These epimeric compounds can be considered as pseudo-enantiomers since they
demonstrate reverse stereochemistry of the helix (P/M) and of the P atom (R P /S P )
but unchanged stereochemistry of the chiral l-menthyl group. The epimeric helicenes
[6]-(P)-endo-53a and [6]-(M)-endo-53b displayed almost identical UV-vis spectra,
blue fluorescence with moderate quantum yield (0.07–0.10), and mirror-image ECD
spectra. Similarly, they exhibited mirror-image CPL spectra with luminescence
anisotropy factor g lum ¼ +8 Â 10
À4 and À 7 Â 10
À4 at 452 nm (excitation at
404–416 nm) for (P)-53a and (M)-53b, respectively (Table 4.8). Note that the
other phosphahelicenes tested underwent photodegradation under the conditions
of the CPL measurement.
In 2018, Tanaka and coworkers reported the enantioselective synthesis of [7]and [9]phosphahelicenes 53c and 53d (Fig. 4.18) by [2+2+2] cycloaddition.
These heterohelicenes displayed modest quantum yields (0.22 and 0.085) and g lum
values (+8.1 Â 10
À4 and + 3.8 Â 10
À4 ) [52].
[6]-(P)-endo-53a
[6]-(M)-endo-53b
P
*l-Men
O
nPr
nPr
P
l-Men*
O
nPr
nPr
n-Bu
n-Bu
P
OMe
O
n-Bu
n-Bu
P
OMe
O
(P)-53c
(P)-53d
Fig. 4.18 CPL-active
phosphahelicenes
Table 4.8 Photophysical data of phosphahelicenes
Compound
λ Abs
max a (nm) λ Em (nm) Φ F (%) Solvent 10
3 g abs 10
3 g lum Ref.
(P)-(+)-53a
436
449
10
CH 2 Cl 2 –
+0.8
[51]
(M )-(À)-53b 438
455
7
CH 2 Cl 2 –
À0.7
[51]
(+)-53c
b
388
487
22
CHCl 3
–
+0.81
[52]
(+)-53d
b
315
502
8.5
CHCl 3
–
+0.48
[52]
a Lowest energy UV-vis band
b
Absolute configuration not determined
74
J. Crassous