Photophysical property trends for a homologous series of bis-ethynyl-substituted benzochalcogendiazoles

Article abstract of DOI:10.1039/c2nj20847d

We report on the preparation and photophysical property study of three homologous benzoheteroarene-ethynylene systems. Significant differences in the series' optical properties indicate a change in the HOMO-LUMO energy as the chalcogen is altered (O, S and Se) which we have examined using TD-DFT methods and shown to be attributable to modification of the HOMO energy. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj20847d

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
NJC
Cite this: New J. Chem., 2012, 36, 550–553
www.rsc.org/njc
LETTER
Photophysical property trends for a homologous series of bis-ethynyl-
substituted benzochalcogendiazolesw
Benjamin A. Coombs,a Benjamin D. Lindner,a Robert M. Edkins,b
Frank Rominger,a Andrew Beeby*b and Uwe H. F. Bunz*a
Received (in Montpellier, France) 4th October 2011, Accepted 22nd November 2011
DOI: 10.1039/c2nj20847d
We report on the preparation and photophysical property study of
three homologous benzoheteroarene–ethynylene systems. Significant
differences in the series’ optical properties indicate a change in the
HOMO–LUMO energy as the chalcogen is altered (O, S and Se)
which we have examined using TD-DFT methods and shown to be
attributable to modification of the HOMO energy.
Highly conjugated linear organic compounds, based on the
arylethynylene motif, have been studied extensively over recent
decades. This is indubitably owing to their facile synthesis and
interesting photophysical properties1 which lends them to significant
Scheme 1 Synthesis of compound 1.
exploitation in the field of organic electronics and sensoring
applications.2
Indeed, numerous materials which feature extended acenes,
functionalized with ethynyl substituents at various positions,
have been reported by our group (ethynyl groups terminated
with H, trimethylsilyl, triethylsilyl and triisopropylsilyl groups).3
In this work we present a systematic study on the change in
photophysical properties as the chalcogenic heteroatom in the
central ring is altered in a series of homologous structures
(see compounds 1–3, Schemes 1 and 2). The compounds
conform to the arylethynylene motif, thus engendering them
with potential as candidates in organic thin film transistors
and OLEDs.
This work presents our first consideration of acenes featuring
benzoselenadiazoles. In fact, comparison of effects caused by
the heteroatom O, S and Se is becoming more common4,5 and
we are considering them due to the potential effects the heavy
and large Se atom will have on the photophysical and structural
properties, as well as for its touted ability to lower the bandgaps
of its polymers, affording them potential in fabricated devices
and applications.6
Scheme 2 Synthesis of compounds 2 and 3.
a Organische Chemisches Institut, Universitat Heidelberg,
¨
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany.
Arylethynylene materials containing the benzothiadiazole
E-mail: uwe.bunz@oci.uni-heidelberg.de; Fax: +49 6221 548 401;
Tel: +49 6221 548 401
unit are relatively simple to prepare, due to the ease of preparing
the key intermediate 4,7-dibromobenzothiadiazole,7 and were
reported recently to display high fluorescence quantum yields.8
Furthermore, incorporation of the benzothiadiazole moiety into
oligomeric and polymeric systems has resulted in materials with
promising application as OPVs and OLEDs.9
b Department of Chemistry, Durham University Science Site,
South Road, Durham, DH1 3LE, UK
w Electronic supplementary information (ESI) available: Detailed
preparation of intermediates and molecular structures. CCDC
846082 (1), 846083 (2), and 846084 (3). For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2nj20847d
c
550 New J. Chem., 2012, 36, 550–553
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Online
The oxygen-containing analogue benzofurazan is less well
known in this field. However it is commonly used as a
‘‘fluorescent handle’’ for labelling of biological species.10
Furthermore, recent work has found benzofurazan-containing
arylethynylenes to exhibit very high quantum yields (up to 96%).11
4,7-Dibromobenzofurazan was made following a literature method9
by bromination of commercially available benzofurazan. This
was isolated in good yield (78%).
Sonogashira cross coupling of 4,7-dibromobenzofurazan
with triisopropylsilylacetylene afforded the target 4,7-bis-
(triisopropylsilylethynyl) benzofurazan (1, Scheme 1).
Various analogues of 2 (Scheme 2) have been reported
previously, as mentioned above. However, the formation of
small acetylene-coupled benzofurazan and benzoselenadiazole
systems is hitherto unknown.
Preparation of benzoselenadiazole is possible via the diamine
used to prepare the sulfur analogue (see ESIw),12 however
coupling of 4,7-dibromobenzoselenadiazole with triisopropylsilyl-
acetylene under Sonogashira conditions was unsuccessful. A
second method (Scheme 2) was attempted via the sulfur analogue,
resulting in successful isolation of 3 in 91% yield.
The photophysical properties of these systems have been
determined and the results are displayed in Table 1. Examination
of the absorption lmax illustrates a clear trend for the progression of
the HOMO energies, attributable to the group VI atom. As shown
in Fig. 1, the absorption maxima exhibit a bathochromic shift as
the heteroatom is changed from O (372 nm) - S (384 nm) - Se
(412 nm) which indicates a decrease in the S1 ’ S0 gap. The
emission maxima also exhibit a bathochromic shift upon
substitution of the heteroatom: O (425 nm) - S (441 nm) -
Se (480 nm). The compounds’ absorption spectra display
similar profiles, with a red-shifted shoulder on the lowest energy
absorption band, assigned as the S1 ’ S0 transition.
Fig. 1 Absorption and emission spectra of compounds 1–3, measured
in n-hexane.
Stoke’s shifts increase slightly through the series, with a
greater increase for compound 3. The presence of the selenium
atom drastically diminishes the oscillator strength for the S1 2 S0
transition in the diazole, resulting in both attenuated extinction
coefficient (e = 12 500 molÀ1 dm3 cmÀ1) and pure radiative
lifetime (t0 = 27.5 ns) relative to the oxygen and sulfur analogues
(e E 20 000 molÀ1 dm3 cmÀ1; jf E 0.8; t0 E 5 ns).
Compounds 2 and 3 display higher energy absorption bands
(S2 ’ S0), with some evidence of vibronic structure; such
detailed fine structure is not observed in the lower energy
absorption bands, attributed to charge transfer (from the
alkynyl-substituents to the central, electron-deficient, hetero-
cycle) broadening this band. Selenium derivative 3 exhibits an
absorption maximum significantly more red-shifted to com-
pound 2 than compound 2 is to 1 (1770 cmÀ1 cf. 840 cmÀ1).
The emission spectra (Fig. 1) for the three compounds are also
similar in profile, with a single high energy shoulder on the
broad emission bands. Additionally they are mirror images of
the lowest energy absorption bands. The ordering of the
emission maxima follows the trend displayed by the absorption
maxima, with the difference between compounds 2 and 3 more
than double that between compounds 1 and 2 (1843 cmÀ1 cf.
853 cmÀ1). Also, upon dissolution in increasing polarity
solvents, a small red shift of approximately 30 nm has been
observed in the emission of all these compounds, although no
change was observed in the absorption maxima.
DFT calculations have been performed (Spartan ’10,13 gas-phase,
B3LYP using 6-311++G(d) basis set) to afford optimised
geometries of these systems, which exhibit similar structures
to those determined using X-ray crystallographic analysis (see
ESIw). TD-DFT energy calculations (Gaussian ’09,14 B3LYP
and CAM-B3LYP using 6-311++G(d)) have afforded predic-
tions of the HOMO–LUMO energies and frequency calcula-
tions have afforded predicted Raman vibrational modes.
From the energy calculations it is shown that the
HOMO–LUMO gap is attributable to the S1 ’ S0 transition
for all three systems. According to the calculations (see Table 2),
the greatest effect upon switching the chalcogen is observed in the
energy of the HOMO, which shows a greater overall change than
the LUMO experiences (0.28 eV cf. 0.14 eV). Furthermore, the
trend exhibited for the change in the HOMO energy calculated
Table 1 Photophysical properties of compounds 1–3, obtained in n-hexane
Compound Abs lmax/nm Em lmax/nm e/molÀ1 dm3 cmÀ1 Stoke’s shift/cmÀ1 Ff (Æ10%) tf/ns (Æ10%) kfa/nsÀ1 (Æ0.14%) knrb/nsÀ1 (Æ10%)
1
2
3
372
384
412
435
441
480
19 900
20 600
12 500
3350
3370
3440
0.76
0.83
0.04
3.4
5.1
1.1
0.22
0.16
0.04
0.07
0.03
0.96
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 550–553 551

View Online
Table 2 Molecular orbitals visualised using Spartan ’10 and Gaussian ’09 (B3LYP/6-311++G(d) and CAM-B3LYP/6-311++G(d))
Compound
1: NON
2: NSN
3: NSeN
LUMO/eV
À3.14
À3.00
À3.09
HOMO/eV
À6.42
À6.22
À6.15
DFT DE/eV
3.28
3.22
3.06
TD-DFT DE/eV (oscillator strength)
Cam-TD DFT DE/eV (oscillator strength)
Observed band-gap/eV
3.29 (0.51)
3.36 (0.65)
3.32
3.23 (0.35)
3.28 (0.50)
3.21
3.06 (0.27)
3.06 (0.39)
3.01
using ab initio methods mirrors that of the absorption and
emission maxima. Predicted molecular orbital plots for these
systems indicate great similarity for the HOMOs. Less similarity
between the homologues is observed for the LUMOs, with
greater electron density being drawn towards the large selenium
atom on the central ring. The linear trend of the HOMO energies
(Se 4 S 4 O) is not maintained in the LUMO energies (S 4
Se 4 O). However, we are pleased to see excellent agreement
between the predicted band-gap energies and those determined
experimentally.
The predicted Raman spectra indicate excellent agreement
with those obtained experimentally (see Fig. 2), further validating
the computed structures. Compound 1 features a higher energy
vibration (1418 cmÀ1) compared to equivalent vibrational modes
at approximately 1352 cmÀ1 for 2 and 1362 cmÀ1 for 3. These
vibrational modes correspond to a stretch in the central ring
system, so is thus clearly an effect of the chalcogen.
In conclusion we have shown that in otherwise identical
systems, the group VI heteroatom exhibits substantial control
over its host molecules’ photophysical properties by modification of
the HOMO energy. Calculations have confirmed this assessment,
also predicting band gaps in excellent agreement with those
determined experimentally. The oxygen and sulfur derivatives
(1 and 2) behave quite differently to compound 3, displaying
similar quantum yields and lifetimes and thus rates of radiative
decay (kf), implying essentially the same mechanism of decay.
The selenium derivative displays the lowest energy absorption
and emission maxima, as well as an attenuation of the
fluorescence quantum yields and lifetimes. The selenium derivative
exhibits a much lower rate of radiative decay (kf), indicating
potentially a different decay pathway.
Fig. 2 Simulated (a) and experimental (b) Raman spectra for
compounds 1–3.
Experimental
The properties of compounds 1 and 2 are so alike that we
question the lack of research into the oxygen-containing
derivatives for material-based applications, echoing Swager’s
earlier comments,9 especially considering their facile preparation.
This work has demonstrated the significant control the
chalcogen has over its parent molecule’s HOMO energy,
and as such research is being continued into larger hetero-
acene systems incorporating selenium. This will enable further
comparisons with sulfur-containing analogues from the
group’s earlier work.3
Syntheses
4,7-Bis(triisopropylethynyl)-benzofurazan (1). 4,7-Dibromo-
benzofurazan (370 mg, 1.33 mmol) was dissolved in a mixture
of THF (15 mL) and Et3N (10 mL) and the solution degassed
by 4 freeze–pump–thaw cycles. Under a flow of nitrogen
triisopropylsilyl acetylene (0.75 mL, 3.33 mmol), Pd(PPh3)4
(46 mg, 5 mol%) and CuI (5 mg, 2 mol%) were added and the
mixture stirred at 65 1C for 16 h, during which time the
solution turned fluorescent yellow/green under illumination
c
552 New J. Chem., 2012, 36, 550–553
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Online
by a long wave (365 nm) UV lamp, and a large amount of
precipitate had formed. The mixture was filtered through a
short silica plug, eluting with hexanes and diethylether. The
collected fraction was reduced in vacuo and the crude product
(orange/brown solid) was chromatographed on a silica gel
column. Using gradient elution of petroleum ether : diethylether
(100 - 75 : 25) the compound was isolated as a yellow, slightly
A. Beeby, J. Phys. Chem. A, 2006, 110, 2114–2121;
(c) P. V. James, P. K. Sudeep, C. H. Suresh and K. G. Thomas,
J. Phys. Chem. A, 2006, 110, 4329–4337; (d) T. Fasina, J. Collings,
J. Burke, A. Batsanov, R. Ward, D. Albesa-Jove, L. Porres,
´
A. Beeby, J. Howard and A. Scott, J. Mater. Chem., 2005, 15,
690–697; (e) J. Siddle, R. Ward, J. Collings, S. Rutter, L. Porres,
L. Applegarth, A. Beeby, A. Batsanov, A. Thompson and
J. Howard, New J. Chem., 2007, 31, 841–851.
2 (a) J. Yang and T. Swager, J. Am. Chem. Soc., 1998, 120,
5321–5322; (b) B. A. D. Neto, A. A. M. Lapis, F. S. Mancilha,
I. B. Vasconcelos, C. Thum, L. A. Basso, D. S. Santos and
J. Dupont, Org. Lett., 2007, 9, 4001–4004; (c) L. Zhao,
I. Perepichka, F. Turksoy, A. Batsanov, A. Beeby, K. Findlay
and M. Bryce, New J. Chem., 2004, 28, 912–918.
1
tacky solid. H NMR (300 MHz, CDCl3): d = 1.17 (s, 42H),
7.46 (s, 2H); 13C{1H} NMR (75 MHz, CDCl3): 11.41, 18.79,
100.47, 103.00, 113.07, 134.63, 149.78; MS (EI): m/z: calcd for
C28H44N2SSi2 480.2963, found: 480.2992.
3 (a) C. Bangcuyo, M. Smith and U. H. F. Bunz, Synlett, 2004, 169–172;
(b) S. Miao, M. Smith and U. H. F. Bunz, Org. Lett., 2006, 8,
757–760; (c) S. Miao, P. V. R. Schleyer, J. I. Wu, K. I. Hardcastle and
U. H. F. Bunz, Org. Lett., 2007, 9, 1073–1076; (d) A. L. Appleton,
S. Miao, S. M. Brombosz, N. J. Berger, S. Barlow, S. R. Marder,
B. M. Lawrence, K. I. Hardcastle and U. H. F. Bunz, Org. Lett., 2009,
11, 5222–5225; (e) A. L. Appleton, S. M. Brombosz, S. Barlow,
J. S. Sears, J.-L. Bredas, S. R. Marder and U. H. F. Bunz, Nat.
Commun., 2010, 1, 1–6; (f) S. Miao, S. M. Brombosz, P. V. R.
Schleyer, J. I. Wu, S. Barlow, S. R. Marder, K. I. Hardcastle and
U. H. F. Bunz, J. Am. Chem. Soc., 2008, 130, 7339–7344.
4 T. Ohulchanskyy, D. Donnelly, M. Detty and P. Prasad, J. Phys.
Chem. B, 2004, 108, 8668–8672.
5 (a) J. Politis, F. Somoza, J. Kampf and M. Curtis, Chem. Mater.,
1999, 11, 2274–2284; (b) R. M. Osuna, R. Ponce Ortiz,
´
T. Okamoto, Y. Suzuki, S. Yamaguchi, V. Hernandez and
4,7-Bis(triisopropylsilylethynyl)-benzothiadiazole (2). 4,7-Di-
bromobenzothiadiazole (310 mg, 1.05 mmol) was dissolved in
a Et3N/THF (1 : 4, 15 mL) solvent mixture and the solution
degassed via three freeze–pump–thaw cycles. TIPS-acetylene
(0.5 mL, 2.2 mmol) was added via a syringe and PdCl2(PPh3)2
(40 mg, 5 mol%) and CuI (10 mg, 2 mol%) added under a flow
of nitrogen. The mixture was stirred at 75 1C for 48 h. The
resulting mixture was filtered through a silica plug, eluting
with pet. ether/ethyl acetate. The solvent was reduced in vacuo
and the residue chromatographed on a silica gel column,
eluting with petroleum ether. The product was isolated as a
lightly yellow solid (460 mg, 88%). 1H NMR (300 MHz,
CDCl3): d = 1.15–1.24 (m, 42H), 7.67 (s, 2H); 13C{1H}
NMR (75 MHz, CDCl3): 11.49, 18.85, 100.40, 102.40, 117.61,
132.82, 154.79; MS (EI): m/z: calcd for C28H44N2SSi2 496.31,
J. T. Lopez Navarrete, J. Phys. Chem. B, 2007, 111, 7488–7496.
´
6 Y.-J. Cheng, S.-H. Yang and C.-S. Hsu, Chem. Rev., 2009, 109,
5868–5923.
7 B. Neto, A. Lopes, M. Wuest, V. Costa, G. Ebeling and J. Dupont,
Tetrahedron Lett., 2005, 46, 6843–6846.
8 (a) M. Akhtaruzzaman, M. Tomura, M. B. Zaman, J.-I. Nishida
and Y. Yamashita, J. Org. Chem., 2002, 67, 7813–7818;
(b) B. Neto, A. Lopes, G. Ebeling, R. Goncalves, V. Costa,
F. Quina and J. Dupont, Tetrahedron, 2005, 61, 10975–10982.
9 (a) J. Bouffard and T. M. Swager, Macromolecules, 2008, 41,
5559–5562; (b) N. Blouin, A. Michaud, D. Gendron, S. Wakim,
E. Blair, R. Neagu-Plesu, M. Belletete, G. Durocher, Y. Tao and
M. Leclerc, J. Am. Chem. Soc., 2008, 130, 732–742;
found: 496.28; elemental analysis calcd for C28H44N2SSi2
67.68, H 8.93, N 5.64%, found: C 67.78, H 9.04, N 5.69%.
C
4,7-Bis(triisopropylsilylethynyl)-benzoselenadiazole (3). To a
solution of 3,6-bis(triisopropylsilyl ethynyl)benzene-1,2-diamine
(0.70 g, 1.48 mmol) in EtOH (13 mL) at 60 1C was added a
solution of selenium dioxide (0.49 g, 4.45 mmol) in hot water
(4 mL). After 2 h the suspension was filtered through a Buchner
¨
(c) D. Muhlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller,
¨
R. Gaudiana and C. Brabec, Adv. Mater., 2006, 18, 2884–2889.
funnel and the collected yellow precipitate was washed with
water. Purification by flash column chromatography gave a deep
yellow solid (silica gel, petroleum ether (100) to petroleum
ether/ethyl acetate, 99: 1) in good yields (730 mg, 91%).
1H NMR (300 MHz, CDCl3): d = 1.14–1.23 (m, 42H), 7.57
(s, 2H); 13C{1H} NMR (75 MHz, CDCl3): 11.53, 18.87, 100.10,
103.05, 119.40, 133.10, 159.65. IR: u = 2942, 2863, 2148, 1460,
994, 881, 801; MS (ESI): m/z calcd for C28H45N2SeSi2
[M + H]+: 545.22823, found: 545.22838, correct isotope
distribution; calcd for C28H44N2SeSi2 C 61.84, H 8.16,
N 5.15%, found: C 61.95, H 8.26, N 5.18%.
10 (a) E. Pazos, O. Vazquez, J. L. Mascarenas and M. Eugenio
´
Vazquez, Chem. Soc. Rev., 2009, 38, 3348; (b) B. Bag and
´
P. Bharadwaj, Inorg. Chem., 2004, 43, 4626–4630.
11 B. A. Coombs, PhD thesis, University of Durham, 2010.
12 R. Yang, R. Tian, J. Yan, Y. Zhang, J. Yang, Q. Hou, W. Yang,
C. Zhang and Y. Cao, Macromolecules, 2005, 38, 244–253.
13 Spartan ’10, Wavefunction, Inc. Irvine, CA.
14 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi,
N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski,
G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich,
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (Bu 771/7-1)
for generous support.
Notes and references
¨
1 (a) A. Beeby, K. Findlay, P. J. Low and T. B. Marder, J. Am.
Chem. Soc., 2002, 124, 8280–8284; (b) S. Greaves, E. Flynn,
E. Futcher, E. Wrede, D. Lydon, P. Low, S. Rutter and
A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz,
J. Cioslowski and D. J. Fox, Gaussian ’09, Revision A.1, Gaussian,
Inc., Wallingford CT, 2009.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 550–553 553