Ferrocenyl-functionalized phenothiazine conjugates: structure–property relationship and electrochemical energy storage studies

Abstract

A set of mono-, di-, and tri-ferrocenyl phenothiazine conjugates 1–3 were synthesized via Buchwald–Hartwig cross-coupling and Pd-catalyzed Suzuki cross-coupling reactions. Phenothiazine served as the central core, and its various sites were explored for functionalization. In the mono-ferrocenyl conjugate 1, ferrocene was linked at the nitrogen atom of the phenothiazine core via a biphenyl spacer, while in the di- and tri-ferrocenyl conjugates, phenyl-linked ferrocene was incorporated at the 3rd and 7th positions of phenothiazine. The photophysical, electrochemical and thermal behavior of these conjugates was analyzed to investigate the effect of structural variations. The di- and tri-ferrocenyl conjugates 2 and 3 showed a bathochromic shift of 50–55 nm as compared to the monofunctionalized ferrocenyl conjugate 1. Electrochemical studies showed two reversible oxidation waves for all conjugates 1–3, attributed to ferrocene and phenothiazine units. Additionally, their electrochemical performance was evaluated by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements using a three-electrode configuration in H₂SO₄ electrolyte, where the conjugates were employed as working electrodes by dropcasting on graphite foil (GF). The 1/GF electrode exhibited predominant capacitive behavior with remarkable stability. The 2/GF electrode demonstrated diffusion-controlled redox activity typical of battery-type behavior. In contrast, the 3/GF electrode showed a combination of capacitive and diffusion-controlled processes, achieving the highest specific capacitance of 160.8 F g⁻¹ at 0.5 A g⁻¹ along with excellent cycling stability. Computational calculations were performed to optimize molecular structures and evaluate frontier molecular orbital energy levels. Single-crystal X-ray diffraction confirmed the structure of 1.

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Introduction

Organic π-conjugated donor–acceptor systems have garnered significant interest due to their potential applications in nonlinear optics (NLO) and organic photovoltaics (OPVs), fueling ongoing research in their design and synthesis.^1–14 π-conjugated moieties featuring donor–π–donor architectures have emerged as promising candidates for applications from organic electronics to molecular sensors. These systems exhibit unique electronic interactions that can be tailored to achieve desired properties, making them versatile in a variety of technological fields.^15–19

Phenothiazine is a tricyclic heterocyclic electron-rich compound.^20,21 It exhibits modifiable photophysical properties and electrophilic substitution at the 3rd and 7th positions, as well as nucleophilic substitution at the N-atom. These features make it an attractive component in material design.^22–28 The well-defined electrochemical characteristics of ferrocene, along with its ability to stabilize the ferrocenium ion,^29,30 make it a valuable component in systems designed for nonlinear optics (NLO) and optoelectronics.^31–38 Multi-ferrocenyl-functionalized systems exhibit enhanced thermal and photochemical stability.^39–48 The ability to modify substituents on the cyclopentadienyl rings allows fine-tuning of electronic properties, expanding ferrocene's utility in advanced materials science, including catalysis, sensors, and molecular electronics. Several research groups have established ferrocenyl and polyferrocenyl frameworks as versatile redox-active systems with broad applications in materials science, catalysis and biomedicine.^49–69

In recent years, electrochemical energy storage systems based on redox-active materials have attracted increasing attention as they effectively bridge the performance gap between traditional electrical double-layer capacitors (EDLCs) and batteries.^70,71 In contrast to EDLCs, which rely solely on electrostatic ion adsorption at the electrode–electrolyte interface, the redox-driven systems store charge through fast and reversible faradaic reactions occurring at or near the electrode surface. This type of charge storage mechanism enables significantly higher specific capacitance, energy density and rate capability, while retaining the excellent cycling stability characteristic of capacitive devices. The superior electrochemical characteristics of redox-active organic and organometallic materials make them promising candidates for advanced energy storage and sensing technologies.^72–75 These materials facilitate rapid charge–discharge processes, efficient energy conversion and prolonged operational stability. Specifically, phenothiazine (PTZ) and ferrocene (Fc) derivatives have emerged as molecular scaffolds for pseudocapacitive electrodes owing to their reversible redox activity, high chemical stability, and facile functionalization.

Phenothiazine can readily participate in reversible one-electron oxidation processes leading to the formation of stable radical cations. This intrinsic redox reversibility allows PTZ-based molecules to function as efficient p-type pseudocapacitive materials.⁷⁶ To enhance their electrochemical response, phenothiazine has been incorporated into π-conjugated donor–π–donor systems or hybridized with conductive carbon supports. For example, PTZ/reduced graphene oxide (rGO) composites have been employed as cathodes in lithium-ion capacitors, achieving significantly improved discharge capacities and cycling stability compared to pristine rGO due to the synergistic combination of pseudocapacitance from PTZ and high conductivity from rGO.⁷⁷ Similarly, PTZ functionalized anthraquinone derivatives have demonstrated excellent capacitive behavior, exhibiting specific capacitance up to 112.32 F g⁻¹ and remarkable cycling stability in symmetric cell configurations.⁷⁸

On the other hand, ferrocene and its derivatives represent another widely studied class of redox-active materials for pseudocapacitive applications.^79,80 The Fe²⁺/Fe³⁺ redox couple in ferrocene provides highly reversible electron transfer characteristics, chemical robustness and structural versatility. When incorporated into conjugated systems or polymer backbones, ferrocene can significantly enhance charge-transfer efficiency, redox reversibility and the overall electrochemical stability of the electrode. In addition, multi-ferrocenyl frameworks and hybrid Fc–carbon composites have exhibited enhanced thermal stability, extended redox window and improved specific capacitance owing to the collective contribution of multiple redox centers.⁸¹ These properties make ferrocene an ideal molecular component for designing multifunctional pseudocapacitive materials.

Inspired by recent advances, we integrate phenothiazine and ferrocene units within a single π-conjugated molecular framework as a rational strategy to achieve synergistic redox activity and enhanced electronic delocalization. The combination of the ferrocene reversible Fe²⁺/Fe³⁺ redox process and the phenothiazine p-type electron-donating character promotes efficient charge transfer and enhances specific capacitance and cycling stability.

In this study, we examine the effect of incorporating ferrocene into phenothiazine-based donor–π–donor systems, with systematic variations in the number and positions of phenyl spacers, i.e., at the N-position and the 3rd and 7th positions of phenothiazine using phenyl-substituted ferrocenyl units.^82–88 The photophysical and redox behaviors of mono-, di-, and tri-ferrocenyl phenothiazine conjugates 1–3 are thoroughly investigated, providing deeper insight into their functional properties and potential applications. Conjugate 3 was previously reported by our group.⁸⁹ Herein, we present a comparative study of 1, 2 and 3 to assess the impact of ferrocenyl substitutions on their electronic and redox characteristics. In this context, the incorporation of ferrocene and phenothiazine moieties into a donor–π–donor framework holds substantial promise.⁹⁰ The synergistic combination of these two components can enhance electronic delocalization and redox properties, potentially broadening their utility in molecular electronics, electrocatalysis, and energy storage.^91–93 This work is aimed to explore the structure–property relationships for potential applications of ferrocenyl-substituted phenothiazine conjugates within a donor–π–donor configuration.

Results and discussion

Synthesis

The ferrocenyl substituted phenothiazine conjugates 1–3 (Fig. 1) were synthesized via Pd-catalyzed Buchwald–Hartwig cross-coupling and Suzuki cross-coupling reactions.^94,95 Ferrocenyl conjugate 1 was synthesized via the Suzuki cross-coupling reaction of 10-(4-bromophenyl)-10H-phenothiazine (PTZ-Br) and Fc-Bpin in the presence of Pd(PPh₃)₄ and Na₂CO₃ in THF : water at 60 °C for 16 h, resulting in a yellowish brown solid in 82% yield (Scheme 1). The precursors 10-(4-bromophenyl)-10H-phenothiazine (PTZ-Br), 3-bromo-10-(4-bromophenyl)-10H-phenothiazine (PTZ-Br₂), 3,7-dibromo-10-(4-bromophenyl)-10H-phenothiazine (PTZ-Br₃), and Fc-Bpin were synthesized via the reported procedure.^96–101

Fig. 1 Molecular structure of ferrocenyl functionalized phenothiazine conjugates 1–3 (* Misra et al., Dalton Trans., 2025, 54, 5906–5920).⁸⁹

Scheme 1 Synthetic route of ferrocenyl functionalized phenothiazine conjugate 1.

The precursors 3-bromo-10-(4-bromophenyl)-10H-phenothiazine (PTZ-Br₂) and 3,7-dibromo-10-(4-bromophenyl)-10H-phenothiazine (PTZ-Br₃) were synthesized by the bromination of 10-(4-bromophenyl)-10H-phenothiazine (PTZ-Br) in the presence of 1.1 and 2.2 equivalents of NBS for 24 h at room temperature in 76% and 82% yield, respectively. The synthesis of compound 3 was previously reported by our group via a sequential Suzuki–Miyaura and Buchwald–Hartwig cross-coupling strategy; however, in the present work, we have employed an alternative synthetic approach involving only the Suzuki–Miyaura cross-coupling reaction.⁸⁹2 and 3 were synthesized via the Suzuki cross-coupling reaction of PTZ-Br₂ and PTZ-Br₃ with Fc-Bpin in the presence of Pd(PPh₃)₄ and Na₂CO₃ in THF : water at 60 °C for 16 h in 62% and 73% yields, respectively (Scheme 2). The ferrocenyl derivatives 1 and 2 were purified by silica gel column chromatography and were characterized using ¹H NMR, ¹³C NMR, and HRMS techniques (Fig. S11–S16).

Scheme 2 Synthetic route of ferrocenyl functionalized phenothiazine conjugates 2 and 3.

Photophysical properties

The photophysical properties of ferrocenyl conjugates 1–3 were investigated using absorption spectroscopy in DCM at room temperature, and the results are summarized in Table 1. The absorption spectra of the conjugates 1–3 show absorption bands in the range of 250–600 nm (Fig. 2). 1 displayed three distinct absorption bands at 260 nm and 303 nm, attributed to π–π* transitions, along with a weaker intramolecular charge transfer (ICT) band around 460 nm. In contrast, the unsymmetrical ferrocenyl conjugate 2 exhibited absorption peaks at approximately 270 nm, 303 nm, and 355 nm corresponding to π–π* transitions accompanied by a red-shifted ICT band near 500 nm.

Table 1 Photophysical and theoretical data of 1–3

Compound	Photophysical data^a			Theoretical data^b			Thermal stability^d
	λ abs (nm)	ε (M^−1 cm^−1)	Optical band gap^a (eV)	HOMO^b (eV)	LUMO^c (eV)	HOMO–LUMO gap	T d (°C)
a Absorbance recorded in DCM at 1 × 10^−5 M conc. λabs: absorption wavelength. ε: extinction coefficient. b HOMO. c LUMO energy levels and theoretical energy band gap calculated from density functional theory. d Decomposition temperatures for 5% weight loss at a heating rate of 10 °C min^−1, under a nitrogen environment.
1	260	6800	3.36	−4.92	−1.19	3.73	458
	303	3000
2	270	81 000	2.83	−4.84	−1.23	3.61	212
	303	84 000
	355	31 000
3	275	66 000	2.62	−4.78	−1.23	3.55	264
	304	88 000
	356	33 000

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Fig. 2 Normalized electronic absorption spectra of 1–3 in dichloromethane (1 × 10⁻⁵ M) at room temperature.

Compared to the mono-ferrocenyl conjugate 1, 2 and 3 displayed a red shift of 52 nm. For conjugates 1–3, a shoulder peak was observed in the lower energy region (450–550 nm) of their absorption spectra. This feature is attributed to an intramolecular charge transfer (ICT) transition, indicating the movement of electronic charge within the molecule upon light absorption. The optical bandgap (E_g) values, calculated from the UV-vis absorption spectra of conjugates 1–3, were found to be 3.36 eV, 2.83 eV, and 2.62 eV, respectively.

FTIR spectroscopic analysis

The IR spectra of the mono, di, and tri-ferrocenyl conjugates 1–3 reveal characteristic stretching frequencies, largely influenced by the structural features and the types of functional groups involved. Both compounds exhibit C–H stretching vibrations around 2800 cm⁻¹ and C=C stretching vibrations in the range of 1400–1600 cm⁻¹, confirming the presence of phenyl rings. The C–N stretching vibration appears between 1200 and 1350 cm⁻¹, while the presence of sulfur is indicated by the C–S stretching vibration at 600–700 cm⁻¹. Additionally, the Fe–C stretching vibrations, characteristic of ferrocenyl moieties, are observed in the 400–600 cm⁻¹ region, reinforcing the organometallic nature of these compounds. These spectral findings collectively support the structural composition and functional group identification of compounds 1–3 (Fig. S1).

Thermal properties

The thermal behaviour of 1–3 was analyzed using thermogravimetric analysis (TGA) at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere (Fig. 3). The mono-ferrocenyl conjugate 1 demonstrated superior thermal stability compared to the di- and the tri-ferrocenyl conjugates 2 and 3. The decomposition temperatures for conjugates 1–3 at 5% weight loss were found to be 458 °C, 212 °C, and 264 °C, respectively, indicating the stability order as 1 > 3 > 2. The mono-ferrocenyl conjugate 1 exhibited two weight loss phases between 350 and 600 °C. In contrast, compounds 2 and 3 exhibited a distinct weight loss around 150 °C, followed by a continuous weight loss between 200 and 600 °C. Ferrocenyl conjugates 2 and 3 showed residual masses of approximately 30% at 500 °C and 525 °C, respectively.

Fig. 3 Thermogravimetric analysis of 1–3 measured with a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere.

Electrochemical properties

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out to examine the redox behavior of ferrocenyl conjugates 1–3. The measurements were conducted at room temperature in an anhydrous dichloromethane solution containing tetrabutylammonium hexafluorophosphate as the supporting electrolyte. A three-electrode setup was employed, consisting of a platinum wire as the counter electrode, a glassy carbon electrode as the working electrode, and Ag/AgCl as the reference electrode. The reversibility of single-electron redox events was assessed using the peak-to-peak separation (ΔE_p = E_pa − E_pc), while the half-wave potential (E_1/2) was calculated as the average of anodic and cathodic peak potentials (E_1/2 = (E_pa + E_pc)/2). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) data for compounds 1–3 are presented in Fig. 4, with corresponding values summarized in Table 2. Oxidation potentials were derived from the CV plots.

Fig. 4 Cyclic voltammograms and differential pulse voltammograms of ferrocenyl conjugates 1–3 in a 0.1 M solution of [N(C₄H₉)₄]⁺[PF₆]⁻ in dichloromethane recorded at a scan rate of 100 mV s⁻¹vs. a saturated Ag/AgCl electrode at 25 °C.

Table 2 Redox properties^a of ferrocenyl conjugates 1–3

Compounds	Wave	E 1/2 (V)	E p (mV)
a Ferrocenyl conjugates 1–3 in dichloromethane at 100 mV s^−1 scan rate vs. Ag/AgCl at 25 °C using a 0.1 M solution of TABF as the supporting electrolyte. E1/2 calculated as Epa + Epc/2 and ΔEp = Epa − Epc.
1	I	+0.58	80
	II	+0.78	90
2	I	+0.49	80
	II	+0.76	70
3	I	+0.50	70
	II	+0.78	60

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The conjugates 1–3 exhibited two reversible oxidation waves, attributed to the ferrocene and phenothiazine units. The ferrocenyl conjugates 1–3 did not exhibit any reduction waves. Specifically, 1 exhibited oxidation waves at E_1/2 = 0.58 V (ΔE_p = 80 mV) and E_1/2 = 0.78 V (ΔE_p = 90 mV), 2 at E_1/2 = 0.49 V (ΔE_p = 80 mV) and E_1/2 = 0.76 V (ΔE_p = 70 mV), and 3 at E_1/2 = 0.50 V (ΔE_p = 70 mV) and E_1/2 = 0.78 V (ΔE_p = 60 mV).

In the di- and tri-ferrocenyl phenothiazine conjugates 2 and 3, each ferrocene unit demonstrates a single oxidation wave at a less positive potential, attributed to their comparable yet distinct environmental interactions. An increase in the number of ferrocene units within these conjugates corresponds to a proportional rise in current intensity, with the tri-ferrocenyl conjugate exhibiting the highest current intensity, followed by the di-ferrocenyl conjugate, and subsequently the mono-ferrocenyl conjugate. This trend confirms the stepwise incorporation of ferrocene units. Moreover, the single phenothiazine unit present in conjugates 1–3 exhibits nearly identical current intensities across all three compounds.

Electrochemical performance of 1/GF, 2/GF, and 3/GF in a three-electrode configuration

The cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements were carried out to evaluate the electrochemical performance of the ferrocenyl conjugates 1, 2 and 3, dropcast on the graphite foil (GF) to form working electrodes 1/GF, 2/GF, and 3/GF respectively. All the measurements were carried out within the potential window of 0.0 to +0.8 V using 1/GF, 2/GF and 3/GF as working electrodes, Ag/AgCl as the reference electrode, Pt wire as the counter electrode and 1 M H₂SO₄ as the electrolyte.

The CV measurements of 1/GF, 2/GF and 3/GF were performed at scan rates ranging from 2 to 10 mV s⁻¹. At 5 mV s⁻¹ scan rate, 1/GF, 2/GF and 3/GF exhibited redox peaks at 0.38 V and −0.25 V, 0.42 V and −0.11 V, and 0.39 V and −0.15 V respectively. The CV of 3/GF shows a larger area under the curve as well as higher current response as compared to 1/GF and 2/GF (Fig. 5a and Fig. S4a, S5a). To investigate the charge storage mechanism, cyclic voltammetry (CV) was performed at scan rates ranging from 2 to 10 mV s⁻¹ within the potential window of 0.0 to +0.8 V. The dependence of current (i, mA) at a fixed potential of 0.4 V on the scan rate (v) was examined using the power law equation (eqn (1)).

i = av^b (1)

where a is a constant and b is the exponent indicative of the charge storage behavior. A b value close to 0.5 indicates battery-type behavior, where the charge storage is predominantly governed by a diffusion-controlled process.¹⁰² In contrast, a b value close to 1.0 is characteristic of surface-controlled capacitive or non-diffusive processes. The b value is determined from the slope of the linear plot of log(i) versus log(v), where i is the current response and v is the scan rate. The calculated b values for 1/GF, 2/GF, and 3/GF were found to be 1.13, 0.39 and 0.54 (Fig. 5b and Fig. S4b, S5b) respectively. The b value obtained for 2/GF indicates a predominantly diffusion-controlled charge storage mechanism, which is characteristic of battery-type behavior. In the case of 2/GF, the charge storage arises mainly from faradaic redox processes involving slow ion diffusion into the bulk of the active material. This suggests that the electrochemical response of 2/GF is governed by kinetically limited redox reactions due to restricted ion mobility or limited accessibility to redox-active sites within the electrode material.

Fig. 5 Electrochemical performance of ferrocenyl conjugate 1/GF in a three-electrode setup. (a) CV curves; (b) linear fitting plot between log(i) and log(v); (c) GCD curves; (d) ratios of capacitive and diffusion contribution at different scan rates; (e) CV curve at a scan rate of 5 mV s⁻¹ with surface-controlled and diffusion-controlled contribution; and (f) cycling stability at 10.0 A g⁻¹ for 2000 cycles.

The b values of 1.13 and 0.54 for 1/GF and 3/GF suggest the pseudo-capacitive behavior. For 1/GF, the b value close to 1.0 indicates dominant capacitive behavior facilitated by fast and reversible surface or near-surface redox reactions, enabling rapid charge transfer. The intermediate b value of 0.54 for 3/GF reflects a mixed charge storage mechanism with contributions from both surface-controlled pseudo-capacitive processes and slower diffusion-limited reactions. The GCD curves of 1/GF, 2/GF and 3/GF were recorded at current densities ranging from 0.5 A g⁻¹ to 10.0 A g⁻¹ (Fig. 5c and Fig. S4c, S5c). At a current density of 0.5 A g⁻¹, 3/GF delivers a specific capacitance of 160.8 F g⁻¹, significantly higher compared to those of 1/GF and 2/GF (Fig. 6 and Table S2).

Fig. 6 Plot of specific capacity vs. current density for 1/GF and 3/GF.

In order to calculate the contribution ratios of the capacitive-controlled process and the diffusion-controlled process, eqn (2) was used.

i = k₁v + k₂v^0.5 (2)

where k₁v and k₂v^0.5 represent capacitive-controlled current and diffusion-controlled current respectively.¹⁰¹

The capacitive and diffusion-controlled contributions were evaluated at scan rates ranging from 2 to 8 mV s⁻¹ for the 1/GF, 2/GF and 3/GF electrodes. At 5 mV s⁻¹, the capacitive contributions of 1/GF, 2/GF and 3/GF were 87%, 19% and 53% which increased to 90%, 23% and 59% at 8 mV s⁻¹. This shift in the capacitive contribution indicates greater dominance of capacitive-controlled pseudo-capacitance at higher scan rates due to improved charge transfer kinetics and enhanced ion transport. The trend is depicted in the bar graph shown in Fig. 5d and Fig. S4d, S5d. To evaluate the long-term charge–discharge cycling stability of 1/GF, galvanostatic charge–discharge (GCD) measurements were carried out at a scan rate of 10.0 mV s⁻¹ for 2000 continuous cycles. After cycling, the 1/GF electrode retained 84.0% of its initial specific capacity (Fig. 5f). The 2/GF and 3/GF electrodes retained 99.0% and 94.8% of their initial specific capacitance, respectively (Fig. S6).

Overall, the ferrocenyl conjugate 3/GF electrode delivers the highest specific capacitance of 160.8 F g⁻¹ at 0.5 A g⁻¹ due to combined capacitive and diffusion-controlled charge storage. In comparison, 1/GF shows stable capacitive behavior, while 2/GF exhibits diffusion-limited redox activity with lower rate performance. A comprehensive comparison of discharge specific capacitance of 1/GF and 3/GF with different electrode materials reported in the literature is provided in Table S3.^103–108

Theoretical modeling

The density functional theory calculations were performed at the B3LYP/6-31G** level to investigate the electronic and structural properties of conjugates 1–3.¹⁰⁹ The HOMOs, LUMOs, and optimized geometry of 1–3 are shown in Fig. 7a. In ferrocenyl conjugate 1, the HOMO is predominantly situated on the phenothiazine moiety, while the LUMO is spread across the phenyl linker and extends toward the ferrocene fragment. Similarly, in conjugate 2, the HOMO remains largely localized on the phenothiazine core and shows extension toward the phenyl-ferrocene unit attached at the 3rd position of phenothiazine. Meanwhile, the LUMO is distributed across the phenyl spacer and extends toward the ferrocene group. In conjugate 3, the HOMO is primarily localized on the phenothiazine core and extends toward both the phenyl spacers and the ferrocenyl units attached at the 3rd and 7th positions of phenothiazine. The LUMO is predominantly distributed over the phenyl linkers bridging the N-position of phenothiazine and the ferrocenyl groups, with further delocalization extending onto the phenothiazine and ferrocenyl moieties.^110,111

Fig. 7 (a) Frontier HOMO and LUMO orbitals and optimized ground-state geometry obtained by DFT calculations for 1–3. (b) Energy level diagram of the frontier orbitals of 1–3 estimated by DFT calculations.

The addition of a ferrocenyl group, along with a phenyl substituent at the 3rd position of the phenothiazine ring, contributes to a further reduction in the energy gap. Computational calculations reveal that the HOMO energy levels for compounds 1–3 are −4.92 eV, −4.84 eV, and −4.71 eV, while the corresponding LUMO levels are −1.19 eV, −1.23 eV, and −1.23 eV. The corresponding HOMO and LUMO energy gaps are 3.73, 3.61, and 3.55 eV, respectively, as illustrated in Fig. 7b. The numerical values of the energy levels corresponding to HOMO−3, HOMO−2, HOMO−1, HOMO, LUMO, LUMO+1, LUMO+2, and LUMO+3 are summarized in Table S1.

TD-DFT (time-dependent density functional theory) calculations were performed for the optimized 1–3 at the B3LYP/6-31G(d,p) level to analyze the electronic transitions related to their absorption spectra. The contribution of the molecular orbitals in the UV/vis absorption spectra was determined using their oscillator strength (f). Fig. 8 depicts the comparison between the experimental and theoretical UV-vis spectra of 3, whereas the corresponding spectra for 1 and 2 are shown in Fig. S3. Table 3 provides a summary of the electronic transitions, including their compositions, oscillator strengths, and corresponding assignments. TDDFT calculations for compounds 1–3 indicate strong absorption features primarily arising from π–π* transitions. These transitions highlight the role of the ferrocenyl and phenothiazine units in modulating the electronic properties of the molecules. Notably, the computational data are in good agreement with the experimental observations.

Fig. 8 TDDFT-predicted (blue) and experimental (pink) UV-vis absorption spectra of 3 in DCM.

Table 3 Theoretical data of ferrocenyl functionalized phenothiazine conjugates 1–3

Compounds	Wavelength (nm)	Composition and molecular contribution	f	Assignment
a Oscillator strength (H = HOMO and L = LUMO).
1	468	H−2 → L+7 (0.38)	0.0035	ICT
	361	H−1 → L (0.35)	0.0265	π–π*
	327	H−1 → L (0.50)	0.2431	π–π*
	393	H−3 → L (0.66)	0.9442	π–π*
	287	H → L+5 (0.53)	0.0679	π–π*
2	469	H−4 → L+9 (0.38)	0.0043	ICT
	378	H → L (0.66)	0.0734	π–π*
	362	H → L+1 (0.55)	0.2599	π–π*
	329	H−1 → L (0.44)	0.3606	π–π*
	327	H → L+2 (0.57)	0.1919	π–π*
3	355	H−5 → L+13 (0.19)	0.0011	π–π*
	331	H → L (0.53)	0.6590	π–π*
	282	H → L+2 (0.46)	0.6616	π–π*

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Single crystal X-ray analysis

The red needle crystals of conjugate 1 were grown through the slow evaporation process using a mixed solution of dichloromethane and hexane at room temperature (Fig. 9). The crystallographic details and refinement parameters are summarized in Table 4. Ferrocenyl conjugate 1 crystallizes in the triclinic crystal system with a space group of P1̄. The single-crystal structure analysis of 1 reveals fourteen different intramolecular interactions distributed among seven distinct units of 1 (Fig. S8a). These 14 intermolecular interactions involve seven distinct units, contributing to the formation of sheet-like structural arrangements with layer-by-layer stacking (Fig. S10).

Fig. 9 Crystal structure of the ferrocenyl functionalized phenothiazine 1.

Table 4 Crystal structure data and structural refinement data for 1

Identification code	1
Empirical formula	C34H25FeNS
Formula weight	535.46
Temperature/K	293(2)
Crystal system	Triclinic
Space group	P1̄
a/(Å)	9.66270 (10)
b/(Å)	9.70060 (10)
c/(Å)	14.0506 (2)
α/(deg)	80.7940 (10)
β/(deg)	77.6050 (10)
γ/(deg)	85.9100(10)
Volume/(Å)^3	1268.82 (3)
Z	2
D x (Mg m^−3)	1.402
F(000)	556.0
μ (mm^−1)	0.701
Θ range for data collection (deg)	0.3 × 0.3 × 0.2
Limiting indices	Mo Kα (λ = 0.71073)
Reflections collected	4.256 to 54.754
Unique reflections	−12 ≤ h ≤ 12, −12 ≤ k ≤ 12, −18 ≤ l ≤ 18
R (int)	40 766
Completeness to θ	5493 [Rint = 0.0397, Rsigma = 0.0277]
Data/restraints/parameters	5493/0/334
GOF on F^2	1.081
R 1 and R2 [I > 2σ(I)]	R 1 = 0.0400, wR2 = 0.1174
R 1 and R2 (all data)	R 1 = 0.0596, wR2 = 0.1267
Largest diff. peak and hole (e A^−3)	0.28/−0.36
CCDC no.	2433585

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The sulfur atom of the phenothiazine moiety exhibits a specific interaction with the hydrogen atom of the neighbouring carbon of the thiazine ring, with a bond length of 2.993 Å. The ferrocene unit displays four significant interactions, while the phenothiazine moiety demonstrates eight. The phenothiazine (PTZ) unit of 1 is sandwiched between two ferrocene moieties, whereas the ferrocene unit is similarly sandwiched between two phenothiazine moieties in an antiparallel configuration.

The crystal packing analysis of 1 demonstrated a complex network of intermolecular interactions, which are pivotal to its solid-state organization. These include C–H⋯π, C–H, H–C, and S–H hydrogen bonding interactions, contributing significantly to the stabilization of the crystal lattice. Non-covalent interactions contribute notably to the overall stability of the molecular structure. Among them, C–H⋯π interactions illustrated in Fig. S7a originate from hydrogen atoms on the phenothiazine, ferrocene, and various phenyl linkers engaging with nearby aromatic rings. In addition, an S–H type hydrogen bonding interaction plays a supportive role by fostering directional contacts between hydrogen and sulfur atoms, thereby improving crystal packing (Fig. S7b). Fig. S8b also highlights the range of torsion angles observed between the phenyl, phenothiazine and ferrocenyl segments in conjugate 1, illustrating their distinct spatial orientations within the molecule.

A comparison between the bond lengths obtained from the DFT optimized structure and those observed in the single-crystal data of ferrocenyl–phenothiazine conjugate 1 shows minor differences (Fig. S9). The comparison between theoretical and experimental data for compound 1 reveals a strong correlation in bond lengths. Specifically, the C–S bonds within the phenothiazine core show minor discrepancies, while the C–N bonds remain nearly equivalent. Additionally, the calculated bond angles align well with those obtained from single-crystal X-ray analysis, as shown in Fig. S9. Notably, the experimentally observed C–S–C angles are slightly larger than those predicted by theoretical models. A similar trend was observed for other bond angles within the phenothiazine framework, which are marginally broader than the corresponding computed values.

Conclusion

In summary, we have designed and synthesized mono-, di-, and tri-ferrocenyl phenothiazine conjugates 1–3. The enhanced conjugation in the di- and tri-ferrocenyl derivatives 2 and 3 resulted in a bathochromic shift compared to the monofunctionalized analogue 1. The mono-ferrocenyl conjugate 1 shows greater thermal stability than the di- and tri-ferrocenyl conjugates 2 and 3. The cyclic voltammetry analysis shows that the ferrocenyl conjugates 1–3 exhibit two reversible oxidation waves. Computational calculations show that the HOMO predominantly resides on the phenothiazine unit, while the LUMO is primarily localized on the phenyl spacer as well as central phenothiazine in all ferrocenyl conjugates 1–3. The electrochemical performance of ferrocenyl conjugates 1–3 was evaluated by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) using graphite foil (GF) electrodes. The ferrocenyl conjugate 3/GF delivers the highest specific capacitance of 160.8 F g⁻¹ at 0.5 A g⁻¹ through a combination of capacitive and diffusion-controlled charge storage. The 1/GF electrode follows a capacitive-controlled mechanism and retains 84% of its capacitance after 2000 cycles. The 2/GF electrode exhibits charge storage dominated by redox processes with slow ion diffusion, which limits its rate performance. Overall, these ferrocenyl conjugates demonstrate moderate energy storage capability with specific capacitance values comparable to those of other molecular redox-active systems. The redox versatility and tunable electronic properties of ferrocenyl functionalized phenothiazine materials make them promising candidates for optoelectronic devices.

Experimental details

Chemicals were used as received unless otherwise indicated. All the oxygen- or moisture-sensitive reactions were carried out under an argon atmosphere, and the reflux reactions were performed in an oil bath. ¹H NMR (400 MHz or 500 MHz) spectra were recorded on a Bruker 400 MHz FT-NMR or a Bruker 500 MHz FT-NMR spectrometer at room temperature. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) downfield from tetramethylsilane (TMS) using the residual protonated solvent as an internal standard {CDCl₃, 7.26 ppm}. The multiplicities are given as s (singlet), d (doublet), t (triplet) and m (multiplet) and the coupling constant, J, is given in hertz. ¹³C NMR (100 MHz and 125 MHz) spectra were recorded on a Bruker 400 MHz FT-NMR or a Bruker 500 MHz FT-NMR spectrometer at room temperature. Chemical shifts are reported in delta (δ) units, expressed in parts per million (ppm) downfield from TMS using the solvent as an internal standard {CDCl₃, 77.16 ppm}. Thermogravimetric analysis was performed on a Mettler Toledo thermal analysis system. UV-visible absorption spectra of all compounds were recorded on a PerkinElmer Lambda 35 instrument in the DCM solution. All the measurements were carried out at 25 °C. HRMS spectra were recorded on a Bruker-Daltonics micrOTOF-Q II mass spectrometer. The cyclic and differential pulse voltammograms (CVs and DPVs) were recorded on a PalmSens 4 electrochemical analyzer in the DCM solvent using glassy carbon as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode. The scan rate was 100 mV s⁻¹ for CV. A solution of tetrabutylammonium hexafluorophosphate (TBAPF₆) in DCM (0.1 M) was used as the supporting electrolyte. DFT calculations were performed using the B3LYP/6-31G(d,p) (B3LYP functional with the 6-31G(d,p) basis set) for C, H, S, O, and N atoms and the LanL2DZ basis set for the Fe atom. Additionally, time-dependent DFT (TD-DFT) calculations were conducted at the B3LYP/6-31G(d,p) level on the optimized structures in dichloromethane. The working electrodes 1/GF, 2/GF and 3/GF were fabricated by preparing a uniform slurry composed of 1, 2 and 3 (active material), carbon black (conductive additive) and a poly(vinylidene fluoride) (PVDF) binder in a weight proportion of 70 : 20 : 10. The mixture was carefully ground to achieve a fine, homogeneous blend and subsequently dispersed in N-methyl-2-pyrrolidone (NMP) as the solvent. The resulting slurry was uniformly applied onto a graphite foil (GF) substrate with a surface area of 1 × 1 cm². The coated electrode was then placed in a hot-air oven at 80 °C and dried for 12 h to ensure consistent film formation and complete solvent evaporation. The final electrode contained approximately 1.8 (±0.2) mg of the active material, forming a stable and well-adhered coating suitable for electrochemical studies. Synthesis and characterization of 1

A mixture of PTZ-Ph-Br (0.100 g, 0.28 mmol), Fc-BPin (0.130 g, 0.33 mmol), Pd(PPh₃)₄ (0.016 g, 0.0014 mmol), and sodium carbonate (0.089 g, 0.84 mmol) was dissolved in THF/water (4/1 v/v) and purged for 10 min. The reaction mixture was then heated to reflux for 16 h under a nitrogen atmosphere. After the completion of the reaction, the reaction mixture was allowed to cool down to room temperature and poured into water. The organic layer was extracted with dichloromethane, and the combined organic layers were washed with saturated brine solution and water and dried over anhydrous sodium sulphate. After filtration and solvent evaporation, the residue was purified by silica gel column chromatography using hexane/dichloromethane (4/1 v/v) as an eluent. A yellow solid of 1 (0.124 g) was obtained in 82% yield. ¹H NMR (500 MHz, CDCl₃) δ: 4.17 (br. s., 5 H), 4.48 (br. s., 2 H), 4.83 (br. s., 2 H), 6.32 (m, J = 8.09 Hz, 2 H), 6.82 (t, J = 7.25 Hz, 2 H), 6.88 (t, J = 7.17 Hz, 2 H), 7.04 (m, J = 7.48 Hz, 2 H), 7.45 (d, J = 8.09 Hz, 2 H), 7.51 (m, J = 7.63 Hz, 2 H), 7.59 (d, J = 7.17 Hz, 2 H), 7.82 (m, J = 8.24 Hz, 2 H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ: 144.3, 140.6, 140.0, 139.1, 131.0, 129.0, 127.0, 126.9, 126.8, 126.7, 122.6, 120.5, 116.3, 82.7, 70.6, 67.1 ppm; HRMS (ESI-TOF) m/z [M]⁺ calculated for C₃₄H₂₅FeNS 535.1052, measured 535.1053.

Synthesis and characterization of 2

A mixture of 3-bromo-10-(4-bromophenyl)-10H-phenothiazine (PTZ-Br₂) (0.100 g, 0.23 mmol), Fc-BPin (0.190 g, 0.50 mmol), Pd(PPh₃)₄ (0.026 g, 0.0023 mmol), and sodium carbonate (0.146 g, 0.13 mmol) was dissolved in THF/water (4/1 v/v) and purged for 10 min. The reaction mixture was then heated to reflux for 16 h under a nitrogen atmosphere. After the completion of the reaction, the reaction mixture was allowed to cool down to room temperature and poured into water. The organic layer was extracted with dichloromethane, and the combined organic layers were washed with saturated brine solution and water and dried over anhydrous sodium sulphate. After filtration and solvent evaporation, the residue was purified by silica gel column chromatography using hexane/dichloromethane (1/1 v/v) as an eluent. A yellow solid of 2 (0.108 g) was obtained in 62% yield. ¹H NMR (500 MHz, CDCl₃) δ: 4.06 (s., 5 H), 4.09 (s., 5 H), 4.33 (br. s., 2 H), 4.37 (br. s., 2 H), 4.66 (br. s., 2 H), 4.72 (br. s., 2 H), 6.32 (d, J = 8.10 Hz, 1 H), 6.36 (d, J = 8.54 Hz, 1 H), 6.84 (t, J = 7.32 Hz, 1 H), 6.89 (t, J = 7.71 Hz, 1 H), 7.06 (d, J = 7.32 Hz, 1 H), 7.12 (d, J = 8.70 Hz, 1 H), 7.30 (s., 1 H), 7.41 (d, J = 8.09 Hz, 2 H), 7.49 (d, J = 8.24 Hz, 4 H), 7.59–7.64 (m, 4 H), 7.87 (d, J = 8.24 Hz, 2 H) ppm; ¹³C NMR (126 MHz, CDCl₃) δ: 144.1, 143.3, 143.3, 140.8, 139.9, 139.1, 138.1, 137.4, 137.2, 135.3, 131.1, 129.0, 127.0, 126.9, 126.8, 126.6, 126.5, 126.2, 125.1, 124.8, 122.6, 120.7, 120.0, 116.4, 116.2, 84.7, 69.7, 69.7, 69.2, 69.1, 66.6, 66.5 ppm; HRMS (ESI-TOF) m/z [M]⁺ calculated for C₅₀H₃₇Fe₂NS 795.1342, measured 795.1344.

Synthesis and characterization of 3

A mixture of 3,7-dibromo-10-(4-bromophenyl)-10H-phenothiazine (PTZ-Br₃) (0.100 g, 0.23 mmol), Fc-BPin (0.285 g, 0.75 mmol), Pd(PPh₃)₄ (0.039 g, 0.0034 mmol), and sodium carbonate (0.219 g, 0.20 mmol) was dissolved in THF/water (4/1 v/v) and purged for 10 min. The reaction mixture was then heated to reflux for 16 h under a nitrogen atmosphere. After the completion of the reaction, the reaction mixture was allowed to cool down to room temperature and poured into water. The organic layer was extracted with dichloromethane, and the combined organic layers were washed with saturated brine solution and water and dried over anhydrous sodium sulphate. After filtration and solvent evaporation, the residue was purified by silica gel column chromatography using hexane/dichloromethane (1/1 v/v) as an eluent. A yellow solid of 3 (0.110 g) was obtained in 73% yield. ¹H NMR, ¹³C NMR and HRMS data are given in ref. 89.

Conflicts of interest

There are no conflicts to declare.

Data availability

Experimental details, UV-visible spectra, TGA, CV and DPV results, DFT and TDDFT calculations, electrochemical performance of 1/GF, 2/GF, and 3/GF in a three-electrode configuration, FTIR results, and single crystal data are provided in the manuscript.

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: theoretical calculations, single crystal X-ray analysis, FTIR, electrochemical performance, ¹H NMR, ¹³C NMR, and HRMS data. See DOI: https://doi.org/10.1039/d5nj04004c.

CCDC 2433585 (1) contains the supplementary crystallographic data for this paper.¹¹²

Acknowledgements

We acknowledge the support of the Council of Scientific and Industrial Research (Project No. 01/3112/23/EMR-II) and the Science and Engineering Research Board (SERB) projects CRG/2022/000023 and STR/2022/000001, New Delhi. We are grateful to the DST-FIST grant for the 500 MHz NMR facility and the Sophisticated Instrumentation Centre (SIC), Indian Institute of Technology (IIT) Indore. N. J. T. is thankful to CSIR (File No. 09/1022(0090)2020-EMR-I), India for the research fellowship. D. G. and V. K. are thankful to IIT Indore for the research fellowship.

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