Solvent and Substituent Effects on Spectral Characteristics and Excited-state Intramolecular Proton Transfer of 2-(2-Aminophenyl) Benzothiazole†

doi:10.7503/cjcu20190127

Abstract

Abstract:

A group of 2-(2-aminophenyl)benzothiazole(APBT) derivatives was synthesized with electron-donating or withdrawing substituent introduced at the nitrogen atom in amino. Solvent and substituent effects on their spectral characteristics and excited state intramolecular proton transfer(ESIPT) were further investigated by ultraviolet absorption spectrum, fluorescence emission spectrum and density functional theory(DFT) calculations. The results showed that, compared to non-polar solvent cyclohexane, the increase of solvent polarity and the formation of hydrogen bond between APBT and solvent molecule cause the maximum absorption peaks and fluorescence emission peaks of APBT red-shift a few nanometers. The ESIPT of APBT was also influenced to some extent by solvent effects. It was found that the charge distributions of nitrogen atom in amino be changed when substituent were introduced in APBT molecules. There were only a single fluorescence emission peak of APBT and its electron-donating substituent derivatives. The peaks were generated by enol tautomer and no ESIPT happened. However, the electron-withdrawing substituent, such as COCH₂Cl, would make the APBT derivatives to occur ESIPT reaction and the dual fluorescence peaks with different wavelength appeared in cyclohexane solution, which should be respectively assigned to enol and keto tautomers. The theoretical calculation of quantum chemistry agreed well with the spectral experimental results.

Key words: Proton transfer, Solvent effect, Substituent effect, Quantum chemical computation

CLC Number:

O641
O621.1

TrendMD:

LI Qing, YI Pinggui, TAO Hongwen, LI Yangyang, ZHANG Zhiyu, PENG Wenyu, LI Yuru. Solvent and Substituent Effects on Spectral Characteristics and Excited-state Intramolecular Proton Transfer of 2-(2-Aminophenyl) Benzothiazole^†[J]. Chem. J. Chinese Universities, 2019, 40(7): 1425.

Figures/Tables 9

Scheme 1 Molecular structures of enol and keto forms of APBT and its derivatives R=CH3(1), H(2, APBT), COCH2Cl(3), COCH2Br(4), COCH3(5), COCH2CH3(6)

Table 1 1H NMR data of compounds 1―6

Compound	Solvent	¹H NMR(500 MHz), δ
1(CH₃)	DMSO-d₆	8.75(d, —NH—, 1H), 8.10(d, 1H), 8.03(d, 1H),7.73(m, 1H), 7.53(t, 1H), 7.44(t, 1H), 7.39(t, 1H), 6.84(d, 1H), 6.72(t, 1H), 2.99(d, 3H)
2(H)	DMSO-d₆	8.09(t, 1H), 8.01(t, 1H), 7.64(t, 1H), 7.52(dd, 1H), 7.42(dd, 1H), 7.34(s, —NH₂, 2H), 7.23(m, 1H), 6.89(t, 1H), 6.65(dd, 1H)
3(COCH₂Cl)	CDCl₃	13.24(s, —NH—, 1H), 8.81(d, 1H), 8.05(d, 1H), 7.93(d, 1H), 7.88(d, 1H), 7.48(m, 2H), 7.44(t, 1H), 7.23(t, 1H), 4.31(s, 2H)
4(COCH₂Br)	CDCl₃	13.06(s, —NH—, 1H), 8.75(d, 1H), 8.12(d, 1H), 7.94(d, 1H), 7.89(dd, 1H), 7.53(m, 2H), 7.45(t, 1H), 7.24(m, 1H), 4.15(m, 2H)
5(COCH₃)	CDCl₃	12.38(d, —NH—, 1H), 8.78(d, 1H), 8.00(t, 1H), 7.92(d, 1H), 7.85(d, 1H), 7.53(t, 1H), 7.48(m, 1H), 7.43(d, 1H), 7.16(t, 1H), 2.33(m, 3H)
6(COC₂H₅)	CDCl₃	12.41(s, —NH—, 1H), 8.82(d, 1H), 8.00(d, 1H), 7.91(d, 1H), 7.84(d, 1H), 7.52(t, 1H), 7.47(t, 1H), 7.43(t, 1H), 2.59(m, 2H), 1.38(m, 3H)

Fig.1 Experimental(A) and calculated(B) absorption spectra of APBT in different solventsa. CYH; b. DCM; c. ACN; d. DMF; e. DMSO. Calculated at TD B3LYP/TZVP theoretical level.

Table 2 Relative energies(EK-E) of keto form, active energies(Ea), solvation energies(Esolv), hydrogen bond energies(EH-bond), frontier molecular orbital energy levels(EHOMO, ELUMO) and their gaps(ΔEL-H) of ground state APBT in different solvents*

Solvent	E_K-E/(kJ·mol^-1)	E_a/(kJ·mol^-1)	$E sol v$ /(kJ·mol^-1)	E_H-bond/(kJ·mol^-1)	E_HOMO/eV	E_LUMO/eV	ΔE_L-H/eV
CYH	85.8	80.6	-6.7	—	-5.668	-1.769	3.899
DCM	84.5	80.6	-15.7	-2.4	-5.726	-1.839	3.887
ACN	84.2	80.6	-18.5	-8.7	-5.621	-1.841	3.780
DMF	84.2	80.6	-18.6	-14.8	-5.580	-1.824	3.756
DMSO	84.1	80.6	-18.8	-17.5	-5.584	-1.803	3.781

Solvent	E_K-E/(kJ·mol^-1)	E_a/(kJ·mol^-1)	$E sol v$ /(kJ·mol^-1)	E_H-bond/(kJ·mol^-1)	E_HOMO/eV	E_LUMO/eV	ΔE_L-H/eV
CYH	85.8	80.6	-6.7	—	-5.668	-1.769	3.899
DCM	84.5	80.6	-15.7	-2.4	-5.726	-1.839	3.887
ACN	84.2	80.6	-18.5	-8.7	-5.621	-1.841	3.780
DMF	84.2	80.6	-18.6	-14.8	-5.580	-1.824	3.756
DMSO	84.1	80.6	-18.8	-17.5	-5.584	-1.803	3.781

Fig.2 Experimental and calculated fluorescence spectra of APBT in different solvents(A) Aprotic solvent: a. CYH; b. Diox; c. ACN; d. DMF; e. DMSO. (B) protic solvent: a. CYH; b. EtOH; c. MeOH; d. H2O. The solid line represents experimental spectra; the dotted line represents calculated spectra at TD B3LYP/TZVP theoretical level.

Table 3 Relative energies(ΔEK*-E*), dihedral angles(A) and oscillator strengths(f) of APBT in different solvents*

Solvent	Δ $E K * - E *$ /(kJ·mol^-1)	A_{N1—C2—C3—C4}/(°)		f
Solvent	Δ $E K * - E *$ /(kJ·mol^-1)	Enol^*	Keto^*	Enol^*	Keto^*
CYH	25.6	0	0	0.399	0.292
Diox	39.2	0.1	0.4	0.398	0.313
ACN	36.8	0	0.1	0.601	0.429
DMF	41.2	-2.6	-13.7	0.588	0.398
DMSO	42.4	-3.0	12.9	0.578	0.400
EtOH	-14.8	-21.0	98.1	0.496	0.001
MeOH	-14.3	-22.7	97.8	0.490	0.001
H₂O	-11.8	-21.9	-97.7	0.510	0.002

Table 3 Relative energies(ΔEK*-E*), dihedral angles(A) and oscillator strengths(f) of APBT in different solvents*

Solvent	Δ $E K * - E *$ /(kJ·mol^-1)	A_{N1—C2—C3—C4}/(°)		f
Solvent	Δ $E K * - E *$ /(kJ·mol^-1)	Enol^*	Keto^*	Enol^*	Keto^*
CYH	25.6	0	0	0.399	0.292
Diox	39.2	0.1	0.4	0.398	0.313
ACN	36.8	0	0.1	0.601	0.429
DMF	41.2	-2.6	-13.7	0.588	0.398
DMSO	42.4	-3.0	12.9	0.578	0.400
EtOH	-14.8	-21.0	98.1	0.496	0.001
MeOH	-14.3	-22.7	97.8	0.490	0.001
H₂O	-11.8	-21.9	-97.7	0.510	0.002

Fig.3 Experimental(a—d) and calculated(a’—d’) fluorescence spectra of APBT and its derivatives(A) a. 1(CH3); b. 2(H); c. 3(COCH2Cl); d. 4(COCH2Br). (B) a. 5(COCH3); b. 6(COC2H5). The solid line represents experimental spectra, dotted line represents calculated spectra at TD PBE1PBE/TZVP theoretical level in cyclohexane.

Table 4 Relative energies(EK*-E*) between keto and enol, selected bond lengths(R), bond angles and NBO charges(Q) of excited state APBT and its derivatives*

Compound	Tautomer	Δ $E K * - E *$ /(kJ·mol^-1)	R_N5—H6/nm	R_N1—H6/nm	∠N1—H6—N5/(°)	Q_N5/e	Q_H6/e
1(CH₃)	Enol^*	28.9	0.104	0.175	143.4	-0.616	0.437
	Keto^*		0.164	0.108	139.5	-0.672	0.468
2(H)	Enol^*	24.0	0.104	0.179	138.7	-0.764	0.436
	Keto^*		0.168	0.106	137.3	-0.309	0.202
3(COCH₂Cl)	Enol^*	-11.2	0.105	0.171	144.8	-0.600	0.453
	Keto^*		0.198	0.103	124.3	-0.673	0.494
4(COCH₂Br)	Enol^*	-8.0	0.105	0.170	145.5	-0.633	0.442
	Keto^*		0.201	0.102	123.0	-0.732	0.477
5(COCH₃)	Enol^*	7.0	0.105	0.171	146.0	-0.606	0.445
	Keto^*		0.183	0.104	132.3	-0.691	0.482
6(COC₂H₅)	Enol^*	10.5	0.105	0.171	146.0	-0.604	0.445
	Keto^*		0.181	0.104	133.6	-0.689	0.480

Table 4 Relative energies(EK*-E*) between keto and enol, selected bond lengths(R), bond angles and NBO charges(Q) of excited state APBT and its derivatives*

Compound	Tautomer	Δ $E K * - E *$ /(kJ·mol^-1)	R_N5—H6/nm	R_N1—H6/nm	∠N1—H6—N5/(°)	Q_N5/e	Q_H6/e
1(CH₃)	Enol^*	28.9	0.104	0.175	143.4	-0.616	0.437
	Keto^*		0.164	0.108	139.5	-0.672	0.468
2(H)	Enol^*	24.0	0.104	0.179	138.7	-0.764	0.436
	Keto^*		0.168	0.106	137.3	-0.309	0.202
3(COCH₂Cl)	Enol^*	-11.2	0.105	0.171	144.8	-0.600	0.453
	Keto^*		0.198	0.103	124.3	-0.673	0.494
4(COCH₂Br)	Enol^*	-8.0	0.105	0.170	145.5	-0.633	0.442
	Keto^*		0.201	0.102	123.0	-0.732	0.477
5(COCH₃)	Enol^*	7.0	0.105	0.171	146.0	-0.606	0.445
	Keto^*		0.183	0.104	132.3	-0.691	0.482
6(COC₂H₅)	Enol^*	10.5	0.105	0.171	146.0	-0.604	0.445
	Keto^*		0.181	0.104	133.6	-0.689	0.480

Fig.4 Excited state geometries of APBT and its derivatives optimized at TD PBE1PBE/TZVP theoretical level in cyclohexane

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