Fig. 3 The location of the highest occupied molecular orbitals (HOMOs)
of fluorinated Alq derivatives obtained by MO calculations.
3
3
However, the F group at the C-6 position of 6FAlq mainly
Fig. 2 (a) Emission of Alq , 5FAlq
3
3
, 6FAlq
3
and 7FAlq
3
thin films
exhibits an electron-withdrawing inductive effect to lower the
HOMO energy level, due to the very low electron density at the C-6
position. Therefore, the red-shift or blue-shift of the absorption
and emission of 5FAlq or 6FAlq can be caused by the narrowed
vacuum-deposited on quartz substrates (y50 nm thick, from left to right)
upon illumination with UV light (365 nm); (b) Photoluminescence spectra
3 3 3 3
of Alq , 5FAlq , 6FAlq and 7FAlq thin films vacuum-deposited on quartz
3
3
substrates (y50 nm thick). The excitation wavelength is 370 nm.
or enlarged HOMO–LUMO energy gap of the two compounds,
respectively. The hypothesis is supported by their HOMO and
LUMO energy levels, obtained from cyclic voltammetry, and their
onsets of absorption in solution (see the ESI{). The HOMO–
and the intensity of the emission peak of 7FAlq
3
are similar as
derivatives
those of Alq . The optical data for the fluorinated Alq
3
3
are summarized in Table 1. All of these observations indicate that
the fluorination position affects greatly the spectral properties.
LUMO energy gaps for 5FAlq
.66 and 2.93 eV, respectively; 0.15 eV smaller and 0.12 eV larger
than that (2.81 eV) of their parent compound Alq
3 3
and 6FAlq are estimated to be
2
3
It has been reported that the light emission of Alq originates
3
.
from the ligand’s electronic p–p* transition from a highest
occupied molecular orbital (HOMO) lying mainly on the
phenoxide ring to a lowest unoccupied molecular orbital
As far as 7FAlq is concerned, the F group at the C-7 position
3
also has a conjugation effect, but the energy gap and the emission
of 7FAlq
hindrance and electrostatic repulsion between the 7-F and 8-O
atoms in 7FAlq may weaken the conjugation effect between the F
group and the phenoxide ring, meaning that fluorination has little
influence on the energy gap and the emission of Alq
3 3
are similar to those of Alq . We speculate that the steric
(
LUMO) located on the pyridyl ring, which was derived from a
7
molecular simulation of the electronic structure of Alq
3
. Generally
3
speaking, the highest electron density of Alq ’s HOMO is located
3
on the phenoxide oxygen and the C-5, C-7 and C-8 positions.
Therefore, it is predicted that an electron-withdrawing or electron-
donating group at these positions will lead to a blue-shift or red-
shift in the absorption and fluorescence spectra. Some reports
support this anticipation, for example, by attaching an acceptor
3
.
It is interesting to notice from Table 1 that fluorination also has
a great effect on the photoluminescence quantum yield of the
fluorinated Alq
stronger coupling of the metal–ligand stretching coordinating to
the electronic transition in Alq may provide additional paths for
non-radioactive decay. Therefore, the tremendous decrease in the
photoluminescence quantum yield (WPL) of 5FAlq is reasonable
3
derivatives. Sapochak et al. suggested that the
3
group of –CN or a donor group of –CH at the C-5 position
3
7,8
causes a blue-shift and red-shift, respectively. However, this
prediction is not suitable in our cases, where the absorption and
8
3
emission of 5FAlq
unchanged.
To understand what happens in the fluorinated Alq
3
are red-shifted while that of 7FAlq
3
is almost
because the conjugation effect makes the coupling of the metal–
ligand stretching stronger and increases the energy loss in the
excited state vibration. On the other hand, the great enhancement
3
derivatives,
we also undertook MO calculations using time-dependent density
functional theory (TD-DFT) with 6-31 G(d) basis sets and the
Becke three-parameter hybrid exchange-correlation functional
in the WPL of 6FAlq is due to the reduced energy loss in the excited
3
state vibration.
In a summary, the emission colour of Alq
the direction of blue light by introducing a F atom in an
appropriate position (6FAlq ). In the meantime, the photolumi-
3
can be tuned towards
(known as B3LYP) to model the HOMOs and LUMOs of these
compounds. The calculated HOMOs are shown in Fig. 3. From
Fig. 3, it is found that, owing to high electron density at the C-5
position and the lone electron pair on the F atom, the F group
3
nescence quantum yield can also be enhanced significantly. This
provides us with a new approach to the design and preparation of
high-performance luminescence materials for blue OLEDs, for
example, by introducing a strong electron-withdrawing group at
C-6 of the phenoxide ring together with an electron-donating
takes part in forming the HOMO of Alq through a conjugation
3
effect, giving rise to the higher HOMO energy level of 5FAlq3.
Table 1 Photophysical data of the fluorinated Alq
3
derivatives
3
group at C-4 of the pyridyl ring in Alq . Work along these lines is
now in progress.
Alq 5FAlq
3
3
6FAlq
3
7FAlq
3
We thank the NSFC (nos. 50225312, 50403022, 50433020) for
financial support.
CHCl
0 nm thick film absorption lmax/nm 392 407
CHCl solution emission lmax/nm 515 547
0 nm thick film emission lmax/nm a 528 550
PL quantum yield in CHCl
PL quantum yield of a 50 nm thick film 1.00 0.091 1.46
3
solution absorption lmax/nm
387 407
375
376
495
505
388
388
515
518
0.70
0.72
5
3
5
Notes and references
3
solution 1.00 0.051 2.62
a
1
{ H NMR spectroscopic and elemental analyses: 5Fq d
H
(500 MHz,
): 8.81–8.83 (q, 1 H), 8.39–8.41 (q, 1 H), 7.49–7.51 (q, 1 H) and 7.05–
.15 (m, 2 H); 6Fq d : 8.72–8.73 (q, 1 H), 8.10–8.12 (q, 1 H), 7.45–7.48 (q,
H) and 6.95–6.99 (m, 2 H); 7Fq d : 8.81–8.82 (q, 1 H), 8.15–8.17 (q, 1 H),
a
CDCl
3
For clarity, the photoluminescence (PL) quantum yield (WPL) of all
compounds is normalized to WPL(Alq ) 5 1.00.
7
1
H
3
H
1
942 | Chem. Commun., 2006, 1941–1943
This journal is ß The Royal Society of Chemistry 2006
Shi, Yue-Wen
