A.R. Bertoti et al. / Journal of Photochemistry and Photobiology A: Chemistry 299 (2015) 166–171
169
to the formation of a long-lived transient with lifetime in excess of
50 s and absorption in the 400 nm region, which can be assigned
m
to the corresponding aryloxyl radical (Scheme 3). A representative
spectrum employing 4- methoxyphenol (1.84 ꢁ10ꢂ3 mol Lꢂ1) as
the hydrogen donor, recorded 5.5 ms after the laser pulse, is shown
in Fig. 4, in which the absorption maximum for the 4-
methoxyphenoxyl radical is centered at 405 nm, fully in accor-
dance with literature data [27,28]. It is worth noting that this
absorption can have some contribution of the ketyl radical derived
from the diazafluorenone since, as described above, this species
also has absorption in this region of the spectrum. For the phenols
containing polar substituents, hydrogen abstraction rate constants
ranging from (4.9 ꢀ 0.1) ꢁ 107 L molꢂ1 sꢂ1, for 3-chlorophenol, to
(1.3 ꢀ 0, 5) ꢁ 1010 L molꢂ1 sꢂ1, for 4-aminophenol, were obtained, as
shown in Table 1. Representative plots for the quenching of 4,5-
diaza-9-fluorenone by 4-methoxy-; 3-methoxy-; 4-chloro- and 3-
chlorophenol are shown in Fig. 8S, Supporting information.
Phenols react with carbonyl triplets faster than alcohols,
alkanes and even 1,4-cyclohexadiene, a very efficient hydrogen
donor [29], with this enhanced reactivity being attributed to the
low bond dissociation energy for O—H and the ease of oxidation of
phenols [30–34].
Fig. 5. Hammett plot of phenolic hydrogen abstraction rate constants (kq) by 4,5-
diaza-9-fluorenone triplet against s+
.
[21] the quenching rate constant approaches the diffusion limit
(kq = (1.1 ꢀ0.4) ꢁ 1010 L molꢂ1 sꢂ1) (Fig. 4 Supporting information,
Table 1). From this set of data on the kinetics of the energy transfer
process we can conclude that the triplet excited state energy of 4,5-
A
linear Hammett plot (Fig. 5) employing the phenolic
hydrogen abstraction rate constants listed in Table 1 and analyzed
using Eq. (3) [35,36] resulted in a value for the reaction constant
r
of ꢂ1.54 ꢀ 0.10 (r = 0.976), similar to those obtained for other
diaza-9-fluorenone is located between 49.3 and 52.4 kcal molꢂ1
.
aromatic monoketones (acenaphthenone [34] and thiochroma-
It is well known that several parameters influence the rate
constant for hydrogen abstraction in photoreduction reactions,
namely the triplet state energy, the nature of excited state (if np*
none [37]) or
a-diketones (acenaphthenoquinone [34], 1,2-
aceanthrylenedione [33] and 1,2-diketopyracene [32]).
kqX
kqH
or pp*), the C—H bonding energy of the hydrogen atom being
transferred, the number of equivalent atoms available for the
reaction, the solvent and the reduction potential of the ground
state ketone. The low values for the quenching rate constant of 4,5-
diaza-9-fluorenone triplet by 1,4-cyclohexadiene (kq = (1.1 ꢀ0.1)
ꢁ 106 L molꢂ1 sꢂ1) (Fig. 6 Supporting information, Table 1) and
other hydrogen donors, such as 2-propanol, cyclohexane and
toluene (Table 1), seem to indicate that this triplet excited state has
pp* character. A comparison of the quenching rate constants for
4,5-diaza-9-fluorenone triplet by these hydrogen donors with
log
¼
sþr
(3)
where kqX is the quenching rate constant for a substituted phenol,
kq is the quenching rate constant for phenol, s+ is a Hammett
H
constant, which depends solely on the nature and position of the
substituent, and
r is the reaction constant, a function of the
reaction under investigation and the conditions under which it
takes place.
The Hammett equation expresses
a general quantitative
those obtained for other fluorenones that are known to have p p
*
relation between the nature of a given substituent and the
reactivity of the reaction center, with the validity of Eq. (2) being
restricted to substituents in the meta- and para-positions of the
triplet configuration, such as 9-fluorenone [24–26], 1,4-diaza-9-
fluorenone [17] and 1,4-diaza-9-benz[b]fluorenone [17] clearly
shows that these values are very close together, which confirms the
benzene ring. The reaction constant (
ty of the reaction to the influence of the substituent. Based on
values, the transition state characteristic with respect to a
developing charge can be obtained. For > 0, a negative charge
is built in the transition state, whereas for < 0 the reaction
develops a positive charge in the transition state. The negative
value for the Hammett reaction constant in the phenolic
r) measures the susceptibili-
pp
* character of the triplet excited state of 4,5-diaza-9-
r
fluorenone. With 1,4- cyclohexadiene the quenching process leads
to the formation of a new transient which was assigned to the ketyl
radical derived from the diazafluorenone, which exhibits absorp-
tion maximum at 390 nm (Scheme 1) (Fig. 2).
r
r
When electron donors such as triethylamine (kq = (1.0 ꢀ 0.3)
r
ꢁ 1010 L molꢂ1 sꢂ1) and DABCO (kq = (6.3 ꢀ 0.1) ꢁ 108 L molꢂ1 sꢂ1
)
hydrogen abstraction by 4,5-diaza-9-fluorenone is in accordance
with the proposed mechanism for this type of reaction, in which a
positive charge on the phenol is developed, as shown in Scheme 3.
In this mechanism, an initial electron transfer from the phenol to
the excited ketone through an intermediate exciplex is proposed,
which is followed by an ultrafast proton transfer, ultimately
resulting in the formation of the radical pair ketyl/phenoxyl
[32–34,37–42].
(Fig. 7S, Supporting information, Table 1) were employed as
quenchers of the 4,5-diaza-9-fluorenone triplet, the formation of a
new transient was also observed (Fig. 3). This species has
absorption maximum at 560 nm and was assigned to the radical
anion derived from this fluorenone, following Scheme 2.
Laser photolysis of 4,5-diaza-9-fluorenone in the presence of
phenol and its derivatives containing polar substituents also leads
O
OH
H
H
H
.
.
hν
ACN
+
+
N
N
N
N
Scheme 1.