Chiral Peropyrene: Synthesis, Structure, and Properties

■ RESULTS AND DISCUSSION

Design and Synthesis of Chiral Peropyrenes. The benzannulation reaction of alkynes has been shown to proceed with Brønsted acids, 13,14 π-Lewis acids, 15 and other electrophiles. 16 Recently, we have reported the successful synthesis of pyrenes, peropyrenes, and teropyrenes through the benzannulation reaction of alkynes promoted by triflic acid (TfOH). 12a We also applied this methodology to the bottom-up solutionphase synthesis of graphene nanoribbons. 17 Herein, we demonstrate that this efficient method works well to provide chiral peropyrenes with tremendous steric hindrance within both bay regions of the molecule (Scheme 1). The synthesis of 1 was achieved using methodology we have previously reported, followed by a double Suzuki cross-coupling reaction to afford the key precursors 2a-2d in modest yields. The modest yields are understandable considering that compounds 2a-2d are sterically hindered with the ethynylaryl substituents being relatively close in space, as seen in the X-ray crystal structure of 2a (Scheme 1). With precursors 2a-2d in hand, the addition of trifluoroacetic acid (TFA) resulted in the clean formation of bis-cyclized products 4a-4d, which were isolated and fully characterized. Monocyclized products 3a-3d were not observed at the end of the reaction. What was interesting was that although there were three possible products from a 2fold alkyne cyclization reaction, only product 4 was observed, where bis-cyclization occurred on the same side of the molecule to make a picene derivative, as seen in the X-ray crystal structure of 4a (Scheme 1). The other two possible products, 5 and 6, were not detected. The formation of compound 4 was also supported by NMR spectroscopy. We conducted a computational study of this acid-mediated bis-cyclization reaction that supported the kinetic preference for the syncyclization path to afford picene product 4 [Figure S2 and Table S1, Supporting Information (SI)].

According to a previous study, 12b we knew that TFA was not acidic enough to evoke a 4-fold cyclization, so we turned to stronger acids, such as methanesulfonic acid (MSA) and triflic acid (TfOH). Only a trace amount of chiral peropyrene was observed when using excess MSA (50-100 equiv) at room temperature, with significant amounts of bis-cyclized product still observed in the reaction mixture. Increasing the temperature did not improve the conversion or yield but rather led to significant decomposition. To our delight, chiral peropyrenes 8 were isolated in moderate yield when a solution of bis-cyclized product 4 was slowly transferred to a CH 2 Cl 2 solution of TfOH at -40 °C. We also observed a small amount of tricyclized product 7 left at the end of the reaction that was not easily separated by chromatography, except 7a.I n c r e a s i n gt h e reaction time or temperature in an attempt to fully convert the tricyclized intermediate 7 to product 8 led to diminished yields. Peropyrenes 8 can possibly exist as three isomers: the (P,P)-and (M,M)-enantiomers, as well as the (P,M)/(M,P)meso isomer. However, the results showed that no meso compound could be detected, which is in contrast to what is observed in other chiral syntems. 7a,c,g This was supported by NMR and high-performance liquid chromatography (HPLC) analysis (vide infra).

Analysis of NMR Spectra. Chiral peropyrene products 8b-8d are highly soluble in common organic solvents, allowing their characterization by NMR spectroscopy. Broad peaks appeared for the bay region aryl protons in the 1 H NMR (CDCl 3 ) spectra of cyclized compound 4b and 8b. It should be noted that broadening was not observed in the reported tetraphenyl-substituted PBIs. 10c This is due to restricted freerotation of the aryl substituents at room temperature (red circles, Figure 2a) within the newly formed diaryl-substituted bay regions. In the case of bis-cyclized intermediate 4, the two aryl substituents terminating the ethynyl groups (blue circles, Figure 2a) are still capable of free-rotation, as evidenced by the relatively sharp signals for those aryl protons in the 1 H NMR spectrum. The four aryl substituents in compound 8 showed broad peaks in the 1 H NMR spectra. Likewise, the 13 C NMR spectra of 8 exhibit four significantly broadened signals, which were assigned to the aryl substituents. To prove our hypothesis, compound 8b was studied by 1 H and 13 C NMR and 1 H- 13 C correlation NMR (heteronuclear single quantum coherence, HSQC) spectroscopy in CDCl 3 at 50 °C[ Figures 2b andS13 and S14 (SI)]. We found that, after increasing the temperature to 50 °C, the broad peaks converged to two relatively sharp peaks in the 1 H NMR spectrum. This was also observed in the 13 C NMR spectrum. Analysis of the HSQC spectrum at 50 °C showed correlations between the broad 1 H and 13 C peaks, greatly simplifying the structure assignment of 8b. Furthermore, compound 8b was studied by variable temperature (VT) 1 H X-ray Crystallographic Analysis. The structure of chiral peropyrene 8b was unambiguously confirmed by X-ray crystallographic analysis of a racemic single crystal (Figure 3a). The X-ray crystal structure clearly showed a twisted πsystem, which is caused by repulsion (on both sides of the molecule) between the two aryl substituents within the same bay region. The distance between two aryl substituents within the same bay region was only 2.9 Å, and the splay angle of the bay region is ∼29°with an end-to-end twist angle of 28°( Figure 3b). A face-to-face molecular packing was found, with an interplanar spacing of 9 Å (Figure 3b).

Separation, Isomerization, Circular Dichroism, and Optical Rotation. The separation of the enantiomers and their characterization with most recent chiroptical spectroscopies available to-date are presented. The data provides insight into the conformational aspects of chiral peropyrenes, allows the assignment of absolute configuration (AC), and demonstrates their utility in applications such as chiral organic light emitters. Excellent separation of the enantiomers of peropyrene 8b was achieved by HPLC using a column packed with a chiral stationary phase based on amylose tris(3,5-dimethylphenylcarbamate) immobilized on silica gel. 18 Elution with hexane/2propanol (98/2 v/v) gave two nicely resolved peaks corresponding to the (P,P)-and (M,M)-enantiomers of 8b, with elution times of 3.2 and 5.2 min, respectively. When monitored by CD detection at 300 nm, bisignate peaks of equal area were observed, as expected for a racemate (Figure 4a). The analytical separation was easily scaled up to the milligram range using a 250 × 10 mm column packed with the same material used in the analytical column. Iterative chromatography with Scheme 1. Synthesis of Axially Chiral Peropyrenes 8 and Possible Intermediates a a Reaction conditions: (i) 1 (2.2 equiv), 1,4-diiodobenzene (1 mmol), Pd(PPh 3 ) 4 (10 mol %), Ag 2 CO 3 (4 equiv), THF, 80 °C, 24 h (2a, 42%; 2b, 39%, 2c, 47%, 2d, 44%). (ii) TFA (50 equiv), CH 2 Cl 2 , rt, 1 h (4a, 97%; 4b, 94%, 4c, 96%, 4d, 95%). (iii) TfOH (20 equiv), CH 2 Cl 2 , -40 °C, 30 min (8a, 28%; 8b, 40%, 8c, 38%, 8d, 35%). For the enantiomer interconversion of 8b, where a stepwise process converts one enantiomer into the unstable meso form and then into the other enantiomer (see the section on the theoretical study), the statistical transmission coefficient f = 0.5 was used in the calculation of the free energy barrier ΔG ⧧ for a single inversion (M,M or P,P to meso) from the rate constant of the overall process. 19 This gave a value of ΔG ⧧ = 28.9 ± 0.1 kcal/mol at 100 °C and is consistent with the computational study (vide infra).

The optical rotation (OR) value [α] 589 25 = +1438 (c = 8.69 × 10 -3 g/L in CH 2 Cl 2 ) is quite large and is similar to that observed in other helicene systems. 20 This large OR value is due to the fact that the first absorption and CD features are close in wavelength to where the OR is measured (vide infra), and thus, according to the Kramers-Kronig (KK) relation, results of OR are intensified. This is caused by the inverse difference in square wavelength from the wavelength of electronic transitions. 21 As a consequence, when the OR is measured for smaller wavelengths, anomalous optical rotatory dispersion (ORD) 21 is observed. In the Supporting Information (Figures S3), we provide the KK curve derived from the electronic circular dichroism (ECD) spectra (vide infra), and we have checked that the experimental values for [α] at 589, 577, 436, 405, and 365 nm show excellent agreement with the theoretical KK curve.

Experimental ECD and absorption spectra are given in the Figure 4b,c for both peropyrene (8b) enantiomers. A nearly constant g factor (Δε/ε) of about 1.2 × 10 -3 is observed for the features detected in the range 460-530 nm: Δε = ε Lε R , where ε L and ε R are the molar extinction coefficients for left and right circularly polarized light, respectively. The longer wavelength features are vibronic; the same happens in the fluorescence and circularly polarized luminescence (CPL) spectra and is confirmed by quantum mechanical calculations (vide infra). Time-dependent density functional theory (TD-DFT) calculations have been conducted for peropyrene 8b, where we substituted the four O-hexyl groups with four Omethyl groups (8a) and assumed the (M,M)-configuration. With the former assumption, we avoided the complication of considering the multiplicity of aliphatic chain conformations, with no prejudice on the chromophoric transitions. As previously reported, when calculating the chiroptical re- sponse, 22 contributions from different conformers must be taken into account: we checked that the orientation of the Omethyl groups has no influence on the peropyrene low-energy electronic transitions and that the phenyl orientation is strictly connected with peropyrene distortion. The following theoretical study of the interconversion confirms that only one structure has appreciable population. This analysis allowed us to obtain the simulated spectrum of (M,M)-8b (Figure 4b), which is superimposed on the experimental spectra. The AC is assigned to the (M,M)-configuration, which corresponds to the second-eluted enantiomer bearing a negative (-) OR sign at 589 nm. This is what is observed for helicene systems, namely, [5], [6], [7], or even higher-membered helicenes. 20,23 To complete the discussion of the observed CD spectra, one may notice that a similar chiroptical response is encountered in perylene bisimides. 24 In particular, the lowest energy transition is characterized by electric and magnetic transition moments that are both parallel to the long axis of the peropyrene moiety. The second observed feature of opposite sign can be associated with two transitions with transition dipole moments parallel to the transverse axis. In both cases, the electric and magnetic dipole transition moments are parallel (or antiparallel), so the calculated sign can be considered correct. The characteristics of the lowest energy transition are maintained in the excited state, giving an analogous description of the CPL band. Transition moments and assignments are reported in Table S3 (SI).

Raman, Fluorescence, and Circularly Polarized Luminescence. Raman spectroscopy is often used to investigate the spectroscopic and structural properties of highly π-conjugated molecular systems. The experimental FT-Raman spectrum of the racemic powder of sample 8b is reported in Figure 5a and compared with results from DFT calculations. The expected G and D Raman features of graphene molecules 1 clearly emerge, and the associated nuclear displacement patterns nicely match those previously observed in another chiral polycyclic aromatic hydrocarbon (2-Br-hexahelicene 23a ) or in the nonplanar perchlorinated hexa-peri-hexabenzocoronene. 25 In addition to perchlorinated hexa-peri-hexabenzocoronene and 2-Br-hexahelicene, the present case of 8b clearly supports the observation that the nonplanarity of these molecules is not hindering the formation of the typical collective Raman modes associated with the graphene cores. This can be taken as the hallmark of πconjugation, which in these systems is just modulated by nonplanarity and not seriously weakened.

Interestingly, because compound 8b has lower symmetry than graphene, the E 2g degeneracy of the G peak is lifted. Hence, longitudinal and transversal G modes are found, which also explains the split G feature observed in the FT-Raman spectrum. On the other hand, the D peak has a very distinct vibrational pattern and it shows up as the strongest Raman line (Figure 5a). The simulated and observed Raman spectrum showed good correspondence. Finally, we anticipate that the analysis of the vibrational modes will be useful for the interpretation of fluorescence and circularly polarized luminescence (CPL) data presented below.

Next we investigated the fluorescence properties of peropyrene 8b, studying the CPL with a home-built apparatus. 26 The results showed that the CPL (Figure 5c) is weak; however, the fluorescence is very intense (Figure 5d). The g(luminescence) factor, (ΔI/I) ≈ 7.7 × 10 -4 {with I =[(I L + I R )/2] being the total intensity of emitted light and ΔI = I L -I R , with I L and I R being the left and right circularly polarized components of emitted radiation}, is slightly smaller than the g factor for absorption (vide supra). The observed Stokes shift is quite low and evident vibronic structuring is observed; the latter interpretation is also backed by the computations commented below. As in most CPL studies, the sign of the single CPL feature is the same as the sign of the longest wavelength ECD band. For helicene systems, 27 two situations are met: one in which the CPL features are sensitive to substituents and the other in which CPL is promoted only by intrinsic helicity. In the former case, CPL is weak; with rotational strengths being very weak, vibronic effects dominate and dictate the observed sign. 27b, 28 In the latter case (see, for example, ref 28a), the CPL is strong due to the presence of heteroatoms within the helical structure, while vibronic effects are almost negligible. In the present case, the intensities of the low-energy absorption/CD bands and emission/CPL signals are quite large. The calculated dipole and rotational strengths are similar in the ground and excited states, coherently with very similar geometries for ground and excited states and small Stokes shift (see Figure S4 and Tables S4 andS5, SI): the g factors are low in both absorption and emission, since the dipole strengths are very large.

The observation that the g-factor is constant throughout the vibronic structures suggests that Franck-Condon terms dominate. 28 This greatly simplifies vibronic calculations, and indeed, the vibronic structure relative to the absorption/ emission related with the lowest lying bright excited state of 8b can be accounted for with a straightforward approach, where the Huang-Rhys (HR) factors are computed with TD-DFT methods and used to determine the relative intensities over the manifold of vibronic transitions (see the SI,g e n e r a l experimental section). Despite the basic displaced harmonic oscillator approximation, certainly less accurate than other treatments including the effects of Dushinsky rotation, 29 this kind of theoretical approach reasonably accounts for the observed line shapes. Even more interesting, among the six modes with sizable HR factors (Table 1), we find those associated with the Raman G and D lines and several observed low wavenumber modes (see the SI, section 3.5). This establishes a connection between the characterization of the vibrational structure and the electronic spectroscopy of the S 1 state of 8b. Results for absorption and emission are reported in Figure S5 and summarized in Table S6 (SI). In the Supporting Information we also report a picture comparing the ground and excited state geometries (followed by a table with bond length values, Table S5); this picture strongly resembles structures of the normal modes reported in Figure 5b, suggesting the role of the D mode in determining the spacing between the observed transitions along the vibronic structure. On the other hand, the low wavenumber modes, bearing sizable Huang-Rhys factors, contribute the most in determining the effective line width of the vibronic structure (see Table S7 and associated discussion, SI). These modes can be described as collective deformations of the molecule as a whole, and some of them resemble other cases investigated in the past, notably the longitudinal and transversal acoustic-like stretching of the molecule. 30 Additional modes characteristic of 8b include the umbrella of the t-Bu groups and the torsion mode of the aromatic core.

Theoretical Study of the Interconversion. As expected, the computational study confirms that the inversion between enantiomers (M,M)-8a and (P,P)-8a proceeds through the achiral intermediate meso-8a, lying 10.0 kcal/mol above the enantiomers 8a, through transition states with an activation energy of 29.2 kcal/mol [Figure 6 and Table S2 (SI)]. The inversion can be explained by a two-step mechanism in which a face-to-face rotation of two methoxyphenyls initially takes place on the one side of the molecule to arrive at the transition state (TS 1 /TS 2 ), and then this is repeated on the opposite side of the molecule to arrive at the enantiomer. The simultaneous rotation of all four methoxyphenyls corresponds to a very high energy path without an intermediate, which is unlikely to be relevant to the inversion of chirality. In the meantime, using the experimental free energy barrier for the (M,M)t o( P,P) interconversion Δ ≠ G ∼ 29 kcal/mol, we can calculate a half-life time for the individual enantiomers to be t 1/2 ∼ 2hatT = 100 °C.