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Direct observation of spiropyran phosphorescence in imidazolium ionic liquids
Chemical Physics Letters 556 (2013) 102–107
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Direct observation of spiropyran phosphorescence in imidazolium ionic liquids Sean P. Naughton, Robyn M. Gaudet, Anne A. Leslie, Amy E. Keirstead ⇑ Department of Chemistry and Physics, University of New England, 11 Hills Beach Road, Biddeford, ME 04005, USA
a r t i c l e
i n f o
Article history: Received 10 August 2012 In final form 16 November 2012 Available online 29 November 2012
a b s t r a c t Emission spectroscopy is used to investigate the photochromism of a spiropyran ester in imidazolium ionic liquids. While the spiropyran exhibits positive photochromism, the ring-opening reaction is slowed such that both fluorescence from the merocyanine form and phosphorescence from the spiro form are observed. These results illustrate the first example of spiropyran phosphorescence in ionic liquids and suggest that this system could be used to design a robust two-color emitting molecular device that depends on the state of the photochrome, where the state can be modulated by internal (the medium) and external (irradiation wavelength) factors. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Reports of studies performed in, and of, room temperature ionic liquids (ILs) continue to raise interesting research questions and present novel applications more than a decade since these materials first appeared in the chemical literature [1–3]. Although these molten salts originally gained attention due to their low volatility, stability under extreme conditions, and potential for use in green chemistry [4,5], a wide array of applications involving ILs have since been reported, including: solvents and catalysts for chemical reactions and separations [6–8], CO2 capture [9,10], electrolytes for photovoltaic cells [11–13], and media for molecular electronic devices [14–16]. Despite the great potential for these materials in a variety of important applications, questions still remain about the physicochemical properties of ILs, including their polarity, heterogeneity, solvation dynamics, bulk vs. microscopic properties, and role of the cation–anion pair [17,18]. Precisely how the IL properties influence molecular guests in terms of their conformations, reactivity, product ratios, and dynamics compared to traditional molecular solvents is an active and deserving area of study. Well-characterized photochemical reactions whose outcomes are sensitive to their environment have been advantageous for investigating ionic liquid properties [19,20]. Spiropyrans [21–23] are a popular class of photochromic molecules because they undergo efficient and reversible transformations in their absorption properties upon exposure to ultraviolet (UV) or visible (vis) light, and exhibit changes in their molecular conformation and dipole that are influenced by the medium. As shown in Scheme 1, spiropyrans exist in a closed, colorless spiro (SP) form in the dark, in nonpolar environments, or when exposed to visible light. Irradiation of the SP form with UV light results in a ring-opening reaction ⇑ Corresponding author. Fax: +1 (207) 602 5993. E-mail address: [email protected] (A.E. Keirstead). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.11.059
at the spiro center to yield the open merocyanine (MC) form that absorbs strongly in the visible region and is planar and highly polar. Using UV and vis light, the interconversion between SP and MC forms can occur repeatedly without decomposition in the absence of oxygen, and is thus characterized as an ‘on’ (MC)-‘off’ (SP) molecular switch. The objective in this Letter was to use this probe system to learn more about ILs as hosts for molecular switches and in turn, how the physicochemical properties of ILs influence the photochromic behavior of spiropyrans. The system selected was the N-methyl ester substituted nitrobenzospiropyran (SPEST) in a series of imidazolium ionic liquids, Scheme 1; this system was chosen because the ILs are commercially available and have been fairly well characterized, and the SPEST has previously been studied in novel media [24–26]. Spiropyrans have proven to be a good probe system for investigating IL properties; indeed, a few groups have reported studies of spiropyrans in IL media in the last few years [27–32]. Although these reports have contributed valuable information to the understanding of IL physicochemical properties, most of the studies focus on the MC isomer with little attention paid to the SP form, and have typically employed a combination of absorption spectroscopy with computational methods. The approach taken here is different: rather than using the absorption properties, emission spectroscopy is used to examine the photochromism and dynamics of the SP-MC interconversion in IL media. Emission spectroscopy is a convenient tool for this investigation partly due to its high sensitivity, but also because the MC and SP forms have very different emission properties: the MC form has a red fluorescence whereas the SP form emits as phosphorescence in the blue-green region (the emission maxima for both are dependent on the medium and on the substitution on the spiropyran core) [33–35]. Furthermore, MC fluorescence is readily observed in both solid and fluid media, whereas SP phosphorescence has only been seen at low temperatures or in rigid media such as solids and glasses [24,35–38]. While the MC
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Scheme 1.
fluorescence and ring-closing are competitive, the lack of SP emission in fluid systems indicates that the ring-opening from the triplet excited state is much faster than phosphorescence (subnanosecond vs. millisecond time regimes); in rigid systems or at low temperature, the ring-opening is slowed so that the SP emission becomes competitive [38]. In this way, observation of SP phosphorescence can inform on the efficiency of the ring-opening process in the spiropyran system, which is dependent on the properties of the material in which it is placed. Zhang et al. [39] have reported on the MC fluorescence of N-methyl nitrobenzospiropyrans in imidazolium ionic liquids and observed both increased fluorescence of the merocyanine and delayed thermal reversion to the SP form compared to that for molecular solvents. In this Letter, we focus on the opposite reaction (i.e., ring-opening from SP to MC) and in the process, observe phosphorescence from the SP form in the ionic liquid media.
2. Experimental 2.1. General Ultraviolet–visible (UV–vis) spectra were collected using a Shimadzu UV-2450 double-beam spectrophotometer and emission/ excitation spectra were obtained using a Jobin–Yvon–Horiba Fluorolog or Cary Eclipse spectrofluorometer; all reported emission and excitation spectra are corrected for detection system response. Ultraviolet irradiation (365 nm) was provided by an 8 W UVP hand-held lamp, and the visible light source was a Cole-Parmer 09790-series white light with a 410 nm bandpass filter. Samples were irradiated at a distance of 5 cm in 1 1 cm quartz cuvettes (Spectrocell). A Mettler-Toledo C20 coulometric Karl-Fischer titrator was used to assess water content and Cannon-Manning semimicro viscometers were used to measure the kinematic viscosity of samples at room temperature. 2.2. Materials 10 ,30 -Dihydro-10 -(3-carbomethoxypropyl)-30 ,30 -dimethyl-6-nitrospiro[2H-1-benzopyran-2,20 -(2H)-indole] (SPEST) was prepared as previously described [26]. Dichloromethane was purchased from Sigma–Aldrich (Chromasolv, HPLC) and dried over 4A molecular sieves. Glycerol was purchased from Sigma–Aldrich (Spectrophotometric grade) and used as received. Absolute ethanol was purchased from Pharmco-Aaper and stored over 4A molecular sieves. Tetrahydrofuran (THF) was purchased from Sigma–Aldrich
(Chromasolv, HPLC), purified using a Vacuum Atmospheres Co. solvent purifier, and stored over 4A molecular sieves. The imidazolium ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4], 98% batch No. STBB3854), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6], >97% batch No. 1393707 V), and 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIM][BF4], >97% batch No. BCBC7263) were purchased from Sigma–Aldrich and purified and dried employing a method adapted from the literature [40,41]. The purity was assessed using UV–vis spectroscopy and the water content of the purified, dried ionic liquid was ca. 50 ppm as measured by KarlFischer titration. 2.3. Sample preparation Samples of SPEST in EtOH (1 104 M) were prepared by directly dissolving the solid in ethanol followed by bubbling with dry nitrogen for 30 min to remove oxygen from the sample. All SPEST-IL samples were prepared in a glove bag under dry nitrogen to minimize exposure of the hygroscopic IL to atmospheric moisture. An aliquot (20–30 lL) of a concentrated stock solution of SPEST in THF was added to 2 mL of the IL in a quartz cuvette to achieve a final concentration of ca. 1 104 M. The sample was mixed, covered with a KimWipeTM, and placed in a vacuum desiccator. The samples were evacuated (105 Torr) for a minimum of 24 h at room temperature in the dark with stirring, then capped with a screw top in the glove bag and back-gassed with nitrogen to ensure that no oxygen was present. The water content of prepared IL samples was measured to be <100 ppm. To prepare the 65% glycerol–ethanol mixture, absolute ethanol was added to glycerol that had been warmed to 35 °C and the mixture was stirred at room temperature until mixing was complete. The SPEST was incorporated as for the IL samples, except that ethanol was used for the stock solution, and the samples were evacuated for less than a minute, just enough to evacuate the headspace before back-gassing with nitrogen. All samples were prepared and manipulated in a minimum amount of ambient light in a temperature-controlled room to curtail photoisomerization and prepared samples were wrapped in aluminum foil and kept in the dark for 24 h with stirring to achieve equilibrium before measurement. 3. Results and discussion Since SPEST has not previously been studied in ILs, its photochromism first needed to be characterized. Figure 1a presents the steady-state absorption spectra of SPEST in the room-temperature
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Figure 1. Steady-state absorption and emission spectra of SPEST (1 104 M) in 100% ethanol (solid lines) and the ionic liquid [BMIM][BF4] (dashed lines) under nitrogensaturated conditions. (a) Absorption spectra. The black lines denote the equilibrium states while the gray lines show the spectra following 365 nm irradiation. (b) Emission spectra. The gray lines show the spectra collected following 365 nm irradiation while the black lines represent spectra collected following visible (>410 nm) irradiation. Samples were excited at 540 nm (EtOH) or 560 nm ([BMIM][BF4]).
ionic liquid [BMIM][BF4] and in deoxygenated 100% ethanol (EtOH), which was used as a comparative non-viscous molecular solvent. Similar to the behavior reported for this compound in molecular solvents [24,25] and that seen here in EtOH, the absorption spectrum of SPEST in [BMIM][BF4] at the equilibrium state shows an absorption maximum at 347 nm and several wavelengths <300 nm, characteristic of a sample enriched in the SP form. A small amount of absorbance is also present at 559 nm, indicating that some MC form is present at equilibrium. This absorption band increases in magnitude following exposure of the sample to 365 nm irradiation, and the short wavelength band shifts to 353 nm. This bathochromic shift is due to the extended conjugation present in the MC form compared to the SP form and is further evidenced by the sample turning a pink color. The spectroscopic data collected for these experiments are summarized in Table 1. The absorption properties of SPEST in the imidazolium ILs [BMIM][PF6], [OMIM][BF4], and in a 65% glycerol–ethanol solution (a comparative viscous molecular solvent) are similar to those obtained in [BMIM][BF4] and EtOH (see Table 1 and Appendix A). Specifically, the absorption spectra acquired for samples at equilibrium mainly show absorption in the UV region, with a band centered at 340–350 nm and a small amount of absorption in the visible, consistent with samples enriched in the SP form. Following UV irradiation, the samples become more pink in color and show increased absorbance in the visible band, with a slight red-shift of the short wavelength absorption band, indicative of samples rich in the MC form. Although the SPEST exhibits positive photochromism in the all of the media studied, some differences in its behavior are evident. First, samples at the equilibrium state show various amounts of the MC form, which is generated thermally in polar or viscous solvents [32]. The sample of SPEST in the glycerol–EtOH mixture showed Table 1 Measured kinematic viscosities (cSt) and spectroscopic data (k, nm) obtained for samples of SPEST in various media. Viscosity (cSt)
EtOH [BMIM][BF4] [BMIM][PF6] [OMIM][BF4] 65% Gly-EtOH
1.7 97 227 417 83
Absorption SP
MC
336 347 347 348 344
352, 353, 351, 356, 348,
545 559 555 566 536
Emission
Excitationa
SP
MC
SP
MC
N/O 470 465 445 N/O
650 641 632 650 638
N/O 400 397 397 N/O
380, 406, 395, 408, 392,
N/O: not observed. a Monitoring at the corresponding emission maximum.
548 566 539 532 492
the most absorbance due to the MC form in the equilibrium state whereas little difference is seen among the other media. This result was also noted visually, as the samples of SPEST in glycerol–EtOH were quite pink when removed from the foil for measurement whereas the other samples were only slightly tinted. Presumably, this enhancement is due to the glycerol–EtOH medium being both polar and viscous (Table 1) so SPEST is driven to its MC state under equilibrium conditions, whereas the pure EtOH is polar but not viscous and the ILs are viscous but not as polar as the alcohol solvents, so the amount of the MC state in these other media is not as great. When the samples were exposed to UV light, all showed a red-shift of the short wavelength band and an increase in the visible band due to formation of the MC form, but the samples of SPEST in the ILs took slightly longer (>1 min) to become saturated in MC form than the samples of SPEST in 100% EtOH and glycerol–EtOH (<1 min). When the MC-enriched samples were irradiated with visible light (>410 nm for two minutes), all samples bleached to a colorless state, but the absorption spectra (not shown) of the samples acquired after this irradiation varied; the samples of SPEST in EtOH and in the glycerol–EtOH mixture both had absorbance in the visible region, while the spectra for the IL samples showed virtually no absorbance in the visible, indicating that a negligible amount of the MC form was present. This result qualitatively suggests that ring-opening from the SP to MC state in an SP-enriched sample occurs more readily (i.e., in the short time delay between irradiation and spectrum acquisition) in molecular solvents than in the SPESTIL samples. Similar delayed photochemistry has been reported for the ring-closing reaction in ILs [39]. Quantitative differences among the absorption properties of the SPEST in the various media are also evident. Specifically, the long wavelength absorption maximum of the MC form ranges from 536 and 545 nm in the 65% glycerol and 100% EtOH samples, respectively, to 555, 559, and 566 nm in the [BMIM][PF6], [BMIM][BF4] and [OMIM][BF4] samples, respectively. Since the MC forms of spiropyrans are known to exhibit negative solvatochromism [42], this result suggests that the ILs are less polar than the alcohol solvents, in agreement with previous reports that the bulk polarities of most ionic liquids are more comparable to organic solvents of medium polarity than short-chain aliphatic alcohols [18,43], and congruent with studies of other spiropyrans in ILs [28,29,39]. Further, the trend observed among the imidazolium ILs is consistent with [BMIM][PF6] being more polar than [BMIM][BF4], and the longer alkyl chain in the [OMIM] cation giving rise to a less polar environment for the spiropyran molecule [3]. Fluorescence spectroscopy was also employed to monitor the photochromism of SPEST in the various media for samples that
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had been enriched in the MC and SP forms. As shown in Figure 1b, 560 nm excitation of a sample of SPEST in [BMIM][BF4] that had been irradiated with UV light gives rise to an emission band centered at 641 nm. This band, which is ascribed to fluorescence from the MC form, decreases in intensity following exposure of the sample to visible light, in line with the behavior observed for SPEST in EtOH here (Figure 1b) and as previously reported [24,25]. Similar behavior was seen for SPEST in the 65% glycerol–EtOH, [BMIM][PF6], and [OMIM][BF4] samples (see Appendix A); the emission maxima of the MC isomer in all five media are listed in Table 1. Although the MC emission is known to be negatively solvatochromic [25], no clear trend with polarity is evident from the data listed in Table 1. Presumably, the influence of polarity competes with viscosity factors, where the higher viscosities increase the time scale for solvent relaxation of the MC excited state,
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resulting in the emissive state being close to the Franck–Condon state and thus a smaller Stokes shift [44]. Zhang et al. [39] have observed a similar result for the N-methyl nitrobenzospiropyran in a variety of IL media. Having confirmed that SPEST exhibits positive photochromism, the next step was to probe for spiropyran phosphorescence as discussed above. To achieve this, the full emission profile of SPEST was measured over time for samples that had been irradiated with UV light to generate a system rich in the MC form. The excitation wavelength for this experiment was selected to be 365 nm, since this wavelength excites both the SP and MC forms of SPEST while minimizing the intrinsic emission of the ionic liquid in the region of interest. Figure 2a presents emission spectra of a sample of SPEST in EtOH (MC-enriched) where spectra were acquired every four minutes (the spectra took approximately one minute to
Figure 2. Emission spectra of SPEST (1 104 M) in (a) ethanol, (b) 65% glycerol–EtOH (c) [BMIM][BF4], (d) [BMIM][PF6], and (e) [OMIM][BF4] where the samples were irradiated with UV light to generate a system rich in MC form prior to measurement. The excitation wavelength was 365 nm and the spectra were acquired in four-minute increments (red to black and direction of arrow). The inset of Figure 2c shows the emission at 470 nm (black) and 640 nm (red) monitored as a function of time and the inset of Figure 2e is an expanded view of the 400–500 nm region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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collect). The spectra show emission at 650 nm due to the MC form, where the emission intensity decreases with sequential scans. This decrease in emission intensity is attributed to conversion of the MC form to the SP form; the ring-closing reaction occurs both photochemically (from the excitation source) and thermally, though the latter is likely a minor contribution as the MC form is quite stabilized in ethanol. On the other hand, no change in emission intensity was observed during sequential scans for the sample of SPEST in the 65% glycerol solution when the experiment was carried out under the same conditions (Figure 2b). Presumably the MC form is so highly favored in the viscous and polar environment that irradiation from the excitation source is insufficient to force the spiropyran to the SP state, or the equilibrium shifts back to an MCenriched state during the four minutes between spectrum acquisition. The thermal relaxation curve obtained for the MC form of SPEST in the glycerol–ethanol solution (Appendix A) indeed suggests that the MC form is thermally robust in this medium compared to ethanol. When the same experiment was carried out for a sample of SPEST in [BMIM][BF4] (Figure 2c), the MC fluorescence monitored at 641 nm showed a decrease in intensity similar to that seen for the SPEST–EtOH sample. In this case, however, a new emission band is observed whose growth at 470 nm is concomitant with the decrease at 641 nm, with an isosbestic point at 575 nm. Similar behavior was observed for samples of SPEST in [BMIM][PF6] (Figure 2d) and [OMIM][BF4] (Figure 2e) where the emission intensities at 632 and 650 nm, respectively, decreased with sequential scans, giving rise to new emission bands at 465 and 445 nm, with isosbestic points present in the spectra. The emission in the 445–470 nm region that is present in the spectra for the SPEST–IL samples can be attributed to phosphorescence from the SP form of SPEST, based on several criteria. First, the emission maxima are in line with the reported phosphorescence spectra for the SP form of various spiropyrans [33,35,37,38]. Second, the time constants for the decay of emission monitored at 640 nm and the growth at 470 nm due to the gradual MC to SP conversion that occurs over the course of the experiment (ca. 1 h) for the SPEST-[BMIM][BF4] sample (inset of Figure 2c) are close (3.3 104 s1 and 2.1 104 s1, respectively), and combined with the presence of an isosbestic point, are a strong indication that one emissive species is giving rise to another. Since the SPEST did not show decomposition or formation of any new species even after prolonged irradiation, the fluorescent MC form is assumed to be converting to the phosphorescent SP form. Some emission from the ionic liquid itself is present in the spectra collected for these samples, for example the small shoulder at ca. 420–430 nm in Figure 2c for [BMIM][BF4]. This emission cannot be avoided as it is due to the intrinsic fluorescence of the imidazolium cations [45,46] and is not the result of chromophoric impurities, especially considering that the ILs were rigorously purified and dried before use. The excitation wavelength of 365 nm was selected in part to minimize emission due to the IL in the region of interest, and the emission profiles of the blank ILs were monitored closely and found to have fluorescence and excitation maxima different than those of the spiropyran (see Appendix A). Thus, the emission that grows in with sequential scans in the SPEST–IL samples is not thought to be due to the IL itself, but is a result of the unique environment provided for the SPEST by the IL. Finally, sequential 400 nm excitation of a sample of SPEST in the fibrous matrix poly(vinylidene fluorideco-hexafluoropropylene) that had been enriched in the MC form was shown [24] to produce a series of emission spectra that closely mirror those shown in Figure 2c–e; that such similar emission profiles were obtained for SPEST in two completely different media but otherwise similar experiments clearly suggests that the shorter wavelength emission band is due to SPEST and not the ionic liquid. To verify the assignment of the emission bands shown in Figure 2 as discussed above, excitation spectra were collected for
samples of SPEST in the various media that had been enriched in the MC or SP forms. The excitation maxima for all samples are compiled in Table 1 and spectra for both forms of SPEST in [BMIM][BF4] are presented in Figure 3. These excitation spectra are similar in shape to the absorption spectra shown for the SPEST-[BMIM][BF4] sample in Figure 1a, supporting this assignment, but the short wavelength bands are significantly red-shifted. Phenomena such as the red-edge effect which is known for IL systems [47] were considered, but since the excitation spectra for the other SPEST–IL systems as well as the SPEST–EtOH and SPEST-65% glycerol samples also showed a similar shift between the absorption and excitation maxima, this result is not a consequence of the medium. Similar shifts between the absorption and excitation spectra have been observed for both the SP [37] and MC [48] forms of spiropyrans, and have been attributed to the molecules absorbing strongly at the shorter wavelengths such that the observed excitation maxima are actually the shoulders at the red-edge of the short wavelength bands. The observation of spiropyran phosphorescence in the SPEST–IL samples indicates that the quantum yield of ring-opening is reduced in the ILs compared to that in molecular solvents. In other words, the ring-opening from SP to MC is sufficiently slowed in the ILs such that phosphorescence competes with photochemistry from the triplet excited state. Previous reports of spiropyran phosphorescence have all employed rigid or glassy media or have involved low temperatures; the ionic liquids employed here were fluid, and all experiments conducted at room temperature. The viscosity of the IL media (Table 1) could contribute to a slowed ringopening, but is not thought to be solely responsible. Gorner et al. have extensively studied the effect of viscosity on the dynamics of spiropyran photochromism, and have indicated that some rigidity is also necessary to observe SP phosphorescence [38,49]. In the present Letter, SPEST was incorporated into a 65% glycerol–ethanol solution to examine the influence of viscosity for the system under investigation; this glycerol–ethanol ratio was selected as it had a kinematic viscosity similar to that of [BMIM][BF4]. In addition to the absence of SP emission in the experiment represented by Figure 2b, a sample of SPEST in the 65% glycerol medium that had been enriched in the SP form and excited at 365 nm did not show emission due to the SP form of SPEST. Thus, both our experimental results and literature reports suggest that factors other than viscosity must contribute to the slowed ring-opening in the IL samples. Additional possibilities to explain these findings could
Figure 3. Excitation spectra of SPEST (1 104 M) in [BMIM][BF4] monitored at 470 nm (black line) after irradiation with visible light to enrich the SPEST in SP form, and at 640 nm (gray line) after irradiation with UV light to enrich the SPEST in MC form.
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include: (i) electrostatic and/or Coulombic interactions between the SP and the ionic liquid that are stronger than those between the SP and the molecular solvents (and/or less stabilization of the transition state via such interactions); (ii) a strong and/or highly ordered solvent cage in the IL media that restricts the ring-opening reaction, and; (iii) solvent reorganization during the ring-opening reaction that is slower for ILs than for the molecular solvents. Precisely how the solute-IL interactions influence the rate and product distributions of organic reactions carried out in these media has provoked much discussion, and all three of these notions have been proposed to explain findings for a wide variety of IL-solute systems [3,50]. In their report, Zhang et al. suggested the delayed thermal reversion of MC to SP in IL media might be attributed to the rearrangement of IL solvent ions, increasing the activation energy for the conformational changes necessary for the ring-closing reaction [39]. Likewise, Coleman et al. have used spiropyrans and spirooxazines to investigate the presence of ‘domains’ in ILs and it is possible that such nanostructuring could play a role here [27]. At this juncture, it is not clear which of these explanations could be best applied to explain the slowed ring-opening that is indicated by the SP phosphorescence observed in the current Letter, although such work is underway. 4. Conclusion
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2012. 11.059. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Our results illustrate that the spiropyran ester SPEST exhibits positive photochromism in the imidazolium ionic liquids [BMIM][BF4], [BMIM][PF6] and [OMIM][BF4], interconverting between the SP and MC forms upon irradiation with UV or visible light. Taking advantage of the negative solvatochromism of the merocyanine form, the ILs are suggested to be less polar than 100% ethanol and a 65% glycerol–ethanol mixture. Notably, the SPEST is shown to be emissive in both its SP and MC forms when in the ILs, in contrast to its behavior in molecular solvents, where only the MC form emits. This result indicates that the ring-opening reaction from SP to MC is sufficiently slowed in the ILs such that phosphorescence competes with photochemistry from the triplet excited state, where the slowed ring-opening is attributed to the unique environment and solute-IL interactions provided by the IL medium. This Letter represents the first reported example of spiropyran phosphorescence in room-temperature ionic liquids. In addition to contributing to the overall understanding of photochromism in ionic liquid media, this system holds potential for a two-color emitting molecular device that depends on the state of the photochrome, where the state can be modulated by internal (the medium) or external (the irradiation wavelength) factors. An additional advantage is that the extreme stability of ILs could protect the molecular switch from degradation, resulting in a more robust system and extending the lifetime of the device. Acknowledgments We gratefully acknowledge the UNE Department of Chemistry and Physics, UNE College of Arts and Sciences, and UNE Vice-President for Research for providing mini-grants and student summer stipends; summer research scholarships for S.P.N. were funded by the Maine Space Grant Consortium. Special appreciation is extended to the Green Family Foundation for their generous support of our research program. Thanks to Lindsey LaPointe (UNE) for help with water content and viscosity measurements. A.E.K. thanks Jerome Mullin (UNE) for assistance with instrumentation, and Sean Vail (SHARP Research Labs) for guidance with SPEST synthesis, and both for helpful discussion. The Jobin-Yvon-Horiba Fluorolog spectrofluorometer used in this Letter was funded by a National Science Foundation Major Research Instrumentation grant (#0923028, PI: Jerome Mullin).
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[17] [18] [19] [20]
[21] [22]
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Direct observation of spiropyran phosphorescence in imidazolium ionic liquids Sean P. Naughton, Robyn M. Gaudet, Anne A. Leslie, Amy E. Keirstead ⇑ Department of Chemistry and Physics, University of New England, 11 Hills Beach Road, Biddeford, ME 04005, USA
a r t i c l e
i n f o
Article history: Received 10 August 2012 In final form 16 November 2012 Available online 29 November 2012
a b s t r a c t Emission spectroscopy is used to investigate the photochromism of a spiropyran ester in imidazolium ionic liquids. While the spiropyran exhibits positive photochromism, the ring-opening reaction is slowed such that both fluorescence from the merocyanine form and phosphorescence from the spiro form are observed. These results illustrate the first example of spiropyran phosphorescence in ionic liquids and suggest that this system could be used to design a robust two-color emitting molecular device that depends on the state of the photochrome, where the state can be modulated by internal (the medium) and external (irradiation wavelength) factors. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Reports of studies performed in, and of, room temperature ionic liquids (ILs) continue to raise interesting research questions and present novel applications more than a decade since these materials first appeared in the chemical literature [1–3]. Although these molten salts originally gained attention due to their low volatility, stability under extreme conditions, and potential for use in green chemistry [4,5], a wide array of applications involving ILs have since been reported, including: solvents and catalysts for chemical reactions and separations [6–8], CO2 capture [9,10], electrolytes for photovoltaic cells [11–13], and media for molecular electronic devices [14–16]. Despite the great potential for these materials in a variety of important applications, questions still remain about the physicochemical properties of ILs, including their polarity, heterogeneity, solvation dynamics, bulk vs. microscopic properties, and role of the cation–anion pair [17,18]. Precisely how the IL properties influence molecular guests in terms of their conformations, reactivity, product ratios, and dynamics compared to traditional molecular solvents is an active and deserving area of study. Well-characterized photochemical reactions whose outcomes are sensitive to their environment have been advantageous for investigating ionic liquid properties [19,20]. Spiropyrans [21–23] are a popular class of photochromic molecules because they undergo efficient and reversible transformations in their absorption properties upon exposure to ultraviolet (UV) or visible (vis) light, and exhibit changes in their molecular conformation and dipole that are influenced by the medium. As shown in Scheme 1, spiropyrans exist in a closed, colorless spiro (SP) form in the dark, in nonpolar environments, or when exposed to visible light. Irradiation of the SP form with UV light results in a ring-opening reaction ⇑ Corresponding author. Fax: +1 (207) 602 5993. E-mail address: [email protected] (A.E. Keirstead). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.11.059
at the spiro center to yield the open merocyanine (MC) form that absorbs strongly in the visible region and is planar and highly polar. Using UV and vis light, the interconversion between SP and MC forms can occur repeatedly without decomposition in the absence of oxygen, and is thus characterized as an ‘on’ (MC)-‘off’ (SP) molecular switch. The objective in this Letter was to use this probe system to learn more about ILs as hosts for molecular switches and in turn, how the physicochemical properties of ILs influence the photochromic behavior of spiropyrans. The system selected was the N-methyl ester substituted nitrobenzospiropyran (SPEST) in a series of imidazolium ionic liquids, Scheme 1; this system was chosen because the ILs are commercially available and have been fairly well characterized, and the SPEST has previously been studied in novel media [24–26]. Spiropyrans have proven to be a good probe system for investigating IL properties; indeed, a few groups have reported studies of spiropyrans in IL media in the last few years [27–32]. Although these reports have contributed valuable information to the understanding of IL physicochemical properties, most of the studies focus on the MC isomer with little attention paid to the SP form, and have typically employed a combination of absorption spectroscopy with computational methods. The approach taken here is different: rather than using the absorption properties, emission spectroscopy is used to examine the photochromism and dynamics of the SP-MC interconversion in IL media. Emission spectroscopy is a convenient tool for this investigation partly due to its high sensitivity, but also because the MC and SP forms have very different emission properties: the MC form has a red fluorescence whereas the SP form emits as phosphorescence in the blue-green region (the emission maxima for both are dependent on the medium and on the substitution on the spiropyran core) [33–35]. Furthermore, MC fluorescence is readily observed in both solid and fluid media, whereas SP phosphorescence has only been seen at low temperatures or in rigid media such as solids and glasses [24,35–38]. While the MC
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Scheme 1.
fluorescence and ring-closing are competitive, the lack of SP emission in fluid systems indicates that the ring-opening from the triplet excited state is much faster than phosphorescence (subnanosecond vs. millisecond time regimes); in rigid systems or at low temperature, the ring-opening is slowed so that the SP emission becomes competitive [38]. In this way, observation of SP phosphorescence can inform on the efficiency of the ring-opening process in the spiropyran system, which is dependent on the properties of the material in which it is placed. Zhang et al. [39] have reported on the MC fluorescence of N-methyl nitrobenzospiropyrans in imidazolium ionic liquids and observed both increased fluorescence of the merocyanine and delayed thermal reversion to the SP form compared to that for molecular solvents. In this Letter, we focus on the opposite reaction (i.e., ring-opening from SP to MC) and in the process, observe phosphorescence from the SP form in the ionic liquid media.
2. Experimental 2.1. General Ultraviolet–visible (UV–vis) spectra were collected using a Shimadzu UV-2450 double-beam spectrophotometer and emission/ excitation spectra were obtained using a Jobin–Yvon–Horiba Fluorolog or Cary Eclipse spectrofluorometer; all reported emission and excitation spectra are corrected for detection system response. Ultraviolet irradiation (365 nm) was provided by an 8 W UVP hand-held lamp, and the visible light source was a Cole-Parmer 09790-series white light with a 410 nm bandpass filter. Samples were irradiated at a distance of 5 cm in 1 1 cm quartz cuvettes (Spectrocell). A Mettler-Toledo C20 coulometric Karl-Fischer titrator was used to assess water content and Cannon-Manning semimicro viscometers were used to measure the kinematic viscosity of samples at room temperature. 2.2. Materials 10 ,30 -Dihydro-10 -(3-carbomethoxypropyl)-30 ,30 -dimethyl-6-nitrospiro[2H-1-benzopyran-2,20 -(2H)-indole] (SPEST) was prepared as previously described [26]. Dichloromethane was purchased from Sigma–Aldrich (Chromasolv, HPLC) and dried over 4A molecular sieves. Glycerol was purchased from Sigma–Aldrich (Spectrophotometric grade) and used as received. Absolute ethanol was purchased from Pharmco-Aaper and stored over 4A molecular sieves. Tetrahydrofuran (THF) was purchased from Sigma–Aldrich
(Chromasolv, HPLC), purified using a Vacuum Atmospheres Co. solvent purifier, and stored over 4A molecular sieves. The imidazolium ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4], 98% batch No. STBB3854), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6], >97% batch No. 1393707 V), and 1-octyl-3-methylimidazolium tetrafluoroborate ([OMIM][BF4], >97% batch No. BCBC7263) were purchased from Sigma–Aldrich and purified and dried employing a method adapted from the literature [40,41]. The purity was assessed using UV–vis spectroscopy and the water content of the purified, dried ionic liquid was ca. 50 ppm as measured by KarlFischer titration. 2.3. Sample preparation Samples of SPEST in EtOH (1 104 M) were prepared by directly dissolving the solid in ethanol followed by bubbling with dry nitrogen for 30 min to remove oxygen from the sample. All SPEST-IL samples were prepared in a glove bag under dry nitrogen to minimize exposure of the hygroscopic IL to atmospheric moisture. An aliquot (20–30 lL) of a concentrated stock solution of SPEST in THF was added to 2 mL of the IL in a quartz cuvette to achieve a final concentration of ca. 1 104 M. The sample was mixed, covered with a KimWipeTM, and placed in a vacuum desiccator. The samples were evacuated (105 Torr) for a minimum of 24 h at room temperature in the dark with stirring, then capped with a screw top in the glove bag and back-gassed with nitrogen to ensure that no oxygen was present. The water content of prepared IL samples was measured to be <100 ppm. To prepare the 65% glycerol–ethanol mixture, absolute ethanol was added to glycerol that had been warmed to 35 °C and the mixture was stirred at room temperature until mixing was complete. The SPEST was incorporated as for the IL samples, except that ethanol was used for the stock solution, and the samples were evacuated for less than a minute, just enough to evacuate the headspace before back-gassing with nitrogen. All samples were prepared and manipulated in a minimum amount of ambient light in a temperature-controlled room to curtail photoisomerization and prepared samples were wrapped in aluminum foil and kept in the dark for 24 h with stirring to achieve equilibrium before measurement. 3. Results and discussion Since SPEST has not previously been studied in ILs, its photochromism first needed to be characterized. Figure 1a presents the steady-state absorption spectra of SPEST in the room-temperature
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Figure 1. Steady-state absorption and emission spectra of SPEST (1 104 M) in 100% ethanol (solid lines) and the ionic liquid [BMIM][BF4] (dashed lines) under nitrogensaturated conditions. (a) Absorption spectra. The black lines denote the equilibrium states while the gray lines show the spectra following 365 nm irradiation. (b) Emission spectra. The gray lines show the spectra collected following 365 nm irradiation while the black lines represent spectra collected following visible (>410 nm) irradiation. Samples were excited at 540 nm (EtOH) or 560 nm ([BMIM][BF4]).
ionic liquid [BMIM][BF4] and in deoxygenated 100% ethanol (EtOH), which was used as a comparative non-viscous molecular solvent. Similar to the behavior reported for this compound in molecular solvents [24,25] and that seen here in EtOH, the absorption spectrum of SPEST in [BMIM][BF4] at the equilibrium state shows an absorption maximum at 347 nm and several wavelengths <300 nm, characteristic of a sample enriched in the SP form. A small amount of absorbance is also present at 559 nm, indicating that some MC form is present at equilibrium. This absorption band increases in magnitude following exposure of the sample to 365 nm irradiation, and the short wavelength band shifts to 353 nm. This bathochromic shift is due to the extended conjugation present in the MC form compared to the SP form and is further evidenced by the sample turning a pink color. The spectroscopic data collected for these experiments are summarized in Table 1. The absorption properties of SPEST in the imidazolium ILs [BMIM][PF6], [OMIM][BF4], and in a 65% glycerol–ethanol solution (a comparative viscous molecular solvent) are similar to those obtained in [BMIM][BF4] and EtOH (see Table 1 and Appendix A). Specifically, the absorption spectra acquired for samples at equilibrium mainly show absorption in the UV region, with a band centered at 340–350 nm and a small amount of absorption in the visible, consistent with samples enriched in the SP form. Following UV irradiation, the samples become more pink in color and show increased absorbance in the visible band, with a slight red-shift of the short wavelength absorption band, indicative of samples rich in the MC form. Although the SPEST exhibits positive photochromism in the all of the media studied, some differences in its behavior are evident. First, samples at the equilibrium state show various amounts of the MC form, which is generated thermally in polar or viscous solvents [32]. The sample of SPEST in the glycerol–EtOH mixture showed Table 1 Measured kinematic viscosities (cSt) and spectroscopic data (k, nm) obtained for samples of SPEST in various media. Viscosity (cSt)
EtOH [BMIM][BF4] [BMIM][PF6] [OMIM][BF4] 65% Gly-EtOH
1.7 97 227 417 83
Absorption SP
MC
336 347 347 348 344
352, 353, 351, 356, 348,
545 559 555 566 536
Emission
Excitationa
SP
MC
SP
MC
N/O 470 465 445 N/O
650 641 632 650 638
N/O 400 397 397 N/O
380, 406, 395, 408, 392,
N/O: not observed. a Monitoring at the corresponding emission maximum.
548 566 539 532 492
the most absorbance due to the MC form in the equilibrium state whereas little difference is seen among the other media. This result was also noted visually, as the samples of SPEST in glycerol–EtOH were quite pink when removed from the foil for measurement whereas the other samples were only slightly tinted. Presumably, this enhancement is due to the glycerol–EtOH medium being both polar and viscous (Table 1) so SPEST is driven to its MC state under equilibrium conditions, whereas the pure EtOH is polar but not viscous and the ILs are viscous but not as polar as the alcohol solvents, so the amount of the MC state in these other media is not as great. When the samples were exposed to UV light, all showed a red-shift of the short wavelength band and an increase in the visible band due to formation of the MC form, but the samples of SPEST in the ILs took slightly longer (>1 min) to become saturated in MC form than the samples of SPEST in 100% EtOH and glycerol–EtOH (<1 min). When the MC-enriched samples were irradiated with visible light (>410 nm for two minutes), all samples bleached to a colorless state, but the absorption spectra (not shown) of the samples acquired after this irradiation varied; the samples of SPEST in EtOH and in the glycerol–EtOH mixture both had absorbance in the visible region, while the spectra for the IL samples showed virtually no absorbance in the visible, indicating that a negligible amount of the MC form was present. This result qualitatively suggests that ring-opening from the SP to MC state in an SP-enriched sample occurs more readily (i.e., in the short time delay between irradiation and spectrum acquisition) in molecular solvents than in the SPESTIL samples. Similar delayed photochemistry has been reported for the ring-closing reaction in ILs [39]. Quantitative differences among the absorption properties of the SPEST in the various media are also evident. Specifically, the long wavelength absorption maximum of the MC form ranges from 536 and 545 nm in the 65% glycerol and 100% EtOH samples, respectively, to 555, 559, and 566 nm in the [BMIM][PF6], [BMIM][BF4] and [OMIM][BF4] samples, respectively. Since the MC forms of spiropyrans are known to exhibit negative solvatochromism [42], this result suggests that the ILs are less polar than the alcohol solvents, in agreement with previous reports that the bulk polarities of most ionic liquids are more comparable to organic solvents of medium polarity than short-chain aliphatic alcohols [18,43], and congruent with studies of other spiropyrans in ILs [28,29,39]. Further, the trend observed among the imidazolium ILs is consistent with [BMIM][PF6] being more polar than [BMIM][BF4], and the longer alkyl chain in the [OMIM] cation giving rise to a less polar environment for the spiropyran molecule [3]. Fluorescence spectroscopy was also employed to monitor the photochromism of SPEST in the various media for samples that
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had been enriched in the MC and SP forms. As shown in Figure 1b, 560 nm excitation of a sample of SPEST in [BMIM][BF4] that had been irradiated with UV light gives rise to an emission band centered at 641 nm. This band, which is ascribed to fluorescence from the MC form, decreases in intensity following exposure of the sample to visible light, in line with the behavior observed for SPEST in EtOH here (Figure 1b) and as previously reported [24,25]. Similar behavior was seen for SPEST in the 65% glycerol–EtOH, [BMIM][PF6], and [OMIM][BF4] samples (see Appendix A); the emission maxima of the MC isomer in all five media are listed in Table 1. Although the MC emission is known to be negatively solvatochromic [25], no clear trend with polarity is evident from the data listed in Table 1. Presumably, the influence of polarity competes with viscosity factors, where the higher viscosities increase the time scale for solvent relaxation of the MC excited state,
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resulting in the emissive state being close to the Franck–Condon state and thus a smaller Stokes shift [44]. Zhang et al. [39] have observed a similar result for the N-methyl nitrobenzospiropyran in a variety of IL media. Having confirmed that SPEST exhibits positive photochromism, the next step was to probe for spiropyran phosphorescence as discussed above. To achieve this, the full emission profile of SPEST was measured over time for samples that had been irradiated with UV light to generate a system rich in the MC form. The excitation wavelength for this experiment was selected to be 365 nm, since this wavelength excites both the SP and MC forms of SPEST while minimizing the intrinsic emission of the ionic liquid in the region of interest. Figure 2a presents emission spectra of a sample of SPEST in EtOH (MC-enriched) where spectra were acquired every four minutes (the spectra took approximately one minute to
Figure 2. Emission spectra of SPEST (1 104 M) in (a) ethanol, (b) 65% glycerol–EtOH (c) [BMIM][BF4], (d) [BMIM][PF6], and (e) [OMIM][BF4] where the samples were irradiated with UV light to generate a system rich in MC form prior to measurement. The excitation wavelength was 365 nm and the spectra were acquired in four-minute increments (red to black and direction of arrow). The inset of Figure 2c shows the emission at 470 nm (black) and 640 nm (red) monitored as a function of time and the inset of Figure 2e is an expanded view of the 400–500 nm region. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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collect). The spectra show emission at 650 nm due to the MC form, where the emission intensity decreases with sequential scans. This decrease in emission intensity is attributed to conversion of the MC form to the SP form; the ring-closing reaction occurs both photochemically (from the excitation source) and thermally, though the latter is likely a minor contribution as the MC form is quite stabilized in ethanol. On the other hand, no change in emission intensity was observed during sequential scans for the sample of SPEST in the 65% glycerol solution when the experiment was carried out under the same conditions (Figure 2b). Presumably the MC form is so highly favored in the viscous and polar environment that irradiation from the excitation source is insufficient to force the spiropyran to the SP state, or the equilibrium shifts back to an MCenriched state during the four minutes between spectrum acquisition. The thermal relaxation curve obtained for the MC form of SPEST in the glycerol–ethanol solution (Appendix A) indeed suggests that the MC form is thermally robust in this medium compared to ethanol. When the same experiment was carried out for a sample of SPEST in [BMIM][BF4] (Figure 2c), the MC fluorescence monitored at 641 nm showed a decrease in intensity similar to that seen for the SPEST–EtOH sample. In this case, however, a new emission band is observed whose growth at 470 nm is concomitant with the decrease at 641 nm, with an isosbestic point at 575 nm. Similar behavior was observed for samples of SPEST in [BMIM][PF6] (Figure 2d) and [OMIM][BF4] (Figure 2e) where the emission intensities at 632 and 650 nm, respectively, decreased with sequential scans, giving rise to new emission bands at 465 and 445 nm, with isosbestic points present in the spectra. The emission in the 445–470 nm region that is present in the spectra for the SPEST–IL samples can be attributed to phosphorescence from the SP form of SPEST, based on several criteria. First, the emission maxima are in line with the reported phosphorescence spectra for the SP form of various spiropyrans [33,35,37,38]. Second, the time constants for the decay of emission monitored at 640 nm and the growth at 470 nm due to the gradual MC to SP conversion that occurs over the course of the experiment (ca. 1 h) for the SPEST-[BMIM][BF4] sample (inset of Figure 2c) are close (3.3 104 s1 and 2.1 104 s1, respectively), and combined with the presence of an isosbestic point, are a strong indication that one emissive species is giving rise to another. Since the SPEST did not show decomposition or formation of any new species even after prolonged irradiation, the fluorescent MC form is assumed to be converting to the phosphorescent SP form. Some emission from the ionic liquid itself is present in the spectra collected for these samples, for example the small shoulder at ca. 420–430 nm in Figure 2c for [BMIM][BF4]. This emission cannot be avoided as it is due to the intrinsic fluorescence of the imidazolium cations [45,46] and is not the result of chromophoric impurities, especially considering that the ILs were rigorously purified and dried before use. The excitation wavelength of 365 nm was selected in part to minimize emission due to the IL in the region of interest, and the emission profiles of the blank ILs were monitored closely and found to have fluorescence and excitation maxima different than those of the spiropyran (see Appendix A). Thus, the emission that grows in with sequential scans in the SPEST–IL samples is not thought to be due to the IL itself, but is a result of the unique environment provided for the SPEST by the IL. Finally, sequential 400 nm excitation of a sample of SPEST in the fibrous matrix poly(vinylidene fluorideco-hexafluoropropylene) that had been enriched in the MC form was shown [24] to produce a series of emission spectra that closely mirror those shown in Figure 2c–e; that such similar emission profiles were obtained for SPEST in two completely different media but otherwise similar experiments clearly suggests that the shorter wavelength emission band is due to SPEST and not the ionic liquid. To verify the assignment of the emission bands shown in Figure 2 as discussed above, excitation spectra were collected for
samples of SPEST in the various media that had been enriched in the MC or SP forms. The excitation maxima for all samples are compiled in Table 1 and spectra for both forms of SPEST in [BMIM][BF4] are presented in Figure 3. These excitation spectra are similar in shape to the absorption spectra shown for the SPEST-[BMIM][BF4] sample in Figure 1a, supporting this assignment, but the short wavelength bands are significantly red-shifted. Phenomena such as the red-edge effect which is known for IL systems [47] were considered, but since the excitation spectra for the other SPEST–IL systems as well as the SPEST–EtOH and SPEST-65% glycerol samples also showed a similar shift between the absorption and excitation maxima, this result is not a consequence of the medium. Similar shifts between the absorption and excitation spectra have been observed for both the SP [37] and MC [48] forms of spiropyrans, and have been attributed to the molecules absorbing strongly at the shorter wavelengths such that the observed excitation maxima are actually the shoulders at the red-edge of the short wavelength bands. The observation of spiropyran phosphorescence in the SPEST–IL samples indicates that the quantum yield of ring-opening is reduced in the ILs compared to that in molecular solvents. In other words, the ring-opening from SP to MC is sufficiently slowed in the ILs such that phosphorescence competes with photochemistry from the triplet excited state. Previous reports of spiropyran phosphorescence have all employed rigid or glassy media or have involved low temperatures; the ionic liquids employed here were fluid, and all experiments conducted at room temperature. The viscosity of the IL media (Table 1) could contribute to a slowed ringopening, but is not thought to be solely responsible. Gorner et al. have extensively studied the effect of viscosity on the dynamics of spiropyran photochromism, and have indicated that some rigidity is also necessary to observe SP phosphorescence [38,49]. In the present Letter, SPEST was incorporated into a 65% glycerol–ethanol solution to examine the influence of viscosity for the system under investigation; this glycerol–ethanol ratio was selected as it had a kinematic viscosity similar to that of [BMIM][BF4]. In addition to the absence of SP emission in the experiment represented by Figure 2b, a sample of SPEST in the 65% glycerol medium that had been enriched in the SP form and excited at 365 nm did not show emission due to the SP form of SPEST. Thus, both our experimental results and literature reports suggest that factors other than viscosity must contribute to the slowed ring-opening in the IL samples. Additional possibilities to explain these findings could
Figure 3. Excitation spectra of SPEST (1 104 M) in [BMIM][BF4] monitored at 470 nm (black line) after irradiation with visible light to enrich the SPEST in SP form, and at 640 nm (gray line) after irradiation with UV light to enrich the SPEST in MC form.
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include: (i) electrostatic and/or Coulombic interactions between the SP and the ionic liquid that are stronger than those between the SP and the molecular solvents (and/or less stabilization of the transition state via such interactions); (ii) a strong and/or highly ordered solvent cage in the IL media that restricts the ring-opening reaction, and; (iii) solvent reorganization during the ring-opening reaction that is slower for ILs than for the molecular solvents. Precisely how the solute-IL interactions influence the rate and product distributions of organic reactions carried out in these media has provoked much discussion, and all three of these notions have been proposed to explain findings for a wide variety of IL-solute systems [3,50]. In their report, Zhang et al. suggested the delayed thermal reversion of MC to SP in IL media might be attributed to the rearrangement of IL solvent ions, increasing the activation energy for the conformational changes necessary for the ring-closing reaction [39]. Likewise, Coleman et al. have used spiropyrans and spirooxazines to investigate the presence of ‘domains’ in ILs and it is possible that such nanostructuring could play a role here [27]. At this juncture, it is not clear which of these explanations could be best applied to explain the slowed ring-opening that is indicated by the SP phosphorescence observed in the current Letter, although such work is underway. 4. Conclusion
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2012. 11.059. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Our results illustrate that the spiropyran ester SPEST exhibits positive photochromism in the imidazolium ionic liquids [BMIM][BF4], [BMIM][PF6] and [OMIM][BF4], interconverting between the SP and MC forms upon irradiation with UV or visible light. Taking advantage of the negative solvatochromism of the merocyanine form, the ILs are suggested to be less polar than 100% ethanol and a 65% glycerol–ethanol mixture. Notably, the SPEST is shown to be emissive in both its SP and MC forms when in the ILs, in contrast to its behavior in molecular solvents, where only the MC form emits. This result indicates that the ring-opening reaction from SP to MC is sufficiently slowed in the ILs such that phosphorescence competes with photochemistry from the triplet excited state, where the slowed ring-opening is attributed to the unique environment and solute-IL interactions provided by the IL medium. This Letter represents the first reported example of spiropyran phosphorescence in room-temperature ionic liquids. In addition to contributing to the overall understanding of photochromism in ionic liquid media, this system holds potential for a two-color emitting molecular device that depends on the state of the photochrome, where the state can be modulated by internal (the medium) or external (the irradiation wavelength) factors. An additional advantage is that the extreme stability of ILs could protect the molecular switch from degradation, resulting in a more robust system and extending the lifetime of the device. Acknowledgments We gratefully acknowledge the UNE Department of Chemistry and Physics, UNE College of Arts and Sciences, and UNE Vice-President for Research for providing mini-grants and student summer stipends; summer research scholarships for S.P.N. were funded by the Maine Space Grant Consortium. Special appreciation is extended to the Green Family Foundation for their generous support of our research program. Thanks to Lindsey LaPointe (UNE) for help with water content and viscosity measurements. A.E.K. thanks Jerome Mullin (UNE) for assistance with instrumentation, and Sean Vail (SHARP Research Labs) for guidance with SPEST synthesis, and both for helpful discussion. The Jobin-Yvon-Horiba Fluorolog spectrofluorometer used in this Letter was funded by a National Science Foundation Major Research Instrumentation grant (#0923028, PI: Jerome Mullin).
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