Ryosuke Asato

Colin J Martin

Seifallah Abid #

Yohan Gisbert #

Fumio Asanoma

Takuya Nakashima

Claire Kammerer #

Tsuyoshi Kawai

Gwénaël Rapenne

François Diederich

A Molecular Rotor Functionalized with a Photoresponsive Brake

A molecular motor which has been previously shown to rotate when fueled by electrons through a STM tip has been functionalized with a terarylene photochrome fragment on its rotating subunit. Photoisomerization has been performed under UV irradiation. VT 1 H-NMR and UV-Vis study demonstrate the rotational motion and its braking action after photoisomerization. The braking action can be reversed by thermal heating. Once the rigid and planar closed form is obtained, the rotation is effectively slowed at lower temperature making this new rotor a potential motor with independent response to electrons and light.

INTRODUCTION

In the last decades, chemists have created many kinds of complex molecular architectures following a monumentalization [START_REF] Balzani | Molecular Devices and Machines -Concepts and perspectives for the Nanoworld[END_REF][START_REF] Rapenne | Design and synthesis of mono-molecular machines[END_REF] strategy. This approach consists of synthesizing molecules for use as elementary building blocks for the construction of more complex systems. Multifunctional nanomachines have emerged as a very dynamic area with their ingenious application based on mechanically interlocked molecules and over-crowded alkenes being recognized by the Nobel committee with the 2016 prize awarded to Jean-Pierre Sauvage, Sir J. Fraser Stoddart and Ben L. Feringa. 3 Other examples including, biomimetic nanomachines [START_REF] Breslow | Biomimetic chemistry: biology as an inspiration[END_REF] such as molecular motors, 5 molecular muscles, 6 molecular walkers [START_REF] Von Delius | synthesis and operation of small molecules that walk along tracks[END_REF] and technomimetic nanomachines 8 like molecular scissors, [START_REF] Muraoka | Light-driven open-close motion of chiral molecular scissors[END_REF] gears, 10 nanowheels, 11 wheelbarrows, 12 nanovehicles, [START_REF]25ºC[END_REF] winches, [START_REF] Gisbert | Modular synthesis of pentaarylcyclopentadienyl Ru-based molecular machines via sequential Pd-catalysed cross couplings[END_REF] elevators [START_REF] Badjić | A molecular elevator[END_REF] and robotic arms have been reported. [START_REF] Kassem | Pick-up, transport and release of a molecular cargo using a small-molecule robotic arm[END_REF] This machinery toolkit provides the basis for the future design of bottom-up nanoscale systems and materials to perform tasks as varied as nanoscale mechanics, information storage and molecular electronics.

In such models, it is very important for a multi-component molecular system to have many independent inputs controlling their motions. In our case, we recently achieved triggering and control over the stepby-step rotation of a molecular motor 2 nm in diameter deposited on a metallic surface. 5f Depending on which part of the rotor subunit is approached by the electron injecting STM tip, we can control the rotation with about 80% unidirectionality achieved. Although this is not sufficient to lead to complete rotational control it has been observed that chirality also plays a very important role in obtaining unidirectionality. [START_REF] Vives | Prototypes of molecular motors based on star-shaped organometallic ruthenium complexes[END_REF] The ruthenium complex used (Figure 1) contains two ligands which are not chiral however once deposited on the surface, the trisindazolyl borate and pentaphenylcyclopentadienide ligands are twisted out of the coordination plane [START_REF] Zhang | A chiral molecular propeller designed for unidirectional rotations on a surface[END_REF] with the ruthenium complex becoming chiral. Aside from the response to electrons, light offers a further interesting independent input to switch on and off the rotation. To achieve this goal, we have designed a dual photo and electro-responsive target motor incorporating a photochromic fragment in the backbone of the rotor. The open form of such photochromes is very flexible and could allow for the rotation of the rotor, whereas once converted to its rigid closed form, the rotation could be hindered leading to rotational slowing and eventual blocking, the photochromic moiety thus acting as a light-induced brake. To devise an efficient target, the most important elements are the design of the linker and the interactions between the motor and the photochrome. In this paper, we present a computational-based molecular design to select the best combination of suitable photochromes and linkers followed by the synthesis and characterization of the selected target. The steric hindrance between the photochrome-substituted rotor and the stator is a key parameter. More precisely, to obtain an efficient brake function, we need to tune the intramolecular interactions to select geometries allowing for blocking of the rotation once linked to the motor. This is proposed to be done by having the rigid closed form of the selected photochrome lie between two of the three legs of the stator part, hindering rotation, while the flexible open form of the photochrome will not interact with the stator, leaving the system free to rotate. Our concept is illustrated in Figure 2. This brake function would be complementary to the rotation of our motor which is induced by electrons and controlled at will in clockwise or anti-clockwise directions. Our motors would then integrate two independent input signals: electrons as the source of energy to rotate and light which could allow or slow the process at will.

RESULTS AND DISCUSSION

Design of the photochromic molecular motor

For the successful integration of the proposed photochromic brake into the existing motor framework we identified a number of contributing factors initially needed to be optimized to allow for its operation, in particular the nature of the photochromic moiety, and the linker between the photochrome and the motor. The latter needs to be short with a controlled directionality to allow the photochrome (i.e. the brake when closed form) to block the rotation by interaction with the stator (the tripodal ligand). To select the most suitable target, Gaussian DFT calculations have been performed using the hybrid dispersive ωB97xD functional 19 to obtain the energy minimized geometries of the open and the closed form of the photochrome-functionalized molecular motor. The optimal target being the one with the largest steric blocking interactions upon photoisomerism between the flexible open form and the rigid closed form.

A number of photochromic moieties with the potential to act as a brake were identified, based upon previous reports of diarylethene and terarylene photochromic properties. 20 Each photochromesubstituted molecular motor was modelized and geometry was optimized in their open and closed photochromic isomers to estimate the changes brought about by ring closure and the resulting effect on rotational motion through steric interactions with the stator part of the molecular motor. These interactions were evaluated qualitatively thanks to the optimized geometries of both isomers of each molecule calculated in the gas phase but giving insightful clues on the ability to rotate and brake of each molecular motor. Videos displaying a 360° rotation of the molecules allowing for a better illustration of the steric hindrances are provided as Supplementary Material to this paper.

The design of the photochromic part was initially considered in terms of the switching efficiency and the geometries obtained before and after. As such, it was decided to use an initial photochromic core framework which can be conjugated to the cyclopentadiene part of the motor (Figure 3, left) rather than one of the peripheral side rings that undergo ring closure (Figure 3, right). To achieve this, a terarylene core was chosen rather than a diarylethene due to the extra rigidity it offers through the central core aromatic ring compared to the more flexible ethynyl core offered by diarylethene. The photochromic terarylene consists of three heteroaromatic rings to form a photoreactive hexatriene framework in its molecular structure, which undergoes reversible photoinduced cyclization to a closed-ring isomer. As previous studies on molecular motors have shown that the aromatic substituents on the cyclopentadiene ring rotate out of the plane, [START_REF] Carella | Synthesis and reactivity of penta(4halogenophenyl)cyclopentadienyl hydrotris(indazolyl)borate Ru(II) complexes : Rotation-induced Fosbury flop in an organometallic molecular turnstile[END_REF] substitution on the core of the terarylene also offers a conformation in the closed form expected to lead to larger steric interactions between the photochromic and stator parts of the molecule which is important to induce blocking of the rotation by the photochromic part in the closed form.

Figure 3. Motor (in blue) with two possible photochromic terarylene substitution patterns (in green). X = C (thiophene) or N (thiazole, with no R2)

We decided to use a combination of thiazole and thiophene rings as the core and peripheral cycles within the photochromic moiety. The presence of both of these substituents is advantageous in terms of conformational preference and is known to maximize the donor-acceptor properties leading to efficient ring opening and closing and higher photochromic quantum yields. [START_REF] Nakashima | Photochromism of Thiazole-Containing Triangle Terarylenes[END_REF] For the initial braking systems it was decided to use methyl substituents (Figure 3, R3) at the positions of photochromic bond formation as this is known to lead to stable, reversible photochromic switching. [START_REF] Kawai | Photochromic amorphous molecular materials based on dibenzothienylthiazole structure[END_REF] Based on these considerations five target photochromic frameworks T1 -T5 (Figure 4) were selected for initial calculations and to screen the effect of substituents present (Figure 3, R1 and R2) upon their incorporation into the motor frameworks MM-T1 -MM-T5. Each was modelled computationally using the Gaussian 09 DFT program operating using the ωB97xD functional and the 6-31G(d,p) basis set 19 for all atoms except ruthenium for which LANL2DZ 24 was used. For the systems containing a thiophene central ring and thiazole peripheral rings (MM-T1 and MM-T2) the absence of a sterically bulky substituent at the thiazole 3-position meant no steric interactions between the photochrome and the stator in either their open or closed isomers was observed (Figure 5). In the case of MM-T1 (see SI) the absence of any interaction with the stator, even in the closed form, lead to the central thiophene ring of the photochrome lying coplanar to the cyclopentadiene ring of the rotor, resulting in the photochrome pointing away from the stator even in the closed isomer. For the three systems containing a thiazole central ring and thiophene based peripheral rings (MM-T3 to MM-T5) the increased steric hindrance resulting from substitution at the thiophene 4-position led to the presence of the desired braking interactions in the closed form. As such, it was decided to use a thiazole as the central core ring connected to the motor and thiophenes substituted at the 4-position as the peripheral photochrome rings.

In the DFT calculations of these systems (MM-T3 to MM-T5), the open form appears highly flexible and shows no apparent interactions between the photochrome and the stator. However, upon switching to the closed form the increased rigidity of the photochrome, in conjunction with the angle between the cyclopentadiene ring and the connected benzothiophene ring of the photochrome (49° for MM-T3), leads to the rigid closed form taking up a position pointing between two of the three legs of the stator. To help to visualize the steric hindrance, a 3D animation movie is available (supporting information) in the case of MM-T3 (open and closed forms). Such geometries are predicted to result in cessation of the rotation of the molecule as the 'brake' is applied upon switching from the open to closed photochromic forms.

To examine the effect of varying the distance between the photochrome and the cyclopentadienyl core and to potentially simplify the synthetic method, a phenyl spacer was inserted between the motor and the photochromic system to give MM-S5, the extended photochrome derivative of MM-T5, and the DFT calculation was repeated. However, in the presence of the spacer both the open and closed forms of MM-S5 show no steric interactions. This led us to conclude that the terarylene photochrome is required to be substituted directly on the cyclopentadiene ring of the rotor to induce rotational blocking further developing the requirements in our molecular design.

Of the remaining designs of molecular motors functionalized with photochromes T3, T4 and T5, MM-T3 (Figure 5) appears to offer the best initial target as a molecular brake for several reasons: 1) It has the lowest amount of interactions between the open photochromic form and the stator, meaning the system will find it easier to rotate prior to application of the light brake. 2) It also contains the highest levels of hydrogen bonding (CH/S and CH/N) between the constituent rings of the photochromic moiety; such interactions are known to lead to higher ring closure quantum yields which will make the braking mechanism more photon efficient. 3) It shows a strong blocking interaction between the closed form isomer and the stator similar to that of other targets with the closed form lying between two legs of the stator. 4) Its relative synthetic simplicity as an initial target molecule compared to MM-T4 and MM-T5. As such we decided to incorporate T3 as the photochromic part of our initial molecular brake motor referred as MM-T3 (Figure 6).

Figure 6. Open (top) and closed (bottom) forms of MM-T3, showing the changes in interaction between

the stator and the photochromic moiety. The -system involved in the photochromic isomerization is indicated in green for the open and red for the closed forms. A 3D animation movie is available as a supporting information to help to ease the visualization of the interactions.

Synthesis of the photochrome-substituted motor

The synthesis of the photochrome-functionalized motor MM-T3 was achieved in five steps with a global yield of 4.5% (Scheme 1 and 2). The synthesis of the required terarylene 4,5-bis(2-methyl-1benzothiophen-3-yl)-1,3-thiazole (T3) was first tackled. To start with, 4,5-dibromo-1,3-thiazole 1 [START_REF] Uzelac | Synthesis of brominated thiazoles via sequential brominationdebromination methods[END_REF] was coupled to 4,4,5,5-tetramethyl-2-(2-methyl-1-benzothiophen-3-yl)-1,3,2-dioxaborolane (2 [START_REF] Nakashima | Photochromism of Thiazole-Containing Triangle Terarylenes[END_REF] ) through Suzuki-Miyaura coupling to give terarylene T3 in a 74% yield after purification by column chromatography. T3 was then lithiated at the 2-position of the thiazole ring using n-butyllithium and reacted with 2,3,4,5-tetraphenylcyclopentadienone to give the tetraphenylcyclopentadienol 4 in 63% yield. Unlike in the precursor T3 only one rotational conformer of 4 is seen in the NMR spectrum, resulting from hydrogen bonding between the alcoholic proton and the thiazole nitrogen favoring one conformation in solution. 4 was then converted to its chloride analog 5 by reaction with thionyl chloride in benzene in a yield of 90%. This chloride analogue consists of three regioisomers present in a nonstoichiometric ratio, leading to highly complex NMR data for 5. [START_REF] Thépot | A convenient synthesis of bromopentaarylcyclopentadienes containing methyl or fluorine substituents[END_REF] The presence of this chlorine allowed us to coordinate the cyclopentadiene ring to the ruthenium atom, with oxidative addition of the triruthenium dodecacarbonyl cluster at the carbon-chlorine bond of all regioisomers of 5 leading to the same metal complex. The cyclopentadienyl ruthenium(II) complex 6 was obtained as a bright yellow solid in 56% yield, with two conformational isomers observed in the 1 H NMR with a 2:1 ratio observed in the methyl peaks.

Finally, coordination of the tris[(ethylsulfanyl)methyl]indazolylborate ligand was performed by reaction of the ruthenium cyclopentadienyl complex 6 with two equivalents of the thallium salt of the hydrotris(indazolyl)borate derivative [START_REF] Erbland | Expedient synthesis of thioether-functionalized hydrotris(indazolyl) borate as an anchoring platform for rotary molecular machines[END_REF] by microwave heating in acetonitrile at 100 °C (Scheme 2). Due to the evolution of carbon monoxide during the reaction, the pressure was released every 10 min and the conversion followed by TLC of the crude mixture. The target photochromic motor MM-T3 was obtained in 20% after 1 hour. Each compound was characterized using NMR and mass spectral data, however the presence of multiple conformational and rotational isomers made complete assignment of the NMR spectroscopic data difficult due to significant broadening of signals.

Scheme 1. Synthesis of the cyclopentadiene rotor functionalized with a terarylene fragment (5).

Scheme 2. Synthesis of the photochromic motor MM-T3 incorporating a terarylene brake.

Photoswitching of the precursor (4)

To examine the effect of illumination upon the motor structure, a 1 H NMR study was carried out. Initially the photochromic switching of the precursors was examined; due to the relative stabilities of the molecules it was decided to look at the alcohol 4 as a representative example. To allow for comparison with MM-T3, this was done in CD2Cl2. Here, sharp signals are observed at 7.68, 7.64 and 6.83 ppm for three of the four photochrome benzothiophene protons, along with a large broadened region between 7.45 and 6.90 ppm masking the fourth benzothiophene signal (Figure 7 top). This broadening of the aromatic region is consistent with what is seen for the photochromic precursor T3 along with observations of similar non-photochromic cyclopentadienyl systems and results from the high rotational barriers for benzothiazole substituents in the terarylene core on the timescale of NMR. The methyl substituents present at the sites of photoinduced bond formation further hinder rotation and as such there are a number of similar but not identical environments for these protons, further broadening the signals. [START_REF] Kammerer | Biomimetic and technomimetic single molecular machines[END_REF][START_REF] Zhang | A chiral molecular propeller designed for unidirectional rotations on a surface[END_REF][START_REF] Erbland | Expedient synthesis of thioether-functionalized hydrotris(indazolyl) borate as an anchoring platform for rotary molecular machines[END_REF] Due to this broadness a precise integration is not possible, however approximate values show an integration of twenty-eight protons relative to the alcohol OH singlet at 4.71 ppm. Along with this, broad signals for the methyl substituents on the photochrome, integrating for six protons, are observed between 1.98 and 1.75 ppm.

As at the illumination wavelengths required both open to closed and closed to open switching can occur, isolation of a solution containing 100% of the closed form was not possible. Instead the photostationary state (PSS) was prepared to allow observation of a solution containing a majority of closed form isomers. Based on previous reports on the efficiencies of ring closing and opening of the photochromic framework a PSS containing greater than 80% closed form is expected. [START_REF] Kutsunugi | Photochromic properties of terarylene derivatives having a π-conjugation unit on central aromatic ring[END_REF] The sample was dissolved in hexane and illuminated for 30 minutes at 254 nm, solvent removed, and the solution redissolved in deuterated dichloromethane to examine the spectrum of the PSS (Figure 7, bottom).

It can be seen that some changes occur in the aromatic peaks, however more significantly there are shifts seen in the methyl peaks between 1.98 and 1.75 ppm, indicating that photochromic ring closure has taken place with the appearance of structural isomers in both the open and closed forms. This is consistent with the absorption spectra of 4 where new bands showing the formation of a photostationary state (Figure 7, inset) including some closed form are observed, which returns to the absorption profile of the open form upon application of light above 400 nm.

Photoswitching of the photochromic ruthenium complex (MM-T3)

Next the 1 H NMR of MM-T3 was run in deuterated dichloromethane. Broad aromatic peaks are again seen between 7.80 and 6.80 ppm, indicating the presence of the cyclopentadiene rotor. Four further aromatic peaks are seen, each integrating for three protons (with two overlapping), for the protons located on the tripodal stator indazolyl fragments. These can be assigned based on previous reports with H [START_REF] Balzani | Molecular Devices and Machines -Concepts and perspectives for the Nanoworld[END_REF] and H 6 (Figure 8, top) appearing downfield at 8.32 and 7.83 ppm and H 3 and H 4 overlapping for a total integration of six protons between 6.96-6.92 ppm. The signals for the aliphatic stator chains can also be observed with H 7 appearing as a singlet at 3.87 ppm, a quartet for H 8 at 2.44 ppm and H 9 appearing as a triplet at 1.26 ppm. Also very broad signals indicating the presence of multiple conformational and rotational isomers are seen between 2.0 and 1.7 ppm for the photochrome methyl substituents. Despite the apparent broadness the expected signals for the cyclopentadienyl rotor, the photochromic brake and the indazolylborate stator can be seen in the 1 H NMR of MM-T3.

To resolve the broad peaks and to observe the effect of thermal cooling on the rotation, the 1 H NMR of MM-T3 was run at 10 K decrements between 293 K and 233 K (Figure 8, see SI for full thermal data). At 273 K the two overlapping stator peaks between 6.96 and 6.92 ppm begin to separate. Upon further cooling to 253 K all four of the aromatic stator peaks split into two resulting from the rotation slowing as the solution cools. Upon rotational 'braking', a 2:1 splitting of these peaks is expected, with two of the stator legs remaining equivalent and the other becoming independent. Indeed, it is stator proton H 1 that points towards the rotor that shows the largest thermal splitting. Upon further cooling to 233 K a further increase in resolution of the aromatic signals is observed, however this is not enough to allow for their further characterization. Such splitting of the stator protons offers a clear measure of the changes in the rotational environment of MM-T3. It is important to note however that unlike in the case of temperature decrease this does not lead to a splitting of the stator peaks. Thus, it can be assumed that while the rotation of the motor may be greatly hindered, it is not completely stopped at room temperature. However, the observation of this broadening even under room temperature NMR conditions, indicates the possibility of obtaining complete light induced rotational 'braking' under different conditions, including those employed for STM analysis. It should be mentioned that no decomposition nor decoordination of the cyclopentadienyl or the tripodal ligand was observed, each fragments having characteristic peaks which did not appeared in any of the NMR spectra of the illuminated MM-T3 '(Figure S13). Next the sample was illuminated using orange (above 550 nm) light in an attempt to return the closed photochrome to its open form, however NMR showed that the sample remained in the closed form even after illumination for 15 min. This can be attributed to a combination of the competition in absorption between the photochrome and the ruthenium center at these frequencies and energy quenching of the ligand-centered excited state by the ruthenium center. Instead the system was left in the dark for seven days to allow the ring opening to occur thermally. NMR analysis after this time showed the successful reversion to the open form, indicating full thermodynamic release of the "brake" over time (Figure 9, bottom).

To further examine the change in MM-T3 with light, its absorption profile before and after illumination was observed and compared with calculated DFT values. In dichloromethane (Figure 10, left, black line) the open form of MM-T3 shows absorption bands between 400 and 600 nm similar to those reported for similar non-photochromic motors. [START_REF] Erbland | Expedient synthesis of thioether-functionalized hydrotris(indazolyl) borate as an anchoring platform for rotary molecular machines[END_REF][START_REF] Carella | Synthesis of new tripodal trifunctionalized hydrotris(indazol-1-yl)borate ligands and X-ray structure of their ruthenium cyclopentadiene complexes[END_REF] TD-DFT calculations show these absorptions as ruthenium centered coming mainly from the metal dominated HOMO-1 orbital level. Upon formation of the photostationary state only small changes are seen, indicating absorption by the ruthenium center is not significantly affected by photochromic ring closure. To monitor these changes, the difference in absorption between the PSS and the open form was calculated (Figure 10

CONCLUSION

In this work, a previously reported molecular motor has been functionalized with a terarylene photochromic fragment and fully characterized. VT-NMR showed that the rotation of the functionalized cyclopentadienyl ligand becomes blocked when the closed form is generated by the application of light and its rotation is drastically hindered. Work is now underway to investigate by STM the complementary functionality of the light to block electron-induced rotation of our motor on surfaces.

EXPERIMENTAL SECTION

Materials and instrumentation

All commercially available chemicals were of reagent grade and were used without further purification.

Column chromatography was carried out on 230-400 mesh silica gel (Aldrich) unless otherwise stated. Compound 1 [START_REF] Uzelac | Synthesis of brominated thiazoles via sequential brominationdebromination methods[END_REF] , 2 [START_REF] Yang | Assessment of the "6-31+G** + LANL2DZ" Mixed Basis Set Coupled with Density Functional Theory Methods and the Effective Core Potential: Prediction of Heats of Formation and Ionization Potentials for First-Row-Transition-Metal Complexes[END_REF] and the thallium salt of hydrotris[(ethylsulfanyl)methyl]indazolylborate [START_REF] Erbland | Expedient synthesis of thioether-functionalized hydrotris(indazolyl) borate as an anchoring platform for rotary molecular machines[END_REF] were synthesized as previously reported.

Microwave reactions were carried out by using a CEM Discover LabMate microwave reactor. TLC was performed on precoated aluminum-backed silica gel 60 UV254 plates (Macherey-Nagel) with visualization by UV irradiation (λ =254, 366 nm). 1 H-NMR (500 MHz) and [START_REF]25ºC[END_REF] C-NMR (125 MHz) spectra were recorded on Bruker Avance III HD 500 MHz, 1 H-NMR (600 MHz) spectra were recorded on JEOL JNM-ECA600 and VT NMR were recorded on JEOL JNM-ECX400P. Residual solvent signals were used as internal references for 1 H and 13 C-NMR spectra which were referenced according to the solvent. IR spectra were recorded with a Nicolet 6700 FTIR-ATR spectrometer. Only selected characteristic peaks were recorded. Recycling preparative GPC were performed on LC-9110NEXT (Japan Analytical Industry). Reverse Phase HPLC were performed on LaChrom Elite (Hitachi). Highresolution mass spectra were obtained with a Waters GCT Premier spectrometer for desorption chemical ionization (DCI/CH4), with a Waters Xevo G2 QTof spectrometer for electrospray ionization (ESI) with a Bruker Autoflex Ⅱ and with a JEOL JMS-Q1000TD and JMS-700 Mstation. UV-Vis absorption spectra were obtained on JASCO V-550, JASCO V-660 and JASCO V-760, Shimadzu UV-26000 spectrometer. Light source of UV (254 nm and 365 nm) was AS ONE SLUV-4.

All calculations were performed with the Gaussian09 package 30 using the hybrid dispersive ωB97xD functional and the 6-31G (d,p) basis set which can represent all atoms up to argon. The LANL2DZ functional operating with a pseudopotential, was used for the ruthenium. Geometry optimization was performed in vacuum and followed by a frequency calculation showed no negative frequencies confirming that resulting structures were in a minimum of the potential map. TD-DFT calculations were performed at the same level of theory for the open and closed form of MM-T3 using the previously optimized geometry as a starting point taking the dichloromethane solvent into account using a PCM model in order to obtain comparable UV-VIS spectra.

Synthesis and characterization

4,5-bis(2-methylbenzo [b]thiophen-3-yl)thiazole (T3) 4,5-Dibromo-1,3-thiazole ( 1) (1 eq., 1.00 g), 4.12 mmol) and 4,4,5,5-tetramethyl-2-(2-methyl-1benzothiophen-3-yl)- (d, 3 J=7.8 Hz, 1H). [START_REF]25ºC[END_REF] C NMR (125 MHz, CD2Cl2, 25ºC) 14.8, 14.9, 90.1(>COH), 121.9, 122.1, 122.3, 122.9, 123.1, 124.1, 124.5, 124.6, 124.9, 126.8, 127.7, 127.9, 128.1 ,128.4, 129.4, 129.9, 130.0, 134.0, 134.2, 134.9, 135.1, 138.1, 138.2, 139.6, 139.7, 140.2, 141.1, 146.5, 171.8 (-CSN). HRMS (DCI/CH4) (m/z): [M] + calcd. for C50H35NOS3 761.1881; found 761.1882. 4) (1 eq., 50 mg, 0.066 mmol) was dissolved in 2 mL of benzene. Thionyl chloride (6 eq., 69 mg, 0.042 mL, 0.58 mmol) was slowly added, and the solution heated to reflux. After heating for 30 min, the solution was allowed to cool down to room temperature. An orange solution was purified by silica gel column chromatography (1:1 dichloromethane:hexane) to give compound 5 as yellowish orange powder. (45 mg, 87%). CD2Cl2,25ºC) shows broad signals in the aromatic region due to three regioisomers present in a non-stoichiometric ratio, see SI (Fig. S4) for the full spectrum   (ppm) 1.51-2.03 (6H, CH3 peaks), 6.62-7.45 (24 H, aromatic signals), 7.57-7.74 (4H, aromatic signals). [START_REF]25ºC[END_REF] C-NMR (125 MHz, CD2Cl2, 25ºC)  (ppm) shows complex signals in the aromatic region due to three regioisomers present in a non-stoichiometric ratio, see SI (Fig. S5) for the full spectrum. 14.5 (CH3), 14.6 (CH3), 15.2 (CH3), 58.4 (>CCl), 121.3, 121.4, 121.8, 121.9, 123.5, 123.6, 124.1, 124.6, 126.5, 127.5, 127.6, 127.7, 127.8, 128.0, 128.3, 128.5, 128.8, 129.0, 129.2, 129.5, 130.0, 130.3, 133.4, 134.2, 134.6, 137.8, 137.9, 140.4, 147.9, 159.2 5) (1 eq., 44 mg, 0.0564 mmol) and triruthenium dodecacarbonyl cluster (0.6 eq., 21.6 mg, 0.0338 mmol) were introduced. Anhydrous and degassed toluene (2 mL) was then added. The mixture was heated to reflux for 2 h under argon. Then the solution was allowed to cool down to room temperature, the organic layer concentrated and purified by column chromatography (1:1 dichloromethane:hexane) to give compound 6 as bright yellow solid (30 mg, 56%). 1 H-NMR (500 MHz, CD2Cl2, 25ºC, TMS) (ppm) shows two conformational isomers. 1.63 (s, 2H, CH3 isomer peak), 1.80 (s, 2H, CH3 isomer peak), 2.06 (s, 1H, CH3 isomer peak), 2.34 (s, 1H, CH3 isomer peak), 6.37-7.84 (28 H, aromatic signals). 13 C NMR (125 MHz,CD2Cl2,25ºC) shows broad signals due to the presence of two conformational isomers, see SI-Fig. S9 for the full spectrum  (ppm) 14.6 (-CH3), 14.8 (-CH3), 77.6 (>CCl), 113.0, 115.4, 121.6, 122.1, 124.4, 124.6, 124.9, 127.6, 127.7, 128.0, 128.2, 128.4, 131.7, 133.7, 138.1, 139.1, 142.1, 144.0, 205.0 (CO), 205.8(CO). IR (ATR, solid): cm -1 2360 (CO), 2342 (CO). In a glove box, in a dried reactor specifically designed for microwave irradiation under argon were successively added a magnetic stirrer bar, chlorodicarbonyl[ (1,2,3,4,5-η)-1-(4,5-bis(2methylbenzo[b]thiophen-3-yl)thiazol)-2,3,4,5-tetraphenyl-2,4-cyclopentadien-1-yl]ruthenium(II) (6) (1 eq., 32 mg, 0.0341 mmol), thallium hydrotris{6-[(ethylsulfanyl)methyl]indazol-1-yl}borate (2.5 eq., 64.4 mg, 0.0815 mmol) and degassed acetonitrile (8.38 mL). The reaction mixture was heated to 100°C six times for 10 min in each heating cycle. A pressure of 5 bar was achieved and it was released between heating cycles. The completion of the reaction was monitored by TLC. The resulting mixture was diluted with dichloromethane and filtered through silica gel. The solvents were removed under reduced pressure and the residue was purified by column chromatography (2:1 dichloromethane:hexane) to afford compound MM-T3 (9.8 mg, 20%) as a brown solid. 1 H-NMR (500MHz, CD2Cl2, 25ºC, TMS)  (ppm)

Figure 1 .

 1 Figure 1. Structure of the parent ruthenium-based molecular motor adsorbed on a gold surface and related electron-fueled unidirectional rotation. The tripodal ligand (stator) is anchored on the surface and the functionalized cyclopendadienyl ligand (rotor) can rotate when electrons are provided by the tip of an STM. Electron injected on one of the four ferrocenes can induce counter-clockwise rotation whereas injection in the tolyl group induce clockwise rotation.

Figure 2 .

 2 Figure 2. Schematic representation of the difference between the open-and closed-forms of the targeted photochrome-substituted molecular motor. Upon application of UV light, the closed-form of the photochrome is obtained preventing the rotation of the rotor part of the motor.

Figure 4 .

 4 Figure 4. Preselected photochromic moieties T1-T5 and S5 for incorporation into the motor backbone. The blue spheres represent the motor fragment.

Figure 5 .

 5 Figure 5. Optimized geometries as side and top views of the open and closed forms of the molecular motors MM-T1 to MM-T5 and MM-S5 functionalized with photochromic moieties T1-T5 and S5. The -systems involved in the photochromic isomerization are indicated in green for the open and red for the closed forms.

Figure 7 .

 7 Figure 7. Photoswitching of the precursor (4) by illumination at 254 nm followed by UV-Vis. (CH2Cl2) spectroscopy (Inset) and by 1 H-NMR (600 MHz) in CD2Cl2. At the top is shown the open form and at the bottom the photostationary state. Assignments were made with the assistance of COSY, NOESY, HMQC and HMBC of the sample run at 500 MHz in CD2Cl2.

Figure 8 .

 8 Figure 8. 1 H-NMR of MM-T3 (Open form) at 293, 273, 253 and 233 K in CD2Cl2 (400 MHz).

Figure 9 .

 9 Figure 9. 1 H-NMR of MM-T3 before (top) and after (middle) illumination along with after seven days (bottom) in CD2Cl2 (500 MHz).

Figure 10 .

 10 Photostationary state

  1,3,2-dioxaborolane (2) (2.3 eq., 2.60 g, 9.47 mmol) were dissolved in 1,4-dioxane (15 mL) and aqueous tripotassium phosphate (2 M, 15 mL) was added dropwise. The solution was bubbled with N2 for 15 min. Then, tetrakis(triphenylphosphine)palladium (5 mol%, 0.230 g, 0.206 mmol) was added and the solution stirred under reflux for 36 hours. After cooling down to room temperature, the solution was extracted by ethyl acetate and washed with water and saturated aqueous solution of sodium chloride. The organic layer was dried with anhydrous sodium sulfate, filtered, and concentrated. Silica gel column chromatography (9:1 hexane:ethyl acetate) gave a white solid (1.1 g, 74%).

[START_REF] Balzani | Molecular Devices and Machines -Concepts and perspectives for the Nanoworld[END_REF]

H-NMR (500 MHz, CD2Cl2, 25ºC)  (ppm) 2.02 (m, 6H), 7.20-7.29 (m, 4H), 7.56 (s 2H), 7.72-7.74 (m, 2H), 9.14 (s, 1H), NMR consistent with previous report.

[START_REF] Kutsunugi | Photochromic properties of terarylene derivatives having a π-conjugation unit on central aromatic ring[END_REF]

  . HRMS (ESI) (m/z): [M+H] + calcd. for C50H35ClNS3 780.162; found 780.167. HRMS (ESI) (m/z): [M+H] + calcd. for C50H35ClNS3 780.162; found 780.167.

ACKNOWLEDGEMENTS

This work was supported by the MEXT Program for Promoting the Enhancement of Research Universities in NAIST, the CNRS and the University Paul Sabatier (Toulouse). It has also received funding from the European Union's Horizon 2020 research and innovation program under the project MEMO, grant agreement No 766864 and from the JSPS KAKENHI grant in aid for Scientific Research on Innovative Areas "Molecular Engine (No.8006)" 18H05419. R.A. thanks NAIST foundation for financial support. Y.G. thanks the French Ministry of National Education for a PhD Fellowship. C.J.M. thanks the JSPS KAKENHI Grant-in-Aid for Early-Career Scientists (19K15312) and G.R. the JSPS KAKENHI Grant-in-Aid for Challenging Research (20K21131). This research was also partly supported by the JSPS KAKENHI Grant Number JP26107006 in Scientific Research on Innovative Areas "Photosynergetics". The DFT calculations appearing in this article were performed using HPC resources from CALMIP (Grant 2020-p20041).


Sheng Gao

Yohan Gisbert

Guillaume Erbland

Seifallah Abid

Claire Kammerer

Alessandro Venturini

Gwénaël Rapenne
email: rapenne@cemes.fr

Barbara Ventura
email: barbara.ventura@isof.cnr.it

Nicola Armaroli

Photophysical properties of 1,2,3,4,5-pentaarylcyclopentadienyl hydrotris(indazolyl)borate ruthenium(II) complexes

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Molecular machines are chemical systems that can undergo mechanical movement on the nanometric scale, upon application of a suitable external stimulus. 1 They can occur in living organisms, but can also be synthesised as so-called artificial molecular machines, 2 which are potentially relevant for applications in nanoscience, 3 biomedicine 4 and material science 5 and have been recently recognised with the Nobel Prize awarded to Sauvage,Stoddart and Feringa. 6 According to the type of movement in spatial dimension, they are classified as linear 7 (e.g., muscle fibers) or rotary 8 (e.g., ATP synthase). Due to their specific movement and structure, some of the rotary machines can be considered as molecular motors 9 or molecular gears. 10 Among artificial molecular machines, only a few are based on coordination complexes even if this is a very versatile and efficient way to assemble mechanical subunits, allowing for the production of a large and diverse range of molecular machines thanks to the vast number of metals and ligands available to tune their molecular architectures and physico-chemical properties. 11 In recent years, gear-shaped molecules have attracted considerable attention from synthetic and physical chemists. 12 In previous work, some of us designed, synthesised and rotated, thanks to the STM tip operating as electron provider 13 or electric-field inducer, 14 a series of star-shaped molecular motors and gears based on ruthenium complexes containing the hydrotris(indazolyl)borate ligands as tripodal platforms. 15 These compounds interestingly exhibited controlled clockwise or anticlockwise unidirectional rotation once anchored on metallic surfaces As tiny movable entities, molecular machines can be stimulated, by chemical, electrical or optical inputs. The latter are preferable because particularly clean, effective, easy to operate and non-invasive. 16 Therefore, it is of capital importance to investigate the photophysical properties of the rotor elements to fully rationalise their mechanical behaviour under illumination.

In this work, we present a detailed investigation into the photophysical properties of two rotor-like Ru(II) complexes, Ar5L1-Ru-S1 and L3-Ru-S1, 17 along with their related ligands/fragments, in tetrahydrofuran (THF) solution both at 298 and 77 K (Scheme 1). In such complexes the metal is coordinated to a hydrotris(indazolyl)borate ligand and a penta(p-bromophenyl)cyclopentadienyl ligand or a penta(m-dimethylphenyl)cyclopentadienyl ligand respectively. 18 Model L3-Ru-(CO)2Br is taken as a reference complex for L3-Ru-S1 bearing only the cyclopentadienyl platform and lacking the scorpionate tripodal unit. L1H, L2H and L3Br are models for the pentaarylcyclopentadienide ligands, as they possess a central cyclopentadiene unit and different substituents on both the peripheral phenyls and the central ring. Finally, S1K and S1Tl are suitable models for the scorpionate tripodal hydrotris(indazolyl)borate ligand, differing only in the counterion, i.e., K + in S1K and Tl + in S1Tl.

Scheme 1. Structures of the two molecular rotors Ar5L1-Ru-S1 and L3-Ru-S1, along with their related ligands or fragments.

The ligand geometry of these Ru(II) complexes is unique. Numerous studies on the photophysical properties of ruthenocene and cyclopentadienyl/arene Ru(II) complexes are reported in the literature 19 and show that these compounds are characterised by a weak and structured emission detectable only at low temperature. On the other hand, some examples of tris(1-pyrazolyl)methane based Ru(II) complexes have been explored, where the tripodal unit is one of the coordinating ligands, accompanied by 2,2'bipyridine, pyridine or 1,10-phenanthroline type ligands. 20 These compounds exhibit from weak to intense luminescence at room temperature, in solution and in the solid state, generally attributed to a MLCT transition not involving the tripodal ligand. To the best of our knowledge, there are no reports on the photophysical behaviour of heteroleptic Ru(II) complexes containing both a cyclopentadienyltype ligand and a hydrotris(indazolyl)borate chelating unit.

We report here a comprehensive photophysical analysis of the present systems, supported by theoretical studies, which is of key importance for the development of molecular rotors based on indazolyl-borate ligands that can be operated by light stimuli or which may exhibit useful photoinduced functions. Notably, this study covers the photophysics of an unusual class of Ru(II) complexes, bearing cyclopentadienyl and indazolyl ligands.

RESULTS AND DISCUSSION

Synthesis of the ligands and complexes

The synthesis of all compounds studied in this paper (Scheme 1) has been previously described. The tripodal thioether-functionalised hydrotris(indazolyl)borate ligands have been prepared as their potassium or thallium salts (S1K and S1Tl). 21 The complex L3-Ru-S1 was synthesised by reaction of S1Tl with bromidodicarbonyl[ 5 -1,2,3,4,5-pentakis(p-bromophenyl)]cyclopentadienylruthenium(II) L3-Ru-(CO)2Br with a yield of 82%, whereas L3-Ru-(CO)2Br was prepared in two-step synthesis from ligand L3Br with the triruthenium dodecacarbonyl cluster Ru3(CO)12, as previously described. 17,21 The synthesis of the complex Ar5L1-Ru-S1 was achieved through the fivefold Suzuki-Miyaura crosscoupling reaction of L3-Ru-S1 with a large excess of 3,5-dimethylphenylboronic acid (4 equiv. per Br centre). The desired product resulting from five consecutive biaryl couplings was obtained in 52% yield which correspond to 88% per coupling reaction. 17 L2H was synthesised by direct palladium-catalysed five-fold arylation of cyclopentadiene according to a literature procedure. 22

Absorption and Emission properties

Preliminary solubility and stability tests were performed in different solvents to evaluate the best medium for the photophysical characterisation of both complexes and ligands and THF was found to be the best choice. L3-Ru-S1, Ar5L1-Ru-S1 and their models L1H, L3Br, S1K and S1Tl were found to be stable at room temperature under ambient light for several days (Figure S1), while L2H and L3-Ru-(CO)2Br showed limited stability. All the spectroscopic measurements were performed using freshly prepared solutions and investigated within 4 hours from preparation.

The absorption spectra of Ar5L1-Ru-S1 and its model ligands L1H, L2H, S1K and S1Tl are gathered in Figure 1 and the corresponding spectral data are collected in Table 1. L1H and L2H exhibit two bands in the regions 200-300 nm and 300-400 nm while S1K and S1Tl absorb only below 320 nm and show similar spectra. The molar absorption coefficient of S1K is slightly lower than that of S1Tl at higher energy. Surprisingly, the molar absorption coefficient of Ar5L1-Ru-S1 is much higher in the UV region (ε = 13.010 4 M -1 cm -1 at 267 nm) when compared to its ligands. Moreover, some new absorption features appear in the 400-500 nm range, indicative of metal-ligand interactions. A detailed theoretical analysis will discuss below these peculiar features, showing that a charge transfer transition from the metal to the * orbitals of the phenyl substituents of the cyclopentadienyl ligand is responsible for the intense absorption in the 250-350 nm region. The normalised absorption and excitation spectra of L1H, L2H, L3Br, S1K and S1Tl are gathered in Figure S3 (in THF at 298 K and 77 K). Except for L2H, the collected excitation spectra, are reasonably matched with the absorption profiles, corroborating the observed emission band. On the contrary, the poor match observed between absorption and excitation profiles of L2H may indicate limited stability of this compound.

Upon excitation at 360 nm, Ar5L1-Ru-S1 exhibits an emission spectrum with a broad band centred at ca. 440 nm under all conditions (Figure 2). To verify the occurrence of wavelength-dependent luminescence behaviour, emission maps were acquired at different excitation wavelengths, from 300 to 395 nm. Figure 3 collects such maps in deoxygenated THF solutions at 298 K, as well as in frozen matrix at 77 K. No significant shift of the emission maximum of Ar5L1-Ru-S1 is detected by varying the excitation wavelength at 298 K, whereas the intensity of the emission band changes following the absorption profile. On the other hand, at 77 K, a wavelength shift from 470 to 450 nm is observed when moving the excitation wavelength from the 300-345 nm to the 345-390 nm region, while the emission intensity remains almost constant. This observed different behaviour might indicate excited states of different nature which are no longer thermally equilibrated in frozen matrix. The emission quantum yield of Ar5L1-Ru-S1 is particularly low, i.e. 0.27% in air-equilibrated THF (Table 2).

Figure 3. Emission maps for Ar5L1-Ru-S1

in deoxygenated THF at 298 K (a and c) and in frozen THF at 77 K (b and d). The maps are depicted both as emission spectra distribution (left) and as contour plots (right). Excitation was moved from 300 nm to 395 nm with 5 nm steps. A spike due to the Raman signal is observed on the high energy side, which is unavoidable due to the weakness of the luminescence bands.

Table 2. Main emission parameters in THF.

In Figure S4 is reported the absorption spectrum of L3-Ru-S1, together with that of the related ligands and intermediates (L3Br and L3-Ru-(CO)2Br) in THF solution. Compared to its reference compounds, L3-Ru-S1 shows new absorption features in the region 300-360 nm, likely due to charge transfer transitions involving the ruthenium centre and the tripodal ligand.

In Figure S5 are collected the normalised absorption and emission spectra of L3Br, L3-Ru-(CO)2Br and L3-Ru-S1 in air-equilibrated and deoxygenated THF at 298 K and in frozen solvent at 77 K; the related absorption and emission parameters are collected in Table 1 andTable 2. L3Br exhibits a weak emission (em = 0.30%) with a maximum around 470 nm in all conditions. L3-Ru-(CO)2Br shows an emission spectrum similar to that of L3Br, with a peak at 468 nm at both 298 K and 77 K, but having emission quantum yield significantly higher (em = 2.67%) than the precursor fragment. The target complex L3-Ru-S1 was found to be virtually non-emissive both at room temperature and at 77K. Therefore, we carried out emission maps only for the intermediate complex L3-Ru-(CO)2Br, which is a relatively strong emitter. Excitation wavelengths were tuned from 300 nm to 395 nm and the results are shown in Figure S6. Both at 298 K and 77 K, the emission maxima and the relative peak intensities change with the excitation wavelengths. An apparent dual emission is observed, with different features when exciting at 300 -350 nm or 350 -400 nm. We tentatively ascribe this behaviour to the presence of some free cyclopentadienyl ligand, which is preferentially excited at 300-350 nm and shows emission features similar to those observed for L3Br upon excitation in the same region and may come from partial decomposition of L3-Ru-(CO)2Br.

Theoretical analysis

In order to rationalise the experimental data described above, we have carried out a computational analysis of the photophysical properties of the systems depicted in Scheme 2 by means of DFT and TD-DFT calculations (see experimental part). In Scheme 2, L1-Ru-S1' and Ph5L1-Ru-S1' represent two model systems of Ar5L1-Ru-S1 where the L1 and Ph5L1 upper ligands and the S1 tripodal ligand are simplified for facilitating the calculations while still enabling a reliable rationalisation. They are respectively made of a cyclopentadienyl group substituted by five phenyls (L1) or five biphenyl (Ph5L1) moieties whereas the tripodal ligand S1' exhibits three methyl substituents instead of the three ethyl in the synthesised tripodal ligand (S1). The phenyl or biphenyl substituents, even if the -systems are not strictly parallel due to steric hindrance, can provide a partial -delocalisation which can stabilise not only the negative charge of the cyclopentadienide core, but also additional charge resulting from light-induced electronic processes. In parallel, we have also investigated L1H and S1' and made additional comparisons with experimental data.

Ground-state geometries

The molecular geometries have been fully optimised by DFT methods, starting from the X-ray data of a parent complex of Ar5L1-Ru-S1. 17 Since we changed the structure to simplify the molecule, we have modified the orientation of the phenyl groups of the upper parts and of the alkyl chain of the thioether legs in order to find two local energetically stable structures (see Experimental Section and Figure S12 A for the overlap between fragment of X-ray structure of a parent complex and calculated structures). In Figure 4 (bottom) are depicted the geometries of L1-Ru-S1' and Ph5L1-Ru-S1'. Upon full optimisation, a perfect superimposition of the two complexes is observed, except for the phenyl and biphenyl substituents of the two different cyclopentadienyl units (Figure S12 B). The average distance between the ruthenium centre and the five carbons of the cyclopentadienyl group are exactly the same in both ruthenium complexes (2.18 Å). Even the average distances between the three nitrogen atoms of the hydrotris(indazolyl)borate ligand directly coordinated to the ruthenium atom are the same (2.20 Å). On the other hand, as expected, Ph5L1 and L1, when analysed separately, exhibit geometries very different from those seen when complexed to the ruthenium centre. This illustrates the well-known steric hindrance between the pentaarylcyclopentadiene unit and the tripodal ligand. 18 Both have D5 symmetry with the phenyls rotated out of the cyclopentadienyl plane for steric reasons (Figure 4 Figure 5 shows the calculated spectra of the two ruthenium complexes depicted in Scheme 2. Ph5L1-Ru-S1' has one strongly dominating absorption feature in the UV with molar absorption coefficients comparable to the experimental values, i.e. between 10 and 1210 4 M -1 cm -1 . The computational spectrum also reproduces well the experimental absorption features between 400 and 500 nm. The five lowest transitions of Ph5L1-Ru-S1' are MLCT bands, i.e. from the ruthenium centre to the cyclopentadienyl ligand (Ph5L1). Conversely, L1-Ru-S1' shows two distinct spectral domains (260-300 nm and 340-380 nm) with much lower molar absorption coefficients (maxima 2-310 4 M -1 cm -1 ), which only partially matches the experimental absorption above 400 nm. The orbitals involved in the lowest energy transitions of Ph5L1-Ru-S1' and L1-Ru-S1' (HOMO-LUMO orbitals, Figure 6 and Figure S7 respectively) clearly evidence charge transfer from the metal to the cyclopentadienyl ligand (MLCT). The HOMO is mainly located on the ruthenium centre and the LUMO is delocalised on the * of the Ph5L1 or L1 phenyl ligands. These transitions peak at 492.6 and 481.1 nm respectively (Table 3). The difference between the two ruthenium complexes can be attributed to the rotating parts L1 and Ph5L1, which have been studied independently (Scheme 2). Ph5L1 exhibits two spectral domains with high molar absorption coefficients (260-300 and 400-440 nm, 8-910 4 M -1 cm -1 ), whereas L1 has only one dominating absorption feature in the UV (320-380 nm) with a maximum molar absorption coefficient of 510 4 M -1 cm -1 (Figure 7). In other words, absorption is more intense in the case of the larger and more delocalised system. The lowest -* transitions, a degenerate pair for each ligand due to their D5 symmetry, are seen with peaks at 438.4 and 396.8 nm, respectively, with small oscillator strength. Picture of the orbitals involved in the transitions indicates a pseudo-charge transfer from the centre to the periphery (Figure S8 and Figure S9). The delocalisation plays an important role in the absorption spectra, but the features of the two ruthenium complexes are not significantly comparable with the spectra of the two different cyclopentadienyl ligands, as shown in Figure 6. L1H exhibits two bands in the regions 200-300 nm and 320-400 nm and S1' has the lowest energy transition below 320 nm (peak at 294.1 nm). The molar absorption coefficient of S1' is slightly lower than that of L1 (Figure 7). The results are in good agreement with experiments even if S1' has been studied without a counterion. For L1H, the lowest energy transition is -* (HOMO-LUMO) and peaks at 370 nm with an oscillator strength of 0.27 (Figure S10). It can be noticed that addition of hydrogen, with the loss of the negative charge of the cyclopentadienyl, makes absorption of L1H and L1 very different (Figure 7, Figure S9 and Figure S10). S1' has two indazolyl cores that are closer to each other than the third. This is due to the interaction of a sulfur atom of the first arm with the aromatic group of the second. Interestingly, the -* transition is essentially related to a charge transfer from the arm not involved in the interaction with the other two (HOMO-LUMO orbitals in Figure S11).

CONCLUSION

The photophysical properties of two star-shaped molecular systems (Ar5L1-Ru-S1 and L3-Ru-S1) and their corresponding ligands / fragments have been investigated in THF solution. The photothermal stability tests for Ar5L1-Ru-S1 and L3-Ru-S1 indicate that these two molecular rotors are stable in THF. New absorption features in the spectra of the complexes with respect to those of the corresponding ligands/fragments are attributed to ruthenium-based charge transfer transitions. A detailed theoretical investigation allowed to establish the role of the * delocalisation on the phenyl groups of the cyclopentadienyl ligand in the absorption transitions. Concerning the luminescence properties, no emission was found for L3-Ru-S1, while Ar5L1-Ru-S1 shows a weak emission with a quantum yield of 0.27% in THF. Overall, these rotors are viable platforms for the creation of photoactivable systems and our current efforts are devoted to this goal.

EXPERIMENTAL Synthesis

All the synthetic details for the preparation of complexes Ar5L1-Ru-S1 and L3-Ru-S1 as well as for the related ligands or intermediates L2H, L3Br, S1K, S1Tl, L3-Ru-(CO)2Br were reported previously. 17,21,22 Photophysical measurements Spectrofluorimetric grade THF (Merck Uvasol®) was used without further treatments. The absorption spectra of the compounds were collected through a Perkin-Elmer Lambda 950 spectrophotometer by using pure THF as reference. Photoluminescence experiments in deoxygenated conditions were carried out into fluorimetric Suprasil quartz gas-tight cuvettes (1 cm), removing oxygen by bubbling argon for 20 minutes. An Edinburgh Instruments FLS920 spectrometer equipped with a Peltier-cooled Hamamatsu R928P photomultiplier tube (185-900 nm) and an Edinburgh 450 W xenon arc lamp (excitation light source) was used to record the emission spectra. The raw spectra were corrected by using a calibration curve supplied by the manufacturer. Photoluminescence quantum yields (em) of the compounds in solutions were obtained from the corrected emission spectra by using an air-equilibrated water solution of quinine sulfate in 1 N H2SO4 as reference (em = 0.546). 23 The excited state lifetimes (τ) were measured by means of a time-correlated single photon counting (TCSPC) HORIBA Jobin Yvon IBH FluoroHub equipment, with a TBX-05C Picosecond Photon Detection Module (300-850 nm) as the detector. Two excitation light sources (pulsed NanoLED) were used to collect the nanosecond range lifetimes, i.e. λexc = 368 nm (pulse duration < 1.4 ns) for L1H, L2H, L3Br, L3-Ru-(CO)2Br and Ar5L1-Ru-S1, λexc = 283 nm (pulse duration < 1.0 ns) for S1K and S1Tl. For the microsecond range lifetimes, a pulsed SpectraLED (λexc = 370 nm, FWHM = 11 nm) was used. The profiles of the luminescence decays were analysed by means of the DAS6 Decay Analysis Software, and the quality of the fit was assessed with the χ 2 value (to be close to unity) and with the residuals randomly distributed along the time axis. Deconvolution of the instrument response function (IRF) has been applied in the fitting procedure. Mono-exponential (Model 1) and bi-exponential (Model 2) fitting functions were used: Model 1: 𝐹(𝑡) = 𝐴 + 𝐵 𝑒𝑥𝑝(-𝑡/) Model 2: 𝐹(𝑡) = 𝐴 + 𝐵 1 𝑒𝑥𝑝(-𝑡/ 1 ) + 𝐵 2 𝑒𝑥𝑝(-𝑡/ 2 )

For Model 2, the weighted amplitudes are expressed as: 𝑓 𝑖 = 𝐵 𝑖  𝑖 / 𝑗 𝐵 𝑗  𝑗 The experimental decay traces and fitting results for the lifetime measurements in the ns range are collected in Table S1. Low temperature (77 K) luminescence spectra were measured placing the sample in a 2 mm inner diameter quartz tube and inserting it into a special quartz cold finger Dewar flask filled with liquid nitrogen. Experimental uncertainties are estimated to be ±8% for τ determinations, ±20% for em, and ±2 nm and ±5 nm for absorption and emission peaks, respectively.

Computational details

The X-ray structure of Ar5L1-Ru-S1 has been used to build the L1-Ru-S1' molecule. Figure S12A shows the overlap between the optimised molecule and a fragment of the X-ray structure of a parent complex. At this optimised molecule we added a second phenyl group to form the Ph5L1-Ru-S1' structure. The X-ray structure shows that the second phenyl is attached in para and rotated out of the plane of the adjacent phenyl of ~±36° (see Scheme 3). Looking in detail in the X-ray structure of Ph5L1-Ru-S1, both conformations with clockwise and counter-clockwise rotation exist in the crystal (Scheme 3) as expected for conformations with similar energies. For example, we tested that the energy difference between the structure where all phenyls are rotated clockwise and structure where they are rotated counter-clockwise is 0.06 kcal/mol. at the m06/6-31g (d,p) level. Consequently, we choose for all phenyls of Ph5L1-Ru-S1' the clockwise rotation being confident that the electronic properties do not change. The starting geometry of S1' was obtained using the GMMX method included in Gaussian 16 package and that of L1H after testing different orientation of the phenyl groups.

Scheme 3. Relative rotation of adjacent phenyls.

Density functional theory (DFT) calculations were carried out using the B.01 revision of the Gaussian16 program package [START_REF] Frisch | Gaussian 16, Revision B.01[END_REF] in combination with the M06 global-hybrid meta-GGA exchange-correlation functional. [START_REF] Zhao | [END_REF]26 The fully relativistic Stuttgart/Cologne energy-consistent pseudopotential with multielectron fit was used to replace the inner-core electrons of the ruthenium metal centre and was combined with the associated triple-ζ basis set (i.e., cc-pVTZ-PP basis). 27 The Pople 6-31G (d,p) basis was adopted for all other atoms to optimise the molecules and 6-31+G (d,p) for the single calculations TD-DFT. 28 All the optimisation procedures were performed using the polarisable continuum model (PCM) to simulate THF solvation effects. [29][30][START_REF] Cramer | Continuum solvation models[END_REF] Frequency calculations were always used to confirm that every stationary point found by potential-energy surface (no imaginary frequencies).

Clockwise

Counter-clockwise

Figure 1 .

 1 Figure 1. Absorption spectra of the compounds Ar5L1-Ru-S1, L1H, L2H, S1K and S1TI in THF solution at 298 K.

Figure 2

 2 Figure2compares the emission spectra of Ar5L1-Ru-S1 and its models L1H, L2H, S1K and S1Tl in THF both at 298 K and 77 K. L1H shows a weak fluorescence (em = 0.75%) at room temperature (298 K) with maximum at 450 nm in THF, which blue-shifts at 430 nm at low temperature (77 K). On the other hand, the fluorescence of L2H with maximum at 389 nm at 298 K, red-shifts by ca. 50 nm when moving to low temperature. S1K and S1Tl are weakly fluorescent at room temperature (em = 2.44% for S1K and 0.69% for S1Tl), with an emission maximum at about 325 nm (Figure2) at 298 K. At low temperature, an intense phosphorescence is observed in both compounds (Figure2and S2), with a vibronic progression of about 1300 cm -1 for S1K and 1400 cm -1 for S1Tl, which are consistent with the symmetric vibrational modes of the phenyl ring.

Figure 2 .

 2 Figure 2. Normalised absorption (black line) and emission spectra of Ar5L1-Ru-S1 and its related ligands/moieties L1H, L2H, S1K and S1Tl in deoxygenated (red line) THF at 298 K and in frozen solvent at 77 K (blue line). Excitation wavelengths are 340 nm, 310 nm and 360 nm for L1H, L2H and Ar5L1-Ru-S1, respectively, and 275 nm for S1K and S1Tl.

Scheme 2 .

 2 Scheme 2. Structures of the calculated molecular rotors L1-Ru-S1' and Ph5L1-Ru-S1', along with their related ligands or fragments. In the calculated series was used S1' with three SMe substituents instead of the three SEt in S1.

  , top).

Figure 4 .

 4 Figure 4. Top: Geometries of the cyclopentadienide ligands Ph5L1 (left) and L1 (right). Bottom: Geometries of the ruthenium complexes Ph5L1-Ru-S1' (left) and L1-Ru-S1' (right)

Figure 5 .

 5 Figure 5. Calculated absorption spectra ( and oscillator strength) of L1-Ru-S1' (red line) and Ph5L1-Ru-S1' (blue line).

Figure 6 .

 6 Figure 6. HOMO -LUMO orbitals of Ph5L1-Ru-S1' (Natural transition orbital, NTO) responsible for the lowest energy transition.

Figure 7 .

 7 Figure 7. Calculated absorption spectra of all the systems depicted in Scheme 2.

Table 1 .

 1 Main absorption parameters in THF at 298 K.

Table 3 .

 3 Lowest calculated electronic transitions and related oscillator strength f.

	Structures	 (nm)	f
	Ph5L1-Ru-S1'	492.6	0.0236
	L1-Ru-S1'	481.1	0.0166
	L1H	370.0	0.2618
	L1	396.8	0.0017
	Ph5L1	438.4	0.0002
	S1'	294.1	0.0312

ACKNOWLEDGEMENTS

This work was supported by the CNRS, the University Paul Sabatier (Toulouse) and the Italian CNR (Project PHEEL). It has also received funding from the European Union's Horizon 2020 research and innovation program under the project MEMO, grant agreement No 766864 and from the JSPS KAKENHI grant in aid for Scientific Research on Innovative Areas "Molecular Engine (No.8006)" 18H05419 and the JSPS KAKENHI Grant-in-Aid for Challenging Research (20K21131). S.G. thanks the China Scholarship Council for his PhD fellowship (file n. 201706870014). Y.G. thanks the French Ministry of National Education for a PhD Fellowship. Dr Colin Martin is warmly acknowledged for his careful reading and improving of our manuscript.