A R T I C L E S
Dutta et al.
systems.20 Interestingly, fusion of dioxole rings to the terminal
positions of acenes, in the place of alkoxy chains, made
relatively little difference in the optoelectronic properties.21
However, donor-acceptor dyads with C60 bismethano-fullerenes
as acceptor showed significantly different photoinduced charge
transfer behavior depending on whether the donor was a TAB
or BDO.22
set. Comparative calculations showed that the dioxole ring is slightly
nonplanar in agreement with experiment,30 thus no symmetry
restrictions were imposed. Vertical optical transition energies and
oscillator strengths were obtained by time-dependent (TD)DFT. All
calculations were carried out within the Gaussian03 program
package.31 Orbital pictures were produced with Molekel 4.3.32
3. Results
In this work we have introduced BDO into the conjugated
backbones of two poly(phenylene ethynylene)s (PPE) and a
poly(phenylene diethynylene) (PEE) resulting in polymers with
localized HOMO and delocalized LUMO, opposite to fairly
commonly observed localized LUMO and delocalized HOMO
in D-A polymers. For example, our calculations (not shown)
confirm the latter case for the benzothiadiazole-containing PPE
of Bunz23 and the cyclopentadienone-containing polythiophene
of Wudl.24 Unlike here, LSS in conjugated polymers is typically
attributed to large conformational differences between the
ground and excited states. The very large effective Stokes shifts
(LSS) demonstrated by the BDO polymers (0.7 to 0.9 eV), may
be useful to minimize self-absorption and light scattering in
optical materials.25 Possible applications include laser dyes,
molecular imaging, scintillators, solar collectors and white light
emitting materials.26-29 In a quantum-chemical approach using
density-functional theory we fully elucidate the reasons for the
electronic and optical properties of the new BDO-based
polymers and compare them to the analogous TAB PPEs.
Synthesis. An alkylated benzo[1,2-d:4,5-d′]bis[1,3]dioxole
(BDO, 3, Scheme 1) was synthesized by modified published
procedures in good yield.20 BDO 3 was intentionally prepared
as a complex mixture of stereoisomers to enhance solubility of
the resulting polymers. R and R′ may be syn or anti about the
benzene ring plane and the 2-ethylhexyl chains are derived from
a racemic starting material. Our first PPEs based on highly
symmetric or slightly lower symmetry BDOs carrying linear
side chains (e.g., BDOFPPE, scheme 2, R ) R′ ) n-hexyl; or
R ) n-hexyl, R′ ) ethyl, not shown) were soluble in common
organic solvents only at elevated temperature. Lithiation of 3
with BuLi/TMEDA and quenching with I2 produced the diodo
BDO 4, from which monomer M1 was synthesized by Pd-
catalyzed Negishi coupling.
Polymer BDOFPPE was prepared via nucleophilic aromatic
substitution,14 with the fluoride-activated silyl-acetylene groups
of monomer M1 acting as nucleophiles and hexafluorobenzene
acting as electrophile (Scheme 2). Multinuclear NMR spectra
1
(19F, H, 13C, Supporting Information) indicate high structural
purity for BDOFPPE, similar to previously reported TAB-based
PPEs. Polymers BDOPPE and BDOPEE were synthesized
using Pd-catalyzed cross- and homocoupling polymerizations,
respectively.
2. Experimental Section
Characterization. Solution photoluminescence (10-8 M THF)
and UV-vis absorption spectra (10-6 M THF) were measured on
a Fluorolog-3 fluorometer and Varian CARY 1 spectrophotometer.
Solid state spectra were recorded from thin films spin-coated from
solution (∼0.5 mg/mL toluene) onto quartz plates.
Optical Properties. The solution UV/vis absorption spectra
(Figure 1a,b) of the polymers TABFPPE, BDOPEE, BDOPPE
and BDOFPPE exhibit similar features, dominated by a strong
absorption maximum (Amax) around 400 nm, similar to many
other PPEs.1,2 Also similar to most PPEs, the separation between
Amax and the photoluminescence maximum (PLmax), or Stokes
shift, is relatively small for TABFPPE (0.15 eV). The absor-
bance spectra of the three BDO-based polymers, however,
contain a more or less pronounced shoulder (A1, Figure 1a, b)
at the low energy side of Amax. This is intrinsic to the diluted
polymers (i.e., contamination and aggregation as a possible
source for A1 were excluded), and is thus assigned to the ground
to first excited state transition (S0fS1). The PL spectra of the
BDO-based polymers are strongly red-shifted against the main
absorption band (Figure 1a). This gives large effective Stokes
shifts, varying between 0.75 and 0.95 eV (Table 1) and larger
than those reported for most conjugated polymers so far.26,27,33
The Stokes shifts for the monomers M1, M2, and M3 are even
larger at 1.45, 1.89, and 1.12 eV, respectively. While the LSS
for M1 and M2 are mainly due the large separation between
Amax and A1, (1.14 and 1.55 eV), the shift for M3 is mainly
due to the large distance between A1 and PLmax (0.72 eV), see
Table 1. The latter is attributed to the larger steric effects of
Electrochemical measurements were performed under nitrogen
atmosphere using a BAS-CV-50W voltametric analyzer and three
electrode cell (platinum working, silver wire counter and pseu-
doreference electrodes). The supporting electrolyte solution (0.1
M (n-Bu)4NPF6, anhydrous CH3CN) was thoroughly purged with
nitrogen before each measurement, and ferrocene/ferrocenium (Fc/
Fc+, E1/2 ) 0.46 V) was used as reference. Polymers were evaluated
as thin films solvent cast directly onto the working electrode while
small molecules were evaluated in dilute CH3CN solution. Detailed
procedures for monomer and polymer syntheses and their charac-
terization are included in the Supporting Information. Monomer
M3 and polymer TABFPPE (Table 1, R ) 3,7-dimethyloctyl) were
prepared as published.14
Theoretical Methodology. The geometry and electronic structure
of the molecules were calculated at the density functional theory
(DFT) level using the B3LYP functional and the 6-311G* basis
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