Nonlinear Optical Response of Some Graphene Oxide and Graphene Fluoride Derivatives

Optofluid. Microfluid. Nanofluid. 2016; 3:53–58

Research Article Open Access

Nikolaos Liaros, Ioannis Orfanos, Ioannis Papadakis, and Stelios Couris* Nonlinear optical response of some Graphene oxide and Graphene fluoride derivatives

DOI 10.1515/optof-2016-0009 1 Introduction Received November 8, 2016; revised December 5, 2016; accepted De- cember 12, 2016 It is about 10 years ago that Optofluidics emerged as a new Abstract: The nonlinear optical properties of two graphene scientific research area, originating from the combination derivatives, graphene oxide and graphene fluoride, are in- of two already established research fields, micro-fluidics vestigated by means of the Z-scan technique employing and optics [1]. Optofluidics aims to combine the advan- 35 ps and 4 ns, visible (532 nm) laser excitation. Both tages of each of these two research areas in the same plat- derivatives were found to exhibit significant third-order form. In fact, Optofluidics aims to combine the appealing nonlinear optical response at both excitation regimes, versatility of using fluids to perform actions arising from with the nonlinear absorption being relatively stronger their micro- or nano-fluidic properties and/or to their op- and concealing the presence of nonlinear refraction un- tical properties in order to replace classical optical com- der ns excitation, while ps excitation reveals the presence ponents made from glass, metals, or plastics. That is why of both nonlinear absorption and refraction. Both nonlin- in the first approach, Optofluidics referred to a class of ear properties are of great interest for several photonics, optical systems employing fluids as optical components. opto-fluidics, opto-electronics and nanotechnology appli- However, the use of fluids possessing nonlinear optical re- cations. sponse or fluids containing materials (e.g. dyes, nanopar- Keywords: graphene oxide, graphene fluoride, nonlinear ticles, etc.) exhibiting nonlinear optical response can of- optical response, nonlinear absorption coefficient, non- fer some more functionalities and open new horizons for linear refractive index parameter, third-order susceptibil- optofluidic devices [1, 2]. For instance, the use of stable ity χ(3) colloidal solutions of some nonlinear optical material can be of interest for some applications as e.g. optical limiters, optical deflectors, etc. Furthermore, the ease of tunability of the nonlinear optical response by simple chemical pro- cesses (as for example by oxidation or reduction, etc.) or through the concentration of the dispersed nonlinear material can provide additional versatility to realize optofluidic devices making use of the combination of the nonlinear optical response of a material and the fluidic aspects. *Corresponding Author: Stelios Couris: Department of Physics, Graphene, the youngest of the carbon allotropes, gen- University of Patras, 26504 Patras, Greece erated an enormous scientific interest almost immediately and Institute of Chemical Engineering Sciences, Foundation for after its discovery, mainly due to the several unprece- Research and Technology-Hellas (FORTH), P.O. Box 1414, 26504 Patras, Greece dented and extraordinary optical, electrical and mechan- Nikolaos Liaros: Department of Physics, University of Patras, 26504 ical properties it exhibits, which have created great ex- Patras, Greece pectations for potential applications in several areas of and Institute of Chemical Engineering Sciences, Foundation for modern technology and nanotechnology. Graphene, be- Research and Technology-Hellas (FORTH), P.O. Box 1414, 26504 ing an atom-thick, two-dimensional carbon layer consist- Patras, Greece Current address: Department of Chemistry & Biochemistry, Univer- ing of hexagonally packed carbon atoms where carbon is sity of Maryland, College Park, MD 20742, USA the principal building block, allows the formation of many Ioannis Orfanos, Ioannis Papadakis: Department of Physics, new stable and structurally fascinating graphene-based University of Patras, 26504 Patras, Greece nanomaterials and nanostructures by means of bottom- and Institute of Chemical Engineering Sciences, Foundation for up and top-down approaches [3–6]. As for the optical Research and Technology-Hellas (FORTH), P.O. Box 1414, 26504 properties of graphene, unlike conventional semiconduc- Patras, Greece

© 2016 Nikolaos Liaros et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. 54 Ë Nikolaos Liaros et al. tor materials, it exhibits an almost constant absorption (of a large energy gap, which is efficiently controlled by tun- ~2.3%) and broadband optical transparency over its entire ing the degree of oxidation, i.e. the sp2/sp3 ratio. The optical spectrum, from UV to IR. Its absorption is deter- ability to tune this ratio allows the continuous tuning of mined only by the fine-structure constant and not by the GO’s band gap, suggesting an efficient way for the tai- properties of the material itself. Furthermore, graphene loring of GO’s NLO response [24, 29]. However, this de- can form easily multilayered structures, and it has been pendence is neither simple nor straightforward because shown that its absorption scales linearly with the num- of the non-stoichiometric nature of GO samples. Another ber of layers [7]. It has also been demonstrated that when way of modifying GO’s NLO response controllably is by the incident light intensity is strong enough, graphene varying the number of graphitic layers of the sample, as exhibits significant broadband saturable absorption (SA), each layer adds about 2.3% of absorption. Moreover, GO due to the Pauli blocking principle, which makes graphene is more easily dispersed in organic solvents compared to suitable for mode-locking applications, e.g. in ultrafast pristine graphene, while it exhibits very good dispersabil- lasers [8, 9], for photodetectors [10, 11], for optical modu- ity in water, forming very stable aqueous colloidal dis- lators [12], in plasmonics [13] and for other optoelectronics persions. This makes it even more attractive in view of structures and devices [14–16]. In addition, graphene has health and environmental safety issues and, in particular, been shown to exhibit important nonlinear optical (NLO) in view of opto-fluidic applications. Furthermore, the pres- responses in a very wide range of frequencies, from mi- ence of hydroxyl and/or carboxyl functionalization groups crowaves and terahertz frequencies [17] up to optical fre- expands significantly the strategies for its further chemical quencies [18, 19]. In particular, concerning the range of derivatization [30]. the optical frequencies, inter-band optical transitions can Graphene fluoride [28] consists of a graphene layer occur at all photon energies. Beyond the typical SA re- with covalently attached fluorine atoms (instead of oxy- sponse of pristine graphene and the optical limiting ef- gen atoms), being a 2-dimensional wide band gap semi- fect due to nonlinear scattering (NLS) [20], its derivatives conductor, exhibiting broadband transparency over its en- have been shown to exhibit several other important NLO tire spectrum, while the stronger the fluorination is, the effects, such as two-photon and multiphoton absorption more the sp2carbon bonds that are transformed to sp3 (TPA) [21, 22], reverse saturable absorption (RSA) [22–24], hybridization. Once more, the ability of tuning the ratio and optical limiting [25–27]. Therefore, graphene exhibits sp2/sp3 can be an efficient way for the continuous tuning excellent photophysical properties and large optical non- of GF’s band gap, thus modifying successfully its NLO re- linearities over a wide range of laser pulse lengths, which sponse [31]. For the improvement of the stability of the GF makes it a highly promising candidate for several poten- aqueous colloids, a non-covalent functionalization of GF tial applications including fast optical communications, sheets (f-GF) with perfluorooctanoate units was used. all-optical switching, optical limiting, etc. The important fact that the studied graphene deriva- In the present work, the NLO responses of two tives dispersed in water or in other organic solvents can graphene derivatives, graphene oxide (GO) and graphene form very stable colloidal suspensions is of particular in- fluoride (GF), are studied under nanosecond and picosec- terest since it denotes the possibility of further exploita- ond visible (532 nm) laser excitation and are compared, tion to integrate optofluidic devices taking advantage of with the goal of shedding more light into the underlying both the high optical nonlinearity of the graphene flakes physical mechanisms of the corresponding NLO response and the fluidity of the liquid medium. in view of their potential for photonics, optoelectronics and nanotechnology applications. The investigation of the NLO response of GO concerns the study of two kinds of 2 Experimental procedures GO, 1-3 layered GO structures, named as SLGO, and few layered GO structures (e.g. up to ~10 layers), denoted as FLGO. 2.1 Preparation of the samples Concerning the GF, the present work studies the NLO response of GF and GF stabilized with perfluorooctanoate groups [28]. The two types of GO samples were prepared and charac- The former graphene derivative, GO, consists of terized according to the procedures described in detail in a graphene layer with attached O-containing groups ref. [32] and references therein and were heavily oxidized (e.g., hydroxyl groups, carboxyl groups, etc.), covalently as evidenced by their respective XPS and Raman spectra. bonded, thus featuring both π-states (sp2-bonded car- The SLGO samples had 1-3 graphene layers, while the FLGO bons) and σ-states (sp3-bonded carbons) and exhibiting ones had up to about 16 layers. The size of the graphene Nonlinear optical response of some Graphene oxide and Graphene fluoride derivatives Ë 55 sheets and the number of layers were controlled through inary (Imχ(3)) and real (Reχ(3)) parts of the third-order non- the centrifugation procedure [33]. GF was purchased from linear susceptibility, χ(3), denoting in a more compact way Aldrich (CF∼0.5) and was used as is. For the preparation the strength of the NLO response of a sample, through the of f-GF samples the approach described in ref. [31] was fol- following relations: lowed. c n2  For the Z-scan measurements, aqueous dispersions of Reχ(3) (esu) = 10−6 0 훾′ (cm2 W) 480 π2 SLGO and f-GF were prepared, while the FLGO and GF were dispersed in DMF. The prepared dispersions were placed and in 1 mm thick quartz cells for the Z-scan measurements. c2n2 Imχ(3)(esu) = 10−7 0 β (cm/W) The UV-Vis-NIR absorption spectra of the prepared disper- 96π2ω sions were measured by spectrophotometer and were care- fully checked for any unwanted changes, like for exam- The details of the Z-scan technique have been presented ple aggregation effects, laser induced photo-degradation, in details elsewhere [34, 35], and only a brief description photo-reduction, etc. All dispersions were found exhibit- will be presented here. So, according to this technique ing remarkable stability in time and under the laser irra- the normalized transmittance of a sample translated along diation conditions employed, their UV-Vis-NIR absorption the propagation direction (e.g. the z-axis) of a focused spectra remaining unchanged for several months. Figure 1, Gaussian laser beam is measured by two different experi- shows some characteristic absorption spectra of the stud- mental configurations, the so-called “open-aperture” and ied graphene derivatives. “closed- aperture” Z-scans. The “open-aperture” Z-scan can exhibit either a minimum or a maximum, suggesting reverse saturable absorption (RSA) or saturable absorption (SA) behavior respectively, while the nonlinear absorption 1.0 coefficient β can be then determined from fitting by the fol- SLGO FLGO lowing equation: 0.8 GF f-GF +∞ [︃ ]︃ ∫︁ βI L (︁ )︁ 0.6 1 0 e 2 T = √ (︁ )︁ ln 1 +  exp −t dt βI0 Le z2 z2 π 1 + 0 1+z2 z2 −∞ 0.4 / 0 Absorbance (a.u.) where I0 is the on-axis peak irradiance, Le is the ef- 0.2 fective thickness of the sample given by relation: Le = (︁ )︁ e−a0 L a a 0.0 1 − 0 and 0 is the sample’s (linear) absorption 400 600 800 1000 1200 Wavelength (nm) coefficient at the laser wavelength and L is the sample’s length (thickness). From the division of the “closed-aperture” Z-scan by Figure 1: UV-Vis-NIR absorption spectra of aqueous dispersions the corresponding “open-aperture” one, the so-called “di- of SLGO and f-GF, and FLGO and GF dispersed in DMF. All spectra correspond to a concentration of 1 mg/ml. vided” Z-scan can be obtained, exhibiting a transmission minimum followed by a post-focal maximum, i.e. a valley- peak configuration, in the case where the sample exhibits self-focusing (i.e., acting as a focusing lens) or a peak- valley configuration in the case of self-defocusing (i.e., act- 2.2 Nonlinear optical measurements ing as a defocusing lens). The difference of the normalized transmittance between the valley and the peak, ∆Tp−v, is For the study of the NLO properties (i.e., the NLO absorp- related to the NLO refraction parameter 훾′ of the sample tion and refraction) of the graphene derivatives the Z-scan through the following relation: technique was used, which is a laser based technique al- √ lowing the simultaneous determination of both the mag- 훾 2 ∆T ′ = 0.25 P−V nitude and the sign of the NLO absorption and refraction kI0Le 0.406 (1 − S) of a sample from a single measurement. These NLO quan- r2 w2 S e−2 a/ a tities are usually expressed in terms of the nonlinear ab- where = 1 − is the linear aperture transmission, sorption coefficient, β (m/W), and the nonlinear refractive ra and wa are the radii of the aperture and the beam radius index parameter, 훾′ (m2/W), which are related to the imag- at the aperture, and k is the wavenumber in vacuum. 56 Ë Nikolaos Liaros et al.

For the experiments, two different laser systems were by the relatively stronger NLO absorption. In fact, this is employed, a 35 ps mode-locked Nd:YAG laser and a 4 ns Q- what ensues under ns excitation, where the nonlinear ab- switched Nd:YAG laser, both operating between 1 to 10 Hz. sorption, being relatively stronger, suppresses the nonlin- Their beams were focused into the sample by means of ear refraction up to its evanescence. In particular, concern- 20 cm focal-length quartz plano-convex lens, while the en- ing the NLO absorption of GOs’ and GFs’, both derivatives ergy per pulse was monitored by means of a calibrated were found exhibiting saturable absorption (SA) behavior joule-meter. The beam waists at the focal plane, for both at very low incident laser intensities, with the nonlinear lasers, were measured with a CCD detector and were found refraction being not measurable under the present exper- to be very close 18 µm at 532 nm. imental conditions, switching quickly to reverse saturable absorption (RSA) with the increase of incident intensity. Since the signals at low intensity were very noisy, intro- 3 Results and Discussion ducing large uncertainties in the determination of the NLO absorption coefficient β, only the β corresponding to the RSA regime is given in the Table. Concerning the NLO ab- For the determination of the nonlinear optical parameters sorption of the two graphene oxides, SLGO and FLGO, it is of the two graphene derivatives, several Z-scan measure- interesting to notice that the increased linear absorption ments were performed on various different concentration of FLGO, due to the number of graphene layers of which dispersions, at several different incident laser intensities. it consists, is the reason for the almost double NLO re- From the analysis of the experimental data the nonlinear sponse it exhibits, under ns excitation, compared to the absorption coefficient β and the nonlinear refraction pa- single layer GO. rameter 훾′ were determined for each measured sample. Another interesting finding resulting from the compar- Then, the third-order nonlinear susceptibility was calcu- ison of the NLO responses of Gf and f-GF is that the func- lated for each derivative. Table 1 summarizes the obtained tionalization of GF results in an important enhancement results. In order to facilitate the comparison the values of of the third-order susceptibility of GF at both excitation the nonlinear parameters presented in the table are all re- regimes, while it changes the sign of the NLO refraction as ferring to a concentration of 1 mg/ml. well. In fact, GF was found exhibiting self-focusing while As can be seen in this Table, the studied GOs and GFs the f-GF was exhibiting self-defocusing. Such behavior im- were found to present sizeable and similar magnitude NLO plies the presence of some electronic state which should response in terms of the third-order susceptibility χ(3). Tak- be located at a lower energy than that of the 532 nm pho- ing into account that the studied samples were prepared tons in the case of GF, while it is pushed at higher energy having similar degree of transformation of their sp2 bonds in the case of f-GF. Similar changes of the sign of the non- to sp3 ones, this reveals convincingly that the ratio sp2/sp3 linear refraction have been reported and discussed else- can be an effective and reliable parameter in order to con- where [36, 37]. trol the NLO response of these graphene derivatives, con- To summarize, it was shown that both GO and GF ex- firming results from previous studies [24, 29]. Another in- hibit significant NLO response, at both ns and ps excitation teresting issue arising from the present results is the ef- regimes, while their response can be further engineered by fect of the pulse length of the laser excitation on the NLO adjusting the number of graphene layers and/or the de- response. As shown, the magnitude of the NLO response gree of oxidation/fluorination of the graphitic sheet. The under 35 ps excitation was found to be about two orders excellent stability of the aqueous dispersions of SLGO and of magnitude lower than that of the corresponding one at f-GF, and the DMF dispersions of FLGO and Gf, along with 4 ns excitation. This finding is consistent with the gen- the different NLO properties they exhibit, makes them ex- eral trend, i.e. excitation with shorter laser pulse results cellent candidates for several applications in optofluidic in lower NLO response, since the response, in this case, technology. arises from phenomena (e.g., electronic excitation, molecular re-orientation, Kerr effect, etc.) which are associated Acknowledgement: The authors acknowledge the sup- with smaller magnitude NLO response and are occurring port of COST Action MP1205 “Advances in Optofluidics: In- in faster time scales as well, as it has been shown for ex- tegration of Optical Control and Photonics with Microflu- ample in the case of fullerenes [34, 35]. Another conse- idics” and the members of the Action for enlightening dis- quence of this situation is that shorter laser pulses give rise cussions. The authors are also indebted to Dr A. Bakan- to weaker NLO absorption, facilitating the measurement dritsos and Dr A. B. Bourlinos for making available the GO of the NLO refraction which otherwise can be concealed Nonlinear optical response of some Graphene oxide and Graphene fluoride derivatives Ë 57

Table 1: Third-order nonlinear optical parameters of 1 mg/ml dispersions of GOs and GFs. SLGO and f-GF were dispersed in distilled water, while FLGO and GF in DMF.

Graphene β 훾′ χ(3) derivative (×10−9 cm/W) (×10−18 m2/W) (×10−13 esu) SLGO 2.08 – 1.0 35ps GF – 0.16 0.24 f-GF 0.8 −1.2 1.4

SLGO 108.4 – 50.1 FLGO 220 – 116.7 4ns GF 12.7 14.7 19.7 f-GF – −34.2 38.6

and GF samples respectively and Prof. R. Zboril for his help Mater., 20(5), (2010), 782. with several material characterizations. [15] H. Zhang, D. Y. Tang, R. J. Knize et al., Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser. Appl Phys Lett, 96, 2010, 111112. [16] T. Gokus, R. R. Nair, A. Bonetti et al., Making graphene lumines- References cent by oxygen plasma treatment, ACS Nano, 3(12), 2009, 3963. [17] L. Ju, B. Geng, J. Horng et al., Graphene plasmonics for tunable terahertz metamaterials. Nat. Nanotechnology, 6, 2011, 630. [1] D. Psaltis, S. R. Quake and C. Yang, Developing optofluidic tech- [18] S. A. Mikhailov, Non-linear electromagnetic response of nology through the fusion of microfluidics and optics, Nature graphene, Europhys. Lett, 79, 2007, 27002. 442, 2006, 381. [19] E. Hendry, P. J. Hale, J. Moger et al., Coherent nonlinear optical [2] Fang, C. et al. Tunable optical limiting optofluidic device filled response of graphene, Phys Rev Lett, 105(9), 2010, 097401. with graphene oxide dispersion in ethanol, Sci. Rep. 5, 2015, [20] L. Yan, Y. Xiong, J. Si et al., Optical limiting properties and mech- art. No 15362. anisms of single-layer graphene dispersions in heavy-atom sol- [3] T. Pichler, Molecular Nanostructures: Carbon Ahead, Nat. Mater. vents, Opt. Express 22(26), 2014, 31837.. 2007, 6 (5), 332−333. [21] Z. B. Liu, Y. Wang, X. L. Zhang et al., Nonlinear optical proper- [4] M. C. Hersam, Progress Towards Monodisperse Single-Walled ties of graphene oxide in nanosecond and picosecond regimes, Carbon Nanotubes, Nat. Nanotechnol. 3(7), 2008, 387. Appl. Phys Lett, 94, 2009, 021902. [5] Z. Li, J. T. Robinson, S. M. Tabakman, K. Yang, H. Dai, Carbon Ma- [22] N. Liaros, P.Aloukos, A. Kolokithas-Ntoukas et al., Nonlinear op- terials for Drug Delivery & Cancer Therapy, Mater. Today, 14(7- tical properties and broadband optical power limiting action of 8), 2011, 316. graphene oxide colloids, J. Phys. Chem. C, 117, 2013, 6842. [6] L. Dai, D. W. Chang, J.-B. Baek, W. Lu, Carbon Nanomaterials [23] G. K. Lim, Z. L. Chen, J. Clark et al., Giant broadband nonlin- for Advanced Energy Conversion and Storage, Small 8(8), 2012, ear optical absorption response in dispersed graphene single 1130. sheets, Nat Photonics, 5, 2011, 554. [7] R. R. Nair, P. Blake, A. N. Grigorenko et al., Fine Structure [24] N. Liaros, J. Tucek, K. Dimos et al., The effect of the degree of ox- Constant Defines Visual Transparency of Graphene, Science, idation on broadband nonlinear absorption and ferromagnetic 320(5881),2008, 1308. ordering in graphene oxide, Nanoscale 8(5), 2016, 2908. [8] Z. Sun, T. Hasan, F. Torrisi et al., Graphene mode-locked ultra- [25] J. Wang, Y. Hernandez, M. Lotya et al., Broadband nonlinear op- fast laser. ACS Nano, 4(2), 2010, 803. tical response of graphene dispersions, Adv. Mater., 21, 2009, [9] H. Zhang, D. Y. Tang, R. J. Knize et al., Graphene mode locked, 2430. wavelength-tunable, dissipative soliton fiber laser. Appl. Phys. [26] M. Feng, H. B. Zhan, Y. Chen, Nonlinear optical and optical limit- Lett., 96, 2010, 111112. ing properties of graphene families, Appl. Phys. Lett., 96, 2010, [10] T. Mueller, F. Xia, P. Avouris, Graphene photodetectors for high- 033107. speed optical communications, Nat Photonics, 4, 2010, 297. [27] N. Liaros, E. Koudoumas, S. Couris, Broadband near infrared op- [11] F. Xia, T. Mueller, Y.-M.Lin, A. Valdes-Garcia, P.Avouris, Ultrafast tical power limiting of few layered graphene oxides, Appl. Phys. graphene photodetector, Nat. Nanotechnol., 4, 2009, 839. Lett., 104(19), 2014, 191112. [12] M. Liu, X. Yin, E. Ulin-Avila et al., A graphene-based broadband [28] A. B. Bourlinos, K. Safarova, K. Siskova, and R. Zboril, The pro- optical modulator, Nature, 474, 2011, 64. duction of chemically converted graphenes from graphene flu- [13] T. J. Echtermeyer, L. Britnell, P. K. Jasnos et al., Strong plas- oride, Carbon 50(3), 2012, 1425. monic enhancement of photovoltage in graphene. Nat. Com- [29] N. Liaros, S. Couris, E. Koudoumas, P.A. Loukakos, Ultrafast Pro- mun, 2:458, 2011. cesses in Graphene Oxide during Femtosecond Laser Excitation, [14] Q. Bao„ H. Zhang„ J.-X. Yang et al., Graphene–Polymer J. Phys. Chem. C, 120(7), 2016, 4104. Nanofiber Membrane for Ultrafast Photonics, Adv. Funct. 58 Ë Nikolaos Liaros et al.

[30] K. P. Loh, Q. Bao, G. Eda, M. Chhowalla, Graphene oxide as a chemically tunable platform for optical applications, Nature Chemistry, 2, 2010, 1015. [31] A. B. Bourlinos, A. Bakandritsos, N. Liaros et al., Water dis- persible functionalized graphene fluoride with significant nonlinear optical response, Chem. Phys. Lett. 543, 2012, 101. [32] R. Y. Gengler, A. Veligura, A. Enotiadis et al., Large-yield preparation of high-electronic-quality graphene by a Langmuir- Schaefer approach, Small, 6, 2010, 35. [33] Y. Zhou, Q. Bao, L. A. L. Tang, Y. Zhong, and K. P. Loh, Hy- drothermal Dehydration for the “Green” Reduction of Exfoliated Graphene Oxide to Graphene and Demonstration of Tunable Op- tical Limiting Properties, Chem. Mater. 21, 2009, 2950. [34] S. Couris, E. Koudoumas, A.A. Ruth, S. Leach, Concentration and wavelength dependence of the effective third-order susceptibility and optical limiting of C60 in toluene solution, J. Phys. B: At. Mol. Opt. Phys. 28(20), 1996, 4537. [35] E. Koudoumas, M. Konstantaki, A. Mavromanolakis et al., Tran- sient and instantaneous third-order nonlinear optical response of C60 and the higher fullerenes C70, C76 and C84, J. Phys. B: At. Mol. Opt. Phys. 34 (24), 2001, 4983. [36] E. Xenogiannopoulou, M. Medved, K. Iliopoulos et al., Nonlinear Optical Properties of Ferrocene- and Porphyrin–[60]Fullerene Dyads, ChemPhysChem, 8(7), 2007, 1056. [37] L. Ðorđević, T. Marangoni, F. De Leo et al., [60]Fullerene– porphyrin [n]pseudorotaxanes: self-assembly, photophysics and third-order NLO response, Phys. Chem. Chem. Phys., 18, 2016, 11858.