Porphyrins with Four Monodisperse Oligofluorene Arms as Efficient Red Light-Emitting Materials

Article abstract of DOI:10.1021/ja039832y

A family of red light-emitting star-shaped porphyrins with four monodisperse conjugated oligofluorene arms was prepared by using two key reactions: Lindsey condensation and Suzuki?Miyaura cross coupling reactions. All porphyrins exhibit high quantum yields (about 0.22) and good solubility in common organic solvents, and form high-quality solid films. Optical studies showed that the star-shaped oligomers could absorb blue light and emit saturated red light via efficient energy transfer from the fluorene segments to the porphyrin core. Copyright

Full text of DOI:10.1021/ja039832y

Published on Web 02/27/2004
Porphyrins with Four Monodisperse Oligofluorene Arms as Efficient Red
Light-Emitting Materials
Binsong Li, Jing Li, Yaqin Fu, and Zhishan Bo*
State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
Received November 27, 2003; E-mail: zsbo@iccas.ac.cn
Chart 1
Organic and polymeric light-emitting materials have attracted
considerable scientific and industrial attention due to their potential
application in large area flat-panel displays.1 For full-color displays
three primary colors, i.e., blue, green, and red light-emitting
materials are required. Green light-emitting materials have already
been achieved with high brightness and efficiency, while red and
blue light-emitting materials, which fully meet the requirements
for commercial application, are scarce.2 The main problem with
red is its relatively low efficiency. Usually red light emitting is
achieved by doping red dyes into host materials with a large band
gap.3 Red dyes are normally small organic molecules, which are
prone to aggregate and end up self-quenching their luminescence.
Organic dyes used as red emission doping materials are limited.
Porphyrin is one, which exhibits reasonable fluorescence efficiency
and good thermal stability. Examples of porphyrins used as red
emitting materials include: tetraphenylporphyrin (TPP)3a,d and
porphyrin platinum (II),3b,c which were used as dopants dispersed
into host matrix such as AlQ3 or poly(9,9-dioctylfluoren-2,7-diyl)s
(POFs). (POFs are very promising blue light-emitting materials with
exceptionally high solution- and solid-state quantum yields and
excellent chemical and thermal stabilities.) Additionally, TPP as
pendant groups attached to PPV polymer backbones with an
alkylene spacer were also investigated.4
Scheme 1 a
The idea in this communication is to bond the porphyrin (guest)
and polyfluorenes (host) with conjugated chemical connections,
which would lead to through-bond energy transfer. The rate of
through-bond energy transfer is normally a hundred times faster
than that of through-space energy transfer (Fo¨ster).6 Furthermore,
these kinds of structures should also help to form homogeneous
amorphous films and prevent the porphyrin rings from aggregating
and self-quenching their fluorescence. For this purpose, we
synthesized a series of well-defined star-shaped porphyrins with
four oligofluorene arms at their meso positions and investigated
the relationships of their structures and optical properties.
The chemical structures of the four-arm oligofluorene porphyrins
1-6 are outlined in Chart 1. The oligofluorene arms not only
conjugate with the porphyrin rings but also provide good solubility
for the whole molecules. The key reactions involved in the synthesis
are “Lindsey condensation” for porphyrin synthesis and Pd-mediated
Suzuki-Miyaura cross coupling (SMCC) reactions for the attach-
ment of the oligofluorene arms. The synthetic routes to porphyrins
1-4 are outlined in Scheme 1.
The synthesis of the monofluorene porphyrin 1 was started from
2-bromo-9,9-dioctylfluorene 7, with treatment of 7 with 1.2-fold
of n-BuLi and followed by quenching with DMF to give 8 in a
yield of 49%. Lindsey condensation of 8 with equimolar amount
of pyrrole in choroform with trifloroacetic acid (TFA) as a catalyst
afforded 1 in a yield of 34%. Porphyrins 2 and 3 were prepared by
SMCC of tetrabromo-monofluorene-porphyrin 5 (vide infra) with
corresponding mono- and bifluoreneboronic acid 9 and 10, respec-
a (a) (i) n-BuLi, -78 °C to rt; (ii) DMF, -78 °C to rt. (b) (i) pyrrole,
TFA, CHCl3, rt, 12 h; (ii) DDQ. (c) Pd(PPh3)4, NaHCO3, THF, H2O, N2,
reflux, 2 d.
tively. Porphyrin 4 was obtained by coupling of tetraiodo-
bifluorene-porphyrin 6 (vide infra) with bi-fluoreneboronic acid 10.
The SMCC reactions were done with Pd(PPh3)4 as a catalyst
precursor in the biphasic system THF/NaHCO3, and afforded the
final products in moderate yields (44-62%). The synthesis of two
key intermediate porphyrins 5 and 6 is shown in Scheme 2.
Monolithium-halogen exchanging of 11 and quenching with DMF
accomplished 7-bromofluorene-2-carbaldehyde 12 in a yield of
61%. Condensation of 12 with pyrrole afforded 5 in a yield of 35%.
The synthesis of tetraiodo-porphyrin 6 started from 11, which was
converted to 2-bromo-7-trimethylsilyl-9,9-dioctylfluorene 13 by
treatment with n-BuLi and followed by quenching with TMSCl.
Halogen-lithium exchange followed by quenching with B(O-i-Pr)3
affording 14 in a 58% yield. SMCC of 12 and 14 in standard
conditions furnished 15 in a yield of 77%. The TMS group was
readily converted to the iodo group by treatment with ICl in CH2-
Cl2. 16 was obtained in a yield of 97%. Condensation of 16 with
pyrrole afforded porphyrin 6 in a yield of 31%. All intermediates
and final products were purified with column chromatography on
silica gel and characterized unambiguously with 1H and 13C NMR
spectroscopy, combustion analysis, or matrix-assisted laser desorp-
tion ionization time-of-flight (MALDI-TOF) mass spectroscopy.
9
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10.1021/ja039832y CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2 a
because the torsion angle between porphyrin and fluorene is smaller
in films than in solution.7 From the efficiency gain and the blue
shift in the emission we would conclude that the degree of
aggregation of the porphyrins was significantly reduced. With an
increase of the number of fluorene units, irradiation of the fluorene
segments at 360 nm resulted in the intensity of fluorene emission
bands between 380 and 500 nm weakening, and the intensity of
the porphyrin Q-band emission at around 658 nm increasing. This
indicated that the efficiency of energy transfer from oligofluorene
arms to the porphyrin core increased with the conjugation length
of oligofluorenes. The remaining emission from oligofluorenes
accounted for the competition between the direct emission from
oligofluorenes and energy transfer to porphyrin. However, energy
transfer become predominant when n g 3, because the emission
intensity from the oligofluorene segments decreased drastically from
1 to 4. The fluorescence quantum yields (ΦF) of 1-4 in toluene
were measured in comparison to TPP (ΦF ) 0.11)5 and range from
0.16 (for 1) to 0.22 (for 4). The values are much higher than many
other porphyrins; however, they do not show a trend and seem to
be similar irrespective of the number of fluorene units.
a (a) (i) n-BuLi, -78 °C to rt; (ii) DMF, -78 °C to rt. (b) (i) pyrrole,
TFA, CHCl3, rt, 12 h; (ii) DDQ. (c) (i) n-BuLi, -78 °C to rt; (ii) TMSCl,
-78 °C to rt. (d) (i) n-BuLi, -78 °C to rt; (ii) B(O-i-Pr)3, -78 °C to rt;
(iii) HCl; (iv) pinacol, CH2Cl2. (e) Pd(PPh3)4, NaHCO3, THF, H2O, reflux,
2 d. (f) ICl, CH2Cl2, 0 °C to rt, 12 h.
In conclusion, a series of star-shaped porphyrins with four
oligofluorene arms were prepared. The side octyl chains on the
fluorene units can efficiently suppress the aggregation of the
porphyrin rings. The star-shaped oligomers showed blue absorption,
but emitted in red due to efficient energy transfer. The efficiency
of light absorption and energy transfer was intensified with
increased conjugated length of the oligofluorene arms.
Acknowledgment. Financial support from The Chinese Acad-
emy of Sciences, the National Natural Science Foundation of China
(20225415), and the Major State Basic Research Development
Program (2002CB613401) is greatly acknowledged. We thank Prof.
Yang Yang for helpful discussion.
Supporting Information Available: Detailed experimental pro-
cedures and characterization of all compounds (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 1. Normalized PL spectra of spin-coated thin films of 1-4 and
TPP.
Porphyrins 1-4 are readily soluble in common organic solvents
such as THF, CH2Cl2, chloroform, and toluene.
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were done with all porphyrins 1-4, and the results fitted the
calculated values very well.
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