Large stokes shift induced by intramolcular charge transfer in N,O-chelated naphthyridine-BF2 complexes

Article abstract of DOI:10.1021/ol302347m

Novel N,O-chelated naphthyridine-BF₂ complexes with push-pull structures have been synthesized and characterized. Spectral investigations on these complexes reveal that photoinduced intramolecular charge transfer occurs and results in a large Stokes shift, which is further supported by density functional theory based theoretical calculations.

Full text of DOI:10.1021/ol302347m

ORGANIC
LETTERS
2012
Vol. 14, No. 20
5226–5229
Large Stokes Shift Induced by Intramolcular
Charge Transfer in N,O-Chelated
NaphthyridineꢀBF2 Complexes
Yun-Ying Wu,†,‡ Yong Chen,*,‡ Gao-Zhang Gou,† Wei-Hua Mu,† Xiao-Jun Lv,‡
Mei-Ling Du,† and Wen-Fu Fu*,†,‡
College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming
650092, P.R. China, and Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, CAS-HKU Joint Laboratory on New Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190,
P.R. China
fuwf@mail.ipc.ac.cn; chenyong@mail.ipc.ac.cn
Received August 22, 2012
ABSTRACT
Novel N,O-chelated naphthyridineꢀBF2 complexes with pushꢀpull structures have been synthesized and characterized. Spectral investigations
on these complexes reveal that photoinduced intramolecular charge transfer occurs and results in a large Stokes shift, which is further supported
by density functional theory based theoretical calculations.
The notable advantages of borondipyrromethene
(BODIPY) derivatives, such as high extinction coeffi-
cients and intense fluorescence emissions ranging from
the blue to near-infrared (NIR) region, are crucially
responsible for enabling them to have potential use in a
wide range of applications involving light harvesting,
energy transport, sensing, or biological imaging.1 Unfor-
tunately, the intense fluorescence of BODIPY dyes in the
solution phase is usually quenched in the solid state be-
cause of intermolecular interactions (e.g., πꢀπ stacking).2
Moreover, most luminescent BODIPY derivatives dis-
play to a limited extent a Stokes shift (<20 nm), which
results in self-absorption of the higher energy part of their
emission spectra and in measurement interference by ex-
citation light and scattered light. Tremendous efforts have
been directed toward addressing this issue, and considerable
progress has been achieved in this context. For example,
Piers et al. explored high Stokes shift anilido-pyridine
BF2 dyes by harnessing a desymmetrized ligand to render
the ground and excited states more energetically distinct.3
Bulky side substituents have been introduced into BOD-
IPY dyes to enlarge the Stokes shift and also to reduce the
πꢀπ stacking in the solid state.4 A large pseudo-Stokes
shift has also been realized by fluorescence resonance
(3) Araneda, J. F.; Piers, W. E.; Heyne, B.; Parvez, M.; McDonald,
R. Angew. Chem., Int. Ed. 2011, 50, 12214–12217.
(4) (a) Zhang, D.; Wen, Y.; Xiao, Y.; Yu, G.; Liu, Y.; Qian, X. Chem.
† Yunnan Normal University.
‡ Chinese Academy of Sciences.
ꢀ
Commun. 2008, 4777–4779. (b) Vu, T. T.; Badre, S.; Dumas-Verdes, C.;
Vachon, J.-J.; Julien, C.; Audebert, P.; Senotrusova, E. Y.; Schmidt,
(1) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932. (b)
Ziessel, R.; Ulrich, G.; Harriman, A. New J. Chem. 2007, 31, 496–501. (c)
Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47,
1184–1201.
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Interscience: London, 1970. (b) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem.
Soc. Rev. 2011, 40, 5361–5388. (c) Yang, Y.; Su, X.; Carroll, C. N.;
Aprahamian, I. Chem. Sci. 2012, 3, 610–613.
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E. Y.; Trofimov, B. A.; Pansu, R. B.; Clavier, G.; Meallet-Renault, R.
J. Phys. Chem. C 2009, 113, 11844–11855. (c) Ozdemir, T.; Atilgan, S.;
Kutuk, I.; Yildirim, L. T.; Tulek, A.; Bayindir, M.; Akkaya, E. U. Org.
Lett. 2009, 11, 2105–2107. (d) Fu, G.-L.; Pan, H.; Zhao, Y.-H.; Zhao,
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7852–7861.
r
10.1021/ol302347m
Published on Web 10/10/2012
2012 American Chemical Society

energy transfer (FRET)5 and through-bond energy trans-
fer (TBET).6 While photoinduced intramolecular charge
transfer (ICT) has been utilized with the aim of achieving
a large Stokes shift in common organic systems, this strategy
has rarely been adopted in BODIPYs and their analogues.7
We are interested in the design and synthesis of naphthyr-
idine-based BF2 complexes with intriguing luminescence
properties. Previously, we reported a bis(BF2) core complex
containing 1,8-naphthyridine units; the complex displayed a
yellow-green emission with a fluorescence quantum yield up
to 0.965 in dichloromethane.8 Similar to most BODIPYs
and their analogues, this naphthyridineꢀBF2 complex ex-
hibited faint photoluminescence in the solid state. By con-
trolling the molecular arrangements and the number of πꢀπ
interactions in the solid state, we then successfully obtained
solid-state emissive 1,8-naphthyridineꢀBF2 complexes.9
One significant deficit in the properties of these dyes is their
generally small Stokes shift. In the present work, we propose
to develop solid-emissive 1,8-naphthyridineꢀBF2 complexes
with large Stokes shifts. Our strategy involves the incorpora-
tion of an electron-donating dimethylamino aryl group into
the 1,8-naphthyridineꢀBF2 moiety, which is anticipated to
result in a pushꢀpull system in view of the fact that the
naphthyridine unit is a well-known acceptor. Indeed, the
newly synthesized donorꢀacceptor compounds (Scheme 1)
were found to exhibit efficient intramolecular charge-transfer
(ICT) emission in polar solvents with relatively large Stokes
shifts and strong solvatochromism. For comparison, com-
plexes 3b and 4b with a relatively neutral group (H for 3b)
or an electron-withdrawing group (COOMe for 4b) on
the p-position of the phenyl ring have also been prepared.
8-naphthyridine or 2-amino-5,7-di(trifluoromethyl)-1,8-naph-
thyridine.10 The raw materials were converted to 1aꢀ4a by
reacting with p-substituted methyl benzoate in dry THF with
sodium hydride as a base; the resulting amido ligands were
treated with BF3 Et2O in dry chloroform in the presence
3
of triethylamine at rt to afford the desired fluorineꢀ
boron complexes 1bꢀ4b in high yields. All compounds
synthesized were fully characterized by multinuclear
NMR spectroscopy, ESI-MS, infrared spectroscopy, and
elemental analyses. It is notable that the strong IR absorp-
tion peaks at ∼1700 cmꢀ1, related to the CdO stretch
vibration of the precursors 1aꢀ4a, vanished after coordina-
tion with BF3 (Figure S1). Details of the experimental pro-
cedures and data for structural characterization were
described in the Supporting Information (SI).
Figure 1. ORTEP diagrams for 1b (left) and 3b (right) showing
30% probability ellipsoids. All H-atoms are omitted for clarity.
Single crystals of complexes 1b, 2b, and 3b suitable for
X-ray diffraction analysis were grown by slow diffusion of
diethyl ether vapors into a dichloromethane solution.
Single crystals of the precursor 3a and 4a were also ob-
tained by evaporating their dichloromethane solutions at rt.
Detailed crystallographic data were summarized in Table S1.
Complex 1b crystallizes in an orthorhombic lattice with-
out solvates. The central B-atom has a slightly distorted
tetrahedron geometry with BꢀF, BꢀN, BꢀO distances of
Scheme 1. Synthetic Routes and Chemical Structures of 1bꢀ4b
˚
1.3795(2), 1.579(3), 1.471(3) A, respectively, and bond angles
around the B-atom ranging from 107.36(1)° to 112.47(2)°.
These values for bond lengths and bond angles are normal for
N,O-chelated BF2 complexes.11 As depicted in Figures 1 and
S2, the entire molecule of 1b is near coplanar, and all portions
except for two F-atoms are located in the same plane; this
suggests the formation of an extended π-conjugation system
and is favorable for intramolecular charge transfer. The
πꢀπ stacking interactions were found to exist along the
As shown in Scheme 1, the naphthyridineꢀBF2 com-
pounds were successfully obtained by a facile two-step
synthetic route starting from 2-amino-5,7-dimethyl-1,
(7) (a) Peng, X.; Song, F.; Lu, E.; Wang, Y.; Zhou, W.; Fan, J.; Gao, Y.
J. Am. Chem. Soc. 2005, 127, 4170–4171. (b) Zhou, Y.; Xiao, Y.; Chi, S.;
Qian, X. Org. Lett. 2008, 10, 633–636. (c) Frath, D.; Azizi, S.; Ulrich, G.;
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Long, C.; Forster, R. J.; Keyes, T. E. Chem. Commun. 2012, 48, 5617–5619.
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Duan, X. M.; Song, H. B. Org. Lett. 2010, 12, 2924–2927.
(9) Quan, L.; Chen, Y.; Lv, X. J.; Fu, W. F. Chem.;Eur. J.
2012 DOI: 10.1002/chem.201201592.
(10) Henry, R. A.; Hammond, P. R. J. Heterocycl. Chem. 1977, 14,
1109–1112.
(11) Hachiya, S.; Inagaki, T.; Hashizume, D.; Maki, S.; Niwa, H.;
Hirano, T. Tetrahedron Lett. 2010, 51, 1613–1615.
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10464–10465. (b) Coskun, A.; Akkaya, E. U. J. Am. Chem. Soc. 2006,
128, 14474–14475. (c) Belman, N.; Israelachvili, J. N.; Li, Y.; Safinya,
C. R.; Bernstein, J.; Golan, Y. J. Am. Chem. Soc. 2009, 131, 9007–9013.
(d) Harriman, A.; Mallon, L. J.; Elliot, K. J.; Haefele, A.; Ulrich, G.;
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Org. Lett., Vol. 14, No. 20, 2012
5227

crystallographic b-axis direction, with a uniform intermole-
˚
cular distance of 3.429 A. For 2b, the bulky CF3 groups
decorated on the naphthyridine ring lead to a slightly less
planar molecular structure and a change in packing modes in
the solid state (Figure S3).
Complex 3b crystallizes into a pale yellow plate with a
space group of C2/c. The crystal structure of 3b reveals a
nonplanar conformation; the dihedral angle between the
naphthyridyl unit and the phenyl ring is 13.83°, which is
comparable to that of 14.98° in its precursor. The striking
difference between 3b and 3a is the bond parameters of the
amido group (Figure S4 and Table S2). The C(10);O(1)
˚
bond length of 1.3039(2) A in 3b is significantly longer with
˚
respect to that of 1.2242(2) A in 3a, but close to a typical
˚
C;O single bond (ca. 1.34 A). In the meantime, the
˚
C(10);N(3) bond distance of 1.3000(2) A, characteristic
of a CdN double bond, is pronouncedly shorter than that
˚
of 1.368(2) A in its precursor. These results, coupled with
IR observations, indicated a transformation from the keto
to enol forms on changing from 3a to 3b.
The absorption and emission spectra of complexes
1bꢀ4b were measured in various solvents; the results are
demonstrated in Figure S5, and the photophysical data are
collected in Table 1. In general, the electron-donating sub-
stituent (NMe2) atthe phenyl p-position induced a remark-
able red shift of the absorption maximum. As shown in
Figure 2, complexes 3b and 4b in dichloromethane exhibit
a structured absorption at 300ꢀ400 nm with a large
extinction coefficient (∼104 molꢀ1 dm3 cmꢀ1), which is
typical for BF2 compounds. In sharp contrast, complexes
1b and 2b display a broad and bathochromic absorption
with λmax at 440 and 480 nm, respectively. The red-shifted
absorption of 1b and 2b can be attributed to originate from
an ICT state, in terms of their strong pushꢀpull struc-
tures.12 Relative to 3b and 4b, large solvatochromic shifts
are observed for 1b and 2b, indicative of the larger differ-
ence in dipole moments of the FranckꢀCondon (FC)
excited state and the ground state in the latter.
The emission spectra of 1b and 2b in dichloromethane
feature a broad and structureless peak, with a λmax (lifetime;
quantum yield) of 537 nm (3.05 ns; 22%) and 642 nm (τ1 =
0.11 ns, τ2 = 2.85 ns; <1%), respectively. The emission
peaks of 3b and 4b in dichloromethane are vibronically
structured with a spacing of ∼1300 cmꢀ1, in accordance with
the CdC and CdN vibrational modes of the naphthyridine
moiety. The emission maxima of 3b and 4b are almost
independent of solvents, while those of 1b and 2b are strongly
dependent on solvents and shift to the low-energy direction
with the increase in solvent polarity (Table S3). The magni-
tude of the solvatochromic shift is found to depend on the
electronic effects of peripheral substituents on the naphthyr-
idine ring. For example, the maxima of the emission spectra
of 1b extend from 448 nm in hexane to 492 nm in toluene and
Figure 2. (Top) UVꢀvis absorption and (bottom) emission
spectra of 1bꢀ4b in dichloromethane measured at a concentra-
tion of ∼1.0 ꢁ 10ꢀ5 mol Lꢀ1. λex = 440 nm for 1b, 480 nm for 2b,
and 350 nm for 3bꢀ4b.
then to 537 nm in dichloromethane. When the electron-
withdrawing CF3 groups are incorporated into the naphthyr-
idine acceptor unit, a more significant solvatochromic shift
from 512 nm (in hexane) to 642 nm (in dichloromethane) is
achieved in 2b. In addition, it should be stressed that 1b and
2b in polar solvents emit weak fluorescence and display fairly
large Stokes shifts (e.g., 97 and 162 nm in dichloromethane,
respectively). The spectral dependence of 1b and 2b on
solvent polarity was further studied on the basis of the
LippertꢀMataga equation,13 and the calculated results reveal
an increase of 15.1 and 16.2 D in dipole moment from the
ground state to the excited state for 1b and 2b, respectively. At
this stage, it should be safe to suggest that 1b and 2b have a
pronounced ICT characteristic of the fluorescent states.14
Another salient feature for complexes 1bꢀ4b is that their
solid-state emissions cover a wide range of 431ꢀ670 nm
(13) (a) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn.
1956, 29, 465–470. (b) Liptay, W. In Dipole Moments and Polarizabilities
of Molecules in Excited Electronic State, in Excited States, Vol. 1; Lim,
E. C., Ed.; Academic Press: New York, 1974.
(12) (a) Zhao, G.-J.; Chen, R.-K.; Sun, M.-T.; Liu, J.-Y.; Li, G.-Y.;
Gao, Y.-L.; Han, K.-L.; Yang, X.-C.; Sun, L. Chem.;Eur. J. 2008, 14,
6935–6947. (b) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev.
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2003, 103, 3899–4032. (c) Demeter, A.; Burces, T.; Zachariasse, K. A.
J. Phys. Chem. A 2005, 109, 4611–4616. (d) Wilsey, S.; Houk, K. N.;
Zewail, A. H. J. Am. Chem. Soc. 1999, 121, 5772–5786. (e) Ward, M. D.
Chem. Soc. Rev. 1997, 26, 365–375.
(14) Chen, R.-K.; Zhao, G.-J.; Yang, X.-C.; Jiang, X.; Liu, J.-F.;
Tian, H.-N.; Gao, Y.; Liu, X.; Han, K.-L.; Sun, M.-T.; Sun, L.-C.
J. Mol. Struct. 2008, 876, 102–109.
5228
Org. Lett., Vol. 14, No. 20, 2012

Table 1. Optical Properties of Complexes 1bꢀ4b at 298 K
λabsa/nm
Stokes
shift/
nm
(ε/molꢀ1 dm3
λema/nm
(τ/ns; Φ)
λemb/nm
(τ/ns; Φ)
compd
cmꢀ1
)
1b
440 (63030) 537
313 (15460) (3.05; 0.22)
252 (14910)
97
162
8
542
(3.70, 1.06; 0.04)
2b
3b
4b
480 (41050) 642
355 (21190) (0.11, 2.85;
257 (15760) <0.01)
384 (41200) 392
366 (36870) (1.40; 0.54)
275 (19330)
670 (0.81, 2.62; 0.01)
431, 450
(0.42, 2.95; 0.16)
389 (30230) 397
370 (32010) (0.53; 0.20)
277 (18990)
8
439, 461, 496
(0.50, 1.31; 0.27)
a In dichloromethane solution. b In solid state. ε = molar extinction
coefficient. Φ= fluorescence quantum yield, calculated using quinine sulfate
as a standard (Φ = 0.546 in 0.5 mol Lꢀ1 H2SO4). τ = fluorescence lifetime.
Figure 3. Normalized solid-state emission spectra of 1bꢀ4b at rt
(λex = 365 nm for 1b, 3b, 4b and 550 nm for 2b).
(Figure 3), while BODIPY dyes barely exhibit fluorescence
in the solid state. The fluorescence maxima for all complexes
are red-shifted with respect to corresponding emissions in
solution. The fluorescence quantum yields of 0.16 for 3b
and 0.27 for 4b are remarkably higher than those for 1b
(Φ = 0.04) and 2b (Φ = 0.01). This possibly originates
from the fact that the molecular structure of the latter is
more planar compared with that of the former. Usually,
high planarity may lead to increasing intermolecular
interactions and result in fluorescence quenching.
For better insight into the observed spectroscopic prop-
erties of 1bꢀ4b, time-dependent (TD) DFT calculations
were performed on these complexes with the Gaussian 03
package.15 All geometrical structures are optimized at the
SCRF(PCM/Bader)-B3LYP/6-311þþG(d,p) level, with
the single-point energy and molecular orbitals calculated
at the TD-DFT(SCRF(PCM/Bader)-B3LYP/6-311þþG-
(d,p)) level. The calculated values for the maximum ab-
sorption wavelengths are in fair agreement with the
experimental results. Figure 4 shows the electron distribu-
tion of the HOMOs and LUMOs for 1bꢀ4b. Although the
HOMO and LUMO densities in 3b and 4b are delocalized
over the whole molecule, the HOMO and LUMO densities
of 1b and 2b are mainly located on the dimethylamino-
phenyl group and naphthridyl-boron moiety, respectively.
This result also further confirmed that the ICT process did
occur from the donor to the acceptor moiety in the latter.
In summary, we developed novel BF2 core complexes con-
taining 1,8-naphthyridine building blocks. The synthesized
complexes show weak to moderate fluorescence in the solid
state. In particular, a large Stokes shift has been achieved for
1b and 2b through the introduction of an electron-donating
group onto the phenyl ring to construct pushꢀpull-type
architecture. The photoinduced intramolecular charge trans-
fer process was analyzed on the basis of the LippertꢀMataga
Figure 4. Molecular orbital energy diagrams and isodensity
surface plots of the frontier orbitals of 1bꢀ4b.
equation and further confirmed by theoretical calculations.
Efforts to improve the solid-state fluorescence quantum yields
and thus to pave the way for usage of these complexes as light
emitters in optoelectronic devices are currently in progress.
Acknowledgment. We thank the Natural Science Foun-
dation of China (NSFC Grant Nos. 21071123, 21001110,
and U1137606), CAS-Croucher Funding Scheme for Joint
Laboratories, and Program for Changjiang Scholars and
Innovative Research Team in University (IRT0979) for
financial support. Y.C. acknowledges Technical Institute
of Physics and Chemistry, Chinese Academy of Sciences
(“100 Talent” Program) for funding support.
Supporting Information Available. Experimental proce-
dures, characterization data of compounds 1aꢀ4a and 1bꢀ
4b, computational details, additional spectral data, crystal-
lographic data (CIF), and complete ref 15. This material is
available free of charge via the Internet at http://pubs.acs.org.
(15) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian:
Wallingford, CT, 2004.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 20, 2012
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