Selective Formation of Negative Curvature - ACS Publications

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#-Extended Corannulene-based Nanographenes: Selective Formation of Negative Curvature Jesús M. Fernández-García, Paul J. Evans, Samara Medina Rivero, Israel Fernandez, David García-Fresnadillo, Josefina Perles, Juan Casado, and Nazario Martín J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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Journal of the American Chemical Society

π-Extended Corannulene-based Nanographenes: Selective Formation of Negative Curvature Jesús M. Fernández-García,† Paul J. Evans,† Samara Medina Rivero,§ Israel Fernández,† David García-Fresnadillo,† Josefina Perles,# Juan Casado,§ Nazario Martín*†,‡ †

Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain.

‡

IMDEA-Nanociencia, Campus de la Universidad Autónoma de Madrid, 28049 Madrid, Spain.

#

Single Crystal X-ray Diffraction Laboratory, Interdepartmental Research Service (SIdI), Universidad Autónoma de Madrid, 28049 Madrid, Spain. §

Departamento de Química Física, Universidad de Málaga, 29071 Málaga, Spain.

ABSTRACT: A geometrically-selective bottom-up synthesis of curved nanographenes is described. The synthetic methodology used involves the extension of the π-system of positively-curved corannulene by a [4+2] cycloaddition reaction followed by cyclodehydrogenation (Scholl oxidation). By selecting the conditions for the Scholl oxidation, the formation of a seven-membered ring that also confers negative curvature to the resulting nanographene can be activated offering two topologically distinct, curved nanographenes from a common precursor. Additionally, the structure-property relationship in these new nanographenes is explored via theoretical, electrochemical, photophysical, Raman and X-Ray crystallographic studies.

INTRODUCTION Graphene has a variety of exceptional chemical and physical properties stemming from its atomically thin mesh of carbon atoms arranged in a 2D honeycomb pattern.1-3 These properties have skillfully been used in a variety of rapidly expanding applications, such as photovoltaics,4 batteries5 and biomedicine,6,7 just to name a few. Pristine single-layer graphene has the advantage of a high carrier mobility at room temperature. However, the zero energy gap between the conduction and the valence bands in graphene limits its utility in electronic applications.8 To overcome this drawback, different geometric strategies to open a bandgap have been used, namely by constraining the area of graphene to form nanographenes,9 by making bilayers10 and by applying strain to the graphene lattice in the form of curvature.11 Furthermore, chemical functionalization is also a valuable alternative strategy to access semiconducting graphene.12 Most commonly, top-down methods are employed for the preparation of nanographenes or graphene quantum dots. These involve the destruction of graphite by oxidation, exfoliation and cutting to generate flakes of single-layer graphene with some defects (oxygen inclusion, lattice vacancies). Although functionally straightforward, these procedures present the disadvantage that they do not allow precise control of the structure, composition and/or size of the generated fragments. As graphene-based electronics become smaller and more efficient, atomistic control of nanographene architectures will be indispensable. Currently, only bottom-up methods, where nanographene is constructed from smaller entities by stepwise organic synthesis, allow the preparation of nanographenes with perfect

control of shapes, sizes, and, therefore, perfectly defined properties.13 One structural attribute that can be controlled in the bottom-up syntheses of sp2 carbon networks is Gaussian curvature.14 Graphene and carbon nanotubes (CNTs) consisting of only hexagons have zero Gaussian curvature. The introduction of pentagons in the hexagonal lattice generates positive Gaussian curvature leading to a bowl shape, as observed in corannulene (Chart 1) and fullerenes. On the other hand, saddle-shaped surfaces with negative Gaussian curvature can be constructed by the inclusion of seven ([7]circulene), eight, or larger membered rings.15 Chart 1. Examples of positive, bowl shape (corannulene) and negative, saddle shape ([7]circulene) curvatures.

corannulene

[7]circulene

While positively-curved polycyclic aromatic hydrocarbons (PAHs) have been widely studied,16-19 the synthesis of negatively-curved PAHs has not been investigated until very recently, 20-30 probably due to the difficulties associated with the construction of seven- or eightmembered rings. Despite this, nanographenes with negative curvature have demonstrated interesting magnetic and electronic properties31 and can be used as anodes in lithium batteries.32 Therefore, we

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were motivated to develop a straightforward synthetic methodology in which nanographene curvature can be controlled.

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Scheme 2. Selectivity of the Scholl reaction. tBu

tBu

RESULTS AND DISCUSSION Herein, we report a selective bottom-up synthesis of positively and negatively curved nanographenes and the elucidation of their structure and properties. We chose corannulene as a starting material for the synthesis of these species because: i) this molecule already possesses a five-membered ring that would confer positive curvature in the resulting nanographene; ii) the synthesis of corannulene has been described on a kilogram scale,33 which makes it a highly accessible compound and iii) the derivatization of corannulene is a wellstudied and high-yielding science. 34-43

tBu

tBu

FeCl 3 MeNO 2 / DCM -50 ºC, 15 min

DDQ, TfOH DCM 0 ºC, 15 min

tBu

5 NM = 986 NM = 976

NM = 974 FeCl 3 MeNO 2 / DCM 80 ºC, 15 min

I. Synthetic Procedure and Reaction Mechanism.

Our strategy is based on the extension of the π-system of corannulene by using a widely-employed methodology to prepare planar nanographenes and nanoribbons, namely a [4+2] cycloaddition of cyclopentadienones to diaryl alkynes followed by dehydrogenative coupling.9 Scheme 1. General synthetic pathway toward curved nanographenes.

IBr

Et 3N 85 ºC, 16 hr.

tBu

3 66%

2 95%

1 tBu

O

tBu

tBu tBu

4 tBu

NM = 976

tBu

tBu

µW, 280 ºC, 40 min tBu

5 50% NM = 986

FeCl 3 MeNO 2 DCM 25 ºC, 15 min

tBu

tBu

NM = 974

52% 42:58

Corannulene 1 (Scheme 1) was easily brominated to yield bromocorannulene 2 upon treatment with IBr,44 which was then extended to (4-tert-butylphenyl)ethynylcorannulene 3 by Sonogashira crosscoupling. This species is able to undergo a [4+2] cycloaddition with cyclopentadienone 4 followed by the release of carbon monoxide. The resulting compound 5 formally joins corannulene and pentaphenyl benzene with tert-butyl groups introduced to improve the solubility of the final product. When FeCl3 in MeNO2 was added to 5 at 25 ˚C, a mixture of two compounds with nominal masses (NM) of 976 and 974 (MALDI-TOF) was formed (Scheme 1). The loss in mass of ten and twelve units indicates the formation of five and six C–C bonds, respectively. Predictably, four C–C bonds can be formed between the phenyl groups of the pentaphenyl benzene moiety in 5, forming four six-membered rings and a planar PAH domain (dibenzo[fg,ij]phenanthro[9,10,1,2,3-pqrst]pentaphene, DBPP). Additionally, a six-membered ring and a seven-membered ring can be formed, fusing the planar region with corannulene resulting in a positively-curved helically chiral product (closure of 6-membered ring, 6, racemate, NM = 976, Scheme 2) or a positively and negatively curved structure (closure of both six- and seven-membered rings, 8, NM = 974). Closure leading only to the seven-membered fusion was not observed.

tBu

tBu

tBu tBu

tBu

tBu

tBu tBu

tBu

Cl tBu

6 69%

tBu

tBu Br Pd(PPh 3)2Cl2 15% CuI 40%

DCM 25 ºC, overnight

tBu

tBu

7 46% NM = 1008

tBu

8 48%

DDQ, TfOH DCM 0 ºC, 15 min quantitative

In order to perform the Scholl oxidation in a selective manner, different reaction conditions were used (Scheme 2). When compound 5 was treated with FeCl3 in MeNO2 at –50 ˚C, only product 6 was produced. On the other hand, raising the temperature to 80 ˚C leads to the formation of a new product 7 (NM = 1008), that incorporates a chlorine atom. The preferential preparation of 8 over 6 was only achieved after changing the oxidant. Thus, the use of DDQ and TfOH at 0 ˚C afforded 8 selectively. Interestingly, these reaction conditions also allowed the quantitative conversion of 6 into 8 as supporting evidence that 8 is formed by the cyclodehydrogenation of 6. For this reason, a structure with only a seven-membered ring closure onto corannulene was not observed. The results above strongly suggest that the formation of the sixmembered ring joining the corannulene and the DBPP moieties is favored over the corresponding seven-membered ring (which is only formed upon more drastic reaction conditions). To gain more insight into this selectivity, DFT calculations were carried out (see computational details in the supporting information). To this end, the final six and seven-ring closures were explored considering the two possible reaction pathways proposed for the Scholl reaction, namely the arenium cation (Figure 1a) or radical cation (Figure 1b) reaction mechanisms,45 where the t-butyl groups were replaced by methyl groups. The formation of the six-membered ring is strongly preferred from both kinetic and thermodynamic points of view, regardless of the mechanism considered, which is fully consistent with the experimental findings. In addition, the inversion barrier of the corannulene moiety in compounds 6 and 8 was also computed. Our calculations indicate that these barriers (12.8 and 9.1 kcal/mol, for 6 and 8, respectively) are comparable to that associated with the inversion of free corannulene (10.2 kcal/mol, see Figure S21 in the supporting information).


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Journal of the American Chemical Society

Figure 1. Computed ring-closure steps during the Scholl reaction. Relative free energies (∆G, at 298 K) and distances are given in kcal/mol and Å, respectively. All data have been computed at the B3LYP-D3/def2-SVP level.

derivative of 8 arising from the forcing conditions of the Scholl reaction with FeCl3 at high temperature. As a result, selective introduction of a chlorine atom onto the corannulene rim carbon nearest to the seven-membered ring is achieved. This minimal detail induces a dramatic change in the structure. A seven-membered ring instead of the [6]helicene gives 7, a more shallow helical structure. In addition, the presence of the seven-membered ring leads to negative curvature and a saddle shape.

Figure 2. X-ray crystal structure of 6 (left) and 7 (right). Hydrogen atoms and solvent molecules have been omitted for clarity.

II. Characterization of Molecular Structures: Curvature and SolidState Interactions.

Structures of compounds 6 and 7 were unequivocally stablished by single crystal X-ray diffraction (Figure 2). Full details of the data collection and structure determination of compounds are presented in the supporting information. In both cases, disordered solvent molecules are located in the interstices (in the case of 6 water molecules, whereas in the case of 7, water and dichloroethane). Nanographene 6, where five new C–C bonds are formed, has a helical structure due to the formation of a [6]helicene and a [4]helicene, and also has positive curvature due to the presence of a five-membered ring in the corannulene fragment. Nanographene 7, which presents an additional C–C bond, can be considered as a chlorinated

a

c

The formation of this seven-membered ring flattens the corannulene fragment as evident in the average bowl depth observed by XRD (6: 0.8586 Å, and 8: 0.8354 Å, Figure S4) and the average POAV angles of the hub carbons (6 (XRD): 7.91˚ 6 (DFT): 8.24˚ 7 (XRD): 7.87˚ 8 (DFT): 8.02˚, supporting information, Section 7). This geometric change also influences the packing of the molecules in the crystal (Figure 3). In the structure of 6, the flat areas (DBPP) approach each other and establish π–π interactions (Figure S5a) which are reinforced by C–H···π interactions with the methyl groups (Figure S5b). Additionally, the corannulene zones establish C–H···π interactions with the neighboring molecule in the direction of the crystallographic axis b (Figure S5c). However, in the structure of 7, planarity does not exist because the negative curvature brought on by the seven-membered ring extends to the rest of the molecule. In this structure, the molecules only establish π–π interactions on the convex sides of neighboring corannulene rings (Figure S6a), and, instead there are numerous C–H···π interactions (Figure S6b), the strongest occurring between neighboring molecules in the direction of the crystallographic axis a.

b

o

b


a


Figure 3. Packing of the molecules in the crystal structures of compounds 6 (left) and 7 (right).

III. Ground Electronic State Characterization: Electrochemical and Spectroscopic Properties.

The electrochemical properties of 6 and 8 were explored by cyclic voltammetry in THF with tetrabutylammonium hexafluorophosphate as the supporting electrolyte and Ag/AgNO3 as reference electrode. Table 1 shows their reduction and oxidation potentials compared to corannulenene and hexa-tert-butylhexa-peri-hexabenzocoronene (HBC).46 Table 1. Oxidation and reduction potentials of corannulenene, HBC, 6 and 8 (in V). Compound

E1red/V

E2red/V

E3red/V

E1ox/V

Corannulene

–2.67

-

-

-

HBC

–2.10

–2.28

–2.66

1.10

6

–1.90

–2.18

–2.84

1.00

8

–1.82

–2.09

-

0.94

a

Measurements made in THF at room temperature using tetrabutylammonium hexafluorophosphate as supporting electrolyte, a glassy carbon as working electrode, platinum wire as counter electrode and Ag/AgNO3 as reference electrode.

Compound 6 presents two quasireversible reduction waves at –1.90 V and –2.18 V along with an irreversible reduction wave at –2.84 V and one quasireversible oxidation wave at 1.00 V. Similarly, 8 also shows two quasireversible reduction waves at –1.82 and –2.09 V and one quasireversible oxidation wave at 0.94 V. Compared to corannulene and HBC, these compounds are better electron acceptors and electron donors, as a consequence of the extension of the π-system. The observed electrochemical trends nicely correlate with the computed energy of the corresponding frontier molecular orbitals. The computed energy of the LUMOs, (6: –2.16 eV ≈ 8: –2.14 eV < corannulene: –1.87 < HBC: –1.72 eV) indicates that the reduction process should be easier for the nanographenes than for HBC or corannulene, as experimentally observed (Figure S20). The Raman spectra can provide simultaneous information about the molecular and electronic structure of the studied compounds. These spectra (Figure 4) all have the well-known G and D graphenic bands which, in the case of these molecular graphenes, are split into several components each. The reference HBC has two well defined G bands at 1612 and 1603 cm–1 that evolve into four bands upon incorporation of the corannulene in 6 or 8 which can be accounted for in terms of: i) reduction of the molecular symmetry on HBC → 6 or 8 and ii) similar ring fusion between corannulene and DBPP in 6 and 8. Both i and ii provoke vibrational mixing between the modes of the two fused moieties and result in the observed splitting of bands in the Raman spectrum.

Figure 4. Solid state 1064 nm FT-Raman spectra of 6, 8, and HBC at room temperature.

Compared to 6, the G bands of 8 have a similar four-band spectral pattern (Figure 4) but with some distinctive frequencies such as that at 1637 cm–1 which is remarkably upshifted by +10 cm–1 (compared to that at 1627 cm–1 in 6). This band is new in the Raman spectrum of 8 and can be described as a corannulene vibrational C−C stretching mode mostly located in the rings at the connection of the corannulene and DBPP, and therefore, structurally sensitive to alteration from one to another. The corannulene G Raman band is slightly affected passing from 1571 cm-1 in 6 to 1573 cm-1 in 8. The inclusion of a seven-membered ring and the change in the fusion mode impart two main effects in the shape of 8 compared to 6: i) Concerning the electronic properties, the system goes from an alternant C=C/C−C mode in the benzene connection in 6 to a non-alternant path in 8. This non-alternant π-delocalization mode is observed in the Raman spectrum with the largest difference of +10 cm– 1 (1627 to 1637 cm–1) likely reflecting the confinement of the electron wavefunction in the environment of the seven-membered ring. This reinforces the double C=C bonds at the expense of weakening the C−C single bonds. ii) Negative curvature arises in the case of 8, and the change in the nature of the connection of the two fragments largely alters the intensity distribution of the D bands in the Raman spectra, which now appear downshifted in 8 with respect to 6. It can be argued that the double curvature overall increases the flexibility of the system and these low frequency C−C stretching skeletal modes correspondingly move to lower frequencies. The UV-Vis absorption spectra of HBC and the molecular nanographenes 6 and 8 are shown in Figure 5. The wavelengths of the absorption maxima and their corresponding absorption coefficients are collected in Table 2.

8

Figure 5. Absorption spectra in CHCl3 at room temperature of HBC, 6 and 8. Inset shows the absorption spectrum of HBC in the Vis region ×10.

6

Table 2. Characterization of the absorption spectra of HBC, 6 and 8.a Compound

λabsmax/nmb (ε/M–1cm–1)c

HBC

445 (1800), 441 (1650), 439 (1600), 390 (46350), 360 (141700), 344 (63750)

6

456 (22000), 436 (21100), 386 (65300), 367 (72500), 346 (73000), 326 (69500), 309 (62600)

HBC


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Journal of the American Chemical Society 8

441 (19650), 437 (19650), 378 (64300), 362 (67700), 358 (67000), 331 (54200), 312 (53100)

a

Measurements made in CHCl3 at room temperature. b Experimental uncertainty: ± 1 nm. c Experimental uncertainty: ± 10%.

The absorption spectrum of HBC displays three sharp peaks in the UV region (390, 360 and 344 nm) with high absorption coefficients (ε > 104) and a weak band in the Vis region with three additional vibronic peaks (445, 441 and 439 nm, ε ∼ 103), in good agreement with previous literature.25,47-51 The vibronic fine structure of HBC in the UV region, a typical feature of polycyclic aromatic hydrocarbons, is lacking in nanographenes 6 and 8. In contrast, the absorption bands of 6 and 8 are multiple and condensed, compared to those of HBC, resulting in seemingly broad bands due to a strong overlap of vibronic modes in the same spectral region. This is a direct consequence of the presence of more condensed rings in the new nonsymmetric molecular graphenes. Thus, the HBC signature is kept in terms of similar values of the absorption maxima in the UV-Vis spectra of 6 and 8 and, especially, the weak band of HBC in the visible region which, remarkably, is a markedly allowed band in the curved nanographenes (ε > 104).52 Time-Dependent (TD) calculations at the PCM(CHCl3)-TDB3LYP-D3/def2-SVP//B3LYP-D3/def2-SVP level were carried out to assign the nature of the absorptions observed in the UV-Vis spectra of nanographenes 6 and 8. The calculations gave a vertical excitation energy of λcalc = 479 and 471 nm, i.e. 2.59 and 2.63 eV for 6 and 8, respectively, which is in reasonably good agreement with the experimental values (Table 2) of λexp = 456 (2.72 eV) and 441 nm (2.81 eV) and in excellent agreement with the optical energy gap obtained from excited state measurements (Table 4), 2.61 and 2.59 eV for 6 and 8, respectively. In both cases, the vertical transition is associated with the promotion of one electron from the HOMO (mainly centered in the DBPP moiety) to the LUMO, which can be considered as a π*-molecular orbital fully delocalized in both the DBPP and corannulene fragments (Figure 6). Therefore, this absorption appearing at ca. 450 nm can be viewed as a π-π* transition which reflects the extension of the π-conjugation in these novel nanographenes.

The emission properties and spectra of HBC, 6 and 8 are collected in Table 3 and Figure 7. Table 3. Characterization of the emission spectra and emission quantum yields of HBC, 6 and 8.a Compound

λemmax / nmb (shoulders)

Φemc

HBC

493 (519, 553)

0.058 ± 0.007

6

494 (522)

0.50 ± 0.05

8

523 (502, 557)

0.25 ± 0.02

a

Measurements made in CHCl3 at room temperature. b Experimental uncertainty: ± 3 nm. c Ar-purged solutions. Emission quantum yields were determined using fluorescein in NaOH 0.1 M as the reference (see supporting information).

When the emission spectra of HBC, 6 and 8 are compared, several differences can be noted. While the emission band of HBC peaks at 493 nm (FWHM = 0.20 eV, i.e. 38.8 ± 2.5 nm), nanographene 6 displays a broader band with a maximum at 494 nm (FWHM = 0.25 eV, i.e. 52.1 ± 1.6 nm). The emission spectrum of 8 displays a red-shifted maximum at 523 nm in an even broader band (FWHM = 0.32 eV, i.e. 70.8 ± 1.0 nm) showing a prominent shoulder at 502 nm. The red-shifted and broader emission band of 8, compared to that of 6 and HBC is a consequence of its more extended π-system and greater flexibility due to its double curvature. This agrees well with the previous discussion regarding the results from Raman spectroscopy.

Figure 7. Normalized emission spectra of HBC, 6 and 8 in CHCl3 at room temperature.

Figure 6. Frontier molecular orbitals involved in the π-π* transition for compounds 6 and 8 (isosurface value of 0.05 au).

IV. Excited Electronic State Characterization: Fluorescence, Quenching by Molecular Oxygen and Excited State Redox Potentials.

Concerning the emission quantum yields determined for HBC and the molecular nanographenes 6 and 8 (Table 3), large differences were observed between the weakly fluorescent HBC molecule and 8 or 6, which show moderate and high fluorescence quantum yields, respectively. Indeed, when the two non-symmetric nanographenes 6 and 8 are compared, we notice that: 8, with a larger π-system, flatter structure, and higher conformational flexibility, shows a lower fluorescence quantum yield consistent with an expectedly higher nonradiative deactivation rate constant depopulating its lowest singlet excited state. On the other hand, 6, with a slightly smaller π-system, less flexible structure and geometry more distorted from planarity, shows a rather high fluorescence quantum yield (50%) indicating more balanced radiative and nonradiative deactivation rate constants in the singlet excited state. 21,22,24,25,28,29,42,53,54 Furthermore,


Journal of the American Chemical Society when the Ar-purged solutions used for the determination of the fluorescence quantum yields were left to equilibrate with air and the emission spectra were recorded, excited state quenching by molecular oxygen was detected in the case of nanographene 6 but not with 8 nor with HBC (Figure 8). In fact, a Stern-Volmer quenching constant (KSV) by oxygen of 215 M–1 was estimated for 6, corresponding to 34% quenching of the excited states under air-equilibrated conditions. That the singlet excited state of 6 is quenched by molecular oxygen, while those of HBC or 8 are not, reflects the fact that the excited state lifetimes are quite different (longer in 6). Fluorescence quenching by molecular oxygen in solution is a diffusion-controlled process, and this excited-state deactivation pathway is efficient only if the lifetime of the excited fluorophore is long enough to allow diffusion and collision of quencher and quenchee, resulting in a high probability of excited- state deactivation and, therefore, strong fluorescence quenching.54 From the previous experimental evidence, a longer emission lifetime may be anticipated for 6 compared to 8 due to the higher nonradiative deactivation 2D Graphrate 4 constant expected for nanographene 8. 250 200

Iem / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

150 100 50 0 480 500 520 540 560 580 600 620 640

λ / nm Figure 8. Emission spectra of 6 (brown, Ar-purged; orange, air-eq.), 8 Lambda / nm vs Iem / a.u. Lambda /and nm vsHBC Iem / a.u. (green, Ar-purged and air-eq.) (blue, Ar-purged and air-eq.) Lambda / nm vs Iem / a.u. in CHCl3 at room temperature. Lambda / nm vs Iem / a.u. Lambda / nm vs Iem / a.u. / nm vs potentials Iem / a.u. The optical energy gap Lambda and redox of the excited states of HBC, 6 and 8 are presented in Table 4. The energy gaps of HBC and nanographenes 6 and 8 are nearly identical within experimental error, although a trend pointing to lower gaps for the nanographenes (with larger π-systems than HBC) can be observed. On the other hand, upon excitation, nanographenes 6 and 8 may be more easily reduced than HBC, while the corresponding excited-state oxidation potentials of HBC and nanographenes 6 and 8 are almost coincident within experimental error and show that they are reluctant to oxidation in their excited state.

Table 4. Optical energy gap and excited state redox potentials of HBC, 6 and 8.a,b Compound

E0–0/eVc

Ered*/V

Eox*/V

HBC

2.65 (2.69)

0.55

–1.55

6

2.61 (2.60)

0.71

–1.61

8

2.59 (2.57)

0.77

–1.65

a

Measurements made in CHCl3 at room temperature. b Experimental uncertainty: ± 5%. c Values in parentheses are the energy gaps calculated

from the corresponding intersections between the normalized absorption and emission spectra of each compound (see supporting information).

The previous results show some important differences in the structure and optoelectronic properties of 6 and 8. These dissimilarities reveal that a fine-tuning of the photophysical properties of these nanographenes may be achieved synthetically by adding a new C–C bond connecting the corannulene and DBPP moieties.

CONCLUSIONS In summary, a bottom-up synthesis of structurally well-defined curved nanographenes from corannulene is described. Different conditions in the final step (Scholl oxidation) allow the selective formation of a seven-membered ring between corannulene and DBPP, which imparts negative curvature to the resulting molecular nanographene. The introduction of negative curvature has a significant impact on the molecular and electronic properties when comparing 6, 8 and HBC with the help of several techniques. Single crystal X-ray diffraction reveals that negative curvature leads to a different packing mode. Raman spectra of 6 and 8 show peaks derived from the moieties of corannulene and DBPP, and indicate wavefunction localization around the seven-membered ring in the saddle point of the curvature. The fusion mode in 6 and 8 converts the nanographenes into redox amphoteric molecules as a result of the extension of the π structure and the convergence of the HOMO and LUMO energy levels. Steady-state emission is considerably enhanced in 6 and 8, and this property is controlled by the degree of molecular flexibility and curvature. DFT modeling nicely confirms the aforementioned experimental findings regarding both the topological and optoelectronic properties of the novel molecular nanographenes. It is, therefore, demonstrated than an apparently slight change in geometry, one cyclization and a change in Gaussian curvature, has large consequences on the properties of 6 and 8 (redox potentials, absorption and emission bands, fluorescence quantum yields, excited-state quenching) which can be related to their potential applications in different fields (energy conversion and storage, sensing, bioimaging or nanomedicine, among others). Therefore, through bottom-up synthetic control, a wise combination of positive and negative curvature in nanographene structures allows the modulation of the properties of these novel nanocarbons. Work is in progress in order to further explore the covalent and supramolecular reactivity of nanographenes 6 and 8, as well as; their photophysics and photochemistry.

EXPERIMENTAL SECTION 1-[(4-tert-Butylphenyl)ethynyl]corannulene 3: To a dry 25-mL schlenk under argon atmosphere with a magnetic stir bar was added a solution of 4-tertButylphenylacetylene (142 mg, 0.89 mmol, 5 equiv.) in freshly distilled Et3N (5 mL), Pd(PPh3)2Cl2 (16 mg, 15%), CuI (10 mg, 40%) and bromocorannulene (60 mg, 0.18 mmol, 1 equiv.). The mixture was stirred at 90 ˚C for 16 h. After cooling, the dark solution was diluted with saturated NH4Cl(aq) (10 mL) and dicloromethane (10 mL), the organic layer was washed with NH4Cl(aq) (10 mL) twice, dried over MgSO4 and filtered. Silica was added to the reaction flask and solvent was removed under reduced pressure to adsorb the mixture which was then purified by silica gel column chromatography with hexane/CH2Cl2 (15:1) affording the product as a yellow solid (48 mg, 66%). 1H NMR (300 MHz, CDCl3) 8.13 (d, J = 8.8 Hz, 1H), 8.05 (s, 1H), 7.88 (d, J = 8.8 Hz, 1H), 7.83 – 7.77 (overlap, 6H), 7.61 (d, J = 8.5 Hz, 2H), 7.44 (d, J = 8.5 Hz, 2H), 1.36 (s, 9H). 13C NMR (75 MHz, CDCl3) 152.1, 136.3, 135,9, 135.8, 135.38, 135.39, 131.7, 131.3, 131.2, 131.18, 131.1, 131.0, 130.6, 127.7, 127.6, 127.5, 127.48, 127.28, 127.7, 126.8, 126.7, 125.6, 121.9, 120.4, 93.6, 87.2, 35.0, 31.7. FT-IR (cm-1) 2960, 832.8, 735.


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Journal of the American Chemical Society HRMS(ESI) [M+Na]+ found: 429.1590, calculated for C32H22Na: 429.1614. [Penta(4-tert-butylphenyl)phenyl]corannulene 5: To a 10-mL microwave vial with a magnetic stir bar was added 3 (36 mg, 0.088 mmol, 1.0 equiv.) and tetra-2,3,4,5-tetrakis[4-(1,1-dimethylethyl)phenyl]2,4-cyclopentadien1-one, (59 mg, 0.10 mmol, 1.1 equiv.). The vial was placed in the microwave reactor and heated to 280 ˚C (in mode “heat as fast as possible to temperature”, stirring 100 rpm, no stirring while cooling) with a hold time of 30 minutes. After cooling, MeOH was added to the vial and it was sonicated. The insoluble portion was filtered on a frit and a pale brown solid was collected as final product without further purification (45 mg, 51%). 1H NMR (300 MHz, CDCl3) 7.71 – 6.67 (overlap, 5H), 7.58 (d, J = 8.7 Hz, 1H), 7.51 (d, J = 8.7 Hz, 1H), 7.32 – 7.24 (overlap, 2H), 6.90 – 6.63 (overlap, 16H), 6.35 – 6.17 (overlap, 4H), 1.12 (s, 9H), 1.09 (s, 18H), 0.79 (s, 18H).13C NMR (75 MHz, CDCl3) 147.9, 147.8, 147.7, 141.7, 141.4, 141.0, 140.9, 138.33, 138.27, 138.0, 137.6, 136.3, 135.7, 135.2, 134.7, 131.64, 131.58, 131.5, 130.78, 130.76, 130.68, 130.66, 130.34, 130.33, 128.1, 127.2, 127.1, 126.70, 126.66, 126.62, 126.60, 125.9, 123.4, 123.3, 34.5, 34.5, 34.2, 31.62, 31.59, 31.3. FT-IR (cm-1) 2962, 2903, 2867, 831, 733. HRMS(ESI) [M+Na]+ found: 1009.5661, calculated for C76H74Na: 1009.5683. 6: To a 25-mL round bottomed flask with magnetic stir bar was added 5 (15 mg, 0.015 mmol, 1 equiv.) in CH2Cl2 (5 mL). The mixture was chilled to –50 ˚C in an acetone/dry ice bath and bubbling of argon was maintained during the reaction. FeCl3 (122 mg, 0.75 mmol 50 equiv.) in MeNO2 (2.5mL, dried over 3Å molecular sieves) was added dropwise. The reaction completion was checked by TLC (15 min). After that, the mixture was quenched with H2O (10 mL), and was extracted with CH2Cl2 (10 mL) twice, the organic layer was dried over MgSO4, filtered and solvent was removed. The mixture was purified by silica gel column chromatography using a hexane/CH2Cl2 (9:1 to 4:1) affording the product as a yellow solid (10 mg, 69%). 1H NMR (300 MHz, CDCl3) 9.80 (s, 1H), 9.31 – 9.19 (overlap, 7H), 8.88 – 8.79 (overlap 3H), 7.93 (d, J = 8.9 Hz, 1H), 7.79 (d, J = 8.8 Hz, 1H), 7.74 (d, J = 8.8 Hz, 1H), 7.68 (d, J = 8.7 Hz, 1H), 7.56 (d, J = 8.7 Hz, 1H), 7.31 – 7.28 (overlap, 2H), 7.15 (d, J = 8.9 Hz, 1H), 1.92 (s, 9H), 1.87 (s, 9H), 1.82 (s, 9H), 1.81 (s, 9H), 1.45 (s, 9H). 13C NMR (75 MHz, CDCl3) 149.7, 149.5, 149.45, 149.4, 148.9, 138.0, 137.2, 136.0, 135.4, 135.1, 133.0, 131.9, 131.2, 131.1, 130.9, 130. 6, 130.51, 130.50, 130.4, 130.2, 129.8, 129.64, 129.60, 129.3, 129.2, 128.4, 127.4, 126.9, 126.8, 126.7, 124.8, 124.2, 124.0, 123.9, 123.6, 123.4, 123.3, 122.3, 121.4, 120.0, 119.9, 119.6, 119.5, 119.1, 119.0, 118.9, 36.0, 35.96, 35.95, 35.93, 35.2. FT-IR (cm-1) 2960, 2902, 2871, 1604, 1472, 1367, 1252, 871, 828. LRMS (MALDI-TOF) 976. HRMS(ESI) found: 976.4990, calculated for C76H64: 976.5003. 7: To a 25-mL round bottomed flask with magnetic stir bar was added 5 (15 mg, 0.015 mmol, 1 equiv.) in 1,2-dichloroethane (5 mL, dried over 3Å molecular sieves). The mixture was warmed to 80 ˚C in an oil bath and bubbling of argon was maintained during the reaction. FeCl3 (122 mg, 0.75 mmol, 50 equiv.) in MeNO2 (2.5 mL, dried over 3Å molecular sieves) was added dropwise. The reaction completion was checked by TLC (15 min). After that, the mixture was quenched with H2O (10 mL), and extracted with CH2Cl2 (10 mL) twice, the organic layer was dried over MgSO4 and filtered. The mixture was purified by silica gel column chromatography using a hexane/CH2Cl2 (12:1) affording the product as an orange solid (7 mg, 46%). 1 H NMR (300 MHz, CDCl3) 9.26 (bs, 1H), 9.09 – 9.05 (overlap, 3H), 8.98 – 8.94 (overlap, 3H), 8.90 (bs, 1H), 8.84 (bs, 1H), 8.35 (d, J = 9.0 Hz, 1H), 8.00 (d, J = 8.7 Hz, 1H), 7.92 (d, J = 2.0 Hz, 1H) 7.84 – 7.79 (overlap, 3H), 7.74 (d, J = 8.7 Hz, 1H), 1.82 (s, 9H), 1.74 (s, 27H), 1.66 (s, 9H). 13C NMR (75 MHz, CDCl3) 150.2, 150.1, 149.9, 149.7, 148.5, 139.3, 138.7, 137.2, 136.7, 136.1, 135.4, 134.6, 132.1, 11.9, 131.6, 131.56, 131.49, 131.3, 130.98, 130.96, 130.62, 130.58, 130.5, 130.43, 130.38, 130.1, 129.8, 129.1, 128.9, 128.7, 128.2, 127.7, 127.6, 127.4, 127.2, 127.0, 126.9, 125.1, 124, 5, 123.8, 123.5, 123.0, 122.9, 121.3, 120.7, 120.2, 119.3, 119.1, 119.06, 119.02, 118.9, 118.8, 118.51, 118.46, 118.1, 35.80, 35.78, 35.75, 35.63, 35.2, 32.04, 32.03, 32.01, 31.9, 31.6. FT-IR (cm-1) 2958, 2903, 2870. 1733, 1606, 1469, 1366, 1256, 1077 830, 766. HRMS(MALDI-TOF) found: 1009.4560, calculated for C76H62Cl: 1009.4540. 8: To a 50-mL round bottomed flask with magnetic stir bar was added 5 (35 mg, 0.035 mmol, 1 equiv.) in CH2Cl2 (15 mL). The mixture was chilled to 0 ˚C in an ice water bath and bubbling of argon was maintained during the reaction. DDQ (350 mg, 1.54 mmol, 44 equiv.) was added in one portion,

then TfOH (3 mL) was added dropwise. The reaction completion was checked by TLC (15 min). After that, the mixture was quenched with H2O (30 mL), and was extracted with CH2Cl2 (30 mL) twice, the organic layer was dried over MgSO4 and filtered. The mixture was purified by silica gel column chromatography using hexane/CH2Cl2 (5:1 to 2:1) affording the product as an orange solid (16 mg, 48%). 1H NMR (500 MHz, CD2Cl2, –5 ˚C) 9.26 (bs, 1H), 9.15 – 9.07 (overlap, 5H), 8.98 (s, 1H), 8.84 (bs, 2H), 8.65 (d, J = 9.0 Hz, 1H), 7.98 (d, J = 9.0 Hz, 1H), 7.92 – 7.83 (overlap, 5H), 7.76 (bs, 1H), 1.79 (s, 9H), 1.76 (s, 9H), 1.76 (s, 9H), 1.74 (s, 9H), 1.58 (s, 9H). 13C NMR (125 MHz, CD2Cl2, –5 ˚C) 150.9, 150.7, 150.4, 149.3, 142.0, 139.6, 137.2, 136.1, 135.3, 133.2, 132.0, 131.7, 131.6, 131.3, 131.2, 131.0, 130.6, 130.5, 130.4, 130.2, 129.5, 129.2, 128.9, 128.8, 128.7, 128.6, 128.4, 127.8, 127.7, 127.2, 125.9, 124.7, 124.3, 124.0, 123.6, 123.2, 122.9, 121.8, 120.5, 120.3, 119.9, 119.8,119.6, 119.4, 119.3, 119.2, 36.1, 36.0, 35.9, 35.2, 32.1, 32.0, 31.5. FT-IR (cm-1) 2959, 2903, 2869, 1727, 1605, 1464, 1366, 1258, 1075, 1018, 945, 905, 798, 731. LRMS (MALDI-TOF) 974. HRMS(MALDI-TOF) found: 974.4825, calculated for C76H62: 974.4852. 8 from 6: To a 50-mL round bottomed flask with magnetic stir bar was added 6 (10 mg, 0.010 mmol, 1 equiv.) in CH2Cl2 (5 mL). The mixture was chilled to 0 ˚C in an ice water bath and bubbling of argon was maintained during the reaction. DDQ (100 mg, 0.44 mmol, 44 equiv.) was added in one portion, then TfOH (1 mL) was added dropwise. The reaction completion was checked by TLC (15 min). After that, the mixture was quenched with H2O (10 mL), and was extracted with CH2Cl2 (10 mL) twice, the organic layer was dried over MgSO4 and filtered. The mixture was purified by silica gel column chromatography using hexane/CH2Cl2 (5:1 to 2:1) affording the product as an orange solid (10 mg, 100%).

ASSOCIATED CONTENT The supporting information is available free of charge on the ACS Publications website. Data for 6 (CIF) Data for 7 (CIF) Synthetic procedures, additional figures/schemes of physical properties and characterization data (PDF), computational details and Cartesian coordinates of all species described in the text.

AUTHOR INFORMATION Corresponding Author

[email protected] Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources No competing financial interests have been declared. J.F.-G., P.J.E., D.G.-F. and N.M. acknowledge support from the European Research Council (ERC-320441-Chirallcarbon), the MINECO of Spain (projects CTQ2017-83531-R) and the Comunidad de Madrid (PHOTOCARBON project S2013/MIT-2841); I.F. thanks the Spanish MINECO-FEDER (Grants CTQ2016-78205-P and CTQ201681797-REDC); J.C. thanks the Spanish MINECO-FEDER (Grants CTQ2015-69391-P).

ACKNOWLEDGMENT We kindly thank Prof. Jay J. Siegel for generously supplying the starting corannulene.

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Selective Formation of Negative Curvature - ACS Publications

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