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diffused to the upper side while the N*-LC diffused to the lower side.9
Thus, a helix gradient distribution is realized accompanied with the
cross-linking of C6M and the formation of a cross-linked network,8 as
shown in Fig. 1b. More importantly, the isomerization of azobenzene
induced by UV irradiation can further expand the reflection wave-
length. It can be found that the HTPs of 2C decreased as the UV
irradiation time is increased, which exhibits a reflection wavelength
of N*-LC red shift, as shown in the ESI.† So the broadband reflection
of the sample shows a red shift owing to the isomerization of 2C
under UV irradiation, as shown in Fig. 1b. However, the restoration
process takes place when the cell is placed under visible light
irradiation for 20 min, which results in the restoration of 2C in the
lower side of the cell where the polymer network is rarefaction as
shown in Fig. 1c, while the 2C remains at the cis isomer at the upper
side of the cell where the polymer network is concentrated,8 so an
even bigger gradient of helix is formed, as shown in Fig. 1c. This can
be confirmed by comparison to the transmittance spectra of sample 2
before irradiation with visible light as shown in the ESI.† With the
fraction of the chiral compound increasing, the reflection wavelength
can be adjusted to the visible light region, which covers 400–800 nm,
as that of the sample 3 shown in Fig. 2. The transmittance at 400 nm
of sample 3 is lower than 50%, which is due to the strong absorbance
of the 2C molecule below 500 nm.
To further confirm the influence of azobenzene isomerization on
the broadband reflection, a series of comparative experiments were
carried out, that is, doping dye, 6C or 2C, into the N*-LC, respectively,
and then polymerizing them under UV irradiation at 40 1C; the
details of the composites can be seen in the ESI.† The content of the
dopant, which is 1.52%, 0.48%, 0.6% respectively, is determined by
the absorptivity.18 As different fractions of compounds were doped,
the absorbance of the mixture is maintained at the same level, so
that the same UV gradient distribution is formed. As shown in Fig. 3,
which shows the transmittance spectra of the samples after irradia-
tion by UV light, it can be observed that the width of transmittance
spectra of the sample doped with 2C is much wider than the one
doped with 6C or dye, which can be attributed to the red shift of the
broadband reflection caused by the isomerization of azobenzene.16
Fig. 4 shows the scanning electron microscopy (SEM) image of
the fractured surface of sample 2. It can be found that the sample
shows a pitch gradient distribution over the thickness of the cell,
that is, a bigger pitch at the upper side while a smaller one at the
Fig. 4 The SEM image of the fractured surface of sample 3.
lower side, as shown in the picture: P1>P2>P3>P4. The above experi-
mental results provide an important support for our explanation.
From the above discussion, it can be found that the broadband
reflection, which covers 1000–2400 nm in the near infrared region
and 400–800 nm in the visible region by adjusting the fraction of
the chiral compound before UV irradiation, can be prepared by
doping the chiral azobenzene compound into N*-LC and then
irradiating by UV light. This work suggests a new direction for
designing broadband reflection and it is promising for the fabrica-
tion of wide-band devices.
This work was supported by the National Natural Science Fund
for Distinguished Young Scholars (Grant No. 51025313), the National
Natural Science Foundation (Grant No. 51333001, 51173003,
51272026, 51273022, 51073096, and 61007016), the Major Pro-
gram of Chinese Ministry of Education (Grant No. 313002), the
Major Project of Beijing Science & Technology Program (Grant
No. Z121100006512002), and the Defense Industrial Technol-
ogy Development Program (Grant No. B1120110006).
Notes and references
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Fig. 3 The transmittance spectra of the sample doped with 0.60(wt)% of 2C,
1.52(wt)% of dye and 0.48(wt)% of 6C.
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