♦ Development of azaporphyrin complexes with main-group elements

Azaporphyrinoids, such as phthalocyanines (Pcs) and tetraazaporphyrins (TAPs) are some of the most well-known and successful artificial dyes and pigments in modern material chemistry. A number of strategies for modifying the π conjugation system of azaporphyrinoids towards fine-tuned optical properties have been proposed to date. In this article, we have proposed a novel "π electron-unmodified" approach by utilizing the unique properties of the "main-group elements", together with their simple synthetic procedures.
Molecular design: Modifications of π conjugation system (i.e. ring expansion, oligomerization, etc) have several dis advantages, such as low synthetic yields, low stabilities under air, and low diversity. Main-group (groups 15 and 16) elements have notable features so that we expected that their introduction into simple azaporphyrin macrocycles could change the absorption spectrum significantly without modifying π conjugation systems.
Phthalocyanines absorbing near-IR light beyond 1000 nm: Based on the frontier MOs of unsubstituted Pc, group 16 (S, Se, and Te) and group 15 (P, As, and Sb) elements were used as peripheral and central (core) substituents, respectively. The resulting Pcs showed a single intense band (Q band) peak beyond 1000 nm. The substantial red-shift appears to originate from pure substitution effects, including central and peripheral groups 15 and 16 elements, and positions of the latter. Their stability under aerobic conditions is comparative to that of CuPc, which is widely used in consumer products. (J. Am. Chem. Soc. 2014, 136, 765; J. Am. Chem. Soc. 2011, 133, 19642.)
Tetraazaporphyrins absorbing light across the entire UV—vis regions: The optical properties of TAP group 15 complexes are markedly different from those of Pc group 15 complexes, having charge-transfer (CT) band between the Soret and Q bands. Moreover, substitution at the peripheral groups of TAP can tune both the position and intensity of the CT band in a rational manner. (Chem. Sci. 2014, 5, 2466.) With respect to another strategy for tuning optical properties, TAP group 15 complexes having hydroxyl groups as axial ligands have been synthesized. Their optical properties across the entire UV—vis region can be altered by the addition of an acid or a base, with alternation of the electronic configuration of the central group 15 elements. (Chem. Commun. 2014, 50, 15101.)
Tetrabenzotriazacorroles as a variant of low-symmetry Pcs: Tetrabenzotriazacorrole (TBC) is known as a π aromatic Pc-corrole analogue, in which one meso nitrogen atom of Pc is missing. We focus on TBC as a variant of low-symmetry Pcs and synthesized μ-oxo phosphorus TBC homo- (Chem. Commun. 2014, 50, 4312.) and hetero- (Macroheterocycles 2014,7, 139.) oligomers. The stacking effect of TBC could be explained conceptually by an exciton coupling model (a model of H-type aggregate). In addition, the low-symmetry structure of TBC showed enhanced efficiency in generating singlet oxygen, so that the molecular symmetry appears to be important in the design of sensitizers for photodynamic therapy.
Conclusion and perspective: The combination of simple macrocycic ligands and main-group elements appears to be an effective strategy for tuning the optical properties. All azaporphyrins in this article are easy to synthesize, robust under ambient air, free from metals and have predictable properties. The novel strategy will open the door to practical applications of functional dyes and pigments.