Schiff Base Ancillary Ligands in Bis(diimine) Copper(I) Dye-Sensitized Solar Cells

Five 6,6′-dimethyl-2,2′-bipyridine ligands bearing N-arylmethaniminyl substituents in the 4- and 4′-positions were prepared by Schiff base condensation in which the aryl group is Ph (1), 4-tolyl (2), 4-tBuC6H4 (3), 4-MeOC6H4 (4), and 4-Me2NC6H4 (5). The homoleptic copper(I) complexes [CuL2][PF6] (L = 1–5) were synthesized and characterized, and the single crystal structure of [Cu(1)2][PF6]·Et2O was determined. By using the “surfaces-as-ligands, surfaces-as-complexes” (SALSAC) approach, the heteroleptic complexes [Cu(6)(Lancillary)]+ in which 6 is the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid)) and Lancillary = 1–5 were assembled on FTO-TiO2 electrodes and incorporated as dyes into n-type dye-sensitized solar cells (DSCs). Data from triplicate, fully-masked DSCs for each dye revealed that the best-performing sensitizer is [Cu(6)(1)]+, which exhibits photoconversion efficiencies (η) of up to 1.51% compared to 5.74% for the standard reference dye N719. The introduction of the electron-donating MeO and Me2N groups (Lancillary = 4 and 5) is detrimental, leading to a decrease in the short-circuit current densities and external quantum efficiencies of the solar cells. In addition, a significant loss in open-circuit voltage is observed for DSCs sensitized with [Cu(6)(5)]+, which contributes to low values of η for this dye. Comparisons between performances of DSCs containing [Cu(6)(1)]+ and [Cu(6)(4)]+ with those sensitized by analogous dyes lacking the imine bond indicate that the latter prevents efficient electron transfer across the dye.


Introduction
The Grätzel n-type dye-sensitized solar cell (DSC) was developed in the early 1990s and converts solar to electrical energy [1][2][3][4]. A coating of nanoparticles of a semiconductor (usually TiO 2 ) on a transparent conducting oxide glass electrode is treated with a dye to form the photoanode of the DSC. The performance of the cell depends on a number of factors, most crucially the dye, the redox couple in the electrolyte, and the electrolyte composition. Over the last 25 years, much effort has gone into optimizing these components in order to improve the overall performance efficiency. Typical dyes include metal-free organic, ruthenium(II) and zinc(II) porphyrin compounds, and the highest photoconversion efficiencies (η) now reach 12-14% [5][6][7][8][9]. The structures of many organic dyes are complex and the associated synthetic strategies and protocols are non-trivial. Thus, the use of metal coordination compounds remains attractive, as they are both photostable and relatively easy to prepare, provided that ligand design and synthesis can be optimized. Although ruthenium(II) dyes represent state-of-the-art, the use of first row d-block metals such as copper [10][11][12][13] or iron [14][15][16][17] is advantageous in terms of sustainability and conforming to the United Nations Sustainable Development Goals (SDGs).
The complexes used as dyes possess ligands that covalently bind to the surface of the semiconductor (the anchoring ligand(s)) and additional ligands, which are used to modify the photonic and electronic properties (the ancillary ligand(s)). In order to achieve efficient electron injection into the semiconductor in a DSC, a dye should exhibit a "push-pull" (or donor-π-acceptor, D-π-A) design, in which the ancillary ligands possess π-donor properties and the anchoring ligands π-acceptor character [18][19][20]. Homoleptic bis(diimine)copper(I) complexes are easily synthesized from reactions of [Cu (MeCN) 4 ] + salts with a 2,2 -bipyridine (bpy) or 1,10-phenanthroline (phen) ligand, but they lack the necessary "push-pull" character. In solution, heteroleptic bis(diimine)copper(I) compounds tend to undergo ligand redistribution leading to mixtures of homo-and heteroleptic species. Two different strategies have been devised to assemble and stabilize heteroleptic bis(diimine)copper(I) complexes for use in DSCs. The HETPHEN (HETeroleptic PHENanthroline) approach utilized by Odobel and coworkers [21,22] employs sterically demanding substituents in the 6,6 -positions of a bpy ligand, or in the 2,9-positions of phen, to isolate heteroleptic [Cu(L)(L )][X] complexes and stabilize them with respect to ligand redistribution in solution. In contrast, we have adopted the "surfaces-as-ligands, surfaces-as-complexes" (SALSAC) approach [23,24], which is extremely versatile and allows heteroleptic copper(I) dyes to be assembled directly on a semiconductor surface by ligand exchange between an anchored diimine ligand and a homoleptic copper(I) complex (Scheme 1). Scheme 1. Schematic representation of the SALSAC ligand-exchange strategy for heteroleptic copper(I) dye assembly.
A class of ligands that has proven beneficial in bis(diimine)copper(I) dyes is based upon 4,4 -styryl-6,6 -dimethyl-2,2 -bipyridine in which the R group in Scheme 2 is systematically varied. Complexes in which R = 4-( n Bu 2 NC 6 H 4 ) or 4-[ n (C 8 H 17 ) 2 NC 6 H 4 ] exhibit absorption spectra dominated by intense intraligand charge-transfer (ILCT) bands with the metal-to-ligand charge-transfer (MLCT) absorption appearing as a low-energy shoulder [25]. With R = CO 2 H, we incorporated the ligands on the left-side of Scheme 2 into homoleptic complexes and established functioning copper(I) dyes in DSCs [26]. The extended conjugation in this family of ligands increases the intensity and results in a red-shift of the MLCT absorption, both of which are beneficial for light-harvesting [26,27]. Daniel, Odobel, and coworkers applied the HETPHEN strategy to incorporate ligands derived from 4,4 -styryl-6,6 -dimethyl-2,2 -bipyridine into heteroleptic copper(I) complexes [27] and followed this with an impressive demonstration of the potential for these dyes in DSCs [22]. Among metal-free organic dyes, a few containing azo spacers have been shown to be effective sensitizers in DSCs [28]. Between the extremes of the C=C and N=N units lies the imine bond, and we were interested to see how heteroleptic copper(I) dyes incorporating Schiff base ligands would perform in DSCs. The ease of preparation of Schiff bases by the condensation of amines with aldehydes or ketones has lead to a myriad of applications, including those in energy-related materials [29]. To the best of our knowledge, there are no examples of Schiff base-decorated bpy ligands being used in copper(I) sensitizers. The reversibility of imine bond formation is an attractive feature in terms of the potential for the structural manipulation and chemical regeneration of degraded surface-bound dyes.

1-5.
Protons H B2 and H B3 were distinguished in each of ligands 2, 3, 4, and 5 by NOESY cross-peaks to the methyl, tert-butyl, methoxy and dimethylamino protons, respectively, and the shift to lower frequencies for the signals for protons H B3 in 4 and 5 ( Figure 1) is consistent with the effects of the electron-donating MeO and Me 2 N groups.  The solution absorption spectra of 1, 2, and 3 ( Figure 2) recorded in CH 2 Cl 2 exhibit intense bands below 350 nm arising from spin-allowed, π*←π transitions. In contrast, the spectra of the methoxy-and dimethylamino derivatives 4 and 5 exhibit intense absorption maxima at 344 and 420 nm, respectively, which are assigned to ILCT transitions, consistent with the "push-pull" character of these compounds. The value of λ max = 420 nm for 5 compares with λ max = 433 nm (also in CH 2 Cl 2 ) for the related Schiff base derivative (Scheme 4) reported by Le Bozec [31].  Figure 3 and are similar to those of the free ligands ( Figure 1 and Figure S6). Signal assignments were made using 2D NMR methods, and the HMQC and HMBC spectra are shown in Figures S27-S36 in the supporting information). The 31 P{ 1 H} NMR spectrum of each compound exhibited a septet at δ −144.4 ppm characteristic of [PF 6 ] − .    6 ] − anion in the asymmetric unit. The second half of each ion is related to the first by a 2-fold rotation axis. Figure 4a shows the structure of the complex cation and selected bond lengths and angles are given in the figure caption. Atom Cu1 is in a distorted tetrahedral coordination environment with the 6-and 6 -methyl substituents of one ligand accommodated over the chelate ring formed by the second bpy unit (Figure 4b). The bpy unit is slightly twisted with an angle of 12.0 • between the planes of the pyridine rings. The two C=N bonds lie approximately in the same plane of the pyridine ring to which each is bonded (torsion angles N3-C13-C3-C4 = -174.2(6) • and N4-C12-C8-C7 = −174.2(7) • ), while the phenyl rings are twisted out of this plane (torsion angles C16-C15-N3-C13 = 147.0(7) • and C26-C21-N4-C12 = 151.8(7) • ). Packing of the [Cu(1) 2 ] + cations involves face-to-face π-stacking of the phenyl ring containing atom C21 and the pyridine ring with N1 ( Figure 5a). The angle between the ring planes is 7.8 • and the centroid...centroid distance is 3.84 Å, parameters that are consistent with efficient packing [33]. The inter-cation π-stacking interactions involve the crystallographically independent ligand with its symmetry generated counterpart, and the packing, therefore, extends throughout the lattice, as shown in Figure 5b.    6 ] are similar and exhibit high-energy absorptions arising from ligand-centered π*←π transitions, in addition to a broad, lower intensity band at 518, 513, and 516 nm, respectively, from the MLCT. In contrast, the spectrum of [Cu(4) 2 ][PF 6 ] (with the peripheral methoxy functionalization) shows an additional absorption at 370 nm arising from ILCT, consistent with the spectral signature of the free ligand 4 ( Figure 2). In [Cu(5) 2 ][PF 6 ], the overlap of the ILCT (461 nm) with the MLCT (shoulder at 530 nm) (compare Figures 2 and 6), and the mixed character of these excited states leads to a significant increase in MLCT intensity and a red-shifting of the absorption. The copper(I) complexes are redox-active and their electrochemical behavior was investigated by cyclic voltammetry (CV). Table 1  (Me 2 bpy = 6,6 -dimethyl-2,2 -bipyridine, TFSI = bis(trifluoromethanesulfonyl)imide) [35]. For [Cu(6,6 -Me 2 bpy) 2 ][TFSI], the original literature value was +0.97 V vs. SHE, the potential is corrected by −0.62 V to formally adjust to Fc/Fc + [36]. Each compound undergoes a series of irreversible ligand-centered reductions (  6 ], if the positive potential scan window is taken past +1.07 or +1.20 V, respectively, through a ligand oxidation process, the return wave for the Cu + /Cu 2+ process is lost and a new ECE (electrochemical-chemical-electrochemical) wave is observed at +0.69 or +0.21 V, respectively. For [Cu(5) 2 ][PF 6 ], the Cu + /Cu 2+ oxidation is irreversible and the oxidation occurs an +0.54 V (Figure 7b). This process is preceded by a partially reversible ligand-centered oxidation at  (Table 1), and we, therefore, assign this to an oxidative process centered on the NMe 2 group. No analogous process is observed in the CV of ligand 2, which is representative of ligands with no redox-active substituents.

Solar Cell Fabrication and Performances
Heteroleptic [Cu(L anchor )(L ancillary )] + dyes were assembled on commercial FTO/TiO 2 electrodes using the SALSAC ligand-exchange strategy [23,24], as depicted in Scheme 1. The phosphonic acid anchoring ligand 6 (Scheme 6) was chosen because it exhibits enhanced binding to TiO 2 with respect to carboxylic acids [24]. Taking both DSC performance and efficient ligand synthesis into account, we have previously established that copper(I) dyes with phosphonic acid or phosphonate anchors [37] containing a phenylene spacer between the anchor and copper-binding domains are favored over those with carboxylic acid (or carboxylate) and cyanoacrylic acid (or cyanoacrylate) anchors [34,38,39]. The solid-state absorption spectra of the dye-functionalized electrodes are shown in Figure 8. Each of [Cu (6)(1)] + , [Cu (6)(2)] + , and [Cu (6)(3)] + has an MLCT absorption maximum at 504 nm, while λ max for [Cu(6)(4)] + is 498 nm. For [Cu(6)(5)] + , the spectrum is dominated by the ILCT absorption (λ max = 438 nm) with a shoulder at ca. 500 nm arising from the MLCT band. Maxima in the solid-state spectra of the heteroleptic dyes ( Figure 8) are blue-shifted with respect to those of the homoleptic complexes in solution ( Figure 6).  The photoconversion efficiencies of the DSCs (sets of three, fully-masked cells for each dye) were measured and cell performance parameters are given in Table 2. For reference, a DSC sensitized with the commercially available ruthenium(II) dye N719 was used. In the right-hand column of Table 2, we give values of the relative efficiency of each DSC relative to the cell with N719 set at 100%. This is a reliable means of comparing DSC parameters recorded, for example, with different sun simulators [40]. The fill-factors (ff ) of the cells lie in the range 65-70%, consistent with well-fabricated DSCs. The use of triplicate cells confirms the reproducibility of the data, lending confidence to the trends observed in performances as the ancillary ligand is varied. Current density-potential (J-V) curves for all the cells are displayed in Figures S37-S41 in the supporting information, and Figure 9 shows J-V curves for the best-performing DSC with each dye.  The highest values of the short-circuit current density (J SC ) are observed for DSCs sensitized with [Cu (6)(1)] + (phenyl substituent) and [Cu (6)(3)] + (tert-butyl substituent). Changing from a phenyl to a 4-methylphenyl group (ancillary ligand 2) results in a decrease in J SC (Table 2 and Figure 9). The differences in J SC are consistent with the EQE (external quantum efficiency) spectra ( Figure 10 The photoconversion efficiencies (η) for dyes containing the Schiff base ancillary ligands with peripheral phenyl, 4-methylphenyl and tert-butyl substituents lie in a similar range (η = 1.26-1.55%) and achieve conversion efficiencies that reach 26% that of N719. The introduction of the electron-donating MeO and Me 2 N groups was expected to be beneficial in terms of the "push-pull" dye design. An overview of bisdiimine copper(I) dyes draws attention to the most efficient ancillary ligands being those with electron-releasing substituents [12]. We have previously described the performances of fully-masked DSCs sensitized with [Cu(6)(7)] + and [Cu (6)(8)] + in which ancillary ligands 7 and 8 are 6,6 -dimethyl-4,4 -diphenyl-2,2-bipyridine and 4,4 -bis(4-methoxyphenyl)-6,6 -dimethyl-2,2-bipyridine (Scheme 7). On going from [Cu (6)(7)] + to [Cu(6)(8)] + , an increase in J SC (4.27 to 4.87 mA cm −2 for the best-performing cells) and η (1.66 to 1.82%) was observed [41]. However, for the Schiff base ancillary ligands, changing the functionality from phenyl to 4-methoxyphenyl on-going from [Cu (6)(1) Table 1). The lower value of EQE max (34%, Figure 10) is consistent with the drop in J SC . A small decrease in V OC is also observed, leading to lower values of solar cell efficiencies (η = 0.99-1.08%). A more substantial decrease in performance is observed when the 4-dimethylamino groups are introduced. On-going from [Cu(6)(1)] + to [Cu(6)(5)] + , values of J SC decrease significantly to 1.49-1.56 mA cm −2 and there is an associated decrease in the maximum EQE to ca. 17% (Figure 10). There is also a detrimental influence on V OC (Figure 9 and Table 2), leading to poor global efficiencies of ca. 0.45%.
Since the performance measurements for DSCs sensitized with [Cu(6)(1)] + , [Cu(6)(4)] + , [Cu (6)(7)] + , and [Cu(6)(8)] + were made under the same conditions (electrodes materials, electrolyte, instrumentation, fully masked), a comparison of the data sheds light on the role of the imine unit in the ancillary ligands. Table 3 compares the data. The pair of dyes [Cu(6)(1)] + and [Cu (6)(7)] + differ only in the presence or absence of the imine spacer, respectively, as does the pair [Cu(6)(4)] + and [Cu(6)(8)] + (see Schemes 2 and 7). In both cases, introducing the imine unit results in a decrease in J SC and V OC , and a loss in overall performance. Thus, not only does the imine spacer hinder the beneficial effects of the electron-releasing methoxy group, it is also disadvantageous in the case of the peripheral phenyl substituent.  [34] were prepared according to the literature.

General Procedure for the Synthesis of the Schiff Base Ligands
Compounds 1-5 were prepared by condensation of 6,6 -dimethyl-[2,2 -bipyridine]-4,4dicarbaldehyde with the appropriate amine according to the following procedure [45]. Anhydrous MgSO 4 (0.4 g) and 1.0 equivalent of 6,6 -dimethyl-[2,2 -bipyridine]-4,4 -dicarbaldehyde were added to a solution of the amine (1.0 mmol, 2.0 eq.) in CH 2 Cl 2 (5 mL) at ambient temperature (ca. 22 • C). The mixture was stirred overnight at room temperature, and was then filtered. The residue was washed with CH 2 Cl 2 and the volume of the filtrate reduced under reduced pressure. The product was precipitated from CH 2 Cl 2 /hexane, was collected by filtration, and was washed with hexane and diethyl ether. Reaction scales and yields are given below.

Compound 1
The method was as above using 6,6 -dimethyl-

General Procedure for the Synthesis of Copper Complexes
[Cu(MeCN) 4 ][PF 6 ] (1 eq.) and the diimine ligand (2 eq.) were dissolved in CH 2 Cl 2 in a round-bottomed flask. The reaction mixture immediately turned red and was stirred for 20 min. After evaporation of the solvent, the product was recrystallized from CH 2 Cl 2 /diethyl ether (v/v 1:10) and the resulting microcrystalline solid was collected by filtration, washed with diethyl ether, and dried under vacuum. Scales of reactions are given below.

Crystallography
Single crystal data were collected on a Bruker APEX-II diffractometer (CuKα radiation) with data reduction, solution and refinement using the programs APEX [46], ShelXT [47], Olex2 [48], and ShelXL v. 2014/7 [49]. SQUEEZE [50] was used for the solvent regions and the electrons removed equated to one molecule of Et 2 O per compound formula unit. The analysis of the structure was made using Mercury CSD v. 4.3.0 [51,52].

DSC Fabrication
FTO/TiO 2 electrodes (Solaronix Test Cell Titania Electrodes, Solaronix SA, Aubonne, Switzerland) were washed with milliQ water and EtOH, heated at 450 • C for 30 min, then cooled to ca. 60 • C. The electrodes were immediately placed in a DMSO solution of 6 (1.0 mM) for 24 h, after which they were removed, washed with DMSO and CH 2 Cl 2 , then dried under N 2 . Each electrode functionalized with 6 was immersed in a CH 2  The working and counter-electrode for each DSC were combined using thermoplast hot-melt sealing foil (Solaronix Test Cell Gaskets, 60 µm, Solaronix SA, Aubonne, Switzerland) and the gap between the electrodes was filled with electrolyte (LiI (0.1 M), I 2 (0.05 M), 1-methylbenzimidazole (0.5 M), 1-butyl-3-methylimidazolinium iodide (0.6 M) in 3-methoxypropionitrile) by vacuum backfilling through a hole in the counter-electrode. This hole was finally sealed (Solaronix Test Cell Sealings and Solaronix Test Cell Caps, Solaronix SA, Aubonne, Switzerland).

Electrodes for Solid-State Absorption Spectroscopy.
The fabrication method described above for the FTO/TiO 2 electrodes was followed, but using Solaronix Test Cell Titania Electrodes Transparent (Solaronix SA, Aubonne, Switzerland).

DSC and EQE Measurements
The DSCs were masked using black-coloured copper sheet with an accurately calibrated aperture smaller than the TiO 2 surface area. Black tape was used to complete the masking at the top and sides of the cells. Performance measurements were made by irradiating the DSCs from behind with a LOT Quantum Design LS0811 instrument (LOT-QuantumDesign GmbH, Darmstadt, Germany, 100 mW cm −2 = 1 sun, AM1.5 G conditions). The simulated light power was calibrated using an Si reference cell. EQE measurements were made using a Spe Quest quantum efficiency setup (Rera Systems, Nijmegen, The Netherlands) with a 100 W halogen lamp (QTH) and a lambda 300 grating monochromator (LOT-Oriel GmbH & Co. KG, Darmstadt, Germany). The monochromatic light was modulated to 3 Hz using a chopper wheel (ThorLabs Inc., Newton, NJ, USA), and the cell response was amplified with a large dynamic range IV converter (Melles Griot B.V., Didam, Netherlands) and measured with a SR830 DSP Lock-In amplifier (Stanford Research Systems Inc., Sunnyvale, CA, USA).

Conclusions
We have prepared and characterized a series of new Schiff base ligands 1-5 bearing bpy metal-binding domains and N-arylmethaniminyl substituents. Their homoleptic copper(I) complexes [CuL 2 ][PF 6 ] were also synthesized. The single crystal structure of [Cu(1) 2 ][PF 6 ]·Et 2 O confirmed a distorted tetrahedral coordination environment; in the Schiff base ligand, the C=N bonds lie approximately in the same plane of the pyridine ring to which each is connected, while the phenyl rings are twisted out of this plane. The solution absorption spectrum of each of [Cu (1) 6 ] results in a substantial increase in MLCT intensity and a red-shift.
Heteroleptic [Cu(6)(L ancillary )] + dyes with L ancillary = 1-5 were assembled on FTO-TiO 2 electrodes and incorporated into DSCs. The best-performing sensitizer is [Cu(6)(1)] + , with a values of η up to 1.51% compared to 5.74% for a DSC sensitized by N719. The introduction of the electron-donating MeO (in 4) and Me 2 N (in 5) groups results in a decrease in J SC and EQE max , and for [Cu(6)(5] + , low values of V OC are also observed. Comparisons between performances of DSCs containing [Cu(6)(1)] + and [Cu(6)(4)] + with those sensitized by related dyes [Cu(6)(7)] + and [Cu(6)(8)] + in which the outer phenyl rings are directly bonded to the bpy domain [41] reveal that not only does the imine spacer hinder the favourable effects of the electron-donating MeO substituent, it is also detrimental in the case of the peripheral phenyl substituent.