Optical Spectra of Phthalocyanines and Related Compounds

This chapter is composed of two parts. The first one describes the interaction between a molecule and light (electromagnetic waves) in the process by which we see the color of an object. Optical absorption and emission, electronic transitions, and molecular orbitals are discussed as graphically as possible within this context. “Functional dyes” are defined, in contrast to traditional dyes in which permanence of their colors is required, as compounds whose color changes according to external stimulus or whose properties unrelated to color are more important. In the second part, applications of phthalocyanines as versatile functional dyes in various industrial and medical fields are discussed. Some examples of their applications, such as optical data storage, electronic photography, photodynamic therapy of cancer, and photodynamic diagnosis of cancer, are illustrated.

In this chapter the prototypical optical absorption spectra of metal-free and metallated phthalocyanines and their theoretical background on the basis of some simple molecular orbital (MO) models are described. Comparison of these spectra with those of porphyrins and other related macrocycles with the tetrapyrrol skeleton suggests that (1) the electronic structures of phthalocyanines are similar to those of porphyrins and (2) the structure of their innermost 16-membered ring is very important for understanding their spectra. Starting from the simplest MO model (a “free electron” circulating along the periphery of an ideal cyclic polyene, C₁₆H ₁₆ ^2- ), the well-known “four-orbital model” is derived without extreme mathematicization but as graphically as possible. The four-orbital model predicts that doubly degenerate LUMOs (lowest unoccupied molecular orbital; e_g) and two nearly degenerate HOMOs (highest occupied molecular orbital; a_1u and a_2u) play a crucial role in determining the spectra of porphyrins and related macrocycles. The appearance of an intense Q band in the spectra of phthalocyanines is interpreted in terms of the disruption of the near degeneracy of the HOMOs. This is due to a higher stability of an a_2u orbital than of the a_1u counterpart owing to the presence of nitrogen atoms at meso-positions. Magnetic circular dichroism spectroscopy is briefly introduced as a powerful tool to investigate degenerate electronic structures of compounds of high symmetry such as porphyrins and phthalocyanines.

In this chapter, some factors that internally and externally affect the optical absorption spectra of phthalocyanine derivatives and related macrocyclic compounds are described and it is illustrated how they contribute to the deviation from the prototypical spectrum. The internal factors include the type and position of substituent(s) on the periphery of the macrocycle, the expansion of the π-conjugation system, the nature of the metal ion in the cavity of the macrocycle (ion size, oxidation number, and coordination geometry), and that of the axial ligand on the central metal ion. The external factors cover acid-base equilibrium, oxidation and reduction on the macrocycle, aggregation and dimerization (exciton coupling and π–π interaction), and solvent effects. In particular, much attention is focused on aggregation and acid-base equilibrium because these phenomena are frequently misunderstood.

In this chapter, optical emission phenomena involving phthalocyanine derivatives and the related macrocyclic compounds are described. In particular, we focus on the fluorescence emitted from the macrocyclic ligand with a brief discussion of other emission phenomena (e.g., phosphorescence, delayed fluorescence, electro-chemiluminescence). Unlike optical absorption, not all macrocyclic compounds luminesce. In this chapter, the factors that cause macrocyclic dyes to be luminescent or nonluninescent are described. Furthermore, we focus on the aggregation and acid-base equilibrium involving these compounds in detail because these phenomena are frequently misunderstood.