2-(2′-Hydroxyphenyl)benzimidazole and 9,10-phenanthroimidazole chelates and borate complexes: Solution- and solid-state emitters

Article abstract of DOI:10.1021/ol400849a

The synthesis, structural, and optical properties of a series of luminescent N-alkylated 2-(2′-hydroxyphenyl)benzimidazole (HBI) or N-arylated 9,10-phenanthroimidazole (HPI) borate complexes are described. The optical properties of these complexes as well

Full text of DOI:10.1021/ol400849a

ORGANIC
LETTERS
2‑(20-Hydroxyphenyl)benzimidazole
and 9,10-Phenanthroimidazole Chelates
and Borate Complexes: Solution- and
Solid-State Emitters
XXXX
Vol. XX, No. XX
000–000
Karima Benelhadj,† Julien Massue,*,† Pascal Retailleau,‡ Gilles Ulrich,*,† and
Raymond Ziessel*,†
ꢀ
Laboratoire de Chimie Organique et Spectroscopies Avancees (ICPEES-LCOSA),
UMR7515 CNRS, Ecole de Chimie, Polymeres, Materiaux de Strasbourg (ECPM),
ꢀ
ꢁ
ꢀ
25 rue Becquerel, 67087 Strasbourg, Cedex 02, France, and Laboratoire de Crystallochimie,
^
ICSN - CNRS, Bat. 27-1 avenue de la Terrasse, 91198 Gif-sur-Yvette, Cedex, France
massue@unistra.fr, gulrich@unistra.fr, ziessel@unistra.fr
Received March 29, 2013
ABSTRACT
The synthesis, structural, and optical properties of a series of luminescent N-alkylated 2-(20-hydroxyphenyl)benzimidazole (HBI) or N-arylated
9,10-phenanthroimidazole (HPI) borate complexes are described. The optical properties of these complexes as well as their corresponding ligands
were evaluated in solution and the solid state. Efficient emission in the blue-green region was obtained with quantum yields up to 91% in CH2Cl2
and 27% in the solid state. These emissions originate from excited state intramolecular proton transfer (ESIPT) for the ligands and from a singlet
excited state for the borate complexes.
The emergence of new and specific technological de-
mands of efficient fluorophores for various applications
ranging from biological labeling to organic electronics is
pushing numerous researchers to investigate new fluores-
cent systems. These new functional fluorescent dyes should
possess overstandard chemical and photochemical stabi-
lity, high absorption coefficient and quantum yields, as
well as a tunability of both absorption and emission bands,
which allow them to fit the application requirements.
Reaching most of the desired features, the boron dipyrro-
methene (BODIPY) family is nowadays one of the leading
fluorescent systems. Its peculiar properties arise from the
rigidification of a cyanine backbone by boron(III)
complexation.1 But they suffer from some drawbacks, such
as very small Stokes’ shifts and weak solid-state emission.
The BODIPY need to be decorated with peripheral bulky
substituents in order to avoid significant aggregation-
caused quenching (ACQ), despite high-quantum yields in
the solution phase.2 The borate chelation strategy has been
recently extended with success to tetrahedral complexes
incorporating N∧N3 or N∧O4 chelates. These compounds
(2) (a) Ozdemir, T.; Atilgan, S.; Kutuk, I.; Yildirim, L. T.; Tulek, A.;
Bayindir, M.; Akkaya, E. U. Org. Lett. 2009, 11, 2105–2107. (b) Lu, H.;
Wang, Q.; Gai, L.; Li, Z.; Deng, Y.; Xiao, X.; Lai, G.; Shen, Z. Chem.;
Eur. J. 2012, 18, 7852–7861. (c) Gai, L.; Lu, H.; Zou, B.; Lai, G.; Shen,
ꢀ
Z.; Li, Z. RSC Adv. 2012, 2, 8840–8846. (d) Vu, T. T.; Badre, S.; Dumas-
Verdes, S.; Vachon, J.-J.; Julien, C.; Audebert, P.; Senotrusova, E. Y.;
ꢀ
Schmidt, E. Y.; Trofimov, B.; Pansu, R. B.; Clavier, G.; Meallet-
Renault, R. J. Phys. Chem. C 2009, 113, 11844–11855.
(3) (a) Quan, L.; Chen, Y.; Lv, X.-J.; Fu, W.-F. Chem.;Eur. J. 2012,
18, 14599–14604. (b) Perumal, K.; Garg, J. A.; Blacque, O.; Saiganesh,
R.; Kabilan, S.; Balasubramanian, K. K.; Venkatesan, K. Chem. Asian.
J. 2012, 7, 2670–2677. (c) Cao, Q.; Xiao, S.; Mao, M.; Chen, X.; Wang,
S.; Li, L.; Zou, K. J. Organomet. Chem. 2012, 717, 147–151. (d) Mao, M.;
Xiao, S.; Li, J.; Zou, Y.; Zhang, R.; Pan, J.; Dan, F.; Zou, K.; Yi, T.
Tetrahedron 2012, 68, 5037–5041.
† ꢀ
ꢁ
ꢀ
Ecole de Chimie, Polymeres, Materiaux de Strasbourg.
‡ ICSN - CNRS.
(1) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932. (b)
Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47,
1184–1207. (c) Boens, N.; Leen, V.; Dehaen, W. Chem. Soc. Rev. 2012,
41, 1130–1172.
r
10.1021/ol400849a
XXXX American Chemical Society

have shown pronounced Stokes’shifts (difference between
the lowest energy band in the absorption spectrum and the
highest energy band in the emission spectrum) due to the
dissymmetrical character of the ground state vs the excited
state.5 Moreover, several examples of boron difluoride
(BF2) complexes have been reported to intrinsically exhibit
intense luminescence as powders, crystals, and embedded
in films.6 Obtaining solid-state luminescent emitters is
propelled by applications ranging from organic light-emit-
ting diodes (OLEDs)7 and luminescent displays8 to solid-
state lasers9 and fluorescent sensors.10 Aprahamian and
co-workers showed that the solid-state fluorescent quantum
yield of BF2ꢀhydrazone complexes depends on several
parameters: (1) the planarity of the molecule, (2) the dipole
moment, and (3) the number of πꢀπ interactions between
neighboring dyes.11 The key factor to design efficient solid-
state molecular emitters is the elimination of parameters
inducing concentration quenching in aggregated states. We
recently reported the synthesis of tetrahedral boron com-
plexes based on 2-(20-hydroxyphenyl)benzoxazole (HBO)
core. These luminescent complexes show interesting optical
properties both in solution and the solid-state.12
Here, we report on the preparation of a series of
N-alkylated 2-(20-hydroxyphenyl)benzimidazole (HBI) or
2-(20-hydroxyphenyl)-9,10-phenanthroimidazole (HPI) and
their corresponding BF2 complexes. To the best of our
knowledge, there is only one report describing related HBI
borate complexes with no photophysical investigations.13 We
show that these chelates and their BF2 complexes display
attractive luminescent properties both in solution and in the
solid state. We also demonstrate that these dyes can be
connected to a BODIPY subunit leading to sophisticated
molecular dyads.
Scheme 1. Synthesis of HBI Dyes 9-11 and Their Borate Com-
plexes 12ꢀ14
Preparation of the substituted HBI 9ꢀ11 and their
corresponding boron complexes 12ꢀ14 is shown in
Scheme 1. To construct the benzimidazole core, o-pheny-
lenediamine A was condensed with 4-substituted salicylal-
dehydes B in refluxing ethanol overnight with acidic
catalysis to provide benzimidazoles 1ꢀ3 in 25ꢀ38% yield.
Protection of the phenol with tosyl chloride in the catalytic
presence of a base was easily achieved in dichloromethane
at rt to give benzimidazoles 4ꢀ6 in 78ꢀ96% yield. Sub-
sequent alkylation using butyl iodide in THF/DMF in the
presence of sodium hydride followed by tosyl deprotection
in basic conditions afforded N∧O chelates 9ꢀ11 which
displayed a distinctive downfield 1H NMR signal for the
H-bonded phenolic proton (see the Supporting Informa-
tion for full characterization). Note that these compounds
are fluorescent in the solid-state due to an intrinsic excited-
state intramolecular proton transfer (ESIPT) process.14
Boron complexation was achieved using an excess of
(4) (a) Kubota, Y.; Tanaka, S.; Funabiki, K.; Matsui, M. Org. Lett.
2012, 14, 4682–4685. (b) Kubota, Y.; Hara, H.; Tanaka, S.; Funabiki,
K.; Matsui, M. Org. Lett. 2011, 13, 6544–6547. (c) Wu, Y.-Y.; Chen, Y.;
Gou, G.-Z.; Mu, W.-H.; Lu, X-.J.; Du, M.-L.; Fu, W.-F. Org. Lett. 2012,
14, 5226–5229. (d) Santra, M.; Moon, H.; Park, M.-H.; Lee, T.-W.; Kim,
Y. K.; Ahn, K. H. Chem.;Eur. J. 2012, 18, 9886–9893. (e) Frath, D.;
Azizi, S.; Ulrich, G.; Retailleau, P.; Ziessel, R. Org. Lett. 2011, 13, 3414–
3417.
BF3 Et2O in 1,2-DCE in the presence of a base. Purifica-
tion by filtration on basic Al2O3 afforded HBI borate
complexes 12ꢀ14 in 52ꢀ98% yield.
3
(5) Araneda, J. F.; Piers, W. E.; Heyne, B.; Parvez, M.; McDonald,
R. Angew. Chem., Int. Ed. 2011, 50, 12214–12217.
(6) (a) Rao, Y.-L.; Wang, S. Inorg. Chem. 2011, 50, 12263–12274. (b)
HPI 15 and 16 were obtained in a one-step reaction15 of
9,10-phenanthrenedione C, benzaldehyde D, and 4-methyl-
or 4-iodoaniline with ammonium acetate in acetic acid at
reflux in 34 and 19% yield, respectively. HPI 15 (R = I) was
then coupled with 4-tert-butylphenylacetylene or ethynyl-
BODIPY under Sonogashira cross-coupling reaction con-
ditions (PdCl2(PPh3)2 10%, CuI 5% in toluene/NEt3) to
yield HPI 18 and 19 in 73 and 56% yield, respectively.
ꢀ
D’Aleo, A.; Gachet, D.; Heresanu, V.; Giorgi, M.; Fages, F. Chem.;
Eur. J. 2012, 18, 12764–12772. (c) Mirochnik, A. G.; Fedorenko, E. V.;
Karpenko, A. A.; Gizzatulina, D. A.; Karasev, V. E. Luminescence 2007,
22, 195–198.
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Chen, C.-H.; Huang, W.-S.; Lin, J. T.; Ke, T.-H.; Chen, L.-Y.; Tsai, M.-
H.; Wu, C.-C. Angew. Chem., Int. Ed. 2008, 47, 581–585. (c) Yuan, W. Z.;
Gong, Y.; Chen, S.; Shen, X. Y.; Lam, J. W. Y.; Lu, P.; Lu, Y.; Wang, Z.;
Hu, R.; Xie, N.; Kwok, H. S.; Zhang, Y.; Sun, J. Z.; Tang, B. Z. Chem.
Mater. 2012, 24, 1518–1528.
(8) Yuan, W. Z.; Lu, P.; Chen, S.; Lam, J. W. Y.; Wang, Z.; Liu, Y.;
Kwok, H. S.; Ma, Y.; Tang, B. Z. Adv. Mater. 2010, 22, 2159–2163.
(9) Samuel, I. D. W.; Turnbull, G. A. Chem. Rev. 2007, 107, 1272–
1295.
(10) (a) Han, T.; Feng, X.; Tong, B.; Shi, J.; Chen, L.; Zhi, J.; Dong,
Y. Chem. Commun. 2012, 416–148. (b) Sun, F.; Zhang, G.; Zhang, D.;
Xue, L.; Jiang, H. Org. Lett. 2011, 13, 6378–6381.
Subsequent boron complexation using BF3 Et2O under
3
basic conditions of 15 (R = Me), 18 (R = ꢀPhCtCPhtBu)
and 19 (R = ꢀPhCtCPhBODIPY) afforded HPI borate
complexes 17, 20, and 21 in 72, 75, and 40% yield, respec-
tively (Scheme 2).
(11) Yang, Y.; Su, X.; Carrol, C. N.; Aprahamian, I. Chem. Sci. 2012,
3, 610–613.
(12) (a) Massue, J.; Frath, D.; Ulrich, G.; Retailleau, P.; Ziessel, R.
Org. Lett. 2012, 14, 230. (b) Massue, J.; Retailleau, P.; Ulrich, G.;
Ziessel, R. New. J. Chem. 2013, 37, 1224–1230. (c) Massue, J.; Frath, D.;
Retailleau, P.; Ulrich, G.; Ziessel, R. Chem.;Eur. J. 2013, 19, 5375–
5386.
(14) (a) Stasyuk, A. J.; Banasiewicz, M.; Cyranski, M. K.; Gryko,
D. T. J. Org. Chem. 2012, 77, 5552–5558. (b) Iijima, T.; Momotake, A.;
Shinohara, Y.; Sato, T.; Nishimura, Y.; Arai, T. J. Phys. Chem. A 2010,
114, 1603–1609 and references cited therein.
ꢀ
(15) Tordera, D.; Pertegas, A.; Shavaleev, N. M.; Scopelliti, R.; Ortꢀ,
~
€
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J. Mater. Chem. 2012, 22, 19264–29268.
B
Org. Lett., Vol. XX, No. XX, XXXX

Scheme 2. Synthesis of HPI dyes 15, 18, and 19 and Their Borate
Complexes 17, 20, and 21
Figure 2. (Left) ORTEP view for 14 showing the atom-labeling
scheme. Thermal ellipsoids are plotted at the 50% level. (Right)
perpendicular view to highlight the distortion at the B atom.
H-atoms are omitted for clarity. B1ꢀN1, B1ꢀO1, C1ꢀN1/
C1ꢀN2, and N1ꢀC7 bond lengths are 1.552(4), 1.453(3), 1.352(3)/
˚
1.348(3), and 1.395(3) A, and the N1ꢀB1ꢀO1, O1ꢀB1ꢀF1/2, and
N1ꢀB1ꢀF1/2 angles are 107.5(2), 108.8(2)/111.6(2), and 109.4(2)/
109.3(2)°.
The X-ray molecular structures of HPI dye 15 and HBI
borate complex 14 are depicted in Figures 1 and 2,
respectively, with selected geometric values.
Photophysical data in solution and in the solid-state
(dispersed in KBr pellets) for all compounds are gathered
in Table 1 (in CH2Cl2) and Table S1, Supporting Informa-
tion (in toluene).
All compounds absorbed in the UV-A region with
relatively high absorption coefficients, as expected for πꢀπ*
transitions of polyaromatics. In the dyad cases (19, 21), an
additional absorption band due to the BODIPY moiety is
observed at ca. 500 nm. More structured vibronic bands are
observed for the HPI family, as expected for extended and
rigid conjugated structures. A hyperchromic effect appears
when electrodonating groups (e.g., diethylamino) are inserted
on the phenol side of HBI dyes.
Two main photophysical phenomena are observed for
the emission. For the HBO ligands, an ESIPT process,
characteristic of hydroxybenzazole family is responsible
for the large Stokes’ shifts ΔSS (Figure 3a,b).14,16 Indeed,
the tautomerization in the excited state leads to the excited
keto form (K*) responsible of the lower energy emission.
The emission derived from the excited enol form (E*) is
weakly observed as a small shoulder in 360ꢀ400 nm region
and is enhanced in solvents with stronger dipolar moments
(Figure 3a and Figure S3, Supporting Information).16 The
emissions of the K* state have a nanosecond scale lifetime,
and the quantum yields are relatively good in the case of 11
(21% and 12% in toluene and CH2Cl2 respectively), due to
the strong donor group on the phenol side and in the case
of the HPI systems 15 (25 and 15% in toluene and CH2Cl2
respectively) (Figure 3). For the latter, the extended delo-
calization and the steric restriction of rotational deactiva-
tion reduce the nonradiative deactivation.17
Figure 1. (Left) ORTEP view for 15 showing the atom-labeling
scheme. Thermal ellipsoids are plotted at the 50% level. (Right)
perpendicular view to highlight the slightly concave distortion of the
pentacyclic platform enforced by the tolyl substitution at N1. C22ꢀ
N1ꢀC2 and C3ꢀN2ꢀC1 angles are 124.23(17) and 105.63(17)°.
In 15, the phenol group is coplanar with the HPI cycle
via an intramolecular H-bond interaction between the
O1H1 hydroxyl group and one of the nitrogen N2 atoms
of the imidazole fragment, with the tolyl group being
almost perpendicular (dihedral angle of 88.0°). This con-
tributes to the overall relative planarity of the 24-atom
platform. In 14, the apparent distortion of the HBI core,
characterized by the meaningful dihedral angle value of
18.0° between the phenolate and the benzimidazole cycles,
is likely due to the butyl and trans diethylamino group
substitution. The boron center displays a fairly regular
tetrahedral geometry. The B1ꢀO1 and B1ꢀN1 bonds
˚
(1.453 and 1.552 A) are similar to the borate complexes
(16) (a) Zhao, J.; Ji, S.; Chen, Y.; Guo, H.; Yang, P. Phys. Chem.
Chem. Phys. 2012, 14, 8803–8817. (b) Das, K.; Sarkar, N.; Ghosh, A. K.;
Majumbar, D.; Nath, D. N.; Bhattacharyya, K. J. Phys. Chem. 1994, 98,
9126–9132.
(17) Nayak, M. K. J. Photochem. Photobiol. A: Chem. 2012, 241, 26–
37.
of benzazole.12 In the crystal structure, the molecular
arrangement is similar to corrugated layers parallel to the
(ꢀ3 0 2) plane (see the Supporting Information for a full
description of the molecular packing).
Org. Lett., Vol. XX, No. XX, XXXX
C

Table 1. Optical Data Measured in Aerated Solution and in the
Solid State
λa(max)
dye (nm) (Mꢀ1.cmꢀ1
ε
λ
em (max)
Δ
b
)
(nm) (cmꢀ1
)
Φ F
τ (ns)
matrix
CH2Cl2
9
338
372
322
333
346
371
334
366
331
352
375
394
363
379
363
383
362
374
14500
17400
39300
19700
32600
71600
25700
28400
21300
77300
20700
74500
387
415
458
465
444
447
382
389
371
394
393
409
472
473
393
429
489
474
513
473
393
473
513
441
3700 0.25 1.04
2800 0.04 0.41/2.90 solidc
9200 0.03 1.75
8500 0.03 1.23/2.93 solid
6400 0.12 1.78
4600 0.04 0.2/0.98 solid
3800 0.32 0.99
1600 0.26 1.69
745 0.65 1.28
3100 0.04 0.28/1.7 solid
1200 0.13 1.94
900 0.07 0.34/1.62 solid
6400 0.15 1.51
5200 0.12 0.46/1.64 solid
662 0.91 1.63
1420 0.17 0.47/1.39 solid
7200 0.08 0.77
5600 0.28 0.22/3.24 solid
430 0.25 2.26
5900
2000 0.53 1.86
2000 0.27 0.63/2.42 solid
430 0.24 2.30 CH2Cl2
3600 0.76 solid
10
11
12
13
14
15
17
18
CH2Cl2
CH2Cl2
CH2Cl2
solid
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
19 362/502
CH2Cl2
solid
CH2Cl2
370
364
373
0.38
20
21 363/502
380
Figure 3. (a) Absorption (plain) and emission (dotted) spectra of
10 (red) and 13 (black). (b) Absorption (plain) and emission
(dotted) spectra of 15 (red) and 17 (black) in CH2Cl2 at rt.
a λ denotes λabs for solution-state absorption or λexc for solid-state
excitation. b Quantum yields determination: in solution, using quinine
sulfate as reference Φ = 0.55 in H2SO4 (1 N), excitation at 366 nm for
dyes emitting below 480 nm or rhodamine 6G, Φ = 0.88 in ethanol,
excitation at 488 nm for dyes emitting between 480 and 570 nm; in the solid-
state at a concentration of around 10ꢀ4 M. c Recorded in KBr pellets.
The borate complexes show efficient emissions (up to
Φ = 91%) with relatively fine and structured emission
bands (Figure 3), a lifetime of ca. 1.5 ns, and a Stokes’ shift
between 350 and 2000 cmꢀ1 (with exception for 12). The
emissions are mirror image of the absorptions (Figure 3),
and no modification was observed in deaerated solvents.
These compounds seem to emit preferentially from a
singlet excited state rather than a low-lying excited state
with a internal charge transfer (ICT) character as pre-
viously observed with HBO-borate complexes.12 As ex-
pected, in light of their emissive properties, the HBI and
HPI borate act both as efficient light-harvesting antennas
in dyads 19 and 21 (see Figures S23ꢀS26, Supporting
Information). The excitonic energy transfer (EET) is
almost quantitative leading to the unique emisson of the
BODIPY subunit when exciting at 360 nm. The efficient
overlap of the antenna emission with the BODIPY absorp-
tion, as well as the short distance between both chromo-
phores explain the strength of the energy transfer.
Interestingly, most of the compounds exhibit a non-negligible
solid-state emission, with ΦF up to 26ꢀ28% for 12 and the
HPI and HPI borate derivatives (see Figures S27ꢀS36,
Supporting Information). For HPI 15, the crystal packing
shows almost no πꢀπ intermolecular orbital overlap, this is
probably one of the factors accounting for a good solid state
quantum yield.17
In summary, we have prepared an original series of
N-alkylated 2-(20-hydroxyphenyl)benzimidazole (HBI)
or 2-(20-hydroxyphenyl)-9,10-phenanthroimidazole (HPI)
and their corresponding BF2 complexes and studied their
optical properties. The ligands exhibit fluorescence which
originates from anESIPT and the borate complexes from a
singlet excited state (no boron decomplexation of the dyes
has been observed during the photophysical studies). They
show intense fluorescence in solution and in the solid state.
These series of compounds are of paramount importance
for the design of efficient blue emitters, suitable for various
applications: as antenna for cassette systems or as blue
solid-state emitters.
Acknowledgment. The Centre National de la Recherche
Scientifique (CNRS) is acknowledged for financial sup-
port and MENRT for PhD grant (K.B.)
Supporting Information Available. Synthetic proce-
1
dures, H and 13C NMR spectra, and crystallographic
and spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX