Mónica Farfán-Paredes

Oscar González-Antonio

Diana E Tahuilan-Anguiano

Jorge Peón

Armando Ariza

Pascal G Lacroix

Rosa Santillan

Norberto Farfán

Physicochemical and computational insight of 19 F NMR and emission properties of meso -( o -aryl)-BODIPYs

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The family of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene derivatives, also known as BODIPYs, are strongly UV-absorbing small molecules that emit relatively sharp fluorescence peaks. [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] These compounds are versatile dyes of considerable interest owing to their photochemical stability, synthetic versatility and good solubility in common organic solvents. Moreover, they show large extinction coefficients and tunable quantum yields. [START_REF] Lakshmi | Synthesis, structure and spectral and electrochemical properties of 3-pyrrolyl BODIPY-metal dipyrrin complexes[END_REF][START_REF] Loudet | BODIPY Dyes and Their Derivatives : Syntheses and Spectroscopic Properties[END_REF][START_REF] Kálai | Synthesis of new, BODIPY-based sensors and labels[END_REF][START_REF] Choi | Aggregation-induced emission enhancement of a meso-trifluoromethyl BODIPY via Jaggregation[END_REF] For these properties, BODIPY-based dyes have been exploited as fluorescent probes, [START_REF] Guo | Highly selective fluorescent OFF-ON thiol probes based on dyads of BODIPY and potent intramolecular electron sink 2,4-dinitrobenzenesulfonyl subunits[END_REF] molecular sensors, [START_REF] Bura | Highly substituted Bodipy dyes with spectroscopic features sensitive to the environment[END_REF] semiconducting materials, [START_REF] Xochitiotzi-Flores | On the structure of meso-substituted F-BODIPYs and their assembly in molecular crystals: An experimentaltheoretical approach[END_REF][START_REF] Singh | Evolution of BODIPY Dyes as Potential Sensitizers for Dye-Sensitized Solar Cells[END_REF][START_REF] Ozdemir | Solution-Processable BODIPY-Based Small Molecules for Semiconducting Microfibers in Organic Thin-Film Transistors[END_REF] biological labelling, [START_REF] Dale | New potent, short-linker BODIPY-ARTICLE[END_REF] viscosity probes [START_REF] Xochitiotzi-Flores | Optical properties of two fluorene derived BODIPY molecular rotors as fluorescent ratiometric viscosity probes[END_REF] and photosensitizers in photodynamic therapy. [START_REF] Awuah | Boron dipyrromethene (BODIPY)-based photosensitizers for photodynamic therapy[END_REF] Another advantage is that it is possible to tune their spectroscopic and photophysical properties functionalizing adequately any position at the core including boron substituents. [START_REF] Ziessel | The chemistry of Bodipy: A new El Dorado for fluorescence tools[END_REF] The BODIPY core has two pyrrole rings fused with a bridging carbon known as the 8 position or meso position and a BF2 unit closing a third ring. The structure is co-planar with the electronic density distributed between three rings, while the two fluorine atoms are in a perpendicular plane. [START_REF] Tram | The synthesis and crystal structure of unsubstituted 4,4difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)[END_REF] Another characteristic of BODIPY dyes related to their photophysical properties is intramolecular rotation. The derivatives with a phenyl ring at meso position have freedom of movement and are considerably less fluorescent in comparison with bulky substituents which have restricted movement. [START_REF] Merkushev | BODIPY associates in organic matrices: Spectral properties, photostability and evaluation as OLED emitters[END_REF] A wide variety of groups have been introduced at the meso position of the BODIPY core in order to study their influence over the photophysical properties, [START_REF] Bañuelos | Modulation of the photophysical properties of BODIPY dyes by substitution at their meso position[END_REF] however, the influence of substituents over fluorine atoms have rarely been studied. [START_REF] Benniston | Molecular rotors based on the boron dipyrromethene fluorophore[END_REF] In this work, different substituents were introduced at the ortho-position of the meso-phenyl moiety in order to study their influence over fluorine atoms by [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR as well as its photophysical properties in different solvents. Computational analyses for electrostatic interactions, hardness/softness and dipole moment were carried out and compared with experimental spectroscopic and photophysical results. Furthermore, the relationship between [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F chemical shifts difference ΔδF of the BODIPY derivatives with the group electronegativity and the σ-Hammett coefficients were investigated. Please do not adjust margins Please do not adjust margins BODIPYs 1-11 were synthesized using the sequence of reactions depicted in Scheme 1. BODIPYs 1-3 were prepared to compare with those that have ortho-substituents at the phenyl moiety.

Results and Discussion

Synthesis and NMR characterization

Additionally, a BODIPY with a bulky group (pyrene) 11 was also synthesized. All compounds were characterized by [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] H and [START_REF] Awuah | Boron dipyrromethene (BODIPY)-based photosensitizers for photodynamic therapy[END_REF] C, Scheme 1. General procedure for synthesis of BODIPY derivatives 1 -11. [START_REF] Dale | New potent, short-linker BODIPY-ARTICLE[END_REF] B and [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR which provided valuable information concerning the symmetry of the molecule.

Compounds 1-3 showed a typical triplet signal in [START_REF] Dale | New potent, short-linker BODIPY-ARTICLE[END_REF] B NMR and a characteristic quartet in [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR due to [START_REF] Dale | New potent, short-linker BODIPY-ARTICLE[END_REF] B- [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F couplings [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] JB,F = 28 Hz, which is in accordance with the literature. [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] The 19 F NMR spectra for these compounds showed that there is a symmetry plane in the molecule and therefore fluorine atoms are equivalent. The introduction of ortho substituents at the phenyl moiety or a bulky group like pyrene restricts the freedom of movement at the meso position and therefore the chemical environment for each fluorine atom is different, as evidenced in [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR. A similar example was reported for a BODIPY having a phenanthrene unit at the meso-position. [START_REF] Benniston | Molecular rotors based on the boron dipyrromethene fluorophore[END_REF] The 19 F NMR spectra of BODIPYs 4-11 showed a doublet of quartets for each fluorine atom due to [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F, [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F coupling (doublet) and [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F, [START_REF] Dale | New potent, short-linker BODIPY-ARTICLE[END_REF] B coupling (quartet). As might be expected, the chemical shift difference between the fluorine atoms (∆δF) varies with the substituent (Fig. 1). The chemical shift is dependent on the electronical environment that surrounds an atom, thus, the fluorine atom in the opposite side of the ortho-substituent is de-shielded and appears to high frequency, while the fluorine atom in the same side of the ortho-substituent is shielded and its signal appears to low frequency. The 11 B-decoupled [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR spectra denoted as [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F{ 11 B} were also recorded showing a single signal at -145.18 ppm for BODIPY 2 with a -OMe group at the para position (Fig. 2a). On the other hand, BODIPY 5 with the -OMe group at the ortho position of the phenyl moiety appears as an AB signal (Fig. 2b). The 19 F{ 11 B} spectrum of 5 showed a doublet corresponding to one of the fluorine atoms at -144.5 ppm (denoted as Fa) and a doublet for the second fluorine atom (denoted as Fb) at -145.6 ppm with a chemical shift difference of 1.1 ppm and a coupling constant [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F, [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F of 2 JFa,Fb = 107 Hz.

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The fact that the rotation of the -OMe substituted phenyl is hindered leads to fluorine atoms that are not equivalent and this kind of compounds can be used to form atropisomers. 20 11 B-decoupled [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR spectra were determined for BODIPYs 4-11 (See supporting information). The obtained chemical shifts and coupling constants of BODIPYs 4-11 are listed in Table 1. Compound 4 with the methyl group in the ortho position of the phenyl moiety showed the smallest chemical shift difference (∆δF = 0.69 ppm), while BODIPY 10 which has a nitro group showed the largest chemical shift difference (∆δF = 2.39 ppm). The [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR spectra in toluene-d8 (ESI † Fig. S31), determined in the range from 30°C to 90°C, show a 0.6 ppm lowfrequency shift but maintained the ∆δF, which means the rotation barrier is > than 16.5 kcal/mol.

Photophysical Properties

Photophysical properties such as absorption maximum (λabs max), emission maximum (λemis max), Stokes shift (Δῡ) and fluorescence quantum yields (ФF) were measured in four different solvents (See Supporting Information, Table S1). The normalized absorption and emission spectra of selected BODIPYs in toluene are shown in Fig. 3.

Regardless of the nature of the meso-aryl substituent, the BODIPYs show an origin band around 500 nm that corresponds to S0 → S1 (π-π*) transition which is slightly redshifted in nonpolar solvents such as hexane and toluene. The shoulder at lower wavelength is due to the 0 -1 vibrational transition. The value of the fluorescence quantum yield is much lower in BODIPYs 1-3 (highest value ФF = 0.11 for 2 in toluene) compared to BODIPYs 4-11 (highest value ФF = 1.05 for BODIPY 4 in toluene, the value greater than one is relative to the standard Rhodamine 6G) . [START_REF] Kubin | Fluorescence quantum yields of some rhodamine dyes[END_REF] The ortho substituents at the phenyl moiety restrict freedom of movement promoting the radiative relaxation pathway. BODIPY 5 with a -OMe group at the ortho position has a fluorescence quantum yield four times larger (ФF = 0.20) than BODIPY 2 with the -OMe group at the para position (ФF = 0.05) in hexane. BODIPY 10 with the nitro group showed the smallest values (ФF = 0.01 or less) due to their highly efficient intersystem crossing that quenches the fluorescence. [START_REF] Collado-Fregoso | Excited-state dynamics of nitrated push-pull molecules: The importance of the relative energy of the singlet and triplet manifolds[END_REF] The fluorescence spectra are strongly dependent on the polarity of the solvent. Thus, the fluorescence quantum yields are quenched in polar solvents due to electron transfer process . The largest Stokes shift is 2688 cm -1 for compound 11 in toluene. A plausible explanation for this result is that the maximum emission in toluene for this compound is 587 nm, this red-shifted emission is due to the possible formation of excimers in pyrene. [START_REF] Winnik | Photophysics of Preassociated Pyrenes in Aqueous Polymer Solutions and in Other Organized Media[END_REF] In the absorption spectra, the band in the 300 -350 nm range shows the characteristic vibronic fine structure of pyrene. Please do not adjust margins Please do not adjust margins Geometry optimizations yielded the global energy minima for the studied BODIPY derivatives, which helped determine that for all the synthesized aryl-substituted compounds, there is a torsion that yields a pronounced dihedral angle between the well-known planar BODIPY moiety and the o-aryl plane. Being both planar portions, that characteristic allows to automatically define the dihedral angle.

For the meso-phenyl-BODIPY 1 whose substituent would be regarded as the reference for the study, that means hydrogen, such dihedral angle (highlighted cyan atoms in Fig. 4) was calculated as 54.9° and is the smallest among all the orthosubstituted systems. The rest of the derivatives show values ranging from 60.6° for fluorine up to 117.7° for the pyrenylsubstituted compound. Fig. 4 gives some examples (compounds 1, 4, 7 and 9) of the calculated structures adding another important feature to the description. The dipole moment vector shows a deviation from the BODIPY plane. This divides the studied species into two sets: the first one includes those derivatives in which the dipole moment vector deviates from the BODIPY plane towards the functional group of interest (H, pyrene, OH and OCH3 described with a negative angle) and the other set contains those derivatives in which this vector deviates opposite to the location of the highlighted functional group (CH3, F, Cl and NO2, described with a positive angle). The remaining compounds can be found within the supporting information (S17-19).

Evaluation of the rotational energy barriers and their influence on the fluorescence quantum yield

Starting from the optimized geometries and the energy minima for ortho-substituted species, and the meso-(p-OMe-phenyl)-BODIPY (2) for comparison reasons, the rotational scan provided the torsion angle at which the energy is maximum, and then a transition state TS optimization was carried out to obtain the actual structure for the rotational TS (TSrot) for obtaining the energy of the rotational barrier for each compound.

The complete series of rotational scans (S20) allowed us to divide the studied species in three groups, one group (compounds 1, 2, 5, 7 and 9) where the dihedral angle for the ground state (GS) conformation was below 90°. For these species, when the systems reach a dihedral angle of 90° this value corresponds to a local maximum during the scan. These local maxima differ only by 2.5 kcal/mol and, therefore, they could be considered conformers in equilibrium. The second group (compounds 4, 10 and 11) where the GS conformation possesses a dihedral angle larger than 90° and possess one true maximum at the TSrot. The final is formed only by the chloridecontaining species, whose GS conformation has a dihedral angle in the vicinity of 90°and presents no local minimum. TSrot structures are shown for selected species in Fig. 5, alongside the rotational barriers for all calculated species, the dihedral angle where the energy maximum is reached, and their corresponding fluorescence quantum yield determined experimentally in this work. The calculated structures evidence that the molecule needs to undergo a deformation from the BODIPY plane in order for the meso substituent to rotate. All other TSrot geometries can be found in the Supporting Information (S30).

A detail not to be disregarded, is that for the o-hydroxyl group two conformers where studied, as shown in Fig. 6; one where the OH points towards the BODIPY core and another one pointing away from it. Energy differences for both, the optimized energy minima (1.14 kcal/mol) and their corresponding TSrot (0.97 kcal/mol) were small and, therefore, considered as equilibrium conformers. From these results there are several points to be considered.

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Please do not adjust margins By plotting the vertical excitations for selected points of the rotational scan and comparing the behavior in energy for both S0 and S1 states (the complete series is available from ESI S20 to S29 together with the plots of the oscillator strengths of the corresponding excitations as a function of dihedral angles) we could observe important elements. It is important to note that in all cases, S1 decreases in energy as the dihedral angle approaches the dihedral angle of maximum energy in the scan, meaning that, as the meso substituent attempts the rotation through the BODIPY plane and the conformation requires more energy to be reached, the energy required for the S0 → S1 decreases, making this transition more energetically available.

But the energy differences between the rotational barrier and its corresponding vertical excitation vary in three different trends.

The first trend is for the species with known rotation through the BODIPY plane within the conditions and timescales of the performed photophysical studies (reinforced by magnetic equivalent fluorine atoms in the BODIPY core). For such compounds, the energy differences are all bigger greater? than 45 kcal/mol. As can be seen in Fig. 7, compound 2 reaches its rotational barrier at 181° in dihedral angle, still has not reached the lowest value for the vertical transition towards S1. This means the phenyl plane rotates through the BODIPY plane first (explaining the equivalence of fluorine atoms in the BODIPY core seen in NMR experiments), and after the phenyl portion rotates through the BODIPY plane, at a dihedral angle of 186° the transition now reaches the lowest point in the curve, having no more geometric hindrance, the molecule can favor the emission process, which explains the enhanced fluorescence quantum yield. This same phenomenon could be observed for the p-aminophenyl computed species.

The next visible trend applies for compounds 4, 5, 8 and 9. These species follow the pattern showed in Fig. 7 for meso-tolyl-BODIPY, 4. The vertical excitations reach a minimum right in the point where the TSrot occurs. But for 5 and 9, the energy difference that goes from the rotational barrier in the electronic GS towards S1 is larger than for 4 and 8, whose values are amongst the lowest, providing an explanation for their high quantum yields. Whilst all these species possess larger values of quantum yield, the lower the difference between S0 (at the TSrot geometry) and S1 (at the dihedral angle of the TSrot geometry), the larger the quantum yield. Compounds 7, 10 and 11 are all particular cases. For the hydroxy-bearing species, as depicted in Fig. 7 the lowest value for the vertical transition S0 → S1 takes place before (dihedral angle of 185°) the TSrot geometry is reached (190°). Which means that the geometry needed for such transition to be most favored is less sterically constrained that the TSrot. The only detail is that the vertical excitation energy is higher than in the case of 4 and 8 and this explains the enhanced quantum yield compared to 1, but not as high as the meso-o-chlorophenyl and meso-tolyl derivatives.

In the case of 10, the comparison between the rotational scan and the vertical excitation energies towards S1, behaves in such a way that the most favored vertical transition occurs at the TSrot geometry and the differences is in the range of the highest emission compounds, which would mean a high quantum yield for nitro-bearing compounds. But, as it is known [START_REF] Collado-Fregoso | Excited-state dynamics of nitrated push-pull molecules: The importance of the relative energy of the singlet and triplet manifolds[END_REF] and we have mention before, the highly efficient intersystem crossing of nitro group keeps this compound outside efficiently emitting.

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The last case is 11, where we can observe that there is severe steric restriction, providing the highest rotational barrier, for this compound, the curve of vertical excitations towards S1, show the existence of four geometries near TSrot that would provide the most favored vertical excitation and the lowest value occurs before the dihedral angle reaches the geometry for TSrot. This vertical excitation is in the range of compounds 5, 7 and 9. It is important to mention that the excited state of the pyrenyl-containing derivative, should possess the largest number of vibrational modes and in spite of having more than a five-fold increase in quantum yield, those vibrational modes give rise to underlying states that hinder the radiative pathways of relaxation from the excited state. The peculiarity observed for meso-pyrene-BODIPY is reflected by the behavior of the oscillator strengths of the calculated vertical excitations, being the only species that when reaching the most favored values for the S0 → S1 transition the oscillator strengths increase in for those values of dihedral angle. This means that for those conformations, the orbital overlap of the HOMO-1 → LUMO transition is more prominent that for the GS conformation, which could contribute to intermolecular interactions with more meso-pyrene-BODIPY molecules or with the solvent that would not be present in any other species.

It is of great importance to mention that the imaginary frequencies for the TSrot are small in magnitude (ranging from -26 cm -1 to -73 cm -1 ) which are not usual for a true TS. This would mean that the potential energy surfaces for electronic GS or excited states would be more complex that what is intuitive [START_REF] Kee | Structural control of the photodynamics of boron-dipyrrin complexes[END_REF] and they would possess underlying states in general. This complexity would call for more sensitive and more in-depth methodologies that would provide a complete description of dynamics and energetics. One fact that supports the complexity involved in describing the energetics of these processes is, for example that the barrier for the other computed para-substituted compounds (BODIPY 2 and 3 for comparison) shows a decrease in rotational barrier for the p-OMe bearing species, but an increase of 66% in quantum yield, which means that for the GS conformation, the rotational process should be even more favored than emission in comparison to the base meso-phenyl-BODIPY 1.

2. Quinoid canonical form and the rotational barrier for different donor groups.

Nonetheless, the electron-donating effect of the p-OMe substituent is enough to create some push-pull effect from the methoxy group towards the BODIPY. The TSrot geometry and energy for the p-NH2 substituted species show that when the electron-donating effect increases, it causes a decrease in rotational barriers as is shown in (S26). This electronic communication can be explained through the quinoid canonical form shown in Scheme 2, which, once in the excited state, should adopt a conformation where rotation is precluded in such a way that a radiative relaxation pathway is preferred, increasing the quantum yield with better electron-donating substituents.

Effect of Electrostatic Features on NMR-Spectroscopic and Photophysical Properties

I. Electrostatic interactions

The computationally obtained structural and physicochemical results show that only the derivatives with para-substitution (1-3) would be energetically allowed to rotate. For the cases in which the ortho substituents in the phenyl are energetically locked, an electrostatic and magnetic interaction between the ortho-substituents and the nearest fluorine atom gives rise to a difference in chemical shifts (∆δF), observable in the [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR spectra. Through a series of computational analyses, we were able to find a series of trends that account for the ∆δF. The first parameter to explore is related to how electrostatic interactions between the fluorine atoms in the BODIPY and the functional group of interest in the phenyl moiety, govern the behavior of ∆δF which, as we can see in the ESP maps (obtained at an isovalue of 0.002 in Fig. 8), can vary from attractive (in the case of those BODIPY derivatives with small ∆δF), to repulsive for the species with large difference in chemical shift; as if when Please do not adjust margins Please do not adjust margins the partial charge in the functional group ortho to the BODIPY becomes more negative, the interaction with the closest fluorine atom increases the shielding with respect to their magnetic behavior, increasing the value of ∆δF as well. The existence of this electrostatic interaction can be further visualized in the contours calculated at an isovalue of 0.002 presented in Fig. 9. These contours were obtained for the plane crossing the main atom in the functional group of interest. It can be seen how for hydrogen, the plane passing through the center of the atom creates level curves indicating the interaction with fluorine, but also with the BODIPY portion of the molecule, and being chemically and electrostatically equivalent for the other ortho hydrogen, this leads to equivalent fluorine atoms and no differences in magnetic behavior. For BODIPY 4 with the methyl group, the carbon and hydrogen atoms are interacting almost solely with fluorine for that level curve and is totally differentiable from the interaction for the remaining ortho hydrogen. The fact that there are level curves for the interaction between the methyl and fluorine, and the attractive character of such electrostatic interaction, provides a small but NMR-detectable ∆δF for the two fluorine atoms. For BODIPY 10 with the nitro substituent, we can observe the particular interaction between the nitro group and the fluorine that produces the ∆δF, and the strongly repulsive character of the interaction sets GIVING? the largest value among the explored series of BODIPY derivatives. Images for the rest of the studied compounds can be found in the Supporting Information (S34-41).

II. Dipole Moment Vectors and Transition Dipole Moments

Let us revisit the values for the deviation angle between the BODIPY plane and the dipole moment vectors shown in Fig. 4 and make a comparison between the angles, the dipole moments of the studied systems, the transition dipole moments for the main excitation of each one of them and their respective values of fluorescence quantum yield (ФF). Calculated dipole moments, the angle regarding the BODIPY plane, and the quantum yield values are given in Table 2. Setting the fluorescence value of 0.03 for the meso-phenyl BODIPY 1 as reference and helped by the rotational barriers studied above, the data shows that, in this case, rotation of the phenyl group is favored over the fluorescence pathway.

As mentioned before, the value for the nitro-containing species decreased threefold, which is typical for nitro-substituted species, since the nitro group quenches fluorescence via intersystem crossing. [START_REF] Collado-Fregoso | Excited-state dynamics of nitrated push-pull molecules: The importance of the relative energy of the singlet and triplet manifolds[END_REF] For all other species the quantum yield is higher, in accordance to the restricted non-radiative relaxation pathways upon introduction of the ortho substituent in the meso-phenyl moiety. [START_REF] Kee | Structural control of the photodynamics of boron-dipyrrin complexes[END_REF] The Fluorescence quantum yield was plotted as a function of dipole moment vector deviation and transition dipole moment (Table 2 and Fig. 10). Positive angles in the deviation from the BODIPY plane, correspond to the species with the highest quantum yields, whilst all the negative angles are for species with 0.3 or lower values for this photophysical feature. This creates a valuable design parameter to estimate an interval of quantum yield by calculating the deviation angle thus providing structure-property relationship arguments. Similarly, three groups can be noticed from the plotted data shown in Fig. 10; the two lowest values (0.01 and 0.03) were explained before, followed by the series of compounds with Please do not adjust margins Please do not adjust margins increased ФF (from 0.16 to 0.31) which correspond to those showing hindered rotation and the low values of transition dipole moment. Finally, the last family of compounds, show a large increase in quantum yield (>0.70) which correspond to hindered rotation and the largest transition dipole moments, leading to a structure-property relationship argument in terms of the behavior of this electronic characteristic.

III. Group electronegativity and Inductive Effects

Having a series of functional groups that share differently the electron density of the phenyl moiety to which they are bonded, gave us a parameter to be considered. Fig. 11 shows how the ∆δF changes as the group electronegativity [START_REF] De Proft | Ab initio determination of substituent constants in a density functional theory formalism: Calculation of intrinsic group electronegativity, hardness, and softness[END_REF] increases for the ortho substituents. Plotting the data, a fairly acceptable correlation can be obtained, showing that the variables are directly proportional. The inductive effect is due to differences in electronegativity between atoms bonded together.

Table 2. Selected data of the different groups.

A Calculated from the geometry optimizations at the B3LYP/def2SVP theory level using the SMD solvation model in hexane B Calculated through TD-DFT computations at the B3LYP/6-311g(d,p) theory level using the CPCM solvation model in hexane.

C Obtained experimentally in this work.

Based on the relation between electronegativity and the ∆δF, σ-Hammett coefficients related to inductive effects (σi) were also taken into account. [START_REF] Takahata | Estimation of Hammett sigma constants of substituted benzenes through accurate density-functional calculation of core-electron binding energy shifts[END_REF] Fig. 12 shows that ∆δF increases as the σ-Hammett coefficients increases.

IV. Functional group hardness/softness

Besides the property of attracting electron density as a functional group, it was important to evaluate the effect of that electron density once distributed in the ortho substituent. The hardness/softness of phenyl-substituents was calculated using reported values and equations. [START_REF] Kaya | A new equation for calculation of chemical hardness of groups and molecules[END_REF] and The results of the NPA we performed, reported as the charge of the ortho functional group (QG) That information can be consulted in Table S2.

With our data treatment we found that monoatomic substituents such as fluoride and chloride are the hardest, followed closely by hydrogen. In the middle of the interval, one finds the hydroxyl and methyl groups. Finally, the nitro group, the methoxy and the pyrenyl are prone to be labelled as soft. It is important to keep in mind that not only the charge located on each functional group is relevant to acquire the term soft or hard, but also the thermodynamic properties of ionization potential and electron affinity which are regarded within the Mulliken-Jaffe parameters. Please do not adjust margins Please do not adjust margins Fig. 13 shows the trends found and the correlations between the hardness and softness of the ortho substituents and the ∆δF.

Looking at the complete panorama of the plotted values there is no clear correlation, but when the data is divided in sections some information is apparent. For the monoatomic substituents, the hard-functional groups attached to the phenyl moiety, there is a clear correlation where the ∆δF decreases as hardness decreases, or as softness increases.

The lack of another central point in the data, that may correspond to nitrile, just to provide an example, poses a limitation for claiming a trend or correlation for that section in the plot. In spite of that, there is the third section of the values that also presents a linear, inversely proportional correlation for ∆δF and softness, which would give different linear behavior depending on the bulkyness of the ortho substituent.

Finally, a parameter we obtained through NPA charge partition, that brings together all studied and measured features, is the calculated charge for the ortho-substituents (QG), which establishes a trend with the rotational energy barriers, the ∆δF and the ФF as shown in Fig. 14. For the rotational energy barriers (Fig. 14a) the absolute value of QG was used, and by doing this, a clear trend where the ortho-substituents with largest charge are the ones with a lower rotational barrier, and as the functional group tends to neutrality, the barrier becomes larger. This same trend is observed for ФF. Eliminating two cases: nitro

Please do not adjust margins Please do not adjust margins for its low fluorescence for reasons discussed above, and pyrenyl because of the nuances of the exceeding amount of vibrational relaxation modes. Fig 14b shows, that as the absolute value of QG becomes larger, the fluorescence quantum yield decreases, meaning that for the substituents close to neutral afforded , we measured the highest values of ФF. This would mean that, when the ortho-substituent is highly charged, it favors some interactions that help to reach the TSrot in nonpolar solvents; and, as we have already explained, when the molecule possesses ortho hindrance, it favors radiative relaxation. Therefore, large values of QG regulate how difficult is to reach the rotational barrier and how much favored the emission properties would be. Fig. 14c shows the final trend established by QG, and it is possible to see that for positive values of QG, ∆δF decreases until the most positively charged species present to BODIPY-fluorine's differentiation NO ENTIENDO ESTO. In contrast, the most negatively charged species are the ones with larges ∆δF values, showing that attractive or repulsive interactions are dictating the magnitude of this magnetic differentiation, as established by the value of QG.

Conclusions

A series of meso-substituted BODIPYs were synthesized. Orthoaryl substitution proved to be a factor of important influence in the spectroscopic and photophysical properties. The magnitude and nature of the electrostatic interaction between orthosubstituents and the fluorine atoms of the BODIPY gives rise to an observable differentiation by [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F-NMR, which is influenced and somewhat established by group electronegativity, hardness/softness and σi-Hammett constant of the ortho substituent. As ∆δF increases, the group electronegativity and the σ-Hammett constants increases. This kind of substitution pattern improved greatly the fluorescence quantum yield compared to the reference mesophenyl-BODIPY in non-polar solvents. Computational studies on the rotational energy barriers and charge partition, provided insight on the TSrot geometries and the close relationship between rotational barriers, ∆δF and fluorescence quantum yields. The study of these parameters showed a structureproperty relationship that provides design arguments for this kind of fluorophores.

Experimental Section

Materials and methods

Chemicals used for the synthesis were reagent grade. Spectroscopic grade solvents were used for all photophysical measurements. 1 H, [START_REF] Awuah | Boron dipyrromethene (BODIPY)-based photosensitizers for photodynamic therapy[END_REF] C, [START_REF] Dale | New potent, short-linker BODIPY-ARTICLE[END_REF] B and [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR spectra were recorded with Bruker 400 MHz, VARIAN Unity Inova 300 MHz and Anasazi 90 MHz spectrometers. Chemical shifts for 1 H and [START_REF] Awuah | Boron dipyrromethene (BODIPY)-based photosensitizers for photodynamic therapy[END_REF] C NMR spectra are referenced relative to the residual protonated solvent. The 11 B NMR chemical shift is referenced relative to BF3•Et2O (δ = 0 ppm), and the [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR chemical shift is given relative to CFCl3 (δ = 0 ppm). Data are listed in parts per million (ppm). UV-visible spectra were recorded on a VARIAN spectrometer. Fluorescence spectra were recorded on a VARIAN spectrophotometer with a slit width of 10nm, at 480 nm excitation wavelength and emission from 490 to 750 nm. The corresponding fluorescence quantum yield (Φf) was calculated according to a standard solution of Rhodamine 6G in ethanol and was determined using the equation below,

𝛷 𝑥 = 𝛷 𝑠 ( 𝐴 𝑠 𝐴 𝑥 ) ( 𝐹 𝑥 𝐹 𝑠 ) ( 𝑛 𝑥 𝑛 𝑠 ) 2
Where Φ is the fluorescence quantum yield, A is the absorbance, F corresponds to the area under the emission curve and n is the refractive index of the solvents used in the measurement. The subscripts x and s represent the tested dye and the standard dye (Rhodamine), respectively.

General Synthesis

Synthesis of dipyrromethanes: To a solution of the corresponding aromatic aldehyde (1 eq.) in pyrrole (4 eq.), was added a catalytic amount of trifluoroacetic acid (TFA). The mixture was stirred at room temperature until total consumption of the aldehyde. The crude product was washed with brine, extracted with dichloromethane, dried over anhydrous Na2SO4 and evaporated to dryness under vacuum. This crude was then purified in column chromatography on silica gel using hexane/ethyl acetate. [START_REF] Gryko | 5-Substituted dipyrranes: synthesis and reactivity[END_REF] CREO QUE VA SIN PUNTO Synthesis of BODIPYs: Into the corresponding dipyrromethane dissolved in CH2Cl2, DDQ (2,3-dichloro-5,6-dicyano-1,4benzoquinone) (1 eq.) was added and the solution was stirred 1 h at room temperature. To this oxidized product, BF3•Et2O (6 eq.) was added under nitrogen atmosphere and stirred for another 15 min, then, triethylamine (3 eq.) was added dropwise and stirring was continued to completion of the reaction and monitored by TLC. The reaction mixture was then washed with brine and extracted with CH2Cl2, the organic layer was combined, dried over Na2SO4 and evaporated to dryness under vacuum to give the crude product. This was further purified by silica gel column chromatography to afford the corresponding BODIPY. 31

Characterization

4,4-difluoro-8-phenyl-4-bora-3a,4a-diaza-s-indacene

(1). Column chromatography using hexane/ethyl acetate (8:2) afforded 1 [START_REF] Kee | Structural Control of the Photodynamics of Boron -Dipyrrin Complexes[END_REF] as a green crystalline solid (60%). Mp: 103 °C. [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] H NMR [400 MHz, CDCl3] (δ, ppm): 7.95 (m, 2H), 7.51-7.60 (m, 5H), 6.94 (d, J = 3.6 Hz, 2H), 6.55 (d, J = 3.6 Hz, 2H); [START_REF] Awuah | Boron dipyrromethene (BODIPY)-based photosensitizers for photodynamic therapy[END_REF] C NMR [100 MHz, CDCl3] (δ, ppm): 147.3, 144.0, 134.9, 133. 7, 131.6, 130.74, 130.44, 128.4. 11 B NMR [160 MHz, CDCl3] (δ, ppm): -0.29 (t, JB-F = 29 Hz). [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR [282 MHz, CDCl3] (δ, ppm): -144.98 (q, JB-F = 29 Hz).

Please do not adjust margins Please do not adjust margins 4,4-difluoro-8-(4-methoxyphenyl)-4-bora-3a,4a-diaza-sindacene (2). Column chromatography with hexane/ethyl acetate (8:2) afforded 2 [START_REF] Betancourt-Mendiola | Scope and Limitations of the Liebeskind-Srogl Cross-Coupling Reactions Involving the Biellmann BODIPY[END_REF] as an orange powder (61%). Mp: 120122 °C. [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] H NMR [400 MHz, CDCl3] (δ, ppm): 7.92 (s, 2H), 7.55 (d, J = 8.8 Hz, 2H), 7.05 (d, J = 8.9 Hz, 2H), 6.98 (d, J = 4 Hz, 2H), 6.56 -6.54 (m, 2H), 3.92 (s, 3H). [START_REF] Awuah | Boron dipyrromethene (BODIPY)-based photosensitizers for photodynamic therapy[END_REF] C NMR [100 MHz, CDCl3] (δ, ppm): 162. 3, 147.7, 143.6, 135.1, 132.6, 131.6, 126.6, 118.4, 114.3, 55.7. 11 B NMR [160 MHz, CDCl3] (δ, ppm): -0.26 (t, JB-F = 28 Hz). [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR [282 MHz, CDCl3] (δ, ppm): -145.19 (q, JB-F = 28 Hz). (3). Column chromatography using hexane/ethyl acetate (8:2) afforded 3 [START_REF] Baruah | BODIPY-based hydroxyaryl derivatives as fluorescent pH probes[END_REF] as a red crystalline solid (46%). Mp: 151 °C. [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] H NMR [400 MHz, CDCl3] (δ, ppm): 7.95 (s, 2H), 7.50 (d, J = 8.6 Hz, 2H), 6.99 (m, 4H), 6.57 (d, J = 2.4 Hz, 2H), 5.82 (s, 1H). [START_REF] Awuah | Boron dipyrromethene (BODIPY)-based photosensitizers for photodynamic therapy[END_REF] C NMR [100 MHz, CDCl3] (δ, ppm): 158.6, 147. 4, 143.5, 134.8, 132.6, 131.4, 126.3, 118.4, 115.6. 11 B NMR [160 MHz, CDCl3] (δ, ppm): -0.25 (t, JB-F = 28 Hz). [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR [376 MHz, CDCl3] (δ, ppm): -144.56 (q, JF-B = 29 Hz). (4). Column chromatography using hexane/ethyl acetate (8:2) afforded 4 [START_REF] Kee | Structural Control of the Photodynamics of Boron -Dipyrrin Complexes[END_REF] as a green solid (10%). Mp: 135 °C. [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] (5). Column chromatography using hexane/ethyl acetate (8:2) afforded 5 [START_REF] Roacho | Synthesis, structural characterization, and spectroscopic properties of the ortho, meta, and para isomers of 8-(HOCH2-C6H 4)-BODIPY and 8-(MeOC6H4)-BODIPY[END_REF] as an orange powder (20%). Mp: 110 -112 °C. [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] (6) (7). Column chromatography using hexane/ethyl acetate (8:2) afforded 7 [START_REF] Kim | Molecular Rotors for the Detection of Chemical Warfare Agent Simulants[END_REF] as a green powder (10%). Mp: 134-136 °C. [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] H NMR [300 MHz, CDCl3] (δ, ppm): 7.89 (s, 2H), 7.4 (td, J = 7.2 Hz, J = 1.7 Hz, 1H) 7.25 (dd, J = 7.7 Hz, J2 = 1.7 Hz, 1H), 7.04 -6.89 (m, 2H), 6.90 (d, J = 3.6 Hz, 2H), 6.49 (d, J = 3.9 Hz, 2H). [START_REF] Awuah | Boron dipyrromethene (BODIPY)-based photosensitizers for photodynamic therapy[END_REF] (8). Column chromatography using hexane/acetone (9:1) afforded 8 as an orange-green crystalline solid (23%). Mp: 103 °C. (9). Column chromatography using hexane/ethyl acetate (9:1) afforded 9 as a red crystalline solid (16%). [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] (10) (11). Column chromatography using hexane/ethyl acetate (9:1) afforded 11 [START_REF] Bañuelos | Modulation of the photophysical properties of BODIPY dyes by substitution at their meso position[END_REF] as a green crystalline powder (20% yield). Mp: 134135 °C. [START_REF] Treibs | Difluorboryl-Komplexe von Di-und Tripyrrylmethenen[END_REF] H NMR [400 MHz, CDCl3] (δ, ppm): 8.3 -8.0 (m, 11H), 6.62 (d, J = 4.2 Hz, 2H), 6.46 (d, J = 3.0 Hz, 2H). [START_REF] Awuah | Boron dipyrromethene (BODIPY)-based photosensitizers for photodynamic therapy[END_REF]

4,4-difluoro-8-(4-hydroxyphenyl)-4-bora-3a,4a-diaza-sindacene

4,4-difluoro-8-(2-methylphenyll)-4-bora-3a,4a-diaza-sindacene

4,4-difluoro-8-(2-methoxyphenyl)-4-bora-3a,4a-diaza-sindacene

4,4-difluoro-8-(2,4-dimethoxyphenyl)-4-bora-3a,4a-diaza-sindacene

4,4-difluoro-8-(2-hydroxyphenyl)-4-bora-3a,4a-diaza-sindacene

4,4-difluoro-8-(2-chlorophenyll)-4-bora-3a,4a-diaza-sindacene

4,4-difluoro-8-(2-fluorophenyl)-4-bora-3a,4a-diaza-s-indacene

4,4-difluoro-8-(2-nitrophenyl)-4-bora-3a,4a-diaza-s-indacene

4,4-difluoro-8-(1-pyrene)-4-bora-3a,4a-diaza-s-indacene

Computational Methodology

All calculations were carried out taking hexane as solvent, trying to match or correlate experimental observations in a solvent with good emission properties and discarding toluene to avoid possible π-stacking interactions with the studied species. Geometry optimizations were carried out through all-electron calculations using the B3LYP hybrid functional at the def2SVP theory level using the SMD solvation approach, to obtain the geometries of lowest energy for the studied species and to analyze important structural details. Regarding the computation of rotational barriers, all geometries were re-optimized with the M06-2X hybrid functional and the def2SVP basis set using the SMD solvation model to refine the computation of energetics. Scans through redundant coordinates for the dihedral angle between the BODIPY and the meso substituents were done using M06-2X/def2SVP approach with the SMD solvation model. Being this combination a proven approach in the field of thermochemistry, energetics, energy barriers, among other features. [START_REF] Mardirossian | How Accurate Are the Minnesota Density Functionals for Noncovalent Interactions, Isomerization Energies, Thermochemistry, and Barrier Heights Involving Molecules Composed of Main-Group Elements?[END_REF][START_REF] Weigend | Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H[END_REF][39] The energy maximum of each corresponding scan was then optimized as a transition state (TS) to determine the real geometry and rotational barrier of such TSs. This was done also with the M06-2X/def2SVP theory level and SMD solvation approach. Time-dependent Density Functional Theory (TD-DFT) calculations were run using the same functional at the 6-311g(d,p) theory level with the CPCM solvation method, to obtain the value for the dipole moment corresponding to the main transition (S0 → S1) for all BODIPY derivatives. After determining the orbital behavior of the S0 → S1 transition, the energetic evolution of S1 was calculated at the M06-2X/def2SVP theory level using the SMD solvation method in hexane, by TDDFT approach to the most representative points in the rotational scan. The vertical transition energies would shape the curve for the energy of the S1 excited state for each species, as a function of dihedral angle, to be compared to the rotational energy barriers. Which is an approach known in the literature to describe similar phenomena to the one studied in the out contribution. 40 Natural Population Analysis (NPA) was performed for all compounds using the B3LYP functional at the 6-31g(d,p) theory level to calculate the charge partition for all atoms in the studied species and thus, calculate hardness/softness of the functional groups in the aryl portions of the BODIPY derivatives. Electrostatic potential (ESP) maps as surfaces and contours were produced to analyze the distribution of electrostatic partition in all the studied molecules, these computations were done using the CPCM solvation model. All calculations were run using the Gaussian 09 software and the GaussView 5.0 visualization suite. 41

Fig. 1 .

 1 Fig. 1. Stacked 19 F NMR spectra of compounds 4-11. Spectra were stacked from smallest to largest chemical shift difference (∆δF).

Fig. 3 .

 3 Fig. 3. Normalized and emission spectra of selected BODIPYs 1 (black), 6 (red), 8 (blue), 10 (green) and 11 (purple) in toluene.

  H NMR [400 MHz, CDCl3] (δ, ppm): 7.93 (s, 2H), 7.45 -7.39 (m, 1H), 7.33 (d, J = 7.9 Hz, 1H), 7.30 -7.23 (m, 2H), 6.71 (d, J = 4.1 Hz, 2H), 6.49 (d, J = 4.0 Hz, 2H), 2.24 (s, 3H). 13 C NMR [100 MHz, CDCl3] (δ, ppm): 147.2, 144.5, 136.5, 135.6, 133.1, 131.2, 130.6, 129.9, 129.8, 125.4, 118.7, 20.1. 11 B NMR [128 MHz, CDCl3] (δ, ppm): 0.31 (t, JB-F = 28 Hz). 19 F NMR [376 MHz, CDCl3] (δ, ppm): -144.87 (dq, JB-Fa = 28 Hz, JF-F = 104 Hz), -145.56 (dq, JB-Fb = 28 Hz, JF-F = 104 Hz).

  H NMR [400 MHz, CDCl3] (δ, ppm): 7.90 (s, 2H), 7.50 (td, J1 = 8.3 Hz, J2 = 1.6 Hz, 1H), 7.31 (dd, J1 = 7.5 Hz, J2 = 1.6 Hz, 1H), 7.08-7.03 (m, 2H), 6.80 (d, J = 4.0 Hz, 2H), 6.48 (d, J = 4.0 Hz, 2H), 3.75 (s, 3H). 13 C NMR [100 MHz, CDCl3] (δ, ppm): 157.2, 144.5, 143.8, 135.7, 131.6, 131.5, 131.1, 122.4, 120.1, 118.1, 111.3, 55.6. 11 B NMR [128 MHz, CDCl3] (δ, ppm): 0.31 (t, JB-F = 28 Hz). 19 F NMR [376 MHz, CDCl3] (δ, ppm): -144.47 (dq, JB-Fa = 28 Hz, JF-F = 104 Hz), -145.63 (dq, JB-Fb = 28 Hz, JF-F = 104 Hz).

  C NMR [75 MHz, CDCl3] (δ, ppm): 153.6, 144.8, 143.0, 135.2, 132.1, 131.6, 120.4, 120.1, 120.0, 119.0, 117.1. 11 B NMR [160 MHz, CDCl3] (δ, ppm): -0.28 (t, JB-F = 28 Hz). 19 F NMR [282 MHz, CDCl3] (δ, ppm): 143.72 (dq, JB-Fa = 28 Hz, JF-F = 104 Hz), 144.94 (dq, JB-Fb = 28 Hz, JF-F = 104 Hz).

  [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F [376 MHz, CDCl3] δ -144.47 (dq, JB-Fa = 28 Hz, JF-F = 105 Hz), -145.67 (dq, JB-Fb = 28 Hz, JF-F = 105 Hz). ES-MS: (C15H10BClF2N2) 283.5 [M + -F].

  H NMR [90 MHz, CDCl3] (δ, ppm): 7.49 (s, 2H), 7.24 -6.77 (m, 4H), 6.51 (d, J = 3.8 Hz, 2H), 6.18 (d, J = 4.0 Hz, 2H). 19 F NMR [84.7 MHz, CDCl3] δ -110.86 (s), -143.88 (dq, JB-Fa = 28 Hz, JF-F = 104 Hz), -145.35 (dq, JB-Fb = 28 Hz, JF-F = 104 Hz).

  [START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F [470 MHz, CDCl3] δ -143.92 (dq, JB-Fa = 28 Hz, JF-F = 104 Hz), -146.32 (dq, JB-Fb = 28 Hz, JF-F = 104 Hz).

  CDCl3] (δ, ppm): 144.51 (dq, JB-Fa = 28 Hz, JF-F = 104 Hz), 145.26 (dq, JB-Fb = 28 Hz, JF-F = 104 Hz).

23

Table 1 .

 1 Chemical shifts and coupling constants between[START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F and 11 B nuclei in CDCl3.

25

1 6 8 10 11 Compound δFa δFb ∆δ JFa,Fb [Hz] δB JB,F [Hz]

	4	-144.87	-145.56	0.69	107		0.31	29
	5	-144.47	-145.63	1.16	107		0.30	29
	6	-144.56	-145.72	1.16	107		0.29	29
		1.0
		0.8
	Normalized Abs.	0.4 0.6
		0.2
		350	400	450	500	550	600
			Wavelength (nm)
								Computational Analysis

  = 29 Hz).[START_REF] Hermanek | B NMR Spectra of Boranes, Main-Group Heteroboranes, and Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms[END_REF] F NMR [282 MHz, CDCl3] (δ, ppm): 144.56 (dq, JB-Fa = 29 Hz, JF-F = 107 Hz), 145.72 (dq, JB-Fb = 29 Hz, JF-F = 107 Hz).

	HRMS-ESI-TOF: Experimental mass for C17H15BFN2O2 m/z
	309.120512; calculated m/z 309.1211 for C17H15BFN2O2; %
	error: 0.361884; [MF] +
	. Column chromatography using hexane/ethyl
	acetate (8:2) afforded 6 as a red powder (25%). Mp: 156-157
	°C. FTIR-ATR (ʋ, cm -1 ): 3108, 2937, 1383, 1255, 1167, 1110,
	1063, 979, 837, 780, 746, 706, 618, 582, 420. 1 H NMR [400 MHz,
	CDCl3] (δ, ppm): 7.87 (s, 2H), 7.24 (d, J = 9 Hz, 1H), 6.83 (d, J =
	3.6 Hz, 2H), 6.59 (d, J = 6.4 Hz, 2H), 6.47 (d, J = 3.6 Hz, 2H), 3.89
	(s, 3H), 3.73 (s, 3H). 13 C NMR [100 MHz, CDCl3] (δ, ppm): 162.7,
	158.7, 144.6, 144.3, 136.4, 132.9, 131.5, 117.9, 115.3, 104.3,
	99.0, 55.6, 55.5. 11 B NMR [128 MHz, CDCl3] (δ, ppm): 0.29 (t, JB-

F

  1 H NMR [400 MHz, CDCl3] (δ, ppm): 7.94 (s, 2H), 7.58 -7.52 (m, 1H), 7.52 -7.45 (m, 1H), 7.43 -7.35 (m, 2H), 6.73 (d, J = 4.1 Hz, 2H), 6.51 (d, J = 4.1 Hz, 2H). 13 C NMR [100 MHz, CDCl3] (δ, ppm): 145.1, 143.5, 135.4, 133.3, 132.5, 131.6, 131.2, 131.2, 130.3, 126.6, 118.9. 11 B NMR [128 MHz, CDCl3] δ -0.68 (t, J = 28 Hz).

  C NMR [100 MHz, CDCl3] (δ, ppm): 146.6, 144.5, 136.6, 132.6, 131.8, 131.4, 130.8, 130.5, 129.1, 128.6, 127.9, 127.9, 127.2, 126.7, 126.3, 126.0, 125.0, 124.6, 124.3, 124.1, 118.9 11 B NMR [160 MHz, CDCl3] (δ, ppm): -0.06 (t, JB-F = 28 Hz). 19 F NMR [282 MHz, Please do not adjust margins Please do not adjust margins

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Acknowledgements

MFP (576515) and OGA (239984) thank CONACYT for their doctoral scholarship. The authors Th would acknowledge financial support from to CONACYT, PAIP and PAPIIP IN222819 DGAPA and the French-Mexican International Laboratory (LIA-LCMMC-CONACYT). Thanks to María Luisa Rodríguez, María Eugenia Ochoa for NMR experiments and Geiser Cuellar for Mass Spectra.

Conflicts of interest

There are no conflicts to declare.

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Qiwu Zhang

Jinfeng Lu

Fumio Saito

Michel Baron

Mechanochemical Solid-Phase Reaction between Polyvinylidene Fluoride and Sodium Hydroxide

Keywords: PVDF, grinding, mechanochemical reaction, defluorination

Mechanochemical solid-phase reaction between polyvinylidene fluoride and sodium hydroxide

INTRODUCTION

Polyvinylidene fluoride (PVDF, [CH 2 CF 2 ] n ) is one of the fluorine resin group, and is distinguished by excellent resistance to heat, aging and abrasion, chemical stability with nontoxicity, unique electrical properties, durability for UV, and gamma radiation. These properties allow us to use it in a wide range such as mechanical, chemical, electrical, agricultural, and food industries. 1,2 Recently, the amount of fluorine resin produced has increased; accordingly, so has the amount of the waste resin been growing. When the waste halogen-bearing resins [3][4][5][6] are burned in a furnace, toxic gases such as HF as well as harmful substances like dioxins are emitted in a flue gas and ashes. Because the emission of such toxic and harmful substances should be prevented, proper techniques are strongly required in waste deposit facilities. A method for adsorbing toxic gases in a thermal treatment of the waste has been developed; however, this is sometimes accompanied with corrosion of devices for adsorbing the gases.

Another route for processing such polymer wastes is a mechanochemical method. [7][8][9] Although it is known that mechanical treatment of polymer will degrade it easily and its molecular weight decreases to a relatively low level, 10 -13 the basic unit (monomer part) remains unchanged after the mechanical treatment. It is different in the mechanochemical process where an active chemical is added into polymer so that not only degradation but a real reaction takes place. In this article, we provide fundamental information on the mechanochemical solid phase reaction between PVDF and NaOH to form organic compounds and NaF. The organic phases after defluorination may be used as fuel, while NaF has a possibility to be a source for fluorine recycling.

EXPERIMENTAL

The PVDF sample (called sample A) used in the experiment was supplied from Dowa Mining Co. Ltd., Japan. NaOH, HCl solution, and ethyl acetate were obtained from Wako Pure Chemical Industries, Ltd., Japan. The PVDF and NaOH samples were mixed as a starting mixture where fluorine is equivalent to sodium in the starting mixture.

A planetary ball mill (Pulverisette-7, Fritsch, Germany) was used to grind the starting mixtures at approximately 700 rpm under atmospheric condition. Four grams of the mixture were charged in a zirconia pot of 45cm 3 inner volume with seven zirconia balls of 15 mm diameter and ground for different periods of time.

Washing operation for the ground samples was conducted as follows: 1 g of sample was dispersed into 50 mL distilled water in a vial with mechanically agitation for 60 min. Subsequently, the suspension was filtered to separate solid and liquid phases. The solid particles were rinsed with 50 mL distilled water two times. The solid particles was called sample B. pH value of the filtrate was regulated to 2 by adding HCl solution, then 40 mL ethyl acetate was added to the filtrate of 250 mL to extract soluble organic phase. The organic sample after the extraction is called sample C. Both samples B and C were dried at 373 K for 2 h in air.

X-ray diffraction (XRD) analysis was conducted on the ground samples for identifying phases in the ground samples. Mg-K␣ radiation was used in Xray photoelectron spectroscopic (XPS) measurement to record spectra for samples A, B, and C. Infrared spectra for these three samples were recorded by means of Fourier transform infrared (FTIR) spectroscopy using a KBr disc method. Fluorine concentration in the filtrate was measured by an Ion-Chromatography (IC) method.

RESULTS

Figure 1 shows XRD patterns of the original PVDF and the mixtures ground for different periods of time. Peaks of NaF are observed in the pattern of the sample ground for 10 min. Intensity of the peak increases relatively with an increase in grinding time. The result suggests that displacement reaction of fluoride anion for OH base proceeds during the grinding to form NaF in the product, although no clear information on dissociation of organic compounds is provided.

Because the reaction product, NaF, is soluble in water, conversion of fluorine from PVDF into NaF can be easily obtained by measuring its fluorine concentration in the filtrate after washing operation. Figure 2 shows yield of fluorine converted from the PVDF sample as a function of grinding time. The yield increases rapidly in the early stage of grinding to about 50% at 10 min, and then rises gradually to about 90% at 60 min. However, no further significant improvement in the yield can be made. Figure 4 shows the O1s spectra of samples B and C. It is interesting to notice that oxygen exists in both samples B and C, and the peak intensity in sample C is much higher than that in sample B. Because oxygen does not exist in the original PVDF, the oxygen in samples B and C should come from NaOH through the mechanochemical reaction. The water-soluble sample C has a much higher amount of oxygen than the insoluble sample B.

Figure 5 shows FTIR spectra of samples A, B, and C. Compared with the pattern of sample A (original PVDF sample), visible peaks in the spectrum of sample B are the same as that of PVDF. The intensity of the peaks has reduced significantly. This indicates that a small amount of unreacted PVDF is left in sample B. As for the pattern of sample C, three new broadened peaks are observed. Combined with the information from XPS, the new peaks could be understood as follows: the peak at about 1700 -1800 cm Ϫ1 comes from carbonyl (CϭO) stretch vibration, and the peak around 1600 -1700 cm Ϫ1 may be related to the stretch vibration of the double carbon binding (CϭC). As for the relatively large peak in the range between 700 -800 cm Ϫ1 , this may be due to the bending vibration of CϭC-H binding.

DISCUSSION

The mechanochemical solid-phase reaction takes place through the substitution of F by the OH base to form a kind of alcohol, [CH 2 -C(OH) 2 ], as given by eqs. ( 1) and (2) expressed as a monomer unit reaction. It is deduced that the displacement reaction for half (one side linked to carbon) of the fluorine given by eq. ( 1) proceeds very easily because nearly 50% of the fluorine is converted into NaF within 10 min of grinding, as shown in Figure 2. The relatively slow increase in fluorine yield in Figure 2 suggests that it has become difficult to replace another half (opposite side linked to carbon) fluorine given by eq. ( 2). High moisture observed in the ground sample over 30 min indicates that dehydration of [CH 2 -CF(OH)] or [CH 2 -C(OH) 2 ] takes place with a progress of substitution reaction of F by the OH base as follows:

͓CH 2 OCF 2 ͔ ϩ NaOH 3 NaF ϩ ͓CH 2 OCF͑OH͔͒ (1) ͓CH 2 OCF͑OH͔͒ ϩ NaOH 3 NaF ϩ ͓CH 2 OC͑OH͒ 2 ͔ (2)
͓CH 2 OCF͑OH͔͒ 3 ͓CHACF͔ ϩ H 2 O ( 3
)
͓CH 2 OC͑OH͒ 2 ͔ 3 ͓CH 2 OCAO͔ ϩ H 2 O (4)
The formation of H 2 O in the ground sample causes strong agglomeration of the fine particles, which hinders the grinding efficiency so that further substitution of F by the OH base expressed as eq. ( 2) proceeds relatively slowly compared to eq. ( 1). This is the main reason why the prolonged grinding does not improve the reaction efficiently. If the water formed is taken out and the strong agglomeration is lightened, the grinding efficiency will be improved considerably to be able to give a nearly complete defluorination for the polymer. The proper method is under consideration.

The substitution of F by the OH base leads to binding of polar base to organic compound, and the dehydration leads to formation of carbon-rich organic phases. The former is a main composition of sample C, and the later is that of sample B. Sample B consists of the unreacted PVDF as well as the carbon-rich compounds. Sample C is the compounds with various types of polar base like the OH base, carbonyl base (CϭO). Therefore, the organic phase in the ground sample is not one pure compound with only one base, but a mixture consisting of different types of chemical bases.

CONCLUSION

Mechanochemical solid-phase reaction between PVDF and NaOH proceeds rapidly in the beginning of the grinding stage to convert nearly half of the fluorine in PVDF into NaF through the substitution of fluorine in PVDF by the OH base. The subsequent dehydration takes place to generate water molecules, which cause the strong agglomeration of the obtained fine particles, decreasing the speed of this substitution reaction. About 90% of fluorine in the PVDF sample can be converted into NaF by this process. This indicates a high possibility of recycling fluorine from waste PVDF polymers.

Figure 1

 1 Figure 1 XRD patterns of the PVDF and NaOH mixture ground for different periods of time.

Figure 2

 2 Figure 2 Yield of the F extracted by washing the ground mixture.

Figure 3 shows

 3 Figure 3 shows C1s spectra of samples A, B, and C. The spectrum for sample A gives two peaks due to the existence of C-C and C-F bindings in the sample. The peak due to C-F binding at higher binding energy broadens and weakens significantly for sample B, and it almost disappears for sample C. These results indicate that C-F binding has been cut off, and the defluorination of PVDF has been achieved.Figure4shows the O1s spectra of samples B and C. It is interesting to notice that oxygen exists in both samples B and C, and the peak intensity in sample C is much higher than that in sample B. Because oxygen does not exist in the original PVDF, the oxygen in samples B and C should come from NaOH through the mechanochemical reaction. The water-soluble sample C has a much higher amount of oxygen than the insoluble sample B.

Figure 3

 3 Figure 3 XPS spectra of the C1s of samples A, B, and C.

Figure 4

 4 Figure 4 XPS spectra of the O1s of the samples.

Figure 5

 5 Figure 5 FTIR spectra of samples A, B, and C.

The authors are grateful to Mr. Choju Nagata, General Manager, Electric Materials Company, Magnetic Materials Business Unit, Dowa Mining Co. Ltd., for generously supplying the PVDF sample. They wish to thank the IFPRI Inc. for providing the financial support to carry out this work.


S Le Person

J R Puiggali

M Baron

M Roques

Near infrared drying of pharmaceutical thin films: experimental analysis of internal mass transport

Keywords: Drying kinetics, Coating, Selectivity, Laser Scanning Confocal Microscopy

Thin polymer films are produced for pharmaceutical purposes from a solution in water and polar solvent. These coatings contain also a heavy solvent with the active substance as solute. A global analysis is presented here of the drying rate in terms of average content for each component, i.e. water, heavy solvent and active substance. The results are used to better the global drying process. The quality of the final product imposes to know the local distribution of the key products. The use of Laser Scanning Confocal Microscopy (LSCM) on two molecules is proposed here; the active substance and the heavy solvent. Combining the two set of results, one is able to determine the cases of maldistribution of the active substance and of eventual crystallisation which in turn will have consequences on storage and successful transdermal delivery of the drug.

Introduction

Modern industries produces more and more thin polymer films and coatings for many purposes, e.g. adhesives, varnishes and paints. In the pharmaceutical industry some films are used in patches for transdermal drug delivery. Drying is the essential unit operation necessary to form the final product. In all cases, mastering of process variable and microscopic aspects of the product quality entails chemical and process engineering and transport phenomena as basic sciences.

After preparation, the coating mixture is spread on a web and submitted to drying in a tunnel or an oven. Frequently, impinging jets and Infra Red Radiation [START_REF] Cohen | A primer on forming coatings[END_REF] accomplish the work in a short drying time (100 s as an order of magnitude). In the end, one must be sure that the selected process and its conditions is able to ensure the right product quality; a limited remanence of the process solvent (generally a mixture of volatile solvents) and a given quality product [START_REF] Wicks | Organic coatings: science and technology[END_REF], i.e. physical and chem-ical homogeneity and an appropriate distribution of active substance.

The tools to design the correct process are pilot plant experiments, bench scale experiments and modelisation of transfers. In this paper, small scale experiments were opted for and an experimental approach of internal transfers. Evidently, the diffusional approach of complex systems containing two immiscible solvents, a shrinking polymeric macromolecule network and an active substance, cannot be tracked from the basic text-book equations. What is modelisable is already intuitively and/or experimentally known. It would take a lot of basic investigation on simpler systems to make a substantial progress on the only problem of cross diffusivities [START_REF] Taylor | Multicomponent Mass Transfer[END_REF]. For all those reasons it has been suggested, in this paper, to track the internal distribution of some key products by Laser Scanning Confocal Microscopy (LSCM). Adding an integral chemical analysis of the film, one is then able to quantify the absolute distribution for films produced under variable conditions.

After few other investigators [START_REF] Verghoot | Confocal laser scanning microscopy: a new method for determination of the morphology of polymer blends[END_REF][START_REF] Li | Surface morphology of a polymer blend examined by laser confocal fluorescence microscopy[END_REF], the use of LSCM for films was advocated because this technique eliminates all informations in the depth but the one from a specific plane. By moving vertically the sample or the objective, one gets a resolution of ca. 1 mm. Not all the molecules are susceptible to be excited by laser beam but we were fortunate to have two in our sample and could then reconstruct all interesting distribution.

The release kinetics of a drug by application onto the skin depends on the appropriate distribution of the drug which in turn depends on the drying process. Hence the selection of a process is based upon the thermosensitivity of the active substance and a homogeneous treatment of the film [START_REF] Potts | Transdermal drug delivery: useful paradigms[END_REF]. It appeared that after a rapid screening that SIR plus convection was the best solution. It is proposed in this paper to validate this first guess by an analysis in depth of the product quality.

Materials and experiences

Product composition

The coating mixture is constituted of: (1) an acrylic adhesive polymer (Pol); (2) three light solvents (Sl i ); (3) one heavy solvent named hereafter excipient (E); (4) one pharmaceutical active substance (AS) reputedly soluble in E. As excipient is essential to the administration of the drug, the drying process must evacuate the light solvent and preserve a maximum amount of E, hence the selectivity requirement. A typical wet coating composition is given in Table 1 together with the molecular weight of the essential products.

The final composition of the coating for each of the solvents, excipient and active substance is also part of the industrial target, the mass distribution within the thickness of the coating was added to this ordinary constraint; these two pieces of information have to be analysed as selective drying. Moreover during drying process the thermal degradation of the active substance must be avoided.

Experimental set-up

In order to substantiate the choice of the best heat supply mode with respect to drying time and quality, the coating drying were studied by convection, conduction, medium and short infra-red respectively (MIR and SIR), each of the last drying modes is always associated with convection. Experiments were carried out in a laboratory set-up mainly composed of two parts, the drying cell and the wind tunnel (see Fig. 1

Drying kinetics

During those experiments the mass of the sample is either continuously recorded or acquired at regular intervals. Moreover the average composition is determined on partially dried coatings by means of chromatographic analysis and coulometry. Then, each point of a curve corresponds to a separate drying experiment carried out at least twice. The initial coating thickness for all the tests was 200 mm.

Drying kinetics reported on Fig. 2 show that the initial drying velocity with infrared is about twice the velocity with other heating modes. Source thermal inertia affects greatly the first two points of the MIR drying curve bringing an abnormal delay. Between conduction and SIR the latter one is far more preferable. Analytical results on films show that the various heating processes can be classified by the remaining ratio of solvents to dry matter (Sl i +E)/Pol, by the ratio of heavy solvent to water E/Sl 1 and by the drying time. In Table 3 it is shown that SIR is vastly preferable in order to keep the heavy solvent inside the product in the minimum drying time. The ratio (Sl i +E)/Pol is not discriminant.

Temperature during SIR drying

Although rapid, and quality respectful, the SIR drying might harm the product because of local over heating. In order to evaluate this danger, It was realised that a thermal study of the coating for different infrared powers, for different thickness and coating supports.

Three thermocouples were embedded in a thick coating of 2-15 mm, an extrapolation of the temperature profile towards the irradiated surface permits to evaluate the highest temperature obtained by irradiation for various drying times and constant infrared power (see Fig. 3).

From Fig. 3, one can infer that 65°C is the maximum temperature reached at the end of the drying process and hence nor alteration nor destruction is to be feared according to the stability diagram of the active substance. Similar results have been obtained by optic pyrometry on similar coatings (acrylic polymers) by Navarri and Andrieu [START_REF] Navarri | High intensity infrared drying study. Part I. Case of thin coated films[END_REF].

It was concluded that SIR drying is the appropriate answer to the drying time and quality constraints.

Thermodynamic of the acti6e phase

During the drying process the thin layer temperature increases as the solvent contents decrease. At the end of the process the coating is cooled at a given AS mass fraction. All these changes in temperature and composition mass and thermal steps are susceptible to produce active substance crystallisation in the thin film and therefore the final quality of the product might be out of bonds. A good knowledge of solubility behaviour of the active substance in the heavy solvent is necessary to avoid this arowback. An analysis of the solvent mass loss coupled with the coating temperature increase is a good tool to propose drying schedules in relation with dried product quality. n solvent i n all solvents -x solvent i , one can evaluates the drying efficiency with respect to water and heavy solvent. In this last relation, n i is the evaporative mass flux of solvent i, n all solvents the total evaporative mass flux and ix i the mass fraction of the solvent i. As indicated in Fig. 6, where S water and S heavy solvent are plotted versus drying time, water evaporates preferentially, regarding to the heavy solvent, at the beginning of the drying process.

The thin film thickness is also plotted on this figure showing that shrinkage occurs essentially when water (major solvent) intensely evaporates [START_REF] Aregba | Une approche du se ´chage de mate ´riaux fortement de ´formables[END_REF][START_REF] Moyne | Mechanism of water transport in drying gels[END_REF]. In Fig. 5 the selective character of drying is demonstrated since 99% of the initial water amount is evaporated in 10 min. In the same time only 2.05% of heavy solvent evaporates; this last evaporation is due to the absence of heavy solvent vapour in the drying air. An important practical consequence of this heavy solvent evaporation is that one must shorten the drying time to maintain most of this solvent inside the product.

Solubility of the active substance in the heavy solvent is obtained by cooling (ambient air flow) of the active phase solution at a given mass fraction (M AS /M AS + M E ) initially unsaturated and homogenised at 75°C with ultra sounds. A thermocouple placed in the solution allows to follow the temperature evolution and, linked with an observation of active substance crystals through a microscope, the saturation temperature is obtained and the solubility curve plotted (Fig. 4).

On the same graph, the AS mass fraction is plotted versus temperature as the SIR drying process of the heavy phase E proceeds. One can notice that no crystallisation problem occurs if the mass fraction is considered in terms of an average value.

In order to show some light on the selective character of the drying operation, purpose of which is not just to remove the moisture, a more accurate study is necessary. Therefore, in relation with the quality and the final product requirements, one has to investigate, more precisely than an average kinetics study, the repartition of the active phase components (heavy solvent and active substance) inside the thin film.

Results

Time e6olution of components

In order to follow the evolution in mass of each component during drying, there is only one way; repeat a given experiment as many times as you need an analysis. This regular dosage is carried out by liquid and gas chromatography and also by coulometry, (complete analysis every 30 s); results are given on Fig.

5.

Using the selectivity coefficient S i of a component i in a solution as defined by Riede and Schlu ¨nder [START_REF] Riede | Selective evaporation of a ternary mixture containing one nonvolatile component with regard to drying processes[END_REF] and used by Wolf [START_REF] Wolf | Selective drying of hygroscopic materials wetted with binary mixtures[END_REF] for hygroscopic materials:S solvent i = Fig. 6. Selectivity factors for the two major solvents, i.e. water (Sl 1 ) and heavy solvent (E ) during drying of a 100 mm thick coating under Short Infra Red drying process. Indication of the shrinkage in terms of reduced thickness.

So far there are only global results that is to say drying curves for each component of the coating. The results are valuable for a good management of the drying operation but failed to give detailed informations on the quality of the product. Again, precise knowledge of the spatial repartition component in the volume is completely necessary to assess the functionality of the final product.

A new non destructive exploration method has been used, tested and validated in order to follow the spatial distribution of the components of the active phase. Laser Scanning Confocal Microscopy (LSCM) can provide this type of informations in as much as the probe molecule is fluorescent under laser beam radiation.

Spatial e6olution of the components

The LSCM allows, without sample preparation, the observation of a given surface called focal surface inside the thickness of the sample, avoiding the destructive slicing, with a much better resolution. Precisely, with a micronic resolution one is able to obtain accurate data on very thin sub-millimetric films.

The principle feature of confocal imaging is that detection is strictly limited to what is in focus; LSCM avoids out-of-focus informations from neighbouring planes.

The substance is irradiated by a laser beam. An intensity of fluorescence, proportional to the quantity of excited substance in the focus plane, is the information in return. Only aromatic molecules and/or elements with double conjugated links are excited by laser beam UV (364) or Visible (633, 543, 488). A map of the average fluorescence relative to the studied component can be obtained on the focus plane. By displacement of the focus plane, one can follow the fluorescence intensity of the component in the thickness of the sample [START_REF] Jinnai | Direct observation of three-dimensional bicontinuous structure developed via spinodal decomposition[END_REF].

Studies were carried out, with the help of Dr Catherine Garnier, at the Macromolecule Physico-Chemistry Laboratory (LPCM-INRA) at Nantes (F). All fluorescence measurements were performed using a microscope Axiovert 135M-Zeiss, an UV 364-Microsysteme laser as the excitation source and a SLM-Zeiss as spectrofluorimeter.

A miniature drying ring obviously had to be brought consisting of the infrared lamp and a blower in the vicinity of the LSCM microscope. Each sample was dried, on a glass plate, for a given length of time (5, 10, 15 min) then sealed over by a plastic film, brought to the microscope and analysed for 120 s during which the relaxation processes are unavailable but limited.

In parallel a chemical dosage allows the determination of the total mass of the fluorescent element in the studied substance. Therefore it is easy to connect the fluorescence informations to quantitative data.

Amongst the constitutive elements of the enduction, only the active substance and the heavy solvent can be excited by a UV364 laser beam.

After the disappearance of water from the thin layer, no further shrinkage occurs and the film consists of polymer and active phase (heavy solvent and active component) the volume of which are simply additive.

It is obvious then to calculate the spatial distribution of the polymer, knowing that of the others components.

Discussion

Because of the fluorescence of the two constituents of the active phase, an analysis of many preparations, with different concentrations, is necessary before studying the real enduction in order to differentiate the respective impact both of the active substance and of the heavy solvent.

Observations were carried out on enductions after 5, 10 and 15 min of drying. Before drying an observation is made in order to assess the degree of homogeneity of the constituents inside the thin layer.

After a 5 min drying analytical dosages (Fig. 5) show that only three components remain in the layer: (1) acrylic polymer; (2) active substance; and (3) heavy solvent. As the thin layer shrinkage is known (Fig. 6), one can transpose the LCSM data in terms of volumic content fields (Fig. 7). The intense moisture removal, through the exposed surface of the layer to the radiation, during the first 3 min of drying (Fig. 7) produces a stress on the polymer skeleton. This stress increases in the water flow direction and as a result the acrylic polymer becomes more and more dense in the upper part of the layer (exposed surface). This intense shrinkage coupled with the polymer compaction causes a displacement of the active phase towards the bottom of the layer (Fig. 7). This instantaneous view of the volumic content fields for the thin layer constituents show a very unstable state. One can easily imagine that the compressive wave must propagates from the top to the bottom of the enduction. Therefore at the end of the drying process the global skeleton structure will be equilibrated.

During the thin layer drying, two main stages are identified on Figs. 8 and9 where the spatial evolution of the heavy solvent and the active substance are reported after 5, 10, 15 min of drying: a first stage (t55 min for the drying condition), where water is intensively removed producing a severe shrinkage of the layer and a compaction of the polymeric skeleton at the top (in fact the exposed surface); as a result, the active phase in solution in the heavy solvent is displaced towards the bottom, a second stage where the volumic content fields of each component equilibrate themselves in the layer due to the combined displacement of the solvent and the relaxation of the polymer skeleton; the discharge of the polymer towards the bottom giving a stable mechanical state is joined to a movement from bottom to top of the active phase with a partial removal of the heavy solvent. Initially, in the thin layer the constituents of the active phase are homogeneously distributed. But the pharmaceutical active substance has a larger steric bulk than the heavy solvent and therefore might react differently to the stresses imposed by the skeleton of the acrylic polymer during the drying process. Then a detailed study of the spatial evolution of the active substance constituents must be carried out in order to prevent risks of separation of the active substance from the active phase.

Combining measurements made by LCSM and solubility data (Fig. 4) of the active substance in the heavy solvent lead one to suppose the existence of the active substance in its crystalline form. This is true for the residual quantity of the active molecules trapped in the retracted skeleton of the adhesive polymer 5 mm under the exposed surface (Fig. 10).

After 5 min of drying the system tends to reequilibrate the mechanical stresses. However, because water is no more present in the system, the polymeric network is not turgescent and the meshes are densely packed. The polymer skeleton acts as a filter for the active substance when the system reequilibrates. By their respective mobility and bulk, the heavy solvent will slip easier than the active substance through the net during this phase of mechanical reequilibrium just as in thin layer chromatography.

Consequently, after 10 min of drying, one can notice (Figs. 8 and9) an inversion of the heavy solvent and active substance concentration profiles inside the enduction. Between the 5th and the 10th min of drying the heavy solvent migrates towards the exposed surface and is removed whereas the active substance, slowed down in its migration, stays in the bottom of the layer. Because of a quicker migration effect than the evaporation one the heavy solvent is stored in the top of the coating. The lack of heavy solvent with respect to the active substance in the bottom of the layer should lead to find, in this location, an important crystallisation. This is shown in Fig. 10 where crystallisation is indicated on a 15 mm thickness in the bottom of the layer.

As the drying process proceeds, the active substance, from its strong affinity with the heavy solvent migrates and homogenises in the enduction thickness. After 15 min of drying, a quasi equilibrium is obtained for the components of the active phase, taking into account the evaporation of the heavy solvent.

Conclusion

The coupling between studies performed, on the one hand on a temporal basis (chromatographic and coulometric analysis), and on the other hand on a spatial basis (LSCM) allows to propose a model of the constituent transports inside the film whose thickness shrinks from 100 to 50 mm during drying.

The schematic description of the behaviour of the acrylic polymer and the solubility curve of the active substance in the heavy solvent allow to explain and to prevent mechanisms of separation of the active phase in the layer thickness. Such separation effects can produce harmful crystallisation phenomena.

These studies on overall and local behaviour of each components of the pharmaceutical thin film are fully sufficient in order to propose drying schedules. Following these lines and the guide suggested by Nadeau and Puiggali J.R. [START_REF] Nadeau | De l'analyse d'un produit a `la conception d'un se ´choir[END_REF], a pilot plant for thin pharmaceutical layer drying was designed [START_REF] Person | Se ´chage Se ´lectif d'Enductions Pharmaceutiques[END_REF].

Fig. 1 .

 1 Fig. 1. Schematic view of the experimental set-up.

Fig. 2 .

 2 Fig. 2. Drying kinetics of a 200 mm thick coating under four drying different processes: pure convection (), combined process: convection plus MIR (x), SIR (), conduction ().

Fig. 3 .

 3 Fig. 3. Averaged temperature of a 100 mm thick coating under Short Infra Red drying (Table 2) obtained by extrapolation of experimental results on 2 -15 mm thick coatings.

Fig. 4 .

 4 Fig. 4. Active substance solubility curve in the active phase (heavy solvent plus active substance) obtained by cooling of a solution initially at 70°C by air at 15°C.

Fig. 5 .

 5 Fig. 5. Mass evolution of each components during drying kinetics of a 100 mm thick coating under Short Infra Red drying process.

Fig. 7 .

 7 Fig. 7. Volumic contents of the water, active phase and polymer during drying of a 100 mm thick coating under Short Infra Red drying process. Indication of the shrinkage in terms of coating thickness before drying and after 5 min of drying.

Fig. 8 .

 8 Fig. 8. Volumic content fields of the heavy solvent at three drying times (300 2, 600 , 900 s x) during drying of a 100 mm thick coating under Short Infra Red drying process.

Fig. 10 .

 10 Fig. 10. Mass fraction fields of the active substance at three drying times (300 2, 600 , 900 s x) during drying of a 100 mm thick coating under Short Infra Red drying process. Indications of the possible zones of crystallisation.

Fig. 9 .

 9 Fig. 9. Volumic content fields of the active substance at three drying times (300 2, 600 , 900 s x) during drying of a 100 mm thick coating under Short Infra Red drying process.

Table 1

 1 Typical wet composition of the coating

	Acrylic polymer (Pol)	Light solvents		Active phase
		Sl 1	Sl 2 Sl 3 Excipient heavy solvent (E)	Active substance (PA)
	Molecular weight	18 (water)	46	76	190	340
	Typical composition (100 g) 52	38	3	4	2	1

Table 2 .

 2

	):

Table 3

 3 Final composition indicators for different heating modes

	Indicators	SIR	MIR	Conduction	Pure convec-
					tion
	Mass ratio	X	:X	0.8X	:0.8X
	(Sl i +E)/Pol
	Mass ratio E/ Y	0.9Y	0.5Y	0.6Y
	Sl 1
	Drying time
	range