Photocalorimetry

The reactions initiated by electromagnetic radiation are said to bephotochemi-cally activated. Note that only the initiation step may require the absorption of one or more photons (a photochemical reaction). Subsequent steps of the mechanism may be dark reactions, proceeding by thermal activation. 

The thermochemical study of photochemical or photochemically activated processes is not amenable to most of the calorimeters described in this book, simply because they do not include a suitable radiation source or the necessary auxiliary equipment to monitor the electromagnetic energy absorbed by the reaction mixture. However, it is not hard to conceive how a calorimeter from any of the classes mentioned in chapter 6 (adiabatic, isoperibol, or heat flow) could be modified to accommodate the necessary hardware and be transformed into a photocalorimeter. 

It is possible that only a fraction of the radiant energy supplied to the calorimeter would be absorbed by the reaction mixture. Part of that radiation can be reflected (Er) and, if the reaction vessel is transparent, another fraction can be transmitted to the surroundings (Et). Furthermore, the electronically excited states of the reactants may decay by luminescence, so more energy (E ) may be lost to the surroundings. If these three contributions are taken into account, equation 10.1 becomes 

To obtain A0bSH we need to evaluate E and the last three terms in equation 10.2. This can be done in a separate experiment (where the same radiant energy E is supplied), by measuring Q for a process whose A0bSH is accurately known, or, more commonly, by measuring Q when a nonphotoreactive substance is contained in the reaction cell. In this case A0bsH = 0 and we have 

For a well-designed set-up, and if the main and reference experiments are performed under the same conditions, it is fair to assume that the reflected energies will be small and that E[ Er. With regard to the transmittance and the luminescence energies, we have to consider two possibilities. If the calorimetric cell is opaque, then these terms will all be zero, that is, 

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Differential photocalorimetry (DPC) is included here since the instrument used is essentially an adaptation of DSC instrumentation. The photocalorimeter comprises a DSC instrument with a UV/visible source mounted on top, such that light of appropriate wavelength or wavelength region from the source is focused onto the measuring head (both reference and sample pans). The most frequent use of DPC is in the study of polymer cure reactions, but it may also be used to follow such as UV degradation. 

For NMR spectroscopic experiments, a thin film of pTrMPTrA was prepared by reacting a quantity of monomer and photoinitiator confined between glass plates with 1 mm separation. The polymerization conditions were the same as those for the photocalorimetry experiments. After 1 hour of UV exposure, the film was removed from the plates and ground to a fine powder using a mortar and pestle. A solid-state 13C NMR spectrum of the powder was obtained immediately, as described below. The remaining polymer powder was divided into two portions, one of which was stored under atmospheric conditions. The other portion was stored under N2. After one week, 13c spectra were again obtained for each of these polymer samples. Both samples were then heated to 280 °C in a vacuum oven and analyzed once more by 13C NMR spectroscopy. 

Polymerization Behavior. Both Fourier-transform infrared spectroscopy (FTIR) and differential scanning photocalorimetry (DPC) were used to characterize the polymerization behavior, curing time, and maximum double bond conversion in these systems. 

Some of these problems can be overcome with a different calorimetric design (see later discussion). Other problems, which are more dependent on the chemistry and physics of the process under study than on the instrumentation, require careful attention. Unnoticed side reactions or secondary photolysis are examples, but one of the most serious error sources in photocalorimetry is caused by the quantum yield values, particularly, as explained, when they are small. Unfortunately, many literature quantum yields are unreliable, and it is a good practice to determine n for each photocalorimetric run. Errors in

inner filter effects, that is, photon absorption by reaction products. 

What can we conclude from all these data Although the two photocalorimetric values are in excellent agreement with each other, these values are problably less accurate than the reaction-solution calorimetric result. In any case, the 4 kJ mol-1 discrepancy is not a cause of concern regarding the general usefulness and reliability of carefully made photocalorimetry experiments. 

C. Teixeira. Photocalorimetry. Methods and Applications. In Energetics of Stable Molecules and Reactive Intermediates-, M. E. Minas da Piedade, Ed. NATO ASI Series C, Kluwer Dordrecht, 1999 105-136. 

A. W. Adamson, A. Vogler, H. Kunkely, R. Wachter. Photocalorimetry. Enthalpies of Photolysis of trans-Azobenzene, Ferrioxalate and Cobaltioxalate Ions, Chromium Hexacarbonyl, and Dirhenium Decacarbonyl. J. Am. Chem. Soc. 1978, 100, 1298-1300. 

Y. Harel, A. W. Adamson. Photocalorimetry. 2. Enthalpies of Ligand Substitution Reactions of Some Group 6 Metal Carbonyl Complexes in Solution. J. Phys. Chem. 1982, 86, 2905-2909. 

Dohrmann JK, Schaaf NS (1992) Energy conversion by photoelectrolysis of water determination of efficiency by in situ photocalorimetry. J Phys Chem 96 4558-4563... 

Phosphorus peroxides, 1039-46 symmetrical, 1041-2 unsymmetrical, 1042-4 Photocalorimetry, ozonides, 165 Photochemistry... 

All the acylated polyimides retained solubility in organic solvents. This gave us reason to believe that photopolymerisation via the double bonds of polyimides could be conducted by the method of differential scanning photocalorimetry. This method, which is widely covered in the literature [73-76], is based on on the principle that heat released during any reaction can be measured. 

Photopolymerization in thin films was carried out at 514 nm, the rate of heat evolution being measured by thin-foil photocalorimetry. The monomer formulation consisted of 85% TMPTA and 15% HDDA, giving a concen-... 

Photopolymerization of the system we have studied appears to proceed by the common mechanism in which termination occurs by reaction between two macroradicals. Analysis of the photocalorimetry traces at different light intensities for our initiator-monomer system shows no evidence for a... 

Photocalorimetry is a technique for determining the ordinary enthalpy (AH) of a reaction but, unlike conventional calorimetry, the reaction is light induced,191 Essentially, the procedure involves measuring the rates of heat production in two irradiated solutions, one containing an absorbing but unreactive substance and the other containing the photosensitive compound. The difference between these rates, per mole of reaction, gives the AH for the photochemical process. 

Photocalorimetry offers a convenient alternative to other methods of AH determination and, in some instances, may be the only practical method. The ligand substitution reactions of robust Werner-type complexes are a case in point. Conventional thermochemical measurements are complicated by the slowness of the substitution process and/or by competing reactions. Some of these same complexes, however, undergo clean photosubstitutions with high quantum yields and thus are excellent candidates for photocalorimetry. Examples include [Cr(NH3)6]3+, [Cr(CN)6]3-and [Co(CN)6]3-.192 Photocalorimetric measurements of AH have also been obtained for isomerization and redox reactions of coordination compounds.193194... 

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