Letter pubs.acs.org/JPCL
Radical Ions of Cycloparaphenylenes: Size Dependence Contrary to the Neutral Molecules Mamoru Fujitsuka,*,† Sachiko Tojo,† Takahiro Iwamoto,‡ Eiichi Kayahara,‡,§ Shigeru Yamago,‡,§ and Tetsuro Majima*,† †
The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan § CREST, Japan Science and Technology Agency, Tokyo 102-0076, Japan ‡
S Supporting Information *
ABSTRACT: Cycloparaphenylenes (CPPs) have attracted wide attention because of their interesting properties owing to distorted and strained aromatic systems and radially oriented p orbitals. For application of CPPs, information on their charged states (radical cation and radical anion) is essential. Here, we measured absorption spectra of the radical cations and the radical anions of CPPs with various ring sizes over a wide spectral region by means of radiation chemical methods. The peak position of the near-IR bands for both the radical cation and the radical anion shifted to lower energies with an increase in the ring size. This trend is contrary to what is observed for transitions between the HOMO and LUMO of the neutral CPP. The observed spectra of the CPP radical ions were reasonably assigned based on time-dependent density functional theory. These results indicate that the next HOMO and the next LUMO levels are important in the electronic transitions of radical ions. SECTION: Spectroscopy, Photochemistry, and Excited States
C
measure the absorption spectra of the radical ion species, radiation chemical methods, that is, γ-ray irradiation and pulse radiolysis, are useful. These methods generate strong oxidation or reduction reagents applicable to various materials, and only one-electron oxidized or reduced solutes can be generated. Furthermore, utilization of a frozen matrix including solute in low concentration (∼mM) excludes formation of products due to intermolecular processes.22 In the present study, we examined the radical cation and the radical anion of [n]CPPs (n = 6, 8, 10, and 12) using radiation chemical methods and theoretical calculations. The absorption spectra of the CPP radical ions over a wide spectral range were successfully obtained, and theoretical calculations were used to assign the observed peaks. The ring size dependence of the absorption spectra was explained. To generate the radical cation of CPPs, a glassy matrix of nbutyl chloride (BuCl) that included CPP as a solute (S) was irradiated with a γ-ray at 77 K. It is well established that γ-ray irradiation of S in BuCl generates the radical cation of S (S•+) according to eqs 1−323
ycloparaphenylenes (CPPs, Figure 1) are typical hoopshaped macrocycles1−3 that have attracted wide attention
Figure 1. Molecular structures of [n]CPPs (n = 6, 8, 10, and 12).
because of their interesting properties owing to distorted and strained aromatic systems and radially oriented p orbitals.4−15 CPPs are also attractive because they can be regarded as the shortest armchair carbon nanotube and are applicable as a host molecule for fullerenes.16−19 For many applications, information about the CPP charged states, that is, the radical cation and radical anion, is indispensable. Very recently, the absorption spectrum of the radical cation of [8]CPP, which was generated by chemical oxidation, was reported by two groups, although there is discrepancy in the assignement of the observed peaks.20,21 This inconsistency probably comes from the oxidation method used to generate the radical cation because control of the oxidation state (radical cation or dication) by chemical oxidation is usually rather difficult. For other CPPs, the absorption spectra of the radical cations have not been reported. Furthermore, there is no reported information on the radical anion species. In order to © 2014 American Chemical Society
BuCl ⇝ BuCl•+ + e−
(1)
e− + BuCl → (BuCl•−) → Bu• + Cl−
(2)
Received: May 8, 2014 Accepted: June 18, 2014 Published: June 18, 2014 2302
dx.doi.org/10.1021/jz5009054 | J. Phys. Chem. Lett. 2014, 5, 2302−2305
The Journal of Physical Chemistry Letters BuCl•+ + S → BuCl + S•+
Letter
Figure 3 shows the absorption spectra of CPPs after γ-ray irradiation in MTHF. The radical anions of CPPs also exhibited
(3)
Figure 2 shows absorption spectra of the radical cations of CPPs generated by γ-ray irradiation. In the case of the radical
Figure 3. Absorption spectra of [n]CPPs (n = 6 (a), 8 (b), 10 (c), and 12(d)) in a MTHF glassy matrix after γ-ray irradiation. Numbers near absorption peaks indicate peak positions in nm units. Blue and red bars indicate oscillator strengths obtained by TDDFT at the UB3LYP/ 6-31G(d) level assuming C1 and D(n/2)d symmetries, respectively.
Figure 2. Absorption spectra of [n]CPPs (n = 6 (a), 8 (b), 10 (c), and 12(d)) in a BuCl glassy matrix after γ-ray irradiation. Numbers near absorption peaks indicate peak positions in nm units. Blue and red bars indicate oscillator strengths obtained by TDDFT at the UB3LYP/ 6-31G(d) level assuming C1 and D(n/2)d symmetries, respectively.
clear absorption bands in the near-IR and UV regions. In the visible region, absorption bands become obvious with an increase in the ring size. In addition, the near-IR band shifts to a lower energy as the ring size increases. Similar spectra were confirmed by pulse radiolysis using DMF as the solvent (Figure S1(b) in the SI). Thus, the absorption spectra of the CPP radical anions resemble those of the radical cations in spectral shape and ring size dependence. To understand the absorption spectra of the radical ions on the basis of molecular orbital (MO) theory, theoretical calculations were performed using density functional theory (DFT). Here, C1, C(n/2)v, and D(n/2)d point group symmetries were assumed for the CPPs because the most stable conformation of neutral CPPs with an even number of phenyl rings is reported to be an alternating zigzag orientation of the paraphenylenes units.12 In Table S1 in the SI, the sum of electronic and zero-point energies calculated for the radical cations and the radical anions of [n]CPPs is summarized. In all cases, the radical ions with C(n/2)v and D(n/2)d point group symmetries are more stable than those with C1 symmetry, and the difference between C(n/2)v and D(n/2)d point group symmetries is rather small (
