9,10-Dihydrophenanthrene with Two Spiro(dibenzocycloheptatriene) Units: A Highly Strained Caged Hydrocarbon Exhibiting Reversible Electrochromic Behavior

The title dispiro hydrocarbon 1 was designed as a new electrochromic material. This multiply clamped hexaphenylethane-type electron donor was prepared from 2,2′-diiodobiphenyl via biphenyl-2,2′-diylbis(dibenzotropylium) 22+ salt. X-ray analysis of 1 revealed a highly strained structure as reflected by an elongated “ethane” bond [bond length: 1.6665(17) Å] and nearly eclipsed conformation. The weakened bond was cleaved upon two-electron oxidation to regenerate the deeply colored dication 22+. The reversible interconversion between 1 and 22+ is accompanied not only by a drastic color change but also by C–C bond formation/cleavage. Thus, the voltammogram showed a pair of well-separated redox waves, which is characteristic of “dynamic redox (dyrex)” behavior. The tetrahydro derivative of 1 with two units of spiro(dibenzocycloheptadiene), which suffers from more severe steric congestion, was also prepared. The crystallographically determined bond length for the central C–C bond [1.705(4) Å] is greatest among the values reported for 9,9,10,10-tetraaryl-9,10-dihydrophenanthrene derivatives.

Hexabenzo [4.4.4]propellaneA [24], which was first reported in 1971, is a multiply clamped HPE in which the central Csp 3 -Csp 3 bond [bond length: 1.563(2) Å] is not greatly expanded [25].In this nonspiro-type clamped HPE, the steric repulsion among the six benzene rings is effectively reduced by adopting a twisted conformation with a torsion angle (α) of about 60 • around the central C-C bond (Figure 1).On the other hand, dibenzodinaphtho [4.3.3]propellaneB [26] has a longer bond [1.612(4)Å] because it has a less-skewed geometry due to the rigid naphthalene planes.This difference in bond length as well as its correlation with torsion/twisting angles (α, θ) could be supported by DFT calculations.On the other hand, Ar 4 DHPs were predicted to have much longer C-C bonds than those in B since the steric hindrance regarding the C 9 -C 10 bond is greater for two unfused benzene rings than for a naphthalene nucleus.
We previously studied the structures and properties of a series of Ar 4 DHPs [27][28][29][30][31][32].When we conducted an X-ray analysis of (4-CH 3 OC 6 H 4 ) 4 DHP (C), we could not obtain accurate values for the bond length or torsion/twisting angles due to the positional disorder of the ethane unit [27].Such disorder has often been observed in globular ethanes (e.g., unclamped HPEs [33] or hexachloroethane [34]), but was not present in the spiro-type Ar 4 DHPs (D and E).Their C 9 -C 10 bond lengths could be accurately determined to be 1.646(4) and 1.635(2) Å by X-ray analyses [29,30].These values are greater than those for other DHPs, and, thus, the spiro ring can be considered to be the key structure for observing a very long bond in Ar 4 DHPs.
Another interesting point regarding Ar 4 DHP is the electrochromic response with "dynamic redox (dyrex)" behavior.Two-electron oxidation induces the formation of cationic chromophores accompanied by fission of the elongated C 9 -C 10 bond.Heterocyclic units (acridan/xanthene) or alkoxy/amino groups have often been incorporated into Ar 4 DHP to raise the HOMO level and to stabilize the corresponding dicationic species.However, under an appropriate molecular design, we envisaged that reversible electrochromic systems could be constructed without the aid of heteroatoms.Herein we report the details of the title spiro-type Ar 4 DHP 1, which is a pure hydrocarbon that can still exhibit reversible electrochromic behavior thanks to the raised HOMO level of 1 as well as the stability of the 14π-dibenzotropylium unit in the bond-dissociated dication 2 2+ .
Molecules 2017, 22,1900 2 of 14 nonspiro-type clamped HPE, the steric repulsion among the six benzene rings is effectively reduced by adopting a twisted conformation with a torsion angle (α) of about 60° around the central C-C bond (Figure 1).On the other hand, dibenzodinaphtho [4.3.3]propellaneB [26] has a longer bond [1.612( 4) Å] because it has a less-skewed geometry due to the rigid naphthalene planes.This difference in bond length as well as its correlation with torsion/twisting angles (α, θ) could be supported by DFT calculations.On the other hand, Ar4DHPs were predicted to have much longer C-C bonds than those in B since the steric hindrance regarding the C9-C10 bond is greater for two unfused benzene rings than for a naphthalene nucleus.We previously studied the structures and properties of a series of Ar4DHPs [27][28][29][30][31][32].When we conducted an X-ray analysis of (4-CH3OC6H4)4DHP (C), we could not obtain accurate values for the bond length or torsion/twisting angles due to the positional disorder of the ethane unit [27].Such disorder has often been observed in globular ethanes (e.g., unclamped HPEs [33] or hexachloroethane [34]), but was not present in the spiro-type Ar4DHPs (D and E).Their C9-C10 bond lengths could be accurately determined to be 1.646(4) and 1.635(2) Å by X-ray analyses [29,30].These values are greater than those for other DHPs, and, thus, the spiro ring can be considered to be the key structure for observing a very long bond in Ar4DHPs.
Another interesting point regarding Ar4DHP is the electrochromic response with "dynamic redox (dyrex)" behavior.Two-electron oxidation induces the formation of cationic chromophores accompanied by fission of the elongated C9-C10 bond.Heterocyclic units (acridan/xanthene) or alkoxy/amino groups have often been incorporated into Ar4DHP to raise the HOMO level and to stabilize the corresponding dicationic species.However, under an appropriate molecular design, we envisaged that reversible electrochromic systems could be constructed without the aid of heteroatoms.Herein we report the details of the title spiro-type Ar4DHP 1, which is a pure hydrocarbon that can still exhibit reversible electrochromic behavior thanks to the raised HOMO level of 1 as well as the stability of the 14π-dibenzotropylium unit in the bond-dissociated dication 2 2+ .

Design and Theoretical Study
The through-bond interaction (TBI) [35] in PAHs is an important factor which modifies the MO levels that determine their electron-donating properties.As postulated based on a simplified s-cis diphenylethane model (Figure 2), the σ-orbital of the ethane bond would be lowered whereas the π +orbital should be raised through TBI.Greater perturbation is expected when the two energy levels are closer.The parallel arrangement of these orbitals is also important for realizing effective TBI.
For Ar4DHP, TBI would be maximized by elongation of the C9-C10 bond, since a longer bond has a higher σ-orbital level to narrow the energy gap toward the π + -orbital.At the same time, an eclipsed

Design and Theoretical Study
The through-bond interaction (TBI) [35] in PAHs is an important factor which modifies the MO levels that determine their electron-donating properties.As postulated based on a simplified s-cis diphenylethane model (Figure 2), the σ-orbital of the ethane bond would be lowered whereas the π + -orbital should be raised through TBI.Greater perturbation is expected when the two energy levels are closer.The parallel arrangement of these orbitals is also important for realizing effective TBI.For Ar 4 DHP, TBI would be maximized by elongation of the C 9 -C 10 bond, since a longer bond has a higher σ-orbital level to narrow the energy gap toward the π + -orbital.At the same time, an eclipsed conformation is desirable to ensure that the orbitals are parallel.The spiro(dibenzocycloheptatriene) units are the ideal skeleton to be incorporated into Ar 4 DHP.In general, Ar 4 DHPs tend to adopt a skewed geometry to reduce the "front strain" [21] among the four aryl groups over the C 9 -C 10 bond.The spiro(xanthene or 10-methylacridane) with the central six-membered ring can force the DHP skeleton to adopt a less-skewed geometry (Figure 1).Here, we propose that spiro(dibenzocycloheptatriene) with a central seven-membered ring is more favorable since it can make the DHP skeleton become nearly eclipsed by the greater steric hindrance between the inner protons on the benzo groups and the DHP plane (Figure 3).The nearly eclipsed conformation would also cause elongation of the C 9 -C 10 bond due to the greater steric repulsion than in the skewed conformation, and, thus, the HOMO level of Ar 4 DHP with two spiro(dibenzocycloheptatriene) 1 would be effectively raised through TBI.
Molecules 2017, 22,1900 3 of 14 conformation is desirable to ensure that the orbitals are parallel.The spiro(dibenzocycloheptatriene) units are the ideal skeleton to be incorporated into Ar4DHP.In general, Ar4DHPs tend to adopt a skewed geometry to reduce the "front strain" [21] among the four aryl groups over the C9-C10 bond.
The spiro(xanthene or 10-methylacridane) with the central six-membered ring can force the DHP skeleton to adopt a less-skewed geometry (Figure 1).Here, we propose that spiro(dibenzocycloheptatriene) with a central seven-membered ring is more favorable since it can make the DHP skeleton become nearly eclipsed by the greater steric hindrance between the inner protons on the benzo groups and the DHP plane (Figure 3).The nearly eclipsed conformation would also cause elongation of the C9-C10 bond due to the greater steric repulsion than in the skewed conformation, and, thus, the HOMO level of Ar4DHP with two spiro(dibenzocycloheptatriene) 1 would be effectively raised through TBI.The tetrahydro derivative 3 with two units of spiro(dibenzocycloheptadiene) was also included in this study as a reference compound; it cannot adopt a similar eclipsed conformation due to severe steric repulsion between the CH2CH2 units in the cycloheptadiene rings.
Density functional theory (DFT) calculations [36] at the B3LYP/6-31G* level predicted that the optimized structure of 1 adopts a nearly eclipsed conformation, as designed.The C9-C10 bond length was estimated to be greater than 1.7 Å, which is much greater than the values previously reported for other Ar4DHPs (Figure 4).The calculated HOMO level of 1 was raised to -5.25 eV, which is much higher than that of (4-CH3OC6H4)4DHP (-5.44 eV).Thus, we designed a stronger electron-donating hydrocarbon than Ar4DHP with four CH3O groups.conformation is desirable to ensure that the orbitals are parallel.The spiro(dibenzocycloheptatriene) units are the ideal skeleton to be incorporated into Ar4DHP.In general, Ar4DHPs tend to adopt a skewed geometry to reduce the "front strain" [21] among the four aryl groups over the C9-C10 bond.
The spiro(xanthene or 10-methylacridane) with the central six-membered ring can force the DHP skeleton to adopt a less-skewed geometry (Figure 1).Here, we propose that spiro(dibenzocycloheptatriene) with a central seven-membered ring is more favorable since it can make the DHP skeleton become nearly eclipsed by the greater steric hindrance between the inner protons on the benzo groups and the DHP plane (Figure 3).The nearly eclipsed conformation would also cause elongation of the C9-C10 bond due to the greater steric repulsion than in the skewed conformation, and, thus, the HOMO level of Ar4DHP with two spiro(dibenzocycloheptatriene) 1 would be effectively raised through TBI.The tetrahydro derivative 3 with two units of spiro(dibenzocycloheptadiene) was also included in this study as a reference compound; it cannot adopt a similar eclipsed conformation due to severe steric repulsion between the CH2CH2 units in the cycloheptadiene rings.
Density functional theory (DFT) calculations [36] at the B3LYP/6-31G* level predicted that the optimized structure of 1 adopts a nearly eclipsed conformation, as designed.The C9-C10 bond length was estimated to be greater than 1.7 Å, which is much greater than the values previously reported for other Ar4DHPs (Figure 4).The calculated HOMO level of 1 was raised to -5.25 eV, which is much higher than that of (4-CH3OC6H4)4DHP (-5.44 eV).Thus, we designed a stronger electron-donating hydrocarbon than Ar4DHP with four CH3O groups.The tetrahydro derivative 3 with two units of spiro(dibenzocycloheptadiene) was also included in this study as a reference compound; it cannot adopt a similar eclipsed conformation due to severe steric repulsion between the CH 2 CH 2 units in the cycloheptadiene rings.
Density functional theory (DFT) calculations [36] at the B3LYP/6-31G* level predicted that the optimized structure of 1 adopts a nearly eclipsed conformation, as designed.The C 9 -C 10 bond length was estimated to be greater than 1.7 Å, which is much greater than the values previously reported for other Ar 4 DHPs (Figure 4).The calculated HOMO level of 1 was raised to −5.25 eV, which is much higher than that of (4-CH 3 OC 6 H 4 ) 4 DHP (−5.44 eV).Thus, we designed a stronger electron-donating hydrocarbon than Ar 4 DHP with four CH 3 O groups.As shown in Figure 5, the HOMO of 1 has the character of (π + -σ), as evidenced by the large orbital coefficients on the ethane bond.Thus, the effective TBI between π + and σ causes the perturbation of both orbital levels, resulting in an increase in the energy of (π + -σ) to become higher than that of π − [next-highest OMO (NHOMO), -5.39 eV].
In the case of the spiro(dibenzocycloheptadiene) derivative, diol 5 was obtained in a similar manner and treated with TMSClO4 [38] in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP).The resulting precursor dication was directly reduced with Zn powder to give the desired compound 3 in 33% yield as a white solid.The low yield of 3 can be partly explained by the formation of byproducts/decomposition products.From the mixture, isomer 6 with a spiro(fluorene) framework was isolated.The eight-membered-ring endoperoxide 7 was also generated as a decomposition pathway of 3. As shown in Figure 5, the HOMO of 1 has the character of (π +σ), as evidenced by the large orbital coefficients on the ethane bond.Thus, the effective TBI between π + and σ causes the perturbation of both orbital levels, resulting in an increase in the energy of (π +σ) to become higher than that of π − [next-highest OMO (NHOMO), −5.39 eV].As shown in Figure 5, the HOMO of 1 has the character of (π + -σ), as evidenced by the large orbital coefficients on the ethane bond.Thus, the effective TBI between π + and σ causes the perturbation of both orbital levels, resulting in an increase in the energy of (π + -σ) to become higher than that of π − [next-highest OMO (NHOMO), -5.39 eV].
In the case of the spiro(dibenzocycloheptadiene) derivative, diol 5 was obtained in a similar manner and treated with TMSClO4 [38] in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP).The resulting precursor dication was directly reduced with Zn powder to give the desired compound 3 in 33% yield as a white solid.The low yield of 3 can be partly explained by the formation of byproducts/decomposition products.From the mixture, isomer 6 with a spiro(fluorene) framework was isolated.The eight-membered-ring endoperoxide 7 was also generated as a decomposition pathway of 3.
When dication 2 2+ was reduced with Zn powder, the newly designed dispiro Ar 4 DHP 1 with two spiro(dibenzocycloheptatriene) units was obtained quantitatively as a white solid (Scheme 1).
In the case of the spiro(dibenzocycloheptadiene) derivative, diol 5 was obtained in a similar manner and treated with TMSClO 4 [38] in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP).The resulting precursor dication was directly reduced with Zn powder to give the desired compound 3 in 33% yield as a white solid.The low yield of 3 can be partly explained by the formation of by-products/decomposition products.From the mixture, isomer 6 with a spiro(fluorene) framework was isolated.The eight-membered-ring endoperoxide 7 was also generated as a decomposition pathway of 3.
X-ray analyses were performed at 150 K by using single crystals of 1 and 3 (Figure 6).Consistent with the results of DFT calculations, 1 adopts a nearly eclipsed geometry.For the DHP skeleton, the torsion angle α over the C 9 -C 10 bond is 23.78 (13) • and the dihedral angle θ for the biphenyl unit is only 3.7 • .Due to the lack of skewing deformation to reduce the "front" strain, the C 9 -C 10 bond is expanded.The length of 1.6665 (17) Å is greater than any value ever reported for Ar 4 DHPs [39].
As indicated by the DFT calculation shown in Figure 4, both of the crystallographically independent molecules of 3 adopt a twisted conformation.In this way, close contact between the two CH 2 CH 2 units over the C 9 -C 10 bond is avoided.However, the molecules still suffer from large steric hindrance between the two spiro(dibenzocycloheptadiene) units.Thus, the bond lengths for the C 9 -C 10 bond [1.705(4) and 1.697(4) Å] of 3 are the highest values reported for the Ar 4 DHP, including 1. X-ray analyses were performed at 150 K by using single crystals of 1 and 3 (Figure 6).Consistent with the results of DFT calculations, 1 adopts a nearly eclipsed geometry.For the DHP skeleton, the torsion angle α over the C9-C10 bond is 23.78(13)° and the dihedral angle θ for the biphenyl unit is only 3.7°.Due to the lack of skewing deformation to reduce the "front" strain, the C9-C10 bond is expanded.The length of 1.6665 (17) Å is greater than any value ever reported for Ar4DHPs [39].
As indicated by the DFT calculation shown in Figure 4, both of the crystallographically independent molecules of 3 adopt a twisted conformation.In this way, close contact between the two CH2CH2 units over the C9-C10 bond is avoided.However, the molecules still suffer from large steric hindrance between the two spiro(dibenzocycloheptadiene) units.Thus, the bond lengths for the C9-C10 bond [1.705(4) and 1.697(4) Å] of 3 are the highest values reported for the Ar4DHP, including 1. X-ray analyses were performed at 150 K by using single crystals of 1 and 3 (Figure 6).Consistent with the results of DFT calculations, 1 adopts a nearly eclipsed geometry.For the DHP skeleton, the torsion angle α over the C9-C10 bond is 23.78(13)° and the dihedral angle θ for the biphenyl unit is only 3.7°.Due to the lack of skewing deformation to reduce the "front" strain, the C9-C10 bond is expanded.The length of 1.6665 (17) Å is greater than any value ever reported for Ar4DHPs [39].
As indicated by the DFT calculation shown in Figure 4, both of the crystallographically independent molecules of 3 adopt a twisted conformation.In this way, close contact between the two CH2CH2 units over the C9-C10 bond is avoided.However, the molecules still suffer from large steric hindrance between the two spiro(dibenzocycloheptadiene) units.Thus, the bond lengths for the C9-C10 bond [1.705(4) and 1.697(4) Å] of 3 are the highest values reported for the Ar4DHP, including 1.

Redox Behavior
To investigate the electron-donating properties and reversibility of the redox behavior of 1, redox potentials were measured by cyclic voltammetry in CH 2 Cl 2 (Figure 7).The one-wave two-electron oxidation peak was observed, which corresponds to the formation of the dication 2 2+ accompanied by C 9 -C 10 bond cleavage.The corresponding reduction peak of 2 2+ appeared at +0.42 V.Such a separation of redox peaks is characteristic of dyrex systems.When scanning was repeated twice, no change was observed in the voltammogram, which indicates highly reversible interconversion between 1 and 2 2+ .

Redox Behavior
To investigate the electron-donating properties and reversibility of the redox behavior of 1, redox potentials were measured by cyclic voltammetry in CH2Cl2 (Figure 7).The one-wave twoelectron oxidation peak was observed, which corresponds to the formation of the dication 2 2+ accompanied by C9-C10 bond cleavage.The corresponding reduction peak of 2 2+ appeared at +0.42 V.Such a separation of redox peaks is characteristic of dyrex systems.When scanning was repeated twice, no change was observed in the voltammogram, which indicates highly reversible interconversion between 1 and 2 2+ .The electron-donating properties of hydrocarbon 1 are striking.The oxidation potential (+0.95 V) is far less positive than that of (4-CH3OC6H4)4DHP (+1.44 V) measured under similar conditions [28].Such a change can be qualitatively accounted for by the different HOMO levels calculated by the DFT method, which demonstrates the validity of our concept for raising the HOMO level of Ar4DHP through TBI without the aid of heteroatoms.

Electrochromic Behavior
Upon the electrochemical oxidation of 1 in CH2Cl2, from colorless to red, an electrochromic response was observed with an isosbestic point at 252 nm.The final UV/Vis spectrum is identical to that of the isolated dication 2 2+ (BF4 − )2 (Figure 8).Regeneration of 1 with a drastic color change from red to colorless was attained by reverse electrolysis of the as-prepared dicationic solution (see supplementary materials Figure S1).This is a rare successful demonstration of electrochromic behavior based on pure hydrocarbon redox species [40].
While 2 2+ was persistent in CH2Cl2, it underwent faci transformation into spiro(fluorene)-type monocation 8 + in CH3CN.When this conversion was followed by UV/Vis spectroscopy at 23 °C, several isosbestic points were observed (Figure 10), from which a reaction rate of 5.0 × 10 −4 s −1 was deduced.Friedel-Crafts-type cyclization also proceeded cleanly in a preparative scale, and 8 + was isolated as a stable salt with 94% yield.The spiro-structure was unambiguously determined by X-ray analysis of the 8 + (SbCl6 − )•(CH3CN) crystal (Figure 9b).The electron-donating properties of hydrocarbon 1 are striking.The oxidation potential (+0.95 V) is far less positive than that of (4-CH 3 OC 6 H 4 ) 4 DHP (+1.44 V) measured under similar conditions [28].Such a change can be qualitatively accounted for by the different HOMO levels calculated by the DFT method, which demonstrates the validity of our concept for raising the HOMO level of Ar 4 DHP through TBI without the aid of heteroatoms.

Electrochromic Behavior
Upon the electrochemical oxidation of 1 in CH 2 Cl 2 , from colorless to red, an electrochromic response was observed with an isosbestic point at 252 nm.The final UV/Vis spectrum is identical to that of the isolated dication 2 2+ (BF 4 − ) 2 (Figure 8).Regeneration of 1 with a drastic color change from red to colorless was attained by reverse electrolysis of the as-prepared dicationic solution (see Supplementary Materials Figure S1).This is a rare successful demonstration of electrochromic behavior based on pure hydrocarbon redox species [40].Quantitative interconversion between donor 1 and dication 2 2+ can be conducted in a preparative manner by chemical oxidation and reduction.Thus, upon treatment of 1 with two equivalents of (4-BrC 6 H 4 ) 3 N +• SbCl 6 − in CH 2 Cl 2 , dication 2 2+ was isolated quantitatively as a stable salt.The X-ray structural analysis of 2 2+ (SbCl 6 − ) 2 •2(CH 2 Cl 2 ) crystal shows that there is a π-π stacking interaction between two dibenzotropylium units [dihedral angle: 12.1 • ; closest C-C contact: 3.31(2) Å] (Figure 9a).The biphenyl unit in 2 2+ is twisted by 70.6 • .While 2 2+ was persistent in CH 2 Cl 2 , it underwent faci transformation into spiro(fluorene)-type monocation 8 + in CH 3 CN.When this conversion was followed by UV/Vis spectroscopy at 23 • C, several isosbestic points were observed (Figure 10), from which a reaction rate of 5.0 × 10 −4 s −1 was deduced.Friedel-Crafts-type cyclization also proceeded cleanly in a preparative scale, and 8 + was isolated as a stable salt with 94% yield.The spiro-structure was unambiguously determined by X-ray analysis of the 8 + (SbCl 6 − )•(CH 3 CN) crystal (Figure 9b).Many dicationic dyes with a biphenyl-2,2 -diyl skeleton adopt a twisted structure similar to that of 2 2+ with a π-π stacking arrangement, for which solvent polarity affects the twisting angle: more acute in a non-polar/less polar solvent and nearly perpendicular in a polar solvent [41,42].The observed solvent effects for the Friedel-Crafts-type cyclization of 2 2+ might be related to this difference in dication conformation depending on the solvent.Facile degradation to the monocation 8 + in CH 3 CN may be related to the largely twisted conformation to facilitate nucleophilic addition of a benzene ring to the cationic center.The lack of further spiro cyclization of 8 + might be related to the ring strain of the 4,8-dihydrocyclopenta[def ]fluorene structure [43].

Crystal Data
Data were collected with a Rigaku Mercury 70 diffractometer (Mo-Kα radiation, λ = 0.71075 Å).The structure was solved by the direct method (SIR2004) and refined by the full-matrix least-squares method on F 2 with anisotropic temperature factors for non-hydrogen atoms.All the hydrogen atoms were located at the calculated positions and refined with riding.

Figure 1 .
Figure 1.Reported structural parameters of Ar4DHPs determined by X-ray analyses.The values obtained by DFT calculations at the B3LYP/6-31G* level in this study are shown in brackets.

Figure 1 .
Figure 1.Reported structural parameters of Ar 4 DHPs determined by X-ray analyses.The values obtained by DFT calculations at the B3LYP/6-31G* level in this study are shown in brackets.

Figure 2 .
Figure 2. Schematic view of through-bond interaction in the diphenylethane model.

Figure 2 .
Figure 2. Schematic view of through-bond interaction in the diphenylethane model.

Figure 2 .
Figure 2. Schematic view of through-bond interaction in the diphenylethane model.