the decrease of the solution concentration upon the secondary aggregation accompanied by the photo-cyclization reaction of 2. The visible light irradiation above
440 nm bleach the solution color and the visible absorption band disappeared
reversibly to give an absorption spectrum identical to that of 2o with transparency
in the visible range, indicating the dissolution of secondary aggregates.
SEM observation after the UV light irradiation clearly demonstrated the formation of secondary aggregates together with an increase in the size of nanoparticles
(Fig. 8.16c). The photo-reaction seems to take place for the molecules on the surface
of nanoparticles in the early stage of photo-conversion. The less solubility of the
colored isomer 2c in the MCH-rich solvent could reduce the colloidal stability of
nanoparticles, leading to the further aggregation and fusion of nanoparticles.
Intermolecular hydrogen bonding interactions are also possible in the colored form
2c, which may also drive the inter-nanoparticle aggregation.
The photoreaction also induced a decrease in the emission intensity in the whole
spectral range (Fig. 8.17a). In the chloroform solution, the emission intensity
decreased linearly as a function of conversion ratio (Fig. 8.17b). The more dramatic
decrease in the emission intensity was observed in the MCH-rich solutions in
comparison to that in the molecularly dispersed chloroform solution. The emission
was almost completely quenched in the small conversion ratio values of 70 and 39%
in chloroform/MCH (1:9) and (1:99) solvents, respectively (Fig. 8.17b). In a similar
manner to the molecularly dispersed state in chloroform, the emission quenching is
due to the FRET process from the excited state of pyrene moiety to the photochrome
ring-closed part in 2c. The energy transfer process also take place in an
intermolecular manner in the aggregates, enhancing the emission quenching efficiency in MCH-rich solvents. The FRET process from the pyrene moiety in the
open-ring state 2o to the ring-closed photochromic part in 2c can apparently
Fig. 8.17 (a) Emission spectral change of D-2o upon UV irradiation in chloroform/MCH (1:99)
solution (1.0 Â 10
À4 M). Before UV irradiation, broken line; at PSS achieved by the excitation at
365 nm, thick black line. (b) Plots of relative emission intensity as a function of a conversion ratio
between D-2o and 2c. Square, in chloroform; circle, in chloroform/MCH (1:9); triangle, in
chloroform/MCH (1:99) (concentration: 1.0 Â 10
À5 M). (c) CPL spectral change of D- and L-2
before (broken lines) and after the UV irradiation at the PSS (solid lines) in chloroform/MCH (1:9)
(concentration: 1.0 Â 10
À5 M)
8 Photo-Switching of Circularly Polarized Luminescence
191
440 nm bleach the solution color and the visible absorption band disappeared
reversibly to give an absorption spectrum identical to that of 2o with transparency
in the visible range, indicating the dissolution of secondary aggregates.
SEM observation after the UV light irradiation clearly demonstrated the formation of secondary aggregates together with an increase in the size of nanoparticles
(Fig. 8.16c). The photo-reaction seems to take place for the molecules on the surface
of nanoparticles in the early stage of photo-conversion. The less solubility of the
colored isomer 2c in the MCH-rich solvent could reduce the colloidal stability of
nanoparticles, leading to the further aggregation and fusion of nanoparticles.
Intermolecular hydrogen bonding interactions are also possible in the colored form
2c, which may also drive the inter-nanoparticle aggregation.
The photoreaction also induced a decrease in the emission intensity in the whole
spectral range (Fig. 8.17a). In the chloroform solution, the emission intensity
decreased linearly as a function of conversion ratio (Fig. 8.17b). The more dramatic
decrease in the emission intensity was observed in the MCH-rich solutions in
comparison to that in the molecularly dispersed chloroform solution. The emission
was almost completely quenched in the small conversion ratio values of 70 and 39%
in chloroform/MCH (1:9) and (1:99) solvents, respectively (Fig. 8.17b). In a similar
manner to the molecularly dispersed state in chloroform, the emission quenching is
due to the FRET process from the excited state of pyrene moiety to the photochrome
ring-closed part in 2c. The energy transfer process also take place in an
intermolecular manner in the aggregates, enhancing the emission quenching efficiency in MCH-rich solvents. The FRET process from the pyrene moiety in the
open-ring state 2o to the ring-closed photochromic part in 2c can apparently
Fig. 8.17 (a) Emission spectral change of D-2o upon UV irradiation in chloroform/MCH (1:99)
solution (1.0 Â 10
À4 M). Before UV irradiation, broken line; at PSS achieved by the excitation at
365 nm, thick black line. (b) Plots of relative emission intensity as a function of a conversion ratio
between D-2o and 2c. Square, in chloroform; circle, in chloroform/MCH (1:9); triangle, in
chloroform/MCH (1:99) (concentration: 1.0 Â 10
À5 M). (c) CPL spectral change of D- and L-2
before (broken lines) and after the UV irradiation at the PSS (solid lines) in chloroform/MCH (1:9)
(concentration: 1.0 Â 10
À5 M)
8 Photo-Switching of Circularly Polarized Luminescence
191