Article pubs.acs.org/JPCC
Photophysical Properties of Porphyrin Dimer−Single-Walled Carbon Nanotube Linked Systems Jinseok Baek,† Tomokazu Umeyama,*,† Satoshi Mizuno,† Nikolai V. Tkachenko,*,‡ and Hiroshi Imahori*,†,§ †
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Laboratory of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FIN-33101 Tampere, Finland § Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ‡
S Supporting Information *
ABSTRACT: Porphyrin dimers were covalently grafted onto electron-accepting single-walled carbon nanotube (SWNT) sidewalls by direct aryl radical addition reaction with an m- or p-phenylene linker with the help of π−π interaction between the porphyrins. A splitting of the porphyrin Soret band and DFT calculations supported the selective formation of the porphyrin dimers on the sidewall of SWNTs. Photoexcitation of the porphyrin dimers on the SWNT resulted in the formation of the exciplex state, which directly decayed to the ground state without yielding the complete charge-separated state. Lifetimes of the porphyrin dimer−SWNT exciplex were longer than that of a porphyrin monomer−SWNT exciplex due to the stabilization by π-electron interaction over two porphyrin rings. In addition, the weaker electronic coupling through the meta-linkage than the para-one may be responsible for the exciplex lifetime of the porphyrin dimer−SWNT with the m-phenylene linker (49 ps) longer than that with the p-phenylene one (24 ps). The results obtained here provide the basic information on the effect of the donor dimerization on the photodynamic behavior of the exciplex state in donor−acceptor linked systems.
■
INTRODUCTION Photosynthesis is one of the most indispensable processes in nature since it efficiently converts light energy to chemical energy. In the initial process of photosynthesis, chlorophylls arranged in a dimeric structure, so-called a special pair, play an important role in initiating a multistep electron transfer (ET) reaction to attain the final long-lived charge-separated state with nearly 100% quantum yield.1 Porphyrins, which have macrocyclic frameworks similar to chlorophylls, are accessible by simple synthetic procedures, and therefore much effort has been devoted to mimicking the dimer structure of the special pair by using porphyrins.2,3 Various covalently or noncovalently assembled artificial systems of porphyrin dimers with acceptors such as fullerenes, quinones, and imides have been constructed so far, revealing that the stabilization of the radical cation, arising from the π−π interaction between the two porphyrin rings, accelerated the photoinduced complete charge separation (CS) and decelerated charge recombination (CR).4−8 Not only fullerenes but also single-walled carbon nanotubes (SWNTs) are also known as an electron-accepting nanocarbon material with unique one-dimensional structures.9,10 Recently, we found that isolated dimers of a model π-aromatic compound, pyrene, were formed exclusively on the sidewall of SWNTs by the direct addition reaction of 4-(1-pyrenyl)phenyl radicals onto the SWNT sidewall.11 The dimer formation is due to the binding of a single aryl group with an © XXXX American Chemical Society
unpaired electron that activates carbon atoms near the binding site, thereby accelerating the addition of the next aryl group at the nearby carbon atoms.12,13 More importantly, the unique intermolecular π−π interaction between the pyrenes may also facilitate the concerted pairwise addition, resulting in the dimer formation.14 Given the intensive intermolecular π−π interaction between the two porphyrin rings, one can anticipate that the special pair mimics, i.e., porphyrin dimers, are grafted on the SWNT sidewall by using the direct addition reaction of the corresponding aryl radicals bearing a porphyrin moiety. In this regard, we also prepared selectively porphyrin monomer− SWNT linked systems with a rigid, short p-phenylene linker by the corresponding two-step reaction: initial addition reaction of the aryl radicals without the porphyrin moiety, followed by the low-yield Suzuki coupling of the porphyrin to the grafted aryl moiety.15−17 They revealed that the photoexcitation of the porphyrin monomer on SWNT yielded an exciplex state, possessing partial positive and negative charges located on the porphyrin and SWNT, respectively. The exciplex rapidly decayed to the ground state without forming the complete charge-separated state. The exciplex state is known to play a Special Issue: Prashant V. Kamat Festschrift Received: August 29, 2017 Revised: September 26, 2017
A
DOI: 10.1021/acs.jpcc.7b08594 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Scheme 1. Synthesis of (ZnP-L)2-SWNT and ZnP-p-ph-SWNT
SHIMADZU NIR-PL system, respectively. A time-correlated single-photon counting (TCSPC) method was employed to measure the fluorescence lifetime using a HORIBA NanoLOGTCSPC with a PPD-850 detector (time resolution; 380 nm; input power, 35 mW cm−2) or monochromatic light through a monochromator (MC-10N; Ritsu, Japan) was illuminated on the modified area of the working electrode (0.20 cm2) from the backside. The light intensity was monitored by an optical power meter (ML9002A; Anritsu, Japan) and corrected for calculation of IPCE values.
■
RESULTS AND DISCUSSION Zincporphyrin (ZnP) dimer−SWNT linked systems with mand p-phenylene linkers (denoted as (ZnP-m-ph)2-SWNT and (ZnP-p-ph)2-SWNT, respectively) were prepared by the direct addition reaction of the in situ generated phenyl radicals bearing a porphyrin unit onto purified SWNT (p-SWNT) sidewalls (Scheme 1, upper), as we have established for the pyrene dimer−SWNT linked systems.11,12,35 The ZnP unit with C
DOI: 10.1021/acs.jpcc.7b08594 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
on thermodynamic considerations, as is the case of the dimeric pyrene-functionalized SWNTs.11 Note that the large binding energy also supports the selective formation of the porphyrin dimers on SWNTs. The optimized dimeric structure of ZnP in the (ZnP-m-ph)2-SWNT exhibits a diagonally slipped-stacked configuration with interplanar distance of ca. 3.2 Å, where the molecular axes of the two porphyrins are parallel and a central Zn atom in one molecule is located on the diagonal direction of the other ZnP molecule (Figure 3a). It is noteworthy that the
Figure 1. UV−visible absorption spectra of (ZnP-m-ph)2-SWNT (red), (ZnP-p-ph)2-SWNT (green), ZnP-p-ph-SWNT (orange), pinBp-ph-SWNT (black), and ZnP-ref (blue, 0.6 μM). The spectra are shown in parallel for comparison. The scale bar denotes the absorbance of 0.1.
Figure 2. Enlarged UV−visible absorption spectra of (a) (ZnP-mph)2-SWNT, (b) (ZnP-p-ph)2-SWNT, (c) ZnP-p-ph-SWNT, and (d) ZnP-ref in the region of 380−500 nm. The spectra in (a) and (b) were fitted with two normal distributed absorptions shown by blue and red dotted lines on the downward-sloping black baseline that corresponds to the absorption of the SWNT moiety. Black dotted lines represent the sum of the blue and red dotted lines on the baseline. The spectrum in (c) is fitted with one normal distributed absorption shown by the blue dotted line on the downward-sloping black line that corresponds to the absorption of the SWNT moiety. Peak wavelengths are shown in the figures. The scale bars denote the absorbance of 0.1.
Figure 3. Structures of the porphyrin dimers in the models of (a) (ZnP-m-ph)2-SWNT and (b) (ZnP-p-ph)2-SWNT calculated by DFT at the RB3LYP-D3/3-21G* level. The H, C, N, and Zn atoms are displayed by a white, gray, blue, and red, respectively. The outer carbon π-bond for upper and lower porphyrin rings is displayed as red and blue solid lines, respectively. The interplanar distance between two porphyrins is (a) 3.2 and (b) 3.3 Å.
The existence of the split Soret band solely in the absorption spectra of (ZnP-L)2-SWNT and its absence in the absorption spectrum of ZnP-p-ph-SWNT support the selective formation of the ZnP dimers that are covalently linked onto the SWNT by the direct aryl radical additions, as demonstrated in the covalently linked pyrene dimer−SWNT system.11 To get insight into the porphyrin dimeric structures on the SWNT sidewall, the binding energies of the porphyrin dimer to the SWNT sidewall were assessed by DFT calculations using the unit cell model of (13,8)-chirality SWNT, which is dominant in the SWNT sample employed in this study.28,29 The binding energy of the two ZnP-m-ph groups to (13,8)SWNT is strongly dependent on the separation distance and orientation between the two ZnP-m-ph units (Figure S6). Functionalization at carbon atoms five C−C bonds distant in the axial direction afforded the most stable configuration based
ZnP pair in the (ZnP-p-ph)2-SWNT also forms an analogous slipped-stacked structure with a slightly longer interplanar distance (ca. 3.3 Å), irrespective of the difference in the linkers (Figure 3b). These optimized structures of the porphyrin dimers were then used to calculate the electronic transition energies (Figure S7). The simulation predicts that the main signals of the Soret bands for the ZnP dimers in both (ZnP-mph)2-SWNT and (ZnP-p-ph)2-SWNT are moderately blueshifted in comparison with that of ZnP-p-ph-SWNT, whereas the additional absorption bands emerge at the longer wavelength. These theoretical predictions qualitatively agree with the experimental results of the UV−visible absorption spectra, verifying the selective formation of the porphyrin dimers on SWNTs (Figure 1). Upon excitation of (ZnP-L)2-SWNT, ZnP-p-ph-SWNT, and ZnP-ref at the Soret band, where the absorbances of the D
DOI: 10.1021/acs.jpcc.7b08594 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C porphyrin are adjusted to be identical, the fluorescence intensities from the ZnP moieties on SWNT are reduced intensively compared to that of ZnP-ref (>99%) (Figure S8). This implies the strong quenching of the singlet excited states of the porphyrin dimer (1ZnP2*) in (ZnP-L)2-SWNT and the monomer (1ZnP*) in ZnP-p-ph-SWNT by the attached SWNT. Note here that the pinB-p-ph-SWNT shows almost no emission in the visible region by the photoexcitation at 427 nm (Figure S8). Furthermore, we monitored the porphyrin emission decays using time-correlated single-photon counting (TCSPC) technique. The fluorescence decay of ZnP-ref exhibits a single component with a lifetime (τ) of 1.7 ns (Figure S9). However, the fluorescence decay curves of (ZnPL)2-SWNT and ZnP-p-ph-SWNT are almost identical with the instrumental response signals, implying that 1ZnP2* and 1ZnP* are quenched rapidly by the tethered SWNT at times that are much shorter than those detected by the present TCSPC system (
