Fig. 3. Spectral behavior of the SKLB1002 coefficient as a function of photon energy, (a) H2TPP and (b) ZnTPP thin films thin films in as-deposited, annealed at 437 K and γ-irradiated with a dose of 10 kGy conditions.Figure optionsDownload full-size imageDownload as PowerPoint slide
Annealing of H2TPP and ZnTPP films at 473 K for 2 h increases the value of the absorption coefficient all over the spectra and causes a significant broadening of the Soret band. A slight red shift of Q-bands is observed which indicates increased π-conjugation due to increased planarity of H2TPP and ZnTPP molecules . Kasha’s exciton model  interprets the red shift as a case of in-line dipoles transition that occur for the long geometrical axes of the molecule parallel to polarization axes, but with transition dipoles polarized along the short axis of a unit molecule. The magnitude of the red shift is consistent with the decrease in the band gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Table 1 illustrates the absorption peaks maxima and the shift in the bands resulting from the annealing temperature and γ-ray irradiation.