molecules Article Molecular Structures and Spectral Properties of Natural Indigo and Indirubin: Experimental and DFT Studies Zixin Ju 1 , Jie Sun 2 and Yanping Liu 1,* 1 Key Laboratory of Textile Science & Technology of Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China; [email protected] 2 Shanghai Naturalism Biological Technology Co., Ltd., Shanghai 201616, China; [email protected] * Correspondence: [email protected]; Tel.: +86-1376-496-1672 Academic Editors: Marcella Guiso and Ilaria Serafin Received: 25 September 2019; Accepted: 21 October 2019; Published: 24 October 2019 Abstract: This paper presents a comparative study on natural indigo and indirubin in terms of molecular structures and spectral properties by using both computational and experimental methods. The spectral properties were analyzed with Fourier transform infrared (FTIR), Raman, UV-Visible, and fluorescence techniques. The density functional theory (DFT) method with B3LYP using 6-311G(d,p) basis set was utilized to obtain their optimized geometric structures and calculate the molecular electrostatic potential, frontier molecular orbitals, FTIR, and Raman spectra. The single-excitation configuration interaction (CIS), time-dependent density functional theory (TD-DFT), and polarization continuum model (PCM) were used to optimize the excited state structure and calculate the UV-Visible absorption and fluorescence spectra of the two molecules at B3LYP/6-311G(d,p) level. The results showed that all computational spectra agreed well with the experimental results. It was found that the same vibrational mode presents a lower frequency in indigo than that in indirubin. The frontier molecular orbital analysis demonstrated that the UV-Visible absorption and fluorescence bands of indigo and indirubin are mainly derived from π π* transition. ! The results also implied that the indigo molecule is more conjugated and planar than indirubin, thereby exhibiting a longer maximum absorption wavelength and stronger fluorescence peak. Keywords: natural indigo; natural indirubin; FTIR; Raman; UV-Visible; fluorescence; DFT 1. Introduction Natural indigo (C.I. 75780), mazarine powder extracted from the indigo plant, is one of the earliest and most popular vat dyestuffs [1,2]. Recently, the demand for natural indigo dye is increasing dramatically for its safety and biodegradability. Natural indigo is now being used by several fashion retail giants such as H&M on a large scale to replace synthetic indigo dye for developing safe, green, and sustainable jeans. Since natural indigo originates from natural resources, it is treble the cost of synthetic indigo dye. Clear evidence of dyeing fabrics with natural indigo is needed to prevent adulteration and protect the interests of consumers [3]. Natural indigo is easily oxidized to indirubin during the extraction process, so the presence of indirubin and other unknown impurities is a key feature of natural indigo dye. In this regard, an in-depth understanding of similarities and differences in the molecular structures features and various vibrational spectra of indigo and indirubin molecules is critical for establishing the authenticity of natural indigo dye. Indirubin (C.I. 75790), an isomer of indigo, which is an effective component of anti-leukemia agent, also has pharmacological effects such as sterilization. The difference in molecular structures of indigo and indirubin gives rise to different physical and chemical properties as well as their Molecules 2019, 24, 3831; doi:10.3390/molecules24213831 www.mdpi.com/journal/molecules Molecules 2019, 24, x FOR PEER REVIEW 2 of 15 Molecules 2019, 24, 3831 2 of 14 Indirubin (C.I. 75790), an isomer of indigo, which is an effective component of anti-leukemia agent, also has pharmacological effects such as sterilization. The difference in molecular structures of indigo and indirubin gives rise to different physical and chemical properties as well as their applications. As shown in Figure1, indigo and indirubin with the molecular formula of C 16H10N2O2 applications. As shown in Figure 1, indigo and indirubin with the molecular formula of C16H10N2O2 have a molecular weight of 262.6 Daltons, both of which have the double indole structure. Indigo is have a molecular weight of 262.6 Daltons, both of which have the double indole structure. Indigo is a planara planar molecule molecule with with a symmetricala symmetrical trans trans structurestructure and and a astrong strong conjugation conjugation effect eff [4]ect, in [4 ],which in which two intramoleculartwo intramolecular hydrogen hydrogen bonds bonds are are formed formed betweenbetween the the adjacent adjacent carbonyl carbonyl group groupss and andthe imino the imino groups.group Indirubins. Indirubin is anis an asymmetric asymmetric structural structural isomer of of indigo indigo by by condensing condensing two twoindole indole rings rings[5]. [5]. In theirIn their production production processes, processes, the the conditions conditions of of reactionreaction should should be be strictly strictly controlled controlled in order in order to avoid to avoid interconversioninterconversion of indigo of indigo and and indirubin indirubin [ 6[6]]. FigureFigure 1. 1.Chemical Chemical structures structures of indigo indigo (a ()a and) and indirubin indirubin (b). ( b). PreviousPrevious works works on on indigo indigo mainly mainly focused focused onon the dyeing dyeing process process optimization optimization and andancient ancient textile textile identificationidentification [7–9 [7].– In9]. orderIn order to to improve improve the the dyeingdyeing performance performance of of indigo indigo and and expand expand its application its application in electronicsin electronics and and other other fields, fields, considerable considerable researchresearch efforts efforts have have been been devoted devoted to the to theDFT DFT studies studies of of indigoindigo and and its derivatives its derivatives [10 [10–14–14]]. The. The earliest earliest investigation on on vibrational vibrational spectra spectra of indigo of indigo has been has been conducted at the HF/3-21G level [15,16]. Later on, recalculations were carried out with more accurate conducted at the HF/3-21G level [15,16]. Later on, recalculations were carried out with more accurate methods or at larger basis sets, such as B3LYP/6-31G(d,p) [11], B3LYP/LANL2DZ/6-31G(d,p) [17], methods or at larger basis sets, such as B3LYP/6-31G(d,p) [11], B3LYP/LANL2DZ/6-31G(d,p) [17], and B3LYP/6-311G(d,p) [18]. A recent study confirmed the assignments of the main experimental and B3LYPfeatures/6-311G(d,p) of the FTIR [and18]. Raman A recent spectra study below confirmed 1700 cm the−1 assignmentsby using B3LYP/6 of the-311++G(d,p) main experimental [19]. 1 featuresTheoretical of the studies FTIR andon the Raman differences spectra between below the absorption 1700 cm− ofby indigo using molecule B3LYP and/6-311 its derivatives++G(d,p) [19]. Theoreticalhave been studies reported on the at diTDff-erencesDFT/B3LYP/6 between-31G(d) the absorption [12], and their of indigo differences molecule in emission and its spectr derivativesa have have beenalso reported been investigated at TD-DFT/ B3LYPat the TD/6-31G(d)-DFT/B3LYP/6 [12], and-31G(d,p) their di levelfferences [13]. The in emission theoretical spectra results have indicated also been investigatedthat the atlowest the TD-DFT excited /singletB3LYP /states6-31G(d,p) of indigo level and [13 its]. Thederivatives theoretical were results derived indicated from the that HOMO the lowest- excitedLUMO singlet transition. states In of addition indigo andto the its studies derivatives based on were a small derived basis set from that the neglected HOMO-LUMO the solvent transition.effect In addition[12,13],to the the effects studies of based environmental on a small solvents basis set and that substitution neglected the patterns solvent were effect assessed [12,13], theat TD effects- of DFT/B3LYP/6-311+G(2d,p) [14]. It was found that indigo showed a strong bathochromic shift as the environmental solvents and substitution patterns were assessed at TD-DFT/B3LYP/6-311+G(2d,p) [14]. dielectric constant of the solvent increases due to the improved stability of the charged separation It wasstructure. found that The indigofluorescence showed quantum a strong yield bathochromic of polymeric indigo shift as is thean order dielectric of magnitude constant lower of the than solvent increasesthat of due indigo to the as improveda result of the stability energy of transfer the charged between separation different ind structure.igo chromophoric The fluorescence units [20] quantum. yield of polymericThe studies indigo on indirubin is an order were of mainly magnitude related lower to the than medical that field, of indigo such as adrug result metabolism of the energy transferprocess between and anti diff-erenttumorindigo mechanism. chromophoric In order to units improve [20]. the efficacy and safety of indirubin, the Theinfluence studies of substituents on indirubin on were the geometry mainlyrelated configuration to the medicaland anti-cancer field, such activity as drugof indirubin metabolism has been process and anti-tumorstudied computationally mechanism. [21 In–24] order. The to optimized improve thegeometric efficacy structure and safety and main of indirubin, vibrational the modes influence of substituentsassignment onof indirubin the geometry were investigated configuration at B3LYP/6 and anti-cancer-31G(d) level activity, but the of indirubinvibrational hasspectra