1. Modulation of Photoswitching Profiles by 10,11 Dialkoxy-methyl Substituents in C2-Symmetric Dibenzosuberane-Based Helicenes
Wen-Ching Chen,[b] Po-Chiao Lin,[b] Chien-Hsiang Chen,[b] and Chien-Tien Chen*[a]
[a] National Tsing Hua University, Taiwan
[b] National Taiwan Normal University, Taiwan
Chem.Eur.J. October 2010

2. Introduction……. Chiral dopant
Photo reversible molecular switches: compounds which are able to switch between electronic configurations when irradiated by light of a specific wavelength
Azobenzene, diarylethenes, dithienylethenes, fulgides(Dialkylidenesuccinic anhydrides), stilbene,spiropyrans etc
Helically shaped overcrowded olefins (helicenes)- reversible change in helicity (P,M)
LC dopants- external impurity (chiral molecules) added to LC
phase of LC may change (nematic to smectic …….)
Chiral dopant
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3. Axially dissymmetric helicenes – chiral dopants
Two main approaches to satisfy requirements are
1. Photoresolution of enantiomeric helicene
2. Trnasform enantiomeric helicenes into corresponding geometrical (diastereomeric) ones by different substituents on geminal position of aromatic ring
J. Am. Chem. Soc , 2000
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4. 6c as chiral dopants in LC – phase change
Present study….
Previous report- C2-symmetric, 10,11-diethyl-dibenzosuberane (DBS)-based helicene (6a’) as chirochromic liquid-crystal (LC) optical Switches
helicenes 6b,c, which possess methoxymethyl (MOM) or n-heptoxymethyl (HOM) groups at both C10 and C11 positions of the seven membered DBS template
To get the conformational effects of the MOM and HOM on the photoswitching selectivity 6 b,c were studied in detail with comparison of 6a’
6c as chiral dopants in LC – phase change
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5. Synthesis……..
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6. Absolute Stereochemistry – by X-ray crystallographic analysis…..
At C1’ of episulphide- S
P-helicene b)
5a-c
10R,11R, 1’S
10S, 11S, 1’R
At C1’ of episulphide- R
M-helicene
Since at C1’ of 5b were found to be S resulting
helicene should P
5b and 6b
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7. Photoisomerization…….
Table 1. Diastereoselective photoisomerization by irradiation of
(10R,11R,P)-6a–c in different solvents with varying wavelengths.
Table 1 shows 6c only
[a] Irradiation time is 1 h
[b] % determined by HPLC analysis on Chiralpak OT(+) column
at 249 nm (isosbestic point) detection wavelength.
[c] de defined as (P-M’-6c)/(P+M’-6c).
[d] Irradiation time is 40 min.
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8. Absorption and CD spectra…..
Figure 2. a) UV spectra of (10R,11R,P)-6c (black)
and (10R,11R,M’)-6c (red) after 290 nm UV, and
the UV difference spectrum (green) in hexane;
b) CD stacked plots of (10R,11R,P)-6c (10 micromolar) in hexane at 258 o C under UV irradiation (290 nm at 0, 10, 20, 30, 40, 50, 60 min).
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9. by the Quantum-Cache program on MOPAC2000-AM1……..
Molecular simulations of (P)-6c and (M’)-6c with conformational permutations
by the Quantum-Cache program on MOPAC2000-AM1……..
Table 2. Minimized energies and three appropriate phenyl torsional
angles for (P)-6c and (M’)-6c.
[a] φ1 means the phenyl torsion between ring A and the central double
bond. [b] φ2 means the phenyl torsion between ring B and the central
double bond. [c] φ3 means the phenyl torsion between ring C and the
central double bond.
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10. Proposed photoswitching cycle…….
ACW db rotn
290 nm
Peq M’eq
1.2 kcal RT RT kcal
Pax M’ax
Conformational flip
Conformational flip
330 nm
CCW db rotn
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11. Photomodulation in LC……….
6b did not give –poor solubility
6c lead to a phase change to cholesteric in nematic LC host (E7)
6c (1wt %) P HTP (+7.2)
6c M’ (-12.1)
6c P (+6.1)
290 nm
330 nm
Figure 3. Polarized optical photomicrographs (40 magnification) of
1 wt% (10R,11R,P)-6c dissolved in E7 at 258 o C in a wedge cell (A: the
initial state; B: after irradiation at 290 nm of A for 3 h; C: after irradiation
of B at 330 nm for 2 h; D: after irradiation at 310 nm for 3 h).
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12. Thanking you Conclusion…
Introduced a series of helicenes with C2-symmetric,10,11-disubstituted-DBS templates and a common 7-bromo-a-tetralin-based bottom fragment
The absolute stereochemistry was determined to be 10R,11R,P assuming chirality transfer from the corresponding (10R,11R,1’S) Episulfides
C10, C11 substituents (ethyl, MOM and HOM) influence conformational flips of ring, as evidenced by CD and HPLC trace experiments
Nematic LC (E7) doped with 6c led to cholesteric mesophases with modulable pitches
Thanking you
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