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Contrasting Photo-Switching Rates in Azobenzene Derivatives: How the Nature of the Substituent Plays a Role

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Contrasting Photo-Switching Rates in Azobenzene Derivatives: How the Nature of the Substituent Plays a Role polymers Article Contrasting Photo-Switching Rates in Azobenzene Derivatives: How the Nature of the Substituent Plays a Role Domenico Pirone 1,2 , Nuno A. G. Bandeira 3 , Bartosz Tylkowski 4 , Emily Boswell 5, Regine Labeque 2, Ricard Garcia Valls 1,4 and Marta Giamberini 1,* 1 Department of Chemical Engineering (DEQ), Rovira i Virgili University, Av. Països Catalans 26, 43007 Tarragona, Spain; [email protected] (D.P.); [email protected] (R.G.V.) 2 Procter & Gamble Services Company n.v., Temselaan 100, 1853 Strombeek-Bever, Belgium; [email protected] 3 BioISI—Biosystems & Integrative Sciences Institute; C8, Faculdade de Ciências, Universidade de Lisboa Campo Grande, 1749-016 Lisboa, Portugal; [email protected] 4 Eurecat, Centre Tecnològic de Catalunya, C/Marcel-lí Domingo, 43007 Tarragona, Spain; [email protected] 5 The Procter and Gamble Company, 8611 Beckett Rd, West Chester Township, Cincinnati, OH 45069, USA; [email protected] * Correspondence: [email protected]; Tel.: +34-97755-8174 Received: 25 March 2020; Accepted: 26 April 2020; Published: 30 April 2020 Abstract: A molecular design approach was used to create asymmetrical visible light-triggered azo-derivatives that can be good candidates for polymer functionalization. The specific electron–donor substituted molecules were characterized and studied by means of NMR analyses and UV-visible spectroscopy, comparing the results with Time Dependent Density Functional (TD-DFT) calculations. A slow rate of isomerization (k = 1.5 10 4 s 1) was discovered for 4-((2-hydroxy-5methylphenyl) i × − − diazenyl)-3-methoxybenzoic acid (AZO1). By methylating this moiety, it was possible to unlock the isomerization mechanism for the second molecule, methyl 3-methoxy-4-((2-methoxy-5-methylphenyl) diazenyl)benzoate (AZO2), reaching promising isomerization rates with visible light irradiation in different solvents. It was discovered that this rate was heightened by one order of magnitude (k = 3.1 10 3 s 1) for AZO2. A computational analysis using density functional (DFT/PBE0) i × − − and wavefunction (QD-NEVPT2) methodologies provided insight into the photodynamics of these systems. Both molecules require excitation to the second (S2) excited state situated in the visible region to initiate the isomerization. Two classic mechanisms were considered, namely rotation and inversion, with the former being energetically more favorable. These azo-derivatives show potential that paves the way for future applications as building blocks of functional polymers. Likewise, they could be really effective for the modification of existing commercial polymers, thus transferring their stimuli responsive properties to polymeric bulky structures, converting them into smart materials. Keywords: photoswitching; azoaryls; azobenzene; UV-Visible; TD-DFT; kinetics; photoisomerization 1. Introduction A great attention during the past decades has been addressed to photochromic molecules in the field of materials science. These organic compounds can be included in many materials, changing the properties of the system when irradiated with light of the appropriate wavelength. The change can affect characteristics like: electronic or ionic conductivity, fluorescence, magnetism, shape, and conformation. The most studied photochromic molecules are azobenzenes [1] which have been widely investigated as promising photo-switchers [2–6] due to their simple preparation Polymers 2020, 12, 1019; doi:10.3390/polym12051019 www.mdpi.com/journal/polymers Polymers 2020, 12, 1019 2 of 20 Polymers 2019, 11, x FOR PEER REVIEW 2 of 21 affect characteristics like: electronic or ionic conductivity, fluorescence, magnetism, shape, and procedures,conformation. good processability,The most studied as well photochromic as high thermal molecules and are chemical azobenzenes stability [1] [ 7which]. However, have been the main interestingwidely characteristicinvestigated as of promisin azobenzeneg photo molecules-switchers is [2 their–6] due reversible to their simple isomerization preparation of theprocedures, double bond, betweengood the processability, thermally stable as wellE as(trans) high thermal configuration and chemical and the stability meta-stable [7]. However,Z (cis) onethe main [8–12 interesting]. This process drivescharacteristic to change theof azobenzene geometry ofmolecules the molecule. is their Structurally,reversible isomerization while the Eof formthe doubl is approximatelye bond, between planar, the Zthe-isomer thermally is three-dimensional stable E (trans) configuration and has and a ground the meta state-stable energy Z (cis) nearlyone [8–12] 0.6. This eV higherprocess drives than the E one [to13 change–15]. This the geometry interconversion of the molecule. between Structurally, the two isomerswhile the makes E form theis approximately azobenzene planar, the most the used organicZ-isomer chromophore is three-dimensional for technical and applications, has a ground including state energy optical nearly waveguides 0.6 eV higher and than shutters the E one [16 [13], optical– 15]. This interconversion between the two isomers makes the azobenzene the most used organic memories [17], photo-active artificial muscles [3–5], photo triggered carriers and membranes [18–22], chromophore for technical applications, including optical waveguides and shutters [16], optical etc. A photostationary state (PSS) will be achieved by a solution containing an azo derivate when memories [17], photo-active artificial muscles [3–5], photo triggered carriers and membranes [18–22], irradiatedetc. A byphotostationary electromagnetic state radiation(PSS) will be of achieved the right by intensity, a solution with containing a steady-state an azo derivateE–Z composition when basedirradiated on the opposingby electromagnetic effects of radiation photoisomerization of the right intensity, and thermal with a relaxationsteady-state backE–Z composition into the E state. The compositionbased on the opposing of the sample effects alsoof photoisomerization depends on the irradiationand thermal wavelength,relaxation back intensity, into the Esolvent-solute state. The interactionscomposition and temperature.of the sample Moreover,also depends the on optical the ir properties,radiation wavelength the response, intensity, time of solvent the isomerization,-solute and theinteractions spectroscopic and propertiestemperature. of anMoreover, azo molecule the optical can be fine-tunedproperties, bythe attaching responseto time the phenylof the rings differentisomerization chemical, and groups the spectroscopic [23]. For example, properties it isof usuallyan azo molecule accepted can that be fineZ–E-tunedthermal by attaching relaxation to may take placethe phenyl by means rings d ofifferent two pathways chemical groups that can [23] be. For competitive example, it (Figure is usually1): accepted rotation, that or inversionZ–E thermal of one relaxation may take place by means of two pathways that can be competitive (Figure 1): rotation, or of the nitrogen atoms [24,25]. Depending on the properties of the substituents bonded to one or both inversion of one of the nitrogen atoms [24,25]. Depending on the properties of the substituents phenyl rings, and on the polarity of the reaction medium, one of these mechanisms of relaxation may bonded to one or both phenyl rings, and on the polarity of the reaction medium, one of these be favoredmechanisms over theof relaxation other [26 may]. be favored over the other [26]. FigureFigure 1. Rotation 1. Rotation and and inversion inversion thermal thermal relaxationrelaxation mechanism mechanismss for for unsubstituted unsubstituted azobenzene azobenzene.. RauRau has has divided divided the the azobenzenes azobenzenes intointo threethree spectroscopic classes classes based based on ontheir their substitution substitution patternspatterns on on the the phenyl phenyl rings:rings: azobenzene azobenzene-type-type molecules, molecules, aminoazobenzene aminoazobenzene-type-type molecules, molecules, and and pseudo-stilbenes,pseudo-stilbenes, respecti respectivelyvely [1]. [The1]. pseudo The pseudo-stilbenes-stilbenes have presented have presented the most efficient the most photo effi-cient response. This behavior must be linked to a mixed photo-stationary state where the Z and E form are photo-response. This behavior must be linked to a mixed photo-stationary state where the Z and E form being constantly interconverted, caused by a shifted and overlapped absorption spectrum of the two are being constantly interconverted, caused by a shifted and overlapped absorption spectrum of the isomers. Consequently, for these compounds, both the forward and reverse isomerization can be two isomers.induced by Consequently, a single wavelength for these of light compounds, in the visible both region. the forwardOn the other and hand, reverse the isomerizationazobenzene-type can be inducedmolecules by a single can isomerize wavelength very ofslowly light from in the the visible Z configuration region. On back the into other the hand, E configuration the azobenzene-type when moleculesbulky cansubstituents isomerize are very introduced slowly from to delay the Z theconfiguration thermal relaxation back into [2 the7,28E]. configurationRecently, the whenstudies bulky substituentsprovided are by introducedJerca et al. revealed to delay that the, using thermal the proper relaxation solvent [27, it, 28is possible]. Recently, to yield the stable studies Z-isom provideders. by JercaT ethis al. behavior revealed can that,
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