The interplay of conformation and photophysical properties in deep-blue fluorescent oligomers

Article abstract of DOI:10.1039/c0cc00211a

We report the synthesis, X-ray crystal structures and photophysics of new donor-acceptor oligomers of fluorene (F) and dibenzothiophene-S,S-dioxide (S) with constrained dihedral angles in the backbone. The materials display bright deep-blue fluorescence and evidence is presented for a planarised intramolecular charge-transfer (PICT) state in the F-S systems.

Full text of DOI:10.1039/c0cc00211a

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The interplay of conformation and photophysical properties in deep-blue
fluorescent oligomersw
Haiying Li,a Andrei S. Batsanov,a Kathryn C. Moss,a Helen L. Vaughan,b Fernando B. Dias,b
Kiran T. Kamtekar,c Martin R. Bryce*a and Andrew P. Monkmanb
Received 3rd March 2010, Accepted 6th May 2010
First published as an Advance Article on the web 25th May 2010
DOI: 10.1039/c0cc00211a
We report the synthesis, X-ray crystal structures and photo-
physics of new donor–acceptor oligomers of fluorene (F) and
dibenzothiophene-S,S-dioxide (S) with constrained dihedral
angles in the backbone. The materials display bright deep-blue
fluorescence and evidence is presented for a planarised intra-
molecular charge-transfer (PICT) state in the F–S systems.
showed no evidence of the unwanted green emission from
fluorenone defects.7–10
Several other groups have recently incorporated dibenzo-
thiophene-S,S-dioxide into small molecules and polymers, to
exploit the high stability, electron affinity and strong fluores-
cence properties of the S unit.11–14 A tris-cyclometalated Ir(III)
complex of a dibenzothiophene-S,S-dioxide derivative exhibits
bright green phosphorescence.15
Studies on p-conjugated oligomers have provided extensive
information which is relevant to the design and development
of polymers for optoelectronic applications.1 In this regard,
linear oligo/polyfluorene derivatives are the benchmark family
of emitters whose properties can be tuned through chemical
modification and copolymerisation.2 The incorporation of
different monomer units into the backbone provides deep
insights into the structure–property relationships in this family
of emitters, especially ground and excited state charge transfer
interactions between donor and acceptor moieties.
From previous studies the structure of the intramolecular
charge-transfer (ICT) state in F–S systems, and some of the
factors governing its formation, remained an open question.
The aim of the present work was to probe this issue and to
develop further the potential of dibenzothiophene-S,S-dioxide
derivatives as fluorophores. We have now incorporated new
functionalised S subunits into oligomers and studied their
fundamental photophysical properties. The specific targets have
the dihedral angles in the backbones constrained by their sub-
stituents into (i) twisted (3–6) and (ii) planarised (ladder-type)
conformations (7).
Thiophene-S,S-dioxide has been widely incorporated into
conjugated oligomers and polymers, including polyfluorenes.3
The sulfone group lowers the LUMO energy,4 which is
beneficial for applications in polymer light emitting diodes
(PLEDs)5 and organic photovoltaics.6 In contrast, dibenzo-
thiophene-S,S-dioxide, which is topologically very similar to
fluorene, has received less attention. We reported a prototype
series of 9,9-dialkylfluorene–dibenzothiophene-S,S-dioxide
(F–S) co-oligomers, e.g. trimer 2.7 Compared to the parent
oligofluorenes, e.g. trimer 1, the F–S co-oligomers showed
improved electron affinity and stability with regard to both
p- and n-doping, while retaining high photoluminescence
efficiencies in both solution and solid state. Dual fluorescence
from both local excited (LE) and intramolecular charge trans-
fer (ICT) states in the F–S co-oligomers and the related
copolymers [p(F–S)] is a special feature imparted by the S
units.8 Regiorandom [p(F–S)] copolymers give improved
performance in PLEDs compared to poly(9,9-dioctylfluorene)
(PFO) under comparable conditions.9 The F–S materials
Only
a limited range of functionalised S units are
known,7–9,11–15 so this study required the synthesis of new
building blocks. The structures of the oligomers used in the
present study are shown in Fig. 1. Compounds 116 and 27 have
a Department of Chemistry, Durham University, Durham,
UK DH1 3LE. E-mail: m.r.bryce@durham.ac.uk
b Department of Physics, Durham University, Durham,
UK DH1 3LE
c Zumtobel LED Division, Green Lane Industrial Estate,
Spennymoor, UK DL16 6HL
w Electronic supplementary information (ESI) available: Synthesis
and characterisation of 3–8; X-ray crystallographic data; photo-
physical data and spectra; and solution electrochemical data. CCDC
768385, 768386, 768387. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0cc00211a
Fig. 1 Structures of oligomers and model compound used in this study.
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been reported previously. Compound 8 is a model 3,7-diaryl
analogue of the planarised compound 7.
the presence of the flexible alkoxy substituent(s). This means
that, for the first time, oligomers have been purified which
contain more S than F units, e.g. the S–F–S trimer 5 and even
the S–S–S trimer 6 show good solubility in organic solvents.
Analogous trimers containing the unsubstituted S unit were
too insoluble to be characterised. Hitherto this reduced
solubility with increasing S content has restricted the com-
position of F–S co-oligomers that could be studied.
The syntheses of 3–6 and 8 were achieved by Suzuki cross-
coupling reactions of the appropriate bromide/dibromide and
boronic ester(acid)/diboronic ester(acid) reagents. The synthesis
of 7 involved a multi-step sequence culminating in a double
intramolecular cyclisation of an appropriately functionalised S
unit—a strategy based on the synthesis of indenofluorenes17
(see the ESIw for details). The X-ray molecular structures of 3, 4
and 6 are shown in Fig. 2. Angles between the ring planes are
presented in Table 1. The bond lengths show no significant
differences between the three structures.
Photophysical data for compounds 1–8 are collated in
Table S1 in the ESI.w Fig. 3 shows the emission spectra of
compounds 1, 2, 4 and 5 in toluene and chloroform solutions;
Fig. 4 shows the spectra of 5 and 6; and Fig. 5 shows the
spectra of 7 and 8.
The fluoro and alkoxy substituents on the S units in 3–6
have two notable effects: (i) a coplanar conformation of the
subunits will be prevented due to peri interactions, leading to a
more twisted backbone structure compared to the oligomer 2;
and (ii) the solubility of the S unit is considerably increased by
The following observations are important.
(i) The PL spectra of the compounds containing both F and
S (i.e. 2–5), or S and Ar units (7, 8) show strong positive
solvatochromism and inhomogeneous broadening; these features
are not observed for homo-F (1) and homo-S (6) compounds.
This is clear evidence for an intramolecular charge-transfer
(ICT) donor–acceptor interaction between F and S units
which is stabilised in polar solvents. This interaction is not
Fig. 3 Normalised photoluminescence spectra of 1, 2, 4 and 5 in
toluene (solid lines) and chloroform (dashed lines). Note the solvato-
chromic shift for 2, 4 and 5, indicative of the ICT state; this is not
observed for 1.
Fig. 4 Normalised photoluminescence spectra of compounds 5
(black) and 6 (red) in toluene (solid lines) and chloroform (dashed
lines).
Fig. 2 X-Ray molecular structures of 3, 4 and 6 (thermal ellipsoids at
50% probability; disorder and all H atoms omitted). Fluorine atoms in
3 and cyclohexylmethyl groups in 4 and 6 are shown green.
Table 1 Angles between the ring planes (1)a
3 (four independent molecules)
4
6
i/ii
36.7
4.4
41.5
78.7
44.2
3.7
39.0
79.7
40.0
4.0
41.1
79.9
46.0
5.5
33.9
81.8
48.9
11.5
56.5
74.8
47.3
13.5
68.7
43.2
ii/iii
iii/iv
i/iv
Fig. 5 Normalised photoluminescence spectra of 7 (red) and 8 (black)
a
For ring notation see Fig. 2.
in toluene (solid lines) and chloroform (dashed lines).
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1–8 are well-fitted by a sum of two exponential functions
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of molecular structure and solvent relaxations on the emission
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(iii) The planarised ladder compound 7 shows by far the
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(ca. 1.7 and 3.8 ns, respectively) (Fig. 6) compared to the
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and model compound 8 (ca. 1.5 and 2.8 ns, respectively). This
is strong evidence for planarisation stabilising the ICT state
(i.e. formation of a longer-lived PICT state).18 For the other
compounds which will have non-planar backbones, there will
be stronger coupling between ground and excited states, hence
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All the compounds exhibit bright deep-blue fluorescence in
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In summary, we have synthesised new donor–acceptor
oligomers with constrained dihedral angles in the backbone.
The materials display bright deep-blue fluorescence. The ICT
interaction is not precluded by the twisted backbone con-
formation, although it is stabilised by the planar conformation
enforced on the ladder system 7. These results provide new
insights into donor–acceptor interactions in conjugated oligo-
mers and demonstrate that dibenzothiophene-S,S-dioxide
derivatives have considerable scope as optoelectronic materials.
We thank EPSRC, Durham University, the UK Technology
Strategy Board (TSB) and Thorn Lighting for funding.
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Notes and references
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