Photoinduced intramolecular charge shift reaction in ammonium N-(3,5-dinitrobenzoyl)-α-phenylglycinate adducts

Article abstract of DOI:10.1016/j.jphotochem.2010.05.005

Ammonium N-(3,5-dinitrobenzoyl)-α-phenylglycinate adducts were synthesized and characterized by using different protonated amines as counter-ion: NH₄⁺, (CH₃-CH₂-) ₂NH₂⁺, (CH₃-CH₂-CH ₂)NH₃⁺, (CH₃-CH₂-CH ₂-CH₂-)NH₃⁺ and (CH ₃-CH₂-)₃NH⁺. A photochemical process was observed under ultraviolet (λ_exc, 254 nm) or solar irradiation, both in solid state and in solution: DMSO, acetone or acetonitrile. 3,5-Dinitrobenzene and carboxylate groups, separated by an N-benzylamide bridge, are present in these adducts, acting as electron acceptor and electron donor, respectively. Spectroscopic analyses (NMR, IR and UV-vis) suggest a photoinduced intramolecular electron transfer. A first-order photochemical kinetics was proposed in DMSO/(n-propylammonium N-(3,5-dinitrobenzoyl)-α- phenylglycinate) solution; such behavior was similar for all adducts studied, probably due to total salt dissociation in solution. In the solid state, however, electron transfer process efficiency is directly proportional to Lewis base (amine) strength of the adduct counter-ion. Decarboxylation is observed after the irradiation process, giving rise to a σ-adduct intermediate, and subsequent formation of benzaldehyde and 3,5-dinitrobenzamide degradation products.

Full text of DOI:10.1016/j.jphotochem.2010.05.005

Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 108–111
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Short Note
Photoinduced intramolecular charge shift reaction in ammonium
N-(3,5-dinitrobenzoyl)-␣-phenylglycinate adducts
Elaine C. de Souza Coelho, Aderivaldo P. da Silva, Daniela M.A. Ferraz Navarro, Marcelo Navarro∗
Universidade Federal de Pernambuco, CCEN, Departamento de Química Fundamental, 50670-901, Recife, PE, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Ammonium N-(3,5-dinitrobenzoyl)-␣-phenylglycinate adducts were synthesized and characterized
by using different protonated amines as counter-ion: NH4+, (CH3–CH2–)2NH2+, (CH3–CH2–CH2)NH3
,
+
Received 7 December 2009
Received in revised form 6 May 2010
Accepted 8 May 2010
(CH3–CH2–CH2–CH2–)NH3 and (CH3–CH2–)3NH+. A photochemical process was observed under ultra-
violet (ꢀexc, 254 nm) or solar irradiation, both in solid state and in solution: DMSO, acetone or acetonitrile.
3,5-Dinitrobenzene and carboxylate groups, separated by an N-benzylamide bridge, are present in these
adducts, acting as electron acceptor and electron donor, respectively. Spectroscopic analyses (NMR, IR and
UV–vis) suggest a photoinduced intramolecular electron transfer. A first-order photochemical kinetics
was proposed in DMSO/(n-propylammonium N-(3,5-dinitrobenzoyl)-␣-phenylglycinate) solution; such
behavior was similar for all adducts studied, probably due to total salt dissociation in solution. In the solid
state, however, electron transfer process efficiency is directly proportional to Lewis base (amine) strength
of the adduct counter-ion. Decarboxylation is observed after the irradiation process, giving rise to a ␴-
adduct intermediate, and subsequent formation of benzaldehyde and 3,5-dinitrobenzamide degradation
products.
+
Available online 15 May 2010
Keywords:
Photoinduced
Electron transfer
Charge shift
N-(3,5-dinitrobenzoyl)-␣-phenylglycinate
␴-Adduct
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
ric effects, configuration and energetics of the bridge. In solution,
the solvent plays a role in the ET process due to polarity variation
The electron transfer (ET) mechanism is a very common event
related to photoinduced processes. Charge transfer (or electron
donor–acceptor) complexes (CTC) have been extensively studied
in fields from ranging biological to materials science [1–3]. Nor-
mally, CTC formation occurs through electron transfer between
vacated HOMO or LUMO orbitals of the donor and acceptor groups
[4]. Intramolecular CTC should occur when a molecule presents
the following fundamentals: an electron donor group (D), an elec-
tron acceptor group (A) and a bridge (B) bonding these two groups
through ␴ and/or ␲ bonds [4,5]. In specific cases, D or A (or both)
can be charged, given a long-lived charge migration species. The
charge transfer process observed in 9-arylacridinium ions has been
described as an interesting example [6,7].
following the electron transfer between species involved [4].
Aromatic nucleophilic substitution reactions have been fre-
quently studied in the presence of a dinitrobenzene electron
acceptor system [10,11]. A CTC intermediate has been proposed
in such reaction mechanisms, formed through a single electron
transfer (SET). In a simplified fashion, we can state that CTC sta-
bility involving nitro-aromatic compounds can be controlled by
the nucleophilicity of the electron donor group, in which strong
nucleophilic groups such as OH−, CN− and MeO−, promote an
intermediate CTC formation, but the equilibrium is displaced to
nucleophilic substitution products [10,11]. In the presence of
amines, CTC formation is favored due to the weak nucleophilic
nature of the amine [12,13]. In other cases, the CTC only can be
obtained under excitation energy such as UV light [1–5,14–20].
Herein, we report the synthesis of stable organic salts derived
from N-(3,5-dinitrobenzoyl)-␣-phenylglycine (DNP) and some
amines [21]. These adducts contain electron donor and acceptor
groups bonded through an N-benzylamide bridge, as displayed in
Scheme 1.
−·
+·
hv
D − B − A −→ (D − B − A)∗ → D − B − A
It has been proposed that the size and nature of the bridge may
control the ET energy, intra- or intermolecular spatial interaction,
environment arrangement and state of the process (solid and/or
solvated) [8,9]. Thus, a large amount of information on the ET pro-
cess has been described, regarding distance dependence, symmet-
2. Experimental
All organic reagents and solvents were purchased from Sigma
Aldrich or Acros. UV–vis and kinetic experiments were carried out
in DMSO, previously dried in the presence of calcium hydride and
∗
Corresponding author. Tel.: +55 81 21267460; fax: +55 81 21268442.
E-mail address: navarro@ufpe.br (M. Navarro).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.05.005

E.C. de Souza Coelho et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 108–111
109
Scheme 1. Ammonium N-(3,5-dinitrobenzoyl)-␣-phenylglycinate adduct struc-
ture.
distilled under vacuum. Nuclear magnetic resonance spectra 1H
NMR and 13C NMR were recorded in a Varian Unity Plus (300 MHz)
spectrometer using acetone-d6, DMSO-d6, or CDCl3 as solvent.
UV–vis spectra were registered in a Varian Cary 50 (Xenon lamp)
or PerkinElmer Lambda 6 (tungsten and deuterium lamp) spec-
trophotometers in the 190–900 nm region, in the presence of the
solvent as background. All measurements were registered in 3 mL
quartz cuvettes. Infrared spectra were registered using a Bruker
FTIR (model IFS66) (1% sample KBr pallets). The decomposition
point was determined on Electrothermal 9100 melting point equip-
ment. Elemental analyses were performed in Carlo Erba (model
EA 1110) equipment. Gas chromatogram/mass spectra were taken
(Shimadzu model QP5050) with a 30-m capillary column, using
a 60–200 ◦C temperature range (10 ◦C min−1). Comparisons with
authentic sample were performed to identify reaction products and
reagents and confirmed by GC/MS. Photoinduced reaction experi-
ments were carried out in a black box. All measurements in solution
samples were performed in 3 mL quartz cuvettes using a 6-W UV
lamp (low pressure Hg, ꢀ = 254 nm) as radiation source. Solid state
sample measurements were performed in round pastil shape con-
taining DNP salts powder under direct irradiation. Syntheses of
adducts are described in the Supplementary data.
Fig. 1. (a) UV–vis spectrum of 5.0 × 10−3 mol L−1 TDNP (DMSO solution) acquired
after successive UV (ꢀexc, 254 nm) irradiations (5 min) in Ar atmosphere. (b) TDNP
color change in solid state after 30 s of UV (ꢀexc, 254 nm) irradiation.
(methyl N-(3,5-dinitrobenzoyl)-␣-phenylglycinate) are not pho-
tosensitive, indicating that the photoinduced process should not
occur through the unpaired electrons of the amide nitrogen [19,20]
or between aromatic groups of the DNP structure. Therefore, the
ET process probably occurs between the carboxylate (donor) and
3,5-dinitrobenzene (acceptor) groups (Scheme 1), in which the N-
benzylamide bridge must support the necessary conformation for
the photoinduced ET. The photochemical effect occurring in the
solid state strongly indicates an intramolecular effect [4].
The UV light irradiation of 5.0 × 10−3 mol L−1 DNP salt solutions
(PDNP, TDNP, DDNP, ADNP, BDNP) dissolved in DMSO, at 5 min
intervals and acquisition of the respective absorbance at 555 nm,
reveals a similar kinetic behavior for CTC formation (Fig. 2), indi-
cating a similar dissociation of the adducts studied in this medium.
Therefore, in solution, the electron transfer should be occurring
only in the anionic portion of the molecule.
3. Results and discussion
The respective ammonium salts were synthesized by
DNP/amine acid–base reaction, in diethyl ether medium:
+
+
NH4
(ADNP),
(CH3–CH2–)2NH2
(DDNP),
(CH3–CH2–
+
+
CH2–)NH3 (PDNP), (CH3–CH2–CH2–CH2–)NH3 (BDNP) and
(CH3–CH2–)3NH+ (TDNP). Satisfactory yields were observed in
the synthesis of the related salts and the respective decomposi-
tion temperatures demonstrate a thermal stability below 130 ◦C
(Table 1).
UV irradiation (ꢀexc, 254 nm) of the related adducts, at the ␲-␲*
transition region of the aromatic groups (ꢀ ∼ 260 nm), both in solu-
tion (DMSO, acetonitrile and acetone) and solid state, causes a color
change from white to purple (Fig. 1), given rise to two new bands
at ꢀ = 406 and 555 nm, and a shoulder at ꢀ = 640 nm, thereby indi-
cating a photochemical phenomenon. DNP and its respective ester
Table 1
Reaction yields, melting and decomposition points for DNP and respective salts.
Entry
Compound
Yield (%)
Decomposition temperature (◦C)
1
2
3
4
5
6
DNP
88
93
74
89
84
66
211–213a
189
170
180
175
ADNP
DDNP
PDNP
BDNP
TDNP
Fig. 2. Irradiation of 5.0 × 10−3 mol L−1 DNP salt solutions (PDNP, TDNP, DDNP,
ADNP, BDNP) in DMSO at 5 min intervals (ꢀexc = 254 nm) and data acquisition at
555 nm.
140
a
Melting point.

110
E.C. de Souza Coelho et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 108–111
Scheme 2. Frontier molecular orbital of the photoinduced ET between nearly
non-interacting D and A groups. Y+ = ammonium counter-ion: in excitation of A
(3,5-dinitrobenzene group), one electron is promoted to LUMO, followed by D (car-
boxylate group) ET to HOMO of A and the CTC is formed.
Fig. 3. Photoinduced reaction monitoring for PDNP solutions (1.25 × 10−3
,
2.98 × 10−3
,
5.0 × 10−3 mol L−1
)
in DMSO: irradiation at 5 min intervals
(ꢀexc = 254 nm) at 555 nm.
Essays were performed to evaluate the kinetics of the colored
intermediate in DMSO solution (Fig. 3). Three different concentra-
tions of PDNP in DMSO (1.25, 2.98 and 5.00 × 10−3 mol L−1) were
irradiated (ꢀexc, 254 nm) using the same procedure described in
Fig. 2. Analysis of the linear coefficient of the curves and cor-
Fig. 4. Color change after 10 min irradiation (ꢀexc = 254 nm) of: ADNP, BDNP, PDNP,
DDNP and TDNP in solid state.
relation of its respective values (1.064, 2.607 and 4.085 × 10−4
)
indicates a first-order behavior for the photoinduced reaction.
However, the colored intermediate undergoes a slow degradation,
due absorbance decrease at the end of the irradiation time [22],
what explains the shape and kinetics observed in the Fig. 3.
(D−) that becomes a radical (D•), after the ET process. Therefore, the
radical anion A−• is neutralized by the ammonium counter-ion (Y+)
and the electronic structure of the CTC ground state corresponds to
a radical anion/radical/cation: [A−•–D•]Y+ [4]. Such mechanism can
be classified as a charge shift as proposed by Kakitami and Mataga
[26].
In DMSO, the kinetics of the ET process was similar for all DNB
salts studied. The counter-ion effect may be neglected likely due the
complete dissociation of the salt in the solution. Different behavior
Therefore, an exciplex intramolecular photoinduced reaction
mechanism should occur, involving an excited state, after UV
irradiation of electrons of A (acceptor group, 3,5-dinitrobenzene)
(Scheme 2). One electron is promoted to LUMO of A, followed by
electron transfer from HOMO of D (donor group, carboxylate) to
the vacant HOMO 1e- of A* and the CTC is formed [23–25]. Thus,
we believe that both reaction partners (A and D) are built into
one molecule, but barely interact. If an electron transfer from D
to A is energetically feasible in the excited state, the product of
such an intramolecular ET reaction is a charge-separated species:
D+•–B–A−•. In the specific case of DNP salts, the D group is an anion
was observed in the solid state and CTC formation was favored in
∼
the following order: TDNP > DDNP > PDNP > BDNP ADNP (Fig. 4).
=
In the ammonium salt structure, the proton is disputed by the
amine and carboxylate groups, which are two different bases
Scheme 3. Mechanism proposed to photodegradation of ammonium N-(3,5-dinitrobenzoyl)-␣-phenylglycinate salts.

E.C. de Souza Coelho et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 108–111
111
(Scheme 1). Therefore, the photoinduced process is favored in the
Appendix A. Supplementary data
presence of stronger Lewis bases (amines), releasing the carboxy-
late pair of electrons for the ET process. Amine basicity works as a
HOMO energy modulator of the carboxylate group in the solid state
(Scheme 2).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.05.005.
It was not observed reversibility after the photoinduced ET
process, neither under visible light irradiation nor heating of the
colored intermediate formed [18], therefore, the photochemical
electron transfer process observed in DNP salts cannot be classified
as a photochromic process [4]. Conventional techniques of analy-
sis were used to evaluate the stability of the CTC intermediate and
identify possible decomposition products. No spectral modification
was observed for NMR analysis after several hours of irradiation
probably due the low quantum yield and elevated extinction coef-
ficient of the observed intermediate, however, the 1H NMR and
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This work was supported by CNPq-INCT/INAMI, CAPES and
PETROBRAS, Brazil.