Time-Dependent Photodimerization of Î±-trans-Cinnamic Acid Studied

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Time-Dependent Photodimerization of #-trans-Cinnamic Acid Studied by Photocalorimetry and NMR Spectroscopy Tamas Panda, and Pance Naumov Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01409 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Crystal Growth & Design

Time-Dependent Photodimerization of -transCinnamic Acid Studied by Photocalorimetry and NMR Spectroscopy Tamas Panda, Panče Naumov* New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates KEYWORDS: calorimetry, dimerization, NMR spectroscopy, photochemistry, solid state

Dedicated to Professor Bill Jones

ABSTRACT: The time-course of photochemical solid-state reactions is routinely monitored by using spectroscopic methods such as NMR or IR spectroscopies, but is comparatively less investigated with thermal methods. In this work, a combination of thermal methods (thermogravimetric analysis and differential scanning calorimetry) was applied together with irradiation with UV light to quantify the conversion and monitor the progress of a well-known photochemical reaction, the [2+2] dimerization of trans-cinnamic acid, and the results are compared with the conversion determined by using 1H NMR spectroscopy. The conversion was correlated with thermodynamic parameters for the reactant such as molar enthalpy, entropy and melting temperature.

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Introduction Photoactive chemical systems are unmatched in their potential for current and future technologies such as storage and transfer of information, nonlinear optics, and so on.1 A subset of of photochemical reactions from a plethora of reactions that proceed in the solid-state are particularly relevant to these applications,2 as they provide means for remote spatial and temporal control over their physical properties in the solid state.3 The kinetics of the forward (photochemical) and reverse (thermal and/or photochemical) processes in these reactions are among the most fundamental parameters required to assess the viability and performance of the related (for instance, photochromic) materials, and are often determined from spectroscopic measurements. Thermal analysis does not appear as the first method of choice to study the progress of a photochemical reaction because the kinetics depends on temperature and excessive heating oftentimes triggers the reverse (thermal) reaction, which renders the determination of the photoconversion by using thermal methods unreliable. In this work we challenged this idea by using a thermally irreversible solid-state photochemical reaction, the [2 + 2] photodimerization of trans-cinnamic acid (-TCA, Scheme 1), a well-known reaction that is much of a guinea pig for explorations into the organic solid-state reactivity.4 The dimerization of the -trans cinnamic acid, where monomers are conveniently arranged for dimerization in a head-to-tail fashion,5 has been extensively studied by single crystal X-ray diffraction,6 atomic force microscopy,7 vibrational spectroscopy8 and solid-state NMR spectroscopy.9‒11 Here, we monitored the progress of this reaction by determining the molar ratio of the reactant and the product independently by two techniques, namely thermogravimetric analysis (TGA) and 1H NMR spectroscopy. The thermal behavior of the reactants and products of


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photodimerization reactions, including that of the trans-cinnamic acid, have been investigated, although not from the view of monitoring the photoconversion from monomer to dimer.12 Differential scanning calorimetry (DSC) was also performed to characterize the reactant (note that due to the technical limitations, only the DSC curves of the melting of the reactant and up to a temperature below its decomposition could be recorded). The thermodynamic parameters (molar enthalpy, entropy and melting/freezing point) of the reactant were correlated with the photo conversion determined by TGA and NMR spectroscopy.

Scheme 1. [2 + 2] photo dimerization of -trans-cinnamic acid (-TCA) to -truxillic acid.

2. Experimental Section 2.1. Materials and methods. The reactant, -TCA (Sigma-Aldrich, 99%) was recrystallized from acetone to obtain the -form, and the crystals were grinded to fine powder. 20 mg of the powdered microcrystals was thinly and evenly spread over a glass slide and exposed to UV light (365 nm) from a medium-pressure mercury lamp (SP-11, Ushio) equipped with an internal heat filter. The light output was placed at a distance of 5 cm from the sample and inclined at an angle of ∼60° to the base. The sample temperature was maintained at room temperature (25 °C). 10 samples were


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prepared on different glass plates, and each sample was exposed to UV light over different time (60, 150, 300, 450, 600, 900, 1500, 1800, 2100 and 2700 s) and analyzed by TGA, DSC and 1H NMR spectroscopy. 2.2. Differential scanning calorimetry (DSC). The differential scanning calorimetric measurements of the samples were carried out on Q2000 instrument (TA Instruments). The samples were taken on a Tzero aluminum pans and heated from room temperature (25 °C) to 140 °C at a heating rate of 10 °C min‒1. 2.3. Thermogravimetric analysis (TGA). The thermogravimetric analysis and simultaneous differential thermal analysis (DTA) were performed with SDT Q600 instrument (TA Instruments) at a heating rate of 10 °C min‒1 using alumina pans. The temperature range for the TGA analysis was 25–500 °C. Dry nitrogen gas was used as carrier gas. 2.4. Nuclear magnetic resonance (NMR) spectroscopy. The NMR spectra were recorded at 25 °C on an Advance 500 spectrometer (Bruker) at working frequency of 500 MHz for the 1H nuclei. All chemical shifts are reported in ppm relative to the signals corresponding to the residual nondeuterated solvent (DMSO-D6:  = 2.5 ppm)

3. Results and Discussion Figure 1a shows a series of TGA traces of -TCA recorded after various times of exposure to UV light (60, 150, 300, 450, 600, 900, 1500, 1800, 2100 and 2700 seconds). As it is inferred from there and from the DTA traces in SI, Figure S1, the thermal behavior of -TCA depends on the irradiation time. The non-irradiated sample (-TCA) shows one TGA step and two DTA peaks.


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The first DTA peak (129 oC) corresponds to melting, and the second (216 oC) to a single-step decomposition. After it was exposed to UV radiation for 60 s, the sample was partially converted to the product. As the irradiation time increases, the two DTA peaks decrease and they finally disappear. Simultaneously, two new endothermic peaks from the product appear in the DTA curve and increase in intensity. The first peak (285 oC, for the pure product) corresponds to melting and the second peak (325 oC, for the pure product) corresponds to a single-step decomposition of the -truxillic acid.

Figure 1. Dimerization of -trans-cinnamic acid (‘TCA monomer’) to -truxillic acid (‘TCA dimer’) monitored by TGA (a,b) and 1H NMR spectroscopy (c,d). (a) TGA traces of pristine and


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UV-irradiated -TCA (the time of irradiation is shown next to each curve). (b) Changes in the mole ratio of the reactant and product with time calculated from the weight loss. (c) 1H NMR spectra of pristine and irradiated -TCA (the time of irradiation is shown next to each spectrum). The akene  and  hydrogens at 6.5 ppm and 7.7 ppm of -TCA (highlighted as blue-shaded regions) disappeared with irradiation time. Simultaneously new peaks appear at 4.3 ppm and 3.8 ppm (green-shaded regions) that correspond to the cyclobutane hydrogens. (d) Changes in the mole ratio of the reactant and product with time calculated from the 1H NMR spectra. Because the decomposition of both reactant and product occur in a single step and are clearly separated in the TGA, the conversion can be calculated from the thermal analysis. During irradiation, it is probable that mixtures and various solid-solutions are formed;13 however, with constant heating, there is no crystallization and no other processes could be detected during irradiation. The weight (Figure S2 in SI) and mole ratios of the monomer and dimer were calculated and are listed in Table 1 and Table S1 in SI. Based on the weight loss, after 60 s of irradiation, the sample was a mixture of 94.4% -TCA and 5.6% -truxillic acid (expressed in mole %). When the irradiation time was increased to 150 s, the conversion of -truxillic acid increased to 15.4%. The conversion of the dimer gradually increased with irradiation time, and as shown in Figure 1a, as reflected in change of the TGA curve from 60 s to 2700 s. Figure 1b shows gradual decrease of the amount of reactant and concomitant increase of the product. The sigmoidal curve in Figure 1b is indicative of JMAK (Johnson-Mehl-Avrami-Kolmogorov) kinetics, as found earlier by Hyes et al.11 The JMAK model describes the kinetics of a phase transition (in this case, photodimerisation) that proceeds by nucleation and growth. In the JMAK equation14-17 y = 1ek(t)n, y is the mole fraction of the photoproduct formed in time t, k is the rate constant, and n is the dimensionality of growth (Avrami exponent). The experimental curve in Figure 1b corresponds to


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an Avrami exponent of (1.12  0.08) and rate constant 10.5  10-4  2  10-4 s1 . The TGA-DTA results confirm that after 2700 s irradiation, -TCA was completely converted to -truxillic acid.

Table 1. Conversion (in mole %) obtained from the TG analysis of irradiated -TCA

UV irradiation time / seconds

-TCA monomer / mole %

-TCA dimer / mole %

0

100  0.1

0.00

60

94.4  0.2

5.5  0.2

150

84.6  0.2

15.3  0.1

300

64.5  0.1

35.4  0.04

450

61.7  0.2

38.2  0.4

600

58.1  0.3

41.8  0.1

900

29.3  0.3

70.6  0.2

1500

18.2  0.2

81.7  0.2

1800

11.6  0.1

88.3  0.3

2100

7.3  0.4

92.2  0.2

2700

1.7  0.3

98.3  0.2

In order to more accurately correlate the photoconversion determined by TGA with the thermodynamic parameters, the samples were also analyzed by using differential scanning calorimetry (DSC). A partially reacted sample contains both reactant and product which, according to the above discussion, melt and decompose separately at temperatures that are very close to the


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melting and decomposition temperatures of the pure reactant or product, respectively. Unfortunately, the decomposition temperatures of the reactant is below the melting temperature of the product, and thus it was not possible to melt both components in order to record the DSC on cooling and to construct the full phase diagram. Moreover, because the reactant decomposes before the product melts, due to technical reasons (chemical decomposition is not compatible with the calorimeter), we were not able to record DSC curves above the melting point of the reactant. Nevertheless, it was possible to record the DSC of the melting and solidification of the reactant— in pure form or in partially reacted samples—and accordingly, the related thermodynamic parameters were selected for correlation with the reaction conversion. Table 2. Molar fusion enthalpy (kJ mol-1), entropy (kJ mol-1 K-1), and melting temperature of pristine and irradiated -TCA. UV Irradiation time / seconds

Melting temperature / C

Fusion Enthalpy / kJ mol-1

Fusion Entropy / kJ mol-1 K-1

0

134.1  0.1

32.9  0.08

0.081

(133.4 C  0.1 C) 60

132.9  0.2

26.6  0.06

0.065

(131.4 C  0.2 C) 150

132.5  0.2

24.2  0.08

0.059

(131.0 C  0.1 C) 300

132.5  0.3

19.6  0.05

0.048

(130.4 C  0.3 C) 450

132.4  0.2

12.6  0.09

0.031

(130.1 C  0.2 C) 600

132.2  0.4

8.1  0.08

0.020


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(129.8 C  0.3 C) 900

132.3  0.2

5.29  0.07

0.013

(129.2 C  0.2 C) 1500

132.0  0.2

3.58  0.08

0.008

(128.4 C  0.3 C) 1800

132.3  0.3

2.61 0.1

0.006

(128.1 C  0.3 C) 2100

131.9  0.2

2.40  0.2

0.006

(128.4 C  0.2 C) 2700

Not visible

NA

NA

Figure 2a shows the changes in the DSC profile of the melting of -TCA before and after irradiation with UV light (SI Figure S4 contains the full DSC profile on heating and cooling over the phase melting/solidification temperature). Pristine -TCA exhibits a single strong endothermic effect at 134 C due to melting. As shown in Figure 2a, the intensity of the peak gradually decreases with irradiation as a consequence of the decreasing amount of the reactant. In line with the TGA and DTA results, after 2700 s the melting peak disappeared, as -TCA was entirely converted to -truxillic acid. The specific and molar fusion enthalpy, entropy and melting temperature were extracted by non-linear integration of the DSC curves, and are shown in Table 2 and SI Table S1. These parameters are plotted with the conversion (in mole %) calculated from the TGA in Figure 2b‒d. These plots show that generally as the reaction progresses the molar enthalpy and entropy of the reactant decrease, as expected from the depletion of the reactant in the partially reacted sample. The melting temperature of pure -TCA, determined by DSC, is 134.1 oC. This temperature decreases about 1 oC after irradiation, due to the presence of the (solid) product which


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acts as impurity. The temperature remains relatively constant in the mixed samples, and at the lowest mole ratio of the reactant of 7.2% where the DSC peak could still be determined (at 2100 s irradiation), it is 131.9 oC.

Figure 2. Dimerization monitored by differential scanning calorimetry (DSC), and correlation with the reaction conversion. (a) DSC of pristine -TCA and samples irradiated with UV light between 60 s and 2700 s. The blue line was added to visualize the shift of the peak that corresponds to melting of the reactant, -TCA. (b) Molar enthalpy of fusion of -TCA plotted as a function of conversion (mole %) obtained by TGA analysis. (c) Molar enthalpy of fusion of -TCA plotted as a function of conversion (mole %) obtained by TGA analysis. (d) Melting temperature of -TCA determined by DSC plotted as a function of conversion (mole %) obtained by TGA analysis.


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A much more reliable approach to independently monitor the reaction progress is to use NMR spectroscopy, and this technique has been regularly applied to determine the conversion of solid-state photodimerizations. We used 1H NMR spectroscopy to assess the accuracy of the conversions obtained by thermal analysis, and also to obtain complementary dataset for correlation with the thermodynamic parameters. Figure 1c shows the effect on the 1H NMR spectra of the dimerization reaction of -TCA with irradiation time (the spectra are shown as individual plots in SI Figure S3). As the sample is irradiated, the alkene  and  hydrogen atoms of -TCA are transformed into cyclobutane hydrogen atoms in the α-truxillic acid. Table 3. Conversion (in mole %) obtained from 1H NMR analysis of irradiated -TCA

UV irradiation time /

TCA monomer / mole %

TCA dimer / mole %

0

100

0.0

60

94.4  0.1

5.5  0.2

150

87.6  0.2

12.3  0.3

300

71.9  0.3

28.0  0.1

450

62.2  0.3

37.7  0.2

600

60.9  0.2

39.0  0.2

seconds


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900

33.1  0.2

66.6  0.3

1500

15.1  0.4

84.4  0.2

1800

8.8  0.3

91.1  0.3

2100

7.0  0.5

92.6  0.2

2700

6.2  0.2

93.7  0.2

This is reflected in gradual decrease in intensity of the peaks at 6.5 ppm ( hydrogen) and 7.7 ppm ( hydrogen) with increasing irradiation time. Simultaneously new peaks appear at 4.3 ppm and 3.8 ppm, which correspond to the cyclobutane hydrogen atoms. The peaks of each spectrum were integrated using an internal standard, and the change of the mole ratio of the reactant and the product with time is plotted in Figure 1d and the conversions are shown in Table 3. We also fitted the conversion determined by NMR spectroscopy (Figure 1d). The average Avrami exponent and rate constant are (1.12  0.08) and 4.0  10-4  0.8  10-4 s1, respectively. These values match closely the respective values obtained based on TGA in Table S3, SI, where the rate constants are listed by NMR and TGA conversion. The exponential trends in the mole ratio determined by NMR spectroscopy (Figure 1d) resemble closely the trends determined by TGA (Figure 1b). Indeed, the linear fit shown in Figure 3d confirms high correlation across the whole range of conversions, with R2 = 0.99289. Figures 3a‒c show the change in thermodynamic parameters with the photoconversion calculated by NMR spectroscopy. These plots resemble closely the plots in Figure 2b‒d, indicating that the conversions obtained from TGA can be used


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similarly to those determined by NMR to make correlations with the enthalpy, entropy and melting temperature of the unreacted reactant.

Figure 3. Correlation of the thermodynamic parameters of the melting of -TCA in pure and partially irradiated samples (determined by DSC) with the conversion obtained using 1H NMR spectroscopy. (a) Specific fusion enthalpy of -TCA plotted with the conversion (mole %) determined by NMR. (b) Fusion entropy of -TCA plotted with the conversion (mole %) determined by NMR. (c) Melting temperature of -TCA plotted with the conversion (mole %)


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determined by NMR. (d) Correlation of the photoconversions determined at various irradiation times by TGA and NMR. Equation of the linear correlation: y = 0.97948x +2.49993, R2 = 0.99289. Conclusions Relative to spectroscopic methods (NMR and IR spectroscopy), thermal methods such as thermal analysis and calorimetry are not commonly used to determine the photoconversions of solid-state photochemical reactions. This is mostly due to technical inconveniences such as longer data acquisition time, but also to some more fundamental obstacles, such as the thermal instability of the reactant, the product or both. Here, we explored the possibility to use thermal methods to monitor the time-course of a well-known solid-state photochemical reaction where the product is thermally stable and does not revert to the reactant by moderate heating. It is demonstrated that thermodynamic parameters related to the melting of the unreacted reactant can be correlated equally well to the conversions obtained from thermal decomposition (TGA) and NMR spectroscopy (1H NMR). More importantly, an excellent correlation was found between the timedependent conversions and the kinetics determined by thermal analysis and NMR spectroscopy. This information could be useful towards future applications of thermal analysis to study photochemical processes as an alternative to spectroscopic methods, for example, in cases where the dissolution of the sample in organic solvent affects the ratio between the reactant and the product and solution-state NMR spectroscopy cannot be applied to monitor the reaction kinetics in the solid-state. ASSOCIATED CONTENT


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Tables and figures with characterization (1H NMR, DSC, TGA-DTA) data of pristine and irradiated of -TCA. The methods for calculation of fusion enthalpy, entropy and melting temperatures of all samples are also presented in the Supporting Information. AUTHOR INFORMATION Corresponding Author * Email: [email protected] (P. N.) Funding Sources This work was financially supported by New York University Abu Dhabi. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was partially carried out using the excellent Core Technology Platform resources at New York University Abu Dhabi. We thank Dr. Liang Li for his help and advice with the TGA and DSC measurements, and Dr. Patrick Commins and Dr. Stefan Schramm for the useful discussions and help with the NMR spectroscopy measurements.

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13. Abdelmoty, I.; Buchholz, V.; Di, L.; Guo, C.; Kowitz, K.; Enkelmann, V.; Wegner, G.; Foxman, B. M. Cryst. Growth Des. 2005, 5, 2210. 14. Avrami, M. J. Chem. Phys. 1939, 7, 1103. 15. Avrami, M. J. Chem. Phys. 1940, 8, 212. 16. Avrami, M. J. Chem. Phys. 1941, 9, 177. 17. Christian, J. W. The theory of transformations in metals and alloys, Part I; Elsevier Science Ltd: Oxford, UK, 2002; Vol. I.


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Time-Dependent Photodimerization of Î±-trans-Cinnamic Acid Studied

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