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Singlet Oxygen Photophysics in Liquid Solvents: Converging on a Unified Picture
journal contribution
posted on 2017-07-21, 18:50 authored by Mikkel Bregnhøj, Michael Westberg, Boris F. Minaev, Peter R. OgilbyConspectusSinglet oxygen, O2(a1Δg),
the lowest excited electronic state of molecular oxygen, is an omnipresent
part of life on earth. It is readily formed through a variety of chemical
and photochemical processes, and its unique reactions are important
not just as a tool in chemical syntheses but also in processes that
range from polymer degradation to signaling in biological cells. For
these reasons, O2(a1Δg) has
been the subject of intense activity in a broad distribution of scientific
fields for the past ∼50 years.The characteristic reactions
of O2(a1Δg) kinetically compete
with processes that deactivate this
excited state to the ground state of oxygen, O2(X3Σg–). Moreover, O2(a1Δg) is ideally
monitored using one of these deactivation channels: O2(a1Δg) → O2(X3Σg–) phosphorescence
at 1270 nm. Thus, there is ample justification to study and control
these competing processes, including those mediated by solvents, and
the chemistry community has likewise actively tackled this issue.In themselves, the solvent-mediated radiative and nonradiative
transitions between the three lowest-lying electronic states of oxygen
[O2(X3Σg–), O2(a1Δg), and O2(b1Σg+)] are relevant to issues at the
core of modern chemistry. In the isolated oxygen molecule, these transitions
are forbidden by quantum-mechanical selection rules. However, solvent
molecules perturb oxygen in such a way as to make these transitions
more probable. Most interestingly, the effect of a series of solvents
on the O2(X3Σg–)–O2(b1Σg+)
transition, for example, can be totally different from the effect
of the same series of solvents on the O2(X3Σg–)–O2(a1Δg) transition. Moreover, a
given solvent that appreciably increases the probability of a radiative
transition generally does not provide a correspondingly viable pathway
for nonradiative energy loss, and vice versa.The ∼50
years of experimental work leading to these conclusions
were not easy; spectroscopically monitoring such weak and low-energy
transitions in time-resolved experiments is challenging. Consequently,
results obtained from different laboratories often were not consistent.
In turn, attempts to interpret molecular events were often simplistic
and/or misguided. However, over the recent past, increasingly accurate
experiments have converged on a base of credible data, finally forming
a consistent picture of this system that is resonant with theoretical
models. The concepts involved encompass a large fraction of chemistry’s
fundamental lexicon, e.g., spin–orbit coupling, state mixing,
quantum tunneling, electronic-to-vibrational energy transfer, activation
barriers, collision complexes, and charge-transfer interactions.In this Account, we provide an explanatory overview of the ways
in which a given solvent will perturb the radiative and nonradiative
transitions between the O2(X3Σg–), O2(a1Δg), and O2(b1Σg+) states.