To evaluate the chiroptical properties of compounds 9 in their S 0 -state structures,
CD and UV-Vis absorption spectra of (R)-9 and (S)-9 were acquired (Fig. 2.19). The
UV-Vis absorption spectra of (R)- and (S)-9 exhibit three main π-πà vibronic
transitions (
1 L a transitions) between 315 and 360 nm that correspond to the pyrene
moieties. Surprisingly, the intensities of the CD bands of (R)-9 were noticeably
weak, and no CD bands of (R)-9 were detected in the 250–300 nm range, which
suggests that, due to its pivotal framework, chiral molecular system 9 acts as a
cryptochiral CPL fluorophore that has no detectable CD signals.
A mechanism for this cryptochirality has been hypothesized. The two pyrene
units in 9 adopt an almost achiral T-shaped structure in the ground S 0 state, resulting
in a θ value of about 80–90
in the axially chiral octahydrobinaphthyl unit, resulting
in an almost undetectable CD spectrum. On the other hand, in the S 1 state, the two
0.0
0.5
1.0
1.5
2.0
2.5
3.0
350 400 450 500 550 600
-60
-45
-30
-15
0
15
30
DC / V
CPL / mdeg
Wavelength / nm
Fig. 2.18 CPL (upper) and
PL (lower) spectra of (R)-9
(red) and (S)-9 (black) in
CHCl 3 (1.0 Â 10
À5
M)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-60
-45
-30
-15
0
15
30
300
350
400
Absorbance
CD / mdeg
Wavelength / nm
Fig. 2.19 CD (upper) and
UV-Vis absorption (lower)
spectra of (R)-9 (red) and
(S)-9 (black) in CHCl 3 (1.0
 10
–5
M)
24
Y. Imai