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Screening supramolecular interactions between carbon nanodots and porphyrins. Alejandro Cadranel, Volker Strauss, Johannes Margraf, Kim A. Winterfeld, Christoph Vogl, Luka #or#evi#, Francesca Pi Arcudi, Helen Hoelzel, Norbert Jux, Maurizio Prato, and Dirk M. Guldi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12434 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017
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Journal of the American Chemical Society
Screening supramolecular interactions between carbon nanodots and porphyrins. Alejandro Cadranel,a Volker Strauss,a Johannes T. Margraf,a Kim Winterfeld,a Christoph Vogl,a Luka Ðorđević,b Francesca Arcudi,b Helen Hoelzel,c Norbert Jux,c Maurizio Prato,b Dirk M. Guldi.a,* a
Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), FriedrichAlexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany. b
Department of Chemical and Pharmaceutical Sciences, INSTM UdR Trieste, University of Trieste. Via Licio Giorgieri 1, Trieste 34127, Italy. c
Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), FriedrichAlexander Universität Erlangen-Nürnberg, Henkestr. 42, 91054 Erlangen, Germany Supporting Information Placeholder ABSTRACT: We present an in-depth investigation regarding the electron accepting nature of pressure-synthesized carbon nanodots (pCNDs) in combination with porphyrins as excited state electron donors. To this end, electrostatic attractions involving negative charges, which are present on the pCND surface, are essential to govern the hybrid assembly, on one hand, and charge separation, on the other hand.
Central to multifunctional energy conversion devices is the 1 synergy of photosensitization and charge separation. Applicable materials should be cheap, non-toxic, and stable towards any environmental conditions. To this end, carbon nanodots (CND) constitute a promising class of photoactive 2,3 materials suitable for photovoltaic applications. Notably, 4 research in this field is still in its infancy. Nonetheless, once fundamental questions concerning synthetic controls, electronic fine-tuning, and long-term photostability have been unlocked, there is plenty of room for designing and imple5 menting CND-based future energy conversion systems. Their complex, partially random structure, offers different sites prone to interact supramolecularly and a palette of 6–10 possibilities for structural modifications. Such a structural versatility impedes, however, the understanding and sys5 tematic modification. In previous studies, we have shown that pressure synthesized CNDs (pCND) bear great potential as electroactive 11–13 building blocks in energy conversion systems. Undoubtedly, among their most interesting features is their bivalent 13 redox activity. For example, when assembled with perylenediimides (PDI) or single-walled carbon nanotubes (SWCNTs), pCNDs donate electrons upon photoexcitation. Charge-separated states in the corresponding electron donor-acceptor assemblies last for several hundred picoseconds. The mode of interactions is dominated by electrostatic forces between, for example, the negatively charged sur-
faces of pCNDs and positively charged PDIs or SWCNTs and is complemented by π-stacking forces.
Chart 1. Porphyrins studied in this work.
In the current work, we present a comprehensive investigation on the electron accepting nature of pCNDs in combination with porphyrins as excited state (ES) electron donors. Porphyrins were chosen by virtue of their light harvesting and electron donating features. Incentives for this work came from altering the driving forces for enabling assembly formation between pCNDs and porphyrins and from probing their potential as photoactive electron donor-acceptor assemblies. Several different pCND / porphyrin combinations were explored to evaluate the contributions stemming from different modes of interactions towards the resulting assemblies. All of the different porphyrins feature the same basic tetraphenyl substitution pattern (Scheme 1): positively 4+ 4+ charged porphyrins (ZnP , H2P ), neutral porphyrins (ZnP, 44H2P), negatively charged porphyrins (ZnP , H2P ), and, 8+ 8+ lastly, bulky, positively charged porphyrins (ZnP , H2P ). We expect to shed light onto the importance of electrostatic versus π-stacking forces in the electron donor-acceptor assemblies. Spectrophotometric absorption titrations were performed for the different porphyrins by adding variable amounts of pCNDs (Figures 1 and S1-S2). Major changes were, for exam4+ 4+ ple, observed in the absorption spectra of ZnP and H2P upon addition of pCNDs (Figures 1 and S1-S2). The Soret band absorptions decrease, which has previously been relat-
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ed to ground state (GS) interactions between pCNDs and 14 positively charged porphyrins. Additional red-shifts on the 4+ 4+ order of 4 nm for ZnP and 6 nm for H2P evolved. All of the aforementioned occur with the formation of isosbestic 4+ 4+ points at 424 / 451 and 411 / 432 nm for ZnP and H2P , respectively. At higher pCND concentrations, all isosbestic points, however, disappear due to the overlap with pCNDs centered absorptions. In contrast, no shifts were observed for neutral porphyrins (ZnP, H2P) and negatively charged 44porphyrins (ZnP , H2P ). For the bulky, positively charged 8+ porphyrins an intermediate scenario is found. For ZnP , a 3 nm red shift is accompanied by a decrease in Soret band absorption intensity and isosbestic points at 427 and 441 nm. 8+ For H2P , a 2 nm red shift is the only appreciable change. 4+ 4+ From a comparison between ZnP / H2P , ZnP / H2P, and 44ZnP / H2P we infer that GS interactions with pCNDs are mainly governed by electrostatic forces, although coordination and π-stacking forces cannot be ruled out to be at work. The latter is revealed in experiments with sterically bulky 8+ 8+ ZnP / H2P . Their bulky nature impedes π-stacking with pCNDs and leaves electrostatic forces as the only interac15 tions.
Of great relevance were the electrochemical experiments regarding the electron donating and accepting properties. In 4+ 4+ 8+ 8+ methanol, first oxidations of ZnP , H2P , ZnP and H2P were seen at +1.19, +1.26, +1.18, and +1.15 V vs Ag/AgCl, respectively (Figure S6). The first reduction of pCNDs evolved 17 at -0.62 vs Ag/AgCl. Considering the first reduction of pCNDs and the first oxidations of the different porphyrins the following charge separated (CS) state energies are calcu4+ 4+ lated: 1.81 (pCND•ZnP ), 1.88 (pCND•H2P ), 1.80 8+ 8+ (pCND•ZnP ) and 1.77 (pCND•H2P ) eV. To validate our CS state hypothesis, we turned to transient absorption spectroscopy. 420 nm (2.95 eV) was chosen as the pump source to selectively excite the Soret band absorptions of the different porphyrins. Reference experiments 4+ 4+ 8+ were conducted with solutions of ZnP , H2P , ZnP , 8+ H2P , and pCNDs, and complemented with solutions of 4418–20 ZnP, H2P, ZnP , H2P and their mixtures with pCNDs.
Further insights into ground-state interactions were gathered in electrochemical measurements with the strongest 4+ interacting assembly: pCND•H2P . The oxidation potential 4+ of H2P shifts anodically upon adding pCNDs (Figure S7). These results reveal an electron density donation from the porphyrin to the nanocarbon structure, which renders the 4+ oxidation of H2P more difficult. We conclude interactions in the GS in the form of a charge-transfer. Excited-state interactions were followed in steady-state fluorescence assays, where the porphyrins were selectively 4+ 4+ excited (Figures 1 and S1-S2). ZnP and H2P give rise to a steadily decreasing fluorescence. The latter transcend into a nearly quantitative fluorescence quenching of around 90% 44(Figure S5). For ZnP / H2P and ZnP / H2P , only subtle dependences of their fluorescence on the pCNDs concentration were derived (Figures 1 and S1-S2). Once again, the 8+ 8+ fluorescence assays with ZnP / H2P are best described as an intermediate scenario yielding a 40% quenching (Figure S5). All attempts to fit the I/I0 vs concentration dependences 16 to 1:1 models failed. This is consistent with the presence of electrostatic forces as the main driving force for the formation of supramolecular associates without a specific stoichiometry. Stern-Volmer plots allow, nevertheless, for a quantitative comparison. Linear fittings of the plots, as they are gathered in Figure 2, render Stern-Volmer constant val4+ 4+ 8+ ues of 0.48 (ZnP ), 0.27 (H2P ), 0.083 (ZnP ), 0.075 8+ 4(H2P ), 0.057 (ZnTPP), 0.037 (H2TPP), 0.017 (ZnP ), and 4-1 0.033 (H2P ) mg L. With the exception of the negatively charged porphyrins, the interactions with pCNDs are stronger for the metallated porphyrins than for the free base por4+ phyrins. KSV increase by 78% (pCND•ZnP versus 4+ pCND•H2P ), 54% (pCND•ZnP versus pCND•H2P), and 11% 8+ 8+ (pCND•ZnP versus pCND•H2P ). The impact is rather small in the case of bulky, positively charged porphyrins. When electrostatic interactions are suppressed, the overall associations are notably weakened. The corresponding association constants for ZnP / H2P with pCNDs are one order of 4+ 4+ magnitude smaller than those for ZnP / H2P . Repulsive 44forces control the interactions between ZnP / H2P and pCNDs.
Figure 1. Absorption (left) and emission (right) spectra of 4+ 8+ -6 ZnP and ZnP (blue, 2 × 10 M) during a titration with pCND (green to dark red) in methanol at room temperature. λex = 565 nm and 570 nm respectively. Final spectra correspond to [pCND] = 4 and 10 mg/L, respectively.
Figure 2. Stern-Volmer plots for the emission quenching of 4+ 4+ 8+ 8+ ZnP and H2P (upper left), ZnP and H2P (upper right), 44ZnP and H2P (bottom left), ZnP and H2P (bottom right) upon the addition of pCNDs in methanol at room temperature. The ES reactivity of pCNDs upon 387 nm (3.20 eV) illumi17 nation was already reported, and our results upon 420 nm (2.95 eV) excitation (Figure S8) are in good agreement with those published. In particular, following photoexcitation,
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Journal of the American Chemical Society core states of pCNDs are populated. The corresponding differential absorption changes include a minimum at 460 nm, a maximum at 520 nm, and a broad absorption in the 600 to 760 nm region. The ES deactivation of pCNDs occurs via different trap states with lifetimes of 6.1 and 71.9 ps– Table S1. From these trap states a 10 ns lived luminescence is 17 the consequence of trap population. Their differential absorption spectra are governed by a 560 nm minimum and < 500 nm as well as > 680 nm maxima. 4+
H2P displays the typical ES features of free base porphyrins in the presence of a small degree of aggregation (Figure S9). Differential absorption spectra and ES lifetimes of the individual components are gathered in Figure S8 and Table S1. In our model, upon excitation of the second singlet ES, ultrafast internal conversion to the first singlet ES takes place. This process is, however, not deconvolutable with the 200 fs resolution. The differential absorption spectra of the first singlet ES includes minima of the Q band absorptions at 520, 555, 594, and 660 nm, next to photoinduced absorptions at 460, 540, 573, 630, and 680 nm. The short singlet ES lifetime of 199.6 ps is assigned to the decay to the GS due to aggregation. Besides, intersystem crossing from the disaggregated porphyrins takes place with a lifetime of 3.6 ns. The accordingly formed triplet ES reveals features that are similar to those of the singlet ES: enhanced absorptions at 465 nm and in the region between 600 and 760 nm. GS recovery evolving from the triplet ES occurs in the microsec44+ ond timescale. H2P and H2P behave similarly to H2P and, as such, they were analyzed with the same model (Figures S11-S12 and Table S1). For example, H2P aggregates decay in 94 ps to the GS, while intersystem crossing to the microsec8+ ond-living triplet occurs in 7.8 ns. The bulky nature of H2P precludes aggregation and, in turn, only intersystem crossing 21 is observed (Figure S10). Table S1 lists all the derived time constants. Upon addition of pCNDs a change in the ES behavior of, 4+ 22 for example, H2P is noted – Figures 4 and S9. Photoexcitation results in population of an initial ES, whose spectral signatures appear red-shifted with respect to those of the 4+ singlet ES of H2P : 490, 555, 585, and 640 nm as well as a broad signal in the 680 – 715 nm region. Importantly, for 4+ H2P in the absence of pCND the 630 nm feature is far more 4+ intense than the 680 nm feature. In pCND•H2P , however, their relative intensities are reversed. Importantly, the 680 – 715 nm region is indicative for the formation of the one4+ electron oxidized form of H2P and, in turn, we ascribe the differential absorption changes to a CS state, that is, •4+ •+ (pCND) •(H2P ) . As recent spectroelectrochemical exper17 iments suggest, the fingerprint absorptions of the oneelectron reduced form of pCND are in the spectral range below 450 nm. This is, however, outside of the detection range of our experimental setup. The spectroscopic features •4+ •+ of the (pCND) •(H2P ) charge separated state are detectable as early as 0.4 ps. As such, its population is connected either directly or indirectly to the photoexcitation. In the earlier scenario, formation takes place instantaneously upon photoexcitation. In the latter scenario, formation of the singlet ES is followed by an ultrafast charge separation. The spectroscopic signatures of pCNDs appear at time delays of around 10 ps, but are absent at the earlier time delays. Taking the aforementioned into concert, the following target model (Figure 3), was developed: Light absorption leads to the pop-
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4+
+
ulation of a (pCND)δ •(H2P )δ charge transfer state (CT, black in Figure 3). Based on the multitude of electron ac13 cepting states in pCNDs, a thermodynamically driven charge separation follows. This branching affords the •4+ •+ (pCND) •(H2P ) CS state (CS, red), which lasts for 34.5 ps before decaying to the GS. In a parallel pathway, the 4+ + (pCND)δ •(H2P )δ charge transfer state depopulates by feeding a third ES (*pCND, blue). This intermediate state resembles that found in pCNDs. We infer a pCND centered ES, which within a few nanoseconds transforms into a fourth ES (**pCND, green). Notably, this model is also applicable to the transient absorption measurements conducted with 4+ pCND•ZnP (Figure S20). 8+
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In the case of pCND•H2P , evidence for the (pCND)δ 8+ + •(H2P )δ charge transfer state is only discernable after a few picoseconds (Figure S17). In other words, it must be formed indirectly rather than directly upon photoexcitation. Such a hypothesis is consistent with the steady-state measurements. These corroborate weaker interactions due to the bulkiness 8+ 4+ of H2P when compared to H2P . Hence, a different target model was applied (Figure S17): The instantaneously formed 8+ state is the H2P singlet ES (S2, black, Figure S17) and trans8+ + forms into the (pCND)δ •(H2P )δ charge transfer state within 500 fs. Characteristics of the latter are features around 8+ + 680 nm. This (pCND)δ •(H2P )δ charge transfer state deactivates in 6.4 ps in a bifurcated pathway. On one hand, •charge separation is observed to lead to the (pCND) 8+ •+ •(H2P ) CS state (CS, blue). After that, charge recombination leads to GS recovery in 37.3 ps – a timescale comparable 4+ to that observed for pCND•H2P . On the other hand, an ES 23 (*pCND, green) is formed, which is pCND centered.
Figure 3. Upper left: Differential absorption 3D map obtained upon femtosecond pump-probe experiments (λex = 4+ 420 nm) of pCNDpy•ZnP in methanol at room temperature. Upper right: Time absorption profiles (open circles) and corresponding fittings (solid lines). Bottom left: Normalized species associated differential spectra – CT: black, CS: red, *pCNDpy: blue. Bottom right: Target model proposed to fit the data. 8+
In pCND•ZnP , the features that corroborate the presence of the porphyrin radical cation also develop during the first few picoseconds (Figure S21). Global analyses revealed a time constant, which is on the order of hundreds of picoseconds, and, which was absent in any of the previously dis-
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cussed cases. We developed a model (Figure S21), which 8+ implied a ZnP centered ES as a product of direct photoexcitation. With a time constant of 700 fs, two different states with similar features > 680 nm are formed. Firstly, we noted •8+ •+ the signatures of the (pCND) •(ZnP ) CS state (CS, red in Figure S21), which decays with 16.6 ps. Secondly, the differential absorption changes of a third ES (S1, blue) resemble 8+ those of the singlet ES of ZnP . Its lifetimes is, however, as short as 310 ps. Therefore, we infer the occurrence of vibrational cooling to form the singlet ES, which intersystem crosses, and, which populates the corresponding triplet manifold (T, green). 8+
the Deutsche Forschungsgemeinschft (DFG) via SFB 952 “Synthetic Carbon Allotropes”.
REFERENCES (1) (2) (3) (4) (5)
8+
Notably, the behavior of pCND•ZnP and pCND•H2P differ as no unambiguous evidence for any intermediate 8+ + (pCND)δ •(ZnP )δ charge transfer state was detected. Similarly, no pCND based ESs are observed and the long-lived ESs are porphyrin-centered. We believe that this trend might result from the energetic difference between the Soret 8+ 8+ band maximum of H2P at 429 nm and of ZnP at 442 nm and the pump wavelength. 420 nm as excitation wavelength forms higher lying singlet ESs, which turn out to be vibra8+ 8+ tionally hotter for ZnP than for H2P . In conclusion, pCNDs and a diverse series of porphyrins have been integrated together into electron donor-acceptor assemblies. Detecting CS states upon photoexcitation underlines the electron accepting character of pCNDs in combination with electron donating porphyrins. Imperative for the electron donor-acceptor assembly formation and for the charge separation activation are electrostatic attractions involving negative charges on the pCND surfaces. Notably, charge recombination is ca. 6-times faster when compared with conjugates, in which nitrogen-doped carbon nanodot 24 (NCND) with smaller amounts of trap states, are used.
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ASSOCIATED CONTENT
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Supporting Information The Supporting Information is available free of charge on the ACS Publications website, including absorption/emission spectra and voltammograms of pCNDpy and pCND, absorp4+ tion/emission titrations of pCNDpy/ZnP and 4+ pCNDpy/H2P , electrochemical titration of ZnP/pCNDpy, and transient absorption experiments for pCNDpy, 4+ pCNDpy/ZnP and pCNDpy/H2P .
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AUTHOR INFORMATION
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Corresponding Author
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[email protected] (21)
ORCID
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Alejandro Cadranel: 0000-0002-6597-4397 Dirk M. Guldi: 0000-0002-3960-1765
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Notes The authors declare no competing financial interests. (24)
ACKNOWLEDGMENT AC thanks a DAAD-ALEARG postdoctoral fellowship and ALN. We acknowledge support by University of Trieste and
Delgado, J. L.; Bouit, P.-A.; Filippone, S.; Herranz, M. A.; Martín, N. Chem. Commun. 2010, 46 (27), 4853–4865. Roy, P.; Chen, P.-C.; Periasamy, A. P.; Chen, Y.-N.; Chang, H.-T. Mater. Today 2015, 18 (8), 447–458. Wang, Z.; Zeng, H.; Sun, L. J. Mater. Chem. C 2015, 3 (6), 1157–1165. Strauss, V.; Roth, A.; Sekita, M.; Guldi, D. M. Chem 2016, 1 (4), 531–556. Miao, P.; Han, K.; Tang, Y.; Wang, B.; Lin, T.; Cheng, W. Nanoscale 2015, 7 (5), 1586–1595. Tetsuka, H.; Nagoya, A.; Fukusumi, T.; Matsui, T. Adv. Mater. 2016, 28 (23), 4632–4638. Liu, J.; Zhao, S.; Li, C.; Yang, M.; Yang, Y.; Liu, Y.; Lifshitz, Y.; Lee, S.-T.; Kang, Z. Adv. Energy Mater. 2016, 6 (9), n/an/a. Arcudi, F.; Đorđević, L.; Prato, M. Angew. Chemie Int. Ed. 2016, 55 (6), 2107–2112. Kwon, W.; Do, S.; Kim, J.-H.; Seok Jeong, M.; Rhee, S.-W. Sci. Rep. 2015, 5 (February), 12604. Arcudi, F.; Dordevic, L.; Prato, M. Angew. Chemie - Int. Ed. 2017, 56 (15), 4170–4173. Strauss, V.; Margraf, J. T.; Clark, T.; Guldi, D. M. Chem. Sci. 2015, 6, 6878–6885. Strauss, V.; Margraf, J. T. T.; Dirian, K.; Syrgiannis, Z.; Prato, M.; Wessendorf, C.; Hirsch, A.; Clark, T.; Guldi, D. M. M. Angew. Chemie Int. Ed. 2015, 54 (28), 1–7. Strauss, V.; Margraf, J. T.; Dolle, C.; Butz, B.; Nacken, T. J.; Walter, J.; Bauer, W.; Peukert, W.; Spiecker, E.; Clark, T.; Guldi, D. M. J. Am. Chem. Soc. 2014, 136, 17308–17316. Stangel, C.; Schubert, C.; Kuhri, S.; Rotas, G.; Margraf, J. T.; Regulska, E.; Clark, T.; Torres, T.; Tagmatarchis, N.; Coutsolelos, A. G.; Guldi, D. M. Nanoscale 2015, 7 (6), 2597– 2608. Notably, despite higher positive charges in ZnP8+ / H2P8+ GS interactions with pCNDs are weaker than for ZnP4+ / H2P4+. Thordarson, P. Chem. Soc. Rev. 2011, 40 (3), 1305–1323. Strauss, V.; Kahnt, A.; Zolnhofer, E.; Meyer, K.; Maid, H.; Placht, C.; Bauer, W.; Nacken, T. J.; Peukert, W.; Etschel, S. H.; Halik, M.; Guldi, D. M. Adv. Funct. Mater. 2016, 26, 7975–7985. Global analysis using a sequential model was employed to derive rates and spectral shapes of the different intermediates. In cases of porphyrin aggregation or supramolecular interaction between pCNDs and the porphyrins, a target analysis was conducted including branching pathways. Van Stokkum, I. H. M.; Larsen, D. S.; Van Grondelle, R. Biochim. Biophys. Acta - Bioenerg. 2004, 1657 (2–3), 82–104. van Wilderen, L. J. G. W.; Lincoln, C. N.; van Thor, J. J. PLoS One 2011, 6 (3). The metallated porphyrins behave like their free base porphyrins (Figures S13-S16). The ES dynamics of ZnP, H2P, ZnP4-, and H2P4- are not significantly altered by the presence of pCNDs (Figures S18S19 and S22-S23). The (pCND)•-•(H2P8+)•+ charge separated state also presents marked features around 680 nm. Finally, the pCNDcentered state presents broad features and deactivates in the nanosecond timescale. Arcudi, F.; Strauss, V.; Đorđević, L.; Cadranel, A.; Guldi, D. M.; Prato, M. Angew. Chemie - Int. Ed. 2017, 56 (40), 12097– 12101.
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