Syntheses and remarkable photophysical properties of 5-(2-pyridyl) pyrazolate boron complexes; photoinduced electron transfer

Article abstract of DOI:10.1039/b309374c

A new series of pyridyl pyrazolate boron complexes 2a-e have been synthesized, in which 2a-c exhibit remarkable dual fluorescence properties due to the photoinduced electron transfer reaction.

Full text of DOI:10.1039/b309374c

Syntheses and remarkable photophysical properties of 5-(2-pyridyl)
pyrazolate boron complexes; photoinduced electron transfer†
Chung-Chih Cheng,a Wei-Shan Yu,b Pi-Tai Chou,*b Shie-Ming Peng,b Gene-Hsiang Lee,b Pei-Chi
Wu,c Yi-Hwa Songc and Yun Chi*c
a Department of Chemistry, Fu-Jen Catholic University, 242, Shin Chuang, Taiwan, ROC
b Department of Chemistry, National Taiwan University, 106, Taipei, Taiwan, ROC
c
Department of Chemistry, National Tsing Hua University, 300, Hsinchu, Taiwan, ROC.
E-mail: chop@ccms.ntu.edu.tw; ychi@mx.nthu.edu.tw
Received (in Cambridge, UK) 5th August 2003, Accepted 28th August 2003
First published as an Advance Article on the web 12th September 2003
A new series of pyridyl pyrazolate boron complexes 2a–e
have been synthesized, in which 2a–c exhibit remarkable
dual fluorescence properties due to the photoinduced
electron transfer reaction.
data reported in other tetrahedrally arranged BPh2 complexes
with chelating ligands of (2-pyridyl)-7-azaindole and (2-pyr-
idyl)-7-indole derivatives.4 Moreover, the phenyl rings are
orthogonal to the 5-(2-pyridyl)pyrazolate ligand, which is
confirmed by the observation of two nearly perpendicular
dihedral angles of 73.9°–77.1°. Their overall molecular struc-
ture resembles that of the 9,9-disubstituted fluorene com-
pounds, which have been widely used as the most promising
blue-phosphor in OLED applications.5
Boron complexes with conjugated light-emitting p-systems
have recently received considerable attention due to their
potential uses in organic light emitting devices (OLED)1 as well
as biomolecular probes.2 It has been a core interest to
chemically modify the ligand chromophores so that the physical
properties can be fine-tuned to achieve a designated function. In
this communication, we report the syntheses and photophysical
studies of a new series of 5-(2-pyridyl) pyrazolate boron
complexes. 5-(2-Pyridyl) pyrazole was selected because it can
function as a chelating ligand upon treatment with a boron
compound such as BPh3, giving a nearly planar, conjugated p-
system that was perfect for studying the photoluminescent
properties.
Pathways leading to the synthesis of 5-(2-pyridyl) pyrazolate
boron complexes are shown in Scheme 1. The required 2A-
(2-pyridyl) pyrazoles (1a–e) were first synthesized using
methods reported in the literature.3 Subsequently, treatment of
1a–e with BPh3 in THF solution affords the target BPh2
complexes 2a–e in good yields of 4 87%. Similar treatment of
1a with B(C6F5)3, followed by recrystallization from a solution
of CH2Cl2 and methanol, affords a colorless crystalline solid 2f
in 48% yield.
As listed in Table 1 the S0 ? S1 (pp*) absorption spectral
features of 2a–e (lmax
~ 315–350 nm) in THF reveal
bathochromic shifts with respect to their free ligands 1a–e (lmax
~ 290–300 nm). The p–p* transition of 2a–e is principally
localized on the planar pyridyl pyrazolate ligands, on which the
HOMOs and LUMOs are mainly located at the pyrazolate and
the pyridyl sites, respectively. The B–N covalent interaction is
believed to exist at the pyrazolate side of the ligand, while the
other N?B dative bonding resides at the opposite, pyridyl
moiety. It is thus anticipated that the electron-deficient BPh2
substituent would exert large stabilization on the pyridyl p-
system due to its electron accepting capability with respect to
A typical crystal structure of 2a is depicted in Fig. 1, along
with the selected metric parameters. It is notable that the boron
atom has a characteristic tetrahedral geometry with angles
N(1)–B(1)–N(2)
= 93.43(13)° and C(10)–B(1)–C(16) =
117.30(15)°. The 5-(2-pyridyl) pyrazolate ligand is chelated to
the boron atom with bond distances, B(1)–N(1) = 1.629(2) Å
and B(1)–N(2) = 1.570(2) Å, for which the N(1)–B dative
distance is found to be distinctively longer than that of the N(2)–
B distance. This observation is consistent with the structural
Fig. 1 The X-ray crystal structure of 2a; selected bond distances: B1–N1 =
1.629(2), B1–N2 = 1.570(2), B1–C10 = 1.596(3), B1–C16 = 1.609(3),
N1–C5 = 1.352(2), N2–C6 = 1.336(2), N2–N3 = 1.336(2) Å.
Table 1 Photophysical properties of 2 in THF (298K).
c
Solvent
THF
Ea (kcal/
mol)
lmax/nma PL lmax/nm t/nsb F1 t/nsb F2
2a
2b
2c
2d
2e
2f
320
315
320
350
340
325
375, 505
365, 488
374, 512
460
430
382
0.53
0.42
0.50
16.4
9.9
6.8
(0.52*, 18.3) 1.31
(0.38*, 14.9) 1.16
(0.48*, 11.5) 1.10
–
–
–
–
–
–
Scheme 1
a Data was taken from the first vibronic peak. b Samples were degassed via
three freeze-pump-thaw cycles. c obtained from the temperature dependent
study in 2MTHF.* denotes the rise time
† Electronic supplementary information (ESI) available: Photophysical
experimental details, the spectral data of all boron complexes, and crystal
data of 2a. See http://www.rsc.org/suppdata/cc/b3/b309374c/
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CHEM. COMMUN., 2003, 2628–2629
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that of the covalent characteristics of the pyrazolate fragment,
resulting in significant red-shift from the corresponding 1a–e
ligands.
2d and 2e exhibited strong, single fluorescence maximized at
460 and 430 nm, respectively in THF (see Fig. 2 for 2e). Similar
results were observed in the solid crystal. Conversely, while the
solid crystal of 2a–c showed only one emission centered at 382,
360 and 370 nm, respectively, dual emission was observed in
solution, consisting of a normal emission band (2a: 375, 2b: 365
and 2c: 374 nm, the F1 band) and an anomalously large Stokes
shifted emission (2a: 505, 2b: 488 and 2c: 512 nm, the F2 band)
in THF. The excitation spectra for both F1 and F2 bands are
identical, which are also effectively the same as the absorption
profile, excluding its origin from traces of impurity. In contrast
to the nearly solvent independent F1 emission frequency, the
fluorescence peak frequency for the F2 band was linearly
proportional to solvent polarity (Df).† Accordingly, a change of
dipole moment of ~ 6.6 Debye between ground and excited
states was deduced.
Fig. 3 Temperature-dependent emission spectra of 2a in 2MTHF at
298–150 K. Insert: The plot for ln(kot) versus the reciprocal of tem-
peratures.
Further insight into the correlation of dual emission proper-
ties was gained from the dynamic studies. As shown in Table 1
the lifetime of the F1 band for 2a was fitted to be ~ 0.53 ns (c2
= 1.02) at 298 K, while the F2 band is apparently composed of
rise and decay components that were fitted to be 0.52 ns and
18.3 ns, respectively (c2 = 1.05). The rise time of the F2 band,
within experimental error, is identical with the decay time of the
F1 band, supporting a precursor-successor type of relaxation
mechanism. As shown in Fig. 3, the F1/F2 ratio for 2a increased
upon decreasing the temperature. The reaction rate monitored
by the decay dynamics of the F1 band or equivalently the rise
dynamics of the F2 band revealed significant temperature
dependence (see insert of Fig. 3). Similar dual emission and
precursor-successor related dynamics were also obtained for 2b
and 2c and the results are listed in Table 1.
The above results can plausibly be rationalized by a
mechanism incorporating a photoinduced electron transfer (ET)
process from the phenyl moiety to the pyrazolate ligand. The
separations from the center of two phenyl fragments to the
center of pyridyl and pyrazolate ring systems are estimated to be
4.6–4.7 Å and 4.9–5.0 Å, respectively. These lengths are
significantly shorter than the typical distance ( ~ 7 Å) that
allows the occurrence of through-space electron transfer. The
reduction peak potential of 5-(2-pyridyl) pyrazolate complexes
2a–b and 2d–e occurred at 21.67, 21.72, 21.76 and 21.86 V,
respectively in CH3CN. The decrease of reduction potential
correlates well with the trend of electron withdrawing properties
of substituents at the pyrazolate ligand. The result, in combi-
taion with the increase of the absorption energy gap being in the
order of 2a ~ 2b ~ 2c > 2d,2e, qualitatively rationalizes the
occurrence of ET in 2a–c. To further support for the phenyl ring
acting as an electron donor in the proposed ET mechanism we
also synthesized 2f in which the absorption gap is similar to 2a,
whereas the phenyl ring is replaced by a perfluorophenyl moiety
to increase the relative oxidation potential. As a result, ET
process is prohibited in 2f, as indicated by a unique, normal
emission band maximum at 382 nm (see Fig. 2).
The drastic difference in photophysics between single crystal
(single, normal emission) and solution phase (dual emission) in
2a–c is intriguing. The logarithm plot of the ET rate6 vs. 1/T is
sufficiently linear (insert of Fig. 3), from which a barrier of 1.31
kcal mol21 and a frequency factor of 1.88 3 1010 s21 were
deduced for 2a. The Ea value obtained for 2a–c (see Table 1) is
on a similar magnitude as the viscosity barrier ( ~ 1.82 kcal
mol21) in 2-methyltetrahydrofuran (2MTHF), indicating that
certain large amplitude motions may couple with the ET
process. Both X-ray and AM1 approaches indicate that the
phenyl rings are nearly orthogonal to the (2-pyridyl) pyrazolate
ligand in 2a–f, of which the configuration may prohibit the ET
process.‡ It is thus tentatively proposed that in solution phase
the ET mechanism may incorporate rotation of the phenyl ring
to an optimum oreintation so that a through-space ET reaction
can take place.
Due to the straightforward syntheses of boron complexes it is
feasible to adjust the D/A strength so that the degrees of charge-
transfer interactions and consequently the luminescence effi-
ciency can be fine-tuned. An ideal boron-ligands system for
devices may exhibit both LE and CT emissions covering an
entire white-light region. Work on other systems such as
imidazoles and triazoles is currently in progress.
Notes and references
‡ CCDC 201833. See http://www.rsc.org/suppdata/cc/b3/b309374c/ for
crystallographic data in .cif or other electronic format.
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6 ket was deduced by kobs 2 knr. The kobs value for 2f was taken as knr due
to its non-ET process. Note lnketT1/2 vs. 1/T is also sufficiently linear
mainly due to the much larger ket value.
Fig. 2 Emission spectra of (a) 2a (-0-), (b) 2b (-5-), (c) 2c (-:-), (d) 2e
(-!-) and (e) 2f (-8-) in THF.
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