Optical and conformational studies on benzobisthiazole derivatives

Article abstract of DOI:10.1021/jp912018h

2,6-Didodecyl-4,8-diphenyl-benzo[1,2-d;4,5-d′]bisthiazole(3) and 2,6-didodecyl-4,8-dipyrrole-2-yl-benzo[1,2-d;4,5-d′]bisthiazole (5) were synthesized, and their optical properties were investigated in solution and in the solid state. Compounds 3 and 5 wer

Full text of DOI:10.1021/jp912018h

J. Phys. Chem. B 2010, 114, 3791–3798
3791
Optical and Conformational Studies on Benzobisthiazole Derivatives
Jun-Gill Kang,*,† Hyun-Joon Kim,† Yong-Kwang Jeong,† Min-Kook Nah,† Changmoon Park,†
Young Ju Bae,‡ Sang Woo Lee,‡ and In Tae Kim*,‡
Department of Chemistry, Chungnam National UniVersity, Daejeon 305-764, Republic of Korea, and Department of
Chemistry, Kwangwoon UniVersity, 447-1 Wolgye-Dong, Nowon-Ku, Seoul 139-701, Republic of Korea
ReceiVed: December 20, 2009; ReVised Manuscript ReceiVed: February 1, 2010
2,6-Didodecyl-4,8-diphenyl-benzo[1,2-d;4,5-d′]bisthiazole (3) and 2,6-didodecyl-4,8-dipyrrole-2-yl-benzo[1,2-
d;4,5-d′]bisthiazole (5) were synthesized, and their optical properties were investigated in solution and in the
solid state. Compounds 3 and 5 were excited with the 325 nm He-Cd laser line to produce blue and green
luminescence, respectively. The luminescence of 5 (Φ ) 14%) was more efficient than that of 3 (Φ ) 5%).
Structural and optical properties were further determined with DFT and ZINDO calculations. The planar
structure of 5 results in π f π* electronic transitions from the pyrrole moiety to the benzobisthiazole frame,
while the twisted geometry of 3 results in luminescence strongly associated with the π f π* transitions
within the benzothiazole frame. The effect of solvent on the luminescence properties of 5 is summarized as
competition between intra- and intermolecular NH · · · N hydrogen bonds.
Introduction
CHART 1: Molecular Structures of 3a, 5a, and 6
Recently, benzobisthiazole derivatives have attracted great
attention because of their conduction properties,1–4 photochromic
properties,5,6 and electroluminescence properties.7,8 However,
most of the research to date has focused on the derivatives in
which electron-rich groups are substituted at the 2 and 6
positions. Until now, the structural and optical properties of 4,8-
disubstituted benzobisthiazole derivatives have not been re-
ported. Recently, we reported the successful synthesis and
structural and optical properties of 2,6-dihexyl-4,8-dithiophene-
2-yl-benzo[1,2-d;4,5-d′]bisthiazole (6, Chart 1) and its copol-
ymer.9,10 The monomer 6, when excited in UV, produced a blue
emission when dissolved in CHCl3 and a green emission from
the solid state. Under the same conditions, its copolymer
produced a yellow emission when dissolved in CHCl3 and a
red emission from a thin solid film. The large red shift in the
emission from the copolymer could be due to the extension of
the π-conjugation. X-ray crystallographic data of the monomer
have indicated that the observed dihedral angle between the
benzobisthiazole frame and the thiophene moiety was only 21°,
suggesting that the crystal stacking and the electronic resonance
effects were sufficient to overcome any steric hindrance: the
Ortep view of 6 is given in Figure S1 (in Supporting Informa-
tion). Some debate, however, exists as to the optical processes
and their relationship to different structural conformations for
the benzobisthiazole derivatives in which electron-rich groups
are substituted at the 4 and 8 positions. To elucidate the
photophysical process of 4,8-disubstituted benzobisthiazole
derivatives, we also synthesized 2,6-didodecyl-4,8-diphenyl-
benzo[1,2-d;4,5-d′]bisthiazole (3) and 2,6-didodecyl-4,8-dipyr-
role-2-yl-benzo[1,2-d;4,5-d′]bisthiazole (5) using the Stille
reaction, and investigated their structural and optical properties
in solution and the solid state. Complementing the experimental
study, density function theory (DFT) and ZINDO calculations
were also performed to determine the configurations of molec-
ular orbitals and transition probabilities for excited states based
on optimized structures. The electronic structures of these 4,8-
bisubstituted benzobisthiazole derivatives were clearly estab-
lished, and a model for the absorbing and emitting levels was
presented. This work has relevance in delineating correlations
between molecular structure and observed photophysical proper-
ties of related 4,8-disubstituted benzobisthiazole derivatives.
Experimental Section
Synthesis. Stille Reaction. All reactions were carried out in
dry nitrogen. DMF was dried over CaH2 and distilled just before
1
use. H NMR spectra were recorded on a 400 MHz JEOL
instrument, and 13C NMR spectra were recorded on a 100 MHz
JEOL instrument. A solution of compound 110 (0.002 mol) in
dried DMF was purged with N2 for 5 min and heated at 30-50
°C to yield a transparent solution. Compound 2 (0.0045 mol)
and Pd(PPh3)4 (0.01 mmol) were added to the solution, and the
mixture was stirred at room temperature for 5 min, heated
quickly to 120 °C, and held at this temperature for 24 h. The
progress of the reaction was monitored by thin layer chroma-
tography(TLC) check and by observing color changes in the
reaction mixture. After the reaction, the mixture was cooled to
room temperature, and solvent was evaporated. The residue was
dissolved in 100 mL of dichloromethane and washed with 100
mL of distilled water (five times). The organic layer was
separated and dried over anhydrous magnesium sulfate. The
solvent was evaporated, and the residue was separated on a
* Corresponding authors. Fax: +81-42-821-6548 (J.G.K.); +82-2-909-
1978 (I.T.K.). E-mail: jgkang@cnu.ac.kr (J.G.K.); itkim@kw.ac.kr (I.T.K.).
† Chungnam National University.
‡ Kwangwoon University.
10.1021/jp912018h  2010 American Chemical Society
Published on Web 02/26/2010

3792 J. Phys. Chem. B, Vol. 114, No. 11, 2010
Kang et al.
SCHEME 1
neutral silica gel column eluted with dichloromethane/hexane
or ethyl acetate/hexane. In particular, we could only measure
1H NMR spectrum of compound 4 because of its low stablility.
Compound 3 was obtained as a white solid in 82% yield.
1H NMR (CDCl3): δ 7.86-7.84 (m, 4H), 7.57-7.53 (m, 4H),
7.49-7.44 (m, 2H), 3.08-3.04 (t, 4H, J ) 8.0 Hz), 1.85-1.78
(quin, 4H, J ) 8.0 Hz), 1.41-1.24 (m, 36H), 0.89-0.85 (t,
6H, J ) 8.0 Hz). 13C NMR (CDCl3): δ 172.81, 148.41, 138.62,
135.76, 129.77, 128.43, 128.24, 128.03, 34.69, 31.47, 29.59,
28.84, 22.49, 14.01. Anal. Found: C, 77.61; H, 9.02; N, 4.33;
S 9.25. Calcd for C44H60N2S2: C, 77.58; H, 8.80; N, 4.11; S,
9.41. mp ) 123 °C.
Compound 4 was obtained as a yellow solid in 60% yield.
1H NMR (CDCl3): δ 7.09-7.08 (d, 2H), 7.03-7.02 (t, 2H),
6.47-6.45 (d, 2H), 3.26-2.22 (t, 2H), 2.03-1.96 (m, 4H),
1.53-1.50 (s, 18H), 1.43-1.26 (m, 36H), 0.90-0.86 (t, 6H).
RemoWal of the Boc Group. Compound 4 (6.88 mmol) was
dissolved in 50 mL of ethanol. A solution of freshly prepared
sodium ethoxide (prepared by reacting 950 mg of Na (41 mmol)
in 20 mL of ethanol) was added, and the reaction mixture was
refluxed over 3 h. The solvent was evaporated, and the residue
was treated with water (100 mL) and extracted with CH2Cl2
(50 mL). The organic extracts were dried over anhydrous
MgSO4, the solvent was evaporated, and the residue was purified
on a silica gel column with a CH2Cl2/Hexane (1:9) eluent.
Compound 5 was obtained in 75% yield.
an ARC 0.5 m Czerny-Turner monochromator equipped with
a cooled Hamamatsu R-933-14 photomultiplier tube. Solution
phase spectra of each compound were dissolved in CH2Cl2 at a
concentration of 5.0 × 10-4 M.
The relative quantum yield of the visible luminescence for
each sample (Φs) was determined by a relative comparison
against a reference of known quantum yield (quinine sulfate in
diluted H2SO4 solution, Φr ) 0.546). The general equation used
in the determination of relative quantum yield is given as
follows:11
Ar(λr) I(λr) ns2 Ds
Φs ) Φr
(
)(
)
2
( )
( )
As(λs) I(λs)
Dr
nr
In the equation, A(λ) is the absorbance, I(λ) is the relative
intensity of excitation light at wavelength λ, n is the average
refractive index of the solvent, and D is the integrated area under
the corrected emission spectrum. The recorded spectra for the
quantum yield were corrected for the spectral response of the
system using an Oriel 45 W quartz tungsten halogen lamp
standard.
Molecular Modeling. 2,6-Hexyl-4,8-diphenyl-benzo[1,2-
d;4,5-d′]bisthiazole (3a) and 2,6-hexyl-4,8-dipyrrole-2-yl-
benzo[1,2-d;4,5-d′]bisthiazole (5a) (Chart 1) were chosen as
model molecules for 3 and 5, respectively, for the comparison
with the X-ray crystallographic data for 6. Molecular geometries
were optimized at the DFT/B3LYP/6-31g(d,p) level using
Gaussian 03.12 On the basis of these optimized geometries,
rotational barriers between the mainframe and the 2,6-substituted
moiety were calculated by changing the dihedral angle between
the two components. Configuration interaction singles (CIS)
calculations were then performed to determine electronic
structures and electronic transitions using semiempirical (ZIN-
DO) method.
1H NMR (DMSO): δ 7.01(s, 2H), 7.02-7.03 (d, 2H),
6.45-6.46 (s, 2H), 3.27-3.23 (t, 4H, J ) 8.0 Hz), 2.03-1,96
(m, 4H, J ) 8.0 Hz), 1.41-1.26 (m, 36H, J ) 8.0 Hz),
0.89-0.86 (t, 6H, J ) 8.0 Hz). 13C NMR (DMSO): δ 172.11,
146.15, 129.24, 128.82, 119.60, 116.85, 109.72, 109.59, 34.38,
31.57, 29.49, 28.95, 28.88, 22.56, 14.07. Anal. Found: C, 73.18;
H 9.06; N, 8.63; S, 9.47. Calcd for C40H58N4S2: C, 72.89; H,
8.80; N, 8.49; S, 9.72. mp ) 135 °C (Scheme 1).
Optical Measurements. Absorption spectra of 3 and 5
dissolved in CH2Cl2 were recorded on a Hitach U-4100 UV-vis
spectrophotometer. For luminescence and excitation spectra
measurements, powdered samples were placed on the coldfinger
of an Oxford CF-1104 cryostat using silicon grease. Excited
light from either a He-Cd laser or an Oriel 1000 W Xe arc
lamp passed through an Oriel MS257 monochromator and was
focused on the sample. Luminescence and excitation spectra
were measured at 90° relative to the excitation beam path with
Results and Discussion
Absorption. Figure 1 shows the absorption spectra of 3 and
5 dissolved in CH2Cl2. The absorption spectrum of 3 exhibits
three bands at 343, 309, and 276 nm. Hereafter, these bands
are referred to as the A-, B-, and C-absorption bands, in order
of increasing energy. The absorption spectra of dissolved hexyl-

Benzobisthiazole Derivatives
J. Phys. Chem. B, Vol. 114, No. 11, 2010 3793
Figure 1. Absorption spectra of 3 (lower) and 5 (upper) dissolved in
CH2Cl2.
substituted 3a and 5a were identical to those of 3 and 5. The
absorption spectrum of pyrrole-substituted 5 was very different
from that of the phenyl-substituted 3. For 5, three characteristic
absorption bands appear in the 250-450 nm region; hereafter,
these bands are referred to as the A-, B- and C-absorption bands,
in order of increasing energy. The A-absorption band, with a
barycenter at 411 nm, was very broad and consisted of at least
three Gaussian components with peaks at 425, 401, and 390
nm. The B-absorption comprised two components with peaks
at 352 and 337 nm. The C-absorption, which peaked at 295
nm, was accompanied by two subpeaks at 285 and 275 nm.
The band shape and the intensity of the C-band were very similar
to that of thiophene-substituted 6. The C-absorption can
therefore be attributed to electronic transitions within the
benzothiazole frame. These results indicate that the A-band of
5 is associated with the substituted pyrrole moiety.
Luminescence and Excitation. Photoluminescence (PL),
excitation spectra, and the quantum yields of 3 and 5 were
measured in CH2Cl2. As shown in Figure 2(a), the PL spectrum
of 3 peaked at 404 nm and exhibited a narrow bandwidth with
a full width at half-maximum (fwhm) of 2590 cm-1. The
excitation spectra of both the 400 and 420 nm emissions spanned
over the 250-370 nm region. The peak position of the excitation
spectrum (348 nm) coincided with that of the A-band. The
Stokes’ shift is approximately 4010 cm-1. The narrow fwhm
and the large Stokes’ shift suggest that the emitting state may
differ from the absorbing state. Figure 2b shows the PL and
the excitation spectra of 5 dissolved in CH2Cl2. Compared with
that of 3, the PL spectrum of 5 was broad (fwhm ) 3390 cm-1)
and exhibited a doublet structure consisting of a high-energy
(HE) emission at 454 nm and a low-energy (LE) emission at
489 nm. The PL quantum yield of 5 (Φ ) 14%) was much
higher than that of 3 (Φ ) 5%). We also measured the excitation
spectra of the HE and LE emissions. The two excitation spectra
differed slightly, as indicated in Figure 2b. The 450 nm
excitation spectrum could be resolved into three characteristic
regions: 370-440, 320-370, and 250-320 nm. The excitation
spectrum of the HE emission resembled the absorption spectrum.
The peak position of the excitation spectrum corresponds to
the center of the A-band. However, the excitation spectrum of
the LE emission (λems ) 510 nm) peaked at 460 nm.
Figure 2. PL and excitation spectra of 3 (a) and 5 (b) dissolved in
CH2Cl2 (5.0 × 10-4 M).
of 5 was very concentration dependent, as shown in Figure 3.
The results for these band analyses are listed in Table S1 in the
Supporting Information. At 5 × 10-6 M, the oscillator strength
(OS) of the LE emission was almost 5 times larger than that of
the HE emission. The OS of both the LE and the HE emissions
increased gradually with concentration, and the ratio of the two
emissions was approximately invariant up to 1 × 10-4 M. Above
this concentration, the relative contribution of the HE emission
increased markedly. The emission color-change response of 5
is visually described in Figure 4: a concentrated solution
displayed a strong blue emission and a diluted solution emitted
a strong green light under 366 nm UV light.
This contrasts with the anticipated effects of π-π stacking,
which typically enhances the degree of LE emission with
increasing compound concentration. Concentration effects were
also evaluated in CCl4, (cis-CHCl)2, and THF. Figure 3 shows
that the concentration dependence of 5 in (cis-CHCl)2 was very
similar to that in CH2Cl2. In CCl4 and THF, however, the HE
and LE emissions were roughly equivalent at low concentration,
and the intensity ratio of the two bands was almost independent
of solution concentration. Solvent effects on luminescence
properties have been well observed for intramolecular proton
transfer (ESIPT) reactions, depending on the polarity of
solvent.13–16 Among the solvents evaluated, THF had the highest
polarity, whereas CCl4 was nonpolar. These results indicate that
the observed solvent effects may be associated more with the
partial charge of the hydrogen atom on the solvent molecule
than with the polarity of the solvent. In both CH2Cl2 and (cis-
CHCl)2, the highly electronegative chlorine atom induces a
partial positive charge on the hydrogen atom, whereas the partial
charge of the hydrogen atom in THF is very low, and CCl4 has
no hydrogen atom.
The doublet structure of the A-band emission from 5 may
be attributed to π-π stacking of the benzobisthiazole frame
and the pyrrole moieties to form a planar structure. The PL
properties of 3 and 5 dissolved in CH2Cl2 as a function of
concentration show that the spectral shape of 3 was invariant
as a function of concentration. Conversely, the spectral shape

3794 J. Phys. Chem. B, Vol. 114, No. 11, 2010
Kang et al.
Figure 3. PL spectra of 5 in CH2Cl2 (a), CCl4 (b), (cis-CHCl)2 (c), and THF (d) at various concentrations (1. 5.0 × 10-6, 2. 1.0 × 10-5, 3. 5.0 ×
10-5, 4. 1.0 × 10-4, 5. 5.0 × 10-4, 6. 1.0 × 10-3 M.
benzothiazole frame. In the oligomer, the pyrrole moiety may
be twisted toward the benzothiazole frame to minimize the steric
hindrance between the two adjacent frames. Solvents with no
H atoms (CCl4) or weakly charged H atoms (THF) do not affect
hydrogen bonding. However, solvents such as CH2Cl2 and (cis-
CHCl)2, which contain highly electropositive H atoms, may form
C-H· · ·N hydrogen bonds between the solvent and the molecule
5. At low concentrations, this could reduce the probability of
oligomer formation. At high concentrations, the frequency of
interactions between individual molecules of 5 increases, and
the oligomeric configuration becomes more favorable. The
doublet structure of the A-band emission from 5 in solution is
attributed to the intramolecular and the intermolecular hydrogen
bond formation appearing as the LE and the HE bands,
respectively.
The luminescence spectra of 3 and 5 were also measured in the
solid state at T ) 10 K and room temperature (RT). Figure 5 shows
that powdered 3, when excited at 340 nm, produced a primary
luminescence spanning from 350 to 540 nm, accompanied by a
weak band from 540 to 650 nm. This weak band appeared only at
low temperature. The overall shape of the solid-state spectra at
RT was similar to that in solution. At 10 K, a vibronic distortion
was evident in the solid state, with an average peak spacing of
Figure 4. Digital photograph of CH2Cl2 solution of sample 5 under
1480 cm-1. This suggests a vibronic distortion associated with the
366 nm UV lamp: (a) 1.0 × 10-3 M and (b) 5.0 × 10-6 M.
C-C stretching of the phenyl group. The excitation spectra of the
440 nm and the 560 nm emissions at 10 K are shown in Figure 5.
Both spectra span from 250 to 420 nm, with a strong peak in the
low-energy region. The high-energy excitation band between 250
and 360 nm was identical to the excitation spectrum obtained in
solution. The low-energy peak, which was not observed in solution,
overlaps with the emission spectrum. It suggests that the absorbing
center responsible for the low-energy peak may be identical to the
Compound 5 may adopt either a monomeric or oligomeric
configuration in solution via the N-H· · ·N intra- or intermo-
lecular hydrogen bond formation, respectively, between the
pyrrole moiety and the benzothiazole frame. Intramolecular
hydrogen bonding in the monomer results in a planar geometry
and allows π conjugation over the pyrrole moieties and the

Benzobisthiazole Derivatives
J. Phys. Chem. B, Vol. 114, No. 11, 2010 3795
Figure 7. Rotational energy barrier between the center and two
neighboring phenyl (square) and pyrrole (circle).
Figure 5. Emission and excitation spectra of 3 (Ems: λexn ) 340 nm;
Exn: λems ) 1.440, 2.560 nm at 10 K).
TABLE 1: Barycenter (nm) and Molar Absorbance of
Absorption Bands Dissolved in CH2Cl2
λabs (ε/1 × 104,
compound
assignment
CH2Cl2)
3
343 (0.9)
309 (1.9)
276 (3.2)
411 (3.6)
352 (2.6)
337 (1.5)
295 (6.4)
π f π* (bisbenzobisthiazole)
π f π* (bisbenzobisthiazole)
π f π* (bisbenzobisthiazole)
π (pyrrole) f π* (bisbenzobisthiazole)
π f π* (bisbenzobisthiazole)
π f π* (bisbenzobisthiazole)
π (pyrrole) f π* (bisbenzobisthiazole)
5
were optimized at the level of DFT/B3LYP/6-31g(d,p). An Ortep
view of 6 is shown in Figure S1 (Supporting Information), and
optimized molecular geometries of 3a and 5a are shown in Figure
S2 (Supporting Information). Selected geometric parameters and
a comparison with empirical X-ray data of 6 are listed Table S2
(Supporting Information). The calculated values pertaining to the
benzobisthiazole frames of 3a and 5a differ from those of 6 by
less than 1%. The absorption and luminescence spectra of 3 and
5, shown above, suggest that the 4,8-disubstituted phenyl or pyrrole
moieties play a key role in determining the optical properties. The
dihedral angles (φ) between the benzobisthiazole frame and the
4,8-disubstituted phenyl or pyrrole moieties determine the degree
of π-orbital delocalization. Starting from the optimized conforma-
tions, φ between the benzobisthiazole frame and the substituted
moiety on one side and -φ for the other part were rotated from 0°
to 180° in 10° increments. As shown in Figure 7, these calculations
indicate global minima of 50° for 3a and 0° for 5a. In 3a, the
substituted phenyl group is almost perpendicular to the mainframe,
indicating that the steric hindrance within the molecule is unfavor-
able for a planar structure. In contrast, 5a forms a planar structure
with a high degree of electronic resonance. The combination of
intrahydrogen bonding (dNH· · ·H ) 2.250 Å) between the NH group
of the pyrrole and the nitrogen atom of the benzobisthiazole frame
and the degree of π-resonance is sufficient to overcome unfavorable
steric hindrances between the benzobisthiazole frame and the 4,8-
disubstituted pyrrole moieties.
Figure 6. Emission and excitation spectra of 5 (Ems: λexn ) 380 nm;
Exn: λems ) 1.520 nm, 2.580 nm at 10 K).
emitting center. Furthermore, the excitation spectrum of the 560
nm emission shows that the low-energy emission between 540 and
650 nm is associated with the triplet state.
Figure 6 shows the luminescence spectra of powdered 5,
excited at 380 nm. As was the case for 3, the low-temperature
spectrum exhibits three well-resolved peaks at 500, 528, and
566 nm. The average spacing between these peaks in the solid-
state spectrum was 1120 cm-1, which is much narrower than
the spacing (1655 cm-1) between the corresponding doublet
observed in emission from CH2Cl2 solution. A strong and unique
vibrational band at 1120 cm-1 appears in the IR spectrum of 5,
which suggests that a multicomponent structure may arise from
vibronic distortions by the pyrrole ring. With increasing
temperature, the intensity of the luminescence and the degree
of vibronic distortion decreased. Figure 6 also shows that at 10
K, the excitation spectrum of the 520 nm emission was very
broad, with a peak at 434 nm. Although the peak position of
this excitation band was almost the same as that observed in
solution, the barycenter of the emission in the solid state was
red-shifted by approximately 50 nm relative to that in solution.
These results suggest that the observed emission from the solid
state corresponds to the low-energy emission from the solution
state, which would indicate that intrahydrogen bonding was
predominant in the solid state. At 10 K, the excitation spectrum
of the 580 nm emission exhibited a sharp peak at 516 nm,
corresponding to the zero-phone transition, which can be
correlated with the emission at 566 nm.
By use of the optimized geometries of the molecules 3a and
5a, the electronic structures and the electronic transitions were
calculated at the level of ZINDO. The results of the molecular
orbital (MO) calculations for 3a are listed in Table 2. The two
highest-occupied molecular orbitals (HOMOs), h1 and h2, were
composed of a linear combination of the px and py orbitals of C
atoms in the benzobisthiazole frame. Note that the molecular
axis is represented by the z-axis. The contributions from the
π-electron systems of the benzobisthiazole frame were 77 and
83%, respectively. The next three HOMOs, h3-h5, correspond
Structural Calculations and Assignments. Quantum me-
chanical calculations were performed to characterize the excited
states and electronic transitions of 3 and 5. Molecular geometries

3796 J. Phys. Chem. B, Vol. 114, No. 11, 2010
Kang et al.
TABLE 2: Some DFT Molecular Orbitals Calculated for 3a
main frame
MO
hartree
8C
2N
2S
phenyl 12C
hexyl 12C
l12
l11
l10
l9
l8
l7
l6
l5
l4
l3
0.099
0.085
0.081
0.078
0.067
0.045
0.033
0.031
0.030
68
46
41
71
73
51
23
0
0
0
3
3
25
4
14
0
0
20
0
9
9
0
0
0
0
8
55
58
19
24
20
14
0
0
0
0
0
0
0
0
0
8
0
0
0
13
3
0
9
2
46
63
100
100
35
30
0
0
28
30
4
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.023
0.011
46
70
91
77
83
6
0
0
24
17
25
l2
l1
-0.007
-0.273
-0.294
-0.318
-0.329
-0.330
-0.337
-0.377
-0.383
h1
h2
h3
h4
h5
h6
h7
h8
14
9
94
100
100
56
8
12
6
4
0
0
0
TABLE 3: CIS Excited States and Predominant Transitions
of 3a
excited
state
oscillator
strength
nm
predominant transition
11Au
337
333
292
284
277
257
241
238
231
225
223
221
215
210
208
206
0.065
0.421
0.941
0.104
0.017
0.462
0.033
0.053
0.096
0.137
0.217
0.271
0.103
0.270
0.331
0.238
h1 f l2, h1, h2 f l1
h1 f l1, h2 f l1
h2 f l1, h1 f l2
h7 f l1
h1 f l5, h3 f l4
h2 f l2
h1 f l9,l7
h1 f l9
h1 f l7
h2 f l9
h1 f l5
h1 f l8
h6 f l1, h1, h2 f l8
h2 f l5, h6 f l1
h2 f l7, h2 f l5, h1 f l8
h6 f l2
21Au
31Au
41Au
51Au
61Au
71Au
81Au
91Au
101Au
111Au
121Au
131Au
141Au
151Au
161Au
to the π-orbitals of the phenyl groups with a contribution of
more than 94%. The lowest unoccupied molecular orbital
(LUMO), l1, is the π-antibonding orbital of the carbon atoms
in the benzobisthiazole frame with a minor contribution from
nitrogen atoms. The contribution of the carbon atoms in the
benzobisthiazole frame is 91%. The next two LUMOs, l2 and
l3, comprise contributions from both the benzobisthiazole frame
and the phenyl groups. The contribution of the phenyl groups
to l2 and l3 is less than 35%. The next two LUMOs, l4 and l5,
correspond to the π-antibonding orbitals of the phenyl groups.
The calculated low-lying excited states and oscillator strengths
of 3a are listed in Table 3. Note that excitations from the X
ground state to all Ag excited states are forbidden under Ci
symmetry. Those transitions were excluded from Table 3.
According to the energy gap and the oscillator strength, the
excited states can be classified into three groups. The first two
Au states were classified into group A and predominantly arise
from the h1 f l2 and the h1 f l1 transitions, respectively. The
X f 1Au transition with f ) 0.065 is weak, whereas the X f
2Au transition with f ) 0.421 is predominant. The X f 2Au
transition is likely the main contributor to the A-band in the
absorption spectrum. Accordingly, the A-band can be attributed
to the π f π* transition of the benzobisthiazole frame. The
Figure 8. Contour plots of the molecular orbitals for 3a and 5a
involved in the A-band excitation and emission.
next three Au states, 3Au-5Au, can be included in group B.
The 3Au excited state occurs predominantly from the h2 f l1
and h1 f l2 transitions. The X f 3Au transition exhibited the
largest calculated oscillator strength, f ) 0.941 and can be
attributed to the B-band in the absorption spectrum. Table 3
shows several close-lying excited states. Among these, the X
f 6Au transition has the largest transition probability with f )
0.462. The 6Au excited state arises predominantly from the h2
f l2 transition. Therefore, the C-band in the absorption
spectrum is composed primarily of the π f π* transition of
the benzobisthiazole frame. The predominant orbital transitions
involved in the A-band excitation and emission are represented
in Figure 8, and the assignments of the absorption bands of 3
are listed in Table 1.
A characteristic feature of the A-band emission from 3 in
solution is that the excitation spectrum does not overlap with
the emission spectrum. The large Stokes’ shift of 4010 cm-1
suggests that the absorbing and the emitting centers differ. A

Benzobisthiazole Derivatives
J. Phys. Chem. B, Vol. 114, No. 11, 2010 3797
TABLE 4: Some DFT Molecular Orbitals Calculated for 5a
TABLE 5: CIS Excited States and Predominant Transitions
of 5a
main frame
2N
pyrrole
hexyl
12C
excited
state
oscillator
strength
MO
hartree
8C
2S
8C
2N
nm
predominant trans.
l11
l10
l9
l8
l7
l6
l5
l4
l3
0.097
0.084
0.083
0.078
0.075
0.070
0.055
0.047
0.019
19
5
0
0
3
0
0
4
21
7
26
3
4
0
0
4
0
0
8
79
77
49
0
0
21
20
14
0
0
0
3
3
0
0
0
2
0
0
22
9
9
82
95
3
0
8
0
0
0
0
0
5
2
13
0
3
3
0
0
0
29
37
13
0
3
0
0
0
28
32
5
0
0
0
0
0
0
0
0
0
0
0
0
11Au
401
370
321
293
278
261
258
249
245
230
228
217
216
215
212
209
0.802
0.098
0.841
0.012
0.101
0.0
0.021
0.058
0.019
0.01
0.126
0.006
0.001
0.695
0.229
0.015
h1 f l1
h1 f l2
h3 f l1
h8 f l3, h7 f l1
h3 f l2
h1 f l7
h1 f l5,l6
h1 f l5,l6
h2 f l3
21Au
45
42
73
29
65
11
66
79
79
36
7
94
3
0
25
12
11
25
31Au
41Au
51Au
62
12
68
5
12
15
64
93
0
68
63
30
0
61Au,
71Au
81Au
91Au
l2
l1
0.007
101Au
111Au
121Au
131Au
141Au
151Au
161Au
h3 f l7
-0.017
-0.245
-0.286
-0.291
-0.337
-0.339
-0.349
-0.369
-0.374
-0.396
h6 f l1, h1 f l6,l11
h2 f l3, h3 f l5
h1 f l9
h6 f l1, h4 f l1
h6 f l1, h1 f l11, h4 f l1
h1 f l9
h1
h2
h3
h4
h5
h6
h7
h8
h9
0
0
0
0
0
transitions indicates that these excited states are associated with
the π f π* transitions of the benzobisthiazole frame and the π
(pyrrole moieties) f π* (benzobisthiazole) transitions, respec-
tively. Accordingly, the C-band can be assigned as the combina-
tion of these two characters. The predominant orbital transitions
involved in the A-band excitation and emission from 5a are
represented in Figure 8, and the assignments of the absorption
bands are listed in Table 1. The 11Au excited state is responsible
for the absorbing and emitting center. Emission from 5 is
predominantly produced via A-band excitation.
26
possible pathway is as follows: the 21Au excited state gains
population through the A-band excitation and then transitions
nonradiatively from the 21Au state to the 11Au. The observed
blue emission then occurs from the 11Au state. In the solid state,
the A-band emission from 11Au state is achieved through direct
excitation mainly. Consequently, the overlap between the
excitation and the emission spectra is very large and results in
the phosphorescence at the low temperature via intersystem
crossing to the 13Au state.
Conclusion
The MO contributions of 5a are listed in Table 4. The h1
HOMO is composed of π-orbitals from the benzobisthiazole
frame and the pyrrole rings. The contribution of the two pyrrole
rings (64%) is much higher than that of the benzobisthiazole
frame (36%). The orbital nature of h1 of 5a is significantly
different from that of 3a. The next two HOMOs, h2 and h3,
correspond to the π-bonding orbitals of the pyrrole moieties
and the benzobisthiazole frame, respectively. The first two
LUMOs, l1 and l2, arise from the linear combinations of the px
and py orbitals of carbon atoms in the benzobisthiazole frame,
with a contribution of 79% each. The compositions of these
two orbitals of 5a are similar to those of 3a. Selected excited
states and the oscillator strengths of 5a are listed in Table 5.
According to the energy gaps and the oscillator strengths, the
1Au-2Au, 3Au-4Au, and 5Au-9Au excited states are respon-
sible for the A-, B-, and C-bands, respectively. In group A, the
two excited states arise from the transitions from the h1 HOMO
to the l1 and l2 LUMOs and are associated with the transition
from the π-system of the pyrrole rings to the π*-system of the
benzobisthiazole frame, respectively. The X f 1Au transition
has a large oscillator strength, with f ) 0.802. The π (pyrrole
moieties) f π* (benzobisthiazole) transitions may be respon-
sible for the A-band and are observed as a doublet with a strong
intensity in the low-energy region compared with the π f π*
transitions of the benzobisthiazole frame as observed with 3.
These transitions are made feasible by the planar structure of
5. The X f 3Au and 4Au transitions can be attributed the B1-
and B2-bands, respectively. The 3Au and 4Au excited states arise
from the h3 f l1 and the h8 f l3 transitions, respectively.
Accordingly, the B1- and B2-bands are associated with the π
f π* transitions of the benzobisthiazole frame. The transitions
in the C group may be classified into the X f 5Au, and the X
f 7Au and 8Au transitions. The predominance of these
The effects of π-electron-rich substituents on the optical
properties of benzobisthiazole derivatives 2,6-didodecyl-4,8-
diphenyl-benzo[1,2-d;4,5-d′]bisthiazole (3) and 2,6-didodecyl-
4,8-dipyrrole-2-yl-benzo[1,2-d;4,5-d′]bisthiazole (5) were evalu-
ated. DFT quantum mechanical calculations indicate that the
dihedral angle between the substituted ring and the benzo-
bisthiazole frame is approximately 50° and 0° for phenyl and
pyrrole substituents, respectively. Due to its nonplanar structure,
the HOMO and LUMO levels of 3 are composed entirely of
the π-character from the benzobisthiazole frame. The observed
blue luminescence is strongly associated with these two orbitals.
The planar structure of 5 allows the π-character of HOMO to
expand over all of the carbon atoms in the pyrrole rings,
constituting a 64% contribution, and over the benzobisthiazole
frame, for the remaining 36% contribution. The observed
greenish yellow luminescence of powdered 5 is strongly
associated with the electronic transitions between the pyrrole
ring and the benzobisthiazole frame.
Acknowledgment. This research was supported by National
Research Foundation (2009-0073199). J.-G.K. and Y.-K.J.
acknowledge the fellowships of the BK 21 program. Y.J.B.,
S.W.L., and I.T.K. acknowledge the technology development
project of new and renewable energies of the Ministry of
Knowledge Economy of the Republic of Korea and a research
grant of Kwangwoon University (2009).
Supporting Information Available: Ortep view (Figure S1)
and optimized (Figure S2), and concentration dependence of
oscillator strengths of the emission bands (Table S1) and
comparison of geometric parameters from optimization calcula-
tion (Table S2). This material is available free of charge via
the Internet at http://pubs.acs.org.

3798 J. Phys. Chem. B, Vol. 114, No. 11, 2010
Kang et al.
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