PERSPECTIVE pubs.acs.org/JPCL
PhthalocyanineCarbon Nanostructure Materials Assembled through Supramolecular Interactions Giovanni Bottari,† Juan A. Suanzes,† Olga Trukhina,† and Tomas Torres*,†,‡ † ‡
Departamento de Química Organica, Universidad Autonoma de Madrid, 28049 Madrid, Spain IMDEA-Nanociencia, Facultad de Ciencias, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain ABSTRACT: The use of self-assembly for the construction of materials based on phthalocyanines and carbon nanostructures—fullerenes, single-walled carbon nanotubes, and graphene—has demonstrated to be a versatile strategy for the preparation of novel, multifunctional systems. Photophysical studies carried out on these photo- and electroactive supramolecular ensembles have revealed the occurrence of an efficient photoinduced electron-transfer process, thus paving the way for the utilization of these materials as active components in optoelectronic devices. This Perspective highlights the recent progress in the preparation of such materials and the potential use of these systems for the construction of nanostructured materials with singular physicochemical properties.
Phthalocyanines (Pcs)4 are two-dimensional macrocycles which, due to their planar and aromatic large structure, are able to selfassemble into stacks through ππ supramolecular interactions.5 The excellent organization properties of Pcs, coupled with their outstanding physicochemical properties have prompted the utilization of these macrocycles as active components in several technological fields with applications ranging from sensors to molecular photovoltaics and from liquid crystals to field effect transistors, to mention a few, all fields in which the organization at the molecular level of these macrocycles often represents a fundamental issue to be addressed.6 Among the Pcs’ most striking features, one can highlight their redox chemistry and their intense absorption in the red/near-infrared (NIR) region of the solar spectrum, with extinction coefficients as high as 200 000 M1 cm1 and high fluorescence quantum yields, which render them ideal light-harvesting antenna systems. These unique physical properties, coupled with the synthetic versatility of these macrocycles, have prompted the utilization of these compounds as multifunctional materials in donoracceptor (DA) ensembles. Among the preferred “companions” employed for the preparation of covalent and noncovalent DA systems based on Pcs, carbon nanostructures such as fullerenes or carbon nanotubes (CNTs) hold a privileged position. The extraordinary electronacceptor properties of fullerenes,7 coupled with their small reorganization energy and their ability for promoting ultrafast charge separation together with very slow charge recombination features, have promoted the incorporation of these carbon nanostructures
T
he preparation of supramolecular architectures in which organic compounds present a high degree of order, which spans from the nanoscopic to the macroscopic level across multiple length scales, is highly desirable and represents a key issue within the fast-growing fields of nanoscience and nanotechnology.1,2 In this context, the utilization of self-assembly appears as an attractive and efficient strategy for the construction of such ordered structures as it can allow preparation of complex, (multi) functional systems in an efficient and controlled way through the use of noncovalent interactions. Among the organic compounds, π-conjugated systems are ideal candidates for the construction of such supramolecular systems due to their excellent self-organization ability.3
The utilization of self-assembly appears as an attractive and efficient strategy for the construction of ordered structures as it can allow preparation of complex, (multi)functional systems in an efficient and controlled way through the use of noncovalent interactions. r 2011 American Chemical Society
Received: February 23, 2011 Accepted: March 25, 2011 Published: April 05, 2011 905
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Figure 1. (a) Molecular structures of mesogenic Pc 2a and nonmesogenic Pc 2b and PcC60 dyads 1ac. (b) Schematic representation of the proposed 1a/2a mesophase formed by self-assembly of Pc 2a and PcC60 dyad 1a.
in a large number of DA systems where photoinduced electrontransfer (PET) processes and solar energy conversion are sought.8 Similarly, the outstanding properties of CNTs9,10 have generated a tremendous interest toward the use of this carbon allotrope in DA ensembles. CNTs, in fact, readily accept electrons, which, in turn, might be transported under nearly ideal conditions through their one-dimensional (1-D) tubular axis. Although these carbon nanostructures possess, per se, optimum electron-acceptor features, this property alone does not necessarily guarantee good “performances” of the resulting carbon-nanostructurecontaining materials. For example, when looking at the use of these carbon nano-objects for photovoltaic applications, one of the hottest fields where this class of materials has been employed, several other parameters besides the electron-acceptor ability need to be taken into account, one of the most important being the nanoand microscopic order of the constituting molecular components within the active layer. In this context, the possibility to functionalize these carbon nanostructures with Pcs, covalently or through supramolecular interactions, is highly desirable because, besides the expected benefits resulting from connecting an excellent lightharvesting molecule with a rich redox chemistry (the Pc) to an exceptional electron acceptor (the carbon nanostructure), it would provide the added value represented by the superior Pcs’ selforganization ability. To date, a large variety of molecular and supramolecular architectures incorporating both Pc and carbon nanostructure moieties have been prepared and the photophysical properties of these systems studied in solution and/or in the solid state.11,12 In most of the cases, the formation of efficient PET processes have been observed for these photo- and electroactive systems, thus paving the way for the utilization of these materials as active components of devices such as organic solar cells.12 On the other hand, scarcer are the examples in which these covalent and noncovalent Pc/carbon nanostructure architectures have been organized via supramolecular interactions over large length scales, although such organization could result in an improvement of some of the chemical and physical properties presented by these self-assembled systems with respect to their molecularly dispersed counterparts. Within the “toolbox” of supramolecular interactions that have been used to self-assemble Pcfullerene systems, the combination of ππ stacking and liquid-crystalline interactions is particularly attractive because, in appropriately substituted Pc macrocycles, it
allows generation of highly ordered, stacked, columnar supramolecular ensembles.13 In this context, mesogenic Pcs have been used for the construction of PcC60 DA supramolecular ensembles. In 2008, the first report on an indirect and easy way to incorporate a series of photoactive PcC60 dyads into a liquid-crystalline architecture appeared.14 The strategy consisted of blending a nonmesogenic PcC60 dyad (1ac, Figure 1a) with a mesogenic, symmetrically substituted octakis(hexadecylthio)Zn(II)Pc 2a. X-ray diffraction (XRD) studies of equimolecular mixtures of Pc 2a and dyads 1a, b, or c resulted in the formation of hexagonal columnar mesophases at ∼6070 °C in which the liquid-crystal columns were composed of an alternating stack of mesogenic Pc and PcC60 dyad (Figure 1b). The use of blends, in which a mesogen is able to induce mesomorphism on a nonmesogenic Pc-based functional material, represents an interesting strategy for the incorporation of photoactive, DA PcC60 systems in a liquidcrystalline architecture. Such an approach, in fact, would help mitigate the synthetic problems related to the preparation and isolation of mesogenic, unsymmetrically substituted Pc compounds. Recently, two reports by Geerts and co-workers15 and Torres and co-workers16 have appeared on the preparation of fully mesogenic PcC60 dyads. In the former report, a series of mesogenic PcC60 dyads (3ad) was prepared by an esterification reaction between unsymmetrically substituted Pcs bearing a terminal alcohol group and a fullerene derivative bearing a terminal acid moiety (Figure 2).15 UV/vis and electrochemical studies on these conjugates did not show any sign of ground-state electronic communication between the acceptor and the donor moieties. The thermotropic properties of these dyads were also studied by polarized optical microscopy (POM) and differential scanning calorimetry (DSC), revealing the formation of liquid-crystalline mesophases in the case of PcC60 ensembles 3c,d. These results suggest that in this series, a long linker is necessary in order to allow the bulky C60 moiety to be accommodated in the columnar liquid-crystalline mesophase formed by the Pc macrocycles. More recently, a mesogenic PcC60 dyad (4) has been reported, which consisted of a hexadodecyl-substituted Zn(II)Pc covalently connected through a flexible spacer to a C60 fullerene via a BingelHirsch cyclopropanation reaction (Figure 2).16 POM and DSC studies on this dyad revealed liquid-crystalline behavior of this ensemble between 80 and 180 °C. Complementary XRD studies showed that PcC60 dyad 4 adopts a rectangular symmetry 906
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Figure 2. Molecular structures of mesogenic PcC60 dyads 3 and 4.
within the columnar mesophase (i.e., Colr), each rectangular unit having a column at its center and four others at its corners. A combination of ππ stacking and DA interactions between symmetric, electron-deficient Pd(II)Pc-bearing alkylsulfonyl groups (2b) and an electron-rich Zn(II)PcC60 dyad bearing six alkoxy substituents at the periphery of the macrocycle (1b) have also been exploited as “primary” supramolecular recognition motifs for the construction of a PcC60/Pc supramolecular triad (1b/2b) (Figure 1a).17 The formation in solution of the DA supramolecular complex 1b/2b was inferred by the analysis of the changes in the absorption and fluorescence spectra of PcC60 dyad 1b upon titration with symmetric Pd(II)Pc 2b, which gives rise to a supramolecular complex 1b/ 2b that has an association constant of ∼5 105 M1 in CHCl3. Job plot method analysis allowed determination of the stoichiometry of the 1b/2b supramolecular triad, which resulted in being 1:1. Transient absorption spectroscopy offers an important tool for the investigation of photoinduced charge-transfer processes in DA systems. This technique, in fact, allows determination of the kinetics of the formation/conversion of photogenerated transient species and has been widely used for the spectroscopic characterization of some of the Pccarbon nanostructure materials presented after. Transient absorption studies on dyad 1b and supramolecular complex 1b/2b showed considerable stabilization of the chargeseparated state of PcC60 1b in the heterocomplex 1b/2b (i.e., from 0.17 to 475 ns) as a consequence of the formation of the DA supramolecular complex, which ultimately helped in delocalizing the radical cationic charge formed over the macrocyclic moiety of the PcC60 dyad between the Zn(II)Pc and the Pd(II)Pc moieties. Although the above-mentioned system does not constitute an example of a nondiscrete organized solid, it clearly shows the importance of DA interactions as a mean to stabilize the lifetime of the charge-separated state in Pc/fullerene ensembles. The formation of long-range, ordered PcC60 nanoaggregates based on the combination of ππ stacking and hydrophilichydrophobic noncovalent interactions has also been reported using an amphiphilic PcC60 dyad (5, Figure 3a).18 Such an amphiphilic system is able to form aggregated species when dispersed in water, as demonstrated by UVvis and light-scattering studies. Further insights into the morphology of these aggregates formed by dyad salt 5 in water were gathered by transmission electron microscopy (TEM) studies in which the formation of uniformly, micrometer long nanorods could be observed (Figure 3b). Interestingly, these nanotubules are reminiscent of those observed for a DA, porphyrinC60 dyad.19 Steady-state and
The excellent organization properties of Pcs, coupled with their outstanding physicochemical properties have prompted the utilization of these macrocycles as active components in several technological fields. transient absorption studies demonstrated that the self-organization ability of the amphiphilic ensemble 5 in water also has a profound influence on the photophysical properties of these 1-D nano-objects. Particularly, transient absorption measurements on these nanotubules of 5 revealed the formation of a long-lived photoinduced charge-transfer product as inferred by the decay analysis of the radical pair species of this DA supramolecular ensemble at 850 (i. e., Pc•þ) and 1050 nm (i.e., C60•) (Figure 3c). For such a system, an impressive stabilization of more than 6 orders of magnitude was observed for the charge-separated lifetime of self-assembled dyad 5 (i.e., 1.4 ms) with respect to a structurally related PcC60 dyad analogous to 5, which lacks the terminal ammonium unit and which is not able to form nanotubules (i.e., ∼3 ns). The use of supramolecular interactions for the “bottom-up” fabrication of PcC60 nanoscale functional systems on solid surfaces has also been recently exploited. A structurally rigid, covalently linked PcC60 conjugate (6, Figure 4a) has been prepared, and its organization ability on highly ordered pyrolytic graphite (HOPG) and graphite-like surfaces has been investigated by using atomic force microscopy (AFM) and conductive AFM (C-AFM), the latter technique being a powerful technique used for measuring electrical properties in nanostructured architectures.20 AFM studies revealed the formation of supramolecular fibers and films as a result of a combination of molecule-tomolecule as well as molecule-to-substrate interactions (Figure 4b), whereas C-AFM studies showed electrical conductivity values as high as 30 μA for bias voltages ranging from 0.30 to 0.55 V. Control experiments revealed that the high electrical conductivity values recorded for the solid-supported, self-assembled PcC60 conjugate are strongly related to the supramolecular order of the dyad within the nanostructures. 907
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Figure 3. (a) Molecular structure of PcC60 dyad 5. (b) TEM image of the nanotubules formed by dyad 5 in water. (c) Differential absorption spectra obtained upon nanosecond flash photolysis (337 nm) of PcC60 dyad 5 in aqueous dispersion with a time delay of 100 ns. (Inset) C60 radical anion decay at 1050 nm. The images in (b) and (c) are reprinted with permission from ref 18. Copyright 2005, American Chemical Society.
Figure 4. (a) Molecular structure of PcC60 conjugate 6. (b) AFM topographic image of dyad 6 drop-casted on HOPG. The image in (b) is reprinted with permission from ref 20. Copyright 2008, Wiley-VCH.
Supramolecular interactions between Pc-based derivatives and single-walled CNTs (SWCNTs) have also been exploited for the preparation of DA Pc/SWCNT materials.2127 The noncovalent functionalization of SWCNTs presents several advantages with respect to the covalent, sidewall derivatization of these carbon nanostructures because it preserves, differently from the latter case, the extended π-network and thus the electronic properties of the CNTs.28 In this context, free-base and Zn(II)Pcs-substituted at their periphery with pyrene (Py) (H2PcPy and Zn(II)PcPy, respectively), a moiety known to adhere effectively to SWCNTs through ππ stacking interactions,29 have been recently prepared and their interactions with SWCNTs probed.21 Titration of a suspension of SWCNTs in THF with variable amounts of H2PcPy (or Zn(II)PcPy) solutions resulted in a drastic change of the absorption and fluorescence of the Pc (i.e., broadening and red shift of the Pc Q-band and quenching of the Pc fluorescence) and SWCNTs moieties (i.e., red shift of the SWCNTs NIR absorption and quenching and red shift of the CNTs fluorescence), thus suggesting that the Py-substituted Pcs are immobilized onto the SWCNTs. AFM experiments carried out on PcPy/SWCNT dispersions supported the previous findings showing the presence of well-dispersed thin bundles of SWCNTs. Complementary Raman measurements carried out on liquid and solid samples of PcPy/SWCNT did not show any change in the G- or D-bands of the SWCNTs with respect to the pristine SWCNT sample, thus suggesting that the immobilization of the PcPy to the nanotube surface does not produce any structural damage of the SWCNTs. Transient absorption
measurements were also carried out on the PcPy/SWCNT material, demonstrating that a rapid charge transfer from the Pc to the SWCNT occurs upon 387 nm photoexcitation. It is important to consider that an equilibrium between bound and unbound PcPy species is taking place for the PcPy/SWCNT ensemble, as suggested by the decrease, upon dilution, of the absorption fingerprints of the Pc Q-band due to the immobilized PcPy and the concomitant increase of the absorptions corresponding to “free” PcPy species. Finally, the photovoltaic properties of these PcPy/SWCNT materials were investigated, revealing stable and reproducible photocurrents with monochromatic internal photoconversion efficiency values for the H2PcPy/SWCNT hybrid material as large as 15 and 23% without and with an applied bias of þ0.1 V respectively. A single-molecule magnet (SMM), a molecular complex that displays slow dynamics of the magnetization at low temperatures and an impressive array of quantum features, constituted by a heteroleptic bis(phthalocyaninato) terbium(III) complex has also been used to functionalize SWCNTs via supramolecular interactions.22 In such system, the supramolecular grafting of a mononuclear rare earth Pc complex to the SWCNTs occurs, as in the previous case, through the ππ stacking of the pendant Py unit of the SMMs to the nanotubes’ surface. A wide range of techniques such as high-resolution TEM, emission spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICPAES), and AFM suggest the formation of the SMM/SWCNT ensemble, with a load of one molecule of SMM per 78 nm of SWCNT length. Magnetization studies on such a SMM/ SWCNT conjugate revealed that the SMM behavior of the 908
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Figure 6. Schematic representation of the noncovalent assembly of a third-generation dendritic Pc onto a SWCNT.
(TPP) macrocycle, either free base or as a zinc complex, linked through its β-pyrrolic position to a Pc.24 The interaction between dispersed SWCNTs and PcTPP dyads was probed initially by means of absorption spectroscopy, which showed, upon titration of the dyad with a SWCNT suspension, a red shift of both the TPP Soret band and the Pc Q-band, clear proof of a mutual interaction between the dyad and the nanotube. Fluorescence studies carried out on the PcTPP dyad alone revealed that upon photostimulation of the TPP unit at 410 nm, a rapid and effective energy transfer from the TPP to the Pc moiety takes place (i.e., complete quenching of the TPP fluorescence and emission from the Pc unit). On the other hand, the same experiment carried out in the presence of a dispersion of the SWCNT resulted also in the quenching of the Pc macrocycle. These findings suggest that the PcTPP dyad interacts strongly with dispersed SWCNTs through the Pc moiety, while the interactions through the TPP macrocycle seem to be intrinsically weak, thus implying a transduction of the singlet excited-state energy from the TPP to the Pc followed by a charge-transfer mechanism to the SWCNTs. This mechanism was finally confirmed by transient absorption studies, which revealed a rapid deactivation of the Pc singlet excited state and the synchronous appearance of the signature of the Pc radical cation and reduced SWCNT species. The formed charge-transfer product is shortlived (i.e., 5 ns) probably due to the short distance between the electron donor (i.e., Pc) and the electron acceptor (i.e., SWCNT) within the supramolecular PcTPP/SWCNT hybrid. Recently, supramolecular interactions between a series of dendritic (i.e., first to third generations), electron-donor freebase Pcs and SWCNTs have also been used for the preparation of dispersible DA, Pc/SWCNT nanohybrids (Figure 6).25 The dendritic Pcs bear an increasing number of oligoethylene glycol end groups, which surround the hydrophobic core and which allow their dispersion both in organic solvents and in aqueous media. The first evidence of the interactions between Pcs and SWCNTs was gathered by absorption studies. Titration of a THF solution of Pcs to a THF suspension of SWCNTs, in fact, resulted in a red shift of both the Pc-centered transitions (at 668 and 703 nm) as well as the near-infrared, SWCNT-centered features, which indicates the immobilization of the Pc macrocycles onto SWCNTs. A similar trend was obtained when the titration experiments were carried out in water. In this latter media, a very stable suspension of the Pc/SWCNT ensemble was obtained in the case of third- and second-generation Pcs, which even after months failed to give rise to any detectable changes, thus suggesting strong electronic interactions between these Pcs and the SWCNTs. AFM, TEM, and Raman spectroscopy further supported the supramolecular functionalization of the Pcs to the
Figure 5. Noncovalent assembly of a SWCNT and a Zn(II)Nc using an imidazolylpyrene bridge.
heteroleptic complex is retained and even improved in the SMM/SWCNT hybrid material. A further sophisticated supramolecular approach represented by a combination of Py/SWCNT ππ stacking and metalligand coordination interactions has also been used for the preparation of a three-component Zn(II)naphthalocyanine (Nc)/Py/SWCNTs system (Figure 5).23,30 Key for the success of the assembly is the presence on the Py derivative of a phenylimidazole ligand which can strongly coordinate to the zinc center of the Nc macrocycle while interacting with the SWCNT surface through the Py moiety, thus acting as a “bridging” unit for such supramolecular assembly. This DA, Nc/Py/SWCNT nanohybrid has been characterized by various physicochemical techniques including TEM, UV/vis, and electrochemical methods. Finally, PET dynamics taking place from the singlet excited Zn(II)Nc moiety to the SWCNT acceptor unit have been demonstrated to give rise to the formation of a Zn(II)Nc•þ/SWCNT• charge-separated transient species, as revealed by steady-state and time-resolved emission studies. More recently, a modification of the PcPy concept seen above, which is based on the ability of the Py unit to “anchor” to the SWCNT surface (vide supra), has been presented, in which the pyrenyl moiety has been replaced by a tetraphenylporphyrin
Among the preferred “companions” employed for the preparation of covalent and noncovalent donoracceptor systems based on Pcs, carbon nanostructures such as fullerenes, carbon nanotubes, or graphene hold a privileged position due to their extraordinary electronacceptor properties. 909
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Figure 7. Molecular structures of Pc-based PPV oligomers 7 and 8.
SWCNT surface. Finally, transient absorption measurements were carried out on the Pc/SWCNT ensemble, which revealed, upon photoexcitation, the formation of a charge-transfer product, namely, the one-electron oxidized Pc and the one-electron reduced SWCNT species, with a lifetime of the charge-separated photoproduct of about 250 ps. Pc moieties covalently linked, in the form of pending arms, to the backbone of poly(p-phenylenevinylene) (PPV) oligomers (7,8ac, Figure 7) have also been prepared, and their interactions with SWCNTs have been studied.26 Oligomers 7,8ac were prepared in the form of an alternating copolymer by Knoevenagel (i.e., 7) or WadsworthHornerEmmons (i.e., 8ac) condensation reactions between Pc-containing diformyl derivatives and appropriate dicyano (in the case of 7) or phosphonate (in the case of 8ac) compounds, obtaining two short (i.e., 7 and 8a containing 4 and 3 repeating units, respectively) and two longer PPV oligomers (i.e., 8b and 8c containing 9 and 16 repeating units, respectively), as confirmed by gel permeation chromatography (GPC) analyses. Several techniques (i.e., AFM, Raman, UVvis, and NIR absorption) demonstrated that only oligomer 7 interacts tightly with SWCNTs, affording stable and finely dispersed SWCNTs’ suspensions, differently from oligomers 8ac, which instead failed to disperse the CNTs. A possible explanation for these results resides in the different electronic character of these two classes of PPV oligomers. The electron-withdrawing cyano substituents in 7, in fact, confer to the PPV-oligomer an n-type character, which results in strong ππ interactions with SWCNTs, which are p-type materials. In oligomer 7, the presence of an ether linkage seems to prevent the electronic communication between the electron-donating Zn(II)Pc units and the n-type, cyano-substituted, conjugated PPV backbone. On the contrary, PPV oligomers 8ac possess a strong p-type character that hampers the interactions with SWCNTs. Transient absorption measurements carried out on the 7/SWCNT ensemble revealed the formation of a metastable charge-separated photoproduct. More recently, the supramolecular interactions between SWCNTs and a long (i.e., 27 repeating units) PPV oligomer (8d), which has the Pc pendant units connected to the PPV backbone through a long spacer, have also been investigated, revealing the occurrence of strong communication between the oligomer and the CNTs (unpublished results). This study demonstrates that the “driving force” for the immobilization of
Figure 8. Frontal view of the proposed supramolecular organization of PcC60 dyad 6 on a 2.5 nm SWCNT on a silicon oxide surface. The image is reprinted with permission from ref 27. Copyright 2010, Royal Society of Chemistry.
these Pc-based PPV oligomers onto SWCNTs is not only represented by the occurrence of n-type/p-type interactions between the CNT and the oligomer (vide supra), but also, the structural flexibility of the oligomer has an important role. The organization properties of a Pc derivative (i.e., PcC60 dyad 6, Figure 4) on SWCNTs grown by chemical vapor deposition directly on surfaces (i.e., hydrophilic silicon oxide wafers) have also been reported.27 In such a study, the strong affinity of dyad 6 for graphitic surfaces (vide supra) coupled to the poor affinity of this conjugate for hydrophilic surfaces such as silica or mica has been exploited in order to promote the organization of 6 on the curved, 1D, graphite-like surface of these SWCNTs. AFM studies revealed that conjugate 6 is able to self-assemble with nanometer precision by means of noncovalent interactions on the outer wall of these SWCNTs. Several control experiments were performed with the aim of identifying some of the key factors responsible for the supramolecular organization of this PcC60 conjugate on these surface-grown SWCNTs. These experiments provided a series of “indirect proofs” which allowed proposal of an organization model for the 6/SWCNT supramolecular heteroarray. In this model, the structurally rigid PcC60 dyads are arranged around the SWCNTs’ 1-D axis, adopting a radial upright position, with the C60 fullerene moieties pointing downward toward the SWCNT interacting with its curved graphitic surface and the Pcs units ππ stacking between 910
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them (Figure 8). The final goal for the preparation of such assemblies is to alter the electrical resistance of these SWCNTs “coated” with nanometer precision by a photoresponsive DA, PcC60 dyad in response to appropriate photonic stimuli. More recently, the first example of a Pc-based system (8d, Figure 7) able to “interact” with graphene, one of the latest entries in the carbon nanostructures’ family and one of the “rising stars” in the field of nanotechnology,31 has been reported.32 Pure, natural graphite (NG) was suspended/exfoliated in a THF solution of oligomer 8d, and upon ultrasonication and centrifugation, the supernatant was subjected to spectroscopic and microscopic investigations. Absorption studies showed, in the presence of graphene, a significant broadening and red shift (i.e., 32 nm) of the Pc’s absorption features of the oligomer, which suggest a strong electronic coupling between the Pc units and graphene. Remarkably, addition of Triton X-100 to the exfoliated NG/8d hybrid replaces 8d from the graphene surface, thus restoring the original absorption of the Pc oligomer. The occurrence of supramolecular interactions between the Pc units of 8d and exfoliated NG was also inferred by comparative fluorescence studies carried out on oligomer 8d and exfoliated NG/8d hybrid, which showed, in the latter system, a strong quenching of the Pc fluorescence (i.e., 60%) compared to pristine 8d. Proofs of the exfoliation ability of oligomer 8d toward NG also came from (i) TEM investigation, which showed for the exfoliated NG/8d hybrid system images typical of mono/few layers of graphene (Figure 9), and (ii) resonant and nonresonant Raman studies, which showed changes in the D, 2D, and G-bands of the hybrid system with respect to the bulk graphite material typical of graphene exfoliation. Insights into the dynamics (i.e., electron transfer versus energy transfer) of the exfoliated NG/8d hybrid were, finally, obtained from time-resolved measurements carried out upon femtosecond flash photolysis exciting either at 387 (where both “free” and “bound” oligomer 8d absorb) or 700 nm (where only the “bound” oligomer species of the exfoliated NG/8d hybrid absorbs). In both cases, the formation of the Pc radical cation and exfoliated NG radical anion species, at 840 and 1290 nm, respectively, was observed as a result of an electron-transfer process from the photoexcited Zn(II)Pc, the electron donor, to graphene, the electron acceptor, giving rise to an electrontransfer photoproduct that has a lifetime of 360 ps. In the case of the 700 nm excitation, the electron-transfer product characteristics evolve with kinetics that are somehow faster than those at the 387 nm excitation (i.e., 2 ps) probably due to the fact that this wavelength excites “directly” the electronically coupled state. The potential use of the exfoliated NG/8d hybrid in solar energy conversion schemes has also been investigated by manufacturing a solar cell prototype, although low photocurrents were recorded for the unoptimized device. This Perspective highlights the use of supramolecular interactions as an efficient strategy for the construction of nanostructured materials based on Pcs and carbon nanosystems, like fullerenes, CNTs, or graphene. The preparation and study of Pcfullerene systems able to self-assemble over large length scales has clearly demonstrated that such organization can often bring about significant changes in some physical properties of these selfassembled systems with respect to their molecularly dispersed counterparts. Moreover, the use of noncovalent interactions as a tool to promote the nano- and microscopic order within fullerenebased DA materials is highly desirable, particularly for molecular photovoltaics and field effect transistor applications, where the
Figure 9. TEM image and selected area electron diffractogram (inset) of exfoliated NG/8d drop-casted onto a carbon-coated copper grid. The image is reprinted with permission from ref 32. Copyright 2011, Wiley-VCH.
order of both the fullerene and the donor components in the solid state is a key issue for achieving high carrier mobilities. The preparation and study of supramolecular assemblies comprised of both Pcs and CNTs is without a doubt another important field of research because it could give rise to novel nanomaterials where the excellent photophysical and redox properties of the Pcs are coupled to the unspoiled electronic properties of these carbon nanostructures. Further work in this field should focus on the possibility of preparing Pc-based systems (polymers or dendrimers) able to selectively bind through noncovalent interactions to SWCNTs of different diameters (which ultimately controls the CNTs’ band gap and redox potentials) or chirality. Similarly, the preparation of watersoluble Pc-based dendrimers able to “interact” with SWCNTs, dispersing them in aqueous media, is another interesting topic that could allow preparation of light-harvesting, DA, SWCNT-based systems able to be processed in environmentally friendly solvents. More recently, Pc/graphene hybrids have been prepared for the first time by means of supramolecular chemistry. This strategy allows one, when using appropriate Pc-based oligomers, not only to effectively exfoliate graphite but, more importantly, to prepare stable DA materials that are endowed with new and even more exciting features than just bare graphene. In this context, it is expected that a fine-tuning of the structural and electronic properties of the Pc-based component will allow preparation of Pc/ graphene ensembles, which could be of great interest for the development of a new generation of optoelectronic devices. Although important advances have been made toward the preparation and study of sophisticated supramolecular Pccarbon nanostructure systems, a lot of understanding is still needed in order to accurately predict and modulate a la carte the physicochemical collective properties of the resulting supramolecular ensembles. Future challenges for these Pccarbon nanostructure ensembles include not only a better comprehension of the self-aggregation ability of these systems but also the implementation of these supramolecular architectures into efficient, solid-state devices, which still represent a major challenge. It can be easily foreseen that scanning force and tunneling 911
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microscopies, as well as optical (ground- and excited-state) characterization techniques will also play a fundamental role.
Although important advances have been made toward the preparation and study of sophisticated supramolecular Pccarbon nanostructure systems, a lot of understanding is still needed in order to accurately predict and la carte the physicomodulate a chemical collective properties of the resulting supramolecular ensembles.
and holds 35 patents. He has been awarded the Janssen Cilag prize for Organic Chemistry 2005 by the Royal Society of Chemistry of Spain. In 2009, he was honored as Doctor Honoris Causa by the Ivanovo State University of Chemistry and Technology, Russia.
’ ACKNOWLEDGMENT We would like to thank our colleagues and co-workers whose names appear in the references of this paper, particularly Prof. Dirk M. Guldi (University of Erlangen-Nuremberg, Germany), for contributions to the area highlighted here. Financial support from MICINN and MEC, Spain (CTQ2008-00418/BQU, CONSOLIDER-INGENIO 2010 CDS 2007-00010 Nanociencia Molecular, PLE2009-0070), COST Action D35, and CAM (MADRISOLAR-2, S2009/PPQ/1533) is acknowledged. G.B. thanks the Spanish MICINN for a “Ramon y Cajal” contract. ’ REFERENCES (1) Special Issue on “Supramolecular Chemistry and Self-Assembly”. Science 2002, 295, 23952421. (2) Special Issue on “Supramolecular Approaches to Organic Electronics and Nanotechnology”. Adv. Mater. 2006, 18, 12271329. (3) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. About Supramolecular Assemblies of π-Conjugated Systems. Chem. Rev. 2005, 105, 1491–1546. (4) de la Torre, G.; Claessens, C. G.; Torres, T. Phthalocyanines: Old Dyes, New Materials. Putting Color in Nanotechnology. Chem. Commun. 2007, 2000–2015. (5) de la Escosura, A.; Martinez-Diaz, M. V.; Thordarson, P.; Rowan, A. E.; Nolte, R. J. M.; Torres, T. DonorAcceptor Phthalocyanine Nanoaggregates. J. Am. Chem. Soc. 2003, 125, 12300–12308. (6) de la Torre, G.; Bottari, G.; Hahn, U.; Torres, T. Functional Phthalocyanines: Synthesis, Nanostructuration, and Electro-Optical Applications. Struct. Bond. 2010, 135, 1–44. (7) Fullerenes: Principles and Applications; Langa, F., Nierengarten, J.-F., Eds.; Nanoscience and Nanotechnology Series; The Royal Society of Chemistry: Cambridge, U.K., 2007. (8) Guldi, D. M.; Illescas, B. M.; Atienza, C. M.; Wielopolski, M.; Martin, N. Fullerene for Organic Electronics. Chem. Soc. Rev. 2009, 38, 1587–1597. (9) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chemistry of Carbon Nanotubes. Chem. Rev. 2006, 106, 1105–1136. (10) Karousis, N.; Tagmatarchis, N.; Tasis, D. Current Progress on the Chemical Modification of Carbon Nanotubes. Chem. Rev. 2010, 110, 5366–5397. (11) Gonzalez-Rodriguez, D.; Bottari, G. Phthalocyanines, Subphthalocyanines and Porphyrins for Energy and Electron Transfer Applications. J. Porphyrins Phthalocyanines 2009, 13, 624–636. (12) Bottari, G.; de la Torre, G.; Guldi, D. M.; Torres, T. Covalent and Noncovalent Phthalocyanine-Carbon Nanostructure Systems: Synthesis, Photoinduced Electron Transfer, and Application to Molecular Photovoltaics. Chem. Rev. 2010, 110, 6768–6816. (13) Simon, J.; Bassoul, P. In Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: Weinheim, Germany, 1993; Vol. 2, Chapter 6. (14) de la Escosura, A.; Martinez-Diaz, M. V.; Barbera, J.; Torres, T. Self-Organization of Phthalocyanine[60]Fullerene Dyads in Liquid Crystals. J. Org. Chem. 2008, 73, 1475–1480. (15) Geerts, Y. H.; Debever, O.; Amato, C.; Sergeyev, S. Synthesis of Mesogenic PhthalocyanineC60 DonorAcceptor Dyads Designed for Molecular Heterojunction Photovoltaic Devices. Beilstein J. Org. Chem. 2009, 5, No. 49. (16) Ince, M.; Martinez-Diaz, M. V.; Barbera, J.; Torres, T. Liquid Crystalline PhthalocyanineFullerene Dyads. J. Mater. Chem. 2011, 21, 1531–1536.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ BIOGRAPHIES Giovanni Bottari graduated with a degree in chemistry from the University of Messina, Italy, in 1999. In 2003, he obtained his Ph.D. from the University of Edinburgh, United Kingdom, working in the preparation and study of stimulus-responsive rotaxanebased systems under the supervision of Prof. D. A. Leigh. The same year, he joined the group of Prof. T. Torres at the Universidad Autonoma de Madrid, Spain, at the beginning as a postdoctoral researcher, benefiting from a two-year Marie Curie Intra European Fellowship, and since 2006, he has served as a “Ramon y Cajal” Fellow. His current research interests include the preparation of phthalocyanine-based donoracceptor molecular materials and their study both in solution and on surfaces. Juan Antonio Suanzes graduated with a degree in chemistry from the Universidad Autonoma de Madrid, Spain, in 2008. The same year, he started his Ph.D. within the group of Professor T. Torres, focusing his research on the preparation and study of donoracceptor phthalocyaninefullerene (empty and endohedral) molecular materials. Olga Trukhina graduated from the Ivanovo State University of Chemistry and Technology, Russia, in 2007 and then became a Ph. D. student under the guidance of Prof. Mikhail K. Islyaikin and Prof. T. Torres, working on expanded porphyrin aza-analogues. In 2010, she started working in the synthesis of phthalocyanine-based molecular nanosystems as a fellow within the group of Prof. T. Torres at the Universidad Autonoma de Madrid, Spain. Tomas Torres is Full Professor of Chemistry at the Universidad Autonoma de Madrid. In addition to various aspects of supramolecular chemistry, his current research interests include the preparation of molecular materials based on phthalocyanines, including applications in solar cells, with a focus on nanotechnology. Prof. Torres has published more than 320 papers and reviews 912
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(17) de la Escosura, A.; Martinez-Diaz, M. V.; Guldi, D. M.; Torres, T. Stabilization of Charge-Separated States in PhthalocyanineFullerene Ensembles through Supramolecular Donor-Acceptor Interactions. J. Am. Chem. Soc. 2006, 128, 4112–4118. (18) Guldi, D. M.; Gouloumis, A.; Vazquez, P.; Torres, T.; Georgakilas, V.; Prato, M. Nanoscale Organization of a Phthalocyanine Fullerene System: Remarkable Stabilization of Charges in Photoactive 1-D Nanotubules. J. Am. Chem. Soc. 2005, 127, 5811–5813. (19) Georgakilas, V.; Pellarini, F.; Prato, M.; Guldi, D. M.; MelleFranco, M.; Zerbetto, F. Supramolecular Self-Assembled Fullerene Nanostructures. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5075–5080. (20) Bottari, G.; Olea, D.; Gomez-Navarro, C.; Zamora, F.; G omezHerrero, J.; Torres, T. Highly Conductive Supramolecular Nanostructures of a Covalently Linked PhthalocyanineC60 Fullerene Conjugate. Angew. Chem., Int. Ed. 2008, 47, 2026–2031. (21) Bartelmess, J.; Ballesteros, B.; de la Torre, G.; Kiessling, D.; Campidelli, S.; Prato, M.; Torres, T.; Guldi, D. M. Phthalocyanine Pyrene Conjugates: A Powerful Approach toward Carbon Nanotube Solar Cells. J. Am. Chem. Soc. 2010, 132, 16202–16211. (22) Kyatskaya, S.; Mascaros, J. R. G.; Bogani, L.; Hennrich, F.; Kappes, M.; Wernsdorfer, W.; Ruben, M. Anchoring of Rare-EarthBased Single-Molecule Magnets on Single-Walled Carbon Nanotubes. J. Am. Chem. Soc. 2009, 131, 15143–15151. (23) Chitta, R.; Sandanayaka, A. S. D.; Schumacher, A. L.; D’Souza, L.; Araki, Y.; Ito, O.; D’Souza, F. DonorAcceptor Nanohybrids of Zinc Naphthalocyanine or Zinc Porphyrin Noncovalently Linked to SingleWall Carbon Nanotubes for Photoinduced Electron Transfer. J. Phys. Chem. C 2007, 111, 6947–6955. (24) Bartelmess, J.; Soares, A. R. M.; Martínez-Díaz, M. V.; Neves, M. G. P. M. S.; Tome, A. C.; Cavaleiro, J. A. S.; Torres, T.; Guldi, D. M. Panchromatic Light Harvesting in Single Wall Carbon Nanotube Hybrids. Immobilization of PorphyrinPhthalocyanine Conjugates. Chem. Commun. 2011, 47, 3490–3492. (25) Hahn, U.; Engmann, S.; Oelsner, C.; Ehli, C.; Guldi, D. M.; Torres, T. Immobilizing Water-Soluble Dendritic Electron Donors and Electron Acceptors—Phthalocyanines and Perylenediimides—onto Single Wall Carbon Nanotubes. J. Am. Chem. Soc. 2010, 132, 6392–6401. (26) Bartelmess, J.; Ehli, C.; Cid, J.-J.; García-Iglesias, M.; Vazquez, P.; Torres, T.; Guldi, D. M. Tuning and Optimizing the Intrinsic Interactions between Phthalocyanine-Based PPV Oligomers and Single-Wall Carbon Nanotubes toward n-type/p-type. Chem. Sci. 2011, 2, 652–660. (27) Bottari, G.; Olea, D.; Lopez, V.; Gomez-Navarro, C.; Zamora, F.; Gomez-Herrero, J.; Torres, T. Ordering PhthalocyanineC60 Fullerene Conjugates on Individual Carbon Nanotubes. Chem. Commun. 2010, 46, 4692–4694. (28) Sgobba, V.; Guldi, D. M. Carbon Nanotubes—Electronic/ Electrochemical Properties and Application for Nanoelectronics and Photonics. Chem. Soc. Rev. 2009, 38, 165–184. (29) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. Noncovalent Sidewall Functionalization of Single-Walled Carbon Nanotubes for Protein Immobilization. J. Am. Chem. Soc. 2001, 123, 3838–3839. (30) D’ Souza, F.; Sandanayaka, A. S. D.; Ito, O. SWNT-Based Supramolecular Nanoarchitectures with Photosensitizing Donor and Acceptor Molecules. J. Phys. Chem. Lett. 2010, 1, 2586–2593. (31) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183–191. (32) Malig, J.; Jux, N.; Kiessling, D.; Cid, J.-J.; Vazquez, P.; Torres, T.; Guldi, D. M. Towards Tunable Graphene/PhthalocyaninePPV Hybrid Systems. Angew. Chem., Int. Ed. 2011, 50, 3561–3565.
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