Surface functionalization of cadmium sulfide quantum confined nanoclusters 6: Evidence of facile electronic communication between remote surface sites

Article abstract of DOI:10.1016/S0277-5387(99)00363-0

The preparation and derivatization of a series of surface functionalized ~ 30 ? cadmium sulfide nanoclusters possessing surfaces of varied electron demand, tethered covalently to an electroactive fluorescent ruthenium polypyridyl complex, are described. Electrochemical analyses of these nanoclusters bearing ruthenium moieties yield a monotonic relationship between the Ru(II/III) oxidation potential and the electron withdrawing nature of the cluster surface, indicating that the electron density at the electroactive ruthenium center is indeed influenced by remote electropositive and electronegative substituents attached to the nanocluster. Further evidence of cluster mediated electronic interactions between remote surface moieties presents itself in the form of a dependence of the integrated ruthenium based MLCT emission intensity on the mole per cent of attached thiolate capping agent possessing an electron withdrawing substituent in the position para to the point of cluster attachment. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00363-0

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 331–341
Surface functionalization of cadmium sulfide quantum confined
nanoclusters
6: Evidence of facile electronic communication between remote
surface sites1
Jonathan G.C. Veinot, Abdiaziz A. Farah, Josie Galloro, Fabio Zobi, Valerie Bell,
*
William J. Pietro
Department of Chemistry, York University, 4700 Keele Street, Toronto, Ont. M3J 1P3, Canada
Received 30 April 1999; accepted 28 September 1999
Abstract
˚
The preparation and derivatization of a series of surface functionalized ;30 A cadmium sulfide nanoclusters possessing surfaces of varied
electron demand, tethered covalently to an electroactive fluorescent ruthenium polypyridyl complex, are described. Electrochemical analyses
of these nanoclusters bearing ruthenium moieties yield a monotonic relationship between the Ru(II/III) oxidation potential and the electron
withdrawing nature of the cluster surface, indicating that the electron density at the electroactive ruthenium center is indeed influenced by
remote electropositive and electronegative substituents attached to the nanocluster. Further evidence of cluster mediated electronicinteractions
between remote surface moieties presents itself in the form of a dependence of the integrated ruthenium based MLCT emission intensity on
the mole per cent of attached thiolate capping agent possessing an electron withdrawing substituent in the position para to the point of cluster
attachment.
q2000 Elsevier Science Ltd. All rights reserved.
Keywords: CdS nanoclusters; Ruthenium(II) polypyridyls; Functionalized nanoclusters; Mesophase materials; Molecular devices
1. Introduction
behavior of the conduction electrons from Bloch wave
mechanics to particle-in-a-sphere wave mechanics. To date,
one fundamental question crucial to the eventual develop-
ment of such proposed devices (i.e. tunnel, negative resis-
tance and coulomb blockade devices, and single electron
diodes and transistors) [11–13] still remains unanswered:
will the quantum size effect lead to a loss of ability of the
semiconductor to mediateelectroniccommunicationbetween
surface groups?
Previously, we demonstrated that the electronic and phys-
ical properties of thiolate-capped CdS nanoclusters are influ-
enced significantly by the characteristics of substituents in
the position para to the point of attachment to the cluster on
an aromatic thiolate capping agent [14–17], and we pre-
sented evidence for facile surface–core communication of
conduction carriers [1]. In this contribution we present the
preparation of a ruthenium polypyridyl complexwhich, when
attached covalently to mixed surface nanoclusters via previ-
ously reported esterification methods [16], serves as an
effective electrochemical and fluorescent probe of cluster
Within the past two decades there have appearednumerous
examples of experimental and theoretical investigations
focused on semiconductor nanoclusters[2–7]. Manyofthese
studies have been predicated on the future applicationofthese
interesting nanoscaled materials to molecular electronic
devices [8–10]. The unique optical and electronic properties
exhibited by semiconductor nanoclusters arisefromthequan-
tum size effect, which results from the splitting of the bulk
semiconductor band structure into discrete, albeit closely
spaced, electronic energy levels and a concurrent widening
of the band gap [7]. This phenomenon becomes manifest
when the size of the cluster approaches that of the exciton of
the semiconductor, and can be thought of as a shift in the
* Corresponding author. Tel.: q1-416-736-5246; fax: q1-416-736-5936;
e-mail: pietro@yorku.ca
1For part 5 in this series, see Ref. [1].
0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd. All rights reserved.
PII S0277-5387(99)00363-0
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mediated electronic communication between remote surface
moieties.
2.3. Preparation of 2-(2-pyridyl)-4-carboxyquinoline
sodium salt (3)
2-(2-Pyridyl)-4-carboxyquinoline was prepared using a
modification of the literature preparation described by Bass
and Morgan [18]. 18 g (0.12 mol) of isatin (2,3-indoline-
dione) and 15 g (0.12 mol) of 2-acetylpyridine were com-
bined and stirred for 30 min. The reaction mixture was
subsequently cooled to 58C and 60 ml of 33% aqueous NaOH
was added with stirring. The ensuing reaction mixture was
warmed to ;608C. The addition of 60 ml of ice wateryielded
a purple–red slurry, which was recovered by filtration and
washed with water. The crude product was dissolved in a
minimum of hot water and decolorized using NORIT acti-
vated carbon pellets. Upon cooling, lightpurpleshinycrystals
of the sodium salt were obtained and dried in vacuo.
2. Experimental
2.1. Chemicals
Water waspurified by passagethroughaSybron-Barnstead
D8902 mixed-bed ion exchange column followed by a
Sybron-Barnstead D8204 activated charcoal filter. Reagent
grade dimethylformamide (DMF), acetonitrileandmethanol
were purchased from Caledon Laboratories (Georgetown,
˚
Ont., Canada). DMF was dried over 4 A molecular sieves
for 12 h and filtered prior to use, while acetonitrile and meth-
anol were used as received. All deuterated solvents used for
1H NMR spectroscopy were purchased from Cambridge Iso-
tope Laboratories (Andover, MA) in sealed ampules and
used immediately upon opening. RuCl3PxH2O, sodium sul-
fide nonahydrate, isatin (2,3-indolinedione), 2-acetylpyri-
dine, and sucrose were purchased from Sigma-Aldrich and
used as received. Cadmium acetate (Aldrich reagent grade)
was recrystallized with concurrent dehydration from glacial
acetic acid and dried at 1008C in vacuo for 24 h. 4-Hydroxy-
thiophenol was obtained reagent grade from Sigma-Aldrich
and distilled at reduced pressure immediately prior to use. 4-
Nitrothiophenol and 2,29-bipyridine were obtained reagent
grade from Sigma-Aldrich, recrystallized from absolute eth-
anol and stored in a nitrogen atmosphere at 58C untilrequired.
Phenolic surface CdS nanoclusters (1) were prepared using
the kinetic trapping method as described previously [16].
Yield 18.43 g (67%). M.p. )3408C (literature)3008C).
2.4. Preparation of 2-(2-pyridyl)-4-carboxyquinoline (4)
17.3 g of 3 were dissolved in 300 ml of water and titrated,
with stirring, to pH 7 using 10% HCl. Off-white crystalswere
recovered by filtration and dried in vacuo.
1
Yield 13.71 g. M.p. )3008C (literature )3008C). H
NMR (DMSO, ppm): d 8.99 (s, 1H), 8.78 (d, 1H), 8.76 (d,
1H), 8.62 (t, 1H), 8.21 (d, 1H), 8.05 (t, 1H), 7.75 (t, 1H),
7.56 (t, 1H). EI-MS (m/z): 250, 206.
2.5. Preparation of cis-[Ru(bpy)2]Cl2 (5)
cis-Ru(bpy)2Cl2 was prepared using a modified literature
preparation as outlined by Meyer and coworkers [19]. 2.54
g of RuCl3PxH2O, 3.78 g (24.20 mmol) of 2,29-bipyridine
and 6.7 g (0.15 mol) of LiCl were refluxed in DMF (40 ml)
under a nitrogen atmosphere for 8 h with vigorous stirring.
The reaction mixture was allowed to cool to room tempera-
ture and then 200 ml of acetone was added. After standing
for 18 h at 48C, a dark solid was recovered, washed three
times with 30 ml portions of water and dried in vacuo. The
recovered product was subsequently Soxhlet extracted with
methylene chloride for a period of 4 days. The purple meth-
ylene chloride solution was concentrated to half its original
volume and the resultant shiny dark purple crystals were
recovered and dried in vacuo.
2.2. Mixed surface X%NO2/Y%OH quantum dot synthesis
(2a–2f)
The synthesis of 2c (16% NO2) will be discussed as a
representative sample for all mixed surface ‘nitro-phenolic’
CdS nanoclusters (2a–2f). Two solutions were prepared in
a 1:1:2 MeOH:MeCN:H2O (v:v:v) solvent mixture. A 20 ml
cadmium acetate solution (1.7 g, 7.4 mmol) was prepared
and degassed thoroughly with N2. A second solution (20 ml)
of sodium sulfide nonahydrate (0.7 g, 2.9 mmol), 4-nitro-
thiophenol (0.2 g, 1.3 mmol) and 4-hydroxythiophenol (1.2
g, 9.5 mmol) was degassed and added to the cadmiumacetate
solution yielding a yellow precipitate. The precipitate was
repeatedly sonicated in water and recovered by centrifuga-
tion. The precipitate was then suspended in acetonitrile,
reprecipitated upon addition of water, recovered by centrif-
ugation, and dried in vacuo for 12 h.
Yield 1.61 g, 3.3 mmol, (14%). M.p. )3008C. 1H NMR
(DMSO-d6, ppm): d 10.00 (d, 2H), 8.30 (d, 2H), 8.13 (d,
2H), 7.91 (dd, 2H), 7.51 (dd, 2H), 7.49 (d, 2H), 6.93 (dd,
2H). UV–Vis (methanol, nm): 364, 524.
Yield 0.8 g. 1H NMR (DMSO, ppm): d 8.6–9.02 (m, 1H),
5.9–7.8 (m, aromatic 5H). FT-IR (KBr, cmy1): 3288 (s, v
br, O–H stretch), 1654 (s), 1920, 1750 (w, 1,4-disub-f
overtone), 1613 (m), 1576 (m), 1489 (vs, NO2 asymmetric
stretch), 1432 (s, ring C_C), 1331 (vs, NO2 symmetric
stretch), 1090 (s), 822 (m), 745 (m, f C–H out-of-plane),
635 (m). UV–Vis (DMF, nm): 404.
2.6. Preparation of [Ru(bpy)2(2-PCQH)]Cl2 (6a)
cis-Ru(bpy)2Cl2 (0.25 g, 0.52 mmol) was dissolved in 10
ml of methanol and a methanol solution (10 ml) containing
AgNO3 (0.241 g, 1.04 mmol) was added. The reaction mix-
ture was stirred for 2 h yielding an orange solution and white
precipitate. The precipitate was removed by filtration and
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333
0.13 g (0.55 mmol) of 4 was added. The reaction mixture
2.9. Electrochemical analysis of 7, 8a–8f
was refluxed for 16 h under a N2 atmosphere. The solvent
was removed in vacuo, and the orange–red residue was puri-
fied by column chromatography on alumina using a 1:1 meth-
anol:chloroform eluent.
Yield 0.31 g, 0.42 mmol (81%). M.p. 280–2828C (dec.).
1H NMR (DMSO-d6): d 8.9 (d, 1H), 8.8 (m, 2H), 8.7 (s,
1H), 8.6 (d, 1H), 8.5 (d, 1H), 8.4 (d, 1H), 8.15 (m, 6H),
8.0 (t, 1H), 7.8 (d, 1H), 7.7 (d, 1H), 7.6 (m, 8H), 7.3 (t,
1H). FT-IR (KBr, cmy1): 3410 (m, br, OH stretch), 3070
(m, br, aromatic C–H stretch), 1610 (s, sharp, C_O stretch).
UV–Vis (methanol, nm): 450, 482 shoulder. ES-MS (m/z):
334.0, 333.5, 333.0, 332.5, 332.0, 331.5, 331.0, 330.5, 330.0,
329.5, 329.0.
2.9.1. Electrode preparation
A homogeneous conductive carbon paste was prepared by
thoroughly mixing 1.2 g of graphite powder with 200 ml of
heavy paraffin oil. Electrodes were subsequently constructed
by thoroughly mixing 100 mg of the standard paste matrix
with 20 mg of the nanocluster to be studied, in subdued light,
until a homogeneous mixture was obtained. A ;2 mm thick
pellet of the paste was formed in the tip of a 1 ml glasssyringe
(diameter ;2 mm) and electrical contact was achieved via
the compression of a silver powder layer which was in turn
addressed with a copper wire.
2.9.2. Differential pulse polarography of 7, 8a–8f
Differential pulse polarography was carried out using a
Cypress Inc. computer controlled potentiostat system in a 0.1
M TBAPF6 acetonitrile solution with a graphite paste work-
ing electrode, platinum wire counter electrode and platinum
wire pseudo-reference electrode. All electrochemical meas-
urements were calibrated using an internal ferrocene stan-
dard. The pulse height, pulse width, and sample time for all
differential pulse polarography measurements were 50 mV,
40.0 ms, and 35.0 ms, respectively. Prior to analysis, all
electrolyte solutions were thoroughly degassed with argon
and all experiments were carried out in an argon atmosphere.
2.7. Preparation of [Ru(bpy)2(2-PCQH)](PF6)2 (6b)
0.2 g (0.27 mmol) of 6a was dissolved in water and the
addition of an aqueous solution of excess of NH4PF6 resulted
in the precipitation of an orange solid. The precipitate was
isolated by centrifugation and washed three times with water.
Yield 0.19 g. M.p. 244–2468C (dec.). 1H NMR (DMSO-
d6): d 9.0 (d, 1H), 8.9 (m, 2H), 8.7 (m, 4H), 8.2 (m, 5H),
8.0 (m, 2H), 7.6 (m, 11H), 7.3 (t, 1H). ES-MS (m/z):
334.0, 333.5, 333.0, 332.5, 332.0, 331.5, 331.0, 330.5, 330.0,
329.5, 329.0.
2.10. Voltammetric analysis of 6a, 6b
2.8. Covalent attachment of 6b to 1, 2a–2f via esterification
(7, 8a–8f)
Cyclic voltammetry was performed with a Princeton
Applied Technologies PAR 263 potentiostat/galvanostat
using an electrode system which consisted of a platinum disk
working electrode (diameter 0.5 mm), a platinum wire
counter electrode and platinum wire pseudo-reference elec-
trode which were separated from the bulk solution by a pol-
ished graphite frit. Samples were prepared in a stock 0.1 M
TBAPF6 solution in electrochemical grade dimethylformam-
ide and degassed with dry argon prior to analysis. All meas-
urements were done at a scan rate of 100 mV sy1 and
potentials were standardized using an internal ferrocene
standard.
The preparation of 7 will be presented as a representative
example of the derivatization of CdS nanoclusters with poly-
pyridyl ruthenium probe through an ester linkage. 30 mg of
6b were refluxed in SOCl2 (10ml)underadryN2 atmosphere
for 3 h to yield a purple solution. The excess solvent was
removed in vacuo, and 100 mg of imidazole in 10 ml of
toluene were added to the purple residue and refluxed for 1.5
h. The toluene was removed and the residue was heated at
608C in vacuo to sublime excess imidazole from the reaction
flask. All procedures involving nanoclusters were carried out
under subdued light. 300 mg of 1 were suspended in 10 ml
of DMF with sonication. The DMF suspension was added to
the reaction flask containing the imidazole derivative of 6b
and stirred for 1 h. The reaction mixture was quenched with
water and the resulting orange solid was isolated by centrif-
ugation and purified through repeated sonication/centrifu-
gation cycles in water, methanol and diethyl ether. The solid
was dried in vacuo for 12 h.
2.11. Spectroscopic analyses
IR spectra were recorded as KBr pellets with a Mattson
1
3000 Fourier transform IR spectrometer. H NMR spectra
were performed on a Brucker ARX 400 MHz nuclear mag-
1
netic resonance spectrometer. H NMR samples were soni-
cated in DMSO-d6 and centrifuged prior to analysis.
Electronic spectra were performed in 1 cm quartz cuvettes
using a Hewlett-Packard 8452 diode array spectrometer.
Solutions for fluorescence studies were thoroughly deoxy-
genated with argon and measurements were made using a
Shimadzu RF-551 spectrofluorometric detector driven by
PC-551 software.
Yield 105 mg. 1H NMR (DMSO-d6, ppm): d 6.0–9.2 (br
M, aromatic and OH protons). FT-IR (KBr, cmy1): 3265
(s, v br, OH stretch) 1874 (w, 1,4-disub-f overtone), 1652
(m), 1582 (m), 1488 (s), 1431 (m), 1383 (w), 1256 (s),
1170 (m), 1092 (w), 821 (m), 763 (w), 635 (w). UV–Vis
(DMF, nm): 310 (shoulder), 430, 505.
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2.12. Transmission electron microscopy
predetermined mole percentages of an electronegative sur-
face moiety (4-nitrothiophenolate) and an electron donating
surface moiety (4-hydroxythiophenolate), we have prepared
a series of CdS quantum dots (2a–2f) with varied surface
electronic properties as well as surface moieties which are
easily derivatized via well established esterification methods
[16].
Transmission electron microscopy (TEM) was performed
using a Phillips EM-301 transmission electron microscope.
Nanocluster samples were prepared by placing a few drops
of freshly sonicated and centrifuged DMF suspension of the
desired cluster (2.5 mg of nanocluster in 3 ml of solvent)
onto J.B.EM Services (Pointe Claire, Dorval, Que.)JBS-183
300 mesh carbon coated copper grids (cleaned in acetone,
chloroform and again in acetone followed by air drying). The
excess solvent was drawn off with a piece of filter paperwhile
holding the grid in non-capillary forceps, leaving aggregates
as well as individual nanoclustersadheringtothegridsurface.
Grids were then air dried at room temperature. Size calibra-
tion was performed using a precision silicon grid of 21600
Proton NMR analyses of 2a–2f show characteristic homo-
geneous peak broadening with superimposed structure simi-
lar to that discussed for other CdS nanocluster systems
indicative of the multiple chemical shift environments and
slow cluster tumbling on the NMR timescale [1,14,15,21].
Regions of the 1H NMR spectra of 2a–2f can be assigned to
aromatic (;6–8 ppm) and phenolic protons (8.5–9.95
ppm). The integration of these broadened regions provides
a suitable determination of the relative percentages of 4-
hydroxythiophenolate and 4-nitrothiophenolate actually
incorporated as surface caps. The 1H NMR spectrum of 2a,
corresponding to 6 mol% 4-nitrothiophenolate, is shown in
Fig. 1. For all mixed surface nanoclusters (2a–2f), 1H NMR
integration ratios indicate that the fraction of 4-nitrothio-
phenolate surface caps is somewhat greater than that defined
by the relative solution mercaptanconcentrationsduringclus-
ter synthesis; thisisexpectedconsideringtherelativeacidities
of 4-nitrothiophenol (pKa(SH)s4.5) [22] and 4-hydroxy-
lines cmy1
.
3. Results and discussion
The covalent attachment of an electroactive transition
metal fluorophore to the surface of a semiconductor nano-
cluster offers an opportunity to probe the electronic interac-
tions between remote surface moieties as mediated by the
quantum dot core. All ‘parent’ CdS nanoclusters were pre-
pared using a modification of the kinetic trapping method
(Scheme 1) thus providing control of the cluster size and
surface composition while facilitating the introduction of
chemically active moieties [1,14–17,20]. We have success-
fully controlled the electron withdrawing/donating proper-
ties of nanocluster organic surface layers through careful
consideration of the surface capping agent composition
employed in the initial reaction mixture. By incorporating
1
thiophenol (pKa(SH)s6.75) [23]. A summary of the H
NMR spectra and surface compositions for 2a–2f are
included in Tables 1 and 2, respectively.
All mixed surface nanoclusters prepared in this work (2a–
˚
2f) lie in a 20–30 A size regime. The diameters of nanoclus-
ters with low percentages of surface tethered f-NO2 (2a)
may be determined by applying tight binding band theory
analysis to data obtained from electronic spectroscopy as
previously described, and are in good agreement with meas-
urements obtained by TEM and X-ray analysis [14–17,24–
26]. Unfortunately, the diameters of clusters with surface
mole percentages of the 4-nitrothiophenolate exceeding
1
Fig. 1. H NMR spectrum of 2a in DMSO-d6 showing distinctive signals
Scheme 1.
from aromatic and phenolic protons.
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Table 1
Summary of 1H NMR analysis of 2a–2f and 8a–8f in DMSO-d6
Table 3
Electronic spectroscopy and cluster dimension data for 2a–2f and 8a–8f
Compound
vs. TMS
Compound
lmax
(nm)
TBT diameter
(A)
TEM diameter
(A)
a
b
˚
˚
2a
2b
2c
2d
2e
2f
8.60–9.05 (br m, 0.94H, OH proton), 6.0–7.3 (br m, 4H,
aromatic proton)
8.60–9.01 (br m, 0.93H, OH proton), 6.0–7.8 (br m, 4H,
aromatic proton)
8.50–9.60 (br m, 0.84H, OH proton), 6.0–7.7 (br m, 4H,
aromatic proton)
8.60–9.20 (br m, 0.78H, OH proton), 5.7–7.8 (br m, 4H,
aromatic proton)
8.70–9.21 (br m, 0.49H, OH proton), 5.8–7.8 (br m, 4H,
aromatic proton)
9.95 (br m, 0.03H, OH proton), 6.7–8.25 (br m, 4H,
aromatic proton)
6.0–9.2 (broad and structured, aromatic and phenolic H)
5.9–9.5 (broad and structured, aromatic and phenolic H)
5.5–9.4 (broad and structured, aromatic and phenolic H)
5.7–10.0 (broad and structured, aromatic and phenolic H)
5.8–10.0 (broad and structured, aromatic and phenolic H)
6.0–10.0 (broad and structured, aromatic and phenolic H)
6.5–10 (broad and structured, aromatic)
2a
2b
2c
2d
2e
2f
340 (shoulder), 410
404
404
404
404
326, 404
365
340 (shoulder)
404
404
24
26"7
27"7
23"7
25"7
26"7
24"7
25"7
28"7
29"7
27"7
26"7
24"7
25"7
7
27
24
8a
8b
8c
8d
8e
8f
404
404
326, 404
7
8a
8b
8c
8d
8e
8f
a Estimated using tight binding band theory.
b
˚
"7 A represents the resolution of the transmission electron microscope.
strong dependence of the metal centered oxidation potential
upon electron donating and withdrawing properties of sub-
stituted fragments affords a means to probe the effectiveness
of a ligand (or nanocluster) to mediate the electronic influ-
ence of a remote functionality on a metal center. Indeed,
Meyer and coworkers [28–31], as well as others [32–35],
have demonstrated that a [cis-Ru(bpy)2] fragment serves as
an effective probe for the electron interaction of a third ligand
with the ruthenium metal center.
Although ruthenium polypyridyl species have previously
been attached to TiO2 nanoparticles by Meyer and coworkers
[36,37], and although these materials exhibited interesting
properties of solar energy conversion with monochromatic
efficiencies of 0.30, no detailed discussion of the electro-
chemical analysis nor the affects of remote substituents on
the ruthenium(II/III) oxidation potential was offered.
Hence, we endeavored to link covalently an electroactive
polypyridyl ruthenium fluorophore to the surface of quantum
sized cadmium sulfide where it serves to probe the extent to
which the semiconductor core mediates electron demand
between surface groups. To facilitate the covalent attachment
of a polypyridyl ruthenium complex to a phenolic surface
CdS nanocluster, it was necessary to replace a single bipyri-
dine ligand of [Ru(bpy)3]2q with a polypyridyl analog
Table 2
Yield and surface composition data for 2a–2f
a
b
Compound
Yield
Theoretical NO2
Observed NO2
(g)
(%)
(%)
2a
2b
2c
2d
2e
2f
0.63
0.67
0.66
0.64
0.58
0.55
1
5
10
15
50
90
6
7
16
22
51
97
a From mole ratio of 4-nitrothiophenol in reaction mixture.
b Determined from 1H NMR analysis.
about 7% (2b–2f) cannot be evaluated reliably using elec-
tronic spectroscopy as the excitonic absorption of the CdS
cluster is obscured by the intense n™p* transition of the f-
NO2 chromophore. For these systems, TEM analysis alone
was performed to evaluate cluster dimensions. A summary
of spectroscopic and TEM determined diameters for 2a–2f
may be found in Table 3.
FT-IR spectra of KBr pellets of 2a–2f show absorptions
characteristic of the pendant surface groups. As expected,
broad absorptions indicative of phenolic functionalities
appear at ca. 3300 cmy1 for 2a–2f, while the spectra lack the
characteristic –SH stretch at 2580 cmy1, thus confirming
exclusive thiolate binding of surface moieties. A complete
summary of the FT-IR spectra for 2a–2f appears in Table 4.
Polypyridyl complexes of ruthenium have been widely
studied [27], and are well known for their stability, long
lived charge transfer excited states, intense phosphorescence
and well characterized electrochemicalproperties.Numerous
examples of correlations between metal centered electro-
chemical potentials and electronic properties of chemical
moieties on substituents on ligands exist in the literature. The
bearing
a carboxylic acid moiety. 2-(29-Pyridyl)-4-
carboxyquinoline (4) offers a carboxylic acid group suitable
for esterification and was prepared using the coupling reac-
tion of 4-acetylpyridine and isatin (2,3-indolinedione) as
described by Bass and Morgan [18].
The 2D-COSY analysis of 4 shows numerous aromatic
signals, the most striking of which is a singlet appearing at
9.0 ppm assigned to the proton in the 3 position. The electron
impact mass spectrum of 4 shows a parent peak at m/zs250
corresponding to the loss of a proton and a fragmentation
pattern including an intense peak at m/zs206 arising from
the loss of –CO2. Scheme 2 illustrates the reaction of 4 with
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Table 4
A summary of FT-IR assignments of KBr pellets of 2a–2f and 8a–8f
Compound
Wavenumber (cmy1
)
2a
2b
2c
2d
2e
3272 (s, br, OH stretch), 1870 (vw, 1,4-disub-f overtones), 1582 (m), 1486 (s, NO2 asymmetric stretch), 1427 (m), 1356 (w),
1231 (s), 1170 (s), 1090 (w), 819 (w), 633 (w)
3250 (s, br, OH stretch), 1875 (vw, 1,4-disub-f overtones), 1578 (s), 1486 (s, NO2 asymmetric stretch), 1425 (m, ring C_C), 1364
(m), 1169 (s), 1086 (m), 1006 (m), 818 (m), 740 (w, NO2 symmetric stretch), 630 (w)
3288 (s, v br, O–H stretch) 1654 (s), 1920, 1750 (w, 1,4-disub-f overtone), 1613 (m), 1576 (m), 1489 (vs, NO2 asymmetric
stretch), 1432 (s, ring C_C), 1331 (vs, NO2 symmetric stretch), 1090 (s), 822 (m), 745 (m, NO2 symmetric stretch), 635 (m)
3440 (s, br, OH stretch), 1874 (vw, 1,4-disub-f overtones), 1625 (m, br), 1573 (m, br), 1498 (m, NO2 asymmetric stretch), 1428
(m, ring C_C), 1331 (m, NO2 symmetric stretch), 1255 (w, br), 1084 (w, br)
3445 (m, br, OH stretch), 3078 (m, br), 1868 (vw, 1,4-disub-f overtones), 1603 (m, sharp), 1573 (m, sharp), 1509 (s, NO2
asymmetric stretch), 1427 (m, ring C_C) 1338 (s, NO2 symmetric stretch), 1180 (w, br), 1110 (w, br), 1084 (w, br), 850 (m,
shrp), 740 (w, NO2 symmetric stretch)
2f
3390 (w, br, OH stretch), 3092 (w, aromatic C–H stretch), 1918, 1788 (vw, 1,4-disub-f overtones), 1646 (w), 1509 (s, NO2
asymmetric stretch), 1432 (m, ring C_C),1340 (s, NO2 symmetric stretch), 1175 (w), 1104 (w), 1082 (m), 846 (m), 736 (m, NO2
symmetric stretch), 678 (w), 529 (w)
7
3265 (s, v br, OH stretch) 1874 (w, 1,4-disub-f overtone), 1652 (m, C_O stretch), 1582 (m), 1488 (s, NO2 asymmetric stretch),
1431 (m, ring C_C), 1383 (w), 1256 (s), 1170 (m), 1092 (w), 821 (m), 763 (w), 635 (w)
8a
8b
8c
8d
8e
8f
3250 (s, v br, OH stretch) 1882 (w, 1,4-disub-f overtone), 1651 (m, C_O stretch), 1579 (m),1489 (s, NO2 asymmetric stretch),
1429 (m, ring C_C), 1258 (s), 1169 (m), 1090 (w),1009 (w), 823 (m), 763 (m), 635 (m), 512 (w)
3240 (s, v br, OH stretch), 1872 (w, 1,4-disub-f overtone), 1654 (m, C_O stretch), 1578 (m),1489 (s, NO2 asymmetric stretch),
1431 (m, ring C_C), 1259 (s), 1172 (m), 1090 (w),1009 (w), 823 (m), 762 (m), 635 (m), 512 (w)
3290 (s, v br, OH stretch), 1880 (w, 1,4-disub-f overtone), 1650 (m, C_O stretch), 1578 (m),1490 (s, NO2 asymmetric stretch),
1428 (m, ring C_C), 1255 (s), 1169 (m), 1088 (w),1006 (w), 822 (m), 753 (m), 635 (m)
3288 (s, v br, OH stretch), 3001 (m, sharp, aromatic C–H stretch), 1878 (w, 1,4-disub-f overtone), 1654 (m, C_O stretch), 1576
(m),1490 (s, NO2 asymmetric stretch), 1432 (m, ring C_C), 1258 (s), 1170 (m), 1090 (w), 912 (w), 823 (m), 745 (m), 635 (m)
3350 (s, v br, OH stretch) 1870 (w, 1,4-disub-f overtone), 1647 (m, C_O stretch), 1575 (m),1503 (s, NO2 asymmetric stretch),
1426 (m, ring C_C), 1272 (s), 1175 (m), 1090 (w), 827 (m), 743 (m), 632 (m)
3380 (s, sharp, OH stretch), 3022 (m, sharp, aromatic C–H stretch), 1888 (w, 1,4-disub-f overtone), 1651 (m, C_O stretch), 1572
(m),1406 (s, NO2 asymmetric stretch), 1429 (m, ring C_C), 1256 (s), 1165 (w), 1090 (m), 1085 (m), 1009 (w), 850 (m), 740
(m), 682 (w), 648 (m), 535 (w)
Scheme 2.
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337
cis-Ru(bpy)2Cl2 yielding ruthenium polypyridyl complex
(6a) bearing a carboxylic acid functionality which may be
exploited as a point of covalent attachment to CdS nanoclus-
ters. The 1H NMR of 6a is illustrated in Fig. 2, and shows a
series of multiplet signals in the aromatic region of the spec-
trum (;7.0–10.0 ppm), all of which have been assigned
with the aid of 2D NMR. The most striking feature of the 1H
NMR spectrum of 6a is a singlet appearing at 8.67 ppm
arising from the unique proton in the 3 position of the 2-29-
pyridylcarboxyquinoline ligand. Peak assignments, chemical
shifts, integration ratios and coupling constants for the 2D-
COSY spectrum of 6a are reported in Table 5.
y1.57 and y1.93 V versus SCE. In addition, an irreversible
chloride oxidation is also observed at 1.18 V versus SCE and
will preclude any accurate measurement of changes in the
Ru(II/III) oxidation potential of probes tethered to function-
alized nanoclusters. Thus, 6a was converted to the corre-
sponding hexafluorophosphate salt (6b) by treatment with
excess aqueous NH4PF6. The electrochemical analysis of 6b
shows a reversible Ru(II/III) oxidation at 1.27 VversusSCE
and three reversible ligand reductions at y1.26, y1.57 and
y1.93 V versus SCE, with the chloride oxidation absent.
Electronic and emission spectroscopy of 6a and 6b shows
the expected MLCT absorptions (450 nm and 482 nm shoul-
der) and MLCT emission maxima (652 nm), respectively
(Fig. 3).
Electrochemical analysis of 6a shows a well defined
reversible Ru(II/III) oxidation process at 1.27 V versus SCE
as well as a reversible 2-29-pyridylcarboxyquinoline reduc-
tion at y1.26 V and two reversible bipyridine reductions at
1
The H NMR spectra of all [Ru(bpy)2(PCQ)]2q func-
tionalized CdS nanoclusters (7, 8a–8f, Scheme 3) in this
Fig. 2. 1D- and 2D-COSY 1H NMR analysis of 6a; detailed peak assignments and coupling constants may be found in Table 5.
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338
J.G.C. Veinot et al. / Polyhedron 19 (2000) 331–341
Table 5
indicating that the cluster core remained intact throughout
1H NMR-COSY assignments for 6a
the derivatization process. The most striking change noted
1
when comparing the H NMR spectrum of the ruthenium
Proton
Chemical shift (d, ppm vs. TMS)
JH–H (Hz)
derivatized CdS functionalized nanoclusters to that of the
parent quantum dots is a loss of definition (Fig. 4(b))
between the phenolic and aromatic signal regions previously
reported for the corresponding parent nanocluster counter-
parts (Fig. 4(a)). The increased complexity in the 1H NMR
suggests that, as expected, the derivatization process caused
an increase in the number of proton environments on the
3
5
6
7
8
8.67 (s)
8.44 (d)
;7.6 (t in m)
7.27 (t)
;7.5 (d in m)
;7.5 (d in m)
8.01 (t)
;7.5 (t in m)
;8.2 (m)
8.80 (d)
;8.2 (t in m)
7.5 (t in m)
7.74 (d)
8.2 (d in m)
7.4 (t in m)
;8.2 (t in m)
8.503 (d)
8.412
7.628
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
7.840
8.396
5.564
1
cluster surface. Table 1 summarizes the H NMR spectros-
copy of 7, 8a–8f.
FT-IR assignments of KBr pellets of 7 and 8a–8f are sum-
marized in Table 2 The appearance of an absorbance at
;1650 cmy1 not present in similar spectra of 1 and 2a–2f
confirms the formation of an ester linkage between the phe-
nolic surface functionalities and [Ru(bpy)2(PCQ)]2q
.
Another significant observation is the appearance of a sharp
absorption at 3381 cmy1, characteristic of a non-hydrogen
bonded –OH, in the FT-IR spectrum of 8f, confirming a loss
of interaction between surface –OH moieties. Fig. 5 shows
the FT-IR spectrum of a KBr pellet of 8f.
8.160
5.484
7.837 (d)
7.6 (t in m)
8.2 (t in m)
8.930 (d)
8.188
8.204
8.589 (d)
The electronic spectra of CdS quantum dots derivatized
with 6b and bearing low molar percentages of 4-nitrothio-
phenolate surface caps (8a) show the characteristicexcitonic
shoulder arising from the semiconductor core, while the
MLCT absorptions of the pendant ruthenium polypyridyl
center are masked by thepresence ofaweakabsorptionshoul-
der arising from the n™p* transition of the –NO2. For clus-
ters with greater than about 7 mol% 4-nitrothiophenolate
caps, both the excitonic shoulder of the cluster core and the
MLCT absorptions of the pendant [Ru(bpy)2(PCQ)]2q are
hidden by the intense absorption attributed to the n™p*
transition of the –NO2 functionality. The electronic spectrum
of 7 is shown in Fig. 6 as a representative example of clusters
having surfaces without –NO2 moieties.
;7.6 (t in m)
8.2 (t in m)
8.806 (d)
8.396
For clusters showing excitonic absorptions, 7 and 8a, it is
possible to approximate the core diameter using a tight bind-
ing band approach as previously described [24–26]. All of
the presented clusters have been found to lie in a size regime
Fig. 3. Emission spectrum of an argon saturated DMF solution of 6b;
lexcs430 nm.
˚
of ca. 27 A. Good agreement wasfound betweentightbinding
band theory approximations and TEM analyses for clusters
2a, 7, and 8a. A summary of cluster dimensions for 7 and
8a–8f may be found in Table 3.
The properties of the covalently attached ruthenium moi-
eties on 7 and 8a–8f offer two independent methods for the
investigation of the electronic processes mediated by the
semiconductor core, emission spectroscopy and electro-
chemistry. All of the presented ruthenium functionalized
nanoclusters (7 and 8a–8f) exhibit MLCT emission
characteristic of the tethered ruthenium moiety (6b) at ca.
652 nm as shown in Fig. 7. Variation of the mole percentage
of 4-nitrothiophenolate surface cap results in a unique rela-
tionship between the mole percentageof4-nitrothiophenolate
surface functionality and the integratedfluorescenceintensity
(Fig. 8(a)). There is an observed rapid decreaseinintegrated
Scheme 3.
work show homogeneously broadened peaks similar to those
observed for all nanoclusters discussed previously (4a–4e),
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J.G.C. Veinot et al. / Polyhedron 19 (2000) 331–341
339
Fig. 4. 1H NMR spectra of DMSO-d6 suspensions of (a) 1 and (b) 7.
Fig. 6. Characteristic excitonic absorption (310 nm) of a CdS nanocluster
and ruthenium based MLCT absorptions (430 and 505 nm) in the electronic
spectrum of a suspension of 7 in DMF.
emission intensity as the mole percentage of 4-nitrothiophen-
olate cap increases from 0 to ;50% followed by a range of
slight decrease in intensity for mole percentages between
Fig. 5. FT-IR spectrum (KBr pellet) of 8f showing C_O and OH stretching
frequencies at 1651 and 3380 cmy1, respectively; detailed assignments may
be found in Table 4.
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340
J.G.C. Veinot et al. / Polyhedron 19 (2000) 331–341
SCE at the core interior. However, we have previouslyshown
that changes in the electronic properties of the clustercapping
agent, as predicted by the Hammett sp parameter, result in
an ultimate change in the electronegativity of the cluster sur-
face [1], consequently shifting the positions of the conduc-
tion and valence band edges at the cluster core/cap interface,
relative to the respective levels in the core interior.
If one considers an electron rich surface (7), the valence
and conduction bands bend up, rendering the surface rela-
tively electropositive (Fig. 9(a)), which in turn increases
the electron density at a pendant rutheniumcenterviaacluster
mediated inductive mechanism, and shiftstheRu(II/III)oxi-
dation potential of the pendant Ru(bpy)2(PCQ) moiety
(1.27 V versus SCE for the free complex; 0.497 V versus
SCE for 7). This dramatic change in Ru(II/III) oxidation
potential for cadmium sulfide nanocluster tetheredruthenium
polypyridyl probes is comparable to that observed for func-
tionalized TiO2 clusters [36,37] and is in good agreement
with the value of 0.59 V versus SCE determined using the
literature analysis of ruthenium based MLCT absorptions as
described by Lever and Dodsworth [35].
Fig. 7. Emission spectrum of a DMF suspension of 8f; lexcs430 nm.
We report here that as the mole ratio of 4-nitrothiopheno-
late cap increases from 0 to 0.97, the oxidation potential of
the pendant [Ru(bpy)2(PCQ)]2q moiety increases from
Fig. 8. Observed dependence of (a) ruthenium based MLCT emission inte-
grated intensity and (b) ruthenium(II/III) oxidation potential on mole per
cent 4-nitrothiophenolate surface functionality for 7, 8a–8f.
;50 and ;97%. From this analysis, it is evident that the
MLCT emission characteristics are effectively quenched by
only a few caps bearing remote electron withdrawing sub-
stituents, indicating that electronic communication mediated
by the semiconductor core is indeed occurring. Interestingly,
electrochemical studies of the same series of ruthenium deri-
vatized nanoclusters exhibit a similar relationship (Fig.
8(b)). Differential pulse polarography of a graphite mull of
7 and 8a–8f shows a broad reversible oxidation process aris-
ing from the pendant ruthenium center which shifts between
0.497 and 0.747 V versus SCE with increased mole percent-
age of 4-nitrothiophenolate surface.
These phenomena may be explained on the basis of a core-
mediated electronic inductive effect. The valence and con-
duction band edges of bulk CdS are 1.36 V and y1.18 V,
respectively, relative to an SCE reference [38]. The quantum
size effect symmetrically widens the observed band gap of
bulk CdS (Egs2.54 eV) to 3.65 eV for ;30 A diameter
CdS nanoclusters, providing a valence band edge of 1.92 V
versus SCE and a conduction band edge at y1.74 V versus
Fig. 9. (a) Electron donating surface caps on Ru-tagged CdS nanoclusters
(7, 8a–8f) result in an upward bending of the valence and conduction bands,
decreasing the cluster surface electronegativity, which in turn results in an
increase of electron density at the ruthenium center lowering the Ru(II/III)
oxidation potential. (b) Electron withdrawing surface caps cause the con-
duction and valence bands to bend downward, consequently increasing the
cluster surface electronegativity and increasing the Ru(II/III) oxidation
potential.
˚
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J.G.C. Veinot et al. / Polyhedron 19 (2000) 331–341
[2] A.P. Alivisatos, J. Phys. Chem. 100 (1996) 13266.
341
0.497 to 0.747 V versus SCE. Applying the inductive model
described above, as the nanocluster surface becomes more
electronegative, thevalenceandconductionbandsbenddown
and the electron density will be depleted at the ruthenium
center (Fig. 9(b)), shifting the Ru(II/III) redox couple to
higher potentials.
The core-mediated inductive model is also consistent with
the observed quenching of the Ru based MLCT emission. In
the case of an electron rich surface (7), the conduction band
edge is bent up above the PCQ p* level, and the excited p*
electron of [Ru(bpy)2(PCQ)]* remains localizedwithinthe
ligand orbital, ultimately relaxing via MLCT emission. Con-
versely, for a relatively electronegative surface, the conduc-
tion band edge bends below the PCQ p* level, resulting in
electron capture and concurrent fluorescence quenching.
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4. Conclusions
We have presented the synthesis and characterization of a
series of CdS nanoclusters having controlled surface electron
demand, and bearing an electroactive ruthenium polypyridyl
fluorophore. Electrochemical and emission studies of these
interesting materials have yielded conclusive evidence that
electronic communication can indeed be mediated by CdS
nanoclusters via a core-mediated inductive mechanism.
These results further suggest the possibility that surfacefunc-
tionalized semiconductor nanoclusters may possess device
activity.
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Acknowledgements
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The authors are indebted to Professor A.B.P. Lever, Dr
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their useful discussions. They also thank Lisa Nelson and
Karen Retheray for assistance with mass spectrometry and
transmission electron microscopy, respectively, and the Nat-
ural Sciences and Engineering Research Council of Canada
(NSERC) for financial support.
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