Semiempirical MO and voltammetric estimation of ionization potentials of organic pigments. Comparison to gas phase ultraviolet photoelectron spectroscopy

A number of organic pigments were identified by semiempirical molecular orbital calculations, using the PM3 method, as having ionization potential (IP) values of 7.0¯9.5 eV. Based on photostability, solubility and commercial availability twelve (quinacridone, isoviolanthrone, indanthrone, indigo, 3,4,9,10 perylenetetracarboxylic dianhydride, bis(p-chlorophenyl)1,4diketopyrrolo (3,4-C) pyrrole, pyranthrone, indanthrene yellow GCN, 16,17-dimethoxyviolanthrone, indanthrene gold orange, 4,4ʹ-diamino-9,9ʹ,10,10ʹ-tetrone [1,1ʹ bianthracene], and N,Nʹ ditridecyl-3,4,9,10-perlenetetracarboxylic diimide) were chosen for further study. The accuracy of the MO calculations was confirmed by experimental measurement of the ionization potentials for eight of the pigments, using gas phase ultraviolet photoelectron spectroscopy. For compounds having at least three fused rings and containing oxygen, nitrogen, or both, the theoretical and experimental IPs have a linear relationship defined by the equation IPexp=0.694IPCalc+1.9049. Lewis acid pigment solubilization (LAPS) was shown to be a viable approach to preparing electrodes for cyclic voltammetry of pigment solid films. The results of the cyclic voltammetry experiments were utilized to formulate the equation Eox (V vs. NHE)=0.5488 IPCalc-3.042, which relates the experimental oxidation potential to the theoretical IP.