ARTICLE pubs.acs.org/JPCA
Delocalization of Positive Charge in π-Stacked Multi-benzene Rings in Multilayered Cyclophanes Mamoru Fujitsuka,† Sachiko Tojo,† Masahiko Shibahara,‡ Motonori Watanabe,§ Teruo Shinmyozu,§ and Tetsuro Majima‡,* †
The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan Department of Chemistry, Faculty of Education and Welfare Science, Oita University, Dannoharu 700, Oita 870-1192, Japan § Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, Hakozaki 6-10-1, Fukuoka 812-8581, Japan ‡
bS Supporting Information ABSTRACT: In the present study, delocalization of a positive charge in π-stacked multi-benzene rings in multilayered para- and meta-cyclophanes, in which benzene rings are connected by propyl chains to form a chromophore array with the face-to-face structure, was investigated by means of transient absorption spectroscopy during the pulse radiolysis using dichloroethane as a solvent. The local excitation and charge resonance (CR) bands were successfully observed. It was revealed that the CR band shifted to the longer wavelength side with the number of the benzene rings. The stabilization energy estimated from the peak position of the CR band showed the efficient charge delocalization over the cyclophanes. Furthermore, the CR bands showed the slight spectral change attributable to the change in distribution of the conformers. The substantially long lifetime of the CR band can be explained on the basis of the smaller charge distribution on the outer layers of the multilayered cyclophanes.
’ INTRODUCTION In the charge transport process in organic molecular materials, the charge carrier is stabilized by surrounding molecules.1 Thus, the stabilized charge in the molecular array exhibits different behaviors from those of the charged molecule isolated in solution or gas phase. The dimer of a molecule can be regarded as the simplest form of the molecular array systems. The charge resonance (CR) band, which is observable with the dimeric molecules of radical ion and neutral molecules, is one of the characteristic properties of the stabilized charge over chromophores.1-5 For the understanding of the critical aspects of charge stabilization, investigation using chromophore arrays with well-defined structure is desirable. Cyclophanes, in which two benzene rings are connected by alkyl chains, can be regarded as one of the important candidates for the study on the charge delocalization, because of their welldefined face-to-face structures.6 In previous papers, we have investigated the CR band of a series of cyclophanes by measuring the transient absorption spectra during the pulse radiolysis.3 It was revealed that the peak position of the CR band strongly depends on the benzene ring-benzene ring distance, i.e., transannular distance. Furthermore, the stabilization energy, which can be estimated from the peak position of the CR band, was expressed as the function proportional to the exponential of the transannular distance. In addition, by employing the γ-ray irradiation to glassy matrix including the cyclophanes, the CR band due to negative charge delocalization was successfully observed, and the r 2010 American Chemical Society
similar transannular distance dependence of the CR band position was also confirmed.5 For more detailed understanding on the charge delocalization over chromophores, investigation on the further stacked chromophore arrays is indispensable. For the benzene clusters in the gas phase, there are several papers on the ionized state of these clusters.7 But delocalization or localization of the charge in these clusters has been discussed for years by several experimental and theoretical studies, because the determination of the detailed structure of these clusters, which has an extensive effect on charge delocalization behavior, is a difficult subject. Kira and Imamura attributed the absorption band, which appeared during the warming process of the γ-ray-irradiated glassy matrix, to the trimer radical cation, although ambiguity on its structure remains.2d Pendant polymers in both solid and solution phases are another important class of materials, in which charge delocalization among the multiple chromophores is apparent.8 In the polymer systems, the number and structure of chromophores contributing to the charge resonance are not clear. To overcome this point, chromophore array systems with well-defined structure are essentially important. Ohkita et al. reported the charge delocalization over three naphthalene rings connected by a single alkyl chain, although it does not limit the structure to the completely Received: November 15, 2010 Revised: December 9, 2010 Published: December 31, 2010 741
dx.doi.org/10.1021/jp110916m | J. Phys. Chem. A 2011, 115, 741–746
The Journal of Physical Chemistry A
ARTICLE
Figure 1. Molecular structures of multilayered cyclophanes.
face-to-face-stacked structure.9 Furthermore, there is no paper on charge delocalization on further stacked chromophore systems with well-defined structure in solution phase. For cyclophanes, some of the present authors have successfully synthesized a series of multilayered cyclophanes, namely, [3.3]para- and meta-cyclophanes (nLPCP and nLMCP, n denotes the number of benzene layers).10,11 Although formation and absorption spectra of the charge-transfer complex were reported for these multilayered cyclophanes,10,11 the charge delocalization behavior has not been examined. Thus, the detailed spectroscopic investigation on the multilayered cyclophane radical cation is quite important to reveal the charge delocalization process. In the present paper, we measured transient absorption spectra of three- and fourlayered para- and meta-cyclophanes (Figure 1) during the pulse radiolysis using dichloroethane as a solvent to form the cyclophane radical cations. The CR bands of multilayered cyclophanes were successfully observed. Their peak positions and transient behaviors are discussed on the basis of the molecular structures estimated by theoretical calculation.
Figure 2. Transient absorption spectra of (a) 3LPCP and (b) 3LMCP (5 mM) in DCE during pulse radiolysis at room temperature. The spectra were obtained at (a) 50 ns (solid line) and 5 μs (dashed line) and (b) 50 ns (solid line) and 500 ns (dashed line) after the electron pulse irradiation.
where chloride and chlorine generated in eqs 2-4 contribute to the formation of a charge-transfer complex with the substrate.3 In the present study, we employed 1,2-dichloroethane (DCE) as a solvent of pulse radiolysis experiments. The typical concentration of the substrate was 5 mM. Since the solubility of 3LMCP in DCE is poor, pulse radiolysis experiment was carried out with solution saturated with the cyclophane (
