Chelation-assisted photoelimination of B,N-heterocycles

Article abstract of DOI:10.1021/ol403550b

Metal-chelation and internal H bonds have been found to greatly enhance the photoelimination quantum efficiency of B,N-heterocycles by 2 orders of magnitude. Green phosphorescent Pt(II)-functionalized 1,2-azaborines have been achieved via photoelimination. A mechanistic pathway for the PE reaction has been established.

Full text of DOI:10.1021/ol403550b

Letter
pubs.acs.org/OrgLett
Chelation-Assisted Photoelimination of B,N-Heterocycles
Soo-Byung Ko,† Jia-Sheng Lu,† and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
S
* Supporting Information
ABSTRACT: Metal-chelation and internal H bonds have
been found to greatly enhance the photoelimination quantum
efficiency of B,N-heterocycles by 2 orders of magnitude. Green
phosphorescent Pt(II)-functionalized 1,2-azaborines have been
achieved via photoelimination. A mechanistic pathway for the
PE reaction has been established.
ncorporating a boron atom in conjugated organic molecules
is known to be highly effective in generating new materials
aim of examining the generality of the PE reaction in metal-
containing systems to develop phosphorescent azaborines, to
shift the PE excitation energy to a longer wavelength, and to
enhance ΦPE, a new BNN system shown in Scheme 2 was
I
with versatile applications including sensory and optoelec-
tronic/photoresponsive materials1−4 and devices.5 In particular,
replacing a C−C unit with an isoelectronic B−N unit in
aromatic compounds leads to the formation of azaborine and its
derivatives,6 which possess interesting electronic, photo-
physical, and chemical properties that are distinct from those
of the isoelectronic C−C analogues, leading to many potential
applications.6,7 Thus, there is great interest in the chemistry of
azaborine and derivatives and the development of efficient
synthetic methods for new B,N-substituted aromatic com-
pounds. We reported recently that B,N-heterocycles such as
BN-1 shown in Scheme 1 can undergo an unusual photo-
Scheme 2. New BNN System
designed, and its Pt(II) chelate complexes were obtained. An
investigation on the new system led to several surprising
findings including the more than 100 times greater ΦPE of the
new system at 350 nm excitation, compared to BN-1, and the
establishment of the reaction pathway shown in Scheme 1. The
details are reported herein.
Scheme 1. Proposed PE Pathway of B,N-Heterocycles
The precursor B,N-heterocycle BNN was designed on the
basis of the consideration that it allows the attachment of a
metal-chelate unit on the backbone via cyclometalation, which
can introduce low energy states such as MLCT, thus potentially
decreasing the excitation energy of the PE. BNN was prepared
by the reaction of 3-methyl-2,2′-bipyridine with n-BuLi at −78
°C in the presence of TMEDA, followed by the addition of
BMes2F (Scheme 3). The metal unit selected for attaching to
BNN is Pt(acac) and the tert-butyl-substituted analogue
Pt(tmhd) (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate),
based on the fact that N,C-chelate Pt(acac) or Pt(tmhd)
compounds can produce bright phosphorescence.5a,b Pt−
BNN1 and Pt−BNN2 were obtained in good yields (85−
88%) by a modified one-pot cyclometalation procedure
elimination (PE) reaction in solution or in doped polymer
films, generating B,N-substituted azaboraphenanthrenes in high
yields.8 This unusual reaction provides a convenient means for
the synthesis of new B,N-substituted aromatic compounds that
would be difficult to synthesize via alternative stepwise chemical
reactions.6,7 Nonetheless, in order to achieve moderate PE
quantum efficiency (ΦPE), the reactions in the previously
reported systems require excitation at 300 nm, which can be
very destructive to certain polymer hosts, thus limiting the use
of the PE method in an in situ generation of patterned films for
organic optoelectronics. Furthermore, the impact of metal ions
on the PE process of B,N-heterocycles is not known. If metal-
containing B,N-heterocycles can undergo a similar PE reaction
as the parent ligand does, it would allow us to access previously
unknown phosphorescent azaborine molecules, which have
potential applications in OLEDs as phosphorescent dopants5
and in phosphorescence-based sensing applications.9 With the
reported previously10 at ambient temperature. BNN and its
1
Pt(II) compounds were fully characterized by H, 11B, and 13
C
Received: December 8, 2013
© XXXX American Chemical Society
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Letter
to a reduction in activation barrier toward photoelimination
and the enhancement of ΦPE.
The photoreactivity of BNN, Pt−BNN1, and Pt−BNN2 was
examined by UV−vis, fluorescence/phosphorescence, and
NMR spectra. As shown in Figures 2 and 3, the absorption
Scheme 3. Synthetic Procedures of BNN, Pt−BNN1, Pt−
BNN2, and Their B,N-Benzoquinolines
NMR spectra and HRMS/elemental analysis. The crystal
structures of BNN and Pt−BNN2 were determined by single-
crystal X-ray diffraction analysis.11
As shown by the crystal structures of BNN and Pt−BNN2 in
Figure 1, the carbon atom (C(21), equatorial) of one of the
Figure 2. UV−vis (top) and emission (bottom) spectra showing the
conversion of BNN and Pt−BNN1 in toluene (1.0 × 10−5 M) to their
corresponding benzoquinoline compounds upon irradiation at 350
nm. Inset photographs show the color change (top) of the samples
before and after irradiation and the emission color (bottom) of BNNa
and Pt−BNN1a in toluene.
Figure 1. Crystal structure of BNN (left) and Pt−BNN2 (right).
mesityl rings is approximately in the same plane with the bpy
unit and the B,N-heterocycle, while the other carbon atom
(C(12), axial) from the second mesityl is approximately
perpendicular to it. The shortest separation distance between
the H atoms of the CH2 group and the carbon atoms of the
mesityls is between C(12) and H(1′) (the syn-H atom). The
bond lengths and angles around the Pt(II) center in Pt−BNN2
are typical and similar to those of the previously reported
Pt(ppy)(acac) and derivatives.5 The key difference between the
structures of BN-1, BNN, and Pt-BNN2 is the torsion angle
(N1CCC2, see Scheme 3) between the two chelate aryl rings
(the bpy unit in BNN and Pt−BNN2, the ppy unit in BN-1),
which decreases from BN-1 (26.7°) to BNN (22.9(2)°) due to
an intramolecular N(2)···H bond (2.48 Å)12 in BNN and to
Pt−BNN2 (10.3(9)°) due to chelation. As a consequence,
there is an increased coplanarity of the chelate unit with the
B(1) and C(1) atoms from BN-1, BNN to Pt−BNN2. Further,
the C(12) and H(1′) distance decreases from BN-1 (2.81 Å),
BNN (2.73 Å) to Pt−BNN2 (2.65 Å), accompanied by more
than 10° decrease of the C(12)−B(1)−C(1)−H(1′) torsion
angle. The optimized structures of BN-1, BNN, Pt−BNN1, and
Pt−BNN2 by DFT show a similar trend (Table S16a,
Supporting Information). On the basis of the structural data,
if the PE process indeed follows the pathway shown in Scheme
1 by eliminating R1 and H′(C(12) and H(1′) in BNN and Pt−
BNN2), it is conceivable that the ΦPE of the reaction would
follow the order of BN-1 < BNN < Pt−BNN1 ≈ Pt−BNN2,
since the increased ring strain of the B,N-heterocycle and the
reduced separation distance between C(12) and H(1′) can lead
Figure 3. Left: absorption spectra of B,N-heterocycles in toluene.
Right: a diagram showing the conversion percentage of the B,N-
heterocycles to their corresponding B,N-benzoquinolines at 350 nm
excitation (1.0 × 10−5 M in toluene).
spectra of Pt−BNN1 and Pt−BNN2 have a distinct low energy
band at 380−450 nm that can be ascribed to MLCT and CT
(mesityl to bpy) transitions, based on TD-DFT computational
data (see the Supporting Information). BNN is nonemissive
while both Pt(II) compounds display very weak green
phosphorescence at ∼495 nm and ΦPhos. ≤ 0.01. Upon
irradiation at 350 nm under N2 atmosphere in toluene or
benzene (1.0 × 10−5 M), the color of the BNN solution
changes gradually from colorless to light brown with the
appearance of a new low energy absorption band at 400−510
nm (ε = 4300 M−1 cm−1) with well-resolved vibrational
features. In the fluorescence spectrum, a blue-green fluorescent
peak appears that grows in intensity with time, as BNN is being
irradiated at 350 nm (λmax = 493 nm, ΦFluor. = 0.60 at 298 K;
478 nm at 77 K). The new species was designated as BNNa, an
analogue of BN-1 and an isoelectronic species of 7,8-
benzoquinoline. Compared to that of 7,8-benzoquinoline, the
low energy absorption band and the fluorescent peak of BNNa
are more than 100 nm red-shifted (see the Supporting
Information), a trend commonly observed in B,N-substituted
B
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Letter
aromatic compounds.6,7 Under 350 nm excitation, both yellow
solutions of Pt−BNN1 and Pt−BNN2 become red-orange with
the appearance of a new low energy band at 440−560 nm (ε =
4300−7600 M−1 cm−1) that was assigned to the new species
Pt−BNN1a and Pt−BNN2a, respectively (Figure 2 and
Supporting Information). In the phosphorescent spectra, the
weak green phosphorescent peak gains intensity with
irradiation time with more than 10-fold increase of the
emission quantum efficiency (λphos = 0.13−0.14, τ = 12.7(3)
−14.3(4) μs). The structures of BNNa, Pt−BNN1a, and Pt−
BNN2a were established by 1D and 2D (NOESY) NMR
spectroscopic analysis, which showed that BNN, Pt−BNN1,
and Pt−BNN2 convert to BNNa, Pt−BNN1a, and Pt−BNN2a,
respectively, in nearly quantitative yields (see the Supporting
Information). The new azaborine compounds were isolated and
characterized by HRMS and NMR. The 11B NMR chemical
shifts of BNNa, Pt−BNN1a, and Pt−BNN2a are at 35.7, 37.5,
and 37.6 ppm, respectively, similar to those previously reported
for 1,2-azaborine compounds6,7 BN-1 and derivatives.8 Efforts
to obtain single crystals of BNNa, Pt−BNN1a, and Pt−BNN2a
for X-ray diffraction analysis were unsuccessful. The UV−vis
absorption and fluorescence spectra of BNN, Pt−BNN1, and
Pt−BNN2 in poly(methylmethacrylate) (PMMA) films display
a change similar to those recorded in toluene upon irradiation,
supporting that the PE reaction of these compounds also occur
in polymer hosts as observed for BN-1 and its derivatives but at
a much lower excitation energy (350 nm) (see the Supporting
Information).
Significantly, despite the presence of the low energy
absorption band at λmax = ∼400 nm for Pt−BNN1 and Pt−
BNN2, excitation at 400 nm only led to very slow and
inefficient PE (ΦPE <0.001), indicative of a minimum excitation
energy threshold required for the PE process. Compared to
BN-1 and its derivatives that require 300 nm excitation to
achieve moderate ΦPE (0.044), the PE of BNN, Pt−BNN1, and
Pt−BNN2 can proceed at 350 nm excitation with an impressive
ΦPE (0.045 for BNN to BNNa, 0.093 for Pt-BNN1 to Pt-
BNN1a, and 0.080 for Pt−BNN2 to Pt−BNN2a). Remarkably,
the ΦPE of BNN is more than 100 times greater than that of
BN-1 (<0.0001) at 350 nm excitation, despite the greater
absorbance of BN-1 at 350 nm (Figure 3). For the Pt(II)
compounds, the ΦPE is even higher, nearly double that of BNN.
Clearly, the greatly enhanced ΦPE at 350 nm for BNN, Pt−
BNN1, and Pt−BNN2 compared to BN-1 cannot be explained
by the absorption intensity difference.
Figure 4. Orbitals involved in the vertical excitation of the B,N-
heterocyclic compounds that are responsible for initiating the PE
process.
orbital localized on the ppy or the bpy unit. The HOMO-4 or
HOMO-7 level of these compounds contains π orbitals of the
chelate unit, the d orbital of the Pt atom (13% for Pt−BNN1
and Pt−BNN2), and most significantly the σ orbitals of the C−
H bond (the CH2 unit) and the B−CMes(axial) bond (C(12))
(27% for BN-1, 28% for BNN, 34% for Pt−BNN1) in addition
to the B−CH2 σ bond. Vertical excitations at energies below
∼3.54 eV have either no or very little C−H or B−C σ bond
contributions. Thus, the photoexcitation of these molecules at
∼350 nm appears to be the minimum energy required to
weaken the C−H and the B−CMes(axial) bond in the PE process,
providing a plausible explanation for the observed minimum
excitation energy threshold. Notably, the σ orbital contributions
of the C−H and B−CMes(axial) bond in HOMO-7 of Pt−BNN1
and Pt−BNN2 are much greater than those in HOMO-4 of
BNN, which could make the photoexcitation of the C−H and
B−C bond more effective. Because there is little difference on
the electronic structure of BN-1 and BNN, the great ΦPE
enhancement of BNN relative to BN-1 at 350 nm excitation
can only be explained by the increased coplanarity of the bpy
unit and the B,N-heterocycle caused by an internal N···H bond
in BNN. The thermal elimination pathway was established by
DFT computations for BN-1 and BNN and is shown in Figure
5. The optimized structure of the transition state involves an H
atom that is midway between C(1) and B(1) atom and a
detached Mes group that is ∼2.0 Å away from the B atom (see
the Supporting Information for details). The computational
TD-DFT calculation results confirmed that the low energy
absorption bands of BNNa, Pt−BNN1a, and Pt−BNN2a in the
420−550 nm region are indeed from a π → π* (HOMO →
LUMO) transition localized on the benzoquinoline ring with a
high oscillator strength. The fluorescence of BNNa is most
likely from the same transition. For Pt−BNN1a and Pt−
BNN2a, the phosphorescence is most likely from the T2 state
involving a π → π* transition of the benzoquinoline ring since
it is necessary to excite these molecules at 395 nm that is
beyond the low energy absorption band (440−550 nm) in
order to observe the phosphorescence.
For the precursor compounds, TD-DFT data indicate the
transition at the 350 nm excitation (3.54 eV) involves HOMO-
4 → LUMO (S6, 96%) for BNN, HOMO-7 → LUMO (S9,
95%) for Pt−BNN1, and Pt−BNN2 (Figure 4). The BN-1
compound has a similar transition as that of BNN and for
comparison purpose is also included in Figure 4. The LUMO
level for BN-1, BNN, Pt−BNN1, and Pt−BNN2 is similar, a π*
Figure 5. A diagram showing the relative energies of the reactant, the
transition state and the products of the PE process.
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data support that the PE reaction has to proceed through the
excited state because of the large activation barrier, presumably
involving a transition state that resembles the one in the
thermal pathway. The activation barrier of BNN was found to
be 8.3 kJ/mol lower than that of BN-1, supporting its greater
PE reactivity.
In conclusion, new and highly phosphorescent B,N-
benzoquinolines have been achieved via a PE process. Metal
chelation and internal H bonds have been found to greatly
enhance ΦPE of B,N-heterocycles by reducing the activation
barrier and lowering the excitation energy, although a minimum
excitation energy threshold was found to be necessary in order
to break the C−H and B−C bonds. The use of metal chelation/
coordination to influence photochromic systems is known to be
an effective approach by influencing the electronic state of the
molecule.13 This work is the first example demonstrating the
impact of metal chelation on PE reactions in which geometric
constraints play a major role.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic details and characterization data for all compounds,
crystal structural data, photophysical data, DFT/TD-DFT
computational data, and NICS values. This material is available
free of charge via the Internet at http://pubs.acs.org.
(12) For similar intramolecular N···H bonds, see: Yokoyama, D.;
Sakaguchi, A.; Suzuki, M.; Adachi, C. Appl. Phys. Lett. 2009, 95,
243303.
(13) (a) Ko, C. C.; Yam, V. W. W. J. Mater. Chem. 2010, 20, 2063−
2070 and references cited therein. (b) Wang, N.; Ko, S.-B.; Lu, J.-S.;
Chen, L. D.; Wang, S. Chem.Eur. J. 2013, 19, 5314−5323.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wangs@chem.queensu.ca.
Author Contributions
†These authors contributed equally to the work reported here.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support. S.W. thanks the
Canada Council for the Arts for the Killam Research
Fellowship.
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