Observable Temperature-Dependent Compaction−Decompaction of

ARTICLE pubs.acs.org/JPCB

Observable Temperature-Dependent Compaction-Decompaction of Cationic Polythiophene in the Presence of Iodide Jian Wang,† Qing Zhang,† Ke Jun Tan,† Yun Fei Long,‡ Jian Ling,† and Cheng Zhi Huang*,† †

Education Ministry Key Laboratory on Luminescence and Real-Time Analysis, College of Chemistry and Chemical Engineering, College of Pharmaceutical Sciences, Southwest University, Chongqing 400715, PR China ‡ Institute of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Hunan 411201, PR China

bS Supporting Information ABSTRACT: Investigation on compaction and decompaction of polymers is very important since it is a fundamental problem in polymer physics. With the aids of atomic force microscope (AFM) and dynamic light scattering (DLS) measurements in this contribution, the temperature-dependent compaction/decompaction transition process of water-soluble cationic polythiophene (PT) was investigated in the presence of KI. The above process is characterized by the red-to-yellow color change and fluorescence recovery and is reversible during the heating-cooling cycles in the range from 25 to 55 °C, indicating that the compaction and decompaction of polymer can be employed as a temperature indicator.

’ INTRODUCTION Since it is a common and interesting phenomenon in polymer conformational transition, the reversible compaction and decompaction process of polymers has been studied quite extensively.1 For example, poly(N-isopropylacrylamide) generally folds into aggregated states when dehydrated at high temperature, and the aggregates then decompact when hydrated at low temperature.2 Similarly, some other conjugated polymers such as polythiophene also undergo conformational transition between compressed aggregates and loose random-coiled species when exposed to some ligands3-12 or under certain environmental conditions,12-15 which can be applied for practical purposes such as drug carriers during the drug delivery or release.16 It is interesting that polythiophene and its derivatives, a class of promising candidates with novel properties such as ionochromic,5,17,18 photochromic,17 thermochromic,12,17-19 and biochromic3,6,8,17 features, have showed high promise in sensing chemistry and biochemistry including inorganic anions,7 ATP,9,11,12 DNA or RNA,4,6-8,20,21 drugs,22 and proteins.6,23 Those novel properties of the polythiophene family are supposed to be related with the conformational transition from nonplanar loose species to planar compacted aggregates (Scheme 1A),4-12 resulting in color and optical changes, and thus have been designed for a variety of sensory devices. Moreover, the introduction of flexible side chains along the backbone (R1 and R2 in Scheme 1A) can not only enhance the r 2011 American Chemical Society

solubility and conjugation but also modulate the properties of polymers.18 On one hand, the presence of the 4-methyl group (R1 in Scheme 1A) in the thiophene ring is essential to form a planar structure. For example, the introduction of the 4-methyl group can make the thermally inactive poly(2-(3-thienyloxy) ethanesulfonate) show thermochromic effect, which presents a decompacted planar loose conformation at low temperature and a compacted nonplanar aggregate conformation at high temperature17 (heating process in Scheme 1A). On the other hand, if the side chain R2 is improper, the conformational transition is also inaccessible. For instance, poly(1H-imidazolium-1-methyl-3-{2-[(4-methyl-3-thienyl)oxy] ethyl}) (PT) is found to be insensitive to the environmental conditions. Namely, the transition between planar and nonplanar conformation can not be achieved during the environmental temperature change. However, when investigating the state chemistry of PT in inorganic aqueous medium, we found that PT exhibits reversible temperature-dependent compaction-decompaction conformational transition in the presence of iodide. That is, the PT molecule gets into some very small particles with planar chiral structure in the presence of KI at room temperature, and the chiral particles can reversibly turn into some nonplanar large Received: July 29, 2010 Revised: October 26, 2010 Published: February 2, 2011 1693

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Scheme 1. (A) Schematic Representation of Polythiophene Conformational Change When Exposed to Ligands or Varying Surroundings and (B) the Observable TemperatureDependent Compaction-Decompaction of PT in the Presence of KI

species at 55 °C with color changing from red to yellow, suggesting that PT undergoes a planar/nonplanar conformational transition. Furthermore, the reversible temperature-dependent compaction/decompaction process can be employed to sense the temperature of surroundings.

’ EXPERIMENTAL SECTION Instrumentation. The absorption spectra were measured with a Shimadzu UV-3600 spectrophotometer (Tokyo, Japan), while the fluorescence and light scattering spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer (Tokyo, Japan). Fluorescence images were obtained by a Nikon Coolpix 4500 camera (Tokyo, Japan) under the irradiation of a WLH204B portable ultraviolet analyzer (Shanghai, China). Dynamic light scattering (DLS) measurements were carried out on a N5 submicrometer particle size analyzer (Beckman coulter, America), and the morphology observations were obtained on a NS-3D atom force microscope (Veeco, USA) with a temperature controller. A Jacso J-810 circular dichroism (CD) spectropolarimeter (Tokyo, Japan) was employed to confirm the conformational change of PT, and a CS501-SP circulatory water bath was used to control temperature. Materials. KI (AR grade) was commercially purchased from Chongqing Chemical Reagent Co. (Chongqing, China). PT was a generous gift of Professor Zhike He of Wuhan University, which has been prepared to detect aspartic acid and glutamic acid.24 Procedures. The concentration of PT in monomer repeat units is 3.0
10-4 M. Into a 1.5 mL plastic tube, 200 μL of 3.0
10-4 M PT and 20 μL of 0.05 M KI were added. The samples were transferred for measurements after the dilution to 400 μL with H2O.

Figure 1. Absorption spectra (a) and CD spectra (b) of PT in the absence or presence of KI at different temperatures. A, PT, 25 °C; B, PT, 55 °C; C, PT þ KI, 25 °C; D, PT þ KI, 55 °C. Concentrations: PT, 1.5
10-4 M; KI, 2.5
10-3 M.

’ RESULTS AND DISCUSSION Interaction of PT with Iodide. It is reported that PT can be used to specifically detect iodide based on the electrostatic interaction and the nature of the side chains.5 Among the anions of I-, Br-, Cl-, F-, S2-, NO3-, SO32-, CO32-, SO42-, ClOand PO43-, only iodide could bring about the color change from yellow to red (Figure S1a in Supporting Information) and fluorescence quenching in the fluorescence images (Figure S1b in Supporting Information) when excited at 365 nm with a portable ultraviolet analyzer. Moreover, similar results can be observed under diluted or concentrated conditions (shown in Figure S2, Supporting Information). Thermochromic Effect of PT Induced by Iodide. Free PT is thermally inactive within the range of 25 to 55 °C, which can be possibly attributed to the steric or repulsive interaction of side chains being so strong that the conformational transition is inaccessible.17,25 Then, the characteristic absorption band at 397 nm (curves A and B in Figure 1a) is believed to be associated with the nonplanar random-coiled (less conjugated) conformation because the twisting of the conjugated backbone can decrease the effective conjugation length, resulting in the blue shift of the absorption band.5-9,12,17,26 Upon addition of KI at 25 °C, however, the color of PT changes from yellow to red (the inserted picture in Figure 1a), and the absorption shifts to 545 nm (curve C in Figure 1a), indicating that the addition of KI results in a more planar conformation and stronger intermolecular π-π* stacking interaction of PT.5-9,12,17,26 That is, KI decreases the interchain and 1694

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The Journal of Physical Chemistry B intrachain repulsive interactions of PT, leading to the conformational conversion from the nonplanar loose random-coiled form (less conjugated) to the planar compressed-aggregated one (highly conjugated) and then resulting in the red-shift of absorption. Once temperature gets increased to 55 °C, however, the color becomes yellow, and the absorption peak shifts to 397 nm again (curve D in Figure 1a), suggesting that the planar compacted PT has been disrupted27 and decompacted into a nonplanar loose species at higher temperature.8,12 A series of measurements were made to understand this temperature-dependent color or spectral transition of PT in the presence of KI. As circular-dichroism (CD) spectra (Figure 1b) show, free PT has a silent CD pattern in the π-π* transition region either at 25 °C or at 55 °C attributed to an achiral randomcoiled conformation of PT backbone. Nevertheless, KI makes PT present the positive Cotton signal at 526 nm and the negative Cotton signal at about 600 nm, suggesting a preferential lefthanded helical arrangement of PT in the KI/PT complex.12 The unique split-type-induced circular dichroism (ICD)28 signals, which are characteristic in the π-π* transition region of the main chains for the complexes of cationic conjugated polymers and anionic targets, obviously show that KI can induce achiral PT into chiral conformation and thus generate the ICD signals. With temperature increasing to 55 °C, however, the ICD vibronic bands of the KI/PT complex completely disappear, implying that the chiral complex formed at room temperature dissociates into random-coiled conformation thoroughly. The light scattering measurements (Figure 2a) with a common spectrofluorometer showed that the conformational transition occurs accompanied with size change.29 It is known that the light scattering intensity (I) of a spherical scattering particle could be calculated as eq 1 if the instrumental parameters of the spectrofluorometer are kept constant29,30 !2 24A2 π2 Nν2 m2 -1 I¼ ð1Þ m2 þ2 λ4 wherein A is the amplitude of the incident light; N is the number of the scattering particles per unit volume; v is the volume of one particle; m is the relative refractive index of the particle versus its surrounding medium; and λ is the incident light wavelength in the medium. The signals at 310 nm are related with Rayleigh light scattering of the aqueous solution measured with an uncorrected spectrofluorometer.29 The strong light scattering of free PT at 474 nm can be attributed to the loose random-coiled conformation with large size. However, KI makes PT compact into small size at 25 °C to reduce the characteristic light scattering signals. At 55 °C, the compacted KI/PT complex gets disordered27 into loose random-coiled species,8,12 leading to the recovery of light scattering signal. Dynamic light scattering (DLS) measurements, which is a good way to measure the size of the particles,15 were used to further confirm the temperature-dependent compactiondecompaction transition. It can be found from Figure 2b that the size of the KI/PT complex decreases from 1350 nm to about 600 nm with increasing KI concentrations for a given amount of PT at 25 °C. Therefore, it can be inferred that KI decreases the intrachain and interchain electrostatic repulsion of PT,5 leading to the compaction and aggregation of PT. At this point, the negative charge of KI is 2.5
10-3 M, while the positive charge of PT is 1.5
10-4 M. Further addition of KI will not cause obvious size changes. Herein, an equimolar ratio was

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Figure 2. Light scattering spectra (a) of PT in the absence or presence of KI. Concentrations: PT, 1.5
10-4 M; KI, 2.5
10-3 M. DLS measurements (b) of PT in the presence of KI at 25 °C. Scattering angle, 90°; PT, 1.5
10-4 M.

not adopted because the solution becomes less stable due to more complete charge neutralization of the complexes.22 To compact PT thoroughly, excessive KI was employed in this investigation. Atomic force microscope (AFM), a powerful tool to observe the morphology of polymers,31,32 can be employed to directly observe the temperature-dependent compaction/decompaction transition of the KI/PT complex. The thicknesses of free PT are about 120 nm at 25 and 55 °C (Figure 3 A,B). Interestingly, fewer particles with a thickness less than 60 nm (Figure 3C) can be found at room temperature upon addition of KI, but many more particles with 120 nm thickness can be seen (Figure 3D) if the KI/PT complex is heated to 55 °C. It has been reported that polymer chains would collapse first and then aggregate,33,34 from which it can be deduced that the backbone of PT has collapsed to yield the smaller aggregated particles. Both AFM and DLS measurements identify that KI compacts PT into smaller size, which accord with the light scattering spectra. However, the discrepancies between the particle size values measured by light scattering and the AFM data should be attributed to the different measurement methods since AFM displays the size of PT in the solid state, while DLS gives the hydration size in the aqueous medium. Moreover, different concentrations of PT were adopted in AFM because so few particles of the KI/PT complex can be observed in the AFM image if PT keeps at low concentration (Figure S3 in Supporting Information). Absorption and emission spectra of PT were employed to further investigate the effect of KI. At 25 °C, the absorption peak (Figure S4a, Supporting Information) gradually shifts to 545 nm 1695

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Figure 3. AFM images (2
2 μm) of PT in the absence or presence of KI at different temperatures. (A) PT, 25 °C; (B) PT, 55 °C; (C) PT þ KI, 25 °C; (D) PT þ KI, 55 °C. Concentrations: PT, 1.5
10-4 M (A and B); 3.0
10-4 M (C and D); KI, 5.0
10-3 M.

Figure 4. Fluorescence spectra of iodide-induced PT at different temperatures (a). Temperature: A-F, 25 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C. And the maximum emission change during the heating-cooling cycles (b). 25 °C (1), 35 °C (2), 45 °C (b), 55 °C (9). Concentrations: PT, 1.5
10-4 M; KI, 2.5
10-3 M.

with increasing KI concentration, which can be ascribed to the delocalization of π electrons along the chain backbone induced by anionic iodide. Furthermore, no shift of absorption could be observed if the concentration of KI is higher than 2.5
10-3 M. Besides the red-shift of absorption spectra, fluorescence spectra also undergo a red-shift when KI is added to the cationic PT solution (Figure S4b in Supporting Information), which can be understood as that KI makes random-coiled nonplanar polymers with flexible chains into planar (highly conjugated) conformation, leading to the red-shift of emission spectra9 and fluorescence quenching in the planar or aggregated form.5,6,17 According with the DLS results, there is no change either in absorption or in fluorescence spectra when KI concentration is higher than 2.5
10-3 M. Therefore, it can be deduced that the spectral change is related with the temperature-dependent conformation and size change of cationic polymers. Further investigations showed that temperature has a strong effect on PT fluorescence signal (Figure 4a) due to the disordering of a planar KI/PT complex into a nonplanar random-coiled conformation upon heating,17,27 leading to the blue-shift of emission. The fluorescence of the KI/PT complex is enhanced and blue-shifted from 610 to 550 nm with increasing temperature from 25 to 55 °C. Owing to the flexibility of the chains,17 the temperature-dependent oscillation of fluorescence-peak wavelength is found to be thermally reversible (Figure 4b) between 610 and 550 nm within the temperature range of 25 to 55 °C. Furthermore, the thermal response shows that the iodide-induced conformational transition of PT is dynamic and sensitive to environmental change (the kinetics is not shown here), which makes it possible to develop an iodide-induced PT-based thermosensitive indicator. 1696

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’ CONCLUSIONS In summary, the temperature-dependent compaction and decompaction of cationic polythiophene induced by KI was investigated. The results above can conclude that KI induces the yellow random-coiled (less conjugated, nonplanar conformation) PT to compact into a red aggregated (highly conjugated, planar conformation) KI/PT complex, with the red-shift of absorption and fluorescence spectra. Upon heating, the compacted KI/PT complex decompacts to the yellow random-coiled polymer with larger size. This process is reversible and can be successfully developed as a visual temperature indicator, which shows a color change in the temperature range 25-55 °C. Moreover, PT is expected to be a general visual model for bionics in terms of compaction-decompaction of biopolymer such as DNA or proteins in cells. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as noted in the text This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86-23-68254659. Fax: þ86-23-68367257. E-mail address: [email protected].

’ ACKNOWLEDGMENT We thank Professor Zhike He in Wuhan University for supplying polythiophene. This work was supported by the National Natural Science Foundation of China (NSFC, No.21035005) and Chongqing Science and Technology Commission for the Chongqing Key Laboratory on Luminescence and Real-Time Analysis (No: 2006CA8006).

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