Key words polycyclic aromatic hydrocarbons - nanographenes - polycyclic aromatic azomethine
ylides - cycloaddition - ullazine
Introduction
Polycyclic aromatic hydrocarbons (PAHs), which can be considered as small cutouts
of graphene, are a remarkable class of organic compounds with unique (opto)electronic
properties.[1 ] A continuous effort in the advancement in the reaction methodology and molecular
design for the development of functionalized PAHs was devoted in order to tune their
(opto)electronic properties and to enable an implementation into organic electronics.[2 ] One efficient pathway to tailor the intrinsic optical and electronic properties
of PAHs is the introduction of nitrogen atoms, which may lead to control over the
energy level of the frontier orbital or the stabilization of charges and radicals.[3 ] For example, the nitrogen-containing PAHs (N -PAHs) with a 16 π-electron ullazine motif have evolved as an attractive class of
PAHs for dye-sensitized solar cells.[4 ] Although the first ullazine derivatives were already reported by Zeller in 1983,[5 ] the synthesis of ullazine-embedded PAHs still remains challenging and is mostly
limited to acid-promoted and metal-catalyzed cyclization reactions.[6 ]
In 2014, our group firstly reported the synthesis of polycyclic aromatic azomethine
ylides (PAMYs, 1 ), which are unique building blocks for the construction of unprecedented N -PAHs via a radical or zwitterionic pathway ([Figure 1a ]).[7 ] Especially, the 1,3-dipolar cycloaddition between PAMY (1 ) and alkenes/alkynes (2 ) with subsequent dehydrogenation is a powerful tool for the formation of ullazine-based
PAHs (3 ).[8 ] Moreover, the 1,3-dipolar cycloaddition with PAMY (1 ) was recently extended to the use of nitriles (4 ) as dipolarophiles and allowed the formation of azaullazine derivatives (5 ) on surface as well as in solution.[9 ] While the cycloaddition of the PAMY building block (1 ) has already been intensively investigated in the last few years, the 1,3-dipolar
cycloaddition of an extended PAMY dimer is not known so far.
Figure 1 (a) The reported 1,3-dipolar cycloaddition between PAMY (1 ) and alkenes/alkynes (2 ) or nitriles (4 ) to ullazine (3 ) and azaullazine-containing (5 ) PAHs. (b) Concept of the double cycloaddition reaction with PAMY dimer (6 ) to a series of alkyl ester-substituted N-PAHs with a laterally extended double ullazine
scaffold DU-1–3 in this work.
In this work, we demonstrate the first cycloaddition of PAMY-dimer (6 ), which allows the synthesis of three novel alkyl ester-substituted N -PAHs with a laterally extended double ullazine scaffold (DU-1 , DU-2 and DU-3 , [Figure 1b ]). The optoelectronic properties of DU-1–3 are comprehensively investigated by UV-Vis absorption spectroscopy, fluorescence
spectroscopy and cyclic voltammetry (CV) as well as supported by theoretical modelling
via density functional theory (DFT) calculations. In particular, the CV of DU-1–3 showed three reversible oxidations waves, which confirmed the electron-rich structure
of the double-ullazine framework. Moreover, spectroelectrochemistry (SEC) measurements
unraveled several cationic species for DU-3 , which were verified by UV-Vis-NIR absorption and electron paramagnetic resonance
(EPR) spectroscopies. Furthermore, the self-organization of DU-1–3 was investigated by polarized optical microscopy (POM) and grazing-incidence wide-angle
X-ray scattering (GIWAXS) measurements. In contrast to linear alkyl ester-substituted
DU-1 and DU-2 (C10 to C12
), the assemblies of the branched alkyl ester-substituted DU-3 (C
7,2 ) showed the highest crystallinity.
Results and Discussion
The target compounds DU-1–3 were synthesized starting from the tetra-alcohol species 7 ([Scheme 1 ]). The synthesis of 7 was carried out according to our previous synthetic route.[10 ] The HCl-induced microwave-assisted cyclization of 7 and subsequent hydride abstraction with tritylium tetrafluoroborate gave the iminium
salt 8 as a crude product.[11 ] Afterwards, the addition of triethylamine (TEA) to the crude iminium salt and corresponding
dipolarophiles (9 , 10 or 11 ) at 60 °C enabled the twofold cycloaddition. The following oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) provided the target compounds DU-1–3 , respectively, as yellow solids in yields from 58% for DU-1 up to 59% for DU-3 and 61% for DU-2 , over two steps.[12 ] All target compounds DU-1–3 were confirmed by NMR spectroscopy in C2 D2 Cl4 (see the Supporting Information, SI). The aromatic protons of DU-1–3 in the 1 H-NMR spectrum are assigned to the expected chemical structure by the assistance of
two-dimensional NMR spectroscopy, respectively (see SI). Furthermore, DU-1–3 were characterized by high-resolution (HR) matrix-assisted laser desorption/ionization
time of flight (MALDI-TOF) mass spectroscopy (MS). In detail, the MALDI-TOF isotopic
distributions of DU-1–3 are in perfect agreement with the simulated patterns (see SI). Due to the different
alkyl ester substitutions, the obtained DU-1–3 showed a different solubility behavior in common organic solvents, such as dichloromethane
(DCM). DU-1 and DU-2 with linear C10 - and C12 -ester substitutions, respectively, provided a low solubility in DCM (0.2 mg/mL).
In contrast, DU-3 with branched C7,2 ester substitutions showed an enhanced solubility of up to 3 mg/mL in DCM. Differential
scanning calorimetry reveals only one peak for all three compounds that is related
to the phase transition from the crystalline to the isotropic phase (SI, [Figure S5 ]). The phase transition temperature decreases with longer and more space-demanding
side chains from 222.4 °C for DU-1 to 206°C for DU-2 and 203.8 °C for DU-3 . Thermogravimetric analysis reveals a high thermal stability of DU-1–3 up to 260 °C (SI, [Figure S6 ]).
Scheme 1 Synthetic route towards DU-1–3 . Reagents and conditions: (a) i) HCl in 1,4-dioxane (4 M), microwave, argon, 1.5 h;
ii) tritylium tetrafluoroborate, acetonitrile, toluene, 90 °C, 2 h. (b) i) TEA, chloroform,
60 °C, overnight; ii) DDQ, toluene, r.t., 3 h.
The optoelectronic properties of DU-1–3 were investigated by UV-Vis absorption and fluorescence spectroscopy in anhydrous
DCM. From the UV-Vis absorption and fluorescence spectra, there were no differences
between DU-1 , DU-2 and DU-3 , suggesting that the different alkyl ester-substituents do not apparently influence
the optoelectronic properties (see SI). Due to the similarity of the optoelectronic
properties, only the branched ethylhexyl ester-substituted DU-3 are exemplarily discussed here. The absorption maximum (λabs ) for DU-3 was observed at 404 nm with two shoulder peaks at 427 and 446 nm (see [Figure 2a ]). The corresponding optical energy gap (ΔE
g ) was estimated from the onset of the UV-Vis absorption spectrum and was calculated
to be 2.68 eV for DU-3 . The time-dependent (TD) DFT calculations at the B3LYP/6-31G(d) level fitted the
experimental UV-Vis absorption result. The fluorescence spectrum of dimer DU-3 revealed a fluorescence maximum at 461 nm with a shoulder at 494 nm ([Figure 2b ]). The fluorescence quantum yields varied in the range from 32.9% for DU-2 up to 37.6% and 37.7% for DU-1 and for DU-3 , respectively.
Figure 2 Optoelectronic investigations are exemplarily shown for the best soluble derivative
DU-3. (a) UV-Vis absorption spectrum of DU-3 in DCM (black line: experiment; red line: simulations; concentration = 10−5 mol/L). (b) Fluorescence emission spectrum of DU-3 in DCM. (c) Cyclic voltammetry of DU-3 in DCM with n -Bu4 PF6 as a supporting electrolyte at a scan rate of 75 mV/s. AgCl-coated Ag-wire was used
as a reference electrode, platinum as a working electrode and Pt/Ti as a counter electrode.
(d) Quantum-chemical DFT calculations of DU-3 with a B3LYP functional and 6-31G(d) basis set.
The electrochemical properties of DU-1–3 were investigated by CV measurements in anhydrous DCM. Due to the different solubility
in DCM, the intensities of the CV measurements of DU-1–3 are diverged (see SI, [Figure S4 ]). Due to the similarity, the CV data of DU-3 with the best solubility in DCM are presented in [Figure 2c ]. Compound DU-3 offered reversible two oxidation waves at 0.3 and 0.5 V versus Fc/Fc+ . In comparison to the first two oxidation waves, the third oxidation peak at 0.78 V
has a much higher intensity, which presumably indicates the overlap of two closely
spaced oxidation processes or a two-electron oxidation. Nevertheless, a reduction
behavior was not observed in the available potential window. The corresponding HOMO
level of DU-3 was estimated to be −5.1 eV by the half-wave potential of the first reversible oxidation.
The LUMO of DU-3 , which was determined from the difference between the optical energy gap and electrochemical
HOMO, is −2.42 eV. DFT and TD-DFT calculations with a Gaussian 09 package were performed
for a deeper grasp of the electronic ground state. The geometry optimization was carried
out via the B3LYP level of theory with the 6-31G(d) basis set. The graphical representations
of the HOMO and LUMO of DU-3 are provided in [Figure 2d ] (DU-1 and DU-2 : see SI, [Figure S11 ]). The HOMO of DU-3 is completely symmetric and equally delocalized over the full π-system. The DFT-calculated
HOMO levels for DU-1–3 are in perfect agreement with the experimental HOMO levels derived from the CV measurements
(SI, [Table S5 ]). In contrast, the distribution of the LUMO shows the main localization between
the two ullazine units.
Due to the highly reversible oxidation behavior of DU-3 , detailed insight into the cationic species was achieved by SEC via in situ CV, UV-Vis-NIR
absorption and EPR spectroscopy in anhydrous DCM (see [Figure 3 ]). At low positive potentials, new absorption bands at 493, 570, 1380 and 1689 nm
are associated to the formation of the radical cation species ([Figure 3a ]). The appearance of the EPR signal confirmed the formation of the radical cation
DU-3•+
. The EPR spectrum of the radical showed a broad unresolved signal with a g -value of 2.0027 ([Figure 3c ]). The DFT calculations demonstrated that the spin density of DU-3•+
is delocalized over two ullazine motifs in agreement with the shape of the HOMO ([Figure 3d ]). During the second redox process, the intensities of bands centered at 493 and
570 nm increased further and the bands peaked at 1380 and 1689 nm are blue-shifted.
The EPR signal intensity showed a two-fold increase during the second oxidation process
([Figure 3b ]), indicating that the dication DU-32+
may have a diradical character. At the potentials of the third redox event in the
CV, the EPR signal intensity decreased and new absorption bands emerged at 531 and
834 nm. This is an indication for the formation of the EPR-silent four positively
charged species DU-34+
.
Figure 3 In situ EPR/UV-Vis-NIR spectroelectrochemistry of the oxidation of DU-3 . (a) UV-Vis-NIR spectra measured during the electrochemical oxidation of DU-3 (blue arrows and numbers: first oxidation; black arrows and numbers: second oxidation;
and red arrows and numbers: third and fourth oxidation). (b) Cyclic voltammogram (black
line) and potential dependence of normalized EPR intensity (blue line). (c) Experimental
EPR spectrum of radical cation species DU-3•+ . (d) DFT-computed spin density distribution of DU-3•+
.
[Figure 4(a–c) ] shows the POM images of DU-1–3 drop-cast films. All layers exhibit birefringence between cross-polarizers and the
light intensity of the entire crystals changes from bright to dark by 45° rotation
of the substrate. DU-1 and DU-2 form a comparable surface morphology with spherulitic domains that are less than
100 micrometers in size. In contrast to DU-1-2 , DU-3 forms much larger polycrystalline structures with diameters larger than 100 micrometers.
This suggests that the branched substituents improve the film crystallinity and molecular
order.
Figure 4 POM images and GIWAXS patterns of DU-1 (a, d), DU-2 (b, e) and DU-3 (c, f) drop-cast films.
To understand the supramolecular organization of the DUs , GIWAXS of the polycrystalline drop-cast film was performed ([Figure 4d–f ]). The GIWAXS patterns reveal significant differences of the three compounds in crystallinity.
All compounds exhibited certain edge-on arrangement on the substrate as indicated
by the maximum intensities of the out-of-plane h00 (according to the Miller index) and in-plane 00l interstack, as well as π-stacking reflections (0k0 ). The out-of-plane interstack distance is closely related to the length of the substituents
and decreased from 2.82 nm for DU-2 to 2.61 nm for DU-1 , and 2.20 nm for DU-3 . This correlation suggests that the long axis of the PAHs is oriented parallel to
the surface. DU-1 and DU-2 reveal a close π-stacking distance of 0.35 nm, but both are poorly ordered in the
in-plane as indicated by a low intensity of the corresponding reflection. Although
the molecular interactions might be reduced due to the steric hindrance of the branched
substituents,[13 ] the π-stacking distance of 0.35 nm for DU-3 remains unchanged. The further equatorial reflection located at qxy
= 0.47 Å−1 and qz
= 0 Å−1 ([Figure 4d ]) is assigned to the in-plane 00l interstack distance of 1.33 nm for DU-1 and DU-2 , and 1.07 nm for DU-3 . Since the theoretical molecular length of 1.62 nm calculated by Cerius[2 ] software is larger, it is assumed that the molecules are arranged in a herringbone
structure. As already observed by POM, the crystallinity of DU-3 is improved in comparison to DU-1-2 as evidenced by the additional high-intensity reflections. In summary of the structural
study, it can be concluded that the introduction of branched side chains in DU-3 enhances the self-assembly and crystallinity.
Conclusions
In summary, we have synthesized a series of alkyl ester-substituted N -PAHs with a laterally extended double ullazine scaffold (DU-1–3 ). As key step for the synthesis, the first example of the double cycloaddition between
PAMY-dimer (6 ) and different electron-deficient dipolarophiles was presented. Interestingly, the
CV measurements for DU-1–3 revealed a highly reversible oxidation potential and confirmed up to three oxidation
waves. The corresponding different oxidized species of DU-3 were investigated by in situ EPR measurements and UV-Vis-NIR spectroscopy. It was
shown that the radical species with the pronounced absorption bands in the NIR region
are formed. Additionally, the self-assembly of the different alkyl ester-substituted
DU-1–3 was investigated by POM and GIWAXS. In contrast to linear alkyl ester-substituted
derivatives (DU-1 and DU-2 ), the branched ethylhexyl ester-substituted compound (DU-3 ) revealed a higher order of the self-assembled structure. We believe that the cycloaddition
of the double-ullazine based PAHs could pave the way for the synthesis of unprecedented
extended N -PAHs or N -doped graphene nanoribbons.
