at position 9 of fluorenes, several 9,9-dialkylated fluorenes
and spirofluorenes were prepared by blocking the reactive
methylene moiety of fluorenes, but they are also found to
be vulnerable to photo- or electrooxidative degradation
leading to emission band broadening and/or with low
electron-hole recombination efficiency.13 Holmes et al.14
recently demonstrated that it is possible to prepare oxidatively
stable polyfluorenes by carefully prefixing the dialkyl
substitution at position 9 of fluorenes. In this paper, we
describe a novel strategy15 which demonstrates that 9-un-
substituted small molecule fluorenes can be stable bright blue
light emitters providing that fluorene/fluorenone systems are
“appropriately functionalized” with donor-acceptor and
chromophoric groups.
Scheme 1
.
Synthesis of Fluorenes (6a-e, 9a-e) and
Fluorenones (7a-e, 10a-e)a
Based on important literature reports on the electrooptical
properties of fluorenes,16,17 we envisaged that an optimistic
strategy is to equip the scaffold with donor-acceptor (D-A)
and chromophoric π-groups for controlling emission char-
acteristics, morphological stability, and/or electron-hole
recombination efficiency. Despite a large number of synthetic
efforts and studies on the fluorene system, to our surprise,
only limited synthetic methodologies are available in the
literature to architect the fluorene framework.18,19 In most
of the reports, commercially available fluorene has been used
as a crucial precursor for preparing mono-, oligo-, or
polyfluorenes, which offers very limited options for sub-
stituent variations.20,21
a Yields are given with respect to their immediate precursors.
We recently developed a new methodology22 for func-
tionalized quateraryls and demonstrated that donor-acceptor
quateraryls are potential candidates for developing small
molecule blue organic light emitting diodes.23 In order to
understand the GED phenomenon, a methodology for the
preparation of fluorenes with donor-acceptor groups was
desirable. Our approach to prepare diversely functionalized
fluorenes is depicted in Scheme 1. The key intermediates
6-aryl-2-oxo-4-methylsulfanyl-2H-pyran-3-carbonitriles
3a-d were prepared from easily accessible precursors
R-cyano-ketene-S,S-acetal 1and various aryl methyl ketones
2a-d as described earlier.24 Our aim to prepare fluorenes
with amine-donor and nitrile-acceptor groups was achieved
by preparing 6-aryl-2-oxo-4-pyrrolidin/piperidin-1-yl-2H-
pyran-3-carbonitriles (4a-e) from 3a-d (see the Supporting
Information) and reacting them separately with 1-indanone
(5) and 2-indanone (8) in the presence of a base, which
furnished 1-aryl-3-pyrrolidin/piperidin-1-yl-9H-fluorene-4-
carbonitrile 6a-e and 4-aryl-2-pyrrolidin/piperidin-1-yl-9H-
fluorene-1-carbonitrile 9a-e, respectively, in excellent yields
(Scheme 1). All of the fluorenes (6a-e, 9a-e) were
efficiently converted to corresponding 9-fluorenones (7a-e,
10a-e) by aerial oxidation in the presence of sodium
hydride.15 A plausible mechanism for the formation of
fluorenes is shown in the Supporting Information.
(11) Liu, L.; Qiu, S.; Wang, B.; Zhang, W.; Lu, P.; Xie, Z.; Hanif, M.;
Ma, Y.; Shen, J. J. Phys. Chem. B 2005, 109, 23366
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(12) List, E. J. W.; Guentner, R.; Freitas, P. S. D.; Scherf, U. AdV. Mater.
2002, 14, 374
.
(13) (a) Grice, A. W.; Bradley, D. D. C.; Bernius, M. T.; Inbasekaran,
The photophysical properties of all of the synthesized
compounds 6a-e, 7a-e, 9a-e, and 10a-e were examined
by UV-vis and photoluminescence techniques (Table 1).
All of the fluorenes (6a-e) substituted at position 1 with
different chromophoric groups (π-groups) such as phenyl,
naphthyl, or pyrenyl showed PL in the blue region (6a-e,
λPL 442-462 nm), while their corresponding fluorenones
(7a-e) showed PL in the yellowish green region (7a-e, λPL
526-529 nm). When we changed the positions of these
donor-acceptor and chromophoric groups as shown in a
series of fluorenes 9a-e, all fluorenes exhibited PL in the
blue region (9a-e, λPL 446-481 nm) but their corresponding
fluorenones showed either blue PL (10c: λPL 460 nm, 10e:
λPL 473 nm) or no PL (10a,b,d) depending on the chro-
M.; Wu, W. W.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 629. (b) Schartel,
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Indian Patent Appl. No. 0838DEL2008, 28 Mar 2008; CDRI ref no. 0053/
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