Bulk optical properties of phthalocyanine pigment particles

Abstract

The optical absorption spectra of small particles of α-, β-, and x-form of metal-free phthalocyanine (H2Pc) and vanadyl phthalocyanine (VOPc) surrounded by a dielectric (polymers) have been studied. The true bulk absorption can be obtained by measuring the difference in transmittance of two films of identical composition but different thicknesses. This approach eliminates effectively light reflection losses. Comparison between the absorption spectra measured by direct vs. difference in transmittance revealed a number of differences: shift in the absorption frequencies, change in absorption intensities, and change in peak-to-valley ratio. These differences are due to light scattering which can be significantly minimized by using the differential transmittance method. The true peak absorption of x-H2Pc particles located at 770 and 615 nm had an absorption coefficient of 35.3 and 34.2 μm−1, respectively. The absorption minimum occurs at 477 nm. The peak-to-valley ratio was 27.The absorption spectra of VOPc particles did not correspond to any one polymorph, but are due to a mixture of two known polymorphs. Some discrepancies between the particle absorption of α-H2Pc or β-H2Pc and the corresponding evaporated film absorption were observed. These discrepancies are attributed to the spread in particle size and also to local agglomeration of the particles.