Triplet state photoacoustic calorimetry

Rios, A. de O., A. Z. Mercadante, and C. D. Borsarelli. 2007. Triplet state energy of the carotenoid bixin determined by photoacoustic calorimetry. Dyes Pigments 74 561-565. 

T = 184kJmol 1 for trans- and cis-stilbene has been obtained by photoacoustic calorimetry [443]. Later, Caldwell and co-workers [444] reported ET = 195 and 178kj mol 1 for trans- and cis-stilbene, respectively. The latter value refers to the relaxed triplet relative to the c state and therefore corresponds to 19 kJ mol 1 difference in enthalpy between the two isomers [444],... 

As noted above, the reactive ES may have a different partial molar volume than the analogous GS. Other than the photoacoustic calorimetry experiments described earlier, there is relatively little direct quantitative information on ES volumes. The magnitude of the AV between the GS and the ES will depend on the excited state orbital parentage. For example, there is little distortion upon forming the doublet LF state of an octahedral Cr(III) complex, so both intrinsic and solvation contributions to AV should be quite small. In contrast, the quartet LF state of Cr(III) and the triplet LF excited states of Rh(III) complexes, both formed by metal centered g( 7 ML) t2g( ML) excitation, would be expected to have substantially positive AVim contributions, but again relatively small differences between ground and excited... 

The ratio of the emission quantum yield and lifetime of the triplet state emission (phosphorescence) yields the product of the intersystem crossing efficiency and radiative decay rate constant. Determination of intersystem crossing efficiencies is generally not straightforward and often techniques other than emission spectroscopy, such as time-resolved photoacoustic calorimetry, are used. 

The photoacoustic calorimetry technique together with the triplet-triplet energy transfer method (see below) has been used to characterise the non-emissive triplet excited state of indigo, that is, to evaluate the intersystem crossing quantum yield and triplet energy values for this compound [20]. 

Empirically, the singlet-state energy is best determined from the intersection between normalised absorption and fluorescence spectra. The same principle could be applied to the triplet energy, but this is not usually feasible because the singlet-to-triplet absorption is very difficult to observe. Alternatively, the triplet energy is estimated from the origin of the phosphorescence spectmm. Non-spectroscopic techniques, such as photoacoustic calorimetry, often provide a better estimate of the triplet energies. 

Brennan, C.M. and Caldwell, R.A., Photoacoustic calorimetry of cycloheptene derivatives. Relaxed triplet energies of 1-phenylcycloheptene and 1,3-cycloheptadiene and the energetics of ground-state cis-trans isomerization, Photochem. Photobiol., 53, 165-168,1991. 

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