allow the movement of the targets to be detected, even when their motion is slow. Therefore, promising materials and techniques that enable clear detection of the local motion of an individual target independent of the persistent emission around the target and autofl uorescence still need to be designed. Photoactivated fl uorescent materials have been developed for highdensity recording and super-resolution imaging. [6][7][8][9][10][11][12] Some systems containing a fl uorescent compound and photochromic compound have also been reported. [8][9][10] The fl uorescence of the emissive materials is switched off and on by effi cient energy transfer from the fl uorescent compound to the photochromic one, which is triggered by photoisomerization of the photochromic compound. If persistent luminescent materials with the ability to be photoactivated can be prepared, the resulting photoactivated persistent emission process will potentially be useful for tracking the local movement of the target independent of other persistent emitting signals around the target as well as background autofl uorescence. However, persistent luminescence is commonly observed from rare metal-doped metal oxides [ 1,2,5 ] and it is still diffi cult to fabricate photoactivated metal oxide particles. Another approach that differs from conventional metal oxide-based persistent emitters is persistent room-temperature phosphorescence (RTP), which has a lifetime approaching one second and has recently been observed from purely organic amorphous [ 13 ] and crystalline materials. [ 14 ] The amorphous materials displaying RTP are composed of a metal-free aromatic guest in an amorphous hydroxyl steroidal host, which is rigid with an oxygen barrier because of its intermolecular hydrogen bonding. The rigidity and oxygen barrier of the steroidal host minimize the nonradiative deactivation from the triplet excited state of the purely organic guest caused by thermal relaxation and oxygen diffusion, resulting in effi cient persistent RTP from the guest species. In such amorphous host-guest materials, the rigidity and oxygen barrier of the amorphous hydroxyl steroidal host around the persistent RTP guest should remain when a small amount of photochromic compound is doped into the host as another guest. Therefore, we anticipate that effi cient energy transfer processes from the guest exhibiting persistent RTP to that exhibiting photochromism should occur in the amorphous hydroxyl steroidal host through photoisomerization of the photochromic compound, which should subsequently trigger on-off switching of persistent RTP.

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TheBiophysicalSociety of JapanGeneral IncorporatedAssociation 8. Masumoto KH. et al, Curr BioL20(24),2199-206.(2010