David O.
Oluwole
a,
Alexey V.
Yagodin
bc,
Jonathan
Britton
a,
Alexander G.
Martynov
c,
Yulia G.
Gorbunova
*cd,
Aslan Yu.
Tsivadze
cd and
Tebello
Nyokong
*a
aDepartment of Chemistry, Rhodes University, Grahamstown 6140, South Africa. E-mail: t.nyokong@ru.ac.za
bDmitry Mendeleev University of Chemical Technology of Russia, Miusskaya sq., 9, 125047, Moscow, Russia
cA. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr., 31, bldg. 4, Moscow, 119071, Russia. E-mail: Martynov.Alexandre@gmail.com
dKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr., 31, Moscow, 119991, Russia. E-mail: yulia@igic.ras.ru
First published on 31st October 2017
Two alternative synthetic approaches affording a low-symmetry A3B-type phthalocyanine 1 bearing two [2′-(2′′-mercaptoethoxy)ethoxy] anchoring substituents were developed. Due to the presence of thiol groups, this phthalocyanine could be conjugated with TOPO-capped (TOPO - trioctylphosphine)-capped CdSe quantum dots (CdSe-QDs) or oleylamine capped silver nanoparticles (Ag-NPs). The nonlinear optical behaviour of starting phthalocyanine, quantum dots, nanoparticles and their conjugates was studied by using an open aperture Z-scan technique, revealing that the grafting of 1 onto the nanomaterials resulted in a significant enhancement of the optical limiting of 1-Ag and 1-CdSe in comparison with the individual components. The conjugate 1-CdSe, being the first example of Pc-based thiol conjugated with quantum dots, revealed superior limiting characteristics with a limiting threshold below 0.18 J cm−2.
Among the organic molecules that have been exploited for OL application, phthalocyanines (Pcs) have proven to be excellent NLO materials.1,7–10 This is due to their excellent photophysical properties and optical nonlinearities provided by the extended π-electron conjugated system, as well as high thermal and chemical stability, wide possibilities of synthesis and structural modification.11 The tuning of the NLO properties of Pcs can be achieved by varying the nature of the central metal in the cavity of the Pc ring, by altering the ring substituents or by lowering the molecular symmetry.12–15 For example, phthalocyanines containing heavy central metals such as In, Ga or Zn show improved NLO behaviour due to increased intersystem crossing as a result of the heavy atom effect.8,16,17 On the other hand, metal free phthalocyanines are also known to possess good NLO behaviour as a result of the lack of symmetry.18
Nanoparticles (NPs) and quantum dots (QDs) are also promising optical materials due to their quantum confinement.19–24 Conjugation of Pcs with nanomaterials improves the NLO behaviour of the conjugates in comparison with the parent species.25–27 Such conjugation typically requires the introduction of certain anchoring groups into the Pcs, and several examples of Pcs conjugated with QDs via amide or ester bonds have been previously reported, however, to the best of our knowledge the conjugates of QDs with thiolated Pcs have not been described. At the same time, thiolation of various organic substrates is a powerful tool for their further conjugation with nanoparticles, metal surfaces, etc.28,29 Thiolated Pcs have been shown to be valuable components of such conjugates, acting as sensors,30 photosensitizers,31 hole injectors,32etc.
In the present study we report the synthesis of the novel low-symmetry phthalocyanine 1, starting from the recently published A3B-type ligand 2,18 bearing two diethyleneglycol chains with terminal OH-groups (Scheme 1). The asymmetry of ligand 2 and its complexes was shown to improve its NLO behaviour in comparison with symmetrical phthalocyanines,18,33 therefore we aimed to study the OL properties of the thiolated derivative 1 and to investigate the influence of its conjugation with various nanomaterials. This was studied herein on the examples the nanoconjugates 1-CdSe and 1-Ag which were formed by the interaction of 1 with trioctylphosphine oxide (TOPO) capped CdSe quantum dots (CdSe-QDs) and oleylamine capped silver nanoparticles (Ag-NPs). The nonlinear optical behaviour of the starting materials and conjugates was studied by using the open aperture Z-scan technique.
Using the first approach, we started the synthesis of the target thiolated phthalocyanine 1 from the previously reported A3B-type ligand 2, which was shown to exhibit prominent NLO properties due to the low molecular symmetry. To introduce the leaving groups, we treated the parent Pc 2 with iodine, triphenylphosphine and imidazole in DCM (Scheme 1). It afforded a diiodide derivative 3, which was isolated in 84% yield after column chromatography. The reaction of 3 with thiourea in the THF/EtOH mixture afforded the corresponding thiouronium salt 4 which was hydrolyzed without isolation with aqueous NaOH solution. The isolation of the dithiol 1 by column chromatography afforded the target compound in a moderate yield (39%). However, the harsh conditions of this reaction might preclude the application of this approach in the case of more sophisticated Pcs, like sandwich complexes, which are less tolerant to hydrolysis in strongly polar media.36–38
With the aim to improve the abovementioned drawbacks, we have proposed another approach, which implied the substitution of iodine atoms with thiobenzoyl fragments and the subsequent mild cleavage of benzoyl protective groups (Scheme 1).39 The treatment of 3 with thiobenzoic acid in the presence of excess of KHCO3 in DMF afforded thiobenzoylated Pc 5 which was isolated in 89% yield. The previously reported procedure of the deprotection of benzoyl groups with hydrazine hydrochloride and sodium acetate in DMF39 failed to furnish the target dithiol 1, however, Pc 5 could be smoothly and quantitatively deprotected by using hydrazine hydrate in the CHCl3/MeOH mixture.
Compounds 1, 3 and 5 were characterized by MALDI TOF MS, 1H-NMR, UV-Vis and FT-IR spectra and elemental analysis, which were in full agreement with the proposed structures (Fig. S1–S9†). 1H-NMR spectroscopy evidenced the non-equivalence of aromatic protons of the phthalocyanine macrocycle in low-symmetry ligands 1, 3 and 5 (Fig. 1, S1 and S5†), which arises from slightly different electronic effects of the butoxy group and the diethyleneglycol chain.18,33 In turn, it results in dissymmetry in the distribution of the electronic density over the ligand, which provides starting Pc 2 and its complexes with the enhanced NLO properties.18,33
Fig. 1 1H-NMR spectra of phthalocyanines 1, 6m and 6o in CDCl3. The numbering of protons is given in Scheme 1. The insets show the resonance signals of aromatic protons. Asterisk marks show the residual water signal. |
Although the presence of the SH-group in 1 was confirmed by FT-IR (a weak band at 2556 cm−1) and NMR spectra (triplet signal at 1.92 ppm and J = 8.2 Hz), in the MALDI-TOF mass-spectrum we observed the set of molecular ions [(M − 2H)n]+ with n = 1–3, which could correspond to the oxidative dimerization of SH-groups under the MS experimental conditions (Fig. S9†).
Moreover, the oxidation of 1 with the formation of disulphide derivatives with S–S bonds proceeded slowly upon the aerobic storage of 1 (Scheme 2). This feature is well-known in thiol chemistry and in our case the formation of disulphides can be facilitated due to the generation of singlet oxygen promoted by the phthalocyanine. Dithiol 1 can undergo the intra-/intermolecular oxidative coupling of SH-groups with the formation of dithiacrown-ether macrocycles.40 In contrast, the benzoylated ligand 5 is air stable, which makes the proposed pathway to the synthesis of thiolated Pcs more convenient in comparison with the methodology involving thiourea.41
We managed to separate the mixture of oxidation products and isolate the essentially pure monomeric phthalocyanine 6m and the mixture of oligomeric species 6o by size-exclusion chromatography on BioBeads S-X1. According to MALDI TOF MS, the fraction 6o contained the species from a dimer to a tetramer. 1H-NMR characterization of the isolated fractions indeed evidenced the disappearance of SH-group resonance in both fractions as well as the appearance of the complicated set of resonance signals in the spectrum of 6m and the splitting of the NH proton signal (Fig. 1). The broad Q-band, which was observed in the UV-Vis spectrum of 6o, is in agreement with the proposed oligomeric structure,42 while the UV-Vis spectrum of 6m was typical of monomeric phthalocyanines (Fig. S10†).
Nevertheless, the formation of disulphides is not an obstacle for further binding to nanomaterials, since they undergo easy reductive cleavage with the formation of metal–sulphur bonds.28–30
The UV-Vis spectrum of 1 alone consisted of well-resolved split Q-bands at 699 nm and 662 nm, which is typical of a monomeric metal-free Pc, but in the UV-Vis spectra of 1-CdSe and 1-Ag, the Pc Q-band regions were dominated by a broad band below 640 nm (Fig. 3). This band might be assigned both to the interaction between the electronic systems of Pc, QDs or NPs, as well as the aggregation of Pc molecules, absorbed on the surface of nanomaterials. The observed enhancement in the UV–Vis absorption spectra of the conjugates (1-CdSe and 1-Ag) around 350 nm to 650 nm compared to compound 1 alone could also be attributed to the presence of CdSe-QDs and Ag-NPs in the respective conjugates which absorb around 350 nm to 650 nm. This is evidenced by using the overlay of UV–Vis absorption for CdSe-QDs, 1 and their conjugates and further corroborated by XRD and TEM (Fig. 4 and 5). The presence of all expected elements was confirmed by EDX spectra (Fig. S11†).
Fig. 3 Normalized UV-Vis spectra of phthalocyanine 1, TOPO capped CdSe quantum dots, oleylamine capped silver nanoparticles alone, conjugates 1-CdSe and 1-Ag in DCM. |
Fig. 5 TEM images and the corresponding size distributions of TOPO-CdSe quantum dots, oleylamine-capped Ag nanoparticles and conjugates 1-CdSe and 1-Ag. |
The XRD pattern of the conjugates resembled the patterns of both 1 and QDs or NPs (Fig. 4 showing the example of 1-CdSe). The XRD diffractogram of CdSe-QDs alone confirmed the presence of three prominent diffraction patterns with 2θ values (2θ = 25.29°, 42.29° and 49.86°) corresponding to the zinc blend crystal and cubic structure at planes 111, 220 and 311, respectively. The size of the CdSe-QDs (3.8 nm) was calculated based on the Debye–Scherrer equation.43 The nanoconjugate 1-CdSe also showed a similar diffraction pattern to the CdSe-QDs, but with additional sharp peaks which could imply an increase in crystallinity upon linking to Pcs. Ligand 1 alone showed crystalline peaks and this is expected for Pcs as reported in the literature.44
TEM measurements were used to characterize both starting QDs and NPs as well as the resulting conjugates (Fig. 5). The size of particles, found by TEM is close to those found by XRD. In the case of 1-Ag the aggregation of nanoparticles upon conjugation was observed which could alter the optical properties of the conjugates.45,46
The imaginary third-order nonlinear susceptibility Im[χ(3)] was calculated from the two photon absorption coefficient βeff; the values are listed in Table 1. Im[χ(3)] is related to the hyperpolarizability γ which measures the interaction of the incident photon with the permanent dipole moments of the Pcs. Im[χ(3)] measures the response time of a NLO material, following the perturbation initiated by the intense laser pulses. An efficient optical limiting material must have high γ and/or Im[χ(3)] as well as βeff. It was shown that the βeff and Im[χ(3)] are improved for 1 in the presence of QDs or NPs. The βeff and Im[χ(3)] values of the conjugates (1-CdSe and 1-Ag) were superior to those of the NPs or compound 1 alone, as shown in Table 1. This trend further lay credence to the synergistic activity that can be achieved when nanomaterials are linked to phthalocyanines.
Compound | β eff (m W−1) | Im[χ(3)] (esu) | I lim (J cm−2) |
---|---|---|---|
a 50% attenuation of the transmitted fluence is not achieved. | |||
1 | 3.61 × 10−8 | 3.09 × 10−11 | 1.35 |
1 -CdSe | 7.30 × 10−8 | 6.26 × 10−11 | 0.18 |
1 -Ag | 5.76 × 10−8 | 4.94 × 10−11 | 0.43 |
CdSe-QDs | 3.00 × 10−9 | 2.57 × 10−12 | |
Ag-NPs | 1.53 × 10−8 | 1.31 × 10−11 |
The investigated OL material displayed a decreasing transmittance as a function of the incident fluence (Fig. 7). At a low incident fluence, the material has a linear transmittance; while at some critical fluence or threshold, the transmittance changes abruptly, and leads to the clamping of the output fluence at a constant value that would presumably be less than the amount required to damage the optical element.5 This critical point is referred to as the threshold limit intensity or fluence (Ilim).5 The lower the Ilim values the better is the material as an optical limiter. The Ilim value decreased (Table 1) for the conjugates as compared to complex 1 alone, particularly indicating a superior optical limiting behaviour for the 1-CdSe composite material. 50% attenuation of the transmitted fluence is not achieved for the nanomaterials which show their deficiency when used alone since their conjugates with Pcs exhibited an improved limiting threshold pivotal for optical nonlinearity.
The improvement of OL in conjugates can be attributed to several factors. In the case of QDs, they absorb at 532 nm where the NLO behaviour was studied, therefore QDs can contribute to the NLO behaviour of Pcs due to the free-carrier absorption (FCA) mechanism, which is usually produced when excitation takes place at the wavelengths where there is linear absorption.26 Second, QDs and NPs may improve the NLO behaviour of Pcs due to the heavy atom effect which improves the triplet state population which directly affects the NLO behaviour. Finally, the aggregation of Pc chromophores in the vicinity of nanomaterials deduced from the UV-Vis spectra may also contribute to the enhancement of OL.45 Further studies might give more information about the synergistic effects which arise from electronic interaction between the phthalocyanine chromophore and quantum dots or nanoparticles.
Dithiol 1 was conjugated with CdSe quantum dots and silver nanoparticles to study their effects on the nonlinear optical characteristics of the phthalocyanine chromophore within the nanoconjugates by the Z-scan technique. The two photon absorption mechanism of optical limiting was confirmed and the characteristic values of Im[χ(3)] and βeff were found. It is evidenced that the combination of 1 with QDs and NPs showed improved NLO behaviour compared to individual components. The mechanisms of this enhancement were discussed.
Dichloromethane (DCM) was distilled over CaH2 under argon. Chloroform was stored over NaHCO3 and distilled over K2CO3 to remove acidic impurities. Tetrahydrofuran (THF) was distilled over Na/benzophenone under argon. Ultrapure water was obtained from a Milli-Q Water System (Millipore Corp., Bedford, MA, USA). Other chemicals (Merck, Aldrich), silica (Macherey Nagel) and BioBeads S-X1 (BioRad) were used as received from commercial suppliers.
The mass spectral data were acquired on a Bruker® Daltonics Ultraflex spectrometer with 2,4-dihydroxybenzoic acid as the matrix. A mass spectrometer operated in a positive ion mode using an m/z range of 500–5000 amu. NMR spectra were recorded with a Bruker Avance 600 spectrometer. NMR spectra were referenced against the residual solvent signal.50
Ground state electronic absorption was measured on Shimadzu UV-2550 and Thermo Evolution 201 spectrophotometers. FT-IR spectra were recorded on Bruker ALPHA FT-IR and Nexus Nicolet spectrometers with universal attenuated total reflectance (ATR) sampling accessories. X-ray powder diffraction patterns were recorded using Cu Kα radiation (1.5405°A, nickel filter) equipped with a Lynx Eye 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. Elemental analyses were carried out on a EuroVector EA-3000 CHNS analyzer. The morphology of nanoparticles was assessed using a transmission electron microscope, ZEISS LIBRA® model 120, operated at 90 kV accelerating voltage. Z-Scan experiments were carried out as described in the literature using a frequency-doubled Nd:YAG laser (Quanta-Ray, 1.5 J/10 ns fwhm pulse duration) as the excitation source. Details have been provided previously.51
Elemental analysis, MALDI TOF MS, NMR and FT-IR measurements were performed at the Shared Facility Centres of the A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, RAS, and the N.S. Kurnakov Institute of General and Inorganic Chemistry, RAS.
Method 2. Thiobenzoylated derivative 5 (12 mg, 8 μmol) was dissolved in the mixture of 2.5 ml CHCl3 and 1.25 ml MeOH, 0.1 ml of hydrazine hydrate was added and the mixture was heated to 40 °C under an argon atmosphere overnight. Upon completion of the reaction, confirmed by MALDI-TOF mass-spectrometry, the reaction mixture was washed with water in a separating funnel and the organic layer was evaporated to dryness, yielding target dithiol 1 with quantitatively cleaved protective groups.
MALDI TOF MS: calculated for C64H82N8O10S2 – 1186.6, found – 1184.5 [M − 2H]+. 1H NMR (600 MHz, CDCl3) δ 8.35, 8.30 and 8.24 (3s, 4H+2×2H, HPc), 4.65 (br s, 4H, α-CH2DEG), 4.59–4.54 (m, 12H, OCH2Bu), 4.21 (t, J = 4.7 Hz, 4H, β-CH2DEG), 3.97 (t, J = 6.5 Hz, 4H, γ-CH2DEG), 2.93 (dd, J = 14.4, 6.5 Hz, 4H, δ-CH2DEG), 2.20 (m, 12H, 2-CH2Bu), 1.92 (t, J = 8.2 Hz, 2H, SH), 1.89–1.85 (m, 12H, 3-CH2Bu), 1.28–1.23 (m, 18H, CH3), −2.98 (s, 2H, NH). UV-Vis (CH2Cl2) λmax (lgε): 700 (4.51), 663 (4.57), 643 (4.46), 419 (4.14), 347 (4.56). FT-IR, cm−1: 3287 (w, N–H stretching), 2930, 2865 (s), 2556 (SH stretching), 1603 (m), 1450 (s), 1380 (m), 1327 (m), 1270 (s), 1195 (m), 1097 (m), 1019 (m), 850 (m). Elemental analysis (%): calc. for C64H82N8O11S2 (1·H2O): C, 63.87; H, 6.87; N, 9.31; S, 5.33. Found: C, 63.65; H, 6.74; N, 9.33; S, 5.24.
(1) |
(2) |
(3) |
In eqn (3), c and n, respectively, are the speeds 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.
At a molecular level, there is a direct correlation of Im[χ(3)] with the hyperpolarizability γ (which provides the nonlinear absorption per mole of the sample) via the relationship shown in eqn (4):8,52
(4) |
(5) |
By definition, the limiting threshold (Ilim/J cm−2) is the input fluence at which the output fluence is 50% of the linear transmission.53,54 The results were found to fit into two photon absorption equation.
Footnote |
† Electronic supplementary information (ESI) available: 1H-NMR, UV-Vis, FT-IR, MALDI TOF MS, EDX data for synthesized compounds and conjugates. See DOI: 10.1039/c7dt03867d |
This journal is © The Royal Society of Chemistry 2017 |