property has been demonstrated by exploiting changes in the electronic and geometric structures of photochromic molecules [16–18]. For example, trans-cis photoisomerization of thioindigo [19] and overcrowded alkene [20] readily controlled the
arrangement of two fluorophore units in an intramolecular manner, modulating their
emission between the monomeric and dimeric (excimer) ones (Fig. 8.1). The photoinduced electrocylization reaction in diarylethenes [17] and fulgido [21] from the
open-ring to the closed-ring form induces a dramatic red-shift of absorption band
from UV to the visible region. This change in the electronic structure often quenches
the emission of fluorophores attached to the photochromic unit based on the energy
transfer with an appreciable spectral overlap between the emission band of
fluorophores and the absorption band corresponding to the closed-ring form of
photochromes (Fig. 8.2). Since most of photochromic reactions, especially the
6π-based electro-cyclization reactions, proceed in a stereospecific manner with
high quantum efficiency, the incorporation of photochromic units in a chiral fluorescent molecule is an effective way to control the chiroptical property including
CPL emission of molecular system by means of light irradiation.
8.2 Quenching of CPL from Chiral π-Conjugated Polymers
by Photochromic Reaction of Diarylethene
The first example of ON-OFF photo-switching of CPL was demonstrated by Akagi
and coworkers [22]. Conjugated polymers based on poly( p-phenylene) and poly
(bithenylene-phenylene) are linked with a photo-responsive diarylethene moiety
Fig. 8.1 Change of the intramolecular interaction between fluorophores in response to the geometrical change of photochromic molecule
Fluorophore
Fluorophore
Emission
UV
vis
Energy
transfer
ON
OFF
Open-ring form
Closed-ring form
Fig. 8.2 ON-OFF emission switching based on a change in the electronic structure of photochromic diarylethene
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T. Nakashima and T. Kawai
arrangement of two fluorophore units in an intramolecular manner, modulating their
emission between the monomeric and dimeric (excimer) ones (Fig. 8.1). The photoinduced electrocylization reaction in diarylethenes [17] and fulgido [21] from the
open-ring to the closed-ring form induces a dramatic red-shift of absorption band
from UV to the visible region. This change in the electronic structure often quenches
the emission of fluorophores attached to the photochromic unit based on the energy
transfer with an appreciable spectral overlap between the emission band of
fluorophores and the absorption band corresponding to the closed-ring form of
photochromes (Fig. 8.2). Since most of photochromic reactions, especially the
6π-based electro-cyclization reactions, proceed in a stereospecific manner with
high quantum efficiency, the incorporation of photochromic units in a chiral fluorescent molecule is an effective way to control the chiroptical property including
CPL emission of molecular system by means of light irradiation.
8.2 Quenching of CPL from Chiral π-Conjugated Polymers
by Photochromic Reaction of Diarylethene
The first example of ON-OFF photo-switching of CPL was demonstrated by Akagi
and coworkers [22]. Conjugated polymers based on poly( p-phenylene) and poly
(bithenylene-phenylene) are linked with a photo-responsive diarylethene moiety
Fig. 8.1 Change of the intramolecular interaction between fluorophores in response to the geometrical change of photochromic molecule
Fluorophore
Fluorophore
Emission
UV
vis
Energy
transfer
ON
OFF
Open-ring form
Closed-ring form
Fig. 8.2 ON-OFF emission switching based on a change in the electronic structure of photochromic diarylethene
178
T. Nakashima and T. Kawai