Combination of absorbance and fluorescence measurements

Since absorbances can only be measured in a certain dynamic range, the dilute solutions used in fluorescence measurements cannot be taken. Therefore the assumption is necessary that the photochemical reaction is governed by the same mechanism for 10 and 10 M solutions. Furthermore the higher concentration forbids an approximation according to eq. (5.111). However, in Section 3.3.11 a principle was introduced substituting the normal time scale by a transformation into a pseudo time (f) In consequence the three equations of eq. (5.147) have to be substituted by 

Taking this time transformation the measured absorbances depend on the time accordingly  

This latter equation is a matrix equation like eq. (4.6). Multi-linear regression allows the determination of the molar absorbance coefficients for any wavelengths which have been measured. The calculated absorption coefficients can be used to allow the determination of the partial quantum yields, (pi and pI by inserting the absorption coefficients into the parameters p, W, and R respectively [177,178,185]. 

The equations derived in Section 4.5.1 were used to examine the photoreaction of stilbene-1 (4,4 -diphenylstilbene, S-1, see Section 5.5.4.2) [178] in more detail. The thermodynamically stable trans-S-l undergoes photoisomerisation and ring closure upon irradiation. Eq. (5.159) was used to calculate (aVS-l) by multi-linear regression [178]. For control, in addition, this absorption coefficient can be calculated by use of the photostationary state using the fluorescence measurement  

The parameters of the reaction cause an error of approximately 3% in b s) by this assumption. At the quasi-stationary state the measured absorbance Ejf (s) allows the calculation of according to 

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