Control of Electron Flow Direction in Photoexcited Cycloplatinated

Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 3819−3824

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Control of Electron Flow Direction in Photoexcited Cycloplatinated Complex Containing Conjugated Polymer−Single-Walled Carbon Nanotube Hybrids Wenjuan Xiong,†,§ Lili Du,†,§ Kin Cheung Lo,† Haiting Shi,† Tomohisa Takaya,‡ Koichi Iwata,‡ Wai Kin Chan,*,† and David Lee Phillips*,† †

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshimaku, Tokyo 171-8588, Japan

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‡

S Supporting Information *

ABSTRACT: Conjugated polymers incorporated with cycloplatinated complexes (P1− Pt and P2−Pt) were used as dispersants for single-walled carbon nanotubes (SWCNTs). Significant changes in the UV−vis absorption spectra were observed after the formation of the polymer/SWCNT hybrids. Molecular dynamics (MD) simulations revealed the presence of a strong interaction between the cycloplatinated complex moieties and the SWCNT surface. The photoinduced electron transfer processes in these hybrids were strongly dependent on the type of the comonomer unit. Upon photoexcitation, the excited P1−Pt donates electrons to the SWCNT, while P2− Pt accepts electrons from the photoexcited SWCNT. These observations were supported by results from Raman and femtosecond time-resolved transient absorption spectroscopy experiments. The strong electronic interaction between the Pt complexes and the SWCNT gives rise to a new hybrid system that has a controllable photoinduced electron transfer flow, which are important in regulating the charge transport processes in SWCNT-based optoelectronic devices. ince first reported in 1991,1 a variety of devices based on single-walled carbon nanotubes (SWCNTs) have been developed for sensing, energy conversion, and storage devices.2−6 The function of SWCNTs can be greatly extended by incorporating different functionalities on their surface.7 The functionalization of SWCNTs with photoactive electron donors and acceptors has been reported, and the excited state and electronic properties of the nanohybrids formed were studied.8 Transient charge carriers were generated by the photoexcitation of donors/acceptors in the conduction/ valence band of SWCNTs upon irradiation of light.8 The use of dispersants as electron donors for SWCNTs has been wellestablished.9,10 When SWCNTs were coupled with metal complex dispersants such as zinc porphyrin11,12 or a conjugated polymer such as poly(3-hexylthiophene) (P3HT),13 the nanotube functioned as an electron acceptor due to its relatively lower conduction band energy. Such an electron transfer process for these hybrids is important to the design of efficient light-harvesting systems.14 On the other hand, there were only a few reports on the use of dispersants as electron acceptors, of which examples include amphiphilic perylenebisdiimides,8,15,16 phenylenevinylene oligomers17,18 and 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) derivatives.19 The SWCNTs acted as electron donors due to the lower-lying lowest unoccupied molecular orbtal (LUMO) levels of the dispersants compared to the SWCNT conduction band. Upon photoexcitation, holes were generated in the SWCNTs as the charge carriers.

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It has been demonstrated that conjugated oligomers10 and polymers20−22 incorporated with porphyrin units could effectively disperse SWCNTs due to the presence of porphyrin units. Other than porphyrin, there was only one example of using a copper(II) complex with planar geometry for the dispersion of SWCNTs.23 Cyclometalated Pt(II) complexes exhibit a nearly planar geometry and highly interesting photophysical properties. In addition, by using different cyclometalated (such as N∧C∧N or C∧N∧C type) or ancillary ligands, the complex formed can be neutral, anionic, or cationic. The optical absorption/emission properties of the complexes can be fine-tuned, and have been successfully applied in light emitting devices.24−27 Due to the π-electronrich ligands and the planar geometry, when cyclometalated Pt(II) complexes are coupled with conjugated systems, it is envisaged that the resulting molecules are promising SWCNT dispersants. Here, we present the use of two conjugated polymers P1−Pt and P2−Pt (Scheme 1) incorporated with cycloplatinated complexes for dispersing SWCNTs. P2−Pt was demonstrated to be an efficient triplet sensitizer,28 and it is of great interest to study its sensitizing properties after coupled with SWCNT. Femtosecond time-resolved transient absorption spectroscopy was applied to probe the dynamics of the photoinduced charge Received: June 3, 2018 Accepted: June 25, 2018 Published: June 25, 2018 3819

DOI: 10.1021/acs.jpclett.8b01713 J. Phys. Chem. Lett. 2018, 9, 3819−3824

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The Journal of Physical Chemistry Letters

very similar spectral features, and its detailed spectroscopic assignments have been reported.28 After forming a dispersion with SWCNTs, both hybrids showed changes in their absorption spectra compared to the nanotube-free polymers. For P1−Pt/SWCNT, the conjugated main chain absorption band shows a shift from 405 to 418 nm, while the shift for the lower energy transitions is not significant. For P2−Pt/ SWCNT, in addition to the shift of the main chain absorption from 420 to 500 nm, the absorption bands centered at 540 and 595 nm show a shift to 620 and 675 nm, respectively. An enhancement in the absorption intensity at 675 nm is observed. The shift of the absorption bands is due to the more hindered rotation between different aromatic units after the formation of the hybrid with SWCNT, resulting in a more effective π-conjugation.10 The P2−Pt absorption at 675 nm shows little shift due to the triplet character of this band (3MLCT−3ILCT),30,31 which possesses a much more spatially confined character than the lowest singlet excited state. Therefore, the planarization of the polymer backbone has less effect on the electronic transition.32,33 For the hybrids, the UV−vis−NIR absorption spectra of P1−Pt/SWCNT and P2− Pt/SWCNT dispersed in THF are presented in Figure S4. From the inset figure, the hybrids only exhibit broad and featureless absorption in the NIR region, indicating that the polymers do not exhibit SWCNTs sorting ability.34 This is consistent with the Raman spectroscopic results (see below). The results presented above suggest that there are strong interactions between the polymers and SWCNTs in P1−Pt/ SWCNT and P2−Pt/SWCNT. Molecular dynamics (MD) simulations were employed to study such interactions in more detail. The snapshots for the MD simulations of the two hybrids (in THF) are shown in Figure 2. The radii of gyration calculated for P1−Pt before and

Scheme 1. Molecular Structures of Polymers P1−Pt and P2−Pt

separation processes in the polymer/SWCNT hybrid systems. Interestingly, these experimental results suggest that P1−Pt donates electrons to SWCNT upon photoexcitation, while P2−Pt accepts electrons from SWCNT. By tuning the relative energy levels between the polymer dispersant and the SWCNT, it is possible to tailor and design electron- or holecarrying carbon nanotube hybrid materials by using the same type of Pt complex monomers. The structures of polymers P1−Pt and P2−Pt are shown in Scheme 1. Detailed synthetic procedures and characterization data are presented in the Supporting Information (SI). Both P1−Pt and P2−Pt are able to disperse SWCNTs effectively, forming homogeneous and stable dispersions in THF. After dispersion and purification, the TEM images (Figures S1 and S2) of the hybrids reveal that the both P1−Pt and P2−Pt present the ability to debundle SWCNTs.8,29 The observed morphology suggests that a layer of polymer is attached on the SWCNT surface. EDX results confirmed the presence of both platinum and sulfur in the hybrids, which further supports the functionalization of the SWCNT platinum-containing polymers (Figure S3). The absorption spectra of P1−Pt, P2−Pt, and the polymer/ SWCNT hybrids in THF are shown in Figure 1.28 For P1−Pt, the absorption due to the main chain at 405 nm and the electronic transitions at lower energy (535−700 nm) due to the mixed intraligand charge transfer (ILCT) and metal-toligand charge transfer (MLCT)-intraligand (IL) transitions of the cycloplatinated complexes are observed.28,30 P2−Pt shows

Figure 2. Snapshots of the MD simulations of (a) P1−Pt/SWCNT and (b) P2−Pt/SWCNT. The fluorene/bithiophene-containing blocks and the cycloplatinated complex blocks are colored blue and brown, respectively. For clarity, THF molecules are not shown.

after attaching onto the SWCNT surface are 30.5 ± 0.4 and 32.1 ± 0.3 Å, respectively. On the other hand, a significant change in the radii of gyration (from 23.4 ± 0.5 to 14.6 ± 0.1 Å) was observed before and after the formation of P2−Pt/ SWCNT. This indicates that the conformation of P2−Pt is more restrictive after attaching onto the SWCNT surface,35

Figure 1. UV−vis absorption spectra of P1−Pt, P2−Pt, P1−Pt/ SWCNT, and P2−Pt/SWCNT (in THF). 3820

DOI: 10.1021/acs.jpclett.8b01713 J. Phys. Chem. Lett. 2018, 9, 3819−3824

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The Journal of Physical Chemistry Letters and is consistent with the difference in the spectral behavior in the absorption spectra as presented above. It was calculated that the interaction energies (Eint) between the polymers and the SWCNT are very similar for both P1− Pt/SWCNT (Eint = −249 kcal/mol) and P2−Pt/SWCNT (Eint = −250 kcal/mol) hybrids (Table S1). However, detailed analysis of the Eint, contact areas (Acontact), and the radial distribution functions (RDFs) between the different moieties in the polymers (Table S2) reveals that the nature of the interactions in P1−Pt/SWCNT and P2−Pt/SWCNT are different. For P1−Pt/SWCNT, the Eint between the Pt complex moieties and SWCNT (−129 kcal/mol) is 8% higher than that between the fluorene units and SWCNT (−120 kcal/ mol). For P2−Pt/SWCNT, the Eint between the Pt complex moieties and SWCNT (−168 kcal/mol) is 2 times higher than that between the bithiophenyl moieties and SWCNT (−81 kcal/mol). This clearly shows the important role of the Pt complex in stabilizing the hybrids, especially for P2−Pt/ SWCNT. Figure S5 shows the RDFs between different moieties and SWCNT. For the Pt complex−SWCNT interactions, the RDFs are bimodal in both hybrids. To scrutinize the details of the interaction between the different moieties and SWCNT, the analysis should be done based on the first peak at 5 Å, which corresponds to the moieties interacting with the SWCNT. A large value of RDFmax or a small value of rmax indicates a strong interaction.36 For P1−Pt/ SWCNT, both the RDFmax and rmax values between the Pt complexes and SWCNT are smaller than those between the fluorene units and SWCNT, which suggests that the interactions between SWCNT−Pt complex and SWCNT− fluorene are of similar magnitude. On the other hand, for P2− Pt/SWCNT, the Pt complex−SWCNT interaction is much stronger than that of the bithiophene−SWCNT (Figure S5b). Regarding the calculated contact area, for P1−Pt/SWCNT, the Acontact between the Pt complex moieties and SWCNT (9 nm2) is slightly smaller than that between the fluorene moieties and SWCNT (9.7 nm2). In P2−Pt/SWCNT, the contact between Pt complex and SWCNT (10.7 nm2) is significantly larger than that of the bithiophene−SWCNT (6.6 nm2). All the results shown above suggest that the interaction between the Pt complex moieties and SWCNT in P2−Pt/SWCNT is stronger than that of P1−Pt/SWCNT. This further supports the claims that the significant spectral changes observed in P2−Pt/SWCNT originate from the stronger interaction in this system. In P2−Pt, the geometry of the cycloplatinated complexes and the more flexible bithiophenyl units render the polymer molecule to wrap on the SWCNTs more effectively,37 while in P1−Pt, the interaction is relatively weaker due to the more rigid fluorene units. The interactions between the polymer main chains and SWCNT were further examined by micro-Raman spectroscopy and presented in Figure 3. Characteristic bands due to the radical breathing mode (RBM), tangential vibrational mode (G band), and disorder mode (D band) of SWCNT are observed.38 The G band of P2−Pt/SWCNT shows an upshift from 1598 to 1602 cm−1, and the G band of P1−Pt/SWCNT shows a downshift from 1598 to 1593 cm−1 after forming hybrids compared to pristine SWCNTs. It has been reported that the G and RBM bands of SWCNTs would shift to higher frequency upon losing electrons and to lower frequency upon receiving electrons.39 The observed band shifts here again suggested a strong interaction between SWCNTs and the polymers.40 Based on the frequency shifts, it is proposed that

Figure 3. Micro-Raman spectra of pristine SWCNTs (black), P1−Pt/ SWCNT (blue), and P2−Pt/SWCNT (red) (excited at 785 nm).

P1−Pt serves as an electron donor and P2−Pt serves as an electron acceptor upon forming hybrids with SWCNTs. In order to study the electron transfer dynamics and to confirm the direction of electron flow proposed above, femtosecond transient absorption (fs-TA) spectroscopic experiments were used to study the photoinduced electron transfer processes in both P1−Pt/SWCNT and P2−Pt/ SWCNT by monitoring the formation and decay of different excited species. By probing the formation of the excited species formed after the charge transfer processes, it is possible to identify the electron flow direction. The UV−vis, fluorescence, and fs-TA spectra of P1 and P1−Pt are shown in Figures S7− S10. The detailed photophysical properties of P2 and P2−Pt have been reported previously.28 For P1−Pt, the absorption bands centered at 470 and 650 nm are assigned to the singlet excited state formed at the Pt complex moieties of P1−Pt, which undergoes a biexponential decay with two time constants (τ1 = 9.8 ps and τ2 = 400 ps), as shown in Figure S9. This behavior is similar to that observed in P2−Pt.28 These two processes are assigned to the intersystem crossing (ISC) and the subsequent relaxation of the triplet excited states. The triplet character can also be confirmed by comparing the lifetimes of the excited species in air-saturated and degassed solutions by the ns-TA experiments (Figure S10), from which the decay time constants obtained in the air-saturated (τ = 161 ns) and degassed solutions (τ = 4.4 μs) are of different order of magnitude. This strongly suggests that the species observed in 3 ns (Figure S9) is triplet in nature. For the polymer/SWCNT hybrids, Figure 4a shows the fsTA spectra of P1−Pt/SWCNT in THF solution. Similar to the

Figure 4. (a) fs-TA of P1−Pt/SWCNT in THF solution acquired after excitation at 400 nm. (b) The kinetics of the decay processes at 490 nm. The solid lines indicate the kinetics fitting to the experimental data points. 3821

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free polymer,28 no initial decay bands were observed at 480 and 650 nm (Figure 5a). The TA spectrum observed at 1 ps (Figure 5a) is dominated by the ground-state bleaching bands of the SWCNTs instead of the Pt complex singlet excited states absorption. By comparing the absorption spectra of P2−Pt/ SWCNT with the TA spectrum of P2−Pt/SWCNT at 1 ps (Figure 5c), the bleaching bands at 500 and 670 nm are assigned to the ground state bleaching of the SWCNTs, while the positive bands at 590 and 730 nm represent the formation of the SWCNT singlet excited states.41 Global analysis of the transient decay dynamics reveals four time constants (Figure 5b): 0.6 ps (62%), 16.3 ps (30%), 408 ps (8%), and a fourth one that exceeds the limit (>3 ns) of the instrument coverage. The processes observed at 0.6 and 16.3 ps have previously been assigned to the exciton−exciton annihilation and singlet exciton recombination.42−44 However, the long-lived component (408 ps) was not observed previously. As shown in Figure 5a, the bleaching bands show little change at 530 nm after 76 ps, while the intensity of the positive bands drops to zero within the time interval from 76 ps to 3 ns. This could be due to the generation of a new species associated with the decay of the singlet SWCNT excited states. To confirm this, the UV− vis spectra of P2−Pt and the transient absorption spectrum of P2−Pt/SWCNT at 3 ns are compared (Figure 5d). The good agreement between these two spectra strongly suggests an electron transfer process from the SWCNTs to P2−Pt, forming radical anions (P2−Pt•−) within 408 ps. The radical anions are long-lived species with a lifetime longer than 3 ns.45 Therefore, it is concluded that P2−Pt accepts electrons from SWNCT after forming hybrids with SWCNTs. The results are in good agreement with the micro-Raman spectral data discussed above. Based on the experimental results obtained, a schematic diagram showing the energetic and kinetic data of different processes is presented in Figure 6. In summary, cycloplatinated complexes containing conjugated polymers based on bithiophenyl and fluorenyl units were utilized as efficient dispersants for SWCNTs. The important role of the platinum complex moieties in stabilizing the polymer/SWCNT hybrids were demonstrated by electronic absorption spectroscopy and MD simulations. The electronic transition energy of the polymer can be varied by using different cyclometalated and ancillary ligands, and the flow direction of the photoexcited electrons in these hybrids can be tailored and designed by varying the comonomer unit. The formation of stable dispersion can also be realized in polar solvent systems if ionic cycloplatinated complexes were used (e.g., the use of N∧N∧N or anionic ancillary ligands). These polymer-SWCNT hybrids may serve as a model for the design of new SWCNT based light harvesting systems in which the

TA spectra of P1−Pt (Figure S9), the initial bands located at 490 and 700 nm at 1 ps are assigned to the singlet excited states localized at P1−Pt. These absorption bands show a red shift compared to those of the pure polymer, and it is suggested to be due to the enhanced ground state absorption for P1−Pt/SWCNT. Subsequently, a noticeable blue-shift of the transient band at 490 nm is observed. This is assigned to the decay of the ground state bleaching.37 Unlike P1−Pt, the kinetics of the decay observed at 490 nm for P1−Pt/SWCNT (Figure 4b) can be fitted by a triexponential function with time constants of 2.7, 52, and 445 ps. The processes occurring at 2.7 and 445 ps are similar to the ISC and solvent relaxation processes as discussed above. For the second process at 52 ps, it is assigned to the electron injection from the triplet excited states of P1−Pt to the SWCNT, resulting in the formation of the charge-separated P1−Pt+/SWCNT•− species.37 In such a case, P1−Pt donates electrons to SWCNT, which is in good agreement with the Raman spectroscopic results. On the other hand, P2−Pt/SWCNT shows remarkably different photophysical properties. Its fs-TA spectra are shown in Figure 5a. Unlike the TA spectra obtained for the nanotube-

Figure 5. (a) fs-TA of P2−Pt/SWCNT in THF solution acquired after 400 nm irradiation; (b) kinetic traces at 730 nm (black triangle), 530 nm (black square) and the respective fitting traces (solid lines, red) based on a global analysis with four exponential functions; (c) comparison between the UV−vis spectra of P2−Pt/SWCNT with the transient absorption spectrum at 1 ps; (d) comparison between the UV−vis spectra of P2−Pt with the transient absorption spectrum at 3 ns.

Figure 6. Schematic diagram showing the photophysical processes in P1−Pt/SWCNT hybrid (left) and in the P2−Pt/SWCNT hybrid (right). 3822

DOI: 10.1021/acs.jpclett.8b01713 J. Phys. Chem. Lett. 2018, 9, 3819−3824

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The Journal of Physical Chemistry Letters electron flow direction and the type of carriers can be controlled. In addition, the photosensitizing system may also be extended to photocatalytic system in which photo-oxidants (holes) or reductants (electrons) can be generated, depending on the types of complexes employed.

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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/acs.jpclett.8b01713.

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Materials, instrumentation, details of synthetic procedure and characterization of compounds, additional TEM images, EDX spectra, UV−vis−NIR absorption spectra, MD simulation results, and transient absorption spectra (Figures S1−S9, Tables S1−S2) (PDF)

AUTHOR INFORMATION

ORCID

Tomohisa Takaya: 0000-0002-6071-4529 Wai Kin Chan: 0000-0002-5898-903X David Lee Phillips: 0000-0002-8606-8780 Author Contributions §

These coauthors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work described in this paper was supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKU700613E, T23-713/11). The computation was conducted in part using the HKU Information Technology Services research computing facilities supported in part by the Hong Kong UGC Special Equipment Grants (SEG HKU09). NAMD was developed by the Theoretical and Computational Biophysics Group in the Beckman Institute for Advanced Science and Technology at the University of Illinois at Urbana−Champaign.

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The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.8b01713 J. Phys. Chem. Lett. 2018, 9, 3819−3824