End-to-End Bent Perylene Bisimide Cyclophanes by Double Sulfur Extrusion

Bending inherently planar π-cores consisting of only six-membered rings has traditionally been challenging because a powerful transformation is required to compensate for the significant strain energy associated with bending. Herein, we demonstrate that sulfur extrusion can achieve substantial molecular bending of a perylene structure to form a substructure of a Vögtle belt, a proposed yet hitherto elusive carbon nanotube fragment. Bent perylene bisimide (PBI) derivatives were synthesized through a double-sulfur-extrusion reaction from the corresponding sulfur-containing V-shaped precursors with an internal alkyl tether. The effect of bending the inherently planar PBI core, which is a recent topic of interest for the design of advanced organic electronic and optoelectronic materials, was investigated systematically. Increasing the curvature leads to a red shift in the absorption and emission spectra, while the fluorescence quantum yields remain high. This stands in contrast with the nonemissive features of previously reported nonplanar PBI derivatives based on conjugative tethers. Detailed photophysical measurements indicated that the increasing curvature with shorter alkyl tethers (i) slightly facilitates intersystem crossing and (ii) significantly suppresses the internal conversion in the excited state of the present bent PBI derivatives. The latter characteristics originate from the restricted dynamic motion associated with the charge-transfer (CT) character between the core chromophores and the N-aryl units.


■ INTRODUCTION
−21 However, examples of the coercive bending of inherently planar π-cores that consist of only six-membered rings, which have zero Gaussian curvature, remain limited because the synthesis of these contorted π-systems requires a powerful transformation to compensate for the significant strain energy associated with their curvature.
The solid-state structures of 10 and 11 were unambiguously determined by using single-crystal X-ray diffraction analysis (Figure 4).In the crystal, 10 and 11 adopt a V-shaped structure similar to that of 9.The interplanar angles between the two naphthalene monoimide units gradually decrease in the order 9 (113°) > 10 (100°) > 11 (94°) due to the smaller bond angle of sulfoxide compared to that of sulfide.The distances between the ortho-substituents of the N-aryl groups  The UV/vis absorption spectra of 9−11 are shown in Figure 5. DNDTBI 9 exhibits a broad absorption band at 450 nm, which was assigned to the charge-transfer (CT) transition from the central sulfur atoms to the naphthalene monoimide units. 53ccordingly, monosulfoxide 10 shows hypsochromically shifted absorption with a shoulder peak tailing to 460 nm.Furthermore, the CT transition completely disappeared in the absorption spectrum of 11, with the remaining sharp absorption bands tailing at 420 nm.
Subsequently, we examined the photoinduced sulfurextrusion reactions of 9−11 in CH 2 Cl 2 (Figure 6).Photoirradiation was performed using a high-pressure mercury lamp equipped with a sharp cutoff filter (λ > 380 nm).Compound 9 underwent the sulfur-extrusion reaction, albeit sluggishly, to afford PBI 3′ in only 19% yield even after an extended photoirradiation period of 3 h.In contrast, the sulfur-extrusion reaction of 10 afforded PBI 3′ almost quantitatively within 900 s.Notably, a broad absorption band appeared at 370−450 nm during the irradiation of 10.This feature resembles the spectrum of DNTBI 1 rather than that of its sulfoxide 2. These results suggest that the sulfoxide unit rather than the sulfide unit of 10 was removed preferentially, which is in agreement with the observed trend for 1 and 2, i.e., the sulfur-extrusion reaction of 2 proceeded more rapidly than that of 1. 38 Furthermore, the sulfur-extrusion reaction of 11 was faster than that of 10, reaching completion to give 3′ in approximately 30 s.After 15 min of photoirradiation, the absorption spectrum is almost identical to that of PBI 3′, indicating nearly quantitative conversion.The lack of isosbestic points can be attributed to the formation of DNTBI sulfoxide 2 as an intermediate.
We also examined the sulfur-extrusion reaction of 11 via electron injection or heating.The cyclic voltammograms of 11 in CH 2 Cl 2 (Figure S39) feature an irreversible reduction wave at −1.38 V in the forward direction.Upon backsweeping, two peaks were observed at −0.97 and −1.16 V, which are identical to those of PBI 3′.Furthermore, spectroelectrochemical measurements of 11 revealed that the absorption spectrum of 11 after electrochemical reduction is identical to that of the radical anion of PBI 3′ (Figure S40).These results indicate that electron injection triggers the extrusion of sulfur from 11, as was observed for the previously reported DNTBI sulfoxide 2. On the other hand, the thermogravimetric analysis (TGA) profile of 11 exhibits no characteristic mass decrease due to sulfur extrusion while DNTBI sulfoxide 2 underwent sulfur extrusion upon heating (Figure S42).
The structures of PBIphanes 5a−5c were unambiguously determined by X-ray diffraction analysis (Figure 7a−c).Structural parameters are summarized in Table 1.In the single crystal, PBIphanes 5a, 5b, and 5c adopt bent structures with nitrogen−nitrogen distances, d, of 10.55, 10.78, and 11.15 Å, respectively.These nitrogen−nitrogen distances are significantly shorter than that of planar PBI 3 (d = 11.3Å) 55 and comparable to those of previously reported contorted PBIphanes 6 (d = 10.25−11.36Å).The end-to-end bend angles, θ, are 33.8°(5a),29.4°(5b), and 19.4°(5c).In the Xray crystal structure of 5b, one methylene unit neighboring an oxygen atom adopts a gauche configuration, which can be attributed to the mismatch due to having an even-numbered alkyl chain as the tethering unit.The π-orbital-axis-vector (POAV) angles, 56 which provide a measure of the degree of orbital hybridization based on structural aspects, were calculated based on the crystal structures of 5a−5c (Table 1), in which the obtained POAV angles at the equivalent carbons were averaged.The POAV angles increase in the order 5c < 5b < 5a, indicating that increased curvature decreases the s-character of the constituent carbon atoms.The deviation from planarity is most pronounced at the imide-substituted carbon atoms (position a).
The strain energies in 5a, 5b, and 5c were investigated using hypothetical homodesmotic reactions at the B3LYP/6-31G(d) level and were calculated to be 22.9, 19.7, and 13.6 kcal mol −1 , respectively (Figure S45).The local strain energies were also visualized using the StrainVis tool (Figure 7d−f). 57The PBIphanes 5a, 5b, and 5c exhibit the greatest local strain at the C−N bond of the N-aryl substituents, which increases with decreasing length of the alkyl linker (5a: 1.61 kcal mol −1 ; 5b: 1.55 kcal mol −1 ; 5c: 1.06 kcal mol −1 ).In the perylene core, the strain is mainly localized at the peripheral bonds around the naphthalene segments, as was also observed in a computational simulation of a Vogtle belt. 57The C−O bonds in 5a−5c also contribute substantially to the accommodation of the local strain.
The solubility value of 5a in CHCl 3 (0.11 mg/mL) is higher than those of 5b (0.027 mg/mL) and 5c (0.045 mg/mL), due to the large molecular curvature.However, these solubility values of PBIphanes 5a−5c are smaller than that of planar N-(2-methoxyphenyl)-substituted PBI 3″ (0.38 mg/mL), which exhibits high solubility due to the existence of rotamers.The relatively low solubility of PBIphanes 5a−5c is attributable to the restricted rotation of their N-substituents.
Aromaticity of PBIphanes.In the 1 H NMR spectra of 5a−5c and untethered planar PBI 3″ in tetrachloroethane-d 2 , the signals of the aromatic protons showed an upfield shift with increasing curvature (5a: 8.64 and 8.62 ppm; 5b: 8.67 and 8.65 ppm; 5c: 8.71 and 8.69 ppm; 3″: 8.74 and 8.72 ppm).DFT calculations reproduced this trend (Table S3).Nevertheless, the nucleus-independent chemical shift (NICS) values at the carbonyl-substituted six-membered rings of 5a−5c and 3″, which represent an averaged value among four rings, are almost comparable (5a: −6.44 ppm; 5b: −6.44 ppm; 5c: −6.52 ppm; 3″: −6.61 ppm).Consequently, we conclude that (i) increasing the molecular curvature in PBI has a negligible effect on the aromaticity and (ii) the observed upfield shifts of the aromatic proton signals originate from the change in orientation toward the neighboring carbonyl group or aromatic ring.
Electrochemical Properties of PBIphanes.The cyclic voltammograms and differential pulse voltammograms of 5a− 5c and 3″ were measured in CH 2 Cl 2 by using 0.1 M Bu 4 NPF 6 as the supporting electrolyte and Ag/AgNO 3 as the reference electrode (Figure S41).The ferrocene/ferrocenium couple (Fc/Fc + ) was used as an internal reference.For 5a−5c and 3″, two reversible reduction waves (5a: −1.03 and −1.27 V; 5b: −1.01 and −1.24 V; 5c: −1.00 and −1.23 V; and 3″: −1.03 and −1.22 V) were observed.These results indicate that the sensitivity of the CV measurements is insufficient to distinguish the subtle effect of molecular bending on the electron-accepting abilities.
Photophysical Properties of PBIphanes.The UV/vis absorption and emission spectra of PBI 3″ and PBIphanes 5a− 5c are shown in Figure 8, and their photophysical parameters are summarized in Table 2. Increasing the structural curvature of the PBI cores results in a red shift of the longest-wavelength absorption peaks from 527 nm (3″) to 544 nm (5a), together with decreased extinction coefficients.This trend contrasts with those of pyrenophane and teropyrenophane, in which increased structural curvature leads to blue-shifted absorption spectra. 26,28These results firmly corroborate the recent argument that the symmetry of the local orbitals in the Kregion (bonding or antibonding) governs the (de)stabilization of the HOMO and LUMO. 50The shortest-wavelength emission peaks also exhibited a bathochromic shift from 535 nm (3″) to 555 nm (5a) with increasing curvature.The PBIphanes exhibited intense emission with quantum yields, Φ F , of 0.84−0.88,which are larger than that of planar PBI 3″ (Φ F = 0.76).It is worth noting here that the previously reported π-tethered contorted PBI derivatives 6 exhibit behavior that is different from that of the present PBIphanes 5a−5c: (i) negligible correlation between the absorption wavelength and molecular bending and (ii) weak emission (Φ F = 0.01−0.32) 41; these differences highlight the fact that the electronic properties of 6 are substantially altered by the πdelocalization onto the tethering π-linkers.
The full widths at half-maximum (fwhm's) of the 0−0 absorption bands of 5a, 5b, 5c, and 3″ calculated from the Gauss fitting are 659, 686, 661, and 601 cm −1 , respectively.The Stokes shifts of 5a, 5b, 5c, and 3″ are 364, 403, 344, and 284 cm −1 , respectively.The larger fwhm and Stokes shift values of PBIphanes 5a−5c compared to those of planar PBI 3″ were attributed to their nonplanar structures.Interestingly, the fwhm and Stokes shift values of 5b are larger than those of 5a and 5c, suggesting that the mismatch of its even-numbered alkyl chain increases the structural flexibility.The photophysical parameters of 5a−5c and 3 are summarized in Table 2. Their radiative decay rates, k r , decrease slightly from 2.1 × 10 8 to 1.8 × 10 8 s −1 with increasing structural curvature.TD-DFT calculations at the CAM-B3LYP/6-31G(d) level indicated that the oscillator strength of the S 0 −S 1 transitions also decreases in the same order (3: 0.96 > 5c: 0.78 > 5b: 0.70 > 5a: 0.67) (Figure S46), which is in agreement with their extinction coefficients.These results can be explained by the decreased HOMO−LUMO overlap with increasing curvature.The increase in structural curvature also decreases the nonradiative decay rates, k nr , from 0.67 × 10 8 to 0.24 × 10 8 s −1 .Transition-absorptionspectroscopy measurements indicated that the photoexcitation of 5b and 5c affords long-lived species with lifetimes of 86 and 125 μs, which were assigned to the triplet excited states (Figure S47).The intersystem crossing (ISC) quantum yield values, Φ ISC , of 5b and 5c are 0.1 and 0.2%, respectively (Figure S48).These Φ ISC values are greater than the reported triplet yield of N,N′-bis(2,5-di-tert-butylphenyl)-substituted PBI 3‴ (Φ ISC < 10 −4 ), 58 indicating that the molecular bending promotes ISC. 59However, the Φ ISC values are in general very low, indicating that the nonradiative deactivation of the bent PBIs 5a−5c mainly proceeds through internal conversion (IC).Using the reported fluorescence yield and decay of PBI 3‴, its IC rate constant was estimated to be k nr = 3 × 10 6 s −1 , which is approximately one-tenth of those of 5a−5c.TD-DFT calculations of 5a−5c also indicated the charge-transfer (CT) character between the core chromophores and the aryl units as the second excited state (S 2 ) in the S 0 -optimized geometries (Figure S46).Thus, the disordered motion in the alkyl linker is anticipated to cause a vibronic admixture between S 1 and S 2 to promote the IC processes to the S 0 states via CT character.Indeed, the Φ F values in toluene (3″: 87%; 5a: 90%; 5b: 86%; 5c:87%) are larger than those in CHCl 3 .Furthermore, the fact that the k nr value of 5c is greater than those of 5a and 5b strongly supports this hypothesis, because the flexibility of the alkyl chain linker increases with increasing length, allowing the orbital hybridization between HOMO and HOMO−1 as shown in Figure S46.In contrast, shorter linkages restrict the aforementioned orbital overlap due to the more rigid geometry.However, the k nr value in 5a is greater than that in 3‴, indicating that the alkyl linkages essentially induce vibronic coupling for the nonradiative processes.

■ CONCLUSIONS
We have described the synthesis and properties of bent PBI derivatives 5a−5c, which feature an internal alkyl chain tethering the two N-aryl substituents.These PBIphanes were synthesized via sulfur-extrusion reactions from corresponding disulfoxide precursors 4a−4c.X-ray diffraction analysis of 5a− 5c demonstrated that their degree of curvature increases with decreasing length of the alkyl tether.The strain energy was evaluated to be as high as 22.9 kcal mol −1 (5a) based on homodesmotic reactions.The 1 H NMR analysis of 5a−5c indicated that the aromatic proton signals were shifted upfield by approximately 0.1 ppm with increasing molecular curvature.This shift is due to the change in orientation toward the neighboring carbonyl group or aromatic ring rather than a change in aromaticity.Electrochemical measurements of 5a− 5c indicated that increasing the molecular curvature in PBI has a negligible effect on electron affinity.In sharp contrast, the increased curvature leads to a red shift of the absorption and emission spectra (by ∼20 nm), while the high fluorescence quantum yields (up to 88%) are maintained.This behavior contrasts with the nonemissive features of previously reported contorted π-extended PBI derivatives 6. Detailed photophysical measurements of 5a−5c indicated that the nonradiative deactivation of the S 1 state via IC is suppressed by the restricted dynamic motion associated with the CT character between the core chromophores and the N-aryl units.Our results demonstrate that the double-sulfur-extrusion reactions enable the construction of curved perylene structures and the effect that the structural curvature exerts on the structural, electrochemical, and photophysical properties of these PBIs.These findings can be expected to find applications in the design of advanced organic materials.

Figure 2 .
Figure 2. Synthesis of PBIs via sulfur-extrusion reactions from the corresponding precursors with inserted sulfur/sulfoxide units.

Figure 4 .
Figure 4. X-ray crystal structures of (a) 10 and (b) 11 with thermal ellipsoids at 50% probability; all hydrogen atoms have been omitted for the sake of clarity.

Table 1 .
Structural Parameters and Calculated Strain Energies for 5a−5c a Nitrogen−nitrogen distance.b End-to-end bend angle.c Strain energy.