Hydrogen-bonding and protonation are fundamental factors
controlling potentials and mechanisms in the
reduction of quinones. These are studied systematically in
benzonitrile, acetonitrile, and dimethylsulfoxide
solutions
by cyclic voltammetry of a series of quinones of increasing basicity
(chloranil to duroquinone), in the presence of
hydroxylic additives of increasing hydrogen-bonding power
(tert-butyl alcohol to hexafluoro-2-propanol) or
acidity
(trifluoroacetic acid). Electrochemical effects are demonstrated
over the complete interaction range, from hydrogen
bonding of reduced dianions to protonation of unreduced quinones.
With increasing concentrations of additives,
three clearly different types of electrochemical behavior are observed
for weakly (I), moderately (II), and strongly
(III) interacting quinone-additive pairs, as follows: (I) Two
well-separated reduction waves, corresponding to formation
of quinone mono- and dianions, shift positively, with no loss of
reversibility. The second wave is smaller, shifts
more strongly, and finally merges with the first. The relative
heights of the waves remain constant. (II) The
positive
shift is accompanied by increasing height of the first peak and
broadening and irreversibility of the second wave.
(III) One or even two, more positively shifted, new prior waves
appear, together with a new anodic wave. In
interpreting these phenomena, the role of hydrogen-bonding is clearly
distinguished from protonation on the basis of
pK
a values of relevant species, effects of
solvent variation, magnitude of potential shifts, and the onset of
irreversibility.
Type I behavior is attributed to stabilization by hydrogen-bonding
of mono- and dianion reduction products; the
number of bonds per quinone ion and bonding equilibrium constants are
estimated from the shifts in peak potentials
with additive concentration. These results are supported by
simulating the experimental cyclic voltammograms using
these parameters. Type III behavior is assigned to initial
hydrogen-bonding or protonation of the quinones. Type
II is attributed to a reduction mechanism involving disproportionation
of primary radicals, assisted by hydrogen-bonding or protonation of the dianion.
In order to clarify mechanisms of excited state
interactions in hydrogen-bonded pairs, we have studied the
kinetics of dynamic quenching of singlet and triplet fluorenone by a
series of alcohols, phenols, and related compounds,
in which hydrogen-bonding power, redox potential, and acidity are
systematically varied. In addition, effects of
solvent basicity or polarity and deuteration help identify the role of
hydrogen-bonding in physical or chemical quenching
processes. Alcohols and weak acids, with high oxidation
potentials, do not quench the triplet, but quench the
singlet
at rates which parallel hydrogen-bonding power. This is attributed
to a physical mechanism, involving vibronic
coupling to the ground state via the hydrogen bond. This is much
stronger in the excited state than in the ground
state, and provides efficient energy dissipation in the radiationless
transition. Phenols, with hydrogen-bonding power
comparable to that of the alcohols but with much lower oxidation
potentials, quench both singlet and triplet by
electron or H-atom transfer, depending on potentials, acidities, and
solvent polarity, as shown by formation of anion
or neutral fluorenone radicals from the triplet. Rates increase
with both decreasing oxidation potential of the phenol
and increasing acidity of the incipient cation radical. Quenching
proceeds via a hydrogen-bonded complex and is
facilitated by proton transfer contributions to the effective excited
state redox potential.
Abstract— The extinction coefficient εT, of triplet benzophenone in benzene has been directly determined by absolute measurements of absorbed energy and triplet absorbance, ΔD0T, under demonstrably linear conditions where incident excitation energy, E0, and ground state absorbance, A0, are both extrapolated to zero. The result, 7220 ± 320 M‐1 cm‐1 at 530 nm, validates and slightly corrects many measurements relative to benzophenone of triplet extinction coefficients made by the energy transfer technique, and of triplet yields obtained by the comparative method.
As E0 and A0 both decrease, ΔD0T becomes proportional to their product. In this situation, the ratio R= (1/A0)(dΔD0T/dE0) = (εT ‐ εG)φT. Measurements of R, referred to benzophenone, give (εT ‐ εG)φT for any substance, without necessity for absolute energy calibration.
Both absolute and relative laser flash measurements on zinc tetraphenyl porphyrin (εT ‐ εG at 470 nm = 7.3 × 104M‐1 cm‐1) give φT= 0.83 ± 0.04.
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