Article pubs.acs.org/JPCC
Understanding Photophysical Interactions of Semiconducting Carbon Nanotubes with Porphyrin Chromophores Hanyu Zhang,† Matthew A. Bork,‡ Kelley J. Riedy,† David R. McMillin,‡ and Jong Hyun Choi*,† †
School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United States Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
‡
S Supporting Information *
ABSTRACT: Donor−acceptor complexes of porphyrins and semiconducting single-walled carbon nanotubes (SWCNTs) are noncovalently assembled using oligonucleotide DNA, and their photophysical interactions are studied for light-harvesting. Five cationic 5,10,15,20-tetrakis(N-methylpyridynium-4yl)porphyrins with a free-base (H2T4) or metal ions at the core (MT4, M = Zn2+, Pt2+, Pd2+, and Cu2+) are explored as donor chromophores as they exhibit species-unique optical signatures, such as fluorescence, phosphorescence, or both. These porphyrins are examined for their abilities to interact with semiconducting carbon nanotubes after photoexcitation. We find that carbon nanotubes efficiently quench the emission properties of porphyrins via charge transfer, which is confirmed by the quenching of singlet oxygen emission generated by porphyrins. Phosphorescence lifetime measurements reveal that the lifetime in the triplet states is largely constant in porphyrins interacting with both DNA alone and DNA-coated SWCNTs, suggesting that photoexcited electrons are transferred to carbon nanotubes from the low-lying singlet state before an intersystem crossing to the triplet state. We demonstrate that the DNA-assembled porphyrin−SWCNT complexes in a photoelectrochemical cell produce stable anodic photocurrents with a conversion efficiency of approximately 1.5%.
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the density-gradient ultracentrifugation (DGU) method,14,15 have demonstrated an ability for charge separation that is superior to unsorted or metallic nanotubes.16 The near-ballistic charge-transport properties of nanotubes may be exploited with noncovalent surface chemistry,17−20 which has not been used often in carbon-nanotube-based donor−acceptor complexes.1,16 Few studies have systematically examined light-harvesting complexes based on semiconducting SWCNTs noncovalently interacting with various porphyrin species; hence, their photoconversion processes are not well-understood. In the present study, we have investigated the photophysical interactions of semiconducting SWCNTs with free-base (H2T4) and metalloporphyrins (MT4), noncovalently assembled by DNA oligonucleotides. Four cationic metalloporphyrins, including ZnT4, PtT4, PdT4, and CuT4, along with H2T4 are explored as photoactive donor molecules (Figure 1). Each of these chromophores shows distinct emission features; H2T4 and ZnT4 exhibit primarily singletstate transitions, whereas PtT4 and CuT4 demonstrate efficient intersystem crossing, displaying phosphorescence from triplet states.10,21,22 PdT4 shows both fluorescence and phosphorescence signatures in its spectrum.6,9,10 From steady-state
INTRODUCTION Porphyrin chromophores tethered to carbon nanotubes have been explored as light-harvesting donor−acceptor complexes.1−4 As effective photosensitizers, porphyrin molecules display several necessary characteristics including strong photoabsorption by the electron-rich π-systems and favorable redox potentials with respect to common electrolytes.5 In addition, the optical properties of porphyrins can be readily tuned by incorporating a metal ion at the core.6−8 We recently have shown that a water-soluble cationic (5,10,15,20-tetrakis(N-methylpyridynium-4-yl)porphyrin)palladium(II) (MT4, M = PdII in this case) exhibits emission signatures from both singlet- and triplet-state transitions, yielding fluorescence and phosphorescence, whereas the metal-free porphyrin (H2T4) displays fluorescence only.9 As a consequence, PdT4 triplet emission may be quenched significantly by oxygen molecules dissolved in solution, while fluorescence signals of H2T4 and PdT4 are not subject to the diffusional quenching. The porphyrin electronic properties can be further tuned by changing the type of metal ions at the center.10 Such tunability has not been explored extensively in donor−acceptor complexes for light-harvesting applications. As a crucial part of donor−acceptor complexes, single-walled carbon nanotubes (SWCNTs) possess desirable traits such as efficient charge separation and high carrier mobility.11−13 Highpurity (∼99%) semiconducting carbon nanotubes, isolated by © 2014 American Chemical Society
Received: April 2, 2014 Revised: May 9, 2014 Published: May 10, 2014 11612
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gently peeling off the PTFE membrane (also see Figure S3, Supporting Information). Optical and Electrochemical Characterization. Steadystate absorption and emission measurements were performed with a PerkinElmer Lambda 950 UV/visible/near-IR spectrophotometer and a Horiba Jobin Yvon Fluorolog 3, respectively. Hypochromism in the absorption spectrum of a porphyrin interacting with DNA or DNA−SWCNTs was determined as the percent decrease of the Soret peak intensity compared to unbound porphyrin Figure 1. Molecular structures of H2T4 (a) and metal-centered MT4 (b), where M = ZnII, PtII, PdII, or CuII.
A − A′ × 100 (%) A
fluorescence measurements, we find that these porphyrins undergo strong charge-transfer interactions with semiconducting nanotubes, which is consistent with our observation of emission quenching of singlet oxygen generated by porphyrins. The excited-state lifetime measurements indicate that photoexcited electrons are transferred to carbon nanotubes from their singlet states rather than their triplet states. The porphyrin− SWCNT-based donor−acceptor complexes all generate stable anodic photocurrents in a photoelectrochemical cell with an efficiency of approximately 1.5% at the excitation wavelengths except CuT4, which has a unique relaxation kinetics.
where A is the absorption of unbound porphyrin at the Soret maximum wavelength and A′ is the absorption of the bound porphyrin at the red-shifted Soret peak wavelength. The excitation wavelengths for porphyrin emission spectra were at their Soret band maxima, which were 422, 435, 401, and 417 nm for H2T4, ZnT4, PtT4, and PdT4, respectively. The emission spectra were corrected by absorption strengths at given excitation wavelengths
EXPERIMENTAL SECTION Materials Preparation. H2T4 tetratosylate was purchased from Frontier Scientific and used without further purification. Metal-centered porphyrins (MT4, M = ZnII, PtII, PdII, CuII) were synthesized using H2T4 as precursors. The detailed synthesis procedures are described in the Supporting Information and were published elsewhere.9,23 The synthesized porphyrin nitrate powders were dissolved in H2O for most of the studies except for the singlet oxygen emission experiments where D2O was used. DGU-purified semiconducting SWCNTs were purchased from NanoIntegris as solid films. These arc-discharge SWCNTs have diameters of 1.3−1.6 nm,16 exhibit E11 features in the near-infrared (near-IR) from 1600 to 2100 nm and display E22 signatures from 900 to 1200 nm (Figure S1a, Supporting Information). These tubes were dispersed in aqueous solution with 1 wt % sodium cholate (SC) through tip sonication for 1 h at 20 W, followed by centrifugation at 15 000 g. The SCdispersed SWCNT solution was subject to surfactant replacement via two-stage dialysis.16 Twenty-four base-long human telomere sequence (5′-(AGGGTT)4 -3′) strands functionalize SWCNTs noncovalently, as confirmed by absorption and resonant Raman spectroscopy (Figure S1, Supporting Information). The molar ratio of DNA to SWCNT is estimated to be roughly 100 strands or 2400 nucleobases per 1 μm of nanotube.16,17,20 DNA-coated SWCNT solution samples were used for optical measurements with porphyrins or deposited onto a polytetrafluoroethylene (PTFE) membrane with a pore size of 0.2 μm. Using vacuum filtration, a porphyrin solution was introduced to the DNA−SWCNT network film on the membrane slowly to allow interaction between porphyrin and DNA, followed by several washing steps to remove excess, free, and weakly bound porphyrins. The membrane film was subsequently pushed against an indium tin oxide (ITO, 70−100 Ω/□, Sigma-Aldrich) coated glass square with constant pressure, which was placed on a hot plate until the film dried fully. Finally, the porphyrin−DNA− SWCNT complex film was transferred to the ITO glasses by
where E represents the measured emission intensity and A is the absorbance. For singlet oxygen emission experiments, PtT4 samples were prepared in 90% D2O with 10% aqueous solution. The concentration ratios of DNA nucleobases and DNA on SWCNTs to PtT4 were estimated to be approximately 2.4:1. Phosphorescence lifetime measurements of PtT4 were carried out using a home-built system including a Hamamatsu R928 photomultiplier tube (PMT) and a dye (2-(1-naphthyl)5-phenyloxazole in toluene) laser pumped by a VSL-337-NDS nitrogen laser. The emission signals were captured with a Tectronix TDS 520 oscilloscope. The obtained signals of the PtT4 sample were fitted with a single-exponential curve, while double-exponential decay functions were used for PtT4 with DNA and DNA−SWCNTs to achieve an optimal fit. In a photoelectrochemical cell, an ITO glass square with an SWCNT−DNA−porphyrin complex was used as a working electrode. Short-circuit photocurrents were measured with a platinum-coated glass as the counter electrode, pinned together with the reference electrode. A Princeton 263A galvanostat recorded photocurrents of deposited SWCNT−DNA−porphyrin films with 5 mM sodium L-ascorbate (NaAsc) in 10 mM mono/disodium phosphate buffer at pH 7.0. A 450 W xenon lamp was used for photoexcitation in conjunction with a Czerny-Turner-type double monochromator with a bandpass of 14.7 nm. The excitation wavelengths for H2T4-, ZnT4-, PtT4-, PdT4-, and CuT4-based complex films were 440, 460, 420, 440, and 440 nm, respectively. The action spectrum of ZnT4-based film was obtained by exciting over a range of 380−800 nm, where the photocurrent was corrected for light intensity at corresponding wavelengths. The film without any porphyrins was excited from 380 to 500 nm with a 10 nm increment. The photoconversion efficiency or internal quantum efficiency (IQE) of the complex films was evaluated by
Ecorrected =
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E 1 − 10−A
IQE = 1240
ISC λ × IP
where Isc [mA/cm2] is the short-circuit photocurrent density measured at the excitation wavelength λ [nm] and Ip [mW/ 11613
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Figure 2. Steady-state absorption (upper row) and emission (bottom row) spectra of porphyrins in 5 mM phosphate buffer as a function of DNA nucleobase-to-porphyrin ratio ranging from 0 (black), 1.2 (red), 2.4 (blue), 12 (brown) to 24 (green): (a, f) H2T4; (b, g) ZnT4; (c, h) PtT4; (d, i) PdT4; (e) CuT4. The excitation wavelengths for H2T4, ZnT4, PtT4, and PdT4 emissions were 421, 437, 401, and 417 nm, respectively. The emission spectra of H2T4, ZnT4, and PdT4 were obtained using 5 nm slit widths for both excitation and emission, whereas 8 nm slit widths were used to record PtT4 emission. The emission spectra are corrected for the absorption strengths at the respective excitation wavelengths.
signature is due to a solvent effect34 and becomes better resolved in less polar solvents, such as ethanol, or with interaction with DNA strands.6 With the molar ratio of DNA/ H2T4 = 12 or higher, two distinct fluorescence peaks at ∼660 and 730 nm are observed. However, the total intensity (i.e., area under the curve) remains roughly constant for all the conditions examined. ZnT4 fluoresces from 600 to 700 nm. The fluorescence signature of free solution ZnT4 molecules is well-defined with two distinct emission peaks at ∼620 and 660 nm compared to that of H2T4. The emission spectra are nearly the same for all DNA/porphyrin ratios from 0 to 24. Despite minor differences in their emission characteristics, both H2T4 and ZnT4 porphyrins demonstrate similar photophysics in that, after excitations to their respective Soret bands (S2), rapid relaxation to lower-lying Q-bands (S1) occurs from which fluorescence emission is generated. Figure 2h shows the emission spectra of PtT4, which are starkly different from those of H2T4 and ZnT4. The PtT4 system undergoes an intersystem crossing efficiently from a singlet to a triplet state, resulting in phosphorescence peaking at ∼680 nm.8 The phosphorescence signal increases with DNA addition, reaching a 4 times greater signal as the DNA/ porphyrin ratio increases from 0 to 24. The increase of the emission signals is attributed to better isolation of porphyrin from oxygen molecules dissolved in buffer that would otherwise interact with the triplet state, forming singlet oxygen and/or superoxide, and thereby diminishing the emission.9,35 The PtT4−DNA interaction reduces the accessibility of the oxygen molecules and protects the porphyrin from emission quenching via energy transfer. The same quenching mechanism is not observed with H2T4 and ZnT4 because their emissions are primarily from singlet states, which have excited-state lifetimes that are significantly shorter than the diffusion time scale of dissolved oxygen.27 Interestingly, PdT4 molecules exhibit both singlet- and triplet-state emissions. Figure 2i shows three distinct emission features of PdT4, where the two peaks at ∼580 and 620 nm correspond to fluorescence, and the signature at ∼690 nm is phosphorescence.6,9,36 A fraction of the excited states emit
cm2] is the light power absorbed by the porphyrins at that wavelength.24
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RESULTS AND DISCUSSION Porphyrin−DNA Interaction. H2T4 and MT4 porphyrin chromophores exhibit strong binding affinities toward DNA oligonucleotides, particularly with the guanine (G)-rich human telomere sequence that is used throughout this study. For example, the dissociation equilibrium constant (Kd) of the 24mer human telomere DNA with H2T4 is approximately 200 nM,25 forming a G-quadruplex structure in the presence of alkali metal ions in a buffer such as K+ and Na+.26 The strong interaction modulates the porphyrin properties, which can be studied by probing porphyrin absorption and emission signatures. Figure 2 shows steady-state absorption and emission spectra of the porphyrins with DNA at various molar ratios of nucleobases to porphyrin ranging from 0 to 24. Absorption spectra (Figure 2a−e) show that all of these porphyrins have prominent Soret bands at 400−440 nm and less prominent Q bands at 500−750 nm. H2T4 displays four bands from Q transitions, while metalloporphyrins only show two peaks due to their increased symmetry.27,28 In the absence of 24-mer DNA, H2T4, ZnT4, PtT4, PdT4, and CuT4 display Soret peaks at 422, 435, 401, 417, and 425 nm, respectively. These Soret bands demonstrate strong hypochromic and bathochromic shifts as the amount of DNA is increased; H2T4, ZnT4, PtT4, PdT4 and CuT4 exhibit 40, 28, 46, 45 and 34% hypochromism and 16, 11, 14, 15, and 9 nm red shifts, respectively, at the DNA/porphyrin ratio of 24:1. This strong optical transduction originates from stacking interactions between nucleobases and the porphyrin core.29−32 The porphyrin molecules can also interact with oligonucleotides via nonspecific electrostatic binding, as four positively charged pyridine groups of the porphyrins bind to negatively charged DNA backbone.31,33 Unlike absorption spectra, the emission spectra of porphyrins with various DNA concentrations show more species-dependent characteristics (Figure 2f−i). In the absence of DNA strands, H2T4 molecules in buffer emit strong fluorescence from 650 to 750 nm (Figure 2f).10 This broadened emission 11614
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Figure 3. Steady-state absorption (upper row) and emission (bottom row) spectra of porphyrins as a function of DNA−SWCNT concentration. While the DNA nucleobases-to-SWCNT ratio is held constant (∼2400), the molar ratio of DNA nucleobase-to-porphyrin is varied from 0 (black), 0.48 (cyan), 1.2 (red), 2.4 (blue) to 4.8 (olive): (a, f) H2T4; (b, g) ZnT4; (c, h) PtT4; (d, i) PdT4; and (e) CuT4. The emission spectra of H2T4, ZnT4, and PdT4 were obtained using 5 nm slit widths for both excitation and emission, whereas 8 nm slit widths were used to record PtT4 emission. The emission signals are corrected for absorption strengths at the excitation wavelengths, which are 421, 437, 401, and 417 nm for H2T4, ZnT4, PtT4, and PdT4, respectively.
fluorescence, while the radiative decay of the triplet state is efficient enough to generate phosphorescence. As seen with H2T4 and ZnT4 in Figure 2f,g, the fluorescence features of PdT4 do not change significantly with the addition of DNA. In contrast, the phosphorescence signal increases significantly with DNA, demonstrating a 5 times enhanced intensity at the DNA/ porphyrin ratio of 24:1 compared with the emission in the absence of DNA. Similar to the PtT4 case, DNA plays a critical role in isolating PdT4 from oxygen molecules that quench the phosphorescence.9 CuT4 also emits phosphorescence, but it is not measured here because its signal is very short-lived.22,37,38 Porphyrin−DNA−SWCNT Interaction. Figure 3 shows absorption and emission spectra of porphyrins interacting with DNA−SWCNTs, where both similarities and differences are observed compared to the porphyrin spectra with DNA. The Soret bands demonstrate hypochromic and bathochromic shifts with increasing DNA−SWCNTs, as seen with porphyrins interacting with DNA. At the DNA/porphyrin ratio of 4.8:1, H2T4, ZnT4, PtT4, PdT4, and CuT4 display 12, 12, 11, 13, and 7 nm red shifts and 41, 38, 47, 60 and 37% hypochromism, respectively. Interestingly, the bathochromism and hypochromism are more prominent with DNA−SWCNTs than with DNA alone, as similar degrees of shifts are observed with DNA− SWCNTs at lower DNA/porphyrin ratios; for example, olive curves in Figure 3a−e (DNA/porphyrin = 4.8) display similar shifts with brown curves in Figure 2a−e (DNA/porphyrin = 12). However, our estimation of the number of DNA per nanotube is somewhat uncertain because the DNA conformation may differ at the SWCNT surface. In fact, we were unable to observe G-quadruplex-induced signals in circular dichroism (CD) spectra from the SWCNT-bound DNA samples. Experiments with Li+ substitution also suggest that the DNA may not be in the G-quadruplex conformation at the SWCNT surface, vide infra. The emission signals of porphyrins with DNA−SWCNTs show important differences from those with DNA alone. While H2T4 and ZnT4 porphyrins interacting with DNA display largely unchanged emission, their fluorescence intensities
gradually decrease with increasing concentration of DNA− SWCNTs. More than half of the fluorescence intensity is quenched at the DNA/porphyrin ratio of 4.8:1 (olive curves in Figure 3f,g). It is also noted that, with increasing DNA− SWCNT concentration, H2T4 fluorescence becomes better resolved (Figure 3f), as seen with porphyrin−DNA (Figure 2f). DNA strands bind strongly to the porphyrin, while solubilizing nanotubes in aqueous solution, and the interaction between porphyrin and carbon nanotubes results in emission quenching. Here, no SWCNT aggregation was apparent under the experimental conditions. Previous studies on porphyrintethered carbon nanotubes showed strong charge-transfer interaction between porphyrins and carbon nanotubes, which suppressed porphyrin emission.3,16,39 Our results suggest that charge transfer to nanotubes may occur from S1 and/or S2. For the S2 state, a charge-transfer process may be driven by the high energy, but S2 has a very fast internal relaxation time.40 In contrast, the lower-lying S1 state may have a comparable time scale with a transfer reaction rate. We show later that our photoelectrochemical measurement provides direct evidence of a charge transfer from S1 to nanotubes. The phosphorescence signals of PtT4 are also modulated by carbon nanotubes, as shown in Figure 3h, where a smaller increase of the emission intensity is observed with DNA− SWCNTs (Figure 3h) compared to DNA alone (Figure 2h). At a DNA/porphyrin ratio of ∼2.4, the emission intensity increases by ∼20% in the presence of SWCNTs, whereas a ∼250% increase is observed with PtT4 with DNA strands alone. The fact that the presence of SWCNTs lessens the increase of PtT4 emission suggests three possible quenching mechanisms. Photoexcited electrons of the porphyrin can be either transferred to SWCNTs before the intersystem crossing or quenched directly from a triplet state (T) by nanotubes. If the DNA conformation changes at the nanotube surface, the PtT4 molecules may also be more exposed to dissolved oxygen molecules. These quenching mechanisms are discussed below in detail with emission lifetime and photoelectrochemical measurements. 11615
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The effects of carbon nanotubes on both fluorescence and phosphorescence emission are also observed with PdT4 (Figure 3i). The fluorescence peaks at ∼580 and ∼620 nm gradually decrease with increasing concentrations of DNA−SWCNTs, displaying approximately half of the original PdT4 emission signal at the ratio of DNA/PdT4 = 2.4 (shown in blue). While the fluorescence signals are quenched with increasing DNA− SWCNTs concentrations, the phosphorescence signature at ∼690 nm is gradually enhanced. However, the extent of increase in the triplet-state emission is significantly less for DNA−SWCNTs than the case with no SWCNTs; for example, 50% vs 400% increases are observed at the DNA/PdT4 ratio of 2.4. These results are consistent with SWCNT-induced fluorescence quenching in H2T4 and ZnT4 and the smaller increase of PtT4 phosphorescence in the presence of carbon nanotubes. The emission of CuT4 in the presence of DNA− SWCNTs was not quantifiable due to its weak intensity. Triplet Quenching Studies. The interactions between porphyrins and SWCNTs were further studied by monitoring near-IR emission of singlet oxygen molecules created by PtT4. Figure 4a shows the singlet oxygen emission in the presence of
from interacting with the oxygen molecules in solution, thereby slowing the buildup of singlet oxygen.9 In comparison, anionic surfactant SC molecules barely changed the emission signals of singlet oxygen even at an orders-of-magnitude higher concentration than the DNA strands (Figure S2, Supporting Information), consistent with effective screening by oligonucleotides. The presence of carbon nanotubes generates significantly greater quenching of the singlet oxygen emission. PtT4 molecules with DNA−SWCNTs demonstrate nearly complete quenching, whereas PtT4 molecules with SC− SWCNTs show approximately 70% quenching at the same nanotube concentrations (Figure S2, Supporting Information). This observation indicates that the nanotube does not greatly alter the porphyrin/DNA interaction. Thus, one would expect an increase of singlet oxygen emission from PtT4 samples with DNA−SWCNTs, for example, if the DNA is unable to protect the porphyrins from interaction with oxygen molecules due to any change of its conformation. However, several mechanisms may be at work in nanotube-induced quenching of singlet oxygen emission. Carbon nanotubes interacting with PtT4 molecules may intercept the photoexcited electrons before they undergo the intersystem crossing or directly quench triplet states, thus preventing singlet oxygen generation.9 It is also possible that SWCNTs can directly interact with the singlet oxygen and quench the emission to some degree.44 To probe the prevalence of nanotube interaction with porphyrins via their singlet state versus triplet state, we performed the excited-state lifetime measurements of PtT4 with DNA and DNA−SWCNTs (Figure 4b). The phosphorescence lifetime of PtT4 alone demonstrates a singleexponential decay function with a decay time of approximately 1.0 μs, which is affected by oxygen molecules in buffer. In the presence of DNA and DNA−SWCNTs, the PtT4 emission lifetime drastically increases due to a better isolation of porphyrins from oxygen molecules in solution. Here, two distinct (fast and slow) components are observed, as the signal is best fitted with a double-exponential decay function. In the presence of the DNA, the fast decay of ∼1.9 μs and the slow decay of ∼9.9 μs are measured. In the presence of DNA− SWCNTs, decay lifetime components of ∼1.1 and ∼9.3 μs are recorded. The fact that both slow components with DNA and DNA−SWCNTs are similar indicates that the presence of SWCNTs does not play significant roles in phosphorescence
Figure 4. (a) Near-IR emissions of singlet oxygen generated by PtT4 in the presence of DNA strands and DNA-coated SWCNTs. (b) Excited-state decay measurements of PtT4 with DNA and DNA− SWCNTs. The phosphorescence lifetime of PtT4 molecules demonstrates a single-exponential decay, whereas those of porphyrins interacting with DNA and DNA−SWCNTs follow a doubleexponential decay function.
PtT4 with DNA and DNA−SWCNTs. Singlet oxygen (1O2), emitting at ∼1270 nm,41,42 is generated by energy transfer from the triplet state of photoexcited porphyrin to molecular oxygen in buffer.9,42,43 The singlet oxygen emission signal diminishes in the presence of DNA, which protects porphyrin chromophores
Figure 5. Photoelectrochemical measurements of porphyrin−DNA−SWCNT films in 5 mM phosphate buffer with 1 mM sodium L-ascorbic acid. (a) Absorption spectra of H2T4-, ZnT4-, PtT4-, PdT4-, and CuT4-based DNA−SWCNT films. The spectra are offset for clarity. (b) Short-circuit photocurrent of PdT4−DNA−SWCNT complexes with photoexcitation at 440 nm. Approximately 0.7 μA/cm2 is recorded with the light turned on, and the photocurrent becomes zero when the light is off. (c) Action spectrum of ZnT4−DNA−SWCNT complexes. Photocurrent signals as a function of excitation wavelength are shown in blue dots, while the absorption spectrum of the complexes in a deposited film is presented in red. The film absorption spectrum is shown after subtraction of the DNA−SWCNT absorption, representing porphyrin signatures only. (d) Photocurrents and the corresponding photoconversion efficiencies of the deposited DNA−SWCNT films with various porphyrins and without any porphyrin. 11616
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the porphyrin−DNA−SWCNT film sample processed with the buffer containing Li+, which is known to destabilize the Gquadruplex conformation46 instead of Na+. We observed the photocurrents comparable to those in Figure 5, suggesting that the G-quadruplex may not be involved in the photoconversion processes. With all our aforementioned observations and discussion, we summarize the photophysical interactions in porphyrin−DNA− nanotube complexes in Figure 6. The most probable scenario
emission and that SWCNTs do not quench the triplet state of the porphyrins directly. Light-Harvesting and Conversion. The interactions between photoexcited porphyrins and SWCNTs are exploited for light-harvesting in a photoelectrochemical cell. Figure 5a shows optical absorption spectra of the assemblies deposited on the ITO working electrode. The Soret bands of these transparent films (Figure S1a, Supporting Information) are clearly present in the spectra, and E22 absorption features of semiconducting SWCNTs are observed from 900 to 1100 nm (Figure S1a).16 These samples typically include approximately 4 μg of SWCNTs that are also responsible for the broad background in the UV and visible range. With ascorbate and its oxidized product, dehydroascorbic acid, as a redox couple in phosphate buffer, porphyrin-based complexes generate consistent anodic photocurrents, where photoinduced electrons are collected at the ITO. Figure 5b shows typical, time-resolved photocurrents from a PdT4−DNA−SWCNT film sample. Irradiation at 440 nm rapidly produces photocurrents at approximately 700 nA/cm2, which becomes zero with the light turned off. These photocurrents originate from the porphyrin photoabsorption as the Soret and Q-band absorption signatures match well with photocurrent signals (Figure 5c). This observation confirms a charge-transfer process from the Qband (S1) to the bound nanotube, while not excluding the possibility of S2 state participation as well. Our photoelectrochemical measurements suggest that the photogenerated electron−hole pairs in porphyrin molecules are separated at the DNA−SWCNT interface and the electrons are collected at the working electrode. The oxidized chromophore molecules are then regenerated by ascorbate, converting into dehydroascorbic acid, which is subsequently reduced at the Pt counter electrode in the photoelectrochemical cell. The photocurrents of H2T4, ZnT4, PtT4, PdT4, and CuT4 complex films and their photoconversion efficiencies are shown in Figure 5d. Here, a DNA−SWCNT film containing no porphyrins was also examined as a control: it did not produce any detectable photocurrents. This observation verifies that neither DNA nor SWCNTs serve as the charge donors in the complexes. The complex films incorporating H2T4, ZnT4, PtT4, and PdT4 have similar photoconversion efficiencies of ∼1.5%. It is clear that the efficiency of the intersystem crossing does not have a significant impact on photoconversion performance as H2T4-, ZnT4-, PtT4-, and PdT4-based films demonstrate similar efficiencies. It is interesting that PtT4 generates a photocurrent comparable to those of H2T4 and ZnT4. The result is striking because PtT4 does not exhibit fluorescence; hence, it is likely to undergo intersystem crossing a 100 or more times faster than H2T4 or ZnT4. In each case, the rate of electron injection from S1 into SWCNT/ITO must compete with intersystem crossing. The explanation may be that, like PdT4, PtT4 has a higher energy S1 state, which, in turn, means the driving force for electron injection ought to be greater. An alternative explanation could be that only a subpopulation of porphyrin is properly disposed for efficient electron injection. The films containing CuT4-based complexes did not produce significant photocurrents (∼40 nA) under the identical experimental conditions. The negligible photocurrent and photoconversion efficiency with CuT4-based complexes could result from an extremely short, excited singlet-state lifetime in CuT4, which is roughly 65 fs.45 Therefore, it is unlikely for electron transfer to SWCNTs to occur. In parallel, we examined
Figure 6. Proposed interaction scheme. The photogenerated electrons in porphyrins may transfer from a low-lying singlet state (S) to the bound carbon nanotube. Once electrons undergo an intersystem crossing to a triplet state (T), they return to the ground state, either emitting phosphorescence or generating singlet oxygen molecules via energy transfer that emit in the near-IR.
upon photoexcitation at the Soret band is that the system rapidly relaxes to the lower-lying singlet state (S1), where an electron may be transferred to nanotubes and converted to photocurrents. The photoelectrochemical measurement results favor a fast electron-transfer mechanism from photoexcited chromophores to SWCNTs before intersystem crossing, consistent with our singlet oxygen emission experiments. Once the intersystem crossing to the triplet state is completed, especially in PtT4 and PdT4, the interaction of the triplet state with oxygen molecules may yield 1O2, which emits phosphorescence in the near-IR spectrum. However, CuT4 likely experiences different relaxation pathways as electrons excited to S2 may bypass S1 and transition to a triplet state,45 which probably explains the weak photocurrents from CuT4− nanotube complexes.
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CONLUSION In this study, we investigated the photophysical processes in the light-harvesting complexes of H2T4 and MT4 (i.e., ZnT4, PtT4, PdT4, and CuT4) porphyrins and semiconducting carbon nanotubes. The porphyrins display strong bathochromic and hypochromic shifts when interacting with 24-mer DNA and DNA-coated SWCNTs. The fluorescence signals of H2T4, ZnT4, and PdT4 are largely unchanged with DNA oligonucleotides, whereas a significant quenching is observed in the presence of SWCNTs. The phosphorescence signatures of PtT4 and PdT4 increase drastically with increasing DNA due to a better exclusion of oxygen molecules that otherwise quench porphyrin emission by generating singlet oxygen via energy transfer. However, the increase becomes less significant in the presence of SWCNTs. Our observation that the lifetime of the triplet state is not affected by the presence of SWCNTs suggests that nanotubes interact with porphyrins primarily via their singlet states before the intersystem crossing by photoexcited electrons. We demonstrate that these electrons can be harvested and converted into electrical current in a photoelectrochemical cell, where ascorbate and dehydroascor11617
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bic acid are used as a redox couple. Stable anodic photocurrents are measured in a similar magnitude with all porphyrin complexes except CuT4, and we estimate conversion efficiencies of ∼1.5% at the excitation wavelengths for all porphyrins but CuT4 under our experimental conditions. Our study suggests that porphyrin derivatives and carbon nanotube species can be further explored to enhance their interactions, which could be useful in designing light-harvesting donor− acceptor complexes.
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ASSOCIATED CONTENT
* Supporting Information S
Experimental details and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation (NSF). J.H.C. gratefully acknowledges an NSF CAREER Award.
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REFERENCES
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