larger than that of pristine tetrathia[9]helicene (Φ F ¼ 0.02). Good CPL properties, with
an anisotropy factor g lum of 3.0 Â 10
À3 were measured for 44. In 2017, Avarvari et al.
reported the synthesis of (P) and (M)-45, corresponding to Pt(diimine)(dithiolene)
connected with a [6]helicene unit [41]. Interestingly, (P) and (M)-45 showed triplet
state CPL activity at room temperature with low g lum values of 3 Â 10
À4
.
4.5 CPL-Active Borahelicenes
Due to the electron-accepting and Lewis acidic character of boron, introducing
one or several boron atoms into carbohelicenes generally results in strongly blueemitting fluorophores. For chemical stability reasons, helicenes incorporating boron
atoms are azaborahelicenes and oxaborahelicenes, i.e., also including N or O atoms.
In 2017, our group prepared enantiopure azabora[n]helicenes 46a-d incorporating
one or two boron atoms and with 6,8 or 10 ortho-fused rings (n ¼ 6, 8, 10, Fig. 4.16)
[42]. These compounds displayed strong absorption between 250 and 450 nm
Table 4.5 Photophysical data of thiahelicenic derivatives
Compound
λ Abs
max a
(nm)
λ Em
(nm)
Φ F
(%)
Solvent
(CPL)
10
3 g abs
10
3
g lum
Ref.
(P)-(+)-41 470
630
–
Dodecane
+10
+10
[37]
(P)-43
390
450
27
THF
À4.7
À0.83
[39]
(P)-44
475
600
30
THF
+6.8
+3.0
[40]
(P)-45
562
715
15
CH 3 CN
~+0.91
b
+0.3
[41]
a Lowest-energy absorption band
b
Taken form [12]
N
F
B
CH 3
CH 3
(rac)-46a
N
F
B
CH 3
CH 3
(rac)-46b
N
B
N
B
H 3 C
CH 3
CH 3
CH 3
F
F
(M)-46d
N
N
B
H 3 C
H 3 C
B
CH 3
CH 3
(rac)-46c
N
B
X
X = H (47), OMe (48), NMe 2 (49)
B
O
O
B O
O
tBu
tBu
tBu
tBu
50
Fig. 4.16 Chemical structures of azabora[n]helicenes 46a–d and 47–49 and oxaborahelicene 50
70
J. Crassous