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The effect of point of substitution and silver based nanoparticles on the photophysical and optical nonlinearity of indium carboxyphenoxy phthalocyanine
Accepted Manuscript Title: The effect of point of substitution and silver based nanoparticles on the photophysical and optical nonlinearity of indium carboxyphenoxy phthalocyanine Authors: David O. Oluwole, Sixolisile M. Ngxeke, Jonathan Britton, Tebello Nyokong PII: DOI: Reference:
S1010-6030(17)30692-5 http://dx.doi.org/doi:10.1016/j.jphotochem.2017.07.032 JPC 10758
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
22-5-2017 25-7-2017 25-7-2017
Please cite this article as: David O.Oluwole, Sixolisile M.Ngxeke, Jonathan Britton, Tebello Nyokong, The effect of point of substitution and silver based nanoparticles on the photophysical and optical nonlinearity of indium carboxyphenoxy phthalocyanine, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.07.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The effect of point of substitution and silver based nanoparticles on the photophysical and optical nonlinearity of indium carboxyphenoxy phthalocyanine David O. Oluwole, Sixolisile M. Ngxeke, Jonathan Britton, Tebello Nyokong* Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa *Corresponding
author. Tel: + 27 46 6038260; Fax: + 27 46 6225109. E-mail: [email protected]. (T. Nyokong)
Graphical abstract
Research Highlights
Indium phthalocyanines were covalently linked to silver containing nanoparticles to form conjugates
triplet state quantum yields of the phthalocyanines improved in presence of silver containing nanoparticles
All conjugates showed improved non-linear optical activity in comparison to the Pc complexes alone.
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Abstract Indium(III) chloride 1,8(11),15(18),22(25)–tetra–(3–carboxyphenoxy) phthalocyanine (1) and indium(III) chloride 2,9(10),16(17),23(24)–tetra–(3– carboxyphenoxy) phthalocyanine (2) were covalently linked to glutathione capped silver nanoparticles (AgNPs–GSH) and silver selenide/zinc sulfide (Ag2Se/ZnS–GSH) quantum dots via amide bond formation. The photophysical and nonlinear optical behaviour of the metallophthalocyanines and their conjugates with nanoparticles were investigated in using the open aperture Z–scan technique. Complex 2 showed enhanced photophysical properties compared to 1. The conjugates revealed improved triplet state quantum yields (except for 1-AgNPs-GSH which afforded lower triplet state quantum yields in comparison to 1) and nonlinear optical activities in comparison to the Pc complexes. The synthesized complexes, nanoparticles and their conjugates could be potential nonlinear optical materials due to their good nonlinear optical activities. Key words: nanoparticles, quantum dots, indium phthalocyanine, nonlinear optics, triplet quantum yields 1. Introduction Nonlinear optical (NLO) devices are being developed in order to protect the eye and other light sensitive objects from intense laser light. NLO materials possess some essential features such as large nonlinear absorption coefficient (βeff), inherent fast response time and efficient optical limiting threshold intensity or
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fluence (Ilim) [1–3]. Among the organic molecules that have been tested for NLO applications, metallophthalocyanines (MPcs) remain one of the most viable materials. The efficient optical nonlinearity of MPcs could be attributed to their extensive 18 π–electron conjugated ring system. In addition, ring substituents on MPcs can be modified, allowing for attachment of other NLO materials such as nanoparticles (NPs) [4,5]. It is also important to note that central atoms and nature of substituents also play an indispensable role in the optical nonlinearity of phthalocyanines [6–10]. On the other hand, silver based nanoparticles (NPs) have been reported to possess NLO properties [11]. AgNPs have found applications in other areas such as in photodynamic therapy [12,13] and as microbial agents [14]. Previous studies in our group have shown that MPcs can form dyad systems with various metal NPs with improved triplet state quantum yields and NLO activities in comparison to the MPcs alone [15–18]. Herein, we report on the covalent linkage of indium(III) chloride 1,8(11),15(18),22(25)–tetra–(3– carboxyphenoxy)phthalocyanine (1) and indium(III) chloride 2,9(10),16(17),23(24)–tetra–(3–carboxyphenoxy)phthalocyanine (2), Scheme 1, to glutathione (GSH) capped silver NPs (AgNPs–GSH) and silver selenide/zinc sulfide quantum dots (Ag2Se/ZnS–GSH QDs). The synthesis of complex 2 has been reported [19], while complex 1 is reported in this work for the first time. The effect of the point of attachment of AgNPs–GSH and Ag2Se/ZnS–GSH QDs on the photophysical and NLO properties of the Pcs will be evaluated. Since both Ag based NPs and Pcs are NLO materials, we expect enhanced NLO
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behaviour of the conjugates through synergistic effect. In addition, the dyad system of the MPc complexes with silver based NPs is expected to afford enhanced triplet state absorption and NLO behaviour due to heavy atom effect. Even though MPcs have been linked to AgNPs [19], this is the first time that Ag based QDs are linked to Pcs. Also core/shell quantum dots show enhanced optical properties compared to the core [20,21], hence core/shell Ag2Se/ZnS– GSH QDs are employed in this work and compared to AgNPs following linking to MPc complexes. The nonlinear optical behaviour of the NPs, Pc complexes and their conjugates were tested in solution (dimethylsulfoxide) using open aperture Z-scan technique at excitation wavelength of 532 nm with 10 ns pulse length and laser energy of ~70 µJ. 2. Experimental 2.1.
Materials
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Ultra-pure water was obtained from a Milli-Q Water System (Millipore Corp., Bedford, MA, USA). Indium(III) chloride, glutathione (GSH), potassium hydroxide, N,N’-dicyclohexylcarbodiimide (DCC), 4-(Dimethylamino)pyridine (DMAP), zinc phthalocyanine, dimethyl formamide (DMF), quinoline and urea were obtained from Sigma Aldrich®. All other reagents and solvents were obtained from commercial suppliers and used as received. Complex 2 [19], oleyamine (OLM) capped AgNPs [22], OLM capped Ag2Se/ZnS QDs [23], and 3– (3,4–dicyanophenoxy)benzoic acid [24] were synthesized as reported in literature.
2.2.
Equipment
FT–IR spectra were recorded on a Bruker spectrum 100 with universal attenuated total reflectance (ATR) sampling accessory. X–ray powder diffraction (XRD) patterns were recorded using a Cu K radiation (1.5405˚A, nickel filter) equiped with LynxEye detector, on a Bruker ® D8 Discover equipped with a proportional counter and the X–ray diffraction data were processed using the Eva ® (evaluation curve fitting) software. The morphology of the NPs were assessed using a transmission electron microscope (TEM), ZEISS LIBRA ® model 120 operated at 90 kV accelerating voltage. Mass spectra data were acquired on a Bruker® AutoFLEX III Smartbeam TOF/TOF
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mass spectrometer operated in positive ion mode using m/z range of 500– 5000 amu. 1H NMR measurement was performed on a Bruker® AMX 600 MHz. Ground state electronic absorption was measured on a Shimadzu ® UV– 2550 spectrophotometer. Fluorescence emission and excitation spectra were acquired on a Varian Eclipse® spectrofluorometer using 360–1100 nm filter. Fluorescence lifetimes were measured using a time correlated single photon counting setup (TCSPC) (FluoTime 300, Picoquant ® GmbH) with LDH–P–670, Picoquant® GmbH, 20 MHz repetition rate, 44 ps pulse width). Details have been provided before [25]. Triplet quantum yields were determined using a laser flash photolysis system. The excitation pulses were produced using a tunable laser system consisting of an Nd:YAG laser (355 nm, 135 mJ/4–6 ns) pumping an optical parametric oscillator (OPO, 30 mJ/3–5 ns) with a wavelength range of 420–2300 nm (NT-342B, Ekspla), as reported in the literature [25]. All Z-scan experiments described in this study were performed using a frequency-doubled Nd:YAG laser (Quanta-Ray, 1.5 J /10 ns fwhm (full width at half maximum) pulse duration) as the excitation source. The laser was operated in a near Gaussian transverse mode at 532 nm (second harmonic), with a pulse repetition rate of 10 Hz and energy range of 0.1 µJ – 0.1 mJ, limited by the energy detectors (Coherent J5-09). The low repetition rate of the laser prevents cumulative thermal nonlinearities. The beam was spatially filtered to remove the higher order modes and tightly focused with a 15 cm focal length lens. The Z-scan system size (l × w × h) used was 600 mm × 300
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mm × 350 mm (excluding the computer, energy meter, translation stage driver and laser system). The liquid samples were placed in a cuvette (internal dimensions: 2 mm × 10 mm × 55 mm, 0.7 mL) and a path length of 2 mm (Starna 21-G-2).
2.3. Syntheses 2.3.1. Indium(III) chloride 1,8(11),15(18),22(25)–tetra–(3–carboxy phenoxy) phthalocyanine (1, Scheme 1) Complex 1 was synthesized as follows: 3–(3,4–dicyanophenoxy)benzoic acid (0.50 g, 1.89 mmol) was weighed into a round bottom flask containing deaerated mixture of InCl3 (0.60 g, 2.71 mmol), urea (0.50 g, 8.33 mmol) and quinoline (4 mL). The reaction mixture was refluxed at 200 °C for 1 h with constant stirring under argon atmosphere. Afterwards, the temperature was lowered to 140 °C and the heating continued for further 5 h. The obtained product (1) was precipitated out of solution with methanol, and ethanol. Complex 1 was finally purified using a silica packed column chromatography with 10% hexane in chloroform and 5% methanol in chloroform. The fine green product was dried in an enclosed fume hood. Yield: 25% (w/w); UV–vis (DMSO): λmax/nm (log Ɛ): 332 (4.54), 638 (4.33), 706 (5.01). IR [(KBr) vmax/cm-1]: 3241 (O–H), 1719 (C=O), 1654 (C=C), 1557 (C–C), 1248 (C–O–C); calculated for C60H32InClN8O12; C 59.70, H 2.67, N 9.28. Found–(C 59.25, H 2.47, N 9.63)%; 1H NMR
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(DMSO-d6): δ, ppm 11.33 (4H, s, COOH), 8.90–8.73 (12H, m, Pc-H), 7.11– 5.29 (16H, m Phenyl-H). MS (MALDI–TOF) (m/z): Calculated for C60H32InClN8O12–1206.09; Found–1209.13 [M+3H]+. 2.3.2.
GSH capped AgNPs and Ag2Se/ZnS QDs, Scheme 2
The synthesis of oleyamine (OLM) capped AgNPs, and Ag2Se/ZnS QDs were done as reported in the literature [22,23]. The functionalization of the NPs with GSH is as follows: the oleyamine capped AgNPs (0.40 g) or oleyamine capped Ag2Se/ZnS (0.40 g) QDs were transferred into separate round bottom flasks containing chloroform (3 mL). GSH (0.25 g, 0.81 mmol) and KOH (1.50 g, 26.74 mmol) were dissolved in methanol (20 mL) and the resulting solution was added to the NPs mixtures. The mixtures were allowed to stir for 2 h to foster the attachment of the thiol moiety of the GSH to the NPs surface via metal– sulfur interaction. The GSH capped NPs were precipitated out of solution using ethanol, purified with methanol and air dried in enclosed fume hood to give AgNPs–GSH and Ag2Se/ZnS–GSH QDs.
2.3.3. Conjugation of complexes 1 and 2 with AgNPs–GSH and Ag2Se/ZnS–GSH QDs, Scheme 3 The conjugation of complexes 1 and 2 with AgNPs–GSH and Ag2Se/ZnS– GSH QDs was as follows: complexes 1 or 2 (0.030 g, 0.025 mmol) were weighed into two flasks, then 2 mL of DMF and DCC (0.015 g, 0.073 mmol) were added to each flask. The reaction mixtures were allowed to stir for 65 h at ambient temperature to activate the COOH group of the
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Pcs. Then, DMAP (0.008 g, 0.066 mmol) and 0.035 g of AgNPs–GSH or Ag2Se/ZnS–GSH QDs were added to each reaction mixture, which were allowed to stir further for 48 h at ambient temperature leading to amide formation between the complexes and GSH capped NPs. The formed conjugates were precipitated out solution with ethanol and successively purified with methanol and air dried in enclosed fume hood. The conjugates are represented as 1–AgNPs–GSH, 1–Ag2Se/ZnS–GSH QDs, 2–AgNPs–GSH, and 2–Ag2Se/ZnS–GSH QDs. 2.4. Photophysicochemical parameters 2.4.1.
Fluorescence (F) and triplet (T) state quantum yields
The fluorescence quantum yields (F) of the complexes and their conjugates with GSH capped NPs, were assessed using a comparative methods reported in the literature [26]. Unsubstituted ZnPc dissolved in DMSO was used as a standard for the MPc complexes and their conjugates (F =0.2) [27]. The triplet (T) quantum yields of the MPc complexes and their conjugates were determined using a comparative method reported in the literature and unsubstituted ZnPc in DMSO was used as a standard (T = 0.65) [28].
2.5. Z-scan measurements
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The optical nonlinearity of the complexes, NPs and their conjugates were investigated using the open aperture Z–scan technique as described in the literature [1], using equation 1. TNorm ( z )
1 I0 [1 eff Leff ( )] z 1 ( )2 z0
(1)
where eff and I0 are the effective intensity dependent nonlinear absorption coefficient and the intensity of the beam at focus, respectively. z and z0 are sample positions with respect to the input intensity and
w02 Rayleigh length (defined by ; λ = wavelength of the laser beam and w0 = beam waist at the focus (z = 0)), respectively. Leff is the effective thickness of the sample and is given by equation 2:
Leff
1 e L
(2)
where is the linear absorption coefficient and L is the thickness of the sample. The imaginary component of the third order optical susceptibility (Im[(3)] in esu) is directly proportional to eff via equation 3 [29,30]:
Im[
( 3)
]
(n 2 0 c eff ) (2 )
(3)
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In equation 3, c and n are the speed of light in vacuum and the linear refractive index respectively. ε0 is the permittivity of free space and λ is the wavelength of the laser light. By definition, limiting intensity (Ilim/J.cm-2) is the input fluence at which the output fluence is 50% of the linear transmission [31,32].
3. Results and discussion 3.1. Characterization of MPcs alone Complex 1 was synthesized by cyclocondensation of 3-(3,4dicyanophenoxy)benzoic acid in the presence of indium(III) chloride and urea in quinoline, Scheme 1. The FTIR, 1H NMR, MALDI-TOF mass spectroscopies and elemental analyses data of the complex were consistent with the predicted structure. The complex showed good solubility in organic solvents. The 1H NMR spectrum of 1 depicted it resonance signal at 11.33 ppm as a singlet with 4 protons (COOH) while the Pc aromatic ring accounted 12 protons with resonance signal at 8.90–8.73 ppm as a multiplet and the phenyl ring gave 16 protons as a multiplet with resonance signal at 7.11 –5.29 ppm. For complex 1, mass to charge ratio of 1206.09 m/z was calculated and 1209.13 m/z was found.
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The electronic absorption spectra of complexes 1 and 2 in DMSO are shown in Fig. 1A. Both non–peripherally (complex 1) and peripherally (complex 2) tetra substituted phthalocyanines have a mixture of four possible structural isomers with the following molecular symmetry: C4h, C2v, Cs and D2h. The different isomers show similar spectra. Complexes 1 and 2 showed no aggregation at concentrations ranging from ~ 1 x 10 -6 to 8 10-6 mol/L in DMSO. The Beer–Lambert law was obeyed at the studied concentrations. The Q–band of 1 was more red shifted than that of 2 due to non–peripheral substitution in the former [33], Table 1 and Fig. 1A. For both the complexes at concentrations less than 10 -6 M, absorption and excitation spectra were similar and mirror images of the emission spectra, Fig. 1C. Slight differences in the maxima of excitation and absorption spectra could be due to differences in equipment used. 3.2. Characterization of NPs and their conjugates The oleylamine stabilized AgNPs and Ag2Se/ZnS QDs were first synthesised followed by phase transfer of the NPs from lipophilic ligand to hydrophilic one. The process was achieved by replacement of the oleylamine with glutathione (GSH) due to the stronger affinity of the thiol in the later for the NPs surface in comparison to the former, Scheme 2. It is noteworthy that residual OLM might still be present on the surface of the NPs. The GSH capped AgNPs and Ag2Se/ZnS QDs are soluble in water whereas the OLM capped derivatives are not. The functionalization of the AgNPs and Ag2Se/ZnS QDs with GSH was
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performed to foster their covalent attachment to COOH substituted complexes 1 and 2. The linkage of the NPs to the complexes was done by activation of complexes 1 and 2 COOH substituents with DCC/DMAP. Upon conversion of the COOH moiety of the Pc complexes to active carbodiimide ester, NPs having NH2 were introduced to foster amide bond formation between the NPs and the complexes, Scheme 3. 3.2.1. UV–vis absorption and emission spectra The formation of the GSH capped Ag 2Se/ZnS QDs is evidenced by broad absorption spectral peaks at 557 nm and 339 nm, Fig. 2. The latter may be attributed to the presence of Ag in Ag2Se/ZnS–GSH QDs. The broad absorbance at 557 nm is typical of QDs for Ag2Se/ZnS-GSH QDs [19]. For AgNPs–GSH, the surface plasmon resonance (SPR) peak is well resolved at 406 nm, Fig. 2. Slight shifts in the Q–bands of complexes 1 and 2 were observed in the conjugates in some cases, Table 1, and Fig. 1B. The presence of the NPs in the conjugates was not clear in Fig. 1B, but will be shown below using XRD. 3.2.2. FT-IR spectra The FT–IR spectra of the Pc complexes alone, NPs and their conjugates are shown in Fig. 3 (using complex 1, 1–Ag2Se/ZnS–GSH QDs and Ag2Se/ZnS–GSH QDs as examples). Complex 1 showed its typical broad carboxylic O–H bands (3241 cm-1), C=O (1719 cm-1), C=C (1654 cm-1), C–
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C (1557 cm-1), and C–O–C (1248 cm-1). Ag2Se/ZnS–GSH depicted three distinct vibrational bands which correspond to OH/NH2 (3241 cm-1), C–O (1572 cm-1), and C–N (1383 cm-1). There was disappearance of the MPcs carbonyl band (C=O) at 1719 cm-1 in the conjugates and slight attenuation of the NPs amine band at 3241 cm -1 and this could be ascribed to possible amide bond formation between the NPs and the MPcs. The appearance of a vibrational band at 1577 cm-1 could be due to the amide band (–HN–C=O). GSH on its own has amide bonds, hence it is difficult to make definite amide bond assignments.
3.2.3. XPS analysis The survey spectra of the NPs, Pc complexes and the conjugates depicted all the expected atomic compositions with their corresponding binding energies, Fig. 4A (using Ag2Se/ZnS–GSH, 1, and 1–Ag2Se/ZnS–GSH as examples). Ag2Se/ZnS–GSH shows the following peaks: Zn 2P (1021 eV and 1044 eV), O 1s (531 eV), N 1s (399 eV), Ag 3d (368 eV), C 1s (285 eV), S 2p (162 eV) and Se 3d (53 eV). The peaks found in Ag2Se/ZnS–GSH are also present in 1– Ag2Se/ZnS–GSH with the appearance of In 3d (445 eV) from complex 1. Table 2 shows that there is an increase in C and N for 1–Ag2Se/ZnS–GSH compared to Ag2Se/ZnS–GSH due to additional C and N from the Pc. The N 1s spectra of Ag2Se/ZnS–GSH QDs (Fig. 4B) afforded two deconvoluted sub peaks at 398.86 eV (–N–H) and 400.55 eV (–N–C) resulting from the glutathione ligand used for the NPs surface functionalization. 1–Ag2Se/ZnS–
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GSH depicted three sub peaks at 398.64 eV (–N–H), 400.17 eV (–N–C) and 401.94 eV (–HN–C=O), Fig. 4B. Of note is the decrease in the peak intensity for –N–H (relative to N-C) peak in 1–Ag2Se/ZnS–GSH QDs in comparison with Ag2Se/ZnS–GSH QDs, Fig. 4B). The decrease in the peak intensity of the latter in the conjugates was due to the covalent linkage of the primary amine of glutathione functionalized NPs with activated carboxylic acid moiety of the complexes which resulted in the formation of an amide (–HN–C=O) bond (Scheme 1). This could have accounted for the observed peak at 401.94 eV (– HN–C=O). GSH alone has an amide bond which is not clear in Fig. 4B.
3.2.4. XRD studies The morphology and size (diameter) of the NPs and their conjugated with MPcs were obtained using an X–ray diffractometry (XRD). The XRD diffractograms of the Ag2Se/ZnS–GSH QDs (as an example) correspond to the orthorhombic phase of bulk Ag2Se. The conjugates showed characteristic diffraction pattern of the NPs with the presence of a broad band around 2 = 11° to 25° which is attributed to the presence of the Pcs [34] in the conjugates, Fig. 5. The sizes of the NPs and their conjugates were estimated using the Scherrer equation (4) [35]:
d
k Cos
4
where λ is the wavelength of the X-ray source (1.5405 Å), k is an empirical constant equal to 0.9, β is the full width at half maximum of the diffraction
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peak and θ is the angular position. The size (diameters) are shown in Table 1. The sizes of the GSH capped NPs increases on conjugation to complexes 1 and 2, most likely due to aggregation. Aggregation upon conjugation could also be due to interaction from adjacent Pc [33]. 3.2.5. TEM images The NPs were spherical and relatively dispersed (Fig. 6) with mean size of 5.3 ± 0.9 nm and 7.9 ± 1.5 nm for AgNPs–GSH and Ag2Se/ZnS–GSH QDs, respectively. The mean size of Ag2Se/ZnS–GSH QDs was higher than that of the AgNPs–GSH due to the presence of the ZnS shell in the former, Fig. 6, Table 1. The sizes from TEM are slightly less than those obtained from XRD.
3.3. Photophysicochemical Parameters 3.3.1. Fluorescence (F) quantum yields and lifetimes (τF) The fluorescence quantum yields and mean lifetimes of the complexes and their conjugates are listed in Table 1. Fluorescence decay curve of 1– Ag2Se/ZnS–GSH QDs is shown in Fig. 7 (as an example). Complexes 1 and 2 afforded mono–exponential fluorescence decay profiles indicating single τF values. Bi–exponential lifetimes were observed for the conjugates which could be attributed to the orientation of the complexes around the NPs. Decrease in the F and τF values of the complexes (insignificant decrease in F for 1– AgNPs–GSH) were observed upon conjugation with NPs and this is thought to
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be due to external heavy–atom effect from the NPs in the conjugates. The larger decrease for Ag2Se/ZnS–GSH QDs is due to the ZnS which provides an extra heavy atom effect due to Zn. The larger quenching by Ag2Se/ZnS–GSH QDs could also be due the larger size of the QDs compared to AgNPs, Table 1. The larger size means more heavy atoms for quenching. The fluoresce behaviour of the Ag2Se/ZnS–GSH QDs (exciting where QDs absorb) was not studied due to lack of signal, most likely due to the heavy atom effect of Ag which will reduce fluorescence. 3.4.2. Triplet quantum yields (T) and lifetimes (τT) Fig. 8 shows the triplet absorption decay curve of complex 2 (as an example). Complex 1 (non–peripherally substituted) gave a larger T value of 0.71 in comparison to 2 (peripherally substituted) at 0.68, Table 1. The larger T value of 1 compared to 2 alone, corresponds to lower F. F and T are competing processes, hence the former is expected to be smaller where the latter is larger. The conjugates of the complexes with NPs afforded improved T values except for 1–AgNPs–GSH which gave T value lower than for complex 1 alone. The observed increase in the T values of the complexes in the conjugates could be due to external heavy atom effect from the NPs, as earlier discussed. The larger increase in the presence of Ag2Se/ZnS–GSH QDs could also be due to their larger size compared to AgNPs, hence more heavy atoms. It has been reported [36] that the planar structure of phthalocyanine becomes significantly distorted by introduction of phenyl substituent at the non-peripheral positions exhibiting
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a saddle-like distorted structure. It has also been suggested that the deformed nature of the Pc could increase in the presence of nanoparticles depending on their proximity to the MPc [37]. It has also been reported that nonplanar deformations enhance nonradiative decay in porphyrins [38]. Thus, the lack of increase in the triplet quantum yields for 1–AgNPs–GSH could be attributed to the deformed structure of the phthalocyanine molecule and the proximity of the core of the NP in AgNPs-GSH compared to 1–Ag2Se/ZnS–GSH (which has a shell), where there is a shell. The deformity is more pronounced at the nonperipheral positions [36], hence 1–AgNPs–GSH (1 non-peripherally substituted) behaves differently from 1–AgNPs–GSH (2 peripherally substituted).
The τT values of the samples ranges from 44 µs to 56 µs. The τT values of complex 1 was lower compared to 2. This is expected because increase in triplet quantum yield should correspond to decrease in triplet lifetime and vice–versa [39]. We observed a decrease in τT values of the conjugates in comparison to the complexes alone even for 1–AgNPs–GSH which showed a decrease in triplet quantum yield compared to complex 1 alone. 3.4. Nonlinear optical properties The optical nonlinearity of the Pc complexes, NPs and their conjugates were measured in DMSO at an excitation wavelength of 532 nm with pulse rate of 10 ns, Fig. 9. The downward dips of curves in Fig, 9 signify reverse saturable absorption (RSA) behaviour resulting from nonlinear absorption of the incident light [4]. The higher the attenuation,
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(decrease in % transmittance), the better the nonlinear absorber. Ag2Se/ZnS–GSH QDs on their own afforded more than 50% decrease in %transmittance, Fig. S1. For metallic NPs alone, a competition between saturable absorption (SA) and reverse saturable absorption (RSA) has been reported [40] depending on the laser energy. Fig. S1 does not show clear SA behavior. Complexes 1 and 2 alone accounted for less than 50% decrease in %transmittance while their conjugates with NPs afforded more than 50% decrease in %transmittance, Fig. 9 which translates to good response. The experimental data for Pcs and conjugates were fitted to two photon absorption (2PA) mechanism. The sequential 2PA mechanism is typical of phthalocyanine. 2PA has been especially effective at producing large nonlinear absorption in phthalocyanines [4]. eff is a measure of 2PA, and the values are listed in Table 1. The conjugates afforded increased eff values compared to complexes 1 and 2 alone. It can be deduced that the complexes (1 and 2) afforded eff values greater than what was reported in the literatures [41,42] for tert–butyl and amino substituted Pcs (311 cm/GW and 222 cm/GW). Similarly, the conjugates formed in this study also showed improved eff values in comparison to Pc and graphene oxide conjugates or Pc and single wall carbon nanotube conjugates (512 cm/GW and 300 cm/GW) [41,42]. For complex 1, the highest increase was obtained in the presence of AgNPs, Table 1, even though this conjugate gave the lowest triplet quantum yield. A direct
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relationship between the triplet excited state absorptions and optical nonlinearity has been reported [12], but according to the current work for 1– AgNPs–GSH afforded the best NLO behaviour.
eff value of Ag2Se/ZnS–GSH QDs alone is larger than that of AgNPs– GSH. QDs are known to contribute to NLO through free carrier absorption (FCA) mechanism. FCA is usually produced when excitation takes place at the wavelengths where there is linear absorption [43]. FCA may be attributed to the excited-state absorption of electrons in the conduction band, and holes in the valence band. As stated above, the presence of Ag in Ag2Se/ZnS–GSH QDs resulted in an absorption at 339 nm. Hence the Ag component contributes only through heavy atom effect since the absorption is far from the 532 nm employed for Z scan. The broad absorbance at 557 nm is typical of QDs hence may be attributed to the Se/ZnS component which has an absorption which encompasses 532 nm and will contribute through both the heavy atom effect and FCA. Thus, Ag2Se/ZnS–GSH QDs which have an absorption covering the 532 nm region, may contribute to NLO through FCA resulting in the observed differences between the effects of Ag2Se/ZnS–GSH QDs compared to AgNPs when alone. For the NPs, AgNPs afforded eff value of 420 cm/GW while the Ag2Se/ZnS QDs accounted for eff value of 3200 cm/GW, Table 1, Figure S1.
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The eff value of AgNPs is improved when combined with complexes 1 and 2. Thus the synergetic effect of complexes 1 and 2 with AgNPs resulted in improved NLO compared to individual components. The eff value of Ag2Se/ZnS–GSH QDs decreases in the presence of complex 1 and increases in the presence of 2. The contribution of FCA following conjugation will affect the eff .
The imaginary third-order nonlinear susceptibility Im[ (3) ] was determined from eff Table 1 and the values followed the trend shown by
eff. Im[ (3) ] characterizes the response time of a nonlinear optical material, following the perturbation initiated by the intense laser pulses. An optical limiting material displays a decreasing transmittance as a function of the incident fluence. At low incident fluence, the material has a linear transmittance; while at some critical fluence or threshold, the transmittance changes abruptly, and leads to clamping of the output fluence at a constant value that would presumably be less than the amount required to damage the optical element. This critical point is known as the threshold limit intensity or fluence I lim, which is typically referred to the energy where 50% transmittance is observed [44]. The conjugates accounted for 50% limiting threshold at 0.68 J.cm-2, 1.61 J.cm-2, 1.24 J.cm-2, and 0.54 J.cm-2 for 1–AgNPs–GSH, 1–Ag2Se/ZnS– GSH, 2–AgNPs–GSH, and 2–Ag2Se/ZnS–GSH QDs respectively, Fig. 10.
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Among the conjugates, 2–Ag2Se/ZnS–GSH accounted for the best optical nonlinearity, Table 1. For the NPs, only the Ag2Se/ZnS QDs afforded 50% attenuation of transmittance at limiting threshold of 0.44 J.cm-2 while the AgNPs failed to give 50% attenuation of the transmittance, Table 1, Figure S2. 4. Conclusion Indium(III) chloride 1,8(11),15(18),22(25)–tetra–(3–carboxyphenoxy) phthalocyanine (1) and indium(III) chloride 2,9(10),16(17),23(24)–tetra– (3–carboxyphenoxy)phthalocyanine (2) were covalently linked to glutathione capped silver nanoparticles (AgNPs–GSH) and silver selenide/zinc sulfide (Ag2Se/ZnS–GSH) quantum dots via amide bond formation. With the exception of 1–AgNPs–GSH, the conjugates afforded improved photophysical behaviour in comparison to complexes 1 and 2. All the conjugates showed improved nonlinear optical behavior compared to Pcs only. The utilization of complexes 1 and 2 with silver based NPs could serve a good synergy in development of viable and efficient nonlinear absorbers. Acknowledgements This work was supported by the Department of Science and Technology (DST) Innovation and National Research Foundation (NRF), South Africa through DST/NRF South African Research Chairs Initiative for Professor
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of Medicinal Chemistry and Nanotechnology (UID 62620) as well as Rhodes University.
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Scheme 1. Synthetic pathways for indium(III) chloride 1,8(11),15(18),22(25)– tetra–(3–carboxyphenoxy)phthalocyanine (1) and indium(III) chloride 2,9(10),16(17),23(24)–tetra–(3–carboxyphenoxy)phthalocyanine (2).
30
Scheme 2. Representative synthetic pathway for the ligand exchange process from lipophilic phase to hydrophilic one using Ag2Se/ZnS–GSH as an example. GSH = Glutathione, OLM = Oleylamine and AT = Ambient Temperature.
31
Scheme 3. Representative covalent linkage pathway for complex 1 to AgNPs– GSH and Ag2Se/ZnS–GSH QDs to form 1–AgNPs–GSH and 1–Ag2Se/ZnS–GSH QDs.
32
Normalised Absorbance
1.0
A
Complex 1 Complex 2
0.8 0.6 0.4 0.2 0.0 300
400
500
Normalised Absorbance
700
800
B
1.0 0.8
600
Wavelength (nm)
Complex 1 1-AgNPs-GSH 1-AgSe/ZnS-GSH
0.6 0.4 0.2 0.0 300
400
500
600
Wavelength (nm)
700
800
33
Normalised Absorbance
Absorption Excitation Emission
1.0
Normalised Fluorescence
C
1.0 0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0 550
600
650
700
Wavelength (nm)
750
0.0 800
Figure 1. Ground state electronic absorption spectra: (A) complexes 1 and 2; and (B) complex 1, 1–AgNPs–GSH, 1–Ag2Se/ZnS–GSH QDs; and (C) Absorption, emission, excitation spectra for complex 1, emission spectrum was recorded at exc. = 620 nm while the excitation spectrum was acquired at exc. = 709 nm. All the measurements were done in DMSO.
Normalised Absorbance
1.0
AgNPs-GSH AgSe/ZnS-GSH
0.8 0.6 0.4 0.2 0.0 300
400
500
600
700
800
Wavelength (nm) Figure 2. Ground state electronic absorption spectra for AgNPs–GSH, and Ag2Se/ZnS–GSH QDs, measured in water.
34
1719 1654
3241
1557
Transmittance (%)
1
1248
3260
1247 1395 1-Ag2Se/ZnS-GSH
1577
3241
1572
1383
Ag2Se/ZnS-GSH
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm-2) Figure 3. FT–IR spectra for complex 1, 1–Ag2Se/ZnS–GSH QDs and Ag2Se/ZnS–GSH QDs.
35
Complex 1
O 1s
N 1s
Intensity (cps)
In 3d
C 1s
(A)
1-Ag2Se/ZnS-GSH
0
200
Zn 2p Zn 2p
Se Auger Se Auger
Ag 3d
S 2p
Se 3d Zn 3p
Ag2Se/ZnS-GSH
400
600
800
Binding Energy (eV)
1000
1200
36 0
0
N-H N-C Envelope
398
400
Binding Energy (eV)
402
Envelope N-H N-C HN-C=O
Intensity (cps)
Intensity (cps) 396
1-Ag2Se/ZnS-GSH
Ag2Se/ZnS-GSH
(B)
404 396
398
400
Binding Energy
402
404
Fig. 4. XPS spectra (A) Survey spectra for complex 1, 1–Ag2Se/ZnS–GSH and Ag2Se/ZnS–GSH; and (B) High resolution spectra of N 1s of 1–Ag2Se/ZnS–GSH and Ag2Se/ZnS–GSH
37
Ag Se/ZnS-GSH 2
Lin (Counts)
1-Ag Se/ZnS-GSH 2
2-Ag Se/ZnS-GSH 2
10
20
30
40
2 (Degree)
50
60
Figure 5. XRD diffractograms for Ag2Se/ZnS–GSH QDs, 1–Ag2Se/ZnS–GSH QDs, and 2–Ag2Se/ZnS–GSH QDs.
38
10
Mean = 5.3 0.9
Particle Number
8
6
A
4
2
0 4
5
6
7
Size Diameter (nm)
8
8
Mean Size = 7.9 1.5
Particle Size
6
B 4
2
0
5
6
7
8
9
Size Diameter (nm)
10
11
Figure 6. Histograms and TEM photo–micrographs for AgNPs–GSH, and Ag2Se/ZnS–GSH QDs.
39
10000
Amplitude (Counts)
8000 6000 4000 2000
Residuals
0 4 2 0 -2 -4 0
5
10
15
20
Time (ns) Figure 7. Fluorescence decay curves for 1–Ag2Se/ZnS–GSH QDs in DMSO.
Change in Absorbance
0.10 0.08 0.06 0.04 0.02 0.00 10
60
110
160
210
Time (µs) Figure 8. Excited triplet state absorption curves for complex 2 in deaerated DMSO.
Normalised Transmittance
1.0 0.8 0.6 0.4
1.0 0.8 0.6 0.4 2 2-AgNPs-GSH 2-AgSe/ZnS-GSH Theoretical Fit
1 0.2 1-AgNPs-GSH 1-AgSe/ZnS-GSH Theoretical Fit 0.0
0.2 0.0 -4
-2
0 Z (cm)
2
4
-4
-2
0 Z (cm)
2
4
Figure 9. Open aperture Z–scan curves for the Pcs and their conjugates. All measurements were done in DMSO at an absorbance of 1.5 at Q–bands and at a pulse rate of 10 ns.
1.0
1.0 1
Transmittance
Normalised Transmittance
40
0.8
0.8
A
B
1-Ag2Se/ZnS-GSH
0.6
0.6 50% Transmittance
0.4
0.4 1-AgNPs-GSH
0.2
0.2 0.01
0.1
1
2 2-AgNPs-GSH 2-Ag2Se/ZnS-GSH 0.01
0.1
1 1.2
Incidence Fluence (J.cm-2)
Figure 10. Transmittance versus incidence fluence for complex 1, 1–AgNPs– GSH, 1–Ag2Se/ZnS–GSH (A), and complex 2, 2–AgNPs–GSH, 2–Ag2Se/ZnS– GSH (B). All measurements were done in DMSO at an absorbance of 1.5 at Q– bands at a pulse rate of 10 ns. The blue solid line corresponds to 50% transmittance.
1.0
0.8
0.6
0.4
0.2
0
41
Table 1: Photophysical and NLO data of Pcs, and their conjugates in DMSO Samplesa
abs
(nm)
F
τF (ns) T
τT (µs) eff
Im[(3)]
(cm/GW) (esu)
Ilim (Jcm-2)
1
706b 0.014 2.26
0.71 49
569
4.80 × 10-12 [d]
1–AgNPs–GSH (14.1)
708b 0.013 0.94
0.68 45
3450
2.96 × 10-11 0.68
1–Ag2Se/ZnS–GSH QDs (15.7) 705b 0.008 0.76
0.92 44
2500
2.14 × 10-11 1.61
2
691b 0.020 2.43
0.68 56
670
5.74 × 10-12 [d]
2–AgNPs–GSH (17.0)
692b 0.017 0.84
0.70 54
2200
1.89 × 10-11 1.24
2–Ag2Se/ZnS–GSH QDs (18.7) 689b 0.014 0.69
0.85 45
3300
2.83 × 10-11 0.54
17.0??)
AgNPs–GSH (8.8)
406c
−
−
−
−
420
3.60 × 10-12 [d]
Ag2Se/ZnS–GSH QDs (13.1)
339c,
−
−
−
−
3200
2.74 × 10-11 0.44
557c values in brackets are the sizes of the NPs alone from XRD in nm. bQ band maxima of the MPcs. cAbsorption band of the NPs. [d] 50% attenuation of transmittance was not obtained a
Table 2: XPS apparent surface composition of Ag2Se/ZnS–GSH QDs and its conjugates Samples
Atomic Concentration (%) C 1s
O 1s
N 1s
Ag2Se/ZnS–GSH QDs (13.1)
85.39
11.43
0.42
1–Ag2Se/ZnS–GSH QDs (15.7)
90.62
4.28
4.96
S1010-6030(17)30692-5 http://dx.doi.org/doi:10.1016/j.jphotochem.2017.07.032 JPC 10758
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
22-5-2017 25-7-2017 25-7-2017
Please cite this article as: David O.Oluwole, Sixolisile M.Ngxeke, Jonathan Britton, Tebello Nyokong, The effect of point of substitution and silver based nanoparticles on the photophysical and optical nonlinearity of indium carboxyphenoxy phthalocyanine, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.07.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
The effect of point of substitution and silver based nanoparticles on the photophysical and optical nonlinearity of indium carboxyphenoxy phthalocyanine David O. Oluwole, Sixolisile M. Ngxeke, Jonathan Britton, Tebello Nyokong* Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa *Corresponding
author. Tel: + 27 46 6038260; Fax: + 27 46 6225109. E-mail: [email protected]. (T. Nyokong)
Graphical abstract
Research Highlights
Indium phthalocyanines were covalently linked to silver containing nanoparticles to form conjugates
triplet state quantum yields of the phthalocyanines improved in presence of silver containing nanoparticles
All conjugates showed improved non-linear optical activity in comparison to the Pc complexes alone.
2
Abstract Indium(III) chloride 1,8(11),15(18),22(25)–tetra–(3–carboxyphenoxy) phthalocyanine (1) and indium(III) chloride 2,9(10),16(17),23(24)–tetra–(3– carboxyphenoxy) phthalocyanine (2) were covalently linked to glutathione capped silver nanoparticles (AgNPs–GSH) and silver selenide/zinc sulfide (Ag2Se/ZnS–GSH) quantum dots via amide bond formation. The photophysical and nonlinear optical behaviour of the metallophthalocyanines and their conjugates with nanoparticles were investigated in using the open aperture Z–scan technique. Complex 2 showed enhanced photophysical properties compared to 1. The conjugates revealed improved triplet state quantum yields (except for 1-AgNPs-GSH which afforded lower triplet state quantum yields in comparison to 1) and nonlinear optical activities in comparison to the Pc complexes. The synthesized complexes, nanoparticles and their conjugates could be potential nonlinear optical materials due to their good nonlinear optical activities. Key words: nanoparticles, quantum dots, indium phthalocyanine, nonlinear optics, triplet quantum yields 1. Introduction Nonlinear optical (NLO) devices are being developed in order to protect the eye and other light sensitive objects from intense laser light. NLO materials possess some essential features such as large nonlinear absorption coefficient (βeff), inherent fast response time and efficient optical limiting threshold intensity or
3
fluence (Ilim) [1–3]. Among the organic molecules that have been tested for NLO applications, metallophthalocyanines (MPcs) remain one of the most viable materials. The efficient optical nonlinearity of MPcs could be attributed to their extensive 18 π–electron conjugated ring system. In addition, ring substituents on MPcs can be modified, allowing for attachment of other NLO materials such as nanoparticles (NPs) [4,5]. It is also important to note that central atoms and nature of substituents also play an indispensable role in the optical nonlinearity of phthalocyanines [6–10]. On the other hand, silver based nanoparticles (NPs) have been reported to possess NLO properties [11]. AgNPs have found applications in other areas such as in photodynamic therapy [12,13] and as microbial agents [14]. Previous studies in our group have shown that MPcs can form dyad systems with various metal NPs with improved triplet state quantum yields and NLO activities in comparison to the MPcs alone [15–18]. Herein, we report on the covalent linkage of indium(III) chloride 1,8(11),15(18),22(25)–tetra–(3– carboxyphenoxy)phthalocyanine (1) and indium(III) chloride 2,9(10),16(17),23(24)–tetra–(3–carboxyphenoxy)phthalocyanine (2), Scheme 1, to glutathione (GSH) capped silver NPs (AgNPs–GSH) and silver selenide/zinc sulfide quantum dots (Ag2Se/ZnS–GSH QDs). The synthesis of complex 2 has been reported [19], while complex 1 is reported in this work for the first time. The effect of the point of attachment of AgNPs–GSH and Ag2Se/ZnS–GSH QDs on the photophysical and NLO properties of the Pcs will be evaluated. Since both Ag based NPs and Pcs are NLO materials, we expect enhanced NLO
4
behaviour of the conjugates through synergistic effect. In addition, the dyad system of the MPc complexes with silver based NPs is expected to afford enhanced triplet state absorption and NLO behaviour due to heavy atom effect. Even though MPcs have been linked to AgNPs [19], this is the first time that Ag based QDs are linked to Pcs. Also core/shell quantum dots show enhanced optical properties compared to the core [20,21], hence core/shell Ag2Se/ZnS– GSH QDs are employed in this work and compared to AgNPs following linking to MPc complexes. The nonlinear optical behaviour of the NPs, Pc complexes and their conjugates were tested in solution (dimethylsulfoxide) using open aperture Z-scan technique at excitation wavelength of 532 nm with 10 ns pulse length and laser energy of ~70 µJ. 2. Experimental 2.1.
Materials
5
Ultra-pure water was obtained from a Milli-Q Water System (Millipore Corp., Bedford, MA, USA). Indium(III) chloride, glutathione (GSH), potassium hydroxide, N,N’-dicyclohexylcarbodiimide (DCC), 4-(Dimethylamino)pyridine (DMAP), zinc phthalocyanine, dimethyl formamide (DMF), quinoline and urea were obtained from Sigma Aldrich®. All other reagents and solvents were obtained from commercial suppliers and used as received. Complex 2 [19], oleyamine (OLM) capped AgNPs [22], OLM capped Ag2Se/ZnS QDs [23], and 3– (3,4–dicyanophenoxy)benzoic acid [24] were synthesized as reported in literature.
2.2.
Equipment
FT–IR spectra were recorded on a Bruker spectrum 100 with universal attenuated total reflectance (ATR) sampling accessory. X–ray powder diffraction (XRD) patterns were recorded using a Cu K radiation (1.5405˚A, nickel filter) equiped with LynxEye detector, on a Bruker ® D8 Discover equipped with a proportional counter and the X–ray diffraction data were processed using the Eva ® (evaluation curve fitting) software. The morphology of the NPs were assessed using a transmission electron microscope (TEM), ZEISS LIBRA ® model 120 operated at 90 kV accelerating voltage. Mass spectra data were acquired on a Bruker® AutoFLEX III Smartbeam TOF/TOF
6
mass spectrometer operated in positive ion mode using m/z range of 500– 5000 amu. 1H NMR measurement was performed on a Bruker® AMX 600 MHz. Ground state electronic absorption was measured on a Shimadzu ® UV– 2550 spectrophotometer. Fluorescence emission and excitation spectra were acquired on a Varian Eclipse® spectrofluorometer using 360–1100 nm filter. Fluorescence lifetimes were measured using a time correlated single photon counting setup (TCSPC) (FluoTime 300, Picoquant ® GmbH) with LDH–P–670, Picoquant® GmbH, 20 MHz repetition rate, 44 ps pulse width). Details have been provided before [25]. Triplet quantum yields were determined using a laser flash photolysis system. The excitation pulses were produced using a tunable laser system consisting of an Nd:YAG laser (355 nm, 135 mJ/4–6 ns) pumping an optical parametric oscillator (OPO, 30 mJ/3–5 ns) with a wavelength range of 420–2300 nm (NT-342B, Ekspla), as reported in the literature [25]. All Z-scan experiments described in this study were performed using a frequency-doubled Nd:YAG laser (Quanta-Ray, 1.5 J /10 ns fwhm (full width at half maximum) pulse duration) as the excitation source. The laser was operated in a near Gaussian transverse mode at 532 nm (second harmonic), with a pulse repetition rate of 10 Hz and energy range of 0.1 µJ – 0.1 mJ, limited by the energy detectors (Coherent J5-09). The low repetition rate of the laser prevents cumulative thermal nonlinearities. The beam was spatially filtered to remove the higher order modes and tightly focused with a 15 cm focal length lens. The Z-scan system size (l × w × h) used was 600 mm × 300
7
mm × 350 mm (excluding the computer, energy meter, translation stage driver and laser system). The liquid samples were placed in a cuvette (internal dimensions: 2 mm × 10 mm × 55 mm, 0.7 mL) and a path length of 2 mm (Starna 21-G-2).
2.3. Syntheses 2.3.1. Indium(III) chloride 1,8(11),15(18),22(25)–tetra–(3–carboxy phenoxy) phthalocyanine (1, Scheme 1) Complex 1 was synthesized as follows: 3–(3,4–dicyanophenoxy)benzoic acid (0.50 g, 1.89 mmol) was weighed into a round bottom flask containing deaerated mixture of InCl3 (0.60 g, 2.71 mmol), urea (0.50 g, 8.33 mmol) and quinoline (4 mL). The reaction mixture was refluxed at 200 °C for 1 h with constant stirring under argon atmosphere. Afterwards, the temperature was lowered to 140 °C and the heating continued for further 5 h. The obtained product (1) was precipitated out of solution with methanol, and ethanol. Complex 1 was finally purified using a silica packed column chromatography with 10% hexane in chloroform and 5% methanol in chloroform. The fine green product was dried in an enclosed fume hood. Yield: 25% (w/w); UV–vis (DMSO): λmax/nm (log Ɛ): 332 (4.54), 638 (4.33), 706 (5.01). IR [(KBr) vmax/cm-1]: 3241 (O–H), 1719 (C=O), 1654 (C=C), 1557 (C–C), 1248 (C–O–C); calculated for C60H32InClN8O12; C 59.70, H 2.67, N 9.28. Found–(C 59.25, H 2.47, N 9.63)%; 1H NMR
8
(DMSO-d6): δ, ppm 11.33 (4H, s, COOH), 8.90–8.73 (12H, m, Pc-H), 7.11– 5.29 (16H, m Phenyl-H). MS (MALDI–TOF) (m/z): Calculated for C60H32InClN8O12–1206.09; Found–1209.13 [M+3H]+. 2.3.2.
GSH capped AgNPs and Ag2Se/ZnS QDs, Scheme 2
The synthesis of oleyamine (OLM) capped AgNPs, and Ag2Se/ZnS QDs were done as reported in the literature [22,23]. The functionalization of the NPs with GSH is as follows: the oleyamine capped AgNPs (0.40 g) or oleyamine capped Ag2Se/ZnS (0.40 g) QDs were transferred into separate round bottom flasks containing chloroform (3 mL). GSH (0.25 g, 0.81 mmol) and KOH (1.50 g, 26.74 mmol) were dissolved in methanol (20 mL) and the resulting solution was added to the NPs mixtures. The mixtures were allowed to stir for 2 h to foster the attachment of the thiol moiety of the GSH to the NPs surface via metal– sulfur interaction. The GSH capped NPs were precipitated out of solution using ethanol, purified with methanol and air dried in enclosed fume hood to give AgNPs–GSH and Ag2Se/ZnS–GSH QDs.
2.3.3. Conjugation of complexes 1 and 2 with AgNPs–GSH and Ag2Se/ZnS–GSH QDs, Scheme 3 The conjugation of complexes 1 and 2 with AgNPs–GSH and Ag2Se/ZnS– GSH QDs was as follows: complexes 1 or 2 (0.030 g, 0.025 mmol) were weighed into two flasks, then 2 mL of DMF and DCC (0.015 g, 0.073 mmol) were added to each flask. The reaction mixtures were allowed to stir for 65 h at ambient temperature to activate the COOH group of the
9
Pcs. Then, DMAP (0.008 g, 0.066 mmol) and 0.035 g of AgNPs–GSH or Ag2Se/ZnS–GSH QDs were added to each reaction mixture, which were allowed to stir further for 48 h at ambient temperature leading to amide formation between the complexes and GSH capped NPs. The formed conjugates were precipitated out solution with ethanol and successively purified with methanol and air dried in enclosed fume hood. The conjugates are represented as 1–AgNPs–GSH, 1–Ag2Se/ZnS–GSH QDs, 2–AgNPs–GSH, and 2–Ag2Se/ZnS–GSH QDs. 2.4. Photophysicochemical parameters 2.4.1.
Fluorescence (F) and triplet (T) state quantum yields
The fluorescence quantum yields (F) of the complexes and their conjugates with GSH capped NPs, were assessed using a comparative methods reported in the literature [26]. Unsubstituted ZnPc dissolved in DMSO was used as a standard for the MPc complexes and their conjugates (F =0.2) [27]. The triplet (T) quantum yields of the MPc complexes and their conjugates were determined using a comparative method reported in the literature and unsubstituted ZnPc in DMSO was used as a standard (T = 0.65) [28].
2.5. Z-scan measurements
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The optical nonlinearity of the complexes, NPs and their conjugates were investigated using the open aperture Z–scan technique as described in the literature [1], using equation 1. TNorm ( z )
1 I0 [1 eff Leff ( )] z 1 ( )2 z0
(1)
where eff and I0 are the effective intensity dependent nonlinear absorption coefficient and the intensity of the beam at focus, respectively. z and z0 are sample positions with respect to the input intensity and
w02 Rayleigh length (defined by ; λ = wavelength of the laser beam and w0 = beam waist at the focus (z = 0)), respectively. Leff is the effective thickness of the sample and is given by equation 2:
Leff
1 e L
(2)
where is the linear absorption coefficient and L is the thickness of the sample. The imaginary component of the third order optical susceptibility (Im[(3)] in esu) is directly proportional to eff via equation 3 [29,30]:
Im[
( 3)
]
(n 2 0 c eff ) (2 )
(3)
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In equation 3, c and n are the speed of light in vacuum and the linear refractive index respectively. ε0 is the permittivity of free space and λ is the wavelength of the laser light. By definition, limiting intensity (Ilim/J.cm-2) is the input fluence at which the output fluence is 50% of the linear transmission [31,32].
3. Results and discussion 3.1. Characterization of MPcs alone Complex 1 was synthesized by cyclocondensation of 3-(3,4dicyanophenoxy)benzoic acid in the presence of indium(III) chloride and urea in quinoline, Scheme 1. The FTIR, 1H NMR, MALDI-TOF mass spectroscopies and elemental analyses data of the complex were consistent with the predicted structure. The complex showed good solubility in organic solvents. The 1H NMR spectrum of 1 depicted it resonance signal at 11.33 ppm as a singlet with 4 protons (COOH) while the Pc aromatic ring accounted 12 protons with resonance signal at 8.90–8.73 ppm as a multiplet and the phenyl ring gave 16 protons as a multiplet with resonance signal at 7.11 –5.29 ppm. For complex 1, mass to charge ratio of 1206.09 m/z was calculated and 1209.13 m/z was found.
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The electronic absorption spectra of complexes 1 and 2 in DMSO are shown in Fig. 1A. Both non–peripherally (complex 1) and peripherally (complex 2) tetra substituted phthalocyanines have a mixture of four possible structural isomers with the following molecular symmetry: C4h, C2v, Cs and D2h. The different isomers show similar spectra. Complexes 1 and 2 showed no aggregation at concentrations ranging from ~ 1 x 10 -6 to 8 10-6 mol/L in DMSO. The Beer–Lambert law was obeyed at the studied concentrations. The Q–band of 1 was more red shifted than that of 2 due to non–peripheral substitution in the former [33], Table 1 and Fig. 1A. For both the complexes at concentrations less than 10 -6 M, absorption and excitation spectra were similar and mirror images of the emission spectra, Fig. 1C. Slight differences in the maxima of excitation and absorption spectra could be due to differences in equipment used. 3.2. Characterization of NPs and their conjugates The oleylamine stabilized AgNPs and Ag2Se/ZnS QDs were first synthesised followed by phase transfer of the NPs from lipophilic ligand to hydrophilic one. The process was achieved by replacement of the oleylamine with glutathione (GSH) due to the stronger affinity of the thiol in the later for the NPs surface in comparison to the former, Scheme 2. It is noteworthy that residual OLM might still be present on the surface of the NPs. The GSH capped AgNPs and Ag2Se/ZnS QDs are soluble in water whereas the OLM capped derivatives are not. The functionalization of the AgNPs and Ag2Se/ZnS QDs with GSH was
13
performed to foster their covalent attachment to COOH substituted complexes 1 and 2. The linkage of the NPs to the complexes was done by activation of complexes 1 and 2 COOH substituents with DCC/DMAP. Upon conversion of the COOH moiety of the Pc complexes to active carbodiimide ester, NPs having NH2 were introduced to foster amide bond formation between the NPs and the complexes, Scheme 3. 3.2.1. UV–vis absorption and emission spectra The formation of the GSH capped Ag 2Se/ZnS QDs is evidenced by broad absorption spectral peaks at 557 nm and 339 nm, Fig. 2. The latter may be attributed to the presence of Ag in Ag2Se/ZnS–GSH QDs. The broad absorbance at 557 nm is typical of QDs for Ag2Se/ZnS-GSH QDs [19]. For AgNPs–GSH, the surface plasmon resonance (SPR) peak is well resolved at 406 nm, Fig. 2. Slight shifts in the Q–bands of complexes 1 and 2 were observed in the conjugates in some cases, Table 1, and Fig. 1B. The presence of the NPs in the conjugates was not clear in Fig. 1B, but will be shown below using XRD. 3.2.2. FT-IR spectra The FT–IR spectra of the Pc complexes alone, NPs and their conjugates are shown in Fig. 3 (using complex 1, 1–Ag2Se/ZnS–GSH QDs and Ag2Se/ZnS–GSH QDs as examples). Complex 1 showed its typical broad carboxylic O–H bands (3241 cm-1), C=O (1719 cm-1), C=C (1654 cm-1), C–
14
C (1557 cm-1), and C–O–C (1248 cm-1). Ag2Se/ZnS–GSH depicted three distinct vibrational bands which correspond to OH/NH2 (3241 cm-1), C–O (1572 cm-1), and C–N (1383 cm-1). There was disappearance of the MPcs carbonyl band (C=O) at 1719 cm-1 in the conjugates and slight attenuation of the NPs amine band at 3241 cm -1 and this could be ascribed to possible amide bond formation between the NPs and the MPcs. The appearance of a vibrational band at 1577 cm-1 could be due to the amide band (–HN–C=O). GSH on its own has amide bonds, hence it is difficult to make definite amide bond assignments.
3.2.3. XPS analysis The survey spectra of the NPs, Pc complexes and the conjugates depicted all the expected atomic compositions with their corresponding binding energies, Fig. 4A (using Ag2Se/ZnS–GSH, 1, and 1–Ag2Se/ZnS–GSH as examples). Ag2Se/ZnS–GSH shows the following peaks: Zn 2P (1021 eV and 1044 eV), O 1s (531 eV), N 1s (399 eV), Ag 3d (368 eV), C 1s (285 eV), S 2p (162 eV) and Se 3d (53 eV). The peaks found in Ag2Se/ZnS–GSH are also present in 1– Ag2Se/ZnS–GSH with the appearance of In 3d (445 eV) from complex 1. Table 2 shows that there is an increase in C and N for 1–Ag2Se/ZnS–GSH compared to Ag2Se/ZnS–GSH due to additional C and N from the Pc. The N 1s spectra of Ag2Se/ZnS–GSH QDs (Fig. 4B) afforded two deconvoluted sub peaks at 398.86 eV (–N–H) and 400.55 eV (–N–C) resulting from the glutathione ligand used for the NPs surface functionalization. 1–Ag2Se/ZnS–
15
GSH depicted three sub peaks at 398.64 eV (–N–H), 400.17 eV (–N–C) and 401.94 eV (–HN–C=O), Fig. 4B. Of note is the decrease in the peak intensity for –N–H (relative to N-C) peak in 1–Ag2Se/ZnS–GSH QDs in comparison with Ag2Se/ZnS–GSH QDs, Fig. 4B). The decrease in the peak intensity of the latter in the conjugates was due to the covalent linkage of the primary amine of glutathione functionalized NPs with activated carboxylic acid moiety of the complexes which resulted in the formation of an amide (–HN–C=O) bond (Scheme 1). This could have accounted for the observed peak at 401.94 eV (– HN–C=O). GSH alone has an amide bond which is not clear in Fig. 4B.
3.2.4. XRD studies The morphology and size (diameter) of the NPs and their conjugated with MPcs were obtained using an X–ray diffractometry (XRD). The XRD diffractograms of the Ag2Se/ZnS–GSH QDs (as an example) correspond to the orthorhombic phase of bulk Ag2Se. The conjugates showed characteristic diffraction pattern of the NPs with the presence of a broad band around 2 = 11° to 25° which is attributed to the presence of the Pcs [34] in the conjugates, Fig. 5. The sizes of the NPs and their conjugates were estimated using the Scherrer equation (4) [35]:
d
k Cos
4
where λ is the wavelength of the X-ray source (1.5405 Å), k is an empirical constant equal to 0.9, β is the full width at half maximum of the diffraction
16
peak and θ is the angular position. The size (diameters) are shown in Table 1. The sizes of the GSH capped NPs increases on conjugation to complexes 1 and 2, most likely due to aggregation. Aggregation upon conjugation could also be due to interaction from adjacent Pc [33]. 3.2.5. TEM images The NPs were spherical and relatively dispersed (Fig. 6) with mean size of 5.3 ± 0.9 nm and 7.9 ± 1.5 nm for AgNPs–GSH and Ag2Se/ZnS–GSH QDs, respectively. The mean size of Ag2Se/ZnS–GSH QDs was higher than that of the AgNPs–GSH due to the presence of the ZnS shell in the former, Fig. 6, Table 1. The sizes from TEM are slightly less than those obtained from XRD.
3.3. Photophysicochemical Parameters 3.3.1. Fluorescence (F) quantum yields and lifetimes (τF) The fluorescence quantum yields and mean lifetimes of the complexes and their conjugates are listed in Table 1. Fluorescence decay curve of 1– Ag2Se/ZnS–GSH QDs is shown in Fig. 7 (as an example). Complexes 1 and 2 afforded mono–exponential fluorescence decay profiles indicating single τF values. Bi–exponential lifetimes were observed for the conjugates which could be attributed to the orientation of the complexes around the NPs. Decrease in the F and τF values of the complexes (insignificant decrease in F for 1– AgNPs–GSH) were observed upon conjugation with NPs and this is thought to
17
be due to external heavy–atom effect from the NPs in the conjugates. The larger decrease for Ag2Se/ZnS–GSH QDs is due to the ZnS which provides an extra heavy atom effect due to Zn. The larger quenching by Ag2Se/ZnS–GSH QDs could also be due the larger size of the QDs compared to AgNPs, Table 1. The larger size means more heavy atoms for quenching. The fluoresce behaviour of the Ag2Se/ZnS–GSH QDs (exciting where QDs absorb) was not studied due to lack of signal, most likely due to the heavy atom effect of Ag which will reduce fluorescence. 3.4.2. Triplet quantum yields (T) and lifetimes (τT) Fig. 8 shows the triplet absorption decay curve of complex 2 (as an example). Complex 1 (non–peripherally substituted) gave a larger T value of 0.71 in comparison to 2 (peripherally substituted) at 0.68, Table 1. The larger T value of 1 compared to 2 alone, corresponds to lower F. F and T are competing processes, hence the former is expected to be smaller where the latter is larger. The conjugates of the complexes with NPs afforded improved T values except for 1–AgNPs–GSH which gave T value lower than for complex 1 alone. The observed increase in the T values of the complexes in the conjugates could be due to external heavy atom effect from the NPs, as earlier discussed. The larger increase in the presence of Ag2Se/ZnS–GSH QDs could also be due to their larger size compared to AgNPs, hence more heavy atoms. It has been reported [36] that the planar structure of phthalocyanine becomes significantly distorted by introduction of phenyl substituent at the non-peripheral positions exhibiting
18
a saddle-like distorted structure. It has also been suggested that the deformed nature of the Pc could increase in the presence of nanoparticles depending on their proximity to the MPc [37]. It has also been reported that nonplanar deformations enhance nonradiative decay in porphyrins [38]. Thus, the lack of increase in the triplet quantum yields for 1–AgNPs–GSH could be attributed to the deformed structure of the phthalocyanine molecule and the proximity of the core of the NP in AgNPs-GSH compared to 1–Ag2Se/ZnS–GSH (which has a shell), where there is a shell. The deformity is more pronounced at the nonperipheral positions [36], hence 1–AgNPs–GSH (1 non-peripherally substituted) behaves differently from 1–AgNPs–GSH (2 peripherally substituted).
The τT values of the samples ranges from 44 µs to 56 µs. The τT values of complex 1 was lower compared to 2. This is expected because increase in triplet quantum yield should correspond to decrease in triplet lifetime and vice–versa [39]. We observed a decrease in τT values of the conjugates in comparison to the complexes alone even for 1–AgNPs–GSH which showed a decrease in triplet quantum yield compared to complex 1 alone. 3.4. Nonlinear optical properties The optical nonlinearity of the Pc complexes, NPs and their conjugates were measured in DMSO at an excitation wavelength of 532 nm with pulse rate of 10 ns, Fig. 9. The downward dips of curves in Fig, 9 signify reverse saturable absorption (RSA) behaviour resulting from nonlinear absorption of the incident light [4]. The higher the attenuation,
19
(decrease in % transmittance), the better the nonlinear absorber. Ag2Se/ZnS–GSH QDs on their own afforded more than 50% decrease in %transmittance, Fig. S1. For metallic NPs alone, a competition between saturable absorption (SA) and reverse saturable absorption (RSA) has been reported [40] depending on the laser energy. Fig. S1 does not show clear SA behavior. Complexes 1 and 2 alone accounted for less than 50% decrease in %transmittance while their conjugates with NPs afforded more than 50% decrease in %transmittance, Fig. 9 which translates to good response. The experimental data for Pcs and conjugates were fitted to two photon absorption (2PA) mechanism. The sequential 2PA mechanism is typical of phthalocyanine. 2PA has been especially effective at producing large nonlinear absorption in phthalocyanines [4]. eff is a measure of 2PA, and the values are listed in Table 1. The conjugates afforded increased eff values compared to complexes 1 and 2 alone. It can be deduced that the complexes (1 and 2) afforded eff values greater than what was reported in the literatures [41,42] for tert–butyl and amino substituted Pcs (311 cm/GW and 222 cm/GW). Similarly, the conjugates formed in this study also showed improved eff values in comparison to Pc and graphene oxide conjugates or Pc and single wall carbon nanotube conjugates (512 cm/GW and 300 cm/GW) [41,42]. For complex 1, the highest increase was obtained in the presence of AgNPs, Table 1, even though this conjugate gave the lowest triplet quantum yield. A direct
20
relationship between the triplet excited state absorptions and optical nonlinearity has been reported [12], but according to the current work for 1– AgNPs–GSH afforded the best NLO behaviour.
eff value of Ag2Se/ZnS–GSH QDs alone is larger than that of AgNPs– GSH. QDs are known to contribute to NLO through free carrier absorption (FCA) mechanism. FCA is usually produced when excitation takes place at the wavelengths where there is linear absorption [43]. FCA may be attributed to the excited-state absorption of electrons in the conduction band, and holes in the valence band. As stated above, the presence of Ag in Ag2Se/ZnS–GSH QDs resulted in an absorption at 339 nm. Hence the Ag component contributes only through heavy atom effect since the absorption is far from the 532 nm employed for Z scan. The broad absorbance at 557 nm is typical of QDs hence may be attributed to the Se/ZnS component which has an absorption which encompasses 532 nm and will contribute through both the heavy atom effect and FCA. Thus, Ag2Se/ZnS–GSH QDs which have an absorption covering the 532 nm region, may contribute to NLO through FCA resulting in the observed differences between the effects of Ag2Se/ZnS–GSH QDs compared to AgNPs when alone. For the NPs, AgNPs afforded eff value of 420 cm/GW while the Ag2Se/ZnS QDs accounted for eff value of 3200 cm/GW, Table 1, Figure S1.
21
The eff value of AgNPs is improved when combined with complexes 1 and 2. Thus the synergetic effect of complexes 1 and 2 with AgNPs resulted in improved NLO compared to individual components. The eff value of Ag2Se/ZnS–GSH QDs decreases in the presence of complex 1 and increases in the presence of 2. The contribution of FCA following conjugation will affect the eff .
The imaginary third-order nonlinear susceptibility Im[ (3) ] was determined from eff Table 1 and the values followed the trend shown by
eff. Im[ (3) ] characterizes the response time of a nonlinear optical material, following the perturbation initiated by the intense laser pulses. An optical limiting material displays a decreasing transmittance as a function of the incident fluence. At low incident fluence, the material has a linear transmittance; while at some critical fluence or threshold, the transmittance changes abruptly, and leads to clamping of the output fluence at a constant value that would presumably be less than the amount required to damage the optical element. This critical point is known as the threshold limit intensity or fluence I lim, which is typically referred to the energy where 50% transmittance is observed [44]. The conjugates accounted for 50% limiting threshold at 0.68 J.cm-2, 1.61 J.cm-2, 1.24 J.cm-2, and 0.54 J.cm-2 for 1–AgNPs–GSH, 1–Ag2Se/ZnS– GSH, 2–AgNPs–GSH, and 2–Ag2Se/ZnS–GSH QDs respectively, Fig. 10.
22
Among the conjugates, 2–Ag2Se/ZnS–GSH accounted for the best optical nonlinearity, Table 1. For the NPs, only the Ag2Se/ZnS QDs afforded 50% attenuation of transmittance at limiting threshold of 0.44 J.cm-2 while the AgNPs failed to give 50% attenuation of the transmittance, Table 1, Figure S2. 4. Conclusion Indium(III) chloride 1,8(11),15(18),22(25)–tetra–(3–carboxyphenoxy) phthalocyanine (1) and indium(III) chloride 2,9(10),16(17),23(24)–tetra– (3–carboxyphenoxy)phthalocyanine (2) were covalently linked to glutathione capped silver nanoparticles (AgNPs–GSH) and silver selenide/zinc sulfide (Ag2Se/ZnS–GSH) quantum dots via amide bond formation. With the exception of 1–AgNPs–GSH, the conjugates afforded improved photophysical behaviour in comparison to complexes 1 and 2. All the conjugates showed improved nonlinear optical behavior compared to Pcs only. The utilization of complexes 1 and 2 with silver based NPs could serve a good synergy in development of viable and efficient nonlinear absorbers. Acknowledgements This work was supported by the Department of Science and Technology (DST) Innovation and National Research Foundation (NRF), South Africa through DST/NRF South African Research Chairs Initiative for Professor
23
of Medicinal Chemistry and Nanotechnology (UID 62620) as well as Rhodes University.
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Scheme 1. Synthetic pathways for indium(III) chloride 1,8(11),15(18),22(25)– tetra–(3–carboxyphenoxy)phthalocyanine (1) and indium(III) chloride 2,9(10),16(17),23(24)–tetra–(3–carboxyphenoxy)phthalocyanine (2).
30
Scheme 2. Representative synthetic pathway for the ligand exchange process from lipophilic phase to hydrophilic one using Ag2Se/ZnS–GSH as an example. GSH = Glutathione, OLM = Oleylamine and AT = Ambient Temperature.
31
Scheme 3. Representative covalent linkage pathway for complex 1 to AgNPs– GSH and Ag2Se/ZnS–GSH QDs to form 1–AgNPs–GSH and 1–Ag2Se/ZnS–GSH QDs.
32
Normalised Absorbance
1.0
A
Complex 1 Complex 2
0.8 0.6 0.4 0.2 0.0 300
400
500
Normalised Absorbance
700
800
B
1.0 0.8
600
Wavelength (nm)
Complex 1 1-AgNPs-GSH 1-AgSe/ZnS-GSH
0.6 0.4 0.2 0.0 300
400
500
600
Wavelength (nm)
700
800
33
Normalised Absorbance
Absorption Excitation Emission
1.0
Normalised Fluorescence
C
1.0 0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0 550
600
650
700
Wavelength (nm)
750
0.0 800
Figure 1. Ground state electronic absorption spectra: (A) complexes 1 and 2; and (B) complex 1, 1–AgNPs–GSH, 1–Ag2Se/ZnS–GSH QDs; and (C) Absorption, emission, excitation spectra for complex 1, emission spectrum was recorded at exc. = 620 nm while the excitation spectrum was acquired at exc. = 709 nm. All the measurements were done in DMSO.
Normalised Absorbance
1.0
AgNPs-GSH AgSe/ZnS-GSH
0.8 0.6 0.4 0.2 0.0 300
400
500
600
700
800
Wavelength (nm) Figure 2. Ground state electronic absorption spectra for AgNPs–GSH, and Ag2Se/ZnS–GSH QDs, measured in water.
34
1719 1654
3241
1557
Transmittance (%)
1
1248
3260
1247 1395 1-Ag2Se/ZnS-GSH
1577
3241
1572
1383
Ag2Se/ZnS-GSH
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm-2) Figure 3. FT–IR spectra for complex 1, 1–Ag2Se/ZnS–GSH QDs and Ag2Se/ZnS–GSH QDs.
35
Complex 1
O 1s
N 1s
Intensity (cps)
In 3d
C 1s
(A)
1-Ag2Se/ZnS-GSH
0
200
Zn 2p Zn 2p
Se Auger Se Auger
Ag 3d
S 2p
Se 3d Zn 3p
Ag2Se/ZnS-GSH
400
600
800
Binding Energy (eV)
1000
1200
36 0
0
N-H N-C Envelope
398
400
Binding Energy (eV)
402
Envelope N-H N-C HN-C=O
Intensity (cps)
Intensity (cps) 396
1-Ag2Se/ZnS-GSH
Ag2Se/ZnS-GSH
(B)
404 396
398
400
Binding Energy
402
404
Fig. 4. XPS spectra (A) Survey spectra for complex 1, 1–Ag2Se/ZnS–GSH and Ag2Se/ZnS–GSH; and (B) High resolution spectra of N 1s of 1–Ag2Se/ZnS–GSH and Ag2Se/ZnS–GSH
37
Ag Se/ZnS-GSH 2
Lin (Counts)
1-Ag Se/ZnS-GSH 2
2-Ag Se/ZnS-GSH 2
10
20
30
40
2 (Degree)
50
60
Figure 5. XRD diffractograms for Ag2Se/ZnS–GSH QDs, 1–Ag2Se/ZnS–GSH QDs, and 2–Ag2Se/ZnS–GSH QDs.
38
10
Mean = 5.3 0.9
Particle Number
8
6
A
4
2
0 4
5
6
7
Size Diameter (nm)
8
8
Mean Size = 7.9 1.5
Particle Size
6
B 4
2
0
5
6
7
8
9
Size Diameter (nm)
10
11
Figure 6. Histograms and TEM photo–micrographs for AgNPs–GSH, and Ag2Se/ZnS–GSH QDs.
39
10000
Amplitude (Counts)
8000 6000 4000 2000
Residuals
0 4 2 0 -2 -4 0
5
10
15
20
Time (ns) Figure 7. Fluorescence decay curves for 1–Ag2Se/ZnS–GSH QDs in DMSO.
Change in Absorbance
0.10 0.08 0.06 0.04 0.02 0.00 10
60
110
160
210
Time (µs) Figure 8. Excited triplet state absorption curves for complex 2 in deaerated DMSO.
Normalised Transmittance
1.0 0.8 0.6 0.4
1.0 0.8 0.6 0.4 2 2-AgNPs-GSH 2-AgSe/ZnS-GSH Theoretical Fit
1 0.2 1-AgNPs-GSH 1-AgSe/ZnS-GSH Theoretical Fit 0.0
0.2 0.0 -4
-2
0 Z (cm)
2
4
-4
-2
0 Z (cm)
2
4
Figure 9. Open aperture Z–scan curves for the Pcs and their conjugates. All measurements were done in DMSO at an absorbance of 1.5 at Q–bands and at a pulse rate of 10 ns.
1.0
1.0 1
Transmittance
Normalised Transmittance
40
0.8
0.8
A
B
1-Ag2Se/ZnS-GSH
0.6
0.6 50% Transmittance
0.4
0.4 1-AgNPs-GSH
0.2
0.2 0.01
0.1
1
2 2-AgNPs-GSH 2-Ag2Se/ZnS-GSH 0.01
0.1
1 1.2
Incidence Fluence (J.cm-2)
Figure 10. Transmittance versus incidence fluence for complex 1, 1–AgNPs– GSH, 1–Ag2Se/ZnS–GSH (A), and complex 2, 2–AgNPs–GSH, 2–Ag2Se/ZnS– GSH (B). All measurements were done in DMSO at an absorbance of 1.5 at Q– bands at a pulse rate of 10 ns. The blue solid line corresponds to 50% transmittance.
1.0
0.8
0.6
0.4
0.2
0
41
Table 1: Photophysical and NLO data of Pcs, and their conjugates in DMSO Samplesa
abs
(nm)
F
τF (ns) T
τT (µs) eff
Im[(3)]
(cm/GW) (esu)
Ilim (Jcm-2)
1
706b 0.014 2.26
0.71 49
569
4.80 × 10-12 [d]
1–AgNPs–GSH (14.1)
708b 0.013 0.94
0.68 45
3450
2.96 × 10-11 0.68
1–Ag2Se/ZnS–GSH QDs (15.7) 705b 0.008 0.76
0.92 44
2500
2.14 × 10-11 1.61
2
691b 0.020 2.43
0.68 56
670
5.74 × 10-12 [d]
2–AgNPs–GSH (17.0)
692b 0.017 0.84
0.70 54
2200
1.89 × 10-11 1.24
2–Ag2Se/ZnS–GSH QDs (18.7) 689b 0.014 0.69
0.85 45
3300
2.83 × 10-11 0.54
17.0??)
AgNPs–GSH (8.8)
406c
−
−
−
−
420
3.60 × 10-12 [d]
Ag2Se/ZnS–GSH QDs (13.1)
339c,
−
−
−
−
3200
2.74 × 10-11 0.44
557c values in brackets are the sizes of the NPs alone from XRD in nm. bQ band maxima of the MPcs. cAbsorption band of the NPs. [d] 50% attenuation of transmittance was not obtained a
Table 2: XPS apparent surface composition of Ag2Se/ZnS–GSH QDs and its conjugates Samples
Atomic Concentration (%) C 1s
O 1s
N 1s
Ag2Se/ZnS–GSH QDs (13.1)
85.39
11.43
0.42
1–Ag2Se/ZnS–GSH QDs (15.7)
90.62
4.28
4.96