Visible-Light Photoinduced Electron Transfer Promoted by Cucurbit[8

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Visible-Light-Induced Electron Transfer Promoted by Cucurbit[8]uril-Enhanced Charge Transfer Interaction: Towards Improved Activity of Photocatalysis Yang Jiao, Jiangfei Xu, Zhiqiang Wang, and Xi Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 13 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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ACS Applied Materials & Interfaces

Visible-Light-Induced Electron Transfer Promoted by Cucurbit[8]urilEnhanced Charge Transfer Interaction: Towards Improved Activity of Photocatalysis Yang Jiao,† Jiang-Fei Xu,†Zhiqiang Wang,†and Xi Zhang*,† † Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China *Email: [email protected]

Keywords: photo-induced electron transfer, charge transfer interaction, host-guest interaction, photoredoxcatalysis, energy levels

Abstract Visible-light-induced electron transfer (Vis-PET) is highly important for optoelectronic devices and photoredox catalysis. Herein, we propose a supramolecular strategy to promote Vis-PET process by charge transfer (CT) interactions. As a proof of concept, a molecular system containing 1,5-alkoxy-substituted naphthalene and viologen moieties were designed to form CT complex in water. The HOMO/LUMO energy gap was lowered by CT interaction, which turned on Vis-PET process to generate viologen radical cations. Moreover, when CT interaction was enhanced by cucubit[8]uril, the efficiency of Vis-PET process wasfurther promoted and the required irradiation wavelength could be further red-shifted by 100 nm. The Vis-PET system exhibited an improved activity of photocatalysis, as supported by the fast photo-reduction of Cytochrome c. This study represents a facile supramolecular way to fabricate radicals with maintained activity under mild conditions, which holds potential to enrich the scope of visible-light photoredox catalysis by the rational utilization of CT systems.

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Introduction Photo-induced electron transfer (PET), as an effective process to generate organic radicals, plays essential roles in the fields of photochemistry and materials science,1-4 particularly in optoelectronic devices, photoredox catalysis and photo-initiated polymerization.5-10 Compared with energetic UV light, visible light possessing natural abundance and few side effects is highly desirable and emerging as a mild and eco-friendly power source.11-14Over the past decades, the addition of photosensitizers (transition-metal complexes, inorganic nanoparticles, etc.)15-20as energy mediators has provided a feasible method to achieve visible-light-induced electron transfer (Vis-PET). Moreover, if there exists a supramolecular way to tune the energy levels by dynamic noncovalent interactions,21-24 one may establish a new route for the construction of metal-free and purely organic Vis-PET systems with tunable, reversible and adaptive properties. In this way, organic radicals with high activity can be fabricated under mild conditions, which may facilitate radical reactions. Charge transfer (CT) interactions are one of the most widely studied noncovalent interactions in supramolecular chemistry.25-31 Through the weak association between an electron-rich moiety (donor, D) and an electron-deficient moiety (acceptor, A), CT interactions lead to the hybridization and modification of the HOMO and LUMO energy levels of both D and A units.32-34In this regard, the formation of CT complex has the potential to narrow the energy gap into the visible-light energy region. This research aims to explore if appropriate CT interactions could be employed to induce Vis-PET processes. Moreover, to overcome the limits of the low binding constants of CT interactions (100–103 M−1),29,

35

host-guest

interactions would be incorporated to enhance the CT interactions,36-39 which may further improve the efficiency of Vis-PET and lower the required energy of the irradiation light. Herein, we propose a supramolecular strategy to achieve and promote the Vis-PET process based on host-enhanced CT interaction in aqueous solution. To this end, a molecular system


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containing one 1,5-alkoxy-substituted naphthalene unit in the middle and two viologen units on both sides (NAPMV) was designed and synthesized (Scheme 1a). When dissolved in water, NAPMV formed intramolecular CT complex, which may be favorable to collect the energy of visible light and generate viologen radical cations via Vis-PET in the presence of triethanolamine (TEOA, sacrificial electron donor).40-41 In addition, cucurbit[8]uril (CB[8]) was introduced into this system to enhance the CT interaction between the naphthalene and viologen units, and in this way the stronger CT interaction would improve the efficiency of Vis-PET. Therefore, we anticipated that the Vis-PET process could be promoted stepwise by increasing the strength of the CT interaction (Scheme 1c).

Scheme 1. a) Chemical structures of the molecules used in this work. b) The photo-induced electron transfer from TEOA (donor) to viologen (acceptor) to form viologen radical cation. c) The Vis-PET process can be promoted stepwise by increasing the strength of the CT interaction.



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Results and Discussion To confirm the formation of the intramolecular CT complex, UV/Vis absorption, fluorescence emission and 1H NMR spectroscopy of NAPMV aqueous solution were carried out and compared with the control compounds, MEV and NAPH. It was clear that neither MEV nor NAPH absorbed above 400 nm, while in the case of NAPMV, a characteristic CT absorption band between 400 nm and 600 nm was observed.28 The NAPMV solution exhibited a deep yellow color, entirely different from the colorless solutions of MEV and NAPH. As indicated by 1H NMR, compared with MEV and NAPH, both of the naphthalene protons and the viologen protons in NAPMV underwent upfield resonance shifts, further confirming the existence of the CT interaction (Figure S2a). The CT absorbance at 432 nm showed a linear dependence on the concentration of NAPMV below 0.7 mM (Figure S1), indicating the intramolecular CT complexation between the naphthalene and viologen units.42-43 Therefore, 0.5 mM of NAPMV solution (1.0 mM for the viologen units) was chosen as the experimental condition to explore the Vis-PET process to ensure the conformation of the intramolecular CT foldamer. The introduction of cucurbiturils with different sizes had opposite effects on the NAPMV CT complex.44-45CB[7] with a smaller cavity can incorporate only one guest molecule, and CB[8] with a larger cavity can incorporate two guest molecules. As shown in Figure 1a and 1c, the NAPMV/(CB[7])2 solution exhibited a slight color and a decrease of the CT absorption band, which provided evidence for weakening CT interaction. In contrast, the NAPMV/CB[8] solution showed a purple color and a bathochromic CT band between 450 nm and 750 nm, indicating a stronger CT interaction. Figure 1b showed the recovery of the fluorescence of the naphthalene chromophore after the introduction of CB[7], but the further quenched florescence after the introduction of CB[8]. This phenomenon further revealed the CT


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interaction was weakened by CB[7] but enhanced by CB[8]. By 1H NMR spectroscopy, the different host-guest complexations between CBs and NAPMV were further confirmed (Figure S2b and S2c) and illustrated in Figure 1d. CB[7] formed 1:1 binary complexes with viologen with high affinity,46-47 thus resulting in the unfolding of the CT complex; while CB[8] with a larger cavity tended to form 1:1:1 ternary complex with naphthalene and viologen moieties, by which the CT interaction was reinforced due to the restriction of D and A units in a limited space.36,

48

Therefore, CB[8] is a suitable supramolecular host for enhancing the CT

interaction in NAPMV.

Figure 1.a) UV/Vis absorption spectra of MEV, NAPH, NAPMV, NAPMV/(CB[7])2 and NAPMV/CB[8] aqueous solutions. b) Fluorescence emission spectra of NAPMV, NAPMV/(CB[7])2 and NAPMV/CB[8] aqueous solutions. c) The photographs of the series of solutions to show their colors. For all the solutions, the concentration of viologen units was controlled as 1.0 mM. d) Schematic representation of the opposite effects of CB[7] and CB[8] on the NAPMV CT complex.



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To study the effect of CT interaction on the Vis-PET process, we performed the white-light (λ > 400 nm) irradiation of MEV and NAPMV solution comparatively, and UV/Vis absorption spectroscopy was used to monitor the generation of viologen radical cations. As shown in Figure 2a, in the case of MEV, little change in absorption was observed after irradiation. However, in the case of NAPMV, the absorption bands peaked at 396 nm and 603 nm, which are characteristic for the viologen radical cation,49-50 increased gradually along with the irradiation (Figure 2b). Moreover, electron paramagnetic resonance (EPR) spectroscopy was carried out to verify the existence of radicals. After the irradiation of NAPMV, a typical EPR signal with g = 2.0028 was observed, representing solid evidence for the generation of viologen radical cations,51-52 whereas the EPR spectrum of MEV showed no signal throughout the irradiation. Therefore, we can conclude that the intramolecular CT interaction turns on the Vis-PETprocess.

Figure 2.UV/Vis absorption spectroscopy was used to monitor the generation of viologen radical cations during the white-light (λ > 400 nm) irradiation of a) MEVand b) NAPMV. c) EPR spectra of MEV and NAPMV after the white-light irradiation.For both solutions, the concentration of viologen units was controlled as 1.0 mM: 1.0 mM for MEV and 0.5 mM for NAPMV.


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We then explored whether the Vis-PET process could be further promoted by CB[8] since the introduction of CB[8] can enhance the CT interaction in NAPMV. To answer this question, the white-light (λ > 400 nm) irradiation of NAPMV/CB[8] was carried out and compared with NAPMV under the same conditions. By determining the molar absorption coefficient of the viologen radical cation (see the detailed calculation process in Supporting Information), we obtained the relationships between irradiation time and radical concentrations for different systems in Figure 3. Throughout the irradiation process of the NAPMV/CB[8] system, the amount of the viologen radical cations increased continuously to reach a concentration as high as 155 µM, 2.4 times higher than NAPMV (65 µM) and 129 times higher than MEV (1.2 µM). It is believed that CB[8] plays a dual role in this promotion. On the one hand, the hostenhanced CT complex led to a broader absorption in the region of visible light and facilitated the formation of excited state, which made an appreciable contribution to the improved VisPET efficiency. On the other hand, CB[8] was reported to encapsulate two viologen radical cations to form a stabilized radical dimer53-54 as the final product, pushing the chemical equilibrium to a higher conversion.

Figure 3. Time-dependent radical concentrations during the white-light (λ > 400 nm) irradiation of MEV (), NAPMV (◆) and NAPMV/CB[8] ().For all the solutions, the concentration of viologen units was controlled as 1.0 mM: 1.0 mM for MEV, 0.5 mM for NAPMV and 0.5 mM for NAPMV/CB[8].



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Besides the improved radical yield under the white-light condition, we expected that the NAPMV/CB[8] system could apply to a milder light source. For this aim, the green-light (λ ~ 550 nm) irradiation of NAPMV and NAPMV/CB[8] was performed comparatively. As shown in Figure 4, there was almost no generation of viologen radical cations during the green-light irradiation of NAPMV because of the weak absorption around 550 nm. However, for NAPMV/CB[8], an effective PET process was driven by the green light, leading to a considerable radical yield. This dramatic difference can be ascribed to the bathochromic absorption of the host-enhanced CT complex. The absorbance around 550 nm was greatly increased to collect the photon energy of green light sufficiently, followed by effective electron transfer. Therefore, by enhancing the CT interaction with CB[8], a milder light source (green light) other than white light can also induce the PET process. Therefore, we conclude that the required irradiation wavelength can be further red-shifted by 100 nm in the case of CB[8]-enhanced CT interaction.

Figure 4. Time-dependent radical concentrations during the green-light (λ ~ 550 nm) irradiation of NAPMV ( ◆ ) and NAPMV/CB[8] ().For both solutions, the concentration of viologen units was controlled as 1.0 mM: 0.5 mM for NAPMV and 0.5 mM for NAPMV/CB[8].


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To elucidate the effect of CT interaction on the Vis-PET process systematically, we estimated the HOMO and LUMO energy levels of MEV, NAPMV and NAPMV/CB[8]. The onset reduction potentials (Eonset, red) were measured by cyclic voltammetry (CV). As an example in Figure 5a, MEV underwent two one-electron reduction steps, where the first Eonset, red is -0.645 V corresponding to the formation of MEV radical cation. In addition, the HOMO/LUMO energy gap (∆E) was evaluated to be 4.19 eV from the data of UV/Vis absorption. Combining these data, the HOMO and LUMO energy levels of MEV were calculated to be -7.93 eV and 3.74 eV, respectively (Figure 5b and 5c).23, 55-56

Figure 5. a) Cyclic voltammogram (CV) of MEV aqueous solution, with Ag/AgCl as the reference electrode. b) Calculation of the HOMO and LUMO energy levels of MEV. c) The summarized HOMO and LUMO energy levels of MEV, NAPMV and NAPMV/CB[8].For all the solutions, the concentration of viologen units was controlled as 1.0 mM: 1.0 mM for MEV, 0.5 mM for NAPMV and 0.5 mM for NAPMV/CB[8].

Compared with MEV, the intramolecular CT interaction in NAPMV raises the HOMO energy level significantly and thus narrows the energy gap to match the energy of visible light, which is responsible for turning on the Vis-PET process. Moreover, when CB[8] is associated with 9 ACS Paragon Plus Environment


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NAPMV, the HOMO energy level can be further raised from -5.94 eV to -5.49 eV, as a result of host-enhanced CT interaction. It should be noted that the HOMO energy levels of both NAPMV and NAPMV/CB[8], even if raised by CT complexation, are still lower than that of TEOA (-4.99 eV),41 which guarantees the spontaneous photo-induced electron transfer processes. Interestingly, the LUMO energy levels show little change after the CT complexation, which suggests that the viologen radical cation generated from NAPMV or NAPMV/CB[8] possesses as high energy as the MEV radical cation. In other words, the different kinds of radical cations have the same activities, regardless of the involvement of CT interaction. On occasion of radical-mediated reactions or polymerizations where the activity of radicals is essential,57-59 the employment of CT interaction may be a feasible way to fabricate radicals under mild conditions while keep their activity. To test the activity of the viologen radical cations formed by Vis-PET, we tried to employ the series of viologen derivatives to conduct a typical photo-reduction process under visible light. Cytochrome c (Cytc) was selected as a model substrate molecule. As a small redox protein involved in the mitochondrial electron transfer chain, Cytc contains a heme group whose central iron atom can be reduced from the Fe(III) to the Fe(II) state.60-62 It was reported that the viologen radical cations could reduce Cytc,63-64 so the kinetic investigation of the reduction process can be a reflection of the activity of the photo-generated viologen radical cations. To this end, MEV, NAPMV or NAPMV/CB[8] respectively, was utilized as the photocatalyst to reduce Cytc under white-light (λ > 400 nm) irradiation. UV/Vis absorption spectroscopy was used to monitor the reduction kinetics. As shown in Figure 6a, along with the photoreduction, the absorption band with a maximum at 548 nm increased gradually, indicating the transformation from Fe(III)-state Cytc to Fe(II)-state Cytc. From the time-conversion relationships in Figure 6b, we found that with the catalysis by MEV, Cytc was reduced very slowly, resulting from the trace amount of viologen radical cations formed by visible-light 10 ACS Paragon Plus Environment

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irradiation. However, when NAPMV served as the catalyst, a significant promotion of the photo-reduction was observed. Moreover, the introduction of CB[8] further accelerated this process. Fitted in terms of first-order reaction, the apparent rate constant in the case of NAPMV/CB[8] was determined to be 0.625 min-1, which was 4.3 times faster than that of NAPMV (0.144 min-1) and 31 times larger than that of MEV (0.0204 min-1). Therefore, we can conclude that the high activity of photo-generated radicals is maintained after the introduction of CT interaction. The enhanced photo-reduction benefits from the improved Vis-PET process by increasing the strength of CT interaction.

Figure 6. a) UV/Vis absorption spectroscopy was used to monitor the photo-reduction process of Cytc during the white-light (λ > 400 nm) irradiation. NAPMV served as the photocatalyst. b) Photo-reduction kinetics of Cytc catalyzed by MEV (), NAPMV ( ◆ ) and NAPMV/CB[8] ().For the series of photocatalysts, the concentration of viologen units was controlled as 1.0 mM: 1.0 mM for MEV, 0.5 mM for NAPMV and 0.5 mM for NAPMV/CB[8]. The concentration of Cytc was 2.0 mM.



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Conclusions In conclusion, we have demonstrated an effective Vis-PET process manipulated by CT interaction in aqueous solution. Through the CT complexation, the HOMO/LUMO energy gap of NAPMV was greatly narrowed to acquire photons of visible light and induced electron transfer reactions. Moreover, by the introduction of CB[8], the host-enhanced CT interaction further promoted the Vis-PET process. This line of research has developed a facile method to fabricate radicals with maintained activity under mild conditions, as supported by the fast visible-light-induced photo-reduction of Cyt c. Considering that CT interactions are relatively universal and well understood, the rational design and utilization of CT systems are of great potential to enrich the scope of visible-light photoredox catalysis and photo-initiated polymerization.

Supporting Information Synthesis of NAPMV; 1H NMR spectra of CT complex; the concentration-dependent UV/Vis spectra of NAPMV aqueous solutions; the thermodynamic analysis of the intramolecular CT interaction; the detailed kinetic analysis of the PET process;the calculation process of HOMO and LUMO energy levels.

Acknowledgements This work was financially supported by the National Basic Research Program (2013CB834502) and the Foundation for Innovative Research Groups of NSFC (21421064). We are grateful to Ms. Xixi Liang (Tsinghua University) for her help on the EPR measurements, and Dr. Kai Liu (Stanford University), Mr. Qiao Song and Mr. Bohan Tang


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(Tsinghua University) for their beneficial discussions. We also thank Prof. Charl F. J. Faul (University of Bristol) for his kind help in polishing the English.

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Visible-Light Photoinduced Electron Transfer Promoted by Cucurbit[8

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