Photoinduced energy- and electron-transfer processes within dynamic self-assembled donor-acceptor arrays

Article abstract of DOI:10.1021/ja026695g

The synthesis and the photophysical properties of a series of noncovalently assembled donor-acceptor systems, dyads, is reported. The presented approach uses an "innocent" coordination compound, a scandium(III) acetyl acetonate derivative, as core and promotor of the dyad formation. Intercomponent photoinduced energy transfer or electron transfer within the dynamic assembly, which yields to a statistical library of donor-acceptor systems, is reported. The assemblies for energy-transfer processes are constituted by an energy donor, Ru(bpy)₃²⁺-based component (bpy = 2,2′-bipyridine), and by an energy-acceptor moiety, anthracene-based unit, both substituted with a chelating ligand, acetyl acetone, that via coordination with a scandium ion will ensure the formation of the dyad. If N,N,N′N′-tetramethyl-2,5-diaminobenzyl-substituted acetyl acetonate ligands are used in the place of 9-acyl-anthracene, intramolecular photoinduced electron transfer from the amino derivative (electron donor) to the Ru(bpy)₃²⁺-unit was detected upon self-assembly, mediated by the scandium complex. The photophysical processes can be studied on the lifetime of the kinetically labile complexes.

Full text of DOI:10.1021/ja026695g

Published on Web 08/29/2002
Photoinduced Energy- and Electron-Transfer Processes
within Dynamic Self-assembled Donor-Acceptor Arrays
Michael Kercher,†,§ Burkhard Ko¨nig,*,† Harald Zieg,† and Luisa De Cola*,‡
Contribution from the Institut fu¨r Organische Chemie, UniVersita¨t Regensburg,
UniVersita¨tsstrasse 31, 93040 Regensburg, Germany, and Institute of Molecular Chemistry,
UniVersity of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands
Received April 26, 2002
Abstract: The synthesis and the photophysical properties of a series of noncovalently assembled donor-
acceptor systems, dyads, is reported. The presented approach uses an “innocent” coordination compound,
a scandium(III) acetyl acetonate derivative, as core and promotor of the dyad formation. Intercomponent
photoinduced energy transfer or electron transfer within the dynamic assembly, which yields to a statistical
library of donor-acceptor systems, is reported. The assemblies for energy-transfer processes are constituted
by an energy donor, Ru(bpy)32+-based component (bpy ) 2,2′-bipyridine), and by an energy-acceptor moiety,
anthracene-based unit, both substituted with a chelating ligand, acetyl acetone, that via coordination with
a scandium ion will ensure the formation of the dyad. If N,N,N′N′-tetramethyl-2,5-diaminobenzyl-substituted
acetyl acetonate ligands are used in the place of 9-acyl-anthracene, intramolecular photoinduced electron
transfer from the amino derivative (electron donor) to the Ru(bpy)32+-unit was detected upon self-assembly,
mediated by the scandium complex. The photophysical processes can be studied on the lifetime of the
kinetically labile complexes.
Introduction:
donor-acceptor dyads. In particular hydrogen bonding, salt
bridges, and hydrophobic interactions have been investigated.14-26
Surprisingly few examples that employ kinetically labile
coordination compounds for assembly of the donor-acceptor
units have been published.27-32 The noncovalent approach offers
some advantages: (i) the synthetic effort is reduced since only
Distance, relative orientation, and the molecular structure that
separates a donor group from an acceptor moiety largely
influence the feasibility of intramolecular electron- and energy-
transfer processes.1-4 To study the effect of these parameters,
most of the effort has been devoted to the synthesis of covalently
linked systems.5-8 Self-assembly is a feature of modern
chemistry,9-13 which has been applied recently to arrange
(14) Ghaddar, T. H.; Castner E. W.; Isied, S. S. J. Am. Chem. Soc. 2000, 122,
1233-1234.
(15) Salameh, A. S.; T. Ghaddar, T. H.; Isied, S. S. J. Phys. Org. Chem. 1999,
12, 247-254.
(16) Chin, T.; Gao, Z.; Lelouche I.; Shin Y.-g.; Purandare, S.; Knapp, S.; Isied
S. S. J. Am. Chem. Soc. 1997, 119, 12849-12858.
* To whom correspondence should be addressed. E-mail:
burkhard.koenig@chemie.uni-regensburg.de and ldc@science.uva.nl.
† Universita¨t Regensburg.
(17) Ward, M. D.; Barigelletti, F. Coord. Chem. ReV. 2001, 216-217, 127-
154.
‡ University of Amsterdam.
(18) White, C. M.; Gonzalez, M. F.; Bardwell, D. A.; Rees, L. H.; Jeffery, J.
C.; Ward, M. D.; Armaroli, N.; Calogero, G.; Barigelletti, F. J. Chem. Soc.,
Dalton Trans. 1997, 727-735
§ Present address: Institute of Molecular Chemistry, University of
Amsterdam, Nieuwe Achtergracht 166, 1018WV Amsterdam, The Neth-
erlands.
(19) Nelissen, H. F. M.; Schut, A. F. J.; Venema, F.; Feiters, M. C.; Nolte, R.
J. M. Chem. Commun. 2000, 577-578.
(1) Balzani, V., Ed. Electron Transfer in Chemistry; Wiley-VCH: Weinheim,
2001.
(20) Nelissen, H. F. M.; Kercher, M.; De Cola, L.; Feiters, M. C.; Nolte, R. J.
M. Chem. Eur. J. Manuscript submitted.
(2) Paddon-Row, M. N. In Stimulating Concepts in Chemistry; Vo¨gtle, F.,
Stoddart, J. F., Shibasaki, M., Eds.; Wiley-VCH: Weinheim, 2000; pp
267-291.
(21) Haider, J. M.; Chavarot, M.; Weidner, S.; Sadler, I.; Williams, R. M.; De
Cola, L.; Pikramenou, Z. Inorg. Chem. 2001, 40, 3912-3921.
(22) Kaifer, A. E. Acc. Chem. Res. 1999, 32, 62-71.
(23) Chang, C. J.; Brown, J. D. K.; Chang, M. C. Y.; Baker, E. A.; Nocera, D.
G. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, 2001; Vol. 3, pp 409-461.
(3) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461.
(4) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163-170.
(5) Paddon-Row, M. N. In Electron Transfer in Chemistry; Balzani, V., Ed.;
Wiley-VCH: Weinheim, 2001; Vol. 3, pp 179-271.
(6) Gust, D.; Moore, T. A.; Moore, A. L. In Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. 3, pp 272-336.
(7) Scandola, F.; Chiorboli, C.; Indelli, M. T.; Rampi, M. A. In Electron
Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001;
Vol. 3, pp 337-408.
(24) Hayashi, T.; Ogoshi, H. Chem. Soc. ReV. 1997, 26, 355-364.
(25) Ward, M. D. Chem. Soc. ReV. 1997, 26, 365-375.
(26) Billing, R.; Rehorek, D.; Henning, H. Top. Curr. Chem. 1990, 158, 151-
200.
(27) Hunter, C. A.; Sanders, J. K. M.; Beddard, G. S.; Evans, S. J. Chem. Soc.,
Chem. Commun. 1989, 1767-1767.
(8) De Cola, L.; Belser, P. Coord. Chem. ReV. 1998, 177, 301-346.
(9) Lehn, J.-M. Science 2002, 295, 2400-2403
(28) De Santis, G.; Fabbrizzi, L.; Licchelli, M.; Poggi, A.; Taglietti, A. Angew.
Chem., Int. Ed. Engl. 1996, 35, 202-204.
(10) Lehn, J.-M. Supramolecular Chemistry; Wiley-VCH: Weinheim, 1995
(11) Fujita, M., Ed. Molecular Self-Assembly. Organic Versus Inorganic
Approaches; Structure and Bonding, Vol. 96; Springer-Verlag: Berlin, 2001.
(12) Sautter A., Schmid D. G., Jung G., Wu¨rthner F. J. Am. Chem. Soc. 2001,
123, 5424-5430.
(29) Di Casa, M.; Fabbrizzi, L.; Licchelli, M.; Poggi, A.; Russo, A.; Taglietti,
A. Chem. Commun. 2001, 825-826.
(30) Hunter, C. A.; Hyde, R. K. Angew. Chem., Int. Ed. Engl. 1996, 35, 1936-
1939.
(13) Wu¨rthner F., Sautter A., Thalacker C. Angew. Chem., Int. Ed. 2000, 39,
1243-1245.
(31) Imahori, H.; Yoshizawa, E.; Yamada, K.; Hagiwara, K.; Okada, T.; Sakata,
Y. J. Chem. Soc., Chem. Commun. 1995, 1133-1134.
9
10.1021/ja026695g CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 11541-11551
11541

A R T I C L E S
Kercher et al.
Scheme 1. Formulas of the Components bpy-L, L-A, and L-D and
a Schematic Representation of Self-Assembled Dyads via the
Coordination to the Scandium(III) Ion
SpectraPro-150s imaging spectrograph equipped with 150 or 600 g
mm-1 grating and tunable slit (1-500 µm) resulting in a 6- or 1.2-nm
maximum resolution, respectively. The data were collected with a
system containing a gaited intensified CCD detector (Princeton Instru-
ments ICCD-576G/RB-EM) and an EG&G Princeton Applied Research
model 9650 digital delay generator. I and I0 are measured simulta-
neously using a double 8 kernel 200 µm optical fiber with this OMA-4
setup. WINSPEC (V 1.6.1, Princeton Instruments), used under Win-
dows, programmed and accessed the setup.
Materials. 4,4′-Dimethyl-2,2′-bipyridine (1),33 4′-methyl-2,2′-bipy-
ridine-4-carboxylic acid (2),34 4′-methyl-2,2′-bipyridine-4-methylester
(3),35 rutheniumbisbipyridine dichloride,36 4-bromomethylene-4′-methyl-
2,2′-bipyridine (6),37 2,5-(N,N,N′,N′-tetramethyldiamino)benzaldehyde
(10),38 and Sc(acac)3
were synthesized according to established
39,40
methods. All solvents employed for photophysical measurements were
of spectroscopical grade and purchased from Aldrich. The benzonitrile
used for the dynamic exchange of ligands was of HPLC grade and
purchased from Aldrich.
Synthesis. 4-(1,3-Butyldione)-4′-methyl-2,2′-bipyridine (4). N-
Isopropylidencyclohexylamine (247 mg, 1.7 mmol) was deprotonated
in THF (20 mL) with 1.7 mmol LDA at 0 °C over a period of 30 min
and slowly added to 365 mg (1.6 mmol) of 3, stirred for 4 h at that
temperature and additional 16 h at room temperature. After neutraliza-
tion with 1 N HCl, the solution was diluted with CH2Cl2 and extracted
several times with saturated aqueous NH4Cl and water. The organic
layer was evaporated to dryness, and the crude product was purified
by column chromatography (silica, CH2Cl2/CH3OH/NH4OH 200:10:
1) to yield 220 mg (54%). 1H NMR (300 MHz, CDCl3) δ 2.27 (s, 3H),
2.45 (s, 3H), 6.39 (s, 1H), 7.17 (m, 1H), 7.75 (m, 1H), 8.26 (s, 1H),
8.56 (m, 1H), 8.74 (s, 1H), 8.78 (m, 1H), 15.76 (b, 1H); 13C NMR (75
MHz, CDCl3, apt) δ 21.41 (-), 26.95 (-), 98.34 (-), 117.94 (-),
120.50 (-), 122.30 (-), 125.38 (-), 142.88 (+), 148.54 (+), 149.28
(-), 150.20 (-), 155.32 (+), 157.36 (+), 178.87 (+), 197.26 (+); IR
(KBr) ν 2922, 1611, 1593, 1545, 1364, 1259, 1079, 831, 780, 841,
668, 514 cm-1; MS (FAB) m/z 255.11 (100) [M + H+], 154.01 (77);
136.03 (62)
substructures are prepared and self-assembled to obtain more
complex architectures. The modular strategy allows the synthesis
of different aggregates from only a few building blocks. (ii)
The electronic interaction can be strongly modulated by solvent,
temperature and concentration of the components. (iii) Electron-
and energy-transfer processes can be studied over reversible
bonds and longer distance, and new information is obtained on
the electronic coupling via different linkages. However, several
disadvantages must also be considered in this approach. The
low association constants often prevent photophysical studies
for which high-dilution conditions are required. Also in many
cases (hydrogen bonds) the use of protic solvent is prevented.
We report here the synthesis and photophysical studies of a
variety of photoactive components and their assembly. In
particular the photoinduced processes in donor-acceptor dyads,
obtained by the assembly of such components via an “innocent”
metal ion, scandium(III), will be discussed. The high association
constants, the possibility of success in many solvents, and finally
the accessibility to many different components for the construc-
tion of our dyads are the most interesting features of our
supramolecular structures. In these dynamic assemblies the
scandium is only a structural motif that holds together an energy
donor or electron acceptor, such as bis(2,2′-bipyridine)[4-
{butane-1,3-dione-1-yl}-4′-methyl-2,2′-bipyridine]ruthenium(II)-
bis(hexafluorophosphate), and an energy acceptor, 9-anthroyl-
acetone, or an electron donor, 3-[2,5-(N,N,N′,N′-tetrameth-
yldiamino)benzyl]-2,4-pentadione (see Scheme 1).
Bis(2,2′-bipyridine)-[4-(1,3-butyldione)-4′-methyl-2,2′-bipyridine]-
ruthenium(II)-bis-(hexafluorophosphate) (5). Bis(2,2′-bipyridine)-
dichloro-ruthenium(II) dihydrate (390 mg, 0.75 mmol) was refluxed
with 189 mg (0.74 mmol) 4-(1,3-butyldione)-4′-methyl-2,2′-bipyridine
in 20 mL ethanol/water (3:1) for 4 h. The solvent was removed in
vacuo and the residue dissolved in 10 mL of water. The remaining
starting material was removed by multiple extraction with CH2Cl2 until
the organic layer stayed clear. The crude product was precipitated as
hexafluorophosphate from water to yield 520 mg (73%). 1H NMR (300
MHz, CD3CN) δ 2.31 (s), 2.59 (s), 6.67 (s), 7.32 (m) 7.39 (m), 7.44
(m), 7.59 (m), 7.75 (m), 7.89 (m) 8.09 (m), 8.55 (m), 8.80 (m), 15.76
(b); MS (ESI) m/z 813.13 (30) [M2+PF6], 334.08 (100) [M2+];
C35H30N6O2Ru calcd 668.147, found 668.16
3-(4-Methylen-4′-methyl-2,2′-bipyridyl)-2,4-pentadione (7). So-
dium acetyl acetate (180 mg, 1.5 mmol) and 4-bromomethylen-4′-
methyl-2,2′-bipyridine (6) (320 mg, 1.2 mmol) were refluxed in THF
(30 mL) for 6 h. The reaction mixture was stirred overnight at room
Experimental Section
Spectroscopy. The UV-vis absorption spectra were recorded on a
Hewlett-Packard diode array 84533 spectrophotometer. Recording of
the emission spectra was done with a SPEX 1681 Fluorolog spectro-
fluorimeter. Lifetimes were determined using a Coherent Infinity Nd:
YAG-XPO laser (1-ns pulses fwhm) and a Hamamatsu C5680-21 streak
camera equipped with a Hamamatsu M5677 low-speed single-sweep
unit. Transient absorption spectroscopy was performed by irradiation
of the sample with a Coherent Infinity Nd:YAG-XPO laser (1-ns pulses
fwhm). The sample was probed by a low-pressure, high-power EG&G
FX-504 Xe lamp. The passed light was dispersed by an Acton
(33) Sasse, W. H. F. Organic Syntheses; John Wiley & Sons: New York, 1973;
Collect. Vol. V, pp 102-107.
(34) McCafferty, D. G.; Bishop, B. M.; Wall, C. G.; Hughes, S. G.; Mecklenberg,
S. L.; Meyer, T. J.; Erickson, B. W. Tetrahedron 1995, 51, 1093-1106.
(35) Wang, G.; Bergstrom, D. E. Synlett 1992, 422-424.
(36) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17, 3334-
3341.
(37) Ciana, L. D.; Hamachi, I.; Meyer, T. J. J. Org. Chem. 1989, 54, 1731-
1735.
(38) Loppnow, G. R.; Melamed, D.; Hamilton, A. D.; Spiro, T. G. J. Phys.
Chem. 1993, 97, 8957-8968.
(39) Sc(acac)3 was prepared by mixing ScCl3 and an excess of 2,4-pentadione
in dry methanol and deprotonation with ammonia gas, upon which the
desired product precipitated out of the solution.
(32) Ko¨nig, B.; Pelka, M.; Zieg, H.; Ritter, T.; Bouas-Laurent, H.; Bonneau,
(40) Gmelin Handbook of Inorganic Chemistry; Springer-Verlag: Berlin; 1981;
Vol. D3, pp 76-96.
R.; Desvergne, J.-P. J. Am. Chem. Soc. 1999, 121, 1681-1687.
9
11542 J. AM. CHEM. SOC. VOL. 124, NO. 38, 2002

PET in Dynamic Self-Assembled Donor−Acceptor Arrays
A R T I C L E S
temperature and evaporated to dryness. The residue was taken up in
CH2Cl2 and washed with diluted acetic acid. Evaporation to dryness
and chromatography (SiO2/CH2Cl2-CH3OH-NH3 (25% in water),
100:5:0.5 (v/v), Rf ) 0.3) yielded 180 mg (54%) of 7 as a yellow oil;
1H NMR (400 MHz, CDCl3) δ 2.09 (s, 6H pentadion-CH3, enol-form),
2.18 (s, 6H pentadion-CH3, keto-form), 2.44 (s, 6H, bipyridine-CH3,
keto- and enol-form), 3.22 (d, 3J ) 7.4 Hz, 2H, bipyridine-CH2, keto-
form), 3.75 (d, 2H, bipyridine-CH2, enol-form), 4.13 (d, 3J ) 7.4 Hz,
1H, pentadion-CH, keto-form), 7.11 (m, 4H, bipyridine-H, keto- and
enol-form), 8.24 (m, 4H, bipyridine-H, keto- and enol-form), 8.55 (m,
4H, bipyridine-H, keto- and enol-form), 16.89 (s, 1H, enol-OH); 13C
NMR (100 MHz, CDCl3) δ 21.13 (+), 23.35 (+), 29.69 (+), 32.70
(-), 33.27 (-), 68.62 (+), 106.74 (Cquat), 120.49 (+), 120.97 (+),
121.98 (+), 122.03 (+), 122.28 (+), 124.06 (+), 124.80 (+), 124.84
(+), 148.16 (Cquat), 148.92 (+), 148.95 (+), 149.39 (+), 149.51 (+),
150.04 (Cquat), 155.61 (Cquat), 156.51 (Cquat), 192.01 (Cquat), 202.55
3.0 Hz, 1H, phenyl-H, enol-form), 6.41 (d, 3J ) 3.0 Hz, 1H, phenyl-
H, keto-form), 6.53 (m, 2H, phenyl-H, keto- and enol-form), 6.99 (d,
3J ) 2.7 Hz, 1H, phenyl-H, keto-form), 7.01 (d, 3J ) 2.7 Hz, 1H,
phenyl-H, enol-form); 13C NMR (100 MHz, CDCl3) δ ) 23.05 (+),
27.74 (-), 29.39 (+), 30.91 (-), 40.88 (+), 40.98 (+), 45.37 (+),
45.59 (+), 68.88 (+), 109.18 (Cquat), 111.28 (+), 112.06 (+), 112.58
(+), 114.88 (+), 120.39 (+), 121.37 (+), 134.79 (Cquat), 135.52 (Cquat),
142.82 (Cquat), 147.60 (Cquat), 167.69 (Cquat), 191.85 (Cquat), 204.35
(Cquat); MS (70 eV), m/z (%) 276 (100) [M+], 233 (16) [M+ - CH3-
CO]. C16H24N2O2: calcd C 69.53 H 8.75 N 10.14; found C 69.46 H
8.82 N 10.10.
General Method for the Assembly of Scandium Complexes. Up
to 10 mg of scandium-tris-acetylacetonate was dissolved with desired
equivalents of ligands in 1 mL of benzonitrile. The solution was
degassed, and a static vacuum of 10-3 Pa was applied. The reaction
flask was left at room temperature while the solvent and all volatile
compounds were collected in a liquid nitrogen-cooled flask. After
complete evaporation of the solvent, the residue was redissolved and
taken to dryness in the same manner twice, to ensure a complete
exchange of ligands.
(Cquat); IR (film) ν 3054, 3007, 2923, 1727, 1595, 1428, 824 cm-1
;
MS (70 eV, EI) m/z 282 (22) [M+], 267 (20) [M+ - CH3], 239 (100)
[M+ - C(O)CH3], 43 (20) [C(O)CH3+].
Bis(2,2′-bipyridine)[3-(4-Methylen-4′methyl-2,2′-bipyridyl)-2,4-
pentadion]ruthenium(II)bis(hexafluorophosphate) (8). Bis(2,2′-bi-
pyridine)dichloro-ruthenium(II) dihydrate (310 mg, 0.6 mmol) and 3-(4-
methylen-4′-methyl-2,2′-bipyridyl)-2,4-pentadione (180 mg, 0.64 mmol)
were refluxed in ethanol/water (3:1, 20 mL). The dark red solution
was evaporated to dryness, and the residue was purified by multiple
gel permeation chromatography steps (Sephadex LH 20, CH3OH),
yielding 360 mg (73%) of 8 (chloride salt) as a dark red solid, mp 248
°C. Counterions were exchanged in water by treatment with aqueous
KPF6 to give 8 (PF6 salt) as an orange residue, mp 172 °C; TLC (SiO2,
Results and Discussion
Design of a Dynamic Self-Assembled Donor-Acceptor
Pair. Acetyl acetonates (acac) are good ligands to complex
trivalent metals ions, leading in the case of scandium(III) ions
to thermodynamically stable (but kinetically labile) coordination
compounds. The association constant in water for the formation
of the hexacoordinated complex is Kâ > 1015 M-1 40
We can
.
1
CH3OH-aqueous NH4Cl-CH3NO2, 7:2:1, Rf ) 0.54); H NMR (400
therefore expect that with such high Kâ complete association
even at high dilution, necessary for photophysical investigations,
occurs. The absorption spectra of Sc(III)(acac)3 shows no bands
at energy below 33500 cm-1. This enables us to build up species
containing energy- or electron donor and acceptor units that
can be selectively excited in the visible region. Scandium(III)
complexes cannot be oxidized, and with a redox potential of
Sc3+/2+ E ) -2.47 V vs Fc/Fc+, the complex will behave as
an innocent spectator in electron-transfer processes between
suitable donor and acceptor ligands coordinated to it.
Sc(III)(acac)3 complexes are kinetically labile. The average
lifetime of the complex is about 5 ms, before an acetylacetonate
is exchanged.41 Therefore, using a statistical approach such
complexes can be dynamically assembled from a reservoir of
available ligands. Depending on the choice of substituted acac
ligands, an entire dynamic library of complexes can be created,
from which some are able to constitute the correct building
blocks for intramolecular energy- or electron-transfer processes.
For our studies we have chosen two different substituents on
the acac ligand, 3-[2,5-(N,N,N′,N′-tetramethyldiamino)benzyl]-
2,4-pentadione as electron donor, L-D, and an acac ligand
containing an anthracene unit, 9-anthroylacetone, L-A, as energy
acceptor (see Scheme 1). The photosensitizer that behaves as
electron acceptor or energy donor is a ruthenium complex, [Ru-
(bpy-L)(bpy)2]2+ (bpy-L ) 1-(4′methyl-[2,2′bipyridinyl-4-yl)-
butan-1,3-dione and bpy is 2,2′-bipyridine). The choice of these
components is dictated by their well-known spectroscopic and
electrochemical properties.
MHz, CD3CN) δ 2.10 (s), 2.13 (s), 2.51 (m), 2.89 (m), 3.00 (m), 7.22
(m), 7.37 (m), 7.52 (m), 7.69 (m), 8.03 (m), 8.34 (m), 8.46 (m); IR
(KBr) ν 2958, 1605, 1483, 1466, 1427, 841, 556 cm-1; MS (ESI) m/z
695 (28) [M+], 261 (100).
3-[2,5-(N,N,N′,N′-Tetramethylamino)benzylidene]-2,4-pentandi-
one (11). 2,5-(N,N,N′,N′-Tetramethylamino)benzaldehyde (10) (500 mg,
2.6 mmol) and 0.24 mL (2.4 mmol) of acetylacetone were combined
with 2-3 drops of piperidine in 25 mL of dry chloroform and refluxed
for 5 h. The mixture was evaporated to dryness. Column chromatog-
raphy (silica, PE/EE 7:3) yielded 400 mg (61%) 11 (Rf ) 0.22) of a
dark-red oil. IR (KBr) ν ) 2980 cm-1, 2941, 2865, 2829, 2789, 1686,
1658, 1505, 1242, 945; 1H NMR (400 MHz, CDCl3) δ ) 2.21 (s, 3H,
pentandione-CH3), 2.41 (s, 3H, pentandione-CH3), 2.65 (s, 6H, di-
methylamino-CH3), 2.83 (s, 6H, dimethylamino-CH3), 6.61 (d, 4J )
2.9 Hz, 1H, phenyl-H), 6.76 (dd, 3J ) 8.8 Hz, 4J ) 2.9 Hz, 1H, phenyl-
H), 7.00 (d, 3J ) 8.8 Hz, 1H, phenyl-H), 7.87 (s, 1H, benzylidene-H);
13C NMR (100 MHz, CDCl3) δ ) 26.73 (+), 31.24 (+), 40.87 (+),
45.24 (+), 114.20 (+), 116.06 (+), 119.40 (+), 127.76 (Cquat), 139.74
(Cquat), 141.24 (Cquat), 144.34 (Cquat), 146.48 (Cquat), 197.17 (Cquat),
204.52 (Cquat); MS (70 eV), m/z (%): 274 (100) [M+], 231 (36) [M+
- CH3CO], 188 (22) [M+ - 2CH3CO].
3-[2,5-(N,N,N′,N′-Tetramethylamino)benzyl]-2,4-pentandione (12).
A solution of 180 mg (0.65 mmol) of 3-[2,5-(N,N,N′,N′-tetramethyl-
amino)benzylidene]-2,4-pentandione (11) and 10 mg of palladium/
carbon (10%) in 50 mL of methanol was hydrogenated at 5 × 106 Pa
hydrogen pressure for 1 h at room temperature. After filtration on Celite,
the methanol was removed in vacuo, and the product was purified via
column chromatography (silica, PE/EE 7:3). Yield: 140 mg (78%) 12
(Rf ) 0.44) of a slightly yellow solid, mp 56 °C. IR (KBr) ν ) 2978
1
cm-1, 2937, 2822, 2781, 1612, 1511, 1191, 947, 811; H NMR (400
MHz, CDCl3) δ ) 1.98 (s, 6H, pentandione-CH3, enol-form), 2.06 (s,
6H, pentandione-CH3, keto-form), 2.50 (s, 6H, dimethylamino-CH3,
keto-form), 2.56 (s, 6H, dimethylamino-CH3, enol-form), 2.77 (s, 6H,
dimethylamino-CH3, enol-form), 2.79 (s, 6H, dimethylamino-CH3, keto-
form), 3.09 (m, 2H, benzyl-CH2, keto-form), 3.61 (s, 2H, benzyl-CH2,
enol-form) 4.07 (bs, 1H, pentandione-CH, keto-form), 6.34 (d, 3J )
In particular the ruthenium complex has (i) an absorption in
the visible where the other components do not absorb, allowing
a selective excitation, (ii) the lowest excited state is luminescent,
(41) Hatakeyama, Y.; H.Kido; Harada, M.; Tomiyasu, H.; Fukutomi, H. Inorg.
Chem. 1988, 27, 992-996.
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A R T I C L E S
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Scheme 2. Synthesis of Ruthenium Tris-bipyridine Functionalized acac Ligands, Ru(bpy)2(bpy-L)
Scheme 3. Synthesis of Anthracene- and Tetramethyl
Phenylenediamine-Functionalized acac Ligands
and with lifetime of the order of hundreds of ns in acetonitrile.42
This will allows us not only to observe even rather slow
processes occurring in the excited states but also to treat the
dynamic assemblies as discrete (static) molecular species in the
excited state, therefore excluding chemistry in this time domain.
Furthermore, because of the relatively easy substitution of the
bipyridine ligands, it was possible to introduce the same type
of chelating ligand, acac, on the Ru-based compound. We have
therefore prepared and studied the Ru(bpy)2(bpy-L)2+ complex,
5, that is able by self-assembly to coordinate a substituted
scandium complex (containing the photoactive acac ligand, L-D
or L-A) to form a suitable dyad for photoinduced energy transfer
or electron transfer processes (see Scheme 1).
Synthesis of the Photoactive Components. Ruthenium tris-
bipyridine complexes have been widely used to study photo-
induced electron- and energy-transfer processes for their unique
photophysical and redox properties.42-44 The chemistry of
bipyridines is very well explored and ensures the availability
of suitable functionalized compounds.45,46 The synthesis of a
ruthenium complex with an acac binding site for scandium(III)
ions is shown in Scheme 2. 4,4′-Dimethyl-2,2′-bipyridine (1)33
was oxidized according to a two-step procedure, reported by
McCafferty et al. with SeO2 and Ag2O.34 The free carboxylic
acid 2 was converted into the methyl ester 335 and compound
bpy-L (4) could be obtained upon addition of lithium-N-
isopropyliden-cyclohexylamine and acidic workup in 54% yield.
The ruthenium complex 5 was formed upon reaction of
ruthenium-bis-2,2’-bipyridine dichloride with bpy-L in a mix-
ture of water and ethanol. The product has been obtained as
PF6-salt in 73% yield.
To achieve a different connectivity between acac and bipy-
ridine 4-bromomethylene-4′-methyl-2,2′-bipyridine (6)37 was
reacted with sodium acac to give 7 in 54% yield. Although the
compound is a suitable ligand to give scandium acac complexes
as confirmed by mass spectrometry, it could not be used to study
photoinduced energy- and electron-transfer processes. Upon
formation of the corresponding ruthenium complex 8 the ligand
shows significant photolability with consequent decomposition.
(42) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Zelewsky,
A. v. Coord. Chem. ReV. 1988, 84, 85-277.
(43) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, C.;
Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L. Chem. ReV. 1994,
94, 993-1019.
(44) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. ReV.
1996, 96, 759-833.
(45) Kaes, C.; Katz, A.; Hosseini, M. W. Chem. ReV. 2000, 100, 3553-3590.
(46) Kro¨hnke, F. Synthesis 1976, 1-24.
9
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PET in Dynamic Self-Assembled Donor−Acceptor Arrays
A R T I C L E S
Scheme 4. Preparation of Substituted Sc(III)(acac)3 Complexes by Dynamic Ligand Exchange
For the synthesis of an acac ligand containing an electron
donor group, L-D, commercially available tetramethyl-phen-
ylendiamine was formylated38 followed by a condensation
reaction with acetyl acetone, to give the benzylideneacetylace-
tonate in 61% yield. Hydrogenation of the double bond led in
78% yield to the desired â-diketone 12 (L-D). The anthracene-
acac conjugate 14 (L-A) was obtained following a procedure
reported by Evans et al.47 by reacting 9-acetylanthracene with
ethyl acetate in the presence of sodium (Scheme 3).
Formation of the Assemblies. For the preparation of the
dyads assembled via a scandium(III)(acac)3 complex a mixture
of the parent scandium(III) tris-acetylacetonate, 15,39,40 ruthe-
nium complex 5, and compound L-D or L-A were mixed in
the appropriate stoichiometry in benzonitrile, and a pressure of
10-3 Pa was applied (see Scheme 4). Benzonitrile is a high-
boiling solvent in which the PF6-salt of the ruthenium complex
is showing good solubility. Applying high vacuum to the
mixture, the solvent and the unsubstituted acetylacetonate, the
only volatile compounds, are slowly evaporated, driving the
equilibrium toward the formation of the scandium complexes
with substituted acac ligands. This procedure yields a statistical
library of different scandium compounds, and assemblies 16,
17, 18, and 19 are obtained when L-A is employed, while
assemblies 19, 20, 21 and 22 are produced if L-D is one of the
reagents, as shown in Scheme 4.
The ligand exchange can be monitored by NMR. The
disappearance of the resonance of the characteristic proton at
carbon 3 of 2,4-pentadione clearly indicates the exchange of
unsubstituted acac ligands. A complete assignment of resonances
is not possible due to the complexity of the mixture.
Assuming simple statistics with equal binding strength for
all â-diketones the relative abundance of different substitution
patterns can be calculated. All complexes were prepared in a
stoichiometry of 1:2 of the ruthenium complex 5 to L-D or L-A,
respectively. The theoretical relative abundance of the scandium
complexes predict the least favorable complex bearing three
ruthenium acac ligands (19) to be obtained in less than 4%.
From this assembly only a luminescence contribution to the
background of the emission spectra of the mixture is expected.
Complexes bearing photoactive units and quenchers form the
majority of all coordination complexes, 66%, and will give
detectable indication of electron- or energy-transfer process.
Assemblies not bearing any ruthenium complexes (18 and 22),
present in 30%, will not contribute to absorption in the visible,
where the excitation is going to be performed, or luminescence
in the 550-800-nm region, where the ruthenium emission is
monitored. Their absorption in the visible (λ > 450 nm) where
(47) Evans, D. F. J. Chem. Soc 1961, 1987-1993.
9
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A R T I C L E S
Kercher et al.
Scheme 5. Scandium(III) Complexes Detected by Mass
Spectrometry
Figure 1. UV-vis absorption spectra and room-temperature emission
2+
spectra (inset) of Ru(bpy)3 (full line), 5 (dashed line) C ≈ 10-5 M, and
Sc(acac)3 (dotted line) C ≈ 10-3 M in acetonitrile solutions.
Table 1. Photophysical Data in Acetonitrile
Absmax/nm
Emmax/nm
Φaerated
τaerated/ns
τdeaerated/ns
Ru(bpy)3
5
455
458
614
624
0.016
0.011
160
150
890
525
Photophysical Properties of 5 and Its Scandium Assembly.
the ruthenium complex presents its metal-to-ligand charge-
transfer (MLCT) bands is negligible. In the case of the
anthracene-substituted acac ligands, the scandium complex 22
bearing three of these ligands is not soluble in benzonitrile and
has been isolated from the mixture. Here we can assume to have
a mixture of three different complexes in which at least one
ruthenium-substituted acac ligand is present. Obviously no
complete quenching of the luminescence of the ruthenium-acac
compounds can be expected by formation of mixed scandium
complexes. The abundance of complexes in the equilibrium
having donor and acceptor ligand, such as 16/17 and 20/21, is
much less than 100%. In addition, a small amount of nonco-
ordinated ruthenium ligands 5 may contribute to an unquenched
background signal.
Mass spectrometry was used to monitor complex formation
and distribution of ligands. In the EI mass spectra of the
equilibrium mixture of Sc(acac)3 (15) with 2 equiv of 12,
molecular ions of the complexes 15 (m/z )342; 20%), 23 (m/z
) 518; 12%), 24 (m/z ) 694; 6%), and 18 (m/z ) 870; 4%),
shown in Schemes 4 and 5, were detected.
The absorption and emission spectra of ruthenium complex 5
2+
and of the reference compound Ru(bpy)3 in acetonitrile
solution are reported in Figure 1. Some photophysical data are
summarized in Table 1. The spectra show the characteristic
1MLCT bands in the visible region that in complex 5 are slightly
red-shifted compared to the absorption of the parent compound.
This can be explained considering that the lowest excited state
involves the transition Ru f bpy-L since the acetylacetonate
is a weak electron-withdrawing group. The direct substitution
of the bpy with the acac moiety provides a good electronic
coupling between the bpy and the acac group.42 The UV region
of the spectrum is dominated by the intense absorption bands
of the bipyridine ligands (300 nm), and by comparison with
the Sc(acac)3 compound, the weak transition centered on the
acac ligands (210 nm) can also be assigned. As already
mentioned the scandium complex does not contribute to the
absorption spectrum in the visible region.
The room-temperature emission spectrum in aerated aceto-
nitrile solution (Figure 1, inset) shows a maximum at 624 nm
also slightly red-shifted, compared with that of the reference
This supports the assumption that all coordination compounds
are present in the equilibrium. The different sensitivity of
detection for each compound in EI-MS does not allow any
quantitative conclusions. With ionization methods such as FAB
or ESI, which allow a much better quantitative analysis, no
scandium-containing complexes could be detected.48
3
complex, and in agreement with the assignment of a MLCT
as the lowest excited state involving a transition from the Ru
to the L ligand. The excited-state lifetime and the emission
quantum yield are reported in Table 1.
Complexation of 5 to Sc(III) ions does not change the
Addition of 1 equiv of 3-(4-methylen-4′-methyl-2,2′-bipy-
ridyl)-2,4-pentadione 7 to the mixture of complexes 15, 18, 23,
and 24 lead to detectable signals of the newly formed complexes
25 (m/z ) 876; 22%) and 26 (m/z ) 882; 8%) in the EI mass
spectrum. This confirms the dynamic character of the mixture
of coordination compounds. The given percentages for the
molecules do not represent absolute abundance in solution. They
only indicate the abundance of the detected fragments in the
mass spectra.
photophysical properties significantly.
Self-Assembly of Energy Donor-Acceptor Dyads. In-
tramolecular Energy Transfer. Upon appropriate choice of
components it is possible to build up, using the scandium as
“assembler”, an energy donor-acceptor dyad. We have chosen
an energy donor such as a ruthenium unit, 5, and as energy
acceptor an anthracene derivative, L-A. The two components
have been previously investigated in covalently linked
More evidence for the formation of scandium complexes with
mixed acac ligands and the dynamic character of the library
will be provided in the photophysical section.
(48) Despite several attempts, we were not able to detect molecular ions of
scandium complexes bearing one or more charged ruthenium-acac ligands
using EI, ESI, FAB, MALDI mass spectrometry techniques.
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PET in Dynamic Self-Assembled Donor−Acceptor Arrays
A R T I C L E S
Scheme 6. Energy Diagram of Ru-Based Component and
9-Acyl-anthracenea
Figure 2. UV-vis absorption spectra of 5 (dotted line), LA (dashed line),
and the assembly of both (Ru-Sc-L-A) (full line) in deaerated acetonitrile
solution. C ≈ 10-5 M.
endoperoxides with singlet oxygen,52 all experiments were
carried out in oxygen-free acetonitrile solutions.
As can be easily seen the spectrum of the adducts (containing
one or two Ru-based moieties or one or two L-A units) is
essentially the sum of the absorption spectra of 5 and L-A. This
indicates that no strong ground-state electronic interaction
between the two chromophores (Ru(bpy)32+ and anthracene) is
observed but does not indicate that the assembly is indeed
formed when the components are mixed together. To have the
proof of the formation of the assembly, steady-state and time-
resolved spectroscopy has been employed.
The emission spectra of 5 and of the assembly show the
characteristic luminescence of the Ru-based component but with
different intensities. As one would expect, in the assembly a
quenching of the emission is observed. This is in agreement
with what was previously reported for covalently linked
systems.49-51 However an accurate evaluation of the lumines-
cence quenching in the mixture is not possible only from the
emission because of the statistical approach employed to build
up the dyad. In fact the presence of free ruthenium complex, 5,
influences the total emission quantum yield, making the correct
evaluation of the quenching impossible. It is interesting to notice,
however, that the presence of free anthracene eventually does
not corrupt our measurements since the fluorescence of the
anthracene is located in a region (400-500 nm) that does not
overlap with the ruthenium emission.
To have a quantitative evaluation of the quenching and to
understand the process, responsible for the decrease in the
emission intensity, time-resolved measurements were performed.
The excited-state lifetime of compound 5, measured under
deaerated conditions, detected at about 620 nm, was 525 ns (see
Table 1). A monoexponential model describes the observed
decay trace. Within the assembly of 5 and L-A through the
scandium complex (adduct 20 or 21) the excited-state lifetime
monitored at 650 nm, shows a biexponential behavior. The long-
lived component has a lifetime identical to that of the un-
quenched complex 5. The short component of the decay trace
is calculated to have a lifetime of 4 ns.
a Full arrows indicate radiative processes, whereas dashed arrows
represent radiationless pathways.
systems,49-51 and the energy-transfer processes from the excited
ruthenium unit to the triplet excited state of the anthracene have
been shown by emission quenching50 and time-resolved spec-
troscopy.49
We expect on the basis of the energetics (see Scheme 6) a
triplet-triplet energy transfer from the ruthenium moiety to the
9-acyl-anthracene, since we are exciting in the 1MLCT band of
the transition metal complex. At such energy in fact, population
of the singlet excited state of the anthracene moiety cannot
occur.
In the assembly process a static distribution of species is
possible, and an interesting library of compounds is obtained.
The only assemblies that will give a photoinduced energy-
transfer process are those containing both, one or two units of
5 and one or two units of L-A (complex 20 and 21, Scheme 4).
For our investigation they will behave identically, and no
attempts were made to separate them. On the other hand the
formation of the assemblies containing only anthracene or
ruthenium complex or both, even though it will make the
measurements more complicated, will not influence the final
results, aiming to investigate the energy-transfer process and
determine its rate.
The absorption spectra of the separate components and of
the assemblies are shown in Figure 2. In the assembly the visible
region is dominated by the already mentioned MLCT bands of
the ruthenium units, and the close UV region by the charac-
teristic absorption bands of the anthracene moiety that also
shows an intense band at 253 nm. Since Ru(bpy)32+ is a known
sensitizer for singlet oxygen and anthracene is known to form
The quenching process can be due in principle to two different
mechanisms: photoinduced electron transfer from the anthracene
to the ruthenium moiety or energy transfer from the excited
(49) Boyde, S.; Strouse, G. F.; Jones, W. E., Jr.; Meyer, T. J. J. Am. Chem.
Soc. 1989, 111, 7448-7454.
(50) Belser, P.; Dux, R.; Baak, M.; De Cola, L.; Balzani, V. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 595-598.
(51) De Cola, L.; Balzani, V.; Belser, P.; Dux, R.; Baak, M. Supramol. Chem.
1995, 5, 297-299.
(52) Klessinger, M.; Michl, J. Excited States and Photochemistry of Organic
Molecules; VCH: Weinheim, 1995.
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A R T I C L E S
Kercher et al.
Figure 3. Transient absorption spectra of the ruthenium-anthracene assembly in deaerated acetonitrile (time frame 50 µs). (Inset) Kinetics of the decay
trace measured at 425 nm.
ruthenium unit to the lowest excited state of the anthracene.
The electron-transfer process can be ruled out because of the
endoergonicity of the process (∆G ) +0.32 V).53 The occur-
rence of energy transfer from the Ru-based to the anthracene-
based component can be explained on the basis of the schematic
energy level diagram (Scheme 6), showing that the energy is
3
transferred from the CT Ru-based excited state to the lowest
triplet excited state of anthracene (T1), which then is radiation-
less deactivated to the ground state. The driving force for this
exoergonic process is ∆G ) -0.30 eV, as calculated from the
involved energy levels.53
In such a process the lowest triplet excited state of anthracene
must be populated, and time-resolved transient spectroscopy has
indeed shown that a strong absorption band is formed after the
laser pulse (2 ns) at about 430 nm which has a lifetime of τ )
Figure 4. Absorption spectra of the ruthenium-anthracene assembly (C
125 µs (Figure 3). As already shown,49 this band is characteristic
≈ 3 × 10-5 M) at t ) 0 (full line) and after 1, 2, 3, and 4 h (broken lines)
of illumination in aerated acetonitrile. (Inset) Emission spectra under same
conditions.
of a triplet-triplet absorption, and the extremely long lifetime
in deaerated solution confirms this assignment.
The efficiency of the energy transfer in the experimental
then may evolve to give other products (indicated by Ru-Sc-
conditions used (C ≈ 10-5 M) excludes any possible bimolecular
X) where the central ring of anthracene has lost its aromatic
character (eq 3):52,54
process. However, to gain further proof that the Ru complex
and the anthracene are linked via the Sc unit a photochemical
experiment was performed. Upon irradiation in aerated solution
Ru-Sc-An + O2 f Ru-Sc-An + (1∆)O2
(1)
with a 250-W Xe-lamp, equipped with an interference filter to
select the 460-nm band of the Ru(bpy)32+, the absorption
spectrum of the assembly changes dramatically (Figure 4). In
particular the disappearance of the anthracene bands at 250 and
340-400 nm is observed. On the other hand, the emission
intensity of the Ru-based component increases up to 50% over
the irradiated period.
The results obtained in aerated solution can be interpreted
by sensitization via the Ru-based 3CT excited state of the Ru-
Sc-anthracene (Ru-Sc-An) assembly, with formation of singlet
oxygen (eq 1), followed by attack of singlet oxygen on an
anthracene ring to form an endoperoxide derivative (eq 2), which
Ru-Sc-An + (1∆)O2 f Ru-Sc-An(O2)
Ru-Sc-An(O2) f Ru-Sc-X
(2)
(3)
The quenching process operated by the anthracene-based
moiety in the photoproducts (eq 3) (where the anthracene
aromaticity has been destroyed and, as a consequence, the T1
level is no longer the lowest excited state, Scheme 6) cannot
occur. This result indicates that the T1 excited state of anthracene
does indeed play a role in the energy-transfer process, quenching
the Ru-based emission observed for Ru-Sc-An and more
importantly that the two units are connected.
(53) Calculated from the E00 of the Ru-based component (17170 cm-1) and the
energy of the lowest triplet state of 9-acyl-anthracene (14700 cm-1) (see:
Murov, S.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd
ed.; Marcel Dekker: New York, 1993).
(54) Schmidt, R.; H.-D. Bauer J. Photochem. 1986, 34, 1-12.
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PET in Dynamic Self-Assembled Donor−Acceptor Arrays
A R T I C L E S
Scheme 7. Energy Diagram of the Assembly Ru-Sc-L-D with a
Schematic Representation of the Photoinduced Electron-Transfer
Processa
Figure 5. Absorption spectra of Ru-Sc-An in acetonitrile (straight line)
and upon addition of 2,4-pentadione (dashed line). (Inset) Emission spectra
under same conditions. C ≈ 10-5 M.
a Full arrows indicate radiative processes, whereas dashed arrows
represent radiationless pathways.
The same experiment has been performed at identical
conditions, but using 1 equiv of 5 and 2 equiv of L-A without
addition of any source of scandium ions. The absorption spectra
show the same changes for the anthracene bands, over time, as
those observed for the assembly. Obviously the ruthenium
moiety still generates singlet oxygen upon irradiation, which
reacts with anthracene to give endoperoxides in solution,
effecting the absorption patterns of L-A. However, the ruthe-
nium luminescence remained unchanged in this experiment
because of no interaction with the anthracene.
Finally further evidence for the dynamic character of the
assembly comes from a ligand competition study. An excess
of 2,4-pentadione was added to a solution of the donor-acceptor
assembly. Since the average lifetime of the scandium complexes
is in the order of ms, a rather fast exchange of ligands was
expected. The excess of 2,4-pentadione will lead to a disas-
sembling of some of the dyad and the displacement of L-A
with unsubstituted acetyl acetonate. The substitution of the
9-acyl-anthracene (quencher) with the “naked” acac would
therefore lead to an increase in the emission intensity since the
deactivation pathway (energy transfer) present in the assembly
has been removed. Indeed the integration of the emission spectra
before and after the addition of the 2,4-pentadione shows a
significant increase of intensity up to 44% upon excitation in
the isoabsorbative wavelength of 460 nm. The absorption spectra
of the solution remains unchanged above 350 nm upon addition
of excess of 2,4-pentadione (Figure 5).
To estimate any bimolecular electron-transfer contribution,
5 and L-D were mixed in a ratio of 1:2 in absence of scandium
ions. The data showed no evidence for bimolecular processes
under the experimental conditions (C ≈ 10-5 M, aerated
acetonitrile) employed. In fact a monoexponential decay, (τ
)150 ns) was observed that, as already discussed, corresponds
to the value of the free ruthenium component 5 (see Table 1).
In the presence of scandium ions under identical experimental
conditions the formation of assemblies (16-19, see Scheme 4)
can occur, and a dyad is formed. Such assembly formation can
be followed spectroscopically since in 16 and 17 a decrease in
the emission intensity of the Ru-based component should be
expected on the basis of a thermodynamically allowed photo-
induced electron transfer from the donor to the ruthenium
component (Scheme 7). Indeed a quenching of the Ru-based
component has been observed. The emission decay, monitored
at 640 nm, becomes biexponential with a long component, due
to the unquenched Ru(bpy)32+ unit, and a short-lived component,
τ ) 10 ns, due to the quenched luminescence. Upon light
excitation an efficient electron transfer from the donor-based
component, L-D, to the excited ruthenium unit (electron
acceptor) is expected on thermodynamical grounds (Scheme 7).
The process is in fact exoergonic (∆G ) -0.41 V)55 and in
acetonitrile at room temperature is expected to be fast for the
assembly. The rate calculated for the forward electron-transfer
From all these results we conclude that within the donor-
acceptor scandium complexes Ru-Sc-An the emission of the
Ru(bpy)32+ moiety is quenched by a fast intramolecular triplet-
triplet energy-transfer process from the Ru-based component
to 9-acyl-anthracene-bearing ligands. The rate constant calcu-
lated from the quenched and unquenched ruthenium excited-
process is ket ) 9 × 108 s-1
.
Oxidized tetramethyldiaminobenzene has a well-known ab-
sorption band between 500 and 750 nm. We have therefore used
time-resolved transient absorption spectroscopy to detect the
formation of the tetramethyl-phenylendiamino radical cation.
Excited Ru(bpy)32+ exhibits two absorption bands (bipyridinium
radical anion), resulting from the transfer of an electron from
the ruthenium to the bipyridine (MLCT transition), as can be
clearly seen in Figure 6a. The absorption at 550 nm unfortu-
nately overlaps with the band expected for the oxidized radical
cation of L-D. Nevertheless, from a comparison between the
state lifetime is ken ) 2.5 × 108 s-1
.
Self-Assembly of Electron Donor-Acceptor Dyads. In-
tramolecular Electron Transfer. Substitution of the “naked”
acac ligand with a 1,4-N,N,N′,N′-tetramethyldiaminobenzene
derivative, L-D, leads to the possibility to build up an assembly
containing an electron donor, L-D and an electron acceptor
complex 5. To have the same statistical complexes as in the
previous section, 5 and L-D were mixed in a ratio of 1:2 in the
presence of scandium ions.
(55) Calculated from the redox potentials of the single components, determined
in 0.1 M TBAPF6-CH3CN against Fc/Fc+ (L-D0/+: -0.07 V.; Ru-
(bpy)32+*/+: 0.34 V).
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A R T I C L E S
Kercher et al.
Figure 6. Transient absorption spectra of 5 (a) and (bpy)2Ru(bpy-L)-Sc-L-D (b), 25 ns time frame, excitation wavelength is 460 nm. (c) Difference
spectra between (a) and normalized (b) frame 1. (d) Decay traces of (bpy)2Ru(bpy-L)-Sc-L-D after 1 (bold line), 2, 5, 10, and 20 ns. (Inset) Lifetime trace
of the radical cation with τ ) 40 ns.
transient absorption spectra of 5 and those of the full assembly
of 5 with Sc and L-D we are able to proof the formation of the
characteristic radical cation of tetramethyl-phenylendiamine
(Figure 6).
shorter time scale (Figure 6d). As can be seen, the first spectrum
does not correspond to the full formation of the radical cation,
since the band is still growing after 2 ns. At longer delays (10,
20 ns) the decay of this species can be monitored, and a lifetime
of τ ) 40 ns was estimated (see inset, Figure 6d). From the
decay of the radical cation absorption, we have calculated the
rate for the back-electron-transfer reaction, kback ) 2.5 × 107
Figure 6a displays the spectra of 5, recorded in acetonitrile
with 25 ns between each frame with minimum instrumental gate
time of 5 ns. The first frame was recorded at 1ns after the laser
pulse. In the spectrum, the negative band at 460 nm is due to
the bleaching of the ground state of Ru(bpy)32+. The strong band
at 375 nm and the weak band above 520 nm are due to the
formed bipyridinium radical anion since a MLCT state is the
lowest excited state. The ratio between these two bands is about
3:1. In Figure 6b we show the spectra of the full assembly
(bpy)2Ru(bpy-L)-Sc-L-D, recorded under identical conditions.
The band above 520 nm is much more dominant in this case,
and the ratio between this one and the 375-nm band rose to
almost 1:1. By normalizing the first frame of these two graphs
to the same intensity of the bipyridinium radical anion band at
375 nm and subtracting them from each other, we obtained the
transient absorption spectrum, displayed in Figure 6c.56 By
comparison with the spectra reported in the literature for the
radical cation of tetramethyl-phenylendiamine,56 it is clear that
the transient in Figure 6c is indeed the same species. To follow
the formation of the transient band at 550 nm (forward electron
transfer) and its decay, that from the emission lifetime should
occur within 10 ns, we performed the same transient spectra in
s-1
Conclusions
.
2+
Substituted â-diketones bearing either Ru(bpy)3 as an
energy-donor or electron-acceptor component, 9-acyl-anthracene
as acceptor moiety for energy transfer, or 1,4-N,N,N′,N′-
tetramethyldiaminobenzene as electron-donating group have
been used to form photoactive dyads around a Sc(III) ion by
self- assembling. Assemblies obtained by coordination with Sc-
(III) ion having Ru(bpy)3-based components and anthracene-
substituted ligands show efficient intramolecular energy transfer
from the excited ruthenium complex to the lowest excited state
of the anthracene-bound component. Within scandium com-
plexes of Ru(bpy)3 and 1,4-N,N,N′,N′-tetramethyldiaminoben-
zene-substituted ligands photoinduced electron-transfer processes
were detected. It is interesting to notice that even though the
energy and electron donor-acceptor systems are not directly
linked, the photoinduced processes are rather fast. The Sc(III)
ion plays only a structural role and is not directly involved in
the process. The use of the dynamic assembly strategy for the
generation of photoactive donor-acceptor dyads provides access
to systems which are not static and react on external stimuli.
(56) A reference spectrum for the tetramethyl-phenylendiamino radical cation
can be found, for example, at: Steenken, S.; Vieira, A. J. S. C. Angew.
Chem., Int. Ed. 2001, 40, 571-573.
9
11550 J. AM. CHEM. SOC. VOL. 124, NO. 38, 2002

PET in Dynamic Self-Assembled Donor−Acceptor Arrays
A R T I C L E S
Such systems are of high complexity, and their study is a
challenge. However, due to their dynamic nature they may offer
advantages for practical applications.
H.Z. thanks the state of Niedersachsen for a graduate fellowship.
We thank Dr. R. M. Williams for helpful discussion and his
help in the preparation of the manuscript. We are grateful to
the MS Service of the University of Fribourg/ Switzerland for
high resolution ESI mass spectra.
Acknowledgment. This work was supported by the Volks-
wagen Foundation. M.K. thanks the Graduate College Sensory
photoreceptors, University of Regensburg, Germany, for support.
JA026695G
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