4,4′-Dimethylazobenzene as a chemical actinometer

Chemical actinometers are a useful tool in photochemistry, which allows to measure the photon flux of a light source to carry out quantitative analysis on photoreactions. The most commonly employed actinometers so far show minor drawbacks, such as difficult data treatment, parasite reactions, low stability or impossible reset. We propose herewith the use of 4,4′-dimethylazobenzene as a chemical actinometer. This compound undergoes a clean and efficient E/Z isomerization, approaching total conversion upon irradiation at 365 nm. Thanks to its properties, it can be used to determine the photon flux in the UV–visible region, with simple experimental methods and data treatment, and with the possibility to be reused after photochemical or thermal reset.


Introduction
The measurement of incident photon fluxes represents a crucial point for the analysis of photoreactions. For this purpose, chemical actinometers [1][2][3][4][5], i.e., chemical species that undergo a photoreaction with known quantum yield, are currently widely used both for UV and visible light. Although they usually provide reliable results by simple measurement and data treatment procedures, chemical actinometers must satisfy several requirements: the system must be simple and well characterized, thermally stable and easy to synthesize or, preferably, commercially available; the photoproduct should be inert; the quantum yields values must be known with high precision, reproducible and ideally independent on the irradiation wavelength, the temperature, the concentration and/or the presence of oxygen; the system should have a high absorption coefficient at the irradiation wavelength; finally, the analytical procedure must be easy and rapid, ideally a simple absorption measurement [1].
Herein we report the complete photochemical characterization of an azobenzene derivative, the 4,4′-dimethylazobenzene (DMAB, Fig. 1). This compound can be easily synthesized, and it displays a clean E-Z isomerization, with a slow thermal back process and an excellent fatigue resistance. Interestingly, thanks to its optical properties, the photoconversion approaches the unity under irradiation at 365 nm. Therefore, we suggest the employment of such compound as a chemical actinometer, its properties allowing an easy manipulation and reliable results.
2 Results and discussion DMAB ( Fig. 1) is commercially available. However, it can be synthesized in good yields, according to well-known procedures [45,46]. In the present study, we synthesized the compounds according to the procedure reported in the SI (Section 1).
Since the absorption spectra of the two isomers are overlapped over the whole UV-vis spectral range, the pure Z isomer cannot be obtained, albeit irradiation at 365 nm affords an almost complete photoconversion (vide infra). Photostationary states (see Table 1, figure S2), are more shifted toward the Z isomer upon irradiation on the π-π* band, up to a maximum of 0.97 at 365 nm. Conversely, irradiation on the n-π* or S 3 bands leads to PSSs less rich in Z isomer, with photoconversion extents up to a maximum of 0.24 at 405 nm [48,49,54,57,59]. Interestingly, the PSS obtained with 254 nm light has a content of Z isomer as low as 11%; therefore irradiation at this wavelength could be considered as a way to speed up the recovery of the E form. However, the fatigue resistance is likely to be lower under irradiation with more energetic light (such as 254 nm); therefore we encourage to recover the E form thermally.
To test the fatigue resistance of DMAB, we performed alternated irradiation at 365 nm and 436 nm. The compounds switched efficiently between the two photostationary states, without any evidence of degradation after more than 100 cycles (figure S3).
When left in the dark, previously irradiated solutions showed a thermally accessible back isomerization process, and the spectrum of the E isomer was recovered in several days (figure S4, SI). The rate constant (k Δ ) is 1.6 × 10 −6 s −1 , which suggests that the thermal back isomerization is substantially negligible in the typical experimental timescales (from few minutes to one day).
Despite the pure Z isomer could not be produced photochemically [43,47], we extracted its absorption spectrum by two spectroscopic methods, which gave comparable results. The first method, reported by Thulstrup, Eggers and Michl (hence the acronym TEM) [60,61], consists in subtracting from the second derivative of the spectrum of the E isomer, the second derivative of the absorption spectrum of a PSS multiplied by an arbitrary value of α. Since the π-π* band of E-DMAB has a vibrational structure-albeit lowand Z-DMAB has none, the right value of α can be found once the subtraction leads to the complete disappearance of the vibrational structure. In our case, we applied the TEM method to the PSS obtained upon irradiation at 365 nm, obtaining α = 97%.
The second method to obtain the photoconversion and the absorption coefficient of the Z isomer, reported by Fischer [62], was applied using the two photostationary states produced under irradiation at 334 nm and 365 nm. The spectrum obtained through this protocol, reported in Fig. 2,  Table 1 Average values of absorption coefficients (ε), photoisomerization quantum yields (Φ EZ and Φ ZE ), and photoconversion extents (α) at the photostationary states of DMAB for the main lines of a medium-pressure mercury lamp a Average of the absorption coefficients values at the two wavelengths, weighted by the contribution of each mercury line (38% at 297 nm, 62% at 302 nm) has been used to calculate the remainder photochemical parameters.

Photokinetic considerations and photoisomerization quantum yields determination
The photoreaction of a T-type photochrome, whose two forms are both photoactive, is described by the following photokinetic equation [2,63,64] In this equation Φ EZ and Φ ZE are the quantum yields of E-Z and Z-E isomerization, respectively, q 0 is the incident photon flux, i are the absorption coefficients of the E-and Z isomers at the irradiation wavelength λ i , f is the fraction of light absorbed by the sample, calculated from the absorbance of the mixture at the irradiation wavelength λ i , according to equation which, in Eq. (1), is multiplied by the contribution of each isomer to the absorbed light, and k Δ ⋅[Z] is the contribution of the thermal process (the optical path of the cuvette is considered 1 cm and henceforth it will be omitted for clarity). It is worth to remark that both the concentrations and the absorbances are time-dependent and prevent the analytical solution of the differential equation.
We measured the photoisomerization quantum yields of DMAB through an apparatus that allowed us to simultaneously irradiate the solution and record the absorption spectra over time. The solution was carefully stirred throughout the whole irradiation, to avoid measurements on non-homogeneous points of the solution. The incident photon flux q 0 was measured with a photodiode. We then fitted the timedependent absorption evolution (see Fig. 3) according to the differential equation (1).
The photoisomerization quantum yields, measured for the main lines of a medium-pressure mercury lamp, are reported in Table 1.
It is worth to remark that, in the fitting process, the most reliable result is the one of the forward reaction, whereas the backward is affected by a larger error. Therefore, in the case of 254 nm, 405 nm and 436 nm, whose photostationary states are shifted to the E form, we minimized the error ( on the Φ ZE values (to be used for actinometry, vide infra), conducting the photokinetic experiments on solutions previously irradiated at 365 nm or 334 nm.
The resulting quantum yield values are in line with similar compounds previously reported in literature [48,49,57]. Upon irradiation on the π-π* band, the E-to-Z quantum yield is around 0.18 and the Z-to-E quantum yield is around 0.40. Both these values are slightly larger upon irradiation on the n-π* band.

Data treatment and recommendations
The application of DMAB as an actinometer requires a faster and easier procedure than a photokinetic fitting. Since the Z-E thermal process is slow enough to be neglected in the experimental timescales, as long as the concentration of the photoproduct is negligible and it does not contribute significantly to the absorbance, the sole process occurring is the E--Z photoisomerization and the Eq. (1) reduces to [2,63] As a general rule, we assume that the absorbance of the photoproduct is negligible as long as it is at least 20 times lower than the absorbance of the reactant, i.e., the following condition is satisfied [64] For short irradiation time intervals and for small variations of absorbance, it can be assumed that f is equal to the average value where f 0 and f 1 are the fractions of light absorbed before and after the irradiation. In these conditions, we can use the Eq. (3) in its discrete form (6) where Δt irr is the time interval of short irradiation. The concentration can be related to the absorbance at a chosen wavelength λ x through the difference of the absorption coefficients at that wavelength, thus The combination of equations (6) and (7), rearranged with respect to time 0, eventually provides If the absorbance at the irradiation wavelength remains above 3 throughout the whole irradiation time (total absorption regime), the fraction of absorbed light equals 1 and the equation simplifies in [2,63,64] In both equations (8) and (9) the absorbance variation in function of the time of irradiation is a straight line, having the absorbance value before irradiation-i.e., at time 0-as intercept and containing the photon flux in the slope. Therefore, q 0 can be calculated from the slope m obtained by a linear fitting of the absorbance over (f·t) or t, respectively.
We propose the following procedures in two absorbance regimes, successfully tested in our laboratory with the ferrioxalate actinometer (see section 4 of the SI for details).
We recommend to work in total absorption regime when possible, as the data treatment is the easiest. Using a solution with concentration above 5 × 10 -4 M is suggested for the range 302-365 nm (procedure A). However, keeping a total absorption regime at the irradiation wavelengths 254 nm, 405 nm, 436 nm throughout the whole measurement would mean that only a spare region of the spectrum is at suitable absorbance values to obtain a reliable measurement. Therefore, in these cases, we recommend to use more diluted solutions and calculate the average fraction of absorbed light for each irradiation step (procedure B). In any case, diluted solutions can be used for all the irradiation wavelengths, provided that average f is taken into account.
To ensure that the concentration of the photoproduct is negligible and the absorbance does not fall below 3 (when working in total absorption regime), we recommend also to keep the photoconversion extent below the 10%.
The E form can be recovered thermally, by leaving the compound at room temperature for at least one month before reuse. To speed up the procedure, we suggest to illuminate it in the visible, where the photostationary states reach an E:Z ratio of around 20:80, or at 254 nm, where this ratio is 11:89, and leave the reaction complete thermally. Irradiate the sample at 365 nm to reach the photostationary state and record the absorption spectrum Calculate q 0 from the obtained slope m For a further simplification of the data treatment, the constant values V, Φ i and Δε at 333 nm (ε E = 27,421 M −1 cm −1 , ε Z = 933 M −1 cm −1 ) can be gathered in a parameter Y, listed in Table 3, thus:

Conclusions
We reported the photochemical characterization of 4,4'-dimethylazobenzene (DMAB), which we suggest to use as a chemical actinometer on account of the advantages listed below: • Availability: DMAB is commercially available; nevertheless, it can be synthesized easily and on large scales, with excellent reaction yields. Other actinom- eters commonly used nowadays are not commercial and require synthetic efforts or purification procedures: Aberchrome is no longer commercially available and must be synthesized; potassium ferrioxalate and Reinecke's salt must be recrystallized to obtain a suitable purity [25,27]; azobenzene is commercially available, but known to be carcinogenic and its use should be discouraged. • Experimental procedure: E-DMAB isomer does not undergo thermal reactions or degradations, thus a blank is not required (as for Reinecke's salt [26] and ferrioxalate [7]). The absorbance recording does not need any dilution (as for Reinecke's salt) [25,27] or addition of reactants (as o-phenanthroline for ferrioxalate or Fe(NO 3 ) 3 for Reinecke's salt). Therefore, the manual errors arising from the procedure are largely minimized using DMAB. • Stability: E-DMAB is stable in solutions kept in the dark; other commonly used actinometers, such as ferrioxalate and Reinecke's salt [7,26], can give thermally the same reaction used for actinometry. Aberchrome solutions cannot be reused after bleaching, since a parasite Z isomer of the open form is accumulated and interferes with the photoreaction [18,19]. Moreover, in case of DMAB, a minor increase in concentration due to the volatility of MeCN can be disregarded, as the data treatment does not require a known concentration value. • Reset: The same solution can be used more than once, by resetting the Z form to the E, either photochemically (at 254 nm or in the visible) or thermally, by leaving the compound at room temperature for 1 month. The excellent fatigue resistance allows to perform measurements on the same solutions up to at least 50 times. • Data treatment: The thermal isomerization is negligible, and, at low photoconversion extent, the absorption contribution of the Z isomer can be neglected too. Consequently, the quantum yield equation can be reconducted to a linear relationship. • Total absorption regime: The absorption coefficient at 365 nm is larger with respect to azobenzene [55], because of the redshift of the π-π* band. If the absorbance at this wavelength is larger than 3, there is still a portion of spectrum below 1 that can be used for the data treatment. This allows to use the compound in total absorption regime up to 365 nm. • Visible region: The PSSs in the visible (405 nm, 436 nm) are less rich in Z isomer, leading to a lower signal if the irradiation is performed starting from the pure E isomer. However, since the PSS at 365 nm is close to the pure Z isomer, the actinometry in the visible can be carried out starting from this PSS, with a  [24][25] or a monochromator SPEX 1681. The lamp power was measured by means of an Ophir PD300-UV photodiode. NIR contribution was measured and subtracted from the total value. Photoisomerization quantum yields and fatigue resistances were determined using a home-made setup, which collects absorption spectra at high rates, under continuous irradiation. A Xenon lamp (75 W) was employed as a probing light and a Hg/ Xe lamp, placed at 90° with respect to the incident beam, was used to photoisomerize the sample, placed in a cuvette and thoroughly stirred. Spectra were recorded every 0.1 seconds, with a spectrograph equipped with a CCD camera (Roper Scientific and Princeton Instruments, respectively). Photokinetic profiles were then fitted using a numerical iterative fitting method implemented in an Igor procedure (Wavemetrics).
Thermal isomerization kinetics measurements were performed on solutions previously irradiated at 365 nm, by monitoring the absorbance variations over time in the dark and at room temperature (298 K). For comparative measurements, potassium ferrioxalate was employed as an actinometer in its "microversion" [2,8].
Acknowledgements This work has been founded by the Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR PRIN 201732PY3X).
Author contributions SS and LC conceived the project, LA and JG synthesized the compound, all the authors conducted the experiments, analyzed the results and reviewed the manuscript.

Conflict of interest The authors declare no competing interests.
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