Communication
be seen. The HOMO and LUMO are no longer delocalized over
the whole molecule, but are mostly localized on the central di-
meric helicene core. It is tempting to correlate these changes
in delocalization of the HOMO/LUMO orbitals to the experi-
mentally observed chiroptical properties, although other ef-
fects, such as differences in charge-transfer states, may also
contribute. A greatly increased p-electron delocalization in the
dimeric chromophore leads to non-linear increase of the ECD
1
[13]
Table 1. Summary of the De values of the B
b
bands for both enantio-
mers of monomeric 3, dimeric 1, and tetrameric 2, and the thiene-2,5-
À6
diyl-bridged dimeric 6, recorded in CH
2
Cl
2
at concentrations at cꢀ10
À1
[molL ] in CH
2
Cl
2
at 296 K.
À1
À1
Compound
De [Lmol cm
]
lmax [nm]
(
(
(
(
(
(
(
(
P)-3
M)-3
209
À220
561
À579
842
À851
332
À283
344
345
368
367
371
370
364
363
P)
M)
2
-1
-1
-2
-2
-6
-6
[6]
band intensities, compared to the monomer. Subsequent re-
duction in p-electron delocalization in the tetrameric chromo-
phore gave a much weaker increase in intensity of the Cotton
effects.
2
P)
4
M)
4
P)
2
M)
2
We subjected (P) -1 to heteroaromatization conditions with
2
[7c,d]
Na S in DMSO/THF 1:1 in the presence of KOH (Scheme 2).
2
Quantitative conversion to thiene-2,5-diyl-linked (P) -6 oc-
2
1
doubling of the number of helicene nuclei. Additionally, the
curred, as was evidenced by the H NMR spectra of the crude
1
lmax values for the Bb bands of the tetrameric system only
show a modest bathochromic shift of 3 nm (Table 1). Interest-
1
[13]
ingly, the second B band of tetrameric 2 displays a Cotton
a
À1
À1
effect of De=716m cm at lmax =314 nm, which corre-
sponds to a substantial bathochromic shift and intensity in-
À1
À1
crease compared to dimeric 1 (De=426m cm at lmax
99 nm).
The increase in the number of helicene nuclei in the oligo-
=
2
mers comes with a linear increase in specific and molar optical
rotations. Indeed, the molar optical rotation doubled from
Scheme 2. Quantitative cyclocondensation of the buta-1,3-diyne-1,4-diyl
moiety in (P) -1 with Na S with simultaneous cleavage of the TIPS groups af-
23
2
À1
2
À1
[
(
f] =18125 [8cm dmol ] in (P)-3 to 40767 [8cm dmol ] in
D
2
2
2
À1
fords thiene-2,5-diyl-linked (P)
2
-6. Reagent and conditions: KOH (0.3 equiv),
P) -1 and again to 82084 [8cm dmol ] in (P) -2. Such a linear
2
4
Na S·9H O (2 equiv), DMSO/THF 1:1, 808C, 1.5 h, quant. (isolated
2
2
increase had already been previously reported for bis-helicenic
yield=96%).
[
14]
terpyridine systems by Crassous and co-workers.
The UV/Vis spectra showed a 2.7-fold increase of the intensi-
ty from (M)-3 to (M) -1 (Figure 3). Tetrameric (M) -2 (blue line)
material. Remarkably, both TIPS groups were cleaved under
the basic conditions. The longest-wavelength Vis absorptions
2
4
showed another large increase in UV/Vis intensity, although
less than a doubling of the Cotton effect is seen upon chang-
ing from dimeric (M) -1 to tetrameric (M) -2. The g-factor (g=
of (P) -6 are substantially bathochromically shifted (by 12 nm)
2
compared to the absorptions of the buta-1,3-diyne-1,4-diyl-
2
4
De/e) plots display strong similarities for all three oligomers be-
tween 240 and 360 nm (Figure S4 in the Supporting Informa-
tion). However, the tetrameric chromophores exhibit a three-
fold higher g-factor intensity at 422 nm, originating from the
new band that appears in the ECD (inset in Figure 2) and UV/
Vis (inset in Figure 3) spectra.
bridged dimer (P) -1 (Figure S5 in the Supporting Information).
2
The thiophene-linked dimer exhibited fluorescence under labo-
ratory UV light at 365 nm, and we further explored this desira-
ble property, which is quite rare for carbohelicenes with un-
[15]
substituted all-benzene backbones. The compound shows a
structured emission band between 440 and 580 nm with two
DFT calculations of ECD and UV/Vis spectra were performed
maxima at lem =457 and 481 nm, attributed to the S symmet-
0
[
9]
[15a]
by using Gaussian 09 (Section S3 in the Supporting Informa-
tion). Structures were optimized at the B3LYP/6-31G(d) level of
theory, with the TIPS replaced by TMS groups. ECD spectra for
the lowest-energy conformer of each of (P)-3, (M) -1 and (M) -2
rical vibrational mode.
The absolute fluorescence quantum
[16]
yield
is 0.25 at lexc =400 nm (Figure 4). Monomeric (P)-3,
buta-1,3-diyne-1,4-diyl-linked dimeric (P) -1, and tetrameric
2
(P) -2 showed expectedly low absolute fluorescence quantum
4
2
4
were calculated at the TD-DFT:cam-B3LYP/6-31G(d) level of
theory incorporating 72 states. A comparison between experi-
mental and theoretical spectra showed good agreement (Sec-
tion S3 in the Supporting Information).
yields of 0.04, 0.1, and 0.06, respectively, with the emission
band (lem =420 and 454 nm) also indicative of the S vibration-
0
al mode (Figure S3 in the Supporting Information). ECD (Fig-
ure S6 in the Supporting Information) and UV/Vis spectral in-
We also calculated the highest occupied (HOMO) and lowest
unoccupied (LUMO) orbitals (B3LYP/6-31G(d); the accuracy of
this basis set was confirmed by comparison with B3LYP/
tensities of the thiophene-bridged dimer (P) -6 were moderate-
2
ly lower than those of (P) -1.
2
A DFT investigation of the HOMO and LUMO orbitals of
DGTZVP for tetrameric (P) -2; Section S3.2 in the Supporting
(P) -6 revealed that the density of the HOMO is localized
2
4
Information). In dimeric (P) -1, the electronic density of these
orbitals was fully delocalized over both helicene moieties. In
around the thiophene core. In the LUMO state, this trend is re-
versed, showing little density on the central thiophene, but on
the outer helicene benzene rings instead. This picture resem-
2
contrast, in tetrameric (P) -2, a clear loss in delocalization can
4
Chem. Eur. J. 2017, 23, 1 – 6
3
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&
&
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