Claus Hierlinger

Heather V Flint

David B Cordes

Alexandra M Z Slawin

Elizabeth A Gibson
email: elizabeth.gibson@ncl.ac.uk

Denis Jacquemin
email: denis.jacquemin@univ-nantes.fr

Véronique Guerchais
email: veronique.guerchais@univ-rennes1.fr

Eli Zysman-Colman
email: eli.zysman-colman@st-andrews.ac.uk

A Panchromatic, Near Infrared Ir(III) Emitter Bearing a Tripodal C^N^C ligand as a Dye for Dye-Sensitized Solar Cells

The synthesis of a new complex of the form [Ir(C^N^C)(N^N)Cl] [where C^N^C = 2-(bis(4-(tert-butyl)phenyl)methyl)pyridinato (dtBubnpy, L1) and N^N is diethyl [2,2'-bipyridine]-4,4'dicarboxylate (deeb)] is reported. The crystal structure reveals an unusual tripodal tridentate C^N^C ligand forming three six-membered rings around the iridium center. The photophysical and electrochemical properties suggest the use of this complex as a dye in dye-sensitized solar cells. Time-Dependent Density Functional Theory (TD-DFT) calculations have been used to reveal the nature of the excited-states.

Introduction

Dye-sensitized solar cells (DSSCs) represent a promising solar cell technology. The majority of champion DSSCs, those showing power conversion efficiencies (PCE) greater than 10%, are based on ruthenium(II) complexes. Iridium(III) complexes, dominant as emitters in electroluminescent devices, [START_REF] Henwood | A Comprehensive Review of Luminescent Iridium Complexes Used in Light-Emitting Electrochemical Cells (LEECs)[END_REF][START_REF] Longhi | Iridium(III) Complexes for OLED Application[END_REF] have to date fared poorly as dyes in DSSCs. [START_REF] Mayo | Cyclometalated iridium(iii)-sensitized titanium dioxide solar cells[END_REF][START_REF] Yuan | Impact of Ligand Modification on Hydrogen Photogeneration and Light-Harvesting Applications Using Cyclometalated Iridium Complexes[END_REF][START_REF] Dragonetti | Simple novel cyclometallated iridium complexes for potential application in dye-sensitized solar cells[END_REF][START_REF] Ning | Novel iridium complex with carboxyl pyridyl ligand for dye-sensitized solar cells: High fluorescence intensity, high electron injection efficiency?[END_REF][START_REF] Baranoff | Cyclometallated iridium complexes for conversion of light into electricity and electricity into light[END_REF][START_REF] Baranoff | Cyclometallated Iridium Complexes as Sensitizers for Dye-Sensitized Solar Cells[END_REF][START_REF] Sinopoli | New cyclometalated iridium(III) dye chromophore complexes for n-type dye-sensitised solar cells[END_REF][START_REF] Sinopoli | Hybrid Cyclometalated Iridium Coumarin Complex as a Sensitiser of Both n-and p-Type DSSCs[END_REF][START_REF] Wang | Iridium (III) complexes with 5,5-dimethyl-3-(pyridin-2-yl)cyclohex-2-enone ligands as sensitizer for dye-sensitized solar cells[END_REF][START_REF] Shinpuku | Synthesis and characterization of novel cyclometalated iridium(III) complexes for nanocrystalline TiO2-based dye-sensitized solar cells[END_REF][START_REF] Gennari | Long-Lived Charge Separated State in NiO-Based p-Type Dye-Sensitized Solar Cells with Simple Cyclometalated Iridium Complexes[END_REF] This is mainly because most iridium(III) complexes are not panchromatic, having absorption spectra that tail off by 550 nm. This induces low short circuit currents in the DSSC and as a consequence poor PCE; typically less than 4%. Indeed, there are very few examples of iridium(III) complexes with significant absorption bands going up to the red or NIR parts of the visible spectrum. [START_REF] Henwood | Unprecedented Strong Panchromic Absorption from Proton-Switchable Iridium(III) Azoimidazolate Complexes[END_REF][START_REF] Hasan | Panchromic Cationic Iridium(III) Complexes[END_REF][START_REF] Tamayo | Cationic Bis-cyclometalated Iridium(III) Diimine Complexes and Their Use in Efficient Blue, Green, and Red Electroluminescent Devices[END_REF][START_REF] Zhao | Series of New Cationic Iridium(III) Complexes with Tunable Emission Wavelength and Excited State Properties:  Structures, Theoretical Calculations, and Photophysical and Electrochemical Properties[END_REF][START_REF] Medina-Castillo | Engineering of efficient phosphorescent iridium cationic complex for developing oxygen-sensitive polymeric and nanostructured films[END_REF][START_REF] Aubert | Linear and Nonlinear Optical Properties of Cationic Bipyridyl Iridium(III) Complexes: Tunable and Photoswitchable?[END_REF][START_REF] Kammer | 1,12-Diazaperylene and 2,11-dialkylated-1,12-diazaperylene iridium(iii) complexes [Ir(C^N)2(N^N)]PF6: new supramolecular assemblies[END_REF][START_REF] Shinpuku | Synthesis and characterization of novel cyclometalated iridium(III) complexes for nanocrystalline TiO2-based dye-sensitized solar cells[END_REF] We recently reported the development of tripodal C^N^C ligands, 2-benzhydrylpyridine and its derivatives, which can coordinate to iridium, forming three six-membered chelate rings through a double C-H bond activation. [START_REF] Hierlinger | An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand[END_REF] When combined with a bidentate diimine ligand such as 4,4'-ditertbutyl-2,2'-bipyridine (dtBubpy), a family of orange-to-red emitting neutral [Ir(C^N^C)(dtBubpy)Cl] complexes was formed with absorption bands tailing off at 600 nm.

Herein, we report an analogous complex showing panchromatic absorption, employing an electron-poor ancillary ligand diethyl [2,2'-bipyridine]-4,4'-dicarboxylate (deeb), and study its use as a DSSC dye.

Results and Discussion

Synthesis

Scheme 1: Scheme for the one-pot synthesis of complex 1.

Compound L1 [START_REF] Hierlinger | An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand[END_REF] and deeb [START_REF] He | A new bipyridyl cobalt complex for reductive dechlorination of pesticides[END_REF] were prepared by literature methods. Complex 1 was obtained as a black solid in 52% yield using a two-step-one-pot protocol wherein a mixture of L1 and IrCl 3 .n6H 2 O in 2-ethoxyethanol/H 2 O (3:1) was heated at reflux for 19 h followed by the addition of deeb and a further reaction time of 6 h (Scheme 1). Complex 1 was characterized by 1 H and 13 C NMR spectroscopy, HR-ESI mass spectrometry, elemental analysis and melting point determination [see Figures S1-6 in the Supporting Information (SI) for NMR and HR-ESI mass spectra].

Crystal Structures

Single crystals of sufficient quality of 1 were grown from CH 2 Cl 2 /Et 2 O at -18°C, and the structure of 1 was determined by single-crystal X-ray diffraction (Figure 1, Table S1). Complex analogous to that seen previously. [START_REF] Hierlinger | An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand[END_REF] The remaining coordination sphere of 1 consists of the deeb N^N ligand and a chloride anion. The arrangement of ligands is unusual, as the chloride coordinates trans to the pyridine of L1 and not trans to a cyclometalated carbon ligand as generally observed in tridentate complexes, [START_REF] Koga | Synthesis, Structures, and Unique Luminescent Properties of Tridentate C∧C∧N Cyclometalated Complexes of Iridium[END_REF][START_REF] Obara | Highly Phosphorescent Iridium Complexes Containing Both Tridentate Bis(benzimidazolyl)-benzene or -pyridine and Bidentate Phenylpyridine:  Synthesis, Photophysical Properties, and Theoretical Study of Ir-Bis(benzimidazolyl)benzene Complex[END_REF][START_REF] Brulatti | Luminescent Iridium(III) Complexes with N∧C∧N-Coordinated Terdentate Ligands: Dual Tuning of the Emission Energy and Application to Organic Light-Emitting Devices[END_REF][START_REF] Ashizawa | Syntheses and photophysical properties of optical-active bluephosphorescent iridium complexes bearing asymmetric tridentate ligands[END_REF][START_REF] Daniels | When two are better than one: bright phosphorescence from non-stereogenic dinuclear iridium(III) complexes[END_REF]

NHE).[34]

The electrochemical properties of 1 were evaluated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in deaerated CH 2 Cl 2 solution at 298 K at a scan rate of 100 mV s -1 using Fc/Fc + as the internal reference and referenced with respect to NHE (0.70 V vs. NHE). [START_REF] Cardona | Electrochemical Considerations for Determining Absolute Frontier Orbital Energy Levels of Conjugated Polymers for Solar Cell Applications[END_REF] The electrochemical data are summarized in Table 1 and where ppy is 2-phenylpyridinato). [START_REF] Chirdon | Tracking of Tuning Effects in Bis-Cyclometalated Iridium Complexes: A Combined Time Resolved Infrared Spectroscopy, Electrochemical, and Computational Study[END_REF] Upon scanning to negative potential, 1 shows a single quasi-reversible reduction wave at -0.94 V ܧ∆( = 99 mV), which is monoelectronic as inferred from the DPV. The electron-withdrawing effect of the ethyl ester groups of the N^N ligand results in a large anodic shift of 610 mV in the reduction wave of 1 compared to R1 (E 1/2 red. -1.58

V vs NHE). [START_REF] Hierlinger | An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand[END_REF] Complex R2 showed two reversible reduction waves in MeCN. The first reduction located at -0.76 V is assigned to the reduction of the deeb ligand while the second one at -1.30 V is due to the reduction of the phenylpyridinato. [START_REF] Chirdon | Tracking of Tuning Effects in Bis-Cyclometalated Iridium Complexes: A Combined Time Resolved Infrared Spectroscopy, Electrochemical, and Computational Study[END_REF] Thus, the reduction of the deeb ligand in 1 is shifted to more negative potentials compared to the same reduction in R2. DFT calculations of the previously reported R1 indicated that both the HOMO and HOMO-1 are close in energy and involve the iridium and chlorine atoms and the two phenyl rings of L1. [START_REF] Hierlinger | An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand[END_REF] As can be seen in Figure 4 the same electron density distribution is found in 1. DFT calculations also show that the three lowest unoccupied orbitals are exclusively localized on the deeb ligand in 1 (Figure 4), while the LUMO+1 is primarily on the pyridyl of L1 in R1, illustrating the stronger accepting character of deeb. The ܧ∆ ୰ୣୢ୭୶ for 1 (2.18 eV) is markedly smaller than that of R2 ܧ∆( ୰ୣୢ୭୶ = 2.33 V). [START_REF] Chirdon | Tracking of Tuning Effects in Bis-Cyclometalated Iridium Complexes: A Combined Time Resolved Infrared Spectroscopy, Electrochemical, and Computational Study[END_REF] Figure 3. Structures of reference complexes R1 and R2.

Photophysical properties

The photophysical data for 1 recorded in CH 2 Cl 2 at 298 K are shown in Figure 5 and the data summarized in Table 2. The absorption profile of 1 differs significantly from that of R1.

Complex 1 shows intense high-energy absorption bands (ε on the order of 3.5 × 10 4 M -1 cm -1 ) below 250 nm, which are ascribed to 1 π-π* ligand-centered ( 1 LC) transitions localized on the deeb ligand. A moderately intense band (ε on the order of 1.5 × 10 4 M -1 cm -1 ) at 319 nm is assigned to a ligand-centered (LC) transition on the deeb with a small CT character (see below).

Weaker bands (ε on the order of 5-6 × 10 3 and 2 × 10 3 M -1 cm -1 ) in the region of 380 -440 nm and tailing to 500 -600 nm are attributed to a mixture of ( 1 MLCT/ 1 LLCT) and spin-forbidden The assignments for complex 1 were confirmed by TD-DFT calculations (see the ESI for technical details). The two lowest singlet states, computed at 623 and 611 nm, present relatively small intensities (oscillator strengths, f, of 0.010 and 0.056, respectively) and mainly correspond to HOMO-1 to LUMO and HOMO to LUMO transitions. As can be seen in Figure 4, this clearly corresponds to a mixed CT process from the metal and the phenyl rings of the C^N^C ligand towards the deeb. The following significant vertical absorption are predicted by TD-DFT at 496 nm (f=0.071), 456 nm (f=0.027) and 443 nm (f=0.084) and these bands can be ascribed to HOMO-2 to LUMO, HOMO to LUMO+1 and HOMO-1 to LUMO+1 transitions, respectively, and therefore all involve strong CT character towards the deeb moiety. The more intense and resolved band at 319 nm experimentally (see Table 2) is computed at 315 nm by TD-DFT (f=0.162) and corresponds to a more LC excitation from a low-lying orbital centered on the deeb (and partly on chlorine atom) towards the LUMO centered on the deeb as well.

Upon photoexcitation at 420 nm, 1 exhibits a broad featureless profile, indicative of an emission with mixed CT character, with a maximum at λ em = 731 nm, an emission that is significantly redshifted (99 nm, 2194 cm -1 ) compared to R1 (λ em = 630 nm). [START_REF] Hierlinger | An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand[END_REF] The red-shifted luminescence is due to the presence of the presence of the π-accepting deeb. The emission of 1 is likewise red-shifted (51 nm, 2194 cm -1 ) compared to that of R2 (λ em = 680 nm). [START_REF] Chirdon | Tracking of Tuning Effects in Bis-Cyclometalated Iridium Complexes: A Combined Time Resolved Infrared Spectroscopy, Electrochemical, and Computational Study[END_REF] The DFT calculations returns an emission of the T 1 state at 762 nm, close to the experimental value, confirming emission from the lowest triplet excited state. The topology of this state, in terms of localization of the excess α electrons, is displayed in Figure 6. As can be seen, the spin density is mostly localized on the Ir and Cl atoms and on the ancillary ligand, the tridentate ligand playing only a minor role in this state. This localization is consistent with the observed red-shift in emission compared to R1 and R2. The measured photoluminescence quantum yield (Φ PL ) of 1 is 0.5%, lower than those of R1 (6%) and R2 (5%). This finding is a logical consequence of the energy gap law, which states that the nonradiative decay rate increases with decreasing emission energy. [START_REF] Bixon | Energy Gap Law for Nonradiative and Radiative Charge Transfer in Isolated and in Solvated Supermolecules[END_REF][START_REF] Caspar | Application of the energy gap law to nonradiative, excited-state decay[END_REF] Among near-infrared emissive cationic Ir(III) emitters with λ em beyond 700 nm bearing diimines as ancillary ligand, most examples exhibit Φ PL values less than 4%. [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF][START_REF] Tao | Efficient Near-Infrared-Emitting Cationic Iridium Complexes as Dopants for OLEDs with Small Efficiency Roll-off[END_REF][START_REF] Hasan | Tuning the Emission of Cationic Iridium (III) Complexes Towards the Red Through Methoxy Substitution of the Cyclometalating Ligand[END_REF][START_REF] Wang | Near-infrared-emitting heteroleptic cationic iridium complexes derived from 2,3-diphenylbenzo[g]quinoxaline as in vitro theranostic photodynamic therapy agents[END_REF][START_REF] Xin | Efficient near-infrared-emitting cationic iridium complexes based on highly conjugated cyclometalated benzo[g]phthalazine derivatives[END_REF] However, NIR-emitting neutral Ir(III) complexes of the form [Ir(C^N) 2 (O^O)] (where O^O a substituted β-diketonate ancillary ligand) employing highly conjugated C^N ligands have reached Φ PL of up to 16%. [START_REF] Cao | Near-Infrared Polymer Light-Emitting Diodes with High Efficiency and Low Efficiency Roll-off by Using Solution-Processed Iridium(III) Phosphors[END_REF][START_REF] Kesarkar | Near-IR Emitting Iridium(III) Complexes with Heteroaromatic β -Diketonate Ancillary Ligands for Efficient Solution-Processed OLEDs: Structure-Property Correlations[END_REF][START_REF] Tao | High-efficiency nearinfrared organic light-emitting devices based on an iridium complex with negligible efficiency roll-off[END_REF] Complex 1 exhibits a multiexponential emission decay, a reflection of the large non-radiative decay rate constant.

Dye-sensitized solar cells (DSSCs)

Sandwich-type solar cells were assembled using 1-sensitised nanocrystalline TiO 2 as the working electrodes, platinized conducting glass as the counter electrode and iodide/triiodide in acetonitrile as electrolyte. The photovoltaic performances of solar cells based 1 and N719, as benchmark sensitizer, are summarized in Table 3. Figure 7 shows the current-voltage characteristics of the dyes under AM 1.5 simulated sunlight (100 mW cm -2 ) and in the dark. J sc is the short-circuit current density at the V = 0 intercept, V oc is the open-circuit voltage at the J = 0 intercept, FF is the device fill factor, η is the power conversion efficiency. The photovoltaic efficiency (η = 0.49%) obtained with 1 is but comparable with results for iridium sensitizers reported elsewhere. [START_REF] Sinopoli | Hybrid Cyclometalated Iridium Coumarin Complex as a Sensitiser of Both n-and p-Type DSSCs[END_REF][START_REF] Sinopoli | New cyclometalated iridium(III) dye chromophore complexes for n-type dye-sensitised solar cells[END_REF][START_REF] Baranoff | Cyclometallated Iridium Complexes as Sensitizers for Dye-Sensitized Solar Cells[END_REF][START_REF] Baranoff | Cyclometallated iridium complexes for conversion of light into electricity and electricity into light[END_REF][START_REF] Dragonetti | Simple novel cyclometallated iridium complexes for potential application in dye-sensitized solar cells[END_REF] Both charge injection from the excited dye into TiO 2 and regeneration by the electrolyte are thermodynamically favourable. The fill factor for the N719 device (0.61) was slightly lower than that typically obtained in optimized devices (0.70-0.75) and the shape of the current-voltage curve is consistent with high series resistance. Procedures used in optimized N719 devices, such as mixed solvents or additives such as chenodeoxycholic acid in the electrolyte or dye bath are likely to improve the performance of the N719 devices, however the conditions chosen were optimal for compound 1. We therefore attribute the reason for the low efficiency for 1 compared to the benchmark Ru dye to be the weak absorption in the visible region, compared to ruthenium-based photosensitizers such as N719. The absorption spectrum of the TiO 2 electrode after immersion in the dye solution is provided in Figure S13 and the spectral response of the DSSC is given in Figure S14. The low Potential / V incident photon-to current conversion efficiency (IPCE < 2%) is consistent with the poor lightharvesting at λ > 500 nm. While these dyes absorb broadly across the visible spectrum, the low ε (ε ~ 2 000 M -1 cm -1 ) compared to ruthenium dyes (ε > 10 000 M -1 cm -1 ) is a limitation to their solar cell performance.

Conclusions

In conclusion, a new panchromatically absorbing, NIR luminescent iridium(III) complexes bearing a tripodal tris(six-membered) chelate ligand has been obtained and comprehensively characterized, including by single crystal X-ray diffraction. The absorption spectrum tails off at 700 nm, much further than most neutral iridium complexes while the emission is significantly shifted into the NIR, with a maximum of 731 nm. DSSCs using 1 as the dye achieved only modest efficiency of 0.49%, comparable to other Ir(III) dyes. This was attributed to the modest absorption coefficient, which leads to weak light harvesting in the visible region and low short-circuit current.

Appendix A. Supplementary data

CCDC 1583853 contains the supplementary crystallographic data for 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. NMR and MS spectra for 1, Supplementary crystallographic data, supplementary electrochemical and photophysical data. Description of the DFT/TD-DFT protocol. Experimental details for the DSSC assembly and testing. Plots of the absorption spectra of 1-sensitized TiO 2 and IPCE spectrum of the DSSC.

1 ,

 1 [Ir(L1)(deeb)Cl], lies in a mirror plane; the pyridyl ring of L1, the iridium(III) and the chloride all lying directly in the plane. The tridentate L1 shows a tripodal chelation motif,

Figure 1 .Figure 2 .

 12 Figure 1. Solid-state structure of complex 1, thermal ellipsoids are drawn at the 50 % probability level. Hydrogen atoms and solvent molecules are omitted for clarity (color code: C = grey, N = purple, O = red, Cl = green and Ir = blue).

the voltammograms are shown in Figure 2 . 21 VFigure 3 ,

 2213 Complex 1 exhibits a quasi-reversible single electron oxidation wave at 1.ܧ∆( ൌ 88 mV), which is assigned to the Ir(III)/Ir(IV) redox couple, with contributions from the two phenyl rings of L1 and the chloro ligand. Compared to [Ir(L1)(dtBubpy)Cl], R1, (E 1/2 ox. 1.04 V vs. NHE)[START_REF] Hierlinger | An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand[END_REF] the oxidation potential in 1 is significantly anodically shifted by 170 mV, reflecting the electron-withdrawing capacity of the ethyl ester groups of the N^N ligand, which modifies the electron density on iridium. However, the oxidation potential of 1 is less positive than that of [Ir(ppy) 2 (deeb)]PF 6 , R2, (E 1/2 ox. = 1.57 V in deaerated MeCN vs NHE,

Figure 4 .

 4 Figure 4. Frontier molecular orbitals of 1 computed through DFT (M06 functional, see the SI for details) and represented using a contour threshold of 0.03 au.

( 3 MLCT/ 3

 33 LLCT) transitions involving the deeb ligand. Iridium(III) complexes often do not show absorption onsets lower in energy than 550 nm;[START_REF] Zhao | Cationic Iridium(III) Complexes with Tunable Emission Color as Phosphorescent Dyes for Live Cell Imaging[END_REF][START_REF] Ertl | Highly Stable Red-Light-Emitting Electrochemical Cells[END_REF][START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] though, there are known examples of neutral Ir(III) complexes showing absorption bands beyond 550 nm.[START_REF] Kesarkar | Near-IR Emitting Iridium(III) Complexes with Heteroaromatic β -Diketonate Ancillary Ligands for Efficient Solution-Processed OLEDs: Structure-Property Correlations[END_REF][START_REF] Brulatti | Luminescent Iridium(III) Complexes with N∧C∧N-Coordinated Terdentate Ligands: Dual Tuning of the Emission Energy and Application to Organic Light-Emitting Devices[END_REF][START_REF] Xin | Efficient near-infrared-emitting cationic iridium complexes based on highly conjugated cyclometalated benzo[g]phthalazine derivatives[END_REF][START_REF] Zhang | Near-Infrared-Emitting Iridium(III) Complexes as Phosphorescent Dyes for Live Cell Imaging[END_REF]

Figure 5 .

 5 Figure 5. The absorptivity (solid line) and photoluminescence spectra (dotted line) of 1 in CH 2 Cl 2 at 298 K (c = 10 -5 M).

Figure 6 .

 6 Figure 6. DFT computed spin density difference plots for the lowest triplet state of 1. Both side and top views are shown and they have been drawn with a contour threshold of 3 × 10 -3 au.

Figure 7 .

 7 Figure 7. Current-voltage curves for DSSCs constructed using 1 (orange) and N719 (red) in the dark (dashed line) and under simulated sunlight (solid line, AM1.5, 100 mW cm -2 ).

Table 1 :

 1 Selected electrochemical properties of complex 1 in degassed CH 2 Cl 2 at a scan rate of 100 mV s -1 with Fc/Fc + as internal reference, and referenced with respect to NHE (Fc/Fc + = 0.70 V in CH 2 Cl 2 );[36,34,37] [36,34,37] [36,34,37]

				PF 6
	N		N	CO 2 Et
		N		N
	Ir	N	Ir	N
	Cl		N	CO 2 Et
	R1
			R2
			Electrochemistry a

a b ∆ E E redox is the difference (V) between first oxidation and first reduction potentials; c E HOMO/LUMO = -[E ox/red vs Fc/Fc + + 4.8] eV.

[START_REF] Cardona | Electrochemical Considerations for Determining Absolute Frontier Orbital Energy Levels of Conjugated Polymers for Solar Cell Applications[END_REF]

Table 2 .

 2 Photophysical properties of complex 1.

	Complex λ abs / nm (ε / M -1 cm	-1 )	a	λ em	b / nm Φ PL b,c / %	τ e d / ns
	1	237 (34819), 319 (14647), 384 (5105),	731	0.5	36 (73 %)
		434 (5607), 504 (2176), 597 (1925)			78 (19 %)
							392 (8 %)
	a Recorded in aerated CH 2 Cl 2 at 298 K; b Recorded at 298 K in deaerated CH 2 Cl 2 solution (λ exc = 420
	nm); c [Ru(bpy) 3 ](PF 6 ) 2 in MeCN as the reference (Φ PL = 1.8% in aerated MeCN at 298 K)[52]; d exc =
	378 nm.

Table 3 .

 3 Photovoltaic performance of 1.

	DYE	J SC cm -2 a / mA	V OC	a / V	FF a	η a / %
	1	0.995	0.67	0.74	0.49
	N719	8.84	0.81	0.61	4.4
	a

Acknowledgements

C.H. acknowledges the Région Bretagne, France for funding. EZ-C acknowledges the University of St Andrews and EPSRC (EP/M02105X/1) for financial support. We thank Umicore AG for the gift of materials. We thank the EPSRC UK National Mass Spectrometry Facility at Swansea University for analytical services. This research used computational resources of 1) the GENCI-CINES/IDRIS, 2) the CCIPL (Centre de Calcul Intensif des Pays de Loire), 3) a local Troy cluster. EAG and HVF thank the ERC for a Starting Grant (p-TYPE, 715354).

A panchromatic absorbing, NIR emitting neutral iridium complexes bearing a tripodal 2-(bis(4-(tert-butyl)phenyl)methyl)pyridinato (dtBubnpy) ligand, a bidentate diethyl [2,2'-bipyridine]-4,4'-dicarboxylate (deeb)] ligand and a chloro ligand is reported. This complex was used as a dye in a dye-sensitized solar cell.


Christian Vassallo
email: cvassall@laas.fr

Anne-Hélène Olivier
email: anne-helene.olivier@univ-rennes2.fr

Philippe Souères
email: soueres@laas.fr

Armel Crétual
email: armel.cretual@univ-rennes2.fr

Olivier Stasse
email: ostasse@laas.fr

Julien Pettré
email: julien.pettre@inria.fr

How do walkers behave when crossing the way of a mobile robot that replicates human interaction rules?

Keywords: Human-robot interaction, Locomotion, Collision avoidance, Gait adaptation, Mobile robot

 A mobile robot was programmed to reproduce the interaction rules of human walkers  We observed the behavior of human walkers crossing the way of this reactive robot

 Contrary to what occurs with a passive robot, the crossing order is preserved  Humans behave with the reactive robot as when crossing another human walker  Making robots move in a human-like way eases their interaction with human walkers

Wordcount is 2997 words.

Introduction

In everyday life, we walk by constantly adapting our motion to our environment. In past work, the relation between the walker and the environment was modeled as a coupled dynamical system. The trajectories result from a set of forces emitted by goals (attractors) and obstacles (repellers) [START_REF] Warren | Behavioral dynamics of visually guided locomotion, Coordination: neural, behavioral and social dynamics[END_REF]. Collision avoidance between pedestrians has also received a lot of attention either using front-on [START_REF] Dicks | Perceptual-motor behaviour during a simulated pedestrian crossing[END_REF] or side-on approach trajectories [START_REF] Huber | Adjustments of speed and path when avoiding collisions with another pedestrian[END_REF][START_REF] Knorr | Influence of Person-and Situation-Specific Characteristics on Collision Avoidance Behavior in Human Locomotion[END_REF][START_REF] Olivier | Minimal predicted distance: A common metric for collision avoidance during pairwise interactions between walkers[END_REF][START_REF] Olivier | Collision avoidance between two walkers: role-dependent strategies[END_REF]. Olivier et al. showed that walkers adapt their trajectory only if a future risk of collision exists [START_REF] Olivier | Minimal predicted distance: A common metric for collision avoidance during pairwise interactions between walkers[END_REF]. This adaptation depends on the order of arrival of pedestrians that defines their order of passage. The first walker that arrives maintains or increases his/her advance by slightly accelerating and changing his/her direction to move away from the other participant. The second one slows down and moves in the opposite direction to reduce the risks of a collision. Huber et al. focused on how trajectories are adapted using speed and heading modifications depending on the crossing angle [START_REF] Huber | Adjustments of speed and path when avoiding collisions with another pedestrian[END_REF]. Future crossing order (who is about to give way or pass first) is quickly and accurately perceived and preserved until the end of the interaction [START_REF] Knorr | Influence of Person-and Situation-Specific Characteristics on Collision Avoidance Behavior in Human Locomotion[END_REF][START_REF] Olivier | Collision avoidance between two walkers: role-dependent strategies[END_REF]. This shows that walkers take efficiency into account since an inversion of the crossing order would result in suboptimal adaptations of higher amplitude. In addition, it was shown that the participant giving way contributes more to solving the collision avoidance [START_REF] Olivier | Collision avoidance between two walkers: role-dependent strategies[END_REF]. Finally, behavior is influenced by the number of pedestrians to interact with and the potential to have social interactions with them [START_REF] Dicks | Perceptual-motor behaviour during a simulated pedestrian crossing[END_REF]. Because humans and robots will have to share the same environment in the near future [START_REF] Goodrich | Human-robot interaction: a survey[END_REF][START_REF] Kruse | Human-aware robot navigation: a survey[END_REF], recent studies focused on tasks involving walkers and a moving robot. Vassallo et al. [START_REF] Vassallo | How do walkers avoid a mobile robot crossing their way?[END_REF] performed an experiment in which participants had to avoid collision with a passive wheeled robot (moving straight at constant speed), crossing perpendicularly their direction. In contrast to a human-human interaction, several inversions of the crossing order were observed, even though this behavior was not optimal. Such a behavior was observed when the walker arrived ahead of the robot with a predictable future crossing distance between 0 and 0.6m but, despite this advance, finally gave way. This result was linked to the notion of perceived danger and safety, and to the lack of experience of interacting with such a robot. Because of its design, the main limitation of Vassallo et al. study [START_REF] Vassallo | How do walkers avoid a mobile robot crossing their way?[END_REF] was its inability to conclude whether the modification of the walker behavior was due to the lack of adaptability of the moving obstacle or solely to its artificial nature. Nonetheless, it was shown in [START_REF] Satake | How to Approach Humans? Strategies for Social Robots to Initiate Interaction[END_REF] that the robot trajectory can be read and understood by humans in a task where a robot moves towards a human to initiate a conversation based on an approach linked to public and social distances. Furthermore, in a face-toface task with a moving robot, humans behave similarly whether they are told or not what the robot trajectory will be [START_REF] Carton | Measuring the effectiveness of readability for mobile robot locomotion[END_REF], showing their ability to actually read the robot motion. Given these results, the question addressed in this paper is: "How would humans behave if they have to cross the trajectory of a robot programmed to replicate the observed human-human avoidance strategy?" Would humans understand that the robot adapts its trajectory and then adapt their own strategy accordingly, or would they give way to the robot as observed in [START_REF] Vassallo | How do walkers avoid a mobile robot crossing their way?[END_REF]?

Materials and methods

Participants

Ten volunteers participated in the experiment (2 women and 8 men). They were 28.8 (±9.5) years old and 1.77m tall (±0.12). They had no known pathology that could affect their locomotion. All of them had normal or corrected sight and hearing. All participants were naïve to the studied situation. Participants gave written and informed consent before their inclusion in the study. The experiment conformed to the Declaration of Helsinki, with formal approval of the ethics evaluation committee of INSERM (IRB00003888, Opinion number 13-124), Paris, France (IORG0003254, FWA00005831).

Apparatus

A C C E P T E D M A N U S C R I P T
The experiment took place in a 40mx25m gymnasium. The room was separated into two areas by 2m high occluding walls forming a gate in the middle (Figure1). Four specific positions were defined: the participant starting position PSP, the participant target PT, and two robot starting positions RSP1 and RSP2, to generate situations where the robot approached from the right or from the left of the participants. Two virtual guidelines ra and rb, parallel to the line (RSP1, RSP2) and respectively located at a distance of 0.5m and 1.0m from the gate, were used as reference for guiding the robot to pass behind or ahead the participant during the avoidance phase. A specific zone between PSP and the gate was named Motion Estimation Zone (MEZ), far enough from PSP to let the participants reach their comfort velocity before they entered the MEZ. The intersection point between the robot and the initial path of the participant was named Hypothetical Crossing Point (HCP) as this is the point where the participant and robot would cross if they do not modify their trajectory.

- -----------------------------Insert figure 1 here ------------------------------

Task

Participants were asked to walk at their preferred speed from PSP to PT passing through the gate. They were told that a robot could be moving beyond the gate and could obstruct them, meaning that the robot could adapt its trajectory according to the participants' one. One experimental trial corresponded to one travel from PSP to PT. We defined tsee, the time at which the participant passed through the gate and saw the robot moving, and tcross, the time of closest approach, when the human-robot distance was minimal (i.e., the "distance of closest approach"). The crossing configuration and the risk of future collision were estimated using the Signed Minimal Predicted Distance, noted smpd, which gives, at each time step, the future distance of closest approach if both the robot and the participant keep a constant speed and direction [START_REF] Vassallo | How do walkers avoid a mobile robot crossing their way?[END_REF]. A variation of smpd means that the participant or/and the robot are performing adaptation. The sign of this function depends on who, between the participant and the robot, is going to pass first: positive if it is the participant and negative otherwise. A change of smpd sign means a switch of the future crossing order.

Recorded data

3D kinematic data was recorded using a 16 infrared cameras motion capture Vicon-MX system (120Hz). Reconstruction was performed with Vicon-Blade and computations with Matlab (Mathworks®). The global position of participants was estimated as the centroid of the reflective markers set on a helmet they were wearing. The stepping oscillations were filtered out by applying a Butterworth low-pass filter (2nd order, dual pass, 0.5Hz cut-off frequency).

Robot Behavior

We used a RobuLAB10 robot from Robosoft (dimension: 0.45x0.40x1.42m, weight 25 Kg, maximum speed ~3 m.s -1 ). The robot reference point was the center of its base. The robot control sequence was the following (cf. Figure1): 1) The robot was at rest at RSP1 or RSP2. 2) The participant crossed MEZ, its arrival time at HCP was estimated. 3) The theoretical speed at which the robot should move to reach HCP at the same time as the participant was estimated. This speed was then further increased (resp. decreased) for the robot to arrive in advance (respectively lately) at HCP, in order to match the expected smpd. This choice was done such that smpd values at tsee were randomly distributed in [-0.9m;0.9m] 4) When the robot had to avoid the human, 2m before reaching HCP, the robot adapted its trajectory by inserting a new way-point on its trajectory, in order to pass behind the walker along the line r_a or ahead the walker by moving along the line r_b, depending on the

A C C E P T E D M A N U S C R I P T
sign of smpd at tsee. 5) When the avoidance phase was over, the robot was controlled to reach its final position.

Experimental plan

Each participant performed 30 trials. The robot starting position (50% from RSP1, 50% from RSP2) was randomized among the trials. To introduce variability, in 2 trials the robot did not move. The participants were not informed about the initial position of the robot nor about the possibility that the robot would not move on every trial. Only the 28 trials with potential adaptations were analyzed.

Analysis

The analysis focused on the time interval during which adaptation was performed. To this end, smpd was normalized in time by resampling the function at 100 intervals between tsee (time 0%) and tcross (time 100%). The quantity of adaptation was defined as the absolute value of the difference between smpd(tsee) (i.e., the initial conditions of the interaction) and smpd(tcross) (i.e., the actual signed minimum distance between the participant and the robot).

Statistics were performed using Statistica (Statsoft®). All effects were reported at p<0.05. Normality was assessed using a Kolmogorov-Smirnov test. Depending on the normality, values are expressed as median (M) or mean ±SD. Wilcoxon signed-rank tests were used to determine differences between values of smpd at tsee and tcross. The influence of the crossing order evolution on the smpd values was assessed by using a Kruskal-Wallis test with post hoc Mann-Whitney tests for which a Bonferroni correction was applied: all effects are reported at a 0.016 level of significance (0.05/3). Finally, we used a Mann-Whitney test to compare the crossing distance depending on the final crossing order.

Results

We considered 279 trials (one has been removed because the robot failed to start). Figure 2 depicts the evolution of smpd for all trials.

- -----------------------------Insert figure 2 here ------------------------------The sign of smpd at tcross showed that participants passed first in 53% of cases, and gave way in the other 47%. Combining this information with the data at tsee, we could evaluate if an inversion of crossing order occurred. The trials have been divided into 4 categories, depending on the relative signs of smpd at tsee and tcross (Pos for positive and Neg for negative): PosPos, PosNeg, NegPos, NegNeg. For example, the PosNeg category contained the trials for which smpd(tsee)>0 and smpd(tcross)<0. smpd categories were distributed among the trials in the following way: PosPos=144 trials (52%), NegNeg=110 trials (39%), PosNeg=22 trials (8%), NegPos=3 trials (1%). All participants had both PosPos and PosPos trials, and 9 out of 10 participants had at least one PosNeg trial. In the remainder of the paper, the NegPos category will not be further considered as it contained only three trials defined as outliers. Examples of corresponding trajectories for each of the 3 remaining categories are depicted in Figure3. Note that in 91% of cases the crossing order was preserved. We only observed ------------------------------Insert figure 3 here ------------------------------Figure4a and 4b show respectively the average evolution of smpd and its time derivative for each category. Based on the sign of the smpd time derivative, we can separate the reaction period during which participants perform adaptations (smpd varies) from the regulation period that follows the collision avoidance (the derivative vanishes, and its sign may even change) as defined in [START_REF] Knorr | Influence of Person-and Situation-Specific Characteristics on Collision Avoidance Behavior in Human Locomotion[END_REF] and [START_REF] Vassallo | How do walkers avoid a mobile robot crossing their way?[END_REF]. The relative duration of the reaction phase for PosPos (55%) and NegNeg (57%) trials was almost the same, while participants were longer to adapt when they decided to give way to the robot in PosNeg (69%) trials.

- -----------------------------Insert figure 4 here ------------------------------Figure5 shows comparison between smpd(tsee) and smpd(tcross) for the 3 categories. For each category, the human-robot distance increased from tsee to tcross so that the risk of collision was reduced. Statistical analysis showed a significant difference of smpd between tsee and tcross for PosPos trials (Msmpdtsee=0.71m, Msmpdtcross=1,08m, Z=9.17, p<0.0001, r=0.76), for NegNeg trials (Msmpdtsee=-0.46m, Msmpdtcross=-1.14m, Z=8.98, p<0.0001, r=0.85) and for PosNeg trials (Msmpdtsee=0.29m, Msmpdtcross=-0.71m, Z=4.11, p<0.0001, r=0.88).

- -----------------------------Insert figure 5 here ------------------------------Finally, the distance of closest approach was influenced by the category (H(2,276)=29.3, p<0.0005). Post-hoc tests showed that the median distance between the robot and participants did not significantly differ between PosPos (M=1.08m) and NegNeg (M=1.14m) trials. However, when an inversion of the crossing order in PosNeg trials occurred, this distance was smallest (M=0.71m).

Discussion

In the current study, results indicated that when a human is crossing the trajectory of a mobile robot which is programmed to replicate the observed human avoidance strategy, strong characteristics of collision avoidance are comparable with the ones of human-human interactions. First, the crossing order is preserved from tsee to tcross in a majority of trials, as observed in human-human interactions [START_REF] Knorr | Influence of Person-and Situation-Specific Characteristics on Collision Avoidance Behavior in Human Locomotion[END_REF][START_REF] Olivier | Minimal predicted distance: A common metric for collision avoidance during pairwise interactions between walkers[END_REF]. However, in 8% of trials, the participants gave way to the robot while they were in position to pass first. Such a behavior was observed when smpd(tsee) was around 0.39m. Above this threshold, participants preferred to preserve their role rather than giving way to the robot. This result is confirmed by the repartition into PosPos and PosNeg categories of trajectories starting from the smpd interval [0.39m, 0.74m], where 94% of trials belong to the PosPos group. This result is in contrast with the one previously observed with a passive robot [START_REF] Vassallo | How do walkers avoid a mobile robot crossing their way?[END_REF], where participants consistently preferred to give way to the robot when the risk of collision was below 0.81m, even though this choice was not optimal. Note that, whether or not an inversion of the crossing order occurred, the trajectories were adapted in order to increase the crossing distance between the human and the robot to reduce the risk of collision. Results show that humans solve the collision avoidance with anticipation, as previously demonstrated during human-human interaction [START_REF] Olivier | Minimal predicted distance: A common metric for collision avoidance during pairwise interactions between walkers[END_REF]. Indeed, Figure 4 A

C C E P T E D M A N U S C R I P T
shows a plateau in smpd values before tcross meaning that the avoidance maneuvers are over before the end of the task. As discussed in the review of Higuchi [START_REF] Higuchi | Visuomotor Control of Human Adaptive Locomotion: Understanding the Anticipatory Nature[END_REF], the anticipatory nature of adaptive locomotor strategies ensures safety navigation during the task. When the participant decides to preserve the crossing order, the task is solved earlier than when a switch of roles occurs, that requires more motion adaptation. The human-human avoidance strategy takes advantage from the configurations of both agents to limit their adaptations [START_REF] Olivier | Minimal predicted distance: A common metric for collision avoidance during pairwise interactions between walkers[END_REF][START_REF] Olivier | Collision avoidance between two walkers: role-dependent strategies[END_REF]. Assuming that both participants have similar locomotion capabilities, a role is assigned to each of them depending on their order of passage, as recalled in the introduction. This high-level strategy is not related to the anthropomorphic walking, it is simply expressed in terms of the trajectory of a representative point (e.g. the waist position and heading) in the horizontal plane of motion. As such, the method can be easily transferred to a wheeled robot. The fact that the robot automatically initiates its avoidance motion by replicating the human strategy allows the human to easily go back to the process usually applied. In this way, the human easily understands the role he/she should play and no conflicting situation occurred in any trials. For this reason, our overall results are comparable to previous findings, which were reported in the case of a human-human interaction. The control of our robot follows a model of shared-avoidance strategy based on the human behavior [START_REF] Vassallo | How do walkers avoid a mobile robot crossing their way?[END_REF]. One conflicting situation might theoretically occur when both agents arrive with a zero smpd (i.e. exactly at the same time) and take the same role. Such a conflicting situation between human walkers was not reported in [START_REF] Vassallo | How do walkers avoid a mobile robot crossing their way?[END_REF] and never occurred in our human-robot experiment. When the human and the robot were approaching the crossing point quite simultaneously, the smpd was checked twice: once at the beginning, based on the measure of the human velocity in the MEZ, and once at tsee. Based on this accurate measurement of the smpd, which is never exactly equal to zero, the robot adopts a role that helps the walker adapt his behavior. For this reason, we never observed any conflicting situation in which the walker would have tried to force the way (NegPos) after the robot had initiated the avoidance. However, the opposite situation (PosNeg), in which the human prefers to give way to the robot though he arrived ahead, was sometimes observed. This cautious behavior does not constitute a conflicting situation that could block both agents. The behavioral similarities observed between human-human and human-robot is in accordance with the study of Carton et al. [START_REF] Carton | Measuring the effectiveness of readability for mobile robot locomotion[END_REF], in which a walker avoids a robot that reproduces an average human trajectory to avoid a face-to-face collision. They showed that giving a human-like behavior to a moving robot gives rise to readable motions that convey intentions. This readability allows humans to minimize their planning effort and avoid the collision earlier and smoothly. In accordance with previous studies [START_REF] Carton | Measuring the effectiveness of readability for mobile robot locomotion[END_REF][START_REF] Dragan | Generating legible motion[END_REF][START_REF] Lichtenthäler | Towards legible robot navigation-how to increase the intend expressiveness of robot navigation behavior[END_REF], our result shows that controlling robots in order to make them behave in a human-like way is a key point to ease human-robot cohabitation.

Conclusion

Our study suggests that when human walkers cross the trajectory of a mobile robot that obeys the observed human-human avoidance rules, they behave closely as when they cross the trajectory of another human walker. This result shows that, for the ease of human-robot collaboration, machines should move by respecting human interaction rules. In future works, as previously investigated in human-human interactions [START_REF] Dicks | Perceptual-motor behaviour during a simulated pedestrian crossing[END_REF][START_REF] Gallup | Visual attention and the acquisition of information in human crowds[END_REF][START_REF] Higuchi | Visuomotor Control of Human Adaptive Locomotion: Understanding the Anticipatory Nature[END_REF][START_REF] Passos | Information-governing dynamics of attacker-defender interactions in youth rugby union[END_REF], it would be interesting to better understand the visual anticipation processes as well as the nature of the visual information underlying such a collaboration. This can be done by using an eye-tracking system to

A C C E P T E D M A N U S C R I P T
couple the adaptations made by the human walkers and the gaze-activity. Also, it would be interesting to evaluate whether the use of a humanoid robot, whose morphology is closer to the one of a human than a wheeled robot, modifies the human behavior. Another direction of research would be to extend this work to the case of multiple walkers interacting with each other at the same time. Would it be possible, if some participants are replaced by robots that behave like humans, to observe the same human adaptation? Finally, the nature of human expectations and presuppositions, that can be linked to the notion of socially-aware navigation (see [START_REF] Rios-Martinez | From proxemics theory to socially-aware navigation: A survey[END_REF] for a review), should have strong influence on the walker behavior. Indeed, in a less controlled context, participants would certainly behave differently than in the framework of a scientific experiment, where the robot is expected to behave safely. An interesting complement of study would then be to lead similar experiment in real-life environment to evaluate the impact of the context to the walker behavior.

  where participants were likely to pass first but adapted their trajectory to finally give way to the robot.

Figure 1 :

 1 Figure 1: Experimental apparatus and task. The robot moves from RSP1 to RSP2 (or vice versa), following the lateral path r_b or r_a to pass respectively behind or ahead the participant.

Figure 2 :

 2 Figure 2: Evolution of smpd normalized in time during the interaction [tsee, tcross] for all the 279 trials. Gray curves represent trials where the initial crossing order was preserved while black curves represent trials where the initial crossing order was changed.

Figure 3 :

 3 Figure 3: Three examples of participant's (P) and robot (R) trajectories during the interaction phase, for PosPos (top), PosNeg (middle) and NegNeg (bottom) categories. The part of the trajectory between tsee (circle mark) and tcross (square mark) is represented in bold line. Corresponding positions along time are linked by dotted lines.

Figure 4 :

 4 Figure 4: (a) Mean evolution (±1 SD) of smpd for each category of trial. (b) Time derivative of the mean smpd. The three vertical segments correspond, for each curve (PosPos, PosNeg or NegNeg), to the time at which the time derivative of the smpd vanishes, i.e., separate the reaction phase (on the left) from the regulation phase (on the right).

Figure 5 :

 5 Figure 5: smpd values for PosPos, PosNeg and NegNeg categories at tsee and tcross. A significant difference in values means that adaptations were made to the trajectory by the participant (***p < 0.001).

Acknowledgements

The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007 -2013) under grant agreement n. 611909 (KoroiBot) and from the French National Research Agency, projects Entracte (#ANR-13-CORD-0002) and Percolation (#ANR-13-JS02-0008).

Conflict of interest

In this work, there are no conflict of interest.

A C C E P T E D


Thomas Biet

Thomas Cauchy

Qinchao Sun

Jie Ding

Andreas Hauser
email: andreas.hauser@unige.ch

Patric Oulevey

Thomas Bürgi

Denis Jacquemin

Nicolas Vanthuyne

Triplet state CPL active helicene-dithiolene platinum bipyridine complexes

Keywords: UV-vis, ECD, photophysical measurements

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

Triplet state CPL active helicene-dithiolene platinum bipyridine complexes

Thomas Biet, a Thomas Cauchy, a Qinchao Sun, b Jie Ding, b Andreas Hauser,* b Patric Oulevey, b Thomas Bürgi, b Denis Jacquemin, c Nicolas Vanthuyne, d Jeanne Crassous e and Narcis Avarvari* a Chiral metal dithiolene complexes represent a family of chiral precursors, which can give rise to molecular materials with properties resulting from the interplay of chirality with conductivity, magnetism, and photophysics. We describe herein the first examples of chiral metal diimine dithiolene complexes, by the use of a platinum(II) centre coordinated by 2,2'-bipyridine and helicene-dithiolene ligands. Straightforward synthesis of racemic and enantiopure complexes allows the preparation of luminescent Pt(bipy) [4] and [6]helicene compounds for which the solid-state structure was determined as well. TD-DFT calculations support the assignment of the low energy bands observed in the UV-vis absorption spectra as mixed metal-ligand-to-ligand charge transfer transitions and confirm that the emission band results from the T1 excited state. Interestingly the enantiopure [6]helicene complexes show CPL activity at room temperature in acetonitrile solutions with anisotropy factors of 3×10 -4 .

Chiral metal dithiolene complexes represent an emerging family of molecular materials where chirality is expected to modulate properties such as conductivity, magnetism, luminescence, etc. 1 For example, differences in conductivity between diastereomeric pairs of anionic Ni(II) bis(dithiolene) complexes with chiral viologene type cations have been noticed, 2 while the first chiral single component conductors based on neutral Au(III) bis(dithiolene) complexes have been recently described. 3 Besides, stable anionic, neutral or cationic species can be easily accessed in metal bis(dithiolene) complexes thanks to the "non-innocent" character of the dithiolene ligands, so that redox modulation of the chiroptical properties can be observed. 4 Although square planar platinum diimine dithiolene complexes have been investigated over more than twenty years especially for their emission properties in solution, [5][6][7][8] and more recently for photocatalytic water splitting, [START_REF] Zheng | Proc. Natl. Acad. Sci[END_REF] no chiral derivative of this heteroleptic family has been yet reported. For example, in Pt(II) 2,2'-bipyridine (bpy) arene-dithiolene complexes such as Pt(bpy)(tdt) (tdt = toluenedithiolate) showing room temperature luminescence in solution arising from a MMLL'CT (mixed metal-ligand-to-ligand charge transfer) transition, 5,6 chirality could be in principle introduced either on the diimine or the benzodithiolene fragments in order to influence the photophysical properties. One of the interests of such complexes relies on the possible observation of circularly polarized luminescence (CPL), which is the differential spontaneous emission of left and right-handed circularly polarized light. [START_REF] Riehl | [END_REF] While chiral lanthanide complexes are generally the most intense CPL emitters, 11,12 several examples of transition metal complexes have been reported as well. [13][14][15] Among them those containing helicene based ligands are particularly interesting, 16,17 as helicenes 18,19 and heterohelicenes 20 are well known non-planar conjugated molecules possessing strong chiroptical properties. Note that CPL active cationic dioxa, azaoxa and diaza [6]helicenes have been recently reported. 21 This work is presenting out investigations on helical dithiolene platinum 2,2'-bipyridine complexes using the hitherto unknown helicene-dithiolate (heldt) ligands, by analogy with the achiral toluene-dithiolate (tdt) previously mentioned. We describe herein the synthesis and the structural characterization of Pt(bpy)([n]hel-dt) (n = 4, 6) complexes together with their chiroptical and photophysical properties supported by DFT calculations. The racemic 2,3-dithiolate- [4] and [6]helicene ligands, generated in situ from the protected precursors (rac)-1a and (rac)-1b respectively, which have recently been used by some of us for the synthesis of TTF-helicenes, 22 have been reacted with Pt(bpy)Cl2 to generate the corresponding complexes. Thus, (rac)-Pt(bpy)( [4]hel-dt) 2a and (rac)-Pt(bpy)( [6]hel-dt) 2b have been isolated as dark purple crystalline solids after column chromatography (Scheme 1 and ESI). As the racemization barrier for [4]helicenes is generally very low, 23 the enantiopure forms have been prepared only for the [6]helicene dithiolene complexes 2b starting from the precursors (M)-1b and (P)-1b separated by chiral HPLC (Fig. S1-S3, ESI). The racemic complexes 2a and 2b crystallize in the centrosymmetric space groups C2/c and P-1 respectively, with both enantiomers (M) and (P) in the unit cell (Table S1, ESI). Worth noting are the helical curvatures (hc) defined by the dihedral angle between the terminal rings of the helicene skeleton amounting to 25.40° and 53.76° for (rac)-2a (Table S2, Fig. S4-S5, ESI) and (rac)-2b (Table S3, Fig. S6-S7, ESI), which are typical values for [4] and [6]helicenes, and the square planar coordination geometry of the platinum centres. DFT calculations in acetonitrile yield dihedral angles 28.06° and 41.68° for 2a and 2b, suggesting that the impact of crystal packing is stronger for the latter compound. The enantiomerically pure complex (M)-2b has been also analysed by single crystal X-ray diffraction thus allowing to confirm that it was obtained from the (-)-1b precursor. (M)-2b crystallized A c c e p t e d m a n u s c r i p t in the orthorhombic system, non-centrosymmetric space group P212121, with four independent molecules in the unit cell (Fig. 1).

Fig. 1 The four independent molecules of complex in the solid state structure of (M)-2b.

The four molecules named as Pt1A -Pt1D slightly differ by the helical curvature values ranging from 58.58° (Pt1C) to 62.36° (Pt1D), while the Pt-S (2.24-2.25 Å) and Pt-N (2.05-2.07 Å) are in the normal range for such complexes (Table S4, ESI). 24 The packing of the molecules is very likely governed by the - stacking interactions occurring along the c direction (Fig. 1 and Fig. S8-S9, ESI). The enantiomeric (P) and (M)-2b complexes represent the first chiral members of the platinum diimine dithiolenes family. As outlined above, Pt(diimine)(dithiolate) complexes are emissive in fluid or frozen solutions. 5,6,25 We have therefore set out to measure in a first time the photophysical properties of the racemic 2a and 2b. In the low energy region the complex (rac)-2a has an absorption band from 450 to 650 nm with a maximum around 550 nm (18180 cm -1 ) and an absorption coefficient  ≈ 6700 M -1 cm -1 (Fig. 2, top). For (rac)-2b the maximum of the corresponding band is around 562 nm and the absorption coefficient  ≈ 3640 M -1 cm -1 (Fig. 2, bottom). This low energy absorption band is typical for Pt(diimine)(dithiolate) complexes and has been assigned to a MMLL'CT transition, as the HOMO has metal/dithiolene character while the LUMO is developed over the unsaturated diimine ligand. 6 The small redshift of the CT transition from 2a to 2b is caused by the slight change in the dithiolate ligand, with a more extended rigid  backbone in the latter, which makes the HOMO energy slightly higher by +0.03 eV in 2b than in 2a. Moreover, the absorption coefficient is smaller for 2b likely because of the more distorted structure. TD-DFT reproduces the experimental trends with a vertical absorption at 556 nm (f = 0.21) for 2a and 562 nm (f = 0.19) for 2b. Both complexes are emissive in fluid solutions of CH2Cl2 when irradiated into the CT bands, showing low energy emission bands at 720 nm (2a) and 715 nm (2b) (Fig. 2 and Fig. S10, ESI, for (rac)-2a in CH3CN). The more distorted structure of 2b might be as well at the origin of the lower emission quantum yield for 2b (0.15%) than for 2a (0.19%) (Table S5, ESI). The perfect agreement between absorption and excitation spectra is proof that the luminescence indeed originates from the two compounds despite the low emission quantum yield. For 2b a luminescence life-time of 124 ns was measured in deoxygenated solution with pulsed excitation at 458 nm (decay curve shown in Figure S11,ESI). With the quantum efficiency of 0.15% this corresponds to a radiative lifetime of around 100 μs, indicating that the emission originates from the T1 state, as is generally the case for Pt(II) complexes. 6 Fig. 2 Absorption, emission and excitation spectra of (rac)-2a (top) and (rac)-2b (bottom) in CH2Cl2. Absorption spectra were measured at concentrations of 2.2×10 -5 M for (rac)-2a and at 8×10 -5 M for (rac)-2b. Emission spectra were measured using the same solutions degassed by nitrogen bubbling for 20 min with excitation at 525 nm for (rac)-2a and at 560 nm for (rac)-2b. Excitation spectra were measured at emission wavelengths of 720 nm for (rac)-2a and 715 nm for (rac)-2b.

To characterize the charge transfer and emission properties, DFT calculations have been performed on 2a and 2b (Fig. S12-S19, Tables S6-S8, ESI). For both compounds, optimized as (M) and (P) enantiomers respectively, the fully relaxed molecular geometries obtained by DFT are in line with those obtained by X-Ray diffraction. In Fig. 3, we represent the electron density difference (EDD) plots corresponding to the transition to the lowest singlet excited-state and the spin density of the lowest emissive triplet state. The EDD representation clearly shows that there is a strong CT from the dithiolene (donor, mostly in blue in Fig. 3) to the diimine (acceptor, mostly in red). The computed CT distance attains 4.0 Å in both 2a and 2b, which is a rather large value. Interestingly, one notices that the metal centre presents both positive and negative density contributions, indicating that it also partially plays the role of an accepting unit. The implication of the metal in the EDD plots is also consistent with a possible intersystem crossing to the T1 state. The spin density of the lowest triplet state is unsurprisingly localized in exactly the same regions as the corresponding S1 state. The emission for the lowest triplet state was estimated in both the vertical and adiabatic approximations. In the former, DFT yields an emission at 802 nm and 815 nm for 2a and 2b, respectively, whereas in the latter that takes into account the difference of vibrational energies, we obtained 759 nm and 760 nm, for the two compounds. These latter values are in good correspondence with the experimental data, with an error smaller than 0.1 eV, confirming that the observed emission is indeed coming from T1. The fact that the vertical values indicate (incorrectly) a small redshift when going from 2a to 2b, whereas the adiabatic energies show essentially no shift, hints that the T1 geometrical relaxation is smaller in the latter compound than in the former. Investigation of the photophysical properties of both enantiomers of 2b has been performed in acetonitrile solutions. Absorption spectra are shown in the top panel of Fig. 4, together with the emission spectrum of (M)-2b. Compared to the spectra in CH2Cl2, the maximum of the absorption is shifted to 530 nm, that is, by 30 nm to lower wavelength. The emission maximum in acetonitrile appears at 720 nm, that is, shifted to higher wavelength by 5 nm. The CD spectra of (P) and (M)-2b in acetonitrile, image mirror of each other, are shown in the bottom panel of Fig. 4. For the CD spectra, TD-DFT indeed yields a weakly positive contribution to the rotary strength for the transition to the lowest excited-state of (P)-2b and a weakly negative contribution to that strength for the lowest excitedstate of (M)-2b, which is consistent with the experimental findings. CPL, representing the differential emission between left and right circularly polarized light and characterized by the anisotropy factor gem = 2(IL -IR)/(IL + IR) at the maximum of the emission band, has been measured for solutions of (M) and (P)-2b in acetonitrile at room temperature (Fig. 4, bottom). As hypothesized through the introduction of the helicene backbone in the dithiolene ligand, the enantiomers of 2b show CPL activity with an anisotropy factor of ±3×10 -4 , which is a typical value for organic, organometallic and coordination complexes in solution excepting the lanthanides. 11,12 It should be mentioned however that this value of CPL anisotropy is for a compound with a luminescence quantum efficiency of only 0.15%, and which represents the first CPL active metal dithiolene complex. In summary, the first helical Pt(diimine)(dithiolene) complexes have been prepared through the introduction of [4] and [6]helicene backbones in the structures of the dithiolene ligand. The solid state structures of the racemic complexes show the presence of both (P) and (M) enantiomers, with helical curvatures typical of [4] and [6]helicenes and packings controlled by  interactions. Enantiopure [6]helicene complexes have been prepared from the corresponding enantiopure precursors separated by chiral HPLC. The complexes are emissive in fluid solutions at room temperature when excited in the MMLL'CT band, the triplet state being responsible for the observed emission band centered at 715-720 nm. DFT calculations support the absorption, emission and CD properties of the enantiopure compounds. The conformationally stable [6]helicene enantiopure complexes show CPL activity. These results underline the interest of helical dithiolene ligands as means to access chirality related combined properties in the derived complexes and open the way towards the preparation of other related compounds by the use of chiral bipyridines in combination with diverse helicene-dithiolenes in order to tune their photophysical properties.

Scheme 1

 1 Scheme 1 Synthesis of Pt(bpy)(hel-dt) complexes 2a-b.

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Fig. 3

 3 Fig.3Top: Density difference plots between the lowest S1 and the S0 state as determined by the TD-DFT on the optimal ground-state geometry. The blue (red) regions indicate regions of loss (gain) of density upon transition. Bottom: spindensity difference plots between the T1 and S0 states considering the T1 state in its optimal geometry. (M)-2a and (P)-2b are displayed on the left and right handside, respectively. See the SI for computational details.

Fig. 4

 4 Fig.4 Absorption spectra of (P)-2b and (M)-2b and emission spectrum of (M)-2b in acetonitrile (2.5×10 -4 M) at T = 298 K, ex = 532 nm (top); CD and CPL spectra of (P)-2b and (M)-2b (bottom).

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  p t e d m a n u s c r i p t

This work was supported in France by the CNRS (GDR 3712 Chirafun), the University of Angers and the French Ministry of Education and Research (grant to T.B.). The investigation was supported in part by the University of Geneva and by the Swiss National Science foundation (grant No 200020_152780). Magali Allain (University of Angers) is warmly thanked for help with the solid state structures.