Synthesis, Photosensitive and Electrochemical Properties of Asymmetric Liquid Crystals Based on Tri-ring Azo-benzoic Acid†

doi:10.7503/cjcu20141138

Abstract

Abstract:

Eight new asymmetric liquid crystals based on azo benzoic acid of two series were prepared, in which 4-alkyl(Me, Et or n-Pr) cyclohexyl carboxylic acid(3a—3c) or 4-alkyl(Me, Et, n-Pr, n-Bu or n-Pe)ben-zoic acid(3d—3h) were esterified with 4'-hydroxy azo benzoic-4-acid. A simple one-step reaction was developed to esterify only the hydroxyl of 4'-hydroxy azo benzoic-4-acid to synthesize target compounds at room temperature. The structure characterizations of all related products were performed using infrared spectrometer(IR), ¹H-nuclear magnetic resonance(¹H NMR), mass spectrometer(MS) and elementary analysis. The liquid crystalline and photosensitive properties were investigated by differential scanning calorimeter(DSC), hot stage polarizing optical microscope(HS-POM) and ultraviolet-visible spectrophotometer(UV-Vis). The HOMO, LUMO energy levels and the energy gaps(E_g) of the compounds were calculated by cyclic voltammetry(CV). All the 8 compounds had liquid crystalline properties and exhibited photosensitivities in methanol solution and in mesophase by irradiation of UV-light. There is an effect of odd-even carbon number alkyl chains on the second peak(π-π^* band of azo) in UV spectra and on energies of frontier molecular orbital was observed. The compound with odd carbon number alkyl chain has lower E_g than its neighboring ones with even carbon number alkyl chain. The result showed that the change tendencies of E_g were in excellent agreement with these of the absorption wavelength in UV spectra. The less was the E_g, the longer was the absorption wavelength(lower energy) of the second peak, and vice versa.

Key words: Liquid crystal, Azo compound, Photosensitivity, Frontier molecular orbital

CLC Number:

O625.65

TrendMD:

ZHENG Minyan, WEI Yongsheng, GENG Wei. Synthesis, Photosensitive and Electrochemical Properties of Asymmetric Liquid Crystals Based on Tri-ring Azo-benzoic Acid^†[J]. Chem. J. Chinese Universities, 2015, 36(5): 899.

Figures/Tables 9

Scheme 1 Synthetic process of target compounds 3a—3h

Table 1 Appearance, yields, melting points, IR, MS data and elemental analysis of target compounds

Compd.	Appea- rance	Yield (%)	m.p./℃	IR(KBr), $ν ˙$ /cm^-1	MS([M⁺], calcd.), m/z		Elemental analysis(%, calcd.)
Compd.	Appea- rance	Yield (%)	m.p./℃	IR(KBr), $ν ˙$ /cm^-1	MS([M⁺], calcd.), m/z		N		C	H
3a	Orange needle	82.4	211.2—212.6	3200—2500(m, O—H), 3069, 2994, 2922, 2882(s, C—H), 1734, 1684(vs, C═O), 1602(m, N═N), 1594, 1559, 1541, 1449(m, ArH), 1428, 1276, 1222, 1201, 1178, 1074, 1010(vs, C—O—C), 879(s, trans-R—N═N—R), 807(w, 1,4-Ar), 688 [w,(CH₂)_n]	366 (366.41)		68.94 (68.84)		6.30 (6.05)	7.83 (7.65)
Compd.	Appea- rance	Yield (%)	m.p./℃	IR(KBr), ν/cm^-1		MS([M⁺], calcd.), m/z		Elemental analysis(%, calcd.)
Compd.	Appea- rance	Yield (%)	m.p./℃	IR(KBr), ν/cm^-1		MS([M⁺], calcd.), m/z		N	C	H
3b	Orange needle	80.2	221.7—222.8	3200—2500(m, O—H), 2950, 2930, 2864(s, C—H), 1755, 1697(vs, C═O), 1603(m, N═N), 1563, 1499(m, ArH), 1427, 1290, 1223, 1200, 1141, 1121, 1009(vs, C—O—C), 868(s, trans-R—N═N—R), 809(w, 1, 4-Ar), 690[w,(CH₂)_n]		380 (380.44)		69.67 (69.46)	6.54 (6.36)	7.49 (7.36)
3c	Orange needle	84.0	208.9—209.5	3000—2500(m, O—H), 2960, 2931(s, C—H), 1755, 1680(vs, C═O), 1603(m, N═N), 1592, 1500, 1449(m, ArH), 1428, 1290, 1224, 1201, 1142, 1122, 1101, 1076, 1010(vs, C—O—C), 869(s, trans-R—N═N—R), 808(w, 1, 4-Ar), 765 [w,(CH₂)_n]		394 (394.46)		70.22 (70.03)	6.73 (6.64)	7.15 (7.10)
3d	Orange needle	81.1	198.0—199.7	3000—2500(m, O—H), 3069, 2994, 2922, 2882(s, C—H), 1734, 1684(vs, C═O), 1602(m, N═N), 1594, 1559(m, ArH), 1498, 1428, 1276, 1222, 1201, 1178, 1141, 1074, 1010(vs, C—O—C), 879(s, trans-R—N═N—R), 812(w, 1, 4-Ar), 776 [w,(CH₂)_n]		360 (360.11)		70.14 (69.99)	4.60 (4.48)	7.93 (7.77)
3e	Orange needle	80.7	200.4—201.9	3000—2500(m, O—H), 3097, 3071, 2968, 2853(s, C—H), 1735, 1685(vs, C═O), 1602(m, N═N), 1559, 1541, 1496, 1458(m, ArH), 1266, 1223, 1195, 1177, 1142, 1068, 1009(vs, C—O—C), 882(s, trans-R—N═N—R), 811(w, 1,4-Ar), 766 [w,(CH₂)_n]		374 (374.39)		70.77 (70.58)	4.90 (4.85)	7.66 (7.48)
3f	Orange needle	85.2	199.5—200.9	3100—2500(m, O—H), 2958, 2929, 2871(s, C—H), 1734, 1684(vs, C═O), 1604(m, N═N), 1495, 1464(m, ArH), 1309, 1267, 1223, 1196, 1141, 1099, 1061, 1010(vs, C—O—C), 882(s, trans-R—N═N—R), 831(w, 1, 4-Ar), 776 [w,(CH₂)_n]		388 (388.42)		71.33 (71.12)	5.30 (5.19)	7.39 (7.21)
3g	Orange needle	88.6	186.3—187.8	3000—2500(m, O—H), 2956, 2927, 2857(s, C—H), 1734, 1681(vs, C═O), 1603(m, N═N), 1595, 1495(m, ArH), 1416, 1379, 1270, 1223, 1198, 1176, 1141, 1100, 1068, 1010(vs, C—O—C), 946, 880(s, trans-R—N═N—R), 846(w, 1, 4-Ar), 775 [w,(CH₂)_n]		402 (402.44)		71.78 (71.63)	5.66 (5.51)	6.75 (6.96)
3h	Orange needle	84.3	187.1—188.5	3000—2500(m, O—H), 2929, 2857(s, C—H), 1786, 1685(vs, C═O), 1602(m, N═N), 1541, 1468(m, ArH), 1417, 1358, 1287, 1255, 1220, 1142, 1101, 1041, 1008(vs, C—O—C), 943, 882(s, trans-R—N═N—R), 817(w, 1, 4-Ar), 721 [w,(CH₂)_n]		416 (416.47)		72.33 (72.10)	5.99 (5.81)	6.88 (6.73)

Table 1 Appearance, yields, melting points, IR, MS data and elemental analysis of target compounds

Compd.	Appea- rance	Yield (%)	m.p./℃	IR(KBr), $ν ˙$ /cm^-1	MS([M⁺], calcd.), m/z		Elemental analysis(%, calcd.)
Compd.	Appea- rance	Yield (%)	m.p./℃	IR(KBr), $ν ˙$ /cm^-1	MS([M⁺], calcd.), m/z		N		C	H
3a	Orange needle	82.4	211.2—212.6	3200—2500(m, O—H), 3069, 2994, 2922, 2882(s, C—H), 1734, 1684(vs, C═O), 1602(m, N═N), 1594, 1559, 1541, 1449(m, ArH), 1428, 1276, 1222, 1201, 1178, 1074, 1010(vs, C—O—C), 879(s, trans-R—N═N—R), 807(w, 1,4-Ar), 688 [w,(CH₂)_n]	366 (366.41)		68.94 (68.84)		6.30 (6.05)	7.83 (7.65)
Compd.	Appea- rance	Yield (%)	m.p./℃	IR(KBr), ν/cm^-1		MS([M⁺], calcd.), m/z		Elemental analysis(%, calcd.)
Compd.	Appea- rance	Yield (%)	m.p./℃	IR(KBr), ν/cm^-1		MS([M⁺], calcd.), m/z		N	C	H
3b	Orange needle	80.2	221.7—222.8	3200—2500(m, O—H), 2950, 2930, 2864(s, C—H), 1755, 1697(vs, C═O), 1603(m, N═N), 1563, 1499(m, ArH), 1427, 1290, 1223, 1200, 1141, 1121, 1009(vs, C—O—C), 868(s, trans-R—N═N—R), 809(w, 1, 4-Ar), 690[w,(CH₂)_n]		380 (380.44)		69.67 (69.46)	6.54 (6.36)	7.49 (7.36)
3c	Orange needle	84.0	208.9—209.5	3000—2500(m, O—H), 2960, 2931(s, C—H), 1755, 1680(vs, C═O), 1603(m, N═N), 1592, 1500, 1449(m, ArH), 1428, 1290, 1224, 1201, 1142, 1122, 1101, 1076, 1010(vs, C—O—C), 869(s, trans-R—N═N—R), 808(w, 1, 4-Ar), 765 [w,(CH₂)_n]		394 (394.46)		70.22 (70.03)	6.73 (6.64)	7.15 (7.10)
3d	Orange needle	81.1	198.0—199.7	3000—2500(m, O—H), 3069, 2994, 2922, 2882(s, C—H), 1734, 1684(vs, C═O), 1602(m, N═N), 1594, 1559(m, ArH), 1498, 1428, 1276, 1222, 1201, 1178, 1141, 1074, 1010(vs, C—O—C), 879(s, trans-R—N═N—R), 812(w, 1, 4-Ar), 776 [w,(CH₂)_n]		360 (360.11)		70.14 (69.99)	4.60 (4.48)	7.93 (7.77)
3e	Orange needle	80.7	200.4—201.9	3000—2500(m, O—H), 3097, 3071, 2968, 2853(s, C—H), 1735, 1685(vs, C═O), 1602(m, N═N), 1559, 1541, 1496, 1458(m, ArH), 1266, 1223, 1195, 1177, 1142, 1068, 1009(vs, C—O—C), 882(s, trans-R—N═N—R), 811(w, 1,4-Ar), 766 [w,(CH₂)_n]		374 (374.39)		70.77 (70.58)	4.90 (4.85)	7.66 (7.48)
3f	Orange needle	85.2	199.5—200.9	3100—2500(m, O—H), 2958, 2929, 2871(s, C—H), 1734, 1684(vs, C═O), 1604(m, N═N), 1495, 1464(m, ArH), 1309, 1267, 1223, 1196, 1141, 1099, 1061, 1010(vs, C—O—C), 882(s, trans-R—N═N—R), 831(w, 1, 4-Ar), 776 [w,(CH₂)_n]		388 (388.42)		71.33 (71.12)	5.30 (5.19)	7.39 (7.21)
3g	Orange needle	88.6	186.3—187.8	3000—2500(m, O—H), 2956, 2927, 2857(s, C—H), 1734, 1681(vs, C═O), 1603(m, N═N), 1595, 1495(m, ArH), 1416, 1379, 1270, 1223, 1198, 1176, 1141, 1100, 1068, 1010(vs, C—O—C), 946, 880(s, trans-R—N═N—R), 846(w, 1, 4-Ar), 775 [w,(CH₂)_n]		402 (402.44)		71.78 (71.63)	5.66 (5.51)	6.75 (6.96)
3h	Orange needle	84.3	187.1—188.5	3000—2500(m, O—H), 2929, 2857(s, C—H), 1786, 1685(vs, C═O), 1602(m, N═N), 1541, 1468(m, ArH), 1417, 1358, 1287, 1255, 1220, 1142, 1101, 1041, 1008(vs, C—O—C), 943, 882(s, trans-R—N═N—R), 817(w, 1, 4-Ar), 721 [w,(CH₂)_n]		416 (416.47)		72.33 (72.10)	5.99 (5.81)	6.88 (6.73)

Table 2 1H NMR data for target compounds

Compd.	¹H NMR(400 MHz, CDCl₃), δ
3a	0.90(s, 3H, CH₃), 1.40—2.97(m, 10H, 10cyclohexyl-H), 7.53, 7.97, 8.01, 8.23(d, J=8.0, 8.4, 8.8, 8.0 Hz, 2H of each, 8Ph-H), 12.96(s, 1H, COOH)
3b	0.89(t, 3H, J=7.6 Hz, CH₃), 1.25—1.26(m, 2H, CH₂), 1.25—2.18(m, 10H, 10cyclohexyl-H), 7.27, 7.64, 7.80, 8.25(d, J=8.0, 9.2, 8.8, 8.0 Hz, 2H of each, 8Ph-H), 12.81(s, 1H, COOH)
Compd.	¹H NMR(400 MHz, CDCl₃), δ
3c	0.88(t, 3H, J=7.2 Hz, CH₃), 1.20—1.27(m, 2H, CH₂), 1.24(m, 2H, CH₂), 1.57—2.60(m, 10H, 10cyclohexyl-H), 7.30, 7.48, 8.01, 8.31(d, J=8.0, 8.4, 8.4, 8.0 Hz, 2H of each, 8Ph-H), 12.73(s, 1H, COOH)
3d	2.41(s, 3H, CH₃), 7.39, 7.41, 8.01, 8.04, 8.32, 8.41(d, J=7.2, 7.2, 8.0, 8.0, 7.2, 7.2 Hz, 2H of each, 12Ph-H), 13.11(s, 1H, COOH)
3e	1.27(t, J=7.2 Hz, 3H, CH₃), 2.74(q, J=7.2 Hz, 2H, CH₂), 7.29, 7.35, 7.41, 8.03, 8.12, 8.25(d, J=8.0, 8.4, 8.4, 8.0, 8.0, 8.0 Hz, 2H of each, 12Ph-H), 12.91(s, 1H, COOH)
3f	0.97(t, 3H, J=7.2 Hz, CH₃), 1.70(m, 2H, CH₂), 2.68(t, J=7.2 Hz, 2H, CH₂), 7.32, 7.34, 7.98, 8.01, 8.11, 8.35(d, J=8.0, 8.4, 8.0, 8.0, 8.0, 8.4 Hz, 2H of each, 12Ph-H), 12.86(s, 1H, COOH)
3g	0.90(t, 3H, J=7.2 Hz, CH₃), 1.29—1.37(m, 2H, CH₂), 1.54—1.65(m, 2H, CH₂), 2.66(t, J=7.2 Hz, 2H, CH₂), 7.29, 7.36, 7.97, 8.02, 8.09, 8.22(d, J=8.4, 8.8, 8.4, 8.8, 8.0, 8.4 Hz, 2H of each, 12Ph-H), 13.01(s, 1H, COOH)
3h	0.90(t, 3H, J=6.8 Hz, CH₃), 1.25—1.44(m, 6H, CH₂CH₂CH₂), 1.64(t, 2H, J=7.2 Hz, CH₂), 7.28, 7.32, 8.03, 8.06, 8.22, 8.28(d, J=8.4, 8.4, 8.0, 8.4, 8.0, 8.4 Hz, 2H of each, 12Ph-H), 13.02(s, 1H, COOH)

Table 3 UV spectrum of target compounds and the maximum time of isomerization

Entry	Compd.	λ_max/nm				f(cis)_t(%)	Time of isomerization/min
		Trans isomer			Cis isomer
		Ⅰ		Ⅱ	Ⅰ	f(cis)_t(%)	Ⅱ	Solution	Mesophase
1	3a	233	347	235	354	11	40	50
	3b	223	330	223	327	55	40	70
	3c	230	334	229	335	38	50	80
2	3d	238	340	238	334	37	70	90
	3e	239	337	240	334	50	70	90
	3f	239	339	239	338	37	60	80
	3g	238	336	239	333	29	60	70
	3h	239	338	238	333	16	50	70

Fig.1 UV-Vis absorption spectra of compound 3b in methanol during trans-to-cis isomerization Fig.1(B) illustrates the small increase of absorption in 430 nm. Time/min: a. 0; b. 10; c. 20; d. 30; e. 40.

Table 4 Redox potentials, EHOMO, ELUMO and energy gaps of target compounds

Entry	Compd.	E_ox/eV		Φ_p/eV	E_red/eV		Φ_n/eV	E_HOMO/eV	E_LUMO/eV	E_g/eV
Entry	Compd.	Ⅰ		Ⅱ			Φ_n/eV	E_HOMO/eV	Ⅰ	Ⅱ
1	3a	0.69	0.66	0.66	-0.49	-0.62	-0.49	-5.06	-3.91	1.15
	3b	0.76		0.76	-0.42	-0.60	-0.42	-5.16	-3.98	1.18
	3c	0.76	0.63	0.63	-0.53	-0.64	-0.53	-5.03	-3.87	1.16
2	3d	0.82	0.65	0.65	-0.47	-0.56	-0.47	-5.05	-3.93	1.12
	3e	0.88		0.88	-0.51	-0.65	-0.51	-5.28	-3.89	1.39
	3f	0.82		0.82	-0.45	-0.54	-0.45	-5.22	-3.95	1.27
	3g	0.83		0.83	-0.54	-0.64	-0.54	-5.23	-3.86	1.37
	3h	0.81	0.58	0.58	-0.50	-0.62	-0.50	-4.98	-3.90	1.08

Table 5 DSC measurement results of target compounds

Entry	Compd.	L/W^a	m.p./℃	ΔH of m.p./(J·g^-1)	d.p.^b/℃
1	3a	4.362	211.2	28.72	218—369
	3b	4.638	221.0	26.54	232—419
	3c	4.967	208.2	23.17	226—425
2	3d	4.110	198.7	25.62	257—413
	3e	4.447	200.3	26.76	246—388
	3f	4.497	199.1	21.82	230—429
	3g	4.549	186.8	40.39	240—393
	3h	4.722	187.9	27.67	230—349

Fig.2 POM images of compounds 3a—3h during heating process(A)—(H) Schlieren textures of 3a, 3b, 3c, 3d, 3e, 3f, 3g and 3h taken at 214, 227, 228, 221, 220, 210, 220 and 220 ℃, respectively.

Fig.3 POM images of texture change of compound 3b under UV radiation at 230 ℃UV radiation time/min: (A) 10; (B) 20; (C) 30; (D) 40; (E) 50; (F) 60.

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