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[求助] 文献翻译:荧光分子开关

A Zinc(II)-Driven Intramolecular Photoinduced
Electron Transfer
Luigi Fabbrizzi,* Maurizio Licchelli,
Piersandro Pallavicini, and Angelo Taglietti
Dipartimento di Chimica Generale, Universita` di Pavia,
via Taramelli 12, I-27100 Pavia, Italy
ReceiVed April 26, 1995
Two-component supramolecules in which a donor (D) and
an acceptor (A) fragment are covalently linked represent an ideal
system for investigating a primary chemical event: electron
transfer. If D (or A) is fluorescent, any photoinduced electron
transfer may be conveniently followed through the quenching
of the fluorescence emission.1 The spacer connecting D and A
plays a crucial role: when it is rigid, the intramolecular electron
transfer process takes place Via a through-bond interaction,
which may be efficient over distances as large as 20 Å.2 When
the spacer is flexible (e.g. an aliphatic chain), the two-component
system may occasionally fold and bring the interacting fragments
into direct contact, according to a statistically controlled process,
allowing the electron transfer process to take place. However,
folding can be favored and properly addressed by making use
of a template. The template is a chemical entity (a molecule,
an ion) able to interact through noncovalent (and reversible)
interactions with the D-A system. If the interaction possesses
some elements of selectivity, the D-A system can be viewed
as a sensor for the template species, signalling the occurrence
of the receptor-substrate binding through fluorescence quenching.
3
In this work we wished to investigate the role of 3d metal
ions as templates for D-A systems and we used as a spacer an
aliphatic polyamine, i.e. a flexible fragment which can coordinate
transition metals. The chosen donor fragment was dimethylaniline
(dma, E°dma+/dma ) 0.78 V Vs SCE, in MeCN
solution), whereas the acceptor and fluorescent subunit was
anthracene (an, E°an/an-)-1.93 V Vs SCE, in MeCN solution).
The energy of the photoexcited state *an (E0-0 ) 3.1 eV) is
large enough to exceed the e[E°an/an- - E°dma+/dma] energy
difference and to guarantee the occurrence of the photoinduced
electron transfer (PET) process on a thermodynamic basis
(¢G°ET ) -E0-0 + e(E°an/an- - E°dma+/dma) ) -0.4 eV). The
an and dma fragments were linked by the tetraamine bridge
-CH2NH(CH2)2NH(CH2)3NH(CH2)2NHCH2-, according to the
synthetic pathway outlined in Scheme 1. The method involves
the following: (i) the Schiff base condensation between
9-anthracenealdehyde and the appropriate linear tetraamine,
present in a 5-fold excess, in order to minimize disubstitution,
followed by hydrogenation of the CdN double bond with
sodium borohydride; (ii) Schiff base condensation at the opposite
-NH2 group of 4-(dimethylamino)benzaldehyde, according to
a 1:1 stoichiometry, and subsequent hydrogenation, to give the
D-A system 2. The procedure appears particularly convenient
and can be generally used to connect any couple of subunits of
varying nature, provided the corresponding aldehyde derivatives
are available. The spacer considered corresponds to the
quadridentate ligand 2.3.2-tet, whose coordinating tendencies
toward 3d metal ions are well documented.4,5 The intermediate
1 was also investigated, for comparative purposes. As a
templating agent, we examined first the ZnII ion, which is nonredox-
active and should not quench *an by an electron transfer
process.
The photophysical behavior of 1 and 2 in solution (MeCN/
water 80:20, v/v) was investigated by carrying out spectrofluorimetric
investigations at varying pH. The reference system
1 was considered first and a solution of 1, containing excess
acid, was titrated with standard base. Under acidic conditions
(pH < 3), the strong and characteristically structured emission
band of anthracene was observed. Fluorescence intensity
decreased until complete quenching during the addition of the
second and third equivalent of base (after the excess acid had
been neutralized). This would indicate that, in the course of
the deprotonation of the tetraammonium fragment, the second
and third proton are released from the two ammonium groups
closest to the anthracene subunit. In fact, following deprotonation,
the lone pair on the nitrogen atom is available for an
electron transfer process to the proximate photoexcited fluorophore
*an, which causes a nonradiative decay and induces
fluorescence quenching. Photoinduced electron transfer in
derivatives of anthrylamines is a well-characterized phenomenon
and constitutes the basis of pH switching of fluorescence.6 The
most efficient quenching effect should be provided by the
secondary amine group adjacent to the anthracene moiety.
Figure 1 illustrates the pH dependence of the fluorescence
intensity, IF, obtained from the titration experiment. From the
IF vs pH plot, a pKA value of 3.9 is assigned to the more acidic
photoactive ammonium group and a pKA value of 7.8 to the
less acidic photoactive ammonium group. Noticeably, such
values correspond to the pKA2 and pKA3 values obtained for 1
(1) Kavarnos, G. J. Fundamentals of Photoinduced Electron Transfer;
VCH Publishers Inc.: New York, 1993.
(2) Winkler, J. R.; Gray, H. B. Chem. ReV. 1992, 92, 369.
(3) Valeur, B.; Bourson, J.; Pouget, J. In Fluorescent Chemosensors for
Ion and Molecule Recognition; Czarnik, A. W., Ed.; ACS Symposium
Series 538; American Chemical Society: Washington, DC, 1993.
(4) Weatherburn, D. C.; Billo, E. J.; Jones, J. P.; Margerum, D. W. Inorg.
Chem. 1970, 9, 1557.
(5) Fabbrizzi, L.; Barbucci, R.; Paoletti, J. J. Chem. Soc., Dalton. Trans.
1972, 1529.
(6) Bissell, R. A.; de Silva, A. P.; Guranatne, H. Q. N.; Lynch, P. L. M.;
Maguire, G. E. M.; McCoy, C. P.; Sundanayake, K. R. A. S. Top.
Curr. Chem. 1993, 168, 223.
Scheme 1
Inorg. Chem. 1996, 35, 1733-1736 1733
0020-1669/96/1335-1733$12.00/0 © 1996 American Chemical Society
through independent potentiometric titration experiments. A
further spectrofluorimetric titration experiment was carried out
on a solution as before, but which contained also 1 equiv of
ZnII. Figure 2 shows the corresponding titration profile.
Noticeably, a fluorescence intensity decrease was observed also
in the present case, whose profile superimposed on that of the
metal-free solution. However, at pH ) 4.5, IF stopped decreasing,
then increased to reach almost the original value. The first
part of the profile reflects the deprotonation of the polyammonium
fragment, with the consequent electron transfer from
one of the proximate amine groups to the photoexcited anthracene
fragment of 2. The following fluorescence increase
should be ascribed to the fact that, at pH ) 4.5, the polyamine
spacer begins to coordinate the ZnII ion, completing metal
complexation at pH g 6. With complexation, the amine lone
pairs are coordinated to the metal center, making their electrons
unavailable for any electron transfer process, which restores
fluorescence. Anthracene fluorescence enhancement following
ZnII-amine interaction has been previously observed with a series
of anthracenyl-substituted polyaza macrocycles.7
Titration of a metal-free solution of the D-A system 2 gave
results similar to those for 1. In particular, the IF Vs pH profile
is nearly coincident with that observed for system 1 (see Figure
1). It should be noted that, in strongly acidic conditions,
electrostatic repulsions between ammonium groups would
stretch the spacer such that the an and dma subunits are well
separated. A distance of 20.6 Å from the aniline nitrogen atom
to the C(9) atom of the anthracene moiety, i.e. that linked to
the spacer, was estimated using MM+ modeling. The spacer
should probably remain extended also in the course of the
deprotonation of the second and third ammonium group, thus
preventing the molecular folding and a through-space interaction
between the *an and dma fragments. Thus, the quenching
mechanism observed with a metal-free solution of 2 should be
the same as for the reference system 1 and should involve an
electron transfer from the proximate secondary amine groups
to the fluorophore.
Conversely, titration of 2 in presence of equimolar ZnII
produced a quite different profile than for 1. In particular, IF
did not stop decreasing, as for reference system 1, but constantly
diminished until complete quenching (see Figure 2).
Such a behavior has to be associated to the coordination of
the tetraamine bridge around the ZnII center, which makes the
D-to-A intramolecular electron transfer process feasible from a
structural point of view. Such a statement is based on the
following points: (i) as the metal-ligand interaction prevents
any electron release from the proximate amine groups, quenching
has to be attributed to the transfer of an electron from dma
to *an; (ii) ZnII tetraamine complexes usually adopt a tetrahedral
coordination; (iii) molecular modeling shows that wrapping of
2 around the metal center, when a tetrahedral mode of
coordination of the tetraamine spacer is imposed, brings the D
and A moieties close enough to interact. Figure 3 shows the
structure of the [ZnII(2)]2+ adduct, as obtained through MM+
modeling. The distance between the aniline nitrogen atom and
the C(9) atom of anthracene is 5.1 Å (compared to 20.6 Å in
the extended metal-free system 2). Shorter distances and more
favorable orientations of the two aromatic moieties of 2 are
obtained by rotating the ó bonds connecting the terminal nitrogen
atoms of the tetra-amine spacer with the carbon atoms of the
-CH2- groups linked to the an and dma subunits. Thus, the
ZnII ion favors the electron transfer process responsible for
(7) Akkaya, E. U.; Huston, M. E.; Czarnik, A. W. J. Am. Chem. Soc.
1990, 112, 3590.
Figure 1. pH dependence of the fluorescence intensity of 1 (triangles)
and 2 (diamonds) in an MeCN/H2O (80:20 v/v) solution.
Figure 2. pH dependence of the fluorescence intensity of 1 (triangles)
and 2 (diamonds), in presence of equimolar ZnII, in an MeCN/H2O
(80:20 v/v) solution. In the case of the reference system 1, ZnII
coordination (pH g 5) prevents the electron transfer from the proximate
amine groups of the spacer and restores fluorescence of the anthracene
subunit. In the case of 2, ZnII coordination brings the dma and *an
fragments close enough to allow electron transfer, which quenches the
fluorescence.
Figure 3. CPK model of the [ZnII(2)]2+ adduct, as obtained through
MM+. The distance between the C(9) atom of the anthracene subunit
and the nitrogen atom of the dimethylaniline fragment is 5.1 Å, to be
compared with the 20.6 Å distance observed in the extended molecule
2.
1734 Inorganic Chemistry, Vol. 35, No. 6, 1996 Notes
quenching, by imposing a favorable stereochemical arrangement
to the D-A system 2.
It may be of some interest to verify the sensing properties of
1 and 2 toward the ZnII ion. In this connection, an MeCN/H2O
solution of 1, adjusted to pH ) 8.1 with morpholine buffer,
was titrated with a standard ZnII solution. Following metal
addition, the fluorescence was progressively restored, to reach
a plateau value at 1 equivalent (see Figure 4). In this sense,
the anthracene substituted tetra-amine 1 represents a further
efficient chemosensor for ZnII of the series introduced by
Czarnik.8 On the other hand, titration with ZnII of a buffered
solution of 2 produced only a slight increase of the fluorescence
intensity and a very modest constant value was obtained after
the addition of 1 equivalent of metal (see Figure 4), discouraging
any use of 2 as a ZnII sensor. However, the equivalence point
is interesting, as it exactly indicates a switching of the
mechanism of fluorescence quenching: from amine-to-anthracene
electron transfer to dimethylaniline-to-anthracene electron
transfer.
The above template approach seems quite general and should
work with any D/A couple, provided that the PET process is
thermodynamically allowed. In this connection, we considered
the D-A system 3, in which the organic donor fragment dma
had been replaced by ferrocene (Fc). Spectrofluorimetric
titration experiments on an MeCN/H2O solution of 3, in the
absence and presence of equimolar ZnII, gave analogous results
as for 2. In particular, the fluorescence of the ZnII containing
acidic solution of 3 was fully quenched with base addition. This
behavior is not surprising, if one considers that Fc is a better
donor than dma (E°Fc+/Fc ) 0.10 V vs SCE in MeCN), which
makes the Fc-to-*an electron transfer process more favorable
(¢G°ET ) -1.1 eV). Thus, also in the present case, ZnII
coordination by the tetra-amine spacer brings the organometallic
donor subunit close enough to the anthracene fragment to permit
electron transfer.
Finally, we wished to investigate other transition metal ions
as further templates for addressing intramolecular electron
transfer in D-A systems like 2. In particular, 3d metals would
appear as versatile candidates because of their capability of
imposing many different stereochemical arrangements, depending
upon their electronic configuration. Spectrofluorimetric
titration of a solution of either 1 or 2 in aqueous MeCN, in
presence of an equimolar amount of CuII, caused a sharp
fluorescence quenching in the pH range 2.5-3.5. At these pH
values, the CuII tetraamine complexes form, which are remarkably
more stable than the corresponding ZnII complexes of 1
and 2. The fact that fluorescence quenching takes place also
with the reference system 1 would indicate that the nonradiative
deactivation of *an occurs Via a direct interaction between the
fluorophore and the metal center. Quenching of *an can take
place through either an energy transfer or an electron transfer
mechanism. On one side, CuII possesses a half-filled 3d level
of low energy (x2 - y2), which can be involved in an energy
transfer process of the Dexter type (double electron exchange).
9,10 On the other hand, the *an-to-CuII electron transfer
process is favored from a thermodynamic point of view (¢EET
)-1.1 eV). In any case, the proximate transition metal center
deactivates *an through a very fast and efficient mechanism,
which, in system 2, precludes the slower intramolecular D-to-A
electron transfer process. NiII has the same effect as CuII,
quenching fluorescence of both 1 and 2 in the 4-6 pH range.
Experimental Section
General Methods. Emission spectra were taken on a Perkin-Elmer
LS-50 luminescence spectrometer (excitation wavelength 364 nm;
maximum emission intensities at 415 nm) and were all uncorrected
for instrumental response. Spectrofluorimetric titrations were performed
on acetonitrile/water (4:1) solutions (50 mL, 5  10-4 M), using
standard HClO4, NaOH, and MII(ClO4)2 solutions.11 The pH scale was
calibrated prior to each titration experiment in aqueous acetonitrile,
through the Gran method.12 Mass spectra were obtained with a Finnigan
TSQ700 instrument.
Materials. Anthracene-9-carbaldehyde (Fluka), ferrocene-1-aldehyde
(Aldrich) and 4-(dimethylamino)benzaldehyde (Aldrich) were used
without further purification. N,N¢-Bis(2-aminoethyl)propane-1,3-diamine
(2.3.2-tet) was prepared as described for the analogous tetraamine
3.2.3-tet,13 distilled at reduced pressure (125 °C; 5  10-2 torr) and
stored over NaOH in the refrigerator.
N-(2-Aminoethyl)-N¢-{2-[(anthracen-9-ylmethyl)amino]ethyl}-
propane-1,3-diamine, 1. 2.3.2-tet (3.2 g, 20 mmol) and anthracene-
9-carbaldehyde (0.81 g, 4 mmol) were dissolved in ethanol (50 mL)
and allowed to react for 36 h at room temperature. Then NaBH4 (1.7
g, 45 mmol) was added portionwise and the resulting solution warmed
at 50 °C for 4 h. Ethanol was distilled off under reduced pressure, the
residue was treated with water (40 mL) and extracted with dichloromethane
(3  30 mL). After the extract was dried over MgSO4 and
solvent removed at the rotary evaporator, a semisolid residue formed,
which was characterized as its tetrahydrobromide salt.
1â4HBr. 1 (0.5 g, 1.43 mmol) was dissolved in ethanol and treated
with excess aqueous 48% HBr. A yellowish precipitate formed, which
was recrystallized from ethanol and dried under Vacuum. Yield: 73%.
Anal. Calcd for C22H34N4Br4 (MR 674.15): C, 39.20; H, 5.08; N, 8.31.
Found: C 39.53; H, 4.95; N, 8.12.
(8) Huston, M. E.; Haider, K. W.; Czarnik, A. W. J. Am. Chem. Soc.
1988, 110, 4460.
(9) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis
Horwood: Chichester, England, 1991.
(10) Suppan, P. Chemistry and Light, Royal Society of Chemistry:
Cambridge, U.K., 1994; p 66.
(11) Fabbrizzi, L.; Licchelli, M.; Pallavicini, P.; Perotti, A.; Sacchi, D.
Angew. Chem., Int. Ed. Engl. 1994, 33, 1975.
(12) Gran, G. Analyst 1952, 77, 661.
(13) Barefield, E. K.; Wagner, F.; Herlinger, A. W; Dahl, A. R. Inorg.
Synth. 1975, 16, 220.
Figure 4. Titration of an MeCN/H2O (80:20) solution of either 1
(diamonds) or 2 (triangles), buffered at pH ) 8.1, with ZnII.
Notes Inorganic Chemistry, Vol. 35, No. 6, 1996 1735
N-{2-[(Anthracen-9-ylmethyl)amino]ethyl}-N¢-[2-(((4-dimethylamino)-
benzyl)amino)ethyl]propane-1,3-diamine, 2. 1 (0.5 g, 1.43
mmol) and 4-(dimethylamino)benzaldehyde (0.213 g, 1.43 mmol) were
allowed to react in ethanolic solution (30 mL) for 36 h at room
temperature. NaBH4 (0.57 g. 15 mmol) was added portionwise and
the resulting solution warmed at 50 °C for 4 h. After ethanol was
distilled off, under reduced pressure, the residue was treated with water
(30 mL) and extracted with dichloromethane (3  30 mL). The solution
was dried over MgSO4 and the solvent was removed at the rotary
evaporator, to give a semisolid residue. MS (ESI): m/z (%): 484
(100%) [M + H+].
N-{2-[(Anthracen-9-yl-methyl)amino]ethyl}-N¢-[2-((ferrocenylmethyl)
amino)ethyl]propane-1,3-diamine, 3. The two-component
system 3 was obtained as a semisolid product through Schiff-base
condensation of 1 with ferrocene-1-aldehyde and reduction with NaBH4,
following the same procedure as for 2. MS (ESI): m/z (%): 549
(100%) [M + H+].
Acknowledgment. This work has been supported by CNRProgetto
Strategico: Tecnologie Chimiche Innovative.
IC950504B
1736 Inorganic Chemistry, Vol. 35, No. 6, 1996 Notes
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