Emission behavior of a symmetrical sexthiophene

Article abstract of DOI:10.1080/15421406.2016.1277497

Detailed absorption and emission behavior were investigated for a sexthiophene molecule in solution, film, and solid. Only strong polarity solvent could result a notable influence on the quench of its fluorescence. In weak and medium polar environment, including in polymer matrix, highly emissive behavior was maintained and exhibit large Stokes shift. More important is its highly emissive in solid state due to the twisted molecular conformation. However, the bandwidth of solid emission (FWHM: 2149 cm^?1) is similar to that of solution emission. The calculated frontier orbital based on TDDFT theory uncovered the underlying signal mechanism furtherly.

Full text of DOI:10.1080/15421406.2016.1277497

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Emission behavior of a symmetrical sexthiophene
Xiaochuan Li, Haiyang Zhang & Young-A Son
To cite this article: Xiaochuan Li, Haiyang Zhang & Young-A Son (2017) Emission behavior
of a symmetrical sexthiophene, Molecular Crystals and Liquid Crystals, 644:1, 240-248, DOI:
10.1080/15421406.2016.1277497
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Date: 06 July 2017, At: 05:03

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Emission behavior of a symmetrical sexthiophene
Xiaochuan Lia,b, Haiyang Zhanga, and Young-A Sonc
aCollaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory
of Green Chemical Media and Reactions, Ministry of Education, Henan Normal University, Xinxiang, Henan, P. R.
China; bHenan Key Laboratory of Green Chemical Media and Reactions, School of Chemistry and Chemical
Engineering, Henan Normal University, Xinxiang, Henan, P. R. China; cBK, Department of Advanced Organic
Materials Engineering, Chungnam National University, South Korea
ABSTRACT
KEYWORDS
Sexthiophene; fluorescence;
highly emissive; solid; large
Stokes shift
Detailed absorption and emission behavior were investigated for
a sexthiophene molecule in solution, film, and solid. Only strong
polarity solvent could result a notable influence on the quench of its
fluorescence. In weak and medium polar environment, including in
polymer matrix, highly emissive behavior was maintained and exhibit
large Stokes shift. More important is its highly emissive in solid state
due to the twisted molecular conformation. However, the bandwidth of
solid emission (FWHM: 2149 cm−1) is similar to that of solution emission.
The calculated frontier orbital based on TDDFT theory uncovered the
underlying signal mechanism furtherly.
Introduction
The organic fluorescence emitters are brighter than that of the inorganic ones due to the
high quantum yield and absorption coefficient. However, the inorganic fluorescence emit-
ters are more stable than that of the organic fluorescence emitters. Generally, the organic
fluorescent dyes are highly soluble in most of organic solvents as the strong interactions
between dye molecule and solvent molecules. The unique emission characters of dye in solu-
tion, film and solid state attractive the interest of chemists, physicists, and biochemists due to
their application in materials science and structural biotechnology. Such potential is devel-
oped greatly and advanced by families of dyes that have been systematically engineered,
such as coumarins, naphthalimide, bisindolylmalimide, perylemediimides, rhodamine, and
F-B complexes [1–16]. Due to the excellent opto-physical properties of the fluorescent dye
families, the mainstream of investigation is focused on the application of organic electron-
ics and photonics, OLEDs, nano-emitters, molecular machine, probes, energy collectors,
bioimaging, and digital design [17–25]. A typical of heterocyclic molecules, comprised of
thiophene, has received particular interest [26, 27]. Thiophene-related oligomers and poly-
mers were well-studied due to the π-conjugated systems and versatile chemistry in elec-
tronic device fabrication. The optical and electronic properties of thiophene derivatives
can be easily tuned in structure variation by the attachment of either different functional
CONTACT Prof. Young-A Son
National University  Gung-dong, Daejeon, -, South Korea; Prof. Xiaochuan Li
yason@cnu.ac.kr
Department of Advanced Organic Materials Engineering, Chungnam
lixiaochuan@htu.cn School of
Chemistry and Chemical Engineering, Henan Normal University East Jianshe Rd. , Xinxiang, Henan, , China.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
241
substituents or microstructures. For instance, the optical properties, packing behavior, and
conductivity of region-regular polythiophenes differ significantly from those of region-
random polythiophenes.
In this paper, we present our design and synthesis of a sexthiophene. The purpose of
this molecule architecture is to investigate its absorption and emission behavior in solu-
tion/film/solid. The single bond connection leads to a highly twisted conformation, which
may resisted to a regular packing. Minimized π-π interaction in oligothiophen systems could
increase the quantum yield and shift the emission wavelength toward longer wavelength
side. It will ignite the strong emission in film or solid and result in broad absorption. The
remarkable opto-physical properties induced by the electronic interaction is deserved to be
investigated.
Experimental
General procedures and materials
The solvents used in the reaction were carefully dried according to the standard pro-
˚
cedure and stored over 4A molecular sieve. All the reagent-grade chemicals were pur-
chased from Sigma-Aldrich CO. LLC./Alfa Aesar-A Johnson Matthey Company (South
Korea) and used without further purification. Melting points were determined on a Mel-
Temp IA9200 digital melting point apparatus in a glass capillary and were uncor-
®
rected. All synthesized compounds were routinely characterized by TLC and NMR. TLC
was performed on aluminum-backed silica gel plates (Merck DC. Alufolien Kieselgel 60
F254).
1H/13C NMR, IR, and high resolution mass spectroscopy
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Brucker AM-
400 spectrometer operating at frequencies of 400 MHz for proton 100 MHz for carbon in
CDCl3. Proton chemical shifts (δ) are relative to tetramethylsilane (TMS, δ = 0) as inter-
nal standard and expressed in parts per million. Spin multiplicities are given as s (singlet),
d (doublet), t (triplet), and m (multiplet) as well as b (broad). Coupling constants (J) are
given in Hertz. Infrared (IR) spectra were run in KBr on an FTS-175C IR spectrometer (υmax
in cm−1). High resolution mass spectra (HRMS) were measured on a Bruker autoflex X III
instrument.
UV-Vis, emission, and emission decay spectra
The absorption spectra were measured with a PERSEE TU-1900 and an Agilent 8453 spec-
trophotometer. Emission spectra were measured with Shimadzu RF-5301PC fluorescence
spectrophotometer. The solvents used in photochemical measurement were spectroscopic
grade and were purified by distillation. The stock solution of compounds (2 × 10−3 M)
was prepared in CH3CN, and a fixed amount of these concentrated solutions were added
to each experimental solution. All the experiments were done repeatedly, and reproducible
results were obtained. Prior to the spectroscopic measurements, solutions were deoxy-
genated by bubbling nitrogen through them. The solid emission spectra were measured
by attaching the solid samples to a black support on an Agilent 8453 spectrophotome-
ter. For the lifetime measurements, a FLS 980 fluorometer (Edinburgh Instruments Ltd.)

242
X. LI ET AL.
Scheme . Synthetic routes of ,,ꢀ,ꢀꢀ-tetra(-methylthienyl)bisthiophene (ST).
was used. Then the lifetime of the excited states was measured by the time-correlated sin-
gle photo counting method (detected at the peak of the PL) under the laser excitation at
405 nm.
Synthesis
The synthetic approach of 4,5,4ꢀ,5ꢀꢀ-tetra(5-methylthienyl)bisthiophene (ST) is shown in
Scheme 1. The synthesis of ST started from commercial available 2-methylthiophene.
Firstly, bromination of 2-methylthiophene proceeded conveniently when treated with NBS
[27–29]. A Grignard reagent was prepared by mixing 2-methylthiophene and magnesium
turnings. Then it was used immediately for coupling reaction with 2,3-dibromothiophene
under standard Kumada conditions and yielded 4. Stannium reagent was prepared by
treated 4 with n-BuLi and chlorotributyltin at 195K. Finally, the target compound ST
could be obtained by self-coupling of 5 using Pd(PPh3)4 (5 mol %) as catalyst, and
Et3N as base in THF. Although ST has been synthesized earlier by Suzuki coupling,
the Stannium reagent self-coupling is more efficient than that of Suzuki methodol-
ogy [28]. And the NMR data (1H and 13C) are well in agreement with the literature
report.
4,5,4ꢀ,5ꢀꢀ-tetra(5-methylthienyl)bisthiophene (ST)
In flame dried Schlenk flask (100 mL), Stannium reagent 5 (3.4g, 6 mmol), Pd(PPh3)4 (50 mg,
0.04 mmol), and DMF (20 mL) were charged under N2 atmosphere. The mixture was heated
to 100°C for 24h. After the starting materials were full reacted, as confirmed by TLC, the
mixture was cooled to room temperature. Then it was poured into distilled water 100 mL and
stirred for 30 min. The mixture was washed extracted with dichloromethane (35 mL × 3).
Then combined organic phase was washed with KF (1 mol/L). After drying over anhydrous
Mg2SO4, the solvent was evaporated in vacuum. The residue was loaded onto the silica gel
column chromatography. Pure sample was obtained with dichloromethane/petroleum ether
(1:6) as the eluent, affording orange powder 1.5 g (90%).
Mp: 123–125°C. IR (KBr, υ, cm−1): 514, 527, 797, 805, 823, 920, 1029, 1158, 1200,
1
1438, 1482, 1520. H NMR (400 MHz, CDCl3): δ (ppm): 7.15 (2H, s), 6.95 (2H, d, J =
4.0 Hz), 6.90 (2H, d, J = 4.0 Hz), 6.68 (4H, d, J = 4.0 Hz), 2.48 (6H, s), 2.47 (6H, s).
13C NMR (100 MHz, CDCl3): δ (ppm): 141.5, 140.3, 134.6, 134.4, 132.3, 130.7, 127.7,
126.7, 126.3, 125.5, 125.3, 15.4. HRMS: [M]+ C28H22S6 requires 550.0046; found [M]+
550.0050.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
243
Table . Optical data of ST in several solvents.
UV-Vis
Fluorescence
FWHMb
Stokes shift
kf ( s−) knr ( s−) ( ν cm−
λabs
λem
εmax
(×)
c
c
Solvent
(–/–a , nm)
(–/–a , nm) ꢀF τFLsol (ns)
(cm−
)
)
cyclohexane /
./.
./.
./.
./.
./.
./.
./.
/ .
/ .
/ .
/ .
/ .
/ .
/ .
.
.
.
.
.
.
.







.
.
.
.
.
.
.
.
.
.
.
.
.
.







toluene
THF
/
/
ethyl acetate /
dioxane /
acetronitrile /
methanol
/
a shoulder band;
bFull width at half maximum is abbreviated as FWHM.
c The rate constants of radiative (kf) and nonradiative (knr) deactivation were calculated from the measured fluorescence
quantum yield (ꢀf) and fluorescence lifetime (τ) according kf = ꢀf/τ and knr = (−ꢀf)/τ.
Result and discussion
The absorption and emission behavior has been investigated in THF previously, which was
reported in a communication without detailed presentation of its properties. Here, the behav-
ior of absorption and emission was reinvestigated in various solvents and film and solid state.
Due to the all-thiophene units configuration, ST exhibited excellent solubility in various sol-
vents, from weak polarity (cyclohexane) to methanol. The fundamental absorption/emission
peaks, absorption coefficient, quantum yields, and Stokes shifts of ST are collected in Table 1.
The UV-vis absorption and fluorescence emission spectra of ST dissolved in a selection of
solvents are shown in Figure 1. The absorption contours of ST are of similar shape with each
other and exhibits typical absorption feature of oligothiophen [27]. It should be noted that
the absorption of ST is slightly different from that of the literature report in THF. Intense
absorption bands were observed around 280–350 nm and a shoulder at the longer wavelength
side. There is no regular trend could be found about the absorption variation induced by the
change of solvent polarizability. The absorption longer than 480 nm is optically transparent.
The molar absorption coefficients (collected in Table 1) of the higher energy absorption band
Figure . Absorption spectra of ST in several solvents (. × − M).

244
X. LI ET AL.
Figure . Emission spectra of ST in several solvents (. × − M). Emission and excitation slit widths were
set to be  nm.
4
−1
−1
−1
−1
r
r
(ranging in 6.0–7.8 × 10 L mol cm ) is almost twice of the lower energy absorption band
4
r
r
(ranging in 3.1–3.5 × 10 L mol cm ). The high/low energy absorption bands could be
attributed to the S0→S1 transition with the bands on the high- and low-energy side corre-
sponding to 0–1 and 0–0 vibration transition, respectively.
Notably, ST is highly emissive in the studied organic media, film and in solid powder state.
Detailed emission properties are listed in Table 1. The emission spectra in several solvents are
presented in Figure 2. In different polarity solvents, the emission profiles of ST are of similar
shape. It is obvious that the emission profiles are composed of high- and low- energy bands
with the former stronger than the latter, suggesting that they should have a common excita-
tion pathway. The intense emission is situated between 469–475 nm, which leads to the blue
fluorescence color. Both the emission maximum and the shoulder bands (∼500 nm) are not
appreciable affected by solvent polarity with only several nanometers variation. The high- and
low-energy emission bands are the 0–0 and 0–1 vibration transitions of a vibronic progres-
sion. The fact that the 0–0 transition is the most intense bands suggests that the geometry of
the ground state and the excited state are quite similar. In solvents of low polarity (from cyclo-
hexane to THF), the low-energy emission band is clearly differentiated. With solvent polarity
increasing from medium (ethyl acetate/dioxane) to high (acetonitrile/methanol), the band
widths of 0–0 and 0–1 vibration transitions in emission increase step wisely and merge into a
single broad band. The variation of FWHM only ranged over 300 cm−1 (2785–3085 cm−1). It
should be noted that the Stokes shift ( ν), calculated from the wavelengths of maximum
intensity, range in 12252–12757 cm−1, which is larger than most of the emissive dyes. In
medium polarity solvents, ST exhibit higher quantum yield than those of low polarity sol-
vents. In high polarity, the faster internal conversion is enhanced, quenching the emission
(low quantum yield).
Another important feature of ST is the highly emissive character in film (5 wt.% in poly-
carbonate) and solid state. And the emission spectra are shown in Figure 3. In film emis-
sion, ST exhibit two emission bands, at 475 and 512 nm with the low energy band slightly
being weighted over the high energy band. Compared to those in solution, the emission of
low energy band slightly bathochromic shift (∼10 nm) due to the tighter packing in polymer
matrix. In film, the monomers contribute completely to the spectrum of dye film. However,

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
245
Figure . Emission spectra of ST in film at  wt.% in polycarbonate and in a solid. Emission and excitation
slit widths were set to be  nm.
the solid emission band shifts ∼10 nm toward longer wavelength side. The most intense emis-
sion band situate at 535 nm with an unnoticeable blue shoulder (476 nm). Both in polymer
matrix and in solid, the twisted molecular conformation avoids the π-π stacking of adjacent
molecules, which is likely to responsible for the quenching of the emission in the solid state
for most aromatic fluorphores [30–32]. Obviously, the most intense band of solid emission
is induced by aggregation. Due to the enhanced 0–1 vibration transition in film, the emis-
sion bandwidth is significantly broadened (FWHM: 4234 cm−1). While in solid emission, the
weakly contribution of monomer, as confirmed by the blue shoulder emission, leads to nearly
half of the film emission bandwidth (FWHM: 2149 cm−1). Due to the red-shift of solid emis-
sion, the Stokes shift is broadened further to 14606 cm−1.
The emission color changes in solution/film/solid were characterized by CIE coordinates
(Figure 4). In solution, the emission color of ST varied from light blue to light green due to
variation of the solvent polarity. The CIE coordinates of ST in solution scattered in a very lim-
ited range, which is in agreement with their emission profile in solution. In film, CIE coor-
dinates of ST moves to green region slightly due to the nearly equal intense of 0–0 and 0–
1 vibration transitions of a vibronic progression. While in solid, the CIE coordinates of ST
moves to green region, which results from the emission of significant aggregated state. The
fluorescence decays of ST collected at their respective emission maxima were fitted to a single
exponential, and the decay profile is shown in Figure 5. The quantum yield and fluorescence
life time increase with increasing solvent polarity. However, the quantum yield and fluores-
cence time decrease in high polarity solvent, which indicates non-emissive decay via inter-
molecular interaction, together with other nonradiative pathways, may act on effectively and
leads to a low quantum yield. In contrast, in solid state, the non-emissive decay is hindered to
a large extent due to irregular tightly packing, which leads to the highly solid emission.
To better understand the geometric, electronic, and optical properties of ST, a comprehen-
sive computational investigation was undertaken using Materials Studio software package. To
reduce the initial run times, the ground state energy-minimized structures were calculated
using DFT and the LDA/DN basis set. Further refinement and optimization of the structure

246
X. LI ET AL.
Figure . CIE  chromaticity diagram of the emissions of TN in solutions, film, and solid state.
were achieved using time-dependent DFT (TDDFT) with B3LYP/DND basis set, as imple-
mented in the Dmol3 package [33, 34]. Figure 6 shows the pictorial presentation of HOMO-
1/HOMO/LUMO/LUMO + 1 with the distinct contributions of each atomic orbital. The
electron density is well-distributed over the whole molecule in HOMO/LUMO + 1. While
the electron density delocalized mainly to two arms in HOMO-1/LUMO. According to the
oscillator strength (f), the transition of HOMO-1→LUMO + 1 and HOMO→LUMO + 1 is
possible with the oscillator strength being 0.52 and 0.28, corresponding to 374 and 397 nm
absorption. It is close to the experimental value mentioned above (∼400 nm) and constitutes
the longest absorption. Due to the weak oscillator strength (0.04), the HOMO→LUMO tran-
sition contributes little to the absorption of ST. the molecule composed by six thiophene rings
Figure . Time-correlated single-photon counting fluorescence decay profile of ST in solution, film, and
solid. A -nm laser pulse was used as the excitation wavelength, and the emission was detected at the
respective emission maxima.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
247
Figure . Electron density distributions and energies of the frontier orbitals of ST.
with single bonding connection, resulting a molecule that has an inversion center (C2h sym-
metry). The molecular orbital (MO) topology indicates that the identical four arms should be
twisted to a certain degree. It must lead to more than one transition between frontier orbital
being involved. In weak polarity solvent (hexane), the dipole moment difference between the
excited and the ground state is more similar to the calculation in vacuum. While in high polar
solvent, the excited state is easy to be stabilized by the polar solvent molecule and lower the
dipole moment. This will lead to quench of fluorescence emission.
Conclusions
In summary, more detailed absorption and emission behavior of a sexthiophene molecule in
solution, solid, and in polycarbonate film were presented. In solution, bright blue emission
was observed with the quantum yield varying from 0.25 to 0.45. In addition, it exhibits a very
large Stokes shift in solution, ranging from 12252 to12757 cm−1. The twisted molecular con-
formation avoids the regular packing in aggregate state and leads highly emissive in solid state.
Therefore, the emission with 5 wt.% in polycarbonate does not lead aggregate-induced emis-
sion peak. Obviously, the aggregate-induced emission in solid will be moved from high energy
emission to low energy emission. Such significantly structure-related emission properties are
remarkable and hold promise for deep implementation in future applications. Further inves-
tigation of their application in optoelectronic devices is current underway in our laboratory.
Funding
This work was supported by the National Natural Science Foundation of China (21272060), Univer-
sity Student Renovation Project (Grant no. 201510476060), and the Program for Innovative Research
Team in Science and Technology in University of Henan Province (15IRTSTHN003). This study was
supported by the Basic Science Research Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Science, ICT and Future Planning (Grant No. 2015021972).

248
X. LI ET AL.
References
[1] Sethan, S. M., & Shah, N. M. (1945). Chem. Rev., 36, 1.
[2] Banerjee, S., Veale, E. B., Phelan, C. M., Murphy, S. A., Tocci, G. M., Gillespie, L. J., Frimannsson,
D. O., Kelly, J. M., & Gunnlaugsson, T. (2013). Chem. Soc. Rev., 42, 1601.
[3] Li, X., Zhou, Y., Zhang, Y., & Son, Y. -A. (2015). Mol. Cryst. Liq. Cryst., 622, 84.
[4] Li, X., Zhao, N., Yu, L., & Son, Y. -A. (2015). Mol. Cryst. Liq. Cryst., 608, 273.
[5] Li, X., & Son, Y. -A. (2015). J. Nanosci. Nanotechnol., 15, 5370.
[6] Chen, C.-T. (2004). Chem. Mater., 16, 4389.
[7] Li, X., Zhao, N., Yu, L., & Son, Y. -A. (2015). Mol. Cryst. Liq. Cryst., 597, 65.
[8] Li, X., & Son, Y. -A. (2014). Mol. Cryst. Liq. Cryst., 601, 182.
[9] Li, X., Zhao, N., Yu, L., & Son, Y.-A. (2015). Mol. Cryst. Liq. Cryst., 600, 14.
[10] Würthner, F., Saha-Möller, C. R., Fimmel, B., Ogi, S., Leowanawat, P., & Schmidt, D. (2016). Chem.
Rev., 116, 962.
[11] Beija, M., Afonso, C. A. M., & Martinho, J. M. G. (2009). Chem. Soc. Rev., 38, 2410.
[12] Loudet, A., & Burgess, K. (2007). Chem. Rev., 107, 4891.
[13] Ulrich, G., Ziessel, R., & Harriman, A. (2008). Angew Chem. Int. Ed., 47, 1184.
[14] Frath, D., Massue, J., Ulrich, G., & Ziessel, R. (2014). Angew Chem. Int. Ed., 537, 2290.
[15] Li, X., & Son, Y. -A. (2014). Dyes Pigments, 107, 182.
[16] Li, X., Ji, G., & Son, Y. -A. (2016). Dyes Pigments, 124, 232.
[17] Yang, J., Yang, D., & Jones, T. S. (2015). Chem. Rev., 115, 5570.
[18] Li, G., Zhao, Y., Li, J., Cao, J., Zhu, J., Sun, X. W., & Zhang, Q. (2015). J. Org. Chem., 80, 196.
[19] Liao, Z., Hooley, E. N., Chen, L., Stappert, S., Müllen, K., & Vosch, T. (2013). J. Am. Chem. Soc.,
135, 19180.
[20] Abendroth, J. M., Bushuyev, O. S., Weiss, P. S., & Barrett, C. J. (2015). ACS Nano., 9, 7746.
[21] Chen, X., Pradhan, T., Wang, F., Kim, J. S., & Yoon, J. (2012). Chem. Rev., 112, 1910.
[22] Schiphorst, J., Kendhale, A. M., Debije, M. G., Menelaou, C., Herz, L. M., & Schenning, A. P. H. J.
(2014). Chem. Mater., 26, 3876.
[23] Qu, D. H., Wang, Q. C., Zhang, Q. W., Ma, X., & Tian, H. (2015). Chem. Rev., 115, 7543.
[24] Tian, H. (2010). Angew Chem. Int. Ed., 49, 4710.
[25] Li, H., & Qu, D. -H. (2015). Sci. China Chem., 58, 916.
[26] Xia, C., Fan, X., Locklin, J., Advincula, R. C., Gies, A., & Nonidez, W. (2004). J. Am. Chem. Soc.,
126, 8735.
[27] Xia, C., Fan, X., Locklin, J., & Advincula, R. C. (2002). Org. Lett., 4, 2067.
[28] Saini, G., Lucas, N. T., & Jacob, J. (2010). Tetrahedron Lett., 51, 2956.
[29] Li, X., & Tian, H. (2005). Tetrahedron Lett., 46, 5409.
[30] Ooyama, Y., & Yoshida, K. (2005). New J. Chem., 29, 1204.
[31] De Halleux, V., Calbert, J. -P., Brocorens, P., Cornil, J., Declercq, J. -P., Brédas, J. -L., & Geerts, Y.
(2004). Adv. Funct. Mater., 14, 649.
[32] Mizukami, S., Houjou, H., Sugaya, K., Koyama, E., Tokuhisa, H., Sasaki, T., & Kanesato, M. (2005).
Chem. Mater., 17, 50.
[33] Delley, B. (1990). J. Chem. Phys., 92, 508.
[34] Delley, B. (2000). J. Chem. Phys., 113, 7756.