Linearly Fused Azaacenes: Novel Approaches and New Applications

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Review

Linearly-fused Azaacenes: Novel Approaches and New Applications Beyond Field-Effect Transistors (FETs) Junbo Li, and Qichun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 20 May 2015 Downloaded from http://pubs.acs.org on May 21, 2015

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ACS Applied Materials & Interfaces

Linearly-fused

Azaacenes:

Novel

Approaches

and

New

Applications Beyond Field-Effect Transistors (FETs) Junbo Lia, Qichun Zhang*a,b a

School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

b

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang

Technological University, Singapore 637371, Singapore

ABSTRACT: Replacing the CH groups in the backbones of acenes with heteroatoms offers scientists greater opportunities to tune their properties, as the type, position, number, and the valence of the introduced heteroatoms have strong effects on the frontier orbital energy levels. When the heteroatoms are nitrogen atoms, all of the resulting materials are called azaacenes. Recently, the synthesis, structure, physical properties and applications of azaacene derivatives have been intensively investigated. This review focuses on recent synthetic efforts (since 2013) towards making novel azaacenes as well as their potential applications beyond field-effect transistors (FETs) including organic light-emitting diodes (OLEDs), memory devices, phototransistors, solar cells, photoelectrical chemical cells, sensors, and conductors. KEYWORDS: polycyclic aromatic hydrocarbons, azaacenes, synthesis, applications, organic electronics, sensing.

1. INTRODUCTION The term acenes, introduced by Clar,1 describes a family of polycyclic aromatic hydrocarbons (PAHs) with linearly-fused benzene rings, which have been shown to be an important class of organic semiconductors due to their potential applications in organic field-effect transistors (OFETs), organic photovoltaic devices (OPVs), organic light-emitting diodes (OLEDs), and organic memories.2-7 In order to further enhance the performance of acene materials, their modification by attaching functionalized substitution groups or increasing the numbers of linearly fused benzene rings have been intensively investigated.8-12, The research in these directions has been summarized in several recent reviews.13-15 Given that the heteroatom-doping method (or ion implantation) using heteroatoms of B, P, Sb, or As has widely been employed in the silicon industry to enhance the electronic properties of silicon, it would be logical to presume that the properties of acenes could also be tuned through their “doping” with heteroatoms. Of course, the concept of “doping” in this context is different from the physical doping of silicon or chemical doping of conducting polymers. Here, the so-called “doping” is a step-by-step synthetic approach using heteroatoms (B, P, O, S, N) to replace CH groups in the backbone of the acenes.16-18 This “doping” will produce a new family of conjugated compounds, oligoheteroacenes, the heterosubstituted analogues of oligoacenes, whose properties are strongly dependant on the types and numbers of the 1

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heteroatoms as well as their positions and orbital hybridizations in the frameworks of the parent compounds.16-18 If all “doping” heteroatoms are nitrogen atoms, the resulting acenes are called azaacenes. Although the synthesis, physical properties, and field-effect transistor (FET) performances of azaacenes up to 2013 have been reviewed by Bunz,19,20 Miao21,22 and Richards,23 there has been considerable recent progress on new approaches to azaacenes and their potential applications (beyond FETs), which are not covered by these earlier reviews. This review will focus on the most recent (since 2013) synthetic efforts to prepare novel azaacenes and their new applications beyond FETs including phototransistors, solar cells, photoelectrochemical cells, organic light-emitting diodes (OLEDs), sensors, memory devices, and conductors.

2. SYTHESIS Unlike acenes, which require numerous complicated steps to form multiple C-C (or C=C) bonds in their frameworks,14 azaacenes require fewer steps to construct the multiple C-N bonds in their backbones. The methods to prepare linear azaacenes (including key precursors such as quinones and N, N-dihydroazaacenes) have been reviewed previously: condensation reactions (including diamines (or tetraamines) with diketones (or tetraketones)24,25 or aromatic 1,2-dihydroxy compounds,26 aldol condensation between aldehydes and cyclohexanedione27), coupling reactions between diamines and activated halides,28-30 Diels-Alder reactions,31 and oxidative coupling reactions.32 Among these methods, condensation reactions (especially reactions between diamines and diketones) and coupling reaction are the most common methods to prepare azaacenes.20 Since diamines (or tetraamines) are generally more readily adjustable in their structures and much more easily synthesized than diketones or dichloro aromatics, much effort has been invested in preparing various diamines (or tetraamines) (Scheme 1) as suitable building blocks for making a variety of azaacenes. Beside condensation reaction, coupling reactions have also been employed as promising methods to approach the synthesis of larger azaacenes which are stable under ambient conditions. In 2011, Bunz’s group obtained three stable azahexacenes (13a-c) through palladium-catalyzed coupling reactions between a substituted diaminoanthracene (5) or diaminophenazine (6) and dichloro-aromatics, followed by oxidation with MnO2 (Scheme 2).33 Interestingly, 13a and 13b have much longer life times in air than their parent compound hexacene. More recently, Bunz’s group successfully prepared four diazaheptacenes through similar coupling reactions (Scheme 3).34 Diazaheptacenes 18, 19a and 19b have been found not to be stable in solution as they are easily dimerized. The more persistent diazaheptacene 19c is stable in solution for at least one hour, which is long enough to permit full characterization. These results made them believe that TIPS-ethynyl groups might not be bulky enough to afford stabilization against dimerization while the larger Si(sec-Bu)3-ethyneyl group used in diazaheptacene (19c) could be a promising candidate to sufficiently protect their azaacenes from dimerization.

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Scheme 1. The structures of the diamines (or tetraamines)

Scheme 2. The synthetic route to azahexacenes via Pd-catalyzed coupling reactions

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Scheme 3. The synthetic route to diazaheptacenes through Pd-catalyzed coupling reactions.

2.1 Condensation reaction Since the synthetic methods to prepare linear azaacenes have been recently reviewed by Bunz et al,20 here we will focus just on recently reported tetraamine building blocks 9 and 10, which have been successfully employed to synthesize some new azaacenes. Four hexazapentacenes (20a-d) have been prepared by us through a simple one-step condensation reaction between tetraamine 9 and commercially available diketones (Scheme 4).35 The as-prepared compounds showed LUMO energy levels as low as -3.6 eV, which suggests that these compounds could be promising candidates for n-channel OFETs. Pyrene-fused hexazaheptacenes36 (21 and 22) and hexazanonacenes37 (23) were also successfully synthesized by this method. Similarly, several tetraazadioxaacene derivatives (24a-h) can be easily obtained (Scheme 5) if tetraamine 9 is replaced by the dioxatetraamine 10.38, 39

4


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H2N H2N

N

NH2 1.5 HCl NH2

N

IBX, CH3COOH

diketones

reflux, 48h

9 R

N

N

N

R

R

N

N

N

R

20a: R = - CH3;

20b: R = -CH2CH3 S

20c: R = -(CH2)4- 20d: R =

N

N

N

N

N

N

21

N

H N

N

N

N

N

N H

N

N

N

22

N

N

N

N

N

N

23 Scheme 4. The structures of compounds 20-23.

5


20-23


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O

R H2N

O

H2N

O

NH2 4 HCl NH2

a-h

R

O

R

N

O

N

R

R

N

O

N

R

10

24a-h

O

O

O

O

O

O

O

O

c

b

a

O O

N

O

O

N

O

e

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d

S O O

O

f

O S g

O h

Scheme 5. The structures of compounds 24a-h.

2.2 Diels-Alder reaction Although the Diels-Alder (D-A) reaction has been widely used to construct larger acenes and twistacenes, employing this method to prepare azaacenes is rare. A likely reason is that the requisite N-containing building blocks are neither commercially available nor easy to prepare. Zhang and co-workers firstly synthesized azapentacenes by employing D-A reactions between tetrabromo-o-xylene and 1,4-anthraquinones.31 The resulting azapentacenes have been shown to be ambipolar organic semiconductors.31 In order to construct larger azaacenes, two new building blocks 25 and 26 for D-A reaction which contain N atoms at different positions have been prepared by our group (Scheme 6).40 Since the similar isoquinolinone (27) had been used by us as an effective building block to construct larger acenes or twistacenes,13, 41-46 it seemed logical that azaisoquinolinones (25 and 26) could be useful building blocks for preparing larger azaacenes. This research direction is now still under investigation in our group. More recently, our group has successfully prepared a series of new diazaacenes (from azanaphthalene to azapentacene) through D-A reactions between in situ generated arynes as dienophiles and substituted 1,2,4,5-tetrazines as dienes (Scheme 7).47,48 Interestingly, the as-prepared diazapentacenes with different substitution groups can self-assemble into different nano/micro structures with various morphologies such as microrods (31a), nanoprisms (31b), and nanobelts (31c). Surprisingly, both 31a and 31b showed an aggregation-induced emission (AIE) effect while compound 31c displayed an aggregation-caused quenching (ACQ) effect (Figure 1).48

6


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Scheme 6. The structures of compounds 25-27.

Ph

N N NH2 Ph n

Ph N N

COOH isoamyl nitrite

Ph

n Ph n=0-3 Ph Ph

Ph N N

N N

N N

N N

Ph 28

Ph

Ph 29 Ph

R

31a: R = N N

Ph

Ph 30

R

31b: R = 31c: R =


7


N S


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

Figure 1. (A) The self-assembly structures of compound 31a-31c; (B) The structures of compounds 31a-31c; (C) The different optical changes of compounds 31a-31c. Reprinted with permission from ref 48. Copyright 2014 American Chemical Society.

3. APPLICATIONS The history of azaacenes can be traced back to well over a century ago. The first protonated azapentacenes (14-dihydro-5,7,12,14-tetraazapentacene (32) and 6, 13-dihydro-6,13-diazapentacene (33)) (Scheme 8) were synthesized by Fischer and Hepp in 189049 and Hinsberg in 1901,50 respectively. However, the possible applications of both compounds were rarely investigated51,52 until Nuckolls, Miao and co-workers reported that compound 33 and its constitutional isomer 34 (Scheme 8) were p-type (hole-transporting) materials with hole mobilities of up to 5×10-5 and 6×10-3 cm2 V-1 S-1, respectively.53 It has since been shown that azaacenes can be p-,26,53 ambipolar27,31 or n-type54 organic semiconductor materials. Among all reported azaacenes, compound 35 (Scheme 8) has the largest electron mobility (up to 3.3 cm2 V-1 S-1).54 Recently, Zhang’s group reported that azapentacenes containing one or two pyridine rings showed good ambipolar OFET properties27,31 while Miao’s group systematically investigated the OFET properties of the non-substituted N-heteropentacenes (including N,N-dihydroazapentacene26 and azaquinones55,56) and silylethynylated N-heteropentacenes.57,58,59 The structure-property relationships of azapentacenes including the effect of N numbers54 and postions57 as well as film surface modification60 on the FET properties of azapentacenes have also been strongly 8


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investigated and reviewed by Miao’s group22. In this review, we will mainly focus on summarizing other reported applications of azaacenes.


3.1 Application in sensors Azaacenes have lone pairs of electrons on the sp2 hybridized N atoms, which can be protonated,61 or coordinate with metal ions,62 or have strong interactions with anions, which make it plausible for rationally designed or modified azaacenes to be used as ion probes In 2012, Bunz and co-workers synthesized a series of water-soluble bis-triazolylphenazine adducts 36a-d and 37a-d.63 The fluorescence of compounds 36a and 37a can be selectively quenched by Ag+ cations in aqueous solution, indicating that these two compounds are good probes to sense Ag+ cations (Figure 2). Note that the halogenation of phenazine cores (such as 36b-d and 37b-d) could weaken the ability of as-prepared compounds to bind metal ions.

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Figure 2. The structures of compounds 36-37 and the Ag+ binding. Reprinted with permission from ref 63. Copyright 2012 American Chemical Society.

Wudl et al reported a pyridone-containing azaacene, 2-methyl-1,4diphenylbenzo[g] isoquinolin-3(2H)-one (38, BIQ) with two tautomers (I and II) (Scheme 9), which was very sensitive to pH value with obvious changes in their color and fluorescence.61 The addition of acid can convert BIQ tautomer I into the aromatic tautomer II, where the latter has a much higher fluorescence quantum yield and a blue-shifted absorption and emission spectrum compared with I. This phenomenon was further observed by our group for the similar compounds 3964 and 40.65 Very recently, we also inserted N atoms into isoquinoline backbones to make materials for sensor applications.40 For example, compounds 25 and 2640 have similar isoquinoline structures but with different N positions. Interestingly, compound 26 (6-azaisoquinolinone) changes in color immediately from yellow to red upon the addition of Fe3+, while 7-azaisoquinolinone (compound 25) shows no color change after the addition of Fe3+ (Figure 3). In addition, the changes in fluorescence for the two probes were also different, which suggests that the positions of N atoms have strong effects on the binding ability of such molecules to metal ions (Figure 3). Moreover, azaisoquinolinone 26 could be further modified through a simple N-methylation to produce a new cyanide ion probe pyridinium-fused pyridinone (41).66 This probe 41 showed a 57-fold fluorescent intensity enhancement upon reaction with cyanide ions in aqueous solution with a very low detection limit (5.38 × 10-8 M) (Figure 4). Theoretical investigations have also been conducted to model the observed remarkable blue-shifted UV-Vis absorption and emission enhancement.


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Figure 3. The Fe3+ detection of compounds 25 and 26. Reprinted with permission from ref 40. Copyright 2013 American Chemical Society.

Figure 4. (A) The mechanism of compound 41 for CN- detection; (B) Fluorescence response of compound 41 (10 mm) upon addition of different anions; (C) Photograph of compound 41 (10 mm) taken under irradiation with UV light after addition of the indicated anions. Reprinted with permission from ref 66. Copyright 2013 Wiley-VCH Verlag GmbH and Co. 11



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Since larger azaacenes possess an electron-deficient backbone, they would be good candidates for anion recognition through anion-π interactions.67 For example, hexazaheptacenes 21 and 22 have been demonstrated to display anion-sensing behavior in DMF36: compound 21 can act as an efficient sensor for the naked–eye detection of F- and H2PO4- anions while compound 22 can selectively sense F- among ten different anions (Figure 5). Continuing on this research, our group developed a novel azaacene 1,2,5,6-tetra(5-hexylthiophene-2-yl)-hexaazapentacene (43) with four thiophene groups attached on both sides of the N-heteroacenes backbone, which could enhance the selectivity of metal cations since the thiophene groups not only have some binding ability with metal ions but also change the electron density of the backbone of the N-substituted heteroacenes.68 In fact, compound 43 has been demonstrated to act as an efficient cation chemosensor for the detection of Cu2+ ion among 15 cations (Ca2+, Cd2+, Co2+, Cr2+, Cu2+, Fe3+, Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Zn2+, Pb2+) (Figure 6), with a detection limit of 1.2×10−6 M. Employing commercially available 2,3-phenazinediamine (44) as a building block, several azaacenes have been synthesized as effective probes to sense metal ions or anions. For example, compound 45 was obtained as an orange powder through a one-pot solid-state reaction between 2,3-phenazinediamine and 3,4,5,6-tetrafluorophthalic anhydride.69 The unexpected product 45 was fully confirmed by single crystal X-ray diffraction analysis which showed strong π-π stacking in the solid state. Our research showed that compound 45 was a good probe to detect F- by UV-vis and fluorescence spectroscopy (Figure 7) with a fast response time. More recently, several 1H-imidazo[4,5]phenazine derivatives (Scheme 10) have also been demonstrated to be effective probes for Fe3+,70 Hg2+,71 and H2PO4-,72,73 respectively.

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Figure 5. (A) UV-vis absorption spectra of DMF solutions of compound 21 in the absence (blank) and presence of F- and H2PO4-; (B) UV-vis absorption spectra of DMF solutions of compound 22 in the absence (blank) and presence of F-. (The inserts show the photos of compounds 21 and 22 with different anions). Reprinted with permission from ref 36. Copyright 2013 Wiley-VCH Verlag GmbH and Co.

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Figure 6. (A) The synthesis of compound 43; (B) UV-vis absorption spectra of DMF solutions of compound 43 in different metal ions; (C) Fluorescence spectra of DMF solutions of compound 43 in different metal ions. (The inserts show the photos of compounds 43 with different cations). Reprinted with permission from ref 68. Copyright 2014 Wiley-VCH Verlag GmbH and Co.

Figure 7. (A) The synthesis of compound 45; (B) The crystal packing of compound 45; (C) UV-vis absorption spectra compound 45 in the absence and presence of different anions; (D) Fluorescence spectra of compound 45 in the absence and presence of different anions. (The inserts show the photos of compounds 45 with different anions). Reprinted with permission from ref 69. Copyright 2013 Elsevier Ltd.


3.2 Applications in Organic Light Emitting Diodes 14


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Nitrogen-containing acenes have become a promising class of materials for applications in OLEDs52 due to their high stability and low LUMO levels which promote electron injection. In 2003, Jenekhe and coworkers reported the synthesis of five n-type organic semiconductors based on 4,9-diphenylanthrazoline (49a-e) (Scheme 11).74 These molecules emitted yellow light from OLEDs with a maximum brightness of 133 cd m-2 and external quantum efficiency of up to 0.07% in ambient air (Table 1). Meanwhile, the same group also investigated the diphenylanthrazolines (49a-e) as electron transport materials with poly(2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) as the emissive layer in OLEDs. The enhancements of the external quantum efficiency and brightness were up to 50 times and 17 times, respectively, when compounds 49a-e were used as the electron transport materials (Table 1). Bunz and coworkers also reported the synthesis and applications of a series of quinoxalines (50a-g) and benzoquinoxalines (51a-g) derivatives (Figure 8).75 Considering materials’ properties such as LUMO levels, thermal stability, solid-state structures, and emission characterization together, three compounds 51a-51c were chosen as emitting layers for OLEDs. The OLEDs based on 51a showed a luminance of 250 cd m-2 at driving voltage of 6.5 V and top luminance of > 1000 cd m-2. Moreover, Bunz et al also compared vacuum deposited and solution-processed OLEDs using 51b and 51c (Figure 8). The TIPs-substituted azaacenes not only had better stability and solubility, which made solution-processed OLED devices possible, but the silyl groups also inhibited undesirable intermolecular interactions to some extent. Although iptycene derivatives have been successfully used as emissive molecules76 or as host materials77 in OLED devices, introducing iptycene units into azaacenes is rare. Very recently, Bunz’s group synthesized three new compounds 52-54 (Figure 9) containing a combination of heteroacene and triptycene units.78 The film containing compound 53 showed the highest fluorescence quantum yield. The OLED performance based on compound 53 (turn-on voltage 3 V, luminance >200 cd/m2 at 7 V) was significantly improved compared to the previously-reported structurally-similar tetracenes79 (turn-on voltage 5 V, luminance ~1 cd/m2 at 12 V). In 2012, our group prepared diazapentacene (57) through a simple condensation between a pyrene diamine (56) and a pyrene diketone (55) (Figure 10).80 The fluorescence quantum yield of compound 57 was 0.59 in CH2Cl2. OLED devices with different doping concentrations (2%, 4% and 40%) of compound 57 in a non-emissive matrix have been fabricated. The best device achieved a brightness of 165 cd m-2 with a current density of 100 mA cm-2 at 15 V (Figure 10). The external quantum efficiency (EQE) dropped consistently as the doping concentration increased, which might be caused by greater intermolecular interactions. In acenes, phenyl substitution has been demonstrated to enhance the stability and solubility as well as reduce intermolecular interactions.41-46 Based on this idea, we constructed two azatetraacenes through [4+2] cycloaddition reactions.81 Interestingly, the diazatetracene (30) shows very low fluorescence while monoazatetracene (58) displays strong fluorescence (Фf = 1) in CH2Cl2, which is higher than that of the classic tetracene rubrene (Figure 11).82 The OLED device based on 58 showed a maximum current efficiency of 6.6 cd A-1 15



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with a turn-on voltage of 5.3V.

Scheme 11. The structures of compounds 49a-e

Table 1. Electroluminescent device properties of dipheylanthrazolines 49a-e. Reprinted with permission from ref 74. Copyright 2003 American Chemical Society. LED a

configuration

Turn-on

Maximum

voltage, V

brightness, 2

cd/m (Vmax)

Current

EQE%

maximum, b

2

mA/cm

MEH-PPV

4

53

500

0.06

49a

16

7

124

0.002

49b

14

32(20)

135

0.006

Power

Device

efficiency

efficiency,

Im/W

cd/A

0.3

1.8

49c

7.5

82(14)

174

0.045

49d

5

46(8)

306

0.040

49e

6.5

27(11)

500

0.008

PVK/49b

8

122(13)

200

0.06

PVK/49d

8

133(13)

229

0.07

MEH-PPV/49a

7

690(18)

43

1.3

MEH-PPV/49b

7

595(18)

15

3.1

2.0

7.0

MEH-PPV/49c

7

579(20)

14.5

2.9

0.9

5.0

MEH-PPV/49d

6

965(17)

116

1.1

0.2

0.8

MEH-PPV/49e

10

468(18)

44

0.6

0.2

1.1

a

ITO/PEDOT/MEH-PPV/Al, ITO/PEDOT/49/Al, ITO/PEDOT/PVK/49b(or49d)/Al, and

ITO/PEDOT/MEH-PPV/49/Al. b Brightness at maximum bias voltage.

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Figure 8. (A) the structures of compounds 50a-g and 51a-g; (B). the device structures of vacuum deposited OLEDs and solution processed OLEDs; (C) Typical JVL polts of OLEDs comprising solution processed 51b (left) 51c (right) emitters in comparision with their vacuum deposited counterparts. Reprinted with permission from ref 75. Copyright 2013 Royal Society of Chemistry.

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Figure 9. (A) The synthetic route to compounds 52-54; (B) the device structure for OLED; (C) The Luminance-voltage (red) and current density-voltage (green) plots of an OLED with 53 as the emitter. Reprinted with permission from ref 78. Copyright 2015 American Chemical Society.

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Figure 10. (A) The synthetic route to compound 57; (B) The Luminance-voltage and current density-voltage plots with different concentrations; (C) The EQE versus current density of the OLED devices. Reprinted with permission from ref 80. Copyright 2012 Wiley-VCH Verlag GmbH and Co.

Figure 11. The structure of compound 58 and its OLED performance. Reprinted with permission from ref 81. Copyright 2015 Elseiver Ltd.

3.3 Applications in organic memory Data storage in organic resistive switching memories is based on a reversible switching between low and high conduction states of the organic active layer, where both states should be stable enough to maintain the information long enough to meet the requirements for a nonvolatile memory.83, 84 Unfortunately, using larger oligoacenes as single active layers seems impractical because most of them are not stable in their low resistance (high conduction) states. The introduction of nitrogen atoms into the backbone of acenes can improve the stability of the molecules in both the ground state and the charged state.19,20 Moreover, both the extended π-conjugated structures and N-H hydrogen bond between neighboring molecules can enhance the 19



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orderly stacking of molecules in solid-state thin films, which may facilitate the charge carrier transport. The N-heteroacenes were firstly investigated by our group as active materials in memory device. Using dioxatetraamine (10) as a building block, two novel heteroacenes 24f and 24g, which contain two different types of heteroatoms O and N, were prepared in 2013. Sandwich-structured memory devices using 24f and 24g as active layers have been fabricated. Both devices showed a typical bipolar resistive switching (RS) behavior in both positive and negative regions (Figure 12).39 Employing diamine (6) as the building block, a novel N-substituted heteroacene 2-(4'-(diphenylamino) phenyl)-4,11-bis((triisopropylsilyl)ethynyl)-1H–imidazo[4,5-b] phenazine (59)85 was easily obtained, in which triphenylamine acts as a donor and the azaacene framework behaves as an acceptor. The sandwich-structure memory devices showed good persistent memory behavior with a high ON/OFF ratio of 103 and good retention of performance in each state (Figure 13). When one more triphenylamine donor was added, the as-prepared tetraazatetracene derivative 2,3-[4,4’-bis(N,N-diphenylamino)benzyl]-5,12-bis[(triisopropylsilyl)ethynyl]-1,4,6,11 -tetraazatetracene (61) displayed rewritable multilevel memory behavior (Figure 14),86 which is probably induced by multielectron intramolecular charge transfer. In order to further achieve ultrahigh density memory devices with the capacity of 3n or larger, we designed a novel larger heteroacene (62) containing nine linearly-fused rings with two electron-withdrawing parts (cyano and pyrazine) (Figure 15),87 which might exhibit multilevel stable conducting states in response to the applied voltage because of the different electron-withdrawing abilities between cyano groups and pyrazine species. In fact, the sandwich-structure memory devices based on 62 exhibited excellent ternary memory behaviors with high ON2/ON1/OFF current ratios of 106.3/104.3/1 and good stability for these three states. This device exhibits a typical write-once read-many-times (WORM) behavior (Figure 15). Moreover, we also successfully combined oxacalix[4]arene and N-heteroacene into one molecule (63), where this as-prepared compound has two different types of electron-withdrawing groups (nitro and pyrazine) (Figure 16).88 The sandwich structure memory devices based on compound 63 were fabricated, which exhibited excellent memory behavior with high ON2/ON1/OFF current ratios of 108.7/104.2/1, low switching threshold voltage of -1.80 V/ -2.87 V, and good stability in all three states (Figure 16).

20


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Figure 12. (A) The molecular structures of compounds 24f and 24g; (B) The device structure;. (C) Bipolar resistive switching (RS) behavior of compounds 24f and 24g; (D) Write-read-erase-read operation of as-fabricated organic RRAM device based on 24f. Reprinted with permission from ref 39. Copyright 2013 American Chemical Society.

21



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Figure 13. (A) The synthesis of compound 59; (B) The memory device structure; (C) Current-Voltage (I-V) characterization of the device; (D) Endurance performance of the devices; (E) Rentention performance of the devices. Reprinted with permission from ref 85. Copyright 2014 Wiley-VCH Verlag GmbH and Co.

Figure 14. (A) The synthesis of compound 61; (B) Current-Voltage (I-V) characterization of the device; (C) Endurance performance of the devices. Reprinted with permission from ref 86. Copyright 2014 Wiley-VCH Verlag GmbH and Co.

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Figure 15. (A) The synthesis of compound 62; (B) The memory device structure; (C) Current-Voltage (I-V) characterization of the device; (D) Stability of the device in three states under a constant “read” voltage of -1 V; (E) Stimulus effect of read pulse of -1 V on the ON, intermediate, and OFF states. Reprinted with permission from ref 87. Copyright 2013 American Chemical Society.

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Figure 16. (A) The synthetic route to compound 63; (B) The structure of memory device; (C) Tapping-mode (5μ 24


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m × 5μm ) AFM topography and typical cross-section profile of an AFM topographic image of the compound 63 film on ITO substrate. (D) and (E) Current-Voltage (I-V) characterization of the device (ITO/63/Al and ITO/63/LiF/Al, respectively); (F) and (G) Stability of the device in three states under a constant “read” voltage of -1 V. Reprinted with permission from ref 88. Copyright 2014 Royal Society of Chemistry.

3.4 Applications in phototransistors A phototransistor is a light-sensitive transistor device, which uses light instead of a gate potential to turn the device’s signal on or off.89,90 Our group firstly demonstrated that azaacenes can be used as active materials in phototransistor devices. Single crystals of a pyrene-fused tetraazaheptacene (64) have been obtained through a PVD method and the crystal structure analysis shows that all carbon and nitrogen atoms in compound 64 are in one plane. Such geometries might benefit for electron or hole transport. Phototransistor devices based on single crystals of compound 64 were fabricated which displayed very good performance in signal amplification under illumination (the so-called photoconductive effect) (Figure 17).91 More importantly, all the phototransistors showed reversible and reproducible performance.

25



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Figure 17. (A) The synthetic route to compound 64; (B) The crystal structure of compound 64; (C) Plot of current (IDS) vs voltages under different light intensities; (D) Dependence of IDS on light intensity at specific voltages; (E) Dependence of IDS on different input light intensities; (F) Time dependence of dynamic photoresponse behavior of compound 64; (g) Stability of single crystal based on phototransistors; (H) The device structure. Reprinted with permission from ref 91. Copyright 2012 American Chemical Society. 26


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3.5 Applications in photoelectrochemical cell Photoelectrochemical cells have been widely used to study inorganic semiconductors.92-95 Logically, such cell should also be powerful tools to probe the properties of organic semiconducting materials. In our group, 1,4,6,8,11,13-hexazapentacene (HAP, 65) has been synthesized and used as an active element in a photoelectrochemical cell.96 The photoelectrochemical tests of HAP were performed in a 22.5 ml ES (extrasil) quartz cell filled with 0.5 M sodium phosphate buffer solution (pH = 7), using an electrochemical workstation (CHI 760E). The photocurrent tests were carried out using a two-electrode set-up, in which, the working electrode was connected with HAP/FTO, while both counter and reference electrodes were connected with a Pt plate counter electrode (Figure 18B). The photocurrent responses were recorded using the amperometric technique under zero-biased (short-circuited) conditions. Figure 18C shows the photocurrent profile of the HAP/FTO electrode recorded under zero-bias (two-electrode, short-circuit) conditions, indicating that HAP is active under visible light (λ> 400 nm) illumination. The repeatable anodic (positive) photocurrent (about 40 nA cm-2) suggests that HAP (65) is an n-type semiconductor. As shown in the inset of Figure 18C, HAP/FTO cells also showed highly repeatable photovoltage (illuminated open-circuit potential) responses (about 10 mV) during the on–off cycles of illumination throughout the measurement.

Figure 18. (A) The synthesis of compound 65; (B) The two-electrode set-up for the photocurrent measurements of HAP/FTO photoanode; (C) Zero-bias photocurrent response of 65 as electrode upon chopped AM 1.5G light illumination (inset shows the photovoltage response). Reprinted with permission from ref 96. Copyright 2014 Royal Society of Chemistry.

3.6 Applications in Solar Cells In 2011, we reported

a

novel,

stable,

27


blue

heteroacene,


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2-methyl-1,4,6,7,8,9-hexaphenylbenz(g)isoquinolin-3(2H)-one (39),64 which has a relatively small π framework and an maximum absorption at 620 nm which is red-shifted compared to that of pentacene (λmax = 582 nm). Interestingly, unlike many similar acenes compound 39 is stable towards light and air, and its solutions showed no noticeable photobleaching when stood under ambient conditions for several days, which suggested it might be a good candidate for solar cells. A simple heterojunction photovoltaic device using 39 as an electron donor and [6,6]-phenyl-C61 butyric methyl ester (PCBM) as an electron acceptor was fabricated. Figure 19B shows that no current response was observed in the dark. However, under the simulated solar illumination (100 mWcm-2 1.5 AM), the devices display an open-circuit voltage (Voc) of 0.35 V, short-circuit current density (Jsc) of 0.27 mAcm-2, and a fill factor (FF) of 0.24 (Figure 19B). Although a relatively-low power conversion efficiency (~0.08%) was observed without further device optimization, our results demonstrated that azaacenes could have some potential applications in solar cells. However, in order to further improve the efficiency of azaacene-based solar cells, more efforts to improve the molecular design need to be conducted.

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Figure

19.

(A)

The

synthetic

route

to

compound

39;

(B)

The

J-V

characteristic

of

an

ITO/PEDOT:PSS/BIQ:PCBM/Al OPV device (Inset: schematic illustration of the solar-cell structure). Reprinted with permission from ref 64. Copyright 2010 Wiley-VCH Verlag GmbH and Co.

3.7 Applications as molecular conductors In 2006, Tadokoro and co-workers reported a conductive metal-coordination polymer based on tetraazanaphthacene (68)97 (Figure 20). In this polymer, tetraazanaphthacene formed radicals to coordinate with Cu ions (mixed valence Cu+/Cu2+) to produce a conductive metal-organic polymer with a conductivity as high as 50 S cm-1 at room temperature. Future research through replacing Cu with magnetic metal ions (Co, Ni, Fe) might generate molecular conductors with strong magnetic properties.

Figure 20. (A) The structure of compound 68 (top) and the crystal structure of 68 and Cu+ (bottom); (B) 29



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Temperature-dependent conductivity of 68 at ambient pressure using the typical four-probe dc method. Reprinted with permission from ref 97. Copyright 2010 Wiley-VCH Verlag GmbH and Co

4. CONCLUSION ADN OUTLOOK Azaacenes, especially azapentacenes, have been intensively investigated over the past decades to elucidate their structure-property relationships. Several groups have previously reviewed the synthesis and OFET properties of azaacenes whereas we have focused on the new approches on the synthesis of linear azaacenes since 2013, and the novel applications of azaacenes beyond FETs. In the field of azaacenes, the coupling reaction between diamines and dichlorides, which was first investigated by Bunz’s group, is one of the most promising methods to obtain larger, stable azaacenes (up to seven fused six-member rings). However, it is still very challenging to make larger azacenes (n>6) because of their poor solubility and extreme unstability towards dimerization and/or protonation. Thus, it remains highly desirable to develop novel methods to construct longer azaacenes. In our research, the classic “clean reaction” strategy has been used to make relatively small azacenes. In order the related building blocks needed for making larger azacenes have been successfully prepared and the synthesis of the acenes is being investigated. Nowadays, to date the applications of azaacenes have mainly focused on OFETs. The azapentacene 35 has shown an excellent electron mobility up to 3.3 cm2 V-1 S-1. However, there exists great scope to investigate other possible applications of azaacenes including solar cells, memories, OLEDs, phototransistors, conductors, photoelectrochemical cells, and sensors. This review has sumarized recent research progress in these directions. We hope that this review will not only be very useful in helping others to understand the relationships between the structures and their properties, but also could be instructive to anyone seeking to design and prepare novel azacenes with enhanced physical properties. According to the evidence from both theoretical calculations and experimental findings, oligoazaacenes and polyazaacenes are electron-deficient systems and the solubility become poorer and poorer with the increased length.98,99 These properties together with their large π-conjugated units, rich nitrogen heteroatoms, and multi-ring aromatic system, could make them good organic anode candidates for lithium-ion batteries (LIBs).100 Opportunities lie for preparing stable and processable oligoazaacenes and polyazoacenes, developing new methods to approach these oligoacences and polyazaacenes for devices applications, and understanding how the number and position of N atoms affect the affinity of lithium ions and the performance of organic devices. Moreover, replaceing CH groups in the backbone of polycyclic aromatic carbons with different types of heteroatoms is also very promising to ehance their performance in organic semicondcutor devices.101,102 AUTHOR INFORMATION Corresponding Author: *Phone: +65-67904705 E-mail address: [email protected]

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ACKNOWLEDGEMENT We thank A/P Andrew Grimsdale for his helpful comments on the manuscript. Q.Z. acknowledges financial support from the Singapore Ministry of Education through the Academic Research Fund Tier 1 (RG 16/12 and RG 133/14) and Tier 2 (ARC 20/12 and ARC 2/13) grants, and the CREATE program (Nanomaterials for Energy and Water Management) funded by the National Research Foundation, Singapore. REFERENCES (1) Clar, E. Polycyclic Hydrocarbons; Academic press: London, 1964; vol 2, pp 206-218 (2) Anthony, J. E. Functionalized Acenes and Heteroacenes for Organic Electronics. Chem. Rev. 2006, 106, 5028–5048. (3) Anthony, J. E. The Larger Acenes: Versatile Organic Semiconductors. Angew. Chem. Int. Ed. 2008, 47, 452–483. (4) Bendikov, M.; Wudl, F.; Perepichka, D. F. Tetrathiafulvalenes, Oligoacenes, and Their Buckminsterfullerene Derivatives: The Brick and Mortar of Organic Electronics. Chem. Rev. 2004, 104, 4891–4945. (5) Sokolov, A. N.; Tee, B. C. K.; Bettinger, C. J.; Tok, J. B. H.; Bao, Z. N. Chemical and Engineering Approaches to Enable Organic Field-Effect Transistors for Electronic Skin Applications. Acc. Chem. Res. 2012, 45, 361–371. (6) Tang, M. L.; Reichardt, A. D.; Wei, P.; Bao, Z. N. Correlating Carrier Type with Frontier Molecular Orbital Energy Levels in Organic Thin Film Transistors of Functionalized Acene Derivatives. J. Am. Chem. Soc. 2009, 131, 5264–5273. (7) Xu, Q. F.; Duong, H. M.; Wudl, F.; Yang, Y. Efficient Single-Layer “Twistacene”-Doped Polymer White Light-Emitting Diodes. Appl. Phys. Lett. 2004, 85, 3357–3359. (8) Chun, D.; Cheng, Y.; Wudl, F. The Most Stable and Fully Characterized Functionalized Heptacene. Angew. Chem. Int. Ed. 2008, 47, 8380–8385. (9) Wantanabe, M.; Chang, Y. J.; Liu, S. W.; Chao, T. H.; Goto, K.; Islam, M. M.; Yuan, C. H.; Tao, Y. T.; Shinmyozu, T.; Chow, T. J. The Synthesis, Crystal Structure and Charge-Transport Properties of Hexacene. Nat. Chem. 2012, 4, 574–578. (10) Payne, M. M.; Parkin, S. R.; Anthony, J. E. Functionalized Higher Acenes: Hexacene and Heptacene. J. Am. Chem. Soc. 2005, 127, 8028–8029. (11) Kaur, I.; Stein, N. N.; Koperski, R. P.; Miller, G. P. Exploiting Substituent Effects for the Synthesis of a Photooxidatiely Resistant Heptacene Derivative. J. Am. Chem. Soc. 2009, 131, 3424–3425. (12) Purushothaman, B.; Bruzek, M.; Parkin, S. R.; Miller, A. F.; Anthony, J. E. Synthesis and Structural Characterization of Crystalline Nonacenes. Angew. Chem. Int. Ed. 2011, 50, 7031–7017. (13) Li, J. B.; Zhang Q. C. Mono- and Oligocyclic Aromatic Ynes and Diynes as Building Blocks to Approach Larger Acenes, Heteroacenes, and Twistacenes. Synlett. 2013, 24, 686–696. (14) Qu, H. M.; Chi, C. Y. Synthetic Chemistry of Acenes and Heteroacenes. Curr. Org. Chem. 2010, 14, 2070–2108. (15) Ye, Q.; Chi, C. Y. Recent Highlights and Perspectives on Acene Based Molecules and Materials. Chem. Mater. 2014, 26, 4046–4056. (16) Jiang, W.; Li, Y.; Wang, Z. H. Heteroarenes as High Performance Organic Semiconductors. Chem. Soc. Rev. 2013, 42, 6113–6127. (17) Stolar, M.; Baumgartner, T. Phosphorus-Containing Materials for Organic Electronics. Chem. Asian. J. 2014, 9, 1212–1225. 31



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and

Optoelectronic

Properties

of

a

New

Twistacene

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Linearly Fused Azaacenes: Novel Approaches and New Applications

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