Knowledge Bank

The optical spectra of free, base porphin doped into anthracene have been recorded at low temperature and consisted of sharp lines (quasilines) and broad continuua. The spectra of one of the two principal porphin site species wert recorded separately by using selective excitation of fluorescence. There are two electronic transitions, usually denoted $Qx$ and $Qy$, in this spectral region. The relative strengths of the transitions from the ground state to these two states was of particular interest in this work. Previous spectral observations of free base porphin in solution indicate that the $Qy$ transition is stronger than that of $Qx$. However, these spectra consisted of only broad components. In the present study, the broad component in the region of $Qx$ was stronger than that in the region of $Qx$ in agreement with previous work. Conversely, in Che quasiline spectra, $Qx$ was stronger than the feature identified as $Qy$, Simple arguments based upon vibronic borrowing in the cyclic polyesne model of porphin indicate that the quasiline spectra are appropriate for assessing the relative transition strengths for free porphin molecules, while the strengths of the broad components are due to the interactions of the lattice or solvent. Thus, in the context of this model, $Qx$ is stronger than $Qy$ for free porphin molecules, which is the case addressed by most quantum theoretical treatments.