Understanding Charge-Transfer Characteristics in Crystalline

Article pubs.acs.org/JACS

Understanding Charge-Transfer Characteristics in Crystalline Nanosheets of Fullerene/(Metallo)porphyrin Cocrystals Bingzhe Wang,†,§ Shushu Zheng,‡,§ Avishek Saha,† Lipiao Bao,‡ Xing Lu,*,‡ and Dirk M. Guldi*,† †

Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials, Friedrich-Alexander University Erlangen-Nürnberg, Egerlandstrasse 3, Erlangen 91058, Germany ‡ State Key Laboratory of Materials Processing, School of Material Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China S Supporting Information *

ABSTRACT: Cocrystals in the form of crystalline nanosheets comprised of C70 and (metallo)porphyrins were prepared by using the liquid−liquid interfacial precipitation (LLIP) method where full control over the morphologies in the C70/(metallo)porphyrins nanosheets has been accomplished by changing the solvent and the relative molar ratio of fullerene to (metallo)porphyrin. Importantly, the synergy of integrating C70 and (metallo)porphyrins as electron acceptors and donors, respectively, into nanosheets is substantiated in the form of a near-infrared charge-transfer absorption. The presence of the latter, as reflection of ground-state electron donor−acceptor interactions in the nanosheets, in which a sizable redistribution of charge density from the electron-donating (metallo)porphyrins to the electron-accepting C70 occurs, leads to a quantitative quenching of the localized (metallo)porphyrin fluorescence. Going beyond the ground-state characterization, excited-state electron donor−acceptor interactions are the preclusion to a full charge transfer featuring formation of a radical ion pair state, that is, the one-electron reduced fullerene and the one-electron oxidized (metallo)porphyrin.

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INTRODUCTION Designing, synthesizing, and probing nanostructured molecular materials with advanced light-harvesting and electron-transfer features are one of the fastest growing research areas. Inspired by natural photosynthesis,1−6 researchers have taken incentives in the area of solar energy conversion and photo(electro)catalysis, electronic devices, sensing, and switching applications.7−9 To achieve higher order architectures, distinct molecular building blocks have been designed through molecular modification and assembled through supramolecular forces.10−15 This enables fine-tuning, on one hand, and optimizing, on the other hand, intermolecular interactions built around van der Waals, hydrogen bonds, electrostatic interactions, π−π stacking, metal−ligand coordination, etc.16−27 Among the many photo- and redoxactive building blocks studied to this date, fullerenes and (metallo)porphyrins stand out as unique electron acceptors and donors, respectively. To realize the full potential of mixed fullerene/(metallo)porphyrin systems in the emerging fields of optoelectronics, molecular wires, bioenergetics, and solar cell transducers, the construction of self-assembled electron donor−acceptor cocrystals remains elusive and an intense research area ever since their initial discovery.28,29 Cocrystals of fullerene complexes featuring amines, hydrocarbons, and sulfides were described in early publications.30−34 In the resulting electron donor−acceptor systems, a wide range of physical properties, including metallic, photoconducting, magnetic, and even superconductive, have been established.35−39 Important for the self-assembly of fullerene © 2017 American Chemical Society

nanoarchitectures are, however, charge-transfer (CT) interactions.40−42 Sizeable intermolecular CT interactions have been corroborated when using electron-donating ferrocenes, (metallo)porphyrins, porphyrazines, tetrathiafulvalenes, etc., by means of characteristic ground-state absorption features.43−48 For synthesizing the complexes of C60, C70, C60O, and C120O with metal free tetraphenylporphyrin as well as with nickel(II), cobalt(II), and zinc(II) tetraphenyl, or octaethyl, porphyrinates, the evaporation method constitutes an early milestone in the field.49−51 Over the years, the preparation of fullerene/ (metallo)porphyrin cocrystals has been optimized, and numerous products have been analyzed in terms of structure, etc.52−56 Yet, inherent defects and lack of control have imposed limits to this methodology. Hasobe and co-workers have introduced a cetyltrimethylammonium bromide (CTAB)-based method to prepare uniform metalloporphyrin hexagonal nanorods, in which the fullerenes are encapsulated along the central axis. Although the resulting nanorods are noncocrystalline, they have been integrated into solar energy conversion schemes.57,58 Little or almost no attention has been directed toward the realization of uniform cocrystals with controllable morphologies in stark contrast to the full-fledged understanding of fullerene and (metallo)porphyrins single crystals.59−78 Likewise, detailed studies regarding electron transfer interactions are scare. Received: June 14, 2017 Published: July 7, 2017 10578

DOI: 10.1021/jacs.7b06162 J. Am. Chem. Soc. 2017, 139, 10578−10584

Article

Journal of the American Chemical Society

Femtosecond transient absorption spectra were obtained with a Ti:sapphire laser system CPA-2101 (Clark-MXR), Inc.) in combination with a Helios TAPPS-transient absorption pump probe spectroscopy detection unit from Ultrafast Inc. The initial laser output wavelength is 775 nm with a pulse width of 150 fs and 1 kHz repetition. The excitation wavelength was generated using NOPAnoncollinear optical parametric amplifier. Transient absorption spectra were measured in the solid state by squeezing the samples in the middle of two quartz glass sheets. Finally, spectra were acquired with a HELIOS (Ultrafast Systems) transient absorption spectrometer.

Herein, we report on the design and characterization of fullerene/(metallo)porphyrin cocrystals using the liquid−liquid interfacial precipitation (LLIP) method79,80 enabling full control over their morphology. Our findings demonstrate sizable ground-state interactions resulting in a shift of charge density from the electron-donating (metallo)porphyrins to the electron-accepting fullerenes. This is rounded off by a full charge separation in the excited state.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Liquid−liquid interfacial precipitation (LLIP) method, in which 5,10,15,20-tetraphenyl-21H,23H-porphine (H 2 TPP) and 5,10,15,20-tetraphenyl-21H,23H-porphine zinc(II) (ZnTPP) as well as C70 are used, was applied to grow C70/(metallo)porphyrin cocrystals. Different C70/(metallo)porphyrin molar ratios and different alcohols, that is, isopropanol, ethanol, and methanol, as antisolvents for precipitation were probed to examine the impact on the morphologies and structures of the C70/(metallo)porphyrin cocrystals. In line with the aforementioned, the cocrystals are designated as C70-H2TPP-1, C70H2TPP-2, C70-H2TPP-3, and C70-H2TPP-4, and C70-ZnTPP-1, C70-ZnTPP-2, C70-ZnTPP-3, and C70-ZnTPP-4; see the Experimental Section for more details. Next, we investigate the morphologies and structures of the corresponding C70/ (metallo)porphyrin cocrystals using different microscopic characterization techniques (Figures 1, 2, S2, and S3).

Materials. Fullerene (C70) was synthesized using a direct current arc discharge method and isolated by high-performance liquid chromatography (HPLC). 5,10,15,20-Tetraphenyl-21H,23H-porphine (H2TPP, >98.0%) and 5,10,15,20-tetraphenyl-21H,23H-porphine zinc(II) (ZnTPP, >98.0%) were purchased from Tokyo Chemical Industry Co., Ltd. Toluene, o-xylene, methanol, ethanol, and isopropanol were available from Sinopharm Chemical Reagent Co., Ltd. Preparation of C70/(Metallo)porphyrin Cocrystals. The C70/ (metallo)porphyrin cocrystals were prepared using the liquid−liquid interfacial precipitation (LLIP) method. Two types of porphyrins of H2TPP and ZnTPP were employed for cocrystallization with C70. Typically, a mixed solution of fullerene C70 and (metallo)porphyrins was prepared by dissolving pristine C70 powder and H2TPP or C70 and ZnTPP in toluene or o-xylene, respectively, with ultrasonication for 45 min. After filtration, the C70/H2TPP (3 mL) or C70/ZnTPP (1.5 mL) solution was mixed with alcohol antisolvent (3 mL) and left overnight at room temperature for precipitation. In our experiments, different kinds of alcohols (methanol, ethanol, and isopropanol) and different initial C70/(metallo)porphyrins molar ratios (1:1 and 1:2) were applied to examine their effects on the morphologies and the properties of the final C70/(metallo)porphyrin cocrystals. The experimental details and the corresponding sample numbers are given in Table 1. We also prepared C70 nanorods for comparison (Figure S1): C70 nanorods were obtained from a solvent system of C70/toluene using the LLIP method.

Table 1. Sample Information samples

(metallo) porphyrin

molar ratio C70/(metallo) porphyrin

antisolvent type

C70-H2TPP-1 C70-H2TPP-2 C70-H2TPP-3 C70-H2TPP-4 C70-ZnTPP-1 C70-ZnTPP-2 C70-ZnTPP-3 C70-ZnTPP-4

H2TPP H2TPP H2TPP H2TPP ZnTPP ZnTPP ZnTPP ZnTPP

1:1 1:2 1:2 1:2 1:1 1:2 1:1 1:1

isopropanol isopropanol ethanol methanol isopropanol isopropanol ethanol methanol

Figure 1. Morphological and structural characterizations of crystalline C70-ZnTPP nanosheets. SEM images for C70-ZnTPP-1 (a) and C70ZnTPP-2 (b); HRTEM images for C70-ZnTPP-1 (c); the inset shows the corresponding TEM image of a single C70-ZnTPP-1 nanosheet; STEM image and STEM mapping images for C70-ZnTPP-1 (d); the scale bars represent 200 nm.

Characterization. The morphologies of the as-obtained C70/ (metallo)porphyrin cocrystals were examined using a Nova NanoSEM 450 field-effect scanning electron microscope (FE-SEM) operating at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), scanning tunneling microscopy (STEM), and STEM mapping images were obtained from a probe-corrected Titan ChemiSTEM (FEI, U.S.) at an acceleration voltage of 200 kV. Powder X-ray diffraction (PXRD) measurements were conducted on Empyrean with Cu Kα source (λ = 1.542 Å) at 40 kV and 40 mA. Single-crystal X-ray diffraction (SXRD) measurements were performed at 173 K on a Bruker D8 QUEST machine equipped with a CMOS camera (Bruker AXS Inc., Germany). UV−vis−NIR spectra were measured on a PE Lambda 750S, using the films composed of C70/(metallo)porphyrin cocrystals. Photoluminescence spectra were obtained from a Bruker VERTEX 70 (samples were excited using a green laser at 514.5 nm with 0.05 mW power).

We observed that the common feature of all C70/(metallo)porphyrin cocrystals is their nanosheet morphology (Figures 1a,b, 2a,b, S2, and S3). Despite their similar morphology, different sizes were derived under different experimental conditions of porphyrin, antisolvent types, and initial molecular ratios. As far as the C70/ZnTPP nanosheets are concerned, from the SEM results, for C70-ZnTPP-1 the average length, width, and thickness are measured as 13.5, 3.0, and 0.6 μm, 10579


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Figure 2. Morphological and structural characterizations of crystalline C70-H2TPP nanosheets. SEM images for C70-H2TPP-1 (a) and C70H2TPP-2 (b); HRTEM images for C70-H2TPP-2 (c); the inset shows the corresponding TEM image of single C70-H2TPP-2 nanosheet; STEM image and STEM mapping images for C70-H2TPP-2 (d); the scale bars represent 200 nm.

Figure 3. SXRD results for C70-ZnTPP-1 (a,b) and PXRD patterns for the C70/(metallo)porphyrin nanosheets (c). A view of the C70·ZnTPP structure revealing C70 and its two nearest ZnTPPs (a) and stacking pattern viewed along the a axis for C70·ZnTPP·2.5C8H10 (b); o-xylene (C8H10) is omitted for the sake of clarification.

respectively, while for C70-ZnTPP-2 the corresponding values are 2.2, 0.8, and 0.3 μm, respectively. For C70/H2TPP nanosheets, the average lengths are 90.0 and 45.0 μm in the cases of C70-H2TPP-1 and C70-H2TPP-2, respectively, while the average width length and thickness are 14.0 and 1.6 μm for C70H2TPP-1 as well as 2.8 and 0.6 μm for C70-H2TPP-2. The above results document that factors such as the porphyrin, the C70/(metallo)porphyrin molar ratio, and the antisolvent influence formation of the nanosheets. Noteworthy, the C70/ (metallo)porphyrin nanosheets are all present as parallelogrammic shapes with angles of about 57.0° between them. HRTEM results further reveal the almost same lattice distances along the same direction for C70-ZnTPP-1 of 1.41 nm (Figure 1c) and for C70-H2TPP-2 of 1.36 nm (Figure 2c). STEM mappings as illustrated in Figures 1d and 2d were conducted to distinguish between the location of fullerenes and (metallo)porphyrins within the nanosheets. Results therefrom provide evidence for the uniform C70 and either ZnTPP or H2TPP distribution throughout the final products and, in turn, substantiation for the similar cocrystallization at the liquid− liquid interface. On the basis of our microscopic characterizations, it is reasonable to conclude that C70 and ZnTPP/ H2TPP undergo similar assembly processes despite different experimental conditions, because of the same stacking mode between the ellipsoidal C70 and planar-like (metallo)porphyrins to form the final parallelogrammic nanosheets. Single-crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) measurements were carried out to determine the molecular packing in the C70/(metallo)porphyrin nanosheets. SXRD results for C70-ZnTPP-1 (Figure 3a−c) demonstrate a triclinic structure (P1̅ space group) with a = 14.46 Å, b = 16.45 Å, c = 17.85 Å, α = 72.68°, β = 87.14°, and γ = 80.26° and a relative fullerene to (metallo)porphyrin ratio of 1:1; see crystallographic data Table S1. The molecular organization within the nanosheets is shown in Figure 3a−c. The PXRD pattern for C70-ZnTPP-1 (Figure 3c) is in good agreement with the SXRD results and shows distinct peaks at

6.43° and 7.67°, which are indexed as (011) and (110), respectively. Moreover, the peak at 6.43° corresponds to a dspacing of 1.38 ± 0.02 nm. A value of 1.38 nm is consistent with the lattice distances derived from the HRTEM images of C70-ZnTPP-1 and prompts a crystal growth along [010], Figure 1c. Interestingly, all C70/(metallo)porphyrin nanosheets possess almost the same PXRD patterns (Figures 3c and S4a,b), which reveal the lack of diffraction peaks stemming from original C70 or (metallo)porphyrin (Figure S4c). Considering similar stacking modes between C70 and ZnTPP or between C70 and H2TPP, all of the final nanosheets share a packing pattern, which is nearly identical to that of C70-ZnTPP1, Figure 3a,b. Next, we performed absorption and fluorescence measurements in the steady-state mode to investigate intermolecular interactions between C70 and the (metallo)porphyrins within the nanosheets. In Figures 4a, the absorption spectra of the C70/(metallo)porphyrin nanosheets are compared to that of C70 nanorods, all in the solid state. In contrast to the C70 nanorods, new absorption peaks appear around 900 nm for C70/ZnTPP nanosheets and at around 800 nm for C70/H2TPP nanosheets (Figure S5). In line with previous studies, these are attributed to charge-transfer (CT) transitions between the electron-donating (metallo)porphyrins and the electron-accepting C70.43−47 Notably, the Soret- and Q-band absorptions of the (metallo)porphyrins are rather weak in the C70/(metallo)porphyrin nanosheets. When turning to the fluorescence spectra, a nearly quantitative quenching of the (metallo)porphyrin features is discernible in the C70/(metallo)porphyrin nanosheets. From the aforementioned results, we conclude that the evolution of a CT state activates a new deactivation channel for the singlet excited state of the (metallo)porphyrins. The latter outperforms the conventional deactivations such as 10580


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Figure 4. Room-temperature absorption spectra (a) and fluorescence spectra (b) of C70 nanorods, (metallo)porphyrins, and C70/(metallo)porphyrin nanosheets deposited on glass substrates upon 514.5 nm laser excitation with a power of 0.05 mW.

Figure 5. Differential absorption spectra (a) obtained upon femtosecond pump probe experiments (532 nm and 500 nJ) of C70-ZnTPP1 nanosheets with time delays between 1 and 400 ps; see figure legend for details. Time absorption profiles and corresponding fits (b) at 440, 581, 650, and 1380 nm monitoring the characteristic absorption features of the ZnTPP ground state, one-electron oxidized form of ZnTPP, and one-electron reduced form of C70, respectively.

radiative and nonradiative processes to the ground or triplet excited state. To shed light onto the nature of the singlet excited-state deactivation, that is, charge separation and charge recombination, time-resolved transient absorption spectra of the C70/ (metallo)porphyrin nanosheets in the solid state were recorded upon femtosecond excitation at 532 nm. An example, C70ZnTPP-1, is given in Figure 5, while the remaining cases are presented in Figures S6−11. As shown, upon photoexcitation, a weak signal appears immediately around 580 nm, which is likely to be due to the singlet excited-state absorption of the (metallo)porphyrins. Because of the weak nature of these features, reasonable lifetimes could only be derived for C70H2TPP-1 and C70-H2TPP-2 as 1.22 ± 0.45 and 1.70 ± 0.15 ps, respectively. Notably, C70-ZnTPP-2, which was prepared with larger ZnTPP to fullerene ratios, shows longer lifetimes. The existence of different structures in sample C70-ZnTPP-2, for example, capsular or interlaced structures, in which fullerenes and/or (metallo)porphyrins are aggregated separately in small domains, should not be ruled out.57,58 Hand in hand with the decay of the singlet excited-state absorption feature of the (metallo)porphyrins, maxima at 480/650 and 1380 nm as well as minima at 440 and 550 nm developed. Importantly, the maxima correlate with the formation of the one-electron oxidized form of the (metallo)porphyrins and the one-electron reduced form of C70.81−84 The minima are a reflection of the ground-state bleaching of the Soret- and Q-band absorptions, which are centered on the (metallo)porphyrins, respectively. Taken the aforementioned into concert, we postulate the rapid formation of a radical ion pair state arising from the singlet

excited state of the (metallo)porphyrins. This finding is in sound agreement with the quantitative quenching seen in the fluorescence spectra. By following the time evolution of the C70 and (metallo)porphyrin centered radical ion pair state features, we note that their decay is biphasic: For example, in C70ZnTPP-1 the two lifetimes are 9.72 ± 3.50 and 173.18 ± 60.66 ps with focus on the one-electron reduced form of C70, while lifetimes of 7.11 ± 2.59 and 116.00 ± 19.16 ps were derived for the one-electron oxidized form of the (metallo)porphyrins. Different from the liquid, in the nanosheets, C70 and the (metallo)porphyrins are arranged in different geometries with face-to-face and face-to-edge arrangements as shown in Figure 3b. It is postulated that charge recombination occurs not only between the two nearest (metallo)porphyrins and C70, which are face-to-face arranged with Zn−C distances of 2.887 and 2.895 Å, but also between the two more remote (metallo)porphyrins and C70 with Zn−C distances of 10.655 and 11.774 Å. An illustration is given in Figure 6. The lifetimes are summarized in Tables S2 and S3 for the corresponding C70/ metalloporphyrins and C70/porphyrins, respectively. Apparently, the type of antisolvents has no dramatic impact on the electron transfer rates. However, the type of (metallo)porphyrins plays a critical role on both charge separation or recombination process. In short, larger driving forces in the C70/(metallo)porphyrin nanosheets, as displayed in Figure 7, evoke larger charge separation and recombination rate constants. Plotting the charge recombination rate constants 10581


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Figure 6. Energy level diagram (a) for the C70/(metallo)porphyrin nanosheets and schematic illustration (b) of the two charge recombination processes. CS, charge separation; CR, charge recombination.

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CONCLUSIONS In this study, we document the preparation of a novel family of uniform C70/(metallo)porphyrin cocrystals yielding crystalline nanosheets. Changing the nature of the (metallo)porphyrin, the relative C70 to (metallo)porphyrin ratio, and the antisolvent brings full control over the sizes and shapes of the resulting nanosheets. With the help of SEM, HRTEM with element analysis, and XRD, morphologies as well as crystal structures are corroborated. From single X-ray crystallography studies, we conclude that the relative molecular ratio between the fullerene and (metallo)porphyrin is 1:1. Steady-state absorption and fluorescence spectroscopies point to strong mutual interactions in the form of a CT in the final nanosheets. Complementary transient absorption measurements confirm rapid charge separation leading to the formation of the one-electron reduced form of C70 and the one-electron oxidized form of the (metallo)porphyrins. Interestingly, we note contributions from face-to-face and face-to-edge arranged C70/(metallo)porphyrin affording two distinctly different lifetimes for the radical ion pair states. The type of solvents and the ratios between fullerenes and (metallo)porphyrins lack, however, any notable impact on either charge separation or recombination. It is the type of (metallo)porphyrin that makes the difference, possibly due to differences in the overall driving forces. As a complement, we determined the attenuation factors β. Our work offers new perspectives for the design, preparation, and study of electron donor−acceptor architectures of high cocrystallinity.

Figure 7. Porphyrin species dependence of charge separation rate constants (a) and distance dependence of charge recombination rate constants (b) in C70-ZnTPP-1, C70-ZnTPP-4, C70-H2TPP-2, and C70H2TPP-3 at room temperature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b06162. Additional SEM images, XRD, absorption, emission, crystallographic, and transient absorption results of C70/ (metallo)porphyrin nanosheets (PDF)

versus the relative Zn−C distances results in dependencies, from which we determined attenuation factors (β) of 0.42 ± 0.13 Å−1 for C70-ZnTPP-1, 0.33 ± 0.05 Å−1 for C70-ZnTPP-2, 0.36 ± 0.09 Å−1 for C70-ZnTPP-4, 0.38 ± 0.28 Å−1 for C70H2TPP-1, 0.31 ± 0.08 Å−1 for C70-H2TPP-2, 0.34 ± 0.14 Å−1 for C70-H2TPP-3, and 0.36 ± 0.26 Å−1 for C70-H2TPP-4.85−88 In light of the latter, we conclude that the charge-transfer proficiencies in the different C70/(metallo)porphyrin nanosheets are comparable.

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Avishek Saha: 0000-0001-8514-0878 10582


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Xing Lu: 0000-0003-2741-8733 Dirk M. Guldi: 0000-0002-3960-1765 Author Contributions §

B.W. and S.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Deutsche Forschungsgemeinschaft (DFG) as part of SFB 953 “Synthetic Carbon Allotropes”, The National Thousand Talents Program of China, NSFC (51472095, 51602112, and 51672093), Program for Changjiang Scholars and Innovative Research Team in University (IRT1014), and Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, is gratefully acknowledged. B.W. acknowledges a fellowship from the Chinese Scholarship Council.

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