Optoelectronics of Organic Nanofibers Formed by Co‐Assembly of Porphyrin and Perylenediimide (2014)

Organic nanofibers are formed by simple ionic co-assembly of positively charged porphyrin (electron donor) and negatively charged perylenediimide (electron acceptor) derivatives in aqueous solution. Two kinds of electron transfer routes between electron donor and electron acceptor under light excitation in nanofibers are confirmed by DFT calculations and experimental data. One-dimensional organic structures are very important due to their potential applications in the fields of electronics, photonics, light-energy conversion, and catalysis.1-8 Out of the many methods to prepare such one-dimensional organic structures, self-assembly is one of the most efficient strategies.9 Numerous organic nanofibers have been achieved through the self-assembly of functional molecules, such as porphyrins and perylenetetracarboxylicdiimides (PTCDI) from solution.1, 4, 5 Porphyrins constitute the ideal building blocks for organic nanostructures due to their attractive electronic, optical, and catalytic properties.10-20 For instance, Shelnutt et al. have reported the preparation of uniform porphyrin nanotubes through the ionic self-assembly of positively and negatively-charged porphyrins.3, 21, 22 On the other hand, PTCDI is an n-type organic semiconductor which has attracted extensive attention over the past decades.4, 11, 23 Recently, one-dimensional nano- to micro- structures and related devices based on PTCDI have been reported.5, 24-27 In order to combine the advantages of porphyrin and PTCDI, complicated porphyrin-PTCDI dyads, triads and even porphyrin-PTCDI4 type functional molecules have been synthesized.11, 28-30 The results demonstrate that when an electron donor component and an electron acceptor component are present in a single molecule, the dyes will display higher charge separation efficiencies under light excitation. However, the covalent synthesis of such complex molecules is highly inefficient and expensive. Fortunately, such complicated multistep covalent synthesis can be avoided33-35 by utilizing the concept of supramolecular chemistry; hierarchical super-structures can be achieved from relatively simple building blocks through weak and non-covalent intermolecular interactions, such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π–π stacking, and electrostatic interactions.31, 32 The employment of noncovalent multicomponent co-assembly may allow us to obtain the desired molecular aggregates more cheaply and efficiently than through covalent synthesis.36 The photo induced electron transfer (PET) from electron donor to electron acceptor is extremely important in many research fields, such as that of optoelectronics, dye sensitized solar cells, and photosynthesis.37-43 For example, electron transfer through the charge transfer chain during the solar excitation of pigments constitutes an essential step in the photosynthesis. In this system, both intramolecular and intermolecular PETs play important roles in achieving efficient charge separation and subsequently photosynthesis.44 To investigate the PET process, porphyrin and PTCDI have frequently been used as model donor-acceptor functional molecules.11, 29, 30, 45-47 However, the study of intermolecular PET is mostly neglected; current research focuses mostly on the intramolecular PET between the covalently connected donor and acceptor components. To remedy this problem, we report the formation of nanofibers from the two-component co-assembly of positively-charged PTCDI derivative and negatively-charged porphyrin through electrostatic interactions. This enables us to observe the intermolecular PET from porphyrin (electron donor) to PTCDI (electron acceptor) via either light excited singlet porphyrin or light excited singlet PTCDI in the obtained ionic co-assembled nanofibers. Herein, two molecules, 5,10,15,20-tetrakis-(4-sulfonatopheneyl)-21,23H-porphyrin (TPPS) and N,N-bis(4-pyridylmethylene)-perylene-3,4,9,10-tetracarboxylic diimide (BPM-PTCDI) (Scheme 1A), were chosen as precursors for the preparation of the ionic co-assembled nanofibers. The choice of these two molecules was compelled by two reasons, the first of which is the matching of energy levels between the selected pairs. The energy levels of both molecules were calculated by the density functional theory (DFT) method using Gaussian03 software; the obtained LUMO and HOMO orbitals can be seen in Scheme 1A. The LUMO energy levels are at –3.01 eV and –3.67 eV versus vacuum for TPPS and BPM-PTCDI, respectively, while the corresponding HOMO energy levels are at –5.72 eV and -6.19 eV, respectively. From this, it can be deduced that the energy gap is about 2.71 eV for TPPS and 2.52 eV for BPM-PTCDI. This result is largely similar to the data estimated from the absorption spectra, which are 2.69 eV and 2.23 eV for TPPS and BPM-PTCDI, respectively. As the LUMO of TPPS is higher than that of BPM-PTCDI, electron transfer from the LUMO of the excited singlet TPPS (Por*) to the LUMO of BMP-PTCDI can occur (Scheme 1B) when the electron is excited from the HOMO of TPPS by light. At the same time, as the HOMO of TPPS is higher than that of BPM-PTCDI, electron transfer from the HOMO of TPPS to the HOMO of the excited singlet BPM-PTCDI (Per*) is also thermodynamically favourable when BPM-PTCDI is excited by light. It is possible that such electron transfer from the electron donor to the electron acceptor may beneficial to the optoelectronic response of the obtained nanofibers. The second reason is the ionic charges of the molecules; TPPS contains four negative charges while BPM-PTCDI possesses two positive charges, making them suitable candidates as the molecular building blocks for ionic co-assembly. It is hypothesized that each TPPS molecule will interact with two BPM-PTCDI molecules to form a sandwich-type complex through electrostatic interaction, and the stacking of these complexes through π-π stacking will result in the formation of one-dimensional aggregates (Scheme 1C). Considering the molecular structure of the precursors, the molar ratio of porphyrin to perylenediimide was kept at 1:2 during the experimental process. The nanofibers formed spontaneously as a red precipitate when the red solution of positively-charged BPM-PTCDI was mixed with the green solution of negatively-charged TPPS. SEM images revealed that the precipitate comprised uniform nanofibers with high aspect ratio (Figure 1A) and the nanofiber width was about 20 nm while the length extended to tens of micro­meters. TEM analysis further confirmed the uniform nanofiber morphology that was resultant from the co-assembly of negatively-charged TPPS and positively-charged BPM-PTCDI through electrostatic interactions (Figure 1B). Both components, TPPS and BPM-PTCDI, are equally important in order for the fiber formation to occur, while SEM images of the cast film from only TPPS or BMP-PTCDI solution display no such uniform nanofiber morphology (Figure 1D and 1E). As a control experiment, when a positively-charged porphyrin, 5,10,15,20-Tetrakis-(N-methyl-4-pyridyl)-21,23H-porphine tetratosylate (TMPyP), was mixed with the positively-charged BPM-PTCDI, the morphology of the obtained aggregates comprised a leaf-like structure (Figure 1C) instead of the desired nanofiber. In addition, the size of the leaf-like structures was not uniform, with the length ranging from about 100 nm to about 1 μm and the width ranging from several tens of nanometers to several hundreds of nanometers. The sharp difference between the morphologies of both aggregates confirmed that electrostatic interaction21 between the positively-charged BPM-PTCDI molecules and the negatively-charged TPPS was essential for nanofiber formation. In order to clarify the molecular interactions in the self-assembly process, UV-vis spectroscopy was employed to investigate the self-assembly behaviour of TPPS and BPM-PTCDI in solution, as shown in Figure 2. When BPM-PTCDI was added to the solution of TPPS, significant changes could be observed in the UV-vis spectra. The peaks at 434 nm and 643 nm, which could be attributed to the Soret band and Q band of porphyrin,48 respectively, demonstrated an obvious decrease with the addition of BPM-PTCDI. At the same time, a broad band located between 450 and 600 nm appeared and increased with the addition of BPM-PTCDI (Figure 2A), which could be attributed to the absorbance of BPM-PTCDI.29, 49 The increase in band intensity is due to the increase in BPM-PTCDI concentration, which resulted in an increase in absorbance. On the contrary, the Soret band of TMPyP did not change with addition of BPM-PTCDI, although the broad absorption band of BPM-PTCDI appeared and increased with increasing BPM-PTCDI concentration (Figure 2B). These experiments clearly illustrated the difference between the self-assembly behaviours of both systems. Strong π–π stacking interactions existed between the TPPS and BPM-PTCDI molecules but not between the TMPyP and BPM-PTCDI molecules upon mixing in solutions. This is because the negatively-charged TPPS would be able to assemble with the positively-charged BPM-PTCDI through attractive electrostatic interactions, but the positively-charged TMPyP would avoid the positively-charged BPM-PTCDI due to repulsive electrostatic forces, thus hindering π–π stacking. In order to conduct a stoichiometric study of the ionic co-assembly, the absorbance at 434 nm was plotted against the molar ratio of BPM-PTCDI to the total of the two dyes. It could be seen from the plot (inset of Figure 2A) that the absorbance decreased with increasing BPM-PTCDI molar ratio in the low molar ratio range, e.g. from 0.2 to 0.6. When the molar ratio is higher than 0.6, the absorbance ceased to decrease with increasing molar ratio. Careful observation of the plot revealed an inflection point at the molar ratio of about 0.64, which was very close to 2/3, meaning that every TPPS molecule interacted with two BPM-PTCDI molecules in the process of ionic co-assembly. This could be explained by considering the molecular structure of the two functional molecules; TPPS contained four negative-charge positions while BPM-PTCDI contained two positive-charge positions, hence the ratio of TPPS to BPM-PTCDI should be 1:2 in order to maintain charge balance. To substantiate the aforementioned hypothesis, significant differences were also observed in the PL spectra when TPPS and TMPyP were separately mixed with BPM-PTCDI. From the PL spectra of TPPS with BPM-PTCDI (Figure 1C), an obvious PL peak can be seen at 660 nm, which could be attributed to the luminescence of TPPS.45 The intensity of the 660 nm peak decreased with increasing BPM-PTCDI, which indicated that the fluorescence of TPPS was quenched by the BPM-PTCDI molecules. A very weak signal at around 545 nm could also be detected at high molar ratios of BPM-PTCDI, which could be attributed to the luminescence of BPM-PTCDI.44 On the other hand, no luminescence quenching was observed when TMPyP was mixed with BPM-PTCDI (Figure 1D). The PL signals, with maximum intensity at 653 nm, could be attributed to the luminescence of TMPyP, the intensity of which does not change with increasing BPM-PTCDI. At the same time, strong fluorescence signals with maximum intensity at 545 nm and obvious shoulder at 587 nm, which could be attributed to the emission from BPM-PTCDI, appeared and increased with increasing addition of BPM-PTCDI. Comparison between the PL spectra of the two mixtures demonstrated that the fluorescence of both TPPS and BPM-PTCDI was quenched upon mixing, while the fluorescence of both TMPyP and BPM-PTCDI remained despite mixing. From this, it could be deduced that PET played an important role in the mechanism of fluorescence quenching. The mechanism of fluorescence quenching in TPPS involved an electron transfer from the LUMO of Por* to the LUMO of BMP-PTCDI ground state, which was similar to that observed in covalently connected porphyrin perylenediimide systems.45 At the same time, electron transfer from the HOMO of ground state TPPS to the HOMO of Per* resulted in the fluorescence quenching of BPM-PTCDI (Scheme 1B).50 Although the resultant charge separation states of both PET processes were the same, the mechanisms were vastly different (Scheme 1B). Firstly, the PET processes were achieved via different excited states, one via Por* and the other via Per*, which led to the fluorescence quenching of TPPS and BPM-PTCDI, respectively. From the fluorescence quenching of both TPPS and BPM-PTCDI, it could be concluded that both types of PET processes occurred simultaneously. As PET from a donor to an acceptor could only occur when both components were sufficiently close to each other,50 fluorescence quenching could not be observed in the case of TMPyP and BPM-PTCDI as electrostatic repulsion would have kept both components apart from each other. From the above observation, it could be deduced that electrostatic interactions imposed a great influence on the PET process from porphyrin to perylenediimide as well as the morphology of the aggregates. Specifically, although a PET process existed in the system of TPPS and BPM-PTCDI, no such PET process could be found in the system of TMPyP and BPM-PTCDI system. At the same time, the morphology of the former system comprised uniform nanofibers of high aspect ratios, while the latter consisted of leaf-like structures. It is common knowledge that PET is beneficial for charge separation, which would manifest as optoelectronic property in the system. Herein, a three-electrode system in Na2SO4 solution was employed to measure the optoelectronic property of the obtained nanofibers. Unfortunately, this method was not suitable for the measurement of optoelectronic response from the TMPyP and BPM-PTCDI film due to the dissolution of TMPyP. In Figure 3, the optoelectronic performance of the cast film from TPPS and BPM-PTCDI was compared with that from BPM-PTCDI only. From the C-V curve (Figure 3A), it could be observed that the current density in the light was significantly higher than that in the dark, which indicated that the optoelectronic response of nanofibers from TPPS and BPM-PTCDI was significantly higher than that of the BPM-PTCDI film. The maximum on-off ratio under the potential of a 0.4 V vs Ag/AgCl electrode reached about 12 for the nanofibers, but was only about 5 for the BPM-PTCDI film. The I-t curve under chopped light irradiation (Figure 3B) displayed stable and repeatable photo-current, which implied the stability of the nanofibers under electrochemical conditions. Meanwhile, the current density obtained from the nanofibers was more than two times higher than that from the BPM-PTCDI film. As the molecular mass of BPM-PTCDI was smaller than that of TPPS, the real optoelectronic response of each molecule within the nanofibers was much higher than that of the BPM-PTCDI film. In conclusion, we have obtained uniform nanofibers with high aspect ratio through the simple solution-based co-assembly of two components, which are donor and acceptor functional molecules. Photoinduced electron transfer from the donor to the acceptor via excited singlet state was observed in the obtained nanofibers. Electrochemical measurement also demonstrated that the obtained nanofibers exhibited very stable and repeatable optoelectronic responses. This work shows the potential of supramolecular materials for energy harvesting, photo-detection, and even nanoelectronics. Materials: 5,10,15,20-tetrakis-(4-sulfonatopheneyl)-21,23H-porphyrin (TPPS) and 5,10,15,20-Tetrakis-(N-methyl-4-pyridyl)-21,23H-porphine tetratosylate (TMPyP) were purchased from TCI (Japan). All other chemicals were from Sigma Aldrich and used as received. Synthesis of N,N-bis(4-pyridylmethylene)-perylene-3,4,9,10-tetracarboxylic diimide (BPM-PTCDI): BPM-PTCDI was synthesized by the imidation of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) with 4-aminomethyl pyridine. In brief, 0.392 g PTCDA and 0.216 g 4-aminomethyl pyridine were dispersed in 30 mL quinoline, and 0.02 g zinc acetate was added as the catalyst. The mixture was heated at 160 °C under stirring for 24 h. After the mixture was cooled to room temperature, the solid product was filtered and collected. The crude product was then dispersed in 100 mL 0.1 M NaOH and stirred for 1h to remove any unreacted PTCDA. Finally, the solid product was further purified through dissolutionin formic acid and precipitation by the addition of 0.1 M NaOH. After filtration and drying in vacuum, 0.385 g dark red solid was obtained (yield = 51%): 1H NMR (400 MHz, DCOOD, 25 °C, TMS): d = 5.27 (s, 4H; methylene), 7.35 (d, 4H; pyridyl), 7.62 (d, 4H; perylene), 7.93 (d, 4H; perylene), 8.69(d, 4H; pyridyl); MS (MALDI-TOF): m/z: 572.45(M+); Calcd for C36H20N4O4 (m/z): 572.15. Preparation of BPM-PTCDI/TPPS Nanofibers: the nanofibers were prepared by ionic self-assembly of the positively-charged BPM-PTCDI and negatively-charged TPPS. For example, 0.2 mL of 1 mM BMP-PTCDI solution in formic acid and 0.1 mL of 1 mM TPPS aqueous solution were added to 2 mL water. After several minutes, the nanofibers were formed as a brown precipitate. Measurement of Photo Activity: the nanofiber dispersion or BPM-PTCDI formic acid solution was first drop-casted onto the FTO surface and dried in air at 60 °C; the resultant FTO was used as a work electrode, Ag/AgCl as the reference electrode, platinum wire as the counter electrode and 0.5 M Na2SO4 as the electrolyte for C-V and I-t curves measurement. A CHI 660D electrochemical station coupled with a 300 W Xe lamp was used to measure the photo-response of the sample. In order to compare the photo current quantitatively, the density of both the films were controlled at about 0.1 mg/cm2 by varying the deposition times. Materials Characterization: the morphology and microstructure of the obtained samples were investigated by FESEM (JEOL JSM-7600F) and TEM (JEOL JEM-2010). The UV-visible spectra were obtained from a UV-Vis spectrophotometer (Shimadzu UV-2550). The PL spectra were recorded from a spectrofluorophotometer (Shimazu RF 5301). Molecular Orbital Calculations: the molecular structures of BPM-PTCDI and TPPS were optimized by applying density functional theory (DFT) with the hybrid B3LYP functional and 6–31G (d) basis set. With the optimized structure, calculations on the electronic ground states of BPM-PTCDI and TPPS were processed at the same theoretical level. All calculations were performed by using Gaussian03 software. The figures were generated using the corresponding GaussView software. The authors thank the financial support from the Singapore National Research Foundation (the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) Programme and NRF-RF2009–04). Y.L. acknowledges the support of National Natural Science Foundation of China (21103134).