Direct photoreduction and ketone-sensitized reduction of nitrospirobenzopyranindolines by aliphatic amines

Article abstract of DOI:10.1021/jp2005698

The photoreduction of 6-nitrospiro[2H-1-benzopyran-2,2′-indoline] (N1) and two derivatives (N2 and N3) by diethylamine or triethylamine (TEA) in solution was studied by pulsed and steady-state photolysis. The quantum yield of coloration of the ring-closed

Full text of DOI:10.1021/jp2005698

ARTICLE
pubs.acs.org/JPCA
Direct Photoreduction and Ketone-Sensitized Reduction of
Nitrospirobenzopyranindolines by Aliphatic Amines
Helmut G€orner
Max-Planck-Institut f€ur Bioanorganische Chemie, D-45413 M€ulheim an der Ruhr, Germany
ABSTRACT: The photoreduction of 6-nitrospiro[2H-1-benzopyran-2,20-indoline]
(N1) and two derivatives (N2 and N3) by diethylamine or triethylamine (TEA) in
solution was studied by pulsed and steady-state photolysis. The quantum yield of
coloration of the ring-closed Sp form, due to photoinduced ring opening, decreases in
acetonitrile with increasing the TEA concentration. The main reason is reaction of
TEA with the triplet-excited open merocyanine form. Quenching of this triplet state
by amines is rather inefficient for N1ꢀN3; the rate constant for triplet quenching by TEA is k6 = (2ꢀ3) ꢁ 106 Mꢀ1
s
ꢀ1. The
secondary transient with an absorption maximum at 420 nm is ascribed to the radical anion. This and the corresponding
R-aminoethyl radical subsequently undergo slow termination reactions, yielding a relatively stable product with a maximum at
420ꢀ450 nm, which is attributed to a ring-opened dihydromerocyanine (MHꢀ). The mechanisms of the two subsequent reduction
reactions are discussed. Using acetone as sensitizer the same dihydroproduct was obtained with the Sp form as acceptor, indicating a
reaction sequence from photogenerated radicals via a ring-opened radical to MHꢀ/MH2. The effect of TEA concentration on the
direct and ketone-sensitized reduction mechanisms was analyzed. Photoreduction by amines, due to competing triplet quenching, is
strongly decreased on admission of oxygen.
Scheme 1
’ INTRODUCTION
The photochromism of spiro[2H-1-benzopyran-2,20-in-
dolines] and several 6-nitro-substituted derivatives (nitroS) has
been intensively investigated.1ꢀ22 For nitroS in polar solvents at
ambient temperature an equilibrium is established between the
ring-closed colorless (Sp) form and the most stable isomer of the
strongly colored open merocyanine forms (trans-M). The col-
oration of nitroS is strongly enhanced upon UV irradiation,1ꢀ5
and this ring-opening photoprocess takes place in the picoseond
range.2 A triplet state is involved in the photoprocesses upon
excitation of the Sp form and at low temperatures also upon
excitation of trans-M.1ꢀ15 The quantum yield (Φc) of coloration
is large in solvents of low polarity; for example, in cyclohexane
and toluene Φc = 0.8 for parent 6-nitrospiro[2H-1-benzopyran-
2,20-indoline] (N1), its 8-methoxy derivative (N2), and its
10-phenyl derivative (N3),4,5 see Scheme 1. When the light is
switched off, the absorbance in the red spectral range decreases
and returns to its original equilibrium value. This thermal
rearrangement defines a thermal relaxation time which depends
on the structure, solvent, and temperature.1,5,15 Structural and
kinetic information has also been obtained by infrared spectro-
scopy.16 Photochemical and thermal reactions of the ring-
opened form with amino acids have been reported for a nitroS
and several spirooxazines.17,18
in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO), and
therefore, involvement of singlet molecular oxygen and a
quenching reaction by DABCO have been considered.23 On
the other hand, relatively little attention has been paid to the
reduction of spiro compounds. From electrochemical studies
with nitroS it is known that the radical anion has an absorption
maximum at 450 nm.28 It is well known that DABCO reacts with
excited states of aromatic molecules via electron transfer.
Electron transfer from an amine may take place also to the triplet
state of nitroS. Moreover, nitrosoaromatics are known to play a
role in the photoreactions of amines with nitroaromatic
compounds.29ꢀ32 It seems therefore desirable to obtain deeper
insight into the photoinduced electron transfer from amines to
nitroS.
Here, the photoreduction of the ring-closed compounds
N1ꢀN3 (Scheme 1) in the presence of diethylamine (DEA)
and triethylamine (TEA) were studied. A major photoproduct
other than the nitroso compound was generated by electron
transfer from the aliphatic amines to the observable triplet state
NitroS have a wide application potential.19ꢀ22 The use of
nitroS as a photochromic system is, however, limited by the
reductive or oxidative photodamage involving free radicals.23ꢀ28
For N2 in air-saturated toluene an oxidative photosplitting has
been reported with 5-nitrovanillin as the major product of the
nitrobenzene part and with 3,3-dimethyloxindol and 1,3,3-tri-
methyloxindol as the corresponding fragments. An increase of
fatigue resistance has been found for a nitroS in aerated toluene
Received: January 19, 2011
Revised:
June 13, 2011
Published: June 20, 2011
r
2011 American Chemical Society
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of the ring-opened merocyanine, which is attributed to a
dihydromerocyanine.
Table 1. Quantum Yields of Coloration and Triplet Forma-
tion and Triplet Lifetimea
compdb
solvent
Φc
ΦT
τT (μs)
’ EXPERIMENTAL SECTION
N1
toluene
0.84
0.16
0.74
0.20
0.63
0.14
0.9
10
12
12
15
12
14
The three nitroS were the same as used previously.5ꢀ8 The
solvents (Merck) were of the purest spectroscopic quality, e.g.,
acetonitrile: Uvasol. TEA was purified by distillation, and DEA
was used as received. The absorption spectra were monitored on
a spectrophotometer (HP, 8453). Molar absorption coefficients
of ε366 = 8 ꢁ 103 and ε604 = 3.3 ꢁ 104 Mꢀ1 cmꢀ1 for Sp and
trans-M of N1 in toluene and of ε580 = 4.8 ꢁ 104 Mꢀ1 cmꢀ1 for
N2 in acetonitrile have been reported.5,7 For photoconversion
the 366 nm line of a 1000 W XeꢀHg lamp combined with a
monochromator were used. The concentration was adjusted
such that the absorbance (in a 1 cm cell) was 1ꢀ3 at λirr.
Conversion was carried out after vigorous bubbling by argon
prior to and during irradiation.
acetonitrile
toluene
0.14
0.77
0.22
0.72
0.16
N2
N3
acetonitrile
toluene
acetonitrile
a In argon saturated at room temperature, λirr = 366 nm, λexc = 308 nm.
b See also refs 5ꢀ8.
An excimer laser (Lambda Physik, EMG 200, pulse width of 20
ns, and energy <100 mJ) was used for excitation at 308 nm. The
absorption signals were measured with two digitizers (Tektronix
7912AD and 390AD). The absorbance of neat TEA at 308 nm is
ca. 1 in a 1 cm cell, but for the much lower TEA concentrations of
<0.1 M used here, this has virtually no effect on the excitation of
the spiro compounds since the contribution of TEA to the total
absorption at 308 nm is below 1%. On the other hand, using
acetone, acetophenone, or benzophenone, the sensitizer con-
centration was adjusted such that the contribution was 50ꢀ95%
of the total absorption at 308 nm. For benzophenone/TEA
(1 mM) without nitroS, the triplet state is the initial transient
and the ketone radical anion is the secondary transient.33 For
benzophenone/2-propanol, the triplet is the initial transient and
the ketyl radical and radical anion were identified as secondary
transients at pH below 8 and at pH 10ꢀ11, respectively.33 In all
cases with ketone, TEA, and nitroS it has been verified that the
initial observable transient is the ketone radical anion, keeping in
mind that the ketone triplet state escapes observation in the
nanoseconds time range. The secondary transient with an absorp-
tion maximum at 420ꢀ440 nm is assigned to the merocyanine
radical, see below. The decay in the absence of oxygen followed
second-order kinetics; the half-life (t1/2 after subtraction of a
constant part in most cases) was found to depend strongly on the
amount of traces of oxygen, and the presented values refer to
those after vigorous bubbling by argon. Note that this colored
product formation can even be slightly enhanced by the analyzing
light (on an even longer time scale). This, however, was prevented
Figure 1. Plots of A560 of N1 in argon-saturated acetonitrile as a
function of the time of irradiation at 366 nm in the presence of 0
(circles), 0.03 (triangles), and 0.2 M (squares) of TEA; (inset) spectra
with 0.2 M TEA prior to (1) and after 50 s of irradiation under either air
(2) or argon (3).
reference redox couple, and are converted into SCE units by
addition of 0.16 V. All measurements refer to 24 °C.
’ RESULTS AND DISCUSSION
Photoreactions of NitroS. Continuous UV irradiation of
various nitroS enhances the coloration with one maximum in
the UV and another (λt) in the red spectral range at
550ꢀ620 nm.1ꢀ9 The quantum yield of coloration is large for
N1ꢀN3 in benzene or toluene, Φc = 0.6ꢀ0.8, but relatively
small, Φc = 0.1ꢀ0.2, in acetonitrile as an example of a nonaqu-
eous polar solvent (Table 1). For N1 and N2 in acetonitrile, a
minor part of the colored trans-merocyanine remains after
thermal relaxation, whereas for N3 only the colorless ring-closed
Sp form is present, i.e., in the latter case equilibrium 1 in Scheme 1
is shifted toward the left side. This occurs also for N1ꢀN3 in
solvents of low polarity with a maximum at λSp = 330ꢀ360 nm.5
For N1ꢀN3 in cyclohexane or toluene, Φc = 0.7ꢀ0.8.5,7,8 For
N2 in toluene Φc = 0.594 and 0.849 have been reported. Other
literature values are Φc = 0.55 for N1 in cyclohexane11 and Φc =
0.88 for N2 in acetonitrile.10 On the other hand, Φc of N1ꢀN3 is
5ꢀ10 times smaller in acetonitrile than in solvents of low
polarity; the main reason is successful internal conversion prior
to photoinduced ring opening.5ꢀ8
by appropriate filters. A Nd laser (GWU Lasertechnik) with λexc
=
354 nm served as second method, but the results presented here
refer to λexc = 308 nm only.
Electrochemical measurements were performed in an airtight
cell under an argon atmosphere in anhydrous acetonitrile with
tetrabutylammonium hexafluorophosphate as the supporting
electrolyte using a potentiostat (EG&G, 273A) with a 0.01 N
AgNO3/Ag electrode in acetonitrile as the reference electrode.
Two glassy carbon disk electrodes embedded in an insulating
polymer were used as working and counter electrodes, respec-
tively. Reversibility of the reduction was determined by examina-
tion of the forward and reverse peak currents at different scan
rates between 0.1 and 3 V sꢀ1. Small amounts of ferrocene were
added after completion of the measurements as internal stan-
dard. The formal potentials were obtained under conditions of
reversibility, are given with respect to the ferrocenium/ferrocene
Plots of the coloration as a function of the time of irradiation at
λirr = 366 nm of N1 and N3 are shown in Figures 1ꢀ3 (circles).
Similar results occur upon pulsed excitation at 308 nm. The
colored merocyanine conformers have a zwitterionic or quinoid
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Scheme 2
Figure 2. Plots of A600 of N3 in argon-saturated toluene as a function of
the time of irradiation at 366 nm in the presence of 0 (circles), 0.03
(triangles), and 0.2 M (squares) of TEA; (inset) spectra with 0.2 M TEA
prior to (1) and after 10 s of irradiation (2) and after 20 h (3) thermal
relaxation.
Figure 3. Plots of A600 (open) of N3 in argon-saturated acetonitrile as a
function of the time of irradiation at 366 nm in the presence of 0
(circles), 0.03 (triangles), and 0.3 M of TEA (squares), thermal
relaxation (full); (inset) spectra with 0.3 M TEA prior to (1) and after
100 s irradiation under air (2) and argon (3).
character, and four conformers with the central transoid segment
can a priori be expected for nitroS after thermal or photochemical
cleavage of the CꢀO bond;1,2,14,16 the most stable merocyanine
isomer is indicated as trans-M. For practical reasons N3 is
favored, as the phenyl substituent strongly accelerates the
thermal ring closure, in contrast to N1, where in acetonitrile
the lifetime of trans-M is ca. 160 s.5 A thermal relaxation is shown
in Figure 3 (full cicles).
3*M state at 450 nm is only 20ꢀ30% of that of the ring-opened
trans-M form at 560ꢀ600 nm.
3
ꢂ
M f Rtrans-M + ð1 ꢀ RÞcis-M
ð4Þ
3
ꢂ
M + O2 f βtrans-M + ð1 ꢀ βÞcis-M + O2
ð40Þ
The previously suggested deactivation reactions of nitroS not
involving amines are briefly summarized (Scheme 2).5ꢀ8,14 The
first observed intermediate upon pulsed excitation at 308 nm is
the triplet state of the open merocyanine (3*M), generated by
steps 2 and 3. It is worth mentioning that the excited singlet
(1*Sp) and lowest triplet (3*Sp) states of the ring-closed form are
not accessible by 10 ns photolysis. The observed 3*M state decays
into the ring-opened ground state trans-M form directly and
indirectly via other isomers. One intermediate (denoted here as
cis-M) has been detected, reactions 4 and 5 (Scheme 2).5,7
Here, 1-R and 1-β are the fractions of 3*M leading to cis-M in
the absence and presence of oxygen, respectively. Quenching
of 3*M by oxygen (step 40) yields the trans and cis merocyanine
forms in amounts which are essentially independent of the O2
concentration, i.e., R ≈ β. The observed triplet state appears
concomitant with the laser pulse; the decay is first order, and
the lifetime is τT = 10ꢀ20 μs in solution at room temperature.
The quantum yield (ΦT) of formation of the ring-opened
triplet state upon UV excitation of the Sp form is substantial for
N1ꢀN3 in nonpolar solvents and decreases in polar solvents.5
The triplet quantum yields and lifetimes are compiled in Table 1.
The spectrum subsequent to TꢀT absorption of N1ꢀN3 in
acetonitrile, showing two bands with maxima at 420 and
560ꢀ610 nm, is due to both the trans- and the cis-merocyanines.
The lifetime of cis-M at room temperature is in the milliseconds
time range.5ꢀ8
Irradiation of NitroS in the Presence of Amines. Coloration
of N1ꢀN3 in argon-saturated acetonitrile, upon irradiation at
366 nm, occurs also in the presence of amines, e.g., at concentra-
tions of 0.002ꢀ0.2 M. Nevertheless, efficient interaction of the
3*M state with TEA can take place. Examples of the absorption
spectra are shown in Figure 1 (inset) for N1 in argon-saturated
acetonitrile in the presence of 0.2 M TEA. Φc decreases with the
TEA or DEA concentrations. Further examples of the photo-
induced absorption spectra in the presence of TEA are shown for
N3 (insets of Figures 2 and 3). The TEA concentration, where
Φc is lowered to 50% of the value in the absence of the amine, is
[TEA]1/2 = 0.07ꢀ0.18 M (Table 2).
It is quenched by oxygen; the rate constant for triplet
9
quenching is k4 = (1ꢀ3) ꢁ 10 Mꢀ1 sꢀ1. The TꢀT absorption
0
spectrum of N2 in acetonitrile, for example, shows two broad
bands with maxima at 450 nm and approximately 560 nm and
levels off at 700 nm. Note that the absorption coefficient of the
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Table 2. Amine Half-Concentrations and Quantum Yields of
Photoproducta
The quantum yield of reduction (ΦH) is similar for N1ꢀN3 and
DEA/TEA, increases with the amine concentration, and approaches
a maximum value of 0.06 (Table 2). The quantum yield Φc
accordingly decreases. Plots of log ΦH as a function of the
logarithmic amine concentrations are shown in Figure 5.
To account for the photoinduced formation of the dihydro-
merocyanine anion of N1ꢀN3 in acetonitrile, electron transfer
(step 6) from an amine (DH2) to the trans triplet state as
acceꢀptor and formation of a radical anion of the merocyanine
(M• ) are proposed, see Scheme 3. The product is eventually
formed by radical termination: one involves the R-aminoethyl
radical, reaction 7. Alternatively, termination and rearrangement
(reaction 8) may occur. Moreover, the deprotonation (step 9) of
the radical cation of the amine is involved.
b
b
compd
solvent
[TEA]1/2 (M) ΦH
[DEA]1/2 (M) ΦH
N1
N2
N3
acetonitrile
acetonitrile
benzene
0.14 (0.1)c
0.12 (0.1)
0.12
0.04
0.04
0.04
0.04
0.1
toluene
0.18
acetonitrile
0.07 (0.09)
0.06
0.08
0.05
a In argon-saturated solution, λexc = 308 nm. b Using [amine] = 0.2 M at
low pulse intensity. c Values in parentheses: obtained from plots of
ΔA580 at 1 ms vs the TEA concentration.
2M•ꢀ + H+ f MHꢀ + trans-M
ð8Þ
ð9Þ
DH2 f DH• + H+
•+
MHꢀ + O2 f M•ꢀ + O2•ꢀ + H+ f Sp=trans-M + HO2
ꢀ
ð10Þ
When the solution is air or oxygen saturated, Φc in the presence
of TEA is nearly the same as in the absence. This is due to triplet
quenching by oxygen (step 40), thereby blocking the electron
transfer pathway initiated by step 6 (Scheme 3).
Scheme 3
Figure 4. Plots of A360 (open) and A440 (full) of N3 in argon-saturated
acetonitrile as a function of the time of irradiation at 366 nm in the
presence of 0.03 (squares), 0.1 (triangles), and 0.3 M of TEA (circles);
(insets) spectra with (a) 0.3 M TEA prior to (1) and after 30 and 100 s of
irradiation (2, 3) and (b) 0.03 M of TEA at 0 (1) and 200 s (2) and 100
min thermal relaxation after air saturation (3).
Reaction 10 accounts for the back-reaction of MHꢀ to the
Sp/trans-M equilibrium. Photoreduction by amines is generally
strongly decreased on admission of oxygen. A thermal back-
reaction of ca. 50% within 100 min after air saturation is
presented for N3 in acetonitrile (inset b, Figure 4). However,
this oxidation is slower when the amine concentration is in-
creased. Another example for an oxidation of the photoproduct is
shown in toluene, curve 3 in the inset of Figure 2.
Time-Resolved Photoreduction. In the presence of both
TEA and oxygen the absorption spectrum at the pulse end is due
to the merocyanine triplet state and the subsequent spectra,
showing two bands with absorption maxima at 420 and ca.
600 nm (Figures 6a and 7a), are due to both the trans and the
Figure 5. Double-logarithmic plots of the quantum yields of coloration
Φc (open) and reduction ΦH (full) as a function of the amine
concentrations for N2/TEA in argon-saturated benzene (triangles)
and N3/TEA in acetonitrile (circles) and N3/DEA (squares).
For a concentration larger than [TEA]1/2, a new species with
maximum at λH = 450 nm appears upon irradiation of N3 in
acetonitrile, as shown in the insets of Figures 3 and 4. For N1
(Figure 1, inset) and N2 the bands at λH = 426 and 450 nm,
respectively, remain after partial thermal relaxation. The yellow
photoproduct is denoted as MHꢀ and ascribed to a nitrophenolate
(dihydromerocyanine anion), see below. Note that p-nitropheno-
late has a band centered at 450 nm.34,35 On the other hand, UV
irradiation leads to a smaller amount of formation of the trans-
merocyanine, i.e., photoreduction competes with photocoloration.
Figure 6. Transient absorption spectra of N1 in (a) air- and (b) argon-
saturated acetonitrile in the presence of 0.2 M of TEA at 30 ns (O), 1 μs
(Δ), 100 μs (b), and 0.1 s ()) after the 308 nm pulse; (insets) kinetics as
indicated.
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Table 3. ReductionPotentialandRateConstantk6 (106 Mꢀ1sꢀ1)
for Triplet Quenchinga
compd
ꢀEred (V)b
k6: TEA
k6: DEA
N1
N2
N3
1.46 (1.38)
1.44 (1.38)
1.45 (1.37)
2.6
2.5
3.0
3.2
3.4
a In argon-saturated acetonitrile at room temperature, λexc = 308 nm.
b Values vs Fc+/Fc. In parentheses vs SCN from ref 28.
N1ꢀN3 are slightly more positive, and therefore, the k6 values
are slightly smaller than with DEA. For other dyes some
deviation from the RehmꢀWeller relationship is not unusual.38
Another quenching reaction was observed for N3 in air-
saturated acetonitrile. It involves the ΔA600 value, which is
constant for more than 1 s but shows a decay in the presence
of DEA. The rate constant for quenching of the trans-M form by
Figure 7. Transient absorption spectra of N3 in (a) air- and (b) argon-
saturated acetonitrile in the presence of 0.2 M of TEA (a) at 30 ns (O),
100 μs (b), 1 ms (2), and 0.1 s ()) after the 308 nm pulse; (insets)
kinetics at 420 nm.
cis ring-opened forms, i.e., triplet quenching by oxygen competes
successfully with that by TEA. The photoinduced changes under
these conditions are faster than in the absence of oxygen and
mimic those in the absence of the amine. In the presence of TEA
(0.03ꢀ0.1 M) formation of the triplet state of N1ꢀN3 in argon-
saturated acetonitrile occurs within the pulse width and the
absorbance at 440ꢀ470 nm follows first-order decay kinetics.
The intermediate, overlapping with TꢀT absorption and mer-
ocyanine formation, has a maximum at 420 nm and a half-life of
5ꢀ20 ms. It should be assigned to the radical anion of the
merocyanine M•ꢀ. The electrochemically generated radical
anion of N1 or N2 has λmax = 430ꢀ455 nm and no strong band
above 500 nm.26,28 From pulse radiolysis of N2 in ethanol a
molar absorption coefficient of ε450 = 4 ꢁ 104 Mꢀ1 cmꢀ1 has
been reported.36 For nitroS, in contrast to various other nitroaro-
matic compounds,29ꢀ32 the ε value of the radical anion of the
acceptor is smaller than that of the triplet state.
The subsequently formed third species shows the 420ꢀ430 nm
band nearly unchanged but only little absorbance in the visible
range. Owing to its amplitude extending to >1 s, this species is a
photoproduct. Examples for N1 and N3 in argon-saturated
acetonitrile are shown in Figures 6b and 7b, respectively. The
signal ΔA580 at 1 ms decreases on increasing the TEA concen-
tration. From this dependence, a half-concentration was ob-
tained. The results are in agreement with the [TEA]1/2 values,
see Table 2.
DEA was found to be approximately 500 Mꢀ1 ꢀ1. This is in
s
contrast to TEA, which does not interact with trans-M at all, in
agreement with the slightly smaller oxidation potential of DEA.
Note that such a reaction cannot be detected in argon-saturated
solution since quenching step 6 decreases the yield ΔA600 of the
trans-M form. Thus, the second quenching reaction by DEA has
no consequences on the overall photoreduction under oxygen-
free conditions.
Radical Reactions. The presence of TEA gives rise to
secondary transients after quenching of the triplet state. At low
amine concentrations (<30 mM) the rate constant kobs, due to
triplet decay, shows a linear dependence on the amine concen-
tration. The slope of this plot is k6 ≈ (2ꢀ3) ꢁ 106 Mꢀ1 sꢀ1 for
the two amines examined (Table 3). This corresponds to a TEA
concentration for 50% triplet quenching of [TEA]1/2
=
30ꢀ90 mM, taking for τT an upper limit of 10 μs. At higher
TEA concentrations an analysis is not possible due to increasing
absorbance of the secondary transients in the appropriate range
of 400ꢀ480 nm.
The transient absorption spectra of N1ꢀN3 in acetonitrile
after triplet decay reveal two or three species: The first with a
maximum at 420ꢀ440 nm and a similar ε value as that of the
TꢀT absorption spectrum is assigned to the merocyanine
radical, one structure is shown in Scheme 4. The radical anion
of N1 absorbs at 400ꢀ500 nm, whereby the ε450 value is almost
as large as that of trans-M.28 The radicals of p-nitrophenol in
aqueous solution have two pKa values, the larger one is at 9.8; the
For N1ꢀN3 in acetonitrile the rate constant k6 for triplet
quenching by TEA is ca. 3 orders of magnitude below the
diffusion-controlled limit. This is consistent with the free energy
change (but not in full agreement) using the RehmꢀWeller
relationship.37
corresponding dianion radical has ε450 = 4 ꢁ 104 Mꢀ1 cmꢀ1 39
.
In the nitroS case, M•ꢀ is converted into a species with a relatively
large εvalue and one characteristic band at λH = 420, 440, and 450 nm
for N1 (Figure 6b), N2, and N3 (Figure 7b), respectively. These
spectra are overlapping with those of the trans- and cis-merocyanines
with a major band at 380ꢀ420 nm and a minor at 550ꢀ620 nm, the
yields of which are reduced on increasing TEA concentration. The
stable species contains the 420ꢀ450 nm band nearly unchanged but
only little absorbance in the visible range. Owing to its amplitude
extending to >1 s, this species is a p-nitrophenolate photoproduct.
From proton transfer studies of the p-nitrophenol/TEA system in
aprotic solvents, the molar absorption coefficient in acetonitrile and
ΔGet ¼ Eox ꢀ Ered ꢀ ET ꢀ EC
Here, Eox is the oxidation potential of the amine, Ered the
reduction potential of the acceptor, ET is the energy of the
quenchable triplet state, and EC is the Coulombic term account-
ing for ion pairing, 0.06 V in acetonitrile. The three nitroS in
acetonitrile exhibit a reversible one-electron reduction at the
same potential of Ered = ꢀ1.44 V. Literature values for N1 and N2
are Ered = ꢀ1.38 and ꢀ1.40 V, respectively.28 For the level of the
3*trans-M state only a rough estimate could be made since it has
to be similar to or lower than that of the fluorescent 1*trans-M
state of 1.7 V. Assuming for ET a value of 1.7 V and the oxidation
potential of DEA with Eox = 0.7 V, it follows that ΔGet is positive
(0.4 V). For the case of TEA with Eox = 0.78 V the ΔGet values of
25 mM of TEA is known to be ε426 = 1.8 ꢁ 104 Mꢀ1 cmꢀ1 35
.
Et3N•+, which in acetonitrile solution has a lifetime of up to a
few microseconds,40 is converted into DH• via deprotonation
step 9. Once the R-aminoethyl radical is formed, a reaction
with spiropyran/merocyanine radicals is expected, step 7
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Scheme 4
Scheme 5
(Scheme 4). Thus, per one initial electron transfer (step 6) to the
3*M triplet state two radicals should be formed. They react via an
intermediate to the corresponding quasi-stable dihydromerocya-
nineanion MHꢀ, step 7. The structuralidentity remains open: two
possible forms are shown in Scheme 4. Such a dihydro species has
not been identified as yet. This assignment would be in analogy
with transients observed for the photoreduction of the triplet state
of nitroaromatic compounds.29ꢀ32
Termination reactions of nitroaromatic radicals could, in
principle, yield nitroso compounds. The observed photopro-
duct should, however, not be attributed to the trans-merocya-
nine nitroso form due to the too large ε value. One could
propose that MHꢀ is a species of the quinonemonoxime
type.41 However, a reversible oxidation of a quinonemonox-
ime has not been reported. There is no obvious reason against
a thermal ring closure of any trans-nitrosomerocyanine,
whereas both structures for MHꢀ shown in Scheme 4 are
expected to be stable as far as oxygen is excluded. Apparently, a
nitroso form is not favored vs the dihydromerocyanine anion
as product, but the main reason behind this finding is not
known as yet.
Figure 8. Absorption spectra of N3 in argon-saturated acetonitrile in
the presence of 1 M acetone and 1 mM TEA (1) and after 10 s irradiation
at 313 nm (2); addition of 1 mM HClO4 (3) and 10 mM of TEA (4).
Most results refer to acetonitrile and the longer lived
intermediates to radical ions. The photoreduction in benzene
or toluene is slightly different. An example is shown in
Figure 2. The reaction of the excited spiro compound with
TEA could lead via electron plus proton transfer (reaction 60)
to SpH• and MH• type radicals and finally (step 70) to
the corresponding dihydromerocyanine/nitrophenol MH2,
Scheme 5. For nitroaromatic compounds, an equilibrium
is operating and should be on the side of the conjugate
acid in nonpolar solvents and of the radical anion side in
polar solvents.30ꢀ32 A similar situation is proposed for
N1ꢀN3.
Figure 9. Absorption spectra of N1 (10) and N3 (1) in acetonitrile in
the presence of 1 M acetone, 1 mM NaOH, and 1 M 2-propanol and
after 9 s irradiation at 313 nm (air saturated, 2,20) and argon-saturated
after 20 s irradiation (3, 30); (inset) absorption spectra of acetone
(0.2 M), TEA (50 mM), and N3 in argon-saturated acetonitrile prior to
(1) and at 30 s irradiation at 313 nm (2) as well as upon air saturation
and 10 min thermal relaxation (3).
Reduction of NitroS via Photogenerated Radicals. UV
irradiation of a ketone in argon-saturated acetonitrile in the
presence of both a nitroS and an aliphatic amine strongly
enhances conversion to the dihydromerocyanine. Here, acetone,
acetophenone, or benzophenone were applied as sensitizers. A
low amine concentration of 1 mM is sufficient for this sensitized
conversion of Sp to MHꢀ/MH2 since the rate constant for
quenching of a ketone triplet state (3*K) by TEA or DEA is close
to the diffusion-controlled limit. An example of radical-induced
conversion of Sp to MHꢀ is shown for N3 in the presence of
acetone and TEA (Figure 8, curve 2 vs 1). Subsequent addition of
HClO4 causes protonation of MHꢀ (curve 3 vs 2). Finally, deproto-
nation of MH2 can be achieved by addition of more TEA (curve 4 vs 3).
The product MHꢀ with a peak at λH = 450 nm disappears at pH
below 8, whereas MH2 is present or appears at pH 10ꢀ11.
Alternatively, an alcohol concentration of 0.1 M is sufficient
for 2-propanol as donor since the rate constant for quenching of
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Scheme 6
strong enhancement of the product yield upon sensitized reduc-
tion is ascribed to the low triplet yield upon direct excitation in
polar solvents.
’ ACKNOWLEDGMENT
The author is grateful to Professor Wolfgang Lubitz for his
support, to Professor Dietrich D€opp, Professor Alexander K.
Chibisov, Dr. Eberhard Bothe, and Mr. Henry Gruen for
stimulating discussions, as well as to Mrs. Petra H€ofer and Mr.
Leslie J. Currell for technical assistance.
Figure 10. Transient absorption spectra of N3 in argon-saturated
acetonitrile in the presence of 1 mM of TEA at 10 μs (0), 100 μs
(b), 1 ms (2), and 0.1 s ()) after excitation of (a) benzophenone and
(b) acetone by the 308 nm pulse; (insets) kinetics at 430 nm.
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The photoreduction of three 6-nitrospirobenzopyranindo-
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