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Edwards KEK, Mermut O, Pietro WJ, Barrett CJ. Optical and computational study of the trans ↔ cis reversible isomerization of the commercial bis-azo dye Bismarck Brown Y. Phys Chem Chem Phys 2023; 25:5673-5684. [PMID: 36734510 DOI: 10.1039/d2cp05010b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
The trans-cis-trans isomerization behaviour of Bismarck Brown Y (BBY) during and after irradiation with visible light, was characterized in detail for the first time by means of optical pump-probe experiments, to study the geometric inter-conversion of bis-azobenzene both in solution and embedded in multi-layered polymeric thin films. The rate constants observed for the thermal cis-trans back isomerization permit a determination of how the thermal isomerization is influenced by its local environment. In both solution and when incorporated into multi-layered thin films, the thermal relaxation observed for the commercial azo dye BBY showed a highly unusual biexponential decay, which clearly demonstrates two distinct isomerization processes. The cis decay showed an anomalous fast isomerization process on the timescale of milliseconds, followed by a slower isomerization process with a cis lifetime on the order of seconds. It was further observed that the faster isomerization process was influenced more by its local environment than was the slower process. The faster isomerization process also displayed a higher rate constant in aprotic solvents such as THF and DMF compared to that observed in protic solvents such as ethanol and water. Additionally, a higher rate constant was observed in solution compared to the multi-layered thin films where motion of the azo molecules was likely more restricted. Following recrystallization of the BBY azo dye, the more expected monoexponential decay was observed for the cis isomer in solution, with a single cis lifetime calculated on the timescale of seconds. This timescale corresponded well to values predicted by density functional theory calculations.
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Affiliation(s)
| | - Ozzy Mermut
- Department of Physics and Astronomy, York University, Toronto, ON, Canada
| | | | - Christopher J Barrett
- Department of Chemistry, McGill University, Montreal, QC, Canada. .,Department of Physics and Astronomy, York University, Toronto, ON, Canada
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Cao C, Zhao Z, Qi Y, Peng H, Fang K, Xie R, Chen W. Effects of alkanolamine solvents on the aggregation states of reactive dyes in concentrated solutions and the properties of the solutions. RSC Adv 2021; 11:10929-10934. [PMID: 35423592 PMCID: PMC8695989 DOI: 10.1039/d0ra10656a] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2020] [Accepted: 02/24/2021] [Indexed: 12/11/2022] Open
Abstract
The aggregation of dyes is a common phenomenon in solutions, particularly concentrated solutions, which seriously affects the dyeing and printing processes. In this study, the effects of alkylamine solvents on the reactive dye aggregation behavior in highly concentrated solutions was studied. Typical cases were conducted with two slightly toxic and environmentally friendly solvents, namely diethanolamine (DEA) and triethanolamine (TEA), and two reactive dyes, namely C. I. Reactive Red 218 (R-218) and C. I. Reactive Orange 13 (O-13). Aggregation states were studied by ultraviolet-visible (UV-Vis) absorption spectroscopy, Gaussian-peak-fitting method and fluorescence spectroscopy. The results showed that both the additives DEA and TEA could reduce the dye aggregation because the solvents, DEA and TEA, can break the iceberg structure and allow easy entry of the molecules into the dye aggregates. Also, the disaggregation caused by DEA was higher as compared with TEA, which may be caused by the weaker hydrogen bond and the relatively smaller steric hindrance effects of DEA. The schematic of disaggregation between R-218 and DEA was also discussed. For R-218, the dimers were disaggregated to monomer, while the higher-ordered aggregates were disaggregated to trimers and dimers for O-13. Moreover, physical properties such as viscosity and surface tension of the solutions were measured. This investigation is instructive for the further dyeing progress with organic bases in the textile industries.
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Affiliation(s)
- Chuangui Cao
- College of Textiles & Clothing, State Key Laboratory of Bio-Fibers and Eco-Textiles, Collaborative Innovation Center for Eco-Textiles of Shandong Province, Qingdao University Qingdao 266071 China
| | - Zhihui Zhao
- College of Textiles & Clothing, State Key Laboratory of Bio-Fibers and Eco-Textiles, Collaborative Innovation Center for Eco-Textiles of Shandong Province, Qingdao University Qingdao 266071 China
| | - Yong Qi
- College of Textiles & Clothing, State Key Laboratory of Bio-Fibers and Eco-Textiles, Collaborative Innovation Center for Eco-Textiles of Shandong Province, Qingdao University Qingdao 266071 China
| | - Hui Peng
- College of Textiles & Clothing, State Key Laboratory of Bio-Fibers and Eco-Textiles, Collaborative Innovation Center for Eco-Textiles of Shandong Province, Qingdao University Qingdao 266071 China
| | - Kuanjun Fang
- College of Textiles & Clothing, State Key Laboratory of Bio-Fibers and Eco-Textiles, Collaborative Innovation Center for Eco-Textiles of Shandong Province, Qingdao University Qingdao 266071 China
- National Manufacturing Innovation Center of Advanced Dyeing and Finishing Technology Tai'an 271001 China
| | - Ruyi Xie
- College of Textiles & Clothing, State Key Laboratory of Bio-Fibers and Eco-Textiles, Collaborative Innovation Center for Eco-Textiles of Shandong Province, Qingdao University Qingdao 266071 China
- National Manufacturing Innovation Center of Advanced Dyeing and Finishing Technology Tai'an 271001 China
| | - Weichao Chen
- College of Textiles & Clothing, State Key Laboratory of Bio-Fibers and Eco-Textiles, Collaborative Innovation Center for Eco-Textiles of Shandong Province, Qingdao University Qingdao 266071 China
- National Manufacturing Innovation Center of Advanced Dyeing and Finishing Technology Tai'an 271001 China
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Dai Y, Yang B, Ding Y, Xu H, Wang B, Zhang L, Chen Z, Sui X, Feng X, Zhong Y, Mao Z. Real-time monitoring of multicomponent reactive dye adsorption on cotton fabrics by Raman spectroscopy. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2020; 230:118051. [PMID: 31958601 DOI: 10.1016/j.saa.2020.118051] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/02/2019] [Revised: 12/27/2019] [Accepted: 01/08/2020] [Indexed: 06/10/2023]
Abstract
Accurate real-time determination of each dye in combination dyeing is critical to the control of dyeing process, which plays an important role in upgrading the dyeing techniques of textile. In this work, Raman spectroscopy was applied to dyeing baths containing multiple dye species of varying structures to quantitatively monitor the dyeing process of each individual dye. Quantitative models were successfully established by partial least squares (PLS) for all combinations of the nine commonly used reactive dyes studied. The correlation coefficients were greater than 0.99, the root mean squared errors of calibration (RMSEC) were less than 0.2650 and the root mean squared errors of prediction (RMSEP) were less than 0.1340, even for the three-component mixture of Reactive Red 239 (RR239), Reactive Yellow 176 (RY176) and Reactive Blue 194 (RB194), which are similar in structures. The model was shown to be valid in the presence of added electrolytes (sodium sulfates). Real-time adsorption monitoring based on the model revealed that the dyes interacted with one another and competed for active sites. The adsorption kinetics obtained by Raman analysis shed light on dye compatibility and could be used to guide the design of dyeing recipe and dyeing process for optimum color reproduction. In addition, in situ monitoring by Raman spectroscopy maybe integrated with real-time on line control of dyeing parameters for fully automated production of dyed fabrics.
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Affiliation(s)
- Yamin Dai
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China
| | - Binfan Yang
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China
| | - Yongsheng Ding
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China
| | - Hong Xu
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China; Lu Thai Textile Co., LTD, Zibo 255000, China
| | - Bijia Wang
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China
| | - Linping Zhang
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China
| | - Zhize Chen
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China
| | - Xiaofeng Sui
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China
| | - Xueling Feng
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China; National Engineering Research Center for Dyeing and Finishing of Textiles, Donghua University, Shanghai 201620, China
| | - Yi Zhong
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China.
| | - Zhiping Mao
- Key Lab of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China; Innovation Center for Textile Science and Technology of Donghua University, Shanghai 201620, China; National Engineering Research Center for Dyeing and Finishing of Textiles, Donghua University, Shanghai 201620, China.
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Boltenkov I, Kolobkova E, Evstropiev S. Synthesis and characterization of transparent photocatalytic ZnO-Sm2O3 and ZnO-Er2O3 coatings. J Photochem Photobiol A Chem 2018. [DOI: 10.1016/j.jphotochem.2018.09.016] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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5
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Catalytic specificity of linear polystyrene-stabilized Pd nanoparticles during Ullmann coupling reaction in water and the associated mechanism. J Organomet Chem 2018. [DOI: 10.1016/j.jorganchem.2017.11.016] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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6
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Specific interactions within micelle microenvironment in different charged dye/surfactant systems. ARAB J CHEM 2016. [DOI: 10.1016/j.arabjc.2015.09.009] [Citation(s) in RCA: 40] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
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Ünal A, Eren B, Eren E. Investigation of the azo-hydrazone tautomeric equilibrium in an azo dye involving the naphthalene moiety by UV–vis spectroscopy and quantum chemistry. J Mol Struct 2013. [DOI: 10.1016/j.molstruc.2013.06.037] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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8
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Abbott LC, Batchelor SN, Moore JN. Structure and Reactivity of Thiazolium Azo Dyes: UV–Visible, Resonance Raman, NMR, and Computational Studies of the Reaction Mechanism in Alkaline Solution. J Phys Chem A 2013; 117:1853-71. [PMID: 23427946 DOI: 10.1021/jp309536h] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Laurence C. Abbott
- Department of Chemistry, The University of York, Heslington, York, YO10 5DD, U.K
| | - Stephen N. Batchelor
- Unilever Research & Development, Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K
| | - John N. Moore
- Department of Chemistry, The University of York, Heslington, York, YO10 5DD, U.K
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Turcu I, Bogdan M. Size Dependence of Molecular Self-Assembling in Stacked Aggregates. 1. NMR Investigation of Ciprofloxacin Self-Association. J Phys Chem B 2012; 116:6488-98. [DOI: 10.1021/jp3034215] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2023]
Affiliation(s)
- Ioan Turcu
- Department of Molecular and Biomolecular
Physics, National Institute of Isotopic and Molecular Technology, 400293 Cluj-Napoca, Romania
| | - Mircea Bogdan
- Department of Molecular and Biomolecular
Physics, National Institute of Isotopic and Molecular Technology, 400293 Cluj-Napoca, Romania
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11
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Resonance Raman and UV–visible spectroscopy of black dyes on textiles. Forensic Sci Int 2010; 202:54-63. [DOI: 10.1016/j.forsciint.2010.04.026] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2009] [Revised: 04/13/2010] [Accepted: 04/13/2010] [Indexed: 11/20/2022]
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12
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Chami F, Wilson MR. Molecular order in a chromonic liquid crystal: a molecular simulation study of the anionic azo dye sunset yellow. J Am Chem Soc 2010; 132:7794-802. [PMID: 20469909 DOI: 10.1021/ja102468g] [Citation(s) in RCA: 106] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We have carried out a detailed atomic simulation study of molecular order within a chromonic liquid crystalline material (sunset yellow) in aqueous solution. Self-assembly occurs in dilute solutions to form stacked aggregates, which show a preference for head-to-tail stacking and antiparallel dipole order. This feature is independent of solution concentration and aggregate size. Stacks are found to be dynamic entities in which rotational transitions (flips) can occur between antiparallel and parallel configurations. At a concentration matching the nematic phase of sunset yellow, the simulations show chromonic columns with a loose hexagonal packing and an intercolumn distance of 2.36 nm. Partial condensation of sodium ions occurs around a chromonic stack, with two preferred binding sites identified for sodium ions, corresponding to strong binding with the oxygens of a sulfonate group and a bridging site between a pair of molecules in a stack. A value for the free energy of binding of a molecule to a stack of 7 k(B)T was obtained for stacks of three and eight molecules, with a slightly larger value (additional 2 kJ mol(-1)) obtained for the dimer binding energy, indicating that aggregation is approximately isodesmic.
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Affiliation(s)
- Fatima Chami
- Department of Chemistry, Durham University Science Laboratories, South Road, Durham DH1 3LE, UK
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13
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Pielesz A. Spectroscopic study of interactions between model direct dyes and cotton. J Appl Polym Sci 2007. [DOI: 10.1002/app.25566] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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14
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Sekiguchi K, Yamaguchi S, Tahara T. Formation and Dissociation of Rhodamine 800 Dimers in Water: Steady-State and Ultrafast Spectroscopic Study. J Phys Chem A 2006; 110:2601-6. [PMID: 16494368 DOI: 10.1021/jp055027p] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
We investigated the fundamental photophysics and photochemistry of a cationic dye rhodamine 800 (R800) in water using steady-state and ultrafast time-resolved spectroscopies. In the ground state, the monomer and dimer coexist in equilibrium, which causes significant concentration dependence of UV-visible (vis) absorption spectra. We determined the equilibrium constant as well as the molar absorption spectra of the monomer and dimer from a global fitting analysis of the UV-vis spectra. The obtained pure dimer spectrum indicates that it is a nonparallel H-dimer. In contrast to the absorption spectra, the steady-state fluorescence spectra do not show any noticeable concentration dependence. The fluorescence lifetime was determined as 0.73 ns regardless of the concentration, and the fluorescence of R800 in water was solely attributed to the monomer. In femtosecond time-resolved absorption measurements, we observed the S(n) <-- S1 absorption bands of the monomer and the dimer, as well as the ground-state bleaching signals. It was found that the S1 dimer dissociates to produce the S1 monomer (and the S0 monomer) or relaxes to the S0 dimer with a time constant of as short as 3.0 ps, which brings about the absence of dimer fluorescence.
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Affiliation(s)
- Kentaro Sekiguchi
- Molecular Spectroscopy Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
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Frantz S, Hübner GA, Wendland O, Roduner E, Mariani C, Ottaviani MF, Batchelor SN. Effect of Humidity on the Supramolecular Structure of Cotton, Studied by Quantitative Spin Probing. J Phys Chem B 2005; 109:11572-9. [PMID: 16852420 DOI: 10.1021/jp050791x] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The effect of water content on the physicochemical properties of the amorphous regions in cotton were investigated by measuring the electron paramagnetic resonance (EPR) of TEMPOL nitroxide radicals, deposited in cotton at different loadings, as a function of the relative humidity (RH) and temperature. Three different components contribute differently to the experimental EPR spectra, corresponding to (a) mobile radicals absorbed in the bulk amorphous region, (b) slow moving radicals adsorbed on the crystallite surfaces in cotton, and (c) aggregated radicals. These components were analyzed by means of computer-aided simulations of the line shapes and simplified line width methods. Polarity and mobility parameters were extracted from the analysis of the spectra. For all loadings and temperatures, the polarity suddenly dropped when the water content fell below approximately 3 wt %, i.e., when water was removed from the bulk amorphous regions. At the lowest loading (2 x 10(-5) mol kg(-1)), the spectra were independent of the RH, and only mobile radicals were observed. At intermediate loading (10(-4)-10(-3) mol kg(-1)) both mobile (fast) and adsorbed (slow) moving radicals were present, the fraction of which depended on the RH. The mobility of the adsorbed and mobile radical signals was smaller at higher loadings, indicating microdomains of different character. The temperature dependence of the rotational correlation times provided the activation energies, which were much lower than in liquids. An equilibrium exists between the mobile and the adsorbed radicals. The temperature dependence of the equilibrium constant, K, gave the enthalpy and the entropy of the adsorption process. At low RH, the enthalpy and the entropy values indicated a simple adsorption process. At 10(-3) mol kg(-1), the values were independent of the RH, but at low loadings the values increased with the increase in the RH, which suggested a displacement of adsorbed water by the radicals at high water content. At loadings above 10(-3) mol kg(-1), signals from radicals strongly interacting via spin exchange were observed, which are assigned to aggregated radicals; simulation of the spectra gave an activation energy of 13 kJ mol(-1) for the spin exchange process. These effects are rationalized on the basis of microdomains of different character within cotton, reflecting the variation in pore sizes (0.5-8 nm) and the relaxation behavior of the cellulose chains.
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Affiliation(s)
- S Frantz
- Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
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Abbott LC, Batchelor SN, Oakes J, Gilbert BC, Whitwood AC, Lindsay Smith JR, Moore JN. Experimental and Computational Studies of Structure and Bonding in Parent and Reduced Forms of the Azo Dye Orange II. J Phys Chem A 2005; 109:2894-905. [PMID: 16833607 DOI: 10.1021/jp045216s] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The structure and bonding of the azo dye Orange II (Acid Orange 7) in parent and reduced forms have been studied using NMR, infrared, Raman, UV-visible, and electron paramagnetic resonance (EPR) spectroscopy, allied with density functional theory (DFT) calculations on three hydrazone models (no sulfonate, anionic sulfonate, and protonated sulfonate) and one azo model (protonated sulfonate). The calculated structures of the three hydrazone models are similar to each other and that of the model without a sulfonate group (Solvent Yellow 14) closely matches its reported crystal structure. The 1H and 13C NMR resonances of Orange II, assigned directly from 1D and 2D experimental data, indicate that it is present as > or = 95% hydrazone in aqueous solution, and as a ca. 70:30 hydrazone:azo mixture in dimethyl sulfoxide at 300 K. Overall, the experimental data from Orange II are matched well by calculations on the hydrazone model with a protonated sulfonate group; the IR, Raman, and UV-visible spectra of Orange II are assigned to specific vibrational modes and electronic transitions calculated for this model. The EPR spectrum obtained on one-electron reduction of Orange II by the 2-hydroxy-2-propyl radical (*CMe2OH) at pH 4 is attributed to the hydrazyl radical produced on protonation of the radical anion. Calculations on reduced forms of the model dyes support this assignment, with electron spin density on the two nitrogen atoms and the naphthyl ring; in addition, they provide estimates of the structures, vibrational spectra, and electronic transitions of the radicals.
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Affiliation(s)
- Laurence C Abbott
- Department of Chemistry, The University of York, Heslington, York YO10 5DD, UK
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Abbott LC, Batchelor SN, Oakes J, Lindsay Smith JR, Moore JN. Semiempirical and Ab Initio Studies of the Structure and Spectroscopy of the Azo Dye Direct Blue 1: Comparison with Experiment. J Phys Chem A 2004. [DOI: 10.1021/jp047234f] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Laurence C. Abbott
- Department of Chemistry, The University of York, Heslington, York, YO10 5DD, U.K., and Unilever Research, Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K
| | - Stephen N. Batchelor
- Department of Chemistry, The University of York, Heslington, York, YO10 5DD, U.K., and Unilever Research, Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K
| | - John Oakes
- Department of Chemistry, The University of York, Heslington, York, YO10 5DD, U.K., and Unilever Research, Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K
| | - John R. Lindsay Smith
- Department of Chemistry, The University of York, Heslington, York, YO10 5DD, U.K., and Unilever Research, Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K
| | - John N. Moore
- Department of Chemistry, The University of York, Heslington, York, YO10 5DD, U.K., and Unilever Research, Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K
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