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Detail Photophysical Studies of Sulfonated Polyaniline in Aqueous Medium Rijia Khatun, Koushik Majhi, Venkanna Meriga, Amit Kumar Chakraborty, and Subrata Sinha J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b06640 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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The Journal of Physical Chemistry

Detail

Photophysical

Studies

of

Sulfonated

Polyaniline in Aqueous Medium Rijia Khatun†, Koushik Majhi†, Venkanna Meriga‡, Amit K. Chakraborty‡, and Subrata Sinha*,† †

Integrated Science Education and Research Centre, Siksha Bhavana, Visva-Bharati,

Santiniketan – 731 235, India ‡

Carbon Nanotechnology Laboratory, Department of Physics, National Institute of

Technology Durgapur, M. G. Avenue, Durgapur – 713 209, India

*E-mail: [email protected]

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ABSTRACT: Sulfonated polyaniline (SPANI) has emerged as a promising polymer in the last few decades due to its solubility in water and relatively moderate conductivity. However, till date, literature data on the optical characterization of SPANI are very limited and preliminary in nature. In the present work, SPANI is synthesized by direct sulfonation of emeraldine salt form of polyaniline with chlorosulfonic acid in an inert solvent. Detail photophysical properties of SPANI are investigated in aqueous medium by using steady state (concentration, temperature, pH and excitation wavelength dependence) and time-resolved spectroscopic techniques. The steady state fluorescence emission measurements are carried out carefully to avoid inner filter effect (especially secondary inner filter effect or reabsorption effect) as well as scattering. Two ground state conformations of SPANI are suggested to exist in aqueous medium. Excitation wavelength dependence of the fluorescence emission spectra is attributed to red-edge effect. All these observations are nicely corroborated by the fluorescence lifetime data of SPANI obtained from time-resolved measurements. All these new findings are extremely important in view of the potential applications of SPANI in polymer optoelectronics.


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INTRODUCTION Polyaniline (PANI) has emerged as one of the most widely studied conducting polymers in the last few decades in view of its many interesting properties. It has a mixed oxidation state as it consists of reduced benzenoid units and oxidized quinoid units, the average oxidation state being given by 1 – y (Scheme 1a).1-3 Depending on the average oxidation state of the main chain, PANI can be obtained in different base states (fully reduced leucoemeraldine with 1 – y = 0, partially oxidized emeraldine with 1 – y = 0.5 and fully oxidized pernigraniline with 1 – y = 1).2 Since photoluminescence in PANI is due to the benzenoid unit,1,3 steady state and time-resolved spectroscopic works have been mostly devoted to the emeraldine base and salt forms of PANI.1,3-5 Only few spectroscopic works

2,3,6

are reported

on the leucoemeraldine base state of PANI probably due to its environmental instability.7,8 The half oxidized (1 – y = 0.5) emeraldine base state (Scheme 1b) is a semiconductor, which consists of an alternating sequence of two benzenoid units and one quinoid unit.1,9 The emeraldine base state of PANI can be non-redox doped with acid to produce the conducting emeraldine salt state of PANI (Schemes 1c and 1d, bipolaron and polaron forms, respectively).1,9 This is a reversible process as the emeraldine salt state of PANI can be converted back to emeraldine base state through treatment with a base.

Again, the

conductivity of PANI (in the emeraldine base state) can be tuned by external protonic doping.4 Apart from good electrical conductivity, PANI (emeraldine base and salt states) has drawn attention of many research groups due to its interesting properties like excellent chemical and environmental stability, redox reversibility and ease of synthesis.5 Based on its conducting or semi-conducting nature, PANI has numerous applications in biosensors,10,11 supercapacitors,12 metal-semiconductor devices,13 actuators,14 secondary batteries,15,16 light emitting diodes,17 photovoltaic devices,18 field effect transistors,19 laser printing,20 anticorrosion coating,21-23 etc.



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In spite of its several interesting properties as mentioned above, PANI has some serious disadvantages like insolubility, infusibility and hence poor processability.24 It is to be noted that solubility of PANI in common solvents (especially in water) is of crucial importance for its various applications. Several research groups reported the synthesis of water soluble PANI derivatives using different approaches, most of which are expensive and not suitable for large scale use.25-29 Till date, sulfonation of PANI, in which the emeraldine salt form of PANI is treated with chlorosulfonic acid in an inert solvent, has been the most common method to improve the solubility and processability of PANI in water.4,30-33 The sulfonated PANI (SPANI) prepared in this method becomes water soluble at all pH values. However, due to the presence of strong electron withdrawing sulfonic acid (-SO3H) groups, the conductivity of SPANI reduces to some extent compared to that of pure PANI.34 Nevertheless, many research groups employed this particular method, due to its relative ease and cost-effectiveness, to prepare water soluble SPANI. The SPANI thus prepared has been characterized (mostly material and electrical) by these research groups

4,30,32,35

in view of its

various potential applications. Only a few literature data are available on the optical characterization of SPANI and its several nanocomposites.24,34 Synthesis and characterization of SPANI (sulphuric acid doped) were carried out by Draman et al.,24 who investigated the effects of doping level on the sensitivity of SPANI in solution of dimethylformamide and film form for oxygen gas detection in terms of fluorescence quenching. Meriga et al.34 prepared composite of SPANI with reduced graphene oxide and found some interesting optical properties of the composite. Agarwalla et al.37 prepared nanocomposite of SPANI with embedded single-walled carbon nanotubes/zinc sulphide nanohybrid fibers and observed that the nanocomposite is a luminescent material of enhanced emission intensity in the visible region of the spectra. However, to the best of our knowledge, detail report on the photophysical properties of


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SPANI is missing in the literature. Therefore, it is extremely necessary to understand the detail photophysical properties of SPANI and its fluorescence emission characterization as a function of various parameters in aqueous medium for commercial applications of SPANI in the field of polymer optoelectronics. In the present work, in order to gain better understanding of the photophysical properties of SPANI (Scheme 2, prepared by direct sulfonation of emeraldine salt form of PANI with chlorosulfonic acid in an inert solvent), detail investigations are carried out on SPANI in aqueous medium by using steady state and time-resolved spectroscopic techniques.

EXPERIMENTAL Chemicals.

Aniline hydrocholoride (SRL India), ammonium persulphate (Merck

India), chlorosulfonic acid (Spectrochem India), 1,2-dichloroethane (DCE) (Spectrochem India) and millipore water (Merck India) were used as supplied without further purification. The solvent (millipore water) was tested before use and no impurity emission was detected in the wavelength region studied. Apparatus. All the measurements were carried out at the ambient temperature (300 K) except mentioned otherwise and repeated several times to check the reproducibility of the data. The Fourier transform infrared (FTIR) spectra were recorded by the standard KBr pellet method by using a Shimadzu FTIR-8400S spectrophotometer. The X-ray diffraction (XRD) data were obtained by using Philips PANalytical X-Pert Pro diffractometer. Electron micrograph of SPANI was recorded by using a scanning electron microscope (SEM, Carl Zeiss Ultra 55, UK) equipped with a field emission (FE) gun operating at 5 KV.



The steady state electronic absorption spectra of the samples were measured by using JASCO V-650 absorption spectrophotometer with 1 cm path length rectangular quartz cuvette. The steady state fluorescence emission spectra of the samples were recorded by using JASCO FP-6500 fluorescence spectrometer. Emission was detected at right angles to the direction of excitation light in order to avoid stray light. It should be mentioned that a large overlapping exists between the steady state absorption and fluorescence emission spectra of SPANI (vide infra). Due to this large spectral overlapping, inner filter effects (primary as well as secondary) are expected to play a significant role (especially at high concentrations) in the shape and relative intensity of the fluorescence emission spectra of SPANI. Inner filter effect is basically absorption of excitation (primary) and emission (secondary) photons by nearby ground state fluorophores in the paths of excitation and emission photons during the fluorescence emission measurements.37 Primary inner filter effect simply reduces the excitation intensity and consequently reduces the overall intensity of the fluorescence emission spectra without changing the shape and energy band positions. However, secondary inner filter effect or reabsorption effect usually causes absorption of the emitted photons in the blue side of the fluorescence emission spectra (depending on the spectral overlap region). This often leads to deformation in the fluorescence emission spectra (both shape and energy positions of the bands) along with a reduction of emission intensity (especially in the blue side of the fluorescence emission spectra). To avoid inner filter effect, front face geometry may be employed by using triangular quartz cuvette so that the excitation and emission photons travel negligible paths through the sample solution. However, this leads to a large amount of scattering, which often overlaps with the fluorescence emission spectra. The scattering may be avoided by keeping the cuvette at 30o or 60o w.r.t. the excitation path. But, it is rather


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difficult to maintain the angle of the cuvette exactly same in all the measurements, especially when relative measurements are carried out. Therefore, in the present work, all the fluorescence emission spectra were recorded by taking the sample in the triangular quartz cuvette (diagonal length = √2 cm) in a geometry as shown in scheme 3. Here, primary inner filter effect could not be avoided. However, as already mentioned, primary inner filter effect merely reduces the overall fluorescence emission intensity to some extent and does not change the shape or energy band positions of the fluorescence emission spectra. The advantage of using such cuvette geometry is that secondary inner filter effect (or reabsorption effect) as well as scattering can be almost completely eliminated and the cuvette geometry can be kept exactly same for all the relative measurements. The fluorescence decay curves of the samples in aqueous medium (taken in usual 1 cm rectangular quartz cuvette) were measured by using a time-correlated single photon counting (TCSPC) spectrophotometer (Horiba Jobin Yovin) with pulsed LED of 280 nm, 295 nm and 340 nm with full width at half maximum (FWHM) of ca. 1 ns and repetition rate of 1 MHz. The fluorescence decays were analyzed using DAS-6 analysis software supplied by Horiba Jobin Yovin. The experimentally obtained fluorescence decay curves were expressed as a sum of exponentials (Equation 1) and analyzed by non–linear least–square iterative convolution method based on Lavenberg-Marquardt algorithm.38 I (t ) = ∑ α i exp( −t / τ i )

(1)

i

Here, αi is the amplitude of the ith decay component associated with fluorescence lifetime τi such that ∑αi = 1. The reduced χ2, Durbin-Watson (DW) parameter and residuals were used to judge the goodness of the fit. Synthesis Procedure of SPANI. SPANI was synthesized following procedures as reported by Ito et al.4 and Agrawalla et al.36 Emeraldine salt was synthesized by chemical oxidation of aniline hydrochloride with ammonium persulfate in aqueous medium. 7 ACS Paragon Plus Environment

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amount of 0.25 M of ammonium persulfate was dissolved in 100 mL of distilled water. An amount of 0.2 M aniline hydrochloride was dissolved in 100 mL distilled water.

The

solutions were added and polymerisation was allowed for 24 h. Emeraldine hydrochloride powder was then collected by filtration and was dispersed in 100 mL of DCE, being stirred and heated to 80 °C for 1 h. The chlorosulfonic acid (8 mL) diluted with 16 mL DCE was added drop-wise for 30 minutes into the dispersion liquid and stirred at 80 °C for 1 h. The produced chlorosulfonated polyaniline was separated by filtration, immersed in 50 mL water and heated to 100 oC for 4 h with stirring to promote its hydrolysis. The resulting greenish solution was cooled to the ambient temperature (300 K) and filtered by vacuum filtration washed by acetone. The filtrate was collected and dried in a vacuum desiccator.

RESULTS AND DISCUSSION Material Characterization of SPANI.

The FTIR spectra of SPANI (see

supplementary figure S1) show quinoid band at 1586 cm-1 and benzenoid band at 1505 cm-1.4,39,40 The peaks at 1304 cm-1 and 1216 cm-1 are attributed to the characteristic C-N stretching and C-N.+ stretching vibrations. The peaks at 1178 cm-1 and 1072 cm-1 are assigned to asymmetric and symmetric O=S=O stretching vibrations, respectively and those at 705 cm-1 and 615 cm-1 are assigned to S-O and C-S stretching vibrations, respectively. A peak appeared at 814 cm-1 for the C-H out-of-plane bending vibration of 1,2,4-trisubstituted aromatic rings, suggesting that –SO3H groups are directly attached to the aromatic rings.4,39,40 The XRD pattern of SPANI (see supplementary figure S2) shows two prominent humps at 2θ equal to 25o and 43.5o. No crystalline peak of SPANI was detected as SPANI is amorphous. These data are in good agreement with those reported earlier.36 The FESEM image of SPANI (see supplementary figure S3) shows its granular structure.


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Concentration Dependent Steady State Measurements.

Figure 1a shows the

steady state electronic absorption spectra of SPANI in aqueous medium at different concentrations. The absorption spectra consist of a relatively sharp peak at 320 nm (3.88 eV), a shoulder at around 440 nm (2.82 eV) and a broad band at 650 nm (1.91 eV). The bands at 320 nm, 440 nm and 650 nm are attributed to π-π* transition due to the benzenoid unit, polaron transition and charge transfer exciton like transition due to the quinoid unit as reported by the earlier research groups.4,30,33 The band at 320 nm (π-π* transition) is related to the extent of conjugation between adjacent phenyl rings in the polymer chain. Interchain photoexcitation leads to charge transfer from the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) so as to form positive and negative polarons, while intrachain photoexcitation leads to the formation of a ‘molecular’ exciton (positive charge on adjacent benzenoid units bound to the negative charge centered on the quinoid unit).30 Based on the steady state absorption spectra of SPANI in water reported by Ito et al.,4 it may be mentioned (without elemental analysis) that the sulfur-to-nitrogen ratio for the presently synthesized SPANI should be between 0.65 to 0.80. The normalised absorption spectra (Figure 1b) show that the energy positions of the extreme blue (320 nm) and red (650 nm) bands remain unchanged with the varying concentration of SPANI. However, a close look (inset of figure 1b) shows that the shoulder type band suffers an apparent red shift from 435 nm to 440 nm accompanied by a regular increase in the absorbance value as the concentration of SPANI is increased from 6.8x10-3 gL-1 to 7.6x10-2 gL-1. Clearly, at low concentration of SPANI, the shoulder type band (low absorbance) is partially masked by the intense band in the blue side of the spectra, thereby showing an apparent peak position of the former band at 435 nm. However, at higher concentration of SPANI, the shoulder type band with enhanced absorbance shows the real energy position at 440 nm. Also, it is expected that



with increasing concentration of SPANI, interchain photoexcitation increases to some extent giving rise to the observed enhancement in the absorbance of the shoulder type band. Figure 2 shows the steady state fluorescence emission spectra of SPANI in aqueous medium at different concentrations. Excitation wavelength was chosen as 300 nm instead of the absorption maximum at 320 nm to avoid Raman peak of water in the fluorescence emission spectra. Like PANI,5 fluorescence emission from SPANI in aqueous medium is ascribed to be due to the benzenoid unit upon photoexcitation by π-π* transition. As the concentration of SPANI is gradually increased from 3.7×10-3 gL-1 to 6.2×10-2 gL-1, the fluorescence emission spectra of SPANI suffer alterations in the overall intensity without any change in the peak energy position (427.5 nm). The fluorescence emission intensity is quite low (Figure 2, curve i) for a dilute solution of SPANI (3.7×10-3 gL-1). As the concentration of SPANI is increased to 3.1×10-2 gL-1 and further to 4.2×10-2 gL-1, fluorescence emission intensity increases continuously (Figure 2, curves ii and iii, respectively) due to increased population of the fluorophores. However, as the concentration of SPANI is increased beyond 4.2×10-2 gL-1, the fluorescence emission intensity starts deceasing (Figure 2, curves iv and v at concentrations 4.5×10-2 gL-1 and 6.2×10-2 gL-1, respectively) due to primary inner filter effect (discussed before). Therefore, concentration dependent measurements of the fluorescence emission spectra of SPANI in aqueous medium exclude the possibility of any excimer/aggregate formation of SPANI at high concentrations. Temperature Dependent Steady State Measurements. Figure 3 shows the steady state absorption spectra of SPANI at different temperatures. With the increase in temperature from 288 K to 333 K, the absorption band in the extreme blue side of the spectra suffers a blue shift of 1.5 nm (from 321 nm to 319.5 nm). Nearly similar shift is observed for the shoulder type band at ca. 440 nm. However, a larger blue shift of ca. 9 nm is observed for the absorption band in the extreme red side of the spectra (from 654 nm to 645 nm) as the


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temperature is increased from 288 K to 333 K. Also, with increasing temperature, the absorbance of the band in the extreme blue side of the spectra increases, while that of the shoulder type band at ca. 440 nm decreases to some extent (absorbance of the band in the extreme red side of the spectra remains nearly unaffected). An isosbestic point is found at 362 nm, which indicates more π-π* transition due to the benzenoid unit at the expense of the polaron band at higher temperature. Again, it is well known that the polarity of a liquid medium decreases with increasing temperature.41 The dielectric constant of water decreases from 82.04 to 66.76 as the temperature is increased from 288 K to 333 K.42 Consequently, solvent-solute relaxation decreases with increasing temperature giving rise to the observed blue shift of 1.5 nm for the band in the extreme blue side of the spectra as well as the shoulder type polaron band. However, a larger blue shift of ca. 9 nm for the absorption band in the extreme red side of the spectra with increasing temperature is attributed to the charge transfer nature of the band (more solvent-solute relaxation). Figure 4 shows the fluorescence emission spectra of SPANI in aqueous medium at different temperatures upon photoexcitation at 300 nm. As the temperature is increased from 283 K to 333 K, fluorescence emission intensity of SPANI reduces gradually accompanied by slight red shift of 1 nm (from 427.5 nm to 428.5 nm) of the peak energy position. Reduction in fluorescence emission intensity at higher temperature is attributed to enhanced collisional quenching (interchain interactions) due to more and more diffusion of the surrounding ground state fluorophores to the singlet excitons. However, the fluorescence emission spectra should suffer a blue shift (like absorption spectra as already mentioned) with increasing temperature (decreasing polarity of the solvent) contrary to the observed red shift (though small). This indicates the presence of two fluorescent (S1 → S0) conformations (energetically lying closely) of SPANI in aqueous medium. It is suggested that at higher temperature, population of one conformation increases to some extent at the expense of the other one via thermal



activation at the crossing of the first excited singlet potential energy surfaces of the two conformations of SPANI. The possibility of the presence of two conformations of SPANI in aqueous medium is discussed in detail in the following sections. pH Dependent Steady State Measurements. In the present work, aqueous solution of SPANI has a pH value 4.5 (low acidic medium). Based on the earlier literature report,30 two possible ground state conformations of SPANI are suggested in acidic (emeraldine salt form of SPANI only) aqueous medium (Scheme 4). It is suggested that in the normal working aqueous solution (low acidic medium) or slightly lower acidic aqueous solution (pH = 4.5 – 6.0), SPANI possesses conformation ‘A’ with a six membered ring formation by H-bonding (weak interaction) between the H-atom of the -SO3H (sulfonic acid) group and N-atom of the >NH (amine) group. However, when the aqueous solution becomes more acidic (pH = 2.0 – 4.0), SPANI may exist in conformation ‘B’ (relatively energetically less stable than the conformation ‘A’ in the ground state) in which the >NH group easily abstracts one H-atom from the solvent to become -NH2+. To verify this conjecture, we measured the steady state absorption and fluorescence emission spectra of SPANI in aqueous medium (acidic) at three pH values: one at normal working condition (4.5), one at less acidic condition (5.9) and one at more acidic condition (2.2). Interestingly, the absorption spectra show a red shift of 4 nm (from 318 nm to 322 nm), while the fluorescence emission spectra show a blue shift of 4.5 nm (from 428.5 nm to 424 nm) as the pH of the aqueous medium is increased from 2.2 to 5.9 (Figure 5). Obviously, as the aqueous medium becomes more and more basic (or less acidic), SPANI converts from energetically less stable (ground state) conformation ‘B’ to more stable conformation ‘A’ giving rise to the observed red shift in the absorption spectra. However, contrary to the expected similar red shift, fluorescence emission spectra show a blue shift with the increase in pH value of the aqueous medium. This is attributed to a decreased conjugation in the polymer backbone at higher pH value with the six membered ring


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formation by H-bonding (vide infra, Scheme 5).30,43 Nevertheless, our pH dependent fluorescence emission measurements confirm the co-existence of two conformations (‘A’ and ‘B’, Scheme 4) of SPANI in acidic aqueous medium (higher population of conformation ‘A’ than that of conformation ‘B’ in less acidic medium and higher population of conformation ‘B’ than that of conformation ‘A’ in more acidic medium). Also, this is in good agreement with the observations in temperature dependent steady state measurements of SPANI in aqueous medium (vide supra). Excitation Wavelength Dependent Steady State Measurements. Figure 6a shows the fluorescence emission spectra of SPANI in aqueous medium upon photoexcitation at different wavelengths (near the π-π* transition due to the benzenoid unit, Figure 1a). Interestingly, as the excitation wavelength is increased gradually from 270 nm to 340 nm, the fluorescence emission spectra of SPANI show a large red shift of 37 nm (from 409 nm to 446 nm). Also, with increase in excitation wavelength from 270 nm to 340 nm, the maximum fluorescence emission intensity first increases up to an excitation wavelength of 290 nm and then starts decreasing continuously as the excitation is made further in the red side of the absorption spectra. Finally, at an excitation wavelength of 340 nm, the fluorescence emission spectra become quite weak in intensity as well as broad in shape. The observed red shift of the peak energy position of the fluorescence emission spectra with increasing excitation wavelength is shown more clearly in figure 6b (see the normalised fluorescence emission spectra and the inset). Again, these data support our earlier conjecture of the possibility of the co-existence of two conformations of SPANI in aqueous medium. Apparently, as the excitation is made in the shorter wavelength side, fluorescence emission is dominated by energetically higher lying (relaxed singlet excited state, less stable) conformation ‘A’, while the energetically lower lying (relaxed singlet excited state, more stable) conformation ‘B’ of SPANI dominates the fluorescence emission at longer excitation wavelengths. This is kind of



dual fluorescence reported by many research groups for several fluorophores under different conditions.44-48 However, if this is the only situation in the present case, one should expect two clear fluorescence bands at some particular condition with an isoemissive point in between the two bands with the varying excitation wavelength. However, in the present case, we observe relatively broad fluorescence emission spectra at all excitation wavelengths with an additional feature of a gradual red shift of the peak energy position with increasing excitation wavelength (Figure 6). Clearly, these observations cannot be explained by the mere co-existence of two conformations of SPANI in aqueous medium. It is to be noted that excitation wavelength dependence of the fluorescence emission spectra (especially the peak energy position) for some particular fluorophores in specific environments has been attributed to red-edge effect by several researchers long time ago.49-62 According to Kasha’s rule,63 fluorescence emission from a particular fluorophore in a particular medium (liquid or solid) usually occurs from the lowest vibrational level of the first excited singlet state to the ground singlet state of the fluorophore. Hence, fluorescence emission is normally independent of the excitation wavelength. However, it has been found by many research groups49-62 that in some specific conditions fluorescence emission from a particular fluorophore does not follow the classical rules. In such a case, when photoexcitation is made at the red (long wavelength) edge of the absorption spectra, fluorescence emission spectra start to depend on the excitation wavelength and shift towards the longer wavelength region. This phenomenon is called red-edge effect (REE). It is suggested that REE occurs not from the violation of the fundamental principles of spectroscopy, but from its operation in some specific conditions, when the ensemble of excited fluorophores is distributed in interaction energy with the surrounding solvent molecules.61 REE usually occurs with the change of excitation wavelength towards the red edge of the absorption band maximum. However, if two conformations of the same


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fluorophore are involved in the absorption as well as fluorescence emission (closely overlapping bands), REE will be present in the fluorescence emissiosn from both the fluorophores. As a result, the overall fluorescence emission spectra will gradually shift to longer wavelengths as the excitation wavelength is increased continuously not only at the red edge of the absorption band maximum, but through the whole range of the absorption band, starting from the blue edge.62 We use this model (Scheme 5) to explain the excitation wavelength dependent fluorescence emission spectra of SPANI in aqueous medium (Figure 6). As shown in scheme 5, at normal working condition (pH = 4.5) in aqueous medium, SPANI exits in two conformations (‘A’ and ‘B’) with the proposed energy level diagram. Of course, in low acidic medium, conformation ‘A’ is expected to be more populated than conformation ‘B’. In general, both ground (S0) and excited (S1) states are considered to have a distribution of interaction energy with the surrounding solvent molecules. Based on our pH dependent measurements, conformation ‘A’ absorbs mostly in the red side of the absorption spectra for a transition to unrelaxed Franck-Condon excited state. Upon photoexcitation at any particular wavelength, emission occurs from a relaxed (solvent-solute relaxation causing Stokes shift) state. Further lowering of the radiative transition energy occurs due to the enhanced conjugation for the conformation ‘B’ compared to that of conformation ‘A’. The most probable case is the excitation of the fluorophores residing at the centre of ground state distribution and emission from the centre of excited state distribution. As dipole-dipole interactions with surrounding solvent molecules are directional, the weakest ground state interactions may become the strongest in the excited state and vice versa. Obviously, a shift to the red excitation edge will select only those fluorophores, which are weaker in the ground state (upper part of the distribution) but stronger in the excited state (lower part of the distribution).62 Such photoselective excitation of fluorophores by low energy photons results in the red shift of the fluorescence emission spectra compared to that excited at the band



maximum. A combined REE shown by both the conformations ‘A’ and ‘B’ of SPANI in aqueous medium (pH = 4.5) gives rise to the observed excitation wavelength dependent fluorescence emission spectra (Figure 6). Time-Resolved Measurements. Fluorescence lifetimes of SPANI were measured by using TCSPC technique at different concentrations as well as at different excitation and emission wavelengths in aqueous medium (Table 1, Figure 7). As expected from steady state measurements, fluorescence lifetime measurements of SPANI reveal nearly similar results irrespective of the concentration of SPANI. Also, fluorescence lifetime of SPANI is always found to consist of three components: a short-lived component (τ1) with sub-ns lifetime, a moderately long-lived component (τ2) with lifetime ca. 3-5 ns and a long-lived component (τ3) with lifetime ca. 8-11 ns. The fractional contribution (f1) of the sub-ns component remains quite low (4% - 7%) in all the measurements. Also, it is to be mentioned that the FWHM of the excitation light sources used in the present TCSPC set-up is ca. 1 ns. Therefore, we refrain to make any comment on the origin of τ1 in the present work. This feature is being investigated more closely by other experimental techniques and the findings will be communicated separately in future. Table 1 shows that at an excitation wavelength of 280 nm and monitoring emission wavelength of 415 nm, the fractional contribution (f2) of the moderately long-lived component is only 4%, while that (f3) of the long-lived component is 91%. As the excitation wavelength is shifted to 295 nm and monitoring emission wavelength is shifted to 425 nm, f2 increases to 12%, while f3 decreases to 82%. Finally, as the excitation wavelength is further shifted to 340 nm and monitoring emission wavelength is shifted to 446 nm, f2 increases to 31%, while f3 decreases to 65%. These features closely agree to our previous steady state observations confirming the presence of two conformations of SPANI (‘A’ and ‘B’) in aqueous medium at normal working condition (pH = 4.5). Accordingly, τ2 is attributed to conformation ‘A’ and τ3 is attributed to conformation ‘B’ of SPANI in aqueous 16 ACS Paragon Plus Environment

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medium. Clearly, as the excitation wavelength is shifted towards the red side of the absorption spectra, population of conformation ‘A’ of SPANI increases gradually at the expense of that of conformation ‘B’.

CONCLUSIONS SPANI is synthesized by direct sulfonation of emeraldine salt form of polyaniline with chlorosulfonic acid in an inert solvent. Material characterizations are carried out by FTIR, XRD and SEM measurements. The steady state absorption spectra of SPANI in aqueous medium consist of three bands: a relatively sharp band at 320 nm due to π-π* transition of the benzenoid unit, a shoulder at 440 nm due to polaron transition and a broad band at 650 nm due to charge transfer exciton like transition of the quinoid unit. The fluorescence emission spectra of SPANI in aqueous medium are measured in such a way that inner filter effect (especially secondary inner filter effect or re-absorption effect) as well as scattering are eliminated. From the concentration dependent measurements of the fluorescence emission spectra of SPANI in aqueous medium, possibility of any excimer/aggregate formation of SPANI at high concentration is ruled out. Temperature dependent fluorescence emission measurements of SPANI in aqueous medium indicate the possibility of the existence of dual fluorescence from two energetically closely lying singlet (S1) excited conformations. Also, pH dependent fluorescence emission measurements clearly suggest two possible ground state conformations (‘A’ and ‘B’) of SPANI in the normal working low acidic aqueous medium (pH = 4.5). The conformation ‘A’ arises due to six membered ring formation by H-bonding between the H-atom of the sulfonic acid group and N-atom of the amine group. On the other hand, the conformation ‘B’ arises due to abstraction of one H-atom from the solvent by the amine group. This is further corroborated by the time-resolved measurements. Again, peak energy position of the fluorescence emission spectra of SPANI in aqueous medium suffers



gradual red shift with increasing excitation wavelength. This feature cannot be explained by the mere co-existence of two conformations of SPANI in aqueous medium. It is suggested that the two conformations of SPANI in aqueous medium lead to a combined REE. This subsequently results in the observed gradual red shift of the fluorescence emission spectra of SPANI in aqueous medium with an increase in the excitation wavelength over the π-π* transition region due to the benzenoid unit. All these new findings help us to gain better physical insights into the photophysical properties of SPANI in aqueous medium, thereby opening up new directions (like color tuning, etc.) in its commercial applications in the field of polymer optoelectronics.

Supporting Information Figures S1 (FTIR spectra), S2 (XRD pattern) and S3 (FESEM image) for material characterization of SPANI are available in the supporting information section.

ACKNOWLEDGMENTS Subrata Sinha acknowledges the Council of Scientific and Industrial Research (CSIR), India (Project No.: 03(1365)/16/EMR-II) for providing financial assistance in the form of grant. SS thanks Dr. Alakananda Hajra (Department of Chemistry, Siksha Bhavana, Visva-Bharati, Santiniketan – 731 235, India) for FTIR measurement. Also, SS thanks Prof. Samita Basu and Mr. Ajay Das (Chemical Sciences Division, Saha Institute of Nuclear Physics, Kolkata – 700 064, India) for fluorescence lifetime measurements.


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Table 1. Fluorescence Lifetime Values of SPANI at Different Concentrations as well as at Different Excitation and Emission Wavelengths in Aqueous Medium Conc. of λEx SPANI

λEm

a

τ1

a

τ2

a

τ3

α2

a

b

α3

(nm) (nm) (ns)

1.6x10-2

a

α1

χ (ns)

(ns)

(gL-1)

8.0x10-3

a

(%)

(%)

280

415

0.42 2.88 10.71 57.28

295

425

0.44 3.20

340

f1

b

f2

b

f3

DW

(%)

37.35 1.01 1.79

0.05

0.04

0.91

48.41 13.21 38.38 1.01 1.93

0.06

0.12

0.82

446

0.56 5.52 11.27 39.87 29.44 30.69 1.02 1.81

0.04

0.31

0.65

280

415

0.47 3.43 10.76 62.73

32.54 1.00 1.82

0.07

0.04

0.89

295

425

0.44 3.27

48.98 12.60 38.42 1.01 1.93

0.06

0.13

0.81

340

446

0.59 5.51 10.98 45.35 25.70 28.95 1.02 1.80

0.05

0.30

0.65

7.60

7.61

5.37

2

4.73

a

Triple exponential fit by using equation 1 (see text).

b

Fractional contribution of ith decay component is fi = αiτ i/(∑ αjτ j) (i, j = 1, 2, 3).



Figure Captions Scheme 1. (a) PANI in mixed oxidation state consisting of reduced benzenoid units and oxidized quinoid units, the average oxidation state being (1 – y); (b) PANI in half oxidized (1 – y = 0.5) emeraldine base state consisting of an alternating sequence of two benzenoid units and one quinoid unit; (c) Emeraldine salt state of PANI (bipolaron form); (d) Emeraldine salt state of PANI (polaron form). Scheme 2. SPANI prepared by direct sulfonation of emeraldine salt form of PANI with chlorosulfonic acid in an inert solvent. Scheme 3. Geometry of the triangular quartz cuvette for steady state fluorescence emission measurements. Scheme 4. Possible conformations of SPANI in aqueous medium at different pH ranges. Scheme 5. Energy level diagram showing REE for the two conformations (‘A’ and ‘B’) of SPANI at normal working condition (pH = 4.5) in aqueous medium. The upward vertical arrows represent light absorption, the downward vertical arrows represent light emission and the wavy arrows represent Stokes shift (plus stabilization in energy due to less conjugation in case of conformation ‘B’ compared to that of conformation ‘A’). Figure 1. (a) Steady state absorption spectra and (b) steady state normalised absorption spectra of SPANI in aqueous medium at a concentration (g L-1) of: (a) 6.8x10-3; (b) 1.6x10-2; (c) 2.0x10-2; (d) 3.2x10-2; (e) 7.6x10-2. Figure 2. Steady state fluorescence emission spectra of SPANI in aqueous medium at a concentration (gL-1) of: (i) 3.7×10-3; (ii) 3.1×10-2; (iii) 4.2×10-2; (iv) 4.5×10-2; (v) 6.2×10-2. Excitation wavelength = 300 nm. Figure 3. Steady state absorption spectra of SPANI in aqueous medium (concentration = 3.2x10-2 gL-1) at a temperature (K) of: (A1) 288; (A2) 303; (A3) 318; (A4) 333.


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Figure 4. Steady state fluorescence emission spectra of SPANI in aqueous medium (concentration = 3.2x10-2 gL-1) at a temperature (K) of: (E1) 283; (E2) 293; (E3) 303; (E4) 313; (E5) 323; (E6) 333. Excitation wavelength = 300 nm. Figure 5. Steady state normalised absorption and fluorescence emission spectra (excitation wavelength = 300 nm) of SPANI in aqueous medium (concentration = 3.2x10-2 gL-1) at different pH values (2.2, 4.5 and 5.9). Figure 6. (a) Steady state fluorescence emission spectra and (b) steady state normalised fluorescence emission spectra of SPANI in aqueous medium (concentration = 3.2x10-2 gL-1) at an excitation wavelength (nm) of: (1) 270; (2) 280; (3) 290; (4) 300; (5) 310; (6) 320; (7) 330; (8) 340. Figure 7. Normalised fluorescence decay curves of SPANI (1.6x10-2 gL-1) in aqueous solution at an excitation wavelength of (a) 280 nm (emission wavelength = 415 nm), (b) 295 nm (emission wavelength = 427 nm) and (c) 340 nm (emission wavelength = 446 nm). Black line: lamp profile; red line: decay curve; blue line: fitted curve. Lower panels show the residuals.



Scheme 1


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Scheme 2



Scheme 3


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Scheme 4



Scheme 5


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Page 33 of 40

2.0

(a)

a b c d e

Absorbance

1.5

1.0

0.5

0.0 300

400

500

600

700

Wavelength (nm)

1.0

(b)

0.45 Norm. Abs.

Normalised Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

0.40 0.35 0.30

0.6

a b c d e 400 440 480 Wavelength (nm)

a b c d e

0.4 0.2 0.0 300

400

500

600

Wavelength (nm)

Fig. 1


700

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Emission Intensity (arb. units)


Page 34 of 40

125

i ii iii iv v

100 75 50 25 0 350

385

420

455

Wavelength (nm)

Fig. 2


490

525

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0.8

0.8

0.6 0.4

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

325

350

375

400

A1-288 K A2-303 K A3-318 K A4-333 K

0.4

0.2

0.0 300

400

500

600

Wavelength (nm)

Fig. 3


700

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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100 E1-283 K E2-293 K E3-303 K E4-313 K E5-323 K E6-333 K

80 60 40 20 0 350

385

420

455

Wavelength (nm)

Fig. 4


490

525

pH 2.2 pH 4.5 pH 5.9

1.0

1.0

0.8

0.8

Absorption Spectra

Emission Spectra 0.6

0.6 300

350

400

Wavelength (nm)

Fig. 5


450

Normalised Emission Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Normalised Absorbance

Page 37 of 40

140

Page 38 of 40

(a)

1 2 3 4 5 6 7 8

105

70

35

0 350

385

420

455

490

525

Wavelength (nm) 1.8 1

(b)

λEm Max (nm)

Normalised Emission Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



2

1.5

3 4

1.2

5

440 430 420 410 275 300 325 350 λEx (nm)

6

0.9

450

7 8

0.6 0.3 350

385

420

455

Wavelength (nm)

Fig. 6


490

525

Page 39 of 40

0

Residuals

Counts

10 10

-1

10

-2

10

-3

4

(a)

Prompt Decay Fit

20

30

20

30

40

50

60

50

60

0 -4

40 Time (ns)

0

10

Prompt Decay Fit

-1

Counts

10

(b)

-2

10

-3

Residuals

10

4

20

30

40

50

60

20

30

40 Time (ns)

50

60

0 -4

0

10

Prompt Decay Fit

-1

Counts

10

(c)

-2

10

-3

10 Residuals

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

20

30

40

50

60

20

30 40 Time (ns)

50

60

0 -4

Fig. 7 39 ACS Paragon Plus Environment


TOC Graphic


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