Radical Ions of Cyclopyrenylene: Comparison of Spectral Properties with Cycloparaphenylene 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: Hoop-shaped π-conjugated molecules have attracted much attention. In this study, the radical ions of [4]cyclo-2,7-pyrenylene ([4]CPY), a cyclic tetramer of pyrene, and [4]cyclo-4,5,9,10-tetrahydro-2,7-pyrenylene ([4]CHPY) were investigated using radia- tion chemical methods, namely, γ-ray radiolysis and pulse radiolysis. The absorption spectra of the radical ions of [4]CPY and [4]CHPY showed clear peaks in the near-IR and UV−vis regions similar to those of [8]cycloparaphenylene ([8]CPP). Theoretical calculations using time-dependent density functional theory provided reasonable assignments of the observed absorption bands. It was indicated that the C4−C5 and C9−C10 ethylene bonds of [4]CHPY do not contribute to the electronic transitions, resulting in absorption spectra similar to those of [8]CPP. On the other hand, it was confirmed that the allowed electronic transitions of the radical ions of [4]CPY are different from those of the radical ions of [4]CHPY and [8]CPP. ■ INTRODUCTION Hoop-shaped π-conjugated molecules have attracted much atten- tion from scientists because of their interesting properties. 1−3 Among these molecules, [n]cycloparaphenylenes ([n]CPPs, where n denotes the number of phenyl rings) (Figure 1 a) have been investigated because of their simple structures and interesting properties, which are different from those of linear oligomers. 4−23 One of the interesting properties of [n]CPPs is the size dependence of the HOMO−LUMO gap, which becomes larger with an increase in the number of phenyl rings. 12,24,25 This tendency is contrary to that for linear oligomers. On the other hand, we confirmed that the absorption peak of the radical ions of [n]CPP tends to shift to lower energies with an increase in the ring size, in contrast to that for neutral molecules. 26−28 Recently, Yamago and co-workers reported the syntheses of [4]cyclo-2,7-pyrenylene ([4]CPY) (Figure 1b), a cyclic tetramer of pyrene, and [4]cyclo-4,5,9,10-tetrahydro-2,7-pyrenylene ([4]CHPY) (Figure 1c). 29 As indicated in Figure 1d, [4]CPY is also a part of the (8,8) armchair carbon nanotube, as is [8]CPP. Some neutral-state properties were also examined. It was indi- cated that the electrochemical properties of [4]CPY are similar to those of [8]CPP, although a spectroscopic study was not per- formed on its oxidized and reduced states. In the present work, the absorption spectra of the radical ions of [4]CPY and [4]CHPY were studied both experimentally and theoretically. From a comparison with the radical ions of [8]CPP, the similarities and differences among [4]CPY, [4]CHPY, and [8]CPP became clear. This information will be important to under- stand the properties of hoop-shaped π-conjugated molecules. ■ EXPERIMENTAL METHODS [4]CPY and [4]CHPY were synthesized according to the procedures in the previous report. 29 Solvents were of the best grades commercially available. The sample for γ-ray radiolysis (1 mM typically) was dissolved in n-butyl chloride (BuCl) or 2-methyltetrahydrofuran (MTHF) to generate radical cations or radical anions, respectively. After several freeze−pump−thaw cycles of the sample solution in a quartz cell (2 mm optical path length), the sample was kept at 77 K to form a transparent glass and then subjected to γ-ray irradiation. The absorption spectra of the γ-ray-irradiated sample at 77 K were measured using a Shimadzu UV-3100PC spectrometer. Pulse radiolysis experiments were performed using an electron pulse (28 MeV, 8 ns, 0.7 kGy per pulse) from a linear accelerator at Osaka University. In the present study, the sample (2 mM typically) was dissolved in 1,2-dichloroethane (DCE) or N,N- dimethylformamide (DMF) in order to generate radical cations or radical anions of the sample, respectively. Transient absorp- tion spectra of the sample were measured by reported procedures. 30 Optimized structures of the radical ions were estimated using density functional theory (DFT) at the UB3LYP/6-31G(d) level. The excitation energies of the radical ions were estimated using time-dependent DFT (TDDFT) at the UB3LYP/6-31G(d) level. All of the theoretical calculations were carried out using the Received: February 5, 2015 Revised: April 13, 2015 Published: April 13, 2015 Article pubs.acs.org/JPCA © 2015 American Chemical Society 4136 DOI: 10.1021/acs.jpca.5b01189 J. Phys. Chem. A 2015, 119, 4136−4141