Coordinative control of photoinduced electron transfer: Bulky carboxylates as molecular curtains

Article abstract of DOI:10.1039/b202815h

An intramolecular photoinduced electron transfer which takes place in a Zn^II polyamine complex can be interrupted through coordination of a bulky carboxylate anion, acting as a curtain.

Full text of DOI:10.1039/b202815h

Coordinative control of photoinduced electron transfer: bulky
carboxylates as molecular curtains
Irene Bruseghini, Luigi Fabbrizzi,* Maurizio Licchelli and Angelo Taglietti
Dipartimento di Chimica Generale, Università di Pavia, Viale Taramelli 12, 27100 Pavia, Italy.
E-mail: luigi.fabbrizzi@unipv.it
Received (in Cambridge, UK) 3rd March 2002, Accepted 8th May 2002
First published as an Advance Article on the web 23rd May 2002
An intramolecular photoinduced electron transfer which
takes place in a ZnII polyamine complex can be interrupted
through coordination of a bulky carboxylate anion, acting as
a curtain.
process to the proximate An fragment. A second spectrofluori-
metric titration was carried out on an identical solution, to
which 1 equiv. of ZnII had been added. An IF decrease was still
observed until pH ~ 4.5, where emission started to increase, as
a result of ZnII complexation to the tetramine (Fig. 1(a), black
triangles). In fact, upon complexation, amine lone pairs become
engaged in the coordination of the metal and are no longer
available for any PeT process: as a consequence, fluorescence is
restored.7 The [ZnII(1)]2+ complex shows a trigonal bipyramidal
stereochemistry, with a solvent molecule (i.e. H2O) occupying
the vacant axial position. The moderate fluorescence quenching
occurring at pH 4 7 has to be associated to the deprotonation of
the axially bound water molecule, which induces the occurrence
an intra-complex PeT process from the metal bound OH2 ion to
the An* subunit.8 The same titration experiment was repeated in
presence of 1 equiv. of triphenylacetic acid (TPA), and gave a
profile perfectly superimposable on that of the second experi-
ment (Fig. 1(a), open triangles).
Electron transfer, the most elementary chemical reaction, plays
a key role in processes of relevance to life.1 During the last
decade, a large number of model compounds for studies on
electron transfer has been provided. Models typically consist of
a photoexcitable fragment, exhibiting for instance acceptor
properties (A), covalently linked to an electron donor (D). In
some cases, the two subunits communicate electronically and
the efficiency of the D-to-A* photoinduced electron transfer
(PeT) can be tuned by varying the through-bond distance
between D and A.2 However, a few examples refer to systems
capable of direct through-space PeT,3 and even fewer examples
have been reported about control and modulation of PeT
processes through the complexation of guest molecules, which
intercalate between the donor and the acceptor.4
Then, the same titration set was carried out with system 2.
Titration of the metal-free solution of the D–A system 2 gave
similar results to those observed for 1, as shown in Fig. 1(b)
(shaded circles). Titration of 2 in presence of an equimolar
amount of ZnII produced a different profile than observed for 1:
in particular, IF kept decreasing even beyond pH = 4.5, where
the ZnII tetramine complex begins to form (Fig. 1(b), filled
circles). We ascribe this behaviour to the metal complexation,
which brings the photoexcited fragment An* and the DMA
subunits close enough to allow the occurrence of an intra-
complex through-space PeT process.5,6 In particular, An*
behaves as an electron acceptor and DMA as a donor. Notice
that the DMA-to-An* PeT process is less efficient that the
amine nitrogen-to-An* process, as indicated by the less
pronounced negative slope of the titration profile in presence of
ZnII, compared to the steeper profile obtained in absence of ZnII.
However, when the titration is performed in presence of TPA, at
pH 4 4.5 a sharp enhancement of fluorescence is observed, with
almost full recovery of the initial emission at pH = 7–8 (Fig.
1(b), open circles). This behaviour is associated to the formation
of the ZnII polyamine complex and to the simultaneous binding
In the past years, we have reported some examples of systems
in which a PeT process can be established by taking advantage
of metal–ligand interactions.5,6
We report here a DA system in which it is possible to address
and modulate a through-space intramolecular PeT process by
means of metal–ligand interactions, taking place in a ZnII
polyamine complex. In particular, we were interested to control
a PeT process from D to A, simply lowering, between D and A,
a molecular ‘curtain’ capable of interrupting the electron
transfer, and to drive this insertion through a coordinative
interaction. In this connection, we have synthesized the
polyfunctional system 2,† in which one anthracene substituent
(An) and two N,N-dimethylaniline (DMA) subunits are ap-
pended, through –CH2– spacers, to the terminal nitrogen atoms
of the tripodal tetramine tren. The precursor system 1,
containing the sole An substituent, was also considered for
comparative purposes.
The photophysical behavior of 1 and 2 in solution (MeOH–
H2O, 4+1, v/v) was first investigated by carrying out spectro-
fluorimetric investigations at varying pH. First, a solution of 1,
containing excess acid, was titrated with standard base. Under
acidic conditions (pH < 2.5), the strong and characteristically
structured emission band of An was observed. Then, on
increasing pH, the fluorescence intensity, IF, decreased until
almost complete quenching at pH 4 8 (Fig. 1(a), grey triangles).
Progressive fluorescence quenching has to be associated to the
fact that the deprotonation of the ammonium groups of 1 makes
the lone pair of each amine nitrogen atom available for a PeT
Fig. 1 Spectrofluorimetric pH-titrations of MeOH–H2O (4+1 v/v) solutions
of (a) 1 (grey triangles); 1 + 1 equiv. Zn2+ (black triangles); 1 + 1 equiv.
Zn2+ + 1 equiv. TPA (white triangles) and (b) 2 (grey circles); 2 + 1 equiv.
Zn2+ (black circles); 2 + 1 equiv. Zn2+ + 1 equiv. TPA (white circles).
1348
CHEM. COMMUN., 2002, 1348–1349
This journal is © The Royal Society of Chemistry 2002

Table 1 LogK values for adduct formation equilibria in MeOH, at 25 °C:
[ZnII(L)]2+ + carboxylate " [ZnII(L)(carboxylate)]+
of the TPA anion to the axial site of the metal complex. While
the bulky carboxylate coordination does not produce any
change in the emission of reference compound [ZnII(1)]2+, in
the case of the [ZnII(2)]2+ binding of the TPA anion brings the
triphenylic moiety amid An* and DMA fragments, thus
disturbing and almost completely removing the intramolecular
eT process. Molecular modelling studies (MM+) indicated that
binding of TPA to ZnII induces a divarication of the An and
DMA substituents, whose reciprocal distance is remarkably
increased. In particular the distance between C(10) of An and
the nitrogen atom of each DMA fragment increases from about
8 Å to 11.5–12.5 Å. However, in absence of crystal structural
data, the proposed stereochemical arrangements must be
considered as an only a hypothesis. The cascade binding of ZnII
and TPA anion to 2 is pictorially illustrated in Scheme 1.
logKa
Anion
[ZnII(2)]2+
[ZnII(1)]2+b
(5.3 ± 0.1)
(4.9 ± 0.2)
(4.7 ± 0.2)
Triphenylacetate
1-Adamantanecarboxylate
Benzoate
Cyclohexylcarboxylate
Acetate
5.6 ± 0.1 (5.5 ± 0.1)
4.5 ± 0.2
5.2 ± 0.1 (5.1 ± 0.2)
(4.4 ± 0.2)b
(4.4 ± 0.2)b
a As determined from fluorescence and UV/Vis (in parenthesis) titrations.
b No emssion changes were detected.
between the anion and the substituents on the polyamine
framework.
By contrast, anion bulkiness has a profound effect in
controlling the efficiency of the intramolecular electron transfer
in [ZnII(2)]2+: in particular, the anion bearing the bulkiest
substituent, TPA, prevents the occurrence of a PeT process from
DMA to An*, so that the excited fluorophore can undergo its
normal radiative decay. This behaviour is ascribed to the fact
that the sterically hindering TPA anion fills the cavity which
interfaces An* and DMA substituents, preventing any occa-
sional contact and extruding solvent molecules. Moreover, the
metal bound TPA anion, even if containing p-fragments, is not
favourably oriented for being permeable to electrons, thus
behaving as an insulating material.
This work has demonstrated that the powerful fluorescent
signal can be switched ON by insulating D and A with an
interfacing molecular fragment, the process being driven by a
metal–ligand interaction. This work has been supported by the
European Union (RTN Molecular Level Devices and Machines)
and by the Italian Ministry of University and Research (PRIN
2001-Dispositivi Supramolecolari).
Scheme 1
In order to corroborate the proposed mechanism, we carried
out further spectrofluorimetric titration experiments, by adding
a variety of carboxylate anions to an MeOH solution of the
[ZnII(2)]2+ complex. Titration profiles are reported in Fig. 2.
On titration with acetate and cyclohexylcarboxylate, the
solution remained poorly fluorescent (open and filled circles in
Fig. 2). A moderate IF increase was observed on titration with
1-adamantanecarboxylate and benzoate (white and black
squares).
On the other hand, titration with the TPA anion (filled
triangles) induced a significant increase of the anthracene
emission. It should be noted that none of the considered
carboxylates induced any IF increase when added to a
methanolic solution of the reference system [ZnII(1)]2+. Non-
linear least-squares analysis of the titration profiles (spectro-
fluorimetric and/or spectrophotometric) confirmed the forma-
tion of 1+1 carboxylate–complex adduct, whose formation
constants K are reported in Table 1. LogK values range between
4.4 and 5.6 and do not seem to correlate with the bulkiness of the
anion substituent, thus excluding steric repulsive effects
Notes and references
† Preparation of 2: a solution of 4-dimethylaminobenzaldehyde (5.1 mmol
in 40 ml of MeOH) was added dropwise over 2 h under magnetic stirring to
a solution of 1 (2.5 mmol in 60 ml of MeOH), whose synthesis was reported
elsewhere.5 The solution was kept at 40 °C for 36 h. Then NaBH4 was
carefully added in small portions and the solution was heated at 50 °C
overnight. The solvent was then removed and the resulting sticky solid was
suspended in 70 mL of water. The aqueous phase was extracted with
CH2Cl2 (3 3 30 mL). The organic phase was dried with MgSO4 and the
solvent was distilled off, giving L as a dark orange oil, which was washed
with several portions of diethyl ether. Yield: 75%. ESMS: m/z (%) 603.4
(100) (M + H+).
1 Electron Transfer in Chemistry, ed. V. Balzani, Wiley, VCH, 2001.
2 K. D. Jordan and M. N. Paddon Row, Chem. Rev., 1992, 92, 395; K. A.
Jolliffe, T. D. M. Bell, K. P. Ghiggino, S. J. Langford and M. N. Paddon
Row, Angew. Chem., Int. Ed., 1998, 37, 916; H. Han and M. B. Zimmt,
J. Am. Chem. Soc., 1998, 120, 8001; T. D. M. Bell, K. A. Jolliffe, K. P.
Ghiggino, A. M. Oliver, M. J. Shephard, S. J. Langford and M. N. Paddon
Row, J. Am. Chem. Soc., 2000, 122, 10661.
3 T. Arimura, S. Ide, Y. Suga, T. Nishioka, S. Murata, M. Tachiya, T.
Nagamura and H. Inoue, J. Am. Chem. Soc., 2001, 123, 10744 and
references therein.
4 J. N. H. Reek, A. E. Rowan, M. J. Crossley and R. J. M. Nolte, J. Org.
Chem., 1999, 64, 6653.
5 G. De Santis, L. Fabbrizzi, M. Licchelli, A. Poggi and A. Taglietti,
Angew. Chem., Int. Ed. Engl., 1996, 35, 202.
6 L. Fabbrizzi, M. Licchelli, P. Pallavicini and A. Taglietti, Inorg. Chem.,
1996, 35, 1733; M. Di Casa, L. Fabbrizzi, M. Licchelli, A. Poggi, A.
Russo and A. Taglietti, Chem. Commun., 2001, 825.
7 A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley, C.
P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97,
1515.
8 L. Fabbrizzi, M. Licchelli, A. Perotti, A. Poggi, G. Rabaioli, D. Sacchi
and A. Taglietti, J. Chem. Soc., Perkin Trans. 2, 2001, 2108.
Fig. 2 Spectrofluorimetric titrations with carboxylates of a MeOH soloution
of [ZnII(2)]2+·. Anions added: acetate (2), cyclohexylcarboxylate (5),
benzoate (-), 1-adamantanecarboxylate (8), TPA (:).
CHEM. COMMUN., 2002, 1348–1349
1349