Knowledge Bank

The visible and near IR electronic absorption spectra of the VOPc and $H2Pc$ solids at room temperature are very broad and show little structure. The electroabsorption spectra, however, are structure-rich and, when properly analyzed, lead to understanding of important physical properties, such as change in the dipole moment and polarizability upon optical excitation. We have recorded the electroabsorption spectra of phases I and II of VOPc and of the X phase of $H2Pc$ under external fields of $-105volts/cm$. Samples were prepared in the form of evaporated thin films and particulates embedded in thin polymer films. Computer fitting of the observed spectra to the theoretical equation reveals that the dipole-moment-increases accompanying some electronic transitions in the VOPc solid are very much greater than that accompanying the Q-band transitions in a structurally very similar molecule, $Fe(III)Cl-tetraphenylporphyrin1$. These excessive dipole-moment, increases in solid may be caused by the mixing in of a state with charge transfer character. The broad electronic absorption spectrum of the VOPc solid was deconvoluted into bands due to both phase I and II. It was also found that at room temperature I-II phase equilibration (in favour of II) continued for several weeks after film evaporation at $77\circ K$.