Wavelength-selective and high-contrast multicolour fluorescence photoswitching in a mixture of photochromic nanoparticles

Sanae Ishida a, Tuyoshi Fukaminato *a, Daichi Kitagawa b, Seiya Kobatake b, Sunnam Kim a, Tomonari Ogata c and Seiji Kurihara *a
aDepartment of Applied Chemistry & Biochemistry, Graduate School of Science & Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan. E-mail: tuyoshi@kumamoto-u.ac.jp; kurihara@gpo.kumamoto-u.ac.jp
bDepartment of Applied Chemistry, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
cInnovative Collaboration Organization, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan

Received 17th April 2017 , Accepted 13th June 2017

First published on 13th June 2017


Herein, we report on wavelength-selective and high-contrast multicolour fluorescence photoswitching between white-, orange-, cyan-, and dark-colours in a mixture of two distinguishable emission-coloured photochromic nanoparticles composed of different pairs of a photoswitching unit and a fluorescence unit.


Stimuli-tunable multicolour fluorescence systems have attracted attention for their potential applications in the fields of optical memory devices, organic light emitting diodes (OLEDs), sensors, inks, displays, and bioimaging.1 Among several strategies to achieve external-stimuli responsive multicolour switching,2–5 photochemical approaches have been very attractive for several applications due to their remote accessibility with high spatial and temporal resolutions. Photochromic materials, which can be reversibly switched for several properties by alternate irradiation with an appropriate wavelength of light, have been often utilized as the key component to achieve the reversible photoswitching of fluorescence properties.6 In particular, diarylethene (DAE) derivatives are the most suitable molecules because of the excellent thermal stability of both their (open- and closed-ring) isomers, their rapid photoresponse, and high fatigue resistance.7 In fact, recently, several researchers have reported the dual colour as well as the multicolour emission photoswitching by utilizing DAE photoswitches. For example, Akagi et al.5c clearly demonstrated reversible emission colour changes between white and RGB (red–green–blue) by a combination of photoresponsive and non-photoresponsive fluorescent polymer nanospheres. However, all of these systems are based on the ratiometric colour change between two different coloured materials or the designated colour change depending on the combination of photoresponsive and non-photoresponsive fluorescent materials, and therefore, the variation of fluorescence colour changes is generally limited.

Ideal photoswitchable multicolour fluorescence systems require selective photoswitching of each colour component and high contrast with a minimum crosstalk between them even in a multicomponent system. However, unfortunately, typical DAE derivatives have broad absorption bands in both isomers and they cannot be completely converted to the closed-ring. These common drawbacks prevent selective and high contrast multicolour fluorescence photoswitching.

In contrast, we recently found a photoswitchable fluorescent nanoparticle (NP) composed of a photochromic DAE and a benzothiadiazole unit exhibited a remarkable nonlinear fluorescence photoswitching behaviour, in which only a small amount of the non-fluorescent closed-ring isomer (quencher) was enough to quench the whole fluorescence signal.8 It is anticipated that this unique nonlinear fluorescence photoswitching property will allow us to overcome the limitations for the selective and high-contrast multicolour fluorescence photoswitching mentioned above. In this study, we prepared two photoswitchable fluorescent molecules PF-1 and PF-2 (Scheme 1), in which each molecule has a different pair of DAE and fluorescence units, and successfully demonstrated wavelength-selective and high-contrast multicolour fluorescence photoswitching between white-, cyan-, orange-, and dark-colours based on the nonlinear fluorescence photoswitching behaviour in their NP state.


image file: c7cc02938a-s1.tif
Scheme 1 Photochromism of PF-1 and PF-2, and molecular structures of model photoswitching units (DAE-1 and DAE-2) and model fluorophores (cyan-BTD and orange-BTD).

In order to achieve the wavelength-selective multicolour photoswitching in a mixture of two components, we have to select two pairs of DAE and fluorescence units. So far, there have been several reports on multicolour (not emissive) photochromic materials using DAE derivatives, such as a fused DAE trimer,9a two- or three-component DAE mixed crystals,9b and multi-component DAE tethering polymers.9c,d Based on these works, we selected DAE-1 and DAE-2 as the switching units, in which each DAE has phenylthiazole- and phenylthiophene-rings as aryl groups, respectively (Scheme 1). As the fluorescence unit, we focused on benzothiadiazole (BTD) derivatives having bulky substituents because of their high emission properties even in the NP state and tunability of emission colours. Consequently, cyan-emission coloured (cyan-BTD) and orange-emission coloured BTD (orange-BTD) derivatives were chosen in this study because a clear white-emission can be observed by mixing these two coloured materials.10 By linking these units covalently, we synthesized two DAE–BTD dyads (PF-1 and PF-2), in which PF-1 is composed of a pair of DAE-1 and cyan-BTD and PF-2 is composed of a pair of DAE-2 and orange-BTD. From our previous work,8 it is revealed that BTD derivatives maintain their fluorescence properties in the NP state even when a DAE unit is connected to the BTD backbone. Therefore, excellent photochromic and fluorescence properties are anticipated for both PF-1 and PF-2 even in the NP state. Detailed synthetic procedures of PF-1 and PF-2 and the experimental details are provided in the ESI.

Fig. 1a and b show the absorption spectra of DAE-1 and DAE-2 in both isomeric states and Fig. 1c and d represent the absorption and the fluorescence spectra of cyan-BTD and orange-BTD units, respectively. It is expected that the photocyclization reaction of DAE-1 upon irradiation with 300–330 nm UV light takes place more efficiently relative to DAE-2, because the absorption coefficient of DAE-1 is much larger than that of DAE-2. When we use 350–400 nm UV light as the irradiation light for the photocyclization reaction, almost similar photoreactivity is expected for DAE-1 and DAE-2 because their photocyclization quantum yields and absorption coefficients in this wavelength region are almost the same.7 However, the absorption coefficients of cyan-BTD and orange-BTD are obviously different in the wavelength region, in which the absorption coefficient of cyan-BTD is approximately two times larger than that of orange-BTD, as shown in Fig. 1c. Therefore, it can be predicted that the apparent photocyclization quantum yield of PF-1 in the 350–400 nm wavelength region becomes small relative to PF-2 and hence the photocyclization reaction of PF-2 preferentially takes place in comparison with PF-1 under irradiation with 350–400 nm UV light. On the other hand, the photocycloreversion reaction of DAE-2 can be selectively induced upon irradiation with longer than 680 nm visible light because the closed-ring isomer of DAE-1 has a very weak (almost zero) absorption band in the wavelength region (Fig. 1b).


image file: c7cc02938a-f1.tif
Fig. 1 Absorption spectra in THF solution of (a) the open-ring isomer of DAE-1 (red line) and DAE-2 (blue line) in the UV (250–400 nm) region, (b) the closed-ring isomer of DAE-1 (red line) and DAE-2 (blue line) in the visible (400–800 nm) region, (c) cyan-BTD (cyan line) and orange-BTD (orange line), and (d) the fluorescence spectra in THF solution of cyan-BTD (cyan line) and orange-BTD (orange line).

Typical absorption and fluorescence spectral changes along with the photochromic reactions upon alternate irradiation with 313 nm and >560 nm light were observed for both PF-1 and PF-2 in THF solution. The detailed results on photochromism and the fluorescence photoswitching properties in THF solution for both molecules are provided in the ESI (Fig. S1 and S3).

Reversible absorption spectral changes along with the photochromic reaction were also observed for both PF-1 and PF-2 NPs upon alternate irradiation with 313 nm and >560 nm light, as shown in Fig. S2a and S4a (ESI). The conversion yield at the photostationary state (PSS) under irradiation with 313 nm light was estimated to be 0.83 for PF-1 and 0.85 for PF-2, respectively. Photocyclization and photocycloreversion quantum yields in NPs of PF-1 were determined to be Φoc = 0.24 and Φco = 3.5 × 10−3, respectively. These values are almost similar to those in THF solution, which indicates that the photoreactivity of PF-1 was maintained even in NPs. Similar trends were observed for PF-2 NPs, as previously reported.9

On the other hand, the fluorescence photoswitching behaviour in NPs was dramatically changed relative to the result in THF solution. In the pure open-ring isomer of PF-1, a clear cyan-coloured fluorescence (Φf = 0.56) was observed with a fluorescence maximum at 490 nm as shown in Fig. S2b (ESI). Upon irradiation with 313 nm light, the fluorescence immediately disappeared and the fluorescence intensity was completely quenched even at a low conversion yield. Upon irradiation with >560 nm light, the fluorescence intensity was recovered perfectly. A similar nonlinear fluorescence ON–OFF photoswitching in the NP state was observed for PF-2 upon alternate irradiation with 313 nm and >560 nm light (Fig. S2, ESI).8

Fig. S5 (ESI) exhibits the correlation plots of the fluorescence intensity of PF-1 and PF-2 as a function of the conversion yield upon irradiation with 313 nm light in THF solution as well as in a suspension of NPs. In THF solution, the fluorescence intensities of PF-1 and PF-2 were linearly changed depending on the conversion yield from the open- to the closed-ring isomer in a reversible manner. On the other hand, remarkable nonlinear fluorescence quenching was observed in NPs of both PF-1 and PF-2, in which the fluorescence was completely quenched when less than 5% of the closed-ring isomer was generated upon irradiation with 313 nm light. From our previous work,8 it was revealed that the nonlinear fluorescence quenching was owing to the efficient intermolecular FRET process from the fluorescence unit to the photogenerated closed-ring isomer in densely packed NPs. The fluorescence signal at 490 nm also returned nonlinearly to the initial level along with the photocycloreversion reaction. The nonlinear fluorescence photoswitching in NPs can be repeated for many cycles (>10 cycles).

To confirm the capability of wavelength-selective fluorescence photoswitching, we checked the wavelength dependence on fluorescence photoswitching of PF-1 and PF-2 in the NP state. Judging from the spectral overlaps of model compounds (Fig. 1), we selected 385 nm and 313 nm UV light as the incident light to induce the fluorescence ON → OFF photoswitching. On the other hand, 680 nm and 560 nm visible light was used to induce the fluorescence OFF → ON photoswitching. It is anticipated that the fluorescence intensity of PF-2 will be preferentially turned off by irradiation with 385 nm light because the contribution of the absorption coefficient of PF-2 in the wavelength region is much larger than that of PF-1, as mentioned above. Fig. 2a indicates the fluorescence intensity change of both PF-1 and PF-2 in the NP state upon sequential irradiation with 385 nm and 313 nm light. Upon irradiation with 385 nm light (150 μW), the fluorescence intensity of PF-2 quickly decreased down to 7% of the initial intensity within 150 s, while that of PF-1 slowly decreased and retained over 60% of the initial fluorescence intensity at the same period. Upon sequential irradiation with 313 nm light (150 μW), the fluorescence intensities of both PF-1 and PF-2 were completely quenched within 30 s. These results indicated that the photocyclization reaction of PF-2 preferentially took place in comparison with PF-1 upon irradiation with 385 nm light. The conversion yields after irradiating with 313 nm light for 30 s were approximately 20% in PF-1 and 10% in PF-2, respectively. The margin was attributed to the difference of the absorption coefficients at 313 nm in their photoswitching units. Continuously, upon irradiation with 680 nm light (500 μW), the fluorescence intensity of PF-2 gradually increased and perfectly recovered up to the initial level, while the fluorescence intensity of PF-1 was completely silent even after irradiation for a long time (Fig. 2b). The fluorescence intensity of PF-1 also started to recover and quickly turned back to the original fluorescence ON level upon sequential irradiation with 560 nm light as shown in Fig. 2b. These results clearly indicated that the fluorescence photoswitching of PF-1 and PF-2 can be selectively modulated by optimizing the irradiation wavelength condition appropriately.


image file: c7cc02938a-f2.tif
Fig. 2 Fluorescence intensity traces in a suspension of PF-1 (black circle) and PF-2 (red circle) NPs (a) from ON to OFF under sequential irradiation with 385 nm and 313 nm light, and (b) from OFF to ON under sequential irradiation with 680 nm and 560 nm light.

Based on this result, we demonstrated wavelength-selective and high-contrast multicolour fluorescence photoswitching in a mixture suspension of PF-1 (1.3 × 10−5 M) and PF-2 (1.1 × 10−5 M) NPs, where each NP was prepared separately and then mixed with each other. From the fluorescence lifetime measurements in the mixture suspensions of PF-1 and PF-2 NPs with several relative concentrations, the FRET process between PF-1 and PF-2 NPs was checked first. In these experiments, two dominant fluorescence lifetimes were observed and the ratios of these components were dependent on the relative concentrations between PF-1 and PF-2 NPs, as shown in Fig. S7 and Table S1 (ESI). These results suggested that no FRET process took place between PF-1 and PF-2 NPs even in the presence of each other and hence white-emission can be easily observed by mixing cyan- (PF-1) and orange-coloured (PF-2) NPs. In the initial state, a broad fluorescence spectrum that covered the whole visible-wavelength region with two peaks at 493 nm and 567 nm was observed and a clear white emission was visualized under excitation with 405 nm light (Fig. 3a). Upon irradiation with 385 nm light, the fluorescence band at 567 nm (orange emission) preferentially decreased and the emission colour clearly changed from white to cyan (Fig. 3b). Upon sequential irradiation with 313 nm light, the fluorescence band at 493 nm (cyan emission) completely disappeared and no emission was observed as shown in Fig. 3c. Subsequently, upon irradiation with 680 nm light, the fluorescence band at 578 nm selectively increased and the emission colour was clearly changed from dark to orange (Fig. 3d). Sequentially, upon irradiation with >560 nm light, the fluorescence band at 493 nm started to recover and the emission colour turned back to white. These emission colour changes can be repeated for many cycles with a high contrast. Such high-contrast emission colour photoswitching was never accomplished in the case of a mixture THF solution of PF-1 and PF-2, as shown in Fig. S6 (ESI). Upon irradiation with 385 nm light to the solution, the orange emission band at 580 nm preferentially decreased in comparison with the cyan emission band at 485 nm, but the orange emission band still remains even at PSS. The cyan emission band remarkably decreased when the solution reaches PSS under sequential irradiation with 313 nm light. However, both orange- and cyan-emission still remained. Since the 100% conversion yield from the open- to the closed-ring isomer did not occur, the fluorescence was not completely quenched in the solution system. These results clearly support the advantage of multicolour fluorescence photoswitching systems based on the nonlinear response in photochromic NPs.


image file: c7cc02938a-f3.tif
Fig. 3 Multicolour fluorescence photoswitching in a mixture suspension of PF-1 and PF-2 NPs upon sequential irradiation with an appropriate wavelength of light: fluorescence spectra (a) before photoirradiation (or after irradiation with >560 nm light to the solution of (d)) (white emission), (b) after irradiation with 385 nm light (cyan emission), (c) after sequential irradiation with 313 nm light (dark), and (d) after sequential irradiation with 680 nm light (orange emission). Excitation wavelength; 405 nm, the dotted-black line in (b–d); the fluorescence spectrum of the initial state. Inset: Photographs of the same cuvettes for the mixture suspension of PF-1 and PF-2 NPs in the corresponding spectrum.

In conclusion, we prepared two fluorescent photoswitchable molecules PF-1 and PF-2, in which each molecule is composed of a different pair of fluorescent and DAE units. PF-1 and PF-2 exhibited a photochromic reaction and a reversible fluorescence photoswitching in solution as well as in a suspension of NPs. In addition, the remarkable nonlinear fluorescence photoswitching owing to the efficient intermolecular energy transfer in NPs was observed for both molecules. This nonlinear fluorescence photoswitching property allows us to modulate the fluorescence intensities of PF-1 and PF-2 NPs selectively by using an appropriate wavelength of incident light, which results in the successful demonstration of the wavelength-selective multicolour fluorescence photoswitching between white-, cyan-, orange-, and dark-colours in a mixture of these two NPs. These results clearly show the potential of photoswitchable fluorescent NPs for multicolour display devices, security inks, or multicolour-labelling in bioimaging.

This work was partly supported by JSPS KAKENHI Grant Numbers JP15H01076 and JP26107013 in Scientific Research on Innovative Areas “Photosynergetics”, JP16H06506 in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation”, and JP15K05464 and JP16K17896. We also acknowledge Prof. Kenji Matsuda's group in Kyoto University for his kind support in fluorescence lifetime measurements.

References

  1. (a) M. Akamatsu, T. Mori, K. Okamoto, H. Sakai, M. Abe, J. P. Hill and K. Ariga, Chem. – Eur. J., 2014, 20, 16293 CrossRef CAS PubMed ; (b) B. W. D’Andrade, R. J. Holmes and S. R. Forrest, Adv. Mater., 2004, 16, 624 CrossRef ; (c) X. Zhang, S. Rehm, M. M. Safont-Sempere and F. Würthner, Nat. Chem., 2009, 1, 623 CrossRef CAS PubMed ; (d) K. Jiang, L. Zhang, J. Lu, C. Xu, C. Cai and H. Lin, Angew. Chem., Int. Ed., 2016, 55, 7231 CrossRef CAS PubMed ; (e) K. Jiang, S. Sun, L. Zhang, Y. Lu, A. Wu, C. Cai and H. Lin, Angew. Chem., Int. Ed., 2015, 54, 5360 CrossRef CAS PubMed .
  2. (a) Y. Sagara, S. Yamane, M. Mitani, C. Weder and T. Kato, Adv. Mater., 2016, 28, 1073 CrossRef CAS PubMed ; (b) Y. Q. Dong, J. W. Y. Lam and B. Z. Tang, J. Phys. Chem. Lett., 2015, 6, 3429 CrossRef CAS PubMed .
  3. (a) J.-H. Kim, Y. Jung, D. Lee and W.-D. Jang, Adv. Mater., 2016, 28, 3499 CrossRef CAS PubMed ; (b) C. Yuan, S. Saito, C. Camacho, S. Irie, I. Hisaki and S. Yamaguchi, J. Am. Chem. Soc., 2013, 135, 8842 CrossRef CAS PubMed .
  4. (a) S. Srinivasan, P. A. Babu, S. Mahesh and A. Ajayaghosh, J. Am. Chem. Soc., 2009, 131, 15122 CrossRef CAS PubMed ; (b) H.-J. Kim, D. R. Whang, J. Gierschner, C. H. Lee and S. Y. Park, Angew. Chem., Int. Ed., 2015, 54, 4330 CrossRef CAS PubMed .
  5. (a) Z. Tian, W. Wu, W. Wan and A. D. Q. Li, J. Am. Chem. Soc., 2009, 131, 4245 CrossRef CAS PubMed ; (b) S. A. Diaz, F. Gillanders, K. Susumu, E. Oh, I. L. Medintz and T. M. Jovin, Chem. – Eur. J., 2017, 23, 263 CrossRef CAS PubMed ; (c) J. Bu, K. Watanabe, H. Hayasaka and K. Akagi, Nat. Commun., 2014, 1, 623 Search PubMed ; (d) M. Bälter, S. Li, M. Morimoto, S. Tang, J. Hernando, G. Guirado, M. Irie, F. M. Raymo and J. Andréasson, Chem. Sci., 2016, 7, 5867 RSC ; (e) S. Kim, S.-J. Yoon and S. Y. Park, J. Am. Chem. Soc., 2012, 134, 12091 CrossRef CAS PubMed .
  6. (a) C. Yun, J. You, J. Kim, J. Huh and E. Kim, J. Photochem. Photobiol., C, 2009, 10, 111 CrossRef CAS ; (b) J. Cusido, E. Deniz and F. M. Raymo, Eur. J. Org. Chem., 2009, 2031 CrossRef ; (c) T. Fukaminato, J. Photochem. Photobiol., C, 2011, 12, 177 CrossRef CAS .
  7. M. Irie, T. Fukaminato, K. Matsuda and S. Kobatake, Chem. Rev., 2014, 114, 12174 CrossRef CAS PubMed .
  8. J. Su, T. Fukaminato, J.-P. Placial, T. Onodera, R. Suzuki, H. Oikawa, A. Brosseau, F. Brisset, R. Pansu, K. Nakatani and R. Métivier, Angew. Chem., Int. Ed., 2016, 55, 3662 CrossRef CAS PubMed .
  9. (a) K. Higashiguchi, K. Matsuda, N. Tanifuji and M. Irie, J. Am. Chem. Soc., 2005, 127, 8922 CrossRef CAS PubMed ; (b) M. Morimoto, S. Kobatake and M. Irie, J. Am. Chem. Soc., 2003, 125, 11080 CrossRef CAS PubMed ; (c) T. J. Wigglesworth and N. R. Branda, Chem. Mater., 2005, 17, 5473 CrossRef CAS ; (d) H. Nishi, T. Namari and S. Kobatake, J. Mater. Chem., 2011, 21, 17249 RSC .
  10. S. Bhattacharya and S. K. Samanta, Chem. – Eur. J., 2012, 18, 16632 CrossRef CAS PubMed .

Footnote

Electronic supplementary information (ESI) available: Synthetic procedures of compounds used in this work and detailed experimental procedures and data. See DOI: 10.1039/c7cc02938a

This journal is © The Royal Society of Chemistry 2017