Singlet Oxygen, Photodynamic Therapy, and Mechanisms of Cancer Cell Death

Figure 2

Molecular orbital diagram of two singlet excited states and a triplet state of oxygen. There are three different states of oxygen molecules, i.e., ³Σ_g⁻, ¹Δ_g, and ¹Σ_g⁺. These states differ in the placement of two electrons in two antibonding orbitals (AOs). In the case of the triplet state (³Σ_g⁻), the two electrons occupy two AOs singly and have the same spin, whereas in the first singlet state (¹Δ_g) the two electrons fill one AO (¹Σ_g⁺) without filling another, which puts the second state in the higher energy state. The first singlet state does not follow Hund’s rule. The second singlet state, in which the two electrons occupy two AOs singly but do not have the same spin, also does not follow Hund’s rule. The ground state oxygen molecule has one unpaired electron in each antibonding orbital and has like spins. When the ground state triplet oxygen (³Σ_g⁻) (b) is excited by the transfer of energy, denoted by E, it changes to singlet oxygen (¹Δ_g) (a), the first excited state. ¹Δ_g has paired electrons in only one antibonding orbital with opposite spin and is unstable and reactive. Even more unstable singlet oxygen (¹Σ_g⁺) (c), the second excited state, is formed by absorbing more energy where two electrons with opposite spin are aligned in two different antibonding orbitals. Usually, ¹Δ_g is more stable in comparison with ¹Σ_g⁺, so the unstable form converts into a more stable ¹Δ_g.

(a)

(b)

(c)