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.
(7) (a) Chen, C.-T. Chem. Mater. 2004, 16, 4389–4400. (b) Lai, M.-Y.;
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,
~
€
(13) Esparza-Ruiz, A.; Hueso-Pena, A.; Noth, N.; Flores-Parra, A.
€
E.; Bolink, H. J.; Baranoff, E.; Gratzel, M.; Nazeeruddin, M. K.
J. Organomet. Chem. 2009, 694, 3814–3822.
J. Mater. Chem. 2012, 22, 19264–29268.
B
Org. Lett., Vol. XX, No. XX, XXXX