C O M M U N I C A T I O N S
In conclusion, an unprecedented series of phase-tunable fluoro-
phores based upon highly photoluminescent BBI salts has been
synthesized. A key structural feature of these fluorophores is that
they incorporate imidazolium moieties, whereupon annulation leads
to desirable luminescent properties. Through judicious choice of
N-substituents and counterions, BBI salts with Td values as high
as 338 °C were obtained for materials that were fluidic below
0 °C. This feature also enabled access to two BBI-based mesogens,
thus introducing a new platform for fluorescent ILC design.
Collectively, these organic salts produced constant blue emission
from solution through cooled glassy states to free flowing liquids.
Considering their high thermal stabilities, amphiphilic properties,
and structural modularity, BBI salts effectively form a new class
of emissive chromophores with promise as processable fluoro-
phores, sensory materials, and models for fundamental photophysi-
cal investigations.
Figure 1. Left: Photoluminescence spectrum of 4•MeSO4 in MeOH (solid
line) and as a thin film (dotted line), each under ambient conditions. Right:
Picture of 4•MeSO4 heated at ca. 80 °C under irradiation from a 365 nm
lamp (5 W).
Acknowledgment. We are grateful to the USARO (W911NF-
05-1-0430, W911NF-06-1-0147), NSF (CHE-0645563), ACS-PRF
(44077-G1), and the Welch Foundation (F-1621) for financial
support. C.S.P. thanks the NSF for funding (DMR-0213918, DMR-
0552399). We also thank Prof. D. L. Gin at CU Boulder for
assistance with LC characterization.
Supporting Information Available: Detailed experimental pro-
cedures and characterization of all new compounds are available. This
Figure 2. PLM images (left: ILC 8a, right: ILC 8b) were obtained as
LC phases and appeared upon cooling from the isotropic melt; magnification
) 100×.
References
maintain intense emission in condensed phases. Photoluminescence
was qualitatively observed from each of the glassy BBI fluorophores
(i.e., 3-7) at temperatures above and below their Tg value. For
example, as depicted in Figure 1, bright blue emission obtained
from a bulk sample of 4•MeSO4 was found to persist at 200 °C, a
temperature well above its Tg. Excitation of an annealed thin film
of this material (obtained via melt-casting) produced a bright blue
emission with a λem of 423 nm, consistent with the λem in solution.
Collectively, these results indicated that BBI-based IL fluorophores
maintained efficient luminescent properties in solution, solid state,
and as flowing liquids.
Having obtained room-temperature fluorescent ILs, we shifted
our focus toward fine-tuning phase control to obtain mesomorphic
fluorophores based on BBIs. This was motivated largely by the
observation that imparting LC behavior to neutral organic fluoro-
phores can greatly improve their performance in electronic ap-
plications.9 Furthermore, the BBI architecture would introduce a
unique structural class of ILCs featuring rigid, polycyclic cationic
cores1i,j similar to dye-based chromonic LCs but photoluminescent
in nature.10
Two BBI-based mesogens, 8a and 8b (see Table 1), were
synthesized, and each was analyzed by differential scanning
calorimetry (DSC), polarized light microscopy (PLM), and variable-
temperature powder X-ray diffraction (VT-PXRD).5,11 Investigation
of the DSC cooling cycles of 8a suggested a broad LC temperature
range that began at 53 °C and extended to 194 °C, at which point
the material became isotropic. Upon cooling from the isotropic melt,
PLM of 8a showed an optical texture indicative of an anisotropic
LC phase (Figure 2 left). VT-PXRD revealed equally spaced
reflections consistent with a smectic phase.5,12 DSC analysis of
8b revealed a narrower but higher temperature LC range (188-
238 °C). The black PLM texture of this material (Figure 2 right)
and its optical transparency in the bulk under normal light were
suggestive of a thermotropic cubic phase.13 Additionally, the
observation of PXRD peaks in the ratios 1/x8, 1/x9, 1/x30, and
1/x35 was consistent with 8b adopting a bicontinuous cubic
phase.5,13 Further elaboration of the LC phases of 8a and 8b is
underway.
(1) (a) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H., Jr. J. Am. Chem.
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Cui, Y. Appl. Phys. Lett. 2003, 83, 1020. (c) Chondroudis, K.; Mitzi, D.
B. Appl. Phys. Lett. 2000, 76, 58. (d) Lo´pez-Martin, I.; Burello, E.; Davey,
P. N.; Seddon, K. R.; Rothenberg, G. ChemPhysChem 2007, 8, 690. (e)
Welton, T. Chem. ReV. 1999, 99, 2071. (f) Paul, A.; Mandal, P. K.;
Samanta, A. J. Phys. Chem. B 2005, 109, 9148. (g) Paul, A.; Mandal, P.
K.; Samanta, A. Chem. Phys. Lett. 2005, 402, 375. (h) Earle, M. J.; Seddon,
K. R. World Patent: WO 2006043110 (2006). (i) Binnemans, K. Chem.
ReV. 2005, 105, 4148. (j) Kato, T. Science 2002, 295, 2414.
(2) For an example of a macromolecular fluorescent IL, see: Huang, J.-F.;
Luo, H.; Liang, C.; Sun, I.-W.; Baker, G. A.; Dai, S. J. Am. Chem. Soc.
2005, 127, 12784.
(3) Sonoluminescence has also been observed from imidazolium salts; see:
Oxley, J. D.; Prozorov, T.; Suslick, K. S. J. Am. Chem. Soc. 2003, 125,
11138.
(4) (a) Boydston, A. J.; Williams, K. A.; Bielawski, C. W. J. Am. Chem.
Soc. 2005, 127, 12496. (b) Boydston, A. J.; Khramov, D. M.; Bielawski,
C. W. Tetrahedron Lett. 2006, 47, 5123.
(5) See Supporting Information.
(6) Starting from 1,5-dichloro-2,4-dinitrobenzene, BBIs 3-8 were synthesized
in high yields via a chromatography-free SNAr-reductive cyclization-
alkylation sequence.5 Where applicable, anion metathesis from the
corresponding BBI diiodide was performed according to literature protocol;
see: Vu, P. D.; Boydston, A. J.; Bielawski, C. W. Green Chem. 2007, 9,
1158.
(7) X-ray crystallographic analysis of 3 revealed infinite rows of dimers which
were attributed to π-π facial interactions between the polycyclic cores
of the molecules. In contrast, solid-state analysis of 6 confirmed that π-π
interactions had been disrupted.5
(8) Notably, the dicationic BBIs can accommodate two dissimilar counterions.
For example, incorporating one BF4 and one MeSO4 anion (i.e., mixing
equimolar amounts of 4•MeSO4 and 4•BF4, or anion metathesis6 of 4•I
with 1.0 molar equiv each of Me3O•BF4 and Me2SO4) produced a BBI
with a Tg of 19 °C.
(9) (a) O’Neill, M.; Kelly, S. M. AdV. Mater. 2003, 15, 1135. (b) Levitsky,
I. A.; Kishikawa, K.; Eichhorn, S. H.; Swager, T. M. J. Am. Chem. Soc.
2000, 122, 2474.
(10) (a) Lydon, J. Curr. Opin. Colloid Interface Sci. 2004, 8, 480. (b) Tortora,
L.; Park, H.-S.; Antion, K.; Finotello, D.; Lavrentovich, O. D. Proc. SPIE
2007, 6487, 64870I-1. (c) Tam-Chang, S.-K.; Helbley, J.; Carson, T. D.;
Seo, W.; Iverson, I. K. Chem. Commun. 2006, 503.
(11) The solution absorption and photoluminescence of compounds 8a and
8b were consistent with their structural analogues 1 and 3, respectively
(see Table 2).
(12) The smectic LC phase layer spacing was found to be less than the
molecular length, indicating some tail interdigitation and/or molecular tilt
in the layers. Similarly, the single-crystal structure of 8a showed a layered
arrangement with overlapping alkyl chains.5
(13) For a recent review on thermotropic cubic mesophases, see: Impe´ror-
Clerc, M. Curr. Opin. Colloid Interface Sci. 2005, 9, 370.
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