Diazonium salts as grafting agents and efficient radical-hydrosilylation initiators for freestanding photoluminescent silicon nanocrystals

Article abstract of DOI:10.1002/chem.201400114

The reactivity of diazonium salts towards freestanding, photoluminescent silicon nanocrystals (SiNCs) is reported. It was found that SiNCs can be functionalized with aryl groups by direct reductive grafting of the diazonium salts. Furthermore, diazonium s

Full text of DOI:10.1002/chem.201400114

DOI: 10.1002/chem.201400114
Communication
&
Diazo Compounds
Diazonium Salts as Grafting Agents and Efficient Radical-
Hydrosilylation Initiators for Freestanding Photoluminescent
Silicon Nanocrystals
Ignaz M. D. Hçhlein,[a] Julian Kehrle,[a] Tobias Helbich,[a] Zhenyu Yang,[b] Jonathan
G. C. Veinot,*[b] and Bernhard Rieger*[a]
Chem. Eur. J. 2014, 20, 4212 – 4216
4212
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
reducing or even at the open-circuit potential conditions.[16,17]
The generally accepted reaction mechanism when no bias is
applied proceeds by a one-electron reduction of the diazoni-
um salt, release of nitrogen, generation of an aryl radical from
the diazonium compound, and formation of a surface-silyl radi-
cal cation. After deprotonation, coupling of the two radicals
gives a robust silicon–carbon bond (Scheme 1).[17] The forma-
tion of silicon-surface radicals during the diazonium reaction
offers yet another path to NC surface modification through the
attachment of a variety of surface groups—diazonium reagents
could reasonably act as radical initiators to induce hydrosilyla-
tion reactions. Previously, porous silicon (p-Si), an electrochemi-
cally etched form of silicon that exhibits very high surface
areas, was functionalized with various compounds in this way
at room temperature with short reaction times.[18] In contrast
to bulk silicon, reports about the reactivity of colloidal SiNCs
with diazonium salts are scarce in literature. To date, only
direct grafting was performed, or the diazonium salts were
used as linking agents, and only comparably large SiNCs (dꢀ
50 nm) that do not exhibit size-dependent properties were in-
vestigated.[19–21]
Abstract: The reactivity of diazonium salts towards free-
standing, photoluminescent silicon nanocrystals (SiNCs) is
reported. It was found that SiNCs can be functionalized
with aryl groups by direct reductive grafting of the diazo-
nium salts. Furthermore, diazonium salts are efficient radi-
cal initiators for SiNC hydrosilylation. For this purpose,
novel electron-deficient diazonium salts, highly soluble in
nonpolar solvents were synthesized. The SiNCs were func-
tionalized with a variety of alkenes and alkynes at room
temperature with short reaction times.
Silicon nanocrystals (SiNCs) exhibit properties that differ sub-
stantially from their bulk counterpart.[1–3] Their optoelectronic
behavior (e.g., photoluminescence) is tunable with size and
surface functionality. This, in combination with their low toxici-
ty and biocompatibility, has garnered SiNCs much attention in
wide ranging applications, such as solar cells, biological
probes, and light-emitting diodes.[4–6]
Most syntheses yield freestanding SiNCs containing a reactive
SiÀH-terminated surface.[7–10] The surfaces of these
particles must be modified, if they are to be useful,
because SiNCs are sensitive toward oxidation and dif-
ficult to disperse in common solvents. In this context,
a variety of hydrosilylation methods have been devel-
oped, involving reaction with unsaturated carbon-
based compounds (i.e., terminal alkenes and alkynes).
These methods generally require high temperatures,
UV radiation, and/or metal-based catalysts.[9,11,12] Fur-
thermore, these reactions can lead to multilayer and
polymer surfaces that can passivate the SiNC sur-
face.[13] The development of new, mild, and metal-
free ways to functionalize SiNCs monolayers and
other surface groups is, therefore, a timely and at-
tractive target.
In part, because well-defined SiNCs have not been
available, the surface chemistry of hydride-terminated
bulk silicon has been more thoroughly investigated
than for its nanocrystalline counterpart.[14] An attrac-
tive and widely applied method to obtain mono- and
Scheme 1. Reaction mechanism of silicon surfaces with diazonium salts. A) Reduction of
the diazonium salt and formation of an aryl radical with release of nitrogen and forma-
tion of Si-surface radical after deprotonation. B) Possible reactions of the Si-centered radi-
multilayers of aryl groups on silicon surfaces involves cal to form SiÀC bonds.
reductive grafting of diazonium compounds.[15] This
assembly can be performed under electrochemically
Herein, we report the first investigation into the reactivity of
several diazonium salts toward photoluminescent quantum
confined SiNCs (d=3 nm). We also demonstrate that the stud-
ied diazonium salts give direct, reductive surface grafting, as
well as radical hydrosilylation initiation.
[a] I. M. D. Hçhlein, J. Kehrle, T. Helbich, Prof. B. Rieger
Wacker-Lehrstuhl fꢀr Makromolekulare Chemie
Technische Universitꢁt Mꢀnchen
Lichtenbergstrasse 4, 85747 Garching (Germany)
Fax: (+49)89-289-13562
The H-terminated SiNCs used in this work were synthesized
by following a known procedure through disproportion of hy-
drogen silsesquioxane (HSQ) giving SiNCs embedded in a sili-
con oxide matrix. The SiNCs were subsequently liberated by
etching with hydrofluoric acid and final extraction in toluene.[7]
Direct reductive grafting of diazonium salts was first investi-
gated by using commercially available 4-nitro (4-NDB) and 4-
bromobenzene diazonium tetrafluoroborate (4-BDB). For these
E-mail: rieger@tum.de
[b] Z. Yang, Prof. J. G. C. Veinot
Department of Chemistry, University of Alberta
Edmonton, Alberta, T6G 2G2 (Canada)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400114. It contains additional transmis-
sion electron microscope (TEM), X-ray photoelectron spectroscopy (XPS),
photoluminescence (PL), and dynamic light scattering (DLS) data, as well
as the experimental procedures.
Chem. Eur. J. 2014, 20, 4212 – 4216
www.chemeurj.org
4213
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
grafting experiments, SiNCs (150 mg etched SiO2/SiNC com-
pound) were suspended in a dry and degassed 0.1m solution
(2 mL) of the diazonium salt in acetonitrile and stirred for 6 h.
In each stage of the reaction, the SiNCs formed turbid suspen-
sions, so the SiNCs could be separated and purified with sever-
al centrifugation and ultrasonication steps in acetonitrile, tolu-
ene, and dichloromethane. FTIR analyses confirmed surface
modification with clear evidence of CÀC ring-stretching bands
(1400–1600 cmÀ1), and in the case of 4-NDB, the spectral signa-
ture of the NO2 moiety (1350, 1500 cmÀ1; Figure 1B and C).
However, strong residual SiÀH bands (2100 cmÀ1) imply the
coverage of the SiNCs is poor. Further supporting partial sur-
face modification, surface oxidation evidenced by SiÀO bands
(1050 cmÀ1), was observed for all samples. The present oxida-
tion of SiNC surfaces is consistent with published reports of di-
azonium-based modification of p-Si.[18]
Clearly, an extensive surface modification of small SiNCs is
not straightforward for this reaction system. It is reasonable
that surface reactivity differences noted for bulk Si and SiNCs
may result from the limited ability of the SiNC to support the
required radical cation intermediate noted in Scheme 1.
Because the surface modification of colloidal SiNCs through
direct reductive grafting of diazonium species is inefficient and
led to substantial surface oxide that can influence the opto-
electronic properties of SiNCs, we turned to the application of
diazonium salts as radical initiators.
Diazonium salts have proven to be efficient radical initiators
of hydrosilylation on p-Si surfaces.[18] It was illustrated above
that reactions suitable for modifying bulk and p-Si are not
always directly transferable to colloidal SiNCs. In this context,
we explore the application of diazonium salts as radical initia-
tors in the surface modification of freestanding SiNCs. Hydride-
terminated SiNCs tend to agglomerate in solution giving
turbid dispersions. Alkyl-surface modification achieved upon
reaction with a-olefins (e.g., dodecene) gave clear colloidal dis-
persions.[5,23] For ease of comparison, dodecene was used for
the present hydrosilylation experiments.
It is possible that the incomplete functionalization is a mani-
festation of the divergent solubility properties of the diazoni-
um salts and the H-terminated SiNCs. Acetonitrile is commonly
used solvent when manipulating diazonium compounds be-
cause of their solubility properties. Unfortunately, H-terminated
SiNCs are hydrophobic/nonpolar and do not disperse well in
this solvent system. In an effort to minimize the impact of sol-
vent polarity and improve SiNC surface coverage, we turned to
an organic-soluble diazonium salt (i.e., 4-decylbenzene diazoni-
um tetrafluoroborate (4-DDB)) known to give toluene-dispers-
ible hydrophobic aryl-grafted Au and Pt nanoparticles.[22] SiNC
modification and product isolation were performed by using
conditions identical to those described herein (see below) for
the other diazonium salts. Unfortunately, the IR spectrum of
the SiNCs showed evidence of limited functionalization, and
SiÀH and SiÀO bands were dominant (Figure 1D).
Diazonium salt induced hydrosilylation was achieved upon
dispersing H-terminated SiNCs (150 mg etched SiO2/SiNC com-
pound) in 3 mL of a toluene solution of dodecene (3 mmol).
The reaction at room temperature was initiated upon addition
of 4-DDB (15 mmol; Note: diazonium salts without long alkyl
chains, such as 4-NDB and 4-BDB, are ill-suited for reactions in
toluene because of their limited solubility). After stirring for 2 h
at room temperature and in complete darkness, the functional-
ized SiNCs were isolated and purified by established antisol-
vent precipitation with methanol. In contrast to the SiNCs con-
taining SiÀH surfaces, the present dodecyl-terminated SiNCs
form a stable clear colloidal dispersion (Figure 2). The resulting
SiNCs showed red photoluminescence (PL) with an emission
maximum at approximately 700 nm, which is typical for their
size.[24]
Figure 2. Unfunctionalized SiNCs in toluene (left), dodecyl functionalized
SiNCs in toluene under visible light and UV radiation (right).
FTIR analysis showed clear evidence of dodecyl surface ter-
mination: the CÀH stretching band (2900 cmÀ1) dominates the
spectrum, whereas substantially weaker features attributable
to trace SiÀH and SiÀO bands are also present (Figure 3A). In
contrast, the reference sample without the addition of the di-
Figure 1. FTIR spectra of A) freshly etched hydride terminated SiNCs; B) re-
acted with 4-NDB; C) with 4-BDB; and D) with 4-DDB.
Chem. Eur. J. 2014, 20, 4212 – 4216
www.chemeurj.org
4214
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Figure 3. A) Dodecyl-functionalized SiNCs with 4-DDB as radical-hydrosilyla-
tion initiator; B) reference sample without the addition of 4-DDB.
azonium salt showed no evidence of functionalization (Fig-
ure 3B).
Having established diazonium reagents do initiate surface
modification of SiNCs, we explored the influence of the diazo-
nium structure on the present reaction. Electron-deficient di-
azonium salts were reported to be more reactive towards ho-
molytic dediazotation.[25] Thus, diazonium salts containing elec-
tron-withdrawing groups are expected to be even more effec-
tive initiators for the present reactions. To test this hypothesis,
the heretofore unknown diazonium salts 2-nitro-4-decyl (2-
NO2-4-DDB) and 2,6-bromo-decyl-diazobenzene tetrafluorobo-
rate (2,6-Br-4-DDB) were synthesized and applied as hydrosily-
lation radical initiators, and their reactivity was compared to
that of 4-DDB and 4-BDB. Conveniently, because the reaction
dispersions become non-opalescent upon functionalization,
the progress of the reaction can be readily monitored qualita-
tively (Figure 4).
As was expected, the more electron-deficient 2-NO2-4-DDB
and 2,6-Br-4-DDB reacted faster than 4-DDB, as was evidenced
by the transparency of the reaction mixture. With 2,6-Br-4-DDB
being the most reactive, the suspension started to turn trans-
parent after only 30 min. Compounds 2-NO2-4-DDB and 4-DDB
needed 60 or 90 min, respectively, to reach this point. No reac-
tion, in comparison to the reference sample without diazonium
salt, was observed with 4-BDB, presumably due to its low solu-
bility in toluene.
Figure 4. Functionalization of SiNCs with dodecene by using A) 4-DDB; B) 2-
NO2-4-DDB; C) 2,6-Br-4-DDB; and D) 4-BDB as radical-hydrosilylation initia-
tors; and E) without addition of a diazonium salt.
Figure 5. Alkenes and alkynes that were used for the hydrosilylation reac-
tions from left to right: dodecyne, vinyl laurate, MMA, styrene, and ethinyl-
TMS.
If the present SiNC reactivity is to find wide application, it is
necessary to demonstrate a broad applicability and functional
group tolerance of the diazonium-salt-induced hydrosilylation.
In this context, we explored reactions with dodecyne, vinyl lau-
rate, methyl methacrylate (MMA), styrene, and ethinyltrimethyl-
silane (ethinyl-TMS) by using 2,6-Br-4-DDB as radical initiator
(Figure 5).
seem to hamper the hydrosilylation reaction. This issue will be
addressed in future work.
All given alkenes/alkynes provided stable colloidal disper-
sions that showed photoluminescence with a maximum at ap-
proximately 700 nm. The FTIR spectra of the functionalized
SiNCs are given in Figure 6 and show the expected bands aris-
ing from the surface groups in question, as well as trace SiÀH
and SiÀO bands.
The reaction times differ for each the compounds with do-
decyne being the most reactive and turning transparent after
only 10 min, whereas vinyl laurate showed the lowest reactivity
and needed 6 h until reaching this point. These differences are
possibly due to different stabilities of the carbon-centered radi-
cals, which are formed during hydrosilylation (Scheme 1B) and
are subject of ongoing investigations. We also noted that al-
kenes containing acidic protons, such as alcohols and amines,
The functionalized SiNCs were further analyzed by using dy-
namic light scattering (DLS) method. Table 1 lists the hydrody-
namic radii of the SiNCs. The values are in accordance with the
size of the SiNCs and literature results.[11]
Chem. Eur. J. 2014, 20, 4212 – 4216
www.chemeurj.org
4215
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Acknowledgements
Z.Y. and J.G.C.V. acknowledge funding from the Natural Scien-
ces and Engineering Research Council of Canada (NSERC),
Canada Foundation for Innovation (CFI), Alberta Science and
Research Investment Program (ASRIP), and University of Alber-
ta, Department of Chemistry. G. Popowich is thanked for assis-
tance with TEM. The staff at the Alberta Centre for Surface En-
gineering and Sciences (ACSES) are thanked for XPS analysis.
B. S. Soller and S. Kissling are thanked for valuable discussions.
I.M.D.H. is grateful for a generous scholarship from the Fonds
der Chemischen Industrie.
Keywords: diazo compounds
nanocrystals · surface grafting
·
hydrosilylation
·
silicon
[1] L. Brus, J. Phys. Chem. 1994, 98, 3575–3581.
[2] Z. Ding, B. M. Quinn, S. K. Haram, L. E. Pell, B. A. Korgel, A. J. Bard, Sci-
ence 2002, 296, 1293–1297.
[3] J. Heitmann, F. Mꢁller, M. Zacharias, U. Gçsele, Adv. Mater. 2005, 17,
795–803.
[4] C. Liu, Z. C. Holman, U. R. Kortshagen, Nano Lett. 2009, 9, 449.
[5] K.-Y. Cheng, R. Anthony, U. R. Kortshagen, R. J. Holmes, Nano Lett. 2010,
10, 1154–1157.
Figure 6. FTIR spectra of the hydrosilylated SiNCs with A) dodecyne; B) vinyl
laurate; C) MMA; D) styrene; and E) ethinyl-TMS.
[6] J. Liu, F. Erogbogbo, K.-T. Yong, L. Ye, J. Liu, R. Hu, H. Chen, Y. Hu, Y.
Yang, J. Yang, ACS Nano 2013, 7, 7303–7310.
[7] C. M. Hessel, E. J. Henderson, J. G. C. Veinot, Chem. Mater. 2006, 18,
6139–6146.
[8] X. Li, Y. He, S. S. Talukdar, M. T. Swihart, Langmuir 2013, 29, 8490–8496.
[9] R. D. Tilley, J. H. Warner, K. Yamamoto, I. Matsui, H. Fujimori, Chem.
Commun. 2005, 1833–1835.
Table 1. Hydrodynamic radii with polydispersities of the functionalized
SiNCs by diazonium salt initiated hydrosilylation determined with DLS.
Reagent
Hydrodynamic radius [nm]
Polydispersity [%]
[10] J. L. Heinrich, C. L. Curns, G. M. Credo, K. L. Kavanagh, M. J. Sailor, Sci-
ence 1992, 255, 66–68.
dodecene
dodecyne
ethinyl-TMS
vinyl laurate
MMA
3.8
4.4
3.6
3.5
2.5
5.3
30.3
32.3
38.9
38.4
70.2
62.6
[11] J. A. Kelly, J. G. C. Veinot, ACS Nano 2010, 4, 4645–4656.
[12] X. Li, Y. He, M. T. Swihart, Langmuir 2004, 20, 4720–4727.
[13] Z. Yang, M. Dasog, A. R. Dobbie, R. Lockwood, Y. Zhi, A. Meldrum,
J. G. C. Veinot, Adv. Funct. Mater. 2014, 24, 1345–1353.
[14] J. M. Buriak, Chem. Rev. 2002, 102, 1271–1308.
[15] J. Pinson, Aryl Diazonium Salts: New Coupling Agents in Polmyer and Sur-
face Science, WILEY-VCH, Weinheim, 2012.
styrene
[16] C. H. De Villeneuve, J. Pinson, M. C. Bernard, P. Allongue, L. De Physique,
J. Phys. Chem. B 1997, 101, 2415–2420.
[17] M. P. Stewart, F. Maya, D. V. Kosynkin, S. M. Dirk, J. J. Stapleton, C. L.
McGuiness, D. L. Allara, J. M. Tour, J. Am. Chem. Soc. 2004, 126, 370–
378.
[18] D. Wang, J. M. Buriak, Langmuir 2006, 22, 6214–6221.
[19] J. M. Chem, S. Yang, G. Li, Q. Zhu, Q. Pan, J. Mater. Chem. 2012, 22,
3420–3425.
[20] S. Yang, Q. Pan, J. Liu, Electrochem. commun. 2010, 12, 479–482.
[21] C. Martin, O. Crosnier, R. Retoux, D. Bꢂlanger, D. M. Schleich, T. Brousse,
Adv. Funct. Mater. 2011, 21, 3524–3530.
[22] F. Mirkhalaf, J. Paprotny, D. J. Schiffrin, J. Am. Chem. Soc. 2006, 128,
7400–7401.
[23] Z. Yang, A. R. Dobbie, K. Cui, J. G. C. Veinot, J. Am. Chem. Soc. 2012, 134,
13958–13961.
[24] M. Dasog, Z. Yang, S. Regli, T. M. Atkins, A. Faramus, M. P. Singh, E. Mu-
thuswamy, S. M. Kauzlarich, R. D. Tilley, J. G. C. Veinot, ACS Nano 2013,
7, 2676–2685.
In summary, we have shown that SiNCs can be functional-
ized with aryl groups by direct reductive grafting of diazonium
salts. However, the coverage of the SiNCs is incomplete, and
oxidation is a strongly occurring side reaction. More effective is
the application of diazonium salts as radical initiators for hy-
drosilylation reactions. For this purpose, new toluene-soluble
diazonium salts were synthesized. Those with electron-with-
drawing groups, such as 2,6-bromo-4-decyl-diazobenzene tet-
rafluoroborate, have shown to be the most reactive. Several al-
kenes and alkynes were hydrosilylated on the SiNCs using this
method. In all cases, photoluminescent stable colloidal SiNC
dispersions were obtained within reaction times of a few
hours. Most importantly, we have demonstrated that the appli-
cation of diazonium salts offers a new, mild, and fast way to in-
itiate hydrosilylation reactions on SiNCs and opens the door to
convenient and predictable surface modification.
[25] C. Galli, Chem. Rev. 1988, 88, 765–792.
Received: January 10, 2014
Chem. Eur. J. 2014, 20, 4212 – 4216
www.chemeurj.org
4216
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim