Dr Adrien T Normand
email: adrien.normand@u-bourgogne.fr

Dr Quentin Bonnin

Stéphane Brandès

Dr Philippe Richard

Paul Fleurat- Lessard

Dr Charles H Devillers

Prof Cédric Balan

ICMUB Pierre Le Gendre
email: pierre.le-gendre@u-bourgogne.fr

Dr Gerald Kehr

Gerhard Erker

Dr S Brandes

Prof G Erker

The Taming of Redox-Labile Phosphidotitanocene Cations

Keywords: transition metal phosphides, titanium, phosphorus ligands, FLP, DFT

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Introduction

Group 4 metal complexes bearing anionic phosphorus ligands have been investigated since the mid-1960s. [1] Numerous studies focused on the reactivity and catalytic applications of Zr phosphinidene (A) and Zr phosphido (B) complexes (Scheme 1, R2P -: phosphido; RP 2-: phosphinidene). [2], [3] By contrast, the corresponding Ti complexes have been somewhat less explored; initially, most reports dealt with the synthesis and characterization of Ti phosphido complexes (B') ─ including P-bridged bimetallic compounds ─ with little emphasis on reactivity. [4] Later reports by Harrod, Stephan and Le Gendre highlighted the importance of Ti phosphido species in catalytic dehydrocoupling and hydrophosphination reactions. [5] Finally, Ti phosphinidene complexes (A') have been studied by Mindiola. [6] We have recently reported on the chemistry of d 0 phosphidozirconocene cations (C). [7] Compared to neutral complexes, these species display enhanced reactivity due to the combination of a coordinatively unsaturated Cp2Zr 2+ fragment with a nucleophilic PR2 -ligand. This reactivity can be exploited in i) the cooperative activation of CO2 and organic substrates; ii) the catalytic hydrogenation of olefins; iii) the synthesis of Zr + / P frustrated Lewis pairs (FLPs) by cycloaddition with alkynes. Unfortunately, we have thus far been unable to isolate these species and conduct more detailed investigations into the nature of the Zr-P bond.

Scheme 1. Group 4 metal complexes with anionic phosphorus ligands.

In parallel to these studies, we have also been exploring the possibility to generate phosphidotitanocene cations (C'). Compared to the Zr analogues, these compounds require a different synthetic approach. Indeed, the salt metathesis route employed with Zr is unfeasible with Ti, since Ti(IV) compounds are prone to one-electron reduction with concomitant production of a phosphinyl radical. [START_REF]This was first observed by Issleib in 1964, see ref 1[END_REF] Scheme 2. Reaction of group 4 metals with alkali metal phosphides (M = Li, Na, K). [START_REF]The phosphinyl radical then recombines to form a diphosphane (R2P-PR2)[END_REF] As a consequence, examples of isolable Ti(IV) phosphido complexes are rather rare, [3a-b], [11] and to the best of our knowledge, experimentally characterized examples of d 0 phosphidotitanocene complexes are unknown. However, we surmised that stable d 0 phosphidotitanocene cations might be obtained by oxidation of readily available neutral Ti(III) precursors containing a pendant phosphine arm (Scheme 3). [START_REF]cationic d 0 phosphidotitanocene complexes [Cp2TiP(SiMe3)2+ and[END_REF] FULL PAPER

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In this paper, we report the successful application of this strategy and we address the question of the nature of the Ti-P bond in C' both experimentally and theoretically (DFT). Finally, we compare the reactivity of a FLP derived from an unsubstituted phosphidotitanocene cation with that of its previously reported Zr counterpart.

Results and Discussion

The synthesis of the target phosphidotitanocene cations can be effected in two steps from previously described [Cp P CpTiCl2] complexes 1. [13] Ti(III) precursors were first prepared on 1-2 mmol scale by reacting 2 eq of lithium phosphide (PPh2Li or PCy2Li) with 1 (Scheme 4). [START_REF]The first equivalent of phosphide acts as a sacrificial reductant[END_REF] Scheme 4. Synthesis of neutral Ti(III) precursors 2a-d.

Compounds 2a-d were obtained as green (2a, 2b, 2d) or brown (2c) solids in moderate yield. They were characterized by EPR spectroscopy, UV-vis spectroscopy, X-ray diffraction and elemental analysis.

Unlike the TiCl2 precursors 1a-d, compounds 2a-d all show an interaction between Ti and the pendant phosphine arm in the solid state (Figure 2). Table 1 provides a summary of relevant metric parameters; data for the analogous compound [Cp2Ti(PPh2)(PMe3] (D) previously reported by Stephan are also included for comparison. [4j] Figure 1. ORTEP drawings of the X-ray structures of 2a-d (ellipsoids drawn at the 30% probability level, hydrogen atoms removed for clarity, except where CH-π interactions are highlighted).

Table 1. Relevant bond lengths [Å] and angles [°] for 2a-d. [a] 2a 2b 2c 2d D [b] Ti-Cp P 2.0556( 12) 2.0713(8) 2.0802( 9) 2.0804(11) 2.088(5) [c] Ti-Cp 2.0528( 14) 2.0527(9) 2.0612( 9) 2.0617 (12) 2.074(4) Ti-P1 [d] 2.6652(8) 2.5990(6) 2.6040(5) 2.6499( 8) 2.636(3) Ti-P2 [d] 2.6636( 8 The most salient structural feature of complexes 2a-d and D is the pseudo tetrahedral geometry of the phosphido ligand. This is reflected in the sum of angles around P1, with values ranging from 319.6( 2)° (2a) to 333.8(1)° (2c). Electronic repulsion between the phosphorus lone pair and the singly-occupied 1a1 orbital of the d 1 metal centre is most likely responsible for this geometry, [2d], [START_REF] Lauher | [END_REF] which should entail a trigonal planar phosphido ligand for the oxidized complexes (vide infra). The absence of πinteractions between the phosphorus lone pair and the Cp2Ti fragment is also evident from i) the orientation of the phosphido ligand, [16] and ii) the relatively long Ti-P2 distances (from 2.6247(6), 2c, to 2.6641(5), 2b) which are close to the sum of covalent radii of Ti and P (2.67±0.11Å). [17] These experimental findings are corroborated by theoretical calculations conducted on 2b (vide infra).

It is interesting to compare the structures of 2b and 2c, since the latter features the considerably more basic and bulkier PCy2 - (compared to PPh2 -) phosphido ligand. First of all, the Ti-P2 distance in 2c is 0.039 Å shorter compared to 2b; the phosphido ligand is also somewhat flatter in 2c, with a larger sum of angles around P2 (+11.3 °). It is tempting to ascribe these observations to increased π-interactions between Ti and PCy2 -, but the orientation of PCy2 -is not adequate for this purpose. A closer inspection of the structure of 2c suggests another explanation, namely CH/π interactions. [18] Indeed, according to Nishio's formalism, 2c contains two Cy hydrogens in region 2 above one of the Ph rings of the phosphane arm, and one in region 1 (see the Supporting Information): therefore, these hydrogens satisfy the criteria for CH/π interactions. [19] As a consequence, the Cy ring is brought closer to the phosphane arm, with concomitant flattening of the phosphido ligand, and shortening of the Ti-P2 distance. The narrower P1-Ti-P2 angle in 2c (79.76(2)°) compared to 2b (85.33(2)°) is also likely a consequence of these interactions. Noteworthy, 2b also features a hydrogen atom in region 1 above one of the phosphane arm's Ph ring, however this is achieved simply by orienting both Ph rings in a perpendicular fashion.

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The X-band EPR spectra of 2a-d were recorded in THF and toluene at 295 K. Due to the extreme moisture sensitivity of these compounds, hydrolysis products were generally observed in increasing amounts over time. The EPR spectrum of 2b in toluene shown in Figure 2 illustrates this phenomenon (see the supporting information for a detailed discussion). Relevant parameters (isotropic g-factor, hyperfine and superhyperfine coupling constants) are gathered in Table 2, along with data for D previously reported by Stephan. [4j] Theoretical values were obtained by DFT calculations (see the Supporting Information for details). Relevant EPR parameters for 2a-d. [a] 2a 2b 2c 2d D [b] g

Calc. [c] 1.9885

A 47/49Ti
Calc. [c] 9.0 9.0 6.2 Calc. [c] 21.0 21.3 -20.9 Calc. [c] 6. [a] spectra recorded in THF (first line) or toluene (second line) at 295 K; a values in 10 -4 .cm -1 [b] literature values, spectrum recorded in toluene at 298 K. [c] Computed in toluene.

Remarkably, the values of g and A for each compound are very similar in THF and toluene, which implies that phosphane coordination is retained in THF. This behaviour is also observed with the d 0 phosphidotitanocene cations (vide infra).

The absolute values of the coupling constant to the phosphido ligand (A 31P -2) range from 4.8 to 16.3.10 -4 cm -1 in THF. These are consistently smaller than the values of the coupling constant to the phosphane arm (A 31P -1), which span a much more restricted range of 21.0 to 22.7.10 -4 cm -1 . These values are consistent with those found by Stephan for D, [4j] and they are a consequence of the higher s character of the Ti-phosphane bond. Indeed, NBO calculations indicate ~40 % s character for the latter, vs ~20 % for the Ti-phosphido bond (see the Supporting Information).

The UV-vis absorption spectra of 2a-d in THF are characterized by local maxima (ε= 570-1410 cm -1 .M -1 ) in the red to near IR region (Figure 3). [20] As TD-DFT is not reliable for open-shell systems, [21] we were unable to confidently analyse these transitions theoretically, and can therefore only speculate as to their exact nature.

We next turned our attention to the synthesis of the target phosphidotitanocene cations 3, following the oxidation route described above (Scheme 3). Thus, Ti(III) complexes 2a-d were oxidized with [Cp2Fe][BPh4] in C6H5Br, following a modification of a previously reported procedure. [13b] Scheme 5. Synthesis of phosphidotitanocene cations 3b-d.

Complexes 3b-d were isolated in good to moderate yield after workup (Scheme 5); however, complex 3a could not be isolated in pure form (the Cp region of the 1 H NMR spectrum showed the presence of impurities, and the mixture could not be purified).

We also attempted the synthesis of the unsubstituted phosphidotitanocene complex [Cp2Ti(PCy2)][BPh4] (3e) following the oxidation route described in Scheme 4. Addition of two equivalents of PCy2Li to [Cp2TiCl2] in C6H5Br followed by oxidation with [Cp2Fe][BPh4] led to a mixture of dia-and paramagnetic species, possibly including the [Cp2Ti(PCy2)(PCy2H)] + cation (see the Supporting Information). When we repeated this experiment in THF, the known compound [Cp2Ti(THF)2][BPh4] was isolated as blue crystals in 23 % overall yield after recrystallization (Scheme 6). [22] This complex was most probably formed by homolysis of the Ti-P bond according to Scheme 2. Scheme 6. Attempted syntheses of an unsubstituted d 0 phosphidotitanocene cation.

In light of the difficulties associated with the oxidation route, we attempted a different approach involving the protonolysis of the Cp2TiMe + cation with PCy2H (Scheme 7). Interestingly, this route did not afford 3e, but instead compound 3f, which contains a coordinated PCy2H ligand. Attempts at preparing 3e by using only 1 eq of PCy2H failed; rather, mixtures of 3f and Cp2TiMe + were obtained. A PPh2 analogue (3g) could also be synthesized, using PPh2H instead of PCy2H.

Scheme 7. Synthesis of d 0 phosphidotitanocene cations stabilized by secondary phosphanes.

Altogether, these results suggest that mixed phosphane / phosphidotitanocene cations are quite stable. Indeed, not only could complexes 3b-f be isolated and studied spectroscopically (NMR, EPR, UV-vis), but X-ray structures were also obtained for 3b, 3c and 3e (Figure 4).

The impact of oxidation is evident upon examination of the solid-state structures of complexes 2b / 3b and 2c /3c (Tables 1 and3). The Ti-P2 distance is 0.30 Å shorter in 3b vs 2b, and 0.26 Å shorter in 3c vs 2c. Additionally, the phosphido ligand in complexes 3 is now much closer to a trigonal planar geometry than in complexes 2, thus bringing the phosphorus lone pair closer to Ti (Σα(P2) = 358.9(3) to 360.0(4)°). Moreover, the orientation of the phosphido ligand (torsion angle φ) also brings the lone pair into the same plane as the empty 1a1, b2 and 2a1 orbitals on Ti. [START_REF]We have previously used phi as an indicator of π interactions between Zr and N or P in zirconocene complexes[END_REF] The unconstrained PPh2H complex 3e is a case in point, with a Ti-P2 bond distance of 2.3611(18) Å (much shorter than the 2.67±0.11Å sum of covalent radii), a perfectly trigonal planar phosphido ligand (Σα(P2) = 360.0(4)°) and an almost perfect alignment of donor-acceptor orbitals (φ = -2.3(3) °). These changes are especially remarkable if one considers that other parameters (e.g. Ti-P1 distances), hardly change upon oxidation. Table 3. Relevant bond lengths [Å] and angles [°] for 3b, 3c and 3e. [a] 3b 3c 3e

Ti-Cp P 2.0471(18) 2.0497( 9) 2.051(3) [c] Ti-Cp 2.040(3) 2.0438( 9) 2.042(3) Ti-P1 [d] 2.5883( 12) 2.6062(5) 2.5868 (17) Ti-P2 [d] 2.3599( 14 Therefore, there is considerable structural evidence to support the hypothesis of π interactions between Ti and the phosphido ligand in complexes 3. This is confirmed through the theoretical analysis of the Ti-P interactions in 2b and 3b (see Computational Details): in 2b, only one lone pair of the phosphido ligand is involved in a Ti-P bond, while both are interacting with the titanium atom in 3b (Figure 5) indicating π interactions in these complexes. [24] This is in line with previous theoretical results obtained on hypothetical d 0 phosphidotitanocene complexes. [START_REF]cationic d 0 phosphidotitanocene complexes [Cp2TiP(SiMe3)2+ and[END_REF] A deeper analysis of the X-ray structures of 3b and 3c reveals yet more interesting features. Firstly, as in the case of 2b and 2c, CH/π interactions are observed between one of the Ph rings of the phosphane arm and CH bonds on the phosphido ligand (Figure 4). Both hydrogens are located in region 1 above the Ph rings (see the Supporting Information). Interestingly, in the case of 3c, this does not seem to induce a narrower P1-Ti-P2 angle as in 2c. Rather, since the Cy ring is engaged in only one CH/π interaction with the Ph ring, both rings adopt a perpendicular orientation, as observed with the Ph rings of 2b and 3b. Examination of the 1 H NMR spectrum of 3b and 3c indicates that this conformation is retained in solution. Indeed, Figure 7 shows the aliphatic region of the 1 H and 1 H{ 31 P} spectra of 3b: the hydrogens of the Cp-CH2 group resonate at 2.87 and 2.04 ppm, with 3 JPH coupling constants of 44.9 and 5.5 Hz (Table 4). [25] Since 3 J coupling between P and H nuclei follows a Karplus relationship, with a maximum corresponding to anti-periplanar arrangement of P-C and C-H bonds, [START_REF] Kühl | The Range of Chemical Shifts, Coupling Constants, and What Influences Each[END_REF] we conclude that the conformation of the ethylene arm of 3b is similar in solution and in the solid state. [START_REF] Janssen | Differences between 3 JPH coupling constants in CpCH2CH2PR2 ligands have been observed before, see for example[END_REF] The same conclusions can also be drawn for 3c.

The CH/π interactions observed in the X-ray structures of 3b and 3c should entail considerably shielded 1 H NMR signals for the hydrogens involved, due to aromatic ring current effects. Indeed, in the case of 3b, the ortho hydrogens of one of the Ph ring of the phosphido ligand resonate as a triplet at 6.25 ppm in CD2Cl2 (500 MHz, 300 K), whereas the same hydrogens resonate above 7.28 ppm in 3d (Figure 8). [28] However, CH-π interactions are not strong enough to prevent the free rotation of the Ph ring of the phosphido ligand. Table 4. Values of θ (XRD) and 3 JPH (NMR) for 3b and 3c . [a] θ1 (°) θ2 (°) 12) 67.08( 16) 43.5 6.1

[a] 1 H spectra recorded at 500 MHz in CD2Cl2, 300 K.

In the case of 3c, a shielded signal (0.00 ppm) is observed in CD2Cl2 (500 MHz, 300 K) for one of the CH2 hydrogens of the Cy ring. Interestingly, a shielded signal is also observed in d8-THF (0.06 ppm) and C6D5Br (-0.17 ppm). The conformation of this complex appears to be frozen in these solvents at 300 K, since only one hydrogen experiences a shielding effect (probably because of steric congestion due to the Cy ring). In summary, there is considerable evidence to suggest that the structures of 3b and 3c are the same in solution and in the solid state. This correspondence indicates that the Cp P CpTi fragment (Cp P = C5H5-CH2CH2PPh2) may be viewed as a preorganized host for CH/π interactions with incoming ligands or substrates. Noteworthy, the neutral d 1 analogues are also preorganized, since the X-ray structures of 2b and 2c show CH/π interactions as well.

The 31 P{ 1 H} NMR spectra of 3b-g are also interesting in a number of ways. First of all, the signals corresponding to the phosphido ligand are very deshielded, around 410 ppm for PPh2 - and 500 ppm for PCy2 -(Table 5). For comparison, the [Cp2ZrPCy2] + cation (C1) gave a more shielded signal at 396.6 ppm (C6D5Br, 202 MHz, 299 K). [7a] In a recent paper, Eisenstein noted that neutral d 0 alkylidene complexes ─ which are valence isoelectronic to cationic d 0 phosphido complexes ─ display considerably deshielded 13 C NMR signals, similar to carbocations, despite their nucleophilic carbene character. [29] The major conclusion of that study was that the observed deshielding of the carbene signal (compared to ethylene) was the result of more efficient coupling between MC (ligand based) and π*MC (metal based) orbitals, which are separated by a narrower gap than the CC and π*CC orbitals of ethylene. Consequently, d 0 alkylidene complexes of 5d metals give less deshielded carbene signals than isoelectronic 4d metals complexes, due to the higher energy of 5d vs 4d empty orbitals. Although this would fall outside the scope of the present paper, it would be interesting ─ in light of the Eisenstein study ─ to investigate whether similar explanations apply to i) the highly deshielded 31 P NMR signal of phosphidometallocene cations 3b-f and C1, and ii) the ~100 ppm difference between Ti (3d) and Zr (4d) complexes.

Table 5. Relevant 31 P{ 1 H} NMR parameters for 3b-g. [a] 3b 3c 3d 3g 3f

δ-P1 [b] (ppm) 45.2 44.9

43.8 43.2 52.3 52.2 23.7 N.A. [c] 39.1 N.A. [c] δ-P2 (ppm) [c] N.A. [d] 500.4 [c] N.A. [d] 2 A. [c] 76.7 N.A. [c] [a] spectra recorded in CD2Cl2 (first line) or d8-THF (second line) at 202 MHz, 300 K; [b] P1: phosphane ligand, P2: phosphido ligand; [c] spectra recorded in C6D5Br; [d] 3e and 3f decomposed in THF.

JPP
Another striking feature of complexes 3b-f, visible in their 31 P{ 1 H} spectra, is the stability conferred by the chelating phosphane arm in a coordinating solvent such as d8-THF. Indeed, complexes 3b-d remain unaltered in this solvent, as evidenced by the similar chemical shifts of P1 and P2 in CD2Cl2 vs d8-THF on the one hand, and the 2 JPP coupling constants on the other hand (64.0 to 71.7 Hz in CD2Cl2 vs 63.0 to 70.6 Hz in d8-THF). In contrast, dissolution of 3e or 3f in d8-THF releases free PPh2H or PCy2H (see the Supporting Information). We measured the UV-vis absorption spectra of 3b-d in THF (Figure 9), and we observed intense bands (ε = 6920-7910 cm - 1 .M -1 ) in the blue region (3b -> 437nm; 3c -> 419 nm; 3d -> 448 nm). DFT calculations were conducted to investigate the nature of this transition. Natural Transition Orbitals (Figure 10) indicate that it corresponds to a LMCT band, mostly centred on the phosphido ligand. [START_REF]ε = 2210-3220 cm -1 .M -1 ) shouldering bands in the UV region (3b -> 348 nm; 3c -> 342 nm; 3d -> 346 nm)[END_REF] Figure 10. Natural Transition Orbitals for the highest energy UV-vis band of 3b.

Since d 0 phosphidotitanocene are usually unstable and readily decompose to give Ti(III) and phosphinyl radicals (Scheme 2), we were intrigued by the apparent stability of complexes 3b-g. The presence of π interactions between the Cp2Ti fragment and the phosphido ligand alone cannot explain this stability, as illustrated by our failure to prepare complex 3e. We reasoned that the additional phosphane ligand in 3b-g might simply provide kinetic stabilization to otherwise thermodynamically unstable entities. To test this hypothesis, we measured the EPR spectra of 3b-g and found that paramagnetic Ti-P species were present in every case. In the case of 3f and 3g, we could positively identify the Cp2Ti(THF)2 + cation as the main paramagnetic species (g = 1.9758, A 47/49Ti = 11.4 x 10 -4 cm -1 ). For complexes 3b-d, the spectra revealed the presence of at least three paramagnetic Ti-P species (see the Supporting Information).

Irradiation of 3g and 3g with a UV lamp for 20 minutes led to mixtures in which only the Cp2Ti(THF)2 + cation was visible by EPR spectroscopy (Figure 11). Analysis of the reaction mixtures by [START_REF]presence of PCy2H in the reaction mixture prevents the phosphinyl radical from dimerizing, instead promoting other termination pathways[END_REF] P{ 1 H} and 11 B{ 1 H} NMR spectroscopy revealed i) the complete disappearance of 3f and 3g, ii) an intact B(C6F5)4 -anion, iii) concomitant formation of (PPh2)2 for 3g, and iv) the apparition of several P-containing species in addition to PCy2H for 3f (some of which was present before irradiation). [START_REF]presence of PCy2H in the reaction mixture prevents the phosphinyl radical from dimerizing, instead promoting other termination pathways[END_REF] For complexes 3b-d, the mixtures after UV irradiation were again complex by EPR spectroscopy, but in each case the BPh4 - anion was also intact, whilst the presence of (PPh2)2 (3b, 3d) or (PCy2)2 (3c) strongly suggested that PR2  radicals had been generated.

Therefore, it appears that d 0 -phosphidotitanocene cations decompose by Ti-P bond homolysis just like their neutral counterparts. Compared to the unsubstituted complex 3e, the additional stabilization provided by the phosphane ligand in 3b-g enables the isolation of these otherwise fleeting species.

In light of these findings, we decided to investigate whether complex 3e could be generated in situ and trapped with a suitable reagent. We had previously shown that the Zr analogue of 3e (C1) reacted with diphenylacetylene to give a Zr + / P frustrated Lewis pair (FLP), [7a] therefore we generated 3e in the presence of this reagent, and indeed complex 4e was isolated in 27 % yield after workup (Scheme 8). Evidence for the formation of 4e initially came from the comparison of its 1 H and 31 P{ 1 H} NMR spectra with those of the previously reported Zr analogue E (containing the MeB(C6F5)3 - anion). [7a] For example, the Cp hydrogens in 4e resonate as a doublet at 6.52 ppm ( 3 JPH = 1.7 Hz) in CD2Cl2, indicative of phosphane coordination to Ti, whilst the 31 P{ 1 H} spectrum shows a signal at -7.6 ppm. Similar spectroscopic features were observed for compound E (Cp: 6.23 ppm, 3 JPH = 0.7 Hz; phosphane : -12.4 ppm). [START_REF]Complex E was characterized in C6D5Br at 299 K[END_REF] Interestingly, the high-resolution ESI mass spectrum of 4e shows the expected presence of the Cp2Ti(C(Ph)C(Ph)PCy2 + cation (with a distinctive cluster of peaks at 553.24758 Da), but the elusive Cp2Ti(PCy)2 + is also observed (375.17211 Da). This suggests some degree of reversibility for the alkyne insertion reaction, at least under the relatively forcing conditions of the mass spectrometry experiment.

A PPh2 analogue of 4e was also prepared by reacting 3e with a four-fold excess of diphenylacetylene. Although the insertion reaction was much slower (72 h vs a few minutes), probably due to the necessity to dissociate PPh2H first, compound 4e (not shown) was isolated in 88 % yield. The preparation of 4e starting from an isolated phosphidotitanocene cation indicates that alkyne insertion probably occurs after oxidation by [Cp2Fe][BPh4] during the synthesis of 4e.

Although 4e is stable enough to be isolated and characterized by NMR spectroscopy, UV-vis spectroscopy, ESI-MS and X-ray diffraction analysis (vide infra), it displays slightly broadened signals by 1 H NMR spectroscopy, suggesting the presence of paramagnetic impurities. Indeed, the EPR spectrum of a solution of 4e in d8-THF indicated the presence of small amounts of [Cp2Ti(THF)2][BPh4] (see the Supporting Information), which again points to the reversible insertion of diphenylacetylene into the Ti-P bond of 3e.

Irradiation of this solution with a UV lamp and monitoring the reaction by EPR spectroscopy revealed that a new Ti-P product was being formed in equimolar quantities with [Cp2Ti(THF)2][BPh4]. Both 1 H and 31 P{ 1 H} spectra showed the complete disappearance of 4e, although the 11 B{ 1 H} NMR spectrum did not evolve during photolysis. Borates are known to decompose photolytically in the presence of electron acceptors, [START_REF] Aj | [END_REF] therefore we initially envisaged a non-innocent role for the BPh4 -anion. However, we could not find any evidence for the oxidation of BPh4 -nor could we identify the diamagnetic byproducts of the reaction.

The reaction was repeated on preparative scale, and complex 4e' was isolated in 40 % yield after workup (Scheme 9). Complexes 4e and 4e' were characterized by single crystal Xray diffraction analysis (Figure 12, Table 5). The metric parameters of these complexes support their formulation as Tialkenyl complexes with a chelating phosphane arm, rather than Ti-carbene complexes bound to an alkylidene phosphonium moiety. [34] The C11-C12 bond distances are consistent with a double bond (4e: 1.344(2) Å; 4e': 1.341(4) Å), whilst Ti-C11 and C12-P distances indicate the presence of single bonds (4e: 2.178(2) Å and 1.813(2) Å; 4e': 2.229(2) Å and 1.798(2) Å). The shorter Ti-P distance in 4e reflects the higher Lewis acidity of Ti(IV) vs Ti(III) (4e: 2.6421(8) Å; 4e': 2.7000(8) Å). In order to determine the redox properties of the 4e/4e', couple, voltammetric analyses were performed in THF, with 0.1 M NaBPh4 as electrolyte. Under these conditions, 4e is reversibly reduced at E1/2 = -0.420 V vs. SCE (Figure 13). The reduced species was attributed to the Ti(III) complex 4e'. Complementary cyclic and steady state voltammetry experiments conducted on a mixture of 4e and 4e' confirmed that these compounds form one redox couple, and can be reversibly transformed into each other (see the supporting information). Finally, we investigated the FLP behaviour of 4e. By contrast with the Zr analogue E, reaction with chalcone or 1 atm CO2 at room temperature in CD2Cl2 did not yield the expected FLP adducts: no reaction was observed in both cases. However, when 4e was reacted with ferrocene carboxaldehyde in CH2Cl2, compound 5e was obtained in 83 % yield after workup (Scheme 10).

Scheme 10. FLP reaction of 4e with ferrocene carboxaldehyde.

Compound 5e exhibits characteristic NMR features in CD2Cl2, indicative of the activation of the CHO moiety by the Ti /P FLP. Firstly, the 31 P{ 1 H} NMR spectrum shows a signal at 32.5 ppm (4e: -7.6 ppm), consistent with the presence of a phosphonium moiety. Secondly, the activated aldehydic proton resonates as a broad signal at 6.12 ppm in the 1 H spectrum, and couples both with the carbons of the ferrocene Cp ring and the CH carbons of the Cy rings in the HMBC spectrum. Finally, the activated aldehydic carbon resonates as a doublet at 80.2 ppm ( 1 JPC = 49.2 Hz) in the 13 C{ 1 H} spectrum.

The high resolution ESI spectrum of 5e displays a single cluster of peaks at 767.25649 Da, which is again consistent with FLP adduct formation.

Single crystals suitable for X-ray diffraction were obtained by slow diffusion of pentane into a CH2Cl2 solution of 5e at -18 °C (Figure 14). Compared to 4e, the Ti-C11 distance (2.231(3) Å) is significantly elongated (+0.053(4) Å). The C11-C12 distance is somewhat elongated (1.361(3) Å, +0.017(4) Å), whilst the C12-P distance is slightly shortened (1.800(3) Å, -0.013(4) Å). Noteworthy, the Fe-O distance (3.521(2) Å) is shorter than the sum of Van der Waals radii (3.94 Å), a feature which was previously observed in a related Zr complex. [35] Finally, complex 4e' failed to react with ferrocene carboxaldehyde (toluene, room temperature), highlighting the

Conclusions

Although d 0 phosphidotitanocene complexes are intrinsically unstable due to their redox-labile nature, we have shown that they can be isolated as phosphane-stabilized cations. This is somewhat counter-intuitive, since d 0 complexes are classified as hard Lewis acids, whilst phosphorus-based ligands fall in the category of soft Lewis bases. It appears that the phosphanestabilized phosphidotitanocene cations reported in this study are unique in many ways: although they are stable enough to be isolated and crystallographically characterized, they readily decompose in the presence of light. Additionally, they have strikingly similar structures in solution and in the solid state, a consequence of the pre-organization of the phosphanefunctionalized metallocenium fragment in a favourable conformation for CH/π interactions. Thus, in contrast to their redox-labile behaviour, they show great structural resilience even in coordinating solvents such as d8-THF. Finally, they give rise to extremely high 31 P NMR chemical shifts, in the 400-500 ppm range, as a consequence of the double-bond character of the Ti-P interaction.

Experimental Section General

General information, detailed reaction procedures, analytical details, and structural data of the new compounds are given in the Supporting Information.

Synthesis

Compound 2a: In an Ar glovebox, a solution of PPh2Li (408 mg, 2.0 mmol) in THF (6 mL) was added to a suspension of complex 1a (487 mg, 1.0 mmol) in THF (8 mL). The reaction mixture turned green almost instantly, and was stirred at room temperature for 30 min. Volatiles were removed in vacuo outside the glovebox, and toluene (15 mL) was added to the residue. The resulting suspension was filtered over diatomaceous earth to remove LiCl, and the filtrate was evaporated. The residue was taken inside the glovebox, and Et2O (15 mL) was added, forming a green precipitate. It was filtered over a sintered glass frit, rinsed with 3x10 mL of Et2O, then 2x10 mL of pentane, then dried on the frit, yielding 2a as a green microcrystalline solid (374 mg, 62 %). Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a toluene solution of 2a at -18°C. Elemental Analysis: calcd for C37H47P2Ti: C, 73.87; H, 7.87. Found: C, 73.17; H, 7.71.

Compound 2b:

A solution of PPh2Li (769 mg, 4.0 mmol) in THF (40 mL) was added to a solution of complex 1b (920 mg, 2.0 mmol) in THF (40 mL). The reaction mixture turned green almost instantly, volatiles were removed in vacuo, and toluene (30 mL) was added to the residue. The resulting suspension was filtered over diatomaceous earth to remove LiCl, and the filtrate was evaporated. The residue was taken inside the glovebox, and a mixture of Et2O (25 mL) and pentane (25 mL) was added, forming a green precipitate. It was filtered over a sintered glass frit, rinsed with 3x4 mL of a 1:1 mixture of Et2O/pentane, then dried on the frit, yielding 2b as a green powder (770 mg, 67 %). Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a toluene solution of 2b at -18°C.

Elemental Analysis: a satisfactory elemental analysis could not be obtained for this compound despite repeated attempts (C content was always low by 5-10 %).

Compound 2c:

In an Ar glovebox, a solution of PCy2Li (408 mg, 2.0 mmol) in THF (6 mL) was added to a solution of complex 1c (461 mg, 1.0 mmol) in THF (6 mL). The reaction mixture turned brown almost instantly, volatiles were removed in vacuo outside the glovebox, and toluene (30 mL) was added to the residue. The resulting suspension was taken back inside the glovebox, filtered over diatomaceous earth to remove LiCl, and the filtrate was evaporated outside the glovebox. The residue was taken inside the glovebox and triturated in Et2O (20 mL), forming a brown precipitate. It was filtered over a sintered glass frit, rinsed with 3x4 mL of a 1:1 mixture of Et2O/pentane, then dried on the frit, yielding 2c as a brown powder (325 mg, 55 %). Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a toluene solution of 2c at -18°C. Elemental Analysis: calcd for C36H45P2Ti: C, 73.59; H, 7.72. Found: C, 73.40; H, 7.60.

Compound 2d:

In an Ar glovebox, a solution of PCy2Li (408 mg, 2.0 mmol) in THF (6 mL) was added to a solution of complex 1c (461 mg, 1.0 mmol) in THF (6 mL). The reaction mixture turned green almost instantly, volatiles were removed in vacuo outside the glovebox, and toluene (30 mL) was added to the residue. The resulting suspension was taken back inside the glovebox, filtered over diatomaceous earth to remove LiCl, and the filtrate was evaporated outside the glovebox. The residue was taken inside the glovebox and triturated in Et2O (20 mL), forming a green precipitate. It was filtered over a sintered glass frit, rinsed with 3x4 mL of a 1:1 mixture of Et2O/pentane, then dried on the frit, yielding 2d as a brown powder (307 mg, 52 %). Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a toluene solution of 2d at -18°C. Elemental Analysis: calcd for C36H45P2Ti: C, 73.59; H, 7.72. Found: C, 73.34; H, 7.59.

Compound 3b:

In an Ar glovebox, 2b (574 mg, 1.0 mmol) and [Cp2Fe][BPh4] (505 mg, 1.0 mmol) were mixed in C6H5Br (5 mL). The reaction mixture gradually turned from green to red over 5 min; residual particles of [Cp2Fe][BPh4] were observed. After decantation, the supernatant was added to 50 mL of pentane under vigorous agitation. A brick-red solid formed, it was filtered over a sintered glass frit, suspended three times in pentane (5 mL) and dried on the frit. The resulting powder was freed from residual C6H5Br and [Cp2Fe] by dissolution in CH2Cl2 (4 mL), filtration over diatomaceous earth, and precipitation as previously. Finally, the obtained product was stirred in toluene (8 mL) for 3h, then filtered and rinsed with toluene and pentane, yielding 3b as a brick-red powder containing 50 mol% of toluene and 15 mol% of pentane (708 mg, 79 %). Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a CH2Cl2 solution of 3b at -18°C. Elemental Analysis: calcd for C60H53BP2Ti(C7H8)0.5(C5H12)0. 15: C, 81.10; H, 6.23. Found: C, 79.88; H, 6.04.

Compound 3c:

In an Ar glovebox, 2c (196 mg, 0.33 mmol) and [Cp2Fe][BPh4] (168 mg, 0 mmol) were mixed in C6H5Br (5 mL). The reaction mixture gradually turned from green to red over 5 min; residual particles of [Cp2Fe][BPh4] were observed, hence the mixture was filtered over diatomaceous earth. The resulting solution was added to 100 mL of pentane under vigorous agitation. An orange solid formed, it was filtered over a sintered glass frit, suspended three times in pentane (5 mL) and dried on the frit. The resulting powder was freed from residual C6H5Br and Cp2Fe by stirring in toluene (20 mL). An oil formed initially, which turned into a powder over 10 min. The solid was filtered over a sintered glass frit, suspended three times in toluene (4 mL) then three times in pentane (4 mL) and dried on the fritt. Complex 3c was obtained as an orange powder containing 66 mol% of toluene (178 mg, 59 %). Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a CH2Cl2 solution of 3c at -18°C. Elemental Analysis: calcd for C60H64BP2Ti(C7H8)0.66: C, 80. 30; H, 7.22. Found: C, 79.40; H, 7.17.

Compound 3d:

In an Ar glovebox, 2d (218 mg, 0.37 mmol) and [Cp2Fe][BPh4] (188 mg, 0.37 mmol) were mixed in C6H5Br (3 mL). The reaction mixture gradually turned from green to red over 5 min; residual particles of [Cp2Fe][BPh4] were observed, hence the mixture was filtered over diatomaceous earth. The resulting solution was added to 100 mL of pentane under vigorous agitation. A brick-red solid formed, it was filtered over a sintered glass frit, suspended three times in pentane (10 mL) and dried on the frit. The resulting powder was freed from residual C6H5Br and Cp2Fe by stirring in toluene (10 mL). An oil formed, the supernatant was discarded, fresh toluene (10 mL) was added and agitation was resumed. A powder formed over a few hours. The solid was filtered over a sintered glass frit, suspended three times in toluene (6 mL) then three times in pentane (10 mL) and dried on the frit. Complex 3d (178 mg, 50 %) was obtained as an orange powder.. Elemental Analysis: C60H64BP2Ti: C, 79.56; H, 7. 12. Found: C, 78.11; H, 7.32.

Attempted synthesis of 3e:

In an Ar glovebox, Cp2TiCl2 (49.8 mg, 0.2 mmol) and PCy2Li (81.6 mg, 0.4 mmol) were mixed in C6H5Br (2 mL). The mixture turned chocolate brown over 5 min. It was filtered over diatomaceous earth, then [Cp2Fe][BPh4] (101 mg, 0.2 mmol) was added and the resulting mixture was stirred for 5 min, during which it turned progressively red. It was filtered over diatomaceous earth in order to remove a green-black insoluble solid. The homogeneous red filtrate was added to 80 mL of pentane under vigorous agitation, to precipitate an orange solid. The precipitate was filtered over a sintered glass frit, suspended three times in pentane (10 mL) and dried on the frit. An orange powder (30 mg) was isolated and analyzed by NMR and EPR spectroscopy. The 1 H and 31 P{ 1 H} NMR spectrum suggested the presence of the Cp2Ti(PCy2)(PCy2H) + cation (compare with 3f) but impurities were also observed. Moreover the 1 H spectrum showed considerably broadened signals, consistent with the presence of paramagnetic impurities. The EPR spectrum revealed a mixture of species.

Compound 3f:

In an Ar glovebox, Cp2TiMe2 (200 mg, 0.96 mmol) and Ph3CB(C6F5)4 (888 mg, 0.96 mmol, 1 eq) were mixed for 5 min in 2 mL of C6H5Br at room temperature. A solution of PCy2H (401 mg, 2.02 mmol) in C6H5Br was added dropwise to the reaction mixture. The resulting deep-red solution was stirred for 3h until a red precipitate was formed. The suspension was added dropwise to 8 mL of pentane under vigorous agitation to complete the precipitation of the desired red compound. The supernatant was removed and the red precipitate was washed twice with 5 mL of pentane. After removal of traces of pentane in vacuo, the red precipitate was dissolved in CH2Cl2 and precipitated from pentane as described above (N.B.: compound 3f slowly decomposes in CH2Cl2, therefore this step must be performed quickly). Removal of volatiles in vacuo yielded complex 3f (1.20 g, 83 %) as a red powder containing 50 mol% of pentane. Elemental Analysis: calcd for C58H55BF20P2Ti(C5H12)0.5: C, 56.39; H, 4.77; Found: C, 57.00; H, 5.00.

Compound 3g:

In an Ar glovebox, Cp2TiMe2 (200 mg, 0.96 mmol) and [Ph3C][B(C6F5)4] (888 mg, 0.96 mmol, 1 eq) were mixed for 5 min in 2 mL of C6H5Br at room temperature. A solution of PPh2H (376 mg, 2.02 mmol) in C6H5Br was added dropwise to the reaction mixture. The resulting deep-red solution was stirred for 3h until a red precipitate was formed. The suspension was added dropwise to 8 mL of pentane under vigorous agitation to complete the precipitation of the desired red compound. The supernatant was removed and the red precipitate was washed twice with 5 mL of pentane. After removal of traces of pentane in vacuo, the red precipitate was dissolved in CH2Cl2 and precipitated from pentane as described above (N.B.: compound 3g slowly decomposes in CH2Cl2, therefore this step must be performed quickly). Removal of volatiles in vacuo yielded complex 3g (980 mg, 83 %) as a red powder. Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of heptane in a bromobenzene solution of the compound at -18°C. Elemental Analysis: calcd for C58H31BF20P2Ti: C, 57.07; H, 2.45; Found: C, 56.71; H, 2.54.

Compound [Cp2Ti(THF)2][BPh4]:

In an Ar glovebox, Cp2TiCl2 (249 mg, 1.0 mmol) and PCy2Li (408 mg, 2.0 mmol) were mixed in toluene (5 mL). After 5 min, the brown reaction mixture was filtered through diatomaceous earth and the filtrate was evaporated in vacuo outside of the glovebox. The residue was dissolved in THF (10 mL) in the glovebox, and [Cp2Fe][BPh4] (485 mg, 0.9 mmol) was added. A blue precipitate appeared gradually; the presence of (PCy2)2 was ascertained by performing no lock 31 P{ 1 H} NMR experiment. After 1H reaction, the reaction mixture was filtered over a sintered glass frit, the precipitate was suspended twice in THF (1.5 mL) and twice in pentane (3.0 mL) and dried on the frit. Complex [Cp2Ti(THF)2][BPh4] was obtained as a blue-green powder (327 mg, 51 %). Material suitable for elemental analysis was obtained by diffusion of pentane into a THF solution (50 mL) of the crude product. Royal blue crystals of [Cp2Ti(THF)2][BPh4] were obtained (152 mg, 45 %). Single crystals suitable for X-ray diffraction analysis were obtained by cooling a saturated solution of [Cp2Ti(THF)2][BPh4] in THF at -18°C. Elemental Analysis: calcd for C36H42PTi: C, 78.64; H, 7. 23. Found: C, 77.45; H, 7.11.

Compound 4e:

In an Ar glovebox, Cp2TiCl2 (498 mg, 2.0 mmol) and PCy2Li (816 mg, 4.0 mmol) were mixed in C6H5Br (8 mL). After 5 min, the brown reaction mixture was filtered through diatomaceous earth onto a mixture of [Cp2Fe][BPh4] (1.01 mg, 2.0 mmol) and diphenylacetylene (356 mg, 2.0 mmol). A green-brown solution was obtained after 5 min, which was added to 120 mL of pentane under vigorous agitation. A green solid formed, it was filtered over a sintered glass frit, suspended three times in pentane (10 mL) and dried on the frit. The resulting powder was freed from residual C6H5Br and Cp2Fe by dissolution in CH2Cl2 (6 mL) and filtration over diatomaceous earth. A blue-green solid was removed, and the resulting homogeneous solution was evaporated to dryness. Toluene (100 mL) was added to the residue, and a green solid formed. The suspension was stirred for 3 h, then filtered over a sintered glass frit. The solid was suspended three times in toluene (15 mL) and three times in pentane (15 mL), and dried on the frit. Complex 4e was obtained as an olive green powder containing 40 mol% of toluene (483 mg, 27 %). Complex 4e slowly degrades over time to give 4e' (vide infra), which explains the impossibility to obtain a satisfactory elemental analysis despite repeated attempts on different batches. Single crystals suitable for Xray diffraction analysis were obtained by slow diffusion of pentane into a CH2Cl2 solution of 4e at -18°C.

Compound 4e':

A solution of 4e (440 mg, 0.48 mmol) in THF (15 mL) was irradiated for 4h30 with a 150 W UV lamp (Heraeus TQ 150). Volatiles were removed in vacuo and the residue was stirred in 20 mL of pentane inside an Ar glovebox for several hours. The resulting suspension was filtered on a sintered glass frit, yielding a blue-green solid and a brown solution. The solution was evaporated to dryness, yielding 4e' as a brown gum (105 mg, 40 %). Single crystals suitable for X-ray diffraction analysis were obtained by cooling a saturated solution of 4e' in pentane at -18°C. Elemental Analysis: calcd for C36H42PTi: C, 78.11; H, 7.65. Found: C, 77.27; H, 8.15. Compound 4g: In an Ar glovebox, compound 3g (400 mg, 0.32 mmol) was dissolved in C6H5Br (3 mL) with an excess of diphenylacetylene (232 mg, 1.3 mmol) at room temperature and stirred for 72 h. The evolution of the reaction was followed by [START_REF]presence of PCy2H in the reaction mixture prevents the phosphinyl radical from dimerizing, instead promoting other termination pathways[END_REF] P{ 1 H} no lock NMR experiments. The starting red solution became green. The compound was precipitated by addition to 6 mL of pentane under vigorous agitation. An oil formed, the supernatant was discarded and the oil extracted twice with pentane. The black-green residue was dissolved in 2 mL of dichloromethane and precipitated as described above. Compound 4g was obtained as a light brown powder containing 30 mol% of pentane after drying in vacuo (350 mg, 88 %). Elemental Analysis: calcd for C60H30BF20PTi(C5H12)0. 3: C, 59.47; H, 2.73; Found: C, 59.73; H, 2.44. Compound 5e: In an Ar glovebox, compound 4e (170 mg, 0.19 mmol) and ferrocene carboxaldehyde (42 mg, 0.20 mmol) were mixed in CH2Cl2 (3 mL). After 5 min, the red reaction mixture was added to 50 mL of pentane under vigorous agitation. A red precipitate formed, which was filtered over a sintered glass frit, rinsed with pentane and dried under vacuum. Compoung 5e was obtained as a brown-red solid (176 mg, 83 %) containing 75 mol% of pentane Single crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a CH2Cl2 solution of 5e at -18°C. Elemental Analysis:

Hydrolysis of Ti(III)-PPh2 complexes

In an Ar glovebox, the Ti(III)-PPh2 complexes (2a: 0.5 mmol; 2b: 0.19 mmol) were dissolved in 4 mL of THF and treated with 0.5 eq of distilled water in THF. The solution turned blue-violet immediately. The volatiles were removed in vacuo, the solid residue was triturated with pentane, filtered and rinsed with pentane. Blue violet powders were obtained in both cases after drying in vacuo (2a: 60 mg; 2b: 46 mg). EPR spectra of 2a and 2b were also recorded in "wet", unstabilized THF (degassed by free-pump-thaw) for comparison purposes.

[Attirez l'attention du lecteur avec une citation du document ou utilisez cet espace pour mettre en valeur un point clé. Pour placer cette zone de texte n'importe où sur la page, faites-la simplement glisser.]

Figure 2 .

 2 Figure 2. EPR spectrum of 2b in toluene showing the apparition of a hydrolysis product over time.

Figure 3 .

 3 Figure 3. UV-vis spectra of 2a-d in THF.

Figure 4 .

 4 Figure 4. ORTEP drawings of the X-ray structures of 3b-g (ellipsoids drawn at the 30% probability level, BPh4 -anion and most hydrogen atoms removed for clarity) and definition of the torsion angle φ.

Figure 5 .

 5 Figure 5. ELF basins in 2b and 3b. Bonding basin are shown in green whilst lone pairs are in red. For clarity's sake, only the basins between Ti and P are shown, and the BPh4 -anion is omitted.

Figure 6 .

 6 Figure 6. Definition of torsion angles θ1 and θ2.

Figure 7 .

 7 Figure 7. Aliphatic region of the 1 H and 1 H{ 31 P} spectra of 3b in CD2Cl2 (500 MHz, 300 K) showing 3 J coupling between P and H nuclei (t: residual toluene signal).

3 JPH

 3

Figure 8 .

 8 Figure 8. Conformational origin of shielded 1 H NMR signals in 3b and 3c (CD2Cl2, 500 MHz, 300 K). BPh4 -anions omitted for clarity.

  un point clé. Pour placer cette zone de texte n'importe où sur la page, faites-la simplement glisser.]

Figure 9 .

 9 Figure 9. UV-vis spectra of 3b-d in THF.

Figure 11 .

 11 Figure 11. EPR spectrum of 3g after 20 min irradiation with a UV lamp

Scheme 8 .

 8 Scheme 8. Synthesis of a Ti + / P FLP by the trapping of a phosphidotitanocene cation.

Scheme 9 .

 9 Scheme 9. Synthesis of 4e by photolysis of 4e.

Figure 12 .

 12 Figure 12. ORTEP drawings of the X-ray structures of 4e and 4e' (ellipsoids drawn at the 30% probability level, hydrogen atoms removed for clarity) Cp1 and Cp2 indicate the centroid of the Cp rings. Selected bond distances (Å) and angles (°). 4e: Ti-Cp1 = 2.0347(9); Ti-Cp2 = 2.0379(9); Ti-P = 2.6421(8); Ti-C11 = 2.178(2); C11-C12 = 1.344(2); C12-P = 1.813(2); Cp1-Ti-Cp2 = 133.83(4); P-Ti-C11 = 63.37(4); Σα(C11) = 359.8(2); Σα(C12) = 359.7(2). 4e': Ti-Cp1 = 2.0562(13); Ti-Cp2 = 2.0623(13); Ti-P = 2.7000(8); Ti-C11 = 2.229(2); C11-C12 = 1.341(4); C12-P = 1.798(2); Cp1-Ti-Cp2 = 133.46(6); P-Ti-C11 = 61.22(6); Σα(C11) = 359.6(3); Σα(C12) = 359.9(3).

  Figure 13. 4e in THF 0.1 M NaBPh4 ([4e] = 5 x 10 -4 M, ν = 100 mV s -1 , working electrode: GC electrode Ø = 3 mm, counter electrode: Pt, Reference electrode: SCE).

Figure 14 .

 14 Figure 14. ORTEP drawings of the X-ray structure of 5e (ellipsoids drawn at the 30% probability level, hydrogen atoms, BPh4 -anion and CH2Cl2 solvate molecules removed for clarity).

  of the d 0 titanocenium cation compared to its neutral d 1 sibling.

  (C5H12)0.75: C, 78.69; H, 7.16; Found: C, 77.88; H, 6.74.

Table 2 .

 2

Acknowledgements

We thank Dr Jean-Claude Chambron and Dr Yves Canac for stimulating discussions on CH--π interactions and recordbreaking 31 P NMR shifts in group 4 metal phosphides, respectively. We thank Dr Michel Meyer for the use of his UV-vis spectrometer. We thank Ms. Marie-José Penouilh-Suzette for measuring the HRMS spectra. Financial support from the Centre National de la Recherche Scientifique (CNRS), Agence Nationale de la Recherche, Deutsche Forschungsgemeinschaft (MENOLEP project), the Conseil Regional de Bourgogne (PARI CDEA program), and the Fonds Européen de Développement Régional (FEDER) is gratefully acknowledged.

data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/datarequest/cif.

UV irradiation of Ti(IV) complexes

The Ti (IV) complexes (3b, 4e: 0.05 mmol; 3c-g: 0.01 mmol) were dissolved in THF or d8-THF and irradiated in sequences of 20 min with a medium pressure Hg lamp (Heraeus TQ 150).

FLP reactions

Compound 4e (43.6 mg, 0.05 mmol) was dissolved in CD2Cl2. Chalcone (10.4 mg, 0.05 mmol) was added and 1 H and 31 P{ 1 H} NMR spectra were recorded, showing only the presence of both reagents.

Compound 4e (43.6 mg, 0.05 mmol) was dissolved in CD2Cl2 and placed in a Schlenk vessel. The vessel was quickly evacuated, and refilled with 1 atm of CO2. The reaction mixture was stirred for 2 h, and 1 H and 31 P{ 1 H} NMR spectra were recorded, showing only the presence of 4e.

Compound 4e' (55.4 mg, 0.1 mmol) was dissolved in toluene. Ferrocene carboxaldehyde (21.4 mg, 0.1 mmol) was added and an EPR spectrum was recorded, showing only the presence of 4e'.

Computational details

All DFT and TD-DFT calculations were carried out with the Gaussian09 code, [36] tightening self-consistent field convergence thresholds (10 -10 a.u.). Geometries were optimized using the B3PW91 functional, with the def2-TZVP basis set for the titanium and the 6-31+G (d,p) basis set for other atoms. [37] Vertical excitations were computed with TD-DFT using the B3PW91 functional, with the def2-TZVP basis set for the titanium and the 6-311++G (d,p) basis set for other atoms. For each complex, 8 states were considered. The solvent effects of THF were included according to the Polarizable Continuum Model (PCM). [38] All orbital isosurfaces have been plotted with the VMD code. [39] The orbital transitions of selected excited states were characterized using the natural transition orbital (NTO) method. [40] Following the benchmark calculations of Hadt et al. [41] and our previous work, [42] EPR g parameters were computed using the GGA BPW91 functional with the aug-cc-pVTZ-J basis set for the Ti atom, [43] and the IGLO-III basis set for other atoms. [44] The A parameters were computed using the hybrid B3LYP functional with the same basis sets. The aug-cc-pVTZ-J basis sets were taken from the EMSL Basis Set Exchange Web site. [45] All parameters were computed in toluene modelled as a PCM.


Jian Yang

Charles H Devillers

Paul Fleurat-Lessard

Hao Jiang

Shifa Wang

Claude P Gros
email: claude.gros@u-bourgogne.fr

Gaurav Gupta

Ganesh D Sharma
email: gdsharma273@gmail.com

Haijun Xu
email: xuhaijun@njfu.edu.cn.

Carbazole Green and Blue-BODIPY Dyads and Triads as Donors for Bulk Heterojunction Organic Solar Cells

Keywords: Green and blue-BODIPY, electrochemistry, bulk heterojunction, power conversion efficiency

Two BODIPY derivatives with one (B2) and two (B3) carbazole moieties were designed, synthesized and applied as electron-donor materials in organic photovoltaic cells (OPV). The optical and electrochemical properties were systematically investigated. These BODIPY dyes exhibit excellent solubility in organic solvents and present high molar extinction coefficients (1.37-1.48 x 10 5 M -1 cm -1 ) in solutions with absorption maxima at 586 nm for mono-styryl group and at 672 nm for di-styryl groups. The introduction of the styryl moieties result in a large bathochromic shift and a significant decrease in the HOMO-LUMO energy-gaps. The BODIPY dyes show relatively low HOMO energies ranging from -4.99 to -5.16 eV as determined from cyclic voltammetry measurements. Cyclic voltammetry measurements and theoretical calculations demonstrate that the frontier molecular orbital levels of these compounds match with PC71BM as the acceptors, supporting their application as donor materials in solution-processed small molecule bulk heterojunction (BHJ) organic solar cells. After the optimization of the active layer, B2:PC71BM and B3:PC71BM based organic solar cells showed the overall power conversion efficiency of 6.41% and 7.47%, respectively. The higher PCE of B3 based OSC is ascribed to the more balanced charge transport and exciton dissociation, better crystalline and molecular packing.

Introduction

Solar cells have attracted considerable attention as renewable and environmentally friendly energy sources due to environmental issues and the increasing energy crisis resulting from the exhaustion of petroleum resources. 1,2 During the development of solar energy conversion technology, silicon solar cells are commercial predominant photovoltaic technologies. 3 However, Organic Solar Cells (OSCs), based on a bulk heterojunction (BHJ) structure typically consisting of a blend of a photoactive polymer or a small molecule as an electron donor and fullerene derivatives as an electron acceptor, are being a promising technology to convert solar energy into electricity. They have shown to be relevant candidates for next generation renewable energy convertors owing to some unique merits such as transparency, low cost, flexibility, copious availability, large area, uncomplicated fabrication, and light weight. [4][5][6][7][8][9] Over the past decades, both Small-Molecule based OSCs (SM-OSCs) and polymerbased organic solar cells have shown a good performance in bulk hetero-junction organic solar cell devices. [10][11][12] Recently, the polymer-based OSCs as donor have made significant progress with an outstanding efficiency of more than 13%, which represents a great progress for future successful commercialization of OSCs. [13][14][15][16] Although the polymer solar cells are very promising in the development of solar energy conversion devices, the drawbacks of polymer OSCs such as chain-length polydispersity, low batch-to-batch reproducibility, structural defects, and possible difficulties in purification still remarkably limit their application in industrial scale. 3,9,17 On the other hand, compared to organic polymers, small organic molecules are preferred to be utilized as photoactive electron donors due to their potential advantages such as facile purification and characterization, definite molecular structure and molecular weight, high purity and good batch-to-batch reproducibility. 9,[17][18][19] In addition, absorption spectra and orbital energy levels of small organic molecules can be finetuned with suitable structural modifications so as to match the device characteristics to a specific given acceptor, which is very difficult in polymer solar cells. 9 Recently, great efforts have been made to raise Power Conversion Efficiencies (PCEs) of small organic molecule OSCs. [20][21][22] To date, OSCs based on a small molecule as a donor have reached efficiencies in the range of 10-12%, 11,21 which has made it closer to commercial viability. [23][24][25] However, new ideal small molecule materials are still in great need to further improve the PCEs and device durability, which are crucial for the commercialization of this technology. 26 Therefore, it is worthwhile to investigate new small organic molecules for application in bulk heterojunction solar cells.

Based on the distinguishing properties of BODIPY and carbazole, in order to develop a promising BODIPY based small molecule for high efficiency OSC, we have designed and synthesized two carbazole-BODIPY based dyes (B2 and B3, Scheme 1) by Knoevenagel condensation reaction. The optical properties, electrochemical behavior and photovoltaic performances of these styryl substituted BODIPY dyes were investigated and the influence of donor's electron donating ability and substituted positions on their electronic and photovoltaic properties were also comparatively investigated.

Scheme 1. Chemical structures of B2 and B3

Employing the PC71BM as acceptor and these two BODIPY based small molecules as donor, the OSCs based on B2:PC71BM and B3:PC71BM BHJ active layers showed overall PCE of 6.41% (Jsc = 13.48 mA/cm 2 , Voc = 0.78 V and FF= 0.61) and 7.47% (16.18 mA/cm 2 , Voc = 0.71 and FF = 0.65), respectively.

Experimental Section

General

All chemicals and solvents were of analytical reagent grade and used directly as received unless otherwise noted. Dry CH2Cl2 was distilled from CaH2 under nitrogen. 1 H and 13 C NMR spectra were collected on a Bruker DRX-600 AVANCE III spectrometer. Chemical shifts for 1 H NMR spectra were recorded in ppm relative to CDCl3 (δ = 7.26 ppm) as the internal standard. Mass spectra data were obtained using a Thermo Fisher LTQ Orbitrap XL (HR-ESI) spectrometer.

N B N F F N B N F F N N B-2 B-3 N

Synthesis

B1 was synthesized according to the reported literature 27 and C1 was synthesized according to the reported literature. 28

Electrochemistry

CH2Cl2 was dried over alumina cartridges using a solvent purification system PureSolv PS-MD-5 model from Innovative Technology and kept under argon. Tetraethylammonium tetrafluoroborate (TEABF4) was purchased from Alfa Aesar and recrystallized in MeOH.

Electrochemical measurements were performed in CH2Cl2 under an Ar atmosphere in a threeelectrode glass cell. Working electrode (WE) was a platinum (Pt) disk (Ø = 1 mm, surface area of about 0.785 mm²). A Pt wire was used as counter electrode (CE). Saturated aqueous calomel (SCE) was used as reference electrodes (RE). The RE was separated from the WE compartment by a double frit comprising an intermediate background solution (0.1 M TEABF4 + CH2Cl2). All the potentials in this manuscript are indicated vs. SCE. In these conditions, when operating in CH2Cl2 (0.1 M TEABF4), the formal potential for the Fc + /Fc couple was found to be +0.52 V vs. SCE. All the electrochemical studies were performed using Autolab PGSTAT 302N potentiostat. Cyclic voltammetry experiments were performed at the scan rate of 100 mV/s.

Synthesis

Synthesis of compounds B2 and B3

In a 50 ml three-neck round bottom flask with a Dean-Stark apparatus, B1 (3 mmol, 1098 mg), C1 (3 mmol, 754 mg), p-toluenesulfonic acid (0.87 mmol, 150 mg), toluene (25 ml) and piperidine (2 ml) were added. After heating to reflux, the progress of the reaction was monitored by TLC, the reaction was stopped until B1 disappeared completely. The mixture was cooled to room temperature, which was washed twice with brine, and extracted three times with dichloromethane. The combined organic phase was dried over anhydrous Na2SO4

Device fabrication and characterization

The solution processed organic solar cells were fabricated on the ITO coated glass substrate.

The ITO coated glass substrates were cleaned in detergent, and subsequently ultra-sonicated in deionized water, acetone and isopropyl alcohol and dried in vacuum oven to remove all the traces of residues. For each molecule, the photovoltaic performance optimization process was started with identifying the donor to acceptor ratio (weight percentage, varying from 1:05 to 1:2) and after that solvent vapor annealing was applied to maximize the performance of the OSCs. The PC71BM was used as acceptor and the total concentration of D:A blend mixture was 16 mg/mL in chloroform. The devices were fabricated by depositing PEDOT:PSS as hole transport layer having thickness of 35-40 nm. The active layer was deposited by spin coating (2500 rpm, 60 s) on the top of PEDOT:PSS layer under ambient conditions. For the solvent vapor annealing (SVA), the optimized (as cast 1:1.5 D/A wt ratio) was exposed to the THF vapors for 40s. A thin layer of PFN was spin coated on the top of the active layer from the methanol solution. The aluminum (Al) electrode was deposited onto the top of PFN layer via thermal evaporation at the pressure less than 10 -5 Torr. The current-voltage characteristics of the OSCs were measured under illumination intensity of 100 mW/cm 2 (AM1.5 G) using a solar simulator and a Keithley 2400 source meter unit. The incident photon to current conversion efficiency (IPCE) measurements were performed using Bentham IPCE system.

Computational details

Quantum mechanics calculations were performed with the Gaussian16 software package. 29 Energy and forces were computed by density functional theory with the M06-2X exchangecorrelation functional, 30 with the Pople 6-31G(d,p) basis sets. Geometry optimizations were conducted with the default criteria. Frequency calculations were performed to confirm that all structures were minimized.

Spectroscopic measurements.

UV-vis spectra were carried out on a Shimadzu UV-3100 spectrophotometer. All the

Results and discussion

Among the various types of small molecule photovoltaic materials, 4,4-difluoro-4-bora-3a,4adiaza-s-indacene (BODIPY) dyes have attracted tremendous interest and have been considered to be excellent photoelectric materials owing to their outstanding photophysical properties, such as high molar extinction coefficients, high fluorescence quantum yields, excellent chemical stability, various redox activity, relatively long excited state lifetimes, high thermal and photo-stabilities, [31][32][33] which have led to their application in many fields including bioimaging, 34,35 nonlinear optics, 36,37 photodynamic therapy, 38 organic light-emitting diode, 39,40 dye-sensitized solar cells (DSSC) 2,41,42 and OSCs. 1 In addition, BODIPY derivatives are easily synthesized, and their photophysical and electrochemical characteristics can be easily fine-tuned though an appropriate synthetic modification on the BODIPY core. [43][44][45][46] For example, the absorption wavelength of the BODIPY dyes can be finely adjusted by introducing different functional groups at the a, b or meso-positions. 27,47 Moreover, BODIPY dyes can also exhibit variable redox chemistry by selectively accessing a group having a corresponding function, and they have been used as both electron donors and acceptors. [48][49][50] Considering these outstanding advantages, BODIPY dyes can served as potent candidates for bulk heterojunction solar cells. 1,7 In 2009, Roncali et al. first reported the use of BODIPY dyes as donor materials and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) as electron acceptor in BHJ-OSCs with a PCE of 1.34%. 51 Since then, continuous progress has been done on BODIPY dyes using them as donors in bulk heterojunction (BHJ) solar cells. 9 These encouraging results suggest that the BODIPY derivatives have emerged as promising photovoltaic materials. On the other hand, carbazole derivatives have attracted considerable attention in the field of optoelectronics owing to their low redox potential, high hole transport properties, coplanar configuration, and strong electron donating ability. 58 The introduction of carbazole donating unit on D-A organic semiconductors, lowering the HOMO energy level, results in high open circuit voltage in OSCs. 65,66 In addition, the introduction of carbazole groups into the BODIPY unit can not only improve the photoelectric properties but also result in the remarkable red shifted absorption spectra and better hole transport. 67 Zhang et al. have designed star shaped carbazole-BODIPY derivatives and employed them as donors in BHJ organic solar cells and reported a moderate PCE of 2.70% with a high Voc of 0.85 V. 68 In 2015, Sharma et al. used three carbazole substituted BODIPYs small molecules as donors along with PC71BM as an electron acceptor to construct solution processed BHJ organic solar cells, which achieved a PCE at 5.08%. 58

Synthesis

BODIPY B1 was synthesized according to the literature. 27 Compounds B2 and B3 were prepared by a Knoevenagel condensation reaction between B1 and p-methoxybenzaldehyde in the presence of 4-methylbenzenesulfonic acid (PTSA) and piperidine in toluene (Scheme 2).

The structures of the synthesized compounds were further confirmed by 1 The cyclic voltammetry curves and differential pulse voltammetry data of B2 and B3 are presented in Figure 1 and Figure 2, respectively. Electrochemical data from Table 1 were used to assess frontier orbital energy levels (Table 2). The HOMO and LUMO energy levels were estimated by using the following equations: HOMO = -(4.8 + E1/2(Ox1)) and LUMO = -(4.8 + E1/2(Red1)) according to Cardona et al. 69 and Wang et al. 70 B1 has already been studied in similar conditions as for B2 and B3 71 and its measured HOMO-LUMO gap was 2.33 eV.

Among B2 and B3 derivatives, B2 exhibits the highest HOMO-LUMO band gap (1.99 eV, Table 1) due to a lower extension of the π-system (see Figure 5). However, this value is lower than the one found for B1 proving the significant π-conjugation between the additional carbazole moiety and the BODIPY moiety. When the BODIPY is connected to two carbazole units (B3), the HOMO-LUMO gap dropped to 1.72 eV. The better LUMO and HOMO offsets between donors (B2 and B3) and PC71BM (higher than the threshold value of ~0.3 eV) is beneficial for the efficient photo-induced charge transfer at D/A interfaces and transport in the BHJ active layers. 72

Optical properties

The photophysical properties of the BODIPY compounds B2 and B3 were measured in CH2Cl2. The normalized absorption and fluorescence spectra of compounds B2 and B3 are shown in Figures 3 and4, respectively. Their photophysical data are summarized in Table 3.

The two BODIPY dyes exhibit classic absorption and emission spectra for BODIPY fluorophores. 73,74 Compound B2 shows the maxima absorption with the sharp S0-S1 (π-π*) transition at 586 nm. By the introduction of the second carbazolyl vinyl group, the main absorption peak of B3 (672 nm) shows a bathochromic shift of 86 nm by comparison with B2 due to the extension of π-conjugation. This is illustrated in Figure 5 in which the HOMO and LUMO densities are shown: in B3 the HOMO is spread on both arms. As the LUMO are mostly localized on the BODIPY moiety, the π-conjugation extension leads to a reduced optical gap for B3 compared to B2. Interestingly, the difference between the optical HOMO-LUMO in B2 and B3 is closed to the one measured in electrochemistry which amounted to 97 nm. In addition, there is a broad but weak absorption in the UV region ascribed to the S0-Sn (n ≥ 2) transitions of the BODIPY moiety for compounds B2 and B3. 75 In summary, B2 and B3 can absorb longer-wavelength light, which suggest there use in light-harvesting systems.

The emission spectra of the two compounds were also recorded in CH2Cl2 solutions (Figure 4) and show perfect mirror symmetry with the absorption spectra with Stokes-shift in the range of 20-39 nm. The fluorescence emission maxima for compounds B2 and B3 are centered at 616 nm and 701 nm, respectively. The fluorescence quantum yields were also determined in CH2Cl2 (Table 3). The fluorescence quantum yields of B2 and B3 are 0.86 and 0.42, respectively. In addition, fluorescence lifetimes (τ) of these two BODIPY dyes were also measured, which show a single exponential decay with a lifetime of approximately 4 ns (Table 3). this may be attributed to the strong aggregation and molecular interaction in the solid state.

The optical bandgap estimated from the onset absorption edge of the thin film absorption is about 1.78 eV and 1.58 eV for the B2 and B3, respectively. The small optical bandgap for B3 may be ascribed to its longer conjugation length as compared to B2 due to the additional carbazole donor unit.

Photovoltaic properties

BHJ OSCs with configuration glass/ITO/PEDOT:PSS/active layer/ PFN/Al were fabricated using B2 and B3 as electron donor material along with PC71BM as electron acceptor material.

Since the relative content of the donor and acceptor in the active layer for OSCs is crucial for obtaining the mixed BHJ morphology and efficient exciton harvesting and formation of sufficient charge transport network. 20,78 Initially, the photovoltaic performance of B2 and B3

were evaluated carefully via the variation of donor to acceptor weight ratios keeping the total concentration of 16 mg/mL using chloroform as solvent. We found that the device with 1:1.5

weight ratio showed the best Power Conversion Efficiency (PCE) about 3.21% and 3.59% for B2:PC71BM and B3:PC71BM active layers, respectively (Table 4). In order to further improve the PCE of the OSCs, we have adopted solvent vapor annealing (SVA) of the active layer having D:A ratio of 1:1.5 with THF for 40 s and found that the PCE has been improved significantly. This is attributed to the improvement in the nanoscale morphology of the active layers. [79][80][81] The current-voltage characteristics of the OSCs based on optimized active layers are shown in Figure 6a and photovoltaic parameters are compiled in Table 4. The OSCs based on the B2:PC71BM and B3:PC71BM active layer after SVA treatment showed overall PCE of 6.41% (Jsc = 13.16 mA/cm 2 , Voc = 0.78 and FF =0.61) and 7.45% (Jsc = 16.18 mA/cm 2 , Voc = 0.71 V and FF = 0.65), respectively. Upon THF treatment, the Jsc and FF of the OSCs increase dramatically, and this may be attributed to the improvement in the nanoscale morphology of the active layer. However, the Voc is reduced after SVA treatment which may be due to morphological changes at the contact interfaces and reduction in the quasi Fermi level gaps. The Voc value for the OSCs based on B2 is higher than that for B3 and this may be attributed to the deeper HOMO energy level of B2, since the Voc is directly proportional to the energy offset between the HOMO of donor and LUMO of acceptor employed in the BHJ active layer of the OSCs. Since the HOMO and LUMO energy levels are about -6.05 eV and 4.10 eV, the DEHOMO (D)-LUMO (A) offset for the B2:PC71BM and B3:PC71BM are equal to 1.065 eV and 0.89 eV, respectively, which is consistent with the trend in the values of Voc in the OSCs. The value of Jsc is higher for B3 than that for B2, and this may be related to the broader and lower bandgap of B3 as compared to B2, leading to improved light harvesting capability of the B3:PC71BM active layer as compared to B2:PC71BM. Moreover, the FF value for the B3 based OSCs is also higher than B2 counterparts, may be originated from the more balanced charge transport as discussed in the later part of the manuscript.

In order to get information about the higher value of Jsc for B3 based OSCs as compared to B2 counterpart, we have measured the incident photon to current conversion efficiency (IPCE) spectra of the OSCs and shown in Figure 6b. The IPCE spectra show two bands i.e. one in the wavelength region of 300-500 nm, which corresponds to the photocurrent generation due to the PC71BM and other beyond 500 nm corresponding to the photocurrent generation due to the donor (B2 or B3), indicating that both donor and acceptor components in the BHJ active layer contribute to the photocurrent generation. As can be seen from the spectra, the IPCE spectrum for B2 based is limited up to 650 nm whereas it is extended up to 790 nm for B3 counterpart. The estimated values of Jsc from the IPCE spectra are about 13.39 mA/cm 2 and 16.12 mA/cm 2 for B2 and B3 based OSCs and are consistent with the values observed in J-V characteristics of the devices under illumination.

The charge transport in the active layer plays an important role for high values of FF and Jsc, therefore, we have measured the hole and electron mobilities in the active layers using the charge collection efficiency (Pcoll) were estimated as Jph/JPhsat under short circuit and maximum power point conditions, respectively. 82,83 The values of Pdiss/Pcoll for the devices based on the B2 and B3 are about 0.907/0.642 and 0.948/0.723, respectively. These observations support that exciton dissociation and collection is more efficient for B3 based OSCs than that for B2 and may be the origin for high values of Jsc and FF for the B3 based OSCs. Generally, at high value of Veff, all the photogenerated excitons are dissociated into free charge carriers and Jphsat mainly depends on the exciton generation rate (Gmax) which is described as Jphsat= qLGmax, where q is the electronic charge and L is the thickness of the active layer. The values of Gmax of the optimized OSCs are about 1.03 × 10 28 m -3 s -1 and 1.12 × In order to get more information about the difference in the recombination processes in the OSCs based on B2 and B3, we have measured the variation of Jsc with illumination intensity (Pin) [84][85][86] as shown in Figure 9a, which if described as Jsc µ (Pin) a , where a is the exponent that describe the degree of recombination. The bimolecular recombination is negligible when the value of a is unity, whereas when a is less than unity there is a competition between the recombination and charge recombination. The value of a for the device based on B2 and B3 are about 0.91 and 0.94, respectively, indicating the bimolecular recombination is less for B3 based device and leading to the high value of FF.

The variation of Voc with Pin for the OSCs are shown in Figure 9b. The Voc is proportional to S [ln (Pin)], where S is the slope extracted from the linear fit of Voc with ln (Pin). When S is equal to 1 kT/q, the bimolecular recombination is the dominant mechanism whereas a larger value of S than 1kT/q indicates that the trap-assisted recombination or germinating recombination may be dominating in the devices. The values of S estimated from the Figure 9b are 1.56 kT/q and 1.38 kT/q for B2 and B3 based devices, indicating that the trap-assisted recombination is less for the B3 based OSCs. Therefore, the higher values of Jsc and FF for the B3 based OSCs can be due to the high exciton dissociation, weak bimolecular recombination and trap-assisted recombination, which are related to the molecular packing crystalline nature of the active layer. stacking distances of 0.408 nm and 0.389 nm, respectively. In addition of these peaks, a weak diffraction peak around 2q = 18° is also observed in both the thin films, which corresponds to PC71BM. [START_REF] Chen | Sharma, and Haijun Xu Two BODIPY derivatives with one (B2) and two (B3) carbazole moieties were designed, synthesized and applied as electron-donor materials in organic photovoltaic cells (OPV)[END_REF] Moreover, both the diffraction peaks are stronger for B3 based thin film as compared to B2 indicating that the degree of crystallinity is more for former than later. The enhanced crystalline nature and reduced p-p stacking distance for the B3:PC71BM active layer may induced the better nanoscale phase separation and beneficial for charge transport and collection. These observations may be the origin for higher FF and greater PCE of OSC based on B3:PC71BM active layer.

Conclusions

Two BODIPY with one (B2) and two carbazole (B3) units were designed and synthesized.

Their optical and electrochemical properties were investigated for their potential application as donor materials for the solution processed bulk heterojunction organic solar cells employing PCBM as electron acceptors. The optimized B2:PC71BM and B3:PC71BM based OSCs showed overall PCE of 6.41% and 7.47%, respectively. The higher PCE of B3:PC71BM than B2:PC71BM is mainly due to the enhanced values of Jsc and FF and may be related to the better charge transport and exciton dissociation. The optical absorption spectra and frontier energy levels of B2 and B3 may also be suitable for non-fullerene acceptors since these two small molecules showed complementary absorption spectra and can be employed for ternary OSCs. We are working in this direction and expecting to achieve higher PCE.

  Spectra and PLQY were measured in Fluoro Log-Ultra-Fast (HORIBA Instrument Inc, Edison) equipped with a 450 W CW Xenon lamp and an Open-Electrode TECooled CCD Detector (Syncerity). Absorption and emission measurements were carried out in 1×1 cm quartz cuvettes with spectroscopic grade solvent, and was kept constant at (298 ± 2) K. An integrating sphere mounted in sample compartment directly for PLQY measure. Nanosecond lifetime and TRES studies were conducted using a TCSPC MCA model equipped with a picosecond photo detector (<200 ps) (PPD850) and picosecond laser (duration is 180 ps, Deltadiode, 100 MHz laser). TRES data were measured by incrementing the monochromator on the emission channel of the time-resolved fluorometer in fixed wavelength intervals at each wavelength. Slices of data were taken in the intensity-wavelength plane to obtain spectra at different times during the decay. Microsecond lifetime decays were collected by a MCS mode on TCSPC HUB (Delta HUB) with a LED source (Spectra LED) as a sample excitation source.

Figure 1 .

 1 Figure 1. Cyclic voltammetry of B2 and B3 in CH2Cl2 0.1 M TEABF4 (c = 10 -3 M, ν = 100 mV.s -1 , WE: Pt, Ø = 1 mm).

Figure 2 .

 2 Figure 2. Differential pulse voltammetry of B2 and B3 in CH2Cl2 0.1 M TEABF4 (c = 10 -3 M, ν = 10 mV.s -1 , WE: Pt, Ø = 1 mm).

Figure 3 .

 3 Figure 3. Normalized UV-vis spectra of compounds B2 and B3 in DCM (upper) solution and (bottom) film

Figure 4 .Figure 5 .

 45 Figure 4. Emission spectra of compounds B2 and B3 in DCM

21

 21

Figure 6 .

 6 Figure 6. (a) Current-voltage characteristics under illumination and (b) IPCE spectra of the OSCs based on optimized B2:PC71BM and B3:PC71BM active layers

4 cm 2 /

 42 devices, respectively. The hole and electron mobility values computed from the space charge limited current (SCLC) model are shown in Figure7aand 7b for hole only and electron only devices, respectively. The hole mobility was found about 8.89 × 10 -5 cm 2 /Vs and 1.07 × 10 -Vs for B2 and B3, respectively for optimized active layers. The electron mobility in the active layers are almost the same about 2.41 × 10 -4 cm 2 /Vs and 2.47 × 10 -4 cm 2 /Vs, respectively, leading to the electron to hole mobility ratio about 3.65 and 2.59, for the active layers based on B2 and B3, respectively. The reduction of electron to hole mobility ratio for B2 based device contributes to the enhancement in the charge transport within the active layer, hence leading to the higher values of Jsc and FF.

Figure 7 .

 7 Figure 7. Dark current-voltage characteristics of the (a) hole and (b) electron only devices based on optimized B2:PC71BM and B3:PC71BM active layers (the solid lines are SCLC fitting)

10 28 m

 28 -3 s -1 for B2 and B3, respectively. The higher value of Gmax for B3 based as compared to B2 counterpart indicates that more excitons are generated in the B3:PC71BM active layer (particularly in B3) due to the broader absorption spectra of B3.

Figure 8 .

 8 Figure 8. Variation of photocurrent density (Jph) with effective voltage (Veff) for the OSCs based on optimized B2:PC71BM and B3:PC71BM active layers

Figure 9 .

 9 Figure 9. Variation of (a) Jsc and (b) Voc with illumination intensity (Pin) for the OSCs based on optimized active layers based on B2 and B3

Figure 10 .

 10 Figure 10. X-ray diffraction (XRD) patterns of the optimized B2:PC71BM and B3:PC71BM thin films

Figure 11 .

 11 Figure 11. TEM images optimized B2:PC71BM and B3:PC71BM with bar scale 200 nm

  7,17,[52][53][54][55][56][57][58][59][60][61][62] In 2016, Liao et al. synthesized and characterized a series of novel BODIPY derivatives with various electron donors at 2,6-positions for BHJ-OSCs, and the best device exhibits 2.15% PCE.63 In 2017, Singh et al. reported a novel boron dipyrromethene based dye decorated with dithiafulvalene wings and its bulk heterojunction solar cells with PC71BM have achieved a PCE of 7.2%.61 In 2018, Li et al. reported three furan-fused BODIPY with increasing perfluorinated alkyl chains on the meso-C of BODIPY core, and an encouraging PCE of 6.4%, with a high FF of 64%, was achieved with C60 as the electron acceptor.

62

Recently, Gros et al. have designed and synthesized two novel small molecules consisting of a BODIPY central core surrounded by two DiketoPyrroloPyrrole (DPP) and two porphyrin units.

3

The resulting molecule BD-tPor presented an impressive PCE of 8.98% when blend with PC71BM as an acceptor for solution processed bulk heterojunction organic solar cells.

3

More recently, Sobral et al. have designed three novel BODIPY-based molecules with different styryl aromatic donor group and constructed OSCs with PC60BM, which lead to a PCE of 2.8%.

64

Very recently, Singh et al. reported two efficient donor materials composed by the dithienosilole/dithienogermole core linked thienyl BODIPY, and the best PCE of 4.58% was achieved.

Table 1 .

 1 Redox potentials vs. Fc + /Fc (E1/2 = (Epa + Epc)/2) of B2 and B3 (CH2Cl2 0.1 M TEABF4, C=10 -3 M, ν = 100 mV/s, working electrode: platinum disk, Ø = 1 mm, counter electrode: Pt wire, reference electrode: SCE); ΔEp (in mV) are given in parentheses.

		Scheme 2. Synthetic route of B2 and B3
	3.2. Electrochemistry
	Potentials	Red1	Ox1	Ox2	Ox3	ΔE(Ox1-Red1)
	V vs. Fc/Fc +					≈HOMO-LUMO gap
	B2	-1.625	0.365	0.645		1.99
		(90)	(80)	(80)
	B3	-1.53	0.19	0.43	1.12 a	1.72
		(85)	(80)	(80)

H NMR and

13

C NMR spectroscopy and HRMS (ESI).

a Irreversible peak, measurable peak potential.

Table 2 .

 2 Frontier orbital energies of B2 and B3 in solution as determined by electrochemical data from Table1.

	Compounds	E1/2(Ox1) V vs. Fc + /Fc	E1/2(Red1) V vs. Fc + /Fc	HOMO (eV) LUMO (eV)	HOMO-LUMO gap (eV)
	B2	0.365	-1.625	-5.165	-3.175	1.99
	B3	0.19	-1.53	-4.99	-3.27	1.72

Table 3 .

 3 The

	B2	586	616	831	1.37	0.86	4.01
	B3	672	701	616	1.48	0.42	3.92
	Concentration: 1×10 -5 M,Compound B2 was excited at 520 nm,Compound B3 was
	excited at 600 nm.

photophysical properties of compounds B2 and B3 in DCM Compound λabs [nm] λem [nm] Δυ [cm -1 ] ε (10 5 ×M -1 ×cm -1 ) ФF τF [ns]

Table 4 .

 4 Photovoltaic parameters of the OSCs based on B2 and B3 as donor and PC71BM as acceptor

	Active layer	Jsc	Voc (V) FF	PCE	µh	µe	µe/µh
		(mA/cm 2 )			(%)	(cm 2 /Vs)	(cm 2 /Vs)
	B2:PC71BM	8.63	0.81	0.46	3.21
	(as cast)				(3.08) a
	B3:PC71BM	10.45	0.75	0.48	3.76
	(as cast)				(3.63) a
	B2:PC71BM	13.56	0.78	0.61	6.45	8.89 × 10 -5 2.41 x 10 -4 2.71
	(SVA)				(6.32) a
	B3:PC71BM	16.24	0.71	0.66	7.61	1.07 × 10 -4 2.47 × 10 -4 2.31
	(SVA)				(7.55) a

a Average of 8 devices

Acknowledgments

Support was provided by the CNRS (UMR 6302), the Université Bourgogne Franche-Comté, the Conseil Régional de Bourgogne through the Plan d'Actions Régional pour l'Innovation (PARI II CDEA) and the European Union through the PO FEDER-FSE Bourgogne 2014/2020 programs. National Key R&D Program of China (2016YFD0600804), Natural Scientific Foundation of Jiangsu Province, P. R. China (No. BK20160922), National Natural Scientific Foundation of China (No. 21301092). C. H. D. thanks the Agence Nationale de la Recherche for funding (ANR-15-CE29-0018-01). The french authors thank the Plateforme d'Analyse Chimique et de Synthèse Moléculaire de l'Université de Bourgogne (PACSMUB, http://www.wpcm.fr) for access to analytical and spectroscopy instrumentation. G. D. S. and G. G. thanks to Department of Science and Technology, Government of India, New Delhi for financial support. Calculations were performed using HPC resources from DNUM CCUB (Centre de Calcul de l'Université de Bourgogne).

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflict of Interest

The authors declare no conflict of interest.