Supplementary Information
Semiconductor nanocrystals dispersed in imidazolium based ionic
liquids: a spectroscopic and morphological investigation
Annamaria Panniello,a Enrico Binetti,a Chiara Ingrosso,a M. Lucia Curri,a Angela Agostiano,a,b Raffaele
Tommasi,c,a and Marinella Striccolia,*
a
b
c
CNR-IPCF Bari Div., c/o Chemistry Department, via Orabona 4, 70126 Bari, Italy
Chemistry Dept., “Aldo Moro” University of Bari, via Orabona 4, 70126 Bari, Italy.
Department of Basic Medical Sciences, Neuroscience and Sense Organs, University of Bari “Aldo
Moro”, P.zza G. Cesare 11, 70124 Bari, Italy.
*Corresponding author. Fax +39 080 5442128; Tel. +39 080 5442027. E-mail address:
m.striccoli@ba.ipcf.cnr.it.
1
Table of content
1. Red Edge Effect of [C2MIM][BF4]
2. Dynamic Light Scattering Experiments
3. FT-IR measurements
4. PL decays of [C4MIM][PF6]
5. Three-exponential fitting parameters of PL decays
6 Average Lifetimes
7. References
2
1. Red Edge Effect of [C2MIM][BF4]
Fig. S1. Normalized PL emission spectra of bare [C2MIM][BF4], excited at 325 nm (blue squares), 350 nm (green dots) and
375 nm (red triangles), respectively. In the inset, the PL peak intensity at constant excitation intensity as a function of the
excitation wavelength is reported
In Fig. S1 the normalized PL emission spectra of [C2MIM][BF4], acquired exciting the sample at three different
wavelengths (325, 350 and 375 nm) are reported. In this range, at increasing excitation wavelength, an enhancement of the
PL intensity (inset of Fig. S1) and a shift of the PL emission maximum towards longer wavelengths are observed (Fig. S1).
Although the excitation wavelength dependence of PL has not been reported for most fluorophores, it has been observed for
few of them in highly viscous or frozen media [1] and in imidazolium ILs.[2-4] Such a phenomenon is generally referred to as
“red-edge effect” (REE) and it can take place in presence of: i) inhomogeneous broadening of the absorption and emission
spectra[1,3] and ii) excited state relaxation, including solvation of the fluorescent state or energy transfer to lower energy
states, in a timescale longer or comparable to the emission lifetimes. [1,3] The REE for the ILs has been ascribed to the
presence of different associated species, originating by long- and short-range organization of the ion pairs constituting the
IL, and emitting from different energy states. [2c, 3-4] In particular, PL spectra of imidazolium ILs exhibit distinct
contributions, namely, a narrow band located in the range 340-360 nm, assignable to the isolated imidazolium ion, and a
lower energy broad band, which can be attributed to the overall contribution of energetically distinct associated forms. [2b-d, 3]
In Fig. S1, when [C2MIM] [BF4] IL is excited at 375 nm, only the emission of the associated forms has been detected. In
this case a PL broad band centered at ~420 nm and a weak, red-shifted shoulder at ~485 nm are observed. PL spectrum
obtained exciting at 325 nm, instead, shows a contribution at ~360 nm, due to the isolated imidazolium ring emission and a
broad band extending into the red, which accounts for the emission of the aggregated structures. The effect of dilution with
apolar (CHCl3) and polar (acetonitrile) solvents reported in Binetti et al.[5] is a further evidence that the emission features
displayed in Fig. S1 are intrinsic properties of the IL. In that work, experimental measurements performed on the same ILs
studied here show remarkable modifications of their emission spectra when these ILs are dissolved in the two organic
solvents. Such evidence, ascribable to a different organization of the ILs in polar and apolar domains, confirms that the
fluorescence of ILs has an intrinsic character, dependent on the ILs chemical structure and that it is not imputable to the
presence of impurities in the IL.
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2. Dynamic Light Scattering Experiments
Fig. S2. Size distribution obtained by DLS of CHCl3 solutions of (A) [C6MIM][BF4] and (B) the corresponding NCs/IL
dispersion.
Dynamic Light Scattering (DLS) measurements were performed at 25°C by means of an LB-550 Horiba particle size
analyzer. A 5 mW laser diode (λ=650 nm wavelength) was operated with the detector angle set at 177° (non invasive back
scattering geometry). The algorithm to determine the particle size from the light signal is based on Fourier-transformed
power spectrum and iterative deconvolution of the relative contribution of variously sized particles in the mixture.
Before performing light scattering experiments, freshly prepared samples were filtered using inorganic membrane filter
Anotop 10 (0.2 μm, Whatman) to remove any interfering dust particles.
Fig. S2 shows the comparison between the mean values of the average size obtained from ten replicates for a given IL,
namely [C6MIM][BF4] (Fig. S2-A), and those obtained from ten replicates for the corresponding NCs/IL dispersion (Fig.
S2-B), both in CHCl3 solutions. Remarkably, DLS measurements carried out on IL chloroform solution reveal an average
size consistent with the assumption of the occurrence of associated forms, which is likely to arise from the self-organization
properties of the polar and apolar domains within the imidazolium IL. In the presence of the colloidal CdSe(ZnS) NCs, the
light scattering measurements show an average size larger than the one estimated for the neat IL in CHCl3 solution, thus
providing further indication of the possible formation of NCs/IL composite in chloroform solution.
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3. FT-IR measurements
Trasmittance (%)
NCs
[C6MIM][BF4]/NCs
[C6MIM][BF4]
SSH-Csp
2
ASH-Csp
2
SSH-Csp
SSH-Csp
3
3
3
BENH-Csp
SSC-N
SSC=N
STRB-F
3500
3000
1500
1000
-1
Wavenumber (cm )
Figure S3 Comparison among FT-IR spectra of the [C6MIM][BF4] IL, the CdSe(ZnS) NCs, and the corresponding
NCs/[C6MIM][BF4] dispersion.
In Figure S5 is reported the comparison among FT-IR spectra of the bare [C6MIM][BF4] IL (purple line), CdSe(ZnS) NCs
(orange line) and the corresponding NCs/IL dispersion (blue line). The characteristic signals of the IL vibrational absorption
have been attributed to the stretching (symmetric stretching, ν SS, and asymmetric stretching, ν AS) and bending (νSS) of the
organic cation and of the inorganic anion. However, the comparison of the bare IL FT-IR spectrum with the spectrum of the
NC/IL dispersion does not reveal any significant variation in the transmittance. Although such a technique could be a very
useful tool for detecting possible mutual interactions between the components of NC/IL dispersions, in this case the IL
signals predominate on that of CdSe(ZnS) NCs, in the range of the investigated concentration.
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4. Effect of the alkyl chain length on PL decays at 480 nm
Fig. S4. PL decays measured in CHCl3 solutions of [C4MIM][PF6] ILs at different emission wavelengths.
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5. Three-exponential fitting parameters of PL decays
Table S1 Three-exponential fitting parameters of PL decays of [C2MIM][BF4] and of the corresponding hybrid with
CdSe(ZnS) NCs as a function of the emission wavelength (excitation wavelength = 375 nm).
[C2MIM][BF4]
λ
(nm)
t1
(ns)
t2
(ns)
t3
(ns)
A1
(%)
A2
(%)
A3
(%)
tAverage
(ns)
χ2
400
1.97±0.03
7.7±0.2
25±1
75.98
21.90
2.13
3.7±0.3
1.26
420
1.68±0.03
6.8±0.3
61±6
82.86
13.00
4.14
4.8±0.6
1.18
485
1.69±0.02
9.9±0.2
53±1
46.37
38.31
15.32
13.0±1.0
1.13
400
0.64±0.06
2.64±0.02
38±2
19.03
76.36
4.61
3.9±0.3
1.15
420
2.0±0.6
5.1±0.2
36±1
72.60
20.75
6.65
4.9±0.7
1.06
485
3.18±0.03
9.9±0.3
55±1
42.18
41.46
16.36
14.4±0.6
1.09
585
2.4±0.6
12.5±0.3
48.6±0.7
11.13
52.96
35.91
24.3±0.8
1.17
[C2MIM][BF4] +
NCs
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6. Average Lifetimes
Table S2 Average Lifetimes of the investigated samples, calculated as weighted average on the three time component values
and their associated amplitudes.[6]
Fig. 3
λ
(nm)
tAverage
(ns)
λ
(nm)
tAverage
(ns)
400
3.7±0.3
400
3.9±0.3
420
4.8±0.6
420
4.9±0.7
485
14.4±0.6
585
24.3±0.8
420
13.8±0.3
585
23.8±0.4
585
24.3±0.8
[C2MIM][BF4]
[C2MIM][BF4] + NCs
485
13.0±1.0
420
4.8±0.1
585
7.2±0.3
NCs
585
29.5±0.5
[C2MIM][BF4] + NCs
420
4.9±0.7
[C2MIM][BF4] + NCs
[C4MIM][BF4] + NCs
7.1 ±0.2
[C4MIM][BF4] + NCs
23.6±0.4
[C6MIM][BF4] + NCs
13.8±0.3
[C6MIM][BF4] + NCs
23.8±0.4
5.4±0.2
[C4MIM][BF4] + NCs
14.4±0.4
[C4MIM][PF6] + NCs
[C6MIM][BF4]
[C6MIM][BF4] + NCs
Fig.4
Fig. 5
Fig. 6
[C4MIM][BF4]
[C4MIM][PF6]
430
430
7.7 ±0.3
17.4±0.4
8
6. References
[1]
[2]
A. P. Demchenko, Luminescence 17 (2002) 19.
(a) A. Paul, A. Samanta, J. Chem. Sci. 118 (2006) 335; (b) A. Paul, P. K. Mandal, A. Samanta, J. Phys. Chem. B 109 (2005) 9148;
(c) P. K. Mandal, A. Paul, A. Samanta, J. Photochem. Photobiol. A: Chem. 182 (2006) 113; (d) A. Paul, P. K. Mandal, A. Samanta,
Chem. Phys. Lett. 402 (2005) 375.
[3] A. Samanta, J. Phys. Chem. B 110 (2006) 13704.
[4] P. K. Mandal, M. Sarkar, A. Samanta, J. Phys. Chem. A 108 (2004) 9048.
[5] E. Binetti, A. Panniello, L. Triggiani, R. Tommasi, A. Agostiano, M. L. Curri ,M. Striccoli, J. Phys. Chem. B 116 (2012) 3512.
[6] O'Connor DV, Phillips D (1984) Time-Correlated Single Photon Counting. Academic Pr, London. doi:ISBN 0125241402
9780125241403
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