Enhanced photoluminescence of sulfur-bridged organic chromophores

Article abstract of DOI:10.1021/ja401383q

A general approach to enhancing the emission quantum yield of several widely studied organic chromophores is presented. The luminescence properties of a series of symmetrical sulfur-bridged chromophores are reported as a function of the oxidation state of the bridging sulfur atom. The photoluminescence quantum yield is significantly enhanced by successively oxidizing the sulfur bridge from sulfide (S), to sulfoxide (SO), to sulfone (SO₂).

Full text of DOI:10.1021/ja401383q

Communication
pubs.acs.org/JACS
Enhanced Photoluminescence of Sulfur-Bridged Organic
Chromophores
Peter R. Christensen,§ Jeffrey K. Nagle,† Aini Bhatti,§ and Michael O. Wolf*,§
§Department of Chemistry, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
†Department of Chemistry, Bowdoin College, Brunswick, Maine, 04011, United States
S
* Supporting Information
ABSTRACT: A general approach to enhancing the
emission quantum yield of several widely studied organic
chromophores is presented. The luminescence properties
of a series of symmetrical sulfur-bridged chromophores are
reported as a function of the oxidation state of the bridging
sulfur atom. The photoluminescence quantum yield is
significantly enhanced by successively oxidizing the sulfur
bridge from sulfide (S), to sulfoxide (SO), to sulfone
(SO2).
Figure 1. (A) Bar graph showing the successive increase in Φf as the
bridging sulfur is oxidized for bridged bithiophene (T2), terthiophene
(T3), naphthalene (Nap), and pyrene (Pyr) species along with the
respective parent arenes in CH2Cl2. (B) Relative absorption-corrected
photoluminescence spectra for 2SOn and T3 showing the increase in
intensity as the sulfur bridge is oxidized.
ighly emissive organic molecules continue to receive a
great deal of attention for their use in organic light
H
emitting devices (OLEDs).1 Solution processing of organic
molecules under ambient conditions may enable low cost, high
throughput manufacturing of devices utilizing these materials
on a wide range of substrates.2 For over two decades
conjugated oligomers and polymers have been targeted as
emissive materials for OLEDs.3 While luminescent polymers
are often better suited for solution processing, the performance
of small molecule OLEDs is typically superior.4 The photo-
luminescence (PL) of conjugated oligomers in solution can be
enhanced by increasing oligomer length;5 for example, the PL
quantum yield (Φf) increases from ∼0.06 for terthiophene (T3)
to 0.34 for sexithiophene (T6).6 A disadvantage of larger
aromatic molecules, however, is their generally poor solubility
and the introduction of nonradiative decay pathways in close-
packed solid state morphologies.7 Alkyl substituents are often
used to improve the solubility of these molecules8 and to
inhibit intermolecular quenching.9 A wide variety of function-
alized thiophene,10 naphthalene,11 and pyrene12 derivatives
have been synthesized both to improve solubility and to
minimize nonradiative decay related to packing of molecules;
however, it is challenging to optimize these parameters without
also hindering intermolecular charge transfer, essential for
efficient OLED operation.13
different photophysical behavior.14,15 In some cases,14a,b the
sulfoxides are less emissive than both the corresponding
sulfones and parent arenes, attributed to nonradiative pathways
including α-cleavage/recombination and/or pyramidal inver-
sion at sulfur. In other cases,14c,15 the sulfones are less emissive.
Enhanced photoluminescence of symmetrically bridged bis-aryl
sulfoxides (Ar−SO−Ar) is unprecedented and differs signifi-
cantly from the behavior of asymmetric aryl sulfoxides (Ar−
SO−R) where R is an alkyl14 or aryl group.15 Here, the
enhanced quantum efficiency with sulfur-bridged chromo-
phores is achieved without diminishing the solubility of these
molecules in common organic solvents, and the lack of bulky
substituents is anticipated to allow for efficient charge transfer
in OLED applications. Furthermore, sulfone-bridged arenes are
also well-known for their high mechanical and thermal
stability16 which is also important for robust OLED
operation.13
Here, we present a new approach for enhancing the solution
and solid-state light emitting properties of a series of organic
chromophores, where each chromophore is bridged symmetri-
cally about a sulfur atom. Successive oxidation of the bridging
sulfur from sulfide (S), to sulfoxide (SO), to sulfone (SO2)
results in a systematic increase in Φf. In most cases the
sulfoxide (SO), and in all examples the sulfone (SO2) bridged
chromophores exhibit photoluminescence quantum yields
greater than the parent arenes (Figure 1). Previously studied
systems, notably, asymmetric sulfoxides and sulfones exhibit
Bithiophene, terthiophene, naphthalene, and pyrene were
bridged symmetrically about a sulfur atom in good yields
Received: February 6, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja401383q | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(synthetic details in Supporting Information). Photophysical
properties of the sulfur-bridged chromophores were examined
as a function of the oxidation state of the bridging sulfur atom
(Table 1). The PL quantum yields (Φf) were calculated using
349 nm) and sexithiophene (λmax = 432 nm).6,19 The extinction
coefficients (ε) for the sulfur-bridged compounds are roughly
twice those of the parent arenes, except for the pyrene
compounds (4SOn) where ε is nearly five times as large as that
of pyrene.
Excited state calculations for 2S, 2SO, and 2SO2 show small
geometry differences from calculated ground state geometries
(see Supporting Information), with increased planarization of
the thiophene rings and decreases in the interannular C−C
bonds seen in all cases. Thiophene oligomers containing S-
oxides and S,S-dioxide rings have been previously found to
maintain planar, or quasiplanar, conformations yielding red-
shifted electronic absorption spectra and enhanced solid-state
emission.20 The C−S−C angle of the bridging sulfur atom
decreases by 0.6° in 2S and by 0.01° in 2SO in the excited
state; however in 2SO2 this angle increases by 1.8° in the
excited state. Thus, as the oxidation state of the bridging sulfur
is increased, the geometry in the excited state prefers a slightly
more linear configuration.
In the sulfoxide and sulfone derivatives, the presence of the
electron deficient bridging sulfur group21 results in the
possibility of intramolecular charge transfer (ICT) playing a
role in the photophysical behavior of these molecules. Fluorene
substituted dibenzothiophene-S,S-dioxides have been previously
shown to have localized excited (LE) states that dominate in
nonpolar solvents, while ICT states are activated in more polar
solvents due to stabilization of the induced excited state dipole
moment.22 To assess the relative ICT character between
sulfide, sulfoxide, and sulfone bridging units, the absorption and
emission energies for 2SOn were plotted (Figure S3) against
the Lippert−Mataga solvent polarizability Δf.23 For compounds
2SO and 2SO2 a linear increase in the Stokes shift was
observed with increasing polarity, while the Stokes shift in
compound 2S shows no trend. In low to moderately polar
solvents (cyclohexane, CHx → dichloromethane, DCM) little
or no broadening in the emission profile is observed; however,
in more polar solvents (DCM → acetonitrile, ACN) a sharp,
nearly linear increase in the full width at half-maximum (fwhm)
is observed for 2S, 2SO, and 2SO2 (Figure S5). The increasing
fwhm and Stokes shift in response to polar media strongly
supports22b,24 the conclusion that ICT is occurring in 2SO and
2SO2 and furthermore that ICT is strongest with the SO2
bridge.
Table 1. Photophysical Data for Compounds in CH2Cl2
Solutions (c ≈ 10−6 M)
absorption
λmax (nm) ε (M−1 cm−1
emission
Φf (298 K)
compound
)
λf (nm)
bithiophene
1S
305
334
325
349
349
374
371
398
277
302
296
298
337
362
351
353
1.3 × 104
3.4 × 104
2.9 × 104
3.2 × 104
2.5 × 104
5.8 × 104
5.3 × 104
4.3 × 104
7.7 × 103
1.3 × 104
1.3 × 104
1.0 × 104
9.7 × 103
4.9 × 104
4.6 × 104
4.3 × 104
365
383
447
430
432
473
479
495
355
368
368
368
372
401
449
432
0.001
0.002 0.001
0.003 0.001
0.45 0.06
0.06
1SO
1SO2
terthiophene
2S
0.012 0.001
0.15 0.02
0.52 0.09
0.02
2SO
2SO2
napthalene
3S
0.012 0.003
0.035 0.001
0.27 0.01
0.38
3SO
3SO2
pyrene
4S
0.011 0.002
0.17 0.05
0.74 0.06
4SO
4SO2
9-cyanoanthracene as a standard. The emission from all
chromophores is short-lived (on the order of nanoseconds)
and attributed to decay from singlet states. The emission
spectra are identical in intensity and appearance when collected
under air or when the solutions are purged with argon. As the
bridging sulfur is oxidized from sulfide (S), to sulfoxide (SO),
to sulfone (SO2), Φf increases with the level of oxidation in all
cases (Figure 1). An enhancement as high as 0.01 to 0.74 is
observed for the series of sulfur-bridged pyrene compounds.
The electronic absorption spectra for all the sulfur-bridged
compounds are red-shifted relative to the spectra of the parent
arenes. The DFT calculated highest occupied molecular orbitals
(HOMOs) and lowest unoccupied molecular orbitals
(LUMOs) for 1SOn (Figure S1) and 2SOn (Figure 2) are
The photoluminescence quantum yields were determined in
solvent mixtures with Δf between 0.0 and 0.3 (Figure 3). In the
least polar solvent (CHx), 2S, 2SO, and 2SO2 all have Φf ≈ 0.1,
the same as that measured for terthiophene (T3) in CHx. For
terthiophene, the low Φf has been attributed to intersystem
crossing (ISC) promoted by the presence of sulfur.5 In
nonpolar solvents it is reasonable that for 2S, 2SO, and 2SO2 a
more localized excited (LE) state on the terthiophene moiety
exists and is deactivated via ISC similarly to terthiophene.
Compound 2S shows a linear decrease in Φf as the solvent
polarity is increased, while Φf values for 2SO and 2SO2 increase
with solvent polarity reaching a maximum in DCM. Excited
state DFT calculations show that a small degree of charge
transfer occurs upon excitation, with the bridging sulfur
becoming more positive in 2S, and less positive in 2SO and
2SO2. Our data show that with increased contribution from
ICT 2SO and 2SO2 are more emissive than in nonpolar
solvents where the LE state is dominant. This is further
evidenced by comparing the solvent dependence of the series
2S, 2SO, and 2SO2 to T3, where only a small variation in Φf is
Figure 2. Frontier molecular orbitals (FMOs) for 2S, 2SO, and 2SO2.
Geometry optimized structures calculated in C2 symmetry for 2S and
2SO2 and Cs for 2SO.
delocalized over both aryl groups. The absorption maxima of
1S, 1SO, and 1SO2 are consistent with two weakly coupled
bithiophene units (λmax = 305 nm) rather than a longer
thiophene oligomer (i.e., tetrathiophene, λmax = 377 nm)5,6,17,18
which would be expected if the sulfur bridge allowed a strong
interaction between the thiophene units. The same comparison
can be drawn between bridged terthiophene compounds
(2SOn), where λmax is between that of terthiophene (λmax
=
B
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Journal of the American Chemical Society
Communication
Figure 4. Thin film solid-state emission using glass substrates for 1S,
1SO, and 1SO2 compared to bithiophene (A) and 2S, 2SO, 2SO2
compared to terthiophene (B) obtained using an integrating sphere.
Relative emission intensities for 1S, 1SO, and 1SO2 (C) and 2S, 2SO,
and 2SO2 (D) on alumina. Also shown are the parent arenes T2 and
T3 on alumina as well as the background spectra (BKG).
Figure 3. Photoluminescence quantum yield (Φf) for 2S (A), 2SO
(B), 2SO2 (C), and T3 (D) as a function of the solvent polarizability
(Δf).
In summary, we have identified a general method to improve
the photoluminescence quantum yield for a series of sym-
metrical sulfur-bridged chromophores, by oxidation of the
bridging sulfur. Solvatochromism and band broadening of the
emission spectra in high polarity solvents indicate charge
transfer is more prevalent in the sulfoxide (SO) and sulfone
(SO2) bridged chromophores. It is proposed that CT states
resulting from oxidized sulfur bridges allow for enhanced
emission. The enhanced photoluminescence combined with the
processability of these soluble compounds is anticipated to be
useful in the fabrication of OLED devices.
observed (Figure 3D). It is also possible that a decrease in ISC
due to reduction in the number of lone pairs on the bridging
sulfur with oxidation, i.e. (π,π*) → (n,π*), plays a role in
enhancing Φf.
1
3
In more polar solvents (Δf > 0.22) a decrease in Φf is
observed for both 2SO and 2SO2. Previous studies have shown
that two different nonradiative pathways, one more prominent
in polar solvents, (related to a twisted intramolecular charge
transfer (TICT) state), and the other more prominent in
nonpolar solvents (attributed to a LE state), were observed in
aminostyryl donor−acceptor dyes.24 Evidence for a TICT state
has also been shown in SO2-bridged aryl molecules.25
Interestingly, the Φf values for 1SO2 (0.45) and 2SO2 (0.52)
exceed those of tetrathiophene, T4 (0.18−0.20), and
sexithophene, T6 (0.34−0.40).5,6,19 The increase in Φf between
T4 and T6 has been attributed to a decrease in kISC with
increasing oligomer length. In the sulfone and sulfoxide bridged
oligomers, the influence of ICT is greater than the effect of ISC
on Φf with increasing oligomer length.5
Solid-state photoluminescence measurements were per-
formed on the bithiophene (1SOn) and terthiophene (2SOn)
series (Figure 4A and B). Thin films were prepared by
dropcasting suspensions (∼1 mg/mL in hexanes) onto
precleaned glass substrates, and quantum yields were
determined using an integrating sphere (Table S7). The
relative solid-state emission intensity was observed to be 1SO <
T2 < 1S < 1SO2 in the bithiophene series and 2S < 2SO2 <
1SO < T3 in the terthiophene series. While the trend in
emission is different than that in solution, a large enhancement
is observed for 1SO2 compared to the parent arene T2. Further
solid-state photoluminescence data were collected using a
modified literature procedure.26 Alumina thin-layer chromatog-
raphy (TLC) plates were cut into 1 cm2 pieces and developed
slowly in 50 μL aliquots of the analyte (∼1 mg/mL, CHCl3) to
achieve uniform coverage. The plates were dried under ambient
conditions and were subsequently subjected to photolumines-
cence measurements. On alumina substrates both 1SO2 and
2SO2 are significantly more emissive than their parent arenes
(Figure 4C and D) under these conditions.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and characterization data (NMR, IR, and
mass spectrometry), absorption and emission spectra, and
details and summary of DFT calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
mwolf@chem.ubc.ca
■
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
■
We acknowledge NSERC for financial support and the
Laboratory for Advanced Spectroscopy and Imaging Research
(LASIR) for facilities access. We would like to thank M. Ezhova
and H. Huang-Hobbs for their assistance and helpful
discussions concerning NMR.
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