Linear Oligoarylamines: Electrochemical, EPR, and Computational

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Linear Oligoarylamines: Electrochemical, EPR, and Computational Studies of Their Oxidative States Hsu-Chun Cheng, Kuo Yuan Chiu, Shih-Hua Lu, Ching-Chin Chen, Yen Wei Lee, Te-Fang Yang, Ming Yu Kuo, Peter Ping-Yu Chen, and Yuhlong Oliver Su J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp512793a • Publication Date (Web): 09 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Linear Oligoarylamines: Electrochemical, EPR, and Computational Studies of Their Oxidative States Hsu-Chun Cheng,† Kuo Yuan Chiu,† Shih Hua Lu,† Ching-Chin Chen,‡ Yen Wei Lee,† Te-Fang Yang,*,† Ming Yu Kuo,*,† Peter Ping-Yu Chen,*,‡ Yuhlong Oliver Su*,†,‡ †

Department of Applied Chemistry, National Chi Nan University, 1 University Road, Puli, Nantou 545, Taiwan

‡

Department of Chemistry, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan Email: [email protected], [email protected], [email protected], [email protected] TEL: (+886)-49-2910960 ext. 4146 (Ming Yu Kuo)

1

ACS Paragon Plus Environment


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ABSTRACT A series of straight-chain oligoarylamines were synthesized and examined

by

electrochemical,

spectroelectrochemical,

electron

paramagnetic

resonance techniques and density functional theory (DFT) calculation. Depending on their electrochemical characteristics, these oligoarylamines were classified into two groups: one containing an odd number and the other an even number of redox centers. In the systems with odd redox centers (N1, N3, and N5), each oxidation was associated with the loss of one electron. As for the systems with even redox centers (N2, N4 and N6), oxidation occurred by taking N2 as a unit. Absorption spectra of linear oligoarylamines at various oxidative states were obtained to investigate their charge transfer behaviors. Moreover, DFT-computed isotropic hyperfine coupling constants and spin density were in accordance with the EPR experiment, and gave a close examination of oligoarylamines at charged states.

KEYWORDS: oligoarylamine, oxidative states, electrochemical, density functional theory, spectroelectrochemical

2


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INTRODUCTION Triarylamines have been investigated extensively as hole transport materials since the first report of the multilayered organic light-emitting diode (OLED) module made by Tang and Van Slyke.1 Because of the facile generation of their cation radicals and stability of charged states, numerous systems that included several kinds of triarylamine units in their oligomeric and dendritic architectures were explored.2–10 Furthermore, those oxidized triarylamine systems were classified mainly into two different functional categories. Some investigations are typical examples of studies of organic mixed-valence molecules as hole transport materials to examine the intramolecular charge transfer (ICT) process,11-12 and some are those of high-spin organic polyradicals to elucidate their stable multiredox properties of the magnetic triarylamine derivatives.13–21 In comparison with the above, both these two cases incorporated “branching points” within dendrimers to link each triarylamine unit. However, the branching points in the high-spin organic polyradicals were located at the ortho or meta positions of a phenyl group to avoid the formation of a closed-shell electronic configuration resulting from the efficient charge transport and to generate multiradical systems. On the other hand, conjugation groups linked at the para position of a phenyl group as an efficient bridge within the ICT cases have been utilized to observe the electron transfer process.11-12 Elucidation of intramolecular 3



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charge transfer is required to shed light on the mechanism. In this study, we synthesized successfully a series of straight-chain oligoarylamines composed of one to six nitrogen redox centers (Scheme 1). Although straight-chain compounds possessing the same or similar moieties (N1–N4 and N6) have been studied before,2,22 we reinvestigated them to exploit their electronic properties systematically and in detail, and compared their electrochemical properties with those of N5.

N

N

N

N

N

N3

N2

N1

N

N

N

N

N

N

N

N

N5

N4

N

N

N N

N

N

N

N6

Scheme 1. Chemical structures of N1-N6.

4


N

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Experimental Section Syntheses of N2, N3, and N4 were carried out according to the methodology reported in the literature.23-26 Synthesis of N2. To a solution of N,N′-diphenyl-phenylene-1,4-diamine (PD, 2 g, 7.7 mmol) and 4-iodobenzene (3.26 g, 16 mmol) in 1,2-dichlorobenzene (20 mL) was added Cu powder (0.25 g, 3.9 mmol), K2CO3 (0.54 g, 3.9 mmol) and 18-crown-6 ether (0.01 g). The reaction mixture was heated in a round bottom flask under nitrogen at 180 °C for 24 hours, and allowed to cool to room temperature. Then, the mixture was diluted with dichloromethane (100 mL) and filtered. The filtrate was concentrated under reduced pressure. The residue was purified with flash column chromatography (CH2Cl2: n-hexane = 1:1) to give N1,N1,N4,N4-tetraphenylbenzene-1,4-diamine (N2, 2.54 g, 80%). 1H NMR（CDCl3 / ppm ）：δ 7.12 (4H, t), 7.26 (8H, t), 7.02 (8H, d), 6.97 (4H, d). Synthesis of N2-Br. To a solution of N2 (2.54 g, 6.16 mmol) in CHCl3 (100 mL) was slowly added NBS (1.30 g, 7.27 mmol). The reaction mixture was stirred at room temperature for 24 hours, and washed with water (50 mL x 2). The organic layer was then concentrated under reduced pressure. The residue was purified with flash column chromatography to produce N2-Br27-28 (2.72 g, 90%). UV/Vis (CH2Cl2) λmax/nm: 315;

5



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MS (FAB+): m/z 492.988 ([M]+) (N2-Br MW: 491.42); 1H NMR (300 MHz, CDCl3): δ 7.323 (2H, d), 7.247 (9H, d), 7.090 (6H, m), 6.967 (6H, m).29 Synthesis of N4. To a solution of N1,N4-bis(4-aminophenyl)-N1,N4-diphenylbenzene -1,4-diamine (N2-(NH2)2, 0.111 g, 0.25 mmol) in 1,2-dichlorobenzene (5 mL) was added Cu (0.191 g, 3 mmol), K2CO3 (0.346 g, 2.5 mmol), 18-crown-6 (0.10 g), iodobenzene (0.17 mL, 1.5 mmol). The reaction mixture was heated at 180 °C for 72 hours under nitrogen, and cooled to room temperature. The mixture was then filtered, and the filtrate was concentrated under reduced pressure. The residue was purified with flash column chromatography (CH2Cl2: n-hexane = 1:2) to give N4 (0.101 g, 54%). 1H NMR (300 MHz, CDCl3): δ 7.25 (t, 12H), 7.21 (t, 6 H), 7.08 (d, 12 H), 6.98 (d, 12H). Synthesis of N5. To a solution of aniline (0.075 g, 0.80 mmol) in toluene (50 mL) was added N2-Br (0.806 g, 1.65 mmol), Pd(OAc)2 (15 mg, 0.016 mmol), BINAP (18 mg, 0.032 mmol) and t-BuONa (160 mg, 1.66 mmol). The reaction mixture was stirred at 180 °C for 48 hours and cooled to room temperature. The mixture was then filtered, and the filtrate was concentrated under reduced pressure. The residue was washed with methanol to give N5 (0.26 g, 29%). UV/Vis (CH2Cl2) λmax/nm: 327; IR (KBr): 3033, 1591, 1491, 1302, 1267 cm-1; MS (FAB+): m/z 914.049 ([M]+) (N5 MW: 914.063); 1H NMR (300 MHz, benzene-d6): δ 7.15-7.10 (m, 8H), 7.10-6.95 (m, 6


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35 H), 6.85-6.80 (m, 8H);

13

C NMR (75 MHz, benzene-d6): δ 148.8, 143.8, 143.6,

129.9, 126.3, 126.0, 125.9, 125.5, 124.6, 124.1, 123.1, 122.9. Anal. Calcd for C66H51N5: C, 86.72; H, 5.62; N, 7.66. Found: C, 86.69; H, 5.63; N, 7.60. Synthesis of N6. To a solution of PD (0.213 g, 0.82 mmol) in toluene (50 mL) was added N2-Br (0.806 g, 1.65 mmol), Pd(OAc)2 (15 mg, 0.016 mmol), BINAP (18 mg, 0.032 mmol) and t-BuONa (160 mg, 1.66 mmol). The reaction mixture was stirred at 180 °C for 48 hours and cooled to room temperature. The mixture was then filtered, and the filtrate was concentrated under reduced pressure. The residue was washed with methanol to give N6 (0.35 g, 40%)22,26. UV/Vis (CH2Cl2) λmax/nm: 338; IR (KBr, cm-1): 3036, 1591, 1493, 1267 cm-1; MS (FAB+): m/z 1081.235 ([M]+) (N6 MW: 1081.35); 1H NMR (300 MHz, CDCl3): δ 7.26 (24H, t), 7.08 (16H, d), 6.99 (20H, m); 13

C NMR (75 MHz, CDCl3): δ 129.3, 125.2, 123.8; Anal. Calcd for C78H60N6: C,

86.64; H, 5.59; N, 7.77. Found: C, 86.59; H, 5.63; N, 7.64.

RESULTS AND DISCUSSION Scheme

2

shows

the

synthetic

protocols

of

N2-N6.

At

first,

N1,N4-diphenylbenzene-1,4-diamine (PD), 4-N-(4-aminophenyl)-4-N-phenylbenzene1,4-diamine (TPA-(NH2)2) , and N1,N4-bis(4-aminophenyl)-N1,N4-diphenylbenzene1,4-diamine

(N2-(NH2)2)

underwent

Ullmann-type

coupling

reactions

with

iodo-benzene, respectively, to provide compounds N2-N4. Treatment of N2 with NBS 7



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I N H2N

Cu / K2CO3

N3

18-crown-6 ether dichlorobenzene

NH2 TPA-(NH2)2

H2N I

N

Cu / K2CO3 N4

18-crown-6 ether dichlorobenzene N NH2 N2-(NH2)2

N

H

N

I Cu / K2CO3

N

18-crown-6 ether dichlorobenzene

N

H

PD

N2 Aniline (0.5 eq.), Pd(OAc)2/BINAP N5 N

t-BuONa, toluene

N

PD (0.5 q.), Pd(OAc)2/BINAP

NBS CHCl3

Br

t-BuONa, toluene

N6

N2-Br

Scheme 2. Synthesis of N2-N6.

furnished

N1-(4-bromophenyl)-N1,N4,N4-triphenylbenzene-1,4-diamine

(N2-Br).

Individual coupling reaction of N2-Br with aniline and PD in the presence of t-BuON

8


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and a catalytic amount of Pd(OAc)2 complex furnished compounds N2 and N6, respectively. The mass spectra were found to match the straight-chain molecules very well, and confirmed the exact structure of N5 and N6 (Tables S1 and S2). Electrochemical properties of N1–N6 were probed by cyclic voltammetry (CV) (Figure 1 and Table 1). Electrochemical oxidation of triphenylamine (N1) in organic solvents is well known to generate a dimer, tetraphenylbenzidine (TPB).30-31 An irreversible oxidation wave at Epa = +1.20 V and three reduction waves at Epc = –0.05, +0.80, and +1.00 V in the reversed scan were observed. After scanning one cycle, another oxidation wave grew at E1/2 = +0.85 V on account of TPB+● generation (Figure 1A). CV results of N2 exhibited two well-defined one-electron reversible redox couples N2/N2+● and N2+●/N22+ at E1/2 = +0.64 and +1.11 V (Figure 1B), respectively.27-29 Oxidation of N3 involved two reversible redox couples at E1/2 = +0.55 and +0.89 V, and one irreversible redox wave at Epa = +1.70 V (Figure 1C). CV results of N4 exhibited three reversible redox waves at E1/2 = +0.52, +0.80, and +1.29 V. Current of the third wave was nearly twice as large as those for the first and second waves (Figure 1D). The third redox wave appears to be an overlap of two redox couples, while the first two redox waves can be assigned to one-electron oxidations. For N5, CV yielded four reversible redox waves at E1/2 = +0.46, +0.67, +0.93, and +1.17 V, and one irreversible wave at Epc = 1.67 V (Figure 1E). Unlike N4, which has 9



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Figure 1. Cyclic voltammograms of 1.0×10-3 M (A) N1, (B) N2, (C) N3, (D) N4, (E) N5, and (F) N6 in CH2Cl2 containing 0.1 M TBAP. Scan rate = 0.1 V/s. Working electrode: GC (Fc+/Fc = +0.55 V).

Table 1. Electrode half potential (E1/2) for the oxidation of oligomers of aniline in CH2Cl2 containing 0.1 M TBAP. Scan rate = 0.1 V/s. vs. Ag/AgCl. Oxidation N1 N2 N3 N4 N5 N6 a

5th

4th

3rd

2nd

1st

---

---

---

---

+1.20 a

------+1.67 a

------+1.17

--+1.69 a +1.29 +0.93

+1.13 +0.89 +0.80 +0.67

+0.64 +0.55 +0.52 +0.46

---

+1.34 b

+0.99 b

+0.77 b

+0.49 b

Irreversible oxidation peak potential (Epa). b Potential obtained from OTTLE

experiment. 10


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two close redox couples, N5 has five well-separated redox centers. In the cyclic voltammogram of N6, CV patterns were similar to those of our previous results.22 Poor solubility affected the performance of the redox waves. Hence, we observed only four poorly resolved oxidative waves at Epa = +0.52, 0.80, 1.01, and +1.40 V. On the other hand, the unobvious reductive waves were at Epc = +0.44, 0.56, 0.74, 0.96, and +1.34 V (Figure 1F). By utilizing controlled potential coulometry (CPC) and differential pulse voltammetry (DPV) to find out the number of electrons involved in each oxidation, we ascertained that only one electron was lost in the first oxidation (Figure S1) and four redox couples were associated with six-electron oxidations.22 Wienk and Janssen conducted electrochemical studies of analogs, but the monomers (TPA) were linked at ortho and meta positions.32 As the ortho and meta positions in a phenyl group are less interactive than the para position, well-resolved redox couples could be observed at the former two sites. In our study, enhancing the number of conjugation systems led to not only a more negative first oxidative potential, due to steric effect2 but also complicated redox couples, due to effective interaction between them. In the previous work,2 N2–N4 systems were monitored in MeCN containing the electrolyte TBAPF6 and were found to exhibit sequential one-electron redox couples. As for our results, N4 had two different CV patterns depending on the chemical environment. It 11



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exhibited three redox waves in CH2Cl2/TABP and four in MeCN/TBAPF6. Solvation and supporting electrolyte are well-known important parameters that influence electrochemical behaviors.33 On the other hand, two electrochemical patterns from N2 to N6 were separated based on the number of odd and even redox centers present in a system. In the system with odd redox centers, electrons were lost one by one. Besides, the third redox potential of N3 and the fifth redox potential of N5 were observed to be over 1.60 V because intramolecular electrostatic repulsion at higher oxidative states obviously pushes the potentials to a more positive value. On the other hand, the systems with even redox centers exhibited different behaviors in the merging of higher oxidation waves. In order to investigate the charge distribution in oxidative states, the optically transparent

thin-layer

electrode

(OTTLE)

method

was

utilized.

The

spectroelectrochemical cell was composed of a 1 mm cuvette, a platinum gauze thin layer as working electrode, a platinum wire as auxiliary electrode, and Ag/AgCl, KCl (sat’d) reference electrode. With oxidation of N1 at +1.20 V, triphenylamine dimer dication (TPB2+) was produced by an irreversible EC reaction.30,34 The specific peaks of TPB at 310 and 352 nm were observed after conducting a spectroelectrochemical experiment. This indicates that the electrochemical process is TPA→TPB2+→TPB+→ TPB (Figure S2).30-31 12


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Spectral changes of N2 were obtained as shown in Figure 2. In the potential range Eappl. = +0.00 – +0.70 V, the absorbance at 312 nm decreased gradually, and new bands grew at 408 and 856 nm, corresponding to N2+●. The intervalence charge transfer band (IV-CT band) at 856 nm was attributed to the interaction between one oxidized redox center and another neutral redox center (Figure 2A). As the applied potential was increased from +1.05 to +1.19 V, the spectrum of N2 dication was obtained (Figure 2B). The result is consistent with those reported in the previous literature.30

Figure 2. Spectral change of compound N2 during the (A) first and (B) second oxidation at various applied potentials in CH2Cl2 containing 0.1 M TBAP.

As for compound N3, the absorbance at 318 nm decreased monotonously, and new bands were formed at 434, 690, and 1224 nm (Figure 3A) in the potential range of Eappl. = +0.00 – +0.61 V, corresponding to the cation radical N3+●. When the applied potential was changed from +0.84 to +0.96 V, bands of the cation radical 13



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N3+● at 434 and 1224 nm disappeared gradually, while new bands grew at 642 and 972 nm (Figure 3B). The lower-energy band of N3+● at 1224 nm (1.01 eV) changed to a high-energy band at 972 nm (1.28 eV) with an isosbestic point at 1118 nm. The positive charge distributed mainly on the central moiety in N3+● then was suggested to be transferred to the outer triarylamine moieties in N32+. The spectral change is due to the electrostatic repulsion of the redox centers.9,17

Figure 3. Spectral changes of compound N3 during the (A) first and (B) second oxidation at various applied potentials in CH2Cl2 containing 0.1 M TBAP.

An investigation of the spectral change of N4 in the potential range Eappl. = +0.00 – +0.75 V showed that the first oxidation results in absorbance decreased at 326 nm and increased at 444, 652, 860, and 1378 nm (Figure 4A). After changing the applied potential from +0.80 to +1.15 V, the absorption bands at 306 and 444 nm decreased in absorbance, but those at 388, 684, 932, and 1284 nm grew at the same time (Figure 4B). In the applied potential range of +1.15 – +1.55 V, the absorbance at 14


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Figure 4. Spectral changes of compound N4 during the (A) first, (B) second, (C) third, and (D) fourth oxidation at various applied potentials in CH2Cl2 containing 0.1 M TBAP.

388 and 1284 nm decreased, and that at 272, 348, and 932 nm increased (Figure 4C). Unlike in case of N4/N4+● and N4+●/N42+ transitions, spectral changes shown in Figures 4C and 4D do not have well-defined isosbestic points. This can be attributed to the mixed absorption due to the overlap of N42+/N43+ and N43+/N44+ redox couples, indicating that no clear transition occurs between two species. When the applied potential was higher than Eappl. = +1.60 V, the absorbance of the bands decreased monotonously and became featureless (Figure 4D). The changed ICT bands from 15



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1378 nm (0.90 eV) in N4+● to 1284 nm (0.97 eV) in N42+ with an isosbestic point at 1475 nm and those from 1284 nm (0.97 eV) in N42+ to 932 nm (1.33 eV) in N43+ with an isosbestic point at 1060 nm can be considered, respectively, as the positive charge distribution in N4+/N42+ and N42+/N43+ transitions to minimize electrostatic repulsion.17 Furthermore, we suppose that the spectral change in Figure 4C was in agreement with the third redox couples containing two close redox reactions in the CV patterns of N4. Figure 5 shows the spectral changes in which a band of neutral N5 at 328 nm decreased and two new bands grew at 446 and 1490 nm, corresponding to a cation radical of N5, in the potential range of 0.00 – 0.58 V (Figure 5A). In the potential range of +0.58 – +0.80 V, the band at 314 nm decreased further, and two bands grew at 410 and 1584 nm (Figure 5B). After applying a more positive potential of up to +0.96 V, the absorbance at 296, 410, and 1584 nm decreased gradually, while new bands grew at 296, 906, and 1374 nm, with an isosbestic point at about 1460 nm (Figure 5C). In the potential range of +0.96 – +1.23 V, the absorbance at 278 and 1374 nm decreased and that at 416 and 896 nm increased (Figure 5D). After applying a potential of +1.76 V, all absorption peaks decreased monotonously (Figure 5E). The changed ICT bands from 1584 nm (0.83 eV) in N52+ to 1374 nm (0.90 eV) in N53+ with an isosbestic point at 1420 nm and those from 1374 nm (0.90 eV) in N53+ to 896 16


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Figure 5. Spectral changes of compound N5 during oxidation of the (A) first, (B) second, (C) third, (D) fourth, and (E) fifth electron at various applied potentials in CH2Cl2 containing 0.1 M TBAP.

nm (1.38 eV) in N54+ with an isosbestic point at 982 nm were obtained after two redox reactions. This indicated that after its first two oxidations in the central moiety, N5 was then transferred to the outer moieties after losing three or four electrons. 9,17 17



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The spectral changes of N6 with applied potential were similar to our previous results.22 As the potential was applied from +0.00 to +0.57 V for N6, the specific band at 338 nm decreased, and new bands grew at 314, 438, and 1420 nm (Figure S3). The ICT band appeared in the first oxidation, indicating a strong interaction between the oxidized redox center and neutral redox centers. In the potential range of +0.70 – +0.90 V, the absorption band at 314 nm diminished gradually, while new bands grew at 396 and 1320 nm (Figure S3B). By applying more positive potentials up to +1.10 V, the absorbance at 1320 nm decreased and new bands appeared at 630 and 910 nm (Figure S3C). In the potential range of +1.25 – +1.50 V, all absorption bands at 430, 590, and 910 nm decreased at the same time (Figure S3D). By the OTTLE experiments, the formal potentials for N6 were estimated to be at +0.49, +0.77, +0.99, and +1.34 V. In high valence states, electrostatic repulsion occurred, followed by the distribution of positive charge from central to outer moieties. 9,17 To understand further the electronic properties of the oxidation states of N2–N6, the EPR spectra of the oxidized N2–N6 were elucidated (Figure 6). The oxidation reagent, silver perchlorate (AgClO4), in CH2Cl2 (0.50 equiv) was used to monitor the radical state of the cation. As shown in Figure 6, the EPR signal displayed a distinct splitting at g ~ 2.0034, corresponding to the formation of common organic molecule radicals. The EPR simulation, highlighted by the red line, was also obtained with 18


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Figure 6. Room-temperature EPR spectra of N2–N6 in CH2Cl2 by experiment (black line) and digital simulation (red line).

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involvement of the nitrogen nuclei in the hyperfine coupling constant (HFCC). N2+● and N3+● exhibit the same results as those in the literature,

9,17

whereas N4+●, N5+●,

and exhibit different patterns . It is noteworthy that the EPR spectrum of N3+● exhibits a similar pattern as that of N5+●. Different HFCCs (ɑN) were determined from the simulations performed with the help of EasySpin, which are reported in Table 2. To our surprise, two decreasing trends in HFCC were identified when the number of odd or even nitrogen atoms was increased. HFCC is proportional to the spin density at the nucleus and directly related to electrons in the s orbital. We figured out the correlation between spin density and HFCC using the density functional theory (DFT) calculation. All calculations were carried out using the GAUSSIAN 03 suit of programs.35 Initially, the function B3LYP, coupled with the basic set 6-31G, provided

Table 2. HFCCs observed by EPR simulation compared to the DFT (B3LYP/6-31G*) calculated data under gas-phase condition. Nitrogen atom A Arylamine radical N2+● (g = 2.0030) N3+● (g = 2.0036) N4+● (g = 2.0038) N5+● (g = 2.0047) N6+● (g = 2.0017)

Expt. ɑ (mT)

DFT ɑ Spin (mT) density

0.56

0.54

0.2266

0.62

0.53

0.49

Nitrogen atom B

Nitrogen atom C

Expt. ɑ (mT)

ɑ (mT)

DFT Spin density

Expt. ɑ (mT)

ɑ (mT)

0.2208

0.28

0.30

0.1252

0.40

0.1645

0.09

0.18

0.0744

0.55

0.37

0.1515

0.27

0.28

0.41

0.29

0.1212

0.21

0.20

Spin density

0.1165

0.05

0.11

0.0481

0.0843

0.09

0.08

0.0350

20


DFT

Page 21 of 29

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gas-phase optimizations and frequency calculations.36-37 A single trend of HFCC and spin density on each cation radical state of N2–N6 was obtained as shown in Table 2, which was different from the experimental results. We did not consider the effect of solvation and basic set, especially on the isotropic HFCCs. In the solution systems, the computed isotropic HFCC depends not only on the geometry, method, and basis set but also on the solvation effect.30,38-39 Furthermore, because the errors due to method and basis set cannot be distinguished easily, systematic studies improving the treatment of correlation and increasing the completeness of the basis set are needed. We selected solvent effects (CH2Cl2) and the self-consistent reaction field (SCRF) method with a polarized continuum model (PCM), and sequentially used the DFT with B3LYP functional coupled with the following basis sets (lanl2dz, TZV, and 6-31G*). The result from functional B3LYP/6-31G* was the best fit of our experimental values, as shown in Tables 3 and S3. The cation radical of N2–N6 also behaved two types and probably determined the properties of the following redox couples. We could observe that not only cyclic voltammograms and absorption spectra, but also electron paramagnetic resonance indicated two types of electrochemical behaviors, from oxidation aspect. As seen in Figure 7, the spin density of N4–N6 for DFT calculation was in good accordance with the spin distribution. 21



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Page 22 of 29

Table 3. HFCCs observed by EPR simulation compared to the DFT (B3LYP/ 6-31G*), considering the solvent effect (CH2Cl2) and using the SCRF method with a PCM. Nitrogen atom A Arylamine radical N2+● (g = 2.0030) N3+● (g = 2.0036) N4+● (g = 2.0038) N5+● (g = 2.0047) N6+● (g = 2.0017)

Expt. ɑ (mT)

DFT ɑ Spin (mT) density

0.56

0.54

0.2248

0.62

0.57

0.49

Nitrogen atom B

Nitrogen atom C

Expt. ɑ (mT)

ɑ (mT)

DFT Spin density

Expt. ɑ (mT)

ɑ (mT)

0.2362

0.28

0.27

0.1150

0.43

0.1788

0.09

0.13

0.0573

0.55

0.44

0.1844

0.27

0.28

0.41

0.37

0.1519

0.21

0.17

DFT Spin density

0.1178

0.05

0.07

0.0280

0.0720

0.09

0.03

0.0149

Figure 7. DFT-computed spin density distribution of (a) N4+●, (b) N5+●, and (c) N6+● (blue: positive spin; green: negative spin; UB3LYP/6-31G).

22


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CONCLUSIONS We have synthesized successfully a series of straight-chain oligoamines and reported the electrochemical, spectroelectrochemical, EPR, and DFT computation results related to their oxidized states. The oligoamines were classified into two categories based on their electrochemical behaviors. The systems with odd redox centers (N3 and N5) were oxidized first on the central moiety, and sequentially lost electron one by one. Those with even redox centers (N2, N4, and N6) were oxidized by N2 skeleton as one unit, which resulted in the merging of oxidation waves. Furthermore, although only one electron was lost for each oxidation, charge was delocalized on the whole linear molecule. Positive charge distribution decreased progressively from the center to the periphery, and was transferred to the periphery moiety in high-valence states. EPR results and DFT calculation suggest that solvation is the major cause of two different oxidative behaviors. The two types of electrochemical behaviors (exhibited by the systems with odd or even redox centers) appeared after the first oxidation. These unusual characteristics provide a strategy to design functional oligoarylamines in the future.

ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology, R.O.C. (NSC 100-2113-M-260-001-MY3, NSC 99-2811-M-260-006), for supporting this work. 23



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SUPPORTING INFORMATION The Supporting Information contains calculated energy results of compounds N2–N6, 1

H and

13

C NMR spectra of N5, mass spectra of N5 and N6, CPC of N6, and

spectroelectrochemical results of N1. This material is available free of charge via the Internet at http://pubs.acs.org/.

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TOC graphic different electrochemical behaviors odd redox centers

even redox centers N2

N

N N

N3

N

N

N

N

N

N

N4 N

N5

N

N6

29


N

Linear Oligoarylamines: Electrochemical, EPR, and Computational

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