The decays of the end-of-pulse species arising from NA and PA
were studied within the pH range 2.9–6.0, the species issued from
IA decaying too fast to be studied. The apparent first order rate
Table 2 Spectral characteristics and bimolecular rate constants of triplets
k/mol−1 dm s
3
−1
+
constants of decay were found to increase linearly with [H ]. In
Triplet from
max/nm
substrate
O
2
H+
5
the case of NA, the linear regression gave k = (8.5 ± 0.5) × 10 +
9
+
−1
(
4.6 ± 0.4) × 10 [H ] s (four experimental data, correlation coef-
3.3 × 109
3.4 × 109
NAp
NAa
435
285/375
9
4.6 × 109
1.7 × 1010
ficient = 0.99). The decay of the transient formed from PA could be
only measured within the pH range 4.7–6. As for NA, we observed
an increase of the decay rate with decreasing pH. Linear regression
1.3 × 10 (NAz)
8
4.8 × 10 (NAa)
PAp
PAa
306/350
290/340
2.4 × 109
8
9.3 × 10 (PAz)
5
10
+
−1
gave k = (1.8 ± 0.5) × 10 + (1.7 ± 0.5) × 10 [H ] s (three experi-
8
1
.8 × 10 (PAa)
mental data, correlation coefficient = 0.96).
IAp
IAa
310/380
290/375
Discussion
Assignment of transients
propose an electron transfer between the zwitterionic triplet and NAp.
The formation of the zwitterionic triplet in a pH range where proton-
ated ground state molecules are dominant is probably due to the depro-
tonation of the protonated triplet rather than by inter-system crossing
of zwitterionic singlet, since no photoreactivity was observed in the
pH range where zwitterions are the dominant forms. The weak pho-
toreactivity of carboxypyridines at pH < pKa1 can be explained by the
small formation of zwitterionic triplet, by the difficulty of the electron
transfer reaction or by a high probability of back electron transfer. The
chemical yield of 40% for hydroxylated products and the formation
of non-aromatic compounds visible on the NMR spectra suggest that
an oxido-reduction occurs between pyridinyl radicals and OH-adduct,
yielding hydroxylated derivatives and reduction products.
The laser-flash photolysis of the three isomers yielded two-band
feature short-lived species at the pulse end. Some of their character-
istics are sumarized in Table 2. Based upon the scavenging effects
of oxygen and methylacrylate, they can be assigned to the excited
+
triplet states. Those observed at pH > 4 react with H with rate con-
stants close to the diffusion-controlled limit. Moreover, their yield
of formation was found to increase with pH, and the curve to closely
follow that of the ground state molecules second ionisation. It can
be concluded that they are the anionic triplets. They are likely to be
produced by excitation of the anionic ground-state molecules, and
the value of pKa2* to be close to that of pKa2. The non-observation
of solvated electrons at pH 4 confirmed this hypothesis. The triplets
arising from NAa, NAp and PAa were found to react with the start-
ing compound. In the case of NAp it could be evidenced that the
triplet gave a secondary transient species.
The long-lived species insensitive to oxygen produced from NA
could not be attributed. It might be related to opening ring products
described in the case of pyridine.7
This secondary species can be assigned using the literature data.
Indeed, the two bands 285/415 nm are reminiscent of those of 3-
2
< pH < 4. In this pH range that corresponds to (pKa1 + pKa2)/2
and in which compounds exist in the zwitterionic form, no transients
were detected and no photoreactivity was observed. It is in accor-
dance with a fast deactivation of zwitterionic singlet excited states.
1
8
carboxypyridinyl radical, 1, (see Fig. 4, dotted line). Subtracting
from the global spectrum that of 1 yielded an absorption band in
the region 300–360 nm resembling that reported for the OH-adduct
1
9
radical (insert of Fig. 4). Taking for the extinction coefficients
pH > 4. Under these pH conditions, IA is photostable while PA
and NA photodimerize with different efficiencies. Results bring
evidence that the triplet is involved in the photolysis again. First,
oxygen both reduces the lifetime of the triplet and the quantum yield
of the reaction. Second, the product × φ increases with increasing
pH in the same pH range as the quantum yield of photolysis does.
Third, IA, which is photostable, has a very short-lived triplet.
The detection of 2 from PA shows that again the initial reaction
of 1 and OH-adduct radical the values reported in the literature
−1
3
−1
−1
3
−1
(7200 mol dm cm at 285 nm and 2000 mol dm cm at 325 nm,
respectively) we computed from the products × φ measured in this
−
1
3
−1
−1
3
−1
work (100 mol dm cm at 290 nm and 40 mol dm cm at 325
nm) that the radicals were both formed with φ = 0.02. It shows
that they are produced in the same pathway. By analogy to these
results, we might expect to observe the 2-carboxypyridinyl radical,
2
, from acidic PA. Unfortunately, 2 exhibits an absorption spectrum
between triplet and ground state molecule is an electron transfer
2
0
(max = 305 and 360 nm) close to that of the end-of-pulse transient
−
(Scheme 2). Molecular calculations showed that the C–CO
2
bond
and its formation cannot be firmly proved. By subtracting the
spectrum of 2 from the transient spectrum measured from acidic PA
order was lower in the charge transfer complex (0.77 and 0.48
for NA and PA respectively) than in the isolated triplet (1.05 and
2
s after the pulse, we measured an absorption in the wavelength
0
2
.98). Moreover, TOC measurements confirmed the CO evolu-
range 300–400 nm that may correspond to the OH-adduct radical by
tion. Since PA is more easily decarboxylated than NA, and also
exhibits a higher quantum yield of photolysis, it seems that the
decarboxylation process is the key step. It probably takes place at
the level of the charge transfer complex competing with the return
to ground-state molecules by reversible electron transfer. Pyridyl
2
0
reference to the literature data (see insert of Fig. 5).
In the case of PAa that shows the highest quantum yield of pho-
totransformation, it was also possible to detect secondary species,
one exhibiting a maximum at 290 nm that is likely to be 2 again,
and another with a maximum at 400 nm that does not resemble any
transient reported in the literature. The non-observation of 1 from
NAa may be due to the overlapping of its absorption spectrum with
that of the end-of-pulse species.
and anionic carboxypyridinyl radicals then escape the cage. The
latter protonates (pK
a
5–6),1 while the pyridyl radical adds to a
8,20
ground state molecule yielding an adduct. The long-lived species
absorbing around 400 nm detected in PA solution might be this ad-
duct. Lastly, a disproportionation reaction between this adduct and
carboxypyridinyl radical occurs, yielding bipyridine and reduction
photoproducts. In the case of PA, a second decarboxylation takes
place, while in the case of NA carboxy-substituted bipyridines are
formed. The structure of photoproducts shows that addition of the
pyridyl radical occurs mainly in the para position, and probably in
the ortho position too, with respect to the carboxy group. It yields
only one photoproduct with PA because of the second decarboxyl-
ation, while it should yield three isomers with NA.
Proposed mechanism
pH < pKa1. The product studies showed that the three isomers have
the same behaviour all yielding hydroxylated derivatives with a chem-
ical yield close to 40%. Based upon the results obtained by laser-flash
photolysis with NA we can propose the mechanism given in Scheme
1. Using the fact that oxygen both quenches the triplet and decreases
the quantum yield of photolysis, it can be deduced that the triplet state
is involved in the reaction. Moreover, the detection of 1 and OH-ad-
duct radical indicates that the reaction between the triplet and NApro-
ceeds through an electron transfer. This reaction appears at a pH where
molecules are protonated. However, an electron transfer between two
protonated molecules seems very unlikely. As an alternative, we may
The stoichiometry of the reaction for PAis one bipyridine deriva-
tive produced for three PA molecules consumed. It is in very good
accordance with the chemical yield found for bipyridine (2 × 28 %)
2
and with the quantity of CO evolved (0.9/8.9 ≈ 2/18). In the case of
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 2 2 5 3 – 2 2 6 1
2 2 5 7