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Novel Aspects Regarding the Photochemistry of AzoDerivatives Substituted With Strong Acceptor Groups Florica Adriana Jerca, Valentin Victor Jerca, Dan-Florin Anghel, Gabriela Stîng#, George Marton, Dan Sorin Vasilescu, and Dumitru Mircea Vuluga J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511925n • Publication Date (Web): 21 Apr 2015 Downloaded from http://pubs.acs.org on April 27, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Novel aspects regarding the photochemistry of azoderivatives substituted with strong acceptor groups Florica A. Jerca†, Valentin V. Jerca†, §*, Dan F. Anghel‡, Gabriela Stinga‡, George Marton#, Dan S. Vasilescu§ and Dumitru M. Vuluga†∞ †

Centre of Organic Chemistry “Costin D. Nenitescu”, 202B Spl. Independentei CP 35-108,

Bucharest 060023, Romania ‡

Institute of Physical Chemistry “Ilie Murgulescu”, Colloid Department, 202 Spl. Independentei

CP 12-194, Bucharest 060021, Romania. #

University “POLITEHNICA” of Bucharest, Department of Organic Chemistry, 1-5 Gh. Polizu

Street, Bucharest 011061, Romania §

University “POLITEHNICA” of Bucharest, Department of Bioresources and Polymer Science,

1-5 Gh. Polizu Street, Bucharest 011061, Romania KEYWORDS: azo compounds, photochromism, isomerization, polymers, photochemistry

ABSTRACT: In this paper, an extended isomerization study of a series of nitro/cyano mono and disubstituted azo-monomers and related azo-poly(methacrylate)s is conducted by UV-Vis and 1H NMR spectroscopy. The experiments, carried out in both toluene and dimethylformamide (DMF), reveal an intriguing behaviour. The monosubstituted azo-derivatives, thermally relax

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monotonously via inversion mechanism, in both solvents. Interestingly, the disubstituted azoderivatives reveal stable Z isomers in DMF, while in toluene they exhibit very fast relaxation, by inversion. The particular stability of Z isomers in DMF is explained in terms of solvent-solute interactions, using quantum chemical calculations and FT-IR spectroscopy. In addition, the information-transmission capability with time of one disubstituted azo-monomer is highlighted. The study reveals that the nature and position of substituents on the azo-moiety, and as well as their interaction with the solvent, have a crucial importance for the Z-E thermal relaxation, and for the displayed photochromic properties.

1. INTRODUCTION The literature background encloses considerable research dedicated to azobenzene-functional materials ranging from fundamental studies1 to practical applications2-6. The investigation of azopolymers remains a hot topic, and the vitality of this field can easily be noticed from the many research papers that continue to appear7-11. The most interesting property of azobenzenes is that the azo bond can undergo a geometric change between the stable E configuration and the metastable Z form, upon irradiation with ultraviolet light or laser. This “molecular photo-switching” is an attractive tool that enables fine-tuning of the optical response for polymer materials that incorporate azobenzenes. Therefore, these polymers can be used as holographic materials12-13, optical storage media14-16, actuators17, etc. It is well known that the appropriate modification of the substitution pattern of the azobenzene core is one of the main factors that allows modulating the thermal relaxation rate of azo-dyes and, therefore, determines the response time of the dye18. The response time of the photochromic dye is a key feature in the material overall performance. Depending on the substitution pattern and local environment, the electronic push-pull structures19 such as pseudo-stilbenes and


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aminoazobenzenes can isomerize from Z configuration back into E configuration very quickly at room temperature18, 20. However, the thermal Z-E isomerization in azobenzene-type molecules is relatively slow2, and it is even possible to elongate the Z isomer lifetime to years, if bulky substituents are introduced to hinder the thermal relaxation. Considerable research has gone into extending the Z lifetime, with the goal of creating bistable photoswitchable systems21. Studies showed that the conformational strain of macrocylic azo compounds can also be used to lock the Z state, where lifetimes of 20 days22, 1 year23, or even 6 years24 were observed. Slow thermal back-isomerising azoderivatives are valuable photoactive basic materials for information storage (memory) purposes25. A molecular-level memory should be stable and easy to write, and its switched form should be stable but readily erasable when necessary. Consequently, it is essential to have a two-state system that exhibits a stable Z form, which does not return to the thermodynamically stable E isomer without an external stimulus. Finding new azoderivatives exhibiting low isomerization rates at room temperature is a challenging point of research and therefore, one of the main goals of the present work. Contrary to the present interest, hydroxy-substituted azobenzenes are a very interesting family of fast thermal Z-to-E azo-dyes26, which have been applied successfully not only for light-driven optical switching applications27, but also as photoactive monomers in elastomers for lightsensitive artificial muscle-like actuators28. The fast relaxation exhibited by this type of azoderivatives is strongly related to solvent-solute interaction. The formation of intermolecular hydrogen bonds between the nitrogen atom of the azo group and the solvent proton, as well as between the -OH group of the azo-dye and the solvent, favours a hydrazone-like electronic distribution with a simple N–N bond, which seems to be the key to the fast thermal isomerization kinetics observed29-34. Studies reveal that the thermal relaxation can be accelerated in the case of


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(E)-4-((4-nitrophenyl)diazenyl) phenol to 4.5 ms, when a nitro substituent is introduced in the 4position, relative to azo bond18. The fast thermal relaxation, in highly conjugated systems, is due to the same azo-hydrazone tautomerism, induced in this case by the push-pull substitution pattern. However, if the hydroxyl group is blocked by esterification, and the azoderivative is altered to an azo-monomer structure, this tautomerism can be decelerated and turned to our advantage as slow thermal back-isomerising azoderivatives18. With this purpose in mind, six different hydroxy-substituted azobenzenes were designed (see Scheme 1a) and then altered to azo-monomer structures (see Scheme 1b). Then, the photoresponsive azo-polymers were obtained through radical copolymerization with methyl methacrylate (see Scheme 1c) and confronted to their azo-monomer counterparts. We report the E-Z-E isomerization kinetics of six nitro/cyano substituted azo-monomers and related poly(methyl methacrylate) copolymers, carried out through UV-Vis and 1H NMR spectroscopy. We discuss, in terms of geometric considerations and solvent-solute interactions, the thermally stable Z isomers yielded in our azobenzene derivatives, and other particular photochemical properties that arise from the substitution pattern of the azo-moieties. 2. EXPERIMENTAL 2.1. Materials. The isomerization studies, were carried out on cyano-, nitro-substituted azobenzene archetype molecules, having different substitution pattern, that have been partially characterized by us elsewhere35. To gain more insight on the relaxation mechanism, C3 (M3) and C4 (M4) were synthesized for this particular study (spectral characterization is given in the supplementary material section). The related azo-polymers, used in our experiments, were obtained by free radical copolymerization, following the recipe from reference36. The azopolymers are characterized by high density of azo-moieties in their side-chains (weight fractions:


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0.49 for P1 and P3, 0.51 for P2 and P4, 0.53 for P5 and P6) and average molecular weights that lay within 5000÷5900 Da range for P2-P6 and 13100 Da for P1. The number average molecular weights (Mn) have been evaluated by SEC with Wyatt Heleos II Multi Angle Light Scattering in-line and batch detector; using N, N’-dimethylformamide as eluent (flow rate 1 mL/min), at 30 °C. 2.2. Spectroscopic measurements. The kinetics of photoisomerization and thermal relaxation were followed by UV-Vis spectrophotometry, on an Ocean Optics High-Resolution Fibber Optic Spectrometer HR 4000, equipped with a Peltier thermostated sample holder. Irradiation was performed with a 25 W mercury lamp equipped with a cut filter for UV radiation at 365 nm, where the power density of the incident light was 30 mW/cm2. Experiments were carried out with diluted solutions (4×10-5 M) in N, N’-dimethylformamide and toluene (spectroscopic grade). The Z-E thermal relaxation of solutions was studied in the dark over a temperature range. For M1, M2, M3 and corresponding polymers, the temperature range was 30, 50, 60, and 80°C. M4, M5, M6 and corresponding polymers were investigated at 0, 10, 20, and 30 °C. After maintaining the polymer solution in the dark overnight, the isomerization was induced by irradiating with a wavelength within the azo’s absorption spectrum (close to its λmax). Kinetics of the Z-E thermal relaxation of the M1-M3 azo-monomers and P1-P3 polymers were also studied by 1H NMR spectroscopy, on a Varian Unity Inova Spectrometer at 400 MHz. 5 wt.% solutions in deuterated DMF were irradiated until photostationary equilibrium was reached, and then the thermal relaxation was followed at four different temperatures 20°C, 40°C, 60°C and 80°C, respectively. The FT-IR spectra of samples in solution were recorded in toluene and DMF using a Bruker Vertex 70 spectrometer fitted with a transmission cell having BaF2 windows, with 50 µm spacer for DMF and 100 µm spacer for toluene. The spectral resolution


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was 1 cm-1 and the spectra were averaged over 64 scans. Spectroscopic-grade solvents toluene and DMF (Sigma–Aldrich) were used to record the spectra in the 1000-4000 cm-1 region. The solution concentration was 0.08 M in toluene and 0.2 M in DMF, respectively. Activation parameters were determined by measuring the temperature dependence of the rate constant and fitting the data with the Arrhenius equation (equation 1) or the Eyring equation (equation 2) to obtain the activation energy, Ea, the enthalpy of activation, ∆H≠, the entropy of activation, ∆S≠ and the Gibbs free energy of activation, ∆G≠. ln ݇ = ln ‫ ܣ‬−

ாೌ

(1)

ோ்

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. ௞

ln ் = −

∆ு ಯ ோ்

+ ln

௞ಳ ௛

+

∆ௌ ಯ

(2)

ோ

where ∆H≠, is the enthalpy of activation, ∆S≠ the entropy of activation, kB is the Boltzmann constant, h is Planck’s constant. The Gibbs free energy of activation, ∆G≠, is given by equation 3: ∆‫ ܩ‬ஷ = ∆‫ ܪ‬ஷ − ܶ ∙ ∆ܵ ஷ

(3)

2.3. Computational Methodology. All the quantum chemical calculations were carried out using GAMESS37 program and the basis set level used was M11/KTZVP38. Solvent effects were included first in an approximate way only. For this purpose we used the polarizable continuum model (PCM) proposed by Tomasi and co-workers39. The quantum mechanical calculations were then performed for the supramolecular cluster composed of one azo-dye molecule (M5) and seven DMF solvent molecules. 3. RESULTS AND DISCUSSION


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To properly highlight the photo-chemical behaviour of our monomers (Mx) and polymers (Px), we used a polar solvent and a non-polar one, dimethylformamide and toluene, respectively. The excess of one of the two isomers was detected by a change in UV absorption spectra and 1H NMR spectra. The UV study aimed to establish the influence of substitution pattern upon the kinetic parameters, thermodynamic parameters, and relaxation mechanism of each azo-moiety in diluted solutions. The NMR study focused on the impact of medium viscosity (concentrated solutions) and host matrix upon thermal back relaxation rate. Due to the distinct substitution pattern of every azo-moiety, we noticed particular E-Z-E isomerization behaviour. 3.1. UV characterization 3.1.1.

Electronic spectra of E-azobenzenes. For the sake of simplicity, although Mx

azobenzenes are 4, 4′-and 4, 2′- substituted compounds, we will name monosubstituted the ones that have one acceptor group and disubstituted the ones that have two acceptors groups. Both the spectroscopy and the related photochemistry of the substituted azobenzenes are complicated due to the low-lying electronic states available and to the two possible geometric forms, Z and E. For these reasons, we investigated first the influence of substitution pattern and strength of acceptor group on to the electronic spectra of the synthesized E-azobenzenes and of the related polymers.


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Scheme 1. Chemical structures of the hydroxy-azobenzenes (A), the azo-monomers (B), and related photo-responsive azo-polymers (C).


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The UV/visible absorption spectra of Mx and related polymers, Px, are characterized by strong π-π* transitions of the E azobenzene moiety at 330-360 nm (depending on the substitution pattern and substituent nature), and a weak absorption at 450-470 nm originating from the forbidden symmetry of n-π* transitions. The absorption maxima of the monomer/polymer series, in DMF solution, depend on the substitution pattern, as showed in Table 1.

Table 1. UV-Vis absorption maxima for Mx and Px in toluen and DMF, respectively.

Code

M1

M2

M3

M4

M5

M6

P1

P2

P3

P4

P5

P6

336

344

339

334

365

359

337

343

336

333

362

360

336

344

39

334

359

345

335

343

334

333

359

347

λmax [nm] (Toluene) λmax [nm] (DMF)

The strength of the acceptor group is demonstrated comparing the absorption maxima of Mx azo-monomers. For 4-monosubstituted azo-monomers, the nitro group, that has a stronger electron withdrawing character ( –M effect)40, induces an 8 nm red shift of absorption maximum as compared to M1 (cyano group). In 2-monosubstituted monomers, the effect is quite the opposite, whereas the nitro group induces a blue shift of λmax. The 10 nm blue shift noticed for M4 can be explained by comparison with the planar structure of M2, since M4 is non-planar due to steric constraints between the 2-nitro group and the azo group N lone pair electrons. The rotation around the double bond requires more energy than the rotation around a single bond. The first excited state of compound M4, in which the azo bond has considerable double bond character, is destabilized relative to its ground state where the bond is closer to a single bond.


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This accounts for the hypsochromic effect in case of M4. In addition, one can conclude that nitro group in the 2-position lowers the energy of the highest occupied molecular orbital (HOMO) and/or raises the energy of the lowest unoccupied molecular orbital (LUMO), thus increasing the π-π* energy gap. The cyano group in the 2-position has opposite effect, but with a lower magnitude, whereas λmax of M3 has a 3 nm red shift as compared to M1. The introduction of cyano substituent in the 2-position, in M5 results in a large bathochromic shift as compared to M2 and M6. Due to its cylindrical geometry, the nitrile substituent mainly raises the electronic conjugation of the system rather than interacting with the azo bond. M6 shows a red shift of 6 nm as compared to M3, and a blue shift of 14 nm towards M5. Two opposite effects can explain this behaviour: i) the second nitro group lowers the energy gap between HOMO and LUMO; ii) the steric constraints induced by the bulkiness of nitro group lead to a non-planar structure, which in the first excited state has lower conjugation than in ground state. The absorption maximum of the monosubstituted azobenzenes is not affected by the change of polarity of the solvent (does not exhibit bathochromic or hypsochromic shifts induced by the change in polarity of the solvent), while for the disubstituted compounds a blue shift is noticed going from toluene to DMF (see Table 1). In the case of related polymer series, we observed the same trends. 3.1.2.

E-Z photoisomerization kinetics. The photoisomerization kinetics of azo-

monomers and polymers was established by monitoring the absorbance of π-π* absorption band. The experimental data were analysed according to equation 4. ஺ ି஺

ln ஺బି஺ ಮ = ݇௜ ‫ݐ‬ ೟

(4)

ಮ


10

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where t is the irradiation time, A0, At and A∞ are the E form absorbance corresponding to the time 0, t and photostationary state, respectively and ki is the rate constant of the E-Z photoisomerization. Figures 1 and S1 show typical UV-Vis spectra of M1 monomer and P4 polymer, respectively, after irradiation at different time periods. Under 365 nm irradiation the intensity of the π-π* absorption band gradually decreases with the irradiation time, while the n-π* absorption band increases, until an equilibrium state is reached. Simultaneously, the absorption maximum of π-π* band is slightly shifted to shorter wavelengths because the molecular conjugation decreases when the Z content increases. With reference to Figure 1, the photostationary state was reached after 9 min and two isosbestic points were observed at 290 and 395 nm.

Figure 1. Changes in the UV-Vis absorption spectra of M1 monomer 4×10-5 M in DMF at 20 °C during E–Z photoisomerization at 365 nm. Inset shows the first-order plot for E-Z isomerization of M1. The plots of ln (A0 - A∞)/(At - A∞) versus irradiation time were linear for all samples (monomers and polymers) as the inset of Figure 1 and S1 shows. In Table 2 the photoisomerization constants obtained are listed. Upon irradiation, all spectra show two isosbestic points, which reveal first order kinetics, uniform photoreactions, and confirm that only


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two absorbing species (E and Z isomers) were present in the reaction medium. From Table 2 one can notice that the ki value is highly sensitive to the nature and number of substituents, and slightly influenced by the substituent position and solvent polarity. Nitro-substituted monomers exhibit a slower photoisomerization rate than cyano-substituted ones, regardless of the substituent position. The same behaviour is noticed both in toluene and in DMF. In toluene, the highest ki value was registered for M5, while for M6 the rate constant could not be measured by UV-Vis spectroscopy, due to the fast thermal relaxation process. In contrast to toluene, the ki value of M5 in DMF was 1500 times smaller. Table 2. Kinetic constants of E-Z photoisomerization reaction of Mx and Px calculated from UV-Vis data at 20° C. Code

ki [s-1]

αZ [%]

Toluene

DMF

Toluene

DMF

M1

1.45×10-2

8.02×10-3

78.7

73.5

M2

8.50×10-3

5.80×10-3

51.1

31.3

M3

1.39×10-2

1.04×10-2

79.3

72.7

M4

9.90×10-3

6.52×10-3

11.9

14.2

M5

2.72×10-2

1.82×10-5

11.6

63.9

M6

-

3.84×10-5

-

85.6

P1

1.03×10-2

6.33×10-3

77.5

72.1

P2

8.00×10-3

2.34×10-3

43.7

27.9

P3

1.32×10-2

9.72×10-3

73.6

68.2

P4

9.40×10-3

6.31×10-3

9.7

10.2

P5

2.31×10-2

1.75×10-5

9.1

61.2

P6

-

3.50×10-5

-

60.5


12

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The related polymers exhibited slightly lower ki values as compared to the monomers. This effect is due to covalent binding of the azo-moiety in the polymer matrix, where the necessary free volume for photoisomerization is more limited. Nevertheless, the Px series preserve the same characteristics as the related monomers. The content of the Z-isomer at the photostationary state (PSS) was determined using equation 5: ߙ௓ =

஺బ ି஺ುೄೄ

(5)

஺బ

where A0 and APSS represent the absorbance at λmax for E isomer before and after irradiation with UV light, respectively. The values calculated for each sample are listed in Table 2. This formula assumes negligible absorption of the cis-isomer at 360 nm. The percentage of the Z isomer (αz), produced upon UV irradiation, depends on the substituent nature and position on the phenyl rings, but the isomerization rate constant is independent of the initial concentration, as pointed out by Weiss41. The low Z content at photostationarity state (PSS) (see Table 2) registered for nitro-substituted monomers is in accordance with other examples in literature42-43. The fast thermal relaxation rate of M4 and M5 in toluene is responsible for the low content of Z isomers registered at PSS (see Table 2). Moreover, in the case of polymers, the content of Z isomer, αZ, was affected by chemical linkage to polymer, leading to slightly lower percentages. 3.1.3.

Thermal Z-E isomerization kinetics. From literature, it is known that the thermal

Z-E isomerization of azo species follows also first-order kinetics44-45. The rate of the Z-E isomerization was monitored in the dark at different temperatures, at or near the absorption maximum of the E isomer. Figures 2 and S2 reveal a gradually recovery pattern, given for P3


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polymer and M2 monomer, respectively. With reference to Figure 2, the absorption band at 346 nm increases in intensity to the starting value before irradiation. The retention of the isosbestic points shows that a real Z-E isomerization occurs thermally. The cycle of E-Z photoisomerization and Z-E isomerization can be repeated several times without significant modification in the intensity of π-π* absorption band.

Figure 2. Z-E thermal relaxation UV-Vis absorption spectra of P3 polymer, 4×10-5 M in toluene at 80 °C. Inset shows the first-order plot for Z-E thermal relaxation of P3. The thermal Z-to-E isomerization rate was determined for azo monomers as well as for corresponding polymers, and summarized in Table 3. For a better understanding, we added the value of kr for the ‘parent’ azobenzene (AB) in Table 3. Table 3. Kinetic constants of Z-E thermal isomerization at 30°C and the thermodynamic parameters calculated with Arrhenius equation. Code

kr [s-1]

A [s-1]

Ea [kJ/mol]

Toluene

DMF

Toluene

DMF

Toluene

DMF

ABa

5.95×10-5

-

-

-

-

-

M1

3.67×10-5

3.67×10-5

91.3

93.5

2.05×1011

4.19×1011


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M2

1.92×10-4

4.30×10-4

80.8

74.9

1.71×1010

3.86×109

M3

3.33×10-5

2.17×10-5

92.3

92.4

2.72×1011

1.85×1011

M4

1.08×10-2

7.15×10-3

75.7

76.3

1.23×1011

1.12×1011

M5

4.59×10-2

-

77.3

-

1.09×1012

-

P1

3.17×10-5

3.17×10-5

93.7

91.4

4.56×1011

1.85×1011

P2

1.65×10-4

5.25×10-4

80.7

68.0

1.38×1010

2.85×108

P3

2.50×10-5

2.17×10-5

92.6

91.2

2.33×1011

1.13×1011

P4

8.14×10-3

7.82×10-3

69.0

76.2

5.85×109

1.06×1011

P5

2.69×10-2

-

62.9

-

1.97×109

-

a

values for azobenzene are in chlorobenzene from reference [50] Asano 1981

The trends enforced by substituent nature, pattern and solvent are: i) All Mx compounds have higher kr values than the azobenzene parent molecule, with the exception of M1 and M3. ii) Nitro group, which has a stronger acceptor effect (-M), accelerates the thermal relaxation in both 2- and 4-substitution. iii) In the case of cyano substitution, the kr value of M3 is a little lower than that of M1, while in the case of nitro substitution we have quite the opposite situation, i.e. kr of M4 (2-substituted) is about two orders of magnitude higher than that of M2 (4-substituted). These results can be explained in the following manner. For CN substitution, only electronic effects influence the stability of the transition state, which should be the same for 2- or 4-position, as it can be seen from resonance structures drawn in Scheme S1. The steric interactions should also be minimized by the cylindrical geometry of the nitrile group. In the second case, it seems that the steric effects induced by the bulky group nitro in the 2-position lower the energy difference between Z form and transition state, leading to a faster thermal relaxation (higher kr value). The situation found


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for cyano substituted azo-derivatives is similar with the ones reported in literature46, where substitution in the 2-position is as effective as in the 4-position. iv) Introduction of two acceptor groups, regardless of their nature, at the same ring leads to a steep acceleration of the relaxation time (high kr) in toluene. Consequently, we were not able to determine the kr value of M6-P6 pair by UV-Vis spectroscopy, not even at lower temperatures (0°C), due to very fast thermal relaxation. However, this situation favours the use of such azobenzene-based photochromic switches in real-time information-transmitting systems as well as optical oscillators. For this type of applications it is essential that the return to the thermodynamically stable E form in the dark occurs as fast as possible, that is, as soon as the optical stimulation is removed the molecule should revert to its initial state18. Figure 3 presents the information-transmission capability of M6 with time at 273 K. The repeatability was tested by submitting the solution to several UV irradiation-dark cycles. It can be noticed that the time response is very fast (milliseconds order) and shows no fatigue during 10 cycles. Although the preliminary results are very promising, thorough investigation by complementary techniques, like laser flash-photolysis, must be done in order to determine the exact response time. Therefore, we underline the potential of M6 and P6 azo-derivatives as promising candidates for photochromic switches.


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Figure 3. Optical density variation for M6, 4×10-5 M in toluene at 0 °C, generated by UV-light irradiation. v) All polymers exhibited the same trends as the monomers, except for P4 and P5. For these two polymers, we noticed a decrease of kr value by half as compared to that of each corresponding monomer. Since we had the same concentration of dye in solution for M4-P4, M5-P5 pairs, this effect can be assigned only to the polymer matrix interactions, which seem to hinder the thermal relaxation. vi) M5 (P5) and M6 (P6) compounds do not thermally relax back to E state in DMF and this particular behaviour is explained in the “Atypical thermal stability of Z isomers” section. 3.2. Thermal Z-E relaxation mechanism. In order to clarify the thermal isomerization pathway of our azo-derivatives we investigated the effect of solvent polarity on kr. Two routes can be possible for thermal relaxation of azobenzenes: inversion and rotation47. These routes can be differentiated on the basis of the dependence of kr on the solvent polarity. The first route proceeds through a transition state (TS) where one nitrogen atom is sp hybridized, whereas rotation proceeds via dipolar TS. For the inversion route very small variation of kr with solvent should be noticed (slightly higher kr values in non-polar solvents, TS is hydrophobic due to the sp hybridization), while for the rotation route the dipolar TS should be stabilized in polar solvents and thus higher values for kr should be obtained. The solvent dependence of kr listed in Table 3 clearly proves that thermal relaxation for M1, M2, M3 and M4 proceeds via inversion route as the polarity slightly affects the rate constants. In the polymer case, kr slightly decreases passing from DMF (polar solvent) to toluene (non-polar solvent). From the viewpoint of substitution pattern, we noticed that for monosubstituted azobenzenes the influence of the acceptor strength on the thermal relaxation in DMF is the same


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as in toluene. As compared to CN group, the nitro group induces an acceleration of kr, regardless of the position on the benzene ring. Placing the nitro group in the 2-position (see M4) also accelerates the thermal relaxation by a factor of 32 and 226 as compared to M2 and M1, respectively. In toluene as well as in DMF, the Arrhenius plots were linear, proving that there is no significant change in the reaction pathway over the examined temperature range (see Figure S3). The 2- and 4- substitution reduces the activation energy as compared to the parent azobenzene. The electron acceptor group lowers the repulsive force between unpaired electrons on the nitrogen atom attached to that ring, increasing the s-character of the C-N bond. This should cause the inversion on that nitrogen atom more easily than on the other nitrogen. The substituents with higher -M effect are more efficient in lowering the activation barrier (nitro as compared to cyano, see Table 3). Placing a CN group in 2-position increases the activation energy by 1 kJ/mol as compared to 4position, possibly due to the smaller contribution of nitrile group (situated in 2-position), to the stabilization of the TS and to the absence of steric interactions. For nitro-substituted compounds the effect is quite opposite, the Ea decreases with 5 kJ/mol for the 2-substituted compound. In general, slower reactions correlate with larger activation barriers. Thus, only when comparing M1 with M2, M3, and M4 we can apply this rule, if we analyse the values listed in Table 3. M4 has the highest kr and consequently the lowest activation energy. Although M5 exhibits the fastest recovery, it has higher activation energy than M4, but lower than M1, M2 and M3. In addition, if we analyse the activation energies of related polymers, one can notice that P1, P2 and P3 show slightly higher Ea than the corresponding monomers. This fact is in good agreement with the thermal relaxation rates.


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Interestingly, for P4 and P5 we have both lower thermal relaxation rates and lower Ea, as compared to related monomers. This indicates that the relatively high rate of isomerization for M5 cannot be explained by differences in the barrier potential, although kr of M5 is almost one order of magnitude faster than that of M4 and two orders of magnitude faster than that of M2. The fact that the relative isomerization rates are not explained by any differences in the barrier potentials is even more obvious when considering P5. It has the lowest barrier potential but do not isomerize faster than M5. Any significant difference in thermal relaxation rates is found in the pre-exponential factors given in Table 3. The lower pre-exponential factor for M4 compensates for its lower-energy barrier, while the high pre-exponential factor of M5 is responsible for its fast relaxation rate. For related polymers (P2, P4 and P5), the same tendency is noticed, whereas the low Ea compensates for the low pre-exponential factors and relaxation rates. In a chemical reaction the pre-exponential factor, A, depends on how often molecules collide and on whether the molecules are properly oriented when they collide (steric factor). From Table 3 we can notice that, even in solution, P5 and P4 have prefactor values smaller than the related monomers, and this can be due to the limited movement of the chromophores bound to the polymer. We determined the activation entropy and enthalpy associated with the thermal Z-E isomerization using Eyring equation48. All compounds have positive enthalpy of activation and negative entropy (see Table 4). Table 4. Thermodynamic parameters as calculated at 25°C using the Eyring equation. Code

∆H≠ [kJ·mol-1]

∆S≠ [J·mol-1·K-1]

∆G≠ [kJ·mol-1]

Toluene

Toluene

Toluene

DMF

DMF

DMF


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ABa

85.8

-

-43.1

-

98.6

-

M1

88.6

90.8

-37.4

-31.5

99.8

100.2

M2

78.1

72.2

-58.1

-70.5

95.4

93.2

M3

89.6

89.7

-35.1

-38.3

100.0

101.1

M4

73.3

73.9

-40.7

-41.4

85.5

86.2

M5

74.9

-

-11.9

-

81.4

-

P1

91.0

88.7

-30.8

-38.3

100.2

100.1

P2

77.9

65.3

-59.8

-92.1

95.8

92.7

P3

89.9

88.5

-36.4

-42.3

100.8

102.1

P4

64.3

73.9

-72.6

-41.9

85.9

86.3

P5

60.5

-

-75.0

-

82.8

-

a

values for azobenzene are in chlorobenzene from reference [50] Asano 1981

The negative entropy of activation indicates a greater degree of ordering in the transition than in the initial state (Z). Comparing M1 with M2, one can notice that nitro group interacts stronger with the solvent than the cyano group, leading to a decrease of entropy in the TS. This interaction is stronger in M2 than in M4. It is probably due to stabilization by conjugation of TS (with respect to the non-planar initial Z form), to the absence of steric effects, and to different strengths of specific solute-solvent interactions. M5 has the highest ∆S≠ value of all monomers and close to zero, it shows a similar degree of order in the TS and in the Z form. For P1, P2, and P3 polymers and M1, M2, and M3 monomers, there are no marked differences between activation parameters. For P4 and P5 there is a decrease in both ∆H≠ and ∆S≠ as compared to the corresponding monomers. A model of cybotactic region (that part of a solution in the vicinity of a solute molecule in which the ordering of the solvent molecules is modified by the presence of the solute molecule)49 can explain the lower activation entropy values obtained in DMF for all azo-monomers. In toluene, the solvent molecules in the


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cybotactic region of the azoderivative are relatively unstructured. As the Z molecules isomerize, the increase in dipole moment of the TS tends to organise the solvent molecule in the region. Thus, the overall entropy of the system will decrease, and so will ∆S≠. In DMF, the molecules in the cybotactic region are already relatively structured. As the Z isomer relaxes, the increased dipole moment of the transition state will not inflict a higher degree of organization upon the structure of the solvent shell as in the nonpolar case. Consequently, ∆S≠ in toluene will be higher than in DMF. Despite the fact that ∆H≠ of M4 in toluene is lower than that of M5, the kr value is 4 times lower at 30 °C. Therefore, the main factor that governs the isomerization rate is ∆S≠, which reflects the difference in the degree of freedom between the ground and the transition states. The relatively higher value of ∆S≠ can be explained by assuming that the isomerization of M5 starts from a structure that has a smaller degree of freedom due to steric constraints imposed by the 2,4-substitution. The thermal relaxation of P4 and P5 is a process governed by entropy and not by enthalpy (see Table 3 and 4), whereas the polymer matrix seems to induce a high level of order in the transition state. In order to reveal the relaxation route for azo-monomers and polymers, we used a linear correlation between the enthalpy and the entropy of activation as described in the literature50. The present results have been combined with those for rotation standard and for inversion standard of Asano et al50 and of other studies51. M1, M3 and P1, P3 related polymers are almost collinear with the azobenzenes that thermally relax by inversion (see Figure S4). Consequently, we can agree that M1 (P1), and M3 (P3) azoderivatives relax back through inversion mechanism; while M2 and P2 in both solvents are in between the lines of the two standards. Comparing the activation enthalpy with the activation entropy of the thermal Z-to-E isomerization, the


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uniqueness of M4 and M5 is further highlighted (see Figure S4). M4 and M5 greatly deviate from the inversion line, indicating that they revert thermally by rotation mechanism. For M4, the kr is not influenced by solvent, which is contradictory. The solvent polarity variation tells us that the thermal relaxation should proceed by inversion, while enthalpy-entropy compensation accounts for the rotation mechanism. If we plot the activation enthalpy vs. activation entropy only for the azo derivatives from our study (see Figure S5) we can notice a linear compensation for M1-M3 and P1-P3, with a high correlation coefficient (R2 = 0.9917), and a fairly ∆H≠- ∆S≠ linear compensation with a correlation coefficient (R2 = 0.9462) for M4-M5 and related polymers. The fact that the rate constants for thermal relaxation of azo derivatives M1-M4 are insensitive to polarity of the medium supports the inversion path, and proves that a clear-cut distinction between thermal relaxation mechanisms based on enthalpy-entropy compensation could not be made in our case, especially for nitro-substituted azo derivatives. The failure can be attributed to the assumption that changes in enthalpy should be partially or fully offset by a similar shift in the entropic component (∆H≠ and ∆S≠ should have the same signs so as ∆G≠ have minimum variation). In our case, for M2 the introduction of a 4-nitro group decreases the activation enthalpy with 7.7 kJ·mol-1, whereas the entropy of activation drops also by 15 J·mol1

·K-1 in toluene. Moreover, we noticed a more pronounced decrease of both parameters in DMF.

This suggests a more polar transition state in DMF, relative to the ground state, for 4-nitro azoderivatives than for azobenzene (negative ∆S≠ value). For M4, the ∆H≠ decreases with 12.5 kJ·mol-1, while ∆S≠ slightly increases with 2.4 J·mol-1·K-1, leading to a decrease of ∆G≠ by 13.1 kJ mol-1. The small ∆S≠ difference points out that the polarity of the TS is similar with that of azobenzene. Probably, the electronic effects induced by nitro groups in TS are surpassed by the steric repulsion between the 2-nitro group and the lone-paired electron of the nitrogen from the


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azo bond, leading to a twist of the phenyl ring and to nonplanarity. Comparing the activation parameters for M5 and azobenzene we noticed a decrease of 10.9 kJ·mol-1 in enthalpy and a significant increase of 31.2 J·mol-1·K-1 in entropy. The entropy increase implies less polar TS than in azobenzene. Consequently, a thermal relaxation by rotation route can be ruled out because a TS with higher polarity is involved in this mechanism. Taking into account the previous discussion, we may safely conclude that the deviation from the inversion to the rotation line noticed for the nitro derivatives is not caused by a change in the thermal relaxation mechanism. In order to elucidate the thermal relaxation route for M5, we determined kr in a variety of solvents with different polarities. The data in Table S1 show no marked influence of solvent on kr. Therefore, we can draw the conclusion that M5 thermal relaxation from Z state to the thermodynamic stable E state proceeds by inversion route.

3.3. Atypical thermal stability of Z isomers. As previously discussed (see Table 3), M5, M6, P5, and P6 azo-derivatives exhibit high thermal stability of the Z form in DMF. Figures 4 and S6 show typical changes in the UV-Vis absorption spectra of M5 monomer and P6 polymer during E–Z photoisomerization in DMF. After removing the UV stimulus, the thermal relaxation did not occur spontaneously, as for the monosubstituted azo-derivatives. To gain more insight into this behaviour we performed some quantum chemical calculations concerning the thermal relaxation of M5. The transition state was determined by the search of a geometry that showed only one imaginary vibration followed by another search through the corresponding module in GAMESS52.


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Figure 4. Changes in the UV-Vis absorption spectra of M5 monomer 4×10-5 M in DMF during E–Z photoisomerization at 365 nm at 20 °C. Inset shows the first-order plot for E-Z isomerization of M5. Taking into account that a simple modelling using the PCM solvent model for DMF did not explain the high stability the Z isomer, we tried to use explicit DMF molecules placed around the M5 molecule. Geometry optimization of Z-M5 in the presence of seven DMF molecules leads to a cluster structure (see Figure S7). All seven DMF molecules interact either with other DMF molecules or with M5 molecule forming a cage-like cluster around the M5 molecule. In this cluster, the Z isomer is confined and stabilized by strong dipole interactions. These physical bonds are formed between the oxygen atom, the carbonyl oxygen of the methacrylate group, and the hydrogen atom from the formamide and methyl groups in DMF. In addition, an interaction between the hydrogen atoms from the methyl group linked to phenyl ring substituted with methacrylate moiety and the oxygen atom from DMF is observed. Another interaction is noticed between the nitrile group and the hydrogen atom from methyl group and the hydrogen atom from the formamide in DMF. In total thirteen such interactions shorter than 3.0 Å are observed. Progressing from Z state to transition state, the breaking of these bonds takes place (see Fig S8 TS cluster). Only five can be observed, and none of them leading to an extended system over the


24

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two aromatic rings. All the interactions of the DMF molecules with methacrylate and nitrile groups disappear. This allows the phenyl ring substituted with the electron withdrawing groups to approach a quasi-collinear arrangement with the azo bond in order to thermally revert by inversion. Moreover, the angle formed by the two rings is 65º in the Z isomer and 87.4º in the transition state showing less conjugation between the two rings in the TS. The azo group is conjugated with the nitrile substituted ring. The bonds between the aromatic ring and the nitrogen from the azo group and also between the two nitrogen atoms in the azo group are shorter than 1.3 Å. In addition, the bond with the methacrylate substituted ring is much longer (1.39 Å). All this leads to an energy barrier between Z and TS of 150 kj/mol, determined by quantum calculations. This value is high enough not to allow thermal relaxation at room temperature. Also, heating the Z form in DMF at 80 °C did not result in the thermal recovery of the E isomer. This proves that the temperature has little or no effect on the intermolecular dipole bonds in the studied interval. The strong and complex interaction between M5 Z isomer and DMF is further highlighted by FT-IR spectroscopy, which is proven to be a powerful tool for investigating hydrogen and dipole bonding in a qualitative and quantitative manner53. The Z-E isomerization was investigated in toluene and DMF, respectively. There are some typical bands which can be identified in E-M5 spectra, such as 2229 (CN), 1581 (N=N), 1342 and 1524 (NO2) cm−1. Figure S9 obviously shows that there is no change in absorption, nor any band shifting in the entire spectrum during irradiation with UV light in toluene. In DMF the situation is completely different. A red shift of the bands corresponding to nitrile (see Figure 5), and nitro functional groups (Figure S10) can be observed. The presence of the isosbestic points, for each case, proves that only two species that vary in concentration are present, namely Z and E isomers.


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Figure 5. Changes of nitrile absorption band in the FT-IR spectrum of M5, 0.2 M in DMF at 20 °C, during E–Z photoisomerization from t=0 to t=85 min. With reference to Figure 5, there are two things worth noting about the 2236 cm-1 band. First, the shift to lower frequency (from 2236 to 2223 cm-1) implies a stronger interaction between the nitrile group of M5 and DMF. Second, the 2223 cm-1 band is significantly broader than the 2236 cm-1 band, which reflects a reasonably wide distribution of hydrogen bond distances and geometries in the cluster. The same behaviour is preserved (13 cm-1 bathochromic shift) in the case of symmetric (1352 cm-1) and asymmetric (1534 cm-1) stretching vibrations corresponding to the nitro group. Consequently, one can expect that DMF has stronger interactions with Z than with E isomer. The interaction between DMF and M5 isomers needs further attention. To this end, we recorded the FT-IR spectrum of 2-amino-5-nitrobenzonitrile and of nitrobenzene in DMF. We found that the bands for nitrile and nitro groups of 2-amino-5-nitrobenzonitrile overlapped with the bands corresponding to M5-Z isomer. These findings prove that the extent of conjugation of M5-Z in DMF is stronger than in M5-E isomer, although we would expect quite the opposite.


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The FT-IR study demonstrates that the complex interactions between DMF and M5 are stronger in the Z state than in the E state. The Z isomer is stabilized by reducing its energy, which translates into an increase of the activation energy of Z-E thermal recovery. The smaller ki value of M5 as compared to the other azo dye monomers can be attributed to the complex dipole-dipole interactions with the solvent, too. M6 has a similar behaviour regarding E-Z-E isomerization in DMF. The same explanations should apply, in terms of stabilization of the two isomers by dipole-dipole interactions with the solvent, as compared to the TS. 3.4. Photochromic properties. Regardless of solvent polarity, the monosubstituted azoderivatives did not present the thermal stability of the Z form as the disubstituted ones. Another intriguing fact is the photochromism exhibited by the disubstituted azo-derivatives in DMF. E-M5 has a green colour, while Z-M5 is yellow-orange. The same photochromic behaviour is noticed for M6 and related polymers in DMF. The other azoderivatives did not exhibit photochromism. Their solutions remain yellow-orange, regardless of the solvent used. The green colour solution of M5 proves that there is a maximum of absorption in the red region of the spectrum. Indeed, a maximum at 616 nm appears in the spectrum of M5 in DMF (see Figure S11). The appearance of the 616 nm band can presumably be because the solute molecules are liable to form a solvated complex with the DMF molecules through an intermolecular dipole interaction54-55. A band at 613 nm was reported by Alizadeh et al56 only in nitro-substituted hydroxy azobenzenes compounds. They concluded that this additional band is a result of an intermolecular charge-transfer transition. This transition involves an electron transfer from the lone pair of electrons at the oxygen atom of the DMF molecules to the antibonding orbital of the OH bond of the phenolic moiety. The appearance of such a band only in the spectra


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of the nitro azo derivatives is due to the high accepting property of the nitro group, which reflects a weakening of the O-H bond because of the decreased electron density on the oxygen atom. In our case, no O-H bond is present, because the hydroxyl group is esterified. This proves that the system involved in the dipole bond is the benzene ring substituted with the strong electron withdrawing groups. We have tried to put in evidence the intermolecular charged-transfer nature of this band by plotting the absorbance of the 616 nm band vs. the molar concentration (see Figure S12). A linear correlation was obtained for this band, instead of a nonlinear relationship, in the studied interval (1×10-5 – 1.2×10-4 M), proving the complex interaction of M5 with DMF. The band at 360 nm displayed also a linear behaviour (see Figure S13). A study on the influence of temperature on the 616 nm band for the E isomer was performed and, as noticed, the effect was minimum (see Figure S14). 3.5.

1

H NMR characterization. The kinetics of thermal Z to E isomerization were followed

by time-course measurements of the signals corresponding to Z isomer for the photoirradiated solutions at different temperatures. Figures 6 and S15 show the 1H NMR spectra of M1 monomer and P1 polymer, before irradiation and at the photostationarity state. A more detailed characterization of 1H NMR spectra is presented in the supplementary material. Because the size of each peak in the NMR spectrum is proportional to the number of protons, the analysis does not require a molecule specific coefficient of absorption to be derived. The concentration of each isomer was ascertained unambiguously with irradiation time.


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Figure 6. 1H NMR spectra of M1 monomer before irradiation and at the photostationarity state, in DMF-d7 solution at room temperature. The isomerization followed first order kinetics in all cases, as was commonly observed when UV-method was employed. The calculated rate constants are given in Table 5.

Table 5. Kinetic constants and activation parameters for Z-E thermal relaxation of M1M3 and P1-P3 in deuterated DMF calculated from 1H NMR experiments. Code

M1

M2

M3

P1

P2

P3

kra [s-1]

4.23×10-4 5.01×10-3 2.69×10-4 4.07×10-4 4.89×10-3 2.55×10-4

krb [s-1]

9.33×10-4 8.92×10-3 5.98×10-4 9.05×10-4 6.90×10-3 5.77×10-4

α [%]

50

17

55

48

15

52

Ea [kJ·mol-1]

81.9

76

82.7

80.2

74.7

81.3

A [s-1]

2.79×109

4.15×109

2.85×109

1.47×109

2.45×109

1.93×109

∆H≠ [kJ·mol-1]

79.1

73.3

78.8

77.5

71.9

75.9

∆S≠

-73.3

-70

-70.5

-78.7

-74.4

-74.8


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[J·mol-1·K-1] ∆G≠ [kJ·mol-1] a

100.9

94.2

99.8

99

94.1

98.2

determined from 1H NMR at 60ºC, bdetermined from UV-Vis at 60ºC.

In all cases, the rate constants, kr, increase with temperature, as they should normally do. Noticeable differences are between cyano-substituted azo-derivatives M1 (P1), M3 (P3) and nitro-substituted azo-derivatives M2 (P2), where the rate constants of nitro-substituted ones are of one order of magnitude higher. This behaviour supports the results found by UV measurements that a stronger acceptor group increases the isomerization rate. For polymers, as compared to the monomer counterparts, a slight decrease of Z-E thermal isomerization rate is most likely induced by the polymeric matrix. These facts can be explained in terms of chain mobility and flexibility of the polymer backbone which does not favour the motion of the thermodynamically meta stable Z form. The Z-E relaxation rate constant determined from 1H NMR, in concentrated solution, is slightly lower than the values obtained by spectrophotometry in diluted solution. The difference could be interpreted by the formation of strong dipolar interactions between the neighbouring Z chromophores, which are more favourable in concentrated than in diluted solution. One can notice that the values calculated for α (the proportion of Z isomer), are lower than those obtained from UV measurements. This is due to the higher concentration required for NMR spectroscopy experiments. In addition, we determined the activation energies and the pre-exponential factors using Arrhenius equation. Employing Eyring equation, the activation entropy and the enthalpy associated with the thermal Z-E isomerization were also calculated (see Table 5). The kr values obtained from 1H-NMR are lower than the ones determined from UV-Vis spectroscopy. Consequently, one can expect higher activation energies. In reality, the result is


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quite the opposite (see Table 5), proving that the isomerization process in concentrated solution is entropy controlled (low pre-exponential factors). This NMR study substantiates that the azo-polymers behave similarly to the corresponding monomer with almost no steric or topological constraints in concentrated solution. 4. CONCLUSIONS We have performed a detailed investigation on the E-Z-E isomerization mechanism for six azo-monomers and corresponding azo-polymers. The effect of solvent polarity on the relaxation rate, and the correlation with the thermodynamic parameters allowed us to conclude that all azoderivatives thermally relax via inversion mechanism. The fascinating part of this study was the particular behaviour of the disubstituted azoderivatives, which revealed stable Z isomers. With the help of theoretical quantum calculation, we have pointed out the nature of the interaction that confines and stabilizes the Z molecules. Seven DMF molecules and an azo-molecule arrange themselves to form a supramolecular cluster, which is the reason for the high thermal stability exhibited by the Z-azoderivatives. In addition, the FT-IR spectra sustained the theory, revealing the complex interactions between DMF and M5, which are stronger in the Z state than in the E state. The same disubstituted azo-derivatives, in a non-polar solvent like toluene, changed their thermal relaxation behaviour to fast thermal Z-to-E azo-derivatives, very much alike to the highly conjugated systems. Even though, P5 does not have much potential for its use in real-time information-transmitting systems, P6 does have such potential for this type of applications, due to the fact that the return to the thermodynamically stable E form in the dark occurs very fast (milliseconds).


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We observed slight differences between the azo-monomers photochemical behaviour and the related azo-polymer structures. The kinetics of the thermal relaxation of polymers, by high resolution 1H NMR, highlighted that the rates of Z-E thermal relaxation are similar with those in diluted solution. This proves that the macromolecular architecture of the matrix is not an impediment that can disturb the desired overall photochemical behaviour of the material, and that would not limit the applicability of these materials. We consider that our azo-polymers have the potential to be used in practical application, such as data storage, due to the two stable states, or real-time transmitting information. Moreover, the same material can be used for any of these applications just by changing the nature of its interaction with the medium. Studies are now in progress to establish if the blocked interconversion is retained in the solid state.

ASSOCIATED CONTENT Supporting Information. UV-Vis spectra of P4 azo-polymer after irradiation at different periods (Figure S1). UV-Vis spectra of M2 azo-monomer thermal relaxation (Figure S2). Mesomeric structures of TS for azo-monomers (Scheme S1). Arrhenius plots for azobenzene monomers (Figure S3). Plot of activation entropy vs activation enthalpy (Figures S4 and S5). Kinetic constants of Z-E thermal relaxation of M5 in different solvents calculated from UV-Vis data (Table S1). UV-Vis spectra of P6 azo-polymer during irradiation with UV-light (Figure S6). Geometry optimization of Z and TS of M5 (Figures S7 and S8). Changes in the FT-IR spectrum of M5 monomer dissolved in toluene and DMF, respectively (Figures S9 and S10). Effect of the concentration on the electronic absorption spectra of M5 monomer in DMF (Figure S11). Plot of


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absorption of 360 nm and 616 nm band vs. concentration for M5 monomer in DMF (Figures S12 and S13). Effect of temperature on the absorption spectra of M5 monomer (Figure S14). 1HNMR characterization of Mx and Px azo-polymers (Figure S15). AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

Author Contributions ∞

The authors acknowledge the contribution of Dumitru Mircea Vuluga being equal to that of the

first and reprint author. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work has been funded by the Sectorial Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132395 and by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PN-II-RU-PD-2011-3-0063. The authors from “Ilie Murgulescu” Institute of Physical Chemistry gratefully acknowledge the support of EU (ERDF) and Romanian Government allowing for acquisition of the research infrastructure under POS-CCE O2.2.1 project INFRANANOCHEM, No. 19/2009.03.01 and the support from PN-IIID-PCE-2011-3-0916 grant.


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Chemical structures of the hydroxy-azobenzenes (A), the azo-monomers (B), and related photo-responsive azo-polymers (C). 194x474mm (300 x 300 DPI)



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Changes in the UV-Vis absorption spectra of M1 monomer 4×10-5 M in DMF at 20 °C during E–Z photoisomerization at 365 nm. Inset shows the first-order plot for E-Z isomerization of M1. 60x46mm (300 x 300 DPI)


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Z-E thermal relaxation UV-Vis absorption spectra of P3 polymer, 4×10-5 M in toluene at 80 °C. Inset shows the first-order plot for Z-E thermal relaxation of P3. 63x50mm (300 x 300 DPI)



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Optical density variation for M6, 4×10-5 M in toluene at 0 °C, generated by UV-light irradiation. 57x41mm (300 x 300 DPI)


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Changes in the UV-Vis absorption spectra of M5 monomer 4×10-5 M in DMF during E–Z photoisomerization at 365 nm at 20 °C. Inset shows the first-order plot for E-Z isomerization of M5. 60x45mm (300 x 300 DPI)



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Changes of nitrile absorption band in the FT-IR spectrum of M5, 0.2 M in DMF at 20 °C, during E–Z photoisomerization from t=0 to t=85 min. 58x42mm (300 x 300 DPI)


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1H NMR spectra of M1 monomer before irradiation and at the photostationarity state, in DMF-d7 solution at room temperature. 63x47mm (300 x 300 DPI)


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