Conformation impact on spectral properties of bis(5,7-dimethyl-1,8- naphthyridin-2-yl)amine and its ZnII complex

Article abstract of DOI:10.1039/b705149b

Novel dual fluorescent compounds, bis(5,7-dimethyl-1,8-naphthyridin-2-yl) amine and its Zn^II complex, were synthesized and crystallographically characterized; their dual fluorescence and two groups of well-structured absorption bands in CH₂Cl₂ were attributed to a molecular conformational equilibrium. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b705149b

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Conformation impact on spectral properties of bis(5,7-dimethyl-
1,8-naphthyridin-2-yl)amine and its ZnII complexw
Yong Chen,a Wen-Fu Fu,*ab Jun-Li Li,b Xi-Juan Zhaoa and Xue-Mei Oua
Received (in Montpellier, France) 4th April 2007, Accepted 5th June 2007
First published as an Advance Article on the web 28th June 2007
DOI: 10.1039/b705149b
Novel dual fluorescent compounds, bis(5,7-dimethyl-1,8-naphthyridin-2-yl)amine and its ZnII
complex, were synthesized and crystallographically characterized; their dual fluorescence and two
groups of well-structured absorption bands in CH2Cl2 were attributed to a molecular
conformational equilibrium.
Introduction
Results and discussion
Dual fluorescence from organic molecules has been actively
investigated in the past few decades.1 Generally, such phenom-
ena result from the formation of an inter- or intramolecular
excited complex2–5 (excimer, exciplex, and so forth), a change
of structure involving ground state or excited state proton
transfer,6–9 or intramolecular charge transfer.10–17 The most
frequently studied types of dual fluorescence involve twisted
intramolecular charge transfer (TICT)11–14 or planar intramo-
lecular charge transfer (PICT)15–17 excited state emission.
Much attention has also been paid to dual fluorescence from
ground state structural changes.18 Previously, Dey and War-
ner supposed that dual fluorescence in 9-(N,N-dimethylami-
no)anthracene (9-DMA) originated from the existence of a
conformation equilibrium in both the ground and excited
states.19 Unfortunately, the absorption spectrum of 9-DMA
didn’t exhibit a marked red-shifted long wavelength band. To
develop a better understanding of the correlation between
photophysical properties and molecular conformation, we
are interested in designing new molecular systems that can
display obvious dual emission and absorption.
Compound 1 (Fig. 1) has a near planar geometry, and the
dihedral angle between the two naphthyridine ring planes is
only 15.51. The bond lengths of N(3)–C(9) and N(3)–C(11) are
1.387 and 1.391 A, respectively, and are partially double bond
in character. This suggests the presence of electronic coupling
between the NH group and the two naphthyridine ring
systems. However, this type of interaction is likely to be
weakened in solution; the lone electron pair of the NH group
is sensitive to its environment and readily interacts with
protons. In solution, the free rotation of the two naphthyr-
idine rings around C–N bonds would impact on the electronic
coupling, resulting in an intramolecular conformational equi-
librium between near planar and twisted conformations in the
ground state. In particular, in protic solvents, due to the
interaction between solvent protons and the lone electron pair
of the NH group, the conjugation would be completely
broken, and the twisted conformation would be prominent,
causing the lone electron pair orbital to be out of the plane of
the p-orbitals of the naphthyridine rings.
The absorption spectrum of compound 1 consists of two
structured absorption bands, the relative intensities of which
are obviously correlated with properties of the solvent (Fig. 2).
In a weak polar solvent, e.g. toluene, the two absorption bands
are of comparable intensity, while in an aprotic solvent, e.g.
CH2Cl2, the low energy absorption bands, ranging from
390–475 nm, decreased, but with a concomitant development
of the high energy absorption bands in the region 300–390 nm.
In the present study, the synthesis and photophysical prop-
erties of bis(5,7-dimethyl-1,8-naphthyridin-2-yl)amine (1) are
reported. It was found that this compound shows both dual
fluorescence and two groups of well-structured absorption
bands. Comparison investigations on the photophysical prop-
erties of compound 1 and two reference compounds (2-(2-
pyridyl)amino-5,7-dimethyl-1,8-naphthyridine (2) and 2-[N-
(2-pyridyl)methyl]amino-5,7-dimethyl-1,8-naphthyridine (3)
(Scheme 1)), as well as the ZnII complex of 1 (4), have shown
that an electronic interaction between the NH group and two
naphthyridine ring systems is responsible for the dual emission
and red-shifted absorption bands.
a Technical Institute of Physics and Chemistry, Graduated University
of Chinese Academy of Sciences, Beijing, 100080, China. E-mail:
wf_fu@yahoo.com.cn; Fax: (+86) 10-6255-4670
b College of Chemistry and Engineering, Yunnan Normal University,
Kunming, 650092, China
w Electronic supplementary information (ESI) available: Figures
S1–S7. See DOI: 10.1039/b705149b
Scheme 1
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wavelength absorption by compound 3 has not been observed,
both in protic and aprotic solvents (Fig. S2w).
It is foreseeable that when rotation of the naphthyridine
ring around the C–N bond is totally prevented, 1 should
mostly display long wavelength absorption bands in aprotic
solvents. Complex 4 affords a chance for us to observe such
phenomena, where 1 acts as a bidentate ligand, its two
naphthyridine rings being fixed by means of coordination to
the photo-inert metal center ZnII. The structural investigation
of 4 by X-ray crystallography shows that the molecule has a
rigid planar geometry, and the dihedral angle between the two
naphthyridine ring planes is less than 51 (Fig. 3). The long
wavelength absorption of 4 is prominent, both in CH3CN and
CH2Cl2 solutions (Fig. S3w). On the contrary, the long wave-
length absorption of complex 4 is fairly weak in CH3OH
compared to those in CH2Cl2 or CH3CN due to interaction
between protic solvents and the lone electron pair, but is
stronger than that of free ligand 1 in CH3OH solution. To
obtain more information, acid titration experiments on com-
plex 4 were performed (Fig. S4w). When HBF4 (3.5 Â 10À3 mol
Fig. 1 Perspective view of 1 with 30% probability ellipsoids. Selected
bond lengths (A) and angles (1): N(1)–C(2) 1.325(2), N(1)–
C(10) 1.367(2), N(2)–C(10) 1.358(2), N(2)–C(9) 1.327(2), N(3)–C(9)
1.387(2), N(3)–C(11) 1.391(2), C(9)–N(3)–C(11) 130.72(1), C(9)–N(3)–
H(3A) 113.4(1), C(11)–N(3)–H(3A) 115.7(1).
With the NH group protonated, the low energy absorption
bands disappeared. Addition of CH3OH to the CH2Cl2 solu-
tion of compound 1 afforded an absorption spectra with an
isosbestic point at 390 nm, weakened low energy absorption
bands and an accompanying increase in the high energy
absorption intensity (Fig. 2). The solution colour changed
from yellow to colorless. On the basis of structural analysis, we
assume that absorption in the longer wavelength region is
associated with a near planar conformation of 1, while the
absorption at the shorter wavelength involves the twisted
conformation.
L
À1 in CH3CN) was added to a CH3CN solution of complex 4,
the long wavelength absorption completely disappeared.
Therefore, the data obtained in the present study shows that
the absorption in the long wavelength region is associated with
electronic excitations, in which both the two naphthyrine ring
systems and the lone electron pair of the NH group partici-
pate, and the absorption at a shorter wavelength involves only
electronic excitations centered within one naphthyridine ring
system.
To verify this tentative assignment, two reference com-
pounds, 2 and 3, were synthesized (Scheme 1). In 2, the
interaction between the naphthyridine ring and pyridine ring
system was similar to that in 1, however, the same interaction
is unlikely in 3 because of the introduction of a methylene
group. In accordance with its structure, it was expected that
the absorption spectra of 2 would display a similar solvent
effect to that of compound 1 (Fig. S1w). Indeed, an investiga-
tion of its absorption spectra revealed that there is weak
absorption in the long wavelength region in aprotic solvents,
indicating the presence of a weak interaction between the
naphthyridine and pyridine rings, while in protic solvents,
long wavelength absorption disappeared. As expected, long
The solvent effect on the fluorescence of compound 1 was in
accordance with that observed in its absorption spectra (Fig.
4). In protic solvent CH3OH, compound 1 showed fluores-
cence emission only in the short wavelength region, while in
CH2Cl2, it displayed two groups of emission bands that were
excitation wavelength-dependent. Upon excitation at 355 nm,
1 displayed a fluorescence emission at around 380 nm with a
tail extending up to more than 500 nm. In the case of
excitation at 420 nm, a longer wavelength emission band
(lmax = 478 nm) developed in the region 450–600 nm. Though
Fig. 3 Perspective view of 4 with 30% probability ellipsoids; all
hydrogen atoms are omitted for clarity. Selected bond lengths (A)
and angles (1): Zn(1)–O(1) 1.947(3), Zn(1)–O(3) 1.961(3), Zn(1)–N(2)
2.068(3), Zn(1)–N(4) 2.048(3), N(4)–Zn(1)–N(2) 89.71(1), O(1)–Zn(1)–
O(3) 103.50(1).
Fig. 2 Absorption spectra changes of compound 1 in CH2Cl2 at
room temperature upon addition of CH3OH. Insert: Absorption
spectra of compound 1 (50 mM) in toluene (solid line), CH2Cl2 (dotted
line) and CH3OH (dash–dot line) at room temperature.
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Fig. 5 Spectrofluorimetric titration of HBF4 to a CH3CN solution of
4 (dash–dot line: [HBF4] = 0). Arrows indicate changes in the
intensities of bands upon acidification. Insert: Normalized emission
spectrum of 4 in CH2Cl2 (solid line) and CH3OH (dotted line) at room
temperature (lex = 360 nm).
Fig. 4 Normalized emission and excitation spectra of 1 in CH2Cl2 at
room temperature. Solid line: emission (lex = 355 nm); dashed line:
excitation (lem = 400 nm); dotted line: emission (lex = 420 nm);
dash–dot line: excitation (lem = 506 nm).
Experimental
the shapes of the fluorescence excitation spectra of the corre-
sponding emission bands are similar, the excitation energies
are completely different to each other. The excitation spectrum
corresponding to the high energy emission at 400 nm in
CH2Cl2 was identical to its short wavelength absorption band,
whereas the excitation spectrum of the low energy emission at
506 nm was markedly red-shifted with respect to that of
compound 1 (Fig. 4). The excitation spectra were measured
with various concentrations of 1 (Fig. S5w), from which there
was no detectable change in the excitation energy at 467 nm,
except for the decrease in intensity of the excitation spectra.
We tentatively suggest that dual fluorescence originates from
two excited states. The small Stokes shift suggests that the
excited state employs a molecular structure similar to that of
the ground state.
General methods
UV-vis absorption spectra were recorded using a Hitachi
U-3010 spectrophotometer. Emission and excitation spectra
were obtained using a Hitachi F-4500 fluorescence spectro-
fluorimeter. 1H NMR spectra were measured on a Bruker
Avance DPX-400 MHz resonance spectrometer using TMS
(SiMe4) as an internal reference at room temperature.
Syntheses
General procedure for the synthesis of 1–3.21,22 A suspension
of 2,4-dimethyl-7-chloro-1,8-naphthyridine,23,24 amine and
powdered KOH in a 2 : 2 : 3 molar ratio in toluene was
refluxed for 24 h. Upon removal of the solvent, the residue was
washed with water until the washings were neutral. The
product was purified by column chromatography over a silica
gel column using CHCl3/CH3CH2OH as the eluent.
1: The amine is 2,4-dimethyl-7-amino-1,8-naphthyridine.25
Yield 0.72 g, 22%; 1H NMR (400 MHz, DMSO-d6, TMS) d =
2.60 (s, 6 H, CH3), 2.62 (s, 6 H, CH3), 7.17 (s, 2 H), 8.28 (d, J
= 9.0 Hz, 2 H), 8.45 (d, J = 9.0 Hz, 2 H) and 10.68 (s, 1 H);
MS(EI): m/z = 330 (M + 1), 329 and 328 (M À 1) (100%).
2: The amine is 2-aminopyridine. 1H NMR (400 MHz,
CDCl3, TMS) d = 2.60 (s, 3 H, CH3), 2.70 (s, 3 H, CH3),
6.95 (t, J = 6.7 Hz, 1 H), 7.02 (s, 1 H), 7.42 (d, J = 8.8 Hz,
1 H), 7.73 (t, J = 7.8 Hz, 1 H), 8.14 (d, J = 8.9 Hz, 1 H), 8.33
(d, J = 4.2 Hz, 1 H), 8.42 (s, 1 H) and 10.90 (s, 1 H).
3: The amine is 2-aminomethylpyridine. 1H NMR (400
MHz, CDCl3, TMS) d = 2.52 (s, 3 H, CH3), 2.64 (s, 3 H,
CH3), 4.98 (d, J = 4.4 Hz, 2 H), 6.30 (s, 1 H), 6.74 (d, J = 8.9
Hz, 1 H), 6.87 (s, 1 H), 7.18 (t, J = 6.1 Hz, 1 H), 7.34 (d, J =
7.8 Hz, 1 H), 7.64 (t, J = 7.6 Hz, 1 H), 7.93 (d, J = 8.9 Hz,
1 H) and 8.56 (d, J = 4.6 Hz, 1H).
Complex 4 also exhibits dual fluorescence emission. At
room temperature, two groups of emission bands with a
vibronic structure are evident in CH2Cl2, and the low energy
emission is prominent, while the high energy emission dom-
inates in CH3OH (Fig. 5). Furthermore, with the addition of
acid (HBF4), the nitrogen atom of the protonated NH group
results in the quenching of the low energy emission and the
enhancement of high energy emission (Fig. 5). The above
results further support the notion that a lone electron pair
interaction with the naphthyridine ring system plays a key role
in the red-shifting of the low energy absorption bands.
In summary, a dual fluorescent compound, bis(5,7-di-
methyl-1,8-naphthyridin-2-yl)amine, has been synthesized
and its spectroscopic properties investigated. This compound
shows two groups of absorption due to the conformational
equilibrium between a near planar molecular geometry and a
twisted geometry. The excitation spectra of the two fluores-
cence bands are different from each other and resemble the
two absorption bands, respectively, which suggests that dual
fluorescence of compound 1 comes from different excited
states of the two different conformations in the ground state.
Spectral investigations on its ZnII complex and reference
compounds 2 and 3 afforded further evidence for the mechan-
ism of dual fluorescence and absorption.
4: A CH3OH solution (10 mL) of Zn(OAc)2 Á 2H2O (0.13
mg, 0.60 mmol) was added to a CH2Cl2 solution (20 mL) of 1
(0.20 mg, 0.60 mmol). The yellow solution was stirred for 5 h
at room temperature and then filtered. Crystals of 4 Á CH2Cl2
suitable for X-ray structural analysis were obtained by vapor
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diffusion of diethyl ether into the filtrate. Yield 0.25 g, 80%;
1H NMR (400 MHz, CDCl3, TMS) d = 2.10 (s, 6 H, CH3),
2.11 (s, 6 H, CH3), 2.50 (s, 6 H, CH3), 6.82 (s, 2 H), 8.24
(d, J = 9.1 Hz, 2 H) and 8.31 (d, J = 9.1 Hz, 2 H).
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CIF or other electronic format see DOI: 10.1039/b705149b
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to the State Key Project (2005CCA06800) for financial sup-
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