Optical absorption of supramolecular aggregates of vanadyl etioporphyrin-III in solutions and thin films

Relative abundance of vanadyl etio-porphyrin-III, VO-EtioP-III, a typical geoporphyrin, reaches its maximum in solid fossil fuels (coals). This determines the interest in studying supramolecular structure and solid-state properties of this compound. Unlike planar etio-type porphyrins, the visible absorption spectra of VO-EtioP-III undergo significant transformation in transition from a solution to solid films, which is associated with formation of supramolecular aggregates. In this work, we compare the spectra of VO-EtioP-III films deposited by thermal evaporation of powder in vacuum (VTE) or by spin-coating of dissolved compound (SC) to prove that they are similar to those observed for binary solutions in which adding water to methanol also initiates aggregation. The absorption spectra of VTE-films are very sensitive to the film thickness and deposition conditions. The spectral studies are supplemented with analysis of the microscopic morphology and conducting properties of the films. While the photoconductivity of solution-processed VO-EtioP-III films suffers due to the inhomogeneity of their structure, the VTE-films behave as good photoconductors with a lateral current response to sunlight up to 10⁵.