Paper
NJC
integrated scattered intensity over a scattering angular range from
201 to 1401 and of intensity–intensity digital photon correlation over
a similar angular range (DLS and depolarized DLS). About
2–3 mL of sample solutions was transferred into a special dust-
free light scattering cell for light scattering measurements. The
scattering cells were held in a brass thermostat block filled with
refractive index-matching silicone oil. The temperature was con-
trolled to within ꢂ0.05 1C. DLS measures the intensity–intensity
time correlation function G(2)(G) in the self-beating mode by means
of a multi-channel (BI-9000) digital correlator. G(2)(G) can be related
to the electric field time correlation function g(1)(t):
Fig. 2 Photographs of the bis-terpyridyl Eu3+-complex L1 methanol
solution at c = 0.2 mg mLꢁ1 with (right) and without (left) UV radiation.
G
(2)(G) = A(1 + b|g(1)(t)|2)
(1)
configuration change of a mono molecule was extended to the
transformation of the assembling microstructures.
Here, A and b are the background (baseline) and a coherence
factor (a parameter depending on the detection coherence),
respectively. The electric field time correlation function, g(1)(t),
was analyzed by the constrained regularized CONTIN method20
to yield information on the distribution G(G) of the characteristic
line width (G) from
Tpy-NH2 was first synthesized using a previously reported
method.21,22 The resulting product was conjugated with phenol
by the diazotization coupling reaction to introduce azobenzene
groups. The chemical structure of compound L1 was characterized
1
by H-NMR spectra shown in Fig. 1a and MALDI-TOF-MS results
ð1
ꢀ
ꢀ
ꢀ
ꢀ
ð1Þ
shown in Fig. S1 in the ESI.† The sharp peaks from 8.66 ppm to
8.83 ppm in Fig. 1a can be ascribed to the protons in pyridine. The
mass spectrum of compound L1 in Fig. S1 in the ESI† shows the
presence of a molecular ion peak with m/z = 989.86, demonstrating
the successful formation of the product. Another compound was
assigned to be compound L2, as shown in Scheme S1 in the ESI,†
and its 1H NMR spectrum is shown in Fig. S2 in the ESI.†
The successful preparation of bis-terpyridyl Eu3+-complex L1
was also investigated by 1H NMR and infrared spectroscopy. As
shown in Fig. 1a, the characteristic peaks of the protons in pyridine
yield a low field shift from 8.66–8.83 ppm to 8.90–9.22 ppm due
to the lower electron density of pyridine after coordination.
Meanwhile, the sharp peaks of protons change into a broad
shape, which is commonly observed after the dynamic inhibition of
the protons.23 As shown in Fig. 1b, the C–N characteristic peak for
pyridine (1584.50 cmꢁ1) weakened after coordination, which was
due to the N-metal interaction in bis-terpyridyl Eu3+-complex L1.24
The presence of europium within bis-terpyridyl Eu3+-complex L1
was demonstrated by X-ray photoelectronic spectroscopy (XPS). The
spectrum shown in Fig. S3 in the ESI† exhibits the presence of C 1s,
O 1s, N 1s and Eu 3d. The deconvolution of the N 1s signal
generates two singlets. The singlet at 389.5 eV comes from the
N–C units; the singlet at 399.1 eV is derived from the N–Eu
units, which indicates the complexation between Eu3+ and
terpyridyl groups (Fig. S3b, ESI†).
g
ðtÞ ¼ GðGÞ expðꢁGtÞdG
(2)
0
Ð
The first and second moments of G(G) are hGi ¼ 1GGðGÞdG
0
Ð1
2
2
and m2 ¼ ðG ꢁ hGiÞ GðGÞdG, respectively. The value of m2/hGi is
0
a measure of the particle polydispersity. If the relaxation is
diffusive, G can be related to the average apparent diffusion
coefficient (Dapp):
2
Dapp(1 + kdc)(1 + fhRg iq2) = G/q2
(3)
Here, kd is the diffusive second virial coefficient, Rg is the radius of
gyration, q is the magnitude of the scattering wave vector, and f is a
dimensionless factor related to hydrodynamic draining, internal
motion, polydispersity, and solvent quality. If the average apparent
diffusion coefficient (Dapp) is known, the apparent hydrodynamic
radius, Rh, can be obtained via the Stokes–Einstein equation:
Rh = kBT/6pZDapp
(4)
Here, kB is the Boltzmann constant, and Z is the viscosity of the
solvent at temperature T. From DLS measurements, we can
obtain the particle-size distribution in solution from a plot of
GꢀG(G) vs. Rh, with GꢀG(G) being proportional to the scattered
intensity of ith particles having an apparent hydrodynamic
radius Rh. DLS measurements were performed at finite concentra-
tions, where inter-particle interactions have been neglected.
After centrifugal purification, bis-terpyridyl Eu3+-complex L1
was suspended in methanol with a final concentration of
0.2 mg mLꢁ1. As shown in Fig. 2, the methanol solution of bis-
terpyridyl Eu3+-complex L1 exhibits the yellowish-brown color of
the azobenzene chromophore.
3 Results and discussion
3.1 Synthesis of bis-terpyridyl Eu3+-complex L1
A long-chain ligand of the coordination compound L1, as
shown in Scheme 1, was synthesized. After being coordinated with
EuCl3ꢀ6H2O, the fabricated bis-terpyridyl Eu3+-complex L1 was
dissolved in methanol and exhibited prominent UV- responsive
emission enhancement. Compound L1 with azobenzene was
3.2 Excitation and emission spectra of bis-terpyridyl
Eu3+-complex L1 and fluorescence variations under ultraviolet
radiation
employed as allosteric groups to offer UV-responsive isomerization. At l = 365 nm ultraviolet radiation, indigo fluorescence was
With the coordination link of each terpyridine group, the observed with the naked eye. The excitation and emission
19358 | New J. Chem., 2019, 43, 19355--19364 This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019