steric hindrance of four OR-phenyl substituents facing each other. The induced twist
is what makes the molecule chiral giving rise to a chiral graphene-type system
[55]. Furthermore, a central C 2 -symmetry axis is preserved, with the system being
comparable to two connected helicenes. The experimental ECD spectrum presents
intense bands of alternating signs at high energies, while at low energy there is a low
intensity CD feature, which has been demonstrated to be shaped by vibronic effects
analyzed theoretically in Ref. [55]. The fluorescence spectrum is quite intense, while
the CPL spectrum is weak, showing vibronic structuring as well, as often typical of
weak CPL spectra. Indeed, in the present case we have g abs ¼ 1.8 Â 10
–3 , while
g lum 10
–3 : thus g lum /g abs 0.5, which is in accord to Mori’s characterization; also,
the Stokes shift is quite small. Similar distorted nanographenes [56] present low g lum
and small Stokes shift.
Table 10.1 Comparison of
experimental g abs , g lum , and
g lum /g abs values and of
calculated electric and
magnetic dipole transition
moments (atomic units) and
angle formed by them (see
Refs. [30, 53]) for HEX,
5N-HEX, and TRIA-HEX.
Calculations are at
CAM-B3LYP/TZVP level
nm
g abs , g lum
g abs /g lum |μ|
|m|
Angle
HEX
abs 414 ~1.3 Â 10
–3
0.16 0.19
96
lum 422
1.4 Â 10
–3
~1.1
0.10 0.07
130
5N-HEX
abs 415
5 Â 10
–3
0.32 0.23
114
lum 430
5.5 Â 10
–3
~1.1
0.40 0.20
161
TRIA-HEX
abs 340
8.3 Â 10
–3
1.07 1.03
39
lum 442
9.1 Â 10
–3
~1.1
0.69 1.50
67
Fig. 10.10 (Left): ECD spectra of the chiral peropyrene, OR ¼ O-hexyl groups (O-methyl for the
calculated spectrum). (Right): CPL spectra of the same compounds. Corresponding fluorescence
has been normalized (see Ref. [55])
236
G. Longhi and S. Abbate