Letter pubs.acs.org/JPCL
Excited States in Cycloparaphenylenes: Dependence of Optical Properties on Ring Length Taishi Nishihara,† Yasutomo Segawa,‡ Kenichiro Itami,‡ and Yoshihiko Kanemitsu*,† †
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
‡
ABSTRACT: Hoop-shaped conjugated molecules, cycloparaphenylenes (CPPs), are simple strings of benzene rings with para linkages that have an ideal quasi-one-dimensional structure without edges. Here, we report optical properties of [n]CPPs (n = 9, 12, 14, 15, 16) clarified by one- and two-photon excitation spectroscopy. We showed that in this system the lowest unoccupied molecular orbital (LUMO) state has the same symmetry as the highest occupied molecular orbital (HOMO) state, and determined the transition energy of the optically forbidden HOMO−LUMO gap. It is found that the ring-length dependence of the HOMO−LUMO transition energy is identical to that of the photoluminescence (PL) energy, and that phonon-assisted transition causes efficient PL. SECTION: Spectroscopy, Photochemistry, and Excited States ince the first theoretical consideration of one-dimensional (1D) systems in 1959,1 there have been many extensive studies on the optical responses of 1D nanostructures.2,3 However, high-quality 1D quantum wires are very difficult to fabricate using molecular beam epitaxy and metal−organic chemical vapor deposition as compared to two-dimensional quantum wells and zero-dimensional quantum dots. Thus, the fundamental properties of 1D quantum wires remain unclear. Chemically synthesized materials such as polymers, oligomers, and single-walled carbon nanotubes have been utilized as 1D quantum wires for experimental studies.4−7 However, long-chain polymers have all the complexity of the real polymers, such as local disorder, defects, impurities, and chain length fluctuation, which can make their optical properties difficult to study. In contrast, oligomers with short chain lengths are well-defined chemical systems and as such can be advantageous for optical studies, since the conjugation chain length can be controlled exactly. In short-chain-length oligomers, however, the edge of the chain acts as a nonradiative recombination center and strongly affects the optical properties.8,9 To avoid this problem, we can instead consider the recently synthesized hoop-shaped conjugated molecules, the cycloparaphenylenes (CPPs), which are simple strings of benzene rings with para linkages that have an ideal quasi-1D structure without edges (Figure 1).10−20 In addition, CPPs
S
represent the shortest sidewall segments of armchair singlewalled carbon nanotubes. CPPs exhibit unique optical properties: for example, an unusual ring-length dependence of the photoluminescence (PL) peak and a Stokes shift of the luminescence.10,21,22 The origin of these unique optical properties of CPPs remains unclear. However, a deep understanding of fundamental physical properties of CPPs is very important for understanding 1D materials and nanocarbon science and technology. To gain deep insight into the optical responses of 1D nanostructures, we need to clarify the excited states of CPPs. In this work, we studied the dependence of the optical properties of [n]CPPs (n = 9, 12, 14, 15, 16) on the ring length by exploiting their advantageous optical absorption and PL using one-photon and two-photon excitation spectroscopy. In all CPPs, the lowest peak of two-photon PL excitation (PLE) is found to be lower than the main peak of optical absorption. The PL peak energy is observed to increase as the number of benzene rings increases, which is very similar behavior to that of the optically forbidden (dipole forbidden) highest occupied molecular orbital to lowest unoccupied moleculat orbital (HOMO−LUMO) transition. This unusual blueshift of the PL energy is attributed to the energy shift of the LUMO state. Strong exciton−vibration couplings are found to enable efficient PL. Figure 2a shows the optical absorption and PL spectra of [n]CPPs (n = 9, 12, 14, 15, 16). All spectra were normalized at their peak intensities. In the absorption spectra, all samples show a maximum peak at 3.66 eV whose energy does not depend on the ring length n. This absorption peak is attributed to the optical transitions between the HOMO and LUMO+1 and between the HOMO−1 and LUMO.21,22 We found that Received: September 21, 2012 Accepted: October 10, 2012 Published: October 12, 2012
Figure 1. [n]Cycloparaphenylene (CPP). © 2012 American Chemical Society
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One- and two-photon excitation spectra were monitored using the PL intensity in a wide spectral region between 1.9 and 3.1 eV in all CPPs. Two-photon absorption occurs when the final state has the same parity as the initial state, while the onephoton absorption transition occurs between opposite symmetry states. The inset of Figure 3 shows the excitation
Figure 3. One- and two-photon excitation spectra of [12]CPP. The inset shows the excitation intensity dependence of the PL intensity. A weak band appears at around 3.2 eV in the one- and two-photon excitation spectra. Figure 2. (a) PL (solid curves) and optical absorption spectra (broken curves) of [n]CPPs (n = 9, 12, 14, 15, 16). (b) Second-order derivatives of the PL and optical absorption spectra of [14]CPP.
intensity dependence of the 2.89 eV PL in [12]CPP under 1.85 eV excitation. The PL intensity is approximately proportional to the square of excitation intensity, which suggests that twophonon absorption produces efficient PL. No significant change in the PL spectrum is observed between one- and two-phonon excitation in any [n]CPP. Thus, the origin of the PL under onephoton excitation is the same as that under two-photon excitation. Figure 3 shows a weak band in the two-photon PLE spectrum of [12]CPP in the energy region between 2.9 and 3.5 eV. A clear band corresponding to the HOMO−LUMO gap energy appears in the two-photon excitation spectrum, and its 3.2 eV peak corresponds to the HOMO−LUMO energy predicted by theoretical calculations.22,27 Similar weak bands are also observed in [14]CPP, [15]CPP, and [16]CPP. The two-photon PLE signal increases rapidly in the high-energy region above 3.55 eV, and there is no peak at around 3.66 eV, where a one-photon absorption peak was observed. This implies the existence of a second two-photon absorption peak above the 3.66 eV one-photon absorption peak. This is also quite consistent with the theoretical calculation mentioned above.22,27 In summary, we have determined for the first time the energy of the optically forbidden HOMO−LUMO transition, i.e., the band gap energy of the 1D ring. The one-photon excitation spectrum is identical to the optical absorption spectrum, with a weak peak at around 3.2 eV. This means that the quantum efficiency of the luminescence is high22 and that nonradiative recombination processes do not affect the optical spectrum, because CPPs have no edge states acting as nonradiative recombination centers. The one-photon excitation peak energy at around 3.2 eV is almost the same as the two-photon excitation peak energy. Moreover, the 3.2 eV peak is also observed in the two-photon excitation spectrum. The optical transition between the HOMO and LUMO states is optically forbidden. However, the strong coupling between the exciton and the phonons (see the multipeaks in the PL spectrum in Figure 2a) allows the radiative HOMO−LUMO recombination. The pure lowest excited state is the optically forbidden one, but phonon scattering allows the radiative recombination to occur from the lowest excited state.
differences in the spectra appear in the low-energy region below 3.5 eV: the low-energy absorption spectrum of [9]CPP is quite different from those of other [n]CPPs. For [9]CPP, a weak peak appears at around 3.1 eV, while for the other CPPs, only the low-energy tail of the main 3.66 eV band appears. These results suggest the existence of a weak absorption band in the low-energy region in all [n]CPP samples.21,22 Under 3.66 eV excitation at the absorption peak, the PL spectrum depends on the ring length of [n]CPPs. The PL peak energy is blue-shifted with increasing ring length. Although similar ring-length dependence of the PL spectrum has been reported,10,21,22 its origin remains unclear. The origin of the unusual blueshift of the PL energy is discussed later. Multipeaks are also observed in the PL spectrum. Similar asymmetry between the optical absorption and PL spectra has been reported in oligo(para-phenylene)s: a broad and featureless band in the absorption spectrum and a clear vibronic multipeak band in the PL spectrum.23 To evaluate the peak energies of the absorption and PL spectra, we calculated the second-order derivatives of spectra. The second-order derivatives of the absorption and PL spectra of [14]CPP are shown in Figure 2b. In the PL spectrum, three peaks are clearly observed at 2.61, 2.78, and 2.96 eV. The energy spacing between these peaks is approximately 180 meV, which corresponds to the phenylene stretching vibration.24 This suggests that strong coupling occurs between the exciton and vibration during the radiative recombination of excitons. These multipeak structures are observed in all CPP samples. On the other hand, in the absorption spectra, the low-energy structure is unclear because of spectral overlap with the strong main peak at 3.66 eV. To clarify the energy structures in the low-energy region, we studied the one-photon and two-phonon excitation spectra of the PL in [n]CPPs. Such nonlinear optical spectroscopy is a powerful tool for understanding the excited states in 1D nanomaterials.5,25,26 3126
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and that phonon-assisted transition causes efficient PL. Our findings provide a good chance to understand the nature of 1D ring excitons and will open new functional nanocarbon devices based on CPPs.
Moreover, at room temperature, the degenerate ground state consists of different symmetry states.28 The resulting different optical transitions contribute the broad 3.2 eV band. Thus, the broad LUMO−HOMO transition band is observed in both the one- and two-photon excitation spectra. In Figure 4, we summarize the ring-length dependence of the optical absorption, two-photon excitation peaks, and PL peaks.
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EXPERIMENTAL METHODS CPPs were synthesized according to reported procedures.29−32 The CPPs were dissolved in chloroform, and quartz sample cells with a path length of 10 mm were used for all optical measurements. For two-photon excitation spectroscopy measurements, the light source was a wavelength-tunable optical parametric amplifier based on a regenerative amplified modelocked Ti:sapphire laser with a pulse duration of 200 fs and a repetition rate of 200 kHz. The laser beam was collinearly focused onto the sample using a lens with a 1000 mm focal length. It was confirmed using the knife-edge method that the laser spot size remained constant through the samples. The PL spectra were measured using a monochromator connected to a charge-coupled device camera. All measurements were performed at room temperature.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
Figure 4. Optical absorption peak, lowest two-photon excitation peak, and two PL peaks as a function of the number of benzene rings n in the [n]CPPs.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Part of this work was supported by KAKENHI (20104006), the MEXT Project of Integrated Research on Chemical Synthesis, and JST-CREST.
The two high-energy peaks of the PL spectra are named PL1 and PL2, respectively. The main optical absorption peak does not depend on the ring length, while both the two-photon excitation and the PL peaks increase with increasing ring length. The blueshift of the HOMO−LUMO gap energy occurs as the number of the benzene rings n increases. A similar trend has been predicted using theoretical calculations.21,22,27 Note that the ring-length dependence of the two-photon PLE peak is very similar to those of the two PL peaks; three curves with the same curvature are illustrated in the figure as a guide to the eye. Furthermore, the PL energy is smaller than the HOMO− LUMO energy, and the efficient PL is not attributed to the radiative recombination processes between the HOMO−1 and LUMO and between the HOMO and LUMO+1, as discussed in refs 21 and 22. This figure clearly shows that the PL energy comes from the LUMO state. Our findings clearly show that the luminescence Stokes shift between the PL and the lowest excited energies does not depend on the ring length. The blueshift of the PL peak energies is attributed to the ringlength-dependent HOMO−LUMO gap energy. Although the optical transition between the HOMO and LUMO states is a dipole forbidden one due to the symmetry between the LUMO and HOMO states, the phonon-assisted optical processes enable luminescence to occur from the excited to the ground states, leading to high fluorescence quantum yields and short fluorescence lifetimes of CPPs.22 In conclusion, we clarified the nature of the lowest excited state of [n]CPPs (n = 9, 12, 14, 15, 16) using linear and nonlinear optical spectroscopy techniques. The transition energy of the optically forbidden HOMO−LUMO gap was determined using the one-photon and two-photon PLE spectra. We found that the ring-length dependence of the HOMO− LUMO transition energy is identical to that of the PL energy
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REFERENCES
(1) Loudon, R. One-Dimensional Hydrogen Atom. Am. J. Phys. 1959, 27, 649−655. (2) Ogawa, T.; Takagahara, T. Optical Absorption and Sommerfeld Factors of One-Dimensional Semiconductors: An Exact Treatment of Excitonic Effects. Phys. Rev. B 1991, 44, 8138−8156. (3) Ando, T. Excitons in Carbon Nanotubes. J. Phys. Soc. Jpn. 1997, 66, 1066−1073. (4) Miller, R. D.; Michl, J. Polysilane High Polymers. Chem. Rev. 1989, 89, 1359−1410. (5) Hasegawa, T.; Iwasa, Y.; Sunamura, H.; Koda, T.; Tokura, Y.; Tachibana, H.; Matsumoto, M.; Abe, S. Nonlinear Optical Spectroscopy on One-Dimensional Excitons in Silicon Polymer, Polysilane. Phys. Rev. Lett. 1992, 69, 668−671. (6) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Exciton Photophysics of Carbon Nanotubes. Annu. Rev. Phys. Chem. 2007, 58, 719−747. (7) Kanemitsu, Y. Excitons in Semiconducting Carbon Nanotubes: Diameter-Dependent Photoluminescence Spectra. Phys. Chem. Chem. Phys. 2011, 13, 14879−14888. (8) Kanemitsu, Y.; Suzuki, K.; Nakayoshi, Y.; Masumoto, Y. Quantum Size Effects and Enhancement of the Oscillator Strength of Excitons in Chains of Silicon Atoms. Phys. Rev. B 1992, 46, 3916− 3919. (9) Kanemitsu, Y.; Suzuki, K.; Masumoto, Y.; Tomiuchi, Y.; Shiraishi, Y.; Kuroda, M. Optical Properties of Quasi-One-Dimensional Thiophene-Based Oligomers. Phys. Rev. B 1994, 50, 2301−2305. (10) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. Synthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop Structures. J. Am. Chem. Soc. 2008, 130, 17646−17647.
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(11) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K. Selective Synthesis of [12]Cycloparaphenylene. Angew. Chem., Int. Ed. 2009, 48, 6112−6116. (12) Yamago, S.; Watanabe, Y.; Iwamoto, T. Synthesis of [8]Cycloparaphenylene from a Square-Shaped Tetranuclear Platinum Complex. Angew. Chem., Int. Ed. 2010, 49, 757−759. (13) Steinberg, B. D.; Scott, L. T. New Strategies for Synthesizing Short Sections of Carbon Nanotubes. Angew. Chem., Int. Ed. 2009, 48, 5400−5402. (14) Bodwell, G. J. Carbon Nanotubes Growth Potential. Nat. Nanotechnol. 2010, 5, 103−104. (15) Jasti, R.; Bertozzi, C. R. Progress and Challenges for the Bottom-Up Synthesis of Carbon Nanotubes with Discrete Chirality. Chem. Phys. Lett. 2010, 494, 1−7. (16) Iyoda, M.; Yamakawa, J.; Rahman, M. J. Conjugated Macrocycles: Concepts and Applications. Angew. Chem., Int. Ed. 2011, 50, 10522−10553. (17) Sisto, T. J.; Jasti, R. Overcoming Molecular Strain: Synthesis of [7] Cycloparaphenlylene. Synlett 2012, 23, 483−489. (18) Bunz, U. H. F.; Menning, S.; Martín, N. Para-connected Cyclophenylenes and Hemispherical Polyarenes: Building Blocks for Single-Walled Carbon Nanotubes? Angew. Chem., Int. Ed. 2012, 51, 7094−7101. (19) Itami, K. Toward Controlled Synthesis of Carbon Nanotubes and Graphenes. Pure Appl. Chem. 2012, 84, 907−916. (20) Omachi, H.; Segawa, Y.; Itami, K. Synthesis of Cycloparaphenylenes and Related Carbon Nanorings: A Step toward the Controlled Synthesis of Carbon Nanotubes. Acc. Chem. Res. 2012, 45, 1378−1389. (21) Iwamoto, T.; Watanabe, Y.; Sakamoto, Y.; Suzuki, T.; Yamago, S. Selective and Random Syntheses of [n]Cycloparaphenylenes (n = 8−13) and Size Dependence of Their Electronic Properties. J. Am. Chem. Soc. 2011, 133, 8354−8361. (22) Segawa, Y.; Fukazawa, A.; Matsuura, S.; Omachi, H.; Yamaguchi, S.; Irle, S.; Itami, K. Combined Experimental and Theoretical Studies on the Photophysical Properties of Cycloparaphenylenes. Org. Biomol. Chem. 2012, 10, 5979−5984. (23) Heimel, G.; Daghofer, M.; Gierschner, J.; List, E. J. W.; Grimsdale, A. C.; Müllen, K.; Belijonne, D.; Bredas, J. L.; Zojer, E. Breakdown of the Mirror Image Symmetry in the Optical Absorption/ Emission Spectra of Oligo(para-phenylene)s. J. Chem. Phys. 2005, 122, 054501/1−054501/11. (24) Colthrup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, CA, 1990. (25) Wang, F.; Dukovic, G.; Brus, L. E.; Heinz, T. F. The Optical Resonances in Carbon Nanotubes Arise from Excitons. Science 2005, 308, 838−841. (26) Maultzsch, J.; Pomraenke, R.; Reich, S.; Chang, E.; Prezzi, D.; Ruini, A.; Molinari, E.; Strano, M. S.; Thomsen, C.; Lienau, C. Exciton Binding Energies in Carbon Nanotubes from Two-Photon Photoluminescence. Phys. Rev. B 2005, 72, 241402(R)/1−241402(R)/4. (27) Wong, B. M. Optoelectronic Properties of Carbon Nanorings: Excitonic Effects from Time-Dependent Density Functional Theory. J. Phys. Chem. C 2009, 113, 21921−21927. (28) Segawa, Y.; Omachi, H.; Itami, K. Theoretical Studies on the Structures and Strain Energies of Cycloparaphenylenes. Org. Lett. 2010, 12, 2262−2265. (29) Omachi, H.; Matsuura, S.; Segawa, S.; Itami, K. A Modular and Size-Selective Synthesis of [n]Cycloparaphenylenes: A Step toward the Selective Synthesis of [n,n] Single-Walled Carbon Nanotubes. Angew. Chem., Int. Ed. 2010, 49, 10202−10205. (30) Segawa, Y.; Miyamoto, S.; Omachi, H.; Matsuura, S.; Šenel, P.; Sasamori, T.; Tokitoh, N.; Itami, K. Concise Synthesis and Crystal Structure of [12]Cycloparaphenylene. Angew. Chem., Int. Ed. 2011, 50, 3244−3248. (31) Segawa, Y.; Šenel, P.; Matsuura, S.; Omachi, H.; Itami, K. [9]Cycloparaphenylene: Nickel-Mediated Synthesis and Crystal Structure. Chem. Lett. 2011, 40, 423−425.
(32) Ishii, Y.; Nakanishi, Y.; Omachi, H.; Matsuura, S.; Matsui, K.; Shinohara, H.; Segawa, Y.; Itami, K. Size-Slective Synthesis of [9]− [11] and [13]Cycloparaphenylenes. Chem. Sci. 2012, 3, 2340−2345.
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