Charge-transfer complexes based on C_2v-symmetric benzo[ghi]perylene: comparison of their dynamic and electronic properties with those of D_6h-symmetric coronene

Abstract

Single crystals of three neutral charge-transfer complexes and a cation radical salt based on a C_2v-symmetric polycyclic aromatic hydrocarbon, benzo[ghi]perylene (bper), were obtained. The 1 : 1 complex with 7,7,8,8-tetracyanoquinodimethane (TCNQ) involves DA-type alternating π-columns, whereas the alternating π-columns in the 2 : 1 TCNQ complex are flanked by another bper molecule. The in-plane rotation of bper in the 1 : 1 complex was significantly suppressed compared with that of coronene in (coronene) (TCNQ), which is associated with the lower molecular symmetry of bper. Whereas the 3 : 1 TCNQ complex involves DDA-type alternating π-columns flanked by another bper molecule, bper molecules in the 3 : 1 cation radical salt with Mo₆O₁₉²⁻ have a columnar structure with a [101]-like charge-ordered pattern associated with intermolecular interactions in the bay region of bper. The dimerisation of charge-rich bper molecules results in an increased energy gap at the Fermi level, and consequently, semiconducting behaviour of the salt has a larger activation energy than that of the isostructural coronene salt in the partially charged state. The lower molecular symmetry of bper also affects the degeneracy of the frontier-orbitals; the energy difference between the HOMO and the HOMO−1 of bper is significantly larger than that of coronene, which is comparable to the intermolecular transfer integrals.

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Introduction

Highly symmetric polycyclic aromatic hydrocarbons (PAHs) are attractive as potential components of high-T_c superconductors owing to their degenerate frontier orbitals^1–3 and are a promising platform for artificial molecular rotors with electronic functionalities in the solid state.^3–6 Because the dynamic and electronic properties of solids largely depend on the molecular and supramolecular structures of the components, a better understanding of the structure–property relationship with a focus on the molecular symmetry of PAHs would allow the rational design of solids with desired functionalities. Over the past few years, our group has prepared a variety of charge-transfer (CT) complexes composed of the smallest D_6h-symmetric PAH molecule, coronene (Scheme 1) as an electron donor (D). Notably, both dicationic⁷ and dianionic⁸ coronene are in a thermally accessible triplet state at room temperature associated with the doubly degenerate HOMO (e_2u) and LUMO (e_1g) levels. The CT complexes range from insulating neutral complexes^9–11 to conducting cation radical salts,^12–14 and especially, some of the latter with π-stacking columns of cationic coronene show semiconducting behaviour with a high room-temperature conductivity (>1 S cm⁻¹). In addition, we found that the in-plane rotational rate of coronene molecules in CT solids can be varied by more than six orders of magnitude (1 kHz to 7 GHz at room temperature) by modifying intermolecular interactions, such as van der Waals, π–π, and hydrogen bonding interactions.

Scheme 1 Molecular structures of coronene and its analogues.

Investigation of complexes composed of coronene analogues and comparison with the corresponding coronene-based CT complexes will provide a deeper understanding of how the high symmetry of coronene affects the dynamic and electronic properties in the solid state. There are three apparent categories of coronene analogues (Scheme 1): (1) D_6h-symmetric polyaromatic hydrocarbons with different π-conjugated sizes (e.g., hexamethylbenzene and hexa-peri-hexabenzocoronene),^15,16 (2) nonplanar PAHs with curved or twisted π-conjugated planes (e.g., [n]circulene (n ≠ 6) and [n]helicene),^17–21 and (3) planar PAHs with reduced symmetries (e.g., 1,2-disubstituted coronene and benzo[ghi]perylene (bper)).^22,23 Within the second group, we have recently realised peculiar π-electron networks in neutral CT complexes by combining corannulene (i.e., [5]circulene)²⁴ or [6]helicene²⁵ as D with planar 7,7,8,8-tetracyanoquinodimethane (TCNQ) analogues as electron acceptors (A's). In search of CT complexes using PAHs in the third group, we focus on C_2v-symmetric bper, which is regarded as a coronene analogue that has lost a benzene ring; unlike 1,2-disubstituted coronene, bper has no bulky substituents and has the higher electron donating ability (E¹_1/2(D) = 1.01 V vs. saturated calomel electrode (SCE)) than that of coronene (1.23 V).²⁶ In the solid state, bulky substituents inevitably inhibit intermolecular π–π interactions, which are indispensable for the formation of a π-conduction network, and the appropriate electron-donating ability allows the formation of stable CT complexes under aerobic conditions. Herein, we report the synthesis and the structural, optical, and transport properties of neutral CT complexes with TCNQ having different compositions (1–3) and a cation radical salt with a hexamolybdate (Mo₆O₁₉²⁻) cluster unit (4), with the support of theoretical calculations. Crystallographic data were also used to assess the thermal fluctuations, which are indicative of in-plane molecular rotations, of bper molecules in the solid state.

Results

(bper)(TCNQ) (1) and (bper)₃(TCNQ) (3) were prepared by the solvent evaporation method, whereas (bper)₂(TCNQ) (2) was obtained preferentially by the co-sublimation method. (bper)₃Mo₆O₁₉ (4) was obtained by the electrochemical oxidation of bper in the presence of an electrolyte, (Bu₄N)₂Mo₆O₁₉ (see Experimental and computational for details). The crystal structures of 1–4 were determined at 100 K (Table 1).

Table 1 Crystallographic data of 1–4

	(bper)(TCNQ) (1)	(bper)2(TCNQ) (2)	(bper)3(TCNQ) (3)	(bper)3Mo6O19 (4)
a R 1 = ∑(|F0| − |Fc|)/∑|F0|. b wR2 = [∑w(F0^2 − Fc^2)^2/∑w(F0^2)^2]^0.5.
Formula	C34H16N4	C56H28N4	C78H40N4	C66H36Mo6O19
Formula weight	480.53	756.87	1033.20	1708.64
Crystal system	Orthorhombic	Triclinic	Triclinic	Triclinic
Space group	P212121	P1̄	P1̄	P1̄
Crystal size (mm^3)	0.49 × 0.09 × 0.06	0.20 × 0.08 × 0.07	0.50 × 0.07 × 0.04	0.56 × 0.08 × 0.04
a (Å)	7.1719(4)	7.1246(8)	10.0725(14)	9.8455(9)
b (Å)	10.8284(5)	12.3495(14)	10.8786(15)	11.0998(10)
c (Å)	29.7864(15)	21.257(3)	12.9332(18)	12.8243(11)
α (deg)	90	78.598(2)	113.110(2)	86.101(1)
β (deg)	90	80.637(2)	105.578(2)	67.763(1)
γ (deg)	90	80.207(2)	90.301(2)	89.093(1)
V (Å^3)	2313.2(2)	1790.4(4)	1245.7(3)	1294.2(2)
Z	4	2	1	1
Temperature (K)	100	100	100	100
d calc (g cm^−3)	1.380	1.404	1.377	2.192
μ(Mo Kα) (mm^−1)	0.083	0.083	0.080	1.499
Reflns used	4716	6295	5057	5122
Refined params	343	541	469	511
R 1 (for I > 2σ(I))^a	0.0386	0.0783	0.0490	0.0204
wR2 (for all data)^b	0.0799	0.1755	0.1189	0.0515
GOF on F^2	1.030	1.003	1.003	1.026
CCDC	1537813	1537814	1537815	1537816

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Charge transfer

Regardless of composition, complexes 1–3 exhibit a CT absorption band at around hν_CT = 12 × 10³ cm⁻¹ in addition to the intramolecular transition bands of the component molecules above 20 × 10³ cm⁻¹ (Fig. 1a). These hν_CT values are lower than those of coronene–TCNQ complexes (14 × 10³ cm⁻¹)⁹ because the electron-donating ability of bper is stronger than that of coronene.^27,28 As shown in Fig. 1b, complexes 1–3 lie on the straight line obtained by applying a least-squares method to the plots of hν_CT against E¹_1/2(D) for the TCNQ complexes of typical PAH molecules (perylene,²⁹ anthracene,³⁰ pyrene,³¹ coronene,^9,32 and triphenylene³³). Bond length analysis based on crystallographic data³⁴ estimates the charge of the TCNQ molecules to be −0.14(3) for 1, −0.23(9) for 2, and −0.17(4) for 3, which are the same within experimental error. Infrared spectra support the neutral nature (Fig. S1 in ESI†).

Fig. 1 (a) Absorption spectra of 1 and 4 measured on KBr pellets (solid lines) together with pristine bper in 10⁻⁵ M chloroform solution (dotted line). Closed and open arrows indicate the bands originating from an intermolecular charge transfer transition between neutral bper and TCNQ and an intramolecular transition of monocationic bper, respectively. Asterisks indicate the band originating from the O–H stretching mode of water molecules absorbed in the KBr powder. (b) Plot of CT energy (hν_CT) against the first redox potential of PAH molecules (E¹_1/2(D)) in TCNQ complexes (closed circle: 1–3, a: (perylene)(TCNQ),²⁹ b: (anthracene)(TCNQ),³⁰ c: (pyrene)(TCNQ),³¹ d: (coronene)(TCNQ),^9,32 e: (triphenylene)₂(TCNQ)(toluene)³³).

Salt 4 exhibits the distinct bands at 8.1 and 13.7 × 10³ cm⁻¹, which are reminiscent of the absorption spectra of the bper monocation isolated in matrices.^35–37 This spectral feature is firm evidence that this salt is the first example of a solid composed of cationic bper. The salt also exhibits a low-lying band with a maximum at approximately 4 × 10³ cm⁻¹, which arises from the intermolecular transition between neutral and monocationic bper molecules,³⁸ as expected from the X-ray diffraction measurements (vide infra).

(bper)(TCNQ) (1). Complex 1 crystallises in a non-centrosymmetric orthorhombic lattice with space group P2₁2₁2₁, and each bper and TCNQ molecule is crystallographically independent. The complex involves DA-type alternating π-stacking columns (Fig. 2a) with a ring-over-bond pattern (Fig. 2b) along the a axis, as in (coronene)(TCNQ) (Fig. 2c).^9,32 Within the column, bper molecules are aligned in the same direction because each unit cell along the a direction contains only one bper molecule. Adjacent bper and TCNQ molecules are arranged in a nonparallel fashion, and the interplanar distance, which is defined as half the distance between bper molecules across a TCNQ molecule, was estimated to be 3.27 Å. It should be noted that the dihedral angle (θ) between bper and TCNQ molecules within the column (3.36°) is apparently larger than that in (coronene)(TCNQ) (2.40°), but the interplanar distance is comparable to that in (coronene)(TCNQ) (3.26 Å).⁹ Along the side-by-side direction, each bper molecule is connected to four adjacent TCNQ molecules (Fig. 2d) through C–H⋯N hydrogen bonds (H1⋯N1: 2.72 Å, H16⋯N1: 2.63 Å, H14⋯N2: 2.75 Å, H12⋯N4: 2.61 Å, H5⋯N3: 2.69 Å vs. sum of van der Waals radii: 2.75 ų⁹).

Fig. 2 (a) Molecular packing of 1 viewed along the b axis in a space-fill representation. Bper and TCNQ molecules appear in green and red, respectively, where carbon and hydrogen atoms in the bay region are darkened. Overlap patterns of adjacent PAH and TCNQ molecules within a column viewed perpendicularly to the molecular plane of PAH in (b) 1 and (c) (coronene)(TCNQ).^9,32 (d) C–H⋯N hydrogen bonds around bper molecule in 1 along the side-by-side direction.

(bper)₂(TCNQ) (2). Complex 2 has lower crystal symmetry (centrosymmetric triclinic P1̄) than 1, and the asymmetric unit contains two bper molecules (denoted as bper-2A and bper-2B) and one TCNQ molecule. In the crystal, the bper-2A molecules construct a DA-type alternating π-stacking column with TCNQ along the a axis, whereas bper-2B, which has a tilt angle with respect to bper-2A of 84.3°, separates the neighbouring columns approximately along the [1 −1 0] direction (Fig. 3a). A coronene–TCNQ CT complex with a 2 : 1 ratio has not been obtained at present, but (perylene)₂TCNQ with the same packing motif was recently prepared by Vermeulen et al.⁴⁰ Within the column, adjacent bper-2A and TCNQ molecules, whose molecular planes are slightly inclined to each other (θ = 0.79°), display a ring-over-bond overlap pattern (Fig. 3b). Along the side-by-side direction, each bper-2A molecule forms short interatomic contacts with two bper-2B molecules through C–H⋯π hydrogen bonds (≈2.9 Å;^39,41,42 the observed C–H⋯C distances are 2.65–2.89 Å) and two TCNQ molecules through C–H⋯N hydrogen bonds (H2⋯N2: 2.66 Å, H6⋯N1: 2.73 Å), as shown in Fig. 3c.

Fig. 3 (a) Molecular arrangement of 2 in a space-fill representation. Bper and TCNQ molecules appear in green and red, respectively, where carbon and hydrogen atoms in the bay region are darkened. (b) Overlap pattern of adjacent bper-2A and TCNQ molecules within a column viewed perpendicularly to the molecular plane of bper-2A in 2. (c) Short interatomic contacts around one bper-2A molecule with surrounding bper-2B and TCNQ molecules in 2. The gray and orange dotted lines show C–H⋯N and C–H⋯C (or C–H⋯π) hydrogen bonds, respectively.

(bper)₃(TCNQ) (3). Complex 3 also belongs to the centrosymmetric triclinic system with space group P1̄ and is isostructural with (coronene)₃(TCNQ).⁹ The asymmetric unit includes one and a half bper molecules (denoted as bper-3A and bper-3B, respectively) and half a TCNQ molecule; bper-3B is disordered over two orientations, which are related by an inversion center, with equivalent occupancy factors of 0.5 (Fig. 4a). As shown in Fig. 4b, the complex involves DDA-type alternating π-stacking columns with a [bper-3A⋯bper-3A⋯TCNQ] repeating unit along the a axis. As in the case of 2, the column is flanked by another bper molecule (bper-3B) with a tilt angle of 69.8° with respect to bper-3A. The ring-over-bond-type overlap pattern between bper-3A and TCNQ (Fig. 4c) is reminiscent of those in 1 and 2, and the molecular planes are slightly inclined to each other (θ = 0.97°). Adjacent bper-3A molecules within the column are oriented in the opposite direction with a ring-over-atom overlap pattern on an inversion center (Fig. 4d), in contrast to the ring-over-bond overlap pattern in pristine bper crystal^43,44 with a “sandwich herringbone structure”⁴⁵ and the same interplanar distance (3.37 Å). A shift along the in-plane direction (2.67 Å) is significantly larger than that in (coronene)₃(TCNQ) (1.50 Å; Fig. 4e) with a longer interplanar distance (3.43 Å).⁹ As shown in Fig. 4f, each bper-3A molecule is connected to two bper-3B molecules through C–H⋯π hydrogen bonds (the observed C–H⋯C distances are 2.70–2.89 Å) and one TCNQ molecule through a C–H⋯N hydrogen bond (H12⋯N1: 2.70 Å).

Fig. 4 (a) Schematic representation of the disorder of bper-3B molecule in 3. (b) Molecular packing of 3 viewed along the b axis in a space-fill representation. Bper and TCNQ molecules appear in green and red, respectively, where carbon and hydrogen atoms in the bay region are darkened. (c) Overlap pattern of adjacent bper-3A and TCNQ molecules within a column viewed perpendicularly to the molecular plane of bper-3A in 3. Overlap patterns of adjacent PAH molecules within a column viewed perpendicularly to the molecular plane of PAH in (d) 3 and (e) (coronene)₃(TCNQ).^9,32 (f) Short interatomic contacts around one bper-3A molecule with surrounding bper-3B and TCNQ molecules in 3. The gray and orange dotted lines show C–H⋯N and C–H⋯C (or C–H⋯π) hydrogen bonds, respectively.

(bper)₃Mo₆O₁₉ (4). Salt 4 is the first cation radical solid based on bper, and the triclinic lattice (space group P1̄) is isostructural with (coronene)₃Mo₆O₁₉.¹³ One and a half bper molecules (denoted as bper-4A and bper-4B, respectively) and half a Mo₆O₁₉²⁻ cluster unit are crystallographically independent; namely, bper-4B and Mo₆O₁₉²⁻ are located on an inversion center. Similar to bper-3B in 3, bper-4B is disordered over two orientations with equivalent occupancy factors of 0.5 (Fig. 5a). Notably, the salt forms a segregated π-stacking column with a [bper-4A⋯bper-4A⋯bper-4B] repeating unit along the a axis (Fig. 5b). Such a cationic column has been reported for several PAH molecules.^46–50 Within the column, adjacent bper-4A molecules oriented in the opposite direction display a ring-over-bond overlap pattern (Fig. 5c) with an interplanar distance of 3.14 Å, which is similar to that in isostructural (coronene)₃Mo₆O₁₉ (3.16 Å; ring-over-atom).¹³ This distance is significantly shorter than those in natural graphite (3.35 Å; ring-over-atom),⁵¹ pristine bper crystal (3.36 Å; ring-over-bond),^43,44 and 3 (3.37 Å; ring-over-atom), possibly due to stabilisation of the partially occupied HOMO of each bper-4A. In contrast, adjacent bper-4A and bper-4B molecules are nonparallel with a dihedral angle of 3.9°; one of the disordered bper-4B molecules is oriented in the same direction as bper-4A, whereas the other is oriented in the opposite direction (Fig. 5d). Notably, the interplanar distance, which is defined as half the distance between two bper-4A molecules separated by a bper-4B molecule, was estimated to be 3.28 Å, which is significantly longer than that in (coronene)₃Mo₆O₁₉ (3.19 Å).¹³

Fig. 5 (a) Schematic representation of the disorder of bper-4B molecule in 4. (b) Columnar structure of bper molecules in 4 in a space-fill representation, where carbon and hydrogen atoms in the bay region are darkened. A and B indicate the bper-4A and bper-4B molecules, respectively. Overlap patterns of (c) adjacent bper-4A molecules and (d) adjacent bper-4A and bper-4B molecules with two possible orientations within a column viewed perpendicularly to the molecular plane of bper-4A in 4. C–H⋯O hydrogen bonds around (e) bper-4A and (f) one orientation of disordered bper-4B in 4 along the side-by-side direction.

Each bper-4A molecule interacts with two adjacent Mo₆O₁₉²⁻ cluster units along the side-by-side direction through C–H⋯O hydrogen bonds (H15⋯O2: 2.67 Å, H16⋯O2: 2.58 Å, H1⋯O9: 2.49 Å, H2⋯O3: 2.50 Å, H3⋯O8: 2.43 Å, H5⋯O2: 2.41 Å vs. sum of van der Waals radii: 2.72 Å;³⁹Fig. 5e). Notably, hydrogen atoms in the bay region of bper-4A are essential for the formation of quadruple hydrogen bonds with an adjacent Mo₆O₁₉²⁻ cluster unit. Fig. 5f shows the short interatomic contacts between one orientation of disordered bper-4B and Mo₆O₁₉²⁻ along the side-by-side direction. Each bper-4B molecule forms C–H⋯O hydrogen bonds with three adjacent Mo₆O₁₉²⁻ cluster units through double or triple C–H⋯O hydrogen bonds (H36⋯O7: 2.68 Å, H34⋯O7: 2.52 Å, H34⋯O4: 2.50 Å, H31⋯O6: 2.63 Å, H30⋯O6: 2.64 Å, H27⋯O7: 2.67 Å, H25⋯O4: 2.37 Å). Unlike the case of bper-4A, hydrogen atoms in the bay region of bper-4B make no contribution to hydrogen bond formation.

Resistivity measurements were performed to investigate the effect of molecular symmetry on the electronic properties. Fig. 6 shows the temperature dependence of the electrical conductivity (σ) of a single crystal of 4, which has a room-temperature value of 5 × 10⁻² S cm⁻¹ similar to that of (coronene)₃Mo₆O₁₉.¹³ However, a marked difference between the two salts was observed upon cooling; namely, the σ value of 4 decreases steeply with decreasing temperature. The activation energy of 4 was estimated to be 0.18 eV, which is more than four times that of (coronene)₃Mo₆O₁₉ (0.04 eV).

Fig. 6 Temperature dependence of the electrical conductivity of a single crystal of 4 (red line) and (coronene)₃Mo₆O₁₉ (black line).¹³

Discussion

Effect of molecular structure on dynamic properties

Solid-state NMR spectroscopy has become a definitive experimental technique for investigating the dynamic properties of molecular rotators in the solid state.^3,5,6 Despite this demonstrated utility, the deuteration of bper is unavailable at the present stage. Another way to assess the dynamic behaviour of molecules is to examine the magnitude and anisotropy of thermal fluctuations based on crystallographic data. Indeed, coronene molecules, whose fast in-plane 6-fold flipping motion was confirmed by the ²H NMR spectra of the deuterated complexes, exhibit large thermal fluctuations along the in-plane direction.^9,11 As shown in Fig. 7a, the in-plane thermal fluctuations of bper molecules in 1 at room temperature are less pronounced than those of (coronene)(TCNQ) (Fig. 7b);⁹ the average mean-square displacement of component carbon atoms, 〈μ²〉, was estimated to be 0.062 Ų for bper in 1 and 0.145 Ų for coronene in (coronene)(TCNQ) at 298 K. Thus, the in-plane rotation of bper is significantly suppressed by the loss of a benzene ring, even though 1 has a larger θ value than (coronene)(TCNQ).

Fig. 7 Thermal fluctuations of (a) bper in 1 and (b) coronene in (coronene)(TCNQ)⁹ at 298 K viewed perpendicular to the molecular plane. Hydrogen atoms are omitted for clarity, and the 50% probability thermal ellipsoids are shown. Temperature dependence of (c) relative 〈μ²〉 values of PAH (bper or coronene) with respect of that of TCNQ (〈μ²〉_PAH/〈μ²〉_TCNQ) and (d) angles between the PAH plane and the crystallographic axis along the side-by-side direction (ϕ) in each CT solid (red circles: 1, black circles: (coronene)(TCNQ)).⁹ The inset shows the arrangement of bper molecules in 1 along the b axis.

To shed light on this phenomenon, we investigate the temperature dependence of the relative 〈μ²〉 value with respect of that of TCNQ in 1, as previously performed for (coronene)(TCNQ).⁹ As shown in Fig. 7c, the coronene complex shows a rapid increase in 〈μ²〉_PAH/〈μ²〉_TCNQ at around 200 K, above which the angle between the coronene plane and the crystallographic a axis (ϕ; Fig. 7d) also steeply increases to accelerate molecular rotation. In contrast, for 1, neither parameter shows such an anomaly in the measured temperature region (100–298 K). The origin of the effect on the dynamic properties of these complexes is not determined from the crystallographic data. However, the difference in the relationship between 〈μ²〉_PAH/〈μ²〉_TCNQ and ϕ provides clear evidence for the importance of intermolecular interactions in determining the dynamic properties. Considering that the decreased axial symmetry of a rotator leads to an increase in the rotational barrier as well as a decrease in the number of energy maxima and minima,⁶ it is apparent that the lower molecular symmetry of bper is a dominant factor in suppressing the displacement that accompanies molecular rotation.

Effect of molecular structure on charge state

The bonds marked by an asterisk in the inset of Fig. 8, which have bonding character in the HOMO level, were found to have similar lengths in pristine bper crystal and neutral CT complexes 1–3 (except for bper-3B), whereas those in bper-4A are longer than them by ca. 0.02 Å (Fig. 8). Considering that the bond lengths in bper-4B are comparable to those observed in the neutral species, bper-4B is in a nearly neutral state and bper-4A must be in a nearly monocationic state to construct a [101]-like charge-ordered column.^52,53 Thus, the low-lying absorption band at around 4 × 10³ cm⁻¹ shown in Fig. 1a can be assigned to electron transfer between cationic bper-4A and neutral bper-4B. Although the reason that the bonds in bper-3B are longer than expected is unclear, density functional theory (DFT) calculations at the B3LYP/6-31+G(d,p) level of theory (Fig. S3 in ESI†) show that the bonds in the bper monocation are elongated by 0.017 Å relative to those in neutral bper, which quantitatively supports the experimental results. First principles calculations within DFT based on the crystallographic data estimate the charges of bper-4A and bper-4B to be +0.82 and +0.37, respectively.⁵⁴

Fig. 8 Lengths of the bonds marked by an asterisk in the inset for pristine bper crystal and 1–4. The pale red horizontal lines mark the estimated values for monocationic (upper) and neutral (lower) bper molecules. The inset shows the HOMO distribution of a bper molecule calculated by DFT methods at the RB3LYP/6-31+G(d,p) level of theory.

Apparently, compared with (coronene)₃Mo₆O₁₉, the removal of a benzene ring from a coronene molecule results in looser molecular packing in 4, as manifested by the orientational disorder of bper-4B. This disorder is an indication that the Coulombic interactions of bper-4B with Mo₆O₁₉²⁻ through hydrogen bonds are weaker than those for bper-4A, which forms quadruple hydrogen bonds with Mo₆O₁₉²⁻ in the bay region. It is possible that the difference in intermolecular interactions plays a key role in the emergence of charge disproportionation, namely, charge-rich bper-4A and charge-poor bper-4B.

To elucidate the relationship between molecular symmetry and the electronic properties, we focus on the interplanar distances within the column as described above. It appears that those in 4 and (coronene)₃Mo₆O₁₉¹³ bear some similarities together with striking differences; namely, the distance between adjacent monocationic bper-4A molecules is similar to that in (coronene)₃Mo₆O₁₉ to stabilise the partially occupied HOMO of bper-4A (3.14 Å in 4vs. 3.16 Å in (coronene)₃Mo₆O₁₉), whereas the distance between bper-4A and bper-4B (3.28 Å) is significantly longer than that in (coronene)₃Mo₆O₁₉ (3.19 Å) possibly due to the neutral character of bper-4B. The intermolecular transfer integrals for 4 at 100 K calculated by the first principles method within DFT (277 meV between bper-4A and bper-4A and 100–159 meV between bper-4A and bper-4B) have a remarkable alternation relative to the case in (coronene)₃Mo₆O₁₉ (257 and 208–219 meV, respectively) owing to the increased distance between bper-4A and bper-4B. As shown in Fig. 9a and b, this alternation inevitably gives rise to decreased band dispersion accompanied by an increased energy gap at the Fermi level (124 meV) compared with that in (coronene)₃Mo₆O₁₉ (32 meV). It appears that the increased energy gap is responsible for the semiconducting behaviour with a larger activation energy, as mentioned above; namely, these salts can be regarded as band insulators. The exceptionally small activation energy of (coronene)₃Mo₆O₁₉ largely depends on the highly symmetric molecular structure of coronene.

Fig. 9 DFT energy band structures of (a) 4 and (b) (coronene)₃Mo₆O₁₉,¹³ calculated by first principles methods using the crystallographic data at 100 K, where T is (0, 1/2, 1/2) and the energy is given relative to the Fermi energy. The coloration of the six highest branches represents the fraction of HOMO (red) and HOMO−1 (blue) components. Red and blue horizontal ticks in the left box indicate the energy levels of the HOMO and the HOMO−1, respectively. (c) Density of states (DOS) of 4 (red) and (coronene)₃Mo₆O₁₉ (black)¹³ near the Fermi energy, where the thick red and black bars indicate the energy gaps.

Effect of molecular structure on orbital degeneracy

One of the most striking effects of molecular symmetry on π-electronic systems, which has an impact on electronic properties, is orbital degeneracy. As reported previously, the energy difference between the HOMO and the HOMO−1 of coronene (Jahn–Teller splitting) in (coronene)₃Mo₆O₁₉ is very small (ca. 0.12 eV; Fig. 9a and c) and comparable to the intermolecular transfer integrals.¹³ It is noteworthy that this value is similar to that in C₆₀ anions (0.06–0.12 eV).⁵⁵ In contrast, the energy difference in 4 (ca. 0.59 eV; Fig. 9b and c) is about five times greater than that in (coronene)₃Mo₆O₁₉ due to the decreased molecular symmetry. This marked difference distinctly indicates that the HOMO and HOMO−1 energy levels in (coronene)₃Mo₆O₁₉ are almost degenerate despite the reduced symmetry of the crystal field (space group P1̄). If cationic coronene molecules could be stacked uniformly in solids, the density of states (DOS) of the HOMO/HOMO−1 band would likely be enhanced because of the orbital degeneracy. Subsequent adjustment of the Fermi level to the high DOS region may produce a new class of organic superconductors within the Bardeen–Cooper–Schrieffer (BCS) framework through electron–phonon coupling.^1–3 Studies along this line are now in progress.

Conclusions

In the present study, the influence of the molecular symmetry of planar PAH molecules on dynamic and electronic properties was investigated by comparing compounds of C_2v-symmetric bper with corresponding compounds of D_6h-symmetric coronene. It was found that the removal of a benzene ring from the coronene molecule significantly suppresses molecular rotation along the in-plane direction in the solid state. In addition, the lower molecular symmetry of bper leads to charge ordering associated with the formation of significant hydrogen bonds with counter anions in the bay region, which is responsible for enhanced alternation of interplanar distances and transfer integrals within the column. These features inevitably result in an increased energy gap at the Fermi level, which corresponds to semiconducting behaviour with a large activation energy. Isostructural cation radical salts composed of PAH molecules with different symmetries enable investigations of the effect of molecular symmetry on the degeneracy of frontier orbitals in the solid state; namely, a comparison with the present bper-based cation radical salt revealed that the doubly degenerate HOMO of coronene molecules is maintained, even though the cation radical salt has a low-symmetry crystal field.

Experimental and computational

Materials and methods

Dichloromethane and pentane were used as received (Wako Chemicals), whereas methanol, chloroform, and acetone were distilled prior to use. Bper was purchased from Aldrich and used without any further purification. TCNQ was purified by sublimation prior to use, and (Bu₄N)₂Mo₆O₁₉ was synthesized and recrystallised from acetone according to the literature procedure.⁵⁶

Synthesis of (bper)(TCNQ) (1). Equimolar amounts of bper and TCNQ (0.017 mmol) was dissolved in 50 mL of dichloromethane/pentane (1 : 1). Natural evaporation of the solution at room temperature yielded ca. 3 mg (0.006 mmol) of black needle-shaped crystals of 1 (yield: 37%).

Synthesis of (bper)₂(TCNQ) (2). Equimolar amounts of bper and TCNQ (0.020 mmol) were ground together in an agate mortar and transferred into a 12 mm-diameter borosilicate glass tube. After sealing under vacuum (5.8 × 10⁻⁵ mbar), the 13 cm-long tube was placed in a two-zone furnace with a temperature gradient from 220 °C (sample side) to 150 °C (opposite side). After heating for 2 days, black thick plate crystals of 2 were grown on the center area in the tube (yield: ca. 50%).

Synthesis of (bper)₃(TCNQ) (3). Equimolar amounts of bper and TCNQ (0.050 mmol) were added to 20 mL of dichloromethane/pentane (1 : 1). After stirring at room temperature, the suspension was filtered to remove any traces of undissolved TCNQ. The filtrate was removed by natural evaporation at room temperature to yield ca. 10 mg (0.010 mmol) of black needle-shaped crystals of 3 (yield: 58%).

Synthesis of (bper)₃Mo₆O₁₉ (4). Bper (0.016 mmol) and (Bu₄N)₂Mo₆O₁₉ (0.022 mmol) were added to the anodic and cathodic compartments, respectively, in an H-shaped cell. After the cell was vacuum-dried, 18 mL of dichloromethane and a few drops of methanol were added to the cell. A constant current (1.0 μA) was passed between the two platinum electrodes. Black needle-shaped crystals of 4 were grown on the anodic electrode over a period of 2 weeks at 15 °C (yield: ca. 50%).

Spectroscopy. UV-Vis-NIR absorption spectra were measured on KBr pellets (3.8–42 × 10³ cm⁻¹) or in a chloroform solution (12–50 × 10³ cm⁻¹) using a Shimadzu UV-3100 spectrophotometer.

X-Ray structural analysis. Single-crystal X-ray diffraction experiments were performed on a CCD-type diffractometer (Bruker SMART APEX II) with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 100 K. A single crystal was mounted on a glass capillary and cooled by a stream of cooled nitrogen gas. The crystal structures were solved by a direct method using the SIR2004 program⁵⁷ and were refined by a full-matrix least-squares method on F² using the SHELXL program.⁵⁸ All non-hydrogen atoms were anisotropically refined. The positional parameters of the hydrogen atoms were calculated under the assumption of fixed C–H bond lengths of 0.93 Å with sp² configuration of the parent atoms.

Resistivity measurements. Electrical resistivity of 4 was measured using a conventional four-probe method using a Keithley 2000 multimeter. Gold wires (ϕ = 10 μm) were attached to a single crystal with carbon paint (DOTITE XC-12). A DC current of less than 10 μA was applied along the stacking direction (crystal long axis).

Theoretical calculations (single molecule). DFT calculations with the B3LYP functional⁵⁹ were carried out with the 6-31+G(d,p) basis set.^60,61 To ensure the reliability of the frequencies, both “Opt = Tight” and “Int = Ultrafine” were specified. The stabilities of the wave functions were confirmed by specifying the “Stable = Opt” keyword in the present DFT calculations. All the computations were performed using the Gaussian 09 program package,⁶² in which the bper molecule lies on the yz plane.

Theoretical calculations (band). Using the experimental structure, we performed the electronic structure calculations within the generalised gradient approximation⁶³ in the framework of the density functional theory as implemented in the quantum-ESPRESSO code.⁶⁴ Ultrasoft pseudopotentials⁶⁵ and the plane-wave basis set with cutoff energies of 30 Ry for wave functions and 150 Ry for charge densities were used. To obtain the transfer integrals, we constructed six maximally-localized Wannier orbitals^66,67 from the six bands at the top of the valence band, which correspond to HOMO and HOMO−1 of each bper molecule. Charge disproportionation was discussed by diagonalising this six-band model.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Numbers JP25288041 and JP16H04139 (YY), JP16H00924 (TK), JP15K17901 and JP17H05153 (Y. Nakano), JP26288035 (HY), JP26110512 and JP16H00964 (HK), and JP23225005 (GS) and by JST PRESTO (TK). Theoretical calculations were partly performed at Research Center for Computational Science, Okazaki, Japan, and the SuperComputer System, Institute for Chemical Research, Kyoto University, Japan.

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Footnote

† Electronic supplementary information (ESI) available: Infrared spectra of TCNQ complexes (Fig. S1), crystal structure of (bper)(GaCl₄) (Fig. S2), Kohn–Sham orbitals obtained from DFT calculations (Fig. S3). CCDC 1537813–1537817 (1–4 and pristine bper). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8qm00112j

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