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efficient reformation of the cis “on” state (ground state), with a quantum yield of
0.37 [ 80 ]. This photochromic reaction is presently the most efficient photoactivation
recorded, and as such is directly accessible for time-resolved pump-probe studies
(Figs. 13 and 14 ).
It has been proposed that photoswitching of the trans “off” state to the bright cis
“on” state of Dronpa involves ESPT as well as photoisomerisation [ 79 , 92 ]. Photo-
switching of Dronpa has been observed at the single molecule level [ 92 ] as well as
in ensembles with picosecond time resolution [ 93 ]. Transient absorption measure-
ments revealed a 4 ps component that was assigned to interconversion from the
neutral “A2” excited state to the anionic I excited state. This phase was assigned
to include ESPT from the twofold H/D KIE [ 93 ]. Recent QM/MM salutations
addressed the possible reaction mechanisms in Dronpa, primarily the order of
the ESPT and photoisomerisation processes [ 90 ]. Li et al. [ 90 ] invoke a twisted
intramolecular charge transfer state in the photoisomerisation reaction, linking
ESPT with trans - cis photoisomerisation in a concerted manner [ 90 ].
The GFP-like protein asFP595 from the sea anemone Anemonia sulcata is another
example of an “RSFP”, having a Met-Tyr-Gly-derived chromophore. Photoswitching
from its non-fluorescent off-state to the fluorescent on-state is done with green light of
568 nm wavelength [ 82 , 84 , 85 ]. Spectroscopic measurements suggest that as for
Fig. 13 Photochromic
behaviour of Dronpa
(22Gm3). Reproduced with
permission from Ando et al.
[ 79 ]“(a) The pH dependence
of 22Gm3 absorbance. (b)
changes in 22Gm3
absorbance. Absorbance
spectra obtained during
irradiation at 490 nm ( black )
and after irradiation at 400 nm
( red ) are displayed. (c)
Normalized excitation ( dotted
line ) and emission ( solid line )
spectra of 22Gm3”
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