NIR Electrochemical Fluorescence Switching from Polymethine Dyes

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Introduction
Interest in stimuli-responsive fluorescence changes has grown rapidly due to a number of applications related to ion sensing, 1, 2 bioanalysis, [3][4][5] fluorescence imaging, 6,7 and reversible control for optical memories. 8,9 ][12][13][14][15] The EF switching is based on the energy transfer between the fluorophore and the electroactive acceptor, or the intrinsic fluorescence quenching of the electroactive fluorophore, so reversible electrochemistry and its correlation with fluorescence is essential to obtain measurable EF switching. 10,13 revious reports have employed poly(oxadiazole)s, 7 poly(methylene-anthracene)s, 16 and various tetrazine derivatives 12,13,[17][18][19] as materials to design reversible and stable EF switching devices.In this context, we recently reported a solid-state EF device in which a tetrazine fluorophore was blended with a solid polymer electrolyte (SPE) to form an electrofluorochromic layer. 17,18 urthermore, multi-color fluorescence switching was achieved by blending a naphthalimide to an electrofluorochromic layer, resulting in a white-blue-dark state of fluorescence. 12However, attempts has not been made to extend the switchable emission to near-infrared (NIR) spectral region, although optical properties in NIR region have attractive advantages in bioimaging, bio-analysis, 20 and night vision devices. 21,22 specially, in the biomedical imaging, the use of NIR emission is a promising approach because it can provide non-invasive and background signal free images. 23Based on these advantages, NIR dyes featuring absorption and emission bands are in the 700-1200 nm spectral range, are currently being studied extensively due to the high interest in various applications ranging from bioimaging to NIR modulation devices and dark field viewing devices. 24,25 herefore, modulation of NIR emission can provide an additional functionality in imaging application, such as selective NIR fluorescence probe, 26 and biomedical diagnosis. 27Also, with dark field viewing application, modulation of NIR emission can be a meaningful signaling device, and signal perturbing device. 28erein, we report NIR EF switching, for the first time, using an NIR emissive polymethine dye, after careful control of the redox reaction of the dyes within the working potential window.A polymethine dye was employed, because it gives fluorescence in NIR region with the excitation of visible to NIR incident light, while they are electroactive.To investigate the relationship of chemical structure on EF switching, a cationic (PM1) and neutral polymethine dye (PM2) were examined under the same experimental condition.Ultimately, the precise control of the applied potential resulted in the optimized ON/OFF switching, while minimizing irreversible decomposition.

Preparation of the EF switching cells
The fluorescence switching devices consisted of an electrolyte that was packed between two ITO electrodes (13 Ω sq -1 ), with Ag wire (d = 0.1 mm) as the reference electrode.The electrolyte solution was prepared by mixing the polymethine dyes (0.01 M) in a 0.2 M concentration of TBAPF 6 /MC solution.The switching device was prepared by assembling the ITO electrode, with Ag wire as the reference electrode, between the working and counter electrodes.The electrolyte was carefully injected into the device through holes drilled into the counter electrode.The holes were sealed by heating with a hot melt 25-μm-thick Surlyn polymer film (Surlyn, Solaronix Meltonix 1170-25).The device was then finally sealed with an epoxy resin. 12

Measurements
Electrochemical measurements for the prepared EF switching cells were obtained using a universal potentiostat [model CHI 624B (CH Instruments, Inc.)].Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed after 5 min of nitrogen purging.UV-Vis spectra were obtained using the spectrometer Lambda 750 (PerkinElmer).Fluorescence spectra were measured using the Model LS55 luminescence spectrometer (PerkinElmer).NIR photography was obtained with a digital camera (IR cut-off filter removed, Power Shot A640, Canon) with a visible light cut-off filter (720 nm, 830 nm cut-off filter), and with 785 nm excitation source (optical power = 3 mW, Su Semiconductor, Korea).The light intensity of excitation source was determined by dividing the optical power by the irradiation area.When recording the fluorescence along with the external voltage, the in-situ fluorescence of the switching device was obtained using a luminescence spectrometer.

Optical and electrochemical properties of the polymethine dyes
The optical properties of polymethine dyes were well matched with the previous report. 25The cationic polymethine dye (PM1) was well soluble in organic solvents and took green color in solution.The absorption and fluorescence bands were maximized in NIR spectral range, as expected from the long electron delocalization between the two electron-donating groups through the hydrocarbon skeleton. 31The maximum absorption band for PM1 was observed at 795 nm with an extremely high extinction coefficient of 370,000 L•mol -1 •cm -1 (Figure 1).The fluorescence appeared as a sharp band in NIR region and maximized at 822 nm.The absorption and emission colors in the NIR region for PM1 were almost imperceptible without the aid of the visible light cut-off filters.On the other hand, a neutral polymethine dye (PM2) took on a pale red hue when dissolved in dichloromethane, showing an absorption max at 508 nm with an extinction coefficient of 52,000 L•mol - 1 •cm -1 , (Figure 1).This absorption resulted in a bright fluorescence band in visible range (max.at 555 nm).Because PM2 is a neutral polymethine dye with an electron-donating indole group, the absorption band appears in the shorter wavelength region compared to the PM1.he transparent green and red solution of PM1 and PM2, respectively, are shown in the photographic images of Figure 1c, which were obtained under room light with a normal camera without visible light cut-off filters.With the aid of a visible cut-off filter, the absorption and emission of the PM1 solution became visible.The NIR images obtained by the digital camera through the cut-off filter with a cut-off < 720 nm and <830 nm displayed vivid blue and bright white color, respectively, for the PM1 solution.As the digital camera presents a processed image, which is white-balanced using a white background, the colors of the output image possibly indicates the color from the transmitted light out of the detectable light.Therefore, the blue color of the PM1 solution (Figure 1c, with a <720 nm cut-off) indicates that the light near    hough the oxi more reversible characteristic c 1.As the mol a t-butyl group ups would make n.On the other at -0.  CV and fluorescence sp ontribute to the for the redu V), which is ẽ oxidation pr oreover, with the neutral ra generated on e, which is kno than the ra re, with some ex ction between and Dye • should n to the formati process is simi of electroche where the col and Dye • is us escence. 39, 44he electroche property of derivative he previous rep ould be a reaso r the seco very.

dary recovery reversibility of y increased, tentials ranging
In order to ac ble EF switchin to a neutral ra equired, as show With the applic ential (1.1 V/0. e quenching t balanced, poss luorescence qu uction redox po nching is main igure 3d, the flu ration times are , the fluorescen n times at each arying from 1. ~1.5, with a 3 ng reversible s 0 % loss (Fi sed on the ox harge during th oxidation of PM ed PM1 was cal n the device (F  S3).Alth  (Figure S3).The amount of oxidized PM2 at first switching step was calculated as 24 % of the whole PM2 in the device.Because PM1 was reversibly switched by 11% of the total amount of PM1, the extra injected/ejected charge would be leading the irreversible decomposition.
Ultimately, we fabricated a reversible NIR EF switching device in a 3-electrode system, and we achieved NIR intensity modulation by precisely controlling the working potential of the reversible redox reaction for PM1 (Figure 4).Upon exposure to a laser light source (785 nm), the switching device showed fluorescence in NIR range.This NIR emission passed through an optical filter to remove background light and was thus pictured by a digital camera.The electrode of the device was connected to a potentiostat in order to control the applied potential (Figure S5).The images in Figure 4c present the NIR switching results from the device containing PM1.The device showed the same vivid blue color (with <720 nm cut-off) and bright white color (with <830 nm cut-off) emissions seen from the PM1 solution.This emission was controlled reversibly by an applied potential, based on electrochemical conversion between the cation to the radical dication, as shown in Figure 4d.This process was reversible for over 100 cycles, but fluorescence loss was also observed due to the side reaction.Although the fluorescence contrast and cyclability of PM1 were relatively smaller than other EF switching devices working in the visible range, 12,15,17 EF switching of the NIR region was observed here for the first time with PM1.

Conclusions
In summary, we have demonstrated the electrochemical switching of NIR fluorescence for the first time, using a polymethine dye (PM1).To investigate the relations between the chemical structures of polymethine dyes and their optical and electrochemical properties, NIR emissive PM1 was compared with a keto group (C=O) substituted analogue (PM2).Due to the conjugation of the methine chain, PM1 showed NIR emissions, under reversible electrochemical reactions.With an applied potential, PM1 undergoes oxidation to a radical dication (Dye •2+ ) and reduction to a neutral radical (Dye • ).Although the radical dication is known to be unstable, because of its possible decomposition reaction such as dimerization and dehydrogenation, PM1 exhibited reversible electrochemical conversion arising from the steric hindrance of the bulky substituent and the reversible reactions between Dye •2+ and the neutral radical Dye • .This is in contrast to the electrochemistry of PM2, which showed irreversible electrochemical reactions due to the conjugation break of the polymethine chain by the group (C=O).A stable electroconversion of PM1 was applied in EF switching.The device showed NIR fluorescence switching with an ON/OFF ratio of ~1.5 and a cyclability of ~100 cycles.These values were relatively small compared to other EF switching, but modulation of the NIR fluorescence was achieved for the first time through precise control of the redox reactions of polymethine dye.We believe that further modification, following this pioneering work, can improve the contrast and cyclability significantly.This would be an interesting goal to realize highly reversible NIR modulating device based on the reversible electrochemical conversion.

Fig. 1
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