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T. Fukaminato et al.
intensity of the center (point 3) and the middle regions (points 2 and 4) maintained
the dark level for a while even after stopping UV light irradiation and then abruptly
started to recover the fluorescence intensity. In the typical linear-response fluorescence photoswitching systems, the fluorescence signal of dark area originated from
the generation of non-fluorescent closed-ring isomers recovers uniformly under irradiation with visible light. Therefore, this result indicated that the other fluorescence
quenching processes should be contributed in the single-crystalline state.
Such unexpected fluorescence recovery behavior in single crystal 3a might be
explained by the following mechanism. The light intensity through an objective
lens has Gaussian distribution and such distribution generates small distribution of
conversion yield in the irradiation area, where the conversion yield becomes high
as the position closes to the center. Such small difference of conversion yield is
almost negligible in typical linear fluorescence photoswitching system such as a
dye-doped polymer film. However, in the nonlinear system, the fluorescence intensity
suddenly and largely changes based on a certain conversion yield. The conversion
yield sequentially reaches to the critical point from outside to center in the irradiation
area because the conversion yield becomes high with closing to the center part. As
a result, the fluorescence recovery behavior with decreasing the size of dark area
was observed. Although further quantitative analysis will be necessary to discuss the
mechanism more detail, from these fluorescence photoswitching behavior, there is
no doubt that the efficient nonlinear fluorescence quenching takes place in the single
crystal 3a.
21.6 Conclusion
We described the giant amplification of fluorescence quenching in photochromic
nanoparticles and a single crystal based on the efficient intermolecular energy
transfer process in densely packed molecular system. These molecular systems show
outstanding state-of-the-art properties in the field of photochromic-fluorescent materials: bright emission, high photostability, and excellent performances in terms of
photoswitching, with very high contrast (10,000:1) and fatigue resistance. Such efficient fluorescent photoswitchable nanoparticles and crystals find potential applications, such as in memory devices, sensors, multicolor displays, and bio-imaging work
with the minimum number of photons [41, 42].
Acknowledgements We deeply appreciate to Prof. S. Kobatake, Dr. D. Kitagawa, Prof. T. Asahi,
Dr. T. Onodera and Dr. Y. Ishibashi for their collaboration. The present work was supported by
JSPS KAKENHI Grant Numbers JP15H01076, JP17H05269 in Scientific Research on Innovative
Areas “Photosynergetics”, JP19H02692 in Scientific Research (B). The author (Sanae Ishida) also
acknowledges to Grant-in-Aid for JSPS Research Fellow Number JP18J23127.
T. Fukaminato et al.
intensity of the center (point 3) and the middle regions (points 2 and 4) maintained
the dark level for a while even after stopping UV light irradiation and then abruptly
started to recover the fluorescence intensity. In the typical linear-response fluorescence photoswitching systems, the fluorescence signal of dark area originated from
the generation of non-fluorescent closed-ring isomers recovers uniformly under irradiation with visible light. Therefore, this result indicated that the other fluorescence
quenching processes should be contributed in the single-crystalline state.
Such unexpected fluorescence recovery behavior in single crystal 3a might be
explained by the following mechanism. The light intensity through an objective
lens has Gaussian distribution and such distribution generates small distribution of
conversion yield in the irradiation area, where the conversion yield becomes high
as the position closes to the center. Such small difference of conversion yield is
almost negligible in typical linear fluorescence photoswitching system such as a
dye-doped polymer film. However, in the nonlinear system, the fluorescence intensity
suddenly and largely changes based on a certain conversion yield. The conversion
yield sequentially reaches to the critical point from outside to center in the irradiation
area because the conversion yield becomes high with closing to the center part. As
a result, the fluorescence recovery behavior with decreasing the size of dark area
was observed. Although further quantitative analysis will be necessary to discuss the
mechanism more detail, from these fluorescence photoswitching behavior, there is
no doubt that the efficient nonlinear fluorescence quenching takes place in the single
crystal 3a.
21.6 Conclusion
We described the giant amplification of fluorescence quenching in photochromic
nanoparticles and a single crystal based on the efficient intermolecular energy
transfer process in densely packed molecular system. These molecular systems show
outstanding state-of-the-art properties in the field of photochromic-fluorescent materials: bright emission, high photostability, and excellent performances in terms of
photoswitching, with very high contrast (10,000:1) and fatigue resistance. Such efficient fluorescent photoswitchable nanoparticles and crystals find potential applications, such as in memory devices, sensors, multicolor displays, and bio-imaging work
with the minimum number of photons [41, 42].
Acknowledgements We deeply appreciate to Prof. S. Kobatake, Dr. D. Kitagawa, Prof. T. Asahi,
Dr. T. Onodera and Dr. Y. Ishibashi for their collaboration. The present work was supported by
JSPS KAKENHI Grant Numbers JP15H01076, JP17H05269 in Scientific Research on Innovative
Areas “Photosynergetics”, JP19H02692 in Scientific Research (B). The author (Sanae Ishida) also
acknowledges to Grant-in-Aid for JSPS Research Fellow Number JP18J23127.