15 Solid-State Fluorescence Switching Using Photochromic …
307
in depth, which corresponds to the recording density higher than 5 Tbit/cm
3 . On
the other hand, Belfield and coworkers accomplished 3D two-photon recording and
two-photon fluorescence readout using polymer film containing a diarylethene and
a fluorene derivative [49, 50]. Two layers separated by 50 μm in depth direction
were written in the polymer film by two-photon absorption of 800 nm light. The
fluorescence of the fluorene derivative was monitored for the readout.
15.2.4 Non-destructive Readout by Intramolecular Electron
Transfer
The fluorescence quenching process as described above is based on the energy
transfer from the excited fluorophore to the diarylethene closed-ring form or the
photochromic reaction of the fluorescent diarylethene itself. However, such a readout
process destroys the recording simultaneously with readout in the optical memory.
Although the influence was minimized using very weak light for the readout and
adopting the diarylethene derivatives with a quite low photocycloreversion quantum
yield, they cannot lead to an essential solution. Here, the intramolecular electron
transfer (IET) process was adopted to accomplish the complete non-destructive
readout for the ultra-high density optical memory. The reduction and/or oxidation
potential of the diarylethene can be altered upon the photochromic reaction. The
changes in the redox potentials of the diarylethene activate or deactivate the pathways
of the IET process between the diarylethene and fluorophore to the charge separation
state. The radiative process of the excited fluorophore can be suppressed when the
IET process occurs. The IET process makes it possible to separate the absorption
spectra of both open- and closed-ring forms of the diarylethene and the fluorescence
spectrum of the fluorophore. Several researchers have tried to design and synthesize
various types of diarylethene–fluorophore dyads to achieve non-destructive readout
in fluorescence photoswitching based on the IET process [51–55].
In 2011, Fukaminato and coworkers successfully demonstrated non-destructive
fluorescence readout of a diarylethene–fluorophore dyad 10a (Fig. 15.7a) in solution and at the single-molecule level based on the IET mechanism [56]. As shown
in Fig. 15.7b, the fluorescence spectrum of the perylenebisimide derivative as the
fluorophore moiety and the absorption spectra of the open- and closed-ring forms of
the diarylethene moiety were completely separated. In addition, energy gaps for the
charge separation in dichloromethane were calculated to be 1.23 and −8.55 kcal/mol
for 10a and 10b by Rehm–Weller equation. It indicates that it is probable that the IET
process takes place only for 10b in the solution. As shown in Fig. 15.7c, only the Φ f of
the closed-ring form decreases with increasing the dielectric constant of the solvent.
As predicted from this result, the fluorescence of dyad 10 can be reversibly switched
by alternating irradiation with UV and visible light in polar solvents. In addition,
the fluorescence intensities of the open- and closed-ring forms did not change even
when irradiated with 532 nm light (2.5 mW/cm
2 ) for 2 h (Fig. 15.7d). These results
307
in depth, which corresponds to the recording density higher than 5 Tbit/cm
3 . On
the other hand, Belfield and coworkers accomplished 3D two-photon recording and
two-photon fluorescence readout using polymer film containing a diarylethene and
a fluorene derivative [49, 50]. Two layers separated by 50 μm in depth direction
were written in the polymer film by two-photon absorption of 800 nm light. The
fluorescence of the fluorene derivative was monitored for the readout.
15.2.4 Non-destructive Readout by Intramolecular Electron
Transfer
The fluorescence quenching process as described above is based on the energy
transfer from the excited fluorophore to the diarylethene closed-ring form or the
photochromic reaction of the fluorescent diarylethene itself. However, such a readout
process destroys the recording simultaneously with readout in the optical memory.
Although the influence was minimized using very weak light for the readout and
adopting the diarylethene derivatives with a quite low photocycloreversion quantum
yield, they cannot lead to an essential solution. Here, the intramolecular electron
transfer (IET) process was adopted to accomplish the complete non-destructive
readout for the ultra-high density optical memory. The reduction and/or oxidation
potential of the diarylethene can be altered upon the photochromic reaction. The
changes in the redox potentials of the diarylethene activate or deactivate the pathways
of the IET process between the diarylethene and fluorophore to the charge separation
state. The radiative process of the excited fluorophore can be suppressed when the
IET process occurs. The IET process makes it possible to separate the absorption
spectra of both open- and closed-ring forms of the diarylethene and the fluorescence
spectrum of the fluorophore. Several researchers have tried to design and synthesize
various types of diarylethene–fluorophore dyads to achieve non-destructive readout
in fluorescence photoswitching based on the IET process [51–55].
In 2011, Fukaminato and coworkers successfully demonstrated non-destructive
fluorescence readout of a diarylethene–fluorophore dyad 10a (Fig. 15.7a) in solution and at the single-molecule level based on the IET mechanism [56]. As shown
in Fig. 15.7b, the fluorescence spectrum of the perylenebisimide derivative as the
fluorophore moiety and the absorption spectra of the open- and closed-ring forms of
the diarylethene moiety were completely separated. In addition, energy gaps for the
charge separation in dichloromethane were calculated to be 1.23 and −8.55 kcal/mol
for 10a and 10b by Rehm–Weller equation. It indicates that it is probable that the IET
process takes place only for 10b in the solution. As shown in Fig. 15.7c, only the Φ f of
the closed-ring form decreases with increasing the dielectric constant of the solvent.
As predicted from this result, the fluorescence of dyad 10 can be reversibly switched
by alternating irradiation with UV and visible light in polar solvents. In addition,
the fluorescence intensities of the open- and closed-ring forms did not change even
when irradiated with 532 nm light (2.5 mW/cm
2 ) for 2 h (Fig. 15.7d). These results