Tuning the intramolecular charge transfer emission from deep blue to green in ambipolar systems based on dibenzothiophene S, S -dioxide by manipulation of conjugation and strength of the electron donor units

Article abstract of DOI:10.1021/jo100898a

The efficient synthesis and photophysical properties of a series of ambipolar donor-acceptor-donor systems is described where the acceptor is dibenzothiophene S,S-dioxide and the donor is fluorene, carbazole, or arylamine. The systems exhibit intramolecular charge transfer (ICT) states (of variable ICT character strengths) leading to fluorescence emission ranging from deep blue to green with moderate to high photoluminescence quantum yields. The emission properties can be effectively tuned by systematically changing the position of substitution on both donor and acceptor units (which affects the extent of conjugation) and the redox potentials of the donor units. The results are supported by cyclic voltammetric data and TD-DFT calculations.

Full text of DOI:10.1021/jo100898a

pubs.acs.org/joc
Tuning the Intramolecular Charge Transfer Emission from Deep Blue to
Green in Ambipolar Systems Based on Dibenzothiophene S,S-Dioxide by
Manipulation of Conjugation and Strength of the Electron Donor Units
Kathryn C. Moss,† Konstantinos N. Bourdakos,‡ Vandana Bhalla,†,§ Kiran T. Kamtekar,^
Martin R. Bryce,*,† Mark A. Fox,† Helen L. Vaughan,‡ Fernando B. Dias,‡ and
Andrew P. Monkman‡
†Department of Chemistry, Durham University, Durham, DH1 3LE, U.K., and
‡Department of Physics, Durham University, Durham DH1 3LE, U.K., §Department of Chemistry,
Guru Nanak Dev University, Amritsar-143005, India, and ^Zumtobel LED Division,
Green Lane Industrial Estate, Spennymoor DL16 6HL, U.K.
m.r.bryce@durham.ac.uk
Received May 7, 2010
The efficient synthesis and photophysical properties of a series of ambipolar donor-acceptor-donor
systems is described where the acceptor is dibenzothiophene S,S-dioxide and the donor is fluorene,
carbazole, or arylamine. The systems exhibit intramolecular charge transfer (ICT) states (of variable
ICT character strengths) leading to fluorescence emission ranging from deep blue to green with
moderate to high photoluminescence quantum yields. The emission properties can be effectively
tuned by systematically changing the position of substitution on both donor and acceptor units
(which affects the extent of conjugation) and the redox potentials of the donor units. The results are
supported by cyclic voltammetric data and TD-DFT calculations.
Introduction
(OPVs),3 organic field-effect transistors (OFETs),4 electrochro-
mism,5 and sensors.6 Studies on π-conjugated oligomers pro-
vide valuable insights into relevant fundamental properties.7
The covalent combination of electron donor and electron
acceptor units has provided a wealth of information about
energy and charge transfer processes.8 For example, the
luminescence of 9,9-dialkylfluorene copolymers9 can be tuned
over a large wavelength range by utilizing intramolecular
The development of conjugated organic materials for optoe-
lectronic devices remains a key objective across many disciplines
of science.1 These materials have applications in organic/poly-
meric light-emitting diodes (O/PLEDs),2 organic photovoltaics
(1) (a) Forrest, S. R.; Thompson, M. E. Chem. Rev. 2007, 107, 923–925.
(Special Issue on Organic Electronics). (b) Handbook of Conducting Poly-
mers, 3rd ed.; Skotheim, T. A., Reynolds, J. R., Eds.; CRC Press: New York,
2007.
(6) Thomas, S. W., III; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107,
(2) (a) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.;
Holmes, A. B. Chem. Rev. 2009, 109, 897–1091. (b) Organic Electrolumines-
cence; Kakafi, Z. H., Ed.; CRC Press: New York, 2005.
1339–1386.
€
(7) (a) Electronic Materials: The Oligomer Approach; Mullen, K.,
Wegner, G., Eds.; Wiley-VCH: Weinheim, Germany, 1998. (b) Martin,
ꢀ
(3) (a) Martı
´
n, N.; Sanchez, L.; Herranz, M. A.; Illescas, B.; Guldi, D.
R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350–1377. (c) Segura,
J. L.; Martın, N. J. Mater. Chem. 2000, 10, 2403–2435.
´
Acc. Chem. Res. 2007, 40, 1015–1024. (b) Hermans, P.; Cheyns, D.; Rand,
B. P. Acc. Chem. Res. 2009, 42, 1740–1747. (c) Cheng, Y.-J.; Yang, S.-H.;
Hsu, C.-S. Chem. Rev. 2009, 109, 5868–5923.
ꢀ
(8) Laquai, F.; Park, Y.-S.; Kim, J.-J.; Bashe, T. Macromol. Rapid
Commun. 2009, 30, 1203–1231.
(4) Katz, H. E.; Huang, J. Annu. Rev. Mater. Res. 2009, 39, 71–92.
(5) (a) Beaujuge, P. M.; Reynolds, J. R. Chem. Rev. 2010, 110, 268–320.
(b) Sonmez, G.; Shen, C. K. F.; Rubin, Y.; Wudl, F. Angew. Chem., Int. Ed.
2004, 43, 1498–1502.
(9) (a) Wallace, J. U.; Chen, S. H. In Polyfluorenes; Springer-Verlag:
Berlin, 2008; Vol. 212, pp 145-186. (b) U. Scherf, U.; List, E. J. W. Adv.
Mater. 2002, 7, 477–487. (c) Leclerc, M. J. Polym. Sci., Part A: Polym. Chem.
2001, 39, 2867–2873.
DOI: 10.1021/jo100898a
r
Published on Web 09/22/2010
J. Org. Chem. 2010, 75, 6771–6781 6771
2010 American Chemical Society

Moss et al.
JOCArticle
CHART 1
charge and energy transfer processes both in solution and in
the solid state.10 Oligo-/polycarbazoles are also a benchmark
family of materials which are widely used for O/PLEDs,
OFETs, and OPVs.11 They are predominantly hole transporters
(p type) with reversible redox properties, strong luminescence,
and a larger band gap than for polyfluorenes. Similarly, aryla-
mines are well documented, mainly due to their high hole-
transporting ability.12 Many optoelectronic applications require
ambipolar compounds: i.e., those able to accept, transport, or
store both holes and electrons.13-15 Intramolecular charge
transfer (ICT) processes16 often play an important role in the
photophysics of ambipolar systems. For example, Kulkarni et al.
demonstrated an increase in ICT emission upon increasing
the strength of the donor unit, comparing N-methylcarbazole
with the stronger donor N-methylphenothiazine in acceptor-
donor-acceptor trimers (acceptor = phenylquinoline).17 Estrada
and Neckers studied fluoren-9-ylidene malononitrile derivatives
as strong acceptors, linked through the 2,7- or 3,6-positions
via alkyne bridges to carbazole donors to provide donor-
acceptor-donor triads.18 Effective electronic communication
was observed through the 3,6-positions to the fluoren-9-ylidene
malononitrile core. However, in contrast with the donor-
acceptor-donor systems described in the present article, the
ICT state in these fluoren-9-ylidene malononitrile derivatives
was a nonradiative deactivation pathway (i.e., nonemissive),
with one of the 3,6-disubstituted oligomers showing total emis-
sion quenching in polar solvents.18
Our group19 and other workers20 have incorporated di-
benzothiophene S,S-dioxide (S) as an electron-deficient
comonomer unit in oligo-/polyfluorene (F) backbones, e.g.
trimer 119a and polymer 219d (Chart 1), which show high
luminescence efficiency. DFT calculations and cyclic vol-
tammetric data established that the lower energy LUMO
levels of the sulfone unit enhance the electron-accepting
properties of 1 and 2.
In solution, F-S co-oligomers and copolymers of type 1
and 2 show a degree of intramolecular charge transfer (ICT)
character and excited state relaxation depending on the nature
of the local environment,: e.g. the solvent polarity.19b,c In
nonpolar solvents, such as hexane or toluene, a well-structured
emission is observed, in agreement with minimal conforma-
tion and solvent relaxations following excitation, whereas in
polar solvents, such as acetonitrile, ethanol, and chloroform,
broad, featureless (and red-shifted) ICT emission is dominant.
The stability of the ICT state in F-S co-oligomers and
copolymers is reduced by more twisted backbone conforma-
tions enforced by substituent effects on the S units.21 Ambi-
polar molecules comprising disubstituted S units as the
acceptor moiety and diphenylamine/triphenylamine donor
moieties have also been studied.22
ꢀ
(10) (a) Beaupre, S.; Leclerc, M. Adv. Funct. Mater. 2002, 12, 192–196. (b)
Yang, R.; Tian, R.; Yan, J.; Zhang, Y.; Yang, J.; Hou, Q.; Yang, W.; Zhang,
C.; Cao, Y. Macromolecules 2005, 38, 244–253. (c) Wu, W.-C.; Liu, C.-L.;
Chen, W.-C. Polymer 2006, 47, 527–538. (d) Zhu, Y.; Gibbons, K. M.;
Kulkarni, A. P.; Jenekhe, S. A. Macromolecules 2007, 40, 804–813. (e)
Kulkani, A. P.; Zhu, Y.; Jenekhe, S. A. Macromolecules 2008, 41, 339–345.
(f) Wu, P.-T.; Kim, F. S.; Champion, R. D.; Jenekhe, S. A. Macromolecules
2008, 41, 7021–7028.
ꢀ
(11) (a) Beaupre, S.; Boudreault, P.-L. T.; Leclerc, M. Adv. Mater. 2010,
22, E6–E27. (b) Zhao, Z.; Xu, X.; Wang, H.; Lu, P.; Yu, G.; Liu, Y. J. Org.
Chem. 2008, 73, 594–602. (c) Blouin, N.; Michaud, A.; Gendron, D.; Wakim,
^
S.; Blair, E.; Neagu-Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc,
M. J. Am. Chem. Soc. 2008, 130, 732–742. (d) Wakim, S.; Aich, B. R.; Tao,
Y.; Leclerc, M. Polym. Rev. 2008, 48, 432–462. (e) Adhikari, R. M.; Mondal,
R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72, 4727–4732. (f) Blouin,
N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295–2300. (g)
Grigalevicius, S.; Ma, L.; Xie, Z.; Scherf, U. J. Polym. Sci., Part A: Polym.
Chem. 2006, 44, 5987–5994. (h) Sonntag, M.; Kreger, K.; Hanft, D.;
Strohriegl, P.; Setayesh, S.; de Leeuw, D. Chem. Mater. 2005, 17, 3031–
3039. (i) Morin, J.-F.; Drolet, N.; Tao, Y.; Leclerc, M. Chem. Mater. 2004,
16, 4619–4626. (j) Thomas, K. R. J.; Liu, J. T.; Tao, Y.-T.; Chuen, C.-H.
J. Mater. Chem. 2002, 12, 3516–3522.
The present work describes new ambipolar oligomers con-
taining S as the core unit. The objective is to tune the emission
properties by combining the strategies of conjugation control
(by systematically changing the substitution pattern between
donor and acceptor) and varying the redox potential of the
donor unit: namely, carbazole (Cz), di-/triphenylamine (DPA,
TPA), and fluorene (F).
(12) (a) Shirota, Y. J. Mater. Chem. 2000, 10, 1–25. (b) Jungermann, S.;
Riegel, N.; Muller, D.; Meerholz, K.; Nuyken, O. Macromolecules 2006, 39,
8911–8919. (c) Kondakov, D. Y. J. Appl. Phys. 2008, 104, 084520.
(13) (a) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953–1010.
(b) Zaumseil, J.; Sirringhaus, H. Chem. Rev. 2007, 107, 1296–1323.
(14) Reviews of electron-transporting materials: (a) Hughes, G.; Bryce,
M. R. J. Mater. Chem. 2005, 15, 94–107. (b) Kulkarni, A. P.; Tonzola, C. J.;
Babel, A.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4556–4573.
(19) (a) Perepichka, I. I.; Perepichka, I. F.; Bryce, M. R.; Palsson, L. O.
Chem. Commun. 2005, 3397–3399. (b) Dias, F. B.; Pollock, S.; Hedley, G.;
Palsson, L.-O.; Monkman, A.; Perepichka, I. I.; Perepichka, I. F.; Tavasli,
M.; Bryce, M. R. J. Phys. Chem. B 2006, 110, 19329–19339. (c) Dias, F. B.;
King, S.; Monkman, A. P.; Perepichka, I. I.; Kryuchkov, M. A.; Perepichka,
I. F.; Bryce, M. R. J. Phys. Chem. B 2008, 112, 6557–6566. (d) King, S. M.;
Perepichka, I. I.; Perepichka, I. F.; Dias, F. B.; Bryce, M. R.; Monkman,
A. P. Adv. Funct. Mater. 2009, 19, 586–591. (e) Dias, F. B.; Kamtekar, K. T.;
Cazati, T.; Williams, G.; Bryce, M. R.; Monkman, A. P. ChemPhysChem.
2009, 10, 2096–2104.
(15) (a) Geramita, K.; McBee, J.; Tilley, T. D. J. Org. Chem. 2009, 74,
820–829. (b) Geramita, K.; Tao, Y.; Segalman, R. A.; Tilley, T. D. J. Org.
Chem. 2010, 75, 1871–1887.
(16) (a) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev. 2003,
ꢀ
(20) (a) Yang, F.; Sun, K.; Cao, Z. J.; Li, Z. H.; Wong, M. S. Synth. Met.
2008, 158, 391–395. (b) Liu, H.; Zou, J. H.; Yang, W.; Wu, H. B.; Li, C.;
Zhang, B.; Peng, J. B.; Cao, Y. Chem. Mater. 2008, 20, 4499–4506. (c) Li,
Y. Y.; Wu, H. B.; Zou, J. H.; Ying, L.; Yang, W.; Cao, Y. Org. Electron. 2009,
10, 901–909.
(21) (a) Liu, J.; Hu, S.; Zhao, W.; Zou, Q.; Luo, W.; Yang, W.; Peng, J.;
Cao, Y. Macromol. Rapid Commun. 2010, 31, 496–501. (b) Kamtekar, K. T.;
Vaughan, H. L.; Lyons, B. P.; Monkman, A. P.; Pandya, S. U.; Bryce, M. R.
Macromolecules 2010, 43, 4481–4488. (c) Li, H.; Batsanov, A. S.; Moss,
K. C.; Vaughan, H. L.; Dias, F. B.; Kamtekar, K. T.; Bryce, M. R.;
Monkman, A. P. Chem. Commun. 2010, 46, 4812–4814.
103, 3899–4031. (b) Gomez, I.; Reguero, M.; Boggio-Pasqua, M.; Robb,
M. A. J. Am. Chem. Soc. 2005, 127, 7119–7129. (c) Mallick, A.; Maiti, S.;
Haldar, B.; Purkayastha, P.; Chattopadhyay, N. Chem. Phys. Lett. 2003,
371, 688–693. (d) Paul, B. K.; Samanta, A.; Kar, S.; Guchhait, N. J. Lumin.
2010, 130, 1258–1267. (e) Mallick, A.; Purkayastha, P.; Chattopadhyay, N.
J. Photochem. Photobiol. C 2007, 8, 109–127. (f) Mahanta, S.; Singh, R. B.;
Kar, S.; Guchhait, N. J. Photochem. Photobiol. A 2008, 194, 318–326. (g)
Chakraborty, A.; Kar, S.; Nath, D. N.; Guchhait, N. J. Phys. Chem. A 2006,
110, 12089–12095.
(17) Kulkarni, A. P.; Kong, X. X.; Jenekhe, S. A. Adv. Funct. Mater.
2006, 16, 1057–1066.
(18) Estrada, L. A.; Neckers, D. C. J. Org. Chem. 2009, 74, 8484–8487.
(22) Huang, T. H.; Lin, J. T.; Chen, L. Y.; Lin, Y. T.; Wu, C. C. Adv.
Mater. 2006, 18, 602–606.
6772 J. Org. Chem. Vol. 75, No. 20, 2010

Moss et al.
JOCArticle
SCHEME 1. Synthesis of 8, 9, and 11a
aReagents and conditions: (i) 2-ethylhexyl bromide, KOtBu, DMF, 130 ꢀC, 55%; (ii) nBuLi, (iPrO)3B, concentrated HCl, THF, -78 to 20 ꢀC, 51%;
(iii) Pd(PPh3)2Cl2, K2CO3 (aq), 1,4-dioxane, 110 ꢀC, 78% for 8, 50% for 9, 78% for 11.
SCHEME 2. Synthesis of 15a
aReagents and conditions: (i) 2-ethylhexyl bromide, KOtBu, DMF, 130 ꢀC, 80%; (ii) nBuLi, (iPrO)3B, concentrated HCl, THF, -78 to 20 ꢀC, 67%;
(iii) 6, Pd(PPh3)2Cl2, K2CO3 (aq), 1,4-dioxane, 110 ꢀC, 78%.
Results and Discussion
products were obtained in low yields. The diarylamine analogue
23 was similarly synthesized from 6. The triarylamine analogue
26 was synthesized by a different route. The diboronic ester
derivative 24 underwent Suzuki cross-coupling (Scheme 5) with
25 to give 26. Both 23 and 26 are bright yellow solids with good
solubility imparted by the n-butyl substituents.
Cyclic voltammetric data on all the Cz-S-Cz, DPA-S-
DPA, TPA-S-TPA, and F-S-F trimers synthesized here
are given in Table 1. The carbazolyl derivatives show irre-
versible oxidation waves at ca. 0.8-1.0 V typically found for
carbazoles.23 These potentials are lower than those of the
reversible oxidation waves observed at ca. 1.2-1.4 V for the
fluorenyl analogues. Even lower oxidation potentials at ca.
0.4 V were obtained for the DPA (23) and TPA compounds
(26), demonstrating a decrease in oxidation potential with an
A series of Cz-S-Cz trimers, as well as their analogous
F-S-F trimers, were synthesized via palladium-catalyzed
Suzuki-Miyaura coupling reactions (Schemes 1-3). The
3,7-dibromo- (linear) and 2,8-dibromo-substituted (bent) S
units 6,19a 7,21c and 1020b were synthesized according to pre-
viously reported procedures. The carbazole boronic acid deri-
vatives 5 and 14 were synthesized in good yields from 4 and 13,
respectively, by a sequence of lithiation, borylation, and hydro-
lysis. The 2-ethylhexyl substituent was chosen to enhance solu-
bility. For the cross-couplings dichlorobis(triphenylphosphine)-
palladium(II) or tetrakis(triphenylphosphine)palladium(0) was
used as a catalyst and trimers 8, 9, 11, 15, 17, and 18 were ob-
tained in moderate to good yields as either white or yellow solids.
N-substituted carbazole trimers 20 and 21 were synthesized via
palladium-catalyzed C-N cross-coupling reactions of carbazole
19 with 6and 10, respectively (Scheme 4). The very low solubility
of 20 and 21 made reaction and workup difficult, and the
€
(23) Brunner, K.; van Dijken, A.; Borner, H.; Bastiaansen, J. J. A. M.;
Kiggen, N. M. M.; Langeveld, B. M. W. J. Am. Chem. Soc. 2004, 126, 6035–
6042.
J. Org. Chem. Vol. 75, No. 20, 2010 6773

Moss et al.
JOCArticle
SCHEME 3. Synthesis of 1, 17, and 18a
aReagents and conditions: (i) 6, Pd(PPh3)2Cl2, K2CO3 (aq), 1,4-dioxane, 110 ꢀC, 86% for 1; (ii) 7, Pd(PPh3)4, Na2CO3 (aq), toluene, 110 ꢀC, 50% for 17;
(iii) 10, Pd(PPh3)2Cl2, K2CO3 (aq), 1,4-dioxane, 110 ꢀC, 21%.
SCHEME 4. Synthesis of 20 and 21a
substituents: e.g., λPL (CHCl3) 1 at 427 nm, 2.90 eV, 8 at
458 nm, 2.71 eV, and 26 at 520 nm, 2.38 eV (Figure 1). This
trend corresponds with the HOMO-LUMO gaps of 3.37 eV
for 1, 3.06 eV for 8, and 2.65 eV for 26 determined by cyclic
voltammetric studies (Table 1). Thus, the fluorene-contain-
ing trimers exhibit deep blue fluorescence which becomes sky
blue for the carbazole analogues and green for the arylamine
analogues (see Figure SI2-8 in the Supporting Information
for a photograph showing the emission colors of compounds
1, 8, and 23). The TPA-based trimer 26 emits at the longest
wavelength of all the compounds in this study (in both
toluene and chloroform), consistent with the stronger donor
enhancing the red shift.
All the oligomers show positive solvatochromism in their
photoluminescence (PL) spectra due to ICT behavior, which
enables the emission color to be tuned by varying the solvent
polarity. Table 2 shows the absorption and emission data in both
toluene and chloroform for all compounds. Compounds 20
and 21 are distinct in that they are bonded directly through the
carbazole N atom rather than via one of the ring carbons and are
considerably less soluble than the other trimers discussed here.
Table 3 gives the Stokes and solvatochromic shift energies.
There appear to be two distinct groups in this study based on
the ICT character of these trimers. Trimers with relatively
strong ICT may be defined by
(i) a lack of fine structure emission in toluene (only one
band value given in Table 2)
(ii) a large positive solvatochromic shift in the emission
band on changing from toluene to chloroform
(>1000 cm-1 in Table 3)
t
aReagents and conditions: (i) 6, Pd2(dba)3, xphos, NaOtBu, BuOH,
toluene, 110 ꢀC, 24%; (ii) 10, Pd2(dba)3, tBu3PHBF4, tBuOH, toluene,
110 ꢀC, 20%.
increase in donor strength (fluorene f carbazole f arylamine).
While trimer 26 showed a reversible two-electron oxidation
wave, indicating that oxidizing one amine group has little
influence on the oxidation of the second amine group, trimer
23 showed two well-separated one-electron reversible waves
(ΔE = 300 mV), implying that oxidation at one amine group
strongly influences the oxidation of the second amine group.
These waves suggest that substantial conjugation exists across
the linear S unit. The reduction potentials observed for the
reversible waves of these trimers are within the range of ca. -1.9
to -2.4 V and arise from the one-electron reduction of the S
unit in all cases. The difluorinated trimers 9 and 17 are more
easily reduced by ca. 0.1-0.3 V in comparison to their parent
analogues 8 and 1, respectively. These data illustrate the
ambipolar nature of these compounds.
(iii) a significantly positive solvatochromatic shift in the
low-energy absorption maximum from toluene to
chloroform (>400 cm-1 in Table 3)
(iv) larger Stokes shifts (5000-6000 cm-1) in both to-
luene and chloroform
The photophysical data for the trimers are summarized in
Table 2. There is a progressive red shift in the absorption and
emission in the series fluorene f carbazole f arylamine, in
line with the increasing electron-donating ability of these
Trimers with relatively weak ICT character may, therefore,
be defined by
(i) the presence of fine structure emission in toluene (two
band values given in Table 2)
6774 J. Org. Chem. Vol. 75, No. 20, 2010

Moss et al.
JOCArticle
SCHEME 5. Synthesis of 23 and 26a
aReagents and conditions: (i) B2(pin)2, Pd(dppf)2Cl2, KOAc, DMF, 90 ꢀC, 26%; (ii) Pd2(dba)3, tBu3PHBF4, NaOtBu, toluene, 107 ꢀC, 44%;
(iii) Pd(PPh3)2Cl2, K2CO3 (aq), toluene, 110 ꢀC, 33%.
TABLE 1. Electrochemical Data and Computed Frontier Orbital Energies
HLG
(CV) (eV)f
HOMO
(calcd) (eV)g
LUMO
(calcd) (eV)g
HLG
(calcd) (eV)h
compd
donor
acceptor
E
ox (V)a,b
Ered (V)a
1
8
9
F
linear
linear
linear, F2
bent
linear
linear, F2
bent
linear
bent
linear
linear
1.20
-2.17
-2.23
-2.14
-2.22
-2.13
-1.91
-2.26
-2.18
-2.19
-2.39
-2.20
3.37
3.06
3.18
2.97
2.94
3.30
3.50
3.14
3.13
2.79
2.65
1.06
0.73
0.84
0.93
0.98
1.18
1.28
1.00
1.16
0.24
0.31
-2.57
-2.86
-2.61
-2.90
-2.63
-2.65
-2.65
-2.42
-2.41
-3.13
-2.78
3.63
3.59
3.45
3.83
3.61
3.52
3.93
3.42
3.57
3.37
3.09
3-Cz
3-Cz
3-Cz
2-Cz
F
0.83c
1.04c
0.75c
0.81c,d
1.39
11
15
17
18
20
21
23
26
F
1.24
N-Cz
N-Cz
DPA
TPA
0.96c,d
0.94c,d
0.40e
0.45
aE1/2 values reported with reference to the ferrocene/ferrocenium couple at 0.00 V using tetrabutylammonium hexafluorophosphate (0.2 M) in DCM
as electrolyte solution. bPoorly defined oxidation wave arising from two simultaneous one-electron-oxidation processes in most cases. cIrreversible wave;
E1/2 values estimated from deconvolution data. dIrreversible reduction wave at ca. -2.0 V observed after oxidation. eWell-defined one-electron reversible
wave with a second reversible wave at 0.70 V. fHOMO-LUMO gap derived from the difference between oxidation and reduction potentials. gCalculated
HOMO/LUMO values converted to the ferrocene/ferrocenium couple: E = -4.8 V - E(HOMO/LUMO) V. hCalculated HOMO-LUMO gap.
(ii) a small positive solvatochromic shift in the emission
band on changing from toluene to chloroform (<1000
cm-1 in Table 3)
For compound 11 spectral shifts due to solvent polarity
are seen in both the absorption and emission spectra, with a
profound effect seen in the emission. The emission spectra
are red-shifted and broadened as the solvent polarity in-
creases (from MCH to methanol), typical of ICT excited
states.24,25 The vibronic structure of the emission spectrum
can be seen in MCH but disappears as the solvent polarity
increases, again indicative of an excited ICT state.26
(iii) little solvatochromatic shift in the low-energy absorp-
tion maximum from toluene to chloroform (<400
cm-1 in Table 3)
(iv) smaller Stokes shifts (3000-4000 cm-1) in both solvents
Compounds with strong CT character are the 2,8-disubsti-
tuted bent trimers 11, 18, and 21, whereas those with weaker CT
character are the 3,7-disubstituted linear trimers 1, 8, 9, 15, 17,
and 20. The linear compounds containing DPA and TPA units
(23 and 26, respectively) are between. The CT strength may be
measured by comparing the emission peak maxima in different
solvents: compound 11 represents strong ICT character, wher-
eas the previously reported compound 1 is representative of
weak ICT character (for details of the solvatochromism of 1 see
ref 19b). Solvatochromic effects were studied in detail for com-
pound 11, and Figure 2 shows the emission spectra in eight
solvents of different polarities (see Figure SI2-19 for the corre-
sponding absorption spectra). Figure 3 depicts the emission
colors of 11 in four solvents.
By plotting the wavelengths of the emission maxima of 11 in
the different solvents as a function of the solvent orientation
polarizability, Δf = (ε - 1)/(2ε þ 1) - 1/2((n2 - 1)/(2n2 þ 1))
(Lippert-Mataga equation),19b the corresponding Lippert plot
is obtained (Figure 4). It can be seen from this plot that the
spectral shift of the emission follows (to a good approximation)
a linear trend. Polar aprotic solvents such as dichloromethane
and acetonitrile which are included in this plot indicate that the
spectral shift is the result of solvent polarity and is not due to
(24) Diwu, Z.; Lu, Y.; Zhang, C.; Klaubert, D. H.; Haugland Photochem.
Photobiol. 1997, 66, 424–431.
(25) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971–988.
(26) Lakowicz, J. R., Ed. Principles of Fluorescence Spectroscopy, 2nd ed.;
Kluwer Academic/Plenum Publishers: New York, 1999.
J. Org. Chem. Vol. 75, No. 20, 2010 6775

Moss et al.
JOCArticle
TABLE 2. Photophysical Properties
λabs (nm),
toluene
ε (M-1 cm-1),
toluene
λabs (nm),
chloroform
λ
PL (nm),
λPL (nm),
chloroform
λex (nm)
for ΦPL
compd
toluene
Φ
PL,a toluene
1
344
366
355
379
352c
377
299
340
339
370
360
55 800
57 100
35 900
34 900
30 700
36 300
62 800
27 000
38 800
53 000
49 100
352
367
355
382
357c
387
300
355
339
371
362
412b
433
427b
440
458
0.90 ( 0.10
0.45 ( 0.05
0.58 ( 0.05
0.25 ( 0.10
0.55 ( 0.05
1.00 ( 0.10
0.59 ( 0.05
0.57 ( 0.05
0.26 ( 0.10
380
8
433b
450
380
380
360
380
380
320
380
360
9
432b
450
454
438
438
11
15
17
18
20
21
408
420b
440
411b
429
425b
440
409
300
320
338
379
323
336
351b
298
380
428b
300
411
47 000
43 700
18 850
14 760
13 000
16 500
9100
20 600
33 600
13 440
35 100
40 500
300
326
350
384
322
334
354c
296
380
436c
302
418
377
387b
431
454
456
442b
424
23
26
482
471
516
520
0.60 ( 0.05
0.75 ( 0.05
375
375
aPhotoluminescence quantum yield. bHighest maximum. cShoulder.
FIGURE 2. Emission spectra for 11 in solvents of different
polarities.
FIGURE 1. Influence of the donor substituents on the absorption
and emission spectra for the linear systems 1, 8, and 26 in
chloroform.
TABLE 3. Solvatochromic and Stokes Shift Energies (in cm-1) for
the Trimers
plots of both 1and 11 suggests, as can be seen from the Lippert-
Mataga equation, that the excited-state dipole moment is larger
than the ground-state dipole moment, which is also confirmed
by the transient spectral shift to lower energies, observed in polar
media from time-resolved data (as discussed in ref 19b). Espe-
cially for 11 the slope is much steeper, which, assuming that the
cavity radii are similar for both compounds, yields a much larger
difference between the dipole moments of the excited and ground
states. This suggests a large charge separation for 11 in the excited
state, which is also compatible with the frontier orbital picture
obtained through DFT calculations (vide infra, Figure 8).
The assumption that both compounds have similar cavity
radii is based on the fact that for both of them the longest axis
is estimated to be ca. 25 A and according to Lippert27 the
cavity radius for nonspherical molecules can be approxi-
mated to 40% of their longest axis.
solvatochromic
Stokes
λabs-λPL
(toluene)
λabs-λPL
(chloroform)
compd
Δλabs
ΔλPL
1
8
9
250
200
350
1250
100
150
1050
500
250
450
400
500
750
550
1700
400
700
1750
850
3450
3800
4000
4900
3750
3950
5550
3650
4900
2600
3100
3950
4350
4200
5350
4100
4550
6250
4000
6300
3550
4700
11
15
17
18
20
21a
23a
26
1650
1350
2000
aTentative low-energy absorption value based on a shoulder.
hydrogen bonding. Figure 4 also shows the Lippert plot of 1 for
comparison (data taken from ref 19b). The slope of the Lippert
(27) El-Gezawy, H.; Rettig, W.; Lapouyade, R. J. Phys. Chem. A 2006,
110, 67–75.
6776 J. Org. Chem. Vol. 75, No. 20, 2010

Moss et al.
JOCArticle
FIGURE 5. Absorption and emission spectra for 8 and 11, represent-
ing linear vs bent trimers, showing solvent effects on the λmax values.
FIGURE 3. Photograph showing the effect of solvent polarity on
the emission color of 11 when excited by a 360 nm laser with 2 ps
pulses (MCH = methylcyclohexane): (left to right) MCH, CHCl3,
EtOH, and MeOH.
FIGURE 6. Absorption and emission spectra for 20 and 21 show-
ing the solvent effects on the maxima values. Bent vs linear
conformations have little effect on the emission maxima values.
As we were completing this paper, Grisorio et al. reported
the N-methylcarbazole analogues of 8 and 11.28 Due to
limited solubility they were unable to study the solvatochro-
mism of these compounds, as we have done for 8 and 11.
Their observation that substitution of S in the 2,8-position
gave more blue-shifted emission in chloroform due to a
reduced conjugation effect, in comparison to the 3,7-isomer,
is consistent with our data discussed above for 8 and 11.
A general trend is that the bent systems have lower fluores-
cence quantum yields compared to their linear isomers: com-
pare 1 and 18, 8 and 11, and 20 and 21 (i.e. linear versus bent,
respectively). The addition of the fluorine substituents increases
the quantum yields ΦPL from 0.45 to 0.58 ((0.05), on comparing
8 and 9 in toluene, presumably due to a decrease in nonradiative
decay pathways caused by C-H bond vibrations.29 A similar
effect is observed for the analogous F-S systems 1 and 17.21c
All time-resolved fluorescence decays could be fitted with two
exponential components whose amplitudes and time constants,
are given in Table 4, along with the average lifetime, the values of
excitation and collection wavelengths, the solvent, and the local
χ2 values of the fitting in each case. The average lifetime was
FIGURE 4. Lippert plots for 1 and 11.
Changing the position of the donor substituents on the S
unit significantly affects the absorption and emission spec-
tra. For 11 (2,8-disubstitution: bent) compared to isomer 8
(3,7-disubstitution: linear), the emission is blue-shifted
(Figure 5). This can be explained by extended conjugative
(para) effects in 8 where four rings are in conjugation,
whereas in 11 only two rings are in conjugation. This effect
is also seen in the analogous F-S systems 1 and 18. However,
the bent vs linear effects of the N-carbazole groups on the
emission maxima in 20 and 21 (Figure 6) are much smaller,
implying that less conjugation is taking place between the
carbazolyl groups and the S unit in the linear isomer.
Analogous 2,8-disubstituted (bent) S units with both DPA
and TPA substituents (without butyl chains) have been
previously reported;22 they exhibit blue-shifted emission in
toluene compared with 23 and 26, indicating similar bent vs
linear effects. A similar effect was reported by Estrada and
Neckers for trimers with the fluoren-9-ylidene malonitrile
acceptor at the core: for the linear trimer the HOMO was
delocalized, whereas for the bent isomer the HOMO was
predominantly localized on the donor units.18
P
P
calculated from the expression30 τav= iAiτi2/ iAiτi, where ai
(29) Babudri, F.; Farinola, G. M.; Naso, F.; Ragni, R. Chem. Commun.
2007, 1003–1022.
(30) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer: Singapore, 2006.
(28) Grisorio, R.; Melcarne, G.; Suranna, G. P.; Mastrorilli, P.; Nobile,
C. F.; Cosma, P.; Fini, P.; Colella, S.; Fabiano, E.; Piacenza, M.; Sala, F. D.;
Ciccarella, G.; Mazzeo, M.; Gigli, G. J. Mater. Chem. 2010, 20, 1012–1018.
J. Org. Chem. Vol. 75, No. 20, 2010 6777

Moss et al.
JOCArticle
and τi are the amplitude and time constant of each exponential,
respectively, and τav is the average lifetime. For all toluene
solutions, except for 1 (see the previous discussion for this
compound),19c both components have positive amplitudes with
the fast components having time constants in the range of
hundreds of picoseconds and the slower components having
time constants in the range of nanoseconds.
component is always below 2%, in agreement with the very
small transient spectral relaxation observed in toluene, a
low-polarity solvent. For compound 11, and also for com-
pound 1,19b additional time-resolved measurements were
performed in ethanol, which is a solvent of high polarity
(Figure 7). The decays collected in the high-energy region of
the spectrum could be fitted with two exponential compo-
nents with positive amplitudes. One component appears as a
fast decay with a time constant of 28 ps, while the slower
component has a time constant of 3.98 ns. The decays
collected in the low-energy part of the spectrum could be
fitted again with two exponential components. However, the
amplitude of the fast one is now negative and its time
constant is 23 ps, which correlates well with the time constant
of the corresponding fast component of the high-energy
region of the spectrum. The latter observation is a good
indication that these components are due to the excited-state
reorganization, controlled by solvent and molecular rear-
rangements, as observed for compound 1.19b The time con-
stant of the slow component (3.93 ns) is also in excellent
agreement with the corresponding one in the high-energy
region of the spectrum. This indicates that the slow compo-
nent is associated with the lifetime of the ICT state of this
compound (11) in ethanol.
The pre-exponential amplitude associated with the faster
component is always small, and the weight of this decay
FIGURE 7. Fluorescence decays for 11 in ethanol collected at 446
and 540 nm.
TABLE 4. Lifetimes of Compounds
compd
solvent
λexc (nm)
λem (nm)
τ2 (ns) (A2)
τ1 (ns) (A1)
χ2
τav (ns)
1a
8
9
11
toluene
toluene
toluene
toluene
ethanol
ethanol
toluene
toluene
toluene
toluene
toluene
toluene
toluene
380
360
360
377
360
360
377
360
363
360
360
381
377
415
433
431
489
446
540
420
431
390
430
425
483
490
0.030 (-0.05)
0.173 (0.12)
0.153 (0.12)
0.098 (0.07)
0.028 (0.87)
0.023 (-0.84)
0.246 (0.09)
0.153 (0.11)
0.129 (0.07)
0.173 (0.18)
0.070 (0.13)
0.154 (0.04)
0.305 (0.07)
1.08 (1.00)
1.47 (0.88)
1.27 (0.88)
1.13 (0.93)
3.98 (0.13)
3.93 (1.00)
1.24 (0.91)
0.92 (0.89)
1.22 (0.93)
3.93 (0.82)
2.68 (0.86)
4.20 (0.25)
1.49 (0.93)
1.12
1.05
1.25
1.04
1.12
1.48
1.05
1.07
1.10
1.45
1.10
1.43
1.02
1.08
1.45
1.25
1.12
3.79
3.93
1.22
0.90
1.21
3.89
2.68
4.17
1.47
15
17
18
20
21
23
26
aData for compound 1 are taken from ref 19c.
FIGURE 8. Frontier orbitals for 8a and 11a. Contour values are plotted at (0.03 (e/bohr3)1/2
6778 J. Org. Chem. Vol. 75, No. 20, 2010
.

Moss et al.
JOCArticle
TABLE 5. Dihedral Angles between Donor (F, Cz, DPA, and TPA) and Acceptor (S) Units in Optimized Ground- (S0) and Excited-State (S1) Geometries
and Energy Differences between These Geometries and Their Planar Forms
compd
S0 dihedral (deg)
S1 dihedral (deg)
Δ(S0-S1) dihedral (deg)
S0 planar energy (kcal mol-1
)
S1 planar energy (kcal mol-1
)
1a
8a
9a
38.3
38.7
39.6
40.1
38.3
40.2
40.7
53.4
54.6
33.6
35.6
22.0
25.0
31.8
34.8
23.6
28.6
27.6
89.3
89.4
56.1
17.7
16.3
13.7
7.8
5.3
14.7
11.6
13.1
-35.9
-34.8
-22.5
17.9
3.9
4.1
5.5
4.9
4.3
5.5
4.8
2.5
2.8
4.4
4.3
3.0
4.3
3.2
11a
15a
17a
18a
20a
21a
23a
26a
25.9
27.9
24.0
25.6
Ground-state (S0) geometry optimizations were carried
out on the 11 trimers at the B3LYP/6-311G** level of theory
without symmetry constraints. Methyl groups were used
instead of ethylhexyl or butyl groups (where present) to
reduce computational efforts, and these model geometries
are labeled with “a” for convenience. Many conformers were
found, but no true minimum for a planar geometry was
located in any trimer. Only the most stable conformers found
are considered here, and their dihedral angles are listed in
Table 5. Comparison of the geometrical parameters between
the optimized model geometry 17a and the experimental
geometry21c of 17 shows very good agreement. The dihedral
angles between the F and S units are 40.2ꢀ in 17a (theory) and
between 36.7 and 46.0ꢀ in 17 (experimental).
TABLE 6. Contributions (%) of the Donor and Acceptor Groups in the
Frontier Orbitals
LUMO
HOMO
acceptor
acceptor
donor
donor
1a
8a
9a
72
84
85
90
74
76
77
93
96
86
78
28
16
15
10
26
24
23
7
29
22
18
13
5
23
19
16
11
36
10
71
78
82
87
95
77
81
84
89
64
90
11a
15a
17a
18a
20a
21a
23a
26a
4
14
22
Planar geometries were optimized using C2v symmetry con-
straints (apart from 23a and 26a, where the tolyl groups at
nitrogen would be unrealistically constrained to be perpendicu-
lar to the S unit) to obtain differences in energy between the
planar and nonplanar geometries. These energy values are given
in Table 5 and would be considered as rotational energy barriers
to planar geometries. For the vast majority of the trimers, the
energy barriers are in the range of only 4.0-5.5 kcal mol-1. The
donor and acceptor units are considered to be freely rotating in
solution at ambient temperatures. The presence of fluorine
atoms in 9a and 17a increases the energy barrier by only
1.4-1.6 kcal mol-1 in comparison to 8a and 1a, respectively.
The geometries of 20a and 21a, however, are computed to have
observed CV and photophysical data. The delocalized LUMOs
suggest that the thiophene S,S-dioxide ring (rather than only
the SO2 unit) acts as the acceptor. The HOMO, LUMO, and
HOMO-LUMO gap (HLG) energies are compared with the
CV data in Table 1, where trends are clearly evident. The
different absolute values are typical when comparing oxidation
and reduction potentials with computed MO energies. The
calculated HOMO energies correspond to the increasing elec-
tron-donating ability of the donor unit from fluorenyl (F) to
carbazolyl (Cz) to arylamine groups (DPA, TPA). The calcu-
lated LUMO and HLG energies are also in broad agreement
with observed reduction potentials and HLGs in Table 1.
The orbital contributions of the acceptor and donor units in
the LUMO and HOMO for each trimer are shown in Table 6.
When the linear and bent analogues of 8a and 11a are com-
pared, it is clear that the HOMO and LUMO characters are
more localized on the donor and acceptor units, respectively, for
the bent isomers. These frontier orbitals are shown pictorially in
Figure 8, where the HOMO and LUMO are considered as
largely π (donor) and π* (acceptor) orbitals, respectively, with
more mixing in both orbitals for 8a. Similar conclusions are
found for the linear/bent pair, 1a and 18a. A slightly different
scenario occurs for 20a and 21a, where the HOMO and LUMO
compositions are similar in both cases and are essentially loca-
lized (Figure 9). This indicates that the conjugation present in
other linear trimers is reduced in compound 20a, due to signi-
ficantly nonplanar orientations of the carbazolyl groups in 20a.
TD-DFT computations were carried out on the S0 model
geometries to assign the nature of the transitions in the absorp-
tion spectra for the trimers, and the results are summarized in
Table 7. In all cases, the observed lowest energy bands are π
(donor) to π* (acceptor) charge transfer transitions in nature.
The charge transfer character of these transitions varies between
43% (1a) and 87% (21a). The bent trimers clearly have greater
considerably larger energy barriers at 25.9-27.9 kcal mol-1
.
Planar geometries of 20a and 21a are unlikely to be present in
solution at ambient temperatures, as hydrogens near the carba-
zolyl nitrogen severely hinder such planar geometries.
The singlet excited-state geometries (S1) were also optimized
without symmetry constraints and, like the ground-state geo-
metries (S0), nonplanar geometries were located as minima. The
dihedral angles are smaller by 5-17ꢀ in all cases, except for
20a, 21a, and23a, where the angles are larger due to unfavorable
steric factors involving the N-carbazolyl and DPA units. The S1
planar geometries were also obtained using C2v symmetry
constraints and reveal slightly smaller energy barriers to planar
forms in comparison to the ground-state geometries. The diffe-
rences in the dihedral angles of 6.6-6.8ꢀ due to the fluorine
atoms in 9 and 17 compared to 8 and 1 in the S1 excited-state
geometries indicate steric effects with the fluorine atoms but, as
shown in their photophysical data and the small energy barriers
to planar forms, these steric effects are not significant.
Molecular orbital calculations on the S0 optimized geome-
tries of the model trimers show that the HOMOs are located on
the donor groups and LUMO on the S unit, in accord with
J. Org. Chem. Vol. 75, No. 20, 2010 6779

Moss et al.
JOCArticle
FIGURE 9. Frontier orbitals for 20a and 21a. Contour values are plotted at (0.03 (e/bohr3)1/2
.
TABLE 7. Comparison of TD-DFT Data with Observed Absorption and Emission Data
λabs (nm)
λem (nm)
ε (M-1 cm-1
obsd
)
calcd
obsd
oscillation strength
transition typea
CT character (%)
calcd
obsd
1a
8a
9a
11a
386
391
406
298
356
331
384
395
321
352
371
425
360
402
316
388
430
324
456
364
379
377
299
340
339
370
360
300
315
338
377
333
351b
298
380
428b
300
411
1.48
0.98
1.07
0.11
0.14
0.21
0.81
1.46
0.24
0.33
0.12
0.41
0.12
0.05
0.27
0.54
0.56
0.87
1.21
57 100
34 900
36 300
62 800
27 000
38 800
53 000
49 100
47 000
43 700
18 850
14 760
13 000
16 500
20 600
33 600
13 440
35 100
40 500
H > L
H > L
H > L
43
62
67
30
81
69
51
53
56
58
81
77
87
84
29
48
50
20
68
462
458
462
406
424
443
443
408
H-4 > Lc
H-1 > L
H-2 > Lþ1
H-2 > L
H > L
15a
458
430
17a
18a
463
405
420
382
H > Lþ1
H > L
H > Lþ1
H > L
20a
21a
23a
530
516
545
437
424
482
H > Lþ1
H-1 > L
H > Lþ4d
H > Lþ1
H > L
26a
H-2 > Lc
H > L
555
471
aH = HOMO, L = LUMO. bShoulder. cLocal transition at S unit. dLocal transition at DPA units.
CT character than their linear analogues (8a vs 11a, 1a vs 18a),
which is reflected in their photophysical properties. In the absorp-
tion spectra for 11, 23, and 26, there are distinct, strong bands at
around 300 nm which are assigned as local π to π* transitions on
the basis of TD-DFT data. Calculated emission data were ob-
tained from TD-DFT computations on S1 optimized geometries.
The emission values listed in Table 7 correspond to the lowest
energy transition in each case and are assumed to be π* (accep-
tor) to π (donor) charge transfer transitions. Figure 10 shows
reasonable correlations between observed and computed values
for both absorption and emission data, where absolute values of
observed and computed data are typically different.
Conclusions
Donor-acceptor-donor S-based oligomers have been
synthesized where the donor strength has been increased in
FIGURE 10. Correlation between observed and calculated absorp-
tion (circles) and emission (triangles) values.
6780 J. Org. Chem. Vol. 75, No. 20, 2010

Moss et al.
JOCArticle
the series fluorene f carbazole f arylamine. Their photo-
physical properties have been investigated in detail, with
particular emphasis on probing the charge transfer character
of the singlet excited state. The substitution of fluorene by
carbazole red-shifts the emission from deep blue to sky blue.
For the arylamine analogues the emission is further red-
shifted into the green region. We have also investigated the
effect of conjugation between the donor and acceptor units
and demonstrated that the para versus meta conjugation
between the successive phenyl rings controls the properties
having a strong effect on the ICT character and emission
yield of the singlet excited state. These results demonstrate a
promising strategy for exploiting ICT states in ambipolar
oligomers to tune the emission across the deep blue-green
spectral region while retaining moderate to high photolumi-
nescence quantum yields. Furthermore, a number of these
oligomers have the potential to be developed into polymers
for use as emissive materials in optoelectronic devices. The
high stability of fluorene, carbazole, and dibenzothiophene
S,S-dioxide derivatives is very attractive in this regard.
acid derivative 5, 14, or 16 (2.2 equiv) were dissolved in 1,4-
dioxane or toluene. The solution was degassed by bubbling
argon through it for 15 min. Dichlorobis(triphenylphosphine)-
palladium(II) (3-5 mol %) or tetrakis(triphenylphosphine)-
palladium(0) (5 mol %) was added and the solution degassed
further. Degassed potassium carbonate solution (2 M) or degassed
sodium carbonate (1 M) was added under argon and the solution
heated to 110 ꢀC for 24 h with protection from sunlight. The resulting
slurry was poured into aqueous sodium chloride solution (5%), and
the organic product was extracted into dichloromethane (DCM).
The organic layer was washed with water until pH 7 and dried over
magnesium sulfate (MgSO4). After removal of the solvent under
vacuum, the residue was purified by column chromatography to
obtain the product.
Acknowledgment. This work was carried out as part of the
TSB-funded TOPLESS project. We thank Dr. G. Williams
(Zumtobel LED Division) for supporting this work, Durham
University for funding (K.C.M.) and for access to its High
Performance Computing facility. We thank the Royal Society
for funding (V.B.).
Supporting Information Available: Text, figures, and tables
giving details of experimental procedures, characterization
data, absorption and fluorescence spectra, fluorescence decays,
solution electrochemical data, and computations. This material
is available free of charge via the Internet at http://pubs.acs.org.
Experimental Section
General Procedure: Palladium-Catalyzed Cross-Coupling.
3,7-Dibromodibenzothiophene S,S-dioxide (6) or 2,8-dibromodi-
benzothiophene S,S-dioxide (10) (1 equiv) and the boronic
J. Org. Chem. Vol. 75, No. 20, 2010 6781