Singlet oxygen generation has attracted dramatic attention as one of the fundamental photochemical functions of phthalocyanine (Pc). Singlet oxygen is widely used in photodynamic therapy (PDT), photooxidation of toxic substances, and photoproduction of important intermediates for other chemicals.

When a photosensitizer is excited by laser, excited triplet state (T₁) is generated. Then, the energy transfer from the excited triplet state (T₁) to triplet oxygen molecule occurs, consequently, singlet oxygen is generated. Thus, the enhancement of the quantum yield of triplet state of photosensitizers is indispensable to the improvement of singlet oxygen quantum yield. Increasing the quantum yield of triplet state requires a change in the spin state. In general, heavy atoms are used due to their large spin-orbit interactions, so-called heavy atom effect. Instead of using heavy atom effect, we focus on improving singlet oxygen generation by using the interactions between organic radicals and Pc molecules as well as lowering the symmetry of π-electron systems. These studies have greatly improved the flexibility in the design of optical functional molecules.

Moreover, we also research on developing solid materials which can generate singlet oxygen.

• N. T. Hong Trang and K. Ishii
  "Photoelectrochemical Oxygen Reduction Reactions Using Phthalocyanine-Based Thin Films on an ITO Electrode "
  J. Phys. Chem. C, 122, 3539-3547 (2018).
• K. Ishii
  "Functional singlet oxygen generators based on phthalocyanines"
  Coord. Chem. Rev., 256, 1556-1568 (2012).
• Kazutaka Ozawa, Kazuyuki Ishii
  "Photophysical and magnetic properties of magnetic silica gel-supported silicon phthalocyanine complexes"
  Phys. Chem. Chem. Phys., 11, 1019 (2009).
• Kazuyuki Ishii, Masahiko Shiine, Yuu Kikukawa, Nagao Kobayashi
  "Synthesis and Photophysical Properties of Silica-Gel-Supported Photofunctional (Phthalocyaninato)silicon Complexes"
  Eur. J. Inorg. Chem., 2008, 2975 (2008).
• K Ishii, H Itoya, H Miwa, M Fujitsuka, O Ito, N Kobayashi
  "Relationship between Symmetry of Porphyrinic π-Conjugated Systems and Singlet Oxygen (¹Δ_g) Yields: Low-Symmetry Tetraazaporphyrin Derivatives"
  J. Phys. Chem. A, 109, 5781 (2005).
• K Ishii, S Takeuchi, S Shimizu, N Kobayashi
  "A Concept for Controlling Singlet Oxygen (¹Δ_g) Yields Using Nitroxide Radicals:?Phthalocyaninatosilicon Covalently Linked to Nitroxide Radicals"
  J. Am. Chem. Soc., 126, 2082 (2004).