Article Cite This: J. Phys. Chem. A 2018, 122, 5162−5167
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Near-Infrared Fluorescence from In-Plane-Aromatic Cycloparaphenylene Dications Yui Masumoto,†,‡ Naoyuki Toriumi,†,# Atsuya Muranaka,*,‡ Eiichi Kayahara,§ Shigeru Yamago,*,§ and Masanobu Uchiyama*,†,‡ †
Graduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Elements Chemistry Laboratory, RIKEN Cluster for Pioneering Research (CPR), Wako-shi, Saitama 351-0198, Japan § Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan Downloaded via TUFTS UNIV on June 15, 2018 at 01:21:43 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Cycloparaphenylenes (CPPs) are hoop-shaped conjugated hydrocarbons corresponding to partial structures of fullerenes or armchair carbon nanotubes. Here, we examined the fluorescence properties of a series of [n]cycloparaphenylene dications ([n]CPP2+, n = 5−9), which have unique in-plane aromaticity. The fluorescence peak positions of the [n]CPP2+s shifted to the longer-wavelength region with increasing ring size, reaching the near-infrared region for those with n > 5. The fluorescence quantum yield of [6]CPP2+ was the highest among the [n]CPP2+s examined in this study, and the value was on the same order as that of carbon nanotubes. The Stokes shifts of [n]CPP2+s were smaller than those of neutral [n]CPPs, which do not have in-plane aromaticity. Theoretical calculations indicate that [n]CPP2+s undergo smaller structural changes upon S0−S1 transition than [n]CPPs do, and this is responsible for the difference of the Stokes shift. Furthermore, molecular orbital analysis reveals that the S0−S1 transition of smaller [n]CPP2+s has an electric-dipole-forbidden character due to HOMO → LUMO/ HOMO → LUMO+1 mixing. The relatively high fluorescence quantum yield of [6]CPP2+ is considered to arise from the balance between relatively allowed character and the dominant effect of energy gap.
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would have a unique “in-plane aromaticity”,12,13 and this prediction was verified in several types of highly substituted C60 derivatives with a (4N + 2) π-electron system (18π-electron, N = 4).14−16 The reactivity, ring current effects, and metallic/ semiconducting properties of carbon nanotubes have been investigated and discussed in relation to aromaticity.17,18 With regard to [n]CPP, we found that a dication of [8]CPP ([8]CPP2+)19 and three other dications ([n]CPP2+, n = 5, 6, 10)20 have in-plane aromaticity (Figure 1c). [n]CPP2+s can be considered to be a (4N + 2) π-electron system with an all-cis-annulene pathway on the basis of their spectroscopic properties, molecular orbital analysis, and nucleus-independent chemical shift (NICS)21 calculations. Some nanocarbons with a curved surface, such as fullerenes, carbon nanotubes, and neutral [n]CPPs, exhibit fluorescence in the visible to near-infrared region.22−28 However, the fluorescence properties of [n]CPP2+s have not been studied so far, though they are likely to be of general interest from both theoretical and practical viewpoints. Herein, we report the fluorescence properties of a series of [n]CPP2+s (n = 5−9). Near-infrared fluorescence was observed for the larger [n]CPP2+s (n = 6−9), and we investigated the relationship between the ring size and the spectroscopic properties both experimentally and theoretically.
INTRODUCTION Nanocarbons with curved surfaces, such as fullerenes and carbon nanotubes, have extraordinary physical and electronic properties, which have led to applications in many areas, including consumer goods, cosmetics, drugs, organic semiconductors, and solar cells.1,2 Cycloparaphenylenes (CPPs), which are hoop-shaped conjugated hydrocarbons consisting of p-substituted benzene, possess a partial structure of fullerenes or armchair carbon nanotubes (Figure 1a). Although CPPs were predicted more than 80 years ago,3 the first successful synthesis was reported only in 2008.4 Since then, several research groups have developed synthetic methodologies for [n]CPPs having different ring sizes (where n represents the number of benzene units).5−11 The smallest currently available CPP, [5]CPP, is a fragment of C60 and armchair [5,5]carbon nanotube, whereas [18]CPP is the largest one synthesized to date. [n]CPPs, fullerenes, and carbon nanotubes have unique radially oriented p-orbitals, in sharp contrast to conventional cyclic π-conjugated molecules where the p-orbitals lie perpendicular to the ring plane (Figure 1b). Planar cyclic π-conjugated molecules with a (4N + 2) π-electron system have special physicochemical properties (aromatic properties). On the other hand, several nanocarbons with radially oriented p-orbitals have also been reported to possess aromatic properties, but their aromaticity is not yet fully understood. Schleyer et al. theoretically predicted that trannulenes (cyclic annulenes in which the C−C double bonds in the ring are exclusively in trans configuration) © 2018 American Chemical Society
Received: April 2, 2018 Revised: May 17, 2018 Published: May 18, 2018 5162
DOI: 10.1021/acs.jpca.8b03105 J. Phys. Chem. A 2018, 122, 5162−5167
Article
The Journal of Physical Chemistry A
Computational Details. All calculations were performed with Gaussian 0930 and Gaussian 1631 program packages. Optimized ground-state (S0) structures were calculated by the restricted DFT method, by means of the hybrid Becke3LYP (B3LYP)32−35 functional. The 6-31G* basis set36−39 was used for all atoms. The structures of [n]CPPs (n = 6, 8) were optimized using D(n/2)d symmetry restriction, whereas no symmetry restriction was applied for the geometry optimizations of [n]CPPs (n = 5, 7, 9). We confirmed that the unrestricted DFT method gave the same results as the restricted method. Molecular orbitals were drawn using GaussView 5.0. Optimized structures for the lowest-energy excited singlet state (S1), excitation wavelengths, and oscillator strengths for the S0 → S1 and S1 → S0 transitions were obtained at the density functional level using time-dependent perturbation theory (TDDFT) approach. The same functional and basis set were used as in the ground-state calculations. Optimized S0 and S1 structures were used for the calculations of S0 → S1 and S1 → S0 transitions, respectively. Contribution of the HOMO → LUMO or HOMO → LUMO+1 transition was calculated by subtracting contribution of the HOMO ← LUMO or HOMO ← LUMO+1 transition. We also calculated properties of the S0 → S1 and S1 → S0 transitions using the TD-DFT approach at the Coulomb-attenuating method (CAM-B3LYP)40 and the semiempirical ZINDO method.41 The results were consistent with those of the B3LYP calculations.
Figure 1. (a) Structures of fullerene, carbon nanotube, and cycloparaphenylene. (b) Two types of arrangement of p-orbitals. (c) Oxidation of [n]CPPs.
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METHODS Experimental Section. General. All reaction conditions dealing with air- and moisture-sensitive compounds were carried out in dry reaction vessels under an argon atmosphere. 1 H NMR (500 MHz) spectra were measured for CD2Cl2 reaction solutions with NOSbF6 and were reported in parts per million (δ) from the residual solvent peak. The electronic absorption spectra were recorded with a JASCO V-670 spectrophotometer. Emission spectra were recorded on a JASCO FP-6600 spectrofluorometer equipped with a Hamamatsu C9940 photomultiplier tube. The electronic absorption and fluorescence spectra of the dichloromethane solution of [n]CPP2+ prepared with Ag[B(C6F5)4] or NOSbF6 were measured using a 1 cm quartz cell under argon atmosphere and those of tetra-tertbuthylnaphthalocyanine29 were measured using the same cell in air. The absolute fluorescence quantum yield (Φf) of tetra-tertbuthylnaphthalocyanine29 was measured using an integrating sphere. Unless otherwise noted, materials were purchased from Aldrich Inc., Wako Pure Chemical Industries, Ltd., Tokyo Kasei Co., and other commercial suppliers and were used after appropriate purification (distillation or recrystallization). Preparation of [n]CPP2+ Solution. Dichloromethane Solution of [n]CPP2+ (n = 7, 8, 9) Prepared with NOSbF6. The solution was prepared according to the reported procedure.20 Representative Example: [7]CPP2+(SbF6−)2. To a suspension solution of NOSbF6 (1.8 mg, 6.8 μmol) in CH2Cl2 (ca. 0.5 mL) was slowly added a solution of [7]CPP (1.2 mg, 2.3 μmol) in CH2Cl2 (ca. 0.5 mL) and stirred for 1.0 h at room temperature. Dichloromethane Solution of [n]CPP2+ (n = 5, 6, 7) Prepared with Ag[B(C6F5)4]. This method was suitable for small rings (n = 5, 6, 7) because of their chemical stability, while oxidation ability of Ag[B(C6F5)4] was not enough to prepare large ring sizes of [n]CPP2+ (n = 8, 9). Representative Example: [7]CPP2+[B(C 6F5) 4−] 2. To a suspension solution of Ag[B(C6F5)4] (4.9 mg, 6.2 μmol) in CH2Cl2 (ca. 0.5 mL) was slowly added a solution of [7]CPP (1.0 mg, 2.6 μmol) in CH2Cl2 (ca. 0.5 mL) and stirred for 1.0 h at room temperature.
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RESULTS The dications [n]CPP2+s (n = 5, 6, 8) were prepared from neutral [n]CPPs by reaction with a chemical oxidant, NOSbF6 or Ag[B(C6F5)4].20,42 Although synthesis of [7]CPP2+ and [9]CPP2+ has not previously been reported, the same reaction conditions were successfully applied to give the desired dications. The 1H NMR spectra of [7]CPP2+ and [9]CPP2+ indicated that the proton signals of the corresponding neutral forms (δ 7.48 ppm for [7]CPP, 7.53 ppm for [9]CPP) were shifted to higher magnetic field upon two-electron oxidation (δ 5.07 ppm for [7]CPP2+, 5.47 ppm for [9]CPP2+), which was consistent with previous findings for other [n]CPP2+s.20,42 Although the 1H NMR signal of [7]CPP2+ was independent of the measured temperature range, that of [9]CPP2+ became sharp as the temperature decreased. This result indicates the existence of a biradical form of [9]CPP2+ at room temperature as is the case with [10]CPP2+ and [12]CPP2+.20 Further, [7]CPP2+ and [9]CPP2+ exhibited intense near-infrared absorption bands at 938 and 1308 nm, respectively, which lay between those of [6]CPP2+ (792 nm) and [8]CPP2+ (1102 nm), and between those of [8]CPP2+ (1102 nm) and [10]CPP2+ (1568 nm), respectively (Figure 2). A linear relationship between the absorption peaks and 1/n was obtained with r2 = 0.998, including the peaks of [7]CPP2+ and [9]CPP2+ (Figure 3a).20 We first measured the fluorescence spectrum of [7]CPP2+ in dichloromethane at room temperature under an argon atmosphere. When the sample solution was excited with a 150 W xenon lamp at the wavelength corresponding to the second lowest energy transition (503 nm), an emission band was observed in the 900−1300 nm region. Because the pattern of the excitation spectrum was consistent with that of the absorption spectrum, the observed emission band was assigned to fluorescence from the dication. The spectroscopic properties of a dichloromethane solution of [7]CPP2+ prepared with NOSbF6 were essentially the same as those of [7]CPP2+ prepared with Ag[B(C6F5)4] (see Supporting Information). The results are consistent with the 5163
DOI: 10.1021/acs.jpca.8b03105 J. Phys. Chem. A 2018, 122, 5162−5167
Article
The Journal of Physical Chemistry A
size. When the fluorescence peak positions were plotted against reciprocal numbers (1/n) of [n]CPP2+s, a linear relationship with r2 = 0.991 was obtained (Figure 3b). Table 1 shows a summary of the spectroscopic data of [n]CPP2+s. The full width at half-maximum (fwhm) of the fluorescence intensity of all [n]CPP2+s was somewhat larger than the value for the S0 − S1 absorption band. Smaller dications [n]CPP2+s (n = 5−7) have relatively small Stokes shifts. The fluorescence quantum yields of [n]CPP2+s were measured with reference to that of tetra-tert-buthylnaphthalocyanine (Φf = 23%, see Supporting Information). The quantum yield of [6]CPP2+ (Φf = 1.8%) was the highest among the [n]CPP2+s (n = 5−9). The value was significantly higher than that of C60 (2 × 10−2 %).22 Other [n]CPP2+s have very low quantum yields (
