Mikhail Yu. Belikov* and
Mikhail Yu. Ievlev
Ulyanov Chuvash State University, Moskovsky pr., 15, Cheboksary, Russia 428015. E-mail: belikovmil@mail.ru
First published on 14th June 2021
Representatives of visible-light-switchable nitrile-rich negative T-type photochromes (NRP) were synthesized. The temperature effect on the thermal relaxation of the photoinduced form of NRP was studied for the first time. It was found that regardless the temperature the reverse thermal reaction in ethanol and 1,4-dioxane proceeded according to a first-order kinetic equation, while an unusual pseudo-zero order of the dark reaction was observed in acetonitrile. Using the obtained experimental data, the activation energy as well as the activation parameters, such as entropy and enthalpy of activation, were calculated for the first time for thermal relaxation of NRP photochromes. It was also shown that an initial form of NRP photochromes bearing a tricyanofuran acceptor was more temperature resistant than the initial form of photochromes with a tricyanopyrrole acceptor.
Ultraviolet or visible light irradiation causes a reversible transformation of photochromes A into a metastable photoinduced form B (Fig. 1). It should be noted that studies of photochromes with the possibility to use visible light only6 as a non-destructive and selective trigger to control the properties of substances and materials are intensified nowadays. Depending on a chromaticity of transformations direct and negative photochromism are distinguished. In the first case, the initial form A absorbs shorter wavelengths than photoinduced form B which is generally deeply colored. In contrary, for the negative photochromes the initial form A absorbs longer wavelengths, and the form B is less colored or completely colorless. It has been noted10,11 that negative photochromes are much less common and therefore poorly studied.
The photoinduced form B can be stable during a long period at dark conditions (P-type photochromism), or it spontaneously returns to the initial state A at room or raised temperature (T-type photochromism).12–14 In the case of T-type photochromes, this feature is important and exciting because it allows to control the stability of the photoinduced form by changing the temperature conditions. This issue has been well studied for known groups of photochromes, such as spiropyrans,15,16 spirooxazines,17 azobenzenes,18,19 diarylethenes,20,21 dihydropyrene,22 dihydroindolizines23 and bindone derivatives.24
Papers15–24 also demonstrate that the analysis of the temperature effect on the rate constant allows to estimate various important energy parameters of the reverse thermal reaction of photochromes, for example activation energy.
Compounds bearing a nitrile-rich acceptor and δ-OH-group in the same molecule are a relatively poorly studied group of negative photochromes (NRP, Fig. 1).25–31
NRP derivatives show photochromism under irradiation by visible light. Their colored initial form is converted into a colorless spiran NRP-S. The reverse reaction occurs when the solutions is stored in the dark, characterizing NRP derivatives as T-type photochromes. The importance of a more detailed study of this group of photochromes is supported by the fact that the possibility for photo-regulation of acidic25,26 and fluorescent27 properties of some representatives has been reported, that indicates the prospects of their practical application.
It should be noted that, despite the reported data on the T-type photochromism of NPR structures, there is no information in the literature about the temperature effect on the process of dark thermal relaxation of the photoinduced NRP-S form. Therefore, the thermodynamic parameters of this process are also unknown. Thus, the aim of this work is to investigate the influence of temperature on the dark reaction rate of photochromes 1, as well as to obtain the data on the thermal stability of NRP photochromes in an individual form.
Also, a previously undescribed compound 1b, bearing a furan ring in the structure as a nitrile-rich heterocyclic unit, was chosen as a second study object. The synthesis of these two compounds 1 was based on the interaction of the corresponding tricyano-substituted acceptors 2a and 2b with ethyl 3-formyl-4-hydroxybenzoate in ethanol in the presence of ammonium acetate under an argon atmosphere (Scheme 1) giving 62–76% isolated yield. Structure of compounds 1 was supported with IR-, NMR spectroscopy and mass-spectrometry.
Fig. 2 shows that after visible light irradiation an absorption band in the visible region is decreased indicating a negative photochromic process for compounds 1 in each solvent (Table 1).
Compound | Solvent | λmaxabs (nm) | ε × 104 (L × mol−1 × cm−1) | Abs | AbsPSS | PCa (%) |
---|---|---|---|---|---|---|
a Photoconversion percentage was estimated using decrease of the main absorption band. | ||||||
1a | EtOH | 410 | 2.92 | 0.730 | 0.376 | 48 |
535 | 0.12 | 0.030 | 0.005 | |||
MeCN | 404 | 3.05 | 0.762 | 0.241 | 68 | |
602 | 0.24 | 0.059 | — | |||
1,4-Dioxane | 406 | 3.28 | 0.820 | 0.547 | 33 | |
1b | EtOH | 424 | 2.32 | 0.579 | 0.072 | 87 |
583 | 0.64 | 0.160 | — | |||
MeCN | 414 | 3.05 | 0.763 | 0.342 | 55 | |
1,4-Dioxane | 415 | 3.54 | 0.886 | 0.494 | 44 |
Depending on the solvent, the photoconversion of the initial form 1 into the photoinduced one is different, for example, for furan 1b it changes from 87% in ethanol to 44% in 1,4-dioxane. Then, the absorption spectra of photochromes 1a,b were registered at various temperatures in each of three solvents before and after irradiation. Fig. 3A shows the absorption spectra of compound 1b in ethanol before and after irradiation in dynamics at 20 °C.
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Fig. 3 UV-Vis spectra (A) and kinetics of the dark reverse reaction, λabs = 424 nm (B) of the compound 1b before and after irradiation (EtOH, C = 2.5 × 10−5 M, 20 °C). |
Fig. 3A shows that the intensity of the band at 424 nm was significantly decreased under irradiation by visible light. After the irradiation had been stopped, the solution was storing in dark conditions and the absorbance was gradually returning to the initial value. Fig. 3B shows first-order kinetics of the dark thermal transformation of compound 1b after irradiation by visible light in ethanol.
Compound | Solvent | Temperature, T (°C) | Rate constanta, k (min−1) or (M × min−1) | Half-life periodb, τ1/2 (min) |
---|---|---|---|---|
a For first-order reactions in ethanol and 1,4-dioxane (min−1), for pseudo-zero order reactions in acetonitrile (M × min−1).b The half-life period (τ1/2) was estimated as 0.693/k for first-order reactions in ethanol and 1,4-dioxane, and as APSS/2k for pseudo-zero order reactions in acetonitrile. | ||||
1a | EtOH | 10 | 0.083 | 8.3 |
15 | 0.180 | 3.9 | ||
20 | 0.351 | 2.0 | ||
25 | 0.708 | 1.0 | ||
30 | 1.381 | 0.5 | ||
MeCN | 10 | 0.38 × 10−6 | 24.1 | |
15 | 0.76 × 10−6 | 12.2 | ||
20 | 1.46 × 10−6 | 6.3 | ||
25 | 2.79 × 10−6 | 3.2 | ||
30 | 4.98 × 10−6 | 1.8 | ||
35 | 8.95 × 10−6 | 1.0 | ||
1,4-Dioxane | 35 | 0.009 | 75.7 | |
40 | 0.017 | 41.1 | ||
45 | 0.030 | 23.2 | ||
50 | 0.057 | 12.1 | ||
55 | 0.102 | 6.8 | ||
60 | 0.175 | 4.0 | ||
1b | EtOH | 5 | 0.144 | 4.8 |
10 | 0.296 | 2.3 | ||
15 | 0.626 | 1.1 | ||
20 | 1.354 | 0.5 | ||
25 | 2.679 | 0.3 | ||
30 | 4.947 | 0.2 | ||
35 | 8.778 | 0.1 | ||
MeCN | 10 | 0.35 × 10−6 | 19.7 | |
15 | 0.69 × 10−6 | 10.1 | ||
20 | 1.48 × 10−6 | 4.7 | ||
25 | 2.64 × 10−6 | 2.7 | ||
30 | 4.82 × 10−6 | 1.6 | ||
35 | 8.51 × 10−6 | 0.9 | ||
1,4-Dioxane | 20 | 0.091 | 7.6 | |
25 | 0.182 | 3.8 | ||
30 | 0.347 | 2.0 | ||
35 | 0.624 | 1.1 | ||
40 | 0.997 | 0.7 | ||
45 | 1.792 | 0.4 |
Investigation of the temperature effect in 1,4-dioxane showed a significant difference in the rate of reverse reaction for compounds 1a and 1b (Fig. 4). Thus, the half-life period (τ1/2) of 1a is 23.2 min and for furan 1b it is 0.4 min only (Table 2) at 45 °C. It should be noted, that kinetics of the reverse thermal reactions of photochromes 1a,b in ethanol (Fig. 3B) and 1,4-dioxane (Fig. 4) were fitted to the first-order equation at each of the studied temperatures.
Acetonitrile solutions of compounds 1a,b demonstrated a comparable with ethanol temperature effect on the rate of the dark reverse reaction. Thus, the half-life period (τ1/2) of furan derivative 1b changes from 19.7 min to 1.6 min ranging the temperature from 10 to 30 °C (Table 2). At the same time, dark thermal reaction in acetonitrile fits to a pseudo-zero order kinetic equation (Fig. 5) regardless the temperature and the type of heterocyclic unit of compounds 1.
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Fig. 5 Kinetics of the reverse thermal transformation of compound 1a (circles) and 1b (squares) in MeCN (C = 2.5 × 10−5 M, 20 °C). |
It is worth mentioning that pseudo-zero reaction order is usually not observed in solutions; therefore, the obtained data are rather unique example. Zero-order reactions are usually found for heterogeneous or enzyme catalyzed processes.32–36 The presence of a rate-limiting step allowing only a small fraction of the reactant molecules to react is typical for such reactions. Thus, the reactant conversion occurs with constant rate and therefore the true reaction order is hidden.
In the case of compounds 1, pseudo-zero reaction order is apparently caused by low deprotonation rate of the photoinduced CH-acid 1S in acetonitrile (Scheme 2), since MeCN is a protonophobia solvent and poorly solvates anions.37 In such solvent as acetonitrile deprotonation of the photoinduced CH-acid 1S becomes a rate-limiting step of the dark reverse reaction unlike other solvents where the reaction rate directly associated with anion 1S− concentration. Therefore, in acetonitrile deprotonation process is followed by the immediate consumption of 1S− and the ring-opening occurs with a constant rate.
The discovery of novel zero-order processes is significant, because such a system could be used, for example, for prolonged drug delivery.38,39
The activation energy (Ea) and frequency factor (A) were calculated from the experimental dark reverse reaction rate constant k values by the Arrhenius equation (eqn (1)), where R is the universal gas constant and T is the absolute temperature. These obtained data are collected in the Table 3. The Arrhenius plot (Fig. 7A) shows a good correlation for both photochromes 1 in different solvents.
![]() | (1) |
Comp. | Solvent | Ea (kJ mol−1) | ΔH≠ (kJ mol−1) | ΔS≠ (J K−1 mol−1) | A (s−1) |
---|---|---|---|---|---|
1a | EtOH | 99.6 | 97.2 | 44.3 | 3.40 × 1015 |
1,4-Dioxane | 101.4 | 98.7 | 2.2 | 2.37 × 1013 | |
1b | EtOH | 98.8 | 96.4 | 52.3 | 8.95 × 1015 |
1,4-Dioxane | 91.3 | 88.8 | 4.7 | 3.02 × 1013 |
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Fig. 7 Arrhenius (A) and Eyring (B) plots for compounds 1a (circles) and 1b (squares) in ethanol (blue) and 1,4-dioxane (black) (C = 2.5 × 10−5 M). |
The activation enthalpy (ΔH≠) and the activation entropy (ΔS≠) were calculated by the Eyring equation, where kB and h are the Bolzmann and Planck constants, respectively.
![]() | (2) |
Fig. 7B shows the Eyring plot with an excellent linearity. We have found that, depending on the solvent and type of the nitrile-rich heterocycle, the activation energy is in the range from 91.3 to 101.4 kJ mol−1 (Table 3). The activation enthalpy of the studying process is in the range of 88.8–98.7 kJ mol−1. More significant differences have been found for the activation entropy values. For the dark reaction in ethanol ΔS≠ is 44.3–52.3 J K−1 mol−1, while in 1,4-dioxane it is much less, namely 2.2–4.7 J K−1 mol−1. A similar dependence has been also shown for the frequency factor (A), it is significantly lower in 1,4-dioxane. The obtained A and ΔS≠ values are consistent with the observed lower τ1/2 of photo-induced form in ethanol.
Data on acetonitrile solutions were not included in the calculations of activation parameters, because the kinetics of reverse thermal process fit to the pseudo-zero order equation and the observed rate constants are not comparable with rate constants of reactions occurred in ethanol and 1,4-dioxane.
In this regard, we evaluated the thermal stability of the synthesized NRP derivatives 1a,b using thermogravimetric analysis for the first time. It was found that furan 1b did not undergo any significant changes upon heating up to 300 °C in an argon atmosphere. A weight loss of 10.4% occurred in the range of 300–320 °C which was accompanied by melting with decomposition (Fig. 8). Pyrrole 1a showed lower thermal stability. It underwent a thermal decomposition in the range of 200–245 °C with a 6.2% weight loss. Such a difference of the TGA curves was probably associated with the presence of a labile aminoalcohol moiety in the structure of pyrrole 1a.
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Fig. 8 TGA curves of compounds 1a and 1b at the heating rate of 10 °C min−1 under an argon atmosphere. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra02979g |
This journal is © The Royal Society of Chemistry 2021 |