Photoinduced Triplet State Electron Transfer Processes From

Institute of Molecular Functional Materials and Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R. China. J. Phys. Chem. C , 2017,...
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Photoinduced Triplet State Electron Transfer Processes From Ruthenium Containing Triblock Copolymers To Carbon Nanotubes Haiting Shi,‡ Lili Du,‡ Kin Cheung Lo, Wenjuan Xiong, Wai Kin Chan,* and David Lee Phillips* Institute of Molecular Functional Materials and Department of Chemistry, The University of Hong Kong, Hong Kong S.A.R. China S Supporting Information *

ABSTRACT: A light-harvesting triblock copolymer incorporated with pyrene and ruthenium complex photosensitizing moieties was synthesized and integrated into the dispersion and surface functionalization of multiwalled carbon nanotubes (MWCNTs) via noncovalent π−π interactions. Molecular dynamics simulation results show that the copolymer interacts with MWCNTs mainly through the pyrene blocks and that the Ru complex moieties far away from the MWCNT could preserve the charge-separated states of the electron donor−acceptor system after photo excitation. This new molecular structure serves as a good model for studying the fundamental photophysics of light harvesting systems based on polymer/carbon nanotube hybrids. Results from femtosecond transient absorption spectroscopy show that the electron transfer process occurs within 383 ps from the Ru complex to MWCNT, which is much faster than the relaxation of the triplet metal-to-ligand charge transfer excited state of the Ru complex. The rapid electron injection process infers that this type of functional metalloblock copolymer/carbon nanotube hybrid material has promising application potentials in solar energy conversion or other light harvesting devices.

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INTRODUCTION Chemical modification and functionalization of carbon nanotube (CNT) provides an effective method for solubility improvement and the introduction of additional functions by varying the functional groups used.1−5 Introducing photo and optical-active organic/inorganic functionalities on CNTs enabled the resulting nanohybrid materials to have a large variety of potential applications in catalysis, sensing, and optoelectronics.6−8 Crucial for device application is to explore the electronic interactions of molecules with CNTs. Therefore, incorporation of CNTs with photosensitizers into the light harvesting systems for the study of photoinduced electron transfer dynamics has become an active area of research. Photoinduced electron transfer processes occurred in the hybrid materials formed between organic polymers9,10 and CNTs, and those between zinc naphthalocyanine/porphyrin complexes11−13 and CNTs have been investigated. In the zinc complex/CNT electron donor−acceptor system, the photo induced electron transfer from the zinc complex singlet excited state to the carbon nanotube acceptor was observed. Photoinduced electron transfer between CdS nanoparticles/CdTe quantum dots and CNTs was also reported.8,14,15The unique photoresponse was attributed to a charge-transfer process between the nanoparticles and CNTs after photoexcitation.16 Compared with small molecules, polymers have a stronger interaction with CNTs, and more stable hybrids can be formed. We previously reported different series of polymer/CNT hybrid materials based on diblock copolymers functionalized with ruthenium complex photosensitizers17 and cationic conjugated polyelectrolyte18 by noncovalent interaction. It © XXXX American Chemical Society

was found that polymers attached on MWCNTs could enhance the photocurrent response compared to unfunctionalized CNTs, which clearly demonstrates the potential of using polymer/CNT hybrid materials in optoelectronic devices. The observation of a fast electron transfer process from organic polymers to CNT surface was also reported by our group.19 All the previous reports on the electron transfer process occurred in CNT hybrids were based on small molecules or organic polymers. There has been no report on the electronic interaction between photosensitizing metallopolymers and CNTs to our knowledge. In the present study, we report the synthesis of a functional triblock copolymer incorporated with pyrene moieties and ruthenium terpyridyl thiocyanato complexes [Ru(tpy) (NCS)3] at the side chain by reversible addition−fragmentation chain transfer (RAFT) polymerization. The copolymer can functionalize multiwalled carbon nanotubes (MWCNTs) by playing roles as dispersing agent and photosensitizer. It was modified based on a previously reported diblock copolymer.17 The use of block copolymers gives a good control to the morphology of the polymers on MWCNT surface, and the position/morphology of different blocks is better-defined. Molecular dynamics (MD) simulations were performed to elucidate the conformation of the polymer/ MWCNT hybrid and to scrutinize the interaction between the polymer and the MWCNTs. It is postulated that the photocurrent generation is a result of a photoinduced electron Received: December 21, 2016 Revised: March 20, 2017 Published: March 20, 2017 A

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mL). The reaction was carried out at 100 °C for 16 h. The solid was collected and washed with methanol. The resulting polymer (100 mg) was then added into a flask with sodium thiocyanate (310 mg) and DMF (4 mL) together under nitrogen. After heating at 130 °C for 24 h in the dark, the solution obtained was cool down and poured into cold methanol (200 mL). Tetrabutylammonium bromide (20 mg) was then added to the solution, and the polymer 7 was obtained as a precipitate. The polymer was washed with methanol for several times, and the solid obtained was dried under vacuum (90 mg, 67% yield). Preparation of Polymers/MWCNTs Hybrids. Polymer 7 (16 mg) was dissolved in DMF (20 mL) and MWCNTs (2 mg) were added to the solution. The mixture was ultrasonicated for 30 min under room temperature. The unmodified MWCNTs which cannot dissolve in DMF were removed by centrifugation for 30 min at 4000 rpm, and the obtained supernatant solution was passed through a PTFE membrane filter with 0.2 μm pore size. The excess polymer was removed by rinsing the membrane with DMF. The polymer/MWCNT hybrid obtained on the membrane surface was redissolved in DMF. The solid was collected by removing the solvent and dried under vacuum (1.45 mg).

transfer process between the photosensitizer and CNTs.16 Understanding the role of the ruthenium complexes in the photocurrent generation process, the nature of the excited states involved, and the dynamics of the electron transfer is of fundamental importance to the design of improved light harvesting systems. Since metal complexes have been widely used as photosensitizers in light harvesting devices, it is of great scientific interest to study the dynamics of the photoinduced electron transfer processes in metal-containing polymers/CNT hybrids to design efficient light harvesting systems.

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EXPERIMENTAL SECTION 4-Cyano-4-(dithiobenzoate)-pentanoyloxy)butylcyano-4-(dithiobenzoate)-pentanoate 1. The chain transfer agent (CTA) was synthesized by a modified literature procedure.20 A mixture of (S)-4-cyanopentanoic dithiobenzoate (0.9 g, 3.4 mmol), 1,4-butanediol (0.15 g, 1.7 mmol), dicyclohexylcarbodiimide (1.05 g, 5.0 mmol), and N,Ndimethylaminopyridine (0.041 g, 0.339 mmol) in dichloromethane (9 mL) was stirred for 1 h under room temperature. The solvent was removed, and the crude product obtained was purified by column chromatography on a silica column using ethyl acetate/hexane 2:5 (v/v) as the eluent to give the target product (0.6 g, 61% yield) as red oil. 1H NMR (400 MHz, CDCl3) δ/ppm: 7.90 (d, J = 7.8 Hz, 2H), 7.57 (t, J = 7.4 Hz, 1H), 7.39 (t, J = 7.7 Hz, 2H), 4.14 (s, 4H), 2.40−2.80 (m, 8H), 1.94 (s, 6H), 1.73 (t, 4H). 13C NMR (100 MHz, CDCl3) δ (ppm): 222.34(s), 171.49(s), 144.49(s), 134.34(s), 128.96(d, J = 7.4 Hz), 127.30(s), 126.68(s), 118.49(s), 64.47(s), 45.75(s), 34.20(s), 29.79(s), 25.62(s), and 24.13(s). EIMS(m/z): 612.12. Polymer 3. Monomer 2 (600 mg, 1.2 mmol), chain transfer agent 1 (23.4 mg, 0.04 mmol) and AIBN (3.3 mg, 0.02 mmol) were dissolved in distilled DMF (4 mL) in a flask. The resulted solution was degassed by three freeze−pump−thaw cycles. The polymerization was carried out at 60 °C for 24 h under nitrogen atmosphere. The reaction mixture was poured into cold methanol (150 mL) to precipitate the homopolymer 3. The precipitation procedure was repeated twice, and the polymer was dried under vacuum (360 mg, 60% yield). Polymer 5. To a dried flask, a mixture of polymer 3 (140 mg), monomer 4 (300 mg, 0.64 mmol), AIBN (1.4 mg, 0.0088 mmol) and DMF (3 mL) was added. The solution was degassed by three freeze−pump−thaw cycles. The copolymerization was carried out at 60 °C for 24 h under nitrogen atmosphere. The reaction mixture was precipitated in cold methanol (250 mL) to give triblock copolymer 5. The polymer was purified by reprecipitation in methanol and dried under vacuum (340 mg, 65% yield). Polymer 6. To a round-bottom flask, polymer 3 (50 mg), RuCl3·xH2O (103 mg) and DMF (2 mL) were added. The reaction was carried out at 100 °C for 16 h under atmosphere in dark. After cool down, the solution was filtrated, and the solid obtained was washed with methanol. The polymer obtained (68 mg), sodium thiocyanate (505 mg), and DMF (4 mL) were then added into a flask together, and the mixture was heated at 130 °C under a nitrogen atmosphere in dark. The resulting solution was poured into methanol (150 mL). Tetrabutylammonium bromide (30 mg) was added to the solution to give polymer 6 as the precipitate. The polymer was purified by washing with methanol, and the solid obtained was dried under vacuum (75 mg, 71% yield). Polymer 7. To a round-bottom nitrogen-purged flask was added polymer 5 (100 mg), RuCl3·xH2O (69 mg) and DMF (3

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RESULTS AND DISCUSSION Polymer Synthesis and Characterization. ABA type triblock copolymer 5 was synthesized by RAFT polymerization using the difunctional chain transfer agent 1 in two steps (Scheme 1). In the first step, terpyridine containing homopolymer 3 was prepared by polymerization of monomer 2 with 4-cyano-4-(dithiobenzoate)-pentanoyloxy)butyl-cyano4-(dithiobenzoate)-pentanoate 1 as the chain transfer agent and AIBN as the initiator. In the second step, polymer 3 was used as the macroinitiator to synthesize polymer 5. The target ruthenium containing polymers 6 and 7 were synthesized by the reaction between polymers 3 and 5 with ruthenium trichloride, replacement of chloride ligands by thiocyanate, and then by the cation exchange with tetrabutylammonium bromide (Scheme 2), yielding polymers 6 and 7, respectively. Homopolymer 6 without the pyrene blocks served as the model polymers in the ultrafast transient absorption experiments. The structural characterization of the polymers was carried out by 1H NMR spectroscopy. The 1H NMR spectrum of polymer 3 (Figure S1c of the Supporting Information) shows typical peak broadening commonly observed in polymers, and the peaks due to the monomer vinyl protons at δ 5.5 and 6.1 ppm disappear. In addition, the peaks due to the two methylene groups (CH2O) at δ 3.8−4.1 ppm, and the peaks in the aromatic region due to the terpyridine unit are consistent with those observed in monomer 2 (Figure S1a). Since the signal due to the methylene protons (OCH2) of chain transfer agent 1 also appears at δ 4.15 ppm, we attempted to estimate the degree of polymerization by comparing the integral of the methylene protons at δ 3.8−4.1 ppm with that of the phenylene aromatic protons at δ 6.9 ppm. The theoretical degree of polymerization of polymer 3 is 15, which was calculated by the ratio between the number of the chain transfer functional group to the monomer. From the 1H NMR spectrum, the degree of polymerization was calculated to be 16, which corresponds to a number-average molecular weight Mn of 7920. The result agrees with that obtained from GPC (Mn(GPC) = 8300, see Table S1), which also revealed a narrow molecular weight B

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7.5−7.9 and 8.4 ppm. The integral of the peak at δ 8.4 ppm was used as the reference to estimate the degree of polymerization and the ratio of the terpyridine/pyrene units in copolymer 5, which was calculated to be 1:2. The monomer ratio obtained also agrees with the ratio of monomers used in the synthesis of polymer 5. It suggests that the degree of polymerization of each block is the same. The Mn of the polymer 5 estimated from 1H NMR (22 960) also agrees with the theoretical value (Mn = 21 520). All these experimental data suggest that the polymerization reactions proceeded with living-like mechanisms. Both polymer 3 and copolymer 5 underwent metal complexation reaction with ruthenium trichloride, which was then followed by a reaction with sodium thiocyanate and cation exchange with tetrabutylammonium bromide, giving polymers 6 and 7, respectively. The degree of metal complexation was estimated by 1H NMR spectroscopy. In the 1H NMR spectrum of polymer 6 (Figure S1e), the protons next to the nitrogen on the pyridine ring show a downfield shift from δ8.7 to 9 ppm compared to the precursor polymer 3 (Figure S1c) due to the metal complexation. Comparing the integral of this peak with the metal free terpyridine peak at δ 6.9 ppm, it is estimated that 87% of the terpyridine units formed complexes with ruthenium. For polymer 7, a similar estimation was carried out from the 1H NMR spectrum (Figure S1f), and the degree of metal complexation was calculated to be 73%. Also, in the 1H NMR spectra of polymers 6 and 7, additional peaks due to the tetrabutylammonium cation at δ 0.9−1.8 ppm are observed. The FTIR spectra of polymers 7 (Figure S2) also confirm the presence of the thiocyanate ligand. An intense peak at 2100 cm−1 was observed and is assigned to the CN stretching band. The UV−vis absorption spectra of polymers 5, 6, and 7 are shown in Figure 1a. Polymer 5 exhibits an intense absorption

Scheme 1. Synthesis of Polymer 3 and Triblock Copolymer 5

Scheme 2. Synthesis of Polymer 6 and Triblock Copolymer 7

Figure 1. (a) UV−vis absorption spectra of polymer 5 in CHCl3, polymers 6, 7, and 7/MWCNT hybrid in DMF solutions. The spectra are normalized at the maximum absorbance. (b) Steady state fluorescence spectra of polymer 5 in CHCl3 and polymer 7 in DMF (excitation wavelength = 400 nm).

band centered at 350 nm, which is assigned to the π−π* transition of the terpyridine ligand and pyrene units. In addition, an absorption band observed at 385 nm is due to the absorption of the pyrene moieties. The absorption features of polymer 6 are similar to those of ruthenium black dye. In addition to the ligand-centered π−π* absorption peak at 330 nm, a broad absorption band spanning from 450 to 700 nm with a shoulder at 390 nm is assigned to the absorption of the MLCT.21 For polymer 7, the optical absorption is essentially the combination of both ruthenium black dye and the pyrene moieties. After incorporating the MWCNT, the absorption intensity increased in the range between 400 and 700 nm due to the broad absorption of MWCNT in the visible region (Figure 1a). Therefore, the electronic excited states of both the ruthenium complex and the pyrene exhibit large electronic

distribution (polydispersity = 1.17). After introducing the pyrene-containing blocks, the 1H NMR spectrum of polymer 5 (Figure S1d) shows additional signals due to the pyrene at δ C

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was estimated that the polymer content in the hybrid is approximately 45%. Figure 2a shows the TEM image of polymer 7/MWCNT hybrid. Pristine MWCNTs often form large bundles and are difficult to disperse in any solvents.25 It can be seen that the majority of polymer 7 dispersed MWCNTs mainly exist as individual tubes. Figure 2b shows the presence of a thin layer of polymer with a thickness of ca. 2−4 nm on the tube surface. EDX experiments showed the presence of ruthenium and sulfur in the hybrid, which supports the functionalization of MWCNT by polymer 7 (see Figure S5). The successful functionalization of MWCNT can also be confirmed by UV−vis absorption spectra (Figure 1a). The absorption spectrum of polymer 7/ MWCNT hybrid shows a very broad absorption spanning between 400 and 700 nm, in addition to the absorption peaks due to the ruthenium complex and pyrene moieties. MD Simulation. The dispersion of MWCNT by polymer 7 is mainly due to the strong interaction between the pyrene units in the copolymer and the MWCNT surface.17,26 A schematic diagram displaying such an interaction is shown in Figure 3. Due to the ionic nature of the ruthenium complexes,

coupling with the MWCNT ground state (see below). Figure 1b shows the steady state luminescence spectra of polymers 5 and 7 upon excitation at 400 nm. Polymer 5 shows an intense emission band centered at 480 nm, which is assigned to the emission of the pyrene excimer.22 However, the intensity of this excimer emission in polymer 7 is significantly reduced. This suggests the excitation of the pyrene units on polymer 7 could be neglected for the limited absorption by the pyrene units at 400 nm. The electrochemical properties of polymer 7 were studied by cyclic voltammetry (CV) (Figure S3). The HOMO and LUMO levels of polymer 7 were estimated by comparing the oxidative and reductive waves observed with those of ferrocene internal standard. Two oxidative waves at 0.6 and 1.0 V and a reductive wave at −1.7 V were observed. With reference to other ruthenium isocyanate complexes reported,23 the first oxidation wave was assigned to the RuII/III couple, while the second one was due to the oxidation of the pyrene moieties. The reductive wave was assigned to the reduction of the terpyridine ligand. The HOMO−LUMO levels of the polymer were estimated to be −5.4 and −3.1 eV, respectively. Therefore, the electron/hole transfer between the ruthenium complex and the MWCNT (work function = −4.3 eV24) is energetically favorable. Preparation of Polymer/MWCNT Hybrid. The preparation of the polymer/MWCNT hybrid was carried out by ultrasonicating a mixture of polymer 7 with multiwall carbon nanotubes in DMF. The polymer 7/MWCNT solution was stable for several months without formation of any precipitate. The difference in color between solutions of polymer 7 and polymer 7/MWCNT hybrid was clearly observed (Figure 2b).

Figure 3. Schematic diagram showing the attachment of polymer 7 on carbon nanotube surface.

the middle block will stay away from the surface of the nanotube, which is less polar in nature. In order to confirm this argument, MD simulations were performed to elucidate the conformation of the polymer 7/MWCNT hybrid and the interaction between them (Figure 4). To analyze the conformation quantitatively, the interaction energy (Eint), radial distribution function (RDFs) and contact area (Acontact) between the MWCNT and different moieties in polymer 7/MWCNT hybrid were calculated and the data are summarized in Table 1. The Eint calculated for the MWCNTpyrene pendant group interaction is −233 kcal/mol, which is due to the strong π−π interaction. Lower Eint (−199 kcal/mol) for the MWCNT-dodecyl chains interactions is mainly due to CH-π in nature. The Eint between both the polymer backbone and metal complexes with the MWCNT are very low (−6.1 and −3.5 kcal/mol, respectively). The calculated Acontact between MWCNT and pyrene pendant groups (13.6 nm2) and dodecyl chains (9.4 nm2) indicate a more effective contact than those of the polymer backbone (0.41 nm2) and metal complexes (0.04

Figure 2. TEM images of polymer 7/MWCNT hybrid in different magnification. The inset photo shows the solutions of polymer 7 (left) and polymer 7/MWCNT (right) in DMF.

The hybrid was characterized by thermal gravimetric analysis (TGA) under a nitrogen atmosphere, with results depicted in Figure S4. The TGA thermogram of polymer 7 shows a twostage decomposition between 200 and 535 °C. The first decomposition process observed at 200 to 255 °C may due to the decomposition of tetrabutylammonium cation, and the second process starting at ca. 300 °C is due to the decomposition of the polymer backbone. For the polymer 7/ MWCNT hybrid, after the initial weight loss due to solvent residue, a decomposition process similar to that of the pure polymer is observed at 300 °C. In addition, a further weight loss at ca. 600 °C was observed. With reference to the TGA thermogram of MWCNT (Figure S3), this could be due to the decomposition of MWCNT. Therefore, from these results, it D

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Figure 4. Snapshot of the MD simulation of the polymer 7/MWCNT hybrid. The pyrene-containing blocks and metal-containing block are colored blue and brown, respectively.

Figure 5. Radial distribution functions between the MWCNT and different moieties in polymer 7.

nm decrease significantly (see Figure S6a, from 0.8 to 30 ps) with little shift in band positions. The presence of two isosbestic points at 520 and 615 nm suggests a torsional relaxation of the excited state species to a more planar structure between the phenylene group and the terpyridine ligand.32−35 The time constant for such a conversion process that gives rise to the 3MLCT excited state is 7.3 ps (Figure S6c), which is similar to the results obtained for the [Ru(4,4′-diphenyl-2,2′bipyridine)3]2+ complex.32 Subsequently, the 3MLCT (Ru-3tpy) excited state decayed with a time constant of 5.6 ns (see Figure S6b,c), which is shorter than those of the carboxylated terpyridyl complexes of tris-thiocyanato Ru(II) (22 ns).21 In order to confirm the assignment for the polymer 6, the fs-TA spectra of a model Ru(tpy) (NCS)3 complex tethered with pyrene unit was collected (Figure S7). The similar spectral features strongly suggest that the photophysical properties of the Ru(tpy) (NCS)3 complex have no significant change after it was incorporated into the polymer. The fs-TA spectra of polymer 7 in DMF are shown in Figures 6a,b. The initial species at 0.8 ps is similar to that

nm2) (Table 1), which suggests that the interaction between the metal complexes and MWCNT is negligible. The RDFs are to estimate the average position between MWCNT and different moieties in polymer 7. The maximum intensities and the distances corresponding to the maximum intensities (rmax) are listed in Table 1, and the corresponding RDFs are shown in Figure 5. The pyrene pendant groups show the smallest rmax (4.1 Å) with MWCNT at maximum RDF, whereas the rmax for the metal complexes is 55.2 Å. The results further confirm an effective interaction between pyrene and MWCNT, and relatively weaker interactions between both polymer backbone and metal complexes with MWCNT.27 A snapshot for the MD simulation of polymer 7/MWCNT hybrid is shown in Figure 4, which provides a vivid picture of the interaction between the polymer and MWCNT. These results confirm that the pyrene moieties have a strong interaction with MWCNT while the ruthenium complexes stay away from the MWCNT surface. Keeping a distance between the electron donor and acceptor can facilitate the formation of the excited states and suppress the charge recombination upon photoexcitation.13,28 Transient Absorption Spectroscopy and Electron Transfer Kinetics. Femtosecond transient absorption (fsTA) spectroscopy was employed to study the photoinduced electron transfer processes between polymer 7 and MWCNT. In order to study the effect of the pyrene moieties in polymer 7 to the photophysical properties of the ruthenium complex moieties, the fs-TA spectrum of model polymer 6 was also collected (Figure S6). After photoexcitation, a 1MLCT [Ru(π)t2g to 1tpy(π*)] excited state29 was formed, and it underwent a very fast intersystem crossing (ISC) process within the instrumental response time (τ ≤ 150 fs) to the triplet 3MLCT (Ru-3tpy) excited state, which is consistent with the results observed in similar type of Ru(II) complexes.30,31 Therefore, the initial species obtained at 0.8 ps (Figure S6a) can be assigned to the 3*MLCT (Ru-3*tpy) excited state of Ru(II) complex in polymer 6. Subsequently, the intensity of the absorption band at 569 nm increases and those at 667 and 458

Figure 6. (a, b) fs-TA spectra of polymer 7 in DMF solution acquired after 400 nm irradiation.

observed in polymer 6, which could be assigned to the 3 *MLCT (Ru-3*tpy) excited state of polymer 7. The transient species observed at 22 ps is assigned to the 3MLCT (Ru-3tpy) excited state after the rotation of the phenylene unit. By

Table 1. Interaction Energy (Eint), Contact Area (Acontact), Maximum RDF Intensity (max RDF) and the Distance between the CNT and Different Moieties at the Maximum RDF Intensities (rmax) for Polymer 7/MWCNT Hybrid Calculated by MD Simulations moiety pyrene pendant groups dodecyl chains polymer backbone metal complexes

Eint (kcal mol−1) −233 −199 −6.1 −3.5

± ± ± ±

4 3 0.7 0.3

Acontact (nm2)

max RDF

rmax (Å)

± ± ± ±

8.8 4.7 3.6 2.5

4.1 18.3 28.0 55.2

13.6 9.4 0.41 0.04 E

0.2 0.2 0.06 0.04

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electronic interaction between the ruthenium complex and the pyrene/MWCNT surface. Figure 7a,b shows the fs-TA spectra of the polymer 7/MWCNT hybrid in DMF solution. Similar to the TA spectra of polymer 7, the absorption bands at 1 ps are assigned to the absorption of the 3*MLCT (3*polymer 7/ MWCNT*) excited state localized at the Ru complexes in polymer 7 (Figure 7a). Due to the increase in ground state absorption by polymer 7/MWCNT compared to polymer 7 (Figure 1a), the transient absorption band of polymer 7/ MWCNT hybrid at 1.2 ps shows an obvious red shift (Figure 7a). Then, the intensity of the band at 568 nm increases slightly while the intensities of the bands at 484 and 700 nm decrease from 1.2 to 5.2 ps. This could be assigned to the formation of the 3MLCT (3polymer 7/MWCNT*) excited state conformer as discussed above. After that, these bands decay rapidly, and a new spectrum appears at the later delay times (Figure 7b). The upward pointing band at 570 nm is due to triplet transient absorption. Disregarding the contribution of this band, the bands at 430 and 510 nm can be associated with the ground state bleaching of polymer 7/MWCNT hybrid by referring to the UV−vis absorption spectra shown in Figure 1a and Figure S8. These results suggest an ultrafast electron injection process from the 3MLCT (3polymer 7/MWCNT*) excited state to the MWCNT directly, leading to the generation of a polymer 7•+/ MWCNT•− species. However, the slow charge recombination process could not be observed by the fs-TA measurement. As shown in Figure 7c, the kinetics at 568 nm can be fitted simultaneously by a biexponential function with time constants of 0.6 and 383 ps, corresponding to the formation of the conformer and the electron injection process from polymer 7* to MWCNT, respectively. The second process is much faster than the relaxation of the 3MLCT excited state. A schematic diagram showing the energetic and kinetic data of all these processes is shown in Figure 8. It is proposed that after excitation at 400 nm, the singlet 1polymer 7/MWCNT* undergoes an ISC process to yield the triplet 3*polymer 7/ MWCNT* species in less than 150 fs, which is followed by the formation of 3polymer 7/MWCNT* conformer in 0.6 ps. Subsequently, an electron injection process takes place at 383 ps to produce the polymer 7•+/MWCNT•− species. The presence of such a very rapid electron transfer process between the photosensitizers and MWCNT is important to the design of new light harvesting systems based on MWCNT hybrids. Potential applications include organic photovoltaic devices and visible/near-infrared light sensors. The use of multifunctional block polymers allows us to fabricate highly stable MWCNT hybrid materials and to vary the photophysical properties of the photosensitizing units, which provides new insights for engineering the electronic structural properties of hybrid polymer/nanotube assemblies.

comparing with the TA spectrum of polymer 6, the change of the transient absorption bands for polymer 7 from 0.8 to 22 ps is less significant. This may be due to the more restricted conformation in triblock copolymer 7 compared to homopolymer 6. It has been reported by our group that pyrene showed two strong transient absorption bands at 370 and 480 nm and a shoulder at 520 nm.19 However, such absorption features were not observed in polymer 7 due to the negligible absorption of pyrene unit beyond 400 nm. In Figure 6b, the absorption bands decay from 22 ps to 3 ns due to the relaxation of the 3MLCT (Ru-3tpy) excited state. The kinetics at 572 nm (Figure 7c) can

Figure 7. (a, b) fs-TA of polymer 7/MWCNT in DMF solution acquired after 400 nm irradiation. (c) Kinetics for both polymer 7 and polymer 7/MWCNT hybrid. The solid lines indicate the kinetics fitting to the experimental data points.

be fitted by a biexponential function with time constants of 11.8 ps and 5 ns, which are similar to those obtained in polymer 6. This further suggests that the pyrene units have no significant effect to the generation of the 3MLCT excited states in polymer 7. A schematic diagram summarizing the formation and decay of all of the transient species observed is shown in Figure 8. After dispersing MWCNT by polymer 7 in DMF solution, the resulting polymer/MWCNT hybrid showed a significant change in its photophysical properties due to the strong

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CONCLUSIONS In summary, triblock copolymers incorporated with pyrene and ruthenium complex photosensitizing moieties were synthesized. The copolymer serves as a model for studies of the photophysics of metallopolymer/CNT light harvesting system. The interaction between the polymer and MWCNT was modeled by MD simulations, which showed that the triblock polymer provided a strong and effective interaction with MWCNT, and the ruthenium complex moieties were kept at a distance away from MWCNT. The lack of direct contact between the ruthenium complex and the MWCNT is critical in prolonging the lifetime of the charge-separated states. The

Figure 8. Schematic diagram showing the photophysical processes in polymer 7 (left) and the charge injection process in the polymer 7/ MWCNT hybrid (right). F

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The Journal of Physical Chemistry C

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photoinduced electron transfer process from the ruthenium complex sensitizers to the MWCNT was probed by ultrafast transient absorption spectroscopy. By comparing the results observed from the polymer/MWCNT hybrid with those obtained from the model polymer, the lifetime of 3MLCT states was significantly shorter (383 ps) because of the rapid electron injection process from the 3MLCT excited state to MWCNT. These new results clearly show the important role played by the MWCNT in the photoexcitation and charge transfer processes. Understanding the dynamics of these photophysical processes provides a basis for designing efficient light harvesting systems for solar energy conversion or other light sensing applications.

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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.jpcc.6b12812. Materials, instruments, and additional characterization data including NMR and FTIR spectra, molecular weight measurement results, TGA thermograms, EDX spectrum (Figures S1−S5 and Table S1), and additional transient absorption spectra (Figures S6−S8) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.K.C.). *E-mail: [email protected] (D.L.P.). ORCID

Wai Kin Chan: 0000-0002-5898-903X David Lee Phillips: 0000-0002-8606-8780 Author Contributions ‡

These authors contributed equally.

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, AoE/P-03/08). Partial financial support from the Hong Kong UGC Special Equipment Grants (SEG HKU07) and the University Development Fund (U of HK) project on “New Ultrafast Spectroscopy Experiments for Shared Facilities” is also acknowledged. The computation was conducted in part using the HKU Information Technology Services research computing facilities that are 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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