Photophysical properties and photoreduction of N-acetyl- and N-benzoylphthalimides

Article abstract of DOI:10.1016/j.chemphys.2011.08.012

The photophysical properties and photoreduction of N-acetylphthalimide (AcP) and N-benzoylphthalimide (BzP), N-3,4,5-trimethoxybenzoylphthalimide (trimethoxyBzP) and N-4-nitrobenzoylphthalimide (nitroBzP) were studied by steady-state and transient techniques. Radicals and their precursor triplet states were detected by flash photolysis. The triplet state properties of AcP and BzP were characterized. In contrast, no triplet absorption was observed with ns-detection for trimethoxyBzP and nitroBzP. Specific products are formed upon electron transfer from triethylamine to the photoexcited acylphthalimides. In addition, H-atom transfer from 2-propanol or other alcohols to the triplet state takes place. The properties of several radical intermediates involved in photoreduction of the acylphthalimides as well as some structure-function relationships are described.

Full text of DOI:10.1016/j.chemphys.2011.08.012

Chemical Physics 392 (2012) 10–15
Contents lists available at SciVerse ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Photophysical properties and photoreduction of N-acetyl- and
N-benzoylphthalimides
László Biczók a, Helmut Görner b,
⇑
a Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary
b Max-Planck-Institut für Bioanorganische Chemie, D-45413 Mülheim an der Ruhr, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The photophysical properties and photoreduction of N-acetylphthalimide (AcP) and N-benzoylphthali-
mide (BzP), N-3,4,5-trimethoxybenzoylphthalimide (trimethoxyBzP) and N-4-nitrobenzoylphthalimide
(nitroBzP) were studied by steady-state and transient techniques. Radicals and their precursor triplet
states were detected by flash photolysis. The triplet state properties of AcP and BzP were characterized.
In contrast, no triplet absorption was observed with ns-detection for trimethoxyBzP and nitroBzP. Spe-
cific products are formed upon electron transfer from triethylamine to the photoexcited acylphthali-
mides. In addition, H-atom transfer from 2-propanol or other alcohols to the triplet state takes place.
The properties of several radical intermediates involved in photoreduction of the acylphthalimides as
well as some structure-function relationships are described.
Received 26 May 2011
In final form 10 August 2011
Available online 30 August 2011
Keywords:
N-acylphthalimides
Photoreduction
Triplet state
Radical
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
The main goal of the present work was to reveal how the vari-
ation of the N-acyl substituent influences the efficiency of the for-
The photophysical and photochemical features of N-substituted
phthalimides and naphthalimides have been studied in great detail
[1–18]. In particular, photocyclization reactions of N-(x-aminoal-
mation and the reactions of the triplet excited species. The
behaviour of N-acetyl (AcP) and N-benzoyl (BzP) derivatives was
compared. To alter the extent of the conjugation with the phthali-
mide moiety, both 4-nitro- and 3,4,5-trimethoxybenzoyl deriva-
tives (nitroBzP and trimethoxyBzP) were examined. The strongly
electron withdrawing character of the nitro substituent is expected
to decrease the electron density of the aromatic ring, whereas elec-
tron donating by the three methoxy groups is anticipated to exert
an opposite effect. The formulas of the investigated substances are
given in Chart 1.
kyl)phthalimides have been intensively investigated [19,20]. The
phthalimides have low fluorescence quantum yields (Uf) and inter-
system crossing is usually the dominant deactivation pathway of
the singlet-excited state [10,13]. N-aryl derivatives form excep-
tions since the transition to the triplet state cannot compete with
the rapid energy dissipation process via internal conversion. For
N-benzyl- and N-phenylethylphthalimides in acetonitrile, the
quantum yield of intersystem crossing is Uisc = 0.2 and 0.4, respec-
tively [21]. A phthalimide benzylation reaction in the presence of
aryl-substituted acetic acids has been employed as precursor in
the formation of phenanthrene lactam alkaloids [21]. The knowl-
edge concerning the photochemistry of the N-substituted phthali-
mides has recently been reviewed [22] and their various
photoprocesses have been discussed [23–41]. The radical anion
can be formed by electron transfer from triethylamine (TEA) to
the phthalimide triplet. An intramolecularly photoinitiated H-atom
transfer has been suggested for N-adamantylalkylphthalimides
[26]. Despite the widespread interest in the light-initiated pro-
cesses of dicarboximides, to the best of our knowledge, no informa-
tion is available on the photochemical and photophysical
properties of their N-acyl derivatives.
2. Experimental
The four N-acylphthalimides were synthesized by the reaction
of phthalimide with the corresponding acid chloride analogously
to that described in the literature [42]. 4-Nitro- and 3,4,5-trimeth-
oxybenzamides were prepared by adding aqueous NH3 solution to
substituted benzoyl chlorides [43]. The other compounds were the
same as used previously or as received. TEA was purified by distil-
lation. The solvents (Merck, trifluoroethanol: TFE, Aldrich) were of
the purest spectroscopic quality and checked for impurities; aceto-
nitrile was Uvasol quality and water was from a Milli-Q (Millipore)
system. The absorption spectra were monitored on a diode array
spectrophotometer (HP, 8453). The molar absorption coefficient
of nitroBzP in acetonitrile is
tion, the 313 nm line of a Hg lamp and a filter were used. Alterna-
e
272 = 1.8 ꢀ 104 Mꢁ1 cmꢁ1. For irradia-
⇑
Corresponding author.
E-mail address: goerner@mpi-muelheim.mpg.de (H. Görner).
tively, irradiation was performed with a 1000 W Xe–Hg lamp and a
0301-0104/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2011.08.012

L. Biczók, H. Görner / Chemical Physics 392 (2012) 10–15
11
AcP R: -CH
BzP
O
N
O
3
3.2. Illumination of N-acylphthalimides
OMe
OMe
OMe
O
R trimethoxyBzP
The curves in Fig. 1 show that the absorption spectrum of nitro-
BzP in acetonitrile is composed of the bands characteristic of the
phthalimide and 4-nitrobenzamide chromophor entities, indicat-
ing very limited conjugation between the two parts of the nitroBzP
molecule. The 4-nitrobenzamide constituent is the major absorb-
ing species above 250 nm and the shoulder at 310 nm results from
the phthalimide moiety. In contrast, the spectrum of trim-
ethoxyBzP fundamentally differs from the sum of 3,4,5-trimeth-
oxybenzamide and phthalimide absorption (inset to Fig. 1). The
lowest energy band is red-shifted by 40 nm, implying an increased
charge transfer character of the lowest excited singlet state (1⁄P).
Introduction of a N-acetyl or N-benzoyl substituent barely changes
the energy and shape of the absorption band of phthalimide.
The absorption spectrum of AcP in acetonitrile shows negligible
alteration upon continuous irradiation at 313 or 254 nm in this in-
ert solvent. In contrast, irradiation of an AcP solution in the pres-
ence of either 2-propanol (e.g. 1–10 M) or TEA (e.g. 0.01 M) leads
nitroBzP
NO2
Chart 1. Structures of AcP and the three BzPs.
monochromator. The relative quantum yield of reduction was ob-
tained by the spectral change, keeping the initial absorbance at
kirr = 313 nm constant. For the determination of the absolute quan-
tum yield of reduction (Ured), we used the aberchrome 540 acti-
nometer [44]. Irradiation of phthalimides in acetonitrile–H2O 3:1
at pH 11 could not be carried out due to a thermal ring-opening
[33]. The photoproducts were analysed on a reverse phase ODS-
3HD column (PerfectSilTarget, 3 l
m, 0.8 ml minꢁ1) with mobile
phase gradient, composed of 0.5% trifluoroacetic acid and either a
1:5 mixture of acetonitrile and water or neat acetonitrile as eluent.
The retention times were tr = 16 and 18 min for AcP and BzP,
respectively. The solutions were bubbled with argon prior to and
during irradiation.
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 (Tek-
tronix 7912AD and 390AD). Relative yields were determined with
optically matched solutions, whose absorbances were typically
to the emergence of
a new absorption around 330–400 nm
(Fig. 2, inset). The results from HPLC analyses show one major
product peak and some secondary products. The slope of the initial
conversion as a function of the irradiation time was used as a mea-
sure of the relative photoreduction yield. An example is shown in
Fig. 2. The Ured values of AcP are compiled in Table 1. Similar re-
sults were found on pulsed UV excitation at 308 nm (not shown).
The Ured values of BzP and trimethoxyBzP are below 0.01. In con-
trast to AcP, virtually no spectral change was observed upon irradi-
ation of BzP in acetonitrile in the presence of either 2-propanol or
0.2–2 (per 1 cm) at the excitation wavelength. The
DA values were
taken as rough measure of Uisc. Phosphorescence of singlet molec-
ular oxygen at 1269 nm was detected after the pulse using a cooled
Ge detector (North Coast, EO 817FP), a silicon filter, an interference
filter and an amplifier (Comlinear, CLC-103). The signal, whose ini-
tial part can be overlapped by fluorescence and/or scatter, was
0.3
A
extrapolated to the end of the 20-ns pulse (I ). The quantum yield
of singlet molecular oxygen formation (UD) was obtained from I
D
3'
2'
1.5
D
0.2
0.1
0.0
values using optically matched solutions (A308 = 0.8) and 2-aceto-
naphthone in benzene with UD = 0.81 as Ref. [45]. When the sol-
vent of the sample and the reference compound differed, UD was
calculated by taking into account the solvent dependence of the
rate constant of the singlet molecular oxygen phosphorescence
emission. For example, the correction factor was taken as 2 and 3
relative to benzene in dichloromethane and acetonitrile, respec-
tively [45]. Electrochemical measurements were performed in an
airtight cell under an argon atmosphere in anhydrous acetonitrile
with tetrabutylammonium hexafluorophosphate, Bu4NPF6, as the
supporting electrolyte at room temperature, using a potentiostat
(EG&G, 273A) with a 0.01 N AgNO3/Ag in acetonitrile as reference
electrode. Two glassy carbon disk electrodes embedded in an insu-
lating polymer served as working and counter electrodes. Revers-
ibility of the phthalimide reduction was determined by
examination of the forward and reverse peak currents at different
scan rates between 50 mV sꢁ1 and 1.6 V sꢁ1. The formal potentials
(E°0) are given as E°0 (Fc/Fc+) vs. the ferrocene/ferricenium reference
redox couple [21,26]. The measurements refer to 24 °C.
A
1.0
0.5
0.0
1
250
300
350
350
λ
(nm)
3
2
1
200
250
300
λ
(nm)
Fig. 1. Absorption spectra of nitroBzP (3), phthalimide (1) and 4-nitrobenzamide
(2) in acetonitrile; inset: spectra of trimethoxyBzP (30), phthalimide (1) and 3,4,5-
trimethoxybenzamide (20).
1.5
1.0
Aobs
A
4
1.0
0.5
3
3. Results
2
350
1
0
0.5
0
400
λ (nm)
3.1. Electrochemical measurements
The phthalimides exhibit a reversible one-electron reduction at
potentials influenced by the substituent linked to the imide nitro-
gen. The potential is E°0 = ꢁ1.74 and ꢁ1.71 V vs. Fc/Fc+ for BzP and
trimethoxyBzP, respectively. Only for the latter compound, we
found an oxidation peak at +1.18 V. Two non-reversible peaks at
E°0 = ꢁ1.72 and ꢁ1.87 V appear for AcP, whereas two reversible
peaks at E°0 = ꢁ1.19 and ꢁ1.535 V were registered for nitroBzP.
0
200
400
Time (s)
Fig. 2. Absorption of AcP at 280 (circles) and 340 (triangles) nm in argon-saturated
acetonitrile in the presence of 1 (full) and 20 mM (open) TEA vs. the time of
irradiation at 313 nm; inset: spectra in the presence of 20 mM TEA at 0, 20, 60 and
300 s, 1–4, respectively.

12
L. Biczók, H. Görner / Chemical Physics 392 (2012) 10–15
Table 1
Quantum yield of reduction of N-acetyl- and N-4-nitrobenzoylphthalimide.a
2
3
2
Donor
TEA
[Donor] (M)
AcP: Ured
NitroBzP: Ured
A
0.001
0.01
0.1
1
9
9
0.004
0.01
0.03
0.01
0.02
0.01
0.02
Aobs
0.04
1
0.1 (0.1)b
0.04
2-Propanol
Methanol
2
1
0.08
0.02
1
300
0
350
400
a
λ (nm)
In argon-saturated acetonitrile solution, kirr = 313 nm.
In parentheses: kexc = 308 nm.
b
0
0
100
number of pulses
Fig. 4. Absorption of nitroBzP at 330 (full) and 400 (open) nm in argon-saturated
acetonitrile in the presence of 0.02 M (circles) and 0.2 M (triangles) TEA as a
function of the number of 308 nm excitation pulses; inset: spectra in the presence
of 0.2 M TEA at 0, 20 and 90 pulses, 1–3, respectively.
1.0
A
4
1.0
0.5
0
Aobs
3
Table 2
0.5
Quantum yield of singlet molecular oxygen formation UD of N-acylphthalimides.a
2
1
300
Solvent
AcP
BzP
TrimethoxyBzP
0.05
NitroBzP
<0.02
250
350
0.04 [0.30]c
0.05 [0.38]
0.03 [0.20]
<0.05 [0.18]
λ
(nm)
Benzene
Toluene
Dichloromethane
Acetonitrile
0.43
0.4
0
0.24
<0.02
<0.02
<0.02
<0.02
0
100
0.4b
Time (s)
a
b
c
In air-saturated solution, kexc = 308 nm.
In TFE UD = 0.3.
In brackets: oxygen-saturated.
Fig. 3. Absorption of nitroBzP at 270 (circles) and 340 (triangles) nm in argon-
(open) and air-saturated (full) acetonitrile in the presence of 15 M 2-propanol vs.
the time of irradiation at 313 nm; inset: spectra at 0, 20, 50 and 90 s, 1–4,
respectively.
3.4. Triplet absorption and decay
TEA. However, prolonged irradiation in the presence of TEA leads
to stable products.
Pulsed excitation of AcP in inert solvents of different polarity,
e.g. benzene or acetonitrile, reveals the absorption of the lowest
triplet state (3⁄P) with maximum at 350 nm and a weak band up
to 500 nm (Fig. 5a). The results are compiled in Table 3. 3⁄P is
formed within the pulse. The triplet lifetime of AcP under argon
The most significant changes in the absorption spectra take
place for nitroBzP. Representative results obtained in argon-satu-
rated acetonitrile containing 2-propanol or TEA are shown in Figs. 3
and 4 (insets). The absorbance at 270 nm (A270) decreases upon
photolysis, and a concomitant rise of product absorption with a
maximum at 340 nm and an isosbestic point at 290 nm were ob-
served. The same major photoproduct is produced in the presence
of 2-propanol or TEA, which is ascribed to the nitrosoBzP or its
reduction product: hydroxylaminoBzP. The Ured values of nitroBzP
are significantly larger than those of AcP (Table 1). The product
pattern in the presence of TEA is more complex, it reveals an
absorption band centered at 400 nm at low conversion. Similar re-
sults with the 400 nm band at low conversion in the presence of
0.2 M TEA were found on pulsed UV excitation of nitroBzP at
308 nm (Fig. 4).
and at low excitation light intensities is typically sT = 5 ls, whereas
T–T annihilation contributes to the decay at higher intensity. Oxy-
gen quenches the triplet state of AcP with rate constants of around
kox = 2 ꢀ 109 Mꢁ1 sꢁ1. The triplet state of BzP in argon-saturated
benzene or acetonitrile has sT = 200 ns. Addition of 50% water to
acetonitrile has virtually no effect on Uisc and sT. The triplet yield
was found to be low for trimethoxyBzP. Pulsed excitation of nitro-
BzP in acetonitrile reveals no transient in the 350–700 nm range.
Moreover, the acetone-sensitized population of the triplet excited
state of nitroBzP failed. Note that the lowest triplet state of N-ada-
mantlyphthalimide was formed by energy transfer after excitation
of acetone as sensitizer [26].
3.3. Phosphorescence of singlet molecular oxygen
Most efficiently singlet molecular oxygen formation was found
when AcP served as a sensitizer. The concentration of O2(1Dg
)
reaches a maximum at the pulse end and the lifetimes are in the
40–80 s range in the four solvents used. The quantum yield of
l
O2(1Dg) formation (UD) in air-saturated solution is around 0.4 in
three solvents and somewhat smaller in dichloromethane (Table 2).
UD is low for BzP and trimethoxyBzP in benzene and virtually zero
in more polar solvents. However, an increase of up to UD = 0.4 was
obtained for BzP in oxygen-saturated solution, indicating that the
substantial quantum yield of intersystem crossing, Uisc is accom-
panied by a short triplet lifetime, see Section 3.4. On the other
hand, UD is below 0.02 for nitroBzP in all air-saturated solvents
examined. It should also be mentioned that no significant fluores-
cence could be detected for the four N-acylphthalimides.
Fig. 5. Transient absorption spectra of AcP in argon-saturated acetonitrile (a)
without additive and (b) in the presence 2 M 2-propanol at 20 ns (s), 1
10 s (h) after the 308 nm pulse (dashed line in b refers to 10 ms); inset: kinetics as
indicated.
ls (D) and
l

L. Biczók, H. Görner / Chemical Physics 392 (2012) 10–15
13
Table 3
Table 4
Results from TT absorption of N-acylphthalimides.a
Properties of the radicals of N-acetyl- and N-4-nitrobenzoylphthalimide.a
Compound
AcP
Solvent
kTT (nm)
sT
(
ls)
D
Ab
Donor
[Donor] (M)
Ar: t1/2 (ms)
Air: t1/2 (ms)
D
Ab
Benzene
Acetonitrile
TFE
Benzene
Acetonitrile
350
350
360
340
350
400c
5
5
5
0.2
0.2
8d
0.65
0.7
0.7
0.75
0.7
<0.01
AcP
TEA
2-Propanol
TEA
0.01
1
0.01
0.1
1
2 (100)c
4 (2)
6
3
1
1
<0.02
1 (1)
<0.003
0.002
0.2
0.1
NitroBzP
0.8
1
0.3
0.2
BzP
2-Propanol
Ethanol
TrimethoxyBzP
1
0.2
a
b
c
a
b
c
In argon-saturated solution, kexc = 308 nm.
Relative triplet yield at kTT, taking benzophenone with
In benzene.
In acetonitrile solution, kexc = 308 nm.
At 400 nm, used as a measure of the yield of radicals.
In parentheses: BzP
D
A = 1 as reference.
d
Upon acetone-sensitized excitation.
3.5. Radicals
The triplet state of the phthalimides in benzene or acetonitrile
reacts with TEA and 2-propanol. For AcP in acetonitrile, the rate
constants are kQ = 3 ꢀ 109 and 1 ꢀ 106 Mꢁ1 sꢁ1, respectively. The
corresponding time-resolved spectra are shown in Figs. 5b and
6b. The triplet state is quenched and a long-lived transient with
absorption maximum at 400 nm is formed. It decays by second-or-
der kinetics with half-lives (t1/2) in the ms time range (Table 4) and
is quenched by oxygen. This process is efficient for the long-lived
species generated by TEA but relatively inefficient for the product
of the reaction with alcohols. Triplet quenching by TEA is sug-
gested to produce the radical anion (Pꢀꢁ), whereas its conjugate
acid (PHꢀ) is formed as a long-lived transient in the presence of
alcohols. A larger TEA concentration yields a faster build-up and
a larger radical yield. Formation of Pꢀꢁ was also detected for elec-
tron transfer from TEA to triplet BzP (Fig. 6a). The delayed build-
up of the radical anion of BzP indicates that Pꢀꢁ is produced not
only in this reaction but also in an electron transfer to ground state
BzP from the TEAꢀ radical, which is formed via deprotonation of
TEAꢀ+. Due to the latter process, the half-life of Pꢀꢁ formation
Fig. 7. Transient absorption spectra of nitroBzP in argon-saturated acetonitrile in
the presence of (a) 9 M 2-propanol and (b) 0.1 M TEA at 20 ns (s), 1 s ( ), 0.1 ms
(d), 1 ms (N) and 10 ms (e) after the 308 nm pulse; insets: kinetics at 400 nm.
l
D
4. Discussion
4.1. Photoprocesses
The photoprocesses of the N-acylphthalimides examined are
triplet reactions. The reactive state of phthalimides is generally
the lowest triplet, in agreement with the literature [3–10,13,19–
22]. Exceptions are 4,5-dimethoxyphthalimides, where Uf is high
[23]. The photoreaction of the singlet-excited species 1⁄P in the
other cases and for the acylphthalimides is unlikely because of
its short lifetime. The quantum yield of fluorescence of N-meth-
ylphthalimide in acetonitrile is Uf < 0.0006 [13] and the fluores-
cence lifetime is 0.185 ns [10]. The quantum yield of singlet
molecular oxygen formation, which can be considered as mini-
mum measure of Uisc, is substantial for AcP (Table 2), where sT is
sufficiently long (Table 3). The low UD for trimethoxyBzP are due
to the low Uisc values. On the other hand, the limiting factor for
(0.4 ls) is larger than sT = 0.2 ls. The radical anion of BzP is extre-
mely long-lived, t1/2 is ca. 0.1 s under our conditions.
No triplet could be observed at 20 ns in the 350–700 nm range
upon laser flashing of nitroBzP in acetonitrile. However, on addi-
tion of TEA (0.001–1 M) a transient with a maximum at 400 nm
and a second band at 680 nm was detected (Fig. 7a and b). It is
formed within the pulse, decays within 1–6 ms and is quenched
by oxygen with a rate constant (kO2) of ca. 1 ꢀ 109 Mꢁ1 sꢁ1. It is as-
signed to the radical anion Pꢀꢁ, see Section 4.3. A larger TEA con-
centration yields a larger radical yield. A similar spectrum was
recorded in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO).
The half-life of Pꢀꢁ in the presence of DABCO is a few
ls, i.e. much
shorter than with TEA, indicating efficient electron back transfer in
the former case. The transient produced from nitroBzP in 2-propa-
nol and ethanol exhibits a similar absorption spectrum. It is as-
signed to the radical PHꢀ. In contrast to Pꢀꢁ, PHꢀ is not markedly
quenched by oxygen (Table 4).
UD of BzP under air is the short triplet lifetime of sT = 0.2 ls under
argon. The exact reason for the fast deactivation from the 3⁄P state
of BzP to the ground state is unclear, but the fast dissipation of the
triplet energy should be due to both flexibility and conjugation of
the two parts of BzP.
4.2. Photoinduced electron and H-atom transfer to triplet AcP and BzP
The triplet state of the phthalimides reacts with TEA and 2-pro-
panol [21,33]. The lowest triplet of AcP in acetonitrile is quenched
by amines, such as TEA, the rate constant of this electron transfer is
kQ = 3 ꢀ 109 Mꢁ1 sꢁ1
. The similar reduction potentials of AcP,
trimethoxyBzP or BzP with respect to N-methylphthalimide, where
E°0 = ꢁ1.7 V vs. Fc/Fc+ in acetonitrile, indicate that the phthalimide
rather than the acyl entity is reduced. The rate constant of H-atom
transfer from 2-propanol to 3⁄AcP is kQ = 1 ꢀ 106 Mꢁ1 sꢁ1. The com-
bination of radicals may yield further products (Scheme 1).
After population of the lowest triplet, step (1), H-atom transfer
from an alcohol to 3⁄P (2) leads to ketyl type radicals: PHꢀ.
Fig. 6. Transient absorption spectra in argon-saturated acetonitrile for (b) AcP in
the presence 0.02 M TEA and (a) BzP and 0.1 M TEA at 20 ns (s), 1
ls (D), 10 ls (h),
0.1 ms (d) and 1 ms (N) after the 308 nm pulse; insets: kinetics at 400 nm.

14
L. Biczók, H. Görner / Chemical Physics 392 (2012) 10–15
2x
pathway a
Competing processes are intersystem crossing to the ground state
and a quenching reaction by oxygen. The latter can lead to singlet
molecular oxygen. The PHꢀ radicals can be scavenged by oxygen,
whereby P is restored and oxygen is converted to hydroperoxyl/
superoxide ion radicals (3)
O
N
O
hν
+DH2
O
N
O
-nitroBzP
O
O
•
OH
OH
N
•
nitroBzP
NO2H
+DH
•
-DH
-D
-H2O
pathway b
O
N
O
O
N
O
O
O
•
•
+DH
+DH
-D
NO
(nitrosoBzP)
NHOH
(hydroxylaminoBzP)
P þ h
m
!
1ꢂP ! 3ꢂP
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
-D
3ꢂP þ DH2 ! PHꢀ þ DHꢀ
Scheme 2. Route of photoreduction of nitroBzP via nitrosoBzP to
hydroxylaminoBzP.
PHꢀ þ O2 ! P þ Oꢀꢁ þ Hþ
2
photolysis, is proposed to have a lifetime as short as around 1 ns or
even slightly less. For 4-nitrobenzoic acid in ethanol, a triplet life-
time of 0.8 ns has been reported [46]. An excited singlet state reac-
tion cannot be excluded, if nitroBzP and TEA form a complex.
Nevertheless, we found that one of the excited states of nitroBzP
reacts with 2-propanol (Fig. 3) and TEA (Fig. 4). One photoreduc-
tion route of nitroBzP leads to nitrosoBzP and possibly to the
hydroxylaminoBzP, see Scheme 2.
The photoreduction properties of several other nitro aromatic
compounds have been reported in great detail [47–53]. The usual
step is the formation of radicals (2Ar–NOꢀ2H or 2Ar–NO2ꢀꢁ) which
undergo disproportionation into Ar–NO2 and the nitroso com-
pound (Ar–NO). A recent work refers to the direct photoconversion
of 1-nitro-9,10-anthraquinone to 1-aminoAQ mediated by donor
radicals [53]. Analogously, the observed radicals of nitroBzP are
proposed to yield nitrosoBzP. Two reduction steps of the Ar–
NOꢀ2H radicals are conceivable, pathway-a due to self-termination
or pathway-b due to reaction with the donor radical. After the dis-
proportionation, a water molecule is eliminated (Scheme 2). Prob-
ably, the mechanism in the presence of TEA is more complex. To
account for the 400 nm band at low conversion (Fig. 4), we tenta-
tively propose formation of a dianion prior to protonation and
water elimination leading to the nitroso product.
2PHꢀ ! P þ PH2
PHꢀ þ DHꢀ ! D þ PH2
The radicals ðHOꢀ2=O2ꢀꢁÞ do not absorb above 300 nm and eventually
produce hydrogen peroxide (not included). Nevertheless, the rela-
tively long t1/2 value of 1 ms for AcP under air with respect to 2–
4 ms in the absence of oxygen (Table 4) indicates that reaction (3)
is slow. If oxygen is fully excluded, several radical disproportion-
ation steps occur, one is self-termination (4) and another cross-ter-
mination (5).
3⁄
For TEA, electron transfer to P (20) leads to the phthalimide
radical anion Pꢀꢁ, which can be scavenged by oxygen, whereby P
is restored and oxygen is converted to hydroperoxyl/superoxide
ion radicals. For oxygen quenching of Pꢀꢁ of AcP an equilibrium
for reaction (30) can be excluded since the rate constant is close
to the diffusion limit. If oxygen is fully excluded, several radical
termination steps occur, one is self-reaction (40) and another
cross-termination (50). Two deprotonation steps (6a), (6b) take
place. The secondary reduction process (7) is illustrated in Fig. 6b
(inset) for BzP
3ꢂP þ DH2 ! Pꢀꢁ þ DHꢀ2þ
ð20Þ
ð30Þ
ð6aÞ
ð40Þ
ð50Þ
ð7Þ
P
ꢀꢁ þ O2 ! P þ Oꢀꢁ
5. Conclusion
2
DHꢀ2þ ! DHꢀ þ Hþ
Introduction of substituted benzoyl moieties at the nitrogen of
phthalimide brings about an entirely different photophysical
behaviour, whereas N-acetyl substitution barely alters the proper-
ties of the parent compound. The triplet state AcP is much longer
lived than that of BzP, and both are efficiently populated. In aceto-
nitrile, they react by H-atom or electron transfer with donors, such
as 2-propanol or TEA, respectively. The electron donating effect of
the three methoxy groups and the electron acceptor character of
the phthalimide moiety provide significant charge transfer charac-
ter for the lowest singlet-excited state of trimethoxyBzP leading to
rapid internal conversion with which intersystem crossing and
bimolecular reactions cannot compete. These factors make
trimethoxyBzP photochemically very stable and no photoproduct
is produced. In the case of nitroBzP, the excitation to the lowest
singlet-excited state takes place in the 4-nitrobenzamide moiety,
leading to similar photophysical and photochemical properties
typical for nitrobenzene derivatives. Photoreduction of nitroBzP
occurs via the radical anion of the nitroarene; a putative triplet
state as radical precursor was not detectable. Due to the formation
of the product which is strongly absorbing at 313 nm, secondary
photoproducts are imminently involved.
2ðHþ þ PꢀꢁÞ ! P þ PH2
ðHþ þ PꢀꢁÞ þ DHꢀ ! D þ PH2
P þ DHꢀ ! Pꢀꢁ þ DHþ
DHþ ! D þ Hþ
ð6bÞ
Note that up to 13 products have been detected in the photoreac-
tion of a naphthalimide with TEA [9]. A reaction scheme is pro-
posed, which was recently used to account for the photoinitiated
domino reactions of N-adamantyl(alkyl)phthalimides [26].
4.3. Photoinduced electron and H-atom transfer for nitroBzP
The significantly lower reduction potentials of nitroBzP,
E°0 = ꢁ1.19 and ꢁ1.54 V, compared to that of BzP, E°0 = ꢁ1.74 V,
indicate that the nitro group rather than the phthalimide entity
is reduced. The triplet state of nitroBzP, albeit not detected by flash
OH
N
OH
N
H
Acknowledgements
O
R
3
O
R
+DH2
2x
•
*
AcP/BzP
•
-AcP/BzP
-DH
We thank Professor Wolfgang Lubitz for his support, Dr. Klaus-
Dieter Warzecha for electrochemical measurements and Mr. Leslie
J. Currell and Mrs. Gabriele Schmitz for technical assistance and the
bilateral program between the Hungarian Academy of Sciences
and the Deutsche Forschungsgemeinschaft. The support by the
O
O
•
hν
+DH
AcP/BzP
other products
Scheme 1. Reduction of the triplet state of AcP or BzP via formation and
combination of the appropriate radicals.

L. Biczók, H. Görner / Chemical Physics 392 (2012) 10–15
15
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