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C: Energy Conversion and Storage; Energy and Charge Transport

Ambipolar Transistor Properties of Charge-Transfer Complexes Containing Perylene and Dicyanoquinonediimines Ryo Sanada, Dongho Yoo, Ryonosuke Sato, Kodai Iijima, Tadashi Kawamoto, and Takehiko Mori J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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The Journal of Physical Chemistry

Ambipolar Transistor Properties of Charge-Transfer Complexes Containing Perylene and Dicyanoquinonediimines †

†

†

†

†

Ryo Sanada, Dongho Yoo, Ryonosuke Sato, Kodai Iijima, Tadashi Kawamoto and Takehiko Mori*,† †

Department of Materials Science and Engineering, Tokyo Institute of Technology, O-okayama,

Meguro-ku, Tokyo 152-8552, Japan

ACS Paragon Plus Environment

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ABSTRACT

In order to investigate charge polarity of transistors based on donor/acceptor cocrystals, charge-transfer complexes of wide donor molecules such as perylene and pyrene are investigated. As-grown single-crystal transistors are deposited from solutions, which attain perfect reproducibility in comparison with the conventional pasted single-crystal transistors. Perylene complexes of dicyanoquinonediimines (DCNQI) with substituents X = CH3, Cl, and Br show hole-dominant ambipolar characteristics.

Owing to the large molecular width, the

donor/acceptor orbitals are not orthogonal, and ambipolar transport is realized. In contrast, pyrene works similarly to ordinary acenes, and not only the tetracyanoquinodimethane (TCNQ) complexes but also the DCNQI complex show only electron-transport. The existence of the short-axis offset in the molecular overlap is the criteria that transforms electron transport to ambipolar transport.


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INTRODUCTION Recently, charge-transfer complexes composed of donor (D) and accepter (A) molecules have been investigated extensively in order to study the doping effect,1 as well as to study conduction mechanism of high-performance DA polymers using well-defined analogous systems.2-5 Along this line, transistors of not a few charge-transfer complexes have been investigated.6-8 Many of these DA complexes have alternating stacks of D and A molecules, and, from the naive consideration, the carrier hopping process between the D highest occupied molecular orbital (HOMO) and the A lowest unoccupied molecular orbital (LUMO) leads to ambipolar charge transport.9,10 In such a case, the electron and hole effective transfer integrals are both given by teff ~ tHL2/E, where tHL is the transfer integral between the D HOMO and the A LUMO, and E is the energy difference between these orbitals. However, electron-only (n-channel) transport has been widely observed, particularly in tetracyanoquinodimethane (TCNQ) complexes (Scheme 1).11-18 This has been explained by considering other orbitals than HOMO and LUMO;19-22 HOMOs of acenes, phenes, and thienoacenes have a horizontal node, and are approximately orthogonal to the TCNQ LUMO (tHL ~ 0). The D next HOMO (HOMO–1) has the same symmetry as the A LUMO, and the large transfer integral makes the A LUMO bandwidth large, which mediates the electron transport. TCNQ does not have a similar "bridge" orbital to mediate hole transport because the HOMO and HOMO–1 levels are too deep. Accordingly, TCNQ complexes show electron-only transport in general. We have, however, found that when dimethyldicyanoquinonediimine (DMDCNQI) is used instead of TCNQ, ambipolar transport is observed.21,23 Since the symmetry of DMDCNQI is lower than TCNQ, this is attributed to the loss of the orthogonality between the D HOMO and


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the A LUMO. If the D HOMO/A LUMO transfer is significant, it is reasonable to show ambipolar transport. However, the acceptor ability of DMDCNQI may be slightly weaker than TCNQ,21 and we cannot entirely exclude the explanation from the acceptor ability. In the present work, we have investigated perylene and pyrene complexes, which have larger molecular widths than the ordinary acenes, and ambipolar transport is expected due to the absence of the D HOMO/A LUMO orthogonality. In addition to the DMDCNQI complex, we have investigated the dichloro- and dibromo-DCNQI (DClDCNQI and DBrDCNQI) complexes (Scheme 1); the latter two are stronger acceptors than TCNQ, and we can exclude the influence of the acceptor ability. These complexes have exhibited hole-dominant ambipolar properties. TCNQ complexes of perylene have been studied for a long time,24-28 but recently revisited frequently.29-35

Transistors of (perylene)(TCNQ) are reported to be electron transporting,

whereas (perylene)2(TCNQ) and (perylene)3(TCNQ) are respectively ambipolar and hole transporting.29 We have also investigated pyrene complexes of TCNQ,36,37 dimethyl-TCNQ (DMTCNQ), and DMDCNQI, but all these complexes have shown only electron transport. The transistor properties are discussed on the basis of the molecular orbitals. NC NC Perylene

Pyrene

H

R CN CN

R H R=H TCNQ R=CH3 DMTCNQ

R NC N

N CN

R R=CH3 DMDCNQI R=Cl DClDCNQI R=Br DBrDCNQI

Scheme 1. Molecular structures.

EXPERIMENTAL SECTION


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DMDCNQI, DClDCNQI, and DBrDCNQI were prepared following the reported procedure.38 Single crystals of the charge-transfer complexes were grown by the slow evaporation method in dichloromethane for (perylene)(TCNQ), (pyrene)(TCNQ), (pyrene)(DMTCNQ), and (pyrene) (DMDCNQI), and in chlorobenzene for (perylene)(DMDCNQI), (perylene)(DClDCNQI), and (perylene)(DBrDCNQI). The donor and acceptor molecules were respectively dissolved in the solvent, and the resulting saturated solutions were mixed. The solvent was slowly evaporated at room temperature, whereas the solutions were kept at –20°C for (pyrene)(TCNQ) and (pyrene)(DMDCNQI).26 After about one week, black needle-like crystals were harvested. The X-ray oscillation photographs of (perylene)(DMDCNQI), (perylene)(DClDCNQI), and (perylene)(DBrDCNQI) were taken using a RIGAKU R-AXIS RAPID II imaging plate with CuK radiation from a rotation anode source with a confocal multilayer X-ray mirror (RIGAKU VM-Spider,  = 1.54187 Å). The X-ray diffraction data of (pyrene)(DMTCNQ) and (pyrene)(DMDCNQI) were collected by a Rigaku four-circle diffractometer (AFC-7R) with graphite-monochromatized MoK radiation ( = 0.71069 Å). The structures were solved by the direct method (SIR 2004) and refined by the full-matrix least-squares method by applying anisotropic temperature factors for all non-hydrogen atoms using the SHELXL programs.39,40

The hydrogen atoms were placed at geometrically

calculated positions.


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Transistors were fabricated onto n-doped Si substrates with a thermally grown SiO2 dielectric layer (300 nm, C = 11.5 nF/cm2) in two different ways. In the first method, the crystals were pasted on the substrate.41 Polystyrene (PS,  = 2.5) with a thickness of 100 nm was deposited on the substrate as a passivation layer by spin coating (3000 rpm and 30 s) a solution of PS (20 mg) in toluene (1 mL),42,43 where the calculated overall capacitance was 7.6 nF/cm2.44 Then, the crystals were pasted on the PS layer using ethanol.41 In the second method, as-grown crystals were used.45,46 The charge-transfer complex was dissolved in chlorobenzene (0.1 wt%) together with polymethyl methacrylate (PMMA, 0.1 wt%,

 = 5.3), and drop-casted on a silicon/SiO2 wafer. After one-hour drying, the substrate was kept in a chloroform vapor overnight.47-49 The overall capacitance including average 360 nm of PMMA was 6.1 nF/cm2, where the PMMA thickness was measured by using Kosaka Surfcorder ET200. After preparing crystals by these two ways, source and drain (S/D) electrodes were made by depositing carbon paste (DOTITE, XC-12) on the two ends of the crystals. Accordingly, the crystal long axes were oriented in the channel direction. The single-crystal X-ray diffraction indicated that the crystal long axes were parallel to the molecular stacking axis, corresponding to the crystallographic a axes. The transistor properties were measured under the vacuum of 10-3 Pa by using a Keithley 4200 semiconductor parameter analyzer. The mobility was estimated from the saturated-region transfer characteristics.

RESULTS AND DISCUSSION

Crystal structures


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Crystal data of (perylene)(DXDCNQI) (X = CH3, Cl, and Br), (pyrene)(DMTCNQ), and (pyrene)(DMDCNQI) are listed in Table 1.

All (perylene)(DXDCNQI) complexes are

isostructural, in which a unit cell contains one D and one A molecules to form mixed stacks along the a axis (Figure 1). The triclinic symmetry is, however, different from the monoclinic symmetry of (perylene)(TCNQ).24,29,30,33 The latter involves two stacks tilted in the opposite directions (Figure 1(a)), whereas all molecules are parallel in the former complexes (Figure 1(b)). The same difference of the column packing happens in pyrene complexes; (pyrene)(DMTCNQ) and (pyrene)(DMDCNQI) have a triclinic parallel structure, but (pyrene)(TCNQ) is monoclinic with alternately tilted columns.36 When viewed perpendicularly to the molecular plane, the TCNQ molecule is located on the lower half of the perylene molecule (Figure 2). This is because the stack is tilted along the molecular short axis. The definitions of the molecular short and long axes are not obvious in perylene, but it is clearly defined in TCNQ. In DXDCNQI, the up/down symmetry is lost, but the C=C and C=N directions are approximately parallel to the perylene molecules. In other words, the phenyl rings of DXDCNQI are approximately parallel to those of perylene. Molecules in these perylene complexes are not displaced along the molecular long axis, so that the

stack

is

not

tilted

in

this

direction.

In

the

crystal

structures

of

bisethylenedithiotetrathiafulvalene (BEDT-TTF) salts, the ordinary stacking structure is called the -phase, in which the molecules are displaced along the molecular long axis.50 In the classifications of the BEDT-TTF salts, the present structures correspond to the "-phase (Figure 1(b)).

The column structure of (perylene)(TCNQ) is the same, but the "side-by-side"

intercolumnar interactions are lost due to the tilted column arrangement (Figure 1(a)). Recently reported polymorphs of (perylene)(TCNQ) ( and ) include two kinds of columns.32 One of the


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columns has the same displacement as the ordinary complex, but the displacement is alternating, so that the column is straight. In another column, the TCNQ molecule is located at the center but largely tilted (Figure 2(b)). These complexes have straight columns. The interplanar spacings of the present complexes are 3.41, 3.34, 3.36, and 3.39 Å for the DXDCNQI complexes (X = CH3, Cl, and Br) and the TCNQ complex, respectively. The structure of (pyrene)(DMTCNQ) is regarded as an ordinary stacking (-phase) structure (Figures 1(c) and (d)).

The molecular long axes of the pyrene and DMTCNQ molecules are,

however, tilted by 9° (Figure 2(g)).

This reminds us (anthracene)(TCNQ),11 in which the

structure is disordered due to the rotation of the anthracene molecule at room temperature, where the tilt angle is not determined definitely. The rotation stops at 100 K, and the tilt angle is fixed at a comparatively large value of 20°. Even in (tetracene)(TCNQ),51 the long axes are slightly tilted (~8°), though recently an entirely straight polymorph has been reported.52 In (pyrene) (TCNQ), the molecules are more tilted (~22°, Figure 2(f)). The overlapping mode of (pyrene)(DMDCNQI) is entirely different; the pyrene molecule is arranged not far from the perpendicular direction to DMDCNQI (Figure 2(h)). The pyrene molecules are tilted by 69° from the horizontal direction, so that the phenyl rings are again approximately parallel to the DMDCNQI ring.

The interplanar spacings of the pyrene

complexes are 3.39, 3.45 and 3.43 Å for the TCNQ, DMTCNQ, and DMDCNQI complexes, respectively. The charge-transfer degree  is estimated from the DCNQI bond lengths and the C=N stretching (Supporting Information). For all perylene and pyrene complexes,  is less than 0.2, and these complexes are essentially regarded as neutral. This is in agreement with the previous estimations for the TCNQ complexes.29,33


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This is also reasonable in view of the weak donor ability of perylene and pyrene (Figure 3). Here, the LUMO levels are estimated from the reduction potentials of acceptors observed in the cyclic voltammetry,53 and the HOMO levels are extracted from the oxidation potentials.54 Since the energy levels of the A LUMO and the D HOMO are by more than 0.5 eV different, the charge-transfer complexes have an essentially neutral character.55 The work function of carbon electrode (–4.8 eV) is close to the LUMO levels of the present acceptors, whereas the HOMO levels of perylene (–5.1 eV) and pyrene (–5.6 eV) are considerably deeper than this.56 Accordingly, in the present transistors, the electron injection is easier than the hole injection.


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Table 1. Crystallographic Data of (Perylene)(DXDCNQI) (X = CH3, Cl, and Br), (Pyrene)(DMTCNQ), and (Pyrene)(DMDCNQI) Crystal

(Perylene) (DMDCNQI)

(Perylene) (DClDCNQI)

(Perylene) (DBrDCNQI)

(Pyrene) (DMTCNQ)

(Pyrene) (DMDCNQI)

Formula

C30H20N4

C28H14Cl2N4

C28H14Br2N4

C30H18N4

C26H18N4

Formula weight

436.51

477.35

544.27

434.50

386.45

Crystal system

Triclinic

Triclinic

Triclinic

Triclinic

Triclinic

Space group

P–1

P–1

P–1

P–1

P–1

Z

1

1

1

1

1

a (Å)

7.3897(2)

7.3392(2)

7.41651(13)

6.896(3)

7.301(2)

b (Å)

8.1488(2)

8.1349(2)

8.17089(15)

9.01(3)

8.087(2)

c (Å)

9.9536(2)

9.9727(2)

9.87887(18)

9.517(2)

8.773(2)

α (deg)

69.7107(8)

67.9300(8)

68.2777(7)

105.15(7)

72.53(2)

β (deg)

77.4637(8)

77.0956(8)

78.4778(7)

101.29(2)

87.38(2)

γ (deg)

88.5772(8)

87.4287(9)

88.3362(7)

95.70(8)

89.74(2)

V (Å3)

547.95(2)

537.34(2)

522.5(3)

553(2)

493.6(2)

 (g/cm3)

1.323

1.475

1.727

1.305

1.300

Total reflns.

6366

6348

5890

3546

3431

Unique reflns. (Rint)

1940 (0.0459)

1931 (0.0427)

1944 (0.0422)

2946 (0.0839)

2889 (0.0407)

R1 (F2>2 (F2))

0.0466

0.0381

0.0321

0.0642

0.0555

wR2 (All reflections)

0.1491

0.1037

0.1006

0.1974

0.1627

GOF

1.072

1.136

1.190

0.979

1.039

Temperature (K)

271

275

271

295

299


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(a)

(c)

(b)

(d)

Figure 1. Crystal structure of (a) (perylene)(TCNQ) viewed along the stacking (//a) axis,23 and (b) (perylene)(DMDCNQI) viewed along the molecular long axis. Crystal structure of (pyrene) (DMTCNQ), (c) viewed along the stacking (//a) axis, and (d) along the molecular long axis.


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Figure 2. Overlap modes of (a) (perylene)(TCNQ),24,33 (b) -(perylene)(TCNQ),33 (c) (perylene)(DMDCNQI), (d) (perylene)(DClDCNQI), (e) (perylene)(DBrDCNQI), (f) (Pyrene)(TCNQ),36,37 (g) (pyrene)(DMTCNQ), and (h) (pyrene)(DMDCNQI).


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Figure 3. Energy levels of the donors and acceptors.

Transistor properties Characteristics of single-crystal transistors are shown in Figure 4, and the transistor parameters are extracted as listed in Table 2. (Perylene)(TCNQ) exhibits only electron transport (Figure 4(a)), where the mobility is a little larger than the previous report (10-3 cm2 V-1 s-1).28 The off current is comparatively large, but this is much better than the previous observation. This suggests existence of a small amount of hole conduction, and to be considered as an intrinsic effect. Therefore, conduction of (perylene)(TCNQ) is basically regarded as electrondominant ambipolar transport, although the hole mobility is too small to be estimated. The DXDCNQI (X = CH3, Cl, and Br) complexes show ambipolar transport (Figures 4(b)-(f)). The pasted and as-grown crystals afford approximately similar mobilities (Table 2). However, pasted crystals frequently do not show transistor properties, though almost all as-grown crystals exhibit transistor properties. Therefore, the latter is more preferable as far as the dissolved charge-transfer complex reproduces the crystals.


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The

DXDCNQI

complexes

show

similar

magnitudes

Page 14 of 34

of

electron

mobilities

to

(perylene)(TCNQ), but the hole mobilities h exceeding 0.01 cm2 V-1 s-1 are larger than the electron mobilities. In particular, (perylene)(DMDCNQI) and (perylene)(DBrDCNQI) show by one order higher hole mobilities than their own electron mobilities and that of (perylene)(TCNQ). The maximum mobilities are slightly smaller than 0.1 cm2 V-1 s-1.

Since

mobilities of

(BTBT)(TCNQ) analogs are around 0.1 cm2 V-1 s-1 (BTBT: [1]benzothieno[3,2-b][1] benzothiophene),12-16 these values are typical of mixed-stack complexes. By contrast, the hole mobility of (perylene)(DClDCNQI) is comparatively small, but this may be ascribed to the enhanced acceptor ability of DClDCNQI. Output characteristics of (perylene)(DMDCNQI) show clear reversed regions, where VG and VD have opposite polarity (Figures 4(c) and (d)).57 It is noteworthy that the threshold voltages are particularly small in the as-grown crystals (< 10 V) in comparison with the pasted crystals (> 10 V, Table 2). The difference of the electron and hole threshold voltages Vth is as small as 0 ~ 10 V. The corresponding values are more than 100 V in ordinary single-component small-molecule materials.57 This is characteristic of the two-component systems. As a result, the "charge neutrality point" is as sharp as graphene transistors. Since the off-current does not drop sufficiently at the charge neutrality point, the apparent on/off ratio is small in the as-grown crystals, but this is not due to the reduced performance because we have observed similar on-current. The small threshold voltages are also the origin of the clear observation of the reversed-region characteristics (Supporting Information). In Figures 4(e) and (f), the transfer characteristics transforms from ~VG2 to ~VG at large |VG|, because VG exceeds VD + Vth, where the transistor goes into the linear region. This also happens due to the small Vth.


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The pyrene complexes have not shown hole transport at all (Figures 4(g) and (h)). The electron mobilities are in the same order as that of (perylene)(TCNQ) (Table 2). Since the electron mobility of (pyrene)(DMDCNQI) is even smaller, we cannot determine whether the comparatively small on/off ratio is due to the remaining hole transport or due to the absolute value of the off current.


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Figure 4. (a) Transfer characteristics of pasted single-crystal transistors of (perylene)(TCNQ). (b) Transfer, and (c, d) output characteristics of as-grown single-crystal transistors of (perylene)(DMDCNQI). Transfer characteristics of as-grown single-crystal transistors of (e) (perylene)(DClDCNQI), and (f) (perylene)(DBrDCNQI).

Transfer characteristics of pasted

single-crystal transistors of (g) (pyrene)(TCNQ), and (h) (pyrene)(DMTCNQ).


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Table 2. Transistor Properties of (perylene)(TCNQ), (perylene)(DXDCNQI) (X = CH3, Cl, and Br), (pyrene)(DMTCNQ), and (pyrene)(DMDCNQI). Complexes (perylene)(TCNQ)

av [max] (cm2 V-1 s-1) 2.2×10–3 [5.9×10–3]

Vth (V) 19

On/off 3×101

Pasted

e

(perylene)(DMDCNQI) Pasted

e

9.8×10–4 [2.1×10–3]

16

2×102

h

1.1×10–2 [3.8×10–2]

–55

7×103

e

1.2×10–3 [1.6×10–3]

1

6×102

h

1.4×10–3 [3.6×10–3]

–15

2×102

e

3.9×10–3 [1.1×10–2]

–21

4×101

h

2.0×10–3 [5.2×10–3]

–22

1×101

e

1.9×10–2 [5.8×10–2]

–2

2×101

h

1.8×10–2 [5.0×10–2]

–5

1×101

e

8.6×10–3 [2.3×10–2]

–8

4×101

h

3.1×10–2 [6.2×10–2]

–20

8×101

e

5.3×10–3 [7.6×10–3]

–5

9×100

h

4.2×10–2 [8.5×10–2]

–10

2×101

As-grown

(perylene)(DClDCNQI) Pasted

As-grown

(perylene)(DBrDCNQI) Pasted

As-grown

(pyrene)(TCNQ)

Pasted

e

3.1×10–4 [1.0×10–3]

34

2×103

(pyrene)(DMTCNQ)

Pasted

e

1.1×10–3 [2.2×10–3]

15

4×103

(pyrene)(DMDCNQI)

Pasted

e

[8.0×10–6]

24

7×100


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Superexchange transfers Transport properties of DA crystals have been discussed on the basis of "superexchange" transfer integrals teff. The LUMO splitting of an A-D-A triad is 2teeff, which gives the A LUMO bandwidth 4teeff mediated by the hybridization of the D orbitals.9,10

The hole transport is

similarly estimated by the HOMO splitting of a D-A-D triad (2theff), which gives the D HOMO bandwidth 4theff due to the hybridization of the A orbitals. If only the hybridization between the D HOMO and the A LUMO is naively considered, teeff and theff ~ tHL2/E are the same. The superexchange transfers are listed in Table 3. Although the perylene HOMO has a horizontal node (Figure 5),22 the TCNQ molecule is located on the lower half of the perylene molecule (Figure 2(a)). Then, the D HOMO and the A LUMO are not orthogonal, and tHL is large (typically about 270 meV in the top lines of Table S1). Therefore, both teeff and theff are large, and ambipolar transport is excepted; the calculations in Table 3 agree with this expectation. This is also confirmed by the partition method, where the hole dominance is explained from the balance of more than two bridge orbitals (Supporting information). Overlap modes of pyrene complexes are different from complex to complex (Figures 2(f)(h)), and we cannot extract a general rule in the pyrene complexes. However in (pyrene) (DMTCNQ), the DA molecules are approximately parallel (~9o, Figure 2(g)), and the D HOMO and the A LUMO are nearly orthogonal (tHL ~ 76 meV) due to the horizontal node of pyrene (Figure 5(b)). The D HOMO–1/A LUMO transfer (336 meV) is overwhelmingly large owing to the stripe symmetry of the D HOMO–1 (Supporting Information). Therefore, the situation is the same as ordinary acene complexes of TCNQ,22 although pyrene is a little thicker molecule than acene and thienoacene molecules. The existence of the horizontal node in the D HOMO as well as the parallel packing is the essential requirements to maintain the orthogonality. This is the


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reason of teeff > theff, in good agreement with the observed electron transport.

The other pyrene

complexes are more tilted, but teeff > theff is mostly satisfied as shown in Table 3, in agreement with the observations.


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Table 3. Superexchange Transfers (meV) Calculated by the Triad and Partition Methods. teeff / theff

Triad method

Partition method

(perylene)(TCNQ)

43 / 42

53 / 90

(perylene)(DMDCNQI)

33 / 43

29 / 71

(perylene)(DClDCNQI)

60 / 68

57 / 107

(perylene)(DBrDCNQI)

65 / 70

63 / 114

(pyrene)(TCNQ)

22 / 21

15 / 10

(pyrene)(DMTCNQ)

37 / 3

54 / 8

(pyrene)(DMDCNQI)

16 / 8

34 / 2


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Figure 5. Molecular orbitals and intermolecular transfers of (a) (perylene)(TCNQ) and (b) (pyrene)(DMTCNQ). Violet and yellow bars represent contributions with opposite signs.


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CONCLUSION In this paper, we have investigated donors with large molecular widths.

The perylene

complexes exhibit considerable molecular displacement along the molecular short axis, and orthogonality between the D HOMO and the A LUMO is lost.22 In such a case, balance of more than two bridge orbitals determines the charge polarity, which depends a little sensitively on the quantitative estimation of the transfer integrals. In contrast to the electron dominant transport in (perylene)(TCNQ), the DXDCNQI complexes exhibit hole-dominant ambipolar transport. Since DClDCNQI and DBrDCNQI are stronger acceptors than TCNQ, the hole dominant transport is not attributed to the acceptor strength, but should be explained from the orbital effect. By contrast, in pyrene complexes, the D HOMO/A LUMO orthogonality is basically maintained due to the absence of the molecular short axis offset, and the D HOMO–1 is the principal bridge orbital similarly to acene complexes. The actual pyrene complexes exhibit electron transport, though the estimation of transfer is not simple owing to the molecular rotation. In the case of wide donor molecules, the existence of the molecular short axis offset is the criteria to distinguish the HOMO–1 mediated electron transport and the D HOMO/A LUMO mediated ambipolar transport. Accordingly, the D HOMO/A LUMO orthogonality and the resulting electron transport is destroyed by the following reasons. (1) Molecular rotation as exemplified by (anthracene)(TCNQ).11,22 (2) Wide molecules such as perylene with short-axis offset. (3) Low-symmetry molecules such as DCNQI.21-23 The perturbation is, however, relatively weak as exemplified by electron transport in (pyrene)(DMDCNQI).


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(4) Donor molecules without a horizontal node like dibenzotetrathiafulvalene (DBTTF) and quarterthiophene (4T).12,22

We have started from such prototypical complexes as

(acene)(TCNQ), but we have to add entirely different combinations of donors and acceptors with different orbital symmetry, though comparatively few complexes in this category are stable enough to show transistor properties. In the above cases, we have to investigate the balance of bridge orbitals. It should be kept in mind that calculations tend to predict small teeff and theff ratios and ambipolar transport. The HOMO–1 mediated electron transport is more robust than the theoretical calculations, so that we have to examine the orbital symmetry according to the above guidelines.

ASSOCIATED CONTENTS Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acs.xxxxxx.xxxxxxx. Description of microscopic images of single crystals, hydrogen bonds, estimation of charge-transfer degree, characteristics of single-crystal transistors together with the operation regions of ambipolar transistors, and partition calculations (PDF) Crystal data (CIF)

AUTHOR INFORMATION Corresponding Author


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*

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E-mail: [email protected]

ORCDI Dongho Yoo: 0000-0003-0886-7533 Ryonosuke Sato: 0000-0002-3471-9158 Tadashi Kawamoto: 0000-0002-5676-4013 Takehiko Mori: 0000-0002-0578-5885

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partly supported by JPSJ KAKENHI Grant Number 16K13974 and 18H02044, and Takahashi Industrial and Economic Research Foundation.


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