Substituent effect on the photophysical and nonlinear optical characteristics of Si phthalocyanine – Detonated nanodiamond conjugated systems in solution

Journal Pre-proofs Research paper Substituent effect on the photophysical and nonlinear optical characteristics of Si phthalocyanine - detonated nanodiamond conjugated systems in solution Refilwe Matshitse, Tebello Nyokong PII: DOI: Reference:

S0020-1693(19)31798-0 https://doi.org/10.1016/j.ica.2020.119447 ICA 119447

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Inorganica Chimica Acta

Received Date: Accepted Date:

20 November 2019 13 January 2020

Please cite this article as: R. Matshitse, T. Nyokong, Substituent effect on the photophysical and nonlinear optical characteristics of Si phthalocyanine - detonated nanodiamond conjugated systems in solution, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119447

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Substituent effect on the photophysical and nonlinear optical characteristics of Si phthalocyanine - detonated nanodiamond conjugated systems in solution

Refilwe Matshitse and Tebello Nyokong* Department Chemistry, Rhodes university, P. O Box, Grahamstown, South Africa Abstract This work reports on the photophysical, nonlinear absorption and optical limiting properties of detonation nanodiamonds (DNDs)-silicon phthalocyanine nanoconjugate systems. Si(IV) phthalocyanines employed are: 2,9(10),16(17),23(24)-tetrakis-(4-pyridyloxy) phthalocyaninato (Si(OH)2TPPc), 2,9(10),16(17),23(24)-tetrakis-(4-tert-butyl) phthalocyanato silicon(IV) hydroxide (Si(OH)2TBPc) and phthalocyanato silicon(IV) hydroxide (Si(OH)2Pc). Pcs were covalently linked to nanondiamonds and investigated using 532 nm laser excitation at 7 ns pulses for their optical limiting properties. Si(OH)2TBPc and Si(OH)2Pc gave larger triplet quantum yields when linked to DNDs, while the value decreased for Si(OH)2TPPc in the presence of DNDs due to aggregation. However all Pcs showed enhanced nonlinear optical properties in the presence of DNDs. DNDsSi(OH)2TPPc and DNDs-Si(OH)2TBPc gave the highest imaginary third-order susceptibility (Im[X(3)]) and hyperpolarizability (𝛾) at 5.19 × 10-8 and 3.7 × 10-8 esu and 2.66 × 10-27 and 1.97 × 10-27 esu, respectively. DNDs-Si(OH)2TBPc nanoconjugates showed lowest limiting threshold (Ilim) value of 0.01 J.cm-2 relative to 0.09 for DNDs-Si(OH)2TPPc. Key words: nanodiamonds, silicon phthalocyanine, photophysics, nonlinear absorption, optical limiting *Corresponding

author. Tel: + 27 46 6038260; Fax: + 27 46 6225109. E-mail: [email protected]. (T. Nyokong)

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1. Introduction Optical limiting materials have the ability to protect sensors such as eyes from laser damage. Optical limiting materials attenuate high intensity incident laser beams to low output intensity. Carbon-based materials, such as carbon black suspensions (CBS) [1], single-walled and multi-walled carbon nanotubes (CNTs) [2], fullerenes [3], graphene [4], and detonation nanodiamonds (DNDs) [5,6] have been used in nonlinear optics (NLO). These materials exhibit large nonlinearity, fast response time, and broadband spectral response [7]. Carbon based materials of interest in this study are DNDs. DNDs have tetrahedral network structures, and comprise of a diamond core (sp3), a middle core (sp2+x) and a graphitized outer layer (sp2) that is often partially oxidized [8]. The presence of sp2 hybridization in DNDs allows for - interactions with other  containing molecules such as metallophthalocyanines (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 [9,10]. These functional groups facilitate the linking of DNDs to other molecules such as MPcs, and in this work Si(OH)2Pc derivatives are covalently linked to carboxyl functional groups in DNDs through hydroxyl moieties on the axial ligands of the SiPc. Since aromatic rings are present in both DNDs and SiPcs, - interactions are also possible. MPcs are known NLO materials because of the extended  conjugation [11-13]. Thus since both MPcs and DNDs are NLO materials, their combination is expected to show enhanced NLO effects through synergetic effect. MPc complexes have been linked to carbon nanomaterials such as graphene quantum dots (GQDs) [14], fullerene [13], graphene oxide (GO) [15], carbon walled nanotubes (CNTs) [16], recently DNDs [17] with 2

improved NLO behavior in most cases. In the reported work [17] 2,9(10),16(17),23(24)tetrakis-(4-pyridyloxy) phthalocyanato silicon(IV) hydroxide (Si(OH)2TPPc) was highly aggregated in the presence of DNDs, which resulted in reduced NLO behaviour for the conjugate. In the current work we compare the NLO of Si(OH)2TPPc with that of other SiPc derivatives namely: 2,9(10),16(17),23(24) -tetrakis-(4-tert-butyl) phthalocyanato silicon(IV) hydroxide (Si(OH)2TBPc) and 2,9(10),16(17),23(24) phthalocyanato silicon(IV) hydroxide (Si(OH)2Pc), Fig. 1. We show that using dilute solution of Si(OH)2TPPc results in improved NLO compared to reported results [17]. Pyridyloxy and tert-butyl substituents used in this study have electron donating characteristics. The bulkier tert-butyl substituents may in addition assist in reducing aggregation. 2. Experimental 2.1. Material Detonation nanodiamonds (DNDs) were obtained from Nanocarbon Research Institute Ltd,

deuterated

dimethyl

sulfoxide

(DMSO-d6),

Si(OH)2Pc,

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. Si(OH)2TBPc is known [18] and can be purchased from Aldrich. The synthesis of Si(OH)2TPPc has been reported [19]. 2.2. Formation of DNDs-MPc nanoconjugate Respective phthalocyanines were covalently linked to the DNDs surface through ester bond, Scheme 1, following literature methods [17]. Briefly, for covalent interaction, DNDs (20 mg) were dissolved in DMSO (3 ml), followed by addition of DCC (0.02 g, 0.098

3

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), Si(OH)2TBPc (25 mg, 0.03 mol) or Si(OH)2Pc (25 mg, 0.04 mol). NHS (0.015 g, 0.13 mmol) was added to each mixture, followed by further stirring for 48 h. It is important to note that non-covalent interaction are also possible when covalently linking Pcs and DNDs. Thereafter, the mixtures 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-Si(OH)2TPPc, DNDs-Si(OH)2TBPc and DNDs-Si(OH)2Pc nanoconjugates were left to dry in a fume hood for 72 h. 2.3. Photophysical parameters Fluorescence (F) and triplet (T) quantum yields of the Pcs and their conjugates were determined in DMSO and using comparative methods described before [20,21]. Unsubstituted ZnPc in DMSO was used as a standard with F = 0.20 [21] and T = 0.65 [22]. 3. Results and Discussion 3.1. Synthesis and characterization DNDs-Si(OH)2TPPc, DNDs-Si(OH)2TBPc and DNDs-Si(OH)2TBPc were synthesized through covalent interaction (ester linkage, Scheme 1) between Pcs and DNDs. Detonation nanodiamonds have been reported to contain carboxyl and hydroxyl moieties on the surface [10,23,24]. Thus COOH groups on DNDs were used for covalent linking to Si(OH)2TPPc, Si(OH)2TBPc and Si(OH)2Pc using DCC and NHS as activating and coupling

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agent, respectively. The ester bond occurred through esterification reaction between the carboxylic moiety on the DNDs and the hydroxyl moiety from ligands of SiPcs. Mass loading ratios were calculated from TGA, using DNDs decomposition as a point of reference following a previously reported method [25]. The mass loadings of Pcs onto each DNDs particle were calculated to be: 289, 38 and 3 g (Pc)/mg DNDs for DNDsSi(OH)2TPPc, DNDs-Si(OH)2TBPc and DNDs-Si(OH)2Pc, respectively, Table 1. Thus, there is higher loading for DNDs-Si(OH)2TPPc most likely due to substituent effect (pyrydyloxy substituent) on the Pc, increasing - interactions in addition to the ester bond. 3.1.1. FTIR spectra The FTIR spectra of Pcs, DNDs and conjugates are shown in Fig. 2. A combination of both - and ester bond formation are possible between DNDs and respective Si(OH)2TPPc, Si(OH)2TBPc and Si(OH)2Pc [17]. The DNDs alone (Fig. 2(a)) showed high intensity COOH signal at 3386 cm-1, C=O and C-H 1636 and 610 cm-1, respectively [24]. The OH and C=N signals of the Si(OH)2TPPc were observed at 3007 and 1510 cm-1, Fig. 2(b). Shifts in the peaks such as C=N peak positions from 1510 cm-1 in the Si(OH)2TPPc alone to 1438 cm-1 (Fig. 2(b)) in the conjugate (DNDs-Si(OH)2TPPc, Fig. 2(c)) confirm changes in the structure. The ester bond was observed at 1620 cm-1 in Fig. 2. Similar shifts and esterbond formation between the rest of the SiPcs (Si(OH)2TBPc; Si(OH)2Pc) and DNDs are shown in Fig. 2 (e) and (g). FTIR cannot unequivocally confirm ester bond formation, however we have proved ester bond formation for DNDs-Si(OH)2TPPc using X-ray photoelectron spectroscopy [17]. 3.1.2. TEM images, DLS size and Zeta potential

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TEM images of DNDs (Fig. 3 (a)) depicts monodispersion of DNDs alone and aggregation upon conjugation of respective Pcs (Fig. 3(b) - (d)), making accurate size determination of the conjugates difficult. Hence, DLS was employed to determine size distribution of respective samples. DNDs alone (Fig. 4(a)) have average size of 2.9 nm. Increases in sizes to 68.1, 78.8 and 91.2 (Fig. 4 (b)-(d)) for DNDs-Si(OH)2Pc, DNDs-Si(OH)2TBPc and DNDsSi(OH)2TPPc, respectively are observed, Table 1. 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 [26-28]. The increase in size following conjugation 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 - stacking [29]. The larger size in DNDs-Si(OH)2TPPc conjugate could be due to the larger loading of the Pcs on the DNDs and smaller sizes for DNDs-Si(OH)2Pc (Fig. 3(d)) corresponding to the least amount of Pcs loaded onto the DNDs (Table 1). Zeta potential (ζ) is an indicator of the stability of nanoparticle (NP) suspensions, and values between -5 to +5 mV indicate fast aggregation [30]. In this study, ζ values of DNDs-Si(OH)2TPPc conjugate (2.41 mV) is much lower than those of DNDs alone (8.80 mV), indicating aggregation as has been observed before for this conjugate [17]. DNDs-Si(OH)2TBPc and DNDs-Si(OH)2Pc showed higher ζ values

(10.56 and 17.23 mV, respectively) due to

decreased Pc loading, suggesting less aggregation.

DNDs showed positive surface

charge irrespective of the predominant carboxylic acid groups on its surface shown in FTIR. Similar observations attribute this to carbon basicity (due to oxygen defects) [31]. 3.1.3. EDX spectra

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Qualitative verification of the elemental composition of the DNDs-Pc conjugate was investigated using EDX as shown in Fig. 5 (a)-(c). The EDX spectra of DNDs alone showed the presence of C and O peaks as expected (Fig. 5(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 reported before [17]. DNDs conjugated to Si(OH)2TPPc showed Si, C, N and O indicative of the presence of SiPc (Fig. 5(b)). Similarly nanoconjugates of DNDsSi(OH)2TBPc and DNDs-Si(OH)2Pc showed the presence of Si, C, N and O peak. 3.1.4. Thermogravimetric Analysis Fig. 6(a)-(g) shows thermogravimograms of DNDs, SiPcs and respective conjugates at temperature range of 50–800 °C in air. Thermal stability and extent of functionalisation upon conjugation of respective SiPcs (Si(OH)2TPPc, Si(OH)2TBPc, and Si(OH)2Pc) to DNDs are amongst some of interesting characteristics obtained from Fig. 6. TGA plots of DNDsSi(OH)2TPPc, DNDs-Si(OH)2TBPc and DNDs-Si(OH)2Pc revealed decreased weight loss compared to Pc alone, thus indicating improvement in stability of the constructed nanoconjugate systems. DNDs alone showed a total weight loss of 100% at 800 °C, Similar improvement in thermal stability of nanosembles of single walled carbon nanotubes (SWCNTs) and functionalised Zn mono carboxy phthalocyanine-spermine has been previously reported [32]. Nitrogen has been reported to form a heat resistant protective network at higher temperatures [33], increasing stability. Since both SiPcs and DNDs have nitrogen, it is possible the increased stability when both are combined is as result of increased nitrogen content and formation of the protective networks stated

7

above. Similar formation of nitrogen protective network due to temperature increase in HIV trans-activator of transcription protein - nanodiamonds- drug doxorubicin (TAT-NDDOX) [33]. In addition, Zn 2,9,16,23-tetrakis[4-(N-methylpyridyloxy)]-phthalocyanine detonated nanodiamond - borondipyrromethenes (ZnTPPcQ-DNDs-BODIPY) has been reported to yield less weight loss compared to BODIPY and ZnTPPcQ alone [34]. Hence, functionalisation of DNDs improved the thermal stability of constructed conjugates. DNDs-Si(OH)2TBPc and DNDs-Si(OH)2Pc showed more stability (less weight loss) at 800 °C when compared to DNDs-Si(OH)2TPPc. The increased weight loss when GO sheets were functionalised with FePc (comparted to GO alone) has been attributed to the decomposition of FePc [35]. The increased weight loss for DNDs-Si(OH)2TPPc compared to DNDs-Si(OH)2TBPc and DNDs-Si(OH)2Pc may be related to the higher loading of the former as discussed above. 3.1.5. Raman Spectra In this study, laser Raman spectral technique is used to determine the quality of assynthesised DNDs and their respective nanoconjugates. The disorder (D) sp3 defects and graphitic (G) sp2 peaks from in plane vibrations were observed at approximately (1363; 1563 cm-1), (1375; 1590 cm-1), (1362; 1551 cm-1) and (1360; 1551 cm-1) for DNDs, DNDsSi(OH)2TPPc, DNDs-Si(OH)2TBPc and DNDs-Si(OH)2Pc (Fig. 7(a)-(d)), respectively. 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). The G bands of the DNDs- Si(OH)2TBPc and DNDsSi(OH)2Pc shifted to lower wavenumbers compared to DNDs while the G band for DNDsSi(OH)2TPPc shifted to higher wavenumbers compared to DND alone. The D bands for 8

DNDs-Si(OH)2TPPc to higher wavenumbers, while

DNDs-Si(OH)2TBPc and DNDs-

Si(OH)2Pc showed small shifts in the D bands. Shifts in the Raman frequencies are often indicative of strong π-electron interactions in hybrid materials [15]. 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 SiPcs to DNDs resulted in increase in ID/IG to 0.18, 1.48, and 0.52 for DNDs-Si(OH)2TPPc, DNDs-Si(OH)2TBPc, and DNDs-Si(OH)2Pc), respectively, compared to ID/IG = 0.01 for DNDs, Table 1. The increase in ID/IG ratio for conjugated DNDs-Pcs compared to DNDs alone is due to increase in defects. The low ID: IG for pristine DNDs confirms the predominance of sp2 carbons. 3.1.6. XRD The X-Ray diffractometer (XRD) patterns for DNDs, Si(OH)2TPPc and DNDs– Si(OH)2TPPc nanoconjugated system (as examples) are shown in Fig. 8 (a)-(c). The XRD peaks for Pc in Fig. 8 (b) at 2 ≈ 16 and 27 with respective indices of 400 and 800 are typical of metallated Pcs [36]. For the DNDs alone the peaks at 44, 75 and 91 are due to 111, 221 and 311 plane of DNDs (NIST number A51588) [37,38]. Conjugation of SiPc to DNDs in Fig. 8(c), showed the presence of both.

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. A broad absorption feature was observed for DNDs in Fig. 9(a). Similar broad absorption has been previously reported to be due to π–π* transitions of 9

aromatic sp2 domains in the material [37]. The ground state optical absorption spectra of 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 corresponding to the Q and the B bands, respectively [29]. Absorption spectra for SiPcs alone shown in Fig. 9(c), (e) and (g) shows Q bands at 670, 677 and 681 nm for Si(OH)2Pc, Si(OH)2TBPc and Si(OH)2TPPc, respectively, Table 1, in DMSO. Red shift in Q bands of substituted SiPcs is attributed to electron donating nature of substituents [39,40].

Upon conjugation of Pcs to DNDs there are red shifts in the Q band to 683, 681 and 687 nm, for DNDs-Si(OH)2TPPc, DNDs-Si(OH)2TBPc and DNDs-Si(OH)2Pc, respectively when compared to SiPcs alone, Table 1. Red shifts are attributed to extended π-network from the DNDs. DNDs-Si(OH)2TPPc shows extreme aggregation as we reported before [17], Fig. 9(f). 3.3. Photophysical and photochemical parameters 3.3.1. Fluorescence quantum yields (ФF) and lifetimes (τ) Pcs alone namely: Si(OH)2TPPc, Si(OH)2TBPc and Si(OH)2Pc have values of F = 0.19, 0.38 and 0.67, Significantly low F values for the Pcs in the presence of DNDs could be because of the electron donating ability of DNDs, since electron donating groups are known to increase intersystem crossing in porphyrins [41], reducing fluorescence. The low F value for DNDs-Si(OH)2TPPc is additionally attributed to aggregation observed in Fig. 9(f). Aggregates are known to convert electronic excitation energy to vibrational energy, resulting in decrease in fluorescence quantum yield of molecules [42].

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Fluorescence lifetimes (τ) for SiPcs 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. Biexponential decay profile were obtained when fitting DNDs-nanoconjugates indicating two lifetimes which could be due to the orientation of the SiPc around the DNDs. Average lifetimes are shown in Table 1. The fluorescence lifetime values lengthened for DNDs-Si(OH)2Pc, and shortened for DNDs-Si(OH)2TBPc and DNDsSi(OH)2TPPc. Increases or decreases in the fluorescence lifetimes may depend on the geometry or distance between the metal and Pc molecule [43].

3.2.2. Triplet quantum yields (T) and lifetimes (τT) Of the SiPcs alone, Si(OH)2Pc gave the lowest T value of 0.24. Higher T values for Si(OH)2TPPc (0.70) and Si(OH)2TBPc ( 0.30) also correspond to their lower F values. F and T are competing processes, where one is low the other is high. Conjugation of Si(OH)2TPPc showed a drastic decrease in T values in the presence of DNDs due to aggregation [17] discussed above. Si(OH)2Pc and Si(OH)2TBPc showed an increase in T values in the presence of DND, as a result of electron donating ability of DNDs to SiPcs [41] discussed above. There is a lengthening of triplet lifetimes of Pcs in the presence of DNDs probably as a result of the protection of the Pcs by the DNDs.

3.3. Nonlinear optical (NLO) studies

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Nonlinear absorption behaviour of SiPcs and respective nanoconjugates were investigated using an open aperture z-scan technique at 532 nm with an excitation pulse of 7 ns at various input energies (20, 30, 60 and 80 μJ) and absorbances (0.1, 0.2, 0.3 and 0.4). All measurements reveal nonlinear absorption (NLA) behaviour (Fig. 11, Fig. S1-S7), with the shapes of the Z-scan profile exhibiting reverse saturable absorption (RSA) signatures in accordance to previously reported studies [44,45]. The transmittance values vary with sample absorbance and the input energy. A higher reduction in transmittance shown by an enhanced dip in RSA profile was observed when SiPcs were in the presence of DNDs than when SiPcs were alone, suggesting that the nanoconjugated systems could be potential optical limiting materials. Under optimal conditions, nanoconjugates (DNDs–Si(OH)2TPPc, DNDs– Si(OH)2TBPc and DNDs–Si(OH)2Pc), showed a dip in transmittance at 43, 14 and 68%, respectively. Respective SiPcs alone (Si(OH)2TPPc, Si(OH)2TBPc and Si(OH)2Pc) showed a dip at ( 49, 83 and 89)% (Fig. 11(a)-(c)). The data were analyzed in the manner reported by Sheik-Bahae et al. and equations are provided in the Supporting Information. Table 2 shows the effective nonlinear absorption coefficient values, Beff, obtained for each sample by fitting the experimental data to the transmittance equations provided in Supporting Information. The Beff values increased for the nanohybrid conjugates compared to Pcs alone. Electron donating groups on the periphery of Pcs have the ability to improve OL effect through an increase in transition dipole moment between the excited states involved in the electronic transition [11]. Hence, Si(OH)2TPPc and Si(OH)2TBPc show better NLO behaviour when compared to unsubstituted Si(OH)2Pc. The π-network from DNDs combined with SiPcs showed better 12

characteristics relative to individual SiPcs, maybe due to the good NLO of DNDs alone. Non-linear absorption properties of DND-H and the DND-NH2 systems have been previously reported to be due to their negative electron affinity (NEA) character, associated with H and N electron donor ability, alternating the band gap to adequate surface dipoles [46]. A plot of input energy versus βeff generally showed decreasing βeff as input energy increases, Fig. 12A, for nanoconjugates. Decreases in βeff as input energy increases indicate sequential two photon absorption (2PA) [47-49]. In Fig.12 B, it is evident that increasing absorbance of nanoconjugates results in an increase in the Beff of optical limiting materials (see also Fig. S1 (a)-(ed)), suggesting that the βeff depends strongly on the number of statistically available 2PA absorbers. Similar linear trends for concentration/absorbance and Beff have been previously reported when Leishman dye [50], and Pc-SWCNTs nanohybrid [51] were used as optical limiting material. Irradiation at 532 nm excites molecules from S0 to either S1 or S2 (the latter with excited state absorption cross-section δ1), Fig. 13. The S2 state is short-lived, hence relaxation to S1 almost occurs immediately. Since the triplet lifetime of the SiPcs and conjugates are much longer than the intersystem crossing lifetime [52], 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. Freecarrier absorption (FCA), which occurs when excitation takes place at the wavelengths where there is linear absorption [51], thus DNDs, with some absorption at 532 nm could be contributing to NLO via FCA in addition to enhancing intersystem crossing in Pcs.

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In this study the effect of absorbance on Beff values in dilute concentrations for DNDsSi(OH)2TPPc when compared to Si(OH)2TPPc alone was evaluated. In our previous study [17] we employed high absorbance of 1, and DNDs-Si(OH)2TPPc gave lower NLO parameters compared to Si(OH)2TPPc. At high concentrations/absorbances, aggregation reduces excited state lifetime and Beff [17,53]. Thus, upon preparation of optical limiting materials in solution, concentration is amongst some of the vital parameters that affect performance of the material. k in Table 2 indicates quantitative evaluation of the ratio of the excited and ground state absorption cross-sections [54]. The absorption contribution resulting from the excited state was evaluated using equations 4 and 5 (Supporting Information). k values shown in Table 2 for the Pcs and conjugates indicate the existence of an excited state (𝛿exc) with higher absorption cross-section than the ground state. The conjugates have larger k values than the Pcs alone. Linear absorption coefficient (𝛼0) for the Pc molecules are presented in Table 2. The 𝛼0 values of investigated Pcs and respective conjugates show remarkable differences at the same absorbance of 0.1 in DMSO (Fig. S9). Materials (especially DNDs-conjugates) with large linear coefficient values showed high NLO characteristics. This is because linear absorption coefficient values are sensitive to structural modification and Pc interactions [55]. The average linear transmittances of Si(OH)2TPPc, Si(OH)2TBPc, Si(OH)2Pc and DNDs at energy and absorbance of 0.1 and 80 µJ is ≈ 93%. Conjugating respective SiPcs to DNDs resulted in a higher transmittance of ≈ 99% shown in Fig. S9, supporting information. Good optical limiting materials are generally known to have linear transmittance that exceed 40% [56]. Hence, SiPcs when conjugated to DNDs have better optical limiting properties. 14

The speed of the response of an optical material to the perturbation due to an intense laser beam is referred to as third-order susceptibility (Im[X3]) and is determined using equation 6 (Supporting Information).

The Im[X3] values for DNDs nanoconjugated

systems of 5.19 x 10-8, 3.7 x 10-8 and 0.96 x 10-8 esu for DNDs-Si(OH)2TPPc, DNDsSi(OH)2TBPc and DNDs-Si(OH)2Pc, respectively, Table 2, and are an improvement compared to the corresponding Pcs alone. DNDs on their own perform the best, 5.57 x 10-8. Hyperpolarizability (𝛾) occurs when the permanent dipole of the molecule interacts with light to cause a bias in the average orientation of the molecule. The 𝛾 (see equation 7 in Supporting Information) in this work lie in the range 10-27 esu and are greatly enhanced when compared to our previous study [17], Table 2, showing the importance of showing low absorbance. The values of Im[X3] and 𝛾 for DNDs-Si(OH)2TPPc, DNDs- Si(OH)2TBPc and DNDs- Si(OH)2Pc in Table 2 showed improvement when compared to SiPcs alone. A good optical limiting (OL) material has the ability to reduce transmittance with increasing incident fluence. The material would assume a linear transmittance at low incident fluence, but changes rapidly at high incident fluence or constant output fluence at thresholds that are constant and less than the amount required to damage the optical element [57]. This critical point is called the threshold limiting intensity or fluence, (Ilim) [58] 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 are experimentally determined using the 15

plots of transmittance against input fluence (Fig. 14(A-C). Ilim values for Si(OH)2TPPc) and DNDs-Si(OH)2TPPc and DNDs-Si(OH)2TBPc were determined to be 0.13, 0.09 and 0.01 Jcm-2, respectively. The transmittances for Si(OH)2TBPc, Si(OH)2Pc and DNDs-Si(OH)2Pc did not drop below 50%, hence there are no values in Table 2. DNDs-Si(OH)2TPPc gave a lower Ilim value of 0.09 when compared to Si(OH)2TPPc alone (0.13), Table 2. A better Ilim value of 0.01 was obtained for DNDs-Si(OH)2TBPc. The enhanced performance of the DNDs-Si(OH)2TPPc and DNDs-Si(OH)2TBPc relative to DNDs-Si(OH)2Pc conjugates may be attributed higher population of the triplet state in the former.

4. Conclusion We have synthesised DNDs-nanoconjugates containing Si(OH)2Pc, Si(OH)2TBPc

and

Si(OH)2TPPc. Despite their respectively low triplet quantum yields of 0.42 for DNDsSi(OH)2TPPc when compared to respective Si(OH)2TPPc (0.70), the nanohybrid showed improved NLO properties at low absorbances. Substituted SiPc nanosembles showed better NLO characteristics when compared to unsubstituted SiPc. Limitting threshold (Ilim) for nanosembles with a more electron donating substituent (DNDs-Si(OH)2TBPc) was better at 0.01 relative to Ilim ≈ 0.09 (DNDs-Si(OH)2TPPc). An indication that the nature of Pc (electron donating) has a great influence on the electron distribution of broadband optical limiting materials. 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 of Medicinal Chemistry and 16

Nanotechnology (UID 62620) as well as Rhodes University. We thank Profs Kimura, Kobayashi and E. Osawa from Shishu University in Japan for the DNDs.

17

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25

NH2 O2N

COOH

OH

+ N

OH

N

N

Si

N

N

N

N

HO

COOH

N

H2 Graphite-like surface sp2

sp3

DCC NHS 48 h H2

G ra ph i te -li k e s ur fa ce

NO2

O O N N HO

N

N

HO HO

N

Si

N

N

N

O

H2N

Scheme 1 (A): Ester covalent linkage between Si(OH)2TBPc (as an example) and DNDs resulting in DNDs- Si(OH)2TBPc nanoconjugate system.

26

(a)

N

(c)

(b)

O

OH

OH

N

O

N

N

N

Si N N N

N

N

N O

N

HO

HO

OH

N

N

N

N

Si

N

N

N

N

N

N

Si

N

N

N

N

O

N

HO

N

Figure 1. The representation of (a) Si(OH)2TPPc, (b) Si(OH)2TBPc, and (c) Si(OH)2Pc

(a) C=O

OH (b)

Intensity (a.u.)

(c)

OH

OH

C-H

CH

C=N

C=N RCOOR' C=O

CH

(d)

(e)

(f)

(g) 4000

3500

3000

2500

2000

1500

1000

500

-1

Raman Shift (cm )

Fig. 2: FTIR spectra of covalent linkage: (a) DNDs, (b) Si(OH)2TPPc alone, (c) Si(OH)2TPPcDNDs, (d) Si(OH)2TBPc alone, (e) Si(OH)2TBPc-DNDs, Si(OH)2Pc-DNDs

27

(f) Si(OH)2Pc alone, and (g)

(b)

(a)

(c)

(d)

Fig. 3: TEM migrographs:(a) DNDs, (b) DNDs- Si(OH)2TPPc, (c) DNDs-Si(OH)2TBPc, and (d) DNDs-Si(OH)2Pc 18

(a) 2.9 nm

3

16

(b) 91,2 nm

14

Number (%)

Number (%)

12

2

10 8 6

1

4 2

0

0

0

5

10

15

20

25

30

35

40

45

50

0

100

200

300

400

Diameter (nm)

500

600

700

800

900

1000

Diameter(nm)

20 18

20

16

(d) 68.1 nm

14

(c) 78.8 nm

Number (%)

Number (%)

15

10

12 10 8 6

5

4 2 0

0 0

200

400

600

800

0

1000

200

400

600

800

1000

Diameter (nm)

Diameter (nm)

Fig. 4: DLS Plots for(a) DNDs, DNDs-Si(OH)2TPPc, (c) DNDs-Si(OH)2TBPc, and (d) DNDsSi(OH)2Pc.

28

Fig. 5: EDX spectra for (a) DNDs, b) Si(OH)2TPPc, and (c) DNDs-Si(OH)TPPc

100

(a)

80

W eight (% )

(b) (g)

60

(e) 40

(c)

20

(f) (d)

0 100

200

300

400

500

600

700

800

o

T emperature ( C )

Fig. 6: Thermogravimetric analysis (TGA) curves for (a) DNDs alone, (b) Si(OH)2TPPc, (c) DNDs-Si(OH)2TPPc, (d) Si(OH)2TBPc, (e) DNDs-Si(OH)2TBPc, (f) Si(OH)2Pc, and (g) DNDsSi(OH)2Pc heated at 20 °C min-1 from 50 to 1000 °C in air.

29

(a)

G

D

(b)

(c)

(d)

1300

1350

1400

1450

1500

1550

1600

1650

Wavelength (nm)

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

(111)

(a)

(221)

Intensity (a.u.)

(b)

(311)

(800)

(400)

(111)

(c) (400) (800)

(221)

20

40

60

80

100

 theta (degrees)

Fig. 8: XRD spectra (a) DNDs alone, (b Si(OH)2TPPc alone, (c) DNDs–Si(OH)2TPPc 30

(a)

(b)

Absorbance (a.u.)

(c)

(d)

(e)

(f)

(g)

300

400

500

600

700

800

Wavelength (nm)

Fig. 9: UV-vis spectra of (a) DNDs alone, (b) DNDs–Si(OH)2Pc, (c) Si(OH)2Pc, (d) DNDs– Si(OH)2TBPc, (e) Si(OH)2TBPc alone, (f) DNDs–Si(OH)2TPPc, and (g) Si(OH)2TPPc alone in DMSO.

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

31

Fig. 11: Open aperture Z-scan signatures of SiPcs, (A) Si(OH)2TPPc, (B) Si(OH)2TBPc, and (C) Si(OH)2Pc and respective conjugates at absorbance of 0.1 and energy of 80 µJ and respective fittings in DMSO.

32

A

40.00

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

βeff (cm GW¯¹)

35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 20

30

40

50 60 Input Energy (µJ)

70

80

B 630.00

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

βeff (cm GW¯¹)

540.00 450.00 360.00 270.00 180.00 90.00 0.00 0.1

0.15

0.2

0.25 0.3 Absorbance (a.u.)

0.35

0.4

Fig. 12: Plots showing (A) input energy and (B) absorbance vs βeff. Each data point for each sample represents an independent Z-scan measurement.

33

Figure 13. Five level energy diagram explaining the dynamics of the excited state population (upward arrows), non-radiative relaxation (dashed arrows) in the studied complexes.

34

Transmittance (%)

(a)

1 0.9

Si(OH)₂TPPc

0.8

DNDs-Si(OH)₂TTPc

0.7 0.6 0.5 0.4 0.3 0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Input fluence (J/cm²) (b) Transmitaance (%)

1 0.8

Si(OH)₂TBPc

0.6

DNDs-Si(OH)₂TBPc

0.4 0.2 0 0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

Input fluence J/cm²

(c)

1 0.9

Transmittance (%)

0.8 0.7 0.6 0.5 0.4 0.3

Si(OH)₂Pc

0.2

DNDs-Si(OH)₂Pc

0.1 0 0

0.005

0.01

0.015

0.02

0.025

0.03

Input fluence (J/cm²)

Fig. 14: Plots of transmittance versus input fluence for (a) Si(OH)2TPPc, (b) Si(OH)2TBPc, and (c) Si(OH)2Pc and their conjugates in DMSO.

35

Table 1: Parameters for the Pcs and their respective nanoconjugates with DNDs in DMSO (where appropriate) Sample

 (nm)a

DLS size (nm)

ξ (mV)b

Pcs loaded Pc(g)/ DNDs (mg)

Raman spectra ID/IG ratio

Фf b

F (ns)

-

2.9

8.80

-

0.01

-

-

Si(OH)2TPPc

681

-

-

-

-

0.19b

2.77b

0.70b

198.7b

DNDs-Si(OH)2TPPc

683

91.2

2.41a

289

0.18b

0.02b

2.69b

0.42b

404.5b

DNDs

ФT

-

T(µs)

-

(637) Si(OH)2TBPc

677

-

-

-

-

0.38

5.09

0.30

372.8

DNDs-Si(OH)2TBPc

681

78.8

10.56

38

1.48

0.04

4.98

0.45

393.8

Si(OH)2Pc

670

-

-

-

-

0.67

5.02

0.24

114.5

DNDs- Si(OH)2Pc

687

68.1

17.23

3

0.52

0.05

5.17

0.37

198.8

a

the peak due to the aggregate in brackets. bValues from reference 17.

36

Table 2: Nonlinear optical properties of DNDs, Si(OH)2TPPc, Si(OH)2TBPc, Si(OH)2Pcs and respective nanoconjugates with DNDs in DMSO at absorbances and energy of 0.1 and 80 µJ, respectively. Sample

𝛼0 (cm-1)

Beff (cm GW-1)

Im[X3]

𝜸

x10-8 (esu)

x10-27 (esu)

𝜹𝒆𝒙𝒄

k ( 𝜹𝒐 )

Ilim (Jcm-2)

DNDs

0.046

239

5.57

28.5

-

-

Si(OH)2TPPc

0.063

19.99

2.46

1.26

150

0.13

DNDs-Si(OH)2TPPc

0.544

30.53

5.19

2.66

177

0.09

Si(OH)2TBPc

0.010

16.5

2.28

1.17

55

0.01

DNDs-Si(OH)2TBPc

0.659

20.50

3.85

1.97

231

-

Si(OH)2Pc

0.001

4.12

0.005

0.002

1.1

-

DNDs-Si(OH)2Pc

0.912

9.77

0.96

0.49

55

-

37