materials of alkylatedphthalocyanines and underivatized C60 on ITO†

Matthew Jurow,a Alessandro Varotto,*b Viacheslav Manichev,a Nikolina A. Travlou,a

Dimitrios A. Giannakoudakisa and Charles Michael Drainac

Clicking thiols onto a core fluorous phthalocyanine (Pc) platform

yields alkyl and fluoroalkyl derivatives. The Zn(II) Pc with 16 thio-

alkanes self-organizes with fullerene C60 into nearly monodispersed

nanoparticles when cast on ITO surfaces. Particle size and surface

density is controlled by varying the Pc/fullerene ratio and concen-

tration. Fluorescence quenching indicates electronic interaction

between the component molecules in the films of nanoparticles. The

strong supramolecular interactions between the Pc with 8 or 16

fluorous alkanes inhibit incorporation of the fullerene.

Organic materials at the nanoscale oen display new,enhanced, and sometimes unpredictable properties; therefore,are studied extensively and used in the preparation of diversephotonic devices such as solar cells and sensors.1–4 Porphyrinsand phthalocyanines (Pc) coupled with fullerenes and carbonnanostructured materials are of interest because theseconstructs can form long-lived charge separated states.5–10 Theability to store and separate charge make these systems idealcandidates for use in photonic devices and solar energy har-vesting.11–14 Pc and fullerene nanostructures are generallyfabricated through time consuming, costly, and difficultsynthetic organic techniques used to form covalent bondsbetween active components.1 For example, Bottari et al.demonstrated that when tri-tert-butyl-Pc is covalently bound toa fullerene derivative and cast onto a surface, conductive bersare formed.15 Fullerenes derivatized with ligands such asamines, thiols and alcohols can coordinate to metallo-Pc withbinding constants on the order of 104 in solution.2,3,10,16

However, once cast on surfaces, the coordinated molecules

w York, 695 Park Avenue, New York, New

ny.edu; Tel: +1 212-650-3791

e, Los Angeles, California, 90045 USA.

ue, New York, New York, 10065 USA

(ESI) available: Control experiments,s, uorescence micrographs, and AFM

364

oen re-organize and/or phase separate depending on thestrength of the relative interactions between the chromophoresand the surface.17 In these systems, modication of fullerenes,including nanotubes, to promote association or covalentbonding diminishes many of their intrinsic advantages aselectron acceptors.18–21

Supramolecular chemistry is oen preferred to the low yieldsynthetic procedures needed to form covalent bonds betweentwo or more chromophores such as porphyrins and Pc.22–28 Thep–p interactions between the curved surface of fullerenes andthe at core of porphyrins drive formation of crystals containingboth,5 but are insufficient to form homogenous blends in lmsor as nanoparticles, e.g. CuPc with C60 results in phase sepa-ration into nanodomains.6 Complex multifunctional systemshave been made to increase interactions between the porphyrindonor and fullerene acceptor.29 Supramolecular materialscomposed of Pc and underivatized fullerenes organized only bydispersion forces are far less studied than covalently attached orcoordinated systems.30–35

Herein we demonstrate the fabrication of Pc/fullerenenanomaterials using simple Pc derivatives by careful tuning ofp–p stacking and van der Waals interactions. The mechanismof formation of self-organized nanoparticles from a blend offullerene C60 and a Pc substituted with 16 thioalkanes36,37 on anindium-tin-oxide (ITO) substrate is discussed. The importantfeatures of this work are: (a) ease of fabrication of the compositenanomaterials by mixing in solution; (b) use of supramolecularforces alone to drive the assembly of the Pc/C60 nanomaterial;(c) the use of pristine fullerene C60 without appendingsymmetry reducing moieties; (d) the ability to produce nearlymonodispersed size-tuneable nanoparticles over large areasusing a solution processed fabrication technique.

A commercially available Pc, ZnPcF16, was treated with a12-carbon thioalkane to afford variously substituted ZnPcspecies that are named according to the number of attachedthioalkanes (Scheme 1). We reported the synthetic details anduse of some of these derivatives to fabricate solar cells else-where.37 Because uorous alkanes on porphyrins enhance the

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Scheme 1 Synthesis of the Pc derivatives by substitution with thioalkanes on acore Pc platform in the presence of a base.

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interaction of these dyes with C60,35 we also examined Pc with 8or 16 uorous alkanes appended to the macrocycle. These dyesare made efficiently by click-type chemistry.†‡

When deposited on ITO electrodes, as either a pure solutionor as a blended solution with C60, the hexadecathioalkyatedPc16S exhibited uniquemorphological characteristics that noneof the lesser-substituted or uorous derivatives demonstrated.When a slide of ITO coated glass is dipped in a solution of Pc16Sin toluene for 1 h and the solvent allowed to evaporate in air, ahigh coverage thin lm is formed (Fig. 1). Both Pc8S and Pc4Sformed only sporadic and amorphous aggregates (Fig. S4†)under the same conditions. AFM analysis consistently shows alm thickness of ca. 10 nm. When immersed for an additionalhour in the dye solution, the slide accumulates more material,

Fig. 1 AFM height analysis of different samples obtained by immersing ITOslides in solutions of 0.3 mM Pc16S with increasing amounts of fullerene C60 for1 h. Images indicate that the formation of the nanoparticles results only at ratiosof Pc/fullerene greater than 1 : 10. All the images are 5 � 5 mm.

This journal is ª The Royal Society of Chemistry 2013

reaching a thickness of approximately 20 nm with moreuniform coverage. The increased thickness, measured by AFM,corresponds to an observed increase in absorbance of theQ-bands by UV-vis spectroscopy (Fig. S6†). Immersions longerthan 2 h or additional immersions increased neither thethickness nor the absorbance of the lms. As anticipated, Pc8S,Pc4S and a tetra-tert-butyl substituted Pc did not form lmsunder these conditions.

Toluene was chosen because both the chromophores andunderivatized C60 can be dissolved at reasonable concentrations.When ITO slides were dipped in a toluene solution containing a>10 : 1 mole ratio of Pc16S : C60, nearly monodispersed particlesformed on the surface aer the solvent was allowed to evaporate(Fig. 2). We hypothesize, based on the following experiments,that the interaction between the chromophores drive theformation of the particles and that the C60 resides amongst the Pcin the nanoparticles. The excess C60 effectively competes forinteractions with the aromatic Pc faces and diminishes the p–p

interactions between the Pc dyes, thereby shiing the equilib-riums towards the materials with both components. While long

Fig. 2 (A) AFM images of nanoparticles obtained from PcS16/C60 ratio 1 : 15,0.3 mM : 4.5 mM (15 � 15 mm), (B) height analysis showing that the nano-islandsare narrowly dispersed and they are ca. 130 nm high, (C) AFM of the same sampleat higher resolution (1.5 � 1.5 nm), (D) size of one particle showing the ca.130 nm height and ca. 800 nm diameter, (E) height and (F) phase of larger 25 �25 mm AFM scan of a similarly prepared sample.

RSC Adv., 2013, 3, 21360–21364 | 21361

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alkyl groups result in relatively weak van der Waals interactions,these are additive. Pc16S forms lms because the van der Waalsforces between the alkanes balance with the p–p stackinginteractions of the Pc macrocycle.

To probe the organization and formation of the observedparticles we studied the effect of changing the Pc16S : C60 ratio insolution on the morphology of cast lms. A large molar excess ofC60 is needed to drive the formation of the particles. WhenPc16S : C60 was premixed in solution at a ratio of 1 : 2 lms weresubstantially less uniform, while at a 1 : 5 ratio we found addi-tional features that may be indicative of phase separation of thecomponent molecules into domains. The nanoparticles becomemore well-dened and the dispersity narrows as the ratio ofPc16S : C60 approaches 1 : 15, at which point the nanoparticlesevenly distribute over the ITO surface (Fig. 2D).

The concentration of Pc16S is a means to control the size ofthe nanoparticles. Particles formed from 0.1 mM Pc16S solutionwere signicantly smaller than those formed from a 0.3 mMsolution (Fig. S8†). Longer time of immersion of the slidescorresponds to a greater surface density of the nanoparticles(Fig. S9†). Even large AFM scans (25 � 25 mm) show completecoverage of the surface (Fig. 2E). Duration of soaking was foundto increase surface coverage until two hours of immersion atwhich point equilibrium was reached. Successive dipping didnot induce more material to adhere to the surface.

Whereas small blue shis of the optical spectrum areobserved in the lms due to H aggregation, disruption of theintra Pc H aggregates and interactions with the fullerenebroadens and red shis the Q band of the Pc in the nano-particles as expected (Fig. S7†). Since the association constantsbetween one porphyrinoid and a fullerene in solution driven byp–p interactions are relatively weak,30 in order to shi theequilibrium towards the formation of the Pc16S/C60 material anexcess of C60 is needed.22,38

Fluorescence spectra were taken of lms prepared fromPc16S : C60 ratios of 1 : 0, 1 : 1, 1 : 5, and 1 : 10 and (Fig. 3).Fluorescence quenching is rst observed at a ratio of 1 : 1Pc : C60, and uorescence is completely quenched at ratios

Fig. 3 Fluorescence of the Pc dye in films on ITO formed by soaking in solutionswith different mole ratios of Pc : C60 demonstrating that the quenching of the Pcincreases as the amount of C60 in the materials increases. The same instrumentsettings were used,† excitation 720 nm. The Pc in solution was 0.3 mM.

21362 | RSC Adv., 2013, 3, 21360–21364

greater than 1 : 5. This data clearly demonstrates an interactionbetween the fullerene C60 and the chromophore molecules. UV-visible and uorescence spectra of the materials in solutionbetween 3 mM and 0.3 mM indicate little or no interactionbetween the Pc and the fullerene and dynamic light scatteringshows no particle above ca. 10 nm. At the ca. 0.2 mM concen-trations used for deposition, DLS and UV-visible spectra indi-cate a wide distribution of aggregate sizes due to the limitedsolubility of the components that are convolved with or maskany interactions between the two, and there is no obviouscorrelation with the narrowly dispersed sizes seen in thedepositions. As oen observed, organization in solution likelydiffers deposited on surfaces. Therefore, it is reasonable toconclude that the particles are forming on the surface duringthe deposition process. To test this hypothesis, we carried outan experiment in which we rst dipped the ITO substrate in asolution containing only Pc16S, let it dry, and then soaked it in asolution containing only fullerene. We found that islands ofmaterials are formed under these conditions, but they areamorphous and likely contain region of each species (Fig. S10†).The morphologies of materials formed by dipping the substratein solution containing only one of the species are very different,yielding thin lms for Pc and much smaller sporadic particles(ca. 15 nm) for the fullerene.

We previously reported that tetraarylporphyrins bearing fouruorous alkanes interact well with C60 mediated by F–p inter-actions to form thin lms with quenched uorescence from theporphyrin.35 Therefore, we synthesized analogous ZnPc byappending 8 and 16 uorous thioalkanes (Scheme 1). Wehypothesized that the uorous alkanes would increase inter-phthalocyanine interactions and also increase the interactionsbetween the dye and C60.

Consistent with our previous report, C60 forms <10 nmclusters and zinc Pc8F144 forms ca. 400 nm thick lms imbed-ded with large islands that are up to 800 nm tall.† Films formedby soaking the ITO slide in the derivative with eight uorousalkanes, Pc8F144, and 15 equivalents of C60 for 1 h form largeislands of the material (Fig. 4) whereas soaking for 2 h results inhigher coverage lms. Lower dye/C60 ratios in the solutionresult in more sporadic coverage with aggregates. UV-visibleand uorescence data of the soak solutions exhibit smalldifferences indicating small interactions between the dye andthe fullerene. The uorous alkane interactions with the C60 areconvolved with the dispersion forces between the Pc to yieldlms with embedded C60 as well as nanoparticle formation.Fluorescence quenching of lms cast from solutions includingfullerene was observed (Fig. S14 and 15†); however the differ-ence in solubility with respect to regular alkane moietiesdictates dissimilar dynamics in the precipitation process of thenanomaterials, which will be object of further studies (see ESI†for more details).

The 16 uorous alkanes on zinc PcF288 render the compoundinsufficiently soluble in solvents that are suitable for C60 tocompare cast lms made similarly to the alkanes. From asaturated solution in THF, PcF288 with C60 forms thin lmscontaining mostly Pc with no spectroscopic or microscopicevidence of signicant incorporation of the fullerene. The

This journal is ª The Royal Society of Chemistry 2013

Fig. 4 AFM analysis of an ITO slide that has been soaked in a toluene solution ofPcF144 : C60 (1 : 15, 0.3 mm : 4.5 mm) for 1 h and rinsed with clean solvent. Imagesare 5 � 5 mm, clockwise from top left: histogram, topology, height profile, phaseimage.

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intermolecular interactions of PcF288, arising from the cumu-lative interactions between the long uorous chains andaggregation of the macrocycles, are sufficiently robust topreclude signicant incorporation of the fullerene. This furtherillustrates that the balance between the interactions of the dyes,C60, and the surfaces, all modulated by the solvent, can be tunedto create hierarchically organize supramolecular materials.39,40

Conclusions

We have synthesized a Pc bearing 16 long alkane moieties topromote self-organization on a surface and demonstrated howto pattern ITO electrode with a blend of this phthalocyanine andfullerene C60 by using a simple electrode immersion method.The same experiment was conducted using clean glass slidesand the results were very similar to what was observed on ITO.The particles are highly monodispersed and changing theconcentration of the chromophores in solution can controltheir size. The facile synthesis on the PcF16 platform can beused with any thiolated moiety to investigate the role of exocy-clic motifs on the properties of nanomaterials.

Acknowledgements

Supported by the U.S. National Science Foundation (NSF CHE-0847997 and 1213962) to CMD and Hunter College scienceinfrastructure is supported by the NSF, including the NationalInstitute on Minority Health and Health Disparities (8G12MD007599) and the City University of New York.

Notes and references

‡ Syntheses of the hydrocarbons Pc are similar to previous reports.37 Syntheses ofthe uorocarbon Pc are described in the ESI.† ZnPc16F272: to ZnPcF16 (50 mg,0.057 mmol) in 19 mL of DMF under nitrogen was added 100 equivalents of3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-hexadecauorodecane-1-thiol (1.65 mL, 5.78

This journal is ª The Royal Society of Chemistry 2013

mmol) and K2CO3 (50 mg, 1.09 mmol). Aer 24 hours reux, the product wasprecipitated with 50 mL of methanol and vacuum ltered. Dark green crystalswere washed with 50 mL of methanol, 200 mL of hot water, 50 mL of acetone, anddried in an oven. All solid state samples were prepared as follows: slides of ITOwere cleaned for 20 minutes in an ozone stream, rinsed with 10 mL of ethanol,dried, rinsed with 20 mL of nanopure water and dried with a nitrogen streamowing through a porous lter. The functional materials were then deposited onthe surface by dipping the slides in a solution containing the chromophores at aspecic concentration/ratio and for an amount of time detailed in the discussion.

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