Mechanism of Photosensitized Generation of Singlet Oxygen during Oxygen Quenching of Triplet States and the General Dependence of the Rate Constants and Efficiencies of O₂(¹Σ_g⁺), O₂(¹Δ_g), and O₂(³Σ_g^-) Formation on Sensitizer Triplet State Energy and Oxidation Potential

Abstract

Rate constants of photosensitized generation of O2(1Σg +), O2(1Δg), and O2(3Σg -) have been determined for a series of ππ* triplet sensitizers with strongly varying oxidation potential (E ox), triplet energy (E T), and molecular structure, in CCl4. We demonstrate that one common dependence on E ox and E T successfully describes these rate constants for the molecules studied here and also for all previously investigated ππ* sensitizers, independently of molecular structure or any other parameter. Photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states can be generally described by a mechanism involving the successive formation of excited noncharge transfer (nCT) encounter complexes and partial charge transfer (pCT) exciplexes of singlet and triplet multiplicity 1,3(T1 3Σ), following interaction of O2(3Σg -) with the triplet excited sensitizer. Both 1,3(T1 3Σ) nCT and pCT complexes decay by internal conversion (ic) to yield O2(1Σg +), O2(1Δg), and O2(3Σg -) and the sensitizer ground state. ic is the rate-limiting step in the nCT channel, whereas exciplex formation is rate determining in the pCT channel. Rotation of the O2 molecule within the solvent cage of 1,3(T1 3Σ) nCT complexes is fast enough to allow for a completely established intersystem crossing (isc) equilibrium, whereas significant noncovalent binding interactions slow rotation and inhibit isc between 1(T1 3Σ) and 3(T1 3Σ) pCT complexes. Upon the basis of this mechanism, we propose a semiempirical relationship that can be generally used to estimate rate constants and efficiencies of photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states in CCl4. The data set includes 127 rate constants for derivatives of naphthalene, biphenyl, fluorene, several ketones, fullerenes, porphyrins and metalloporphyrins, and other homocyclic and heterocyclic aromatics of variable molecular structure and size. It is suggested that the general relationship presented here can be used for the optimization of the singlet oxygen photosensitization ability of many molecules, including those used in biological and medical applications, such as the photodynamic therapy of cancer.

Identification and reactivity of the triplet excited state of 5-hydroxytryptophan

“…An overall mechanism is shown in Scheme 1, emphasising the apparent paradox of 5HT being involved in both formation and quenching of singlet oxygen. The efficiency of the sensitisation of singlet oxygen by a triplet state organic molecule can be influenced by several factors (30,45). These include the triplet state energy and oxidation potential (E ox ) of the sensitiser that may favour a charge transfer process (charge transfer is negligible if the E ox of the sensitiser is greater then 1.8 V vs SCE, (30)).…”

The Effect of Solvent Polarity on the Balance between Charge Transfer and Non-Charge Transfer Pathways in the Sensitization of Singlet Oxygen by ππ* Triplet States

“…The overall rate constant k D of product formation is calculated from k T Q according to eq 2. The single rate constants k T 1Σ , k T 1∆ , and k T 3Σ are then obtained by eqs 3-5: [23][24][25][26] * E-mail: R.Schmidt@chemie.uni-frankfurt.de; Fax: ++49-69-798-29709.…”

“…[23][24][25][26][27][28] This finding led to the conclusion that the formation of O 2 ( 1 Σ g + ), O 2 ( 1 ∆ g ), and O 2 ( 3 Σ g -) proceeds from 1 (T 1 3 Σ) and 3 (T 1 3 Σ) encounter complexes with negligible CT character () nCT complexes) being in a fully established intersystem crossing (ISC) equilibrium, see the left-hand side of Scheme 1. [23][24][25][26][27][28] Internal conversion (IC) of the 1,3 (T 1 3 Σ) nCT complexes occurs with rate constants k ∆E 1Σ , k ∆E 1∆ , and k ∆E 3Σ () k ∆E P ) to lower-lying nCT complexes 1 (S 0 1 Σ), 1 (S 0 1 ∆), and 3 (S 0 3 Σ), which dissociate to the respective products. The IC is ruled by the energy gap relation log(k ∆E P /m) ) f(∆E) of eq 6 where ∆E is [23][24][25][26]28 The k T P /m data of sensitizers with nonnegligible CT interactions deviate, however, significantly from this polynomial to larger values.…”

“…The IC is ruled by the energy gap relation log(k ∆E P /m) ) f(∆E) of eq 6 where ∆E is [23][24][25][26]28 The k T P /m data of sensitizers with nonnegligible CT interactions deviate, however, significantly from this polynomial to larger values. [23][24][25][26][27] In this case, a second deactivation path is opened for 1,3 (T 1 3 Σ) nCT complexes which leads via 1,3 (T 1 3 Σ) exciplexes with CT character () CT complexes) irreversibly also to the formation of O 2 ( 1 Σ g + ), O 2 ( 1 ∆ g ), and O 2 ( 3 Σ g -), see the righthand side of Scheme 1. Interestingly, no ISC equilibrium exists for 1,3 (T 1 3 Σ) CT complexes in contrast to what has been found for the 1,3 (T 1 3 Σ) nCT complexes.…”

Upconversion Nanoparticles for Light-Activated Therapy

“…Meanwhile, the second excited singlet state of molecule oxygen, O 2 ( 1 R g þ ), can also be formed in competition with O 2 ( 1 Δ g ) and rapidly deactivated to the relatively long-lived O 2 ( 1 Δ g ) species in solution [36,40]. The first excited state O 2 ( 1 Δ g ) has an energy of 94 kJ mol −1 above the ground state O 2 ( 3 R g À ) [40], thus requiring that photosensitizers have a triplet-state energy higher than 94 kJ mol −1 . The singlet oxygen ( 1 O 2 ) has a lifetime of 0.01-0.04 μs in biological environment, corresponding to a short diffusion distance of 0.01-0.02 μm [41].…”