Nonlinear optical-active ferrocene conjugated Y-shaped imidazole donor–π–acceptor [(D–π)₂–IM–π–A] compounds for dye-sensitized solar cells using non-corrosive copper complexes as a redox mediator

Abstract

Two new bis-ferrocene (Fc) conjugated Y-shaped imidazole (IM) based donor–π–acceptor [(D–π)₂–IM–π–A] type “push–pull” compounds [(Fc–CH=CH)₂–IM–C₈H₁₇–C₆H₄–R] {R = COOC₂H₅ (1); COOH (2)} have been synthesized for second-order nonlinear optics (1, ester) and dye-sensitized solar cell (2, acid) applications. The redox potentials of the compounds were examined using cyclic voltammetry, which shows one-electron charge transfer from the ferrocene to ferrocenium ion (Fe²⁺ ⇌ Fe³⁺), and these potentials were employed to calculate the related energy gap. The second-order nonlinear optical (NLO) properties of the ester compound 1 were explored using the Electric-Field-Induced Second Harmonic generation (EFISH) technique in CHCl₃ solution with an incident wavelength of 1907 nm, and it shows an interesting μβ_EFISH value of −780 × 10⁻⁴⁸ esu. Besides, the carboxylic acid compound 2, adequately functionalized for anchoring to TiO₂, was tested as a photosensitizer in dye-sensitized solar cells (DSSCs) with non-corrosive Cu(I)/Cu(II) complexes as the redox mediator. Although the photovoltaic performance of 2 attached to TiO₂ in a DSSC device is rather limited (J_SC = 1.51 mA cm⁻², V_OC = 404 mV, FF = 42.7, η = 0.26%), it confirms the potential of ferrocenyl conjugated D–π–A systems as photosensitizers. In addition, frontier molecular orbital levels, ground and excited state dipole moments (μ_g and μ_e), and the origin of electronic absorption spectra were studied by using density functional theory (DFT) with the B3LYP method using 6-31+G** as the basis set.

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1 Introduction

Energy demand is a worldwide problem. In order to develop an alternative source for non-renewable energy, dye-sensitized solar cells (DSSCs) were first reported by O'Regan and Grätzel using ruthenium(II) (polypyridyl) complexes as photosensitizers; they now reach photon-to-current conversion efficiencies of up to 14% with nanocrystalline TiO₂.¹ Sensitizers are essential as DSSC components because they control light harvesting and charge separation abilities, which are necessary for better efficiencies.^2,3 Remarkably, by changing the chromophore and anchoring groups of sensitizers, it is possible to increase their molar extinction coefficient and solar light harvesting capacity, thus improving the photocurrent and efficiency of DSSCs.^4,5 A number of metal-based sensitizers with notable power conversion efficiencies (PCE) have been reported, such as Ru-based dyes,^6–9 Zn porphyrins,¹⁰ and perovskites.¹¹ However, these devices have some drawbacks, particularly the Ru-based dyes, due to their high costs and the low abundance of metals and their toxicity in nature.¹² Therefore, it is necessary to explore alternative, low cost, eco-friendly new dyes. Hence, the design of photosensitizers in DSSCs is even today a challenging task.

Thus, in the last two decades, researchers have been interested in the use of ferrocene-based complexes for DSSC applications, due to the versatile oxidation states (Fe²⁺ ⇌ Fe³⁺) of iron in the ferrocene moiety; in fact, ferrocene is responsible for the reversible one-electron redox reaction and can act as a low-cost, non-toxic donor in DSSCs. The redox potential of the dyes can be tuned by varying the substitution on cyclopentadienyl rings.¹³ Thanks to these aspects, various ferrocenyl derivatives have been studied in DSSCs as possible photosensitizers. Thus, ferrocenyl bearing Ni(II) and Cu(II) dithiocarbamate complexes (3.87 and 2.80%),¹⁴ ferrocenyl substituted triphenylamine based donor–acceptor dyes (3.65%),¹⁵ ferrocene appended porphyrin derivatives (0.0081%),¹⁶ ferrocene diketopyrrolopyrrole based acceptors (6.44%),¹⁷ ferrocenyl benzimidazoles (5.81%),¹⁸ Y-shaped biferrocenyl quinoxaline (0.81%),¹⁹ ferrocene-based cyanovinyl compounds (0.18 and 0.10%),^20,21 ferrocene appended azine spacers (4.04%)²² and π-extended ferrocenyl dyes (0.68%)²³ with different anchoring groups have been investigated using I⁻/I₃⁻ as a redox mediator and their efficiencies were compared with the standard N719 dye under similar experimental conditions, showing the potential of ferrocene derivatives for DSSCs. However, to date, no ferrocene-based Y-shaped imidazole compound has been studied as a sensitizer.

Nevertheless, heterocyclic scaffolds have been widely used in donor–acceptor (D–A) systems because they provide high chemical and thermal robustness.²⁴ Among them, imidazole moieties act as a good auxiliary electron donor or acceptor because of their high electron deficiency originating from two asymmetric nitrogen atoms that lower the π* level of the conjugated system.²⁵ Thus, imidazole-based organic dyes are used as molecular sensitizers in DSSCs.²⁶ Ferrocenyl-based imidazole molecules appear to be of great interest in this field, because the conjugated framework offers improvement in the charge-transfer transition from the auxiliary electron donor to first electron donor.¹⁸ The Y-shaped imidazole (IM) molecules [(D–π)₂–IM–π–A], with the two donors at the peripheral C4/C5 positions of the imidazole and the acceptor at the C2 position, show high charge transferability due to the presence of electron deficient nitrogen in the imidazole unit. Compared with linear molecules the Y-shaped molecules have better intramolecular charge transfer (ICT) behaviours due to the presence of more electron donor or acceptor units.^25,27 Literature reports are available on organic Y-shaped imidazole compounds for DSSCs,²⁸ light-emitting diodes (OLEDs),²⁹ nonlinear optics (NLO),²⁶ and two-photon absorbers.³⁰ We have reported some ferrocene conjugated Y-shaped imidazole compounds for their second-order NLO properties,³¹ but their photovoltaic applications as dyes in DSSCs have never been investigated. To develop new imidazole-based organometallic dyes for DSSCs, herein we report a new Y-shaped ferrocene conjugated imidazole dye (D₂–π–A) as a sensitizer with electron-deficient nitrogen units as an additional electron acceptor (2) and a parental complex (1) with interesting second-order nonlinear optical (NLO) properties (Chart 1).

Chart 1 The molecular structure of the Y-shaped ferrocene conjugated imidazole compounds 1 and 2.

Generally, the I⁻/I₃⁻ redox couple is used as a redox mediator in DSSCs,³² but this couple has some drawbacks, for example I₂ is volatile and its long-term cell sealing is quite complicated. The I₃⁻ reduction at the counter electrode is obtained by the DSSC cathode only by the platinum coating, but it is unstable, and it faces photovoltage loss due to the mismatching with the dye redox potential. In addition, I⁻/I₃⁻ is a highly corrosive material and it will corrode most metals and the light harvesting efficiency of the dye is limited due to the dark colour of I₃⁻.³³ These are the serious disadvantages in using metal grid collectors for upgrading DSSCs to large areas. To overcome these problems, some of us introduced the Cu(II)/Cu(II) redox couple for DSSC applications.³⁴ The mechanism of a Cu redox couple is similar to that of “blue copper proteins” due to a distorted tetragonal geometry, which is an intermediate geometry between Cu(I) and Cu(II) and therefore leads to fast electron exchange.^35,36 In this respect, we have explored heteroleptic Cu(I)/Cu(II) complexes as redox mediators in DSSCs and the efficiencies start from less than 0.1% to reach a maximum of 3.0%.^37,38 In the literature, reports are available with ferrocene conjugated donor–π–acceptors as sensitizers using I⁻/I₃⁻ and cobalt(II/III) electrolyte as redox mediators,^14,19 but non-corrosive Cu(I)/Cu(II) redox couples have not been reported yet. So, to the best of our knowledge, there is no report on ferrocene-based sensitizers in dye-sensitized solar cells using copper complexes as redox mediators.

Besides, it is worth pointing out that a ferrocene complex was the first reported NLO-active organometallic species.³⁹ It was shown that the ferrocene moiety plays an important role as an electron-donor in charge transfer processes, when it is attached to an acceptor moiety in push–pull NLO molecules.^39,40 Clearly organometallic compounds are an attractive and growing class of second-order NLO-phores, because they provide several advantages over the inorganic and organic molecules, owing to the presence of NLO active electronic charge-transfer transitions of the metal to ligands at relatively low energy and of high intensity, tunable by virtue of the nature, oxidation state, and coordination sphere of the metal center.⁴⁰ Several investigations have been carried out with ferrocene-based molecules in the field of NLO with different acceptor groups such as metal carbonyls, porphyrinoids and heteroarenes. An increase in the strength of the electron-withdrawing moiety and in the length of the π-bridge between the donor and the acceptor groups leads to an increase in the β values.⁴¹ Recently, we have investigated Y-shaped biferrocene conjugated quinoxaline and imidazole compounds, demonstrating their strong second-order NLO response by using the Electric Field Induced Second Harmonic (EFISH) technique in solution. These complexes found application as polymethylmethacrylate composite films with a good second harmonic generation (SHG) response.³¹

Considering the aforementioned information, we have synthesized two new ferrocene conjugated Y-shaped heterocyclic complexes (Chart 1), starting from the same scaffold: ester (1) and acid (2); they both present an alkyl chain, in order to prevent aggregation of the molecules and charge recombination which would decrease the photovoltaic properties. The linear optical properties have been investigated using UV-visible spectroscopic techniques and the nonlinear optical behavior of compound 1 was studied using the EFISH technique in solution. The DSSC properties of compound 2, with an appropriate anchoring group, was investigated with different electrolytes (solutions containing I⁻/I₃⁻ or homoleptic copper(I/II) complexes bearing two 2-n-butyl-1,10-phenanthrolines) and compared with the standard N719 dye. In addition, the absorption band and the electron transition effect of the imidazole auxiliary acceptor were investigated theoretically by using density functional theory (DFT) and time-dependent DFT (TD-DFT) at the B3LYP/6-31+G** level. As previously reported, we confirm that it was not possible to measure, with the EFISH technique, the second-order NLO response of complexes bearing a –COOH moiety, such as complex 2 of the present paper, due to dissociation of –COOH.⁴²

2 Experimental sections

2.1 Materials and methods

Chemicals were procured from TCI and Sigma Aldrich Chemical & Co. The commercially available solvents were used after purification using a distillation process. According to the reported procedure, (1E, 5E)-1,6-bisferrocenyl-hexa-1,5-diene-3,4-dione was synthesized.⁴³ The product was purified using the column chromatographic method with silica gel (AVRA, 60–120 mesh), hexane and ethyl acetate as solvents. The Debus-Radziszewski reaction was followed to prepare the ferrocene conjugated Y-shaped imidazole ester compound 1.

2.2 General characterization

The NMR spectra were recorded on a BRUKER (400 MHz) spectrometer. Chemical shifts were reported in δ (ppm). The ESI-mass spectra were recorded on LC/MS, 6230B4 Time of Flight (TOF), Agilent technologies. The elemental analysis was carried out using CHNS Elemental Analyzer-PerkinElmer-2400 CHNS/O Series. FT-IR spectra were obtained using a SHIMADZU IR Affinity-1 instrument equipped with a high-sensitivity DLATGS detector; spectra were recorded as KBr discs. Electronic absorption spectra were recorded in dichloromethane, in a 1 cm² quartz cuvette at room temperature, using a JASCO V-670 UV-Visible spectrophotometer. Electrochemical measurements were recorded using a CHI620E electrochemical analyser; they were performed in 5 × 10⁻³ M solution in dichloromethane with 0.1 M tetrabutylammonium perchlorate (TBAP, Aldrich) as the supporting electrolyte at a scan rate of 100 mV s⁻¹. Platinum wire was used as the counter electrode, glassy carbon as the working electrode and an Ag/AgCl electrode as the reference electrode.

2.3 Synthesis of Y-shaped imidazole ester (compound 1)

The ferrocene conjugated Y-shaped imidazole ester 1 was synthesized by condensation between (1E,5E)-1,6-bisferrocenyl-hexa-1,5-diene-3,4-dione (1 mmol), ethyl 4-formylbenzoate (1 mmol) and n-octyl amine (1 mmol), in the presence of ammonium acetate and acetic acid, under reflux for 3 hours. After completion of the reaction, the solvent was evaporated to give a crude product, that was further purified by column chromatography using hexane and ethyl acetate (8 : 2). Yield: 411 mg, 55%. C₄₄H₄₈Fe₂O₂N₂: calcd C, 70.60; H, 6.46; N, 3.74; found C, 70.62, H, 6.50, N, 4.04. ESI-MS: exact mass: 748.2415, found mass: 748.2425. ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.16 (d, J = 8 Hz, 2H, C₆H₄), 7.71 (d, J = 8 Hz, 2H, C₆H₄), 7.27 (d, J = 16 Hz, 1H, CH), 6.87 (d, J = 16 Hz, 1H, CH), 6.70 (d, J = 16 Hz, 1H, CH), 6.54 (d, J = 16 Hz, 1H, CH), 4.51 (s, 2H, H_β C₅H₄), 4.47 (s, 2H, H_β C₅H₄), 4.44 (q, J = 14.4Hz, 2H, –O–CH₂), 4.36 (s, 2H, H_α C₅H₄), 4.25 (s, 2H, H_α C₅H₄), 4.23 (s, 5H, C₅H₅), 4.15 (s, 5H, C₅H₅), 3.96 (t, J = 7.6 Hz, 2H, N–CH₂), 1.45 (t, J = 7 Hz, 3H, CH₃), 1.17 (m, 12H, C₆H₁₂), 0.87 (t, J = 6.8 Hz, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 166.2 (1C, C=O), 146.9 (1C, N–C), 137.9 (1C, p-C), 135.4 (1C p-C), 132.3 (–N–C), 130.5 (2C o-C), 129.8, 129.0, 128.9, 126.3 (2C m-C), 117.6, 112.2, 84.3 (1C, C_ipso), 83.2 (1C, C_ipso), 69.5 (2C, C_α C₅H₄), 69.4 (5C, C₅H₅), 69.3 (5C, C₅H₅), 68.7 (2C, C_α C₅H₄), 66.8 (2C, C_β C₅H₄), 66.7 (2C, C_β C₅H₄), 61.2 (1C, –O–CH₂), 44.9 (1C, N–CH₂), 31.7 (1C, CH₂), 30.6 (1C, CH₂), 29.0 (1C, CH₂), 28.9 (1C, CH₂), 26.5 (1C, CH₂), 22.6 (1C, CH₂), 14.3 (1C, CH₃), 14.1(1C, CH₃). FT-IR: 3084(w) ν(C–H arom), 2918(s) ν(C–H aliph), 2839(s) ν(C–H aliph), 1713(s) ν(C=O), 1641(m) ν(C=C), 1494(m), 1470(s) ν(C–H methyl), 1405(s), 1255(m) ν(C–O),1105(s) ν(C=C Fc), 1035(s), 1006(s), 924(s) ν(C–C Fc), 824(s) ν(CH arom), 695(s), 492(s) cm⁻¹. UV-visible data λ_max (CH₂Cl₂) = 245 (π–π*), 363 (n–π*) and 461 (d–d) nm, and molar extinction coefficient ε_max = 15 791, 12 736 and 2305 M⁻¹ cm⁻¹.

2.4 Synthesis of Y-shaped imidazole acid (compound 2)

The ferrocene conjugated Y-shaped imidazole acid 2 was synthesized by hydrolysis of compound 1 in the presence of 1N NaOH solution in methanol under reflux for 3 hours. After completion of the reaction, the solvent was removed and 20 mL of water was added to this residue and acidified at pH 2 using HCl. The solid was filtered, washed with water and dried to obtain compound 2. Yield: 532 mg, 74%. C₄₂H₄₄Fe₂O₂N₂: calcd C, 70.01; H, 6.16; N, 3.89; found C, 69.97; H, 6.01; N, 3.90. ESI-MS: exact mass: 720.2102, found mass: 720.2114. ¹H NMR (400 MHz, CDCl₃) δ (ppm): 8.18 (d, J = 7.6 Hz, 2H, C₆H₄), 7.71 (d, J = 7.6 Hz, 2H, C₆H₄), 7.37 (d, J = 16 Hz, 1H, CH), 6.81 (d, J = 15.6 Hz, 1H, CH), 6.70 (d, J = 15.6 Hz, 1H, CH), 6.52 (d, J = 15.6 Hz, 1H, CH), 4.51 (s, 2H, H_β C₅H₄), 4.47 (s, 2H, H_β C₅H₄), 4.36 (s, 2H, H_α C₅H₄), 4.23 (s, 2H, H_α C₅H₄), 4.22 (s, 5H, C₅H₅), 4.09 (s, 5H, C₅H₅) 3.96 (t, J = 7 Hz, 2H, N–CH₂), 1.24 (m, 12H), 0.86 (t, J = 6.8 Hz, 3H, CH₃). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 169.1 (1C, C=O), 146.9 (1C N–C), 136.7 (1C p-C), 133.5 (1C p-C), 133.1, 132.3 (–N–C), 130.2 (2C o-C), 129.1, 128.6, 127.7 (2C m-C), 116.4, 111.7, 84.1 (1C, C_ipso), 82.9 (1C, C_ipso), 69.6 (2C, C_α C₅H₄), 69.5 (5C, C₅H₅), 69.3 (5C, C₅H₅), 68.9 (2C, C_α C₅H₄), 66.9 (2C, C_β C₅H₄), 66.7 (2C, C_β C₅H₄), 44.9 (1C, N–CH₂), 31.7 (1C, CH₂), 30.5 (1C, CH₂), 29.0 (1C, CH₂), 28.9 (1C, CH₂), 26.5 (1C, CH₂), 22.6 (1C, CH₂), 14.1 (1C, CH₃). FT-IR: 3431(b) ν(OH), 3084(w) ν(C–H arom), 2918(s) ν(C–H aliph), 2839(s) ν(C–H aliph), 1703(s) ν(C=O), 1641(m) ν(C=C), 1494(m), 1470(s) ν(C–H methyl), 1405(s), 1316(w) ν(C–H aliph), 1255(m) ν(C–O), 1105(s) ν(C=C Fc), 1035(s), 1006(s), 924(s) ν(C–C Fc), 824(s) ν(CH arom), 695(s), 492(s) cm⁻¹. UV-visible data λ_max (CH₂Cl₂) = 231 (π–π*), 371 (n–π*) and 453 (d–d) nm, and molar extinction coefficient ε_max = 9193, 12 500 and 3194 M⁻¹ cm⁻¹.

2.5 Theoretical calculations

The electronic structures and the molecular properties of ferrocene conjugated Y-shaped imidazole compounds 1 and 2 were investigated using Density Functional Theory (DFT) for understanding the bonding patterns, electronic charge and molecular orbital energy distributions. The geometries of the synthesized compounds in the gas phase were optimized using Becke's three-parameter and the Lee–Yang–Parr functional at the B3LYP level.⁴⁴ The B3LYP level of computational calculations were carried out using the 6-31+G** basis set to determine the global minimum energy structure of all the dyes and their molecular properties were calculated. The computation calculations were performed using the Gaussian 16 Revision C.01 program⁴⁵ and the frontier molecular orbital density plots were visualized using the GaussView 6.1.1 program.⁴⁶

2.6 EFISH measurements

The Y-shaped imidazole ester compound 1 was studied using the EFISH technique⁴⁷ in CHCl₃ solution at 10⁻³ M concentration, with a non-resonant incident wavelength of 1907 nm, obtained by Raman-shifting the fundamental 1064 nm wavelength produced by a Q-switched, mode-locked Nd³⁺:YAG laser manufactured by Atalaser. The apparatus used for EFISH measurements is a prototype made by SOPRA (France). The values of μβ_EFISH reported are the mean values of 16 measurements performed on the same sample. The sign of μβ is determined by comparison with the reference solvent (CHCl₃).

2.7 Fabrication and evaluation of solar cells

TiO₂ electrodes were prepared by spreading (doctor blading) a colloidal TiO₂ paste (20 nm sized; “Dyesol” DSL 18NR-T) onto a conducting glass slide (FTO, Hartford glass company, TEC 8, with a thickness of 2.3 mm and a sheet resistance in the range 6–9 Ω cm⁻²) that had been cleaned with water and EtOH treated with a plasma cleaner at 100 W for 10 min, dipped in a freshly prepared aqueous TiCl₄ solution (4.5 × 10⁻² M), at 70 °C for 30 min, and finally washed with ethanol. After first drying at 125 °C for 15 min, a reflecting scattering layer containing >100 nm sized TiO₂ (“Solaronix” Ti-Nanoxide R/SP) was bladed over the first TiO₂ coat and sintered until 500 °C for 30 min. Then, the glass coated TiO₂ was dipped again into a freshly prepared aqueous TiCl4 solution (4.5 × 10⁻² M), at 70 °C for 30 min, then washed with ethanol, and heated once more at 500 °C for 15 min. At the end of these operations, the final thickness of the TiO₂ electrode was in the range of 8–12 μm, as determined by SEM analysis. After the second sintering, the FTO glass coated TiO₂ was cooled at about 80 °C and immediately dipped into a CH₂Cl₂ solution (1.5 × 10⁻⁴ M) of the selected dye at r.t. for 18 h. The dyed titania glasses were washed with EtOH and dried at r.t. under a N₂ flux. Finally, the excess TiO₂ was removed with a sharp Teflon penknife, and the exact active area of the dyed TiO₂ was calculated by means of microphotography. A 50 μm thick Surlyn spacer (TPS 065093-50 from Dyesol) was used to seal the photoanode and a platinized FTO counter electrode. Then, the cell was filled up with the desired electrolyte solution (see the details reported in Table 3). The photovoltaic performance of the cells was measured with a solar simulator (Abet 2000) equipped with a 300 W xenon light source; the light intensity was adjusted with a standard calibrated Si solar cell (“VLSI Standard” SRC-1000-RTD-KG5). The current–voltage characteristics were acquired by applying an external voltage to the cell and measuring the generated photocurrent with a “Keithley 2602A” (3A DC, 10A Pulse) digital source meter. For a given complex and configuration, at least four different devices were made and characterized on different days; the difference between the average and the highest or lowest efficiency values was usually lower than 5%.

3 Results and discussion

3.1 Synthesis of compounds 1 and 2

The ferrocene conjugated Y-shaped imidazole ester compound 1 was obtained via the Debus-Radziszewski condensation reaction. (1E,5E)-1,6-bisferrocenyl-hexa-1,5-diene-3,4-dione reacted with ethyl 4-formylbenzoate and n-octyl amine in the presence of ammonium acetate and acetic acid under reflux conditions. The obtained ferrocene conjugated Y-shaped imidazole ester 1 was further hydrolyzed with 1N NaOH solution in methanol to give compound 2, as shown in Scheme 1.

Scheme 1 The synthesis of compounds 1 (ester) and 2 (acid).

3.2 Characterization of compounds 1 and 2

The purity of compounds 1 and 2 was examined using ¹H and ¹³C NMR spectra, determined in CDCl₃ at room temperature; the spectra are shown in Fig. S1–S4 (ESI†). In the ¹H NMR spectra of compounds 1 and 2, the ferrocene protons of the two unsubstituted cyclopentadienyl rings (η⁵-C₅H₅) appear as a singlet in the region 4.1–4.2 ppm, whereas the substituted cyclopentadienyl ring (η⁴-C₅H₄) is at 4.3–4.5 ppm. The aliphatic CH protons are resonating at 0.8–1.9 ppm and the –N attached CH₂ protons are at 3.9 ppm as a triplet. In the case of compound 1, the ester CH₂ protons appear at 4.4 ppm as a quartet. The phenyl protons of compounds 1 and 2 appear as two doublets at 7.69–8.16 ppm. In the ¹³C NMR spectra of compounds 1 and 2, the ferrocene carbons resonate at 66.7–69.2 ppm and C_ipso carbon of the substituted cyclopentadienyl ring are in the range of 82.9–84.3 ppm. The carbonyl carbon (–C=O) appears at 166.3 (1) and 169.1 (2) ppm and aliphatic carbons are resonating in the range at 14.0–44.8 ppm. In compound 1, the oxygen attached carbon appears at 61.2 ppm and all other phenyl carbons for compounds 1 and 2 appear in the range of 126–138 ppm. In the FT-IR spectra of compounds 1 and 2, the C=O stretching is around 1700 cm⁻¹. The aromatic C–H stretching vibrations are observed at 3084 cm⁻¹ and the aliphatic C–H stretching vibrations are in the range of 2840–2918 cm⁻¹ for both compounds 1 and 2. The –OH band is at 3431 cm⁻¹, which confirms the formation of acid in compound 2, and the spectra are shown in Fig. S5 (ESI†). The mass spectra of compounds 1 and 2 show a peak with the expected isotopic pattern at m/z = 748.2425 (for 1) and 720.2114 (for 2), respectively, and the obtained data coincide with the calculated data, as shown in Fig. S6 and S7 (ESI†).

3.3 Electrochemical properties

The redox behavior of compounds 1 and 2 was studied in dichloromethane solution with 0.1 M supporting electrolyte [N(C₄H₉-n)₄]ClO₄ (TBAP) at a scan rate of 100 mV s⁻¹. Platinum wire was used as the counter electrode, glassy carbon as the working electrode and Ag/AgCl electrode acted as the reference electrode. The cyclic voltammograms of compounds 1 and 2 are shown in Fig. 1. The ratio of anodic to cathodic current is (i_pa/i_pc) close to unity and it is quasi-reversible for the electrochemical assessment. The oxidation potentials of compounds 1 and 2 are observed in the range of 0.66 to 0.73 V, which indicates the one-electron charge transfer from the ferrocene to ferrocenium ion (Fe²⁺ ⇌ Fe³⁺). The observed half-wave potential (E_1/2 = 0.579 for 1 and 0.661 V for 2) and peak separation (ΔE = 0.159 for 1 and 0.145 V for 2) values of compounds 1 and 2 are different from the parent ferrocene (E_1/2 = 0.44 V and ΔE = 0.71 V for FcH/FcH⁺ reversible system).⁴⁸ In addition, compounds 1 and 2 exhibit higher halfwave potentials than the bisferrocenyl imidazole⁴³ and Y-shaped imidazole aldehyde compounds reported in the literature,⁴⁹ due to an increase of the electron withdrawing ability of the COOH group. The half-wave potential (E_1/2) of compound 2 is higher and shifted towards a more positive potential than 1 due to the presence of the electron-withdrawing carboxylic acid group, which leads to a more effective charge transfer process 2.¹⁸ Besides, it is necessary to know the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. These parameters are useful to understand the regeneration of dyes and the electron transfer process in DSSCs.⁵⁰ The HOMO and LUMO energy level were derived by the formula E_HOMO = −(E_ox + 4.4) and E_LUMO = E^optical_g + E_HOMO,⁴⁸ as shown in Table 1. The energy gaps of the compounds are 3.07 (1) and 2.89 (2) eV, which follows the same trend as that obtained with the theoretical (DFT) calculations.

Fig. 1 Cyclic voltammogram of 1 and 2 in the presence of 0.1 M TBAP supporting electrolyte at 0.1 mV s⁻¹ in 10⁻³ M CH₂Cl₂ solution.

Table 1 Cyclic voltammetry data and experimental HOMO, LUMO and optical bandgaps

Compound	E pa (mV)	E pc (mV)	i pa/ipc	E 1/2 (mV)	ΔE^a (mV)	E HOMO (eV)	E LUMO (eV)	λ ^onset (nm)	E ^opticalg ^c (eV)
a Calculated as the HOMO and LUMO level obtained from cyclic voltammetry using EHOMO = −(Eox + 4.4). E^optical onset values obtained from the oxidation peak in the cyclic voltammogram. ELUMO = E^opticalg + EHOMO. b Calculated as λ^onset values from the absorption spectra in CH2Cl2 solvent. c Calculated as the optical bandgap calculated from the absorption onset/edge using the equation (E^opticalg) = 1240/λ^onset.
1	659	500	1.3	579	159	−5.27	−2.23	403	3.07
2	734	589	1.2	661	145	−5.26	−2.37	429	2.89

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3.4 Optical properties

The electronic absorption spectra of compounds 1 and 2 were obtained in dichloromethane (CH₂Cl₂) solution at 10⁻⁵ M (see Fig. 2) and different concentrations (see Fig. S8, ESI†). In these spectra, we observe three major transitions, within the range of 200–700 nm. The higher energy absorptions are 245 nm (ε = 15 791 M⁻¹ cm⁻¹) and 231 nm (ε = 9193 M⁻¹ cm⁻¹) for compounds 1 and 2, due to the π–π* transition and originate from a ligand-centered transition.⁴³ The medium energy n–π* transition of compounds 1 and 2 at 363 (ε = 12 736 M⁻¹ cm⁻¹) and 371 nm (ε = 12 500 M⁻¹ cm⁻¹) respectively, can be described by metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT) due to intramolecular charge transfer (ICT).⁵¹ The low-energy less intense electronic absorption band at 461 nm (ε = 2305 M⁻¹ cm⁻¹) for 1 and 453 nm (ε = 3194 M⁻¹ cm⁻¹) for 2 (d–d transition assigned to ¹E_1g ← ¹A_1g) is due to the degenerate transition of Fe(II) for metal–ligand charge transfer (dπ–π*).⁵² It should be pointed out that the introduction of a carboxylic acid group produces a noticeable lowering of the ε values of the absorption bands with respect to the corresponding ethyl ester. These spectra can be compared with those of the parent ferrocene unit: the wavelengths are red-shifted due to the presence of the extended π-conjugation of an imidazole moiety, which enables the improved charge transfer process of the compounds, leading to a bathochromic shift⁵³ (see Fig. 2).

Fig. 2 Absorption spectra of 1 and 2 in CH₂Cl₂ solution at (1 × 10⁻⁵ M).

3.5 Second-order nonlinear optical properties of compound 1

The second-order NLO properties of compound 1 were investigated using the Electric Field Induced Second Harmonic (EFISH) generation technique,⁴⁷ working in CHCl₃ at a 10⁻³ M concentration with a non-resonant incident wavelength of 1.907 μm, obtained by Raman-shifting under high hydrogen pressure by a Q-switched, mode-locked Nd³⁺:YAG laser. This method is a valuable alternative to Hyper-Rayleigh scattering (HRS), which suffers from the restriction of possible overestimation of the quadratic hyperpolarizability values due to multiphoton fluorescence. EFISH can offer direct information on the intrinsic molecular NLO properties through eqn (1)

γ_EFISH = (μβ_λ/5kT) + γ(−2ω, ω, ω, 0) (1)

where, μβ_λ/5kT is the dipolar orientational contribution of nonlinearity to the molecule, and γ(−2ω, ω, ω, 0), the third order polarizability at frequency ω of the incident light, is the purely electronic cubic contribution to γ_EFISH which can usually be ignored, when studying the second-order NLO properties of dipolar molecules. Compound 1 shows a good μβ_EFISH value of −780 × 10⁻⁴⁸ esu. To estimate the quadratic hyperpolarizability (β_EFISH) value of compound 1, it is necessary to know the dipole moment (μ). Here, we have calculated a theoretical dipole moment in the gas phase as well as in the solvent phase (CH₂Cl₂) with the B3LYP/6-31+G** level of theory. There is a slight difference in the gas and solvent phases, as reported in Table 2, indicating that there is no significant solute–solvent interactions. Compound 1 shows a negative value of μβ_EFISH, corresponding to a negative value of Δμ_eg (the difference between the excited and ground state dipole moments) upon excitation based on the two-level model.^40d,42 A slightly higher negative value of μβ_EFISH (−1000 × 10⁻⁴⁸ esu) was reported for a ferrocene conjugated Y-shaped imidazole compound, due to a higher ground state dipole moment (5.1 × 10⁻¹⁸ esu). Compound 1 shows a very large β_EFISH value (−211 × 10⁻³⁰ esu), which is slightly higher (1.2 times) than that previously reported for other ferrocene imidazole compounds.^31b

Table 2 Photophysical and second order NLO properties of compounds 1 and 2

Compound	λ ^MEmax ^a [nm (eV)^−1] ε^a (×10^5) [M^−1 cm^−1]	λ ^LEmax ^a [nm (eV^−1)] ε^a (×10^5) [M^−1 cm^−1]	HOMO^b (eV)	LUMO^b (eV)	λ max [nm (eV)]	μ gas/DCM^b (×10^−18 esu)	μβ EFISH (×10^−48 esu)	β EFISH (x10^−30 esu)
a Experimental data (ME = medium energy, LE = low energy). b Theoretical calculations using TD-DFT. c In anhydrous CHCl3, the estimated uncertainty in the EFISH measurement is 10%. d β EFISH was calculated using the computed μDCM value.
1	363(3.41)/1.27	461(2.68)/0.23	−4.99	−1.90	372 (3.33)	3.2/3.7	−780	−211
2	371(3.34)/1.25	453(2.73)/0.31	−4.98	−2.06	390 (3.17)	3.6/4.8	—	—

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3.6 DFT studies

Density functional theory (DFT) was used to analyze the molecular geometries and electronic properties of the ferrocene conjugated Y-shaped compounds 1 and 2. Unfortunately, the single crystal data of compounds 1 and 2 were not obtained due to the poor diffraction quality. The synthesized compounds were optimized in the gas phase and the geometrical parameters were determined using the DFT-B3LYP functional with 6-31+G** basis set. For transition metals, the B3LYP function is known to produce reasonably accurate geometries and the molecular geometries of compounds 1 and 2 are shown in Fig. S9 (ESI†). The optimized structure has accurate geometry in agreement with previously reported Y-shaped structures.^31b,49

The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energies and absorption maxima (λ_max), were calculated using the DFT-B3LYP functional with the 6-31+G** basis set, and the values are reported in Table 2. Based on the computed results, it is clear that the HOMO and LUMO orbital distribution was developed through the charge exchange from the bi-ferrocenyl donor and imidazole with the acceptor unit. In compounds 1 and 2, the HOMO is mainly localized on the ferrocene moiety and the π-spacer double bond along with a part of the imidazole unit and the calculated HOMO energy level for compounds 1 and 2 are −4.99 eV and −4.98 eV, respectively. The LUMO is localized in both the imidazole unit and the para-position of the aromatic ring in the presence of ester (1) and acid (2), as shown in Fig. 3. Here, we notice that the imidazole ring participates in the HOMO as well as in the LUMO levels and it contributes to the balanced charge transfer in compounds 1 and 2.

Fig. 3 Schematic orbital energy levels of 1 and 2 at the B3LYP/6-31+G** level of theory in the gas phase. Energy gaps and orbital distribution of the HOMO and LUMO are also shown.

Furthermore, we have carried out Time-Dependent Density Functional Theory (TD−DFT) using the B3LYP function with the 6-31+G** basis set for compounds 1 and 2 to assign the definite transitions observed in the experiments (Table 2). The theoretically observed absorption spectra for compounds 1 and 2 are shown in Fig. S10 (ESI†): the lower-energy (LE) band is at 474 nm (1) and 483 nm (2) whereas the higher-energy (HE) band is at 408 nm (1) and 390 nm (2). The LE, with less intense electronic transition, is mainly described by HOMO → LUMO excitations (more than 73%). This excited state has large oscillator strengths, because it is dominated by a one-electron transition with a large transition dipole moment. The HE transitions, more intense, are described by HOMO → LUMO+1 (more than 36%) for compounds 1 and 2; these excitations can be assigned as n → π* due to the electronic transition within the molecules.⁴⁹ The electronic structures, excited energies, oscillator strengths and orbital transitions of compounds 1 and 2 are listed in Tables S1 and S2 (ESI†).

3.7 Dye-sensitized solar cells with compound 2

Dye-sensitized solar cells were prepared using FTO glass coated TiO₂ sensitized with bis-ferrocenyl Y-shaped imidazole compound 2 as the photoanode, platinized FTO as the counter electrode and an electrolyte solution containing I⁻/I₃⁻ or homoleptic copper(I/II) complexes bearing two 2-n-butyl-1,10-phenanthrolines (E1/E2, Fig. 4) as the redox couples, already reported and successfully employed by some of us.⁵⁴

Fig. 4 Chemical structures of the investigated copper-based redox mediator.

The newly synthesized bis-ferrocenyl Y-shaped imidazole compound 2 was employed as a dye in DSSCs. In fact, it has a COOH group that might provide good anchorage on the TiO₂ surface. In dye-sensitized solar cells, the ability to generate electricity is determined by various components, such as TiO₂, dyes, redox mediator and Pt surfaces. Among these, the redox mediator plays an important role in the power conversion efficiency of a DSSC as it involves regeneration of the dye molecules. From a general point of view we can say that the widely used I⁻/I₃⁻ redox mediator can cause some problems among which the most important are: the long-term stability of the devices, compromised by the corrosive power of the oxidant I₂ towards the sealing material and the competition with the dye in the light harvesting, due to the absorption characteristics of I₃⁻.^32,55 To overcome these problems, some non-corrosive Cu(I)/Cu(II) redox couples have been proposed for DSSCs, leading to a fast electron exchange.³⁵ The results of the investigated DSSCs are presented in Table 3 together with those obtained with the Ru(II) benchmark N719; in the same table, the details of the composition of the electrolyte solutions employed are also reported whereas the corresponding (J–V) curves are shown in Fig. 5. The best PCE obtained for compound 2 is 0.26%, reached with I⁻/I₃⁻ based electrolyte, while in the presence of the standard Cu(I)/Cu(II) electrolyte composition⁵⁴ the PCE decreases up to 0.16% due mainly to a significant drop in J_SC, from 1.51 to 0.95 mA cm⁻² (see Table 3). We tried to limit this inconvenience by modifying the copper-based electrolyte composition. An increase of the LiTFSi concentration up to 0.2 M leads to an improvement in both J_SC and PCE which reaches the values of 1.18 mA cm⁻² and 0.19%, respectively, but which remain far from the values obtained with the I⁻/I₃⁻ based electrolyte. The values of J_SC obtained with compound 2 do not exceed 1.51 mA cm⁻² and we can explain this result considering the very low absorption in the visible region (Fig. 2) and the electron transfer between the acceptor and ferrocene units, as confirmed by the completely quenched fluorescence, which leads low photon conversion efficiency.^56,57 Finally, the low values of PCE, obtained with compound 2, compared to those obtained, in this study with the bench mark N719, have also been caused by a low V_OC: around 0.4 V, regardless of the electrolyte used, mainly attributable to the electrochemical characteristic of 2, as well as from FF values not higher than 43% probably due to bad interfaces inside the devices. The lower value of V_OC for compound 2 is due to the higher level of ground state oxidation potential (0.73 V), which enhances the electron back transfer from the conduction band to the oxidized dye.¹⁵

Table 3 Main photovoltaic parameters^a of DSSCs based on compound 2

Entry	Dye	Solvent	Electrolyte	J SC (mA cm^−2)	V OC (V)	FF (%)	η (%)
a Values measured in the absence of any mask. b 0.65 M N-methyl-N-butylimidazolium iodide, 0.04 M iodine, 0.025 M LiI, 0.28 M t-BuPy in a 15/85 (v/v) mixture of valeronitrile/acetonitrile. c 0.17 M Cu(I), 0.017 M Cu(II), 0.1 M LiTFSI in CH3CN. d 0.17 M Cu(I), 0.017 M Cu(II), and 0.2 M LiTFSI in CH3CN. e 0.6 M N-methyl-N-butylimidazolium iodide, 0.03 M iodine, 0.1 M guanidinium thiocyanate, and 0.5 M t-BuPy in a 15/85 (v/v) mixture of valeronitrile/acetonitrile.
1	2	CH2Cl2	I^−/I3^−^b	1.51	0.40	42.7	0.26
2	2	CH2Cl2	Cu(I)/Cu(II)^c	0.95	0.40	40.6	0.16
3	2	CH2Cl2	Cu(I)/Cu(II)^d	1.18	0.41	39.1	0.19
	N719	EtOH	I^−/I3^−^e	19.1	0.70	65.5	8.77

---

Fig. 5 J–V characteristics of DSSCs in the presence of 2 as the dye and various electrolytes (see Table 3).

The HOMO levels of compound 2 is found to be −5.26 V vs. Ag/Ag⁺ in dichloromethane, which is higher than the redox mediators [−4.4 V (I⁻/I₃⁻) and −4.9 (E1 and E2)] vs. Ag/Ag⁺ in CH₂Cl₂).^54a This implies that the oxidized dyes produced from respective electron injection into the CB of TiO₂ will favourably accept electrons from redox mediators. The schematic energy level diagram of TiO₂ and dye with the redox mediators are presented in Fig. S11 (ESI†), based on the absorption spectrum and electrochemical data, which indicates that compound 2 has appropriate electronic energy levels to be deployed as promising sensitizers in DSSCs. In addition, the energy gap between compound 2 and the Cu-based redox mediator is 0.3 eV, which is sufficient for the dye regeneration process.^17,58

Table S3 (ESI†) summarizes the DSSC performances of previously reported ferrocene based sensitizers, which indicate that the device performance of the sensitizers reported in the presented investigation is comparable with that of the previously reported derivatives. Significantly, compound 2 shows much higher photoconversion efficiencies than those previously reported for a ferrocene conjugated multi-donor system in the presence of a I⁻/I₃⁻ redox mediator (η = 0.015%),⁵⁶ probably due to the presence of two ferrocene units. Moreover, the power conversion efficiencies are higher than those obtained with a ferrocene-based porphyrin derivative tested in the presence of a cobalt(II)/cobalt(III) electrolyte system (η = 0.008%),¹⁶ a ferrocene-modified zinc phthalocyanine (η = 0.003%),⁵⁷ ferrocenyl-chalcones (η = 0.211%),⁵⁹ ferrocenyl dyes with different alkyne units as π-spacers (η = 0.115%),²⁰ a bifunctional ferrocene-based cyanovinyl (η = 0.10%),²¹ and ferrocene methoxy phenyl conjugated multi donor systems (η = 0.015%),⁵⁶ all of them examined using a iodide/triiodide (I⁻/I₃⁻) electrolyte system.

4 Conclusion

In this work, we have synthesized new biferrocenyl conjugated imidazole compounds with two different terminal functional groups: ester (compound 1) and anchoring carboxylic acid (compound 2). These compounds were characterized with the aid of different spectroscopic and analytical techniques. The second-order nonlinear optical (NLO) properties of compound 1 were determined using the EFISH technique. The related β_EFISH value is −211 × 10⁻³⁰ esu, slightly higher than that of previously reported ferrocenyl imidazole compounds. The photovoltaic performances of compound 2 with a carboxylic anchoring group was carried out in DSSCs using both a conventional iodide/triiodide (I⁻/I₃⁻) electrolyte system and non-corrosive Cu based electrolyte. This dye may become a promising starting point to develop DSSCs employing low-cost, non-toxic components such as iron-based dyes and copper-based electrolytes. In addition, the experimentally observed HOMO and LUMO values are in good agreement with the theoretical calculation using the DFT-B3LYP method with 6-31+G** as the basis set. The ester compound 1 is a good second-order NLO-phore in solution and could be a precursor for the preparation of second harmonic generation active polymeric films. Higher DSSC efficiencies could be achieved by using effective ferrocenyl conjugated D–π–A systems, which absorb more in the visible region, and this is currently in progress in our laboratory.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from the DST Indo-Italian Joint Project (no. INT/Italy/P-15/2016(SP). S. P. thanks the Indian Council of Medical Research (ICMR) for the Senior Research Fellowship (SRF) [File No. 45/30/2020-BIO/BMS]. The authors thank VIT for providing “VIT SEED GRAND (File No. SG20210122)” for carrying out this research work. The authors gratefully acknowledge the VIT-SIF for providing the instrumental facilities.

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Footnote

† Electronic supplementary information (ESI) available: Characterization data (¹H and ¹³C NMR spectra, ESI-mass data); FT-IR, computational studies (DFT/TD-DFT). See DOI: https://doi.org/10.1039/d3nj03668e

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