Near edge X-ray absorption fine structure (NEXAFS) at the C and N K edges and UV photoelectron spectroscopy (UPS) at variable excitation energy (21.2 and 120 eV) have been used to characterize the electronic structure and the molecular orientation of chloroaluminum phthalocyanine (ClAlPc) thin films deposited on glass or SnO2 substrates and subjected to chemical transformations. Transformation H occurs when the films are soaked in an aqueous acidic solution, while transformation I is observed if I3-/I- is added to the solution. It is demonstrated that the protonation of the aza nitrogen atoms of ClAlPc happens during both transformations. It modifies the valence band of the compound but has no effect on the unoccupied levels of the material. Besides the protonation, the uptake of I3- in the phthalocyanine film during transformation I leads to the formation of a charge-transfer complex between the phthalocyanine and the iodine species with a modification of both the occupied and unoccupied levels of the material. The band gap is reduced by 0.3 eV and 1.5 eV for transformations H and I, respectively. No orientation-dependent effect is observed when the chloroaluminum phthalocyanine is deposited on SnO2. On the contrary, strong polarization effects are observed in the NEXAFS spectra of phthalocyanine deposited on a float glass substrate. In the as-deposited film, the molecules are lying almost flat on the substrate surface. They are switching to a perpendicular to the substrate orientation after transformation H, while they are becoming randomly oriented after transformation 1. Despite the amorphous structure found after transformation I, the photoelectrochemical activity of the film is three times as large as the photoactivity of the untreated ClAlPc. This increase of photoactivity is related to the formation of a charge-transfer complex and is explained in terms of a better interaction between the film and the redox (I3-/I-) in solution. Bromoaluminum phthalocyanine (BrAlPc) films were also probed by NEXAFS. All modifications observed for ClAlPc were also obtained for BrAlPc.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry