Photocontrolled transparent–opaque transition of a thermosensitive
homopolymer solution based on an azobenzene–cyclodextrin system
Minxi Yin, Meina Zhang, Gang Yuan, Chunyu Wang, Liyan Wang*
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012,
China
Electronic supplementary information
1. 1H NMR spectrum of Azo-Py+
Figure S1. 1H NMR spectrum of Azo-Py+ in D2O at 298K.
2. Stoichiometry of the Inclusion Complex of PEPAAm and α-CD
The stoichiometry of a host–guest complex can be determined by the method of continuous
variations (Job's method). In this case, α-CD acts as host, and the repeating unit of PEPAAm is the
corresponding guest. Cloud point is taken as the property that is closely related with the concentration of
complex. Equation 3 in the reference 33 was used:
1
Cc = (Tc−T0)C0/(Tc0−T0)
(S1)
C0 is the initial concentration of the repeating units of PEPAAm. Cc is the equilibrium
concentration of the repeating units of PEPAAm complexed with α-CD. Tc0 is the cloud point of
PEPAAm when all the repeat units of PEPAAm are complexed with α-CD (it is an imaginary state). Tc
is the actual cloud point of the PEPAAm solution containing different amounts of α-CD. Because
(Tc0−T0) is a constant, the concentration of the complex Cc is proportional to (Tc−T0)C0. An appropriate
Job’s plot for the present system would be (Tc−T0)C0 as a function of the molar concentration fraction of
α-CD. We measured the cloud points of a series of mixed solutions of PEPAAm and α-CD, in which the
sums of the host concentration and guest concentration are all equal to 6.36×10−2 mol/L. Since C0 is
proportional to molar concentration fraction of guest (Xg), we plot (Tc−T0)Xg as a function of the molar
concentration fraction of α-CD, as shown in Figure S2. When the molar concentration fraction of α-CD
equals 0.5, (Tc−T0)Xg has a maximum, i.e. the concentration of complex is maximal. This result
indicates that the stoichiometry of inclusion complex of α-CD and a repeating unit is 1:1.
Figure S2. Job’s plot for inclusion complex of PEPAAm and α-CD. For these mixed solutions of
PEPAAm and α-CD, the sums of the host concentration and guest concentration are all equal to
6.36×10-2 mol/L.
3. Determination of the Inclusion Complexation Constant with NMR Titration
We measured the chemical shift of Ha of 3.18 mmol/L PEPAAm solutions with different molar
concentrations of α-CD with tetramethylammonium chloride (TMAC) as the reference. As shown in
2
Figure S3, the chemical shift of Ha of PEPAAm shift from 1.109 to 1.130 as the concentration of α-CD
increase from 0 to 57.25 mmol/L. This phenomenon suggests the formation of inclusion complex of
PEPAAm and α-CD. The NMR titration data were fitted by Equation S2, of which the deduction
procedure was similar to that of Equation 5 in the reference 33. It is necessary to state that Equation S2
contains an approximation, and please find a better equation for determining association constants in the
review article written by P. Thordarson (Chem. Soc. Rev., 2011, 40, 1305–1323).
δ is the chemical shift of Ha of PEPAAm containing different concentrations of α-CD. δ0 is
chemical shift of Ha of PEPAAm in the absence of α-CD. δc0 is the chemical shift of Ha of PEPAAm
when all the repeat units of PEPAAm are complexed with α-CD (it is an imaginary state). D0 is the
molar concentration of α-CD. Keq is the inclusion complexation constant. The solid line in Figure S3 is
the fitting curve using Equation S2. The obtained fitting parameter is Keq = 6.5±1.1 L/mol. The
coefficient of determination (R2) is 0.999.
Figure S3. Increase of the chemical shift of Ha of PEPAAm (in D2O at 298K) with increasing
concentration of α-CD, and curve fitting with Equation S2 (solid line). The concentration of repeat unit
of PEPAAm was maintained at 3.18 mmol/L.
4. Calibration Curve of UV Absorbance versus Mole Fraction of trans-Azo-Py+
A solution of 14.1 mmol/L Azo-Py+ was prepared, and the same molar concentration of α-CD was
3
added. The UV-vis spectrum of the solution was recorded after light irradiation. As shown in Figure S4a,
the absorption band at around 325 nm decreased gradually, and meanwhile the band at around 430 nm
increased slightly as the UV irradiation went on. This spectral variation clearly shows the isomerization
of the azobenzene group from the trans form to the cis form. The solution was subsequently irradiation
with visible light. As shown in Figure S4b, the absorption band at around 325 nm increase gradually,
and meanwhile the band at around 430 nm decrease slightly. This indicates the isomerization of the
azobenzene group from the cis form to the trans form. In addition, two isoabsorptive points exist at 237
nm and 278 nm in UV-vis spectra.
Figure S4. UV-vis absorption spectra of Azo-Py+/α-CD solution under UV irradiation (a) for 0 min, 2
min, 4 min, 7 min, 9 min, 11 min and subsequently under irradiation with visible light (b) for 0 min, 2
min, 4 min, 6 min, 8 min.
Beer's law was applied to analyze the isomerization of Azo-Py+ by UV−vis spectroscopy. In the
following discussion, the absorbance of α-CD was ignored, which is very small between 210 and 700
nm.
The absorbance of Azo-Py+/α-CD solution at 325 nm is given by
A325 = (εt,325Xt+εc,325Xc)lc = (εt,325Xt+εc,325(1- Xt))lc
(S3)
c is the concentration of Azo-Py+. l is the optical path length. εt,325 and εc,325 are the molar
absorption coefficient of trans-Azo-Py+ and cis-Azo-Py+ at 325 nm, respectively. Xt and Xc are the
molar fraction of trans-Azo-Py+ and cis-Azo-Py+, respectively.
The absorbance of Azo-Py+/α-CD solution at isoabsorptive point 237 nm is given by
A237 = (εt,237Xt+εc,237Xc)lc = ε237lc
(S4)
In order to eliminate the effect of the error of concentration on A325 in future experiments, we
obtained Equation S5 using Equation S3 and S4.
4
(S5)
The mole fraction of trans-Azo-Py+ (Xt) can be determined by 1H NMR. As shown in Figure S5,
after UV irradiation, the peak at 5.80 ppm was assigned to the protons of methylene of trans-Azo-Py+,
and the peak at 5.54 ppm was assigned to the protons of methylene of cis-Azo-Py+. Thus the mole
fraction of trans-Azo-Py+ can be calculated using the areas of the two peaks in a 1H NMR spectrum
after UV irradiation.
Figure S5. 1H NMR spectra of Azo-Py+ before UV irradiation (a) and after UV irradiation (b).
We carried out the following experiments to obtain a calibration curve. Firstly, we prepared a
solution of 14.1 mmol/L Azo-Py+ containing α-CD with the same molar concentration in D2O, and
irradiated the solution with UV light. Then, the mole fraction of trans-Azo-Py+ (Xt) was determined by
1
H NMR. Meanwhile, 5 μL of the solution was diluted to 2 mL with water, and the UV−vis spectrum of
the dilute solution was measured to calculate the value of A325/A237. We obtained a series of Xt and
corresponding value of A325/A237 of the solution under UV irradiation for different time periods. As
shown in Figure S6, a calibration curve was established by curve fitting the data using Equation S5. The
calibration curve can be used to determine mole fraction of trans-Azo-Py+ from a UV−vis absorption
5
spectrum.
Figure S6. Calibration curve of A325/ A237 versus Xt.
5. GPC Trace of PEPAAm
By analyzing its GPC trace (Figure S7), molecular weights of PEPAAm are obtained, Mw =
1.25×104; Mn = 9.13×103. Polydispersity index equals 1.37.
Figure S7. GPC trace of PEPAAm.
6