Photophysical and nonlinear optical characteristics of pyridyl substituted phthalocyanine - Detonation nanodiamond conjugated systems in solution

Diamond & Related Materials 94 (2019) 218–232

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Photophysical and nonlinear optical characteristics of pyridyl substituted phthalocyanine - Detonation nanodiamond conjugated systems in solution Refilwe Matshitse, Samson Khene, Tebello Nyokong

T

⁎

Department Chemistry, Rhodes university, P. O Box, Grahamstown, South Africa

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanodiamonds Pyridyl phthalocyanine Photophysics Nonlinear absorption Optical limiting

In this study photophysical, nonlinear absorption and optical limiting properties of detonation nanodiamonds (DNDs)-phthalocyanine nanoconjugate systems containing: 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyaninato (H2TPPc), 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyanato zinc(II) (ZnTPPc) and 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyanato silicon(IV) hydroxide (Si(OH)2TPPc), were investigated in dimethylsulfoxide solution. Pcs were non-covalently linked to nanondiamonds (also covalently linked for Si(OH)2TPPc) and investigated using 532 nm laser excitation at 10 ns pulses for their optical limiting properties. Complexes that have higher triplet state absorption also possessed enhanced nonlinear optical behaviour following reverse saturable absorption mechanism. Superior optical performance is observed when the Pcs had a central metal with axial ligands conjugated to DNDs in solution. Nanoconjugate of DNDs-Si(OH)2TPPc and respective Pc in solution gave the highest imaginary third-order susceptibility (Im[X(3)]) and hyperpolarizability (γ) at 2.91 × 10−8 and 3.17 × 10−8 esu and 3.88 × 10−28 and 4.22 × 10−28 esu, respectively, with Ilim value of 0.47 and 0.39 J·cm−2.

1. Introduction Nanodiamonds (NDs) are interesting carbon-based nanomaterials as a result of their outstanding mechanical performance [1], chemical resistance [2], versatile surface chemistry [3,4], biocompatibility [5], stability [6], low toxicity [7] and unique optical and electrical properties [8,9]. Due to these properties, NDs have found applications in many areas including in wear-resistant polymers, metal coating [2,10], lubricant additives [1], health care products [11], and nonlinear optics (NLO) [12]. On the other hand, metallophthalocyanines (MPcs) have found applications in many areas including in NLO, photocatalysis, electrochemical sensors, photodynamic therapy (PDT), dye-sensitized solar cells (DSSC), and semiconductors amongst others [13–16]. MPcs have received considerable attention as NLO materials due to their large nonlinearities, inherently fast response time, broadband spectral response, ease of processing, thermal stability, and extensive delocalized π electron systems [17–23]. The presence of an extended π electron conjugation system can result in a significant increase in the triplet state population, which leads to improved reverse saturable absorption (RSA) at 532 nm for nanosecond laser pulses [17,24]. NLO materials strongly attenuate optical beams to specific threshold levels under conditions of intense irradiation [25]. Multiphoton absorption, reverse saturable absorption (RSA), nonlinear scattering, and nonlinear ⁎

refraction areis the dominant mechanisms responsible for NLO behaviour [26]. Both MPcs and NDs are NLO materials and they are combined in this work for the first time for NLO applications. Nanodiamonds synthesised using detonation method (detonation nanodiamond, DNDs) are employed in this work. It has been previously reported that properties of NDs are dictated by the surface functionalities, diamond core or a combination of both. DNDs have tetrahedral network structures, and comprise a diamond core (sp3), a middle core (sp2 + x) and a graphitized outer layer (sp2) that is often partially oxidized [27]. The presence of sp2 hybridization in DNDs allows for π-π interactions with other π containing molecules such as MPcs, and this is employed in this work. In their pristine state, DNDs contain several functional groups present on the surface including amine, amide, alcohol, carbonyl, and carboxyl [28,29]. These functional groups facilitate the linking of DNDs to other molecules such as Pc, and in this work a Si(OH)2Pc is linked to DNDs via an ester bond in addition to possible π-π interactions. MPc complexes have been linked to other carbon nanomaterials such as graphene quantum dots (GQDs) with improved NLO behaviour [30–32], but MPc have never been linked covalently or non-covalently to DNDs. DNDs and GQDs (in their pristine state) have different functional groups and are expected to influence NLO behaviour of Pcs differently. The combination of DNDs and Pcs explored in this work, is

Corresponding author. E-mail address: [email protected] (T. Nyokong).

https://doi.org/10.1016/j.diamond.2019.03.013 Received 23 January 2019; Received in revised form 5 March 2019; Accepted 18 March 2019 Available online 20 March 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.

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nanoconjugates was measured using a Malvern Zetasizer Nanoseries, Nano-ZS90 (containing 633 nm helium neon laser). Elemental compositions of the NPs and the nanoconjugates were qualitatively determined using energy dispersive X-ray spectroscopy (EDX), INCA PENTA FET coupled to the VAGA TESCAM operated at 20 kV accelerating voltage. Triplet state quantum yields were determined using a laser flash photolysis system consisting of an LP980 spectrometer with a PMT-LP detector and an ICCD camera (Andor DH320T-25F03). The signal from a PMT detector was recorded on a Tektronix TDS3012C digital storage oscilloscope. The excitation pulses were produced by a tunable laser system consisting of a Nd:YAG laser (355 nm, 135 mJ/4–6 ns), pumping an optical parametric oscillator (OPO, 30 mJ/3–5 ns) with a 420 to 2300 nm (NT-342B, Ekspla) wavelength range. Triplet lifetimes were determined by the exponential fitting of the kinetic curves using the ORIGIN 6 Professional software. The absorbance value used for triplet state studies was fixed at 1.5 and the solution degassed by bubbling argon for 30 min prior to measurements. All Z-scan analysis were performed using frequency doubled Nd:YAG laser (Quanta-Ray, 1.5 J/10 ns fwhm pulse duration) as the excitation source. The laser was operated in a near Gaussian transverse mode at 532 nm (second harmonic) with low repetition rate of the lasers to prevent cumulative thermal nonlinearities, details have been provided before [37]. All z-scan profiles were determined at the same absorbance of 1.

Fig. 1. Structure of the Pcs used in this work.

2.3. Formation of DNDs-MPc nanoconjugate

expected to show enhanced NLO effects through synergetic effect. Constructed DNDs nanoconjugates will be further optimised by investigating the effect of central metal using unmetallated and metallated phthalocyanines namely: 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyaninato (H2TPPc), 2,9(10),16(17),23(24)-tetrakis(4-pyridyloxy) phthalocyanato zinc(II) (ZnTPPc) and 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyanato silicon (IV) hydroxide (Si(OH)2TPPc), Fig. 1. The pyridyloxy substituent was chosen due to its bulkiness which should prevent aggregation.

Respective phthalocyanines were adsorbed onto the DNDs surface through non-covalent (π − π stacking, Scheme 1) and covalent (ester bond, Scheme 2) interaction, the latter according to previously reported method for amide bonding with slight modification [31]. Briefly, for non-covalent interactions, ZnTPPc (27 mg, 0.05 mmol) and H2TPPc (24 mg, 0.03 mmol) were each separately mixed with DNDs (27 mg) in 5 mL of DMSO and sonicated for 4 h, followed by overnight stirring for 48 h. For covalent interaction, DNDs (20 mg, 0.047 mmol) were dissolved in DMSO (3 mL), followed by addition of DCC (0.02 g, 0.098 mmol) to activate the carboxylic acid moiety. The reaction mixture was stirred for 48 h and followed by addition of Si(OH)2TPPc (25 mg, 0.03 mmol) and NHS (0.015 g, 0.13 mmol) and the reaction mixture was further stirred for 48 h. It is important to note that noncovalent interaction are also possible when covalently linking Si (OH)2TPPc and DNDs. Thereafter, the mixtures (both non-covalent and covalent) were centrifuged at 3500 rpm for 5 min in ethanol to precipitate the DNDs-Pc conjugates out of the solution and remove unreacted Pc derivatives or DNDs. The resulting DNDs-H2TPPc, DNDsZnTPPc and DNDs-Si(OH)2TPPc nanoconjugates were left to dry in a fume hood for 72 h.

2. Experimental 2.1. Material Detonation nanodiamonds (DNDs) were obtained from Nanocarbon Research Institute Ltd., deuterated dimethyl sulfoxide (DMSO), ethanol, N,N′-dicyclohexylcarbodiimide (DCC), and N-hydroxysuccinimide (NHS) were from Sigma Aldrich. All other reagents and solvents were used as received from commercial suppliers. The syntheses of H2TPPc, ZnTPPc and Si(OH)2TPPcs were as reported in literature [33,34]. 2.2. Equipment

2.4. Photophysical parameters Ground state electronic absorption spectra were recorded with Shimadzu UV-2550 spectrophotometer. Varian Eclipse spectrofluorimeter with 360–1100 nm filter was employed for recording excitation and emission spectra. Fluorescence lifetimes were measured using a time correlated single photon counting (TCSPC) setup (Fluo Time 300, Picoquant GmbH), details have been provided before [35]. X-ray diffraction (XRD) analysis was performed on a Bruker D8 Discover diffractometer, equipped with a Lynx Eye detector, under Cu K-α radiation (λ = 1.5405 Å). X-ray photoelectron spectroscopy (XPS) analysis were conducted with an AXIS Ultra DLD (supplied by Kratos Analytical) using Al (monochromatic) anode equipped with a charge neutralizer, details have been provided before [36]. Raman spectra were recorded with Bruker vertex 70-Ram II Raman spectrometer (1064 nm Nd:YAG laser and liquid nitrogen cooled germanium detector). The morphology of the nanoconjugates was determined using Zeiss Libra 120 model transmission electron microscope (TEM) at 80 kV. Particle size distribution (dynamic light scattering, DLS) of the

Fluorescence (ΦF) and triplet (ΦT) quantum yields of the Pcs and their conjugates were determined in DMSO and using comparative methods described before [38–41]. Unsubstituted ZnPc in DMSO was used as a standard with ΦF=0.20 [39] and ΦT=0.65 [40]. Triplet lifetimes were determined by exponential fitting of the kinetic curves using Origin Pro 8 software. 3. Results and discussion 3.1. Synthesis and characterization DNDs-H2TPPc, DNDs-Si(OH)2TPPc and DNDs-ZnTPPc were synthesised through non-covalent interaction (π-π stacking, Scheme 1) between Pcs and DNDs. Detonation nanodiamonds have been reported to contain carboxyl and hydroxyl moieties on the surface [29]. Thus, additionally for Si(OH)2TPPc conjugate the COOH groups on DNDs were 219

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Scheme 1. Non-covalent interaction of different central metals Pcs to DNDs resulting in DNDs-Pc nanoconjugate systems.

Si(OH)2TPPc. Fig. 2B(c) shows the FTIR spectrum upon covalent linkage of Si(OH)2TPPc to DNDs. The DNDs alone showed high intensity OH signal at 3386 cm−1, a small C]O stretching at ~1700 cm−1 as a shoulder to the adsorbed water OH bending at 1636 cm−1 [43]. The CeH peak is observed at 610 cm−1. The OH and C]N signals of the Si(OH)2TPPc were observed at 3007 and 1510 cm−1, respectively. A drastic reduction intensity and shift to lower wavenumbers for DNDs peaks at 3386 cm−1 to 3324 cm−1 was observed upon covalent linkage of Si(OH)2TPPc to DNDs, suggesting conversion of most OH group in COOH in DNDs to ester bond hence decreasing the OH signal. The splitting of the C]O peak around 1600 cm−1 in the conjugate can be associated with the presence of RCOOR′ stretch arising from the linkage of the Si(OH)2TPPc to the DNDs, indicating covalent bonding from hydroxyl group of the Pc to the carboxylic moiety of the DNDs. Moreover, shifts in the C]N peak positions from 1510 cm−1 in the Pc to 1438 cm−1 in the conjugate (DNDsSi(OH)2TPPc) (Fig. 2B(c)) also confirms the functionalisation of DNDs by Si(OH)2TPPc. The XPS wide scan spectra (Fig. 3A(a)–(c)) showed all the expected elements for DNDs, Si(OH)2TPPc and DNDs-Si(OH)2TPPc at respective binding energies with C1s (285 eV), N 1s (398 eV) and O 1s (530 eV). There are additional Si2p peaks for Si(OH)2TPPc (Fig. 3A(b)) and the conjugate (Fig. 3A(c) due to the presence of the central Si atom in the phthalocyanine. Table 2 shows that DNDs consist mainly of C (95.46%)

used for covalent linking to Si(OH)2TPPc (ester linkage, Scheme 2) using DCC and NHS as activating and coupling agent, respectively. Loading of the Pcs onto the DNDs was investigated following previous studies using absorption instead of fluorescence [42]. This involves comparing the Q band absorbance intensities of the conjugate (DNDs-Pc) with that of the respective Pc before the conjugation. Equal masses (mg) for Pc and DNDs-Pc conjugates where separately weighed and dissolved in the same volume of the solvent. The mass loadings of Pcs onto each DNDs particle were calculated to be: 470, 500, 800 μg (Pc)/mg DNDs for DNDs-H2TPPc, DNDs-ZnTPPc and DNDs-Si (OH)2TPPc, respectively, Table 1. Thus there is higher loading for DNDs-Si(OH)2TPPc most likely due to the presence of both π-π interactions and the ester bond.

3.1.1. FTIR spectra and X-ray photoelectron spectroscopy (XPS) The FTIR spectra following π-π stacking of Pcs as well as covalent linkage of Si(OH)2TPPc to DNDs are shown in Fig. 2. For π-π stacking, a slight shift in C]N peak from 1670 cm−1 (for Pc alone) to 1656 cm−1 (for the conjugate, using ZnTPPc and its conjugate as examples in Fig. 2A) upon interaction with DNDs and peak broadening around 3200 cm−1 associated with OH stretching in the DNDs. For Si (OH)2TPPc (Fig. 2B) both π-π and ester bond formation are possible. The ester bond occurred through esterification reaction between the carboxylic moiety on the DNDs and the hydroxyl moiety from ligands of 220

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Scheme 2. Ester covalent linkage between Si(OH)2TPPc and DNDs resulting in DNDs-Pc nanoconjugate systems.

Table 1 Parameters for the Pcs and their respective nanoconjugates with DNDs in DMSO (unless otherwise stated). Sample

λ (nm)a

DNDs H2TPPc DNDs-H2TPPc ZnTPPc DNDs-ZnTPPc Si(OH)2TPPc DNDs-Si(OH)2TPPc

– 675 675 675 673 684 666 (610)

a b

DLS size (nm)

ξ (mV)b

Pcs loaded Pc (μg)/DNDs (mg)

Raman spectra ID/IG ratio

2.9 – 7.5 – 28.2 – 32.5

8.80 1.16 2.02 1.28 1.76 1.85 2.41

– – 470 – 500 – 800

0.01 – 0.67 – 0.17 – 0.18

The peak due to the aggregate in brackets. ξ values were determined in hexanol. 221

Фf

b

– 0.57 0.08 0.17 0.08 0.19 0.02

τF (ns)b – 3.07 2.58 2.70 2.69 2.77 2.69

ФT

τT (μs)

0.20 0.39 0.33 0.49 0.70 0.42

119.9 98.9 176.4 139.0 198.7 404.5

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Fig. 2. FTIR spectra of (A) non-covalent interaction: (a) ZnTPPc and (b) DNDs–ZnTPPc and (B) covalent linkage: (a) Si(OH)2TPPc alone, (b) DNDs and (c) DNDs-Si(OH)2TPPc.

with some O (3.04%) and N (1.46%). The %O and %N increase in the presence of the Pc since the Pc has both these elements. High resolution spectra obtained for the C1s component is shown in Fig. 3B((a)–(c)) for DNDs, Si(OH)2TPPc and respective conjugate. Upon the deconvolution of the C1s spectra, Fig. 3B(a), the DNDs reveals the presence of four components at 282.8, 283.6, 285.8 and 286.9 eV assigned as indicated in Fig. 3B(a). Pc alone (Fig. 3B(b)) also has four components. The C1s spectrum of the DNDs-Si(OH)2TPPc nanohybrid was deconvoluted into five peaks (282.7, 283.7, 285.1, 286.4 and 288.2 eV), Fig. 3B (c). The new peak at 288.2 eV is assigned to COOR, due to the ester bond between the hydroxyl moiety of Si(OH)2TPPc and carboxyl moiety in DNDs. Hence, XPS proves the formation of the ester bond. 3.1.2. TEM images, DLS size and zeta potential Fig. 4(a–d) shows TEM images of DNDs and their conjugates with Pcs. For DNDs alone, monodispersity is observed in Fig. 4(a), however aggregation (Fig. 4(b)–(d) occurs upon conjugation of Pc, making accurate size determination difficult. Hence, DLS was employed to determine size distribution of respective samples. For the DNDs alone (Fig. 5(a)), the average size of 2.9 nm was obtained. It has been reported that NanoCarbon Research Institute, Japan (where we obtained the DND) produces 3 nm-sized positively charged colloidal species [44], this is confirmed with DLS in this work. In addition, DNDs of size ranges 1–12 nm have been reported previously using high-resolution transmission electron microscopy (HRTEM) and electronic structure modelling based on density-functional tight-binding (DFTB) theory and ultracentrifugation experimental isolation [45,46]. The size increases to 7.5, 24.4 and 50.8 nm (Fig. 5(b–d)) upon conjugation to the respective Pcs, Table 1. The increase in size could be due to aggregation. Aggregation is probably due to interactions between the Pcs on adjacent NPs via π-π stacking since Pcs are known for their π-π stacking [47]. The larger size of the DNDs-Si(OH)2TPPc conjugate could be due to the larger loading of the Pcs on the NPs. Zeta potential (ζ) is an indicator of the stability of nanoparticle (NP) suspensions, values between −5 to +5 mV indicate fast aggregation [48]. In this study, ζ values of conjugates are much lower than those of DNDs alone, indicating aggregation as shown by TEM images in Fig. 4. Previous reviews on zeta potential of nano-drug delivery systems have shown that despite the low ζ values, the suspensions are stable [48,49]. DNDs showed positive surface charge irrespective of the predominant carboxylic acid groups on the ND surface shown in FTIR. Similar observations attribute this to carbon basicity (due to oxygen defect) [50]. As stated above, it has been reported that NanoCarbon Research Institute produces positively charged colloidal species [44]. 3.1.3. EDX spectra Qualitative verification of the elemental composition of the DNDsPc conjugate was investigated using EDX as shown in Fig. 6(a–d). The EDX spectra of DNDs alone showed the presence of C and O peaks as expected (Fig. 6(a)). The N peak is not observed for DNDs alone since C and N are next to each other in the periodic table, and presence of major carbon composition in these samples overlaps with the nitrogen peak, which could be found in small percentages when compared to carbon. However XPS has proved the presence of N as discussed above. DNDs conjugated to H2TPPc only showed C and O indicative of unmetallated Pc (Fig. 6(b)). However, nanoconjugates of DNDs-ZnTPPc and DNDs-Si (OH)2TPPc showed the presence of Zn, Si, C and O peaks (Fig. 6(c, d)). 3.1.4. Raman spectra In this study, laser Raman spectral technique was used to determine the quality of as-synthesised DNDs and their respective nanoconjugates. 222

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Fig. 3. (A): Wide scans of (a) DNDs, (b) Si(OH)2TPPc (c) DNDs-Si(OH)2TPPc conjugate, (B): High resolution C 1 s spectra of (a) DNDs, (b) Si(OH)2TPPc, and (c) DNDs-Si(OH)2TPPc.

The ID: IG (sp3:sp2) ratio is generally considered as a quality parameter to determine the extent of functionalization of the carbon nanomaterials. This is because the G-band is not affected by defects, whereas the D-band is enhanced by the presence of sp3 defects in the sp2 lattice. Conjugation of Pcs to DNDs resulted in increased DNDs defects as judged by increase in ID:IG ratio for conjugated DNDs-Pcs compared to DNDs alone, Table 1. The low ID: IG for pristine DNDs confirms the predominance of sp2 carbons.

Table 2 Atomic composition for DNDs and Si(OH)2TPPc alone and DNDs-Si(OH)2TPPc. Sample

Atomic concentration (%) C

DNDs Si(OH)2TPPc DNDs-Si(OH)2TPPc

O 95.46 61.72 73.88

3.04 36.5 24.04

N 1.46 1.77 14.09

3.1.5. XRD The X-ray diffraction (XRD) patterns for DNDs–H2TPPc, DNDs–ZnTPPc and DNDs-Si(OH)2TPPc nanoconjugates when compared to Pcs and DNDs alone are shown in Fig. 8. The XRD patterns of the Pc at 2θ = (15–30)° is typical of H2Pc alone with typical lattice indices 400 and 600 [54]. Metallated Pcs (ZnTPPc and Si(OH)2TPPc) and respective conjugates (DNDs–ZnTPPc and DNDs–Si(OH)2TPPc) show suppression of the XRD peaks between 2θ = 21° and 30° due to the interference of the phthalocyanine ring and metalation of the centre. Electron density due to central metal substitution in phthalocyanine has been reported to modify tilt angle which alters the intensities in XRD phases [54]. For the DNDs alone the peaks at 45, 73 and 92° are due to 111, 221 and 311 plane of DNDs (NIST number A51588) [55], Fig. 8(a), and they showed no significant shift following linking.

The D and G bands were observed at 1369 and 1592 cm−1, respectively for DNDs alone (Fig. 7(a)). These characteristic Raman bands result from the E2g tangential vibrational mode of the sp2 bonded carbon (G band), and the disordered A1g breathing vibrational mode of the aromatic sp3 carbon rings (D band), respectively. The D bands of the DNDsH2TPPc and DNDs-Si(OH)2TPPc shifted to lower wavenumbers compared to the DNDs alone. The G bands of all the nanoconjugates when compared to DNDs alone, showed little or no change (Fig. 7(a–d)), except for DNDs-H2TPPc, where there was a shift to higher wavenumbers. Shifts in the Raman frequencies are often indicative of strong π-electron interactions in hybrid materials [51]. Raman spectra shown in Fig. 7 were obtained by excitation at 1064 nm. Similar Raman spectra on vacuum annealed DNDs when excited at 514 nm have been previously reported [52]. However, there were discrepancies in the Raman spectra on DNDs by Korepanova et al. when excited at 200, 355 and 532 nm [53]. Raman spectroscopy of DND is related to the laser irradiation wavelength used as an excitation source [52,53]. Hence the variation in the Raman spectra.

3.2. UV–vis absorption and emission spectra Fig. 9 shows UV–vis absorption spectra of DNDs, Pcs and their conjugates, separately dispersed in DMSO. Broad absorption feature

Fig. 4. TEM micrographs: (a) DNDs, (b) DNDs-H2TPPc, (c) DNDs-ZnTPPc, and (d) DNDs-Si(OH)2TPPc. 224

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Fig. 5. DLS plots for (a) DNDs, (b) DNDs-H2TPPc, (c) DNDs-ZnTPPc, and (d) DNDs-Si(OH)2TPPc.

Fig. 6. EDX spectra for (a) DNDs, (b) DNDs-H2TPPc, (c) DNDs-ZnTPPc, and (d) DNDs-Si(OH)2TPPc. 225

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G (d) 1592

Intensity (a.u.)

D 1281 (c) 1592 1369

(b)

1353

1611

(a) 1592

1369 1700

1600

1500

1400

1300

1200

-1

Raman Shift (cm )

Fig. 8. XRD spectra (a) DNDs alone, (b) H2TPPc alone, (c) DNDs–H2TPPc, (d) ZnTPPc alone, (e) DNDs–ZnTPPc, (f) Si(OH)2TPPc alone and (g) DNDs–Si (OH)2TPPc.

Fig. 7. Raman spectra for (a) DNDs alone, (b) DNDs-H2TPPc, (c) DNDs-ZnTPPc, and (d) DNDs-Si(OH)2TPPc.

in DNDs–Si(OH)2TPPc which could have resulted in more association between the Pcs hence resulting in aggregation. Aggregation was confirmed for DNDs–Si(OH)2TPPc since there was a decrease in the peak due to the aggregate (high energy peak) as the concentration was decreased, Fig. S1 (Supporting information) and a slight increase in the peak due to the monomer (low energy peak). For all the Pcs alone, lack of aggregation was confirmed with Beer-Lambert law being obeyed in the concentration range shown, Fig. S2 (Supporting information, using H2TPPc as an example).

was observed for DNDs in Fig. 9. Similar broad absorption has been previously reported [55]. These broad absorption peaks result from π–π* transition of aromatic sp2 domains in the material. The ground state optical absorptions in Pcs are dominated by two major absorption bands in the visible or near infrared (IR) (670–1000 nm) and the UV (325–370 nm) regions of the spectrum corresponding to the Q and the B bands, respectively [47]. Absorption spectra for Pcs alone in Fig. 9 showed narrow monomeric Q band at 675, 675 and 684 nm, for H2TPPc, ZnTPPc and Si(OH)2TPPc, respectively, Table 1, in DMSO. The Q band for H2TPPc does not show the normal splitting of the Q band that is typical of free-based phthalocyanines, showing instead a single sharp Q band. The lack of splitting of the Q band in H2Pcs is a result of the basicity of the solvents. It has been documented that in basic solvents such as DMSO, the inner pyrrole hydrogens are acidic enough to dissociate and the Pc becomes symmetrical and thus possesses an unsplit Q band [47]. There were no significant shifts in the Q band for H2TPPc and ZnTPPc following conjugation to DNDs. Aggregation for DNDs–Si(OH)2TPPc nanoconjugated system was observed, Fig. 9. Cofacial stacking mode of Pcs (the so-called H stacks) give rise to a blue shift of the Q-band (due to the aggregate) relative to the Q-band of the monomer (low energy peak). The other stacking mode (very rate in phthalocyanines) in which the molecules are offset relative to each other leads to slipped stacks (J-stacks) gives rise to a red shifted Q-band [47]. A blue shifted peak is observed in Fig. 9 for DNDs–Si(OH)2TPPc, suggesting H aggregation. Aggregation is due to interactions between the Pcs on adjacent NPs via π-π stacking cofacially. The aggregation observed for DNDs–Si(OH)2TPPc could be a result of the larger loading, Table 1. Most likely also is the presence of both the ester and π-π bonds

3.3. Photophysical and photochemical parameters 3.3.1. Fluorescence quantum yields (ФF) and lifetimes (ττ) Of the Pcs alone, H2TPPc, has the largest ΦF value of 0.57, while ZnTPPc and Si(OH)2TPPc have values of 0.17 and 0.19, respectively. The high value for H2TPPc is due to the lack of a central metal. Heavy metals such as Zn enhance the intersystem crossing to the triplet state, hence lowering fluorescence [56]. There is drastic decrease in ΦF values for the Pcs in the presence of DNDs. This could be because of the electron donating ability of DNDs, since electron donating groups are known to increase intersystem crossing in porphyrins [57], reducing fluorescence. The decrease in fluorescence peak intensity for the conjugates compared to Pcs alone confirms quenching, Fig. S3 (Supporting information), though very weak peaks for DNDs–Si(OH)2TPPc and Si(OH)2TPPc. As stated above DNDs–Si(OH)2TPPc is highly aggregated and has the lowest ΦF value. Aggregates are known to convert electronic excitation energy to vibrational energy, resulting in decrease in fluorescence quantum yield of 226

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followed the same trend as the ΦF values, this is expected since the two are related. 3.3.2. Triplet quantum yields (ΦT) and lifetimes (τT) Of the Pcs alone, H2TPPc gave the lowest ΦT value of 0.20 corresponding to the high ΦF values since the two are competing processes. The higher ΦT values for Si(OH)2TPPc and ZnTPPc also correspond to their lower ΦF values. The highest value of ΦT is obtained for Si (OH)2TPPc at 0.70. For H2TPPc and ZnTPPc, there is an increase in ΦT values in the presence of DNDs, corresponding to the decrease in ΦF values. There is a lowering of the ΦT value for DNDs–Si(OH)2TPPc compared to Si (OH)2TPPc alone, this could be due to aggregation as shown by the UV–vis spectra. DNDs-H2TPPc and DNDs-ZnTPPc showed shorter triplet lifetimes (at 98.9 and 139.0 μs, respectively) compared to the corresponding Pcs alone (at 119.9 and 176.4, respectively). Thus, there is a shortening of triplet lifetimes of Pcs in the presence of DNDs corresponding to increase in triplet yields [56]. Comparing DNDs–Si (OH)2TPPc to Si(OH)2TPPc, an increase in triplet lifetime for the former corresponding to the decrease in triplet quantum yield. 3.4. Nonlinear optical (NLO) studies Nonlinear absorption behaviour of respective DNDs-nanoconjugates was investigated using an open aperture Z-scan technique with an excitation pulse of 10 ns at input energy of 30 μJ at 532 nm. Linear absorption coefficient (α0) for the Pc molecules are presented in Table 3. The α0 values of investigated Pcs and respective conjugates in Table 3 show remarkable differences at the same absorbance in DMSO (Fig. 9). Pcs with various central metals and Pc mass loadings could be responsible for the observed differences. α0 values have been previously reported to be sensitive to structural modification and Pc interactions [59]. The average linear transmittances of H2TPPc, ZnTPPc, Si (OH)2TPPc and respective conjugates of H2TPPc and ZnTPPc is ≈98% (Fig. S4, Supporting information). However, conjugate of DNDs-Si (OH)2TPPc showed less transmittance of ≈94%. Z-scan profiles of Pcs alone and DNDs-H2TPPc, DNDs-ZnTPPc and DNDs-Si(OH)2TPPc nanoconjugate systems, Fig. 11 show typical RSA characteristics and the measurements showed nonlinear absorption (NLA) behaviours. A higher reduction in transmittance shown by an enhanced dip in RSA profile was observed when H2TPPc and ZnTPPc were in the presence of DNDs than when alone (only a very small increase for the unmetalated derivative, showing the importance of metalation), suggesting that the nanohybrids could be potential optical

Fig. 9. UV–vis spectra of DNDs, H2TPPc, DNDs-H2TPPc, ZnTPPc, DNDs-ZnTPPc, Si(OH)2TPPc, and DNDs-Si(OH)2TPPc in DMSO. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

molecules [58]. Fluorescence lifetimes (τF) for Pcs and respective nanoconjugates were obtained from fitting the fluorescence decay data in Fig. 10(a) and (b). Pcs alone showed mono–exponential decay profiles indicative of one fluorescence lifetime. Bi-exponential decay profile were obtained when fitting DNDs-nanoconjugates indicating two lifetimes which could be due to the orientation of the Pc around the NPs. Average lifetimes are shown in Table 1. The fluorescence lifetime values

Fig. 10. Typical fluorescence lifetimes of (a) Si(OH)2TPPc and (b) DNDs-Si(OH)2TPPc. 227

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Table 3 Nonlinear optical properties of DNDs, H2, Zn and Si(OH)2 TPPcs and respective nanoconjugates with DNDs in DMSO. Sample

α0 (cm1)

Beff (cm GW−1)

Im[X3] (esu)

γ (esu)

k(

δexc ⎞ ⎟ δo

Ilim (Jcm−2)

⎠

DNDs H2TPPc DNDs-H2TPPc ZnTPPc DNDs-ZnTPPc Si(OH)2TPPc DNDs-Si(OH)2TPPc

11.52 13.10 17.95 10.43 31.7 2.59 1.73

22.0 41.0 58.5 42.8 60.9 136 125

−8

1.43 × 10 1.50 × 10−8 2.04 × 10−8 1.53 × 10−8 2.13 × 10−8 3.17 × 10−8 2.91 × 10−8

−28

1.27 × 10 1.33 × 10−28 1.89 × 10−28 1.82 × 10−28 2.62 × 10−28 4.22 × 10−28 3.88 × 10−28

– 22.3 55.6 35.4 87.0 342 306

– – – – – 0.39 0.47

limiting materials. The opposite was the case for DNDs–Si(OH)2TPPc compared to Si(OH)2TPPc, where the latter showed an enhanced dip in transmittance. The poor performance of DNDs–Si(OH)2TPPc could be due to aggregation discussed above. DNDs alone showed a smaller dip at 82% (Fig. S5: Supporting information), compared to say DNDsZnTPPc at 53%. This suggests that DNDs alone are not as good NLO material as to when combined with Pcs. It has been observed under the laser irradiation with low energy, saturable absorption (SA) occurs for GQDs [60] due to ground state bleaching. Fig. S5 shows that the Z-scan profiles for DNDs alone at different energies exhibited SA to RSA profiles, which became more defined with decrease in energy. However the SA to RSA behaviour persisted. Nonlinear scattering and the nonlinear absorption are the dominant mechanisms of optical limiting DNDs, with the former being observed for larger nanoclusters [61,62]. Table 3 shows the effective nonlinear absorption coefficient values, Beff, obtained for each sample by fitting the experimental data to the transmittance equations reported in previous studies [63], and provided in Supporting information (Eqs. (S1), (S2) and (S3)). The Beff values increased from 41.0 and 42.8 cm/GW in H2TPPc and ZnTPPc to 58. 5 and 60.9 cm/GW for the corresponding nanohybrid conjugates, respectively. Though both Si(OH)2TPPc and its conjugate showed RSA behaviour, the Pc alone display enhancement in RSA as well as Beff compared to the conjugate, which could be attributed to effect of aggregation. Aggregation has been known to reduce the excited state lifetime and hence the effective nonlinear optical absorption coefficient [64]. The larger Beff values of DNDs-ZnTPPc and DNDs-Si(OH)2TPPc when compared to DNDs-H2TPPc correspond to the larger triplet quantum yields of the former. Similar trends were observed in Beff values of Pcs alone. The most logical mechanism which explains observed RSA observed in phthalocyanines and conjugates when using the nanosecond laser is based on a five-level model, Fig. 12. Irradiation at 532 nm excites molecules from S0 to either S1 or S2 (the latter with excited state absorption cross-section δ1). The S2 state is short-lived, hence relaxation to S1 almost occurs immediately. Since the triplet lifetime of the Pcs and conjugates are much longer than the intersystem crossing lifetime [31], there will be more transfer of molecules from S1 to the T1. Subsequent absorption of laser radiation will result in further excitation of molecules in T1 to T2 with an excited absorption cross section of δ2. Similar mechanism has been previously reported in carbon based nanomaterials conjugated to phthalocyanines [30,37]. The DNDs absorb at 532 nm, their contribution towards RSA is in the improvement of triplet quantum yields which in turn improves NLO behaviour [65]. The contribution of DNDs could also be due to nonlinear scattering as stated above [61,62]. DNDs are also expected to contribute to NLO behaviour of Pcs due to the free-carrier absorption (FCA) mechanism [66,67]. FCA is usually produced when excitation takes place at the wavelengths where there is linear absorption. RSA from nanosecond laser pulses has been previously reported to be due to one-photon, two-photon, three-photon absorption (1PA, 2PA, 3PA) mechanisms, a combination of both or all three [68]. It is known that for organic materials, the contribution due to 3PA to the NLO response typically occurs at longer wavelength (using a laser tuned at

Fig. 11. Open aperture Z-scan signatures of (A) H2TPPc, (B) ZnTPPc and (C) Si (OH)2TPPc when alone and in the presence of DNDs together with respective fittings in DMSO.

228

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Fig. 12. Five level energy diagram explaining the dynamics of the excited state population (upward arrows), non-radiative relaxation (dashed arrows) in the studied complexes.

(determined using Eq. (S7), Supporting information). The values of Im [X3] and γ for DNDs-H2TPPc and DNDs-ZnTPPc in Table 3 showed improvement when compared to Pcs alone. For DNDs-Si(OH)2TPPc, there is a decrease in Im[X3] and γ compared to the Pc alone due to aggregation. However, at low laser energy of 24 μJ shown in Supplementary information (Fig. S6), Si(OH)2TPPc and DNDs-Si(OH)2TPPc further reveals higher difference of the NLO response between the DND conjugate and the Si(OH)2TPPc. Previous studies report on a complex dependence on simultaneous action of electronic and secondary vibrational nonlinearities at high intensities [72]. Of note however is that at 30 μJ DNDs-Si(OH)2TPPc and Si(OH)2TPPc have the highest Beff, Im [X3] and γ in Table 3 compared to the rest of the corresponding conjugates and Pcs despite the aggregated nature of DNDs-Si(OH)2TPPc. Coordination of an axial ligand to central metal has been previously reported to enhance NLO properties by introducing a dipole moment perpendicularly oriented with respect of the Pc ring plane, which alters the electronic structure of the macrocycle, and introduces new steric effects modifying the packing properties of Pcs and respective conjugate [73]. The γ in this work lie in the range 10−28 esu which are much higher than the reported 10−29 esu reported for ZnPc derivative on pristine GQDs [30], Table 4, again showing the superiority of DNDs in improving the NLO behaviour of Pcs. The γ values in this work are in the range of those obtained for doped GQDs [30], Table 4 [20,30,74]. A good optical limiting (OL) material displays reduced transmittance with increasing incident fluence. This type of device has a linear transmittance at low incident fluence, but abruptly changes at higher incident fluence or there is a threshold at which the output fluence becomes a constant value that should be less than the amount required to damage the optical element [75]. This critical point is called the threshold limit intensity or fluence, (Ilim) [76] which is a very important parameter in optical limiting measurements. The Ilim value may be defined as the input fluence at which the transmittance is 50% of the linear transmittance. While there is currently no defined optimal range for Ilim values, it is generally accepted that good nonlinear optical material performs better at a low value of Ilim, as this means that the limiting would occur at a lower intensity, allowing for more cautious protection of sensors. The values of Ilim can be experimentally determined using the plots of transmittance against input fluence (Fig. 13(A–C). The Ilim values for H2TPPc, ZnTPPc and their respective nanoconjugates could not be determined since the transmittance did not drop below 50% of the linear transmittance (Fig. 13A and B). DNDs-

Table 4 Comparison of synthesised Pc nanoconjugate systems with the best reported Pccarbon composites. Samplea

Ilim (Jcm−2)

γ (esu)

Reference

DNDs-H2TPPc DNDs-ZnTPPc DNDs-Si(OH)2TPPc ZnPc-GQDs ZnPc-NGQDs ZnPc-SNGQDs CuPc-C60 InPcCl-SWCNT

– – 0.47 0.61 0.58 0.55 > 200 0.21

1.89 × 10−28 2.62 × 10−28 3.88 × 10−28 7.01 × 10−29 3.34 × 10−28 5.03 × 10−28

This work This work This work [30] [30] [30] [74] [20]

5.78 × 10−27

a GQDs = graphene quantum dots, SNGQDs = sulfur/nitrogen doped GQDs, SWCNT = single walled carbon nanotubes.

wavelengths much > 530 nm) [67]. The fitting equation used for a 3PA transmittance data, suggests negligible contribution [67]. In our work a laser of 532 nm laser was used, hence, the contribution of 3PA is expected to be negligible. Merit coefficient (k) in Table 3 indicates quantitative evaluation of the ratio of the excited and ground state absorption cross-sections [68,69]. The absorption contribution resulting from the excited state was evaluated using Eqs. (S4) and (S5) (provided in Supplementary information). k values (Table 3) for the Pcs and conjugates indicate the existence of an excited state (δexc) with higher absorption cross-section than the ground state. For H2TPPc and ZnTPPc, the conjugates have larger k values than the Pcs alone, the opposite is true for Si(OH)2TPPc as observed for Beff values above. The third-order susceptibility (Im[X3]) measures the speed of the response of an optical material to the perturbation initiated by an intense laser beam [70] and can be determined using Eq. (S6) in Supporting information. The Im[X3] values for DNDs nanoconjugated systems of 2.04 × 10−8, 2.13 × 10−8 and 2.91 × 10−8 esu for DNDsH2TPPc, DNDs-ZnTPPc and DNDs-Si(OH)2TPPc, respectively, Table 3, correspond with previous study [71]. DNDs on their own perform the worst, Table 3. When exposed to light, the permanent dipole of the molecule interacts with light to cause a bias in the average orientation of the molecule, resulting in induced hyperpolarizability (γ). Nonlinear optical properties of a material are directly dependent on the γ value 229

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linked to C60 and resulted in Ilim > 200 J/cm2 [74] (Table 4). This Ilim value is much larger than reported in this work showing the importance of DNDs when compared to C60. Physical and photophysical properties of a material system, such as: absorption band, particle size, and aggregation state have been reported to strongly influence optical limiting performance [26]. 4. Conclusion We have synthesised DNDs-nanoconjugates containing H2TPPc, ZnTPPc and Si(OH)2TPPc phthalocyanines. The presence of DNDs in constructed nanoconjugated resulted in enhanced triplet quantum yields for H2TPPc and ZnTPPc of 0.39 and 0.49, respectively. Low triplet quantum yields of 0.42 was observed for DNDs-Si(OH)2TPPc nanohybrid when compared to its respective Si(OH)2TPPc (0.70). Z-scan data were fitted taking into account both the nonlinear and excited state absorption processes. All the DNDs-nanoconjugated systems and the corresponding Pcs followed reverse saturable absorption through two photon absorption mechanism at the excitation wavelength of 532 nm. Electron donating properties of DNDs and presence of a central metal with axial ligand for DNDs-Si(OH)2TPPc and corresponding Pc showed improvement NLO properties when compared to conjugates of DNDs-H2TPPc and DNDs-ZnTPPc and respective Pcs. Limiting threshold (Ilim) values for DNDs-H2TPPc and DNDs-ZnTPPc were undeterminable because the input fluence at which the transmittance is 50% of the linear transmittance was < 50%. Limiting threshold for DNDs-Si (OH)2TPPc was shown to be 0.47 and 0.39 for Si(OH)2TPPc. Acknowledgements This work was supported by the Department of Science and Technology, Republic of South Africa (DST) Innovation and National Research Foundation (NRF), South Africa through DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology (UID 62620) as well as Rhodes University. We thank Profs Kimura, Kobayashi and E. Osawa from Shinshu University in Japan for the DNDs. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.diamond.2019.03.013. References [1] O. Shenderova, A. Vargas, S. Turner, D. Ivanov, M. Ivanov, Nanodiamond-based anolubricants: investigation of friction surfaces, Tribol. Trans. 57 (2014) 1051–1057. [2] V.N. Mochalin, Y. Gogotsi, Nanodiamond-polymer composites, Diamond Related Mater. 58 (2015) 161–171. [3] V. Vaijayanthimala, D.K. Lee, S.V. Kim, A. Yen, N. Tsai, D. Ho, H. Chang, O. Shenderova, Nanodiamond-mediated drug delivery and imaging: challenges and opportunities, Exp. Opin. Drug Deliv. 12 (2015) 735–749. [4] E.K.H. Chow, D. Ho, Sci. Cancer nanomedicine: from drug delivery to imaging Transl. Med. 5 (2013) 216RV4 (12 pages). [5] O.A. Shenderoava, V. Grichko, Nanodiamond U.V. Protectant Formulations, U.S. Patent 8753614, UV Protective Coatings; US Patent 9296656 (2005) 2005. [6] A.S. Barnard, Stability of Nanodiamond, Ultrananocrystalline Diamond, Elsevier, UK, (2006) pp. 117–154. [7] V.N. Mochalin, O. Shenderova, D. Ho, Yury Gogotsi, The properties and applications of nanodiamonds, Nat. Nanotechnol. 7 (2011) 1–13. [8] A. Chaudhary, J.O. Welch, R.B. Jackman, Electrical properties of monodispersed detonation nanodiamonds, Appl. Phys. Lett. 96 (2010) 242903–1–242903-3. [9] M.W. Doherty, N.B. Manson, P. Delaney, F. Jelezko, J. Wrachtrup, L.C. Hollenberg, The nitrogen-vacancy colour centre in diamond, Phys. Rep. 528 (2013) 1–45. [10] V.Y. Dolmatov, Detonation nanodiamonds: synthesis, structure, properties and applications, Usp. Khim. 76 (2007) 376–397. [11] A.M. Schrand, S.A.C. Hens, O.A. Shenderova, Nanodiamond particles: properties and perspectives for bioapplications, Crit. Rev. Solid State Mater. Sci. 34 (2009) 18–74. [12] S. Josset, O. Muller, L. Schmidlin, V. Pichot, D. Spitzer, Nonlinear optical properties of detonation nanodiamond in the near infrared: effects of concentration and size

Fig. 13. Input fluence (I0) versus transmittance curves of (A) H2TPPc, DNDsH2TPPc; (B) ZnTPPc, DNDs-ZnTPPc and (C) Si(OH)2TPPc, DNDs-Si(OH)2TPPc nanoconjugate systems in DMSO solvent.

Si(OH)2TPPc in Fig. 13C showed a larger Ilim value of 0.47 when compared to Si(OH)2TPPc alone (0.39), Table 3. The lack of drop below 50% of the linear transmittance for H2TPPc, ZnTPPc and their respective nanoconjugates could be due to non-covalent interaction of two different molecules with phase separation problems associated with mixtures such as DNDs-H2TPPc and DNDs-ZnTPPc. However, covalently linked material function as combined molecules because the working dynamic range is extended through the covalent linkage between Pc and DNDs. Broadband optical limiting materials are obtained from a combination of individual molecules that possess optical limiting behaviour linked covalently [73]. Such material results in broadened optical limiting band, extended working range and extensively reduced limiting thresholds, that translate to better optical limiting properties. Similar broad band optical limiting materials with phase stability have been previously reported when CuPc was covalently 230

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