Zigzag-Edged Polycyclic Aromatic Hydrocarbons from Benzo[ m ]tetraphene Precursors

: A series of zigzag-edged polycyclic aromatic hydrocarbons (PAHs) ( Z1-Z3 ) were synthesized from 2,12-dibromo-7,14-diphenyl-benzo[ m ]tetraphene ( 9 ) as a versatile building block. Their structures were unambiguously confirmed by laser desorption/ionization time-of-flight mass spectrometry, 1 H NMR, Raman, and Fourier-transformed infrared (FTIR) spectroscopies as well as scanning tunneling microscopy. The fingerprint vibrational modes were elucidated with theoretical support. The edge-and size-dependent optical properties were characterized by UV-Vis absorption and fluorescence spectroscopy and DFT calculations. Moreover, ultrafast transient absorption spectroscopy revealed distinct modulation of the photophysical properties upon π -extension from Z1 to Z2 , the latter having a gulf edge.


Introduction
The studies of polycyclic aromatic hydrocarbons (PAHs) are evolving towards the development of organic materials for various applications such as sensing, [1] (optoÀ )electronics, [2] spintronics, [3] and energy storage, [4] considering their versatile electronic structures and physical properties.Many endeavors have been devoted to investigating size and edge effects on their electronic properties. [5]Typically, two types of edge structures have been considered, namely, zigzag and armchair (Figure 1a), [2b,6] while some other types, such as the cove and gulf, further contribute to the structural diversity. [7]3b,8] Hence, the efficient synthesis of PAHs with zigzag edges and investigation of their structure-property relationships is of high relevance.
For the synthetic exploration of PAHs, the development of concise and efficient modular methods is desirable. [9]To this end, we have used 2,12-dibromo-7,14-diphenylbenzo[m]tetraphene (9) as the building block, which is structurally related to the "U-shaped" monomers employed for the onsurface synthesis of zigzag graphene nanoribbons [7c,8c] (Figure 1b).A long and branched alkyl chain is introduced on the skeleton to improve the solubility of the final PAH products.Through debromination or coupling reaction of 9, followed by oxidative cyclodehydrogenation, a series of unprecedented report the synthesis and characterizations of three PAHs with zigzag edges, Z1, Z2, and Z3 through this modular approach (Figure 1c).While Z1 and Z3 consist of zigzag and armchair edges, the latter being a dimer of the former, Z2 possesses zigzag, armchair, and gulf edges.Z1-Z3 were characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), NMR, FTIR, and Raman spectroscopies with theoretical supports, and scanning tunneling microscopy (STM).UV-Vis absorption and fluorescence spectra revealed significant red-shift upon πextension from Z1 to Z2, and then to Z3.Moreover, ultrafast transient absorption (TA) spectroscopy revealed significant differences in the photophysical properties of Z1 and Z2 despite their structural similarity.

Synthesis and characterization of Z1-Z3
For obtaining Z1, Z2, and Z3, the required precursors 10, 11, and 12, respectively, were prepared from compound 9 (Scheme 2).First, debromination of 9 by treatment with two equivalents of n-butyllithium afforded compound 10 in 98 % yield.Subsequently, 10 could be readily transformed into the desired product Z1 through the oxidative cyclodehydrogenation using 2,3-dichloro-5,6-dicyano-p-benzoquinone/trifluoroacetic acid (DDQ/TFA) (Scheme 2a) in 85 % yield. [14]MALDI-TOF MS analysis of Z1 displayed a signal at m/z = 790.25,which was consistent with the expected value of 790.54 (Figure 2, black plot).Moreover, the isotopic distribution of Z1 was in perfect agreement with the simulated pattern, which provided strong evidence for the formation of Z1.
Precursor 11 was next prepared through the Suzuki coupling of 9 with phenylboronic acid in 71 % yield.The cyclodehydrogenation of 12 with DDQ/TFA afforded target Z2 in 81 % yield.On the other hand, the synthesis of Z3 was achieved in four steps (Scheme 2b): 1) compound 9 was treated with one equivalent of n-butyllithium to give a mixture of 9, 10, and their mono-bromo derivative; 2) This mixture was subjected to a Yamamoto coupling to furnish a mixture of oligomers, including dimer 12, trimer, and tetramer; 3) The oligomers were then separated by preparative recycling size-exclusion chromatography (SEC), providing dimer 12 in 16 % yield; 4) The subsequent oxidative cyclodehydrogenation of 12 gave Z3.Similar to Z1, MALDI-TOF MS analysis of Z2 and Z3 exhibited mass peaks at m/z = 938.66 and 1575.03 in line with the theoretical molecular mass of 938.58 and 1575.05,respectively (Figure 2, blue and red plots).Their isotopic distributions were also in agreement with the simulated patterns, which validated the successful formation of Z2 and Z3.Generally, extended PAHs suffer from strong aggregation in solution, which results in the broadening of peaks in 1 H NMR spectra.Nevertheless, the well-resolved 1 H NMR spectrum of Z1 could be recorded at room temperature in dichloromethane-d 2 (Figure 3), and those of Z2 and Z3 at 393 K in 1,1,2,2tetrachloroethane-d 2 (see Supporting Information, Figure S26-27).Because of the extended π-system, some proton signals on the aromatic backbone appeared downfield and shifted to a range of δ = 8.0-9.5 ppm.Moreover, the signal of the proton at the "gulf" position of Z2 was more strongly downfield shifted to δ = 10.4 ppm, which indicates the intense deshielding effect by neighboring aromatic rings.With the help of 2D HÀ H Correlation Spectroscopy (COSY) NMR, aliphatic and aromatic signals of Z1 and Z2 could be assigned, further verifying the successful synthesis of the model compounds (see Supporting Information, Figure S4).

Raman and Fourier-transform infrared analyses of Z1 and Z2
Raman and FTIR spectra of Z1 and Z2 were studied to obtain insights into the vibrational modes of such PAHs with zigzag, armchair, and gulf edges (Figure 4).The Raman spectra of PAHs are generally dominated by collective CÀ C stretching vibrations of the honeycomb structure, which are named D and G in the literature. [15]Correspondingly, the Raman spectra of Z1 and Z2 exhibit these characteristic signals around 1300 cm À 1 and 1600 cm À 1 (Figure 4a).The multiplicity of D and G signals is a   typical property of PAHs resulting from the coupling of characteristic collective CÀ C stretching coordinates with the inplane CÀ H bending vibrations at the edge of the molecule. [16]ven though it is difficult to identify specific edge markers, i. e., peaks originating from specific edge structures, it is clear from the comparison of the Raman spectra of Z1 and Z2 that the manifold of peaks in the D and G regions, as a whole, can be regarded as the fingerprint of the molecular structures.DFT calculations on Z1 and Z2 provide simulated Raman spectra which nicely correlate with the experimental results and allow the identification of all minor vibrations involving different normal modes related to collective breathing modes and ring stretching modes.Inspection of the normal modes associated with the intense Raman peaks reveals the expected nuclear displacements for D and G modes (see Supporting Information, Figure S1).
The infrared signals of PAHs in the CH out-of-plane (opla) bending region (980-650 cm -1 range) are informative of the molecular structure and edge topology. [17]The comparison of the theoretical and experimental spectra of Z1 and Z2 shows a good agreement, as reported in Figure 4b.Deviations between the theory and the experiment are presumably due to the role of intermolecular interactions in the solid state since the calculations were performed for isolated molecules.The pres-ence of alkyl substituents could also induce an effect on the signals through contributions arising from the manifold of possible conformers.Nevertheless, to limit the computational cost, only the fully trans-planar conformations of the substituents were considered in the DFT calculations (see Supporting Information, Figure S2) and we focused on the CH-opla markers.In the IR spectra of Figure 4b, the a 1 feature of Z1 can be assigned to the rocking modes of the alkyl chains; b 1 is the rocking mode in Z2.The remaining bands labeled in the FTIR spectra of Figure 4b are all due to opla vibrations of CH bonds.Several signals (i. e., a 2 , a 3 ', a 3 ", a 4 , and a 6 in Z1 and b 2 and b 3 in Z2) are characteristic of the specific edge configurations, whereas the weak a 5 band is assigned to the lateral phenyl substitution.
Adopting the mode naming described in the literature, [17][18] and with reference to the pattern of the normal modes, the b 2 signal in Z2 can be described as QUATRO and b 3 as DUO (Figure 4c, insert).The a 2 and a 3 ' bands of Z1 can be described as TRIO and a 4 as DUO.The remaining two modes (a 3 ", a 6 ) have a more complex pattern of in-phase/out-of-phase CH-opla contributions.All these Raman and FTIR characterizations of Z1 and Z2 show good agreement with corresponding DFT calculations, providing additional structural evidence and also marking the strength of vibrational spectroscopy for precisely characterizing PAHs with different structures.

Scanning tunneling microscopy analysis of Z3
Except for the structural confirmation through 1 H NMR measurement, the STM analysis of Z3 also allowed the visualization of its molecular and self-assembled structures at the solutiongraphite interface. [19]Z3 was selected for STM analysis due to its large footprint which is expected to stabilize the self-assembled monolayers via π-π interactions with the graphite surface.Figure 5a displays a typical STM image of the Z3 monolayer formed at the highly oriented pyrolytic graphite (HOPG)/1,2,4trichlorobenzene (TCB) interface, which displays rows of bright rectangular units stacked end-to-end.Each rectangular unit corresponds to the aromatic backbone of a single Z3 molecule.
The dimensions of the aromatic core extracted from calibrated STM images (length ~2.3 nm, width ~1.1 nm) are in good agreement with those obtained from a model optimized by molecular mechanics (Figure 5b).The bright rows are periodically separated by relatively darker troughs.The distance between the bright adjacent rows (unit cell vector b) indicates that the darker region hosts the tetradecyl chain, i. e., the shorter decyl branch of the 4-decylhexadecyl group.The longer dodecyl arm may be partially adsorbed or alternatively backfolded in the supernatant solution.The alkyl chains could not be resolved, possibly in view of their mobility on the time scale of STM imaging.The molecular model displayed in Figure 5b depicts the arrangement of Z3 molecules within the selfassembled network.The empty regions in the molecular model could be occupied by the partially adsorbed dodecyl arm of the chain or by mobile TCB molecules (both not shown in the molecular model for the sake of clarity).
The excitation spectra of three PAHs were also recorded, which were in good agreement with their absorption spectra, corroborating the purity of the obtained samples (see Supporting Information, Figure S3).Z1, Z2, and Z3 display fluorescence with emission maxima at 459, 488, and 514 nm, respectively, under excitation at their absorption maxima (Figure 6).Their photoluminescence spectra revealed many sharp bands, which apparently pointed to the symmetry-forbidden transition from the first excited singlet state to the ground state, similar to that of the previously reported PAHs with partial zigzag edges. [22]ansient absorption spectroscopy of Z1 and Z2 The ultrafast transient absorption (TA) spectroscopy was employed to gain a deeper insight into the photophysical properties of Z1 and Z2 (Figure 7), which showed distinct static optical spectra despite the structural similarity.8e,24] Nevertheless, the precise structure-property relationship is still largely unclear, for which Z1 and Z2, sharing the same zigzag-edged (sub)structure, offer an intriguing comparison.To minimize the intermolecular aggregation, Z1 and Z2 were blended into a polystyrene solution (40 mg/mL in toluene) with a concentration of 0.1 mg/mL for the TA measurements.Although the signal intensity of Z1 is relatively lower than that of Z2 due to the reduced oscillator strength at the selected excitation wavelength (400 nm), Z1 exhibits a broad negative signal that can be associated with the excited-state absorption (S 1 !S i ), with possibly a hint of positive SE signal at 470 nm (0'-0 transition) that is strongly overwhelmed by the negative absorption signal.In contrast, Z2 displays a positive signal centered around 500 nm, which can be attributed to SE (0-0') transition and is clearly more prominent than in Z1.Although the damping of SE in Z2 is ultrafast and occurs in ~300 fs, this observation indicates that the zigzag edge alone does not bring about the occurrence of SE, which is caused by the π-extension, forming a gulf edge, in this case of Z1 vs. Z2.This phenomenon could be attributed to an increased electron localization and reduced non-radiative recombination upon the π-extension, while the possible appearance of hypothesized localized topological emissive states [25] is still under investigation with other PAH systems.

Conclusion
In summary, we synthesized three zigzag-edged PAHs, Z1, Z2, and Z3, from a common "U-shaped" intermediate compound 9.The most prominent feature of the resultant PAHs is edge-and size-dependent electronic properties, and their optical energy gaps moderately lower from 3.19 eV to 2.29 eV upon the πextension.This work offers a deeper understanding of the structure-property relationships of PAHs with the common zigzag-edged (sub)structure, revealing the modulation of vibrational, optoelectronic, and photophysical properties.7d,26]

Experimental Section
Essential Experimental Procedures/Data are available in the Supporting Information material of this article.

Figure 3 .
Figure 3. 1 H NMR spectra of Z1 in dichloromethane-d 2 at 25 °C; inset: The assignment of aliphatic and aromatic protons of Z1.

Figure 4 .
Figure 4. (a) Experimental (top traces) and simulated (bottom traces, frequency scaled by 0.98) FT-Raman spectra of Z1 and Z2 (1064 nm excitation).(b) Experimental and simulated FTIR spectra of Z1 and Z2 in the fingerprint out-of-plane CH bending region.Simulated spectra have been frequency scaled by 0.98 and are reported below each corresponding experimental spectrum.(c) Representation of normal modes associated with the FTIR spectra features labeled in panel b.The size of blue/red circles on the molecular sketches is proportional to positive/negative nuclear displacements along the out-of-plane direction.Inset: Representation of SOLO, DUO, TRIO, and QUATRO.

Figure 5 .
Figure 5. STM characterization of the compound Z3.(a) STM image showing a self-assembled network of Z3 formed at the TCB/HOPG interface (C Z3 = 6.3 × 10 À 5 M).The dashed rectangle highlights the aromatic backbone of a single Z3 molecule.The graphite symmetry axes are displayed in the lower left corner.The unit cell is overlaid on the STM image and the parameters are a = 2.3 � 0.2 nm, b = 2.7 � 0.1 nm, α = 84.5 � 3°.Imaging parameters: I set = 75 pA, V bias = À 1.125 V. (b) Molecular model showing the arrangement of Z3 molecules within the self-assembled network.The dodecyl arm of the branched chain is plausibly backfolded in the supernatant liquid phase and is not shown in the molecular model.

Figure 7 .
Figure 7. TA spectra as a function of pump-probe delay for (a) Z1 and (b) Z2suspended in a polystyrene solution (40 mg/mL in toluene) with a concentration of 0.1 mg/mL.The excitation wavelength was 400 nm for both molecules.