The Journal of Physical Chemistry B
Article
Scheme 3. General Procedure for Synthesis of Ligands 1 and
2
intensity occurs when forming [(tpy) −Zn] (the complex with
2
a 2:1 ligand-to-metal ratio). The subsequent formation of tpy−
2+
Zn (←(tpy) −Zn + Zn ), however, often exhibits a smaller
2
fluorescence response and could be in the opposite direction
(
e.g., fluorescence increasing when forming 1a−Zn). The
finding could be a valuable guide for optimizing the
performance of tpy-containing materials.
The fluorescence spectrum at −196 °C reveals information
about the LE state because the molecules are frozen to its
ground-state conformation, which eliminates (or minimizes)
molecular reorganization in the excited state. The low
temperature also minimizes the solvent effect on the ICT as
orientation of frozen solvent molecules is less likely on the time
scale (<10 ns) of the excited state. By monitoring the
fluorescence response at different temperatures, the study
detects the transition from the LE state to the ICT-enabled
state, which is associated with a large spectral red shift. The
results point to that ICT plays an important role in the
emission properties of tpy−Zn complexes. Therefore, the
ability to generate a strong ICT in 1a is responsible for its zinc
binding-induced fluorescence quenching (via formation of 1a−
Zn) and its emission shift to a longer wavelength. Through the
study, the low-temperature fluorescence is shown to be a useful
tool for elucidation of the ICT mechanism, which is common
in luminescent materials. It should be noted that the
temperature-dependent ICT is based on the gradual permission
of the molecular motion in the excited state (through
controlling the rigidity of the molecular environment).
Detection of the temperature-dependent ICT in 1a−Zn thus
points out that the observed emission may not be associated
with the simple MLCT as the local rigid binding geometry
between Zn2+ and terpyridine may not be affected, and we do
not expect a large spectral shift at different temperatures.
δ 8.98−8.42 (m, 6H), 8.03−7.72 (m, 4H), 7.34 (dd, J = 7.4, 4.8
Hz, 2H), 6.82 (d, J = 8.9 Hz, 2H), 3.04 (s, 6H).
Compound 1b was synthesized by reaction of 2-acetylpyr-
1
idine with 4-diphenylamino-benzaldehyde. H NMR (300
MHz, CDCl ) δ 8.76−8.70 (m, 2H), 8.19 (d, J = 6.8 Hz,
3
2
7
7
H), 8.04 (d, J = 51.8 Hz, 2H), 7.86 (td, J = 7.5, 1.6 Hz, 2H),
.58 (d, J = 8.7 Hz, 4H), 7.47 (ddd, J = 7.5, 4.8, 1.2 Hz, 2H),
.31−7.28 (m, 4H), 7.13 (s, 4H), 7.01 (s, 2H).
Compound 1c was synthesized by reaction of 2-acetylpyr-
1
idine with 4-methyl-benzaldehyde. H NMR (300 MHz,
CDCl ) δ 8.73 (s, 2H), 8.67 (d, J = 7.9 Hz, 2H), 7.88 (t, J =
3
8
.0 Hz, 2H), 7.83 (d, J = 8.1 Hz, 2H), 7.44−7.35 (m, 2H), 7.34
(d, J = 4.7 Hz, 2H), 7.32 (d, J = 7.8 Hz, 2H), 2.43 (s, 3H).
Compound 2a was synthesized by reaction of 1-(6-methyl-
pyridin-2-yl)-ethanone (A) with 4-dimethylamino-benzalde-
1
hyde (B). H NMR (300 MHz, CDCl ) δ 8.69 (s, 2H), 8.43
3
(
2
6
d, J = 7.8 Hz, 2H), 7.86 (d, J = 8.7 Hz, 2H), 7.74 (t, J = 7.7 Hz,
H), 7.19 (d, J = 7.5 Hz, 2H), 6.85 (d, J = 8.7 Hz, 2H), 3.05 (s,
H), 2.68 (s, 6H).
Compound 2b was synthesized by reaction of 1-(6-methyl-
pyridin-2-yl)-ethanone (A) with B, 4-diphenylamino-benzalde-
1
hyde (B). H NMR (300 MHz, CDCl ) δ 8.21 (s, 2H), 7.98 (d,
3
J = 7.3 Hz, 2H), 7.88 (d, J = 15.8 Hz, 2H), 7.74 (t, J = 7.6 Hz,
2
4
H), 7.57 (s, 2H), 7.32 (d, J = 7.2 Hz, 4H), 7.15 (d, J = 7.5 Hz,
H), 7.07 (s, 2H), 7.02 (s, 2H), 2.67 (s, 6H).
Compound 2c was synthesized by reaction of 1-(6-methyl-
ASSOCIATED CONTENT
■
*
S
Supporting Information
pyridin-2-yl)-ethanone (A) with 4-methyl-benzaldehyde (B).
1
H NMR (300 MHz, CDCl ) δ 8.70 (s, 2H), 8.44 (d, J = 7.8
3
Hz, 2H), 7.82−7.77 (m, 2H), 7.73 (d, J = 7.7 Hz, 2H), 7.33 (d,
UV−vis of compounds 1 and 2 and their zinc complexes.
1
J = 7.8 Hz, 2H), 7.20 (d, J = 7.6 Hz, 2H), 2.67 (s, 6H), 2.44 (s,
H NMR titration of 1a with ZnCl (PDF)
2
3H).
Spectral Titration. All of the solvents for the fluorescence
AUTHOR INFORMATION
experiments were analytic grade, which were purchased from
Fisher Scientific and used without further purification. The
ligand solutions in DMSO (10 mmol/L) were prepared and
■
*
used as stock solutions. The 1 mM ZnCl solution was obtained
2
Notes
by dissolving zinc dichloride in DMSO. All UV/vis and
fluorescence titration experiments were performed using 10 μM
ligands in ethanol or aqueous solution (pH 7.4, 10 mM HEPES
The authors declare no competing financial interest.
2
+
ACKNOWLEDGMENTS
buffer) with varying concentrations of Zn at room temper-
■
1
ature.
This work was supported by the NIH (Grant No.
R15EB014546-01A1). We also thank the Coleman endow-
ment from the University of Akron for partial support and Mr.
Nick Alexander for assistance in acquiring the mass spectra.
CONCLUSIONS
■
In conclusion, several tpy ligands with different substituents
have been synthesized, and their zinc binding characteristics
have been examined by using spectroscopy at room and low
temperature. The study shows that the tpy ligands react readily
REFERENCES
■
(
1) Wild, A.; Winter, A.; Schlu
̈
tter, F.; Schubert, U. S. Advances in
with the added ZnCl , forming the zinc complex with a 2:1
2
the Field of π-Conjugated 2,2′:6′,2′-Terpyridines. Chem. Soc. Rev.
2
+
ligand-to-metal ratio (i.e., tpy −Zn) when the Zn concen-
2
2011, 40, 1459−1511.
2
+
tration is low. Further addition of Zn will lead to complex
tpy−Zn with a 1:1 ligand-to-metal ratio. The spectral evidence
also indicate that a relative large decrease in fluorescence
(
2) Andres, P. R.; Schubert, U. New Functional Polymers and
Materials Based on 2,2′:6′,2?-Terpyridine Metal Complexes. Adv.
Mater. 2004, 16, 1043−1068.
F
J. Phys. Chem. B XXXX, XXX, XXX−XXX