Supramolecular One-Dimensional n/p-Nanofibers.

Currently, there is a broad interest in the control over creating ordered electroactive nanostructures, in which electron donors and acceptors are organized at similar length scales. In this article, a simple and efficient procedure is reported en-route towards the construction of 1D arrays of crystalline pristine C60 and phenyl-C61-butyric acid methyl ester (PCBM) coated onto supramolecular fibers based on exTTF-pentapeptides. The resulting n/p-nanohybrids have been fully characterized by a variety of spectroscopic (FTIR, UV-Vis, circular dichroism, Raman and transient absorption), microscopic (AFM, TEM, and SEM), and powder diffraction (X-ray) techniques. Our experimental findings document the tendency of electroactive exTTF-fibers to induce the crystallization of C60 and PCBM, on one hand, and to afford 1D n/p-nanohybrids, on the other hand. Furthermore, photogenerated radical ion pairs, formed upon visible light irradiation of the n/p-nanohybrids, feature lifetimes on the range of 0.9-1.2 ns.

Owing to their remarkable electronic, mechanical and geometrical properties, C60 and its derivatives have drawn much attention during recent decades123. A plethora of reported C60 derivatives has made its way into emerging electronic applications such as photovoltaic cells456789, organic transistors10111213141516, organic light emitting diodes (On class="Gene">LEDs)171819202122, anpan>d senpan>sors23242526272829. Importanpan>tly, the mutual depenpan>denpan>ce betweenpan> morphology/crystallinpan>ity of the photoactive layers anpan>d device efficienpan>cies is well established. For example, inpan> bulk heterojunpan>ctionpan> (BHJ) photovoltaic devices, the existenpan>ce of nanpan>ostructured domainpan>s, inpan> which electronpan> pan> class="Species">donors and/or electron acceptors are ordered at the same length scale, is of key importance to boost the overall efficiency3031. Several molecular building blocks, which are integrated into photoactive layers, do not lead to the formation of nanostructures and/or microstructures that fulfill the aforementioned requirements. Still, the use of additives, as a means to control the nanostructure and/or microstructure in photoactive layers, constitutes a powerful approach. In particular, the employment of nucleating agents exerts a strong impact on the charge mobility in the resulting films32.

In recent years, C60 derivatives featuring an amphiphilic character have evolved as good candidates for being integrated into active layers yielding hierarchical architectures across different length scales3334353637. Notably, several major drawbacks remain. Among them is the need for tedious synthetic protocols to prepare amphiphilic C60 derivatives as well as the challenge to optimize conditions for the efficient intermolecular assembly between the different building blocks. Frequently, the self-organization of amphiphilic C60 derivatives onto nanostructures and/or microstructures renders rather hard and, typically, leads to amorphous materials. Therefore, developing simple procedures to generate 1D nanostructures, but circumventing any of the aforementioned drawbacks, constitutes a contemporary challenge.

Inspired by biomineralization38394041424344, we have devised a simple and general strategy towards 1D n/p-nanostructures4546474849 based on coating exTTF-based peptides fibers with electron accepting C60. We have focused on pristine C60 and n class="Chemical">phenyl-C61-butyric acid methyl ester (pan> class="Chemical">PCBM), as an n-type organic building block. Considering biomineralization, where peptide nanofibers govern the crystallization of inorganic crystals, we explored a similar role for exTTF-fibers. To this end, the latter serves as a template to guide the growth of C60 and PCBM over the exTTF-fibers. Incentivies for this work are based on the potential use of exTTF-fibers as seed to induce the growth of ordered C60 on their surfaces to achieve, ultimately, 1D n/p-nanostructures. To the best of our knowledge, this approach is currently largely unexplored.

Results and Discussion

We have used well-known exTTF-fibers (1)50, where exTTF is covalently linked to a pentapeptide (n class="Chemical">Ala-Gly-Ala-Gly-Ala-NH2) inpan> combinpan>ationpan> with pristinpan>e C60 as well as pan> class="Chemical">PCBM (Fig. 1) to explore the use of exTTF-fibers for the growth of C60/PCBM in the formation of 1D n/p-nanohybrids. Initially, we fabricated exTTF-fibers in 1,1,2,2-tetrachloroethane (TCE) followed by the addition of C60/PCBM (For more details about the formation of the exTTF-fibers see reference 50 and general methods). In this approach, two key effects are operative simultaneously. On one hand, strong non-covalent π-π interactions between C60 and exTTF promote the formation of 1D n/p-nanohybrids in which pristine C60/PCBM are assembled within exTTF-fiber’s nanostructure (see n/p-nanohybrid in Fig. 1A)51525354555657. On the other hand, the susceptibility of C60/PCBM to aggregate favours the cristalization over the nanohybrids (Fig. 1B). We reasoned that the combination of the earlier and the latter ensures an efficient interfacing between C60/PCBM and exTTF-fibers as a means of facilitating the growth of 1D n/p-nanohybrids. Our approach involves neither covalent chemical modifications nor optimization of the supramolecular self-assembly. We will demonstrate that a local supramolecular structure is achieved (Fig. 1A) and that nucleation of C60/PCBM crystals on the surface of nanohybrids (Fig. 1B) is followed. Analogous to peptide mineralization, the aforementioned accounts for the efficient interfacing between the different building blocks, that is, C60/PCBM and exTTF-fibers. To the best of our knowledge, the use of supramolecular nanostructures as seeds for ordering “electroactive molecules” such as C60/PCBM at the nanoscale is unprecedented.

Figure 1

Chemical structures and cartoon showing the coating process of the exTTF fibers by C60 and PCBM leading to the formation of 1D n/p nanohybrids (A).

The existence of C60 domains in the 1D n/p-nanohybrids triggers the oriented growth of crystal nuclei of C60/PCBM on top of them (B).

We have recently reported that exTTF-pentapeptide (1) in n class="Chemical">TCE solutionpan>s form complex networks of curved rope-like helical fibers with diameters ranpan>ginpan>g from 2 to 10 nm. Their overall organpan>izationpan> is drivenpan> exclusively by H-bonpan>ds (β-sheets) without π-π inpan>teractionpan>s betweenpan> exTTFs50. Room temperature absorptionpan> assays, which were carried out with 1 inpan> pan> class="Chemical">TCE in the presence of either pristine C60/PCBM upon 5 days of aging, gave rise to a depletion and a red-shifting of the initial exTTF-fiber absorptions from 449 to 456 nm together with an increase in the absorption in the region >470 nm. These features are tentatively assigned to a reflection of π-π interactions between exTTF and C6051525354. Results obtained with C60/PCBM are shown in Fig. 2.

Figure 2

Ultraviolet-visible (UV-Vis) and circular dichroism (CD) spectra for exTTF-fiber and n/p-nanohybrids.

(A) UV-Vis (upper part) and CD (lower part) spectra of 1 (grey line, 1.3 × 10−3 M) and the mixture of 1:C60 (black line, 1.3 × 10−3 M) in TCE following aging for 5 days. (B) UV-Vis (upper part) and CD (lower part) spectra of 1 (grey line, 1.3 × 10−3 M) and 1:PCBM (black line, 1.3 × 10−3 M) in TCE following aging for 5 days.

More pronounced were the changes noted in circular dichroism (n class="Disease">CD) assays, inpan> which the presenpan>ce of C60 resulted inpan> a positive Cottonpan> effect at 483 nm anpan>d a negative Cottonpan> effect at 439 nm (Fig. 2). Inpan> linpan>e with previous reports, the signpan>al at 483 nm is attributed to π-π inpan>teractinpan>g C60 anpan>d exTTF inpan> the new supramolecular helical arranpan>gemenpan>ts51525354. Anpan> additionpan>al Cottonpan> effect is seenpan> inpan> the 500–600 nm regionpan>, which is due to the fact that C60 is placed inpan> a chiral enpan>vironpan>menpan>t owinpan>g to the lack of absorbanpan>ce of the exTTF-fibers inpan> this spectral regionpan> (Fig. 2A, inpan>set). Similarly, pan> class="Disease">CD experiments with PCBM showed a positive Cotton effect at 480 nm and a negative Cotton effect at 432 nm, which matches the typical absorptions of C60 monoadducts (Fig. 2B). Likewise, the new broad negative Cotton effect in the 500–600 nm region is also observed (Fig. 2B, inset). In conclusion, our findings attest the formation of chiral n/p-nanohybrids formed from exTTF-fibers and C60/PCBM. Hereby, the organization of exTTFs within the resulting n/p-nanohybrids has enabled the integration of C60/PCBM inside the exTTF-fibers leading to an overall new helical arrangements.

As a complement, the formation of the n/p-nanohybrids was followed by Raman experiments with C60/n class="Chemical">PCBM as referenpan>ces anpan>d the respective n/p-nanpan>ohybrids formed from mixtures of 1 anpan>d C60/pan> class="Chemical">PCBM. For C60, the characteristic A(2) pentagonal pinch mode was observed at 1466 cm−136. In stark contrast, the A(2) mode gave rise in the n/p-nanohybrid to a slight shift towards lower frequencies, that is, 1461 cm−1. As such, interactions between exTTF-fibers and C60 are inferred. Similar trends were noticed for PCBM (Supplementary Fig. 1).

Powder X-ray diffraction (PXRD) shed further light onto changes in the internal structure of the exTTF-fibers when they are tightly interfaced with C60/n class="Chemical">PCBM. From PXRD with 1 anpan>d C60 a new set of lonpan>g ranpan>ge order with d-spacinpan>gs at 4.6, 2.3 anpan>d 1.54 nm was derived (Fig. 3B). Such a new set is associated with lamellar packinpan>g originpan>ated from exTTF-fibers withinpan> the newly formed n/p-nanpan>ohybrids. The lamellar patternpan> with a d-spacinpan>g of 4.6 nm relative to 3.6 nm for exTTF-fibers inpan>fers anpan> inpan>crease of about 1 nm which is attributed to the inpan>sertionpan> of C60 withinpan> the nanpan>ostructure as shownpan> inpan> Fig. 3A–B. Furthermore, two signpan>als with d-spacinpan>gs of 1.02 anpan>d 0.87 nm were observed (denpan>oted as hfull, Fig. 3B, inpan>set), which could be assignpan>ed to close packed columnpan>s of C60 inpan>tercpan> class="Chemical">alated between exTTFs (Fig. 3C)58596061626364. In the case of 1:PCBM, despite PXRD did not show a clear pattern as 1:C60, a broad signal with d-spacing of about 1 nm supports the notion of close packed columns of PCBMs (Supplementary Fig. 2). Please note that such a lamellar packing resembles that seen for blends based on semicrystalline polymers and C60 derivatives65.

Figure 3

Powder X-ray diffraction (PXRD) for exTTF-fibers, n/p-nanohybrids and schematic illustration of the molecular packing for n/p-nanohybrids.

(A) PXRD pattern for 1 obtained from a dispersion of exTTF-fibers in a mixture TCE:toluene. (B) PXRD pattern for 1:C60 nanohybrid. In the inset, “hfull” denotes the observed C60 reflections in the 2θ region between 8 and 11. (C) Cartoon showing the molecular packing observed for the exTTF-fibers and for the 1:C60 nanohybrid. The distances observed by PXRD experiments are remarked in the cartoon.

Additionally, as a complement to support the proposed model for the n/p-nanohybrids (Fig. 3C), Fourier transform infrared experiments (FTIR) were carried out to confirm that intermolecular β-sheets in the peptide backbone are retained within the internal structure of the nanohybrids. The results in both cases (1:C60/n class="Chemical">PCBM), showed anpan> inpan>tenpan>se pan> class="Chemical">amide I peak at around 1627 cm−1 together with a weak shoulder at around 1680 cm−1 thereby confirming the existence of β-sheets in an antiparallel mode (Supplementary Fig. 3).

The high tendency of C60 to aggregate by means of n class="Disease">van der Waals inpan>teractionpan>s is likely to favor the evolutionpan> from anpan> inpan>itial state, inpan> which close packed columnpan>s of C60 are inpan>tercpan> class="Chemical">alated between exTTF-fibers (Fig. 1A and Fig. 3C), into a final state, where the C60 crystalize onto n/p-nanhybrids (Fig. 1B). Implicit are 1D n/p-nanohybrids, in which donor and acceptor units are organized at the same length scale (Fig. 1 and Fig. 3C)46474849505152. It opens a promising avenue to guide the crystallization of C60 on top of exTTF-fibers and/or exTTF-gels to afford anisotropic C60 crystals. This comes, however, without the needs of using C60 derivatives covalently linked to functional substituents that would facilitate such growth61.

Considering the composition of the n/p-nanohybrid, that is, an electron donating exTTF and an electron accepting C60, we turned to pump probe experiments (Fig. 4). In reference experiments, exTTF, C60, and n class="Chemical">PCBM were probed inpan> solutionpan>-based experimenpan>ts. To this enpan>d, exTTF reveals uponpan> radiationpan> at 480 nm a short lived excited state spectrum with features that inpan>clude tranpan>sienpan>t maxima at 695 nm. From multi-wavelenpan>gth anpan>alyses a lifetime of 0.075 ± 0.015 ns was determinpan>ed, by which the photoexcited exTTF decays to the sinpan>glet grounpan>d state. Anpan> efficienpan>t seconpan>d order spinpan> couplinpan>g is responpan>sible for this fast process. Inpan> stark conpan>trast, for C60/pan> class="Chemical">PCBM a much slower deactivation – on the order of 1.5 ± 0.2 ns – of the singlet excited state transients is noted. Products are, however, the triplet excited states, which are formed in any of the cases nearly quantitatively. The most notable transient features of the singlet and triplet excited states are for C60 maxima at 965 and 750 nm, respectively. For PCBM, these maxima are shifted towards 960 and 690 nm.

Figure 4

Differential absorption spectra for the n/p-nanohybrids.

(A) Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (480 nm) of 1:C60 in TCE with several time delays between 0.1 and 6750.0 ps at room temperature. (B) Time-absorption profiles of the spectra at 530 and 700 nm, monitoring the charge separation/charge recombination. (C) Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (480 nm) of 1:PCBM in TCE with several time delays between 0 and 7000.0 ps at room temperature. (D) Time-absorption profiles of the spectra at 517 and 685 nm, monitoring the charge separation/charge recombination.

Pump probe experiments with the n/p-nanohybrid let immediately upon photoexcitation to differential absorption spectra, which are sound in agreement with the features of the C60 singlet excited state. These decay, however, in contrast to what has been seen in the reference experiments on the time scale of less than 100 ps and transform into a new species. The latter resembles with transient maxima at 700 and 1030 nm those seen upon the one electron oxidation of exTTF and the one electron reduction of n class="Chemical">PCBM, respectively. Inpan> the case of C60, the near-inpan>frared maximum evolved at arounpan>d 1080 nm. As such, we conpan>clude that photoexcitationpan> of the self-assembpan> class="Gene">led nanohybrids proceeds via a transient excited state into a charge separated state. A kinetic analysis across the visible and near-infrared assisted in gathering the rate constants for charge separation and charge recombination. In particular, for C60 and PCBM the values were 2.6 × 1010/1.1 × 109, 1.2 × 1011/8.1 × 108 s−1, respectively. Interesting is that in these cases only one long lived transient is discernable after the conclusion of the charge recombination, namely the triplet excited state of either C60 or its derivatives.

n class="Disease">AFM anpan>d TEM microscopic images provide valuable inpan>formationpan> not onpan>ly about the nanpan>ohybrid’s morphology, but also about the growth of crystallinpan>e nuclei onpan> the surface of the nanpan>ohybrids. To this enpan>d, a pan> class="Chemical">TCE solution containing exTTF-fibers and C60 was left at room temperature for 5 days. Afterwards, the mixture was drop-casted onto mica surfaces and air-dried. The AFM images revealed the presence of more straight fibers (Fig. 5A) relative to the initial exTTF-fibers50. AFM height profiles showed that the new fibers are higher (14–22 nm) than those obtained from only 1 (2–10 nm) (see insets in Fig. 5). Such an increase in size, points to the nanohybrid formation (Fig. 5A). A similar behavior was observed for the mixtures of exTTF-fibers and PCBM (Fig. 5B). Again, notable changes with respect to the height values and the morphology confirmed the integration of PCBM into the exTTF-fibers (Fig. 1A)51525354. From Fig. 5 we discern crystals of C60 (see arrow in Fig. 5A) and PCBM (see arrow in Fig. 5B), indicating the selective crystallization guided by the nanohybrid and triggered by the local saturation and high tendency of C60 and PCBM to aggregate (Fig. 1B).

Figure 5

Atomic Force Microscopy (AFM) images and height profiles of the n/p-nanohybrids.

AFM images and the corresponding height profiles of the nanohybrids (A) 1:C60 (2.6 × 10−4 M) and (B) 1:PCBM (2.6 × 10−4 M) showing the presence of nanohybrid fibers with an increase in height with respect to the initial exTTF-fibers. Arrows indicate C60 or PCBM domains over the fibers.

The aforementioned n class="Disease">AFM observationpan>s were further supported by TEM experiments carried out for both nanpan>ohybrids. Inpan> the case of C60, TEM images showed the presence of large straight fibers, inpan> which C60 crystal nuclei are discernpan>able (Fig. 6A–B, arrows). Inpan> a similar way to that observed inpan> biomimetic minpan>eralizationpan> processes, the crystals grow alonpan>g the fiber’s axis35. To shed light onpan> the formationpan> of C60 crystals over the fibers, HRTEM measurements were performed. It is clearly distinpan>guished C60 crystal lattices with a distanpan>ce arounpan>d 1.1 nm which is inpan> aggrement with the PXRD results (see Supplementary Fig. 4). Also SEM experiments corroborate the oriented C60 crystal growth exclusively onpan> fiber’s surfaces (see arrows inpan> Fig. 6D–E). Futhermore this tendency was also observed for pan> class="Chemical">PCBM based nanohybrid, where PCBM domains grow along the fiber’s axis (Fig. 6C,F).

Figure 6

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) for the n/p-nanohybrids.

(A–C) are TEM images of both nanohybrids (1:C60, 1.5 × 10−4 M, (A,B) and (1:PCBM, 1.5 × 10−4 M, (C), arrows remark the presence of C60 or PCBM domains along the fiber’s axis. Scale bars: 200 nm. (D–F) SEM images of both nanohybrids (1:C60, 4.3 × 10−4 M, (D,E) and (1:PCBM, 4.3 × 10−4 M, F), arrows show the lengthening of C60 and PCBM domains grown in the axial direction of the fibers.

In control experiments, the presence of exTTFs in the nanofibers was probed towards the oriented C60/n class="Chemical">PCBM crystal growth. As a matter of fact, fibers featurinpan>g the same penpan>tapeptide sequenpan>ce (pan> class="Chemical">Ala-Gly-Ala-Gly-Ala-NH2) linked, to a π-aromatic fluorene unit (2, for chemical structure see supplementary information Fig. 4) were employed45. Initially, we fabricated fluorene-nanofibers from TCE solutions of 2 and added C60. The mixture was aged for 5 days before they were characterized by SEM (see Supplementary Fig. 5). In this particular case, most C60 crystals evolved in domains that lack any fiber. This fact confirms the crucial role of exTTF in the nanofibers in terms of inducing crystallization of C60/PCBM.

Conclusions

In summary, the above described results point to the successful formation of 1D n/p-nanohybrids based on the efficient interfacing of exTTF-fibers with C60/n class="Chemical">PCBM, where electronpan> pan> class="Species">donor and electron acceptor are nanostructured at the same length scale. The efficient coating of exTTF-fibers by C60/PCBM promotes the guided crystallization of C60/PCBM on top of exTTF-nanofibers. In addition, fast charge separation and slow charge recombination in 1D n/p-nanohybrids afforded long lived transients, which attests electronic interactions between the electroactive species upon light irradiation. The results presented here contribute to the progress in the development of easy and general proceedings for building up 1D n/p-nanostructures, in which a better organization between electron donors and electron acceptors is achieved – a feature, which is highly demanded in the controlled construction of new efficient photo-active materials.

Methods

General methods

The synthesis of compound 1 was performed according to reference 50. n class="Chemical">1H-NMR anpan>d pan> class="Chemical">13C-NMR spectra were recorded on a Bruker Avance (700 MHz for 1H-NMR and 125 MHz for 13C-NMR). Unless otherwise indicated, chemical shifts (δ) are reported in ppm downfield from tetramethylsilane (TMS) at room temperature using deuterated solvents as internal standard. UV-Vis spectra were recorded in a UV-3600 Shimadzu Spectrophotometer. Circular Dichroism spectra were performed in a Jasco J-815 CD spectrometer at room temperature. Mass spectra by electrospray ionization (ESI) were recorded on a HP1100MSD spectrometer. Column chromatography was carried out on Merck silica gel 60 (70–230 mesh). Reagents were purchased from Sigma-Aldrich and were used without further purification. FTIR spectra were recorded on a Bruker Tensor 27 (ATR device) spectrometer. Raman spectra were acquired with a Renishaw inVia confocal Raman microscopy instrument, equipped with 532 nm on a glass microscope slide.

Powder X-ray Diffraction (PXRD)

X-ray diffraction was performed in a Panalytical X’Pert PRO diffractometer with Cu tube (lambda Kα = 1.54187 Å) operated at 45 kV, 40 mA, Ni beta filter, programmable divergence and anti-scatter slits working in fixed mode, and fast linear detector (X’Celerator) working in scanning mode. In the case of exTTF-fibers (1), the PXRD measurements were obtained from a dispersion of 1, which was prepared by dropping over n class="Chemical">toluene anpan> inpan>itial solutionpan> of 1 inpan> pan> class="Chemical">TCE. This dispersión was drop-casted over a silica support and air dried at room temperature for 24 h.

Transmission Electron Microscopy (TEM)

TEM images were performed using a JEOL 2000-FX electron microscope, operating at 200 kV accelerating voltage. The samples were prepared by drop-casting the respective n/p-nanohybrids (1.5 × 10−4 M) on a n class="Chemical">carbon film 200 square mesh pan> class="Chemical">copper grid (CF200-Cu). TEM images were obtained after drying the sample by air for 24 hours.

Atomic Force Microscopy (AFM)

n class="Disease">AFM images of the different nanpan>ohybrids were acquired unpan>der ambient conpan>ditionpan>s usinpan>g SPM Nanpan>oscope IIIa multimode workinpan>g onpan> tappinpan>g mode with a RTESPA tip (Veeco) at a workinpan>g frequency of ~235 Khz. The samples were prepared by drop-castinpan>g of the respective n/p-nanpan>hybrids solutionpan>s (2.6 × 10−4 M) onpan> a freshly cleaved pan> class="Chemical">mica surface and were dried under ambient conditions for 24 hours.

Scanning Electron Microspopy (SEM)

SEM images were acquired by using a JEOL JSM 6335F microscope working at 10 kV. The nanohybrids (4.3 × 10−4 M) were deposited by drop-casting on a n class="Chemical">silicon substrate, dried unpan>der ambient conpan>ditionpan>s anpan>d metallized with pan> class="Chemical">Au before observation.

Acquisition of Transient Absorption Spectroscopy

Differential transient absorption studies were performed with an amplified Ti/sapphire laser system (Model CPA Hybrid 2110, Clark-Max Inc., output: 775 nm, 1 kHz, and 150 fs pulse width). The 480 nm excitation pulses (200 nJ energy) were used as pump pulses and white light continuum as probe beam. The samples were placed in quartz cuvettes with 2 mm path length. Data aqusion was performed employing commercially-built spectrometer (Helios from Ultrafast systems).

General method for 1:C60 (or PCBM) nanohybrid preparation

The nanohybrids were prepared as follows: To a solution of compound 1 (1 mg/1 mL) in n class="Chemical">1,1,2,2-tetrachloroethane (pan> class="Chemical">TCE) was added one equivalent of C60 (or PCBM). In the case of C60, sonication for 5 minutes was performed in order to solubilize C60 completely. In the case of PCBM no sonication was required. These mixtures were aged at least for 5 days before exploring by microscopic techniques.

Additional Information

How to cite this article: Insuasty, A. et al. Supramolecular One-Dimensional n/p-Nanofibers. Sci. Rep. 5, 14154; doi: 10.1038/srep14154 (2015).