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Article

Exploring Structure-Property Relationships in a Family of Ferrocene-Containing, Triphenylamine-Based Hybrid Organic Dyes

by
Tavneet K. Singh
,
Reese A. Grandy
,
Emma S. Dennis
,
Anja S. B. Schouten
and
Bryan D. Koivisto
*
Department of Chemistry and Biology, Toronto Metropolitan University, 350 Victoria St., Toronto, ON M5B 2K3, Canada
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(12), 6001; https://doi.org/10.3390/app12126001
Submission received: 9 March 2022 / Revised: 30 May 2022 / Accepted: 9 June 2022 / Published: 13 June 2022
(This article belongs to the Special Issue Organic and Hybrid Photovoltaics)
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Abstract

:

Featured Application

A series of ferrocene-containing bis-triphenylamine dyes have been developed and tested in dye-sensitized solar cell (DSSC) applications.

Abstract

In this work, a new family of triphenylamine-based dyes equipped with ferrocene (Fc)-containing donors have been synthesized, characterized, and studied within dye-sensitized solar cells (DSSC). With the objective of designing a redox-robust iron-containing dye that effectively sensitizes titania, a family of five structurally related dyes containing ferrocene were prepared. The physicochemical properties of all dyestuffs were studied using UV-Vis and electrochemical measurements. Different cross-coupling strategies resulted in the ability to modestly tune the Fc/Fc+ redox potentials of the dye. Despite the Fc-containing dyes having optoelectronic properties consistent with the non-ferrocene parent dye, the performance of the dyes in devices was dismal and decreased with the number of appended ferrocenes. While this finding was consistent with previous attempts to explore Fc-TPA DSSC dyes, our spectroelectrochemical data supports the hypothesis that the ferrocene component of the dye is oxidized by the electrolyte, ultimately decreasing the dye’s ability to be a suitable sensitizer. While these dyes are not suitable for DSSC applications, they might find applications in other photo-induced integrated devices where charge recombination is minimal.

1. Introduction

In the development of novel technologies, one of the objectives of materials science is to replace expensive chemistry components with low-cost chemical building blocks [1,2]. This has been particularly true in the development of the dyestuffs used in dye-sensitized solar cells (DSSCs) [3,4,5,6,7,8]. From the first reported high-efficiency DSSC [9], there have been active pursuits to replace ruthenium with less costly metals, or to develop dyes that are entirely metal-free [10]. In this pursuit, Zn-porphyrin DSSC dyes have exhibited some of the highest DSSC performances [11,12], but other first row metal-based hybrid (organic/inorganic) dyes remain unexplored. While perovskite-based solar cells now dominate next-generation PV literature [13] owing to their superior performance, the DSSC remains a unique manifold to examine the optoelectronic properties of organic molecules and screen their potential applications in integrated devices [14].
Owing to their low cost, redox activity, and position in the periodic table (above ruthenium), iron-containing DSSCs are particularly attractive, but have been considerably less efficient in DSSC applications [15]. Ferrere et al. [16] reported an iron-containing bipyridyl complex analogous to N3 and observed charge injection into the titania (TiO2) semiconductor upon photo-excitation. However, other works using time-resolved electronic absorption spectroscopy studies [17,18] have shown that these iron complexes have extremely short metal-to-ligand charge-transfer (MLCT) lifetimes and rapid internal conversions competing with charge injection, ultimately lowering their photovoltaic efficiencies. However, a renaissance of iron complexes is being observed thanks in part to the development of N-heterocyclic carbene-Fe based DSSC dyes [19,20,21,22,23].
Moving away from iron-based complexes as sensitizers, ferrocene-containing hybrid organic dyes have also received considerable attention [24]. Ferrocene (Fc) has been incorporated into organic photovoltaic materials in several ways, including as a donor (D) or acceptor (A) in bulk heterojunction solar cells [25,26,27], as a redox mediator and hole transport material in DSSCs [28,29,30,31], and as a dye substituent in a myriad of DSSC dye architectures [32,33,34,35,36,37,38,39,40,41,42]. In each of these cases, the electron-rich and redox-active behavior of ferrocene is the motivation for its use. However, despite the ubiquitous nature of ferrocene as a redox-active donor, since the report of L1 (Figure 1; 1a), or ‘Sun’s dye’ in 2007 [43], triphenylamine (TPA) donors have not been successfully conjugated with ferrocene to make DSSC dye candidates [44,45,46,47]. Considering the extensive use of both ferrocene and triphenylamine as donors in the development of DSSC dyes, there are remarkably few reports that see ferrocene attached to TPA as a tandem donor system [48,49,50,51]. Herein, we report a new TPA-Fc dye architecture and examine its potential for optoelectronic applications using DSSC motifs.
Our group has previously reported a variety of D-π-A DSSC dye motifs including those containing Fc [39] and TPA [52] donors, and most recently we reported a family of bipodal bis-triphenylamine dyes [53,54] with the form D-π-D-A (Figure 1; 2a) that are our most efficient dyes yet. As such, we were intrigued by the work of El-Zohry et al. [50,51], who modified Sun’s dye with ferrocene (Figure 1; 1c), the only other example of Fc-TPA dyes. In their work they observed that the performance of the Fc-modified dyes was poorer than ferrocene-free dyes in DSSC applications, owing to a faster recombination process (using IR transient absorption studies). They concluded that the twisted intramolecular charge-transfer (TICT), a key feature benefitting TPA dye performance, was disrupted by the intramolecular electron transfer from ferrocene, leading to the deleterious enhanced electron recombination processes.
Building on the work by El-Zohry et al., we sought to examine the effect of appending ferrocene to our family of dyes (Figure 1; 2b–e). We hypothesized that there would be an improvement in performance because our bis-TPA dyes should mitigate adverse recombination, as the ferrocene is attached to the distal (from the anchors/semiconductor) TPA that is farthest from the TiO2 surface. In addition, we anticipated that the twisted excited state would be preserved in at least one of the TPA units upon photoexcitation. To this end, we present the synthesis, physicochemical properties, and device performance of a series of ferrocene-functionalized triphenylamine dyes for DSSC applications. As will be discussed herein, despite the rational design, the presence of ferrocene remains deleterious in the device performance of this family of dyes.

2. Materials and Methods

2.1. General Methods

General synthesis considerations and DSSC cell fabrication procedures can be found in the supporting information. Included below are the synthetic details and characterization of all the novel compounds reported herein. The synthesis of 1b has also been included in the supporting information (Scheme S1). As described above, 1a [43] and 1c [51] have been previously reported and have been included in this study as benchmark compounds for device performance comparison.

2.2. Synthesis

Scheme 1 highlights the synthetic steps used to make dye Family 2. Starting from 3a [54] (Scheme 1A), bromination with 2 equivalents of N-bromosuccinimide (NBS) yielded compound 4. Careful attention was given to the stoichiometry of NBS, as over-bromination yielded an observed side product. Molecule 4 was then converted to the ferrocene derivatives 3b and 3c using a Suzuki cross-coupling with ferrocene boronic acid.
It should be stressed that there are few examples of TPA-ferrocene compounds in the literature, and we think we know why. The cross-coupling (both Sonogashira and Suzuki) with ferrocene and TPA is very challenging; it took several efforts by former students before we landed on a successful reaction condition that yielded the desired product. In our hands, the need for a phase transfer catalyst (tetrabutylammonium tetrafluoroborate), toluene/water, and specifically potassium carbonate were extremely important for the success of the reaction. In addition, the Pd(dppf)Cl2 catalyst seemed to be another piece needed to carry out these reactions. In the case of 3b and 3c, stoichiometry was also carefully controlled, but mixtures of di-ferrocenyl (3c) and dibromide (3b) were found in the product distribution of each reaction regardless. The partial cross-coupling is believed to be a result of the highly electron-rich TPA (and Fc-modified TPA) having a lower rate of oxidative addition with respect to the aryl halide. As a result of incomplete cross-coupling, careful chromatography was needed to isolate the desired compound. After achieving this, 3b and 3c were reacted further using a Knoevenagel condensation with cyanoacetic acid to yield compounds 2b and 2c, respectively.
The syntheses of 2d and 2e were done using a combination of Suzuki and Sonogashira cross-coupling reactions. 5d and 5e were prepared similarly to each other after Sonogashira coupling of ethynyl-ferrocene [55] with tribromotriphenylamine using varied stoichiometry and careful chromatography. 5d and 5e were then cross-coupled with boronic ester (6 [56]) to yield 3d and 3e, respectively. Again, both 3d and 3e underwent a Knoevenagel condensation with cyanoacetic acid to yield compounds 2d and 2e, respectively.
We were quite surprised that prior to this work, TPA-alkynylferrocene derivatives 5d and 5e had not been previously reported. After working with these compounds, we have discovered why. Ferrocene-alkyne-TPA (all of our d and e) derivatives are not particularly stable in solution. Chromatography was our principal route for purification, but there was regular attrition in yield because samples needed to be frequently recrystallized. In addition, we needed to use ascorbic acid washes during extractions. All these steps were critical for our physicochemical and structural characterization. While it is not surprising to see electron-rich alkynes undergoing several side reactions including alkyne hydroalkoxylation and benzannulation, we did begin to question their utility in materials chemistry applications after handling these dyes and precursors.

2.3. Experimental Details

General procedure for the synthesis of dyes (2a–e): Aldehydes (3a–e, respectively, 0.2 mmol) were dissolved in a minimal amount of CHCl3:heptane (1:1). The solution was sparged for 30 min. A few drops of piperidine were added and the mixture was sparged for an additional 5 min. Cyanoacetic acid (0.051 g, 0.6 mmol) was added before refluxing the solution for 4 h. After cooling to room temperature and allowing the red precipitate to settle, the excess solvent was decanted off. The remaining precipitate was dissolved in CHCl3 and washed with 1.2 M HCl, and the organic layer was dried in vacuo to yield a dark orange/red solid. See below for the characterization of 2b–e.
4,4′-((4′-((4-bromophenyl)(4-ferrocenylphenyl)amino)-[1,1’-biphenyl]-4-yl)azanediyl) bis(4,1-phenylene))bis(2-cyanoacrylic acid), 2b: (110 mg, 0.117 mmol, 59.0%). 1H NMR (400 MHz, DMSO-d6): δ = 8.24 (Ha, s, 2H), 8.05 (Hb, d, 3JHH = 8 Hz, 4H), 7.76 (He, d, 3JHH = 8 Hz, 2H), 7.65 (Hd, d, 3JHH = 8 Hz, 2H), 7.52 (Hc, d, 3JHH = 8 Hz, 4H), 7.27 (Hf, d, 3JHH = 8 Hz, 2H), 7.23 (Hh, Hm, m, 4H), 7.10 (Hg, d, 3JHH = 8 Hz, 2H), 7.03 (Hi, Hn, m, 4H), 4.73 (Hj, s, 2H), 4.35 (Hk, s, 2H), 4.08 (Hl, s, 10H).
(2E,2’E)-3,3’-(((4’-(bis(4-ferrocenylphenyl)amino)-[1,1’-biphenyl]-4-yl)azanediyl)bis(4,1-phenylene))bis(2-cyanoacrylic acid), 2c: (140 mg, 0.133 mmol, 66.0%). 1H NMR (400 MHz, DMSO-d6): δ = 8.24 (Ha, s, 2H), 8.04 (Hb, d, 3JHH = 8 Hz, 4H), 7.75 (He, d, 3JHH = 8 Hz, 2H), 7.66 (Hd, d, 3JHH = 8 Hz, 2H), 7.52 (Hc, d, 3JHH = 8 Hz, 4H), 7.28 (Hf, d, 3JHH = 8 Hz, 2H), 7.23 (Hh, d, 3JHH = 8 Hz, 4H), 7.10 (Hg, d, 3JHH = 8 Hz, 2H), 7.03 (Hi, d, 3JHH = 8 Hz, 4H), 4.74 (Hj, s, 4H), 4.34 (Hk, s, 4H), 4.07 (Hl, s, 10H).
4,4’-((4’-((4-bromophenyl)(4-(ferrocenylethynyl)phenyl)amino)-[1,1’-biphenyl]-4-yl)-azanediyl)-bis(4,1-phenylene))bis(2-cyanoacrylic acid), 2d: (64 mg, 0.066 mmol, 50.8%). 1H NMR (400 MHz, DMSO-d6): δ = 8.20 (Ha, s, 2H), 8.03 (Hb, d, 3JHH = 8 Hz, 4H), 7.74–7.65 (He, m, 4H), 7.50 (Hc, d, 3JHH = 8 Hz, 4H), 7.27 (Hf, d, 3JHH = 8 Hz, 2H), 7.24–7.12 (Hh, Hm, Hg, m, 4H), 7.03–6.99 (Hi, Hn, m, 4H), 4.56 (Hj, s, 2H), 4.35 (Hk, s, 2H), 4.27 (Hl, s, 10H).
(2E,2’E)-3,3’-(((4’-(bis(4-ferrocenylethynyl)phenyl)amino)-[1,1’-biphenyl]-4-yl)azanediyl)bis(4,1-phenylene))bis(2-cyanoacrylic acid), 2e: (75 mg, 0.068 mmol, 34.7%). 1H NMR (400 MHz, CDCl3): δ = 7.76 (Ha, s, 2H), 7.60–7.45 (Hb, He, Hd, m, 8H), 7.40–7.30 (Hf, Hc, m, 6H), 7.20–7.14 (Hh, Hg, m, 6H), 7.07 (Hi, d, 3JHH = 8 Hz, 4H), 4.49 (Hj, s, 4H), 4.24 (Hl, Hk, s, 14H).
4,4’-((4’-((4-bromophenyl)(4-ferrocenylphenyl)amino)-[1,1’-biphenyl]-4-yl)azanediyl)dibenzaldehyde, 3b: 4 (87.1 mg, 0.124 mmol), ferrocene boronic acid (1 eq., 28.5 mg, 0.124 mmol), and tetrabutylammonium tetrafluoroborate (0.4 eq., 16.3 mg, 0.050 mmol) were dissolved in toluene:water (8:2, 50 mL). The mixture was sparged for 30 min. Next, K2CO3 (4 eq. 56.6 mg, 0.41 mmol) was added and the solution was left to sparge for an additional 5 min. The palladium catalyst, Pd(dppf)Cl2 (0.1 eq, 10.1 mg, 0.0138 mmol) was added. The solution was refluxed under nitrogen for 12 h. The crude mixture was dried in vacuo. The residue was solubilized in dichloromethane (DCM) and washed with ascorbic acid; the organic layer was collected, dried over MgSO4, and the solvent removed. The crude product was purified using a silica column (Rf = 0.22) with DCM as eluent to yield an orange solid after solvent removal. (71.5 mg, 0.088 mmol, 71.3%). 1H NMR (400 MHz, CDCl3): δ = 9.93 (Ha, s, 2H), 7.82 (Hb, d, 3JHH = 8 Hz, 4H), 7.61 (He, d, 3JHH = 8 Hz, 2H), 7.53 (Hd, d, 3JHH = 8 Hz, 2H), 7.44–7.38 (Hf, H i, m, 4H), 7.26 (Hc, d, 3JHH = 8 Hz, 4H), 7.21–6.99 (Hj, Hg, Hh, Hk, m, 8H), 4.63 (Hl, s, 2H), 4.33 (Hm, s, 2H). 4.11 (Hn, s, 5H).
4,4’-((4’-(bis(4-ferrocenylphenyl)amino)-[1,1’-biphenyl]-4-yl)azanediyl)dibenzaldehyde, 3c: 4 (990 mg, 1.41 mmol), ferrocene boronic acid (2.4 eq., 780 mg, 3.39 mmol), and tetrabutylammonium tetrafluoroborate (0.41 eq., 190 mg, 0.577 mmol) were dissolved in toluene:water (8:2, 50 mL). The mixture was sparged for 30 min. Next, K2CO3 (3.3 eq., 0.65 g, 4.60 mmol) was added and the solution was left to sparge for an additional 5 min. The palladium catalyst, Pd(dppf)Cl2 (0.1 eq., 0.120, 0.164 mmol), was added. The solution was left to reflux under nitrogen for 12 h. The crude mixture was dried in vacuo. The residue was solubilized in DCM and washed with ascorbic acid; the organic layer was collected, dried over MgSO4, and the solvent removed. The crude product was purified using a silica column (Rf = 0.18) with DCM as eluent to yield an orange solid after solvent removal. (1100 mg, 1.20 mmol, 85.3%). 1H NMR (400 MHz, CDCl3): δ = 9.93 (Ha, s, 2H), 7.82 (Hb, d, 3JHH = 8 Hz, 4H), 7.63 (He, d, 3JHH = 8 Hz, 2H), 7.53 (Hd, d, 3JHH = 8 Hz, 2H), 7.43 (Hc, d, 3JHH = 8 Hz, 4H), 7.33–7.20 (Hf, Hg, Hh, m, 8H), 7.11 (Hi, 3JHH = 8 Hz, 4H), 4.63 (Hj, s, 4H), 4.33 (Hk, s, 4H), 4.11 (Hl, s, 10H). 13C NMR (100 MHz, CDCl3) δ 190.43, 151.86, 147.25, 145.24, 144.08, 138.42, 133.97, 133.36, 131.33, 128.06, 127.55, 127.32, 127.15, 127.00, 124.37, 123.59, 122.86, 85.53, 69.65, 68.86, 66.27.
4,4’-((4’-((4-bromophenyl)(4-(ferrocenylethynyl)phenyl)amino)-[1,1’-biphenyl]-4-yl)-azanediyl)-dibenzaldehyde, 3d: 5d (260 mg, 0.425 mmol), 6 (180 mg, 0.421 mmol) and tetrabutylammonium tetrafluoroborate (0.4 eq., 55 mg, 0.167 mmol) were dissolved in toluene:water (8:2, 50 mL). The mixture was sparged for 30 min. Next, K2CO3 (3.3 eq., 190 mg, 1.37 mmol) was added and the solution was left to sparge for an additional 5 min. The palladium catalyst, Pd(dppf)Cl2 (0.1 eq., 35 mg, 0.048 mmol), was added. The solution was refluxed under nitrogen for 12 h. The crude mixture was dried in vacuo. The residue was solubilized in DCM and washed with ascorbic acid; the organic layer was collected, dried over MgSO4, and the solvent removed. The crude product was purified using a silica column (Rf = 0.17) with DCM as eluent to yield an orange solid after solvent removal. (270mg, 0.324 mmol, 77.0%). 1H NMR (400 MHz, CDCl3): δ = 9.93 (Ha, s, 2H), 7.82 (Hb, d, 3JHH = 8 Hz, 4H), 7.61 (He, d, 3JHH = 8 Hz, 2H), 7.53 (Hd, d, 3JHH = 8 Hz, 2H), 7.44–7.38 (Hf, i, m, 4H), 7.26 (Hc, d, 3JHH = 8 Hz, 4H), 7.21–6.99 (Hj, Hg, Hh, Hk, m, 8H), 4.52 (Hl, s, 2H), 4.27 (Hm, Hn, s, 7H). 13C NMR (100 MHz, CDCl3) δ 190.59, 152.01, 146.57, 146.33, 144.52, 138.28, 134.96, 132.64, 132.59, 131.52, 131.46, 128.35, 127.96, 127.23, 125.84, 124.69, 123.91, 123.60, 123.04, 118.42, 116.08, 88.12, 85.69, 71.45, 70.07, 68.90.
4,4’-((4’-(bis(4-(ferrocenylethynyl)phenyl)amino)-[1,1’-biphenyl]-4-yl)azanediyl)dibenzaldehyde, 3e: 5e (400 mg, 0.539 mmol), 6 (230 mg, 0.538 mmol), and tetrabutylammonium tetrafluoroborate (0.4 eq., 70 mg, 0.21 mmol) were dissolved in toluene:water (8:2, 50 mL). The mixture was sparged for 30 min. Next, K2CO3 (250 mg, 1.81 mmol) was added and the solution was left to sparge for an additional 5 min. The palladium catalyst, Pd(dppf)Cl2 (0.1 eq., 40 mg, 0.055 mmol), was added and the solution was left to reflux under nitrogen for 12 h. The crude mixture was dried in vacuo. The residue was solubilized in DCM and washed with ascorbic acid; the organic layer was collected, dried over MgSO4, and the solvent removed. The crude product was purified using a silica column. The product was run through a silica column using DCM to purify it (Rf = 0.16). The resulting product was an orange solid. (250 mg, 0.26 mmol, 48.2%). 1H NMR (400 MHz, CDCl3): δ = 9.94 (Ha, s, 2H), 7.83 (Hb, d, 3JHH = 8 Hz, 4H), 7.63 (He, d, 3JHH = 8 Hz, 2H), 7.54 (Hd, d, 3JHH = 8 Hz, 2H), 7.42 (Hc, d, 3JHH = 8 Hz, 4H), 7.33–7.20 (Hf, Hg, Hh, m, 8H), 7.10 (Hi, 3JHH = 8 Hz, 4H), 4.53 (Hj, s, 4H), 4.28 (Hk, Hl, s, 14H). 13C NMR (100 MHz, CDCl3) δ 190.63, 152.06, 146.62, 146.58, 144.54, 138.38, 135.02, 132.65, 131.56, 131.49, 128.39, 127.95, 127.27, 124.98, 123.95, 123.07, 118.51, 88.14, 85.71, 71.48, 70.11, 68.93.
4-bromo-N-(4-bromophenyl)-N-(4-(ferrocenylethynyl)phenyl)aniline, 5d: tribromotriphenylamine (530 mg, 1.10 mmol) was dissolved in minimal tetrahydrofuran (THF):NEt3 (1:1, 14 mL) in a microwave vial. The solution was sparged for 30 min. Ethynyl-ferrocene (235 mg, 1.11 mmol) was added dropwise, and the solution was then sparged for an additional 5 min before CuI (0.1 eq., 20 mg, 0.11 mmol) and Pd(dppf)Cl2 (0.1 eq., 90 mg, 0.12 mmol) were added to the reaction mixture. The microwave vial was then sealed, and the reaction was heated to 65 °C for 12 h. The reaction mixture was dried in vacuo and the crude product was dissolved in DCM and washed with saturated aqueous NH4Cl and ascorbic acid solution. The product was dried over MgSO4 and then the solvent was removed in vacuo. The product was isolated by column chromatography using heptane: DCM (4:1) as the eluent (Rf = 0.26). The product was an orange solid. (200 mg, 0.33 mmol, 29.7%). 1H NMR (400 MHz, CDCl3): δ = 7.39–7.31 (Hg, Hh, m, 6H), 7.00–6.88 (Hf, Hi, m, 6H), 4.50 (Hj, s, 2H), 4.25 (Hl, Hk, s, 7H). 13C{1H} NMR (100 MHz, CDCl3): δ 146.44, 146.17, 132.71, 132.64, 126.05, 123.53, 118.60, 116.25, 88.21, 85.61, 71.57, 70.27, 69.11.
4-bromo-N,N-bis(4-(ferrocenylethynyl)phenyl)aniline, 5e: tribromotriphenylamine (640 mg, 1.33 mmol) was dissolved in minimal THF: NEt3 (1:1, 14 mL) in a microwave vial. The solution was sparged for 30 min. Ethynyl-ferrocene (2.1 eq., 590 mg, 2.79 mmol) was added dropwise and the solution was then sparged for an additional 5 min before CuI (0.1 eq., 30 mg, 0.16 mmol) and Pd(dppf)Cl2 (0.1 eq., 110 mg, 0.15 mmol) were added to the reaction mixture. The microwave vial was then sealed, and the reaction was heated to 65 ºC overnight. The reaction mixture was dried in vacuo and the crude product was dissolved in DCM and washed with saturated aqueous NH4Cl and ascorbic acid solution. The product was dried over MgSO4 and then the solvent was removed in vacuo. The product was isolated by column chromatography using heptane:DCM (4:1) as the eluent (Rf = 0.25). The product was an orange solid. (300 mg, 0.40 mmol, 30.4%). 1H NMR (400 MHz, CDCl3): δ = 7.40 (Hg, Hh, d, 3JHH = 8 Hz, 6H), 7.08–6.96 (Hf, Hi, multi, 6H), 4.53 (Hj, s, 4H), 4.28 (Hk, Hl, s, 14H). 13C{1H} NMR (100 MHz, CDCl3): δ 146.44, 146.15, 132.73, 132.63, 126.05, 123.51, 118.58, 116.26, 88.22, 85.65, 72.27, 70.32, 69.40.

3. Results

Physicochemical studies presented in this work include cyclic voltammetry, spectroelectrochemistry, density functional theory (DFT) (applied to the dyes), and absorption/emission spectra. These data have been collated in Table 1. The solubility of the ferrocene-containing cyanoacetic acid derivatives (2b–2d) in DCM and acetonitrile is poor. While we were able to dissolve sufficient sample into solution for optical measurements (in DCM), the electrochemical and spectroelectrochemical characterization reported herein was done using the more soluble aldehyde precursors (3a–3e). As can be seen from Figure S1, and consistent with our previous work [54], only modest changes are observed electrochemically when comparing the dyes and their aldehyde precursors. Thus, we use the assumption that aldehydes are a reasonable proxy for electrochemical comparison.

3.1. Optical Properties of the Dyes

All optical properties of the dyes and precursors discussed herein were collected in DCM solutions and are collated in Table 1. In addition, absorption and fluorescence spectra are included in the supporting information (Figure S2). All the dyes (and aldehyde precursors) exhibit weak to moderate fluorescence behavior in solution, consistent with previous observations [54]. Upon conversion of the aldehyde precursors (3a–e) into the dye derivatives (2a–e), there is a significant broadening and bathochromic shift in the UV-Vis absorption spectra for all dyes. Since the Knoevenagel condensation does little to perturb the highest occupied molecular orbital (HOMO) energies of the dye electrochemically (vide infra, Figure S1), it is reasonable to conclude that this bathochromic shift is a result of a decrease in the lowest unoccupied molecular orbital (LUMO) energy in the dye family. This can be attributed to extended conjugation. Figure 2a contrasts the experimental absorption behavior of the symmetrical dyes 2a, 2c, and 2e, and Figure 2b depicts the calculated frontier molecular orbitals involved in dye absorption. At first glance, there is little difference between the observed absorption of dye 2a and 2c, but significant differences (blue shifted, with a low energy tail) are observed for dye 2e.
To better understand the nature of these optical behaviors in Family 2, time-dependent density functional theory (TDDFT) calculations were performed (summarized in Figure 2b) to determine the dominant optical transitions. Ground-state geometries were optimized with the B3LYP hybrid functional. All optimizations were calculated without any symmetry constraints using the 6-311G(d,p) basis set with the Gaussian 16 (rev C.01) software package [57]. Predicted absorption spectra (by TDDFT) had a much broader spectral envelope that would be well suited for panchromatic absorption, but that was not observed experimentally. In each of the dyes studied, the HOMO-LUMO optical transitions are not significant contributors to the absorption spectra because of insufficient wavefunction overlap between these orbitals and a resulting weak transition oscillator strength. The basis set and functional used in the TDDFT accurately predicted the optical properties of dye 2a, but were less effective at predicting the spectra for the ferrocene containing dyes. Figure 2a shows the predicted transitions overlayed with the experimental findings, but the calculated dominant transitions predicted for both ferrocene derivatives were weak (oscillator strength below 0.15) with HOMO-LUMO+1 transitions being the most dominant. When compared to the experimental spectra, it may rationalize the low-energy tails, but otherwise, the calculated spectra do not match well with experimental work. One possible reason for a lack of intense transitions calculated by TDDFT is a lack of molecular orbital overlap between the ground and excited states in the ferrocene derivatives.
Perhaps another way to rationalize the blue-shifted spectrum of 2e is to use electrochemical arguments and differences between the TPA donors considering the nature of Fc attachment. Usually our family of D-π-D-A dyes exhibits a more intense, red-shifted absorption profile when electron density differences between the two TPA donors are maximized. As seen from Table 1 and elaborated further in our electrochemical studies, when considering the alkyne substitution on the distal TPA, the sp-hybridized alkyne carbon is more electronegative than the sp2 carbon. As a result (observed using cyclic voltammetry), the distal TPA oxidation is increased by about 30 mV in the alkyne ferrocene derivatives 3d and 3e when contrasted with 3b and 3c, respectively. This would lower the TPA donor difference, and blue-shift our absorption profile.
Perhaps one other notable feature is that the TDDFT calculations see the LUMO+2 orbitals as possessing some electron density on the ferrocene unit for 2c and 2e; this could be a possible route to dye deactivation in the excited state.

3.2. Electrochemical Properties

Table 1 includes a summary of the cyclic voltammetry (CV) data collected in DCM (100 mV/s; 0.1 M NBu4PF6; 2 mM aldehyde concentration; calibrated vs. normal hydrogen electrode (NHE) using octamethylferrocene as an internal standard). As previously suggested, reversible electrochemistry was not observed in the dye family (2b–e). This has been attributed to a lack of solubility of the dye during oxidation and subsequent electrodeposition. Changing the supporting electrolytes or solvents yielded no improvement, and therefore the CV measurements were done using the aldehyde precursors (3a–e). As mentioned previously, TPA donors show good electrochemical reversibility and the addition of ferrocene provides a rich electrochemical cyclic voltammogram with three reversible oxidation waves; electrochemical reversible reductions were not observed (Figure 3). Consistent with our previous findings, all the dye precursors in Family 2 (3a–e) exhibit two well-resolved electrochemical reversible oxidations at higher potentials, owing to the presence of two electronically unique redox-active TPA units [54]. While it is certainly an over-simplification, the oxidation wave at lower potential is assigned to the more electron rich distal (relative to titania) TPA and the second oxidation is centered on the electron deficient proximal TPA unit with the aldehydes attached. The low potential wave (for 3b–3e) is assigned to the ferrocene subunits added to the dye.
Figure 3a visually highlights the oxidation behavior for the directly bound, mono- and bis-functionalized Fc-derivatives 3b and 3c, respectively. Relative to 3a, the only significant change is the presence of ferrocene-containing groups and their oxidation at lower potentials. There is also a decrease in the resolution between the two TPA oxidation waves owing to the increase in the oxidation potential of the distal TPA unit. As ferrocene itself oxidizes at the lowest potential, the directly attached ferrocene unit(s) would serve as an electron-withdrawing ferrocenium cation post-oxidation, thus pushing the second oxidation (TPA-centered) wave to a higher potential. A similar result is observed for ferrocene-alkyne derivatives in Figure 3b. Another feature that both Figure 3a and Figure 3b highlight is the effect of double-ferrocene substitution versus mono-substitution. Comparing the ferrocene oxidation waves 3b to 3c (and 3d to 3e, respectively), one can see a doubling of the ferrocene (Fc)/ferrocenium (Fc+) wave intensity upon double substitution. This suggests that both Fc/Fc+ oxidations happen simultaneously at the same potential. This also suggests that the two oxidations happen independently and there is minimal electronic communication between the ferrocenes through the π-system of the TPA.
There are also some noticeable differences between Figure 3a and Figure 3b. The Fc/Fc+ oxidation wave potential of ferrocene alkyne derivatives 3d and 3e is approximately 100 mV higher than 3b and 3c, respectively. This is likely due to the inductive electron withdrawing effect of the alkyne, as previously discussed. One other notable feature is the pseudo-reversibility of 3c during the third oxidation wave. This could suggest that there may be a quinoidal intermediate that becomes increasingly unstable above this potential, or that there is a salting-out of a highly charged species. However, it is unclear why it is only observed in this aldehyde.

3.3. Device Performance

The photovoltaic performance of the dyes in Family 1 and Family 2 compared to both benchmarks, 1a and 2a, are displayed in Table 2. DSSC cell fabrication and testing details (dark and illuminated J-V curves and electrochemical impedance spectroscopy) have been included in the supporting information. Device construction was done by hand, in ambient conditions, so multiple cells were made and averages and standard deviations from these cells are used in our table below. The number of cells that make up the averages and standard deviations are shown as column ‘N’ in Table 2.
A quick glance at the data in Table 2 and the story is clear; all ferrocene derivatives drastically underperformed when compared to our benchmark dyes, and the device performance was inversely proportional to the amount of ferrocene present. This was a little disappointing particularly for 2c, which exhibited similar optical and electrochemical behavior to 2a. While the DSSC dye interface represents a complicated collection of competing rates (recombination, injection, regeneration, etc.), it is clear that the inclusion of ferrocene units in this family of dyes follows what was previously reported by El-Zohry et al. [50,51]. In their work (see cited entries in Table 2), there was a significant decrease in performance when conjugating ferrocene to the organic dye scaffold of 1a. The recombination rates for 1c (TiO2 to Fc+) were determined to be 300 ps, while for 1a (TiO2 to TPA+), the rate was 12 ns. The authors concluded that this faster recombination rate for the ferrocene derivatives was a result of the twisted intramolecular charge transfer state being disrupted by the intramolecular electron transfer from ferrocene, leading to poor device performance. We anticipated that our dyes possessing two TPA rings and greater charge separation would mitigate this deleterious behavior, but our device data is consistent with El-Zohry and this seems to suggest that our ferrocene dyes (2b–2e) are also unsuitable for DSSC devices. However, the ferrocene potentials are different for our new family of dyes, and spectroelectrochemical measurements suggest there is another reason for our dyes’ deleterious behavior in devices.

3.4. Spectroelectrochemistry

While it would be easy to conclude that this family of dyes behave similarly to the family presented in the work by El-Zohry et al. [50,51], we postulated that there may be another deleterious factor involved in the performance of our devices. The presence of ferrocene as a redox relay was designed to stabilize the TPA via donation of electron density to the TPA upon photoexcitation. As a result, ferrocenium would be generated in the photo-oxidized state. However, in our device fabrication, we use the Z1137 I−/I3− electrolyte. Iodides are both coordinating and nucleophilic, so we hypothesized that another possible pathway to poor device performance could be dye reactivity with the electrolyte with either the ambient or photo-oxidized conditions. In previous work, combining ferrocenium and tri-iodide redox couples showed no deleterious effects [58]; however, owing to the complex nature of the electrolyte additives and because no attempt was made to strictly remove oxygen from the electrolyte or fabrication process, the potential for ferrocenium to react [2,58,59] was seen as a realistic possibility. To examine this further, we performed spectroelectrochemistry experiments (Figure 4) on aldehyde 3c in acetonitrile and separately in the electrolyte mixture.
While there was some pseudo-reversibility observed in 3c at higher oxidation potentials (>1.3 V vs. NHE, Figure 3a), this dye precursor had the most promising optical properties (similar to 3a—Figure 2a), so we elected to study this system spectroelectrochemically in acetonitrile (Figure 4a). Acetonitrile was chosen to parallel the I−/I3− electrolyte solvent used in the fabrication of our DSSC devices. Figure 4a shows the gradual increase in the absorption of the ferrocenium cation over time at the oxidation potential of 0.9 V vs. NHE. This was an expected observation based on the electrochemistry previously discussed. The <400 nm region of the spectra has rich spectroelectrochemistry as well, but the behavior here is not particularly relevant for the argument herein. In Figure 4b, a similar experiment is shown, but in addition to 3c, a roughly 10-fold amount of I−/I3− electrolyte was also added. While the trends in the <400 nm portion of the spectra are complicated by electrolyte absorption (see Figure S3 as a control), the key observation is that after an initial increase in ferrocenium absorbance from oxidation (Figure 4b; ~500 nm), the ferrocenium signal starts to decrease and the spectral envelope broadens. As the only variable/change in the experiment is the addition of the electrolyte Z1137, this would suggest that something in the electrolyte is contributing to dye instability in the oxidized state. It is conceivable that this adverse reactivity could extend to the curing or operational period of the device, suggesting that the dye could be quenched before the device is even tested.

4. Discussion and Conclusions

The original goal of this work was to investigate a new family of ferrocene-containing DSSC dyes based on our previously reported D-π-D-A dye architecture. Despite previous reports by El-Zohry et al. [50,51], where they observed poor performance in devices using ferrocene-modified dyes (e.g., 1c), we hypothesized that our dyes would mitigate these deleterious effects on back-electron transfer owing to their bis-TPA dye architecture and extended charge separation. El-Zohry et al. attributed poor performance to a quenched twisted intramolecular charge-transfer (TICT) behavior (vide supra), however we surmised that an extended π-system in our bis-TPA architecture could overcome this. To this end, we prepared directly bound ferrocene (2b, 2c) and the alkyne ferrocene derivatives (2d, 2e).
Based on TDDFT calculations the optical behavior of both the directly-bound and alkyne-connected Fc derivatives looked promising (pre-synthesis) with panchromatic absorption envelopes; however, that was not realized experimentally. The directly bound ferrocene derivatives 2b and 2c have a similar absorption profile as 2a, whilst the ferrocene alkyne derivatives (2d and 2e) have a less desirable, blue-shifted spectrum. Future work will examine other basis sets in the TDDFT calculations to try and make better predictions for these hybrid dye systems.
The electrochemical behavior for 2a–2e seemed consistent based on the structural modifications. The alkyne ferrocene derivatives (2d and 2e) have a slightly more positive Fc/Fc+ oxidation potential versus NHE, owing to the electron withdrawing nature of the alkyne bridge. This is pleasing because it suggests a degree of tunability of the ferrocene oxidation potential through chemical modification. Generally, there appeared to be a lack of electronic communication within the dye that manifested in the redox active components behaving independently, with notable exceptions being a minor shift in the distal TPA oxidation. This was more likely due to the electrostatic electron withdrawing effect of the oxidized ferrocenium cation rather than communication between the electrophores.
Device performance of all ferrocene-containing dyes presented herein was dismal, and drastically underperformed relative to the non-ferrocene containing benchmarks 1a and 2a. Despite the success of using iron-based electrolytes [60,61,62] and other ferrocene-modified dyes motifs [33,34], our poor device performance is consistent with rapid TiO2-Fc+ recombination observed for other TPA-based dyes containing ferrocene [50,51]. However, examining the stability of the ferrocene in a model electrolyte environment using spectroelectrochemical measurements seems to suggest that the stability of ferrocene-modified organic dyes is also a potential problem. In our case, without rigorous attempts to avoid oxygen during device preparation, one possible decomposition pathway could be degradation of the ferrocenium by molecular oxygen or electrolyte additives, as discussed previously [63].
Finally, perhaps the ultimate limitation of using ferrocene dyes involves our choice of electrolyte. As elaborated in Figure 5, the regeneration of oxidized dye with iodide leads to the formation of the diiodide radical (I2−•), and the redox potential of this couple must also be considered when determining the driving force for dye regeneration [64]. As such, it is possible that ferrocene is being oxidized by the I2−• species and converting into ferrocenium either in ambient conditions or when the dye is in the excited state [65]. Regardless, when considering this family of ferrocene-containing dyes, the lack of sufficient driving force for regeneration also impedes device performance. As such, perhaps using other electrolytes or tuning the redox potential of the ferrocene could be explored to overcome this deleterious behavior.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app12126001/s1, Scheme S1: Synthesis of 1b; Figure S1: Aldehydes as reasonable proxies for dyes in cyclic voltammetry; Figure S2: UV-vis absorption and Fluorescence emission in DCM for dyes and their precursors; Figure S3: Spectroelectrochemistry of Z1137 I−/I3− electrolyte in acetonitrile (MeCN, 7.0 × 10−3 mol/L) using an optically transparent thin layer electrochemical (OTTLE) cell. Figure S4. IV curves for DSSCs under full spectrum light (sun) and in the absence of light (dark). Figure S5. Resistance and capacitance values for DSSCs under full spectrum light (sun) and in the absence of light (dark). Figure S6. Nyquist and Bode plots for DSSCs under full spectrum light (sun) and in the absence of light (dark). Reference [51] is cited in the Supplementary Materials.

Author Contributions

Conceptualization, T.K.S. and B.D.K.; data curation, T.K.S. and B.D.K.; formal analysis, T.K.S. and R.A.G.; funding acquisition, B.D.K.; investigation, T.K.S., R.A.G., E.S.D. and A.S.B.S.; methodology, T.K.S., R.A.G., E.S.D. and B.D.K.; project administration, B.D.K.; supervision, B.D.K.; visualization, B.D.K.; writing—original draft, T.K.S.; writing—review and editing, T.K.S., E.S.D., A.S.B.S. and B.D.K. All authors have read and agreed to the published version of the manuscript.

Funding

T.K.S and B.D.K would like to thank Mitacs Canada and Detect-Tech Inc. for financial assistance for this research.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Acknowledgments

R.A.G. would like to thank the NSERC URSA program and the Faculty of Science for scholarship support. E.S.D. would like to acknowledge the Inorganic Chemistry Exchange Program for the administration of her research exchange. E.S.D. and A.S.B.S. would like to thank Koivisto Materials Consulting Inc. for financial support of their research. B.D.K would like to thank the Faculty of Science at Toronto Metropolitan University for their equipment grant needed to carry out this research.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Family 1 based on Sun’s Dye where 1a [43] and 1c [51] have been previously reported. 1b with one ferrocene unit is reported herein. Family 2 is based on a previously reported family (2a [54]) of bipodal dyes, and herein we report TPA dyes modified with both ferrocene and ferrocenyl-alkyne moieties.
Figure 1. Family 1 based on Sun’s Dye where 1a [43] and 1c [51] have been previously reported. 1b with one ferrocene unit is reported herein. Family 2 is based on a previously reported family (2a [54]) of bipodal dyes, and herein we report TPA dyes modified with both ferrocene and ferrocenyl-alkyne moieties.
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Scheme 1. Part (A): the synthesis of Fc-TPA dyes 2b and 2c using Suzuki cross-coupling. 2a and 3a were previously reported [54]. Part (B): the synthesis of Fc-alkyne-TPA dyes 2d and 2e using Sonogashira/Suzuki cross-coupling strategies.
Scheme 1. Part (A): the synthesis of Fc-TPA dyes 2b and 2c using Suzuki cross-coupling. 2a and 3a were previously reported [54]. Part (B): the synthesis of Fc-alkyne-TPA dyes 2d and 2e using Sonogashira/Suzuki cross-coupling strategies.
Applsci 12 06001 sch001
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Figure 2. Optical behavior of dyes 2a, 2c, and 2e in DCM: (a) experimental UV-Vis absorption data for dyes 2a (black), 2c (red) and 2e (green). TDDFT calculated dominant transitions (dashed lines) leading to the observed absorptions for 2a are shown; (b) frontier molecular orbitals for 2a, 2c, and 2e (left to right). Oscillator strengths provided for each dominant transition calculated from TDDFT.
Figure 2. Optical behavior of dyes 2a, 2c, and 2e in DCM: (a) experimental UV-Vis absorption data for dyes 2a (black), 2c (red) and 2e (green). TDDFT calculated dominant transitions (dashed lines) leading to the observed absorptions for 2a are shown; (b) frontier molecular orbitals for 2a, 2c, and 2e (left to right). Oscillator strengths provided for each dominant transition calculated from TDDFT.
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Figure 3. Cyclic voltammetry comparisons for novel dye precursors 3b–e, when contrasted with 3a: (a) comparing the electrochemical effect of direct ferrocene addition; (b) comparing the electrochemical effect of ferrocene units added via alkynes.
Figure 3. Cyclic voltammetry comparisons for novel dye precursors 3b–e, when contrasted with 3a: (a) comparing the electrochemical effect of direct ferrocene addition; (b) comparing the electrochemical effect of ferrocene units added via alkynes.
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Figure 4. Spectroelectrochemistry of 3c in acetonitrile (MeCN, 7.0 × 10−4 mol/L) using an optically transparent thin layer electrochemical (OTTLE) cell. The sample was held at a suitable voltage for oxidation of the ferrocene unit, but not the TPA (0.9 V) for the durations listed above: (a) for aldehyde 3a there is an increasing ferrocenium absorption at ~500 nm over the duration of the experiment; (b) 3a and Z1137 I−/I3− electrolyte—after an initial increase in ferrocenium concentration, we start to see a decrease in the ferrocenium intensity. This may suggest that the ferrocenium cation is not stable in the Z1137 I−/I3− electrolyte over time. The absorption values have not normalized and a control spectroelectrochemistry experiment (under identical conditions) for the Z1137 I−/I3− electrolyte can be found in the supporting information, Figure S3.
Figure 4. Spectroelectrochemistry of 3c in acetonitrile (MeCN, 7.0 × 10−4 mol/L) using an optically transparent thin layer electrochemical (OTTLE) cell. The sample was held at a suitable voltage for oxidation of the ferrocene unit, but not the TPA (0.9 V) for the durations listed above: (a) for aldehyde 3a there is an increasing ferrocenium absorption at ~500 nm over the duration of the experiment; (b) 3a and Z1137 I−/I3− electrolyte—after an initial increase in ferrocenium concentration, we start to see a decrease in the ferrocenium intensity. This may suggest that the ferrocenium cation is not stable in the Z1137 I−/I3− electrolyte over time. The absorption values have not normalized and a control spectroelectrochemistry experiment (under identical conditions) for the Z1137 I−/I3− electrolyte can be found in the supporting information, Figure S3.
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Figure 5. Device architecture and energetics. (a) Approximate energy levels of the dye, ferrocene moiety and electrolyte based on the data in Table 1 and literature [64,66]; (b) fabricated representative devices for dyes 2a, 2c, and 2e with color and intensities consistent with their absorption spectra in Figure 2a.
Figure 5. Device architecture and energetics. (a) Approximate energy levels of the dye, ferrocene moiety and electrolyte based on the data in Table 1 and literature [64,66]; (b) fabricated representative devices for dyes 2a, 2c, and 2e with color and intensities consistent with their absorption spectra in Figure 2a.
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Table
Table 1. Physicochemical properties of DSSC dyes and novel aldehyde precursors reported herein.
Table 1. Physicochemical properties of DSSC dyes and novel aldehyde precursors reported herein.
MoleculeUV-Vis λmax nm
(ɛ × 104 M−1 cm−1)		Fluorescence Emission λmax nmE1/2 a (V vs. NHE)
1a285 (2.75) 480 (2.43)640 1.22
1b 337 (3.85)496 (3.77)6350.66 1.34
1cb 345 (4.22)505 (3.37)6900.65 1.32
2a307 (2.34)400 (2.68)462 (3.34)571 1.141.36
2b 361 (0.97)460 (0.58)510
2c342 (2.66)408 (sh, 2.50)465 (3.09)509
2d 352 (1.23)450 (0.92)509
2e301 (1.82)370 (3.32)441 (1.67)504
3a240 (2.65)365 (4.66) 483 1.121.35
3b240 (3.50)370 (5.10) 4740.651.221.38
3c245 (5.05)370 (6.15) 4860.661.231.39
3d245 (5.50)370 (8.25) 4900.771.251.40
3e245 (2.27)370 (3.32) 4760.771.261.40
5d 320 (3.81)440 (0.25)5070.77 1.35
5e 330 (3.54)450 (0.24)5080.78 1.38
7c 325 (2.31)415 (2.48)6570.67 1.34
a Data collected using 0.1 M NBu4PF6 in DCM solutions at 100 mVs−1 and referenced to an octamethylferrocene [OFc]/[OFc]+ internal standard. Calibrated at 0.225 V vs. NHE for Ofc. b 1c was previously reported by El-Zohry et al., but the physicochemical data reported here was collected on our freshly prepared sample for comparison. c 7 is an aldehyde precursor and its synthesis is reported in the supporting information.
Table
Table 2. Photovoltaic performance of DSSCs based on the ferrocene-based dyes compared to benchmarks 2a bichromic-bipodal dyes.
Table 2. Photovoltaic performance of DSSCs based on the ferrocene-based dyes compared to benchmarks 2a bichromic-bipodal dyes.
DyeVOC (V)JSC (mA/cm2)FFη (%)N a
1a0.65 ± 0.0043.97 ± 0.5700.58 ± 0.0262.71 ± 0.3453
1ab0.69 ± 0.00210.4 ± 0.10.76 ± 0.505.5 ± 0.1-
1b0.33 ± 0.0440.22 ± 0.1180.21 ± 0.1170.028 ± 0.0043
1cb0.57 ± 0.0032.40 ± 0.10.53 ± 0.00080.70 ± 0.1-
2a0.70 ± 0.0134.10 ± 0.0970.71 ± 0.0773.70 ± 0.4836
2b0.49 ± 0.1410.70 ± 0.2540.49 ± 0.1540.30 ± 0.0967
2c0.26 ± 0.0280.17 ± 0.0250.21 ± 0.0260.018 ± 0.0026
2d0.49 ± 0.0140.69 ± 0.0870.70 ± 0.3070.43 ± 0.0086
2e0.30 ± 0.0330.11 ± 0.0370.37 ± 0.1500.022 ± 0.0046
a N is the number of simultaneous test cells prepared that make up the mean and standard deviation. The I−/I3− electrolyte combination used was Z1137. See supporting information for device fabrication details. b Additional entries for 1a and 1c were previously reported by El-Zohry et al. [50,51].
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Singh, T.K.; Grandy, R.A.; Dennis, E.S.; Schouten, A.S.B.; Koivisto, B.D. Exploring Structure-Property Relationships in a Family of Ferrocene-Containing, Triphenylamine-Based Hybrid Organic Dyes. Appl. Sci. 2022, 12, 6001. https://doi.org/10.3390/app12126001

AMA Style

Singh TK, Grandy RA, Dennis ES, Schouten ASB, Koivisto BD. Exploring Structure-Property Relationships in a Family of Ferrocene-Containing, Triphenylamine-Based Hybrid Organic Dyes. Applied Sciences. 2022; 12(12):6001. https://doi.org/10.3390/app12126001

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Singh, Tavneet K., Reese A. Grandy, Emma S. Dennis, Anja S. B. Schouten, and Bryan D. Koivisto. 2022. "Exploring Structure-Property Relationships in a Family of Ferrocene-Containing, Triphenylamine-Based Hybrid Organic Dyes" Applied Sciences 12, no. 12: 6001. https://doi.org/10.3390/app12126001

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