Stephania Abdallah

Rana Mhanna

Justo Cabrera-González

Dr Rosario Núñez
email: rosario@icmab.es

Amine Khitous

Fabrice Morlet-Savary

Olivier Soppera

Davy-Louis Versace

Jean-Pierre Malval
email: jean-pierre.malval@uha.fr

Reversible Optical Data Storage via Two-Photon Micropatterning of o-Carboranes embedded Switchable Materials

Keywords: Two-Photon lithography, o-carborane initiators, optical data storage

Two-photon polymerization (2PP) constitutes a powerful light-triggered 3D stereolithography allowing the fabrication of micro-or nanostructures with spatially encoded functionalities. In the present report, we take benefit of this programmable property in order to develop of a photoresist authorizing the direct 2PP-lithography of luminescent switchable µ-structures. In this single step processing method, we highlight the pivotal role of a quadrupolar o-caborane initiator comprising a 9, 10-disubstituted anthracene core capable to chemically entrap oxygen inhibiting species during the free radical two-photon initiation step. Such an O2 sequestration approach not only enhances the two-photon initiation performance of the polymerization but also generates a non luminescent endoperoxide by-product embedded in the polymerized macrostructure. Interestingly, a simple heating of the final materials promotes the endoperoxide thermolysis resulting in the recovery of the luminescent o-carborane dye. This original luminescence turn OFF/ON property exhibits an excellent fatigue resistance through a large number of alternated thermal and light stimuli cycles. The potential of this direct laser writing strategy for reversible data storage applications is finally demonstrated by two-photon patterning of a series of matricially organized µ-structures used as rewritable binary optical memories which can be easily encoded on demand.

INTRODUCTION.

Two-photon lithography (2PL) has emerged as one of the most promising additive manufacturing techniques which enables the micro-and nanofabrication of intricate architectures [START_REF] Lay | Two-Photon-Assisted Polymerization and Reduction: Emerging Formulations and Applications[END_REF][START_REF] Barner-Kowollik | 3D Laser Micro-and Nanoprinting: Challenges for Chemistry[END_REF][START_REF] Lafratta | Multiphoton Fabrication[END_REF] . This maskless printing approach implies the raster scanning of a tightly focused fs-pulse laser within a photoresist which gives rise to the customized micro / nanostructures through a layer-by-layer slicing procedure. The high writing accuracy of 2PL stems from the intrinsic spatial confinement of the two-photon polymerization (2PP) reaction due to a primary two-photon absorption step which only occurs within an unsupported floating volume concentric to the excited focus. As a result, the elemental polymerized pixel can be generated with a drastically reduced dimension falling far below that of the optical diffraction limit. Therefore, 2PL not only displays a long range penetration depth into the photoresist (typically higher than 300 µm) but also constitutes a non diffraction limited technique which authorizes the fabrication of objects with arbitrary 3D geometries and complex nanoscale details. These two decisive advantages definitively make 2PL a truly 3D lithography able to spatially control the programmation of specific functionalities into the photopatterned materials [START_REF] Ding | Controlled Microstructural Architectures Based on Smart Fabrication Strategies[END_REF] . With this respect, a very large panel of advanced functional materials has emerged from 2PL [START_REF] Lay | Two-Photon-Assisted Polymerization and Reduction: Emerging Formulations and Applications[END_REF][START_REF] Barner-Kowollik | 3D Laser Micro-and Nanoprinting: Challenges for Chemistry[END_REF][START_REF] Ding | Controlled Microstructural Architectures Based on Smart Fabrication Strategies[END_REF][START_REF] O'halloran | Two-Photon Polymerization: Fundamentals, Materials, and Chemical Modification Strategies[END_REF] . For instance, the unprecedented properties of metamaterials in mechanic [START_REF] Schaedler | Ultralight Metallic Microlattices[END_REF] , acoustics [START_REF] Mei | Dark acoustic metamaterials as super absorbers for low-frequency sound[END_REF] , magnetics [START_REF] Linden | Magnetic Response of Metamaterials at 100 Terahertz[END_REF][START_REF] Kenanakis | Three-Dimensional Infrared Metamaterial with Asymmetric Transmission[END_REF] or optics [START_REF] Kenanakis | Three-Dimensional Infrared Metamaterial with Asymmetric Transmission[END_REF][START_REF] Silva | Performing Mathematical Operations with Metamaterials[END_REF][START_REF] Torgersen | Hydrogels for Two-Photon Polymerization: A Toolbox for Mimicking the Extracellular Matrix[END_REF] directly derived from the appropriate arrangement of a periodic pattern or lattice which is designed with a very fine adjustment of the size, shape and orientation. In this precise case, 2PL is perfectly adapted for modulating the metamaterials response since the related properties basically derived from a geometry tuning issue [START_REF] Silva | Performing Mathematical Operations with Metamaterials[END_REF] . Another very popular strategy to spatially program the material consists in seeding a latent functionality either into the cross-linkable monomer or into the two-photon initiator both constituting the fundamental components of the photoresist. The latent property can then be activated after the 2PL fabrication. For instance, reconfigurable microstructures that can dynamically respond to distinctive local stimuli (pH, solvent, temperature) are fabricated following this construction guideline with the added value specifically centred on the cross-linking monomer. For instance, hydrogel / organogel photoresists clearly constitute one of the most popular materials toolboxes [START_REF] Torgersen | Hydrogels for Two-Photon Polymerization: A Toolbox for Mimicking the Extracellular Matrix[END_REF] whose stimuliresponsive properties have been extensively employed in plethora of micro objects such as drug delivery devices [START_REF] Xing | Two-photon polymerization microfabrication of hydrogels: an advanced 3D printing technology for tissue engineering and drug delivery[END_REF] , soft microrobots [START_REF] Huang | 3D Printed Microtransporters: Compound Micromachines for Spatiotemporally Controlled Delivery of Therapeutic Agents[END_REF] , biomimetic self-actuators [START_REF] Hippler | Controlling the shape of 3D microstructures by temperature and light[END_REF] , stimuli-responsive shape-shifting µ-objects [START_REF] Lv | Humidity-responsive actuation of programmable hydrogel microstructures based on 3D printing[END_REF][START_REF] Sun | Ingenious humidity-powered micro-worm with asymmetric biped from single hydrogel[END_REF] . The second alternative 2PL method consists in implementing the added value into the two-photon initiating system. In this bottom-up approach, the two-photon initiator not only promotes the cross-linking reaction but disseminates concomitantly a novel functionality within the native macrostructure. For instance, the fabrication of conductive metallic structures without any extra deposition is inspired from this methodology where the two-photon initiator is also involved in a photoreduction reaction with a metallic salt incorporated in the photoresist [START_REF] Lay | Two-Photon-Assisted Polymerization and Reduction: Emerging Formulations and Applications[END_REF][START_REF] Blasco | Fabrication of Conductive 3D Gold-Containing Microstructures via Direct Laser Writing[END_REF] . With this 'one pot' seeding approach, the resulted µ-object simply inherits its functionality from that of its related 2P-initiator leading to the fabrication of plasmonic resonators [START_REF] Tabrizi | Functional Optical Plasmonic Resonators Fabricated via Highly Photosensitive Direct Laser Reduction[END_REF] , SERS µ-sensors [START_REF] Lee | Direct Metal Writing and Precise Positioning of Gold Nanoparticles within Microfluidic Channels for SERS Sensing of Gaseous Analytes[END_REF] , catalytic µ-reactors [START_REF] Xu | On-chip fabrication of silver microflower arrays as a catalytic microreactor for allowing in situ SERS monitoring[END_REF] , bactericidal photopatterned surfaces [START_REF] Versace | Highly Virulent Bactericidal Effects of Curcumin-based µ-Cages Fabricated by Two-Photon Polymerization[END_REF] or electrochemilumiscent devices [START_REF] Mhanna | Micropatterning of electrochemiluminescent polymers based on multipolar Ru-complex two-photon initiators[END_REF] . In line with this direct laser writing method, we report herein a two-photon activable resist incorporating a multipolar ocarborane initiator with a dual functionality which combines 2PP initiation with a fluorescence ON/OFF switching through alternative thermal and light stimuli. Note that psubstituted carboranes are very robust compounds with high thermal and chemical stability that make them attractive for a wide field of applications [START_REF] Núñez | Electrochemistry and Photoluminescence of Icosahedral Carboranes, Boranes, Metallacarboranes, and Their Derivatives[END_REF] such as heat-resistant materials 24- 28 , thermo or mechanochromism [START_REF] Naito | o-Carborane-Based Anthracene: A Variety of Emission Behaviors[END_REF] , color-tunable devices [START_REF] Mukherjee | Boron clusters in luminescent materials[END_REF][START_REF] Ferrer-Ugalde | Fluorescence of New o-Carborane Compounds with Different Fluorophores: Can it be Tuned?[END_REF][START_REF] Peterson | Carborane-Containing Poly(fluorene): Response to Solvent Vapors and Amines[END_REF] among others. In this report, we extend for the first time the applied potentials of o-carborane derivatives toward two-photon lithography. As depicted in Scheme 1, we present two distinctive dyes with a quadrupolar (ACAR) or octupolar (XCAR) architecture. Each system integrates an organic multibranched core playing the role of a p-electron relay constellated with bulky o-carboranes subunits at its external rims.

Scheme 1.

Molecular structures of the o-carboranes along with their multipolar geometries.

These icosahedral boron clusters are well-known to be strong electron-withdrawing groups with a highly polarizable s-aromatic character [START_REF] Núñez | Electrochemistry and Photoluminescence of Icosahedral Carboranes, Boranes, Metallacarboranes, and Their Derivatives[END_REF] which should efficiently promote a longrange p-electronic delocalization over their entire multipolar structure with subsequent benefit effects on the two-photon absorption cross-sections [START_REF] Terenziani | Enhanced Two-Photon Absorption of Organic Chromophores: Theoretical and Experimental Assessments[END_REF][START_REF] Katan | Effects of (Multi)branching of Dipolar Chromophores on Photophysical Properties and Two-Photon Absorption[END_REF] . We first provide a detailed investigation of the linear and non linear photophysical properties of these multibranched systems and compare their respective two-photon initiating reactivity. We then demonstrate how an additional side reaction inherent to the quadrupolar anthracene-based compound can be judiciously oriented to spatially encode at µm-scale a luminescence commutating functionality into the final materials.

RESULTS AND DISCUSSION.

The one-(1PA) and two-photon (2PA) absorption spectra of the molecules in dichloromethane (DCM) have been superimposed in Figure 1 for a direct comparison of their respective absorption bands. Table 1 gathers their corresponding spectroscopic data. In the low energy region, the 1PA spectrum of ACAR is dominated by a single absorption band with a moderate intensity (eMAX ~ 18 000 M -1 cm -1 ) positioned in the 350-500 nm range. In the same spectral region, the 2PA spectrum clearly displays two well separated bands with a main intensity band at l2PA~ 710 nm exhibits a maximum 2PA cross-section (dMAX) of ca.

21 ± 3 GM. These two 2PA bands presumably echo two distinctive electronic transitions (i.e.

S0→S1 and S0 →S2 transitions [START_REF] Chaari | Luminescence properties of carborane-containing distyrylaromatic systems[END_REF] ) located in the same energy region than those observed for the 1PA band. It should be noted that the lowest energy 2PA band has a very weak intensity with dMAX ~ 3 GM. This low 2PA ability observed for the S0 → S1 transition agrees with the 2PA dipole selection rule indicating that the lowest energy transition of a A-p-A quadrupole is theoretically twophoton forbidden. As shown in Figure 1, the 1PA band of XCAR is blue shifted as compared to that of ACAR but presents much higher intensity of about 7-fold that of its quadrupolar homologue. This 1PA band is very broad and structured presumably due to the occurrence of multiple S0→Sn transitions [START_REF] Cabrera-González | Tetrakis{[(pdodecacarboranyl)methyl]stilbenyl}ethylene: A Luminescent Tetraphenylethylene (TPE) Core System[END_REF] . Similarly to ACAR, the 2PA spectrum of XCAR does not match its 1PA one. This spectral effect is typically observed for centrosymmetric D2d octupolar structures [START_REF] Savel | Metalinduced dimensionality tuning in a series of bipyrimidine-based ligands: a tool to enhance two-photon absorption[END_REF][START_REF] Akdas-Kilig | Luminescence, and Nonlinear Optical Properties of New Bipyrimidine-Based Multifunctional Octupoles[END_REF][START_REF] Zhang | Theoretical Study of One-and Two-Photon Absorption Properties of Octupolar D2d and D3 Bipyridyl Metal Complexes[END_REF][START_REF] Lambert | Synthesis and Nonlinear Optical Properties of Three-Dimensional Phosphonium Ion Chromophores[END_REF] for which the S0 → S1 transition is also theoretically forbidden. As a consequence, the 2PA band positioned at 720 nm should be assigned to the S0 → S2 transition.

The corresponding dMAX is moderate with a value of about 95 ± 13 GM.

eabs The fluorescence spectra of ACAR and XCAR in DCM are also presented in Figure 1. In both cases, the emission spectrum consists in a broad and structureless band located in the 500-750 nm range leading to a very large Stokes shift of about 7500 to 8000 cm -1 . This large spectral shift is a clear evidence of a strong electronic and/or geometrical change between ground and emitting states. In addition, the dyes are weakly emissive. This is especially true for the four-branched derivative with a fluorescence quantum yield (Ffluo) of 6-times lower than that observed for its two-branched homologue (see Table 1). This loss in emissivity is typically observed for p-conjugated multibranched chromophores [START_REF] Terenziani | Enhanced Two-Photon Absorption of Organic Chromophores: Theoretical and Experimental Assessments[END_REF][START_REF] Katan | Effects of (Multi)branching of Dipolar Chromophores on Photophysical Properties and Two-Photon Absorption[END_REF][START_REF] Katan | Two-Photon Transitions in Quadrupolar and Branched Chromophores: Experiment and Theory[END_REF] . It can be assigned to a large amplitude electronic relaxation at S1 state, leading to an electronic symmetry breaking between the unrelaxed Franck-Condon state giving rise to an electronic delocalization that implies a full delocalization over the entire p-conjugated structure and the relaxed emitting state, whose excitation is localized more specifically on a single branch of the dye. Moreover, the molecular dimensionality of the system amplifies the efficiency of this non radiative deactivation process. This latter effect can be nicely exemplified here by comparing the nonradiative-to-radiative rate constants ratios ( knr / kr ) which are multiplied by a factor ~ 7 on going from ACAR to XCAR (see Table 1). Interestingly, the excitation localization on a single branch of the dye can be also accompanied by a concomitant conformational change of the molecular structure. In order to demonstrate this interesting property, we measured the emissive conformers at the ground state since the excitation spectra recorded from both emission bands perfectly match each others suggesting that the two emitting species derive from the same S1 state. It is well known that excited states relaxation is both influenced by the solvent dielectric constant as well as the solvent viscosity [START_REF] Grabowski | Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures[END_REF] . However for 2MTHF, the relative impact of these two parameters can be sequentially discriminated depending on the temperature range. In the high temperature region, the fluorescence bathochromic effects observed for both compounds should be attributed to the enhancement of the medium polarity since the dielectric constant of 2MTHF continuously increases from 7 to 18 upon cooling [START_REF] Furutsuka | Dielectric Constant of 2-Methyltetrahydrofuran[END_REF][START_REF] Goes | Temperature Dependence of Charge-Transfer Fluorescence from Extended and U-shaped Donor-Bridge-Acceptor Systems in Glass-Forming Solvents[END_REF][START_REF] Bublitz | Effective Polarity of Frozen Solvent Glasses in the Vicinity of Dipolar Solutes[END_REF] .

/
In low temperature region nearby the glass-transition of 2MTHF, this dielectric constant drastically drops to 2. ACAR and XCAR have been employed as two-photon initiators for free radical polymerization. With this respect, they have been associated with an aliphatic amine reactant such as the N-methyl diethanolamine (MDEA). In this bi-component approach [START_REF] Fouassier | Photoinitiator for Polymer Synthesis-Scope, Reactivity and Efficiency[END_REF] (see Figure 3), a photoinduced electron transfer (PeT) can be promoted between the twophoton excited chromophore acting as an electron acceptor species and MDEA. Assuming that the photoinduced electron transfer proceeds from the S1 state, the free energy (DGeT) associated to the mechanism can be estimated according to the Rehm-Weller equation: The generated a-aminoalkyl radical exhibits a very high reactivity upon addition reactions [START_REF] Fischer | Factors Controlling The Addition of Cabon-Centered Radicals to Alkenes-An Experimental and Theoretical Perspective[END_REF] on acrylates C=C bonds leading to a very efficient initiation process for free radical polymerization. The formation of MDEA • appears to be a crucial species for the 2PP efficiency and can be identified by ESR-spin trapping measurements. The writing speed performances of our o-carborane-based photoresists have been compared to each other. For this purpose, a series of square frames with 20 µm side were two-photon patterned by gradually increasing the direct laser writing speed in order to estimate the maximum value of the fabrication speed which guarantees a good quality of the model structure. For each formulation, Figure S4 shows a typical transmitted micrograph of each µ-structure with its corresponding writing speed. According to such a qualitative screening test previously proposed by Liska et al. [START_REF] Lunzer | Beyond the Threshold: A Study of Chalcogenophene-Based Two-Photon Initiators[END_REF][START_REF] Li | A Straightforward Synthesis and Structure-Activity Relationship of Highly Efficient Initiators for Two-Photon Polymerization[END_REF][START_REF] Li | Synthesis and structure-activity relationship of several aromatic ketone-based two-photon initiators[END_REF][START_REF] Pucher | Structure-Activity Relationship in D-p-A-p-D-Based Photoinitiators for the Two-Photon-Induced Photopolymerization Process[END_REF] , it can be inferred that the photoresist with XCAR authorizes a maximum writing speed of about 400 µm s -1 which is ~1.8-fold higher than that measured for ACAR formulation in the same conditions. In order to compare more precisely the 2PP performances of ACAR and XCAR, we have also evaluated the polymerization energy threshold (Eth) of their respective isomolar photoresists. This photonic parameter corresponds to the minimum local absorbed-energy density necessary to observe an effective polymerization of the resin. From a practical point of view, the method simply consists in writing several polymer lines into a drop of each resin upon focusing the fs-laser beam using a 40x, 0.95-NA objective. By gradually decreasing the laser excitation power, we can then evaluate the minimum deposited energy below which the 2PP is inefficient. Figure 5 typically shows SEM images of symmetrical patterns of polymer lines written into each formulation at 800 nm. From the progressive disappearance of the polymer lines as function of the laser excitation power we can estimate that the 2PP energy thresholds (Eth) for ACAR and XCAR formulations are about 42 µJ and 20 µJ, respectively. It is also worth noting that the width of the polymer lines generated for each energy threshold displays an average value in the 240-250 nm range for both formulations as depicted in the insets of Figure 5. According to the formalism proposed by Baldacchini et al. [START_REF] Baldacchini | Acrylic-based resin with favorable properties for three-dimensional two-photon polymerization[END_REF] , this 2PP threshold energy (Eth) can be directly connected to the quantum yield for the generation of primary radicals (Fi)

DGeT = Eox -Ered -E00 + C (1)
owing to the following relationship 3, 58 :

(

where C is the concentration of the 2P initiator, [R0] denotes the local concentration of primary radicals necessary to reach the gelation point. This concentration is expected to be invariant for a given acrylate formulation. Considering the fact that the 2PA cross-sections ratio between XCAR and ACAR is about ~ 10 at 800 nm, it can be estimated that ACAR displays a Fi value which is ~ 2.3 times higher than that of XCAR. Despite a much weaker 2PA cross-section, ACAR exhibits a higher initiation efficiency than XCAR. More precisely, the reaction sequence consecutive to the excitation of ACAR is likely to generate a much substantial amount of MDEA • available for the initiation reaction. In all these reaction sequences, it should be underlined that a significant part of MDEA • is consumed by the competing inhibition reaction with oxygen (O2). Indeed, this latter species which is initially present in the resin is extremely reactive toward the a-aminoalkyl radicals. For instance, the [ ]

2 0 . . th i E C R F µ d
inhibition rate constant of MDEA • by oxygen is typically two orders magnitude higher than that of the free radical addition onto acrylate C=C bonds [START_REF] Ligon | Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization[END_REF] . In this context, any side reactions allowing the local consumption of dissolved O2 would indirectly enhance the photoinitiation performance. We are precisely in this case for ACAR. This chromophore indeed comprises a 9, 10-disubstitued anthracene subunit, which is prompt to trap oxygen molecule via a wellknown [4 + 2] photo cycloaddition [START_REF] Aubry | Reversible Binding of Oxygen to Aromatic Compounds[END_REF][START_REF] Clennan | Synthetic and mechanistic aspects of 1,3-diene photooxidation[END_REF] . Similarly to the 9,10-diphenyl anthracene [START_REF] Aubry | Reversible Binding of Oxygen to Aromatic Compounds[END_REF][START_REF] Fudickar | Reversible Light and Air-Driven Lithography by Singlet Oxygen[END_REF] , the first step of the mechanism (see Scheme 2) should imply the photogeneration of a singlet oxygen ( 1 O2) through a triplet-triplet energy transfer between the excited triplet state of ACAR and the oxygen molecule ( 3 O2).

Scheme 2.

Global mechanism of the [4+2] photoinduced cycloaddition of 1 O2 on ACAR as well as the thermolysis of the generated endoperoxide product.

The quantum yields for singlet oxygen generation (FD) have been evaluated for both o-carboranes in ethanol using the well known photoxidation method involving the 1,3diphenylisobenzofuran (DBF) used as a very efficient 1 O2-sensitive indicator [START_REF] Derosa | Photosensitized singlet oxygen and its applications[END_REF] . In this methodology, Rose Bengal (RB) was also employed as a reference photosensitizer (FD r = 0.68 in ethanol [START_REF] Derosa | Photosensitized singlet oxygen and its applications[END_REF] ). Isoabsorbant solutions of o-carboranes and RB were irradiated at 480 nm in presence of DBF. Figure S5 typically shows the absorbance changes of DBF at 425 nm for different short irradiation times. The value of FD / FD r can be then derived from the ratio of the corresponding slopes at the initial time of irradiation. According to this photoxidation method, ACAR presents a quantum yield for singlet oxygen generation of about 0.05 whereas the corresponding FD for XCAR is ~ 3-fold lower. Therefore, the excitation of ACAR should lead to a more consumption of dissolved O2 as compared to the corresponding photoreaction with XCAR. Interestingly for ACAR, the photogenerated singlet oxygen can be chemically trapped according to a second step reaction depicted in Scheme 2. This second reaction sequence consists in a cycloaddition of 1 O2 into the bridgehead meso positions of ACAR with a consecutive production of an endoperoxide. In order to demonstrate the high sensitivity of ACAR to oxygen as compared to XCAR, the photolysis of the dyes has been performed in acetonitrile solutions where the concentration of dissolved O2 was alternatively reduced and increased upon N2 or air-bubbling. Figure 6 shows the absorbance time-dependent changes of ACAR and XCAR at their respective lMAX during the photolysis reactions at lirr. = 365 nm.

First, one clearly observes that the absorption spectrum of XCAR remains invariant irrespective to the presence or absence of oxygen. This is a clear indication that XCAR does not undergo any photobleaching under our irradiation condition. Under our heating conditions, the thermal recovery half-time is about 30 min in DMSO (see

Figure S6B

). As a consequence, ACAR constitutes a very interesting two-photon initiator able to meanwhile implement a switchable emissive property into the two-photon patterned materials. As an illustrating example, Figure 7 shows the fluorescence images from two 2D

µ-grids composed of periodic 5 x 5 µm squares two-photon fabricated using ACAR and XCAR formulations. The XCAR-based µ-structure is used as a reference system. The fluorescence emission collected from the µ-grids presumably stems from the remaining dyes embedded into the polymer. As depicted in Figure 7A, a complete photobleaching of the dyes is achieved when illuminating the µ-structures during 3 min at 365 nm using a 40x, 0.65-NA objective (I365 nm = 350 kW cm -2 ). These two samples were then heated at 130 °C during 5 min. While XCAR µ-structure does not display any fluorescence turn-on, a clear recovery of the luminescence signal is nicely observed for the ACAR-embedded µ-grid. It should be also noted that further heating of the XCAR sample even at higher temperature (e.g. 160 °C) did not lead to any fluorescence return. Note that this irreversible photobleaching indirectly highlights the crucial role of the disubstituted anthracene group within ACAR structure. This molecular subunit indeed guarantees the reversible formation of the endoperoxide. Multiple cycles of alternative light-and thermal-stimuli were applied to the ACAR µ-grid. As shown in Figure 7B, the switchable materials exhibit a very good resistance fatigue after 10 cycles. Figure S7 shows much more ON/OFF cycles (e.g. 40 cycles) highlighting the robustness and the nice reversibility of the observed commutation. Hence one clearly demonstrates that this original two-photon stereolithography approach allows the 'one pot' direct laser writing of switchable luminescent µ-devices. To illustrate the promising applied potential of our micropatterned strategy in the domain of optical data storage, a 2D matrix composed by an array of 3 x 3 µ-slabs was two-photon patterned using ACAR formulation.

Each of these nine square slabs with 5 µm side and 1 µm thick can be considered as an independent optical memory unit whose luminescent binary state can be switched via thermal (ON) or light (OFF) stimuli allowing the reversible encoding of the 2D matrix.

CONCLUSION.

The linear and nonlinear photochemical properties of two multibranched o-carboranes chromophores have been thoroughly investigated with the objective to account for their two-photon induced reactivity which was oriented toward 3D stereolithography applications.

More particularly, the comparison of the dyes 2PP performances lays the stress on an intrinsic superior reactivity of the quadrupolar o-carborane derivative as compared to the octupolar homologue, which paradoxically displays a 10 times higher 2PA cross-section. The design of the π-system of ACAR bearing carborane acceptor moieties origins a triplet excited state that makes ACAR an excellent triplet oxygen scavenger. This unexpected effect stems from an original side reaction specific to the anthracenic quadrupole which can trap the surrounding oxygen via a [4 + 2]-cycloaddition. The subsequent local consumption of the dissolved oxygen in the resin parallely reduces the oxygen inhibition role toward the free radical twophoton polymerization. We demonstrate that this cycloaddition reaction not only improves the 2PP efficiency but also generates an embedded endoperoxide product within the micropatterned polymer. This endoperoxide can regenerate the luminescent o-carborane quadrupole upon heating resulting in a fluorescence 'switch-on' of the fabricated µ-structures.

Stability of these compounds for materials applications is essential, and the present of boron clusters enhance this robustness. Based on this simple 'one pot' processing approach where all the added value is implemented on the anthracene-based o-carborane dye, we have thereby developed an original optical data storage strategy allowing the direct laser writing of switchable materials with robust writing-reading-erasing properties.

EXPERIMENTAL SECTION.

4.1 Materials. The synthesis and the characterization of ACAR and XCAR which have been previously described elsewhere [START_REF] Chaari | Luminescence properties of carborane-containing distyrylaromatic systems[END_REF][START_REF] Cabrera-González | Tetrakis{[(pdodecacarboranyl)methyl]stilbenyl}ethylene: A Luminescent Tetraphenylethylene (TPE) Core System[END_REF] are also detailed in the supporting informations.

Polyethylene glycol diacrylate monomer (PEGDA, Mn 575) and pentaerythritol triacrylate (PETIA), N-methyl diethanolamine (MDEA), n-tetrabutylammonium hexafluorophosphate (TBAPF6), 1,3-diphenylisobenzofuran (DBF) and Rose Bengal (RB) were purchased from Aldrich. All the solvents which are spectroscopic grade were also purchased from Aldrich. Where S is the detected two-photon excited fluorescence integral area, c the concentration of the chromophores, and F is the fluorescence quantum yield of the chromophores. η is the collection efficiency of the experimental set-up and accounts for the wavelength dependence of the detectors and optics as well as the difference in refractive indices between the solvents in which the reference and sample compounds are dissolved. The measurements were conducted in a regime where the fluorescence signal showed a quadratic dependence on the intensity of the excitation beam, as expected for two-photon induced emission. For the calibration of the two-photon absorption spectra, the two-photon excited fluorescence signal of each compound was recorded at the same excitation wavelength as that used for standards (i.e. lexc = 782 nm for fluorescein). The laser intensity was in the range of 0.2-2 x 10 9 W/cm 2 .

The experimental error on the reported cross section is 15 %. 4.7 Two-Photon lithography. The 3D lithographic microfabrication was carried out using a Zeiss Axio Observer D1 inverted microscope. The two-photon excitation was performed at 800 nm using respectively a mode-locked Ti: Sapphire oscillator (Coherent, Chameleon Ultra II: pulse duration: ~140 fs; repetition rate: 80 MHz). The incident beam was focused through a 0.95 NA objective (40 X) which leads to a radial spot size 450 nm at lexc = 800 nm (1/e 2 Gaussian). A drop of the resin is deposited on a cover slip which is mounted on a 3D piezoelectric stage allowing the translation relative to the laser focal point. The intensity of the entering laser is controlled with the use of an acousto optic modulator. The displacement of the sample and all photonic parameters (i.e. excitation power and irradiation times) are computer-controlled. The 3D microstructure is finally obtained by washing away the unexposed monomer resin using ethanol.

NOTES.

The authors declare no competing financial interest.

ASSOCIATED CONTENT

Figure 1 .

 1 Figure 1.One-(full lines) and two-photon (circles) absorption spectra of the dyes in DCM. The fluorescence spectrum of each compound in DCM has been also added.

  of the compounds in a glass forming solvent such as 2methyltetrahydrofuran (2MTHF).Figure 2 shows the fluorescence bands evolution of the dyes in 2MTHF by decreasing temperature. From 300 K to 80 K, the fluorescence quantum yields of ACAR and XCAR are respectively amplified by a factor 5 and 17 concomitantly with the decrease of their Stokes shift by a factor 4 and 2.5.

Figure 2 .

 2 Figure 2.Fluorescence spectra of ACAR and XCAR in 2MTHF over temperature range 80-300 K. Insets: Arrhenius plot for the nonradiative-to-radiative rate constants ratio.

Figure 2 .

 2 6 and remains constant whereas the viscosity enhances by 17 orders of magnitude up to 3.5 x 10[START_REF] Lee | Direct Metal Writing and Precise Positioning of Gold Nanoparticles within Microfluidic Channels for SERS Sensing of Gaseous Analytes[END_REF] Pa s -1 . As a consequence, the strong fluorescence enhancement observed in low temperature region should be mainly attributed to a viscosity-induced hindering effect which drastically hampers any conformational changes at S1 state. With this respect, a trans-to-cis photoisomerization hypothesis can be first excluded since the absorption spectra of our stilbenyl chromophores remain perfectly invariant upon UVirradiation in N2-degassed solutions. Similarly to the twisting relaxation mechanism established by Lapouyade et al.[START_REF] Letard | Structure-photophysics correlations in a series of 4-(dialkylamino)stilbenes: intramolecular charge transfer in the excited state as related to the twist around the single bonds[END_REF][START_REF] Lapouyade | Photophysics of donor-acceptor substituted stilbenes. A time-resolved fluorescence study using selectively bridged dimethylamino cyano model compounds[END_REF][START_REF] El-Gezawy | Solvatochromic Behavior of Donor-Acceptor-Polyenes: Dimethylamino-cyano-diphenylbutadiene[END_REF] for donor-acceptor substituted stilbenes, we proposed that the photoinduced conformational change observed for ACAR and XCAR proceeds via an intramolecular twisting mechanism from a local excited state with a nearly planar stilbene conformation to a perpendicular one implying the rotation of the external o-carborane benzyl moiety around the central C-C bond as illustrated in Scheme 1 (Θ ≈ 90°). This twisted geometry is expected to present a forbidden optical transition which accounts for the non-or weakly emissive character of this relaxed excited state. The proposed precursor-successor mechanism is in line with the dual emission of ACAR whose band intensities reversely evolve by decreasing temperature. The Arrhenius plots of ln (knr / kr) vs. 1 / T are also presented in For both dyes, the correlations are sufficiently linear in the low temperature region to derive activation energies in the 1.6-1.8 kcal mol -1 range. These values nicely coincide with the viscosity barrier of 2MTHF (~ 1.82 kcal mol-1 49 ) and corroborates the barrierless character of these twisting conformation changes.

  Figure S3). Therefore DGeT can be estimated to values of -0.17 eV for ACAR and -0.14 eV for XCAR. These negative driving forces reasonably suggest that both PeT are thermodynamically allowed. Interestingly, the consecutive oxidation of MDEA generates an unstable amine-centred radical cation (MDEA •+ ) which rapidly deprotonates to form a neutral radical species 51 (MDEA • ).

Figure 3 .

 3 Figure 3. (Left side) Two-photon polymerization mechanism based on a bi-component free radical initiation strategy. (Right side) (a) ESR spectrum of spin trap adducts generated after irradiation of ACAR or XCAR mixed with MDEA in presence of PBN. (lirr. = 365 nm. Solvent: tert-butyl benzene) (b) Simulated spectrum.

Figure 3

 3 typically shows the ESR-spectra of spin adducts which were produced during the irradiation of each bicomponent system (dye + MDEA) in presence of a-phenyl-N-tertbutylnitrone (PBN) used as a spin trap reactant. Both ESR spectra are dominated by the signal of the α-aminoalkyl adduct of PBN showing an hyperfine coupling constants (aN = 14.4 and aH = 2.2 G) which agree very well with those previously reported 53 .To demonstrate the applied potential of our multibranched o-carborane derivatives for multiphoton 3D stereolithography applications, each bi-component system has been associated with an acrylate-based formulation comprising 67 mol% of a poly(ethyleneglycol)diacrylate monomer (PEGDA), where the carborane-based molecules were perfectly soluble at mM range and 33 mol% of pentaerythritol triacrylate cross-linker (PETIA). The addition of this latter trifunctional monomer should guarantee a better mechanical and geometrical integrity of the two-photon patterned structures. The formulations were prepared at isomolar concentration of dyes (1.5 mM) and with 3 %-w of MDEA. Figure4shows several SEM images depicting various 3D µ-structures with arbitrary geometries fabricated at 800 nm: i) A polymer boat rudder with a 10 µm diameter fabricated using ACAR formulation. ii) A 15 µm diameter pumpkin-like object fabricated based on XCAR formulation using the same condition iii) A 35 µm-high screw with 1 µm pitch photopatterned with the same XCAR formulation. Interestingly, each µ-structure displays a surface roughness which illustrates the point-by-point fabrication procedure. Indeed, each 3D microstructure is obtained through a layer-by-layer assembly process obtained by z-stacking of several fused photopatterned planes. In the same manner, these polymerized planes are fabricated by two-photon curing along the scanning paths of the laser beam. The minimum polymerised volume element ('voxel: volume pixel') generated after the beam translation should fused with its adjacent voxel to ensure the mechanical integrity of the slice. It is precisely such voxel overlapped effects that create roughness at the external surface of the 3D microstructure.

Figure 4 .

 4 Figure 4. SEM micrographs of various two-photon polymerized µ-structures fabricated at 800 nm using ACAR (A) and XCAR (B-C) formulations (scale bars = 5 µm).

Figure 5 .

 5 Figure 5. SEM micrographs of 2PP lines fabricated for distinctive laser excitation powers (lex = 800 nm). The energy value in red color denotes Eth for each formulation. Inset : Magnified SEM images of the polymerized lines generated at Eth. Formulation: Isomolar concentration of dye (1.45 mM) and 3 %-w MDEA. The resin is an acrylatebased mixing of PEGDA (67 mol%) and PETIA (33 mol%).

Figure 6 .

 6 Figure 6. Time-dependent changes of the absorbances of the dyes in acetonitrile solutions in presence or absence of dissolved O2. Insets: Evolution of the absorption spectrum of the dyes for distinctive irradiation times. (lirr. = 365 nm, P = 120 mW).

Figure 7 .

 7 Figure 7. A. Comparison of the fluorescence switch processes between ACAR-and XCARbased two-photon patterned µ-grids upon a consecutive light-triggered (lirr. = 365 nm during 3 min.) and thermal (130 °C during 5 min) cycle. B. Multiple fluorescence turn-off and turn-on cycles for ACAR µ-grid upon alternative light-and thermalinduced stimuli (lirr. = 365 nm, 3 min // D = 130 °C, 5 min)

Figure 8 .

 8 Figure 8. Fluorescence images of a 3 x 3 matrix composed of switchable ACAR-embedded µ-slabs (5 µm side) during several distinctive writing-erasing cycles. The writing step corresponds to the irradiation of selected µ-slabs at lirr. = 365 nm during 3 min. The erasing step consists in 5 min heating of the glass coverslip sample at 130 °C.

4. 2 4 . 3 4 . 4 4 . 6

 2434446 photomultiplier from Hamamatsu with high sensitivity photon-counting mode. The decays were fitted with the iterative reconvolution method on the basis of the Marquardt/Levenberg algorithm[START_REF] Connor | Time correlated single photon counting[END_REF] . Such a reconvolution technique allows an overall-time resolution down to 0.2 ns.The quality of the exponential fits was checked using the reduced c 2 (£ 1.2)

4. 8 4 . 9

 849 Fluorescence image microscopy. The multiple cycles relative to the fluorescence turn-on / turn-off from the two-photon patterned µ-structures were monitored using fluorescence microscopy. The glass coverslip with the µ-structures was positioned on an Olympus IX73 inverted microscope equipped with a 75 W Xe lamp housing. The sample was then excited from the bottom of the sample at lexc. = 365 nm through a 40X, 0.65-NA objective using an Olympus U-FUN fluorescence mirror unit composed of a 365 nm band pass filter (BP360-370), a dichroic mirror (DM410) and long pass filter (BA420IF). The epifluorescence signal from the photopatterned sample is collected by the same objective, passes through the same fluorescence mirror unit and collected by a CMOS camera (ORCA-Flash4.0 from Hamamatsu). For each light-triggered fluorescence turn-off step, the timedependent emission decrease from the excited sample was recorded through a series of timelapse epifluorescence images. For each fluorescence turn-on step, the sample was first heated during 5 min at 130 °C on a heating plate then similarly repositioned on inverted microscope for the next turn-off step. Cyclic voltammetry. The redox potentials of the compounds were measured by cyclic voltammetry using a Radiometer Voltalab 6 potentiostat. All measurements were performed at 300 K in N2-saturated dichloromethane (DCM) with 0.1 M of TBAPF6 used as supporting electrolyte. The cyclic voltammograms (CVs) were recorded using a three-electrode cell with a platinum disk as working electrode. A platinum wire was used as auxiliary electrode. A saturated calomel electrode (SCE) used as a reference was placed in a separate compartment with a salt bridge containing the supporting electrolyte. Ferrocene (Fc) was used as an internal reference (considering EFc/Fc+ = + 0.53 V in DCM vs. aqueous SCE 71 ).

  Information. Fluorescence spectra of ACAR and XCAR in high and low temperature regions of 2MTHF. Cyclic voltamogramms of o-carboranes and MDEA in dichloromethane. Screening test structure to evaluate the maximum two-photon writing speeds of ACAR and XCAR formulations. Determination of quantum yields singlet oxygen generation of o-carborane derivatives in ethanol. Fluorescence and absorption spectra recovery of ACAR by heating a solution of its endoperoxide in DMSO at 130°C. Multiple ON/OFF fluorescence switches from an ACAR embedded µ-grid during alternative light-and thermal-induced stimuli. Synthesis and characterisation of ACAR and XCAR.

Table 1 .

 1 Spectroscopic data of o-carborane compounds in DCM.

		10 3 M -1 cm -1	labs / nm	lfluo / nm	E00 (a) / eV	Ffluo tfluo / ns	kr (b) / 10 9 s -1	knr (b) / 10 9 s -1
	ACAR	18.5	417	605 2.70 0.12 1.23	0.10	0.72	7.2
	XCAR	132.2	347, 395 (s) 577 2.75 0.02 0.64	0.03	1.53	51
	(a) E00 ≈ ½ hc(nabs + nfluo) in glassy matrix of 2MTHF. (b) kr = Ffluo / tfluo , knr = (1-Ffluo) / tfluo.

TABLE OF CONTENTS ENTRY.

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ACKNOWLEDGMENTS

This work was financially supported by MICINN (PID2019-106832RB-I00 and the Severo Ochoa Program for Centers of Excellence for the FUNFUTURE CEX2019-000917-S project) and by Generalitat de Catalunya (2021-SGR00442)