The conformational behavior of 2o in solution was characterized by means of
1 H
NMR, CD, fluorescence, and CPL measurements. The
1
H NMR study suggested the
operation of intramolecular interactions including hydrogen bonding between the
amide groups and π-π stacking between the pyrene units. Together with this result,
the strong excimer-like emission observed at 500 nm and the suppressed monomer
emission band at 390 nm (Fig. 8.7a) clearly support the preferential adoption of a
compactly folded conformation as depicted in Fig. 8.6. The CD measurement
confirms the chirality induction in the secondary structure of the main framework
in 2o (Fig. 8.8b). The positive- and negative-first Cotton effect signals for D- and
Fig. 8.7 (a) Fluorescence and (b) CPL spectral change of 2o upon UV (365 nm) irradiation in
chloroform. Traces in (a) were measured with an irradiation interval of 5 s. Dashed line plots in (b)
correspond to CPL spectra at the photostationary state (PSS) under UV irradiation
300 400 500 600 700 800
Wavelength/ nm
0
0.4
0.2
0.6
Absorbance
(a)
300 400 500 600 700 800
Wavelength/ nm
/ cm -1
M -1
0
40
- 40
0
4
-4
CD/ mdeg
D-form
L-form
D-form
L-form
D-form
L-form
(b)
(c)
Fig. 8.8 Absorption (a) and CD (b, c) spectral change of 2 in chloroform (8.7 Â 10
À6 M) upon UV
irradiation. (a) Solid line, L-2o; dashed line, at photo-stationary state (PSS). (b, c) Solid lines, 2o;
dashed lines, at PSS
182
T. Nakashima and T. Kawai