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 arechemical 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 properties temperature. of an Moreover, azo molecule the optical can be fine-tunedproperties, by the attaching responseto time the phenyl of 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 can substituents 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, be enhanced using the by proper the nature solvent, and itposition is possible of substituents to yield stable on theZ -isomers.aromatic rings This [29] behavior. can beThis enhanced last feature by of thethe azobenzene nature and-type position molecules of substituents is a very promising on the tool aromatic that allows rings fine [-29tuning]. This of last featurethe of optical the azobenzene-type response. Understanding molecules isthe a very isomeri promisingzation kinetics tool that of allows different fine-tuning substituted of the azo optical- derivatives in solution is of great importance in order to gain control over the response time to light response. Understanding the isomerization kinetics of different substituted azo-derivatives in solution irradiation and design photo responsive materials for precise applications. For instance, these azo is of greatsystems importance can be used in as order moieties to gain to bring control stimuli over-responsive the response polymers time into to lighta broad irradiation range of already and design photo responsive materials for precise applications. For instance, these azo systems can be used as moieties to bring stimuli-responsive polymers into a broad range of already existing commercial polymeric families, thus transferring their stimuli responsive properties to polymeric bulky structures with the aim of enhancing materials performances. Polymers 2020, 12, 1019 3 of 20

The idea of our work was to design, synthesize, and study novel asymmetrical azobenzene compounds with a precise substitutional pattern, bearing one reactive site. The specific properties derived from their substitutional pattern and the single reactive site could make them good candidates for the chemical modification of commercial polymers. The main goal was to create a proper photosensitive active group that, by means of its reactive site, could be grafted to a wide range of polymers, thus giving them stimuli-responsive properties. We focus our attention on the study of the E–Z–E isomerization kinetics of two related novel azo-derivatives (shown in Scheme1). To the best of our knowledge, the synthesis of these two compounds has not been reported in the literature. These systems were well-designed to achieve an isomerization when irradiated by visible light and a thermal back isomerization when heated above room temperature. We investigated the conversion process in silico with a static approach (time dependent DFT) and the photodynamics of isomer conversion through a multi-configurational wavefunction methodology (QD-NEVPT2) in order to have a complete understanding of the properties of such systems. In the course of our investigation, some unusual and unexpected photochemistry of these asymmetrical ortho substituted derivatives were discovered. These findings can provide new insights regarding azobenzene isomerization as well as the possibility to reveal new applications for a well-known organic molecule. For instance, through a reactive site such as a carboxylic group, these photoactive moieties can be conveniently grafted to several classes of commercial polymers, such as polyvinyl alcohols (PVAs) and polyepichlorohydrin (PECH), thus conferring upon them light responsiveness [19]. In this way, commercial polymers could behave as novel smart materials.

2. Materials and Methods

2.1. Materials Briefly, 4-Amino-3-methoxybenzoic acid (TCI, Tokyo Chemical Industries, 99% pure, TCI Europe, ≥ Boereveldseweg 6 - Haven 1063, 2070 Zwijndrecht, Belgium), 4-methylphenol (p-Cresol) (TCI, Tokyo Chemical Industries, 98% pure), methyl iodide (CH I, 141.94 g/mol), sodium hydride (NaH, 50% ≥ 3 dispersion in mineral oil), sodium hydroxide (NaOH) and sodium nitrite (NaNO2) were purchased from Merck Spain (Merk Spain, Calle de María de Molina, 40, 28006 Madrid, Spain) and used without further purifications. Tetrahydrofuran (THF) was pre-dried over calcium hydride and distilled from sodium benzophenone ketyl, according to standard procedures. Dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and dioxane, spectrophotometric grade, were purchased from Merck and used as received.

2.2. Characterization Subsequently, 1H NMR and 13C NMR spectra were recorded at 400 and 100.4 MHz, respectively, on a Varian Gemini 400 spectrometer (Varian, Inc. 3120 Hansen Way, Palo Alto, CA, USA) with proton noise decoupling for 13C NMR. The chemical shifts were given in parts per million from TMS (Tetramethylsilane) in 1H NMR spectra, while the central peak of the solvent was taken as the reference 13 13 in the case of C NMR spectra. The C NMR spectra of the polymers were recorded at 30 ◦C, with a flip angle of 45◦, and the number of transients ranged from 20,000 to 40,000 with 10%–20% (w/v) sample solutions in deuterated dimethyl sulfoxide (DMSO-d6). A pulse delay time of 5 s for the 1H NMR spectrum was used. Quantitative 13C NMR spectra were performed using deuterated dimethyl sulfoxide (DMSO-d6) at 30 ◦C with a pulse delay time of 5 s. Photoisomerization was investigated using a UV-1800 Shimadzu Spectrophotometer (SHIMADZU EUROPA GmbH, Albert-Hahn-Strasse 6-10, 47269 Duisburg, Germany) with a scanning rate of 3000 to 2 nm/min. The kinetics of photoisomerization and thermal relaxation were followed by the same instrument. The samples for UV-visible spectroscopy were prepared via dilution of the sample into adequate solvent (DMSO, DMF, Dioxane) to give a final concentration of 7.4 10 5 M in red light room (λ > 600 nm), to assure that they were predominantly × − in E form. After maintaining the azo dye solutions in the dark overnight, the spectra were taken in a Polymers 2020, 12, 1019 4 of 20 Polymers 2019, 11, x FOR PEER REVIEW 4 of 21

UV-gradeovernight, quartz the spectra cell with were optical taken beamin a UV path-grade of 1 quartz cm before cell with and afteroptical irradiation beam path with of 1 acm desk before lamp equippedand after with irradiation a 11 W wi Ath E27 a desk ID60 lamp LED bulbequipped (Jedi with Lighting a 11 iDualW A E27 bulb, ID60 TAO LED UK, bulb 4 Deanfield (Jedi Lighting Court, LinkiDual 59 bulb Business,TAO Park,UK, 4 Deanfield Clitheroe, Court, Lancs, Link UK). 59 During Business the Park, test Clitheroe, the sample Lancs, was UK placed). During at 2 the cm test from thethe light sample source. was The placed optical at 2 signalcm from output the light of white source. light The was optical measured signal by output an Ocean of whi Opticste light USB2000 was Miniaturemeasured Fiber by an Optic Ocean Spectrometer Optics USB2000 (StellarNet, Miniature Inc., Fiber 14390 Optic Carlson Spectrometer Circle, Tampa, (StellarNet, FL, USA) Inc., in 14390 a dark roomCarlson at room Circle, temperature, Tampa, FL, USA for more) in a detailsdark room the at measured room temperature spectrum, isfor shown more details in Figure the measured S1 in the SI (3890spectrum lux at is 2 cmshown and in 1030 Figure lux atS1 8 in cm, the from SI (3890 the source, lux at respectively).2 cm and 1030 The lux solutionat 8 cm, wasfrom analyzed the source, after 5 minrespectively). irradiation. The The solution process was was analyzed repeated after several 5 min times irradiation since. the The photostationary process was repeated state (PPS) several was observed.times since The theZ– photostationaryE thermal relaxation state of(PPS) solutions was observed. was studied The afterZ–E irradiatingthermal relaxation the samples of solutions reaching thewas photostationary studied after irradiating state with the the samples iDual lamp reaching and thenthe photostationary heating them in state a TCC-100 with the Thermoelectrically iDual lamp and Temperaturethen heating Controlled them in a TCC cell-100 holder Thermoelectrically (Aragonesses 13,Temperature Polig. Industrial Controlled 28100, cell holder Alcobendas, (Aragonesses Madrid, Spain)13, Polig. for theIndustrial analysis 28100, at di Alcobendas,fferent temperatures. Madrid, Spain) The for instruments the analysis collected at different the temperatures. spectra each 2The min instruments collected the spectra each 2 min for 30 min. The temperatures used for the experiment for 30 min. The temperatures used for the experiment were: 30, 40, 50, and 60 ◦C. Background was performedwere: 30, 40, on each50, and of 60 the °C. solvents Background used bothwas performed before and on after each light of the irradiation solvents andused confirmed both before that and the after light irradiation and confirmed that the solvents absorption spectrum did not affect the region solvents absorption spectrum did not affect the region between 250 and 600 nm. between 250 and 600 nm. 2.3. Synthesis of 4-((2-hydroxy-5-methylphenyl) diazenyl)-3-methoxybenzoic acid (AZO1) and Methyl-3-methoxy-4-((2-methoxy-5-methylphenyl)diazinyl)benzoate2.3. Synthesis of 4-((2-hydroxy-5-methylphenyl) diazenyl)-3-methoxybenzoic (AZO2) acid (AZO1) and Methyl-3- methoxy-4-((2-methoxy-5-methylphenyl)diazinyl)benzoate (AZO2) For the synthesis of 4-((2-hydroxy-5-methylphenyl) diazenyl)-3-methoxybenzoic acid (AZO1), 12 mmolFor of the 4-Amino-3-methoxybenzoic synthesis of 4-((2-hydroxy acid,-5-methylphenyl) 80 mL of water, diazenyl) and 7- mL3-methoxybenzoic of 35% hydrochloric acid (AZO1), acid were 12 mmol of 4-Amino-3-methoxybenzoic acid, 80 mL of water, and 7 mL of 35% hydrochloric acid introduced in a 100 mL beaker and mildly heated at 40 ◦C to get the amine chlorohydrate (Scheme1). were introduced in a 100 mL beaker and mildly heated at 40 °C to get the amine chlorohydrate The resulting suspension was then cooled to 0 ◦C in an ice bath. Meanwhile, 1.35 g (19.6 mmol) (Scheme 1). The resulting suspension was then≈ cooled to ≈ 0 °C in an ice bath. Meanwhile, 1.35 g (19.6 sodium nitrite were dissolved in 2.8 mL of water and added dropwise to the first suspension to obtain mmol) sodium nitrite were dissolved in 2.8 mL of water and added dropwise to the first suspension the corresponding diazonium salt (solution A). Solution B was prepared by dissolving 1.616 g (15 mmol) to obtain the corresponding diazonium salt (solution A). Solution B was prepared by dissolving 1.616 p-Cresol in 14 mL of water with 2.10 g (52.5 mmol) of sodium hydroxide. Solution B was then carefully g (15 mmol) p-Cresol in 14 mL of water with 2.10 g (52.5 mmol) of sodium hydroxide. Solution B was added dropwise to solution A at 2–3 C. A dark red precipitate was obtained. The crude product then carefully added dropwise to solution◦ A at 2–3 °C. A dark red precipitate was obtained. The was then re-dissolved in 200 mL of a 10% aqueous solution of sodium hydroxide, the solution was crude product was then re-dissolved in 200 mL of a 10% aqueous solution of sodium hydroxide, the diluted with 800 mL of water and then 20 mL 37% hydrochloric acid were added carefully. The product solution was diluted with 800 mL of water and then 20 mL 37% hydrochloric acid were added precipitatedcarefully. The as aproduct pure red precipitated solid which as a was pur thene redfiltered solid which and driedwas then in oven filtered at 45and◦C; dried yield in 58%, oven m.p.: at 262.745°C;◦C yield (deg.) 58%, m.p.: 262.7 °C (deg.)

SchemeScheme 1. Synthetic1. Synthetic paths path ofs 4-((2-hydroxy-5methylphenyl) of 4-((2-hydroxy-5methylphenyl) diazenyl)-3-methoxybenzoic diazenyl)-3-methoxybenzoic acid (AZO1) acid and(AZO1) methyl-3-methoxy-4-((2-methoxy-5-methylphenyl) and methyl-3-methoxy-4-((2-methoxy-5-methylphenyl) diazinyl) diazinyl) benzoate benzoate (AZO2). (AZO2).

Polymers 2020, 12, 1019 5 of 20

For the methylation of the hydroxyl groups of the previous molecule and the production of methyl-3-methoxy-4-((2-methoxy-5-methylphenyl) diazinyl) benzoate (AZO2), to 7.4 mmol of AZO1 in 50 mL of dry THF, 0.83 g of NaH were added (60% dispersion in mineral oil) followed by 1.73 mL (27 mmol) of methyl iodide. The mixture was stirred at 80 ◦C for 24 h. The solvent was evaporated with a rotavapor and the product was recrystallized from 2-propanol, obtaining an orange fine powder. Yield: 49%, m.p.: 138.4 ◦C. The spectral characterization of both compounds is summarized below. AZO1: 1H NMR (DMSO-d6) δ: 12.3 (s, 1H, OH), 7.8 (d, J = 8.3 Hz, 1H), 7.7 (d, J = 1.5 Hz, 1H), 7.6 (dd, J = 3.8, 1.2 Hz, 1H), 7.6 (d, J = 1.6 Hz, 1H), 7.3 (m, 1H), 6.9 (d, J = 8.4 Hz, 1H), 4.0 (s, 3H), 2.3 (s, 3H).13C NMR (DMSO-d6) δ: 167.0 (s); 155.9 (s); 151.9 (s); 142.1 (s); 134.6 (s); 129.3 (s); 118.5 (s); 114.1 (s); 56.7 (s); 20.3 (s). AZO2: 1H NMR (DMSO-d6) δ: 7.7 (d, J = 1.3 Hz, 1H), 7.5 (dd, J = 8.2, 1.4 Hz, 1H), 7.4 (d, J = 8.2 Hz, 1H), 7.3 (dd, J = 8.4, 1.9 Hz, 1H), 7.2 (d, J = 1.8 Hz, 1H), 7.1 (d, J = 8.5 Hz, 1H), 4.0 (s, 3H), 3.9 (s, 3H), 2.3 (s, 3H). 13C NMR (DMSO-d6) δ: 169.5 (s); 156.4 (s); 155.2 (s); 144.9 (s); 142.6 (s); 142.1 (s); 133.4 (s); 129.8 (s); 121.8 (s); 117.0 (s); 115.8 (s); 113.9 (s); 113.7 (s); 56.5 (s); 56.2 (s); 20.5 (s).

2.4. Computational Methods Density functional calculations were performed using the ADF [30] 2017 103 modelling suite, employing the Perdew, Burke, and Ernzerhof hybrid functional (PBE0) [31–33]. The basis sets consisted of all electron triple zeta Slater type orbitals with one polarization function (TZP). The COnductor like Screening MOdel (COSMO) dielectric continuum solvation scheme was employed in the calculations with default appropriate parameters for dimethylformamide, dimethyl sulfoxide and dichloromethane [34]. Geometry optimizations did not involve any symmetry constraints. Time dependent (TD-PBE0) runs for the calculation of UV-visible transitions required 60 singlet-singlet excitations [35]. Additional multiconfigurational self-consistent field calculations [36] were performed using the ORCA 4.0.1 software package [37,38]. The PES scans were carried out by relaxing the geometries in vacuo at the state average (six singlets) CASSCF (4,3) level to a minimum at every scan point. At each of the scan points the wavefunction subsequently underwent a perturbation through the Quasi-Degenerate N-Electron Valence Perturbation Theory (QD-NEVPT2) approach [39,40] in the Nakano formulation [41]. The Ahlrichs def2-TZVP contracted Gaussian type basis set [42] was employed for every element making use of the resolution of the identity (RI) Coulomb integral approximation scheme with the def2-TZVP/C correlation density fitting basis set [43]. Using the state average CASSCF wavefunction, a non-SCF CI run was performed for the ground (S0) and second excited (S2) states which then underwent a density matrix analysis using the natural bond orbital scheme (NBO 6 program [44]). The natural localized molecular orbitals were generated, and their bond orders computed using the procedure devised by Schleyer et al. [45].

3. Results & Discussions

3.1. Molecular Design of the Azo-derivatives: Azo compounds are usually synthesized with procedures like: azo coupling reaction (coupling of diazonium salts with activated aromatic compounds), Mills reaction (reaction between aromatic nitroso derivatives and anilines) and Wallach reaction (transformation of azoxybenzenes into 4-hydroxy substituted azo-derivatives in acid media) [46]. They are mostly produced with symmetric structures that have two potential reactive sites that offer the possibility to combine them with other monomers. In this way, polymers capable to react when irradiated by an electromagnetic irradiation can be produced. They are typically sensitive to UV light irradiation and they can have unstable Z form that can convert back to the more stable E form within picoseconds. Polymers 2020, 12, 1019 6 of 20

A preliminary study on the possible substitution pattern for the novel azobenzene derivatives synthesized in this work was performed. The objective was to achieve a well-designed asymmetric structure to:

Produce a molecule with only one reactive site for further modifications and be used as a side-chain group. • Move the excitation wavelength of the E-Z isomerization from the UV region of the absorbance • spectrum to the visible light one. Enhance the stability of the Z form once obtained after irradiation with white light. • To successfully synthesize the first compound AZO1, an azo coupling reaction procedure was chosen with p-Cresol and 4-amino-3-methoxybenzoic acid as precursors. In this specific electrophilic aromatic substitution reaction, a phenol with a methyl group in para position was selected as the activated arene. The substitution occurs stereotypically in para to the activating group when a bulky substituent is selected as the attacking species. However, ortho substitution takes place when the para position is already occupied by another substituent. The 4-amino-3-methoxybenzoic acid was chosen to give the molecule the desired asymmetry and a reactive site for further modifications. In addition, with the presence of an electron donor group on the aromatic ring, like the methoxy one, we aimed to move the isomerization wavelength to the visible region of the absorption spectrum to trigger the isomerization with white light irradiation [47]. Lastly, it was reported that the methyl group could enhance the stability of the Z-isomer prolonging its half-life in the darkness [48]. The conditions of the reaction were carefully controlled both in pH and temperature due to the instability of the diazonium salts formed during the process. A mildly acidic or neutral solution is usually needed when an aromatic amine is involved in this reaction: nevertheless, if the solution is too acidic, the concentration of free amine becomes too small and the reaction does not occur. On the other hand, the phenol is usually not sufficiently active for the reaction, but, the more reactive phenoxide ion is a better initiator to generate the target azobenzene, when slightly alkaline conditions are present. Nonetheless, neither amines nor phenols can react in moderately alkaline solution because the diazonium ions are converted into the corresponding diazo hydroxide [49]. From the account given by Bandara et al. [50] regarding a specific set of azobenzene derivatives, the hydroxyl groups can hinder the isomerization by forming hydrogen bonds with the azo group, or other substituent on the opposite ring. We therefore envisaged to apply this same idea for the present work. The AZO1 compound was thus methylated to give AZO2 (Scheme1). This modification generates a structure with two electron donor groups in ortho position on both the phenyl rings, changing the conjugation of the electrons on the –N=N– double bound. This modification was needed given the small isomerization conversion registered for AZO1 E-isomer when irradiated by white light, as shown in the following sections. The novel azobenzene compounds were obtained with reasonably good yields and purities. Their structures were characterized by 1H–NMR, 13C–NMR, and UV-Vis spectroscopy.

3.2. H–NMR and 13C–NMR Characterizations: In Figure2 1H–NMR spectra of AZO1 and AZO2 molecules dissolved in deuterated DMSO are shown. The proton spectra exhibited some similarities. The peaks corresponding to the aromatic hydrogens are highlighted in the insets showing the δ region between 7.0 ppm and 8.0 ppm. The peaks related to the hydrogen atoms of the methoxy and methyl groups are present in the region from 2.0 to 4.0 ppm. A peak corresponding to the –OH of the carboxylic group is visible at 12.18 ppm (Figure2A). After the conversion to methyl ester, this peak disappeared as it is evident in the Figure2B. On the other hand, a peak corresponding to the methyl ester group is present at 3.91 ppm. All the peaks of the aromatic rings were shifted after the methylation, but their number remained unchanged as expected. The small peak at 3.33 ppm is characteristic of the residual water in d-DMSO. From these evidences, one can conclude that the methylation of the AZO1 molecule was successfully accomplished. AZO2 Polymers 2020, 12, 1019 7 of 20

spectrum in d-DMSO at 30 ◦C at the PSS is shown in Supplementary Figure S7, and for AZO1, no changes were measured by NMR analyses after irradiation. Polymers 2019, 11, x FOR PEER REVIEW 7 of 21

1 FigureFigure 2. 1H–NMR 2. spectrumH–NMR comparison spectrum comparison of 4-((2-hydroxy-5-methylphenyl)diazenyl)-3-methoxybenzoic of 4-((2-hydroxy-5-methylphenyl)diazenyl)-3- methoxybenzoic acid (AZO1) and methyl-3-methoxy-4-((2-methoxy-5-methylphenyl)diazinyl) acid (AZO1) and methyl-3-methoxy-4-((2-methoxy-5-methylphenyl)diazinyl) benzoate (AZO2): benzoate (AZO2): A) and B), respectively. (A) and (B), respectively. The 13C–NMR chemical shifts of the molecules AZO1 and AZO2 are given in Table 1. From the 13 Thedata C–NMRshown, it is chemical evident that shifts two of new the signals molecules appear AZO1 after the and met AZO2hylation are of given AZO1. in They Table can1. Frombe the data shown,related itto isthe evident two additional that two carbon new atoms signals present appear in the after AZO2 the structure methylation after AZO1 of AZO1.methylation. They can be related to the two additional carbon atoms present in the AZO2 structure after AZO1 methylation. Polymers 2019, 11, x FOR PEER REVIEW Polymers 2019, 11, x FOR PEER REVIEW 8 of8 21of 21 Table 1. 13C NMR chemical shifts comparison of AZO1 and AZO2 molecules.

AZO1 AZO2 1 118.2 113.3 2 135.1 133.0 3 134.2 129.4 4 128.1 113.6 5 141.7 142.2 6 151.7 144.5 96 138.1151.7 141.7144.5 109 155.5138.1 154.8141.7 111010 155.5 113.7155.5 154.8 116.6154.8 121111 113.7 128.9113.7 116.6 156.0116.6 13 122.0 121.4 1212 128.9128.9 156.0156.0 14 116.1 115.4 171313 122.0 56.3122.0 121.4121.4 55.8 181414 116.1 166.7116.1 115.4 169.1115.4 211717 56.3 19.956.3 55.8 20.155.8 221818 166.7166.7 - 169.1169.1 56.1 23 - 55.8 2121 19.919.9 20.120.1 2222 - - 56.156.1 2323 - - 55.855.8

3.33.. 3UV/Vis. UV/Vis Spectra Spectra Interpretation Interpretation: : TheThe UV UV–visible–visible absorption absorption spectra spectra of ofthe the azo azo-derivatives-derivatives exhibit exhibit two two characteristic characteristic absorption absorption bandsbands. The. The first first characteristic characteristic absorption absorption band band is ispresent present around around 3 3 303–032–320 0nm nm, while, while the the second second absorptionabsorption band band is ispresent present around around 412 412–325–325 nm. nm. ComparingComparing the the absorption absorption spectra, spectra, it itis isevident evident that that the the methoxy methoxy groups groups attached attached on on both both the the phenylphenyl rings rings of ofAZO2 AZO2 are are blue blue shifting shifting the the λ 1λ of1 of26 2 nm6 nm in inDMF DMF (T (abTable le2). 2 This). This can can be be explained explained by by thethe lower lower conjugation conjugation effect effect that that they they induce induce on on the the –N=N –N=N– bond– bond electrons. electrons.

TableTable 2. 2.Wavel Wavelengthength maxima maxima and and experimentally experimentally measured measured rate rate constants constants for for isomerization isomerization (k i (k), i), reversereverse isomerization isomerization (k r(k) andr) and the the content content of ofthe the Z- Zisomer-isomer at atthe the PSS PSS (α Z(α). ZAZO1). AZO1 and and AZO2 AZO2 optical optical propertiesproperties comparison. comparison. * = * = the the obtained obtained value value is is not not reliable reliable because because it it is is fa lling falling within within the the experimentalexperimental uncertainty uncertainty of ofthe the instrument instrument (see (see text). text).

−1 −1 SampleSample SolventSolvent λ1 λ(nm)1 (nm) λ2 λ(nm)2 (nm) ki k(si (s)− 1) kr k(sr (s)− 1) αZα (%)Z (%) AZO1AZO1 DMFDMF 415415 330330 1.51.5  10 10−4 −4 * * 3.23.2 AZO2AZO2 DMFDMF 389389 320320 3.083.08  10 10−3 −3 3.773.77  10 10-6 -6 45.745.7 ToTo understand understand the the optical optical properties properties of ofthe the two two compounds compounds, an, an in -insilico-silico analysis analysis was was carried carried out out withwith the the time time dependent dependent density density functional functional (TD (TD-DFT)-DFT) approach. approach. A ADFT DFT geometry geometry optimisation optimisation (see (see computationalcomputational methods) methods) was was performed performed for for the the ground ground state state for for both both structural structural isomers isomers of ofAZO1 AZO1 andand AZO2. AZO2. The The simulated simulated electronic electronic vertical vertical transitions transitions of ofboth both molecules molecules are are displayed displayed on on Figure Figure 3 3 andand these these will will occur occur in inthe the frontier frontier orbital orbital region region mapped mapped out out in inFigure Figure 4. 4The. The calculated calculated bands bands (TD (TD- - PBE0)PBE0) compare compare favourably favourably well well to tothe the experimental experimental ones ones, presenting, presenting a redshifta redshift in inthe the region region of of20 20–50–50 nm.nm. The The relative relative intensities intensities are are incorrectly incorrectly described described, likely, likely due due to tosolvent solvent interaction interaction effects. effects. Polymers 2020, 12, 1019 8 of 20

3.3. UV/Vis Spectra Interpretation: The UV–visible absorption spectra of the azo-derivatives exhibit two characteristic absorption bands. The first characteristic absorption band is present around 330–320 nm, while the second absorption band is present around 412–325 nm. Comparing the absorption spectra, it is evident that the methoxy groups attached on both the phenyl rings of AZO2 are blue shifting the λ1 of 26 nm in DMF (Table2). This can be explained by the lower conjugation effect that they induce on the –N=N– bond electrons.

Table 2. Wavelength maxima and experimentally measured rate constants for isomerization (ki), reverse isomerization (kr) and the content of the Z-isomer at the PSS (αZ). AZO1 and AZO2 optical properties comparison. * = the obtained value is not reliable because it is falling within the experimental uncertainty of the instrument (see text).

1 1 Sample Solvent λ1 (nm) λ2 (nm) ki (s− ) kr (s− ) αZ (%) AZO1 DMF 415 330 1.5 10 4 * 3.2 × − AZO2 DMF 389 320 3.08 10 3 3.77 10-6 45.7 × − ×

To understand the optical properties of the two compounds, an in-silico analysis was carried out with the time dependent density functional (TD-DFT) approach. A DFT geometry optimisation (see computational methods) was performed for the ground state for both structural isomers of AZO1 and AZO2. The simulated electronic vertical transitions of both molecules are displayed on Figure3 and these will occur in the frontier orbital region mapped out in Figure4. The calculated bands (TD-PBE0) compare favourably well to the experimental ones, presenting a redshift in the region of 20–50 nm. The relative intensities are incorrectly described, likely due to solvent interaction effects. The band maxima corresponding to the electronic excitations are listed in Tables3 and4 for AZO1 and AZO2, respectively. The most intense transition in the visible region can be assigned to the second excited state for both E-AZO1 and E-AZO2, each showing oscillator strengths of 0.57 and 0.91 respectively. The conventional assignment of distinctly separate n π* and π π* transitions is → → not rigorously valid in the case of E-AZO1 since there is an admixture of both transitions in the first Polymerstwo bands. 2019, 11, x FOR PEER REVIEW 9 of 21

Experimental Calc. TD-PBE0

250 350 450 550 250 350 450 550 UV-Vis spectrum of E-AZO1 UV-Vis spectrum of E-AZO2 FigureFigure 3. 3. MeasuredMeasured and and calculated calculated absorption absorption spectra spectra of ofE-AZO1E-AZO1 (left (left) and) and E-AZO2E-AZO2 (right (right) in) dimethylformamide.in dimethylformamide.

Polymers 2020, 12, 1019 9 of 20

Polymers 2019, 11, x FOR PEER REVIEW 10 of 21

E-AZO1 E-AZO2

Energy

76a - LUMO (π*); -2.9 eV 84a – LUMO (π*); -2.7 eV

75a – HOMO (π2); -6.4 eV 83a – HOMO (π2); -6.4 eV

74a – HOMO-1 (n2); -7.0 eV 82a – HOMO-1 (n2); -6.8 eV

73a – HOMO-2 (n1); -7.2 eV 81a – HOMO-2 (n1); -7.0 eV

72a – HOMO-3 (π1); -7.4 eV 80a – HOMO-3 (π1); -7.4 eV

Figure 4. Kohn-Sham molecular orbitals relevant to the electronic transitions of E-AZO1 and E-AZO2 Figure 4. Kohn-Sham molecular orbitals relevant to the electronic transitions of E-AZO1 and E-AZO2 in the visible region. in the visible region.

The band maxima corresponding to the electronic excitations are listed in Tables 3 and 4 for AZO1 and AZO2, respectively. The most intense transition in the visible region can be assigned to the second excited state for both E-AZO1 and E-AZO2, each showing oscillator strengths of 0.57 and 0.91 respectively. The conventional assignment of distinctly separate n→ π* and π→ π* transitions is not rigorously valid in the case of E-AZO1 since there is an admixture of both transitions in the first two bands.

Polymers 2020, 12, 1019 10 of 20

Table 3. Calculated excitation energies, band maxima assignments and oscillator strengths (f osc) of E-AZO1 (experimental values from UV/Vis analysis are shown in brackets).

Excited State # Main Character Excitation (nm) f osc π π* (48%) 2→ 489 S1 n π* (37%) 0.34 1→ (415) n π* (12%) 2→ π π* (51%) 2→ 441 S2 n π* (36%) 0.57 1→ (399) n π* (11%) 2→ n2 π* (76%) 2 S3 → 379 6.9 10 n π* (23%) × − 1→ 338 S4 π π* (98%) 0.33 1→ (330)

Table 4. Calculated excitation energies, band maxima assignments and oscillator strengths (f osc) of E-AZO2 (experimental values from UV/Vis analysis are shown in brackets).

Excited State # Main Character Excitation (nm) f osc

n2 π* (58%) 3 S1 → 494 9.6 10 n π* (37%) × − 1→ 448 S2 π π* (98%) 0.91 2→ (389) n1 π* (59%) 2 S3 → 372 6.6 10 n π* (39%) × − 2→ 333 S4 π π* (98%) 0.30 1→ (320)

The first and second electronic transitions of E-AZO1 correspond to two maxima calculated to be at 489 and 441 nm and have a majority of π π* character. The other band with significant oscillator 2→ strength in this compound is due to a π π* transition occurring at 338 nm. 1→ The spectral features of E-AZO2 are altogether significantly different in that n π* transitions do → not admix with the π π* type excitations. Since the former transitions are symmetry forbidden, they → have a very low oscillator strength while the latter show up as very strong intensity bands. To broaden the study on the optical properties of the two moieties, the samples were dissolved in different solvents, according to the solubility of the different molecules. Figure5 shows the dipole moments (in Debye) of each solvent compared to the wavelengths of the main peaks (λ1, λ2, λ3) of the absorption spectra of the azo-derivatives. For AZO1 molecule, a broader range of solvents could be tested. The first peculiar aspect is that AZO1 main peak (λ1) is splitting its signal passing from a more polar solvent like dimethyl sulfoxide to a less polar one like dichloromethane. This can be ascribed to the different mechanisms of excitation followed by the electrons involved in the azo double bond when irradiated by light. In more polar solvents the signal linked to this excitation are overlapped (λ1), then their wavelength split (λ1 and λ3) being better resolved when the polarity of the solvent decreases. The second crucial aspect is highlighted by the hypsochromic shift of λ1 and the bathochromic shift of λ2 of AZO1 molecule in polar solvents. This can be related to the stabilization of the electronic cloud of AZO1, that is favoured by the presence of a less polar solvent like dioxane. This, in fact, can decrease the conjugation effect of the electrons of the azo double bond, shifting lambda to lower wavelengths. AZO2 λ1 is slightly influenced by the solvent polarity, a bathochromic shift is registered passing from a less polar solvent to a more polar one [51]. The UV-Vis spectrum was calculated for E-AZO1 in various implicit solvent dielectric media to determine how the effect that polarity shifts the visible band wavelengths. As can be seen from the values in Supplementary Table S4, the values of λ1 tend to increase when passing from vacuum to the solvated environment. Polymers 2020, 12, 1019 11 of 20

Neglecting dispersion interactions between solvent and solute represents the limiting case of solvation in non-polar media. However, the value of λ1 changed only a few for the used selection of solvents, with varying degree of polarity differences. The observed trend for DMSO is not reproduced by the calculation and in fact the value is slightly higher with respect to CH2Cl2 and DMF, but the change is so small that barely any significance can be inferred from it. The failure to account for the experimental trend is a sign that explicit interactions between the solute and the solvent, which are not consideredPolymers 2019 at, 11 this, x FOR level PEER of theory, REVIEW are essential to explain the subtle band shifts. 12 of 21

FigureFigure 5. 5.Band Band absorption absorption maxima maxima of of AZO1 AZO1/AZO2/AZO2 correlated correlated with with solvent solvent dipole dipole moment moment (Debye). (Debye). 3.4. E–Z Photoisomerization For AZO1 molecule, a broader range of solvents could be tested. The first peculiar aspect is that AZO1The main photoisomerization peak (λ1) is splitting kinetics itsof signal azo-derivatives passing from was a establishedmore polar bysolvent monitoring like dimethyl the absorbance sulfoxide ofto S 2 aabsorption less polar band. one like Figure dichloromethane.6 shows the UV–Vis This spectra can be of ascribed the AZO1 to the and different AZO2 molecules mechanism afters of irradiationexcitation atfollowed different by times. the electrons Under white involved light in irradiation the azo double the intensity bond when of the irradiated S2 absorption by light. band In graduallymore polar decreases solvents with the signal the irradiation linked to time,this excitation while the are S4 overlappedabsorption band(λ1), then increases, their wavelength until the photo-stationarysplit (λ1 and λ3) statebeing is better reached. resolved Both when the solventthe polarity polarity of the and solvent substituent decrease natures. The influence second crucial the compositionaspect is highlighted of the PSS. In by fact, the comparing hypsochromic the isomerization shift of λ1 and of both the molecules, bathochromic there shift are big of di λ2ff oferences AZO1 thatmolecule are easy in to polar point solvents. out. First, This only can a slightbe related change to isthe visible stabilization in AZO1 of absorption the electronic spectra cloud (Figure of AZO1,6A). ThethatE-Z is conversionfavoured by rate the is presence about 3.3 of % a after less 4 polar h 30 minsolvent of irradiation like dioxane. compared This, in to fact, 45.7% can of AZO2decrease after the 50conjugati min of irradiation.on effect of The the kinetic electrons constant of the of azo the isomerizationdouble bond, forshifting the two lambda molecules to lower differ wavelengths. by an order ofAZO2 magnitude λ1 is slightly (Table2 influenced). by the solvent polarity, a bathochromic shift is registered passing from a less polar solvent to a more polar one [51]. The UV-Vis spectrum was calculated for E-AZO1 in various implicit solvent dielectric media to determine how the effect that polarity shifts the visible band wavelengths. As can be seen from the values in Supplementary Table S4, the values of λ1 tend to increase when passing from vacuum to the solvated environment. Neglecting dispersion interactions between solvent and solute represents the limiting case of solvation in non-polar media. However, the value of λ1 changed only a few for the used selection of solvents, with varying degree of polarity differences. The observed trend for DMSO is not reproduced by the calculation and in fact the value is slightly higher with respect to CH2Cl2 and DMF, but the change is so small that barely any significance can be inferred from it. The failure to account for the experimental trend is a sign that explicit interactions between the solute and the solvent, which are not considered at this level of theory, are essential to explain the subtle band shifts.

3.4. E–Z Photoisomerization The photoisomerization kinetics of azo-derivatives was established by monitoring the absorbance of S2 absorption band. Figure 6 shows the UV–Vis spectra of the AZO1 and AZO2 molecules after irradiation at different times. Under white light irradiation the intensity of the S2 absorption band gradually decreases with the irradiation time, while the S4 absorption band increases, until the photo-stationary state is reached. Both the solvent polarity and substituent nature

Polymers 2019, 11, x FOR PEER REVIEW 13 of 21 influence the composition of the PSS. In fact, comparing the isomerization of both molecules, there are big differences that are easy to point out. First, only a slight change is visible in AZO1 absorption spectra (Figure 6A). The E-Z conversion rate is about 3.3 % after 4 h 30 min of irradiation compared toPolymers 45.7%2020 of, 12AZO2, 1019 after 50 min of irradiation. The kinetic constant of the isomerization for the12 two of 20 molecules differ by an order of magnitude (Table 2).

(A) (B)

Figure 6. UV/VisUV/Vis Absorption changes upon irradiation of visible light. From From left left to to right, right, ( AA)) AZO1 AZO1 and ( B) AZO2 absorption spectra.

Table5 5lists lists the the photoisomerization photoisomerization rate rate constants constants calculated calculated from from UV–Vis UV experiments–Vis experiments on AZO2 on AZO2in different in different solvents solvents at 303K. at It 303K. can be It observedcan be observed that AZO2 that kAZO2i increases ki increases going from going a lessfrom polar a less solvent polar solventlike dioxane like to dioxane a more polarto a more one, such polar as DMSO. one, such This as evidence DMSO. can This be evidencerelated to the can photoisomerization be related to the photoisomerizationmechanism. When rotationmechanism. or inversion-assisted When rotation or rotation inversion pathway-assisted is preferred, rotation pathway an acceleration is preferred, of the anphotoisomerization acceleration of the rate photoisomerization in polar solvents israte noticed, in polar due solvents to the stabilizationis noticed, due of to the the dipolar stabilization transition of thestate. dipolar A change transition in the state. isomerization A change in mechanism the isomerization can be mechanism responsible can for be the responsible lower percentage for the lower of Z percentagefraction registered of Z fraction when registered AZO2 is dissolved when AZO2 in dioxane is dissolved [51]. in dioxane [51].

Table 5.5. AZO2 E-ZE-Z photoisomerizationphotoisomerization rate constants at 303K. In this order: kii, direct isomerization kinetic constant; constant; kr k, rthermal, thermal relaxation relaxation kinetic kinetic constant; constant; αZ, contentαZ, content of the ofZ-isomer the Z-isomer at the PSS at the(photo PSS-

stationary(photo-stationary state); τ state);1/2, halfτ-1li/2fe, half-lifeof Z-isomer of Z -isomerin the dark. in the dark.

Solvent 1 1 α (%) τ (s) Solvent ki (sk−i (s)−1) kr (s k−1r)(s − ) αZ (%) Z τ1/2(s) 1/2 DMF 3.08 10 3 3.77 10 6 45.7 1.84 105 DMF 3.08× − 10−3 3.77  10×−6 − 45.7 1.84  105 × DMSO 6.38 10 3 3.14 10 6 48.3 2.21 105 × − × − × DIOX DMSO 8.516.3810  104 −3 3.14 2.14 10−610 6 48.3 16.72.21  105 3.24 105 × − × − × DIOX 8.51  10−4 2.14  10−6 16.7 3.24  105 3.5. Z–E Thermal Relaxation 3.5. ZThe–E T kinetichermal Z–ERelaxationthermal relaxation of AZO1 and AZO2 were investigated at several temperatures in orderThe to kinetic determine Z–E itsthermal thermodynamic relaxation parameters. of AZO1 and However, AZO2 inwere the investigated case of AZO1, at thermal several temperaturesrelaxation was in impossible order to determine to study its due thermodynamic to the hindered parameters. isomerization. However, A rough in the estimation case of AZO1, of the thermalkinetic constant relaxation relative was impossible to Z-E thermal to study relaxation due to ofthe AZO1 hindered was isomerization. attempted from A therough data estimation obtained of in theDMF: kinetic unfortunately, constant relative the obtained to Z-E thermal value is relaxation not reliable of becauseAZO1 was it isattempted falling within from the data experimental obtained inuncertainty DMF: unfortunately, of the instrument the obtained (for sake value of explanationis not reliable the because tentative it isfitting falling is within shown the in experimental the SI with a 2 uncertaintyvalue R = 0.040). of the instrument The rate of Z–E(for sathermalke of explanation relaxation ofthe AZO2 tentative molecule fitting was is shown monitored in the in SI the with dark a valueat the R absorption2 = 0.040). The maximum rate of Z of–E its thermalE-isomer. relaxation The thermal of AZO2Z to moleculeE isomerization was monitored rate, summarized in the dark at in theTable absorption6, was determined maximum for of AZO2 its E-isomer. at di fferent The thermal temperatures Z to E and isomerization solvents. The rate, process summarized was found in Table to be first order in all the studied solvents.

Polymers 2019, 11, x FOR PEER REVIEW 14 of 21

6, was determined for AZO2 at different temperatures and solvents. The process was found to be first order in all the studied solvents. Polymers 2020, 12, 1019 13 of 20 Table 6. AZO2 kr (in s−1) constants in different solvents at different temperatures.

1 TableT 6. (K)AZO2 kr (in s− DMF) constants in diff DMSOerent solvents at different DIOX temperatures.

× −6 × −6 × −6 303.15T (K) 3.77 10 DMF 3.14 10 DMSO 2.14 10 DIOX × −5 6 × −5 6 × −6 6 313.15303.15 1.18 3.77 10 10− 1.02 3.14 10 10− 6.23 2.14 10 10− × 5 × 5 × 6 323.15313.15 3.22 1.18× 10−5 10− 3.01 ×1.02 10−5 10− 1.73 ×6.23 10−5 10− × 5 × 5 × 5 323.15 3.22 10− 3.01 10− 1.73 10− × ×−5 × −×5 × −5× 333.15333.15 5.59 5.59 10 10 5 6.93 6.93 10 10 5 3.81 3.81 10 10 5 × − × − × − Calculation of the first-order rate constants at various temperatures allowed us to estimate ‡ thermodynamicCalculation activation of the first-order parameters rate such constants as the activation at various energy temperatures (Ea), activation allowed enthalpy us to estimate(ΔH ), and activation entropy (ΔS‡) for thermal Z to E reaction using Arrhenius and Eyring equations thermodynamic activation parameters such as the activation energy (Ea), activation enthalpy (∆H‡), (equations shown in Appendix A). We used the linear correlation between the enthalpy and the and activation entropy (∆S‡) for thermal Z to E reaction using Arrhenius and Eyring equations entropy(equations of activation shown in to Appendix determineA). the We mechanism used the linear of AZO2 correlation thermal between relaxation. the enthalpy In all the and solvents, the entropy the Arrheniusof activation plots to were determine linear, the proving mechanism that there of AZO2 is no thermal significant relaxation. changeIn inall the the reaction solvents, pathway the Arrhenius over theplots examined were linear, temperature proving range. that there AZO2 is no compound significant has change a positive in the enthalpy reaction of pathway activation over and the negative examined entropytemperature for all range.the solvents AZO2 used compound (see Table has a7). positive enthalpy of activation and negative entropy for all the solvents used (see Table7). Table 7. AZO2 thermodynamic activation parameters. Table 7. AZO2 thermodynamic activation parameters. Solvent Dipole Moment (D) A (s−1) Ea (kJ mol−1) ΔH‡ (kJ mol−1) ΔS‡ (J mol−1 K−1) Solvent Dipole Moment (D) A (s 1)E (kJ mol 1) ∆H (kJ mol 1) ∆S (J mol 1 K 1) DMF 3.86 6.57 × 10− 7 76.6a − 74.0‡ − −‡104.09 − − DMF 3.86 6.57 107 76.6 74.0 104.09 DMSO 4.1 3.41 × ×109 87.2 84.5 −71.25− DMSO 4.1 3.41 109 87.2 84.5 71.25 × − DIOXDIOX 0.45 0.45 2.222.22 × 10108 8 81.381.3 78.6 78.6 −94.0394.03 × − AZO2 in DMF has the highest ΔS‡ absolute value and the lowest ΔH‡, and therefore exhibits the fastest AZO2thermal in relaxation DMF has the[52,53]. highest This∆ isS‡ consistentabsolute value with andthe calculation the lowest ∆thatH‡ ,will and be therefore shown exhibitsbelow. In the orderfastest to understand thermal relaxation the dynamic [52,53 photochemical]. This is consistent and thermal with theprocesses calculation involved that in will the be isomerisation shown below. ofIn AZO1 order and to understand AZO2 a choice the dynamicof appropriate photochemical computational and thermal strategy processes was paramount. involved inThe the employment isomerisation ofof density AZO1 functional and AZO2 theory a choice is ofunsuitable appropriate in this computational instance because strategy there was is paramount.no guarantee, The that employment at every stageof density in the transformation functional theory within is unsuitable the specific in thisexcited instance state becausehypersurface, there isth noe electron guarantee, wavefunction that at every ofstage the inground the transformation state remains within singly the determinan specific excitedtal. stateThe hypersurface,best methodology the electron is to wavefunctionadopt multi- of configurationalthe ground state self-consistent remains singly field determinantal. theory [36,54] The with best a methodology posteriori perturbational is to adopt multi-configurational corrections for a goodself-consistent description field of theorydynamical [36,54 electron] with a posterioricorrelation perturbational such as the correctionsNEVPT2 approach for a good [40,55] description (see of Computationaldynamical electron Methods). correlation such as the NEVPT2 approach [40,55] (see Computational Methods). TwoTwo conventional conventional mechanisms mechanisms usually usually adopted adopted to todescribe describe the the isomer isomer change change involve involve the the either either thethe twisting twisting of of the the (N(N=N=N–C)–C) angleangle (inversion)(inversion) or or a rotationa rotation of theof the planes planes that that make make up the up (C-Nthe =(C-N–C) N=Ndihedral–C) dihedral (rotation). (rotation). Only the Only latter the was latter found was to exhibitfound moreto exhibit favourable more energiesfavourable for theenergies photochemical for the photochemicalconversion. Thisconversion. contradicts This the contradicts hydrogen the bonding hydrogen inhibition bonding scenario inhibition as presented scenario as by presented Bandara et byal. Bandara [50], since et al. rotation [50], since of the rotation molecule of around the molecule the –CNNC– around dihedral the –CNNC planes– doesdihedral not necessarilyplanes does require not necessarilythe termination require of the the termination intramolecular of the hydrogen intramolec bondular with hydrogen the adjacent bond hydroxyl with the group. adjacent An hydroxyl alternative group.explanation An alternative is given explanation below to account is given for below the slower to account rates offor photo-switching. the slower rates of photo-switching. TheThe alternative alternative twisting twisting scenario scenario may maybe found be found in the insupporting the supporting information information section. More section. elaborateMore elaborate reaction reaction coordinates coordinates combining combining angular angular and dihedral and dihedral changes changes have also have been also studied been studied for unsubstitutedfor unsubstituted azobenzene azobenzene [56]. [56]. AsAs seen seen from from Tables Tables 3 3and and 44 the the second second excited excited state state (S (S2) 2presents) presents the the highest highest oscillator oscillator strength strength forfor both both AZO1 AZO1 and and AZO2: AZO2: this this will will be be the the most most efficient efficient conversion conversion pathway. pathway. A Arelaxed relaxed potentialpotential energyenergy surface surface (PES) (PES) scan scan of the of –CNNC– the –CNNC dihedral– dihedral at the QD-NEVPT2(4,3) at the QD-// NEVPT2(4,3)//CASSCFCASSCF (4,3)/def2-TZVP (4,3)/def2-TZVP level of theory level is presented of theory in is Figure presented7 and in in Figure Figure 87 for and AZO1 in Figure and AZO2,8 for AZO1respectively. and AZO2, Therein respectively. the energies Therein of eachthe energies of the lowest of each six of singlets the lowest were six monitored singlets were throughout monitored this throughoutreaction coordinate. this reaction The coordinate. main composition The main of co themposition states at theof the beginning states at of the the beginning process (E -isomer)of the is qualitatively described with different colours. The curves are qualitatively similar to what was previously reported for azobenzene [57]. Polymers 2019, 11, x FOR PEER REVIEW 14 of 21

6, was determined for AZO2 at different temperatures and solvents. The process was found to be first order in all the studied solvents.

Table 6. AZO2 kr (in s−1) constants in different solvents at different temperatures.

T (K) DMF DMSO DIOX 303.15 3.77 × 10−6 3.14 × 10−6 2.14 × 10−6 313.15 1.18 × 10−5 1.02 × 10−5 6.23 × 10−6 323.15 3.22 × 10−5 3.01 × 10−5 1.73 × 10−5 333.15 5.59 × 10−5 6.93 × 10−5 3.81 × 10−5 Calculation of the first-order rate constants at various temperatures allowed us to estimate Polymers 2019, 11, x FOR PEER REVIEW 15 of 21 thermodynamic Polymersactivation 2019 parameters, 11, x FOR PEER such REVIEW as the activation energy (Ea), activation enthalpy (ΔH‡), 15 of 21 and activation entropy (ΔS‡) for thermal Z to E reaction using Arrhenius and Eyring equations process (E-isomer) is qualitatively described with different colours. The curves are qualitatively (equations shownPolymerspr ocessin Appendix2020 (E, 12-isomer), 1019 A). isWe qualitatively used the linear described correlation with differentbetween the colours. enthalpy The and curves the are qualitatively14 of 20 similar to what was previously reported for azobenzene [57]. entropy of activationsimilar to determineto what was the previously mechanism reported of AZO2 for thermal azobenzene relaxation. [57]. In all the solvents, the Arrhenius plots were linear, proving that there is no significant change in the reaction pathway over the examined temperature range. AZO2 compound has a positive enthalpy of activation and negative entropy for all the solvents used (see Table 7).

Table 7. AZO2 thermodynamic activation parameters.

Solvent Dipole Moment (D) A (s−1) Ea (kJ mol−1) ΔH‡ (kJ mol−1) ΔS‡ (J mol−1 K−1) DMF 3.86 6.57 × 107 76.6 74.0 −104.09 DMSO 4.1 3.41 × 109 87.2 84.5 −71.25 DIOX 0.45 2.22 × 108 81.3 78.6 −94.03 AZO2 in DMF has the highest ΔS‡ absolute value and the lowest ΔH‡, and therefore exhibits the fastest thermal relaxation [52,53]. This is consistent with the calculation that will be shown below. In order to understand the dynamic photochemical and thermal processes involved in the isomerisation of AZO1 and AZO2 a choice of appropriate computational strategy was paramount. The employment of density functional theory is unsuitable in this instance because there is no guarantee, that at every stage in the transformation within the specific excited state hypersurface, the electron wavefunction of the ground state remains singly determinantal. The best methodology is to adopt multi- configurational self-consistent field theory [36,54] with a posteriori perturbational corrections for a good description of dynamical electron correlation such as the NEVPT2 approach [40,55] (see Computational Methods).FigureFigureFigure 7. 7.7.QD-NEVPT2(4,3) QDQD--NEVPT2(4,3)//SA6NEVPT2(4,3)//SA6//SA6-CASSCF--CASSCFCASSCF (4,3) (4,3)/def2(4,3)/def2/def2-TZVP--TZVPTZVP potential potentialpotential energy energyenergy surface surfacesurface scan scanscan of ofof the thethe six sixsix lowest lying singlet states of AZO1. The S2 and S3 have an avoided crossing at approximately Two conventionallowestlo westmechanisms lying lying singlet singlet usually states states adopted of of AZO1. AZO1. to describe The The S 2 S2and andthe S isomer3 S3have have an change an avoided avoided involve crossing crossing the either at at approximately approximately (CNNC) = 139° and some 8.6 kcal/mol above the S2 minimum. the twisting of the (CNNC)(N=N(CNNC)–C)= =139angle 139°◦ and and(inversion) some some 8.6 8.6 kcal kcal/molor /amol rotation above above the ofthe Sthe 2S2minimum. minimum.planes that make up the (C- N=N–C) dihedral (rotation). Only the latter was found to exhibit more favourable energies for the photochemical conversion. This contradicts the hydrogen bonding inhibition scenario as presented by Bandara et al. [50], since rotation of the molecule around the –CNNC– dihedral planes does not necessarily require the termination of the intramolecular hydrogen bond with the adjacent hydroxyl group. An alternative explanation is given below to account for the slower rates of photo-switching. The alternative twisting scenario may be found in the supporting information section. More elaborate reaction coordinates combining angular and dihedral changes have also been studied for unsubstituted azobenzene [56]. As seen from Tables 3 and 4 the second excited state (S2) presents the highest oscillator strength for both AZO1 and AZO2: this will be the most efficient conversion pathway. A relaxed potential energy surface (PES) scan of the –CNNC– dihedral at the QD- NEVPT2(4,3)//CASSCF (4,3)/def2-TZVP level of theory is presented in Figure 7 and in Figure 8 for AZO1 and AZO2, respectively. Therein the energies of each of the lowest six singlets were monitored throughout this reaction coordinate. The main composition of the states at the beginning of the

FigureFigureFigure 8. 8. 8.QD-NEVPT2(4,3) QDQD--NEVPT2NEVPT2(4,3)//SA6(4,3)//SA6//SA6-CASSCF(4,3)--CASSCF(4,3)/def2CASSCF(4,3)/def2/def2-TZVP--TZVPTZVP potential potential potential energy energy energy surface surface surface scan scan scan of of of the the the six six six lowest lying singlet states of AZO2. S2 and S3 states cross at approximately (CNNC) = 150 and lowestlowest lyinglying singletsinglet statesstates ofof AZO2.AZO2. SS2 andand SS3 3 statesstates crosscross atat approximatelyapproximately (CNNC)(CNNC) == 150°and150°and◦ 6.66.6 6.6 kcal/mol above the2 S minimum. kcal/molkcal/mol aboveabove thethe SS2 minimum.minimum.2 The photochemical process originates with the promotion of S S (π1 π*1) which can allow the 0 02 2 11 11 TheThe photochemicalphotochemical processprocess originatesoriginates withwith thethe promotionpromotion ofof S→S0SS2 (π(π π*π*)) whichwhich cancan allowallow thethe system enough flexibility to distort the dihedral once the π bond strength has a formal bond order of systemsystem enoughenough flexibilityflexibility toto distortdistort ththee dihedraldihedral onceonce thethe ππ bondbond strengthstrength hashas aa formalformal bondbond orderorder ofof zero.zero. However, However, due due to to orbital orbital polarisation polarisation there there remains remains a a marginally marginally bonding bonding interaction interaction and and some some zero. However, due to orbital polarisation there remains a marginally bonding interaction1 and some thermal energy is still required to break it. This barrier is computed to be 36.0 kJ mol−−1 in the case thermalthermal energyenergy isis stillstill requiredrequired toto breakbreak itit.. ThisThis barrierbarrier isis computedcomputed toto bebe 36.036.0 kkJJ molmol−1 inin thethe casecase ofof of AZO1 and 27.4 kcal mol 1 for AZO2 and takes place at (CNNC) = 135 and (CNNC) = 143 − ◦ ◦ Polymers 2019, 11, x FOR PEER REVIEW 14 of 21

6, was determined for AZO2 at different temperatures and solvents. The process was found to be first order in all the studied solvents.

Table 6. AZO2 kr (in s−1) constants in different solvents at different temperatures.

T (K) DMF DMSO DIOX 303.15 3.77 × 10−6 3.14 × 10−6 2.14 × 10−6 313.15 1.18 × 10−5 1.02 × 10−5 6.23 × 10−6 323.15 3.22 × 10−5 3.01 × 10−5 1.73 × 10−5 333.15 5.59 × 10−5 6.93 × 10−5 3.81 × 10−5 Calculation of the first-order rate constants at various temperatures allowed us to estimate thermodynamic activation parameters such as the activation energy (Ea), activation enthalpy (ΔH‡), and activation entropy (ΔS‡) for thermal Z to E reaction using Arrhenius and Eyring equations (equations shown in Appendix A). We used the linear correlation between the enthalpy and the Polymers 2020, 12, 1019 15 of 20 entropy of activation to determine the mechanism of AZO2 thermal relaxation. In all the solvents, the Arrhenius plots were linear, proving that there is no significant change in the reaction pathway over respectively. One metricthe examined to measure temperature the π symmetry range. AZO2 bond strengthcompound from has the a positive electronic enthalpy density of is activation the and negative natural localized molecularentropy for orbital all the (NLMO) solventsbond used order(see Table taken 7). from the CASCI(4,3) density matrix. A comparative analysis of the azo bond orders in ground (S0) and excited (S2) states is displayed on Table8. It may be seen that the calculated πTablebond 7. order AZO2 is thermodynamic identical from activation the outset parameters. (S ). The S S 0 0→ 2 excitation suppresses the π bond strength differently in AZO1 and AZO2. In AZO1, the π bond −1 −1 ‡ −1 ‡ −1 −1 remnant is stronger thanSolvent in AZO2 Dipole thereby Moment justifying (D) theA additional(s ) Ea (kJ energetic mol ) requirementΔH (kJ mol for) theΔS (J mol K ) dihedral rotation and renderingDMF AZO1 kinetically3.86 more6.57 inert × in 10 the7 S2 state.76.6 74.0 −104.09 DMSO 4.1 3.41 × 109 87.2 84.5 −71.25 Table 8. NLMO bond orders in the two states S0 and S2 at the ground state CASCI (4,3)/def2-TZVP level. DIOX 0.45 2.22 × 108 81.3 78.6 −94.03 π π AZO2NLMO in DMF(N–N) has the Bond highest Order Δ S0S‡ absoluteNLMO value(N–N) and Bond the lowest Order S Δ2 H‡, and therefore exhibits the 3 AZO1fastest thermal relaxation 0.951 [52,53]. This is consistent with7.84 the10− calculation that will be shown below. In × AZO2 0.934 4.43 10 3 order to understand the dynamic photochemical and thermal× − processes involved in the isomerisation of AZO1 and AZO2 a choice of appropriate computational strategy was paramount. The employment Disregardingof thermal density contributions functional theory to the is transition unsuitable state in this energies, instance i.e., because∆E (S )there∆G is (Sno )guarantee, and that at every ‡ 2 ≈ ‡ 2 assuming a high quantumstage in yieldthe transformation for the S2 excitation, within the the Eyring specific equation excited yieldsstate hypersurface, a ki(AZO1)/k ith(AZO2)e electron wavefunction ratio of 3.1 10 2 usingof the the ground above state calculated remains values singly which determinan is reasonablytal. The close best to experimental methodology one is to adopt multi- × − (4.8 10 2). Overall,configurational a 9 kJ mol 1 energyself-consistent difference field leads theory to a [36,54] decrease with in thea posteriori reaction rateperturbational constant corrections for a × − − close to two ordersgood of magnitude. description of dynamical electron correlation such as the NEVPT2 approach [40,55] (see Towards moreComputational acute angles one Methods). hallmark feature [57] of azobenzene derivatives comes to the fore, which is the conicalTwo intersection conventional (CI) mechanisms allowing S 2usuallyto configurationally adopted to describe admix the with isomer S0 leading change involve the either to the cis isomer, inthe the twisting region of 80 the◦ < (–CNNC–)(N=N–C) angle< 100 (inversion)◦ as the lone or a pairs rotation (n) become of the planes degenerate that make up the (C- 1 with the π orbitals.N=N The–C) energy dihedral gap (rotation). in this nonadiabatic Only the latter coupling was foun (H02d) isto 98.3exhibit kJ mol more− infavourable AZO1 energies for the 1 and 83.1 kJ mol− inphotochemical AZO2. From conversion. the Landau–Zener This contradicts relation [58 the], ithydr is knownogen bonding that the probabilityinhibition scenario of as presented H2 /α transition betweenby the Bandara two adiabatic et al. surfaces[50], since S0 rotationand S2 can of the be givenmolecule by e −around02 where the –αCNNCis a collection– dihedral planes does not of constants incorporatingnecessarily nuclear require velocity, the termination and the di offf erencethe intramolec in reactionular coordinatehydrogen bond slope with in each the adjacent hydroxyl 2 adiabatic state. H02group.is the squaredAn alternative energy explanation gap between is thegiven two below states to at account the avoided for the crossing. slower rates As the of photo-switching. 1 1 values of H02 are considerableThe alternative for both moleculestwisting scenario (98.3 kJ molmay− bein AZO1found andin the 83.1 supporting kJ mol− in information AZO2), section. More the probability of radiationlesselaborate reaction transition coordinates is practically combining zero. Thus,angular the and S dihedralS transition changes will have require also been studied for 0← 2 the emission of oneunsubstituted photon at 1217 azobenzene nm (AZO1) [56]. and 1437 nm (AZO2) wavelengths (infra-red region). The potential energyAs landscapes seen from alsoTables aff ord3 and an 4 overview the second of theexcited thermal state back-isomerization (S2) presents the highest which oscillator strength 1 has the values +74.9for andboth+ AZO174.4 kJ and mol AZO2:− . These this valueswill be arethe inmost good efficient agreement conversion with thepathway. relaxation activation energies determinedA relaxed experimentally potential evenenergy neglecting surface the(PES) necessary scan thermalof the corrections–CNNC– (zerodihedral at the QD- point, vibrational, rotationalNEVPT2(4,3)//CASSCF and translational). (4,3)/def2-TZVP level of theory is presented in Figure 7 and in Figure 8 for In light of theAZO1 current and data, AZO2, the presence respectively. of the Therein –OH group the energies in AZO1 of cannot each of be the considered lowest six to singlets play were monitored a direct role in thethroughout rapid back isomerizationthis reaction ofcoordinate. AZO1 if one The considers main composition this neutral of tautomer the states alone at the as beginning of the alluded to in the literature [50]. In view of the acidic properties of AZO1, all the possible tautomeric zwitterionic isomers were optimized at the PBE0/TZP level and their optical properties assessed (see Supporting Information section). Our results show that a low energy tautomer may play a role in the isomerization process if the following equilibrium occurs (Scheme2). This zwitterionic isomer should be more favored in a polar 1 solvent medium [∆E(DMF) = 3.6 kcal mol− ]: Our results show that the presence of the hydroxy group does indeed influence the conversion process in AZO1, but not by creating strong hydrogen bonds with nitrogen but by instead allowing for the existence of diazo-phenolate that has distinctly unique photodynamic properties (Supplementary Figure S6). Polymers 2019, 11, x FOR PEER REVIEW 16 of 21

AZO1 and 27.4 kcal mol−1 for AZO2 and takes place at (CNNC) = 135° and (CNNC) = 143° respectively. One metric to measure the π symmetry bond strength from the electronic density is the natural localized molecular orbital (NLMO) bond order taken from the CASCI(4,3) density matrix. A comparative analysis of the azo bond orders in ground (S0) and excited (S2) states is displayed on Table 8. It may be seen that the calculated π bond order is identical from the outset (S0). The S0S2 excitation suppresses the π bond strength differently in AZO1 and AZO2. In AZO1, the π bond remnant is stronger than in AZO2 thereby justifying the additional energetic requirement for the dihedral rotation and rendering AZO1 kinetically more inert in the S2 state.

Table 8. NLMO bond orders in the two states S0 and S2 at the ground state CASCI (4,3)/def2-TZVP level.

NLMO π(N–N) Bond Order S0 NLMO π(N–N) Bond Order S2 AZO1 0.951 7.84×10−3 AZO2 0.934 4.43×10−3

Disregarding thermal contributions to the transition state energies, i.e., ΔE‡(S2) ≈ ΔG‡(S2) and assuming a high quantum yield for the S2 excitation, the Eyring equation yields a ki(AZO1)/ki(AZO2) ratio of 3.1  10−2 using the above calculated values which is reasonably close to experimental one (4.8  10−2). Overall, a 9 kJ mol−1 energy difference leads to a decrease in the reaction rate constant close to two orders of magnitude. Towards more acute angles one hallmark feature [57] of azobenzene derivatives comes to the fore, which is the conical intersection (CI) allowing S2 to configurationally admix with S0 leading to the cis isomer, in the region 80° < (–CNNC–) < 100° as the lone pairs (n) become degenerate with

−1 the π orbitals. The energy gap in this nonadiabatic coupling (퐻02) is 98.3 kJ mol in AZO1 and 83.1 kJ mol−1 in AZO2. From the Landau–Zener relation [58], it is known that the probability of transition −퐻2 /훼 between the two adiabatic surfaces S0 and S2 can be given by 푒 02 where α is a collection of constants incorporating nuclear velocity, and the difference in reaction coordinate slope in each 2 adiabatic state. 퐻02 is the squared energy gap between the two states at the avoided crossing. As the −1 −1 values of 퐻02 are considerable for both molecules (98.3 kJ mol in AZO1 and 83.1 kJ mol in AZO2), the probability of radiationless transition is practically zero. Thus, the S0S2 transition will require the emission of one photon at 1217 nm (AZO1) and 1437 nm (AZO2) wavelengths (infra-red region). The potential energy landscapes also afford an overview of the thermal back-isomerization which has the values +74.9 and +74.4 kJ mol−1. These values are in good agreement with the relaxation activation energies determined experimentally even neglecting the necessary thermal corrections (zero point, vibrational, rotational and translational). In light of the current data, the presence of the –OH group in AZO1 cannot be considered to play a direct role in the rapid back isomerization of AZO1 if one considers this neutral tautomer alone as alluded to in the literature [50]. In view of the acidic properties of PolymersAZO1, 2019all the, 11 , possiblex FOR PEER tautomeric REVIEW 14 of 21 zwitterionic isomers were optimized at the PBE0/TZP level and their optical properties assessed (see Supporting Information section). 6, was determined for AZO2 at different temperatures and solvents. The process was found to be first Our results show that a low energy tautomer may play a role orderin the inisomerization all the studied process solvents. if the Polymersfollowing2020 equilibrium, 12, 1019 occurs (Scheme 2). This zwitterionic isomer should be more favored in a16 polar of 20 Table 6. AZO2 kr (in s−1) constants in different solvents at different temperatures. solvent medium [ΔE(DMF) = 3.6 kcal mol−1]: T (K) DMF DMSO DIOX 303.15 3.77 × 10−6 3.14 × 10−6 2.14 × 10−6 313.15 1.18 × 10−5 1.02 × 10−5 6.23 × 10−6 323.15 3.22 × 10−5 3.01 × 10−5 1.73 × 10−5 333.15 5.59 × 10−5 6.93 × 10−5 3.81 × 10−5 Calculation of the first-order rate constants at various temperatures allowed us to estimate

thermodynamic activation parameters such as the activation energy (Ea), activation enthalpy (ΔH‡), Scheme 2. Tautomeric equilibrium between two structural isomers (A, C) of AZO1. Scheme 2. Tautomeric equilibrium between two structural isomersand activation (A,C) of AZO1. entropy (ΔS‡) for thermal Z to E reaction using Arrhenius and Eyring equations

4. Conclusions (equations shown in Appendix A). We used the linear correlation between the enthalpy and the entropy of activation to determine the mechanism of AZO2 thermal relaxation. In all the solvents, the In this work, two novel asymmetrical azobenzene derivativesArrhenius were successfully plots were synthesized,linear, proving that there is no significant change in the reaction pathway over characterized, and studied. A molecular design was done in orderthe to planexamined and create temperature moieties range. with AZO2 compound has a positive enthalpy of activation and negative one reactive suitable site for polymer functionalization. The moleculesentropy were setfor toall have the solvents electron-donor used (see Table 7). groups that permit isomerization when irradiated by visible light irradiation and a sufficiently stable Z-isomer with a thermal relaxation at temperatures above 303K. A wide-ranging study on theTableE–Z–E 7. AZO2 thermodynamic activation parameters. isomerization mechanism of the azo-derivatives was performed. The definite substitutional pattern was found to change the specific photochemistry of the molecules. ASolvent hindered Dipole isomerization Moment was (D) A (s−1) Ea (kJ mol−1) ΔH‡ (kJ mol−1) ΔS‡ (J mol−1 K−1) discovered in the case of AZO1 molecule, subsequently analyzed by theDMF TD-DFT calculations.3.86 6.57 × 107 76.6 74.0 −104.09 On the other hand, by methylating AZO1 to AZO2, the photochemistry of AZO2 was found to DMSO 4.1 3.41 × 109 87.2 84.5 −71.25 follow the bibliography trends. AZO2 isomerization occurs under visible light irradiation, giving × 8 a stable Z-isomer at room temperature, with thermal relaxation rate constantsDIOX increasing0.45 at higher 2.22 10 81.3 78.6 −94.03 temperature, as expected. The interaction between solvent and chromophoreAZO2 plays in aDMF role inhas determining the highest ΔS‡ absolute value and the lowest ΔH‡, and therefore exhibits the the rate of this process and therefore should be considered in futurefastest applications. thermal relaxation [52,53]. This is consistent with the calculation that will be shown below. In The rate of photo-switching was experimentally determined toorder be k (AZO1) to understand= 1.5 the10 dynamic4 s 1 and photochemical and thermal processes involved in the isomerisation i × − − k (AZO2) = 3.08 10 3 s 1 using DMF as solvent. of AZO1 and AZO2 a choice of appropriate computational strategy was paramount. The employment i × − − By means of TD-DFT calculations, the molecules were studied andof compared.density functional The calculated theory bandsis unsuitable in this instance because there is no guarantee, that at every (TD-PBE0) relate well to the experimental ones, presenting a minor systematicstage in redshift.the transformation The conventional within the specific excited state hypersurface, the electron wavefunction assignment of distinctly separated n π* and π π* transitions wasof found the toground be not rigorouslystate remains valid insingly determinantal. The best methodology is to adopt multi- → → the case of E-AZO1 since an admixture of both transitions in the first twoconfigurational bands was found. self-consistent The spectral field theory [36,54] with a posteriori perturbational corrections for a features of E-AZO2 are different in that n π* transitions do not admixgood with description the π π* typeof dynamical excitations. electron correlation such as the NEVPT2 approach [40,55] (see → → The dynamic photochemical and thermal processes involved inComputational the isomerisation Methods). of AZO1 and AZO2 was studied by means of multi-configurational self-consistent fieldTwo theoryconventional with a mechanisms posteriori usually adopted to describe the isomer change involve the either perturbational corrections. The rotation mechanism of the planesthe that twisting make upof the (C–N(N=N=–C)N–C) angle (inversion) or a rotation of the planes that make up the (C- dihedral angle was found to be the most favourable for the E-Z isomerN=N– change.C) dihedral The photo-chemical(rotation). Only the latter was found to exhibit more favourable energies for the process originates with the electronic promotion of S0 S2 and an energeticphotochemical barrier conversion. in the S2 surface This contradicts the hydrogen bonding inhibition scenario as presented → 1 was found along the reaction coordinate valued at +36.0 and +27.4by kJBandara mol− foret al. AZO1 [50], andsince AZO2 rotation of the molecule around the –CNNC– dihedral planes does not respectively. This difference in thermal barriers for the S2 state explainsnecessarily why require AZO1 the is kineticallytermination of the intramolecular hydrogen bond with the adjacent hydroxyl more inert to isomerisation with respect to AZO2. Another routegroup. for AZO1 An alternative conversion explanation is through is given below to account for the slower rates of photo-switching. dihedral rotation of the proton tautomer but the ground state of theThe Z isomer alternative in this twisting instance scenario is may be found in the supporting information section. More thermodynamically unstable and rapidly switches back to the E isomer.elaborate This reaction is consistent coordinates with thecombining angular and dihedral changes have also been studied for spectral evidence. unsubstituted azobenzene [56]. While the topology of the hydrogen bonds in the ortho positionAs seen may from be relevantTables 3 inand the 4 the second excited state (S2) presents the highest oscillator strength photo-switching of certain azo derivatives, this feature is entirelyfor reliant both on AZO1 the natureand AZO2: of the this most will be the most efficient conversion pathway. favourable reaction coordinate. In light of previous results [53], this showsA relaxed that subtlepotential interplays energy surface (PES) scan of the –CNNC– dihedral at the QD- between substituent groups and the aryl rings pose difficulties in theNEVPT2(4,3)//CASSCF modular design of tuneable (4,3)/def2-TZVP azo level of theory is presented in Figure 7 and in Figure 8 for compounds since their interconversion shows up as system specific.AZO1 and AZO2, respectively. Therein the energies of each of the lowest six singlets were monitored The photo-switching in AZO1 is inhibited due to a stronger throughoutπ bond order this hindering reaction rotationcoordinate. The main composition of the states at the beginning of the around the N–N bond axis. Further studies will be required to attempt to draw systematic functional group trends within the same mechanistic scenario. Polymers 2019, 11, x FOR PEER REVIEW 14 of 21

6, was determined for AZO2 at different temperatures and solvents. The process was found to be first order in all the studied solvents.

Table 6. AZO2 kr (in s−1) constants in different solvents at different temperatures.

T (K) DMF DMSO DIOX 303.15 3.77 × 10−6 3.14 × 10−6 2.14 × 10−6 313.15 1.18 × 10−5 1.02 × 10−5 6.23 × 10−6 323.15 3.22 × 10−5 3.01 × 10−5 1.73 × 10−5 333.15 5.59 × 10−5 6.93 × 10−5 3.81 × 10−5 Calculation of the first-order rate constants at various temperatures allowed us to estimate thermodynamic activation parameters such as the activation energy (Ea), activation enthalpy (ΔH‡), and activation entropy (ΔS‡) for thermal Z to E reaction using Arrhenius and Eyring equations (equations shown in Appendix A). We used the linear correlation between the enthalpy and the entropy of activation to determine the mechanism of AZO2 thermal relaxation. In all the solvents, the Arrhenius plots were linear, proving that there is no significant change in the reaction pathway over the examined temperature range. AZO2 compound has a positive enthalpy of activation and negative entropy for all the solvents used (see Table 7).

Table 7. AZO2 thermodynamic activation parameters.

Solvent Dipole Moment (D) A (s−1) Ea (kJ mol−1) ΔH‡ (kJ mol−1) ΔS‡ (J mol−1 K−1) DMF 3.86 6.57 × 107 76.6 74.0 −104.09 × 9 Polymers 2020, 12, 1019 DMSO 4.1 3.41 10 87.2 84.5 17 of 20 −71.25 DIOX 0.45 2.22 × 108 81.3 78.6 −94.03 AZO2 in DMF has the highest ΔS‡ absolute value and the lowest ΔH‡, and therefore exhibits the To conclude, in this work a molecular design approach was used to plan and produce two novel fastest thermal relaxation [52,53]. This is consistent with the calculation that will be shown below. In moieties, suitable candidates for applications that aim to take advantage of visible light irradiation. order to understand the dynamic photochemical and thermal processes involved in the isomerisation Combined with TD-DFT calculation, a comprehensive study of their properties was achieved giving of AZO1 and AZO2 a choice of appropriate computational strategy was paramount. The employment also more information about the important role of the substituents that must be selected during the of density functional theory is unsuitable in this instance because there is no guarantee, that at every planning phase of these molecules. These findings can provide new insights regarding substituted stage in the transformation within the specific excited state hypersurface, the electron wavefunction azobenzene isomerization as well as the possibility to reveal new applications for a well-known of the ground state remains singly determinantal. The best methodology is to adopt multi- organic molecule. configurational self-consistent field theory [36,54] with a posteriori perturbational corrections for a Supplementary Materials:good descriptionThe following of are dynamical available onlineelectron at http: correlati//www.mdpi.comon such as/2073-4360 the NEVPT2/12/5/1019 approach/s1, [40,55] (see Figure S1—(A) EmissionComputational spectra of white, Methods). blue, green, yellow, red lights emitted from the tuneable Jedi Lighting iDual lamp employed as irradiating source, and (B) corresponding illumination values measured at 2 and 8 cm from the light source; FigureTwo S2—QD-NEVPT2 conventional mechanisms//CASSCF(4,3) usually/def2-TZVP adopted Potential to describe energy surface the isomer scans ofchange AZO1 involve the either (top) and AZO2 (bottom)the twisting relative of to the the (NNC)(N=N–C) angle angle twisting (inversion) to the linear or a position,rotation Figureof the S3–planes Tight that grid make up the (C- scan of AZO1 (left) and AZO2 (right) evidencing the two (avoided) crossings of S and S , Table S1—Relative N=N–C) dihedral (rotation). Only the latter was found to2 exhibit3 more favourable energies for the energies of the zwitterionic isomers of AZO1 as calculated through PBE0/TZP (DMF); Table S2—Calculated UV/Vis absorption spectrumphotochemical of E-AZO1-C conversion. at the PBE0 This/TZP contradicts (DMF) level the of hydr theory;ogen Figure bonding S4—Calculated inhibition UV-Vis scenario as presented spectra of the two tautomersby Bandara (convoluted et al. [50], with since a Gaussianrotation functionof the molecule at FWHM around=60); Figure the –CNNC S5—Active– dihedral space planes does not composition of the statenecessarily average CAS(4,3)SCFrequire the wavefunctiontermination of E-AZO1-C;the intramolec Tableular S3—Active hydrogen space bond composition with the of adjacent hydroxyl E-AZO1-C (first point in PES of Figure S3 below); Figure S6—Potential energy surfaces (QD-NEVPT2/def2-TZVP) with respect to dihedralgroup. rotation An alternative in AZO1-C; explanation Figure S7—QD-NEVPT2 is given below//CASSCF(4,3) to account/def2-TZVP for the slower Potential rates energy of photo-switching. surface scans of AZO1-CThe relative alternative to the R twisting (NNC) angle scenario twisting may to be the found linear position;in the supporting Table S4—Calculated information section. More 1 band absorption wavelengthselaborate reaction for the AZO1 coordinates system; Figurecombining S8— Hangular NMR AZO2 and dihedral spectrum changes at PPS in have d-DMSO also been studied for solution at 30 C; Figure S9—First-order kinetic of the Z-E thermal relaxation of AZO2 in DMF solution at 30 C; ◦ unsubstituted azobenzene [56]. ◦ Figure S10—Tentative fitting of AZO1 thermal relaxation kinetic constant in DMF solution at 30 ◦C. As seen from Tables 3 and 4 the second excited state (S2) presents the highest oscillator strength Author Contributions: Conceptualization, M.G. and D.P.; methodology, M.G. and D.P.; Modelling, N.A.G.B., formal analysis,for both N.A.G.B.; AZO1 and investigation AZO2: this D.P; will writing—original be the most efficient draft preparation, conversion D.P. pathway. and N.A.G.B; writing—review and editing,A relaxed M.G. and potential B.T.; visualization, energy B.T.;surface supervision, (PES) M.G.,scan E.B.,of R.L.the and–CNNC R.G.V.;– projectdihedral at the QD- administration, E.B,NEVPT2(4,3)//CASSCF R.L. and R.G.V.; funding acquisition, (4,3)/def2-TZVP M.G. and level R.L. of All theory authors is have presented read and in agreed Figure to 7 the and in Figure 8 for published version of the manuscript. AZO1 and AZO2, respectively. Therein the energies of each of the lowest six singlets were monitored Funding: This researchthroughout was funded this by reaction Horizon2020 coordinate. ITN Marie The Curie main grant co agreementmposition no. of 675624. the states NAGB at would the beginning of the like to thank Fundação para a Ciência e Tecnologia for funding of computational research facilities through project grants PTDC/QUI-QFI /31896/2017 and PTDC/QUI-QFI/29236/2017, and additionally to the BioISI research unit (UID/MULTI/04046/2019). Acknowledgments: The authors would like to thank Ramón Guerrero for NMR experiments. Conflicts of Interest: The authors declare no conflict of interest.

Appendix A

Appendix A.1 Theory and Calculations The photoisomerization experimental data were analyzed according to Equation (A1).

A0 A ln − ∞ = kit (A1) At A − ∞ where t is the irradiation time, A0, At and A are the E form absorbances corresponding to the time 0, t ∞ and photostationary state (PSS), respectively, and ki is the rate constant of E–Z photoisomerization. In the case of thermal relaxation, the kinetics follow Equation (A2), where kr is the rate constant of the Z–E relaxation, A0, At and A are the Z form absorbances corresponding to the time 0, t and ∞ photostationary state and t is the relaxation time.

A0 A ln − ∞ = krt (A2) At A − ∞ The content of the Z-isomer at the PSS was determined using Equation (A3).

A0 APSS αZ = − (A3) A0 Polymers 2020, 12, 1019 18 of 20

where A0 and APSS represent the absorbances at λmax for E isomer before and after irradiation with UV light, respectively. Activation parameters of the thermal back isomerization were determined by measuring the temperature dependence of the rate constant and fitting the data with the Arrhenius equation (Equation (A4)) or the Eyring equation (Equation (A5)) to obtain the activation energy, Ea, the enthalpy of activation, ∆H‡, and the entropy of activation, ∆S‡.

E ln k = ln A a (A4) − RT where k is the thermal rate constant, R is the universal gas constant, T is the temperature in Kelvin, A is the steric factor and Ea is the activation energy.

k ∆H k ∆S ln = ‡ + ln b + ‡ (A5) T − RT h R where ∆H‡ is the enthalpy of activation, ∆S‡ the entropy of activation, kb is the Boltzmann constant, h is Planck constant.

References

1. Rau, H. Photoisomerization of azobenzenes. Photochem. Photophysics 1990, 2, 119–141. 2. Wang, L.; Dong, H.; Li, Y.; Xue, C.; Sun, L.-D.; Yan, C.-H.; Li, Q. Reversible Near-Infrared Light Directed Reflection in a Self-Organized Helical Superstructure Loaded with Upconversion Nanoparticles. J. Am. Chem. Soc. 2014, 136, 4480–4483. [CrossRef] 3. Yamada, M.; Kondo, M.; Mamiya, J.; Yu, Y.; Kinoshita, M.; Barrett, C.J.; Ikeda, T. Photomobile Polymer Materials: Towards Light-Driven Plastic Motors. Angew. Chem. Int. Ed. 2008, 47, 4986–4988. [CrossRef] [PubMed] 4. Yin, R.; Xu, W.; Kondo, M.; Yen, C.; Mamiya, J.; Ikeda, T.; Yu, Y. Can sunlight drive the photoinduced bending of polymer films? J. Mater. Chem. 2009, 19, 3141–3143. [CrossRef] 5. Deng, W.; Li, M.-H.; Wang, X.; Keller, P. Light-responsive wires from side-on liquid crystalline azo polymers. Liq. Cryst. 2009, 36, 1023–1029. [CrossRef] 6. Dong, L.; Feng, Y.; Wang, L.; Feng, W. Azobenzene-based solar thermal fuels: Design, properties, and applications. Chem. Soc. Rev. 2018, 47, 7339–7368. [CrossRef][PubMed] 7. Silong, S.; Lutfor, M.R.; Rahman, M.Z.A.; Yunus, W.M.Z.W.; Haron, M.J.; Ahmad, M.B.; Yusoff, W.M.D.W. Synthesis and characterization of side-chain liquid-crystalline polyacrylates containing azobenzene moieties. J. Appl. Polym. Sci. 2002, 86, 2653–2661. [CrossRef] 8. Baba, K.; Ono, H.; Itoh, E.; Itoh, S.; Noda, K. Kinetic study of thermal Z to e isomerization reactions

of azobenzene and 4-dimethvlaniino-40-nitroazobenzene in Ionic Liquids [1-R-3- methyliniidazolium bis(trifluoromethylsulfonyl)imide with R = Butyl, Pentyl, and Hexyl]. Chemistry 2006, 12, 5328–5333. [CrossRef] 9. Garcia-Amorós, J.; Martínez, M.; Finkelmann, H.; Velasco, D. Kinetico-Mechanistic Study of the Thermal

Cis-to-Trans Isomerization of 4,40-Dialkoxyazoderivatives in Nematic Liquid Crystals. J. Phys. Chem. B 2010, 114, 1287–1293. [CrossRef] 10. Dhammika Bandara, H.M.; Burdette, S.C. Photoisomerization in different classes of azobenzene. Chem. Soc. Rev. 2012, 41, 1809–1825. [CrossRef] 11. Spiridon, M.C.; Jerca, F.A.; Jerca, V.V.; Vasilescu, D.S.; Vuluga, D.M. 2-Oxazoline based photo-responsive azo-polymers. Synthesis, characterization and isomerization kinetics. Eur. Polym. J. 2013, 49, 452–463. [CrossRef] 12. Jerca, F.A.; Jerca, V.V.; Kajzar, F.; Manea, A.M.; Rau, I.; Vuluga, D.M. Simultaneous two and three photon resonant enhancement of third-order NLO susceptibility in an azo-dye functionalized polymer film. Phys. Chem. Chem. Phys. 2013, 15, 7060–7063. [CrossRef] 13. Tamai, N.; Miyasaka, H. Ultrafast Dynamics of Photochromic Systems. Chem. Rev. 2000, 100, 1875–1890. [CrossRef][PubMed] Polymers 2020, 12, 1019 19 of 20

14. Dürr, H.; Bouas-Laurent, H. (Eds.) Photochromism: Molecules and Systems; Elsevier: Amsterdam, The Netherlands, 1990. 15. El’t´sòv, A.V.; Sviridov, Y.E.; Whittall, J.W. Organic Photochromes; Consultants Bureau: New York, NY, USA, 1990. 16. Bang, C.-U.; Shishido, A.; Ikeda, T. Azobenzene Liquid-Crystalline Polymer for Optical Switching of Grating Waveguide Couplers with a Flat Surface. Macromol. Rapid Commun. 2007, 28, 1040–1044. [CrossRef] 17. Gibbons, W.M.; Shannon, P.J.; Sun, S.-T.; Swetlin, B.J. Surface-mediated alignment of nematic liquid crystals with polarized laser light. Nature 1991, 351, 49–50. [CrossRef] 18. Tylkowski, B.; Giamberini, M.; Underiner, T.; Prieto, S.F.; Smets, J. Photo-Triggered Microcapsules. Macromol. Symp. 2016, 360, 192–198. [CrossRef] 19. Tylkowski, B.; Peris, S.; Giamberini, M.; Garcia-Valls, R.; Reina, J.A.; Ronda, J.C. Light-Induced Switching of the Wettability of Novel Asymmetrical Poly(vinyl alcohol)-co-ethylene Membranes Blended with Azobenzene Polymers. Langmuir 2010, 26, 14821–14829. [CrossRef][PubMed] 20. Trojanowska, A.; Marturano, V.; Bandeira, N.A.G.; Giamberini, M.; Tylkowski, B. Smart microcapsules for precise delivery systems. Funct. Mater. Lett. 2018, 11, 1850041. [CrossRef] 21. Del Pezzo, R.; Bandeira, N.A.G.; Trojanowska, A.; Prieto, S.F.; Underiner, T.; Giamberini, M.; Tylkowski, B. Ortho-substituted azobenzene: Shedding light on new benefits. Pure Appl. Chem. 2018, 91, 1533–1547. [CrossRef] 22. Pirone, D.; Marturano, V.; del Pezzo, R.; Prieto, S.F.; Underiner, T.; Giamberini, M.; Tylkowski, B. Molecular Design of Microcapsule Shells for Visible Light-Triggered Release. Polymers 2019, 11, 904. [CrossRef] 23. Nicolescu, F.A.; Jerca, V.V.; Stancu, I.C.; Vasilescu, D.S.; Vuluga, D.M. New Organic–Inorganic Hybrids with Azo-dye Content. Des. Monomers Polym. 2010, 13, 437–444. [CrossRef] 24. Whitten, D.G.; Wildes, P.D.; Pacifici, J.G.; Irick, G. Solvent and substituent on the thermal isomerization of substituted azobenzenes. Flash spectroscopic study. J. Am. Chem. Soc. 1971, 93, 2004–2008. [CrossRef] 25. Merino, E.; Ribagorda, M. Control over molecular motion using the cis–trans photoisomerization of the azo group. Beilstein J. Org. Chem. 2012, 8, 1071–1090. [CrossRef][PubMed] 26. Marcandalli, B.; Liddo, L.P.-D.; Fede, C.D.; Bellobono, I.R. Solvent and substituent effects on thermal cis–trans-isomerization of some 4-diethylaminoazobenzenes. J. Chem. Soc. Perkin Trans. 1984, 2, 589–593. [CrossRef] 27. Tanino, T.; Yoshikawa, S.; Ujike, T.; Nagahama, D.; Moriwaki, K.; Takahashi, T.; Kotani, Y.; Nakano, H.; Shirota, Y. Creation of azobenzene-based photochromic amorphous molecular materials—synthesis, glass-forming properties, and photochromic response. J. Mater. Chem. 2007, 17, 4953–4963. [CrossRef] 28. Rau, H.; Rötger, D. Photochromic Azobenzenes Which are Stable in the Trans and cis Forms. Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. A 1994, 246, 143–146. [CrossRef] 29. Jerca, F.A.; Jerca, V.V.; Anghel, D.F.; Stinga, G.; Marton, G.; Vasilescu, D.S.; Vuluga, D.M. Novel Aspects Regarding the Photochemistry of Azo-Derivatives Substituted with Strong Acceptor Groups. J. Phys. Chem. C 2015, 119, 10538–10549. [CrossRef] 30. Velde, G.T.; Bickelhaupt, F.M.; Baerends, E.J.; Guerra, C.F.; van Gisbergen, S.J.A.; Snijders, J.G.; Ziegler, T. Chemistry with ADF. J. Comput. Chem. 2001, 22, 931–967. [CrossRef] 31. Adamo, C.; Barone, V. Toward reliable density functional methods without adjustable parameters: The PBE0 model. J. Chem. Phys. 1999, 110, 6158–6170. [CrossRef] 32. Adamo, C.; Barone, V. Toward chemical accuracy in the computation of NMR shieldings: The PBE0 model. Chem. Phys. Lett. 1998, 298, 113–119. [CrossRef] 33. Ernzerhof, M.; Scuseria, G.E. Assessment of the Perdew–Burke–Ernzerhof exchange-correlation functional. J. Chem. Phys. 1999, 110, 5029–5036. [CrossRef] 34. Klamt, A.; Schüürmann, G. COSMO: A new approach to dielectric screening in solvents with explicit expressions for the screening energy and its gradient. J. Chem. Soc. Perkin Trans. 1993, 2, 799–805. [CrossRef] 35. Casida, M.E. Time-Dependent Density Functional Response Theory for Molecules. In Recent Advances in Density Functional Methods; World Scientific: Singapore, 1995; Volume 1, pp. 155–192. 36. Roos, B.O. The Complete Active Space Self-Consistent Field Method and its Applications in Electronic Structure Calculations. In Advances in Chemical Physics; John Wiley & Sons, Ltd: Hoboken, NJ, USA, 2007; pp. 399–445. 37. Neese, F. Software update: The ORCA program system, version 4.0. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2018, 8, e1327. [CrossRef] Polymers 2020, 12, 1019 20 of 20

38. Neese, F. The ORCA program system. WIREs Comput. Mol. Sci. 2012, 2, 73–78. [CrossRef] 39. Angeli, C.; Borini, S.; Cestari, M.; Cimiraglia, R. A quasidegenerate formulation of the second order n-electron valence state perturbation theory approach. J. Chem. Phys. 2004, 121, 4043–4049. [CrossRef] 40. Angeli, C.; Cimiraglia, R.; Evangelisti, S.; Leininger, T.; Malrieu, J.-P. Introduction of n-electron valence states for multireference perturbation theory. J. Chem. Phys. 2001, 114, 10252–10264. [CrossRef] 41. Nakano, H. Quasidegenerate perturbation theory with multiconfigurational self-consistent-field reference functions. J. Chem. Phys. 1993, 99, 7983–7992. [CrossRef] 42. Schäfer, A.; Horn, H.; Ahlrichs, R. Fully optimized contracted Gaussian basis sets for atoms Li to Kr. J. Chem. Phys. 1992, 97, 2571–2577. [CrossRef] 43. Neese, F. An improvement of the resolution of the identity approximation for the formation of the Coulomb matrix. J. Comput. Chem. 2003, 24, 1740–1747. [CrossRef] 44. Glendening, E.D.; Landis, C.R.; Weinhold, F. NBO 6.0: Natural bond orbital analysis program. J. Comput. Chem. 2013, 34, 1429–1437. [CrossRef] 45. Reed, A.E.; Schleyer, P.V.R. Chemical bonding in hypervalent molecules. The dominance of ionic bonding and negative hyperconjugation over d-orbital participation. J. Am. Chem. Soc. vol. 1990, 112, 1434–1445. [CrossRef] 46. Merino, E. Synthesis of azobenzenes: The coloured pieces of molecular materials. Chem. Soc. Rev. 2011, 40, 3835–3853. [CrossRef][PubMed] 47. Dong, M.; Babalhavaeji, A.; Samanta, S.; Beharry, A.A.; Woolley, G.A. Red-Shifting Azobenzene Photoswitches for in Vivo Use. Acc. Chem. Res. 2015, 48, 2662–2670. [CrossRef][PubMed] 48. Nishioka, H.; Kashida, H.; Komiyama, M.; Liang, X.; Asanuma, H. Incorporation of methyl group on azobenzene for the effective photo-regulation of hybridization and suppression of thermal isomerization. Nucleic Acids Symp. Ser. 2006, 50, 85–86. [CrossRef] 49. Djedaini-Pilard, F.; Barbot, F.; Len, C.; Hamon, F. Azobenzenes—synthesis and carbohydrate applications. ResearchGate 2009, 65, 10105–10123. [CrossRef] 50. Bandara, H.M.D.; Friss, T.R.; Enriquez, M.M.; Isley, W.; Incarvito, C.; Frank, H.A.; Gascón, J.; Burdette, S.C. Proof for the Concerted Inversion Mechanism in the trans cis Isomerization of Azobenzene Using Hydrogen → Bonding To Induce Isomer Locking. J. Org. Chem. 2010, 75, 4817–4827. [CrossRef] 51. Jerca, V.V.; Jerca, F.A.; Rau, I.; Manea, A.M.; Vuluga, D.M.; Kajzar, F. Advances in understanding the photoresponsive behavior of azobenzenes substituted with strong electron withdrawing groups. Opt. Mater. 2015, 48, 160–164. [CrossRef] 52. Asano, T.; Okada, T. Thermal Z-E isomerization of azobenzenes. The pressure, solvent, and substituent effects. J. Org. Chem. 1984, 49, 4387–4391. [CrossRef] 53. Wazzan, N.A.; Richardson, P.R.; Jones, A.C. Cis-Trans isomerisation of azobenzenes studied by laser-coupled NMR spectroscopy and DFT calculations. Photochem. Photobiol. Sci. 2010, 9, 968–974. [CrossRef] 54. Shepard, R. The Multiconfiguration Self-Consistent Field Method. In Advances in Chemical Physics; John Wiley & Sons, Ltd: Hoboken, NJ, USA, 2007; pp. 63–200. 55. Angeli, C.; Cimiraglia, R.; Malrieu, J.-P. N-electron valence state perturbation theory: A fast implementation of the strongly contracted variant. Chem. Phys. Lett. 2001, 350, 297–305. [CrossRef] 56. Casellas, J.; Bearpark, M.J.; Reguero, M. Excited-State Decay in the Photoisomerisation of Azobenzene: A New Balance between Mechanisms. ChemPhysChem 2016, 17, 3068–3079. [CrossRef][PubMed] 57. Gagliardi, L.; Orlandi, G.; Bernardi, F.; Cembran, A.; Garavelli, M. A theoretical study of the lowest electronic states of azobenzene: The role of torsion coordinate in the cis–trans photoisomerization. Theor. Chem. Acc. 2004, 111, 363–372. [CrossRef] 58. Persico, M.; Granucci, G. Photochemistry: A Modern Theoretical Perspective; Springer International Publishing: Berlin/Heidelberg, Germany, 2018.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).