Visible light induced hydrogen production over thiophenothiazine-based dye sensitized TiO₂ photocatalyst in neutral water

Abstract

In this study, we report the design and synthesis of a series of organic sensitizers, namely, (E)-3-(10-butyl-8-(methylthio)-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (UP1), (E)-3-(10-pentyl-8-(methylthio)-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (UP2) and (E)-3-(10-hexyl-8-(methylthio)-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (UP3), and their application for photocatalytic hydrogen production. The as-prepared sensitizers show pronounced light harvesting capability under a wide visible region (400–650 nm) and exhibit relatively strong fluorescence at 625–628 nm with lifetimes of 0.59–0.62 ns. The extinction coefficient of the dyes increases by increasing the different substituents of methine chain on the nitrogen of the thiophenothiazine ring. Alkyl substitution in the sensitizers led to desirable electrochemical behaviour, which facilitates the effective electron injection from the dye to TiO₂. The resulting thiophenothiazine@TiO₂–Pt composites exhibited high hydrogen production efficiency from water splitting at neutral conditions. The UP3 sensitized Pt–TiO₂ (UP3@PT) photocatalyst showed hydrogen evolution up to 1048 μmol (TON 1397) with an excellent apparent quantum yield, ∼50%, which is much higher than the other photocatalysts such as UP1@PT, UP2@PT and pure TiO₂. Operational parameters, such as the effects of the substitution in dye molecules, dye concentration, activation temperature of TiO₂ and pH of the reaction medium, were explored. Theoretical studies and experimental measurements corroborated that, the addition of methine unit in the sensitizer enhanced the efficiency in dye@TiO₂ composite via reduced charge recombination and increased light capture.

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Introduction

To address the increasing global demand for energy and to reduce the depletion of fossil fuels, an attractive scientific and technological solution is to convert solar energy into fuel by means of photocatalytic hydrogen evolution via water splitting. Ever since the report on photo-induced water splitting on TiO₂ electrode under UV irradiation by Fujishima and Honda¹ was published, enormous attention has been focused on photocatalytic H₂ evolution over semiconductors.² Among the metal oxide semiconductors reported, TiO₂ is most widely used for solar energy conversion due to its (i) appropriate band structure for water splitting, (ii) significant photostability and (iii) low cost and facile preparation method.³ However, it can only harvest the less abundant UV component of the solar spectrum with low efficiency because of its wide band gap ranged over 3.2–3.02 eV. Therefore, it is of great need to develop effective means to broaden the light absorption region and to improve the charge separation efficiency of the catalyst.

Dye sensitization is a promising route to enable the effective utilization of visible-light for photo-induced hydrogen production.⁴ In contrast, organic dyes are of core interest for acquiring efficient light-harvesting capability with many advantageous characteristics over noble metal complexes. These characteristics include (i) high molar extinction coefficient, (ii) diversity of molecular structures, (iii) diverse way to design donor–π–bridge–acceptor (D–π–A), (iv) the availability of versatile functional molecules associated with the tuning of the electronic and chemical properties,⁵ and (v) facile synthesis as well as low cost and environmentally friendly. Recently, we have reported impressive hydrogen production from water splitting using organic dye sensitized TiO₂ photocatalyst.⁶

Phenothiazines are well known photosensitizers with electron-rich amine and sulphide moieties. The interesting aspect of this class of compound is the intriguing stability of the cationic radical formed by the oxidation of one electron.⁷ Thus, it can be anticipated that phenothiazine based sensitizers possess significant stability in photo-induced electron transfer from the dye to TiO₂. Based on these characteristics, diverse redox devices have been developed for efficient photon energy conversions.^8,9 However, the changes in optical and electrochemical behaviour due to the introduction of an alkyl chain on the nitrogen atom of the thiophenothiazine core and its effect in sensitized H₂ generation is still under sporadic attention.

Herein, we have selected thiophenothiazine as an electron donor on the basis of the following reasons: (i) the heterocyclic compound contains electron-releasing nitrogen and sulphur heteroatoms, (ii) the phenothiazine ring is nonplanar and therefore can inhibit both the molecular aggregation and the formation of intermolecular excimers.¹⁰ Moreover, the use of alkyl chains in the molecular design of these new dyes is not only beneficial to suppress dye aggregation, but can also help to prevent the fast charge recombination of the conduction band (CB) of TiO₂ and oxidised dye. The present study reports the synthesis of three new organic sensitizers derived from the systematic N-alkylation of thiophenothiazine, and its application in highly efficient solar-induced photocatalytic H₂ evolution in the presence of triethanolamine (TEOA) as a sacrificial electron donor (Fig. 1). The changes in the optical and electrochemical properties were evaluated with the change in alkyl chain length. It was also found that the H₂ production efficiency is the function of different operational parameters, namely, concentration of dye, activation temperature of TiO₂ and pH of the reaction medium.

Fig. 1 Illustration of the visible-light-driven water splitting of thiophenothiazine dye sensitized TiO₂.

Experimental section

Materials and reagents

In the present study, all chemicals used were of analytical grade and were purchased from commercial sources. Importantly, 2-methylthiophenothiazine and 1-bromoalkane were purchased from Sigma-Aldrich, USA, and used without further purification. Solvents used as reaction media were purchased from local sources and used after distillation. The used hexane (Hex) refers to the fraction boiling in the range of 60–70 °C. Reactions were monitored using analytical TLC plates (Merck, silica gel 60 F254, 0.25 mm) and compounds were resolved with ultraviolet light. Silica gel (100–120 mesh) was used for column chromatography.

Synthesis

The synthetic approach for the targeted dyes is shown in Scheme 1. N-Alkylation of the 2-methylthiophenothiazine followed the Vilsmeier–Haack formylation with POCl₃ and N,N-dimethyl formamide (DMF), which resulted in an aldehyde intermediate. Knoevenagel condensation with cyanoacetic acid in the presence of piperidine resulted in the production of 4, 5 and 6 as sensitizers UP1 (R = N butyl), UP2 (R = N pentyl) and UP3 (R = N hexyl) (80% yield), respectively. The crude products were purified by column chromatography.

Scheme 1 Synthesis of UP1, UP2 and UP3 dyes. [i] R-Br, KOH, KI, DMSO, rt, 5 h; [ii] POCl₃, DMF, 80 °C, 3 h; and [iii] cyanoacetic acid, piperidine, reflux, 8 h.

Preparation of Pt–TiO₂ (PT)

To improve the H₂ production efficiency, 1 wt% Pt was deposited onto the surface of commercial TiO₂ (cTiO₂) using the previously reported method.^5e In a 100 ml round-bottomed flask, TiO₂ (1.0 g, Sigma-Aldrich, anatase) was dispersed in 25 ml methanol. Thereafter, aqueous solution of H₂PtCl₆ (0.25 ml, 8 wt% aqueous solution) was added into the methanolic suspension of TiO₂ and the reaction mixture was irradiated by a 400 W xenon lamp for 30 minutes. The resultant light gray colour Pt–TiO₂ composite was retrieved by centrifugation, washed three times with excess methanol and dried under vacuum at 60–70 °C.

Adsorption of dye molecules on the semiconductor surface

Dyes were separately dissolved in acetonitrile–ethanol solution (1 : 1 v/v, 10 ml).^5a Then, cTiO₂ was dispersed into the resulting solution and the mixture was continuously stirred at room temperature in the dark for 24 hours. After completion of the reaction, the mixture was filtered, washed with ethanol and then dried in air. These final composites were denoted as UP1@PT, UP2@PT and UP3@PT. The reference dyes were also adsorbed on TiO₂ by a similar method and labelled EY@PT and N719@PT.

Photocatalytic experiments

Photocatalytic H₂ generation experiments were carried out in a doubly jacketed Pyrex glass reactor with a flat optical window and external cooling jacket. All the experiments were carried over 20 ml aqueous suspension of 10 mg photocatalyst containing 10 vol% of TEOA as the sacrificial electron donor (SED). The solution was adjusted to the desired pH using 1 M hydrochloric acid. It was then air sealed with a rubber septum. Before light irradiation, dissolved air was removed by 20 min high vacuum followed by purging with Ar gas. A 450 W xenon arc lamp (New Port, USA and working at 400 W) using cutoff filter (λ > 400 nm) was used for irradiation on the suspended photocatalyst of the aqueous reaction mixture in the photoreactor, which was kept under constant stirring during the reaction. The resulting evolved gases were analyzed by gas chromatography using a Perkin Elmer Clarus 580 GC equipped with molecular sieve 5 Å column, thermal conductivity detector (TCD) and argon as the carrier gas.

The apparent quantum efficiency (AQE) was estimated by the following eqn (1)

The average photon flux of the incident light was determined on an optical power/energy meter, New Port, model: 842 PE. All the photocatalytic hydrogen generation experiments were performed under the simulated solar light illumination of intensity ∼2.0 Sun (1.95 W cm⁻²).

Results and discussion

Characterization

The characterization techniques used in this study are powder X-ray diffraction (XRD) for determination of phase composition, and transmission electron microscopy (TEM) for morphological and structural studies. The optical properties of the dyes were explained by UV-vis, fluorescence and time dependent photoluminescence spectroscopy. Electrochemical properties were studied by cyclic voltammetry (CV); moreover, optical properties of dye@TiO₂ composites were measured by diffuse reflectance spectroscopy (DRS). Dye@TiO₂ attachments and surface interactions were affirmed by Fourier transform infrared (FTIR) spectroscopic studies. Moreover, the as-synthesized dyes (UP-series) were fully characterized by NMR, ESI-MS and FTIR, the results of which have been incorporated in ESI.†

Physical properties

The absorption spectra of the as-synthesized sensitizers in a 2.5 × 10⁻⁴ M methanolic solution are depicted in Fig. 2 and corresponding data are collected in Table 1. Owing to their similar structures, all three dyes, UP1, UP2 and UP3, showed quite similar broad absorption over the range of 420–460 nm (Fig. 2a) in the wide visible region and they appeared deep orange to red in colour. The strong absorption was due to the intra-molecular charge transfer (ICT) transition from thiophenothiazine donor to the cyanoacrylic acid acceptor moiety.¹¹ The high electron donating hexyl moiety may lead to a lower energy ICT transition, which is attributable to the red shifted absorption maxima in UP3. The slightly blue shifted absorption maxima of UP2 could be ascribed to the intermolecular aggregation of the dye. Such phenomenon has been found to be common in many other organic sensitizers.¹² The molar extinction coefficient, ε, of the synthesized dyes ranged from 14 000, 16 000 and 16 080 M⁻¹ cm⁻¹ for UP1, UP2 and UP3, respectively. The red shift in the absorption spectrum and higher molar extinction coefficient could increase the light harvesting ability and enhance the photocatalytic activity of hydrogen generation.

Fig. 2 (a) Absorption spectra. (b) Emission spectra. (c) Fluorescence lifetime spectra of UP1, UP2 and UP3 dyes in methanol (2.5 × 10⁻⁴ M). (d) DRS of cTiO₂, UP1@T, UP2@T and UP3@T.

Table 1 Optical and electrochemical properties of the dyes

Dye	λmax^a (nm)/εmax (M^−1 cm^−1)	λem^b (nm)	λmax^c (nm) (TiO2)	τ^d (ns)	E0–0^e (eV)	Eox^f (V)	HOMO^g (eV)	LUMO^h (eV)
a Absorption.b Emission spectra were measured in CH3OH solution (2.5 × 10^−4 M) at room temperature.c Absorption spectra of TiO2 (DRS) were obtained by measuring the dye adsorbed on the TiO2 surface.d Lifetime decay of dyes.e E0–0 was estimated from the transition energy measured at the onset of absorption spectra.f The oxidation potential of the dyes in CH3CN with 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) as an electrolyte.g HOMO value calculated using potential value of oxidative waves.h LUMO value was calculated by HOMO + E0–0.
UP1	433/14 000	625	471	0.62	2.85	2.25	−5.97	−3.12
UP2	425/16 000	626	481	0.59	2.91	1.56	−5.96	−3.05
UP3	455/16 800	628	489	0.61	2.76	1.76	−6.15	−3.43

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The emission spectra of the dyes were recorded in methanol and are incorporated in Fig. 2b. The wavelength of the emission peaks was not strongly affected by modifying the alkyl groups, but UP3 was slightly red shifted in comparison with UP1 and UP2, which possibly originated from the longer chain length in UP3. The fluorescence lifetime spectra of the sensitized dyes ranged from 0.59 to 0.62 ns (Fig. 2c and Table 1) and are significant for the stable electron transfer from the excited dye to TiO₂. The DRS result (Fig. 2d) fully complements the UV-vis spectra.¹³ The red shifts coupled with spectral broadening arose due to the electronic interactions between the adsorbed dye and TiO₂.

For efficient electron injection, the excited redox potential of the dye should be more than the CB edge of TiO₂. To evaluate the thermodynamic basis of this electron transfer processes, CV was used to assess the energy levels of the sensitizer. CV of the dyes was carried out in an acetonitrile solution containing 0.1 M tetrabutylammonium tetrafluoroborate (TBABF₄) as the supporting electrolyte (Fig. S4 ESI†). The redox potential of the dyes was measured from their corresponding peak potential and the values are incorporated in Table 1. The calculated LUMO values are −3.67 eV, −3.11 eV and −3.43 eV, which are below the vacuum level for UP1, UP2 and UP3, respectively, and are suitable for the spontaneous electron injection from the excited dye molecule to CB of TiO₂.¹⁴ A quasi reversible wave was observed during the cathodic oxidation at around 0.7 V, which may be attributed to the formation of dye/dye⁺ couple and is consistent with the typical redox process of the phenothiazine core.¹⁵ Moreover, in the negative potential region, the electron transfer follows quite similar trends for all the dyes, though the alkyl substitution has significant effects on the optical and electrochemical properties.

The surface interactions between the dye and TiO₂ have been clarified on the basis of FTIR studies. Significant changes in peak intensity and peak position were detected. This indicates the strong attachment of the carboxylate groups of the dyes onto the TiO₂ surface. The peaks at 1622, 1668 and 1677 cm⁻¹ for –C=O were found in UP1, UP2 and UP3 dyes, respectively, and they disappeared after being sensitized on the TiO₂ surface. It is interesting to note that peaks at 1604, 1606 and 1633 cm⁻¹ for the O–C–O group appeared for dye/TiO₂ composites as UP1@T, UP2@T and UP3@T, respectively, which corroborates the formation of the ester-like linkage of the carboxyl group between the dyes and TiO₂. The corresponding results have been incorporated in ESI (Table S1 and Fig. S1†).

XRD was carried out to investigate the changes in the phase composition and crystallinity of TiO₂ nanoparticles on calcination at different temperatures over the range of 450–750 °C. As shown in Fig. 3, 2θ value of 25.2° corresponds to the (101) plane of anatase phase, which becomes more intense due to increasing crystallinity with temperature. Pristine TiO₂ showed a slight amorphous nature; however, at 450 °C, a pure crystalline anatase phase was obtained. No significant change was observed in the XRD pattern when the temperature was increased from 450 to 550 °C. Rutile peaks were not found in the TiO₂ activated at 550 °C, which may due to the lower concentrations of the corresponding phase in the bulk composite.^16a A complete transformation from anatase to rutile was observed at 750 °C (details in ESI†). As shown in Fig. 4a and b, the FESEM images of Pt–TiO₂ composite exhibited an irregular-shaped morphology.^16b It can be envisioned from Fig. 4c that the black spots of Pt are visible on TiO₂ surface. In Fig. 4d, the lattice distances of 0.341 nm and 0.228 nm correspond to the (101) and (111) planes of anatase TiO₂ and Pt, respectively.¹⁷ This confirms the successful deposition of Pt onto the TiO₂ surface.

Fig. 3 PXRD patterns of cTiO₂ at different temperatures.

Fig. 4 (a and b) FESEM images (c) TEM image and (d) HRTEM of Pt–TiO₂.

Water splitting reaction

To assess the performance of these dyes for photocatalytic water splitting reactions, the H₂ production of dye@Pt–TiO₂ system containing dyes UP1–UP3 was investigated in 20 ml TEOA–water mixture (1 : 9). Moreover, to optimize the dye concentration and pH of the medium, a series of experiments were carried out and the results are incorporated in Fig. 5. Dye concentrations were screened between 0.25 and 2 μmol/10 mg Pt–TiO₂. The dye adsorption on the Pt–TiO₂ particle was complete because the filtrates obtained after the adsorption treatment were transparent. The catalytic activity increases with increasing amount of the dye and reaches the maximum at a dye amount of 1.5 μmol/10 mg Pt–TiO₂ and decreases at 2 μmol/10 mg Pt–TiO₂ (Fig. 5a). Typically, the fraction of incident light absorbed by the dye increases with increasing loading of the dye and saturates at a certain level, because in general, a higher sensitization effect is expected at higher surface concentrations. However, excessive surface concentration of dyes, particularly those with carboxylic acid groups as an anchor, is found to decrease the sensitization effect. Above this concentration, the photocatalytic activity starts to decrease primarily due to the agglomerating nature of the dyes¹⁸ and later on due to the reduction of the penetration depth of incident light.¹³ A similar deactivation appears to occur in UP1 and UP2 for photocatalytic hydrogen evolution.

Fig. 5 Photocatalytic activities of the UP1@T, UP2@PT and UP3@PT composites with (a) different concentrations, pH = 7.0 and (b) different pH (4.0, 7.0 and 10.0) for water splitting. Reaction conditions: 20 ml of 10 vol% aqueous TEOA (2 ml) solution, 10 mg catalyst.

For the photocatalytic hydrogen evolution, the adsorption of the dye molecule onto the TiO₂ surface is required. It provides the electron-transfer path from the dye to semiconductor surface.¹⁹ FTIR results confirmed that the carboxyl group of the dye molecules are fastened on the TiO₂ surface by ester-like linkage (see ESI Fig. S1†). The linkage between the dye and TiO₂ depends on the environmental pH.²⁰ As shown in Fig. 5b, all the dye adsorbed TiO₂ composites exhibited the highest activity of hydrogen evolution at pH 7. The lower photocatalytic activity in acidic and basic medium is primarily due to the hydrolysis of the ester linkage of dye@TiO₂ at pH 4 and 10, respectively. Moreover, at pH 4, the surface titanols were protonated, and at pH 10 the carboxyl groups of the dye were deprotonated. Thus, the ester linkage was not formed effectively. Subsequently, the pH value can also influence the existing state of the sacrificial electron donor as in an acidic solution, the protonation of TEOA occurs, which leads to the slow regeneration of the oxidized dye molecule.²¹

The higher TON values of UP1–UP3 could be due to the stability of the cationic species of the sensitizer, as shown in their corresponding redox behaviour. Moreover, the H₂ production efficiency of the sensitizers was dependent on the alkyl groups on nitrogen. It can be envisioned from the results that UP3 with the longest alkyl chain achieved TON 1397, which was found to be much higher than the H₂ generation observed for reference dye N719 and EY under similar experimental conditions after 10 h irradiation. This observation indicates that the longer alkyl chain plays a key role in the stabilization of oxidized dye species during the photocatalytic process. Recently, it has been reported that the longer alkyl chain increases the efficiency of ruthenium based dye sensitized solar cells.²²

It can be noted that the sensitized H₂ generation is also greatly dependent on the annealing temperature of TiO₂. The photocatalysts wherein TiO₂ was activated at 550 °C showed the maximum activity. In particular, UP1@PT, UP2@PT and UP3@PT achieved TON 860, 1250 and 1397, respectively, after 10 h under the specified reaction conditions (Fig. 6 and Table 2 and Fig. S5 ESI†). As shown in Table 2, UP3@PT showed superior performance over UP1@PT and UP2@PT. The lowering of the photocatalytic activity with the increase in calcination temperature may be due to the large decrease in the specific surface area of TiO₂ (Table 3).^16a In addition, a significant increase in the crystallinity was observed on increasing the calcination temperature (Fig. 3), resulting in a higher probability of charge carrier recombination at the bulk traps. This suggests that a good control of crystallite size is required in order to prevent any charge carrier recombination. Apart from the effect of specific surface area and crystallinity, pure TiO₂ photocatalyst also underwent the anatase-to-rutile phase transformation with increasing activity. Since the rutile phase has a lower flat band potential compared to NHE potential (H⁺/H₂ level) than the anatase phase,¹⁶ this leads to a smaller driving force of the rutile TiO₂ for water reduction to produce hydrogen than the anatase TiO₂.

Fig. 6 (a) Photocatalytic activities of the different catalysts for water splitting, (b) H₂ production yield of different photocatalysts after 6 h. Reaction condition: 20 ml of 10 vol% aqueous TEOA (2 ml) solution, 10 mg catalyst, pH-7.0.

Table 2 Photocatalytic performance of the dyes

Catalyst	Amt. of H2^a (μmol)	TON^b	TOF^c (h^−1)	AQE (%)
a H2 evolution after 10 h.b TON (turn over number) = (2 × amount of H2)/amount of dye after 10 h.c TOF (turn over frequency) values after 10 h. Reaction conditions: 10 mg photocatalyst (1.5 μmol dye) in 20 ml of 10 vol% neutral aqueous TEOA (2 ml) solution.
UP1@PT	645.67	860	86	30
UP2@PT	938.05	1250	125	44
UP3@PT	1048.05	1397	140	49
EY@PT	306.36	408	41	14
N719@PT	121.32	161	16	6

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Table 3 Photocatalytic activity of UP3@PT^a composite

Calcined temperature	H2 yield (μmol)	AQY (%)
a UP3@PT composite (cTiO2 calcined at different temperatures) condition: 10 mg photocatalyst (0.1 μmol dye) in 20 ml of 10 vol% neutral aqueous TEOA (2 ml) solution, irradiation time 10 h under visible light (λ ≥ 400 nm).
cTiO2	322	15
cTiO2 (450 °C)	401	19
cTiO2 (550 °C)	1048	50
cTiO2 (750 °C)	166	8

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Stability of UP3 dye

Furthermore, the stability of UP3@PT composite at optimal experimental conditions has been investigated and tested for six cycles. As shown in Fig. S6 (ESI†), the yield of H₂ evolution is nearly the same in the first and second cycles. Thereafter, it declines in the third cycle and maintains almost the same level up to the 6^th run, wherein the H₂ production yield was 490 μmol. The decreased rate of hydrogen evolution may due to the consumption of SED and the partial decomposition of the dye. Furthermore, from 4–6 run, the composite did not exhibit any significant loss of activity, indicating its prominent stability during the photocatalytic H₂ production. Moreover, in the recyclability reaction at pH 10, the rate of H₂ production significantly decreased after the second run (Fig. S7†) and at the 6^th run, the photoactivity was almost two times lower than that observed at a neutral pH. It is noteworthy that at higher pH, phenothiazine based dyes undergo decomposition or dimerization,²³ which may be inhibited by the alkyl groups attached to nitrogen atom in the phenothiazine ring.

Computational studies

To gain further insight into the differences in the H₂ generation performance of these UP dyes, density functional theory (DFT)²⁴ calculations were performed at B3LYP/DND (dmol3) level for the geometry optimizations. The frontier MOs of these dyes reveals that HOMO–LUMO excitation moves the electron density distribution from the phenothiazine moiety to the cyanoacrylic acid (Fig. 7). Moreover, it has already been established that the alkyl groups on nitrogen can induce a favorable orientation of dyes on TiO₂, which may result in efficient electron injection from excited dyes to TiO₂.

Fig. 7 HOMO and LUMO states of UP1, UP2 and UP3.

Conclusions

To sum up, a series of new and inexpensive donor–acceptor sensitizers containing thiophenothiazine ring were prepared and they showed efficient photocatalytic performances in visible-light-driven water splitting. Increasing the methine unit (–CH₂–) on the nitrogen of the thiophenothiazine ring expands the spectral response of the dye and dye sensitized semiconductor from 430 to 455 nm and 470 to 490 nm, respectively. The as-prepared dyes showed good donating ability and excellent surface protection onto the TiO₂ surface, which led to a good photocatalytic performance and efficient inhibition of charge recombination. Importantly, UP3@PT photocatalyst showed better photophysical properties and photocatalytic activity than UP1@PT, UP2@PT. We significantly achieved high photocatalytic activity in UP3@PT up to 1048 μmol (TON 1397) with an excellent apparent quantum yield, ∼50% after 10 h light irradiation. This study may open up new insights to improve the photocatalytic conversion efficiency and holds promise for the future design of novel and highly efficient photocatalysts.

Acknowledgements

The authors acknowledge the solar energy network project TAPSUN, NWP-56 for the financial support. We also acknowledge the support of Dr D. Chatterjee, HOD, Chemistry & Biomimetics Group and Prof. Dr P. Pal Roy, Director, CSIR-CMERI for endless encouragement in the research. A. Tiwari and I. Mondal acknowledge AcSIR for Ph.D. enrollment.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterization data of new compounds, cyclic voltammograms of dyes. See DOI: 10.1039/c5ra03039k

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