1. Enhanced nonlinear optical responses of Zinc diaminopyrimidin-2-ylthio phthalocyanine conjugated to AgxAuy alloy nanoparticles
Owolabi M Bankole and Tebello Nyokong*
Rhodes University, Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
INTRODUCTION
The invention of modern lasers has promoted a growing interest in the development of optical limiting devices against laser-induced damage of light sensitive materials. The unique optical and electronic properties of phthalocyanines and metallic nanoparticles make them suitable candidates for photo-driven applications like nonlinear optics [1,2]. It is expected that composites of phthalocyanine-Au-Ag nano-alloys will give rise to materials with enhanced photophysical and optical limiting (OL) behavior compared to individual molecule. The present work describes for the first time the enhancement of photophysical and OL response of zinc diaminopyrimidin-2-ylthio phthalocyanine (2), in presence of AgxAuy alloys. The conjugates of 2 and Ag-Au are termed 2-Ag1Au3 and 2-Ag1Au3 corresponding to the mole ratio of Ag and Au atoms in the alloy mixture.
RESULTS
EXPERIMENTAL
Sample
Absorption (nm) 2
689 (Q band)
2-AgxAuy
Ag1Au3
482 (501)a
Ag3Au1
423 (444)a
Table 1. Absorption spectra data for 2,
2-Ag3Au1 and 2-Ag1Au3 in DMSO.
a SPR due to AgxAuy in the conjugate with 2.
Scheme 1. Synthetic pathway for ZnPc (2)
Scheme 2. Synthesis of AgxAuy
alloy nanoparticles
Figure 1: TEM images of Ag3Au1,
Ag1Au3,2-Ag3Au1 and 2-Ag1Au3.
Figure 2: EDX spectra of Ag3Au1,
2-Ag3Au1, Ag1Au3 and 2-Ag1Au3
Figure 3: UV spectra of Ag3Au1, Ag1Au3, in hexane
(left) and 2, 2-Ag3Au1 and 2-Ag1Au3 in DMSO (right)
Figure 1 shows TEM images of monodisperse and nanosphere shaped alloy nanoparticles with average particle size of 9 ± 1 nm. Significant aggregation
due to π–π stacking of 2 was observed following conjugation to AgxAuy.
Figure 2 shows the elemental compositions of bimetallic NPs and the conjugates verified by energy dispersive spectroscopy (EDS). Spectra of AgxAuy alone show peaks due to Ag and Au as expected. Additional peaks due to Cl, C and O are from HAuCl4·3H2O, diphenyl ether and oleic acid used in the preparation of NPs. In addition to peaks of AgxAuy alone, the EDS spectra of 2-AgxAuy showed nitrogen, sulphur and zinc peaks confirming the presence phthalocyanine macrocycles on bimetallic NPs
Figure 3A shows the UV absorption spectra of AgxAuy with intense SPR bands positioned at 423 nm and 482 nm for Ag3Au1 and Ag1Au3, respectively. The SPR peaks of the alloys are very close to the SPR positions of pure AgNPs and AuNPs, depending on the composition of Ag or Au present in the alloys.
Absorption spectra of 2 and the conjugates showed monomeric behaviour in DMSO, figure 3B.
There is No shifts in the Q-bands of 2 after conjugation to AgxAuy Table 1.
Scheme 3: Immobilization of ZnPc (2) on AgxAuy via N or S-Au bond
2-Theta
Planes
38.50
111
44.10
200
65.10
220
78.00
311
82.30
222
22.50
002
Sample ΦF
F (ns) ΦT
τT (µs) 2
0.122
2.45
0.831
350
2-Ag1Au3
0.051
2.36
0.945
251
2-Ag3Au1
0.046
2.51
0.918
223
Table 2: FCC planes of 2 and
bimetallic NPs
Table 3: Summary of the photophysical properties
of 2 and 2-AgxAuy
Figure 4: XRD patterns of Ag3Au1, 2-Ag3Au1,
2, Ag1Au3 and 2-Ag1Au3
Figure 5: Full XPS scans for NPs surface functionalized with
Phthalocyanine macrocycles.
The XRD patterns of AgxAuy showed crystalline well-defined peaks assigned to 111, 200, 220, 311 and 222 planes of face centered-cubic of metallic gold and silver, figure 4. The additional broad peaks in XRD spectra of 2-AgxAuy near 220 is an indication that the phthalocyanine is present in the composites. Summary of the positions of the planes are listed in Table 2.
Table 3 shows summary of the photophysical parameters of the studied samples. Enhanced triplet-triplet absorptions of 2 was observed in presence bimetallic nanoparticles
Figure 5 shows the wide scan of 2 and its conjugate to bimetallic NPs. The wide scan of 2 shows presence of S2p, S2s, C1s, N1s and Zn2p signals consistent
with compositions of Znphthalocyanine. These peaks are also presence in the XPS spectra of 2-AgxAuy alongside the spectra of Au4f at 83.5 eV and Ag3d at 368 eV. This confirmed
successful formation of 2-AgxAuy.
Fig., 6A shows core-levels of N1s spectra of 2 at (N–C from pc ring) and eV (-NH2). The peaks of -NH in 2-Ag1Au3 shifted to higher BE at ~ eV (protonated amine), suggesting successful conjugation of 2 to AgxAuy NPs.
Figure 6B, S2p3/2 BE of 2 at eV shifted to higher BE at and eV for 2-Ag1Au3 and 2-Ag3Au1, respectively. The BE at ~ 162 corresponds to thiolated species immobilized on bimetallic NPs in 2-AgxAuy.This shows that S also played a role in the formation of the Pc–NPs conjugates.
Figure 6: (A) N1s and (B) S2p deconvoluted XPS spectra to show the binding sites of bimetallic NPs to N or S-of phthalocyanine 2
Sample
𝛽 𝑒𝑓𝑓
(cm GW-1)
I m [  (3) ]/esu
𝛾 (esu)
Ilim
(J cm-2) 2
209
4.91 x 10-11
2.15 x 10-29
0.76
2-Ag3Au1
910.5
2.14 x 10-9
9.35 x 10-28
0.31
2-Ag1Au3
1108.8
2.60 x 10-9
1.14 x 10-27
0.28
Table 4: Nonlinear optical parameters of 2 and 2-AgxAuy in DMSO at 532 and 10 ns pulses..
Figure 7: OA Z-scans for 2, 2-Ag1Au3 and 2-Ag3Au1
I00 ~55.4 MW cm−2, Absorbance = 0.5 in DMSO.
Figure. 8. Concentrations (absorbance) dependence on βeff
for 2-Ag1Au3 and 2-Ag3Au1. Ioo ≈ 55.4 MW cm−2 in DMSO.
Figure 9. Output fluence (Iout) versus input fluence (Iin) for
2, 2-Ag1Au3 and 2-Ag3Au1 in DMSO. (I00) ≈ 55.4 MW cm−2
Fig. 7 shows plots of transmittance against the Z-positions for the samples at an absorbance of ca 0.5A measured in 0.2 cm cuvette. At Z=0, the minimum transmission values of 73.9%, 49.3% and 51.5% were respectively measured for 2, 2-Ag1Au3 and 2-Ag3Au1 in DMSO. The samples were found to exhibit reverse saturable absorption (RSA) behaviour; corresponding to positive nonlinear absorptions. The transmittances are lower for the conjugates compared to unconjugated Pc. Figure 8 shows the variation of the concentration (absorbance) of 2-AgxAuy solution from 0.5 to 2.5 at constant laser intensity of ~55.4 MW cm−2. Both composites revealed that the eff increases linearly as the absorbance of the sample increases. That is OL ability of the samples increase with increasing concentration.
Figure 8 shows the variation of the concentration (absorbance) of 2-AgxAuy solution from 0.5 to 2.5 at constant laser intensity of ~55.4 MW cm−2. Both composites revealed that the eff increases linearly as the absorbance of the sample increases. suggesting direct relationship between the nonlinear optical responses and the concentration of each sample
Fig. 9 clearly showed that all the samples including unconjugated Pc deviated from linearity. The deviation of the composites, 2-AgxAuy were much lower pronounced compared to 2 alone. Hence, the conjugates showed better NLO responses to the input intensity.
The optical liming parameters such as 𝐼 𝑚 [  (3) ], 𝛽 𝑒𝑓𝑓 and 𝛾 reported for 2-AgxAuy composites in this work are remarkably larger than the corresponding values of 2 alone, Table 4. Also, 2-Ag1Au3 is better than 2-Ag3Au1 as NLO material evidenced from the reported parameters.
CONCLUSIONS
Enhanced photophysical and optical limiting responses of the new phthalocyanine and the conjugates in DMSO were observed. NLA effects due to RSA are direct consequence of increased triplet-triplet absorption due to the presence of bimetallic nanoparticles. The conjugates are suitable materials for potential applications in optical limiting processes.
REFERENCES
[1]. Y. Chen, M. Hanack, Y. Araki, O. Ito, Chem. Soc. Rev., 2005, 34,
[2] I. Papagiannouli, P. Aloukos, D. Rioux, M. Meunier and S. Couris, J. Phys. Chem. C, 2015, 119, 6861−
ACKNOWLEDGEMENTS
This work was supported by the Department of Science and Technology (DST) and National Research Foundation (NRF), South Africa through DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology (to T.N.).