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3.2 Temperature effects on trans-2-NCA photoreactivity

The effect of temperature on trans-2-NCA photoreactivity was studied, as described above, storing at 38-40 °C a test and a control sample wrapped in aluminium;15 the behaviour of the test sample was identical to that observed for irradiation at room temperature (20-25 °C), while the control sample did not show any change in absorbance (Figure 5 and Table 3).

No decomposition products due to temperature were detected by HPLC analysis, both in the test and in the control sample, confirming that trans-2-NCA photoreactivity was not affected by temperature; this allows trans-2-NCA to be used as actinometer also in commercial light cabinets, provided with Xe-arc lamps without mirrors, where high temperatures (~ 40 °C) can be reached.11

Figure 5. Absorbance at 440 nm of NCA 0.5% solutions exposed to Xe-arc lamp at room temperature (C) and at 38-40°C (A), close to a control sample wrapped in aluminium foil (B).

Figure 5. Absorbance at 440 nm of NCA 0.5% solutions exposed to Xe-arc lamp at room temperature (C) and at 38-40°C (A), close to a control sample wrapped in aluminium foil (B).

Table 3. Regression lines of Abs vs UV-A dose (J/cm2) from Xe-arc lamp, at different temperatures.

r Slope x 10"2 sd x 10'2 Y-int x 10"2 sd x 10'2

3.3 Long-term stability

In order to study the long-term stability of the proposed solution, trans-2-NCA solution lots, stored in the dark at room temperature as well as refrigerated, were subjected to UV/Vis spectrophotometric and HPLC analyses. No absorbance changes or degradation products were detected in solutions stored up to 1 month, though an opalescence was observed, mainly in the refrigerated solutions, due to a decrease of solubility at low temperature. It was therefore suggested that the same lot of trans-2-NCA solution could be used as actinometer at least for two-three weeks, if stored in the dark and refrigerated (to limit the solvent evaporation); a possible precipitate is redissolved at room temperature.

3.4 Photodegradation mechanism

The photodegradation mechanism of trans-2-NCA, responsible for the absorbance increase in the visible region after UV exposure, was studied by HPLC and GC.

The analyses of photoexposed solutions showed the formation of a new chromatographic peak, at a retention time lower than that of NCA (peak 2 in Figure 6), whose area increase was found linearly correlated to the UV-A dose, up to ~ 30 J/cm2. This photoproduct was identified as the NCA as-isomer by GC-MS and !H NMR data; its mass spectrum was analogous to that of trans-2-NCA, while NMR spectrum showed different olefinic hydrogen coupling constants, according to the general rule of cis-trans isomerism.

Figure 6. HPLC chromatograms of trans-2-NCA solutions exposed to different UV-A dose levels: A) 0.00 J/cm2, B) 8.50 J/cm2, C) 25.5 J/cm2. Stationary phase: reverse phase C18, 5pm (150 x 4.6 I.D.); mobile phase: ammonium acetate buffer (pH 7.0, 0.01M)-CHsCN 70:30 (v/v); flow rate: 0.8 mUmin; UV detection at 240 nm.

Figure 6. HPLC chromatograms of trans-2-NCA solutions exposed to different UV-A dose levels: A) 0.00 J/cm2, B) 8.50 J/cm2, C) 25.5 J/cm2. Stationary phase: reverse phase C18, 5pm (150 x 4.6 I.D.); mobile phase: ammonium acetate buffer (pH 7.0, 0.01M)-CHsCN 70:30 (v/v); flow rate: 0.8 mUmin; UV detection at 240 nm.

The quantum yield ((p) of this photoisomerisation was calculated according to the following equation:

_ n° of moles of photoproduct (per unit time) n° of moles of absorbed photons (per unit time)

and was found to be 0.15.

The number of moles of photons absorbed by the NCA solution was determined by the "micro-version" of potassium ferrioxalate actinometry (Fischer, 1984), at the excitation wavelength of 313 and 365 nm.

Two aditional photoproducts, at longer retention times (3 and 4 in Figure 7), were detected in significant concentration in solutions exposed to high UV-A doses (>30 J/cm2).

Their GC-mass spectra were the same and suggested the compounds to be the cis and trans forms of 2-nitrosocinnamic acid methyl ester. The molecular ion peak (m/z 191) of 2-nitrosocinnamic acid methyl ester and a peak at m/z 161, due to the loss of nitroso radical (NO), were observed in the mass spectrum. The characteristic methylcinnamate fragmentation pattern was also identified by two important peaks (m/z 130 and m/z 102), associated with the expulsion of the methoxyl radical (-OCH3) and of the ester group (-CO2CH3), respectively.16

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