Quenching by Oxygen of the Lowest Singlet and Triplet States of Pyrene and the Efficiency of the Formation of Singlet Oxygen in Liquid Solution under High Pressure

Abstract

Quenching by oxygen of the lowest singlet (S1) and triplet (T1) states of pyrene at pressures up to 400 MPa in liquid solution was investigated. The rate constant of the S1 state, k q S, decreased significantly with increasing pressure, while that of the T1 state, k q T, was nearly independent of pressure at the lower pressure region (<100 MPa) and decreased monotonically with further increase in pressure. The activation volume for k q S, ΔV q S⧧, at 0.1 MPa fell in the range of 12−16 cm3/mol, depending on the solvents examined, whereas that for k q T, ΔV q T⧧, was nearly zero. It was found that both the activation volumes, ΔV q S⧧ and ΔV q T⧧, are significantly smaller than those determined from the pressure dependence of the solvent viscosity, η, ΔV η ⧧ (22−25 cm3/mol). For the quenching of the S1 state, the large difference between ΔV q S⧧ and ΔV η ⧧ was interpreted in terms of the competition of the quenching with diffusion, and k q S was separated into the contributions of the rate constants for diffusion, k diff, and for the bimolecular quenching in the solvent cage, k S,bim. For the quenching of the T1 state, it was found that k q T/k diff increases over 1/9 and approaches 4/9 with increasing pressure. The oxygen concentration dependence on the quantum yield for the formation of singlet oxygen, ΦΔ, in methylcylohexane (MCH) was measured at pressures up to 400 MPa in order to separate ΦΔ into the contributions of the S1 and T1 states. From the results, together with those of the pressure dependence of the quantum yield of T1 state, the branching ratio for the formation of singlet oxygen in the T1 state, f Δ T, was found to decrease with increasing pressure. The oxygen quenching of the T1 state from these results was discussed by using the mechanism that involves the encounter complex pairs with singlet, triplet, and quintet spin multiplicities.

Efficient Intersystem Crossing in Heavy-Atom-Free Perylenebisimide Derivatives

“…The triplet state quantum yield (Φ T ) of compound PBI-6 was determined as 51.3%, using the ground-state bleaching method (see Experimental Section for details). This Φ T value of PBI-6 is similar to those reported for other heavy-atom-free PBI derivatives. , It should be pointed out that this value is not always well approximated by the singlet oxygen quantum yields (Φ Δ ). , …”

“…61,73 It should be pointed out that this value is not always well approximated by the singlet oxygen quantum yields (Φ Δ ). 74,75 PBI-6 was used as a triplet photosensitizer to initiate intermolecular triplet−triplet energy transfer (TTET, Figure 6a−i and Figure S26a−i and Figure S27a−f). 76−79 To this end, we selected B-1 as triplet acceptor, a choice justified by, on the one hand, its low T 1 state energy level (1.06 eV, which is smaller than that of PBI-6, 1.34 eV), and on the other hand, its very red-shifted absorption band compared to PBI-6.…”

Visible-light harvesting pyrene-based MOFs as efficient ROS generators

“…Among various photochromic compounds, pyrene and its derivatives with the four-ring-fused planar electron-enriched skeleton are well-known for their unique optical and photoelectronic properties 38. They have been demonstrated to possess a low energy triplet state as well as significant O 2 -induced enhanced intersystem crossing (EISC), which are favourable to generate ROS, especially singlet oxygen ( 1 O 2 ), through energy transfer 39–41. Several pyrene-based MOFs have been investigated for 1 O 2 generation 37,42–44.…”

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(1Σg+), O2(1Δg), and O2(3Σg-) Formation on Sensitizer Triplet State Energy and Oxidation Potential

“…On the basis of these results, it was suggested that O 2 quenching of excited triplets exclusively proceeds via singlet encounter complexes, and that no isc takes place between the complexes of different multiplicity. However, later studies provided clear evidence for k T Q values in excess of 1/9k diff 6,17,[22][23][24][25][26][27][28] and even of 4/9k diff , [29][30][31] thus showing that complexes of singlet, triplet, and quintet multiplicity can significantly contribute to the overall mechanism. Moreover, a large number of studies showed that, for easily oxidizable molecules, rate constants correlate with the ease of oxidation of the sensitizer, which means that O 2 quenching of T 1 states proceeds, at least partially, via exciplexes.…”