Synthesis and properties of [8]-, [10]-, [12]-, and [16]cyclo-1,4-naphthylenes† †Electronic supplementary information (ESI) available: Detailed experimental procedures, computational studies, and spectral data for all compounds. See DOI: 10.1039/c6sc04048a Click here for additional data file. Click here for additional data file.

The synthesis and properties of various [n]cyclo-1,4-naphthylenes ([n]CNs, n = 8, 10, 12, and 16) are described.


Introduction
Cycloparaphenylenes 1 (CPPs, Fig. 1), i.e., ring-shaped aromatic hydrocarbons consisting exclusively of paraphenylenes, have recently received much attention on account of their highly symmetric structures and the unique electronic properties that arise from their radial p-conjugation modes. CPPs have also been proposed as building blocks for carbon nanotubes (CNTs), given that CPPs represent the shortest sidewall segment of armchair CNTs. 2 Since the rst synthesis of [9]-, [12]-, and [18] CPPs by Bertozzi and Jasti in 2008, 3a CPPs of various size, i.e., [5]- [18]CPPs have been synthesized by the Jasti, 3 Itami, 4 and Yamago 5 research groups. While the amounts of synthesized CPPs were very small in the beginning, synthetic improvements allowed syntheses on the gram-scale, 3e,5g and several CPP sizes are now commercially available. 6 With CPPs of various sizes in hand, unprecedented sizedependencies of the physical properties of CPPs have been revealed and size-specic applications of the CPPs have been discovered. For example, the HOMO-LUMO gap of the CPPs increases with increasing CPP sizes, which stands in contrast to the size-dependency of linear oligoparaphenylenes. As a consequence, the [n]CPPs show size-dependent uorescence properties. 5b,7 The selective incorporation of fullerenes into [n]CPPs (n ¼ 10 and 11) was possible because the fullerenes t into the cavity of these CPPs. 3e,8 The uniform porous structure of [12]CPP renders it a discrete adsorbent of H 2 O, MeOH, and EtOH. 9a The diameter-controlled growth of CNTs has been achieved by using [9]-and [12]CPPs as seeds for chemical vapor deposition methods. 9b Strategies for the size-selective synthesis of CPPs have widely been applied to the synthesis of related ring-shaped 10 or cage-shaped p-conjugated molecules. 11 That is, the controlling the size of CPPs has allowed signicant innovations in this eld.
Among the derivatives of CPPs, cyclo-1,4-naphthylenes (CNs) are simple but intriguing molecules. CNs consist of naphthalene rings and can be considered as p-extended CPPs generated by benzannulation of all the benzene rings in CPPs. Our group previously reported the synthesis of [9]CN, and the unique structural properties of [9]CN attributed to the sterically hindered integration of naphthalene rings were uncovered. 12 However, due to the lack of CNs of other sizes, the size-dependent properties and the odd-even effect in CNs have so far remained unexplored. Moreover, CNs are potential precursors for the so-called Vögtle's belts, a type of carbon nanobelts proposed by Vögtle in 1983. 13 Since CNs contain all the carbon content required for Vögtle's belts, ideal cyclodehydrogenation reactions between the peri positions of neighboring naphthalene rings can convert CNs into the corresponding Vögtle's belts. 1m,14 The rigid structures of Vögtle's have been proposed to be thermally stable seeds for the perfectly controlled growth of CNTs. The synthesis of CNs of varying size is thus an attractive research target in synthetic organic chemistry, as CNs are an interesting new family of radially p-conjugated molecules, and potentially also important for the tailored synthesis of CNTs.
Herein, we report the synthesis of even-numbered [n]CNs (n ¼ 8, 10, 12, and 16) and their size-dependent photophysical properties, especially the multi-color uorescence. The rotation barrier of the naphthalene rings in [10]CN was determined experimentally and theoretically.

Synthesis of [8]-, [10]-, [12]-, and [16]CNs
Our synthetic strategy for the generation of CNs is shown in Fig. 2. Previously we synthesized [9]CN through a nickel-mediated cyclotrimerization of the ternaphthyl-convertible L-shaped unit and a subsequent reductive aromatization. 12 To obtain CNs of various sizes, we designed extended L-shaped units that can be converted into quater-or quinquenaphthylene units. A widely applied strategy in the synthesis of CPPs 3-5 is the combination of L-shaped units with linear units that allows synthesizing macrocycles of varying size. A palladium-catalyzed Suzuki-Miyaura cross-coupling 15 or a nickel-mediated homocoupling 16 produced the desired macrocycles as precursor to CNs. In contrast to the synthesis of [9]CN, not only cyclic trimers but also cyclic dimers and tetramers were obtained.
In order to generate cyclic oligomers, 2 was subjected to Suzuki-Miyaura cross-coupling reaction conditions (Scheme 1): 2 was dissolved in a mixed solvent system (toluene/EtOH/H 2 O; 25 mM) and heated to 90 C for 26 hours in the presence of 10 mol% Pd(PPh 3 ) 4 and 5 equiv. of K 2 CO 3 . The products were puried by preparative recycling gel permeation chromatography (GPC) to afford a mixture of cyclic oligomers. Because further purication proved difficult, the mixture was subjected to the reductive aromatization using granular lithium. As a result, [8]-, [12]-, and [16]CN were successfully isolated in 0.1%, 1.5%, and 0.4% yields, respectively (yields with respect to 2). This result indicates that the cross-coupling of 2 produced the corresponding cyclic dimer, trimer, and tetramer in low yields, whereas the homocoupling of 1a only afforded the cyclic trimer. 12 The other L-shaped unit, 3, is also a precursor for the synthesis of CNs. Treatment of 3 (1.0 mM) with Ni(cod) 2 (2.2 equiv.) and 2,2 0 -bipyridyl (2.2 equiv.) in DMF at 85 C furnished a mixture of coupling products. Aer the following reductive aromatization, [10]CN was isolated in 0.5% yield over two steps. 18 This result indicates that the mixture of coupling products contained a small amount of cyclic dimer.
The 1 H NMR spectra of [8]-, [10]-, [12]-, and [16]CNs in CD 2 Cl 2 displayed a clear size-dependency (Fig. 3a). These evennumbered CNs showed simple spectra with three types of proton signals assigned to the 2,3-(red), 5,8-(green), and 6,7-positions (blue) of the naphthalene units. These spectra indicated highly symmetric structures for [8]-, [10]-, [12]-, and [16]CNs in solution, which stands in contrast to that of [9]CN. Highly symmetric conformations (such as D 4d for [8]CN, Fig. 3b) were optimized by DFT calculations as the most stable ones, and these were in good agreement with the highly symmetric NMR spectra. The NMR signals for the hydrogen atoms at the 2,3-positions (red) of the naphthalene units shied upeld with decreasing size of [n]CN. DFT calculations indicated that the size-dependent upeld shis should mainly arise from the through-space shielding of neighboring naphthalene rings, which increases as the bent angles of naphthalene rings increases (for details, see ESI †). A similar trend was observed in [n]CPPs (n ¼ 8-18). 1k

]CN), green ([9]CN), light blue ([10]CN), and deep blue ([12]-and [16]CNs) can be obtained (also see
To investigate the origin of the size-dependency of the photophysical properties of CNs, time-dependent (TD) DFT calculations were carried out on [n]CNs (n ¼ 8, 10, 12, 14, and 16) at the B3LYP/6-31G(d) level of theory. For all these CNs, similar orbital shapes, as well as degeneracy and transitions were observed. The energy diagrams of the six frontier molecular orbitals (HOMOÀ2 to LUMO+2) and a pictorial representations of these six orbitals for [12]CN are shown in Fig. 5a. Whereas the HOMO and LUMO of [12]CN are delocalized over the entire ring, the HOMOÀ1, HOMOÀ2, LUMO+1, and LUMO+2 are delocalized albeit separated into two sections on opposing sides of the rings. The associated energy diagram reects the degeneracy for the HOMOÀ1/HOMOÀ2 (À5.07 eV) and LUMO+1/ LUMO+2 (À1.57 eV) pairs. Considering the shape of the orbitals, the occupied and unoccupied orbitals should represent the p and p* frontier orbitals of the conjugated poly-1,4naphthylenes. The TD DFT calculations indicated that two characteristic energetically low-lying transitions originate from the set of six orbitals: one transition is a forbidden HOMO / LUMO transition with an oscillator strength (f) of 0.00 (excited state 1), while the other is a degenerate transition, in which both the HOMOÀ1 / LUMO and HOMO / LUMO+1 excitations are mixed with a high f value of 1.46 for [12]CN (excited states 2 and 3). All these transitions are p-p* transitions. In the absorption spectrum of [12]CN, the excited states 2 and 3 should correspond to l abs1 , while the shoulder peak l abs2 should correspond to excited state 1. The forbidden transition may be due to a deformation away from high symmetry on account of a dynamic conformational change. Fig. 5b shows the molecular orbital energies for [8]- [16]CNs. With increasing size of the [n] CNs, the HOMO levels increased and the LUMO level decreased, whereas the energies of the HOMOÀ1 and LUMO+1 levels remained largely unaffected. This size-dependency is very similar to those of CPPs. 7 The origin of this behavior may be ascribed to two factors: (1) the lack of energy dependence of the frontier molecular orbitals on the conjugation length, and (2) the substantial effect of bending and twisting of the naphthalene rings on the orbital energies, as in the case of CPPs. 7a

Kinetic study on the conformers of [10]CN
In our previous theoretical examination of [9]CN, we predicted that the rotation of the naphthalene rings in [9]CN was affected by the ring strain. However, experimental information on the dynamic behavior of [n]CNs had remained elusive. Herein, we successfully obtained a conformational isomer of [10]CN during The highest absorption maxima. c The longest absorption maxima determined by a peak separation method. d Emission maxima upon excitation at l abs1 . e Absolute uorescence quantum yields determined by a calibrated integrating sphere system within 3% error. the synthetic study, which allowed us to determine the isomerization barrier of [10]CN experimentally for the rst time.
The synthesis of this [10]CN isomer is shown in Scheme 2. When the mixture containing cyclic dimer of 3 was reduced for 2 h less than under optimized conditions (12 hours), a 1 H NMR spectrum of lower symmetry, including two high-eld shied doublets of doublets at 6.29 ppm and 6.53 ppm, was observed. Considering that this product showed the same mass number as the highly symmetric [10]CN (D 5d -[10]CN) and that it was converted into D 5d -[10]CN upon heating at 120 C, the product should be a conformer of [10]CN. Supported by the simulation of the 1 H NMR chemical shi by DFT calculations at the GIAO B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level of theory (see ESI for details †), we assumed that this isomer might be C s -[10]CN, in which one naphthalene ring is oriented towards the inside of the CN ring. The high-eld shi of the two protons marked by blue and green circles in Scheme 2 might thus be due to the shielding effect of the ring current of the neighboring naphthalene rings.
The dynamics of the isomerization from C s -[10]CN to D 5d -[10]CN was studied theoretically and experimentally. Initially, we calculated the isomerization pathway from C s -[10] CN to D 5d -[10]CN. As shown in Fig. 6a (bottom), two rotation modes are possible for 1,1 0 -binaphthyl, i.e. ortho-peri and periperi. For the isomerization of C s -[10]CN, we also found two pathways, corresponding to the ortho-peri (path A) and peri-peri mode (path B). For each pathway one intermediate and two transition states were found, and the highest transition state for each pathway is shown in Fig. 6a. Isomerization barriers of 30.6 and 30.2 kcal mol À1 were calculated for path A and path B, respectively. Considering that the rotation via the ortho-peri mode is much more favorable for 1,1 0 -binaphthyl (25.4 and 34.3 kcal mol À1 for ortho-peri and peri-peri, respectively), 12,19 the observed destabilization of the ortho-peri mode and the stabilization of the peri-peri mode in [10]CN may be due to the ring strain in [10]CN. Although it is difficult to determine which pathway is the favorable isomerization pathway from C s -[10]CN to D 5d -[10]CN, we expect that the isomerization should occur under mild heating conditions. Accordingly, it should be possible to determine the isomerization barrier by monitoring the decreasing integration of C s -[10]CN in the 1 H NMR spectra in 1,1,2,2-tetrachloroethane-d 2 using coronene as the internal standard. The rst-order rate constants k (s À1 ) of the conversion at various temperatures were estimated using the following equation:  (Fig. 6b). The Eyring plot based on these data provided the following activation parameters DH ‡ ¼ 27.7 kcal mol À1 , DS ‡ ¼ 2.0 cal mol À1 K À1 , and DG ‡ 298 K ¼ 27.1 kcal mol À1 . These values are consistent with the theoretically predicted isomerization barriers.

Conclusions
We have achieved the synthesis of [n]CNs (n ¼ 8, 10, 12, 16) and uncovered their intriguing structural and photophysical  properties. Nickel-or palladium-mediated couplings of the extended L-shaped units 2 and 3, followed by reductive aromatization of the coupling products afforded [n]CNs (n ¼ 8, 10,12,16). The size-dependent properties of these CNs were examined by UV-vis absorption and uorescence spectroscopy. Theoretical studies supported a unique inuence of the number of naphthalene rings in [n]CN on its structural and photophysical properties. A kinetic study on the thermal conversion of the C s -symmetric conformer of [10]CN (C s -[10]CN) into the most stable D 5d -symmetric conformer (D 5d -[10]CN) indicated that the ring strain substantially affects the rotation barrier of the naphthalene rings in [10]CN. With this series of CNs in hand, further attempts to synthesize carbon nanobelts from CNs via sequential cyclodehydrogenation reactions are currently ongoing in our laboratory.